EPA
United States
Environmental Protection
Agency
Office of
Research and
Development
Industrial Environmental Research
Laboratory
Cincinnati, Ohio 40^68
EPA-600/7-77-062
June 1977
DEVELOPMENT STATUS AND
ENVIRONMENTAL HAZARDS
OF SEVERAL CANDIDATE
ADVANCED ENERGY SYSTEMS
Interagency
Energy-Environment-
Research and Development
Program Report
-------
RESEARCH REPORTING SERIES
Research reports of tie Office of Research and Development, U.S. Environmental
Protection Agency, have been grouped into nine series. These nine broad cate-
gories were established to facilitate further development and application of en-
vironmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The nine series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
6. Scientific and Technical Assessment Reports (STAR)
7. Interagency Energy-Environment Research and Development
8. "Special" Reports
9. Miscellaneous Reports
This report has been assigned to the INTERAGENCY ENERGY-ENVIRONMENT
RESEARCH AND DEVELOPMENT series. Reports in this series result from the
effort funded under the 17-agency Federal Energy/Environment Research and
Development Program. These studies relate to EPA's mission to protect the public
health and welfare from adverse effects of pollutants associated with energy sys-
tems. The goal of the Program is to assure the rapid development of domestic
energy supplies in an environmentally-compatible manner by providing the nec-
essary environmental data and control technology. Investigations include analy-
ses of the transport of energy-related pollutants and their health and ecological
effects; assessments of, and development of, control technologies for energy
systems; and integrated assessments of a wide range of energy-related environ-
mental issues.
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.
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EPA-600/7-T7-062
June 1977
DEVELOPMENT STATUS AID EJTV1RONMENTAL
HAZARDS OF SEVERAL CANDIDATE
ADVANCED ENERGY SYSTEMS
Morris M. Penny
Sidney V. Bourgeois
Lockheed Missiles & Space Company, Inc.
Huntsville Research & Engineering Center
Huntsville, Alabama 35807
Contract No. 68-02-1331
Task Order No. 8
EPA Project Officer
William Cain
Power Technology and Conservation Branch
Industrial Environmental Research Laboratory
Cincinnati, Ohio 1*5268
INDUSTRIAL ENVIRONMENTAL RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OHIO 1+5268
-------
DISCLAIMER
This report has been reviewed by the Industrial Environmental Research
Laboratory, Cincinnati, U.S. Environmental Protection Agency and approved
for publication. Approval does not signify that the contents necessarily
reflect the views and policies of the U.S. Environmental Protection Agency,
nor does mention of trade names or commercial products constitute endorsement
or recommendation for use.
ii
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FOREWORD
When energy and material resources are extracted, processed and used,
changes are produced in the existing environment that in many instances are
undesirable. These undesirable changes resulting from both substances and
effects comprise what we define as pollution. Pollution of air, land and water
may adversely affect our aesthetic and physical well being. Protection of our
environment requires that we recognize and understand the complex interaction
between our industrial society and our environment.
The Industrial Environmental Research Laboratory-Cincinnati (lERL-Ci)
assists in developing and demonstrating new and improved methodologies aimed
at minimizing, abating and preventing pollution from industrial and energy-
related activities.
Several advanced energy concepts are currently being considered as
future sources of energy. Concurrent development of environmental protection
measures and energy conversion technology will avoid possible delay of com-
mercialization. This document presents a review of the development status
and anticipated environmental hazards of several of these advanced energy
concepts.
David G. Stephan
Director
Industrial Environmental Research Laboratory
Cincinnati
iii
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ABSTRACT
The report gives a review of the development status of several advanced
energy concepts and discusses the primary environmental hazards of each
system.
Systems reviewed include potential new sources of energy and improved
energy conversion. Each system is evaluated with respect to its development
status, and estimates made as to when each will begin to contribute signi-
ficantly to U.S. energy needs. Appraisals were made of the environmental
impact of each system including assessment of the adequacy of pollution
control technology and potential gross ecological impact. The overall con-
clusion is that each energy system has a negligible or mild direct environ-
mental impact when compared with conventional fossil fuel and nuclear
systems, but that indirect impacts for some of the energy systems could be
severe and need further study to quantify their impact. Considering both
the expected environmental impact and period of technology break through/
commercialization, the following order of R&D priorities on the candidate
energy systems has been developed: high temperature turbines, ocean thermal
gradients, windmills, magnetohydrodynamics, metal vapor (potassium) Rankine
topping cycles, hydrogen fuel cells, thermionics, electrogasdynamics, and
thermoelectric conversion.
This report was submitted in fulfillment of Contract No. 68-02-1331 by
Lockheed-Huntsville Research and Engineering Center under the sponsorship of
the U.S. Environmental Protection Agency. This report covers the period
December 1975, to February 1976.
iv
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CONTENTS
Section Page
FOREWORD iii
1 INTRODUCTION 1
2 OVERVIEW 3
Z.I Development Status and Projected Environmental 3
Hazards
2.2 Federally Funded Research 6
2.3 Environmental R&D Recommendations 6
3 MAGNETOHYDRODYNAMICS 12
3.1 Description of the System 12
3.2 Efficiency 13
3.3 Size Limitation 13
3.4 Development Status 17
3.5 Anticipated Contribution to U.S. Energy Needs 18
3.6 Costs and Benefits 18
3.7 Appraisal of Environmental Aspects 19
3.8 MHD Research 19
4 HYDROGEN FUEL CELLS 22
4.1 Description of the System 22
4.2 Efficiency 24
4.3 Size Limitations 25
4.4 Development Status 25
4.5 Costs and Benefits 28
4.6 Anticipated Contribution to U.S. Energy Needs 29
4.7 Environmental Considerations 30
4.8 Research and Development Considerations 31
V
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Section Page
5 OCEAN THERMAL ENERGY CONVERSION 33
5.1 Description of the System 33
5.2 Efficiency 33
5.3 Size Limitations 35
5..4 Development Status 35
5.5 Anticipated Contribution to U.S. Energy Needs 36
5.6 Costs and Benefits 36
5.7 Appraisal of Environmental Aspects 37
5.8 Evaluation of Research and Development Requirements 38
6 WIND POWER 39
6.1 Description of the System 39
6.2 Efficiency 43
6.3 Development Status 43
6.4 Anticipated Contribution to U.S. Energy Needs 44
6.5 Cost and Benefits 45
6.6 Appraisal of Environmental Aspects 46
6.7 Evaluation of Research and Development Requirements 47
7 TURBINES 48
7.1 General 48
7.2 Steam Turbines 49
7.3 Gas Turbines 58
8 THERMOELECTRIC CONVERTERS 66
8.1 Description of the System 66
8.2 Efficiency 67
8.3 Size Limitations 70
8.4 Development Status 70
8.5 Anticipated Contribution to U.S. Energy Needs 71
8.6 Costs and Benefits 71
8.7 Appraisal of Environmental Considerations 72
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Section Page
9 THERMIONIC CONVERTERS 73
9.1 Description of the System 73
9.2 Efficiency 73
9.3 Size Limitations 75
9-4 Development Status 75
9.5 Anticipated Contribution to U.S. Energy Needs 77
9.6 Costs and Benefits 78
9.7 Appraisal of Environmental Aspects 78
10 POTASSIUM VAPOR TOPPING CYCLES 80
10.1 Description of the System 80
10.2 Efficiency 82
10.3 Size Limitation 82
10.4 Development Status 82
10.5 Anticipated Contribution to U.S. Energy Needs 83
10.6 Costs and Benefits 84
10.7 Environmental Appraisal 84
11 ELECTROGASDYNAMICS 85
11.1 Efficiency 87
11.2 Size Limitations 87
11.3 Development Status 88
11.4 Anticipated Contribution to U.S. EnergyJNeeds 88
11.5 Costs and Benefits 88
11.6 Appraisal of Environmental Aspects 89
11.7 Research and Development Considerations 89
12 RECOMMENDED ENVIRONMENTAL R&D PRIORITIES 90
FOR CANDIDATE ENERGY SYSTEMS
REFERENCES 96
vll
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LIST OF FIGURES
Number Page
1 Unit Operations for an MHD Power Plant 14
2 Electrochemical Oxidation of Hydrogen in a Fuel Cell 23
3 Schematic of Closed Cycle Rankine Engine Showing 34
Typical OTEC Parameters
4 MARK-I 20 kW Wind Generator on Single Pole Support 41
5 MARK-III 60 kW Wind Generator on Stayed 100 ft pole 41
and MARK-V 100 kW Wind Generator (520 kW Machine)
on 100 ft Tower
6 Proposed Wire Rope and Kingpost Wind Generator 42
System Single Bank: 9.6 MW/mile, Double Bank:
19.2 MW/mile
7 Operation of a Heat Engine 50
8 Basic Components of a Rankine Cycle Heat Engine as 50
Used in Steam Turbine Power Plants
9, Rankine Cycle with Reheat 52
10 Rankine Cycle with Regeneration 53
11 Regenerative Cycle Gas Turbine 59
12 Nuclea Cycle Gas Turbine 61
13 Combined Cycle Gas Turbine 63
14 Simple Thermocouple 66
15 Efficiency of a Thermoelectric Generator 68
16 Schematic of a Thermionic Energy Converter 74
17 Flow Diagram for Potassium Binary Vapor Cycle 81
Power Plant Fueld by a Molten Salt Reactor
18 Schematic of the EGD Basic Operation 85
Numb e r
1
2
3
LIST OF TABLES
Overview of the Development Status and Environmental
Impact of the Candidate Advanced Energy Systems
Ongoing Federally Funded Research of Candidate
Advanced Energy Sources
Recommended Environmental RfkD Priorities
for Candidate Advanced Energy Systems
Page
4
7
9
vili
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LIST OF TABLES (Continued)
Number
4 Recommended Time Phase for Environmental 10
R&D of Candidate Advanced Energy Systems
5 MHD Power Plant Process Steps Descriptions 15
6 MHD Research Levels Outside the U. S. (Ref, 14) 21
7 Fuel Cell Efficiencies 24
8 Thermoelectric Materials 69
ix
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Section 1
INTRODUCTION
The objective of this study is to review the development status and
anticipated primary environmental hazards of candidate advanced energy
systems. Systems evaluated included potential new sources of energy,
improved energy conversion devices, and direct energy conversion
systems. Potential new sources of energy considered were ocean thermal
energy conversion (OTEC) and wind power. Improved energy conversion devices
considered were magnetohydrodynamics (MHD), hydrogen fuel cells,
potassium topping cycles and high temperature turbines. The
direct energy conversion devices considered were thermionic, thermoelectric
and electrogasdynamic systems. Advanced energy systems undergoing
similar evaluations in other studies, but not considered here, include coal
conversion, oil shale, geothermal, breeder and fusion nuclear power, energy
from wastes, solar, biomass, and tidal energy.
This type of study is required to ensure that environmentally acceptable
systems will result from the search for viable alternate energy sources to
our nation's dwindling domestic fossil fuels. Integrating environmental con-
trol into the design of a developing technology should prove to be cheaper and
more effective than retrofitting pollution control devices to existing utility
or industrial facilities. Concurrent development of environmental protection
measures and energy conversion technology will also avoid possible delay of
commercialization which would occur if existing and probable pollution regu-
lations are ignored until the completion of development.
For the candidate energy systems considered in this study, the environ-
mental research and development priorities and timing will be presented from
-------
a compilation of the following information:
An evaluation of the current development status of the technology
An estimate of when the subject technology may begin to contribute
significantly to U.S. energy needs
An appraisal of the environmental impact of the technology of each
system when applied as operating systems including assessment of
adequacy of control technology, potential gross ecological impacts,
etc.
A summary of ongoing federally funded research.
This information was obtained from the existing technical literature and/or
by interviewing knowledgable personnel from government, industry and
educational institutions. The major published source relied upon for gen-
eral information was Ref. 1, whereas the principal interviews were with
personnel from the Energy Research and Development Administration (ERDA),
Electric Power Research Institute (EPRI) and principal investigators in the
various technology development efforts. Descriptions of on-going federally
funded research were obtained primarily from ERDA and the Smithsonian
Science Information Exchange (SSIE).
The overall conclusions of this investigation are that each of the energy
systems considered has a negligible or mild direct environmental impact when
compared with conventional fossil fuel and nuclear systems, but that indirect
impacts could be severe and need further study to quantify their impact.
The sections that follow are: a summary of technology development
status, environmental impact and research and development needs; detailed
discussions of each advanced energy system; and finally, a concluding section
which presents the conclusions and recommendations of this study.
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Section 2
OVERVIEW
The development status and anticipated primary environmental hazards
of the following advanced energy systems has been reviewed:
MHD (Open and Closed Cycle)
Hydrogen Fuel Cells
Potassium Topping Cycles
High Temperature Turbines (Open and Closed Cycle)
Ocean Thermal Gradient
Wind
e Thermionic
Thermoelectric
Electrogasdynamic
A summary of ongoing federally funded research of these systems has also
been compiled. Based on the preceding information, recommended environ-
mental R&D priorities for these candidate energy systems have been identified.
2.1 DEVELOPMENT STATUS AND PROJECTED ENVIRONMENTAL HAZARDS
A concise overview of the development status and an environmental
appraisal of each energy system is given in Table 1. Although each energy
system under consideration usually possesses several distinct variations
and alternatives, the particular system chosen in Table 1 was considered
the most likely to provide the earliest, significant contribution to U. S.
energy needs (i.e., open cycle MHD in Table 1, rather than closed cycle
MHD). The particular systems chosen for inclusion in Table 1 are described
in the following sections.
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Table 1
OVERVIEW OF THE DEVELOPMENT STATUS AND ENVIRONMENTAL IMPACT OF THE CANDIDATE ADVANCED ENERGY SYSTEMS
Energy System
MHD (Open Cycle)
Hydrogen Fuel Cells
Ocean Thermal Energy
Conversion
Wind Power
High Temperature
Turbines
Steam
Gas
Thermoelectric
Converters
Thermionic
Converters
Potassium Vapor
Topping Cycle
Electr og asdynamic
Converters
Development Status/
Power Capacity
Experimental/18-32 MW
Experimental/26 kW
Conceptual/ 100 MW
Experimental/100 kW
Experimental (GEJ/2MW
Production
Production
Experimental/Low
Power
Experimental/Conceptual
(Utility Application
22 MW)
Experimental/250 kW
Exper irnent al/ 3 0 W
Efficiency
55-60%
55-70%
2,5%
30-37%
50% at 2800F
38-40%
Simple Cycle 30%
Combined 46%
Regenerative 40%
4-6%
1-15%
20 -25%
50-55%
Unknown
Most Probable
Method of Utilization
Baseload (Topping
Cycle)
Peak Power Demand
Energy Storage
Special Purpose
Energy Storage
Baseload
Baseload
Peak Power Demand
Intermediate
Special Purpose
Special Purpose
Topping Cycle
Topping Cycle
Special Purpose
Earliest
Widespread
Commercial
Utilization
2000
1995
1990-2000
2000
Unknown
2000
1980
Unknown
2050
1990
2000
Percent of U.S.
Energy Needs
Unknown
Unknown
1-5%
1-20%
Un kn own
80%
-25%
Unknown
Unknown
Unknown
-15%
Unknown
Environmental Appraisal
Direct
Emissions
Moderate
Negligible
to Moderate
Negligible
to Moderate
Negligible
Moderate
Moderate
Negligible
Negligible
Moderate
Negligible
to Moderate
Negligible
NSPS Status*
Yes
Unknown
Unknown
Yes
Yes
Yes
Yes
Yes
Yes
Indirect
Moderate
Moderate
Moderate
to Severe
Moderate
Moderate
Moderate
Moderate
Negligible
Negligible
Negligible
Meets present NSPS emission rate limitations for coal fired boilers.
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The categories of development status considered in Table 1 ranged
from conceptual, experimental, demonstration and production (these are
with respect to a utility size power plant). Conceptual stage projects refer
to those in which only paper studies, detailed design and/or limited laboratory
scale testing of components are underway. Experimental systems consisted
of those which had progressed to bench scale or pilot plant testing of the full
system or where major subsystems were undergoing large scale demonstration
tests. Demonstration status refers to those systems undergoing semi-works
or full-scale testing of an integrated plant containing all the subsystems,
instrumentation, and process control of a commercial unit. A unit is con-
sidered ready for commercial application if all development and demonstration
phases have proved its technical and possible economic feasibility, and if
several units are in full-time use.
Appraisal of environmental impacts are rated as negligible, moderate
or severe. Where possible, these descriptors are in comparison with the
impact associated with New Source Performance Standards (NSPS) or other
regulations which apply to utilities with conventional (500 to 1000 MW) coal
fired steam plants.
Distinction between direct and indirect impacts consists of the following:
Direct impacts result from activities on the energy production site
(e.g., coal storage, crushing, combustion, and ash and flue dis-
posal for MHD).
Indirect impacts have their environmental effect at the site of other
activities required to support the advanced energy development
(e.g., emissions from CdS photovoltaics used as collectors of
solar energy), or off site considerations such as weather modification,
and more general considerations such as aesthetics and land use.
Predictions of wide scale commercial utilization were primarily based
on Refs. 1, 9 and 19 and upon conversations with key ERDA and EPRI personnel.
Significant, widespread utilization is defined as 1% of total U. S. consumption.
-------
2.2 FEDERALLY FUNDED RESEARCH
Ongoing, federally sponsored research in the nine candidate energy
systems is summarized in Table 2. One-hundred forty-four studies
totaling over $26 million were being performed by six federal agencies
(ERDA, NSF, DOD, NASA, EPA and FEA). Wind energy was receiving
the greatest funding and number of studies in 1975, while thermoelectric
conversion was receiving the least funding. Eight percent of the total number
of these studies were identified as devoted to environmental considerations.
2.3 ENVIRONMENTAL RfeD RECOMMENDATIONS
All of the advanced energy systems considered in this study are
summarized in Tables 3 and 4 with respect to their anticipated environmental
impact and R&D requirements. Several of the systems were found to have
a fairly negligible effect on the environment when a single unit is considered.
However, in the case of windmills and ocean thermal conversion systems,
a cluster arrangement of many plants will be required to produce a sizable
amount of electrical energy. This clustering arrangement could have an
adverse impact on the local environment.
Table 3 lists the probable emission source and the environmental
impact and R&D requirements, while Table 4 summarizes the time frame
for the RfeD studies. These projections were obtained by considering the
present development status, projected timing of widespread commercialization,
adequacy of the environmental data base, expected environmental impact and
adequacy of the existing pollution control technology.
As shown in Table 1 most of the systems are either in the experimental
or feasibility stages of design at the present time. These efforts should be
augmented currently with the suggested R&D efforts if the studies are to
significantly impact the preliminary and final design stages. Thermionic
and electrogasdynamic direct energy converters operating as a. base load unit
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Table 2
ONGOING FEDERALLY FUNDED RESEARCH O.F CANDIDATE
ADVANCED ENERGY SOURCES
Energy System
Wind
Subtotal
Magnetohydrodynamic s
Subtotal
High Temperature
Turbines
Subtotal
Ocean Thermal
Gradient
Subtotal
Sponsoring
Agency
ERDA
NSF
NASA
FEA
ERDA
DOD
NSF
NASA
DOD
NSF
EPA
NASA
NSF
NASA
ERDA
Number
of Studies
15
36
3
1
55
11
12
6
1
30
1
1
2
3
7
13
1
11
27
Environmentally
Related Studies
0
1
0
0
1
1
0
0
0
1
0
0
2
0
2
2
0
1
3
Federal ^
Funds, $10
6320
2830
430
4Q
9620
5110
1050
500
?
6660
3910
120
50
40
4120
1440
?
1360
2800
Continued
The primary source of this information is the Smithsonian Science Information Exchange (SSIE).
Not all federal agencies supply notice of research projects to SSIE, therefore the above information
is probably incomplete. Other sources of information include contacts with agency or contractor
personnel.
1975 grant and contract funds.
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Table 2 (Continued)
Energy System
General or Multi-
System Studies
Subtotal
Thermionic
Subtotal
Potassium Topping
Cycles
Electrogasdynamic s
Subtotal
H2 Fuel Cells
Subtotal
Thermoelectric
Subtotal
Sponsoring
Agency
NSF
ERDA
EPA
ERDA
NASA
ERDA
DOD
NSF
NASA
NSF
EPA
DOD
NASA
ERDA
Total
Numb e r
of Studies
2
3
J_
6
1
4
5
1
5
1
6
1
1
2_
4
1
2
J^
4
144
Environmentally
Related Studies
1
2
J_
4
0
0
0
0
0
_0
0
0
0
J^
1
0
0
J)
0
12
Federal
Funds, $1CT
1120
925
25
2070
450
30
480
300
220
40
260
140
30
20
190
90
?
40
130
26,630
oo
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Table 3
RECOMMENDED ENVIRONMENTAL R&D PRIORITIES FOR CANDIDATE
ADVANCED ENERGY SYSTEMS
Energy System
MHD
Hydrogen Fuel Cell
Windmills
Ocean Thermal Energy
Conversion
Potassium Topping
Cycles
Gas Turbines
Thermionic
Electrogasdynamics
Type of Emission
Alkali Salt Seed
NOX
Leachate Sludge
Undersea Plumes,
Spills, Leaching
Potassium
NO
X
SO , NO , Fly Ash
x x
Control
Technology Needs
Seed Collection
Combustion Mods.
Define Control
Technology Needs
1
To be Determined
Define Control
Technology
Required
Combustion Modi-
fication, etc.
High Temperature
Control Technology
Indirect Effects
Effect on Animal and Plant
Life
To be Determined
Weather Modification
Land Use
Aesthetic
Addition of Nutrients to the
Surface Waters
Effect of Cold Water on Surface
Marine Life and Weather
Shipping Lane Interference
Effect on Animal and Plant
Life
Smog
Acid Rain
Similar to Conventional Coal
Fired Steam Plants
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Table 4
RECOMMENDED TIME PHASE FOR ENVIRONMENTAL R&D OF CANDIDATE ADVANCED ENERGY SYSTEMS
Energy System
MHD
Hydrogen Fuel Cells
Windmills
Ocean Thermal Energy
Conversion (OTEC)
Potassium Topping Cycles
High Temperature
Gas Turbines
Thermionic
Elect rogasdynamic
Thermoelectric
R&D Task
Seed Collection Device
Efficiency, NOX Control
Control Technology
Definition
Environmental Assessment
of Weather Modification
Environmental Assessment
of Undersea Plumes and
Upwelling
Environmental Assessment
to Identify Emission Sources
Rates and Control Technolog
Design Most Effective Con-
trol Technology for Low
Grade Petroleum Fuel
Define High Temperature
Control Technology Re-
quirements
Unknown
Technology
Breakthrough
1985
Existing
Existing
1990
1980
,
y
Existing
1976
1976
Unknown
Earliest Expected
Widespread
Commerical Use
2000
1995
2000
1990
1990
1980
2050
2000
Unknown
R&D Task
1976
1976
1976
1976-1977
1976
1976
1976
1976
Unknown
Current Development Status
for Utility Power Generation
Experimental Units Have Been
Demonstrated
Demonstration Unit in Operation
Demonstration Unit in Operation
Conceptual and Feasibility Studies
Experimental Unit Under Con-
struction
Production
Experimental
Conceptual
Current Efforts Limited to Space
Applications Due to Extremely
Low Efficiency
Order of R&D Priorities
High Temperature Gas Turbines, OTEC, Windmills
MHD, Potassium Topping Cycles, Hydrogen Fuel Cells,
Thermoionic, Eletrogasdynamics, Thermoelectric.
Not necessarily for utility or electrical power applications.
-------
are considered to be impractical. However; operating as a topping cycle,
these systems can quite possibly be utilized to increase the thermal efficiency
of the power plant. As noted in Table 4, thermoelectric conversion, as a result
of very low efficiencies, is considered impractical for utilization in any manner
in the production of electrical power on a commercial basis.
11
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Section 3
MAGNETOHYDRODYNAMICS
3.1 DESCRIPTION OF THE SYSTEM
The magnetohydrodynamic (MHD) generator produces electrical energy
directly from thermal energy. It is a heat engine that combines the features
of the turbine and generator of the conventional steam plant by replacing the
rotating wire conductor with an electrically conductive fluid. Three variations
of the MHD concept have received consideration for power generation:
Open Cycle System; The working fluid is generated by combusting a
fossil fuel. The combustion products are made conducting by seeding
with an easily ionizable element, such as an alkali metal, and by ele-
vating the combustion gas temperature by feeding the combustor with
preheated air. The gases are accelerated to high velocities by con-
verting their thermal energy to kinetic energy through the use of a
subsonic to supersonic expansion nozzle. The magnetic field is posi-
tioned in the supersonic portion of the nozzle. The gases are then
cleaned and allowed to exhaust from a stack.
Closed Cycle System: The closed cycle system is conceptually the
same as the above open cycle system. The primary difference be-
tween the systems are: (1) the conduction gas is circulated in a closed
loop. Since the working fluid is never lost there is more latitude avail-
able in choosing the working fluid and in obtaining electron densities
that give sufficient conductivity, and (2) the thermal energy is provided
by a nuclear reactor or by an externally fired source.
Liquid Metal System; In this system a liquid metal is mechanically
pumped through the magnetic field.
The present state of MHD technology indicates the open cycle system is
the most promising "near term" candidate energy system. Both the open
and closed cycles are considered to offer sufficient energy generation
potential to warrant further development. The open cycle system is closer
to development than the closed cycle system (Ref. 1). The closed cycle
12
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system has basic problems of enthalpy extraction, generator and diffuser
efficiency which must be solved. The liquid metal system has primarily
been oriented toward space power systems where the elimination of rotating
machinery is the objective.
The abundance of domestic coal makes it the most attractive candidate
fuel for MHD power plants. Cesium is technically attractive for seeding the
gases in the MHD generators. However, economics indicate that potassium
is the material that will most probably be used. The MHD energy system that
will be considered in the following discussion is the open cycle (Fig. 1, Table 5)
which is coal fired and potassium seeded.
3.2 EFFICIENCY
MHD power systems have potentially higher efficiencies than conven-
tional steam and other turbogenerator type energy conversion systems.
First generation plants are envisioned to operate in the binary cycle mode.
The MHD system would serve as a topping cycle to a conventional steam
plant where the predicted overall plant efficiency is in the range of 46 to 50%.
The ultimate efficiency is projected to be in the range of 55 to 60% (Ref. 2).
The closed cycle MHD system appears to be capable of efficiencies of 50%
when operating at temperatures on the order of 2900F. The liquid metal MHD
power system is predicted to have overall efficiencies competitive with those
of modern steam systems when operating at the same maximum cycle tem-
perature and shonld have efficiencies approaching 50% at an operating tem-
perature of 1600F (Ref. 1).
3.3 SIZE LIMITATION
Magnetohydrodynamic generators become more efficient with increase
in size. This is because friction effects and heat losses become less signifi-
cant as the MHD ducts become larger (i.e., surface-to-volume ratio decreases).
The size limitations for the MHD central station power plants will be dependent
on limitations of supporting equipment such as pumps, heat exchangers, etc.
13
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Raw Materials
Conversion/Combustion
Products/Byproducts
Coal
Supply
&,
Coal
Pulverizer
and
F
KZ CO, Seed Makeup
icacr
>_-/
Đ !^
Đ
MHD
Combustor
j
/ N
I
Ū1
/*r\
(
(
<* s
9
High Ternp.
Air Heater
3
)
MHD
Generator
i
g
Steam
Generator
Đ
DC-AC
i
I
(M)
(18)
Low Ternp.
Air
Steam Turbine
AC Generator
Đ
!
fl
Switch
Yard
jĐ
J)
1
Đ
1
r
(20)
1
a
-4>i
" 1 uw
Seed Recovery L^-i^j Stack
System
(23) (??)
Đ
"
Claus
Plant
j
A
Đ
Fig. 1 - Unit Operations for an MDH Power Plant
-------
Table 5
MHD POWER PLANT PROCESS STEPS DESCRIPTIONS
Station Description
1 5.15 x ID5 kg Coal/hr or 3.96 x 109 kcal/hr
Coal Analysis: 8% Ash
1.1% Sulfur
7683 cal/gm
2 1210 kg/hr K~ CO., Seed Makeup
& 3
3 Pulverized Coal, 70% Through 200 Mesh
4 40,200 kg/hr K- CO3, Seeding Material
5 5.55 x 105 kg/hr Seeded Coal
6 Solid Waste: 30,900 kg/hr of Slag Rejected, 807 kg/hr
K, CO Lost
8
7 Combustor Heat Rejected 5.54 x 10 kcal/hr
8 Combustor Air Feed at ~1370°K
9 Air and Fuel for High Temperature Air Heater
1.0 Plasma at 2480°K
11 DC Current From MHD Unit
12 AC Current From DC-AC Inverter
13 MHD Exhaust Gases at ~1700°K
14 Steam at ~800°K
15 AC Current
16 Heat Rejected from Steam Turbine
17 2,300 MW Electrical Power Output
18 MHD Exhaust Gases
19 Heated Air (Low Temperature)
20 Air Supply
21 MHD Exhaust Gases
22 Recovered KZ CO3, 39,000 kg/hr
(Continued)
15
-------
Table 5 (Concluded)
Station Description
23 Solid Waste:
10,000 kg/hr of Fly Ash
403 kg/hr of KZ CO3
24 H_S from Seed Recovery System
Ģi
25 Usable Sulfur -5600 kg/hr
26 Exhaust Gases
27 Gaseous and Particulate Waste
860 kg/hr NO
LJ
24 kg/hr SO2
515 kg/hr Particulates
16
-------
3.4 DEVELOPMENT STATUS
MHD technology has developed primarily over the past 17 years.
Initial developments were made in the United States, however major efforts
are now being conducted in other countries, particularly the U.S.S.R.
The MHD generator will serve as the upper portion of a binary genera-
tion cycle. A conventional steam turbine driven generator will serve as the
bottoming portion of the cycle. This development of MHD technology has
centered not only on MHD generator components such as superconducting
magnets, combustors, seeding techniques, etc., but also on components of the
overall power generation system.
Two large experimental MHD generators for short duration operation
were built in the middle 1960s to provide an understanding of the MHD process
and to provide experimental data for prediction of MHD generator perform-
ance. Both were built by Avco under Department of Defense funding. The
larger one, called the MK-V produced an output of 32 MW. The smaller one,
located at Arnold Engineering Center, Tullahoma, Tennessee, had an output
of 18 MW (Ref. 3). The smaller unit, called the LORHO, is presently being
modified for performance demonstration experiments (Refs.4 and 5). It is
scheduled to be'operational by mid-1976.
Several smaller units with power levels of a few kilowatts have pro-
vided design data and information regarding long-term operation of MHD
generators under conditions simulating those in commercial power genera-
tion. These efforts have primarily been carried out by Avco, the University
of Tennessee Space Institute and Stanford University (Ref. 3).
Efforts relating to the overall MHD system, seed recovery, and the
environmental aspects of the system are being conducted by the ERDA's
Pittsburgh Research Center (Refs.6 and 7).
The U.S.S.R has an extensive MHD development program in progress.
Their U-25 plant is a natural gas fired pilot plant with a designed output of
17
-------
25 MW. It includes a dc to ac inverter and feeds power into the Moscow
grid system. In addition to this facility the U.S.S.R. has three smaller
facilities for research on specific aspects of the system. The U.S.S.R. is
presently designing a 200 to 600 MW commercial demonstration generator
(Refs.3 and 8).
MHD efforts are also underway in Japan. The most significant of these
efforts is the development of a small pilot plant. Other countries that have
recently started MHD research are Germany and Poland.
3.5 ANTICIPATED CONTRIBUTION TO U.S. ENERGY NEEDS
MHD is presently at the stage of development that a demonstration plant
must be constructed before MHD can progress to the point of making signifi-
cant contributions to the U.S. energy needs. MHD units for emergency and
peaking power may be available as early as the 1980's, however, it is the
consensus of experts that it will be around the year 2000 before MHD will
make significant contributions to the U.S. energy needs (Ref. 9).
3.6 COSTS AND BENEFITS
Although the technical feasibility of MHD power plants has been demon-
strated, the concept is still in the developmental stage, and thus very little
information has been developed on the projected economic benefits to accrue
from central station MHD power plants. The higher efficiencies projected
for the various MHD systems must provide sufficient fuel cost savings to
compensate for the capital costs of the MHD systems. Most of the economic
studies carried out have been for open-cycle plasma -stseam systems. The
first generation open-cycle MHD topping systems for electrical generating
plants may conceivably compete successfully with conventional steam sta-
tions in areas of high fuel costs. Future nuclear power plants would have
an economic advantage over open-cycle fossil-fueled MHD power plants only
in areas where fossil fuel is relatively expensive.
18
-------
3.7 APPRAISAL OF ENVIRONMENTAL ASPECTS
The effluent problems associated with MHD power plants are essen-
tially those associated with the energy source (i.e., fossil, or nuclear fuel).
For coal fired systems these have been found to be SO_, NO , particulates
C* X
and thermal. The MHD concept has the potential to reduce thermal discharge
and conserve fuel supplies when compared with conventional power plants
because they are predicted to have improved energy conversion efficiency.
The total quantity of thermal emissions will be reduced in inverse proportion
to the improvement in efficiency.
Seed material must be removed and recovered from the effluent gases
for environmental as well as economic reasons. Seed materials being con-
sidered are alkali metal salts whose release to the environment as finely
divided salts would be undesirable. Furthermore, the cost of these materials
dictate that they be recycled for economic operation. The SO_ emission is
Ģt
expected to be about 5 ppm which is orders of magnitude below the NSPS for
fossil fuel fired facilities. Experimental studies have shown that 99.8% of
the sulfur in 2.2% sulfur coal can be removed (Ref.4). The NO emission is
X.
expected to be about 135 to 300 ppm (Ref. 13) or possibly lower depending on
the cool down rate of the gases. In present experimental facilities as much
as 95% of the particulates have been removed from the effluent gases (Ref. 13).
The open cycle MHD system is estimated to meet or exceed EPA requirements
for SO-, and NO at costs that are predicted to be below burning 2% sulfur coal.
L* X
The need to reclaim seed material for the MHD system has provided
for high efficiency particulate control. Experimental data indicates that
particulate emissions will not be a problem. Consequently, it appears that
emission control technology for MHD systems is adequate.
3.8 MHD RESEARCH
A substantial number of government and non-government agencies have
assumed active roles in the U.S. commercial MHD electric power development
19
-------
effort. The Energy Research and Development Administration, through its
MHD Project Office, has been assigned the lead role in the national program,
in which capacity it has established a centrally organized national research
program on MHD involving a large number of U.S. institutions, e.g., Avco
Everett Research Laboratory, University of Tennessee Space Institute,
Massachusetts Institute of Technology, Stanford University, General Electric
Company, Westinghouse Corporation, MEPPSCO, Inc., Argonne National Lab-
oratory, National Bureau of Standards, STD Corporation, Arnold Engineering
Development Center, Fluidyne, Pittsburgh Energy Research Center, and others,
Other U.S. Government agencies such as the National Science Foundation, the
National Aeronautical and Space Administration, and the Office of Naval Re-
search, are also funding MHD research directly, as is the Electric Power
Research Institute (Ref. 13).
In calendar year 1974, the MHD program was significantly expanded
through the availability of a total worldwide budget of $7,750,000 in FY 74 and
$12,500,000 in FY 75 (Ref. 14). Table 6 gives the funding in MHD research
outside the U. S.
20
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Table 6
MHD RESEARCH LEVELS OUTSIDE THE U.S. (REF, 14)
Country
Financing
Institutions
Remarks
Switzerland
1968-1972: FS 2.1 million
Commission federale pour
I1 encouragement de la
recherche scientifique.
Government aid tem-
porarily suspended; new
possibilities being studied.
Research carried out at
Institut Battelle, Geneva.
Direct conversion into
electricity by closed
cycle process.
Japan
Yen 6.4 billion from
1966 to 1975
MITI (Ministry of Inter-
national Trade and
Industry).
Research on long-term
operation, heat exchanger,
seed recovery, heat-proof
materials, superconducting
magnets; this work is
centered on a 1,000 KW MHD
test plan.
Netherlands
Universities
Sweden
SK 0.4 million p.a.
AB Atomenergi
Exploratory work on open-
cycle MHD systems.
-------
Section 4
HYDROGEN FUEL, CELLS
4.1 DESCRIPTION OF THE SYSTEM
The fuel cell (Fig. 2) is a device that produces electrical energy from
the controlled electrochemical oxidation of fuels. The basic components of
a simple hydrogen-oxygen fuel cell are the electrodes (anode and cathode)
and the electrolyte, which can be either acidic or basic. The reactants are
normally consumed only when the external circuit is completed, allowing
electrons to flow and the electrochemical reaction to occur. When the exter-
nal circuit is completed, an oxidation reaction, yielding electrons, takes
place at the anode and a reduction reaction, requiring electrons, occurs at
the cathode. The electrodes provide electrochemical-reaction sites and
also act as conductors for electron flow to the external circuit.
Continuous operation of the cell necessitates the removal of heat, water
and any inert material that enters the cell with the reactants. Reaction
kinetics are usually enhanced by the incoroporation of a catalyst, such as
platinum, on the high surface area electrode surfaces. Power is produced
as long as fuel and oxidant are supplied to the fuel cell and the external
circuit is closed, allowing current to flow.
Hydrogen fuel cells are not currently envisioned as primary energy
sources (i.e., baseload power generators). Their application appears rather
to be in peak load demand application, or for dispersed generation of electri-
cal power in residential or small community sites, or at electrical substations.
Two modes of hydrogen generation are currently under development
(Ref. 15) for fuel cell operation: (1) steam reforming of hydrocarbons and
coal, and (2) electrolysis of water.
22
-------
FUEKHJ
POROUS
ANODE
POROUS
CATHODE
SPENT FUEL AND
WATER VAPOR
1 WAW '
ELECTRON FLOW-*-
SPENT
OXIDANT
OXIDANT (02)
Fig. 2 - Electrochemical Oxidation of Hydrogen in a Fuel Cell
23
-------
Steam reforming is an alternative mode of fossil fuel combustion.
Effluent from the fuel cells provides heat to reform hydrocarbons, such as
natural gas, and yields a hydrogen-carbon dioxide mixture. Heat produced
in the cells is also used to preheat the water used in the reforming reaction.
If given proper treatment, hydrogen can be used directly in the fuel
cells as can the fuel gas from coal gasification.
The electrolysis of water to produce hydrogen and oxygen is an energy
storage and/or transmission application. This method is quite suitable for
utilization in conjunction with nuclear power plants which operate most effi-
ciently when generating in the base load power demand mode. During nonpeak
power demand periods, excess power from the nuclear plant is utilized to produce
hydrogen and oxygen from the electrolysis of water which could be then used in
fuel cells during later peak demand power period to reconvert to electrical energy.
4.2 EFFICIENCY
The theoretical maximum efficiency of a fuel cell is a function of the
fuel and oxidant used. Where systems are integrated, as with a reformer,
the theoretical efficiency is based on the primary feed material rather than
on the fuel that is electrochemically oxidized. Projected reference efficiency
limits (Ref. 1) based on laboratory investigations and system studies are given
in Table ?.
Table 7
FUEL CELL EFFICIENCIES
Fuel
Hydrogen
Methane
Coal
Cell
Voltage
1.23
1.06
1.02
Theoretical
Cell Efficiency
0.83
0.92
1.00
Projected System
Efficiency, 1980
0.65
0.30 - 0.55
0.70
24
-------
Gross efficiency is the product of the theoretical maximum efficiency and
the ratio of the operating voltage to the theoretical voltage. For hydrogen-
fuel cells, this efficiency today is 0.54 to 0.61.
Because of extensive heat transfer and mass transfer interactions,
subsystem efficiencies cannot be multiplied to determine the overall effi-
ciency of integrated fuel cell power system. The present published efficiency
of conversion of chemical energy from natural gas fuel to ac electrical energy
for the 12.5 kW Pratt & Whitney system (Ref. 15) is 40 to 45%. The large
central-station version of this system is projected to have an overall effi-
ciency of nearly 55%. The Westinghouse high-temperature system concept
was estimated to be able to operate at a projected efficiency of 58% for the
100 kW size and at nearly 70% for 1000 MW based upon dc output.
4.3 SIZE LIMITATIONS
Several fuel cell power generation systems in the 10 to 20 kW range
have been constructed and operated. The modular construction of fuel cells
and power-conditioning equipment allows a nearly direct proportional scaling
into the multi MW range. Plumbing, wiring and fault-isolation equipment
requirements are also nearly proportional to the system power capability.
Fuel conditioning and control equipment have a scaling factor of 0-9. Sys-
tems can be demonstrated in small sizes, and full-scale systems can then
be produced by conventional engineering techniques. Systems using fuel
reforming or high-temperature cells are significantly more efficient in large
sizes (> 100 kW) due to the reduction in external surface area per unit volume.
4.4 DEVELOPMENT STATUS
Several fuel cell power generation systems in the 10 to 20 kW range
have been constructed and operated. The modular construction of fuel cells
and power conditioning equipment allows a nearly direct proportional scaling
into the multi-MW range. No control stability or complexity problems are
introduced in paralleling fuel cell banks to construct large systems. In fact,
25
-------
overall system reliability is improved through load sharing in multistack
systems and as a result of the capability to replace modular units on a pro-
grammed basis.
Two approaches are currently being followed in the development of the
hydrogen fuel cell application to electrical power plants. These are for
central station (i.e., base load) application (Ref. 1) and for dispersed genera-
tion of electrical power in residential sections, small communities or at
electrical substations. Work on the central station application is still in the
laboratory and system study phase. Westinghouse, until 1970, was engaged
in development work (Ref. 16) for the Office of Coal Research and had
developed a preliminary design for a 100 kW system based on gasification
of coal and a high-temperature (1870F) zirconia electrolyte fuel cell. The
system uses high-temperature materials in the construction of the fuel cell.
A porous nickel anode, a stabilized zirconia electrolyte and a porous, tin-
doped, indium-oxide cathode are deposited on a 0.5 inch diameter porous,
stabilized zirconia tube with appropriate cell interconnections. The total
system consists of fuel cell battery tubes assembled into banks, a coal gasi-
fier and ancillary equipment. Cell banks which operate at 1850F are physi-
cally located in the fluidized-bed coal gasifier for maximum heat recovery.
System development did not proceed beyond the preliminary design stage and
is currently suspended.
Pratt & Whitney has a major program for dispersed generation using
natural gas reformers and low-temperature (< 250F) fuel cells of the phos-
phoric acid and potassium hydroxide electrolyte types. The Institute of Gas
Technology has been doing complementary work using low-temperature
phosphoric acid and higher-temperature (2200F) molten carbonate electro-
lyte cells. All the above fuel cells will also operate on the fuel formed from
coal gasification. This work has been sponsored by segments of the gas
industry, American Gas Association, and most recently by the Edison Electric
Institute (EEI).
26
-------
Fuel cells of the type under development by Pratt & Whitney are of the
plate and frame type. Simple components produced in high volume are assem-
bled into series stacks and either bolted or bonded together. Flow passages,
a porous catalyzed nickel anode, an electrolyte-saturated matrix and a porous
catalyzed cathode make up the unit cell. Phosphoric acid is used as the electro-
lyte by Pratt & Whitney, platinum -rhodium alloy as the anodic catalyst, and
platinum as the cathodic catalyst.
In the Pratt & Whitney system, the cells operate at about 230F. This
system burns the effluent from the fuel cells to provide heat to reform hydro-
carbons, such as natural gas, yielding a hydrogen/carbon-dioxide mixture.
(Heat produced in the cells is also used to preheat the water used in the re-
forming reaction.) Given proper pretreatment, hydrogen can be used
directly in the cells as can the fuel gas from coal gasification. Thermal and
electrical output from nuclear reactors can be used to produce hydrogen and
oxygen via the electrolysis of water. In this manner fuel cells have the po-
tential to become an integral part of electrical systems (Ref. 17) for the dis-
persion of electrical power. However, it is not known at this time if this
approach is being seriously investigated.
Although there have been many successful programs resulting in num-
erous fuel cell systems for specialized applications, dominant uncertainties
remain with respect to commercial power applications because of a lack of:
ŧ Detailed engineering design of low-cost systems
Detailed design of fuel cells for high-volume production
and long life
Demonstration of the costs, lifetimes, efficiencies, and
operational parameters of the projected systems.
Fuel cell systems have been manufactured to date on a limited production
basis for space application. Currently five organizations are capable of
producing such systems in quantity (Ref. 1): Pratt & Whitney, General
27
-------
Electric, Westinghouse, Union Carbide Corporation and Alsthom. None is
actively marketing commercial systems of significant size. Pratt & Whitney
recently conducted extensive field tests of its 12.5 kW natural gas system.
4.5 COSTS AND BENEFITS
Since no large fuel cell power systems have been built, an estimate of
the costs is somewhat speculative. Costs, however, have been projected for
the coal-fired, high-temperature system by taking into account research and
development progress to date and comparing unit costs of various elements
with similar items in a coal-fired, steam-turbine power plant. By assuming
that the cost of electricity produced by a coal-fueled fuel cell system is equal
to that from a steam turbine system, the allowable capital costs for the fuel
cell system can be projected. The result of these assumptions and calcula-
tions is to suggest that a coal-fueled fuel cell system can produce competitively
priced electricity if it can be built for a total capital cost of $294 to $374 per
kilowatt electrical. The three critical items are the fuel cell, power inverters
and spare parts. Each of these has projected cost ranges that will allow reach-
ing the cost target.
The key item is the cost of the fuel cells. The cost range allocated, $60
to $80 per kilowatt electrical, corresponds to a manufactured cost of $7.00 to
$9-30 per pound based on the materials requirements. Total materials costs
for these thin-film, solid-electrolyte fuel cell assemblies have been estimated
to be about $21 per kilowatt electrical ($2.45 per pound), leaving an allowable
margin for manufacturing and assembly of $39 to $59 per kilowatt electrical
($4.55 to $6.85 per pound). These allowable manufacturing costs show reason-
ably good agreement with independent direct estimates.
28
-------
The major projected advantage of fossil-fueled, fuel-cell systems for
central station power generation is that they operate at a higher conversion
efficiency than is possible with any system presently in use. This higher
efficiency results in a lower rate of fossil fuel reserve depletion, reduced
air pollution and no thermal pollution of natural bodies of water. Projected
economics of central station fuel cell power systems show equivalent capital
costs and lower operating costs. For dispersed generation of electrical
power using fuel cells, the capability for gaseous fuel storage at the point of
usage allows a degree of freedom not found in present electrical distribution
systems. Coupling a hydrogen fuel cell system, to a nuclear powered hydrogen
production facility offers several additional potential advantages:
Improved load factor for the nuclear plant because it is producing
a storable fuel
Enhanced hydrogen supply for use in the chemical and metallurgical
process industries as well as for heat in homes and industrial plants,
compared with that currently available from hydrocarbon sources
Pollution free generation of electricity at points of use.
4.6 ANTICIPATED CONTRIBUTION TO U.S. ENERGY NEEDS
The system being developed by Pratt & Whitney offers the most pro-
mise for any near term contribution to U.S. energy needs. The major effort
by Pratt & Whitney started in 1967 with the first phase becoming a six year
$50 million program. This effort, presently sponsored by 31 gas utilities
that make up the Team to Advance Research for Gas Energy Transformation
(TARGET) group, has the goal of developing fuel-cell systems using reformed
natural gas (methane) as fuel. The developmental work is being done by
Pratt & Whitney, who in May 1971 demonstrated a 12.5 kW system supplying
all of the electrical energy to a home in Connecticut. This was the first of
60 test installations of various capacities planned, about 50 of which had been
installed as of 1 June 1974 (Ref. 1). More than 4000 hours of automatic
operation were demonstrated before refurbishing was required.
29
-------
Pratt & Whitney, in December 1973, announced a $42 million coopera-
tive program with nine electric utilities to develope a 26,000 kW fuel cell.
This level of power is sufficient to provide electricity- for a community of
about 20,000 people, and the manufacturer has estimated that deliveries of
these units can begin as early as 1978. The application is dispersed power
generation, as with the TARGET program, but in this case the unit of power
is larger. The fuel will likely be distillate oils at first, with heavier oils
coming later (Ref. 18).
When the constraints of economics and operating lifetime are imposed,
the feasibility of wing fuel-cell systems for central station or dispersed power
generation is undetermined. The most probable application of hydrogen
powered fuel cells will be in peak power demand applications. However, fuel-
cell technology is no longer receiving significant funds (Ref. 1). The result
is that this energy source is unlikely to contribute significantly to alleviating
the U.S. energy needs until 2000 (Ref. 9).
4.7 ENVIRONMENTAL CONSIDERATIONS
Central station systems using fuel cells will produce chemical pollutants
similar to those obtained by conventional combustion of the same fuels. The
fuel cell, however, is particularly sensitive to the same pollutants, primarily
sulfur, now causing concern in conventional steam turbine generator plants.
This sensitivity will require extensive fuel pretreatment to eliminate con-
taminants prior to electro-chemical oxidation. For an equivalent electrical
power output, the higher operating efficiency of fuel cell systems will result
in a reduction of the total quantity of fuel required and a reduction in the
quantity of material discharged in the emission of nitrogen oxides because
of the reduced temperatures to which the air streams are exposed. Waste
heat rejection is not a significant problem with fuel cell power systems since
most of the waste heat is used in the fuel gasification or reforming process.
Excess heat is rejected to the atmosphere, and cooling water is not required.
30
-------
Large numbers of low-temperature fuel cells could have some impact
on the catalyst material market and on the natural reserves. However, the
catalyst is not consumed except for processing losses, and the total quantity
available will be relatively unchanged. The total effect of this utilization of
catalyst materials is unknown. (This is a problem common with certain
pollution-control equipment being considered for internal combustion engine
powered automobiles.)
Increased utilization of dispersed generation of electrical power, made
possible by the high efficiency of relatively small fuel cell systems, should
have a positive effect on the environment, particularly in urban areas. Gas
transmission by buried pipeline requires less land for an equivalent amount
of energy transmitted, but the total environmental impact of buried pipelines
has not been thoroughly evaluated. The remote locations envisaged for fuel
processing plants and the chemical removal of sulfur at these plants should
result in a positive environmental effect.
Potential water pollution effects will be limited principally to production
of leachate from the fuel cells and sludge disposal from the electrolysis pro-
cess. However, these waste streams are amenable to treatment using estab-
lished technology.
Indirect effects relate principally to land use and facility construction
on remote sites. These will have to be examined on a per case basis.
4.8 RESEARCH AND DEVELOPMENT CONSIDERATIONS
Before the use of hydrogen as a fuel increases substantially significant
changes in production and consumption technologies are likely (Ref. 19). Con-
trol requirements should include:
Establishment of emissions and effluent standards covering the
materials suggested for use in high efficiency electrolyzers,
high temperature thermal decomposition of water and fuel cells
31
-------
Establishment of design guidelines to steer development away
from selection of materials that require large amounts of energy
or that have large known detrimental environmental impact during
their production process.
To ensure that adequate environmental protection measures are incor-
porated in the development of hydrogen for energy storage and transportation,
a periodic review of this development should be carried out. Should environ-
mental or economic attractiveness imply a more rapid move toward deploy-
ment of hydrogen technology, a continuing review would be desirable.
32
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Section 5
OCEAN THERMAL ENERGY CONVERSION
5.1 DESCRIPTION OF THE SYSTEM
At many places in the tropical and subtropical regions of the world,
ocean surface temperatures are in the 75 to 85F temperature range. The
warm surface layer circulates toward the poles where it cools. It then flows
back along the deep ocean trenches. In these lower ocean layers (approxi-
mately 2000 feet below the surface) the water temperatures are 35 to 45F.
This temperature difference between the surface and lower depths has been
shown (Refs.20 and 21) to be adequate to drive a Rankine cycle heat engine.
Both open and closed cycles have been considered. The first plant
designed and operated off the coast of Cuba was a French-built open Rankine
cycle. It's operation depended upon evacuating a chamber to 0.03 atmos-
pheres at which the warm sea water would flash vaporize. The resulting
low density steam drove a turbine and then condensed in a second evacuated
chamber by direct contact with cold sea water falling like rain. A plant con-
structed in this manner designed to develop significant (100 MW) power would
require turbines with very large rotors and matching ducts (Ref. 22). A
recent study (Ref. 23) indicates that the forces on the turbine blades could be
met by low mass structures similar to sailplane wings, which need not be
expensive.
To most investigators use of the closed Rankine cycle appears to be the
more favorable approach. Instead of evaporating the sea water to drive a
turbine, a working fluid operating in a closed loop (Fig. 3) is used to drive
the turbine. This is accomplished by evaporating the working fluid at 70F
via a heat exchanger with the warm sea water. Candidate working fluids are
water vapor, ammonia, propane and Freon (Ref. 1). Ammonia or propane are
currently receiving the most attention. For example, ammonia has a vapor
33
-------
,1 Electric power *
{ output
_-
NH- vannr 1
Warm water
inlet
'"°F> Eva
Warm water
exhaust
(71'F)
(--7.5-9 aim) [Turbine
(6B'F) 1 generator \i
~~~~J I'.,
porator Cor. dens
t c- !
XX NH-, liquid '50'F)
Pump
H3 vapor
5-6atm|
Colci water exhaust
er
(41'F)
Fig. 3 - Schematic of Closed Cycle Rankine Engine Showing Typical
OTEC Parameters
pressure of 8.7 atmospheres at a temperature of 70F so that the turbine can
be two orders of magnitude smaller than in the open cycle (Ref. 22). After
driving the turbine the ammonia is recondensed at 51.2Fby heat exchange
with 40F water drawn from the lower ocean depths.
5.2 EFFICIENCY
The closed cycle systems operate on an overall ocean temperature
difference of only 68F (+7F depending on plant location, pipe depth and per-
haps the season). Temperature differences in the evaporator and condenser
(from sea water to working fluid) are 9F each. Thus only 18F is left for driving
the turbine. This yields a theoretical Rankine cycle efficiency (defined as the
working fluid's enthalpy change through the turbine divided by its enthalpy
change through the evaporator) of only 3.3% (Ref. 22). However, when the
pumping power required for the warm sea water, the cold sea water and the
working fluid and the efficiencies of the pumps, turbines, and generators are
taken into account, the theoretical 3.3% drops to a real net efficiency of only
about 2.5%.
34
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5.3 SIZE LIMITATIONS
Size does not presently appear to be a problem. Because of relatively
small temperature differences, the evaporation and condensation steps in the
cycle require about 4 to 6 million square feet of heat exchange surface per
100 MW of capacity (Ref. 24). Final assemblies are predicted to be about the
size of a modern supertanker tunnel on end, and they would float partially above
water or be completely submerged. Existing shipyards have the capabilities
to construct the assemblies which could be completely assembled at the ship-
yard and towed to sea or be assembled at sea.
5.4 DEVELOPMENT STATUS
The use of ocean thermal gradients to produce electrical energy on a
commercial basis is still in the conceptual design stage. Conceptual designs
for working plants have been drawn up, but some problems need more study
before a prototype design can be considered. The biggest problems are
centered on the heat exchangers for handling the low-quality thermal energy
inherent in the concept. Current studies are focusing on increasing the heat
transfer efficiency in order to reduce their size and cost and on forestalling
problems of biofouling in the marine environment (Ref. 29).
ERDA currently has 30 contracts with companies, universities and
government laboratories. The objective is to yield enough information and
advances in design to erect a pilot plant by late in this decade and a 100 MW
prototype before the mid-1980s. Two contractors (TRW Systems and Lockheed
Missiles & Space Company) are currently performing conceptual preliminary
design for Rankine-cycle based units.
35
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5.5 ANTICIPATED CONTRIBUTION TO U.S. ENERGY NEEDS
The relatively low thermal efficiencies and necessity to transmit power
from offshore virtually preclude the use of OTEC facilities for commercial
power generation. The most attractive thermal gradients are found along the
earth's tropical belt where the existing demand for electric power in general
is lowest. However, OTEC plants might find their main function as power
sources for chemical process facilities (Ref. 24). As an energy storage device
the OTEC power output can be used in the production of hydrogen which can be
shipped to shore for later conversion to electrical energy or other uses.
Ammonia can also be produced.
Energy produced via OTEC facilities for widespread commercial use
is predicted by the year 2000 (Ref. 19).
5.6 COSTS AND BENEFITS
If the costs of heat exchangers can be kept down ERDA feels that the
concept is attractive despite its low thermal efficiency. Projections of cost
(including transmission to shore) indicate it has the potential to be competitive
with other means of electrical power generation. TRW foresees operating
costs of 35 mills/kWh for its prototype and 20 mills for production units com-
pared to 28 to 33 mills for conventional electricity generation. Lockheed
figures are 36 and 21 mills, respectively, with construction costs about $200
million per platform or $l,250/kW. TRW feels that units from its design
would cost about $l,000/kW.
The chief benefit to be derived is that the initial costs are the primary
costs since no fuel need be purchased to supply energy to the system. Cur-
rent projections for ammonia indicate a demand of 10 million tons per year
by 1985, but new plants may not be able to get natural gas from domestic
sources. Twenty-one OTEC plants of 500 MW size could supply the required
energy to manufacture 10 million tons per year of fertilizers and chemicals.
Other energy-intensive products that could be -made at sea include aluminum
36
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(from alunina produced from bauxite on shore), magnesium, methanol, sun-
thetic and liquid hydrogen (Ref. 22). For the long term, liquid hydrogen might
become the largest volume product of tropical OTEC plants.
5.7 APPRAISAL OF ENVIRONMENTAL ASPECTS
The OTEC concept as an energy-producing system has received little
attention with respect to its Impact on the environment. Preliminary indica-
tions are that the major influence on the environment will be from indirect
effects, though some direct effects should be investigated, including leaching
of materials, spills of working fluid, upwelling and undersea plumes from
evaporators and condensers.
No appreciable environmental impacts from effluents are foreseen for
OTEC plants supplying electric power to shore or making liquid ammonia or
liquid hydrogen at sea. For production of other products, the question of
effluent wastes will have to be addressed. The effect of the undersea plumes
from evaporators and condensers is not known, nor is the potential value of
upwelling as a source of nutrients for mariculture. Leaching of materials
or spills of the working fluid can occur from an OTEC plant, but the effect
is not currently known.
Indirect effects from the OTEC plant include interference with shipping
lanes and ecological conditions. Interference with shipping lanes 'may occur
as a result of the number of units and the shear size of each unit required to
produce a desired amount of energy. Ecological aspects which must be con-
sidered are the effects on warming of surface films and plant breakwater
marine life in the area along with littoral drift.
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5.8 EVALUATION OF RESEARCH AND DEVELOPMENT REQUIREMENTS
Efforts in research and development should be directed as follows:
Establish siting limitations for clustered plants
Assess value of upwelling as a source of nutrients for mariculture
Assess effect of leaching of materials and spills of the working
fluid
Assess effects of discharged undersea plumes from the evaporators
and condensors.
The operation of an ocean thermal conversion plant will alter the local
surface temperature, the temperature gradient to depth, and possibly in-
crease local turbulence and currents. These effects, plus the possible
biological "pasture" or the artificial surface film (if used to increase surface
temperature) will affect the local albedo and the overall energy balance in
the vicinity of the plant. The net effect is currently unknown.
ERDA's present plans call for a 100 MW sized prototype hull with a
25 MW power module to be completed by 1983. A demonstration plant con-
sisting of the same hull but refitted with four advanced power modules is
scheduled for completion in 1985.
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Section 6
WIND POWER
6.1 DESCRIPTION OF THE SYSTEM
Man's use of the wind dates back many centuries (Ref. 26). The ancient
Chinese and other eastern peoples used windmills to pump water and employed
sails to drive ships. The Crusaders carried the windmill concept from the
Middle East to Europe. JLater, the windmill became an important part of
rural America, especially in the Midwest. Water pumping was the primary
application, but many windmills were employed to generate electricity for
farm lighting. Lead-acid storage batteries were used to store energy for
use during calm periods. Electrification of rural areas along with the avail-
ability of low cost, internal combustion engine powered pumps led to the de-
cline of the windmill; however, some are still in use in remote areas.
The kinetic energy of the winds can be used to produce mechanical
energy or electric power. The potential amount of wind energy available is
very large. For example, the estimate (Ref. 27) has been made that the energy
potential of the winds over the continential United States, the Aleutian arc, and
g
the eastern seaboard is equivalent to 10 MW. The most promising regions
for wind power applications (Ref. 28) are the New England and Middle Atlantic
coasts, along the Great Lakes, Gulf Coast and Aleutian Islands and through the
Great Plains, Rocky Mountains and Cascade Mountains.
The capability of a wind machine to exchange momentum is a function
of its aerodynamic shape and size; its location in the wind stream, whether
its blades are fixed in pitch, controlled in pitch to maintain a synchronous
shaft speed, or controlled in pitch to maximize momentum, exchange; and
whether there is any interference with optimum wake expansion. The best
machines are the so-called modern high-speed propeller machines with
39
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three or two very-high-aspect-ratio, thin, carefully twisted blades. The
less efficient the machine however, the easier it is to start at a low cut-
in speed, and the smaller the machine, the lower the cut-in-speed. One
can immediately see where cost becomes related to very high efficiency,
but machines of very good efficiency can be simple and thus relatively inex-
pensive and reliable.
The most simple configuration is probably that of a single modern high
speed wind generator atop the tallest western cedar pole that can carry it in
the expected wind regime, with account taken of storm and ice loadings. The
best location for the most simple configuration is in a clear area. If the ter-
rain is wooded, the machine will produce well, depending upon how much clear-
ance is provided between tree tops and the swept diameter. From the most
simple configuration one can progress down the path of added complexity and
cost and added productivity of grouping machines. Where the winds are light
to moderate, arrays of large numbers of small machines can produce a signif-
icant annual yield, whereas larger machines might not. L/arger machines are
more economical when the winds are moderate to strong. Concepts for the
placement of large numbers of small-to-medium-size wind generators, which
share support cost, lead to: (1) structural space array on top of towers, and
(2) cable-suspended arrays that would be analogous to hydroelectric dams.
Step-by-step transition from a single machine atop a pole to a large number
of machines in cable suspension systems can be seen in Figs. 4, 5 and 6.
Traditionally, a wind conversion system comprises a support tower, a
rotor, a step-up transmission, and an energy converter, such as an electrical
generator or a water pump. A control system can adjust rotor blade pitch and
rotate a platform at the top of the tower to keep the rotor facing into the wind.
The transmission and energy converter usually surmount the tower; but in
some designs the rotor joins a generator or water pump at the base of the
tower through a set of bevel gears and a long vertical shaft.
Horizontal axis rotors are the best developed and understood and aero-
dynamically the most efficient (Ref. 28). However it appears that, for the near
40
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MARK-I
Fig. 4 - MARK-I 20 kW Wind Generator on Single Pole Support
f
Wind
direction
MARK-I 11
MARK-V
Fig. 5 - MARK-III 60 kW Wind Generator on Stayed 100 ft Pole
and MARK-V 100 kW Wind Generator (520 kW Machine)
on 100 ft Tower
41
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Fig. 6 - Proposed Wire Rope and Kingpost Wind Generator System
Single Bank: 9.6 MW/mile, Double Bank: 19.2 MW/mile
42
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future at least, such rotors will not exceed about 150 feet in diameter and gen-
erate no more than a fewmegawatts of power in typical sites with favorable winds,
6.2 EFFICIENCY
No machine can extract all the power in a wind stream. The theoretical
limit for rotor is 59.3% of the power in the wind stream passing through the
area swept out by the rotor blades. A well designed rotor actually extracts
only up to 40 to 45% of the power of the wind (Ref. 28). When the rotor drives
an electrical generator through a gear-type transmission, the maximum power
output runs between 30 and 35% of the wind power.
The vertical axis lift type Darrieus rotor is an old concept which has
never been developed extensively; but is now being considered for both small
and large systems. As its principal benefit the vertical axis windmill offers
a higher efficiency (35 to 37%)- Moreover it is omnidirectional and does not
have to face into the wind. It can therefore use a more compact and less-
expensive tower than a horizontal axis windmill of the same capacity.
6.3 DEVELOPMENT STATUS
In 1890, the Danish government funded a program to improve windmill
performance and develop new concepts. From this program came the first
windmill-electric generator. Windmills developed during this period supplied
an equivalent of 200 MW of power. During the 1930' s interest in windmills was
high in Russia, Germany, France England and the U.S. In 1931 the Russians
built a 100 kW system which fed power into a large network until it was de-
stroyed in World War II.
The largest electrical system ever operated was the Smith-Putman ma-
chine built in Vermont during World War II. The plant was designed to feed
power into existing networks and was run a total of 1100 hours during four
years. In March 1945 one blade broke at a place where a weakness was known
to exist but was not corrected due to war-related shortages. Because there
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was no gurantee that the extra expenditure and experiments would provide a
cost effective plant, the program was terminated in November 1945.
The first large-scale wind turbine (Ref. 28) generator built under cur-
rent Federal funding resides at the NASA-Lewis facility at Plum Brook, Ohio.
It has a horizontal axis downwind rotor 125 feet in diameter designed for a
rated capacity of 100 kW in 18 mph winds. The rotor operates at a constant
speed, in winds over 18 mph, by feathering the blades. The blades are fully
feathered in wind speeds greater than 60 mph and the system is designed to
withstand 150 mph winds.
This initial 100 kW experimental system will test components and sub-
systems and will be used to collect performance data to aid in designing other
wind generators, of all sizes. Performance data collected will include energy
and power output at various wind speeds; loads, stresses and vibrations in
components such as blades, hubs and tower; and the stability and effectiveness
of the control systems.
6.4 ANTICIPATED CONTRIBUTION TO U.S. ENERGY NEEDS
Experience has shown that wind energy conversion systems can be built
and operated successfully. Accurate projections of potential wind power market
penetrations cannot be made until studies have answered the basic questions of
costs, service life, maintenance requirements and applications. However, it
is possible to make some preliminary estimates. It has been estimated that
about 20% of the electrical power demands in the year 2000 could be supplied
by wind power systems (Refs. 1 and 28). This is a somewhat optimistic view
which corresponds to the maximum available estimated wind energy. The most
pessimistic estimate is approximately 1% of the total yearly electrical needs.
It seems plausible that a sizable export market could be developed,
since fossil fuel availability and costs are far more severe in other countries
(Ref. 28). This would mean a favorable impact on the U.S. balance of trade.
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Whether the windmill can provide any significant energy generation in
the future depends on the resolution of two problems (Ref. 1); environmental
disturbance and economics.
6.5 COST AND BENEFITS
The best cost estimates based on experience show that wind plant
capital costs per kilowatt of installed capacity may eventually equal costs for
equivalent-capacity fossil fuel plants (Ref. 29).
Current costs of fossil fuel and nuclear power plants range from 400
to $800 per kW (1974 dollars) with the cost of some future nuclear plants pro-
jected to reach $1000 per kilowatt. Costs of large wind turbines are expected
to vary according to the number produced annually as follows (Ref. 1): 1 to
100 units - $300 per kW; up to 1000 units - $250 per kW, and; up to 20,000
units $100 per kW. In terms of average capacity at a favorable (30% use
factor), the preceding figures correspond to $1167, $833 and $333 per kW,
respectively. The result is that the price of power (in mills/kWh) delivered
by wind turbine generators, including fixed charges plus operation and main-
tenance cost will become more competitive with the price of power delivered
by conventional fossil and nuclear fueled plants (including costs of fuel, pol-
lution controls and waste disposal).
If wind tuxbine generators can be reduced in cost to $500 per kW
(average), then wind-generated electricity on a cost basis could replace con-
ventional fuels (but not conventional prices) at the following minumum prices
for such fuels (Ref. 1):
Coal - $29 per ton
Nuclear $146 per pound of U,Og.
For most large power systems, where the mix of fuel sources is
expected to contain more and more nuclear energy, the outlook for wind as
a supplemental source of energy is not promising. For smaller systems and
isolated communities that depend primarily on oil fired power plants, the
-------
prospects for wind energy applications as a supplemental source of electric-
ity are promising (Ref. 1) if low cost turbines can be developed.
6.6 APPRAISAL OF ENVIRONMENTAL ASPECTS
Wind power systems produce no air or water pollution and little noise.
Only a quiet swishing sound can be heard when one stands under a 15 meter
propeller-type rotor. However, the "visual pollution" imposed by the wind
plants and transmission lines to feed into a power network could be a problem
if care is not taken to make the plants aesthetically appealing. Potentially,
there could be a land use problem also because a tremendous number of wind
plants would be needed to make a significant contribution to the country's
future energy needs. Each plant would not require much land; but when a
large number (with transmission lines) are placed small distances apart to
supply most of the power in a region, the land rights, zoning regulations and
reactions of the local citizenry could raise substantial problems.
One estimate (Ref. 1) for wind plants indicated that 350,000 square miles
of the Great Plains would be suitable for wind turbines. To supply 15% of the
total energy supply in the year 2000 will require towers centered on each square
mile, each tower being 600 feet high and containing an array of 20 machines
with 50 foot diameter blades. The required land area is approximately equiv-
alent to the combined areas of North Dakota, South Dakota, Nebraska, Kansas
and Oklahoma. Actually, the machines would occupy only a small portion of
the area, but electrical interconnections and access roads would require ad-
ditional land. This is just an example which indicates the number of wind
plants required to contribute a significant portion of energy. More realistic-
ally wind mills will contribute energy to smaller communities and more iso-
lated regions as previously discussed.
Large numbers of densely concentrated wind power generators might
alter local wind patterns (Ref. 1) and consequently local weather. This poten-
tial effect has not been assessed. The obvious environmental disturbance will
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be that of aesthetics. Structures of the size and numbers required for large
scale power generation may not be palatable to many people.
Regarding weather modification, the effect might be analogous to an
increase in surface roughness of the terrain. Conditions may exist for which
the effects upon temperature, precipitation, or wind patterns would be signifi-
cant. It has even been speculated that the addition of angular momentum to the
atmosphere may have some relationship to tornado frequency.
Also, the rotating blades may adversely affect the bird population, with
a corresponding increase in the insect or rodent population. The "scarecrow"
effect may deter some types of wildlife from entering the area. The turbine
noise, infrasonic pressure waves, and altered wind patterns could impact
wildlife and domestic grazing animals.
6.7 EVALUATION OF RESEARCH AND DEVELOPMENT REQUIREMENTS
The effect of a dense concentration of windmills on local weather pat-
terns is not known. Current power output capability of an individual wind
plant virtually dictates a concentration of several plants to supply the power
for a community of any size. It is not known if the operation of the windmill
will alter the local wind pattern (induced local turbulence from the wash off
the turbine blades) or not. Consequently the first order of priority is to de-
termine the effect on local wind patterns. This can be accomplished in a
large industrial wind tunnel such as the one located at Colorado State University.
Environmental chambers such as this have the capability to model the terrain
and examine the effect of structures placed within the terrain. Consequently
from scale model testing such as this, the effect of such factors as plant
height, blade design, plant density, material requirements and their manu-
facture, etc., can be studied. ERDA's schedule calls for a 10 MWg facility
to come on line in 1981. Large scale production of wind turbines is slated to
begin in 1983. Under optimum conditions, a rated wind energy conversion
capacity of 15,000 MW is predicted for 1985.
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Section 7
TURBINES
7.1 GENERAL
Turbines provide a means to convert the energy contained in a gas
stream to mechanical energy via expansion of the gas stream through the tur-
bine. Turbines generally operate in conjunction with a compressor located
upstream of the turbine, but driven by the turbine. The function of the com-
pressor is to compress the gas stream to a high pressure before heat addition
and expansion through the turbine. The excess mechanical energy, above that
required to drive the compressor, derived from the turbine is then used to drive
a generator to produce the electrical power.
Turbines are used in both open and closed systems. In an open system
a high temperature gas stream from a combustion device passes through the
nozzle and turbine and is then exhausted external to the system. A closed
system differs from the open system in that the working fluid continually
circulates through the system and energy is supplied externally through a
heat exchanger. Li a conventional steam plant a boiler via a heat exchanger
provides high pressure, high temperature steam to the turbine which transfers
mechanical energy to the generator. A condenser is then used to extract
heat from the steam and return low pressure water to the boiler.
A combined cycle is one which employs some combination of an open
and closed cycle operating to produce electrical power. A discussion on
turbines utilized by power generation plants naturally divides into steam and
gas turbine categories. Steam turbines are utilized in closed systems while
gas turbines, depending on the application, are utilized in both open and closed
systems. Consequently the following discussion addresses steam and gas
turbine applications to electrical power generation.
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7.2 STEAM TURBINES
7.2.1 Description of the System
The steam turbine is a heat engine that takes heat from a high temp-
erature source, converts it into mechanical energy, and rejects waste
heat at a lower temperature. A representation of the operation of a heat
engine is shown in Fig. 7. According to the Second Law of Thermodynamics,
to convert all of the transferred heat into mechanical energy is impossible;
that is, a heat engine cannot be 100% efficient. The most efficient heat
engine operating within the constraints of the Second Law is one that follows
a theoretical concept known as the Carnot cycle. While the features of
this concept are not attainable in an operating heat engine system, the
cycle is useful as a standard in evaluating the performance of actual heat
engines.
Steam turbine energy systems are based on the Rankine cycle, a
practical modification of the Carnot cycle. In the Rankine cycle, heat from
the energy source (fossil fuel combustion gases or nuclear fuel) is trans-
ferred to water at high pressure in a boiler and produces high pressure,
high temperature steam. The steam enters the turbine where it expands to
a low-pressure, low-temperature steam and in so doing does work against
the turbine blades, causing the turbine shaft to rotate which in turn drives
an electrical generator. After the thermal energy in the steam has been
converted to mechanical energy in the turbine, the discharged (spent) steam
is converted back into water in a condenser. The water is then pumped back
into the boiler and starts the cycle over again. The heat removed in the con-
denser is rejected to the environment through the use of cool bodies of water
(i.e., lakes, ponds or rivers) or of cooling towers. This cycle is shown in
Fig. 8.
Modifications to the Rankine cycle which improve its thermal efficiency
use the concepts of regeneration and reheat. In the reheat process, a portion
of the steam that has partially expanded to an intermediate pressure in the
49
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HIGH TEMPERATURE BODY
HEAT TRANSFERRED
TO HEAT ENGINE
MECHANICAL ENERGY
OUTPUT
HEAT REJECTED
FROM HEAT ENGINE
LOW TEMPERATURE BODY
Fig. 7 - Operation of a Heat Engine
HIGH PRESSURE
HEAT INPUT
TO CYCLE
(FUEL)
HIGH TEMPERATURE STEAM
BOILER
TURBINE
PUMP
HIGH PRESSURE
WATER
CONDENSER
LOW PRESSURE
WATER
GENERATOR
ELECTRICAL
ENERGY
MECHANICAL ENERGY
OUTPUT TO GENERATOR
LOW PRESSURE
LOW TEMPERATURE
STEAM
HEAT REJECTED
FROM CYCLE
Fig. 8 - Basic Components of a Rankine Cycle Heat Engine
as Used in Steam Turbine Power Plants
50
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turbine is reheated in the boiler and then returned to the turbine to complete
the expansion process (Fig. 9). The regenerative process extracts a fraction
of the steam from the turbine after partial expansion and uses it to heat the
water leaving the condenser before it enters the boiler. The device where
this heat exchange occurs is called a feedwater heater. A number of feed-
water heaters are generally used in modern systems. This process is shown
in Fig. 10.
Typical steam power plants will use both reheat and regeneration. The
extent of reheat and regeneration for a particular plant will be determined by
economic considerations, principally the fuel cost. The light water reactor
(LWR) nuclear plants in operation today, for the most part, use the regenerative
process only because the temperatures available in LWRs are not particularly
economical for reheat purposes.
7.2.2 Efficiency
The maximum efficiency for a heat engine is the Carnot cycle efficiency,
which is a theoretical efficiency that cannot be achieved in practice but which
serves as a measure of performance for actual cycles. The Carnot efficiency
* S r-p rp
H " L
n
TH
where
n = Carnot cycle efficiency
TT = low temperature of heat rejection, R
J_i
T = high temperature of heat addition, R
The equation shows that theoretical efficiency is improved by increasing the
heat addition temperature, TT,, and decreasing the heat rejection tempera-
rl
ture, TT . Steam turbine systems typically operate between a maximum
i-i
temperature of 1000F (1460R) and minimum temperature of 70F (530R). A
Carnot cycle operating between these temperature limits of heat addition
and heat rejection would have an efficiency of 65%.
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BOILER
TURBINE
REHEAT STEAM
D-
PUMP
CONDENSER
Fig. 9 - Rankine Cycle with Reheat
52
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BOILER
EXTRACTION STEAM
TURBINE
CONDENSER
FEEDWATER HEATER
Fig. 10 - Rankine Cycle with Regeneration
53
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Actual steam turbine plant efficiencies for units in the 1000 MW range
are on the order of 38 to 40% for fossil-fueled and HTGR units and 31 to 34%
for BWR and PWR units. Improvements to these efficiencies through the use
of additional stages of reheat and regeneration are not economically practical
at the present time, because the increased investment costs offset the operating
savings .
Use of higher steam temperatures and pressures to improve efficiency
of fossil units is limited because: (1) metals currently used are near their
metallurgical limit, and (2) metals that can withstand more extreme steam
conditions are too costly to be economical and have a limited lifetime. This
is referred to in a new approach by General Electric (Ref. 30) in which the
turbine is water cooled, p&rmitting turbine operating temperatures on the
order of 2800F. Efficiency is expected to be increased to about 50%.
7.2.3 Size Limitations
The size of steam turbine units is expected to increase above the present
maximum of about 1300 MW in order to reduce capital, operating, and mainte-
nance costs on a per kilowatt basis. Although these large units will require
some improvements in turbine, generator and boiler design, no major problems
are expected.
Factors that may have an effect on plant size are cooling water and land
area requirements. Because larger units require greater amounts of cooling
water and regulations are being introduced that limit the amount of heat that
can be discharged into natural bodies of water, sources of cooling water for
large plants have become a problem. Greater land area requirements are a
result of larger coal and ash storage areas, flue gas cleaning equipment, and
cooling facilities for the condensed cooling water. Dry cooling towers are
expected to occupy a position of greater prominence.
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7.2.4 Development Status
It is believed that there are not likely to be many major improvements
in steam turbine technology. Advanced blade technology, seals, and moisture-
removal techniques as well as lower-cost, high-temperature alloys are areas
receiving current attention.
A major problem in further development of high temperature turbines
is the fact that alloys currently used in turbines are just about at their tem-
perature limits. One line of research is directed toward finding substitute
turbine blade material such as ceramics and composites for the alloys.
General Electric is currently developing a new high-temperature turbine
(Ref. 30) that should be available to utilities before turbines made from new
materials. The GE turbine relies on water instead for cooling vanes and blades.
A utility-sized version producing 180 MW is expected to operate at 2800F.
Thus far, the experimental device has produced 2 MW at temperatures be-
tween 2800 and 3500F and pressures greater than 200 psi.
Water is circulated through the turbine blades through small channels
close to the surfaces of the blades thus protecting the metal parts from over-
heating. Water is ejected from the tips of the spinning blades. The water
ends up in a collecting device surrounding the turbine shroud and is returned
to a heat exchanger for recycling.
7,2.5 Anticipated Contribution to U.S. Energy Needs
Approximately 78% of the electric generating capacity in the U.S. in
1970 was based on steam turbine energy systems, and this percentage is
expected to increase slightly by the year 2000. The remaining 22% of capacity
was supplied by hydroelectric (~15%) and gas turbine and diesel electric (~7%)
power systems.
The kinds of service provided by the various types of plants can be
classified in terms of base, intermediate or peaking load. Base-load units
55
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are large, efficient units that operate continuously at or near their full
capacity. Typical annual capacity factors (percentage of annual output if
operated continuously at maximum capacity) are around 80%. Intermediate
load units are smaller, less efficient and typically are required to shut down
and start up daily as demand varies. Capacity factors vary from 20 to 60%.
Peak load units provide power for short periods of the day, when the demand
for electricity is at its maximum, and have capacity factors of 20% and less.
Steam turbine systems are predominantly used for base load and inter-
mediate load service. Base load service is provided by large fossil fueled
and nuclear units, whereas intermediate service is provided by either older
or small fossil fueled units, originally designed for base load, or newly de-
signed fossil fueled units built specifically for this service.
New peaking service is now generally provided by pumped storage,
gas turbine or diesel energy systems, rather than steam, turbine systems,
because of the quick startup requirements and the economics involved.
7.2.6 Costs and Benefits
The preponderance of the electric generating capacity of the United
States today is based on the utilization of the Rankine cycle, which attests to
its relative economics. The very wide range of conditions for which an
individual unit may be designed (i.e., varying construction conditions, varying
labor productivity) leads to significant cost differences of plants installed at
different locations within the nation. Environmental control costs will also
add substantial amounts to the basic costs of the plant.
Disadvantages exist with the steam turbine energy system that are
prompting investigations into alternative electric generation schemes.
Principal factors are its associated adverse environmental effects and the
desire for higher efficiencies than can practically be obtained from a steam
Rankine cycle alone. Low efficiencies result in higher rates of (1) consumption
56
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of limited fuel reserves; (2) air pollution; and (3) thermal pollution. The
indirect method of electric generation - energy transformations from
chemical or nuclear to thermal, from thermal to mechanical, and from
mechanical to electrical - along with the large and complex equipment used
is also considered a system disadvantage when compared with other genera-
tion concepts.
Not withstanding these considerations, the steam turbine system is
currently the most economical and technologically developed energy system
available to the electric power industry.
7.2.7 Appraisal of Environmental Aspects
The principal means of operating steam turbines in power plants is
by coal fired boilers to produce high pressure and high temperature steam.
Typical coal fired power plants emit significant quantities of particulates
(fly ash) and noxious gases such as sulfur oxides, nitrogen oxides, carbon
monoxide and hydrocarbons (Ref. 1). Heat rejected by the condenser cooling
water to rivers and lakes is considerable; in 1964, for example, of all
industrial cooling water used in the U.S., 81% was used by electric power
plants. These aspects have received considerable attention over the past
few years. The main consideration is what effects steam power plants cur-
rently under development and design have on the environment.
In the foreseeable future, the primary means of steam generation for
the steam driven turbine will be a fossil fuel fired boiler with primary em-
phasis on increasing efficiency of the turbine. An advancement in the thermal
efficiency provides the double environmental gain of reducing thermal and
other waste emissions to the environment and conserving resources of natural
fuel.
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7.3 GAS TURBINES
7.3.1 Description of the System
The gas turbine system has the function of converting input chemical
energy of fuel into heated, compressed gas that expands while doing work on
rotating blades similar to the steam turbine. The mechanical output is
coupled to a generator shaft which in turn generates electrical power. Com-
ponents of this system include a compressor, a combustion chamber, and one
or more turbines together with heat exchangers, as called for by cycle design
(Fig. 11). In the simplest cycle, no heat exchangers are employed. An im-
portant characteristic of the gas turbine is the essential requirement for a
clean (no particulates or corrosive components) gas flow through the turbine,
forcing the need for a clean burning fuel or a source of high temperature
thermal energy, such as a nuclear reactor, where the fuel element coolant
is the high pressure heated gas for the turbine expansion.
I
One of the salient characteristics of a gas turbine is its requirement
for a clean fuel so that the gas flow through the turbine is neither erosive
(from particulates) nor corrosive (from vanadium, sodium, potassium, lead
and sulfur compounds). Calcium is also troublesome because it forms hard
deposits. All of these elements are contained in residual fuel oils the low
cost residue of the petroleum refining processes that produce the distillate
fuel oils (diesel and kerosene) and gasoline. As a consequence of this, com-
paratively clean residual and crude fuel oils must be carefully selected, and
these must then be treated further before fuel oil products can be used for
gas turbine operations. In addition, the growing need for unlimited oil re-
sources for other applications makes this energy source questionable for
large scale use in the electric power industry.
Gaseous fuels present no problems of this nature. Natural gas as dis-
tributed by utilities is an ideal fuel, but its scarcity also mitigates against
use for electric power generation. Considerable attention is being given to
the possibility of using high or low Btu gas derived from coal gasification,
58
-------
Exhaust
Gas
Combustion
Chamber
Compressor
Generator
Air
Fig. 11 - Regenerative Cycle Gas Turbine
59
-------
and there does not seem to be any technical problem in doing so. Coal gasi-
fication systems must provide economical techniques for removing sulfur. The
gas turbine and the combined cycle plant might adapt very well to this type
of fuel.
As discussed previously, high temperature helium gas cooled thermal
nuclear reactors coupled to steam turbine converters (Fig. 12) are now being con-
sidered by the utilities as a viable alternative to water cooled reactors, and
a number of units have been ordered. As a result, increased emphasis may
be placed on the closed cycle helium gas turbine rather than the steam turbines
as the energy conversion system.
i
7.3.2 Efficiency
Gas turbine plants now available have the following efficiencies:
Ŧ Simple cycle, 27%,
Combined cycle, 36 to 38%,
Regenerative cycle, 34%.
With currently available materials and turbine-cooling technology, commercial
designs should be available in the 1975-77 period having better thermal effi-
ciencies, by a factor of 1.1 or more, which could make the combined cycle
competitive with the best available conventional steam plants. By 1980,
further evolutionary progress is expected to yield improvements resulting in
a 1.2 miltiplier on present day thermal efficiency performance.
7.3.4 Size Limitations
One great advantage of the gas turbine cycle engine is that it lends
itself to the concept of modular design and factory fabrication. The result
*
is substantial economies in lead time and in costs for field erection. Another
advantage of the modular concept is that good partload fuel economy can be
realized by shutting down one or more units when only part of the total capacity
60
-------
Regenerator
Precooler
' y Generator
Coolino Water
J
Fig. 12 - Nuclear Cycle Gas Turbine
-------
is needed. Multiplicity of units also affords improved reliability and avail-
ability, as maintenance can be done to a single unit with only a partial re-
duction in capacity. These capabilities are highly desirable for plants used
for the midload service range.
7.3.5 Development Status
The outstanding advantages of the gas turbine for aircraft propulsion
has produced the research and development effort that led to the improved
aerodynamics of flow path design, metal alloys allowing high turbine inlet
temperatures, and improved methods of cooling turbine blades and nozzles.
The fallout of this technology has greatly improved the position of the gas
turbine and has led to its acceptance for peak load central station power
service.
7.3.6 Anticipated Contribution to U.S. Energy Needs
Today the simple cycle gas turbine prime mover is favored for new
equipment to accommodate the peak portion of the electrical power demand.
Fast start, low initial cost, and short delivery time are features desired for
peak-load plants and are met by gas turbine units. An important variation
of the simple cycle system is the combined gas turbine and steam plant.
Here the hot exhaust from the power turbine is used to generate steam in
an unfired boiler. The steam is used in a conventional system to generate
50% more power without additional fuel (Fig. 13). The combined cycle thermal
efficiency is comparable with that of a modern steam plant and is being used
by some utilities for serving intermediate system loads. One forecast is that
by 1980 the gas turbine and the combined gas turbine and steam power plant
could be providing some 25% of the power requirements of the electric utility
industry in meeting peak and intermediate load demands. Gas turbine cycles
are expected to be used in high temperature gas cooled reactor (HTGR) and gas-
cooled fast reactor (GCFR) systems.
On the basis of present technology, the role of the gas turbine prime
mover as an electric power producer through 1990 should be largely in
62
-------
Boiler
Fuel
Exhaust
Gas
Air
Generator
Cooling Uater
Generator
Fig. 13 - Combined Cycle Gas Turbine
63
-------
peaking and intermediate load operation, where it will contribute possibly
as much as 30% of the power capability and about 15% of the total electrical
energy production. Developments in gas turbine technology that improve
efficiency might make this system more attractive for base load service.
Gasification of coal could enhance the gas turbine's position by making it a
possible alternative for base load service.
7.3.7 Costs and Benefits
Low initial cost is an area that has made gas turbine energy systems
particularly attractive to utilities. The efficiency has been of lesser impor-
tance for peaking service, but whether this will hold in the future, because
of the dwindling supply of clean fuel, is largely unknown. The relative sta-
tion costs and performance levels of gas turbine plants are as follows:
Thermal Efficiency
Type $/kW (%}
Simple Cycle 90 27
Regenerative Cycle 100 34
Combined Gas and Steam 150 37
Turbine
The comparable fossil fueled steam turbine plant figure is $180 per
kilowatt for a plant without sophisticated environmental controls; this cost
could escalate to over $300 per kilowatt when environmental controls are
added. The efficiency of modern steam turbine plants is about 39%.
The cost advantages of the gas turbine cycles arise primarily from the
elimination of a fired, high pressure boiler with its superheater, reheater and
regenerative feedwater heaters. These steam-generating components cost
about 40 to $50 per kilowatt. Though a boiler is incorporated in the combined
cycle, its cost is only about 15 to $20 per kilowatt, as it is an unfired heat ex-
changer operating at a low pressure (less than 1000 psi).
64
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Further advantages of the gas turbine are lower construction costs,
about $5 less per kilowatt, and lower interest and escalation charges by about
$20 less per kilowatt, due in part to much shorter field erection times. Pro-
jections for the future indicate that the gas turbine plant advantage in base
line cost figures will increase further. This conclusion results from the fact
that available technological improvements, which may be incorporated into the
1975-1980 designs, will increase specific power by 40% or more. The result
is that a. given size (and cost) of turbomachinery has a higher kilowatt rating.
Moreover, there will be significant gains in thermal efficiency, although per-
centagewise not as much as for the rating gain.
7.3.8 Environmental Considerations
The site requirements for gas turbine fossil fuel plants are modest in
acreage. The noise levels are low as the high frequency noise typical of
turbomachinery may be acoustically treated at low cost.
Stack gas pollutants are virtually nil insofar as carbon monoxide and
hydrocarbons are concerned. As a low sulfur, low ash fuel is a requirement
for the turbine operation, fly ash and sulfur dioxide emissions are also
negligible. However, a present problem area is the stack effluent of oxides
of nitrogen (NO and NO7). The technique now used to treat the problem is to
L*
inject demineralized water into the combustion chamber, at a mass flow rate
comparable to the fuel rate, for loads above 40% of rating. Most gas turbine
manufacturers feel that they will be able to offer combustion chambers that
will reduce oxides of -nitrogen without the added complexity of water injection.
A set of emission standards is needed (such as the maximum values
applying to conventional steam plants) to provide realistic design targets
for research to reduce the oxides of nitrogen.
Simple cycle and regenerative cycle plants, relative to the combined
gas turbine and steam cycle plants, do not have an extensive requirement
for cooling water. The combined gas turbine steam cycle has cooling water
requirements about 40% or less than those of a conventional steam plant.
65
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Section 8
THERMOELECTRIC CONVERTERS
8.1 DESCRIPTION OF THE SYSTEM
A thermocouple, is a device consisting of two dissimilar conductors
joined together to form two junctions and a closed electrical circuit. As
long as the temperatures of the two junctions are not equal, a current will
flow in the circuit. This effects was discovered in 1822 by T. J. Seebeck
(Ref. 31). The Seebeck effect suggests the potential for direct conversion
of heat to electricity without the use of moving parts (Fig. 14).
CONDUCTOR A
Tl
HEAT-~~ V°s HEAT
SOURCE/ ^ ^^ \SINK
CONDUCTOR B
Fig. 14 - Simple Thermocouple
Since a device that utilizes the Seebeck effect is a heat engine, it is
subject to the usual laws of thermodynamics and its maximum efficiency is
the Carnot efficiency. However, losses always limit a practical device to
efficiencies that are some fraction of the Carnot efficiency. For a thermo-
electric generator with common metal junctions, and even with a temperature
difference of several hundreds of degrees (between hot and cold junctions),
this fraction of Carnot efficiency is about 0.1%. For the best metal junction,
66
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one formed of antimony and bismuth operating below their melting points, the
efficiency is about 1%. To be considered as a replacement for a Rankine cycle
(conventional steam cycle) plant, the efficiency must be nearer to 50% of Carnot.
It is obvious that materials other than the common metals must be used if
thermoelectric power generation is to be economically feasible for large scale
applications. A number of materials, elemental and compound, are of interest.
In certain cases these materials are semiconductors. Many compounds have
been studied but only tellurides of Pb, Bi,. Ag, Ge, Sb and Sn [ e.g., PbTe, Bi,Te,
t* J
GeTe-AgSbTe (TAGS), PbSnTe, BiSbTe] and SiGe have been used extensively in
practical devices. These 'materials have potential efficiencies in the range of
11 to 27%. The efficiency of practical devices will be lower.
8.2 EFFICIENCY
Table 8 lists several materials in use in thermoelectric devices as well
as some of the parameters pertinent to efficiency calculations. The product
ZT. (T is the mean operating temperature and Z is a term called the figure
of merit) will determine the percent of Carnot efficiency obtainable. The effi-
ciency obtainable from an operating couple is found using the information in
Table 8 and Fig. 15. No 'material listed, operating with a sink temperature of
27 C and a source at the maximum allowable temperature, can approach an
efficiency of 20%.
The efficiencies shown in Fig. 15 have been calculated for ideal conditions.
Some representative numbers for actual devices as taken from Table 8 show
the severity of the materials and engineering limitations for thermoelectric
generators (TEGs). A number of TEGs used primarily for space applications
(where efficiency is not necessarily the most important consideration) have
efficiencies of less than 6% and most are in the 4 to 5% range. TEGs for
terrestrial applications have efficiencies generally in the range of 4 to 6%.
Recognizing the reduction in efficiency in a real device, figures of merit
above 5 x 10"3 are necessary to attain overall system efficiencies of about 10%.
Whether or not this Z is obtainable is open to serious question. A paper pub-
lished in 1967 by Ure, a well-known worker in the field of thermoelectricity,
67
-------
30
20
z
tU
O
u.
10
Z=5xlO-3,
Z=4xlO-3,
Z=3xlO-3
Z=10-3
0 100 200 300 400 500 600 700
DIFFERENCE BETWEEN HOT AND COLD JUNCTION TEMPERATURES
Fig. 15 - Efficiency of a Thermoelectric Generator
68
-------
Table 8
THERMOELECTRIC MATERIALS*
Materials
Bi9 Te,
^ 3
BiSt) A T c_ ~
J_.*l^ŧ--^ .ŧ. v^y j-
Bi2Te2Se
PbTe
Ge Te (+Bi)
ZnSb
AgSbTe2
InAs(-t-P)
CeS^Ba)
CugTe3S
Ge-Si
Ge-Si
Melting
Point
(C)
575
904
725
546
576
940
930
Type
n or p
P
n
n or p
P
P
P
n
n
n
P
Z
max
(figure of
merit)
2.0 x 10"3
3.3 x 10"3
2.3 x 10"3
1.2 x 10"3
1.6 x 10"3
1.2 x 10"3
1.8 x 10"3
6.0 x 10~4
8.0 x 10"4
1.5 x 10"3
9.0 x 10"4
6.0 x 10"4
Temp.
for zmax
(C)
27
27
27
27
527
227
427
627
927
827
627
627
Max.
Operating
Temp. (C)
177
177
327
627
627
327
627
827
1027
927
927
'fD.A. Wright, "Thermoelectric Generation," in Direct Generation of Elec-
tricity, K.H. Spring (ed.), Academic Press, New York, 1965.
69
-------
states that a ZT of about 2 to 2.5 seems to be an upper limit. To obtain a
ZT of 2.5, operating between 27C and 727C, a material would need a Z of 3.84
x 10~3. The efficiency would be about 28%. With a Z of 5 x 10"3 and opera-
tion between the same temperatures, the efficiency would be 32%.
8.3 SIZE LIMITATIONS
A thermoelement is inherently a low power device. By appropriate
series/parallel electrical arrangements, higher power outputs can be obtained.
This modular system lends itself to the construction of high power systems,
but still has a very low output for its size and weight.
A 150-W solar powered TEG would use 480 couples, with a weight of
1.62 Ib for a power density of 94 W/lb for the elements alone. In the actual
generator, this drops to 11.3 W/lb (Ref.32). A radioisotope thermoelectric
generator for use in a Transit navigational spacecraft, TRIAD I, has a power
density of 1.2 W/lb for the total assembly. For the thermoelectric panels,
the power density is about 6 W/lb (Ref. 33).
8.4 DEVELOPMENT STATUS
Extensive effort has been devoted to the development of thermoelectric
materials -with a high figure of merit, especially those materials that operate
at higher temperatures and efficiency. Silicon-germanium alloys are con-
sidered as especially promising for operating temperatures near 1000C, and
these materials are under active investigation (Ref. 33).
The techniques of joining the couples to the metal plates to form junc-
tions and terminals are fairly well established, although these joining tech-
niques are more art than science. Each new combination of materials
introduces new problems that 'must be solved before the thermoelectric ma-
terial can be used in a practical generator. Additional problems are often
introduced by the brittle nature of most of the useful 'materials.
70
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8.5 ANTICIPATED CONTRIBUTION TO U.S. ENERGY NEEDS
The main benefit of the thermoelectric generator is that it has no moving
parts which will tend to increase its reliability and long life. The modular
construction of a TEG allows a variety of power levels to be easily obtained
for a given basic couple.
The low efficiency and low power output per couple, together with high
unit costs, will probably limit the application of TEGs to small special-purpose
power sources. The present economics are unacceptable for central-station
power application and the low efficiency would create severe drain on our energy
resources.
8.6 COSTS AND BENEFITS
Present costs for small fossil fueled TEG systems are about 25,000 to
$30,000 per kilowatt electrical. This cost is more than 50 times the cost of
a large conventional power plant. There is no large obvious reduction in unit
cost that can be projected for increasing plant size since many small elements
are required and the high unit cost still prevails. Mass production techniques
applied to TEGs would, however, tend to reduce unit costs below what they are
today.
An example of the cost of a thermoelectric generator can be found in
the catalog of an established supplier of thermoelectric devices (Ref. 34). A
20-W TEG that operates between 125 and 25C requires 26 modules, each con-
taining 31 couples. Since the cost of each module is $60 (1971 catalog price),
the cost of the TEG is $78 per watt. More recently, different suppliers offer-
ing other types of thermoelectric devices have quoted prices in the range of
$40 per watt. There do not appear to be any benefits accruing from TEG for
commercial electric power generation.
71
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8.7 APPRAISAL OF ENVIRONMENTAL CONSIDERATIONS
Since a TEG is a thermal conversion device with no moving parts, the
only pollution results from the heat source. Naturally, being a thermal
engine governed by the laws of the thermodynamics, heat will be rejected to
the surroundings.
The low efficiency of the TEG means more thermal energy must be re-
jected to the environment. Conversely, for the same useful power emitted,
more fuel is consumed. With existing low efficiencies, energy sources will
be depleted at a faster rate than is now the case. For central station plants
this is unacceptable.
72
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Section 9
THERMIONIC CONVERTERS
9.1 DESCRIPTION OF THE SYSTEM
The principle of operation of thermionic devices is based on the emission
of electrons from metals at high temperatures. This phenomenon was first
investigated by Thomas Edison and was subsequently used as the basis of the
conventional vacuum tube.
A thermionic converter is a device that contains an electron emitter and
collector in a sealed envelope at reduced pressure. The emitter is heated, in-
creasing the energy of the free electrons in the metal and causing them to travel
faster. This increased kinetic energy allows the electrons to escape from the
open surface of the hot emitter and to 'move through an intervening space to the
cooler electron collector. With no external circuit connections, a potential
difference (voltage) will develop between the collector and emitter. When con-
nected to an external circuit, the potential difference will cause a current to
flow (Fig. 16). In a thermionic converter with reasonable spacing between the
emitter and collector, some of the emitted electrons do not have enough energy
to reach the collector, so they form an "electron cloud" (or space charge) which
tends to repel subsequent electrons and hence limit the available current. In
order to achieve reasonable power density, a low pressure ionized vapor (usually
cesium) is introduced to neutralize the space charge.
9.2 EFFICIENCY
The theoretical efficiency of a thermionic converter is limited by emitter
and collector temperatures. As in any heat engine, the theoretical efficiency
is seldom attained.
73
-------
EMITTER COLLECTOR
ELECTRONS /
HEAT
IN
_ ^^ŧ
*" ^^^^^r**^
/*
-*-
-^__-^
-^ x~^-
>
Đ/
I
Đ-
A^ -
^-^
Đ-
Đ--
Đ-
e-
e-
Đ-
1
- > ŧ-
__ŧ._ŧ.
^->
I > 1 1 > 11
->-->-.
~ ^~ *
rŧ j*ŧ *i^
--ŧ->--
- > > -
;^^";;
\
\
V
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1
V Ĩ
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* i
* t 1
1 1
XS *
w '
XS\\
HEAT
OUT
x-> , *-*,,
^ ^_
^
-------
9.3 SIZE LIMITATIONS
Power systems that utilize thermionic converters will consist of indi-
vidual units connected in series and parallel combinations to produce the
voltage and current requirements for the various applications. Construction
will be modular, and the unit size selected will depend on a number of considera-
tions. Thermionic module size in the TVA Bull Run Plant analysis was set at
22 MW. Consideration is currently being given to applications in modified
fossil fueled central station boilers with plant electrical capacities in the
hundreds-of-MWe range.
9.4 DEVELOPMENT STATUS
Thermionic converter systems can be used with thermal inputs from
any source, including solar and nuclear power. However, from the stand-
point of central station power application, the 'major interest in thermonic
conversion is as a topping unit for fossil fueled plants. Thermionic con-
verters are most efficient at high temperature, and they match the heat-
source properties of a fossil fueled plant well. Central-station nuclear
power reactors are not suitable for thermoionic applications since it is not
practical to incorporate these conversion systems within the core of the re-
actor, and neither the water cooled nor the sodium cooled reactors operate
at high enough coolant temperature to consider locating the thermionic con-
verter outside the reactor.
Although, in concept, the thermionic converter is a relatively simple
device, building long-lived efficient thermionic converters is no easy task.
The electrodes must operate close to one another and at high temperature
so that the level of power generated is sufficient for practical applications.
Also, the high operating temperature leads to high efficiencies. For example,
the emitter may operate at 1880F and the collector at 918F. Under these con-
ditions the theoretical efficiency is 41%; however, practical devices will never
achieve this ideal efficiency. A high potential efficiency, as well as the feature
75
-------
of having no moving parts, makes thermionic energy conversion worthy of
further consideration as a topping system with more conventional power
cycles (Ref. 38).
With the exception of the concept developed for application to the TVA
Bull Run coal plant, little has been done until recently in evaluating thermionic
power systems applied to central station power, particularly not to coal fired
plants designed to meet EPA pollution standards. Present program efforts
are focusing on these applications again. The state of the art is primarily
based on the AEC/NASA program. Based on this work, thermionic devices
are technically feasible but need further development to extend their lifetime.
The experimental work of the 1960s identified most of the problem areas
in converter design and operation except those of the economics of central
station power application. The cesium environment and high operating tem-
peratures can cause emitter vaporization, thermal warping, insulator shorting
and seal failures. For space applications, the main problems were concerned
with achieving the following:
Lifetime of at least 5 years
ŧ Reproducible and stable thermionic converter performance
ŧ Demonstration that any electrical arcing that might occur
is not destructive to the cell and will not result in excessive
power losses
Qualification to expected shock and vibration environments
Ŧ Simplication of fabrication methods and lower costs.
The use of chemical vapor deposition as a technique for cladding con-
verter emitters with tungsten has been successful in establishing stable long
term performance. Adoption of fine grained, high density alumina with niobium
skirts brazed with a V-60/Nb-40 alloy may eliminate the insulator problems.
Finally, the introduction of oxygen into the converter may reduce operating
temperatures and improve the overall performance. Both lower cost materials
76
-------
and fabrication methods are required. For topping cycles, research is
focused on achieving high efficiency and lower costs at lower and more
practical operating temperature ranges.
9.5 ANTICIPATED CONTRIBUTION TO U.S. ENERGY NEEDS
Thermionic converters have several potential applications ranging from
a cardiac pacemaker that operates in the 0.1-mW range to a modified (topping)
thermodynamic cycle for a central station power plant that operates at 1000
MW. Thermionic devices, which are coupled with nuclear heat sources, are
especially attractive for long range and long duration space applications
because of their basic simplicity, the absence of moving parts, and their rela-
tively higher efficiency as compared with other space power generators.
Interest in the thermionic device as a topping unit for conventional
central station power plants rests on its potential for increasing overall plant
efficiency. Furnace temperatures which are not normally usable in conven-
tional boilers and steam turbines, because of metallurgical limitations, can
be effectively used with thermionic converters to increase the overall efficiency
of the cycle. An analysis carried out for the Tennessee Valley Authority (TVA)
Bull Run coal fired plant shows that thermionic topping might result in an in-
crease of station output from 914 to 1139 MW and a gain in plant efficiency
from 41.3 to 50.6%.
A thermionic energy conversion system has the potential to improve
fossil fueled plant efficiency from the present 40% to possibly 50%. The
system should be particularly adaptable to coal plants in which the combustion
chamber temperature is well above the normal working temperature of the
steam turbine. Insufficient studies are available to establish requirements
of thermionic power systems as applied to new coal plants that will meet EPA
standards. At present, low cost, reliable converters have not been developed,
and thermionic topping cycles for coal fired steam turbine power plants cannot
be justified on an economical basis.
77
-------
A survey conducted by Chemical Engineering (Ref. 19) indicates the
breakthrough in thermionic converters should occur by about 1982. The
economic feasibility of the concept and its commercial and widespread use
is predicted by about the year 2050.
9.6 COSTS AND BENEFITS
With the exception of certain terrestrial and hydrospace applications,
the cost of thermionic converters for producing power has not been assessed.
The value of a thermionic converter for a fossil fueled plant can be estimated
based on the incremental efficiency produced by a topping cycle operating with
no degradation of the steam plant performance. Using a capital cost of $300
per kilowatt-electrical and a fuel cost of 50 cents per million Btu for a coal
fired steam plant, the purchase price for each of the thermionic modules could
be as high as 15cents/W and still be economically competitive. Present costs
for these devices are considerably higher than this, and current research is
directed at achieving significant cost reductions.
The increase in plant efficiency and the apparent ease in incorporating
the thermionic modules in the boiler unit of a fossil fueled plant would suggest
that this is a fruitful route to follow.
9.7 APPRAISAL OF ENVIRONMENTAL ASPECTS
The operation of a thermionic generator produces no additional pollutants
other than those normally present from the particular heat source used. It is
important to note, however, that the use of thermionic topping in conventional
central station power plants would increase the overall plant efficiency. The
topping device, in principle, utilizes all the heat supplied to it with 100% effi-
ciency because its rejected heat is at a temperature above the normal steam
cycle operating temperature. Thus, the increase in overall plant efficiency
results in less thermal energy rejected to the surroundings for the equivalent
electrical power production.
78
-------
There are no new known environmental effects introduced with a therm-
ionic converter system. With the higher efficiency projected for a thermionic
system, the pollutants normally produced by the energy source being used will
be diminished for equivalent amounts of electrical energy generated.
79
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Section 10
POTASSIUM VAPOR TOPPING CYCLES
10.1 DESCRIPTION OF THE SYSTEM
Electrical utilities generate most of their electrical energy in fossil N
fuel fired Rankine cycle steam turbine plants. Some of the low melting point
metals, such as potassium (melting point 144F), when vaporized, can be used,
like steam, as the working fluid to drive a turbine. The principal advantage
of "liquid metals" as the working substance in a power plant is their high boil-
ing or vaporizing temperature at a modest boiler pressure. (For example,
potassium boils at 1400F at 1 atmosphere in contrast to water boiling at 662F
at 2400 psia,) Mercury, which boils at 907F at 100 psia, can also be con-
sidered as a working fluid. The lower boiling pressure allows, in principle,
an acceptable boiler cost in spite of the higher boiling temperature. While
the liquid metals possess advantages relative to water in the boiler portion
of the plant, water has the advantage in the condenser. This difference re-
sults from the liquid metal vapor densities being so low which makes the
condenser (and low pressure end of the turbine) excessively large and costly.
This difference can be resolved by combining a liquid metal Rankine cycle
with the water Rankine cycle. In this concept the metal vapor condenser,
now operating at acceptable vapor densities, serves as a boiler for the water
cycle. Thus, while each individual cycle is not of high thermal efficiency,
the binary cycle has a high efficiency because the energy rejection from the
high temperature topping cycle is used again in the boiler of the lower tem-
perature water cycle (Ref. 1).
An example of a potassium topping cycle for a conventional steam turbine
system is shown in Fig. 17 (Ref. 39). The heat source for this particular binary
cycle is a molten salt breeder reactor.
80
-------
oo
S-l, FUEL-TO-SALT HEAT EXCHANGER
S-2, SALT-TO-POTASSIUM BOILER
B, SUPERCRITICAL-PRESSURE STEAM SENERATOR
RH-I, REHEATER I
RH-2, REHEATER 2
POTASSIUM-TO-
STEAM BOILER
FUEL NO, 133: LiF-BeFz-UF4-TI>F., [67-18.5-0.5-14mole f,}
SALT NO. 14: NoF-KF-LiF-UF, (W.9-4J.5-44.5-U mclsiu
Đ
c
RH-2
) Ŧ
BH-!
P
, j
>d
3 C
i
f
3
-'S.
\
Đ
1050
4CKX
*
7
<'
? t
786
Hi!
10 5C
104
F,
psio
F
Vpsio
?05
265
. . V
10 tC
251
F,
psio
i 553-F,410.0 psio
l_ r _!__-_
FEED W&TER HEATERS
Đ
STEAM
COHIOENSER
Đ
MOLTEN-SALT REACTOR
Fig. 17 - Flow Diagram for Potassium Binary Vapor Cycle Power Plant
Fueled by a Molten Salt Reactor
-------
10.2 EFFICIENCY
For a coal fueled plant with a boiler efficiency of 90%, the thermal
efficiency of a potassium steam binary cycle is estimated to be 50 to 55%
or more, over the range of turbine inlet temperatures of 1400 to 1800F.
Thus, binary power cycles, with a potassium topping cycle on a steam cycle,
possess the potential of a higher energy conversion efficiency than the single
fluid steam cycle. Such systems would probably produce lower cost power
in plants of large capacity rather than small and would operate more effi-
ciently at design capacity than at part load. Consequently, potassium binary
cycle plants should find application primarily as base load plants.
10.3 SIZE LIMITATION
There are no inherent limitations to the size of the mercury or potas-
sium topping cycle plants, because as is currently done in steam plants,
*
capacity can be increased by using multiple units in a parallel flow arrange-
ment (Ref. 1).
10.4 DEVELOPMENT STATUS
The potassium topping cycle has potential for use above about 1400F.
There is no previous history of use of potassium topping cycles in utility
power plants, but potassium topping cycles for central station power have
been studied as far back as the early 1960s. More recently, a potassium
topping cycle has been proposed by Oak Ridge National Laboratory for use
with the molten salt nuclear reactor (Ref. 39). A three-fluid (i.e., tenary
cycle) system involving a gas turbine in addition to the potassium and steam
cycles has also been suggested. Alternative fossil fuels considered for this
system included coal, oil and gas (Refs.40 and 41). Others have also studied
a potassium-steam binary cycle of more conventional design using coal as a,
fuel. At present, except for the HTGR, nuclear heat sources for the potassium
cycle are nonexistent. Use of the potassium topping cycle with an HTGR has
apparently not been investigated. The need to develop high temperature
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furnaces and boilers as part of a program to bring potassium topping cycles
to fruition is recognized.
During the 1960s, various agencies of the government were engaged in
the development of the technology for potassium (and cesium) Rankine space
power systems. Several turbines were built and operated on potassium vapor.
The largest of these were 250 and 340 hp. The turbine efficiencies were meas-
ured and found to be about 75%, confirming design predictions. The blades and
disks were, for the most part, fabricated of nickel based alloys. Potassium
boilers, condensers and pumps in relatively small sizes have been successfully
tested.
The scale-up from current research and development experience for
the turbine rating is in the range of 300- to 1000-fold. Thus, turbine blade
manufacturing techniques using appropriate alloys must be developed for the
very large blade sizes required. Similarly the turbine seal, which must ex-
clude oxygen (air) from the potassium loop, must also be scaled up success-
fully. Oak Ridge National Laboratory, under a grant from the National Science
Foundation, began the construction of a potassium boiler module in 1974. This
is a several megawatt capacity unit designed to operate at the 1550F level.
The ERDA has recently begun funding of paper studies to utilize fluidized bed
coal combustion as the energy source.
In summary, neither mercury nor potassium Rankine topping cycles
are now being offered commercially. The mercury system was developed
at one time, and the potassium system is under active investigation. Manu-
facturing facilities and background capability for the equipment in this type
of system would be available from a number of well established manufacturers.
10.5 ANTICIPATED CONTRIBUTION TO U.S. ENERGY NEEDS
Due to the nature of the problems facing the potassium topping cycle
energy system, and the relatively small amount of attention the problem
appears to have received, it is not possible at this time to accurately estimate
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when this energy system may begin to contribute significantly to U.S. energy
needs. Widespread use could possibly occur by the 1990s if much larger re-
search and development funds are invested.
10.6 COSTS AND BENEFITS
No meaningful information exists on the costs of a potassium topping
cycle for fossil fuel plants. Clearly, because of the increased complexity,
the plant capital costs -will be higher than a conventional steam plant but
these could be offset by higher plant efficiency. Detailed plant and equipment
design studies are needed to develop more reliable cost data.
The major advantage of the binary cycles using potassium with steam
is that of increased conversion efficiency. The benefits that stem from an
increase in efficiency (such as reductions in fuel consumption, waste heat re-
lease, and production of pollutants) will apply to both fossil fired and nuclear
plants. The disadvantages (higher capital and maintenance costs and increased
complexity of the plant and its operation) will also be applicable to both. The
extent to which the advantages will outweigh the disadvantages is unknown.
10.7 ENVIRONMENTAL APPRAISAL
The reduction of fossil fuel consumption due to higher efficiency auto-
matically reduces the quantity of most of the air pollutants produced per unit
of electrical energy generated. Likewise, the waste heat discharged by the
plant will be considerably curtailed. Accidental discharge of large quantities
of potassium to the environment would be harmful to vegetation and animal life
in the immediate area of the plant. Runoff of potassium wastes into ground-
water, streams, lakes or oceans could be detrimental. At low concentrations,
potassium will not be hazardous since it is a normal constituent of foods.
Fail safe, 100% efficient seals or the use of suitable scrubbing equipment would
have to be developed to prevent the release of sizable quantities of potassium
from a power plant.
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Neutral Particles
Jb-~
Ionizer
Power
Supply
Fig. 18 - Schematic of the EGD Basic Operation
loses kinetic energy which shows up as electricity at the output terminals
when the electrons are reunited with the gas at the collector.
The basic difference between the EGD and the magnetohydrodynamic
(MHD) generators in that the MHD generator operates by accelerating a
85
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highly ionized gas through an electromagnetic field which induces an emf and
a current in the gas similar to the armature windings of a conventional gen-
erator passing through its magnetic field. The MHD generator produces a
low voltage, high current dc power while the EGD generator produces a high
voltage, low current dc power (Ref.43).
Flow velocities in the EGD generator can be supersonic or subsonic.
Increasing the flow velocity from subsonic to supersonic speeds permits the
tube length-to-diameter ratio to be increased which presumably allows more
energy to be extracted from the gas. This increases the output somewhat;
however, the relative amount of energy extracted from the gas does not change
because more energy is required to accelerate the gas (Ref.43). That is, the
efficiency remains about the same.
The longer length-to-diameter ratio of the tube allows a longer distance
between the collector electrode and the corona electrode so that a larger voltage
difference can be sustained without exceeding the breakdown voltage of the gas.
In this way, the voltage can be increased. Expansion of the gas through a
supersonic nozzle can cool the gas to such an extent that vapor droplets are
condensed out and these droplets can become the charged particles. However,
for large power plants, most experimenters (Ref.43) feel that the subsonic
converter holds more promise. The charged particles in a subsonic EGD
generator are typically particles of dust (i.e., fly ash). These particles carry
a number of charges on their surface, rather than a single charge captured
by the condensing vapor droplet.
The consensus is that the EGD converter must be a slender tube.
There must be several stages of the tubes in series to achieve the re-
quired efficiency with the gas flow at a subsonic velocity and seeded with
small (~0.2jLX diameter) particles (Ref.43). Gas density must be as high or
higher than normal atmospheric density and the temperature must not be so
high that charges leak through the insulating walls.
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11.1 EFFICIENCY
The overall thermal efficiency of the EGD converter depends strongly
on the particle size, the corona charging field and the dust loading. With a
dust loading of 0.4 and particles of 2jU diameters, the overall system effi-
ciency has been predicted (Ref.43) to be 33%. If the same loading is main-
tained and the particle sizje decreased to IjU, the efficiency is expected to
increase to 48%. Approximately 10% mass of participates from conventional
pulverized coal combustion are less than 4jU in diameter.
11.2 SIZE LIMITATIONS
Even though the EGD is quite simple in principle there are some funda-
mental limitations. The first is associated with the required EGD force to
oppose the flow of the moving gas. The force is comparable to the force pro-
duced by a turbine. That force is given by the product of three factors: (1)
the charge on an electron; (2) the longitudinal electric field; and (3) the number
of ions per cubic foot of the gas. Now, the maximum longitudinal electrical
field is limited by the breakdown strength of the gas so that knowing this
number, the minimum number of ions per cubic foot can be calculated and an
estimate of the tube length made. However, the ion density has an associated
electrostatic field due to the presence of the ions and the balancing charges
from which they were separated. If the space in the tube is large; the field
produced by the ions themselves can easily exceed the breakdown strength
of the gas. This says in essence that for a given chamber size and gas pres-
sure only so many ions can be accommodated. In EGD generators this is
referred to as the radial electrostatic field since in the geometry of the EGD
channel, the ions are separated from their balancing charges (electrons) in
the radial direction only. This radial field is zero at the center of the channel
and increases with increasing channel diameter. The induction of radial ion
drift imposes a limitation on tube length since the ions cannot be transported
many diameters down the tube before they drift into the wall.
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11.3 DEVELOPMENT STATUS
The EGD energy converter is still in the experimental development
stage. Small scale experiments were conducted in the late 1960s under
contract to the Office of Coal Research and in cooperation with the Foster
Wheeler Corporation (Ref.43). A converter was designed as a coal fired
EGD test facility with a thermal capacity close to half a megawatt. The
configuration contained two channels in series, each 32 mm in diameter and
100 mm long. Each channel produced 30 W at 500,000 V (Ref.43). Efficiency
or power density were not calculated. The final outcome of the program is
not presently known.
11.4 ANTICIPATED CONTRIBUTION TO U.S. ENERGY NEEDS
A significant portion of the research and development work on the EGD
in the U.S. appears to be directed toward applications other than large com-
mercial power plants, primarily where compactness, mobility and/or non-
moving parts are required. Typical applications include power generation
for electrostatic spray guns used in paint and coating operations, space ve-
hicle power generation and medical therapy requiring a room to be filled
with aerosols.
The future of the EGD as a large scale power generator is unknown at
this time. Data published by Chemical Engineering (Ref. 9) indicate that the
development of electrog as dynamic generators will show significant progress
in the early 1980s with a breakthrough in large scale designs being achieved
by 1985 with significant output being delivered by the year 2000.
11.5 COSTS AND BENEFITS
Preliminary cost analysis (Ref.43) indicates that electrical power via
an electrogasdynamic generator can be delivered by high voltage dc trans-
mission at a cost comparable to nuclear power generation. These predictions
are based on the assumption of a capital cost saving over conventional coal
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fired plants of approximately 30% and an overall thermal efficiency in the
40 to 50% range.
A prime advantage of the EGD generator is that it does not require
cooling water. Heat rejection is accomplished by exhausting the combustion
gases into the atmosphere. This can be important since a principal factor
in determining the location of large steam powered plants is the availability
of cooling water for condensing the stream. If the power station was a coal
fired EGD baseload generator, for example, it could then be located wherever
the coal was cheapest regardless of water supply.
11.6 APPRAISAL OF ENVIRONMENTAL ASPECTS
As a large scale power generator, the associated environmental impacts
will depend on the method of gas production for the generator. Heat will prob-
ably be rejected to the atmosphere, thus eliminating the need for large bodies
of cooling water and the accompanying thermal pollution of the water supply.
Exhausting the combustion gases to the atmosphere should not present
a problem (Ref.43). Open cycle systems will have to maintain pollution con-
trol of the seeding particles which are expected to be of the order of 4/0. or
less. Problems (if any) presented by the exhaust gases cannot be assessed
at this time since typical operating pressures and temperatures have not been
established.
11.7 RESEARCH AND DEVELOPMENT CONSIDERATIONS
Research and development requirements for control technology should
be assessed when the preliminary systems studies for large power plants are
being conducted.
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Section 12
RECOMMENDED ENVIRONMENTAL R&D PRIORITIES
FOR CANDIDATE ENERGY SYSTEMS
Most of the advanced energy systems addressed in this report are in
the stage of development where the pollution potential has not been seriously-
considered. This has resulted in part because in many instances only the
basic technical feasibility of the system has been studied. One objective of
this study has been to identify those areas where the potential for pollution
exists in order that pollution control technology (if required) can be developed
integrally with the energy conversion technology.
Most of the advanced energy systems examined will use either the con-
ventional coal fired combustion or other low grade fuel to supply thermal
energy in the foreseeable future. Each of these systems characteristically
has an improved thermal efficiency in comparison to the conventional steam
power energy generator. The net result is a reduction in the thermal energy
rejected by the system and hence a reduction in thermal pollution. The im-
proved efficiency also has the added advantage of requiring less fuel, and
hence less pollution, to produce an equivalent amount of energy by current
energy producing plants. However, each system was invariably found to
have some unique feature that will require an evaluation of the potential
adverse environmental effects.
MHD; The magnetohydrodynamic energy converter is projected to
utilize an alkali metal as a seed material for a high energy gas stream with
the seed material being collected for reuse. Seed material candidates, such
as potassium, can have an adverse effect on the environment when present
in large concentrations. Large concentrations can occur from normal opera-
tions with inadequate particulate control devices.
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An evaluation of the collection efficiency (the size distribution of emis-
sions versus collection device efficiency) should be made and a worst case
analyses conducted to determine the expected concentration levels emitted
during normal operation. The next step is to determine the need for estab-
lishing emission standards and at the same time assess the need for further
development of the seed collection technology.
The basic MHD technology has already been developed with construction
of a demonstration plant expected in the mid 1980s. To-mo'st effectively im-
pact the design of these systems an environmental assessment should be con-
ducted during the 1976-77 time period and an evaluation made of the adequacy
of the seed collection technology.
Also, NO control technology is required for MHD units because of the
jĢ.
emphasis on high temperature combustion. This area of technology is currently
receiving attention (Ref. 6); but further research and a continuing assessment
of MHD NO control technology development is warranted. In particular, EPA
5i
should be involved in emission data collection and assessment of ERDA's
planned MHD demonstration plants.
An indirect effect which should be investigated is the increase in pollu-
tion and energy consumption (if any) associated with the manufacture of large
quantities of seed materials. This can be conducted during the .same time
frame as the environmental assessment of the unit operations,
Hydrogen Fuel Cells; Environmental studies with respect to the utiliza-
tion of hydrogen fuels in utility size power plants should be directed toward
the discharge of waste and wastewater. Most probable discharges will be
leachate from the fuel cells and sludge generated by the electrolysis process.
A demonstration plant is currently being put into operation, An environ-
mental assessment of the unit operations should be conducted during the 1976-77
time frame to define the emission sources and the expected emission rates.
An assessment of the adequacy of existing control technology can then be made.
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Ocean Thermal Energy Conversion; OTEC plants will be designed to
move large quantities of water through the system and consequently will be
rather large structures physically. This, coupled with the requirement for
a large number of plants to produce a significant amount of energy, provides
some unique environmental situations that are not apparent from a cursory
examination. Cold water from the ocean depths used in the condensing phase
will be exhausted at a temperature of 45F (a differential with surface water
temperature of approximately 30F). The warm water from the exhaust
phase will be exhausted at about 74F which is approximately 3F cooler than
the inlet surface water. These two streams (undersea plumes) will mix,
and coupled with the plumes from adjacent plants, will probably stretch for
many miles before equilibrating with the surface water. Two possible con-
sequences of this are modification of the local weather patterns and an ad-
verse effect on the local marine life (i.e., plankton, fish, etc.). Neither of
these effects is readily accessible at this time. Recommended research
topics include an evaluation of the size of the area affected by the undersea
plumes and the associated water temperatures. This can be accomplished
with the state of the art of numerical flow analyses. The effect of excess
nutrients (probably beneficial) brought to the surface by the cold water up-
welling can be studied from samples taken from the ocean depths and the
effect correlated with the amount of water expected to be used in the con-
densing operation. The effect of spills and leaching is not known so that an
assessment of control technology needs is required at this time.
The widespread use of the OTEC system is projected by the year 2000.
However, rather extensive conceptual and feasibility studies are currently
being conducted. Due to the unknown adverse effect of the cold undersea
plumes, studies should be initiated as soon as possible to define the inter-
action of the undersea plumes with warm surface waters.
Windmills; Windmill power plants are also unique in that on an indi-
vidual basis, each unit probably will have a negligible effect on the environ-
ment. However, to produce power for commercial and community use, large
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numbers of windmills will be clustered. The potential then exists for weather
modification and adverse indirect effects (land use and aesthetics). The tech-
nology to produce power from windmill plants currently exists. The demon-
stration unit at NASA's Plumbrook facility has been constructed to provide
data for the design of commercial class power generation plants. Conse-
quently, to most effectively contribute to the design of commercial systems,
the environmental assessments should be initiated in the current year.
The potential for weather modification can be studied in an environ-
mental chamber. The most profitable approach is to parametrically investi-
gate, windmill size (height, etc.), blade design, clustering and terrain.
Potassium Topping Cycles; The primary environmental consequence
of a potassium topping cycle is the accidental discharge of large quantities
of potassium wastes into groundwater, streams, lakes or oceans. Fail-safe
seals or suitable scrubbing equipment may have to be developed to prevent
the release of sizable quantities from a power plant.
The projection of widespread use of the topping cycle is by the year
1990. An appraisal of the effect of the discharge of large quantities of
potassium and the state of associated control technology should be initiated
during the 1976-77 time period.
Gas Turbixies; The basic gas turbine technology is currently well de-
veloped and being extended to high temperature operation to increase the
thermal efficiency. Gas turbines require a low sulfur, low ash fuel for turbine
operation so that fly ash and SOx emissions are negligible. However, a pre-
sent stack effluent is oxides of nitrogen (NO and NO2). The technique now
used to treat the problem is to inject demineralized water into the combustion
chamber which lowers the flame temperature. For the higher operation tem-
peratures desired new methods of NOx control will be required; for instance
combustion process modification.
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A set of emission standards for gas turbines is needed (such as the
maximum values applying to conventional steam plants) to provide realistic
design targets for research to reduce the oxides of nitrogen.
Research studies on control technology should be initiated during, 1976
to define possible candidate schemes which can then be assessed with respect
to the most favorable design applications.
Thermionic and Electrogasdynamic Direct Energy Converters: These
systems are still in the basic device development stage. Neither converter
appears to have much potential as a base load generator. However, utilized
as the upper portion of a topping cycle, these systems can increase the thermal
efficiency of the power plant. Operating in this manner, the converter would
use the flue gas from the coal fired steam plant as the converter working fluid.
The environmental impact of the converter operating in this capacity is
not known at the present time. Consequently, an environmental assessment
should first be conducted to identify any potential environmental problems.
These can then be categorized and recommended control technology studies
formulated where necessary to provide adequate pollution control.
Thermoelectric Converter: The extremely low thermal efficiencies
of thermoelectrics virtually preclude their being used as power generators
in utility size power plants. The required power output means more fuel
consumed than competitive systems, with more heat rejected to the environ-
ment and consequently energy sources depleted at a faster rate than is now
the case. For central power plants this is unacceptable so that no meaningful
R&D work is currently envisioned with respect to utility size power generators.
Summary: In conclusion, the advanced energy systems considered in
this study are receiving technology development funds at an increasing rate
while at the same time are receiving token attention with respect to environ-
mental control. Due to the many benefits of integral development of both
94
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energy conversion technology and environmental control technology, it is
therefore recommended that substantial pollution control and assessment
studies be initiated for these systems within the next two years. Con-
sidering both the expected environmental impact and period of technology
breakthrough/commercialization, the following order of R&D priorities on
the candidate energy systems has been developed: high temperature turbines,
ocean thermal gradients, windmills, magnetohydrodynamics, metal vapor
(potassium) Rankine topping cycles, hydrogen fuel cells, thermionics, electro-
gasdynamics, and thermoelectric conversion.
It has also been obvious during the course of this survey that a great
deal of emphasis is and will be spent on energy conservation technologies
for the industrial and other sectors of the U,S. economy. Very little evi-
dence was found on environmental assessment considerations for these
energy conservation technologies. Thus it is recommended that a study,
similar to the present analysis, be conducted on energy conservation.
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REFERENCES
1. "Liquid Metal Fast Breeder Reactor Program Proposed Environmental
Statement, Vol. Ill," WASH-1535, U.S. Atomic Energy Commission, Vol.3
of 7, December 1974.
2. Hals, F., and W. Jackson, "Systems Analysis of Central Station MHD Power
Plants," 5th International Conference on MHD Power Generation, Munich,
West Germany, April 1971.
3. Kantrowitz, A., and R. J. Rosa, "MHD Power Generation," American Physical
Society Topical Conference on Energy, Chicago, February 1974.
4. Ring, L. E. et al., "Design of an MHD Performance Demonstration Experi-
ment," 13th Symposium on the Engineering Aspects of MHD, Stanford Uni-
versity, March 1973.
5. Ring, L. E. et al., "A Status Report on the MHD Performance Demonstration
Experiment," 13th Symposium on the Engineering Aspects of MHD, Stanford
University, March 1973.
6. Bienstock, D. et al., "Air Pollution Aspects of MHD Power Generation,"
13th Symposium on Engineering Aspects of Magnejohydrodynamics, Stan-
ford University, 1973.
7. Bienstock, D. et al., "Environmental Aspects of MHD Power Generation,"
Intersociety Energy Conversion Conference, Boston, 1971.
8. Dicks, J.B., "MHD Central Power: A Status Report," Mech.Eng., May
1972, p. 14.
9. Rosenzweig, M.D., "Timetable Now Emerges for Technical Breakthroughs,"
Chemical Engineering, November 1975, pp. 124-126.
10. McDermit, J.H., "A Preliminary Environmental Assessment for an MHD
Energy System," LMSC-HREC TR D496562, Lockheed Missiles & Space
Company, Huntsville, Ala., October 1975.
11. Mori, Yasuo et al., "Reduction of NO Concentration in MHD Steam Power
Plant System," 13th Symposium on Engineering Aspects of Magnetohydro-
dynamics, Stanford University, 1973.
12. Jackson, W.D.,and R.V. Shanklin, III, "Coal Fired MHD for Central Station
Power Generation," Proc. Frontiers of Power Technology, Oklahoma State
University, 1973.
13. The U.S. MHD Electric Power Development Program, 1975 Annual Report
of the Office of Coal Research.
96
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14. Energy R&D, Organization for Economic Cooperation and Development
Paris, France, 1975.
15. Lueckel, W. J., and P. J. Farris, "The FCG-1 Fuel Cell Power Plant for
Utility Use," presented at the summer meeting of the IEEE Power Engi-
neering Society, July 1974.
16. Brown, J. T. et al., "1970 Final Report Project Fuel Cell - Research
Development," Report No. 57, for Office of Coal Research, Department
of the Interior, August 1970.
17. Gregory, D.P./D.Y.C. Ng and G.M. Long, "The Hydrogen Economy,"
The Electrochemistry of Cleaner Environments, Plenum Press New
York, 1972.
18. lammartino, N.R., "Fuel Cells: Fact and Fiction," Chem.Eng.. 81(11)-
62, 64, May 1974.
19- Hughes, E., E. Dickson, R. Schmidt, "Control of Environmental Impacts
from Advanced Energy Sources," EPA-600/2-74-002, March 1974.
20- Claude, G.. "Power from Tropical Seas," Mech.Eng., 52(12), 1930,
pp. 1039-1044.
21. Anderson, J., "Economic Power and Water .from Solar Energy," ASME
Technical Paper No. 72-WA/SOL-2, November 1972.
22. Dugger, G., "Ocean Thermal Energy Conversion," Astronautics and Aero-
nautics, November 1975, p. 58.
23. Brown, C., and L. Wechsler, "Engineering an Open Cycle Power Plant for
Extracting Solar Energy from the Sea," Paper OTC 2254, Offshore Tech-
nology Conference, Houston, Texas, May 1975.
24. Kohn, P., "Ocean Thermal Gradients Beckon Energy Planners," Chemical
Engineering, February 1976, p. 53.
25. Dugger, G., Ocean Thermal Energy Conversion," Solar Energy for Earth -
An AIAA Assessment, Chap. 10, April 1975.
26. Mitre Corporation, "System Analysis of Solar Energy Programs," Report
MTR-6513, Bedford, Mass., December 1973.
27. "Energy for a Technology Society - Principles, Problems, Alternatives."
28. Savino, J., and F. Eldridge, "Wind Power," Aeronautics and Astronautics,
November 1975, p. 53.
29. Proceedings of the Second Workshop on Wind Energy Conversion Systems,"
' NSF/ERDA, August 1975.
30. "Steam Turbine Tolerates 1900°C," Industrial Research,! December 1975,
p. 19-
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31. Seebeck, T. J., "Evidence of the Thermal Current of the Combination
Bi-Cu by Its Action on Magnetic Needle," Abt. d. Konial., Akak. d. Miss.
Berlin, 1822-23.
32. Angrist, S. W., Direct Energy Conversion, Allyn and Bacon, Inc., Boston,
Mass., 2nd ed., 1971, pp. 165-179.
33. Proceedings of the 7th Intersociety Energy Conversion Engineering Con-
ference, Americal Chemical Society, Washington, D.C., 1972, pp. 130-235,
534-539.
34. The Cambion Thermoelectric Handbook, 1971 Edition, Cambridge Thermi-
onic Corporation, Cambridge, Mass, p. 1815.
35. "ISOMITE (BATTERIES)," Bulletin of the Donald W. Douglas Laboratories,
Richland, Washington, undated.
36. Holland, J. H., "Thermionic Fuel Element Development Status Summary,"
p. 1060 in Proceedings of the 7th Intersociety Energy Conversion Eng^-
neering Conference, San Diego, September 1972, American Chemical
Society, Washington, D.C., 1972.
37. Thermionic Topping Converter for a Coal Fired Power Plant," Office of
Coal Research, R &D Report No. 52, U.S. Department of the Interior,
Washington, D.C., Undated.
38. Morris, J. F., "Performance of the Better Metallic Electrodes in Cesium
Thermionic Converters," Proceedings of the 7th Intersociety Energy Con-
version Engineering Conference, San Diego, Calif., September 1972, p. 1050,
American Chemical Society, Washington, D.C., 1972.
39. Fraas, A. P., "A Potassium-Steam Binary Vapor Cycle for a Molten Salt
Reactor Power Plant," J. Engr. Power, October 1966, pp. 355-367.
40. Fraas, A.P., "Preliminary Assessment of a Potassium-Steam-Gas Vapor
Cycle for Better Fuel Economy and Reduced Thermal Pollution," Oak Ridge
National Laboratory, Report ORNL-NSF-EP-6, August 1971.
41. Fraas, A. P., "Fluidized Bed Coal Combustion System Coupled to a Potassium
Vapor Cycle," presented at the AIChE Annual Meeting on New Coal Combustion
Techniques, New York, 29 November 1972.
42. Lawson, M., and H. Ohain, "Electro Fluid Dynamic Energy Conversion Pre-
sent Status and Research Areas," J. Engr. Power, Transactions of the ASME,
April 1971, p. 201.
43. Gourdine, M., "Electrogasdynamics," Science and Technology, July 1968,
p. 50.
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
EPA-600/7-77-062
3. RECIPIENT'S ACCESSION-NO.
Development Status and Environmental Hazards
of Several Advanced Energy Systems
5. REPORT DATE
June 1977 issuing date
6. PERFORMING ORGANIZATION CODE
Morris Penny and Sidney Bourgeois
8. PERFORMING ORGANIZATION REPORT NO.
ERFORMING ORGANIZATION NAME AND ADDRESS " "
Lockheed-Huntsville Research & Engineering Center
P.O. Box 1103
Huntsville, Alabama 35807
10. PROGRAM ELEMENT NO.
EHE624B
11. CONTRACT/GRANT NO.
68-02-1331, Task 8
12. SPONSORING AGENCY NAME AND
INDUSTRIAL ENVIRONMENTAL RESEARCH LABORATORY
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, Ohio 1+5268
13. TYPE OF REPORT AND PERIOD COVERED
Final 12/75-2/76
14. SPONSORING AGENCY CODE
EPA/600/12
15. SUPPLEMENTARY NOTES
EPA project officer for this report is William Cain
16. ABSTRACT ' ' " '^~" i
The report gives a review of the development status of several advanced energy
concepts and discusses the primary environmental hazards of each system.
Systems reviewed include potential new sources of energy and improved energy
conversion. Each system is evaluated with respect to its development status, and
estimates made as to when each will begin to contribute significantly to U.S.
energy needs. Appraisals were made of the environmental impact of each system
including assessment of the adequacy of pollution control technology and potential
gross ecological impact. The overall conclusion is that each energy system has a
negligible or mild direct environmental impact when compared with conventional
fossil fuel and nuclear systems, but that indirect impacts for some of the energy
systems could be severe and need further study to quantify their impact. Con-
sidering both the expected environmental impact and period of technology break-
through/commercialization, the following order of R&D priorities on the candidate
energy systems has been developed: high temperature turbines, ocean thermal
gradients, windmills, magnetohydrodynamics, metal vapor (potassium) Rankine
topping cycles, hydrogen fuel cells, thermionics, electrogasdynamics, and
t.hp-pTnr.PlPftHp ponversion. . _ _
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
e. COSATI Field/Group
Environment
Pollution
Assessment
Power Supplies
Energy Conversion
Electric Power Plants
Estimates
Forecasting
Impact
Conversion
Energy Sys terns
10A
10B
8. DISTRIBUTION STATEMENT
Unlimited
19. SECURITY CLASS (ThisReport)
Unclassified
109
20. SECURITY CLASS (Thispage)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
99
aUAOOfflmUfflTPWIITIIIG OFFICE 1977- 757-056/647Z
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