vvEPA
United States
Environmental Protection
Agency
Industrial Environmental Research
Laboratory
Research Triangle Park NC 27711
EPA-600/7-80-099
May 1980
A Demonstration of
Beneficial Uses of Warm
Water from Condensers
of Electric Generating
Plants
Interagency
Energy/Environment
R&D Program Report
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RESEARCH REPORTING SERIES
Research reports of the 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 m 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.
EPA REVIEW NOTICE
This report has been reviewed by the participating Federal Agencies, and approved
for publication. Approval does not signify that the contents necessarily reflect
the views and policies of the Government, nor does mention of trade names or
commercial products constitute endorsement or recommendation for use.
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.
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EPA-600/7-80-099
May 1980
A Demonstration of Beneficial Uses
of Warm Water from Condensers
of Electric Generating Plants
by
LL Boyd (U. of MM), G.C. Ashley, J.S. Hietala,
R.V. Stansfield, and T.R.C. Tonkinson
Northern States Power Company
414 Nicollet Mall
Minneapolis, Minnesota 55401
Grant No. S-803770
Program Element No. EHE624
EPA Project Officer: Theodore G. Brna
Industrial Environmental Research Laboratory
Office of Environmental Engineering and Technology
Research Triangle Park, NC 27711
Prepared for
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Research and Development
Washington, DC 20460
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PREFACE
Northern States Power Company has been involved with the development of
beneficial uses of power plant waste heat since 1970. As construction of
the two-unit coal-fired Sherburne County (Sherco) station progressed, accom-
panied by an intensified energy crisis, waste heat utilization appeared to
be economically feasible. With its closed-cycle cooling system and adequate
condenser outlet temperatures, Sherco was selected as the site for a warm
water greenhouse heating demonstration.
The University of Minnesota Agricultural Experiment Station and the
United States Environmental Protection Agency joined forces with NSP in
developing a program. The resulting Sherco Greenhouse Project is an excellent
example of cooperation by an educational institution, a government agency,
and private industry in meeting the challenge of a national energy problem.
n
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CONTENTS
Preface ii
Abstract iv
Figures v
Tables vi
Abbreviations vii
Acknowledgments viii
Introduction 1
Conclusions and Recommendations 2
Description of Facilities 3
1. Structural Facilities 3
2. Air Heating System 5
3. Backup Heating System 5
4. Soil Heating System 7
5. Cooling/Ventilation System 7
6. Greenhouse Water Circulation System 8
7. Control System 10
8. Data Collection System 11
9. Heating System Performance and Operating Experience 11
10. Severe Cold Weather Performance Data 12
11. Electric Energy Consumption 23
12. Waste Heat Availability . 32
13. Operating Problems 42
14. Greenhouse System Operation 43
General Horticultural Considerations 44
1. Soil 44
2. Irrigation 44
3. Fertilization 45
4. Pest Control 45
5. Harvesting and Marketing 46
6. Detailed Cultural Methods 46
Tomatoes 47
Snapdragons 49
Roses 49
Lettuce 49
Qeraniums 49
Containerized Tree Seedlings 50
Nursery Stock 51
Economic Feasibility 52
Heat Delivery Cost and Pricing Concepts 5^
Heat Exchange System Cost 55
Operating Cost Comparison to Conventional Systems 56
Appendices
A. Background %
B. Pilot Project ^
ill
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FIGURES
Number Page
1 Heating and Cooling System Schematic 4
2 Trane Air Handler 6
3 Modine Unit Heater 6
4 Soil Heating System Headers f . . 8
5 Cooling Water Circulation System Schematic 9
6 Unit I Gross Electrical Output and Condenser
Outlet Water Temperature January 9, 1977 13
7 Heating System Performance January 9, 1977 14
8 Wind Speed January 9, 1977 16
9 Solar Radiation January 9, 1977 17
10 Heating System Performance January 9, 1978 18
11 Heating System Performance January 10, 1978 19
12 Heating System Performance February 2, 1978 20
13 Air Heating System Performance January 9, 1977 21
14 Soil Heating System Performance January 9, 1977 22
15 Typical Warm Water Temperatures 24
16 Typical Greenhouse Air Temperature 25
17 Condenser Outlet Water Temperature Distribution January, 1978 26
18 Average Greenhouse Air Temperature Distribution January, 1978 27
19 Typical Warm Water Temperature Variation With Unit Load ... 28
20 Monthly Electric Power Consumption of Heating Fans and Pumps . 30
21 Monthly Capacity Factors of Greenhouse Heating System Units . 31
22 Monthly Electric Power Consumption of Greenhouse Boilers
During Unit I Outages 35
23 Pipeline Losses Between Unit 1 Condenser and Greenhouse
January 8, 1978 37
24 Pipeline Losses Between Unit II Condenser and Greenhouse
January 10, 1978 38
25 Connection at Cooling Tower
26 Warm Water Distribution System Schematic 40
27 Sherco Power Plant and Greenhouse Complex 64
iv
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TABLES
Number Page
1 Sherco Greenhouse Electrical Power Consumption 32
2 Warm Water Availability 33
3 Summary of Warm Water Availability and Temperature 34
4 Cropping Chronology 47
5 Yields of Marketable No. 1 and No. 2 Tomatoes 47
6 Yield Vs. Density for Tropic Tomato - Spring 1977 48
7 Growing to Flowering Time - Geraniums 50
8 Crop Yields 50
9 Waste Heat System - Materials Cost Summary 55
10 Comparative Operating Costs of Waste Heat Versus
Conventional Heating 57
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LIST OF ABBREVIATIONS AND SYMBOLS
ABBREVIATIONS
British thermal unit
British thermal units per hour
British units per hour per square foot
British thermal units per pound
centimeter
cubic feet per minute
cubic meters per second
degrees Celsius
degrees Fahrenheit
foot
foot-Candles
gallon
gallons per min
gallons per min
gallons per square foot
gigajoule (one billion joules)
gram
hectare
horsepower
hours
inch
kilogram
kilogram per cubic meter
kilojoule per kilogram
kilowatt
kilowatt-hour
liters per second
liters per square meter
megawatt
megawatt-hours
meter
million British thermal units
ounce
ounces per cubic feet
parts per million
pound
quart
square foot
square meter
SYMBOLS
calcium
calcium carbonate
carbon dioxide
magnesium
temperature differential
Btu
Btu/hr
Btu/hr-ft2
Btu/lb
cm
ft3/nrin
m3/s
°C
°F
ft
ft-C
gal
gal/min
gpm
gal/ft2
GJ
gm
ha
hp
hrs
in
kg a
kg/m3
KJ/kg
kw
kwh
1/s
1/m2
MW
MWH
m
MBtu
oz
oz/ft3
ppm
Ib
qt
ft2
m2
Ca
CaC03
C02
Mg
AT
VI
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ACKNOWLEDGMENTS
The authors gratefully acknowledge the contributions and support given
to the Project by the following individuals and organizations:
Project Employees:
Patricia Adamson, Assistant Horticulturist
Roberta Brehmer, Assistant Horticulturist
Ron Svehla, Assistant Horticulturist
University of Minnesota Staff:
Keith Huston, Director Agricultural Experiment Station
Evan R. All red, Agricultural Engineering
Alvin A. Aim, Forest Resources
Donald G. Baker, Soil Science
George R. Blake, Soil Science
David W. Davis, Horticulture Science
Arnold M. Flikke, Agricultural Engineering
James V. Groth, Plant Pathology
Kenneth A. Jordan, Agricultural Engineering
Sagar V. Krupa, Plant Pathology
Frank Lang, Electronics Technician
Harold Pellet, Horticulture Science
Francis L. Pfleger, Plant Pathology
Paul E. Read, Horticulture Science
Elwin L. Stewart, Plant Pathology
James B. Swan, Soil Science
Richard E. Widmer, Horticulture Science
Harold F. Wilkins, Horticulture Science
Commercial Greenhouse Operators:
Gordon Bailey, Bailey Nurseries
Leonard Busch, Len Busch Greenhouse
Al Hermes, Hermes Floral
Don Hermes', Roseville Greenhouse-Becker
Jim Hermes, Hermes Floral
Tom Hermes, Roseville Greenhouse-Becker
Tom Lange, VEG, Inc.
Don Rosacker, Hans Rosacker Floral
Richard Young, Northgro, Inc.
Richard Zelinka, Forjay, Inc.
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Northern States Power Company:
D. W. Angland, Executive Vice President
E. C. Glass, Director, Planning & Research
L. C. Weber, Manager of Research
Pat Gordon, Plant Manager, Sherco
Wayne Kaplan, Communications Representative
Environmental Protection Agency:
James Chasse
Alden Christiansen
vili
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INTRODUCTION
In July 1978, the total electric generating capability of United States
power plants was 540,000 megawatts (MW).1 Assuming that there will be a 3-4%
compound annual growth rate, a modest forecast by most utility standards,
energy availability by the late 1990's must be doubled. This means that
before the year 2000, the nation's power demands will be more than one
million MW.
Rapidly increasing demand for electric power requires that each day for
the next 20 years, an average of 74 MW of electric power generation will be
placed in service in this nation. A high proportion, 80%, of the added
generating facilities will be thermal plants, with large volumes of warm
water discharged from the condensers.
Using present technology, generating plants are only 35-40% efficient.
Putting it another way, nearly two-thirds of the energy required for plant
operation is not converted to electricity. Most of the unused thermal energy
is rejected by the cooling system and is called waste heat, although the
rejected heat is a consequence of the second law of thermodynamics and is not
willful waste.
All steam-electric generating stations require huge quantities of cool-
ing water to dissipate waste heat. To protect the environment, most future
power plants will be required to install a closed-cycle system of cooling,
utilizing cooling towers or cooling ponds. Temperatures of circulating
cooling water leaving the condenser in a closed-cycle design are likely to
be in the 24°-49°C (75°-120°F) range, considerably higher than the condenser
discharge of "once-through" cooling plants. A closed-cycle 1000 MW plant
requires cooling water circulated at the rate of 25,240 1/s (400,000 gpm),
rejecting nearly 52.8 X 106 GO (50 X 1012 MBtu) of waste heat each year.
A truly energy conscious society must recognize the tremendous waste of
energy this represents on a national scale. New ways to recapture waste
heat should' be explored to make the best possible use of non-renewable
resources: coal, oil, natural gas, and uranium.
National Electric Reliability Council - July, 1978, Review
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CONCLUSIONS AND RECOWENDATIONS
The Sherco Greenhouse project has satisfactorily demonstrated that
power plant condenser discharge water, with a minimum temperature of 29.4°C
(85°F), can be used to supply the total heating requirements of greenhouses
in a northern climate. The demonstration has proven the technological and
economic feasibility of this application of power plant waste heat.
At the same time, it is acknowledged that: (1) the capital investment
in a warm water heating system is slightly higher than the investment in a
conventional heating system and (2) electric power costs will be greater
with a warm water heating system than with a conventional heating system.
If the results of this project are to be successfully applied at other
power plant sites, it is recommended that:
(1) Reliability of warm water service be considered by selecting
power plants with two or more generating units.
(2) The individual greenhouse owners provide an adequate backup
heating system in the event of a warm water supply outage.
(3) The area at the power plant reserved for greenhouses be as
close as practicable to the source of warm water.
(4) The greenhouse structure should employ energy saving features
such as a double-layer roof and sidewalls.
Northern States Power Company is presently leasing land to private
greenhouse operators for the purpose of building waste heat greenhouses. It
is intended that the greenhouse owners build and manage their own structures
and equipment. The Company will provide a supply of warm water at a fair
and reasonable rate reflecting: (1) the capital investment required to
deliver the warm water, (2) the operating and maintenance expense connected
with the pipeline, and (3) the electric energy necessary to pump the warm
water from the power plant to the greenhouse complex.
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DESCRIPTION OF FACILITIES
STRUCTURAL FACILITIES
The Sherco Greenhouse is a conventional arch frame, gutter-connected
style. The 14 modules, or bays, are each 5.2 m (17 ft) wide by 29.3 m
(96 ft) long and represent a covered surface area of 0.21 ha (0.53 acre).
The gutter clearance height is 2.4 m (8 ft) with the longitudinal axes of
the bays oriented north-south. This type of greenhouse was chosen for the
demonstration project primarily because of the lower initial capital cost
and because of the wide acceptance of the structural type among greenhouse
operators and growers.
The roof covering of the greenhouse is a double layer, air-inflated
ultraviolet-resistant polyethylene. The double layer of polyethylene was
determined to be the most viable roof covering because of lower initial
capital cost, high popularity with growers and primarily because of the
higher resistance to heat transfer. Replacement of the roof covering was
performed on an annual basis mainly due to the nature of the project with
regard to potential failure during the winter heating season. The only
problems experienced with the polyethylene roof covering were slight and were
associated with damage due to ice formation coupled with high winds during
winter sleet storms.
The east and west side walls and south end wall are covered with an
outside layer of reinforced corrugated fiberglass panels and an inside
layer of polyethylene. The north end wall is covered with an outside and
inside layer of polyethylene. The lower portion of the polyethylene on the
north wall is raised to act as an outside air intake for the evaporative pad
cooling system as shown in Figure 1.
The greenhouse crops and mechanical equipment were arranged to provide
north-south bench and row orientation. East-west main walkway access was
provided on the south end of the greenhouse and on the north end under the
structures used to support the heat exchanger units.
A crop support system was added as a retrofit design, independent of the
basic greenhouse structure. The rose support system consists of three layers
of galvanized welded wire fabric which extends the full 25.9 m (85 ft) length
of each bench. The crop support system for the tomato plants utilizes a wood
member frame for supporting overhead horizontal steel pipes. The tomato
plants were supported with nylon twine.
The headhouse adjoining the greenhouse is a 12.2 m (40 ft) by 18.3 m
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FOR
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Figure 1 Heating and Cooling System Schematic
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(60 ft) steel frame building. The headhouse accommodates space for a pro-
duction work area, a boiler room, a data acquisition and control room, an
office and restroom, a data acquisition and control room, an office and
restroom area, a walk-in cooler and a mechanically vented chemical storage
cabinet. The headhouse acts as a central facility for all horticultural and
mechanical system operations.
AIR HEATING SYSTEM
The heating system employed in the Sherco greenhouse consists of an air
heating system and a soil heating system. Warm water is circulated through
an underground piping grid to provide crop root zone temperature control and
also through finned-tube, forced air heat exchangers.
The air heating system was designed to provide all of the make-up heat
requirement of 586 KW (2.0 X 105 Btu/hr) at the design conditions of -34.4°C
(-30°F) ambient and 10°C (50°F) greenhouse air dry bulb temperature. The
water to air heat exchange is accomplished by use of commercially packaged
fan/finned-tube heat exchanger units.
Twelve of the 14 greenhouse bays are equipped with 2.24 KW (3 hp) centri-
fugal air handlers in combination with draw-through finned coils as shown in
Figures 1 and 2. The rated heat transfer capability of the coils is 44.5 KW
(1.52 X 105 Btu/hr) with 29.4°C (85°F) entering water and 10°C (50°F) enter-
ing air conditions. At the design water flow rate of 1.87 1/s (29.6 gal/min)
and the design air flow rate of 3304 1/s (7000 ft3/min), the coil outlet
conditions of 21.2°C (70°F) air and 23.9°C) (75°F) water temperatures are
experienced. Heated air is uniformly distributed by overhead 0.76 m (30 in)
diameter perforated polyethylene ducts which extend the full length of the
bay.
The two center bays of the greenhouse are equipped with a horizontal
discharge (non-ducted) unit heater in each end, oriented to discharge air
toward the opposite end heater. These unit heaters, as shown in Figure 3,
have circumferential finned-tube coils with 746 W (1 hp) propeller fans.
At the design air flow rate of 3260 1/s (6900 ft3/min) and water flow rate of
1.0 1/s (16 gal/min), these units will effect a heat transfer rate of 17.6 KW
(6.0 X 105 Btu/hr) from 29.4°C (85°F) entering water to 10°C (50°F) entering
air. The unit heaters were installed to permit comparative indications of
the economic or technical performance benefits inherent in either type of
heat exchanger unit.
BACKUP HEATING SYSTEMS
The greenhouse is equipped with an alarm system which monitors three
parameters critical to the environment control system operation. Project
personnel are notified of alarm conditions caused by greenhouse air tempera-
tures below 10°C (50°F) (in the event of mechanical equipment failure), by
supply water temperatures below 23.9°C (75°F) (in the event of an unscheduled
power plant outage) or by an electric power interruption.
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Figure 2 Trane Air Handler
Figure 3 Modine Unit Heaters
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Since the greenhouse electric power is not supplied directly by the
Sherco plant, two 390 KW (1.28 X 106 Btu/hr) electric hot water boilers
serve as a standby heat source in the event of simultaneous, two-unit plant
outage. These boilers were used during the 1975-76 heating season to pro-
vide heating by simulating circulating water temperatures, before the first
generating unit was operating commercially.
In the event of an electric power interruption during the heating
season, an emergency heating system is utilized. A 10 KW propane-fueled
electric generator provides emergency power for lights and for portable
propane heater operation. Six 103 KW (3.5 X 105 Btu/hr), propane-fired,
forced-air heaters are deployed in the greenhouse to provide heat until
normal electric service conditions are restored. In both electric power
interruptions experienced during the past three heating seasons, scant use
of the emergency system was necessary because of prompt restoration of normal
electric power service.
SOIL HEATING SYSTEM
The soil heating system was installed primarily to provide crop root
zone temperature control capability to enhance plant response. While soil
heating does provide a small portion of the makeup heat requirements, the
air heating system is designed for the total heating required because of the
inherent slow thermal response time of the soil heating system.
The installed soil heating system (Figure 4) consists of 2.54 cm (1 in)
diameter polyethylene pipes spaced 0.61 m (24 in) on centers and buried
approximately 30.5 cm (12 in) below grade. Each bay of the greenhouse con-
tains a grid of 8 pipes and the 14 grids are header-connected to form two
systems of seven grids each. The soil heating system is supplied circulation
water in parallel with the air heating units (Figure 4) on a demand basis.
The soil heating design water temperature differential is 5.6°C (10°F)
with 29.4°C (85°F) supply water at the design flow rate of 0.063 1/s (1 gal/
min) per pipe. The temperature differential estimate of 5.6°C (10°F) was
based on a 10°C (50°F) soil surface temperature and a 15.6°C (60°F) root zone
temperature.
COOLING/VENTILATION SYSTEM
The greenhouse cooling system, as shown schematically in Figure 1,
functions fndependently of the heating system. The conventional evaporative
cooling system utilized consists of a 0.91 m (36 in) high cellulose pad
mounted vertically to extend the length of the north wall of the greenhouse.
Potable water is distributed and recirculated through the pads at a rate of
7.57 1/s (120 gal/min). Air is drawn through the wetted pads by fourteen
0.91 m (36 in) propeller type exhaust fans, with one fan located in the
south end of each bay.
The total design cooling air flow rate is 9.2 X 104 1/s (1.95 X 105 ft3/
min) or 6.6 X 103 1/s (13,900 ft3/min) per bay.
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Figure 4 Soil Heating System Headers
All fans and the evaporative pad system are operated automatically with
the outside air intake of the pad system fully open during periods when
cooling is required. During the moderate temperature transition periods from
heating to cooling, ventilation with preheating is provided by operating the
centrifugal air handlers with the outside air intake open as shown in
Figure 1. During periods with ambient temperature below freezing, the re-
quired minimum ventilation is provided by manual operation of exhaust fans
drawing outside air through two 0.6 m (24 in) wall louvers located in the
two center bays of the greenhouse. As discussed more completely in the
control system description, ventilation for humidity control within the
greenhouse is provided as a manual function.
GREENHOUSE WATER CIRCULATION SYSTEM
As shown schematically in Figure 5, water circulation through the green-
house system is provided by primary circulation pumps. From the main supply
pipeline, water is pumped through the greenhouse heat exchangers by four
3.73 KW (5 tip) pumps, each of which is capable of circulating 9.47 1/s
(150 gal/min). Two pumps operate in parallel per zone on a demand basis, as
explained in the control system description. The soil heating system is
equipped with one 0.75 KW (1 hp) secondary booster pump per zone, which
operates on a demand basis.
8
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V
A
t.
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t t
MAIN WARM WATER SUPPLY
MAIN WARM WATER RETURN
\WARM WATER ^ 85° F
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ING
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Figure 5 Cooling Water Circulation System Schematic
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The design water flow rate of 25.6 1/s (406 gal/min) for air heating
and 7.1 1/s (112 gal/min) for soil heating, or a combined total circulation
flow rate of 32.7 1/s (518 gal/min), are based on the maximum design water
temperature differential of 5.6°C (10°F).
CONTROL SYSTEM
The greenhouse environmental control function is predicated upon simple
on-off thermostat control, with dry bulb temperature being the only auto-
matically controlled variable.
Each of the 14 greenhouse bays are equipped with separate heating and
cooling thermostats. For control function purposes, the 14 bays are sub-
divided into two distinct control zones of seven bays each. One zone is
further equipped with one additional heating and cooling thermostat per bay
and time clock switching to provide differential day and night temperature
control setpoints.
A central control and status indication panel provides automatic heating
or cooling operation mode selection and selection of outside or return air
for heating. Selection of either outside or return air for heating is pro-
vided on a manual basis to afford humidity and temperature control in each
zone of the greenhouse. This selection is a manually controlled function
because of infiltration heat loss and heat exchanger coil freezing problems
associated with automatic outside air intake dampers.
The air heating thermostat in any bay initiates the starting of primary
pumps for water circulation through the heat exchange coils as well as
starting the fan(s) associated with that particular bay. The starting of
centrifugal air handlers initiates the delayed opening of return or outside
air dampers to prevent rapid inflation damage to the polyethylene air dis-
tribution ducts.
A primary starting circuit is incorporated into the control system to
provide determinant starting times. That is, the first heating thermostat
in a zone starts one of the 3.73 KW (5 hp) primary circulation pumps. The
second 3.73 kw (5 hp) pump in the particular zone is started, to operate in
parallel with the first pump, when a prescribed number of heating thermostats
"made" are activated. When the heating requirements of the zone are satis-
fied, both pumps are stopped and the initial pump starting sequence 1s
reversed to average overall pump operation time.
The soil heating thermostat in each zone is integrated with the seven
air heating thermostats to provide primary control. In addition to primary
pump starting, the soil heating thermostats provide starting signals for
secondary (0.75 KW/zone (1 hp/zone)) soil heating system booster pumps.
The cooling system control is automatically segregated from heating
control. The cooling system is controlled by a thermostat in each bay
starting and stopping the appropriate exhaust fans.
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DATA COLLECTION SYSTEM
The greenhouse is fully instrumented with an automatic data collection
system to provide a complete environmental history and to establish energy
balances to assess the technical feasibility of the warm water heating system.
A microprocessor-control led data acquisition and recording system scans 227
data transducers at one-half hour intervals. The data are recorded on paper
tape and magnetic tape and are computer processed to provide time function
relationships of pertinent environmental and water conditions.
Temperatures are measured at 10 individual points in each of the 14 bays
of the greenhouse. Three dry bulb temperatures are monitored at each of
three random locations in each bay. The three temperatures measured at each
location include one soil temperature, at random depths from 10.2 cm (4 in)
to 30.5 cm (12 in) below grade, one air temperature below the plant canopy
and one air temperature near the top of the plant canopy. Dew point is also
measured at one location in each bay.
The ambient conditions of air, dry bulb temperature and dew point, are
monitored at one location outside the greenhouse. Total incident solar
radiation is monitored at one location inside the greenhouse and at one
location outside the greenhouse at rooftop elevation.
The balance of the points monitored include greenhouse main supply and
return water temperatures, soil heating system supply and return water
temperatures, and supply and return air and water temperatures on four heat
exchangers. Manual readings were also recorded daily of total greenhouse
water flow, soil heating water flow and electrical power consumption of
circulating water pumps, heating fans and ventilating fans.
Air dry bulb and soil temperatures are measured with 24-gauge, copper-
constantan thermocouples, air dew point is measured with heated lithium
chloride electrical hygrometers and water temperatures are measured with
immersion type copper-constantan thermocouples. Solar radiation is measured
by star pyranometers and is recorded as hourly integrated values.
HEATING SYSTEM PERFORMANCE AND OPERATING EXPERIENCE
Operation of the warm water heating system began in December, 1975.
Since the Sherco Greenhouse was operational before the start up of Sherco
Generating Unit I, condenser waste heat temperatures were simulated by the
use of electric boilers. In the 1975-76 heating season, the primary heating
objective was to verify the heating system design. Water flow rates, air
flow rates, temperature drops, greenhouse heat demand, and other variables
were observed and recorded manually for comparison with design values.
Several simulation runs using 29.4°C (85°F) water confirmed the design capa-
bility of the heating system to transfer the heat necessary to maintain
greenhouse air temperatures in an acceptable range of 13-16°C (55-60°F) on
the coldest nights, -29°C (-20°F), experienced.
11
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In September, 1976, the Sherco Greenhouse was first supplied heat
directly from Sherco Generating Unit I. The heating objective during the
winter of 1976-77 was to verify that the waste heat available from the oper-
ating power plant was adequate for heating the Sherco Greenhouse. By utiliz-
ing an automatic data logger, 227 data points were monitored and recorded
at 1/2 hour intervals throughout the heating season, including all heat
transfer and greenhouse environmental parameters of interest. This compre-
hensive data set proved beyond all doubt the technical feasibility of
utilizing actual power plant waste heat to provide a suitable greenhouse
environment for the production of floral and vegetable crops.
In the third year of operation, the 1977-78 heating season, the main
technical objective was to solve some heat exchanger and pipeline fouling
problems identified in the previous heating season. Of course, additional
heating system performance data were taken and analyzed to add to,the body of
experience previously gained. Of particular interest in the 1977-78 heating
season was the observation of the variation of condenser outlet water temper-
atures with generator unit loading. Prior to the 1977-78 heating season,
the Sherco Generating Plant had only one operating unit. With the addition
of Unit II, the station began to operate as a load following rather than
baseload facility.
In order to completely document the technical performance of the Sherco
Greenhouse Project, particularly the heating system performance, the balance
of this section of the report is divided into three parts, each dealing with
a specific aspect of the technical performance of the heat source, the heat
delivery, and heat transfer systems.
SEVERE COLD WEATHER PERFORMANCE DATA
The greenhouse heat loss reached near design conditions when outside
ambient air temperatures fell to -29 to -34°C (-20 to -30°F). Oftentimes
when this occurred, the waste heat warm water available from the power plant
was significantly warmer than the design temperature of 29.4°C (85°F). For
example, Figure 6 shows the condenser outlet water temperature available to
the pipeline for delivery to the Sherco Greenhouse on January 9, 1977. This
date was chosen because it was the coldest day ever recorded during the
project. The corresponding generator unit output shown in Figure 6 is
typical of baseload operation of the Sherco Generating Plant. As indicated
in Figure 6, a 24-hour average condenser outlet water temperature of 37.6°C
(99.6 F) was recorded. At the same time, the average water temperature
measured at the greenhouse inlet was 36°C (96.2°F) shown in Figure 7. The
average ambient air temperature on January 9, 1977 was -31°C (-23.8°F),
with a high of -23°C (-10°F) and a low of -42°C (-42.9°F) at 8:00 a.m. The
heat transfer systems in the greenhouse were able to maintain an average
greenhouse air temperature of 15°C (59.5°F) with an 8:00 a.m. low of 14°C
(58°F) using warm water available at 33°C (91.1°F).
The complete air and soil heating system AT averaged 3°C (5.3°F) while
the inside to outside air AT averaged 46°C (83.3°F). The 24-hour average
heat transfer rate based on the total design water flow rate of 32.7 1/s
(518 gpm) was 402 KW (1.4 X 106 Btuh).
12
-------
750
700
»—
D_
Oi
O
-------
LU
QL
op OQ
120
no
100
90
80
70
60
50
40
30
20 J
10
0
-10 _
-20
-30
-40
-50
-- 10
-40
-30
--10
-20
--30
40
WARM WATER SUPPLY (96.2)*
WARM WATER RETURN (903
GREENHOUSE AIR (59.5)
AMBIENT AIR (-23.8)
*Numbers In parentheses are
daily averages in °F
02 4 6 8 10 12 14 16 18 20 22 24
TIME, hr
Figure 7 Heating System Performance January 9, 1977
14
-------
Wind and solar radiation data for January 9, 1977 are presented in
Figures 8 and 9 to complete the 24-hour environmental history. The wind
velocity data were those recorded by the National Weather Service at
St. Cloud, Minnesota about 20 miles northwest of the Sherco Greenhouse.
Solar data were measured inside and outside the greenhouse with pyranometers
and the hourly integrated values were recorded. From these data, it is
obvious that January 9, 1977 was not a completely typical cold day with 40%
cloud cover from sunrise to sunset; but, the light winds were fairly typical.
Additional severely cold weather performance data are plotted in Figures
10 and 11. The dates selected represent the two coldest days on record in
the 1976-77 and 1977-78 heating seasons. January 10, 1977 was nearly as
cold as January 9, and the second coldest day recorded during the project.
It differed only slightly from the previous day in that clear skies prevailed
from sunrise to sunset. Again, greenhouse air temperatures were held above
14°C (58°F) during darkness hours when ambients dipped to -33°C (-28°F)
using waste heat available at 37°C (98.5°F), 24-hour average temperature.
Figure 10 indicates the same data for January 9, 1978, which coincident-
ally was the coldest day of the 1977-78 heating season. It may be noted that
the waste heat was available at slightly higher temperatures, but the green-
house air temperature was actually lower. No abnormal environmental con-
ditions were noted to explain the apparent difference from previous data.
It is possible that reduced temperature set points on the greenhouse
thermostats controlled the temperature to the lower than expected value.
Figure 12 is the same data for February 2, 1978. Here, satisfactory
space temperatures were maintained in the greenhouse when the waste heat
source water temperature was 32°C (90°F), near the design water temperature,
and ambient outside air was -20°C (-4°F).
More insight into heating system performance can be gained by a detailed
look at the performance of the air heating and soil heating systems separately.
Figure 13 shows the finned-tube heat exchanger inlet and outlet water and air
temperatures for the 24 hours of January 9, 1977 as measured at the heat
exchanger located in the easternmost bay of the greenhouse. The inlet air
temperature to the heat exchanger was essentially the average air temperature
in that particular bay of the greenhouse. It can be seen that the water was
being cooled approximately 4°C (8°F) while air was being heated 9°C (16°F).
Both the waterside and airside AT's were below design values. The logarithmic
mean temperature difference of the heat exchanger averaged 14°C (25.3°F)
with an average heat transfer rate of 24.4 KW (117,450 Btu/hr) at the design
water flow rate. This measured performance was 60% of the design value as
a result of heat exchanger tubeside fouling. However, due to the average
warm water temperature being 6°C (11.2°F) above design, the potential adverse
consequences on greenhouse air temperature were avoided.
Typical soil heating performance data are shown in Figure 14 for January
9, 1977. The soil temperature plotted is the average of 42 soil temperatures
measured at depths varying from a 2.5-30 cm (1-12 in) through the total
2123 m2 (22,848 ft2) growing space. The water AT in the soil heating system
was relatively constant at 3°C (6°F). Since water flow in the system was
15
-------
02 4 68 10 12 14 16 18 20 22 24
TIME, hr
Figure 8 Wind Speed January 9, 1977
16
-------
Btu/hr-ft2
TOO
KJ/hr-m2
EXPECTED /
V/
8
10 11 12 13 14 15 16
TIME, hr
17
Figure 9 Measured Hourly Average Total Solar Radiation January 9, 1977
Sunrise to Sunset Sky Cover = 40%
Wind Speed = 5.75 miles/hr from East
18
-------
UJ
Q.
°F
120
no
100
90
80
70
60
50
40
30
20
10
0
-10
-20
-30
-40
-50
L 40
- 30
10
- 0
- -10
--20
WARM WATER SUPPLY (101.2)*
WARM WATER RETURN (92.3)
20 GREENHOUSE AIR (58.3)
AMBIENT AIR (-10.4)
--30
-40 *Numbers in parentheses are daily averages in °F
02 4 6 8 10 12 14 16 18 20 22 24
TIME, hr
Figure 10 Heating System Performance January 9, 1978
18
-------
UJ
OL
°F
120 _
no _
100 _
90 _
80 _
70 _
60 _
50 _
40 .
30 .
20 .
10 .
0 ,
-10 _
-20 .
-30 ,
-40 <
-50
WARM WATER SUPPLY (98.5) *
- 30
- 20
10
- 0
- -10
- -30
WARM WATER RETURN (92.8)
GREENHOUSE AIR (59.2)
AMBIENT AIR (-18.8)
_40 *Numbers in parentheses are daily averages in °F
—I 1 1 1
0 2 46 8 10 12 14 16 18 20 22 24
TIME, hr
Figure 11 Heating System Performance January 10, 1977
19
-------
CC.
°F
120
no
100
90
80
70
60
50
40
30
20
10
0
-10
-20
-30
-40
-50
WARM WATER SUPPLY (97.1)*
-30
WARM WATER RETURN(91.2)
20
GREENHOUSE AIR (63.
-- 10
- 0
--10
AMBIENT AIR (-1.
-20
"-30
-40 *Numbers in parentheses are daily averages in °F
0 24 6 8 10 12 14 16 18 20 22 24
TIME, hr
Figure 12 Heating System Performance February 2, 1978
20
-------
°F
110
100
90
80
70
60
50
40
- 40
- 30
SUPPLY WATER (96.2)*
RETURN WATER (88.1)
OUTLET AIR (74.5)
INLET AIR (58.9)
10
*Numbers in parentheses are daily averages in °F
02 4 6 8 10 12 14 16 18 20 22 24
TIME, hr
Figure 13 Air heating System Performance January 9, 1977
21
-------
Q-
&
°F
no
100
90
- 40
80 _
70
60
50
40
SUPPLY TEMPERATURE (96.2)
RETURN TEMPERATURE ( 89.0)
- 20
SOIL TEMPERATURE (61-
-- 10
*Numbers in parentheses are daily averages in °F
0 2 46 8 10 12 14 16 18 20 22 24
TIME, hr
Figure 14 Soil Heating System Performance January 9, 1977
Sunrise to Sunset Sky Cover = 40%
22
-------
also constant, the design flow rate of 7 1/s (112 gpm) gave a nearly constant
heat transfer rate of 98.4 KW (336,000 Btu/hr) or 46 watts/m2 (14.7 Btu/hr-
ft2). However, the measured water flow in the soil heating system was
normally about 5 1/s (80 gpm); thus, a heat transfer rate of 70.3 KW
(240,000 Btu/hr) or 33 watts/m2 (10.5 Btu/hr-ft2) was actually achieved.
This contrasts with the air heating system design heat transfer rate of
295 watts/m2 (92 X 106 Btu/hr-ft2) of greenhouse floor area. The rise in
soil temperature after 10:00 a.m. in Figure 14 is apparently due to higher
water temperatures in the heating grid, higher air temperatures in the
greenhouse, and higher solar radiation heat flux. Thus, soil heating
amounted to a little more than 10% of the total heat transferred, but some
of this heat was lost to the earth and did not contribute to heating the
greenhouse air.
Besides the coldest day data, the monthly overall performance during
January, 1977 and January, 1978 has been analyzed. Figures 15 and 16
present the distribution of warm water and average greenhouse air tempera-
tures for the month of January, 1977. Because Sherco Generating Unit I was
operating more or less base loaded, 29.4°C (85°F) water was available 99.6%
of the time. With this warm source water, it was straightforward (in spite
of heat exchanger fouling) to maintain greenhouse air temperatures above
13°C (55°F) 99.9% of the time.
Figure 17 shows the waste heat temperature distribution available for
greenhouse heating January, 1978. These data were taken from plant operating
logs and are a composite of the operating data for the two-unit station.
In January, 1978 an interconnected pipeline system was in operation so that
waste heat could be made available from either generating unit in the event
of unit outages. By careful manual selection of the waste heat source, it
was possible to provide waste heat at a temperature above 29.4°C (85°F),
73% of the time, even though Unit I and Unit II produced warm water greater
than 29.4°C (85°F) only 50% and 65% of the time, respectively. With the
available waste heat, air temperatures in the Sherco Greenhouse were main-
tained above 13°C (55°F) 86% of the time (Figure 18). The difference
between January, 1977 and January, 1978 resulted from the change in plant
operation from a single-unit baseload station to a two-unit load following
station. When operating in an electric system load following mode, the
gross electrical output of each unit can vary from 700 MW in the daytime
down to a minimum load of about 300 MW in the nighttime. This electrical
load variation can result in an 8°C (15°F) change in condenser outlet water
temperature as illustrated in Figure 19.
ELECTRIC ENERGY CONSUMPTION
Of particular concern in the design of mechanical systems to utilize
waste heat is the operating electrical energy requirement. In order to
assess the overall feasibility of waste heat utilization for greenhouse
heating, careful records of electrical energy consumption were maintained.
Manually read electric meters were installed to measure separately the
electric energy consumption of the Sherco Greenhouse air heating system fan
motors, water circulation system pump motors, cooling system fan motors, and
23
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100
80--
ro
c
3
IO >
-M id
(U
Q.
60..
40--
20 „_
20
Greater Than 85°F
99.6% of the Time
in January, 1977
30
40
60
70
80
90
100
I
no
120
Figure 15 Typical Warm Water Temperatures
-------
ro
01
100-
to
-C
0) 0)
+•> S-
(O 3
S- (O
t!3 J-
O)
•r- 0)
i*- -o
O 0)
0) 3
0>
o
5-
0)
0.
80-
60-
40-
20-
10
4-
Greater than
55°F 99.9% of
the Time in
January 1977
20
I
40
50
60
70
80
90
100 °F
Figure 16 Typical Greenhouse Air Temperature
-------
ro
OC
UJ
O.
25
20
10
40
732 of
Time
Temperature
Greater than
85°F
T
50
i
60
i
70
i
80
I
90
100
110
120
130
TEMPERATURE, °F
Figure 17 Condenser Outlet Water Temperature Distribution January, 1978
Composite of Unit I and Unit II
-------
ro
of
Time
Temperature
Greater Than
55°F
30 40 50 60 70 80 90 100 110
TEMPERATURE, °F
Figure 18 Average Greenhouse Air Temperature Distribution January 1978
-------
120 -
I- 50
rv>
00
ae.
1
UJ
Q_
UJ
110
100
90
80
70
60
- 40
_ 30
"- 20
50 -- 10
100
I
200
300
i
400
i
500
600
700
GENERATOR UNIT LOAD, MW
Figure 19 Typical Warm Water Temperature Variation with Unit Load
-------
miscellaneous electrical usage. Back up heating energy supplied by electrical
boilers was determined by subtracting all the separate readings from the
Sherco Greenhouse master service meter.
Figure 20 shows the monthly total power consumed by the heating fans
and circulating water pumps in the greenhouse during the 1976-77 heating
season. Beginning in October the curve shape is characteristic of the heat-
ing degree days associated with each month of operation. It should be
noted that the weather turned abnormally cold in November with 1249 heating
degree days compared to a normal of 1050. In contrast, February was abnorm-
ally warm with 1283 heating degree days as opposed to a normal of 1448. One
may notice that the peak electric energy use of the pump motors is nearly
80% of that of the fan motors even though the installed pumping capacity is
one half that of the fans. This is due mainly to two factors: the method
of flow control (parallel pumping) and over capacity in the pumping system.
The sharp decline in pumping energy consumed in February resulted from
using half of the installed pump capacity, in addition to the change in
the weather conditions. The sharp rise in March was due to continuous
operation of one half of the pump capacity to facilitate heat transfer
measurements.
Using the previous data, Figure 21 was generated to show what percent of
the installed heating system capacity was used on a monthly average basis.
These data proved useful in estimating the electric energy use that commercia1
operations might expect. Again, it may be noted that the water pumping
capacity factors are high, compared to the fans because only four pumps can
start or stop; whereas, the air heating system can start or stop 14 fans.
Table 1 summarizes the annual electrical use of all the greenhouse
systems. For 1976-77, the period covered is October 1, thru May 31, while
in 1977-78 a complete year starting and ending on June 1 is presented. The
amount of electrical energy consumed by the warm water heating system fans
and pumps was greater in 1977-78 due to the longer period covered and due to
the differences in warm water temperatures available from the power plant.
The total energy consumed by the fans and pumps in 1977-78 is estimated to be
8.5% of the total energy required to heat the greenhouse during the year.
Thus, the warm water heating system had an annual ratio of heat delivered to
electric energy consumed of 11.8, which would be equivalent to a heat pump
COP of the same value. The instantaneous ideal rated heat output to electric
energy input of the Sherco Greenhouse heating system is 12.7.
The energy consumed by the backup heating boilers was necessitated by
pipeline outages, reduced condenser outlet water temperatures, and generator
unit outages.
A total of 642 hours of operation without waste heat available were
recorded from October 1, 1976 to April 25, 1977, at which time the pipeline
was shut down for maintenance. Therefore, with an overall availability of
warm water waste heat of 75% for the period October through May, backup
electrical energy during the first heating season amounted to 249,661 kwh or
about 16% of the total calculated annual energy required. Figure 22 shows
29
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15
10
o
Q.
O
i—<
OL
O
0
FANS
OCT NOV
DEC JAN
MONTH
FEB MAR APR
MAY
Figure 20 Monthly Electric Power Consumption of
Heating Fans and Pumps (1976 - 1977)
30
-------
oc
P
o
o
>-
80
70
60
50
40
30
20
10
CIRCULATING PUMPS
OCT NOV
DEC JAN
MONTH
FEB MAR APR MAY
Figure 21 Monthly Capacity Factors of Greenhouse
Heating System Units (1976 - 1977)
31
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TABLE 1. SHERCO GREENHOUSE ELECTRICAL POWER CONSUMPTION
10/1/76 - 5/31/77 6/1/77 - 6/1/78
Use KWH KWH
Warm Water Pumps 48,647 52,846
Heating Fans 63,766 79,188
Cooling Fans 4,506 10,813
Miscellaneous Use 37,220 56,160
Boilers & Well Water Pump 249,661 555.793
Total 403,800 754,800
the characteristic monthly energy use by the Sherco Greenhouse boilers during
times of generator unit outages during the 1976-77 heating season. It should
be noted that 32,500 kwh of the supplemental heat was used during May when
the pipeline was taken out of service. Thus, the adjusted supplemental heat
discounting May was 217,161 kwh, or 13.9% of the calculated annual heat energy
requirement.
The second heating season was similar to the first except that almost
twice as much backup heat was required. This was due to several factors,
the most important being fouled heat exchangers, lower average condenser
outlet water temperatures, pipeline failures, and pipeline unavailability in
August, September, and October, 1977 necessitated by commercial expansion of
the waste heat pipeline system. Based on degree day data, an estimated
151,025 kwh were used for heating during these three months. Hence, the
adjusted supplemental heat consumed was 404,768 kwh or about 32% of the
annual greenhouse heat requirement. Since this percentage of supplemental
heat addition would likely be considered unacceptable by commercial waste
heat users, it should be noted that a commercial greenhouse operator, who
used the same waste heat source with unfouled (clean) heat exchangers during
the same heating season, required only a 10% supplemental heat addition.
WASTE HEAT AVAILABILITY
The pipeline installed to transmit the condenser waste heat 1067 m
(3500 ft) each way to the Sherco Greenhouse and back to the Unit I cooling
tower basin went into service on September 16, 1976. The main 30.5 cm
(12 in) diameter pipeline operated flawlessly until April 25, 1977 when it
was taken out of service for cleaning and inspection. However, a 15 cm
(6 in) diameter service pipeline that brings warm water into the greenhouse
from the main pipeline failed three times during the 1976-77 heating season.
These failures resulted in the loss of waste heat service to the greenhouse
for 75.0, 21.5, and 29.0 hours during the months of December, February, and
March, respectively. Warm water service availability to the greenhouse
due to these failures was reduced to 96.6%. However, the overall avail-
ability of waste heat at the greenhouse was only 87.9% due to the combined
effects of both pipeline failures and generator unit outages. Table 2
32
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summarizes the Unit outages and pipeline failures month-by-month for the 1976-
77 heating season.
During 1977-78 heating season warm condenser water was supplied to the
Sherco Greenhouse and to commercial greenhouse operators. In order to do
this, the pipeline network was expanded and an interconnection to Unit II
was made. Thus, waste heat could be made available from either generating
unit by manual operation of valves at the point of interconnection of the
two units. Normally these valves were operated when operating personnel were
notified of a generator unit outage. With this feature it was possible to
have a constant supply of waste heat available to the pipeline except for
the time it took to switch the valves during unscheduled generator unit
outages, and except for a single simultaneous two-unit outage which occurred
in May. For a period of 56 hours, Unit I was out of service for annual
maintenance and Unit II was intentionally scheduled out for weekend mainten-
ance.
TABLE 2. WARM WATER AVAILABILITY
Month
September 16, 1976
October
November
December
January
February
March
April 25, 1977
Total or Average
Sherco
Unit I
Outages
(hrs)
32.0
229.5
0
7.5
168.0
78.5
0
0
515.5
Pipeline
Failures
(hrs)
0
0
0
75.0
0
21.5
29.0
0
125.5
Overal 1
Availability
of Waste Heat
@ Greenhouse
(percent)
90.5
69.2
100.0
88.9
77.4
85.1
96.1
100.0
87.9
Table 3 summarizes the weekly overall availability of waste heat at a
commercial operator's greenhouse for the period November 2, 1977 to May 31,
1978. Prior to November 2, 1977 the pipeline system was out of service for
construction to extend service to commercial operators. While the pipeline
continued in service after May 31, 1978, the greenhouse heating season was
essentially over. As indicated, for a period of 4848 operating hours an
overall availability of 96.4% was achieved.
On a weekly basis during the first heating season, there were only four
weeks out of 30 when warm water was unavailable for part of the time. Un-
available service to the commercial greenhouse operations was primarily due
to pipeline failures. Two pipeline failures occurred during the heating
seasons: one was the failure of a service line from the main pipeline into
a commercial floral operation and the other failure involved a main 30.5 cm
33
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TABLE 3. SUMMARY OF WARM WATER AVAILABILITY
AND TEMPERATURE
85°F or
Service Availability Greater
Dates (Hrs) (%) (%)
11/2 - 11/8 156 100 95
11/9 - 11/15 168 100 93
11/16 - 11/22 168 100 92
11/23 - 11/29 168 100 57
11/30 - 12/6 168 100 69
12/7 - 12/13 80 47.6 84
12/14 - 12/20 158 94.0 84
12/21 - 12/27 168 100 74
12/28 - 1/4 168 100 40
1/5 - 1/11 168 100 96
1/12 - 1/18 168 100 98
1/19 - 1/25 168 100 97
1/26 - 2/1 142 84.5 95
2/2 - 2/8 168 100 81
2/9 - 2/15 168 100 98
2/16 - 2/22 168 100 47
2/23 - 3/1 168 100 99
3/2 - 3/8 168 100 87
3/9 - 3/15 168 100 82
3/16 - 3/23 168 100 90
3/24 - 3/29 168 100 92
3/30 - 4/5 168 100 86
4/6 - 4/12 168 100 87
4/13 - 4/19 168 100 92
4/20 - 4/26 168 100 85
4/27 - 5/3 168 100 88
5/4 - 5/10 112 66.7 91
5/11 - 5/17 168 100 92
5/18 - 5/24 168 100 94
5/25 - 5/31 168 100 97
Totals or Averages 4848 96.4
34
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OL
LU
OCT NOV DEC JAN FEB MAR APR MAY
MONTH
Figure 22 Monthly Electric Power Consumption of Greenhouse
Boilers During Unit I Outages (1976 - 1977)
35
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(12 in) diameter plastic pipeline. In the former case, a four-day outage
occurred; in the latter case, a 26-hour outage occurred. The period of un-
available service indicated in May was during the previously mentioned two-
unit outage of the generating station.
It is also noteworthy that warm water at the power plant condenser out-
let was available at a temperature greater than 29.4°C (85°F) for 85.4% of
the time. However, the actual temperature delivered to the Sherco Greenhouse
was somewhat lower due to pipeline heat losses. Figures 23 and 24 illustrate
typical piping heat losses from the condenser outlet to greenhouse inlet for
warm water taken from Units I and II, respectively. The temperature drop is
greater during daylight hours when pipeline flows are low. During periods
of higher flows, the AT averages about 1.2°C (2.2°F) when the Unit I source
supply is used and 3.1°C (5.6°F) when the Unit II source supply is used
The difference in pipeline heat loss for the two sources results from the
differences in pipe diameter, pipe material, and total transmission distance
Based on a nominal pipeline flow rate of about 63.1 1/s (1000 gpm), the
pipeline heat loss from the Unit I source was about 300 watts/m (312 Btu/hr-
ft). This loss rate is about twice that calculated for the desiqn basis
with 29.4°C (85°F) water temperature. y
OPERATING EXPERIENCE
Construction work on the Sherco Greenhouse began in August of 1975 The
greenhouse and the mechanical equipment were installed to provide a commer-
cially oriented demonstration project, which employed systems and practices
currently accepted by the commercial greenhouse industry.
The exception to this, of course, was the installation of the warm
water air and soil heating systems. The greenhouse system was constructed
by union craft labor under the supervision of project personnel.
The heating system was made operational by November, 1975. During the
heating season of 1975-76, the greenhouse heating proved to be satisfactory
under a simulation scheme. That is, warm waterwas provided by two - 390 KW
(1.28 X 106 Btu/hr) electric boilers at a temperature simulating the antici-
pated winter design minimum condenser outlet cooling water temperature of
29.4°C (85°F). The first year of operation involved the technical verifi-
cation of the heating system performance as designed. Preliminary tests
indicated that the heating system design was adequate in all conditions
experienced, and any problems associated with the initial operation of the
various system components were diagnosed and remedied.
On the basis of the successful first year of operation, the connecting
pipeline was installed during the summer of 1976 to utilize condenser reject
heat from Unit I, which began commercial operation in the spring of 1976
The greenhouse circulation water pipeline source is at a cooling tower supnlv
riser pipe as shown in Figure 25, with circulation flow through the pipeline
being provided by the available head pressure at the tap point as developed
by the Sherco main circulation pumps.
36
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Ul
0£
1
90
CONDENSER OUTLET WATER
SIX-INCH SUPPLY WATER
0 24 6 8 10 12 14 16 18 20 22 24
TIME, hr
Figure 23 Pipeline Losses Between Unit I Condenser
and Greenhouse January 8, 1978
37
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CL.
°F
110
105
- 40
100
95
90 _
85
80
-_ 35
CONDENSER OUTLET WATER
SIX INCH SUPPLY WATER
0 2 46 8 10 12 14 16 18 20 22 24
TIME, hr
Figure 24 Pipeline Losses Between Unit II Condenser
and Greenhouse January 10, 1978
38
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Figure 25 Connection at Cooling Tower
The connecting 1067 m (3500 ft) greenhouse supply and return pipelines
consist of 30.5 cm (12 in) diameter low pressure polyvinylchloride pipe.
The pipeline is uninsulated and is buried at a nominal depth of 1.52 m (5 ft).
The above ground piping at the cooling tower is insulated and heat traced to
minimize heat loss and freezing problems during periods when no flow exists
in the pipeline. The initial water flow capability of this pipeline was
100 1/s (1600 gal/min) after completion of installation in September, 1976.
In the Summer of 1977, an interconnection between the two generating
units was made to provide commercial waste heat service with two-unit
reliability. The Unit II supply connection is at a cooling tower riser,
similar to the Unit I connection, and a pipeline interconnects to the pre-
viously existing pipeline from the Unit I tower.
Figure 26 shows in schematic form the Sherco warm water distribution and
service pipeline system as it existed during the 1977-78 heating season. As
indicated, the installed Unit II connecting pipeline is a 45.7 cm (18 in)
diameter cast iron pipe which is uninsulated and buried to a nominal depth
of 1.52 m (5 ft). This portion of the pipeline was intentionally oversized
to be capable of delivering sufficient flow to heat up to 5.7 hectares (14
acres) of greenhouses at the present development portion of the Sherco site.
Also indicated in Figure 26 is the method of providing warm water service
to the commercial customer. The service connections are installed as a
typical utility service, where flow from the main pipeline enters the service
supply pipe, passes through a recording flow meter and then is distributed by
the user's pumps to the greenhouse heating system prior to being returned to
the main return pipeline.
39
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I
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OLE
•«<
jo,
I,1-'! j
Li/ '
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/ >< »
TERMINATION -
i^j
31
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-4
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CHLOKIN/XTION
-------
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.veto
FITTING AND
MM watt oimtmnitm MO SPNICI HNIM JTSTW >M.«I
4* Httt
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Figure 26 Warm Water Distribution System Schematic
-------
OPERATING PROBLEMS
Coil Fouling
In November of 1976, after about two months of heating with condenser
cooling water, the overall heat transfer capability of the finned-tube coils
in the Sherco Greenhouse appeared to be reduced by about 40% from design
conditions. Further investigation indicated that waterside coil tube fouling
was the reason for loss of performance. Chemical analyses of samples of
material deposits from various system components indicated that the fouling
was caused by a microbial-based slime. Remedial action in the form of
chemical or mechanical cleaning was recommended.
Mechanical cleaning of the coils was investigated, but found to be an
unworkable solution. This was due to the difficulty in accessing the tubes
in the particular coils used in the Sherco fan/coil units and also to the
presence of spiral-wound turbulence-inducing wires on the inside surface on
the coil tubes.
Various chemical cleaning programs were also investigated and attempted.
Routine chlorination was determined to be the most effective means of slime
accumulation control. The removal of the heat exchanger deposits was further
complicated by the formation of iron oxide deposits on the interior walls of
the carbon steel distribution piping in the greenhouse. It was determined
during the 1977-78 heating season that acid cleaning of the greenhouse
internal circulation system was the most viable option, and this procedure
was adopted as routine maintenance is required.
Based on the experience of the Sherco Greenhouse during 1976-77, general
recommendations on system component materials were offered to the commercial
customers who installed systems during the summer of 1977. The recommenda-
tions were that the finned-tube coils be fully accessible for mechanical
cleaning and that distribution piping be of a non-oxidizing, easily cleaned
material.
Pipeline Fouling
Concurrently with the fouling of the heat exchangers, a marked decrease
in flow capability of the main pipeline was exhibited. The original 100 1/s
(1600 gal/min) capability was reduced by about 40% to 60 1/s (1000 gal/min)
over the first two months of pipeline operation.
The cause of this flow reduction was indicated again as microbial based
slime accumulation on the interior of the pipe walls. It was found that the
generating plant's routine chlorination to prevent condenser tube fouling
was not effective in preventing fouling of the greenhouse system, because of
the location in the circulation system where the condenser water is diverted
for the greenhouse system.
The successful solution to the removal of pipeline slime deposits in-
volved mechanical cleaning using polyurethane "pigs" which were propelled
42
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through the pipe by water pressure. After this mechanical cleaning operation,
the original full flow capability was restored.
The experience in pipeline and waterside coil fouling impacted the
design of the commercial service pipeline as installed in 1977. The pipeline
source connections have fittings for mechanical cleaning access. Remote
chlorine injection systems were also installed at both the Unit I and II
source connections to allow for routine supplemental chlorination to prevent
slime formation. Inspection of the commercial grower's heat exchangers and
piping system has indicated that the routine chlorination program is effect-
ive in preventing slime deposition.
Circulating Water Pump Wear
During the heating season of 1977-78, two instances of seal failure on
two of the four Sherco Greenhouse circulation pumps were encountered. Upon
inspection of the seals, it was indicated by the equipment vendor that while
the pumps had been in service for three heating seasons, the exhibited
deterioration was unusual. It was further suggested that the mechanical seal
wear was due to the abrasive action of a particulate material suspended in
the circulation water. Inspection of the pump impeller and housing indicated
no apparent signs of abrasion or wear.
GREENHOUSE SYSTEM OPERATION
The general operation of the greenhouse environmental control system is
quite straightforward. With the mechanical equipment automatically con-
trolled, the system as designed requires minimal observation and attention
on the part of the greenhouse operator. As is substantiated by the two
commercial growers at the Sherco site, the mechanical maintenance routine
is equal to or less demanding than that of a conventionally heated greenhouse
system.
43
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GENERAL HORTICULTURAL CONSIDERATIONS
SOIL
The greenhouse was constructed on a soil similar to Hubbard loamy coarse
sand. This soil exhibits low water holding capacity, low cation exchange
capacity and low pH. It was disturbed and reshaped during construction.
It was necessary to add pulverized dolomitic limestone, phosphate potash,
and nitrogen to a 0-15 cm (0-6 in) depth prior to crop installation. Coarse
textured sphagnum moss peat was added to the rose beds at a 1:20 peat-soil
ratio to improve the water holding capacity. This had the effect of lowering
the pH to 5.0-5.5 and subsequent additions of dolomitic and hydrated lime on
the soil surface have not been successful to date in raising the pH to a
6.0-6.5 optirflum.
The geraniums were potted in a 3:2:2 and the cinerarias in a 1:1:1 soil-
peat-perlite mix containing 5.3 kg/m3 (5.36 oz/ft3) of treble superphosphate
(0-46-0).
IRRIGATION
The irrigation water was obtained from a ground source. No cooling tower
water was used for irrigation. The water quality generally was as follows:
pH 7.7, total hardness 213 ppm (as CaCOa), Ca 55 ppm, and Mg 13 ppm. Any
potential phytotoxic ions were at insignificant levels.
Irrigation water was applied to the roses using a Gates perimeter system
fitted with 180° flat spray nozzles. It was possible to water about 93.6 m2
(1000 ft2) simultaneously. A soil tensiometer was employed to assist in
estimating soil moisture. Water was turned on at a 10 centibars tension and
applied at 12.2 1/m2 (0.3 gal/ft2) at each watering. Irrigation frequency
was subject to seasonal adjustment.
The cut flower and vegetable crops were irrigated with Viaflow trickle
irrigation tubing. The tubing was arranged so that each row of plants was
adjacent to a water source.
The potted geraniums were grown on the surface of the heated soil. Sub-
irrigation was supplied by capillary mat underlaid with a sheet of poly-
ethylene film. Each mat was 1.22 m (4 ft) wide and wetted by means of three
lines of trickle irrigation tubing.
The containerized nursery stock was irrigated with 360° spray nozzles
mounted on metal rods and elevated about 0.3 m (1 ft) above the container
surface.
44
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FERTILIZATION
Soil fertility levels were determined for soluble salts, pH, nitrate-
nitrogen, potassium, phosphorus, calcium, and ammonium-nitrogen on a routine
basis at the University of Minnesota Soil Testing Laboratory. Periodic
foliar analysis was performed on the roses and on other crops when visual
deficiency or toxicity symptoms developed. Fertilizing during crop growth
was accomplished by injecting soluble fertilizer into the irrigation lines
with a single head injector set at a 1:100 dilution ratio.
The rates of macro and secondary elements were added according to the
results of periodic soil tests. Soluble microelement mix was added once to
each crop with a low nutrient requirement and twice to crops with high
demand. Specific microelements were applied when visual deficiency symptoms
were evident. Potted geraniums had a 8 gm (4.6 oz) per pot of Osmocote
14-14-14 mixed into the potting soil after steaming.
Carbon dioxide enrichment was supplied from three propane-fired gener-
ators. A timer automatically actuated these units two hours prior to sunrise.
The C02 level at sunrise was 1500-2000 ppm and was allowed to gradually
decrease to ambient during the day.
PEST CONTROL
Steam was not available so, during the July crop changeover, Vapam was
used for soil fumigation. The soil was presaturated with water and the
Vapam applied at 1 1/10 m2 (1 qt/100 ft2) with a hose and suction device.
The soil was water saturated again after Vapam application. The area was
allowed to dry for 4-5 days, rototilled, and then spot planted with tomato
plants. If no phytotoxicity was observed, crop planting into ground beds
was resumed. This treatment held weed growth to a minimum and appeared to
have reduced damping off in the snapdragon transplants.
Two attempts were made at introducing Encarsia formosa into the tomato
crop for biological control of whitefly, Trialeurodes yaporariorum (Westwood).
Both attempts proved unsuccessful as the Encarsia population could not expand
as rapidly as the whitefly population. Endosulfan 50 WP was used for control
after the Encarsia failed.
The two-spotted spider mite, Tetranychus urticae (Koch), infestations
occurred mostly on the roses and was controlled with Pentac 50 WP.
Aphids, Macrosiphum spp,on flowering crops were controlled with Aldicarb
and on food crops with MaTathion 25 WP.
Because of the high air flow from the heating system, botrytis, Botrytis
cinerea, did not present much of a problem for most crops. Botrytis infected
the leaves and stems of mature tomato plants after cooling pad usage was
started.
The rose varieties Sonia and Belinda appeared most susceptible to
powdery mildew, Sphaerotheca spp. Applications of Pipron LC and Parnon LC
45
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were used for control.
Rhizoctonia presented the greatest problem in lettuce and geranium.
Benlate was used on geranium and nothing was used on lettuce. Few damping
off problems were encountered with snapdragons.
HARVESTING AND MARKETING
The Spring 1976 tomato crop was picked at the light pink stage (10-20%
ripeness) and bulked into 13.6 kg (30 Ib) boxes. These were transported to
a Minneapolis retail store where they were graded and packed. Some were
retailed at the store and the balance were marketed to other retail stores.
Bulk packing and transporting at this stage of ripeness caused excessive
bruising and cracking. Better market quality could be obtained if the fruit
were allowed to mature on the vine to between 60 and 80% ripeness.
The Spring 1977 tomato crop was graded at the site into U.S. No. 1, 2,
and unclassified according to U.S. Standards for Grades of Greenhouse
Tomatoes. Grade No. 1 was sized into nested plastic separators with 3 X 5,
4X5,5X5,6X5 and 6X6 count. Two layers of separators were placed
into a 9.1 kg (20 Ib) carton. The holding quality of the product under these
conditions was much improved. The packed tomatoes were not stored in a
cooler but delivered on the same day to another local tomato grower who sold
the product through his established market channels.
The Fall-Winter 1976 leaf lettuce was cut, washed and packed into 2.26
kg (5 Ib) boxes. Bibb lettuce was packed 24 per box. An excessive amount
of labor was required to wash out sand deposited in the plants from the soil.
Growing through a plastic film mulch would have done much to prevent this.
The finished product was delivered both to a wholesale produce house and also
directly to a supermarket.
Blooming geraniums and cinerarias were placed into flats and delivered
directly to retail outlets.
The snapdragons were bunched into tens by color and cut to about 1 m
(39 in) in length. A plastic sleeve was pulled over the floral spike and
the bunches placed in buckets containing preservative. The bunches were
transported and sold directly to the wholesale market.
Cut roses were bunched in bulk, placed into buckets containing preser-
vative, and taken to a local grower who performed the grading and marketing.
Monthly grading reports showing the quantity and average price for each
grading category were kept for the five cultivars.
The selling price for all the produce varied according to the seasonal
supply and demand. None of the crops were grown for a contract price.
Demand was good most of the time and quality was generally good.
DETAILED CULTURAL METHODS
Table 4 lists the major crops grown and the cropping schedule.
46
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1976
1976
1976
1976
1977
1977
1977
1977
1977
1977
1978
1978
1978
1978
1978
TABLE 4.
MONTH
January
June
July
August
December
January
February
May
July
October
November
January
February
May
June
July
CROPPING CHRONOLOGY
CROP
Tomatoes started
Green peppers started
Roses planted
Tomatoes terminated
Green peppers terminated
Snapdragons started
Lettuce started
Geraniums started
Snapdragons terminated
Tomatoes started
Lettuce terminated
Geraniums terminated
Tomatoes terminated
Snapdragon transplants started
Freesia started
Tree seedlings started
Cinerarias transplants started
Geraniums started
Cinerarias terminated
Freesias terminated
Geraniums terminated
Tree seedlings terminated
Snapdragons terminated
End of project
TOMATOES
Several cultivars of tomato were pi anted in the two spring crops to
determine their yield potential. Table 5 shows the yield of marketable
No. 1 and 2 tomatoes.
TABLE 5. YIELD OF MARKETABLE NO. 1 AND 2 TOMATOES
Cultivar kg (Ib) per plant
SPRING 1976
Ohio MR-13
Ohio-Indiana Hybrid 0
Super M
Tropic
Eurocross
Tuckcross 520
2.8
3.2
3.2
3.4
4.0
4.3
(Continued)
47
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TABLE 5 (continued)
Cultivar kg (Ib) per plant
(9.2)
(9.6
SPRING 1977
Vendor 4.2
Terrific 4.3
Vendor 76 4.5 (9.9)
Bruinsma 5.6 (12.3)
Eurocross BB 5.6 (12.4)
Michiana 5.8 (12.8)
Tropic 6.0 (13.2)
Better Boy 6.5 (14.4)
Tuckcross 520 6.7 (14.8)
Two factors could account for the difference in yield: plant density
and soil fertility.
The area per plant for the Spring 1976 crop was 0.24 m2 (2.60 ft2),
whereas for the Spring 1977 crop it was 0.37 m2 (4 ft2). In addition, the
general soil fertility of the 1976 crop was too low and the 1977 crop was
more optimal.
It should be noted that the differences between the cultivars are only
an indication since a totally randomized plot and statistical analysis was
not performed.
The Spring 1977 crop was planted primarily to Tropic because the vege-
tative growth rate of the vine was less vigorous than other red-fruited
cultivars and red fruit is preferred. Tropic was planted at the following
densities: one bay at 0.32 m2 (3.4 ft2) per plant, three bays at 0.37 m2
(4 ft2), and one bay at 0.43 m2 (4.6 ft2). Table 6 shows the yield per plant
for marketable grades No. 1 and 2. As would be expected, a reduction in
plant density improved the per plant yield. However, on a per bay basis, it
appears that either a lower density with a high yield or a high density with
a low yield produced about the same weight yield per bay.
TABLE 6. YIELD VS. DENSITY FOR TROPIC TOMATO - SPRING 1977
Spacing Yield
m2 (ft2) per plant kg (Ib) per plant kg (Ib) per bay
0.32 (3.4) 5.2 (11.5) 2176 (4798)
0.37 (4.0) 5.6 (12.4) 2009 (4430)
0.43 (4.6) 6.8 (15.1) 2138 (4714)
48
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SNAPDRAGONS
This crop was grown because there are few local cut snapdragon producers
left, and there appeared to be a demand in the local market. This projection
proved to be generally true except for the period between Thanksgiving and
Christmas when the demand was very low. The crop was generally apportioned
into 50% white, 25% yellow, and 25% pink. Seedlings in group 3 flowering
classification included about 10% novelty (double flower, butterfly, bicolor)
for spring cutting. All snaps were grown as single stem cuts as these gen-
erally had better stem and flower spike quality, were more uniformed and
flowered earlier. Attempts to program the beds to flower in a sequential
order to insure a steady cutting supply proved unworkable. No facilities for
photocontrol were available so the bed was cut when it had sufficient floret
count and replanted when empty. The only significant problem encountered was
a tendency for boron deficiency to occur in the sandy soil. The single layer
of Vexar nylon netting tended to shift horizontally when the plants reached
mature height. Additional staking and tying of the netting helped solve this
problem.
ROSES
Plants were pinched at 1 cm (0.4 in) diameter to provide a heavy yield
for Valentine's Day and Mother's Day. At other times selective pruning was
practiced to remove unproductive and weak shoots. During June and July the
plants were pruned back to 2-3 of the lowest prominent buds produced over
the previous year to control plant height. Marketable flowers were generally
cut with two, five-leaflet leaves left below the cut.
Night temperature thermostat setting minimums were 15.6°C (60°F) for
Cara Mia and Golden Fantasie, and 16.7°C (62°F) for Sonia, Belinda and Coed.
The day maximum settings were generally 5.5°C (10°F) higher than the night.
LETTUCE
Discussions with potential buyers for this product indicated a prefer-
ence for looseleaf cultivars that could be used as salad components and
liners. Grand Rapids H5-4 was chosen because of its tip burn resistance and
Waldman's for its dark green color appeal. Several beds of Summer Bibb were
planted in August to supply a limited market. All varieties fared well
through the first crop, but the second yield crop was reduced due to a
rhizoctonia infection.
GERANIUMS
Spinter type, F, hybrid geraniums, including Deep Red, Scarlet, Salmon,
Snowdon Cheri, Heidi, Rosita and Ringo, were chosen for seed propagation.
The night air temperature was maintained at 15.6°C (60°F) for the first 10
weeks after transplanting and then elevated to 18°C (65°F) until sale.
Table 7 shows the growing time required from seeding to first petal show.
49
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TABLE 7. GROWING TO FLOWERING TIME - GERANIUMS
Cultivar Weeks to Flower
Heidi, Snowdon
Ringo, Rosita
Salmon, Cheri, Scarlet, Deep Red
15
16
17
TABLE 8. CROP YIELDS
Quantity Growing Yield Per
Sold Area Growing Area
Kg (lb) m2 (sq ft2) m2 (sq ft2)
11421 (25381) 1062 (11,424) 23.75 (2.22)
Crop
Duration
6 mo
Crop
Tomato
1976
Tomato
1977 11605 (25791)
Leaf Lettuce 1557 (3460)
Snapdragons
Geraniums
Roses
5489 bunches
(w/bunch)
6000 pots
113,082 bunches
(25/bunch)
929
437
494
139
410
(9,996)
(4,704)
(5,312)
27.6
7.8
11.1
(2.58)
(0.73)
(1.0)
Bunches
(1,500) 4218 (4 pots)
(4,410) 275.8 30.17
Bunches
6 mo
6 mo
11 mo
6 mo
24 mo
CONTAINERIZED TREE SEEDLINGS
A bay of containerized tree seedlings was planted and maintained by
Bailey Nurseries, Inc., Newport, MN. The soil was leveled, sprinkled and
compacted with a roller to obtain a fairly level, flat surface and covered
with a 4-mil sheet of clear polyethylene. The planting consisted of 57,000
Mugho Pine (Pinus mugo). 2,000 Colorado Spruce (Picea punoens), 2,000 Black
Hills SpruceTPTcea glauca 'Densata') and 2,000 Austrian Pine (Pinus nigra).
The containers used were 32 cm (12.6 in) on a side with 100 cells per unit.
The cells measured 2.8 cm (1.1 in) diameter and 15 cm (5.9 in) deep. These
were filled with a peat-perlite mix, seeded with 3-4 seed per cell and
covered with a 0.5 cm (0.2 in) layer of number 2 crushed granite poultry grit.
The containers were placed on the polyethylene sheet to prevent root pene-
tration into the soil and also to allow air pruning of any emerging root tips
through the bottom drain holes. After emergence the cells were thinned to
two plants per cell, then two months later thinned to one plant per cell.
When the day length shortened to 12 hours, cool white standard 40-watt
fluorescent lights were used to extend the photoperiod to 24 hours. The
light intensity measured from 2690 to 3770 lux (250-350 ft-c) at the container
surface.
50
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The seeds germinated well and growth was vigorous except for December,
January and February. No insect or disease problems were encountered. The
containers were removed the latter part of May. By this time the growing
medium was well root-bound and could easily be removed by simply pulling at
the base of the stem. The average stem lengths in May were as follows:
Mugho Pine 15.24 cm (6 in), Colorado and Black Hills Spruce 25.4 cm (10 in),
and Austrian Pine 28 cm (11 in).
Two minor crops consisting of freesia (Freesia hybrida) and green bell
peppers (Capsicum Annuum) were grown for triaTTReitner crops fared very
well economically but growing conditions were adequate.
No yield trials were conducted for the soil heating system since it was
impractical to implement a totally controlled situation where valid compari-
sons could be made. It is intuitively felt, however, that the soil heating
was beneficial. Verification of soil heating benefits can be obtained by
reviewing the literature.
NURSERY STOCK
A propagation trial of deciduous and evergreen ornamental nursery stock
was conducted from November 1976 through June of 1977. Hardwood cutting
material was rooted directly in the sandy soil using the soil heating system
for bottom heat and the trickle irrigation tubing as a moisture source. The
tops were hand misted. Hardwood tree seeds were also planted in the soil.
Most material germinated and rooted very well.
Removal of the material showed well developed root systems. Shoot
growth rates were very rapid as would be expected in greenhouse conditions.
CONCLUSION
The cultural practices followed in the greenhouse were, by and large,
standard for this area. From a horticultural standpoint, it was felt that
the most advantageous aspects of this heating system were the high air flow
which tended to reduce foliar fungal diseases and the soil heating which
improved the root environment.
This high air flow did cause foliar burn in the roses during late spring
when the plants were tall. This problem could have reduced by 2.9 m (10 ft)
gutter height instead of the 2.3 m (8 ft) used.
51
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ECONOMIC FEASIBILITY
A stated goal of the Sherco Greenhouse project was to demonstrate the
economic feasibility of utilizing waste heat for greenhouse heating in
Northern climates. In order to do so, careful attention was paid to cost
throughout the project design, construction, and operation. The data gathered
during the Sherco Greenhouse project, along with experience of commercial
greenhouse operators who now utilize waste heat at Sherco, support the con-
clusion that economic feasibility has been demonstrated. In order to main-
tain the proper perspective, it should be remembered that only the economic
feasibility of heating with waste heat has been proven. That is, waste heat
has been shown to be competitive on a current cost basis with oil heat. In
making the foregoing statement, it is assumed that the waste heat available
in condenser cooling waters has no economic value as viewed by the utility.
Furthermore, it is assumed that utilities are willing to sell waste heat to
users at their operating cost plus a reasonable return on investment to
supply waste heat some distance from the power plant. With these assumptions
then, what remains to be proven is that it is economically feasible to re-
locate a vast greenhouse industry to selected power plant sites with suitable
and reliable waste heat. This will depend on a number of site and market
related variables, but some of the most important considerations have been
identified in this project and are addressed here.
HEAT DELIVERY COST AND PRICING CONCEPTS
The cost to deliver heat from the condenser outlet of a power plant to
greenhouse customers depends on certain general variables such as distance,
total acreage served, pipeline design philosophies, etc. and some site
specific variables such as type of soil, climatic conditions, and method of
connection to the cooling water system. All of the information presented
here is based on the site-specific characteristics of Sherco. While it may
be possible to generalize from this experience, generalization should be
made carefully with due consideration to the key variables that might differ
from site to site.
A 20.3 cm (8 in) tap and valve in a 183 cm (72 in) steel riser pipe at
Unit I tower was installed along with a 30.5 cm (12 in) diameter plastic
pipeline, extending 1067 m (3500 ft) each way to the Sherco Greenhouse site
and back to the cooling tower basin in the summer of 1976. The total cost
of this installation, which also included a 15.2 cm (6 in) plastic service
line 30.5 m (100 ft) long from the main 30.5 cm (12 in) pipeline into the
Sherco Greenhouse and interconnection with the greenhouse heating system,
was $84,000. This pipeline was capable of delivering 101 1/s (1600 gpm)
from Unit I riser pipe to the Sherco Greenhouse site. The following summer
52
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it was decided to expand the pipeline system to provide waste heat as a
utility service to commercial customers. To do so, a 35.6 cm (14 in) tap
and valve were installed on a 183 cm (72 in) steel riser at the Unit II
cooling tower. This tap fed into a 45.7 cm (18 in) cast iron pipeline that
went 305 m (1000 ft) each way to interconnect to the previously built 30.5 cm
(12 in) line. Included in this construction also was the cost of extending
the 30.5 cm (12 in) pipeline some 213 m (700 ft) one-way distance and two
service connections with meters to two commercial greenhouse sites. Also
included was the cost of an automatic chlorination injection system to
control pipeline bacterial fouling. The total cost of this work was $141,000.
The new pipeline was intentionally oversized so that it could accommodate
future expansion of up to about 442 1/s (7000 gpm) with some additional future
construction. To date then, $225,000 has been invested in pipelines to serve
waste heat to present and anticipated customers. Based on this experience
and engineering estimates of the cost to further expand waste heat service
for the 442 1/s (7000 gpm) initial development, a total investment in pipe-
lines of nearly $600,000, or about $43,000 for each .4 hectare (1 acre) is
projected. Using this investment and typical utility financing, the .4
hectare (1 acre) user must pay about $6,000-$8,000 per year for twenty years
to completely amortize the investment. This cost is the major cost of
serving waste heat to the user, but fortunately, it is a fixed cost once the
investment is made. After twenty years, continued operation of-the pipeline
should be possible, in which case this large fixed cost would no longer need
to be charged.
In addition to the capital investment in pipelines, there are operating
costs associated with the pumping energy required to deliver the flow to the
user and return it, as well as the chemical cost for chlorination, and over-
all pipeline network operation and maintenance. The calculated energy
required to deliver warm water about 1.2 km (4000 ft) at Sherco amounts to
.032 kwh/m3 (.12 kwh/1000 gal). This is based on low pipeline velocities
that will eventually increase, increasing the energy requirement in the
future. At current NSP internal cost for electricity, the pumping energy
costs .0484/m3 (.184/1000 gal). For a typical .4 hectare (1 acre) plastic
greenhouse, the pumping energy would cost $270 per year. Chlorine for
bacterial slime control now costs about $550/year which prorates to .0444/m3
(.167 4/1000 gal).. Operating labor and maintenance will be determined with
additional experience, but the present allowance for other O&M is .1724/m3
(.6534/1000 gal). Actual maintenance experience in the 1977-78 heating
season indicated a cost of .3544/m3 (1.344/1000 gal) which is considered
excessive but typical of a first year of operation. Thus, the total oper-
ating cost now stands at .2644/m3 (1.04/1000 gal) which works out to about
$1500 per year for a typical .4 hectare. (1 acre) greenhouse.
The total capital and operating cost (experienced and projected) for a
5.7 hectare (14 acre) complex at Sherco then amounts to about $9200 per year
for a .4 hectare (1 acre) greenhouse. On the basis of the annual energy
required by the greenhouse, this amounts to a cost of $.96 GJ ($1.02/16*
Btu.
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Several points are noteworthy concerning this calculated cost of waste
heat. First, this is the current cost, not a twenty-year average cost. In
the future cost escalation of pipeline O&M will cause this figure to rise,
though a doubling of the O&M cost would only increase the current cost by
16%. Secondly, the calculated cost per unit energy required by the green-
house is based on a calculated annual greenhouse heat loss of 9548 GJ/year
(9050 X 106 Btu/yr), which does not include boiler losses for comparable
useful heat derived from combustible fuels. And, finally, the instantaneous
heat extracted from the waste heat source is based on a AT of 8.3°C (15°F),
which is only half of the available heat in the waste heat stream since the
power plant condenser temperature rise is 16.1°C (29°F). It would be possible,
therefore, in some applications to reduce the unit cost of heat by achieving
a higher AT in the greenhouse heating system. This assumes, of course, that
waste heat is sold on the basis of cost of delivery of warm water and not on
the basis of value of heat extracted by the user.
The latter possibility suggests that the philosophy adopted by utilities
for waste heat pricing can impact the economic feasibility. The impact,
whether adverse or beneficial, depends obviously on the philosophies adopted.
NSP has decided to attempt to price the first units of waste heat sold on a
projected future average cost of serving a defined, in this case 5.7 hectare
(14 acre), complex of greenhouses. What this means is that the return on
investment deemed acceptable by NSP will not be achieved until the pipeline
network installed is fully utilized. The early year losses are initially
sustained as part of the development cost of commercializing the concept, but
hopefully will be recovered later after the pipeline network is fully
amortized. This pricing philosophy is promotional in that it provides
service at less than cost for the number of years it takes to fully develop
the installed pipeline system. However, the price signal is realistic since
it reflects expected future costs so that large increases in price need not
occur in the future.
In addition to the general cost of service pricing philosophy, NSP has
decided to offer waste heat to commercial users on the basis of long-term,
fixed-price contracts. These contracts, currently offered for ten-year
periods, are seen as mutually beneficial to NSP and the greenhouse operator.
They benefit NSP because of take or pay provisions that guarantee repayment
of the large fixed costs associated with the waste heat service, and they
immensely benefit the greenhouse operator because energy costs are no longer
subject to arbitrary price changes or general inflation. While only the
fixed cost of service (i.e., pipeline investment cost) is covered by these
contracts, this still stablizes energy cost for the grower because fixed
costs are over 80% of the cost of serving waste heat.
The current NSP pricing philosophy has been implemented in the form of
a demand and use rate structure for waste heat, or warm water service as it
is called. The demand portion of the rate now stands at $3.42/l/min ($12.95/
gpm) of peak average 15-minute flow recorded by NSP on the customer premises
during an entire heating season. The use portion is now .264
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rate which reflects fixed costs will be held constant for the ten-year
duration of the service agreement contract. After ten years the demand rate
offered for contract renewal will be that prevailing in the year of renewal.
During the period of the service agreement, the variable use charge will be
changed yearly according to cost of service data.
HEAT EXCHANGE SYSTEM COST
In order to utilize low temperature condenser waste heat, greater heat
transfer surface areas and larger volumes of heated air must be distributed
than in conventional greenhouse heating systems. Obviously, this produces
an increase in the cost of the heating system, initially, and in the cost of
operating the system due to the increased electric energy requirements.
The actual capital costs for all air heating system materials delivered
to the Sherco Greenhouse in mid 1975 was $46,662 or $21.98/m2 ($2.04/ft2).
Table 9 presents a breakdown of the actual cost for Sherco along with an
estimate for a .4 hectare (1 acre) commercial greenhouse facility in 1977
dollars. The materials include everything needed for the complete erection
of the air heating system and water circulation system including all sheet-
metal, dampers, ducts, piping, valves, fittings, flowmeters, wiring, and
controls. In the case of the estimate for a commercial facility, a complete
back up heating system is included in the cost estimate. The reduction in
cost for the commercial system compared to Sherco results from design im-
provements that were made as a result of the Sherco experience. It should be
noted that the costs presented do not include a soil heating system which is
not considered a necessary, though perhaps a desirable, part of the basic
waste heat heating system.
TABLE 9. WASTE HEAT SYSTEM
Materials Cost Summary (Actual Dollars)
Estimated
Sherco Greenhouse Commercial
Actual1 Greenhouse2
Mid 1975 Mid 1977
Fan Coil Units and Accessories 21,656 47,000
Circulating Mater Pumps, Piping,
and Valves 6,764 5,000
Electrical and Controls 18,242 11,000
Back Up Heaters and Fuel Storage - 11,000
TOTAL 46,662 74,000
$/m2 21.98 18.59
Sherco Greenhouse Area = 2123 m2 (22,848 ft2)
2Commercial Greenhouse Area = 3980 m2 (42,840 ft2)
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The cost to install the waste heat system was not specifically separated
from the total labor cost to erect the entire Sherco Greenhouse facility.
However, because of the highly prepackaged nature of most of the heating
system equipment, it is believed that installation labor would not exceed 25%
of the total heating system materials cost. While this percentage may seem
low, it should be considered that an entire greenhouse can be erected for
about $10.76/m2 ($l/ft2). And, 25% of the heating system materials cost
amounts to $4.52/m2 ($.42/ft2) of greenhouse floor area. In addition, recent
commercial experience has shown that an entire waste heat greenhouse facility
could be erected for little more than $6.56/m2 ($.61/ft2) labor cost, exclud-
ing electrical work and some other minor subcontract items. Using an esti-
mated installation cost of 25% of total materials yields a total investment
of about $92,500 for a .4 hectare (1 acre) greenhouse complete waste heat
system. This figure must then be compared to the cost of the conventional
heating system displaced. Estimates vary from about $10.26 - 21.52/m2
($1.00 - 2.00 ft2) of heated floor area for the Minnesota climatic conditions.
Using the mid-range of the estimates, a conventional heating system serving
a .4 hectare (1 acre) greenhouse would cost about $65,000. The apparent
capital cost difference between a conventional system and the waste heat
system is about $27,500 for the .4 hectare (1 acre) unit. This differential
capital investment must be amortized over the life of the heating system,
which is believed to be 20 years, and considered as an added cost of owning
and operating the waste heat system.
OPERATING COST COMPARISON TO CONVENTIONAL SYSTEMS
The operating costs for a greenhouse facility using waste heat depend on
the price charged for waste heat, the availability and temperature of the
waste heat, the cost of electricity to operate the heating system, and the
cost of backup or supplemental fuel. During the operation of the Sherco
Greenhouse Project, accurate records were kept and the cost of all elements
are known. However, because the Sherco Greenhouse heating system was designed
before the cost of waste heat was known, it probably uses a higher peak flow
rate than can be justified on a commercial basis. The Sherco Greenhouse uses
the same total flow for air and soil heating as does a commercial facility
which is almost twice as large. However, the commercial greenhouse was de-
signed when the cost of waste heat was known. Therefore, the operating
experience and projections based on the commercial facility are presented here.
The comparative operating costs of utilizing waste heat versus conven-
tional heating are indicated in Table 10. The figures presented are based on
a .4 hectare (1 acre) double plastic greenhouse located at the Sherburne
County Plant site. The total operating costs on an annual basis for the
waste heat system in 1978 are estimated to be $17,000 which includes $8,000
for waste heat, $7,000 for electricity, and $2,000 for standby propane.
A typical conventional greenhouse using No. 2 fuel oil has an estimated
heating cost of $28,000 per year. There is an apparent savings of $11,000
per year by utilizing the waste heat system in 1978 dollars. But, the big
advantage of the waste heat system is more apparent in the future as fuel
costs escalate. The waste heat system would realize an escalation in the
cost of propane and electricity, but the waste heat costs would not increase
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very much due to the fact that the pipeline investment had been made 20 years
previously. In 1998, at a compound escalation of 6% for all utilities and
variable operating costs, the waste heat system shows a cost advantage of
$51,000 per year. These comparative operating costs are projections based
in part on operating experience and anticipated operating experience at the
Sherco site.
The actual operating costs for all utilities for the .4 hectare (1 acre)
commercial floral range totaled $3.98/m2 ($.37/ft2) for the period November-
May. This compared to $7.00/m2 ($.65/ft2) for a glass greenhouse owned by
the same company. While part of the savings is attributable to the fact that
one greenhouse is double polyethylene and the other is glass, probably only
30% of the savings results from that difference. The remaining savings,
about $.86/m2 ($.08/ft2), directly result from the use of warm water for
heating. This actual experience in the first year of commercial operation of
the warm water heating system falls considerably short of the projected annual
savings of roughly $11,000 for a .4 hectare (1 acre) unit, but the greenhouse
with which the comparison was made received 62% of its energy as interruptible
natural gas. Had the other greenhouse been heated only with oil, the compara-
tive savings would be about $13,000.
TABLE 10. COMPARATIVE OPERATING COSTS OF WASTE HEAT
VERSUS CONVENTIONAL HEATING
- 1978 Costs - - 1998 Costs -
Waste Heat Conventional Waste Heat Commercial
Fuel Cost
Standby Propane 2,000 6,000
#2 Fuel Oil 31,000 95,000
Electricity 8,000 1,000 26,000 4,000
Waste Heat Cost 8,000 10,000
Total Per Acre
Per Year 18,000 32,000 42,000 99,000
Basis: 12 Fuel Oil @ $.45/gal
Propane Cost @ $.43/gal
Electricity Cost @ $.035/kwh
Escalation @ 6%
In terms of overall commercial acceptance of waste heat, there are other
considerations that are very important. One of these is the distance from
the market. Most new electric power stations are located some distance from
major metropolitan centers for reasons of air quality and other considera-
tions. Therefore, the source of waste heat is not close to the market for
greenhouse products. The increased transportation costs must be considered
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by the prospective greenhouse operator in determining whether he can relocate
to a power plant site to use waste heat. A second consideration is the cost
of land and the manner in which land acquisition is handled. At Sherco, NSP
owns all of the land adjacent to the power plant. The land on which the
commercial greenhouses are located is leased land. Therefore, they will
never benefit from the appreciation in land values which in the past has
generated significant long-term capital gains for greenhouse owners. Thus,
in cases where land will be owned by a utility and leased to greenhouse
operators, significant savings in operating costs are necessary to offset the
disadvantage of leased land. Another consideration may be the local property
tax structure. Generally speaking, the value of a power station impacts
local property tax rates to the extent that property taxes tend to be low in
areas in which power plants are located. This can have a beneficial effect
on the commercial greenhouse operation.
While it appears economically feasible to provide waste heat service to
greenhouse customers and it appears reasonable for the greenhouse customers
to make additional investments in heating systems to save in future heating
costs, it is still too early to predict whether this trend will hold true at
many power plant sites in the U.S. or whether this will prove to be a
relatively unique situation.
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Appendix A
BACKGROUND
Northern States Power Company began its investigation of beneficial use
of waste heat from its power plants in 1970. At that time, all of the
Company's generating facilities employed once-through cooling. Some of the
plants used "helper" cooling towers to meet thermal effluent standards.
However, none of the stations were operated on a closed-cycle basis.
A nuclear plant under construction at that time near Red Ming, Minnesota
(Prairie Island) was designed for closed-cycle operation. A coal-burning
plant in Sherburne County, also closed-cycle, was on the drawing board. The
warmer temperatures expected from closed-cycle design encouraged research
engineers at the Company in their efforts to investigate waste heat appli-
cations.
The Agricultural Experiment Station of the University of Minnesota was
invited in December, 1970, to participate in discussions concerning the
waste heat problem. Possible agricultural enterprises where warm water might
be economically attractive were suggested. The response from University
scientists was enthusiastic. From that time on, the Company and the Univer-
sity worked in a close partnership to explore all possibilities.
The University formed two three-man teams comprised of agricultural
engineers, horticulturists and soil scientists to study along with others at
the University, at NSP and elsewhere, the best ways to use the then unused
heat in the cooling water. NSP provided financial support for the study.
It soon was evident that little use could be made of the warm water from
the once-through cooling generating plants. Discharge water temperatures in
winter when it could be used for heating were approximately 10°C (50°F). or
about the permitted 10°C (18°F) above the ambient temperature of the Intake
water at 0°C (32°F). Most heating applications for enclosed plant production
or residences required temperatures of 15.5eC-21.1°C (60°-70°F). Even use
for soil heating showed little promise because few crops grow well at temper-
atures of 10°C (50°F) or lower. Frost protection was considered but soon
rejected because water droplets cool to ambient air temperature if they
travel 4.6-6.1 meters (15-20 ft) through the air. The main consideration in
frost protection is the release of heat, 334,934 KJ/kg (144 Btu/lb), by the
water as it changes state from liquid to solid, I.e. to ice.
The review of heating requirements for various applications and the
projection that all, or nearly all, future steam electric generating plants
would operate on a "closed-cycle" basis eventually led to the warm water
greenhouse project. NSP's closed-cycle designs called for condenser d1s-
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charge temperatures of no less than 29.4°C (85°F). If water temperature re-
ductions during delivery from condensers to the use point could be limited to
0.5-1.7°C (1-3°F), then an operating differential between the source water
and the use environment of 5.5°C-11.1°C (10°-20°F) could be expected. It
appeared that adequate heat transfer could take place for heating structures,
if equipment were properly sized and operated.
Some of the research projects and demonstration work studied by the
teams of University agricultural scientists included:
(1) The Environmental Research Laboratory, University of Arizona
and its "total energy" pilot study at Puerto Penasco, Mexico
(2) Oak Ridge National Laboratories greenhouse work led by
Dr. Sam Beall and William Furlong
(3) Commercial vegetable greenhouses in Cleveland, Ohio and
Houston, Texas
(4) Wright Roses, Cranbury, New Jersey
(5) Eugene Water and Electric Board, Eugene, Oregon
(6) Federal Water Quality Laboratory, Corvallis, Oregon
(7) Dr. L. L. Boersma, Oregon State University
(8) TVA, Muscle Shoals, Alabama
Most of these visits and preliminary studies took place in 1972 and
1973.
It was at this time that a joint decision was made by NSP and the
University of Minnesota to seek financial support from the U.S. Environmental
Protection Agency for a greenhouse demonstration.
The objectives of the project were:
(1) Utilize energy that is being wasted as a basis for an overall program
to conserve natural resources.
(2) Demonstrate methods, using readily available equipment, of economically
and reliably heating and managing a greenhouse with waste warm water
from the condenser of a power plant.
(3) Encourage private operators to build their own greenhouse at the power
plant site to take advantage of a low-cost form of energy.
(4) Determine the costs and problems associated with building, operating,
and maintaining a warm water supply and return pipeline at a multi-
unit plant.
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Early in the development of the greenhouse waste heat project, consider-
ation was given to the use of power plant warm water for cooling purposes as
well as heating. It was reasoned that if cooling a structure with warm water
proved to be practical, greenhouses could be used as supplementary horizontal
cooling towers. This concept was investigated in a pilot project which will
be described later in this report.
One of Northern States Power Company's stated goals: "To contribute
to the quality of life where we serve by: (1) demonstrating active steward-
ship of the environment and of the use of natural resources, and (2) sharing
our varied corporate resources with those who need help" was accomplished by
this project.
The NSP Sherburne County Generating Plant (Sherco), located 45 miles
northwest of the Twin Cities, was the site selected for the project. Sherco
Is a two-unit coal-fired plant with a total rated output of 1360 NW. It has
closed-cycle mechanical draft wet cooling towers for thermal pollution control
and a limestone scrubber system for air pollution control. The first unit
began commercial operation in May, 1976, with the second unit going on line
in May, 1977. The condenser cooling water flow rate for each unit 1s 15.77
m3/s (250,000 gpm) with a design cooling water temperature differential, AT,
of 16.1°C (29°F). The winter design minimum temperature of the water at the
condenser outlet 1s 29.4°C (85°F). This 1s the design temperature of the
warm water for the greenhouse heating system.
The Company owns more than 1200 hectares (3000 acres) of land adjacent
to the power plant. Space has been allocated for: (1) a greenhouse develop-
ment, (2) a 137 hectare (340 acre) University Sand Plain Agricultural Ex-
periment Station Farm and (3) an Industrial park.
The soil in the area is a loamy, coarse sand, and the topography Is
nearly level. Excellent drainage conditions permit greenhouse construction
without drain tiles. The soil conditions and topography make the Install-
ation of warm water supply and return pipelines connecting the greenhouse
to the power plant relatively easy.
In 1973, with the energy crisis beginning to surface, the emphasis on
waste heat projects began to shift from thermal pollution control to energy
utilization and conservation. It was becoming Increasingly difficult to
justify, economically, construction of greenhouses at power plant sites in
sufficient numbers to significantly reduce thermal pollution. For example,
It would require 405 hectares (1000 acres) of greenhouses to use the heat
rejected by a 1000 MW plant under conditions of operation at the Sherco
plant.
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Appendix 8
PILOT PROJECT
While the application for EPA funding was pending, NSP, with the Univer-
sity's commitment of technical assistance, developed a small pilot project to
"prove the concept" of heating greenhouses with waste heat in a northern
climate.
Previous research work conducted in the U.S. had concentrated on using
evaporative cooling pads with saturated air to provide both latent and
sensible heat transfer for greenhouse heating. In this research work, it
had been found that high humidity occurred in the greenhouse itself; there-
fore, the potential existed for increased disease problems in similarly
heated greenhouses. In addition, the difference in climate in Minnesota
compared to Southern states where the other greenhouse research work was
being conducted suggested that a heating system design suitable in one area
might not be suitable in another.
Because of the considerable concern regarding whether an evaporative
approach would work, it was decided to design a dry finned-tube heat exchange
system of sufficient size to transfer the required heat from 29.4°C (85°F)
water to heat a 204 m2 (2,200 ft2) greenhouse which was owned and operated
by a commercial florist in Minneapolis. In addition to the finned-tube
system, an evaporative system was installed in series to allow total flexi-
bility of operation for either heating or cooling with the evaporative
system, heating only with the dry finned-tube, or heating with both the
evaporative system and the finned-tube system. In addition, the pilot
research greenhouse heating system was designed to operate in a constant
heat rejection mode. Outside air could be introduced automatically so that
the heat exchange equipment had a constant inlet air volume and temperature
and a constant inlet water volume and temperature. Thus, a constant heat
load would appear on the heating system regardless of the actual heat demand
of the greenhouse.
The pilot research greenhouse was designed in the fall of 1973 and put
into operation in the spring of 1974. The greenhouse was operated in the
heating mode in March, April and May of 1974, and evaporative cooling tests
with warm water were run in the summer of 1974. Additional heating system
tests were run in the winter of 1974-75. As a result of the operating ex-
perience in the pilot research greenhouse, it was concluded that using very
warm water, up to temperatures of 46.1°C (115°F) on an evaporative cooling
pad, resulted in no measurable sensible cooling of the ventilation air for
the greenhouse. While large heat rejection rates could easily be achieved,
there was no particular benefit to the greenhouse, since the resulting
greenhouse temperature (but not humidity) could have been achieved just as
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easily by mechanical ventilation only. As a matter of fact, a more satis-
factory environment could be achieved by using recirculating well water (at
lower temperature than warm condenser water) on an evaporative cooling system
rather than trying to reject waste heat.
The heating experiments, on the other hand, revealed that the 29.4°C
(85°F) warm water source temperature was adequate to maintain greenhouse
temperatures above 12.8°C (55°F) throughout the 1974-75 winter heating
season. It was tentatively concluded at that time that the required area of
finned-tube heat exchanger was not excessive for installation directly in
the greenhouse, but that the fan power requirement was relatively high. It
was also concluded that temperature drops of more than 10°C (18°F) would be
very difficult to achieve with finned-tube exchangers without adversely
affecting the internal air temperature of the greenhouse. It was further
concluded, based on rather limited tests of direct evaporative heating of
the greenhouse, that the resultant fogging and high humidity conditions
would not be acceptable to commercial floral operators. It should be re-
membered that the research work reported was conducted in an existing
commercially-owned and operated greenhouse, so there was not complete freedom
to conduct tests that would result in conditions in the greenhouse considered
adverse to the production of a commercial crop. Nevertheless, the overall
conclusion of the pilot research project was that waste heat properly utilized
with finned-tube heat exchanger systems could create an environment that was
comparable with that obtained with other heat sources currently used in the
greenhouse Industry. It was believed that such a system could be designed,
installed, and operated at a cost that might be competitive with the cost of
alternative fuels in the future.
It was also concluded that the notion of a constant heat rejection rate
permitting greenhouse functioning as a horizontal cooling tower for the
power plant was impractical since it required continuous operation of a fan
and air handling system considered to be an excessive energy consumer in the
first place. This consideration [in conjunction with the considerations that
it would take more than 405 hectares (1000 acres) of greenhouses to reject
all of the heat from a 1000 MW power station and that evaporative cooling
with waste heat from a power plant provided no particular benefit for the
greenhouse] suggested that the idea of greenhouses replacing cooling towers,
perhaps, could and should be set aside until such time as the basic concept
of utilizing waste heat for greenhouse heating could be demonstrated to be
commercially viable and of commercial interest. From this experience, then,
the foundation was laid for the development of the more commercialized system
designs utilized in the Sherco Greenhouse project.
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en
jjSfiMi'iiii*1.-
Figure 27 Sherco Power Plant and Greenhouse Complex
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
REPORT NO.
E£A-600/7-80-099
'TITLE AND SUBTITLE
Demonstration of Beneficial Uses of Warm Water
from Condensers of Electric Generating Plants
3. RECIPIENT'S ACCESSION NO.
6. PERFORMING ORGANIZATION CODE
. REPORT DATE
May 1980
AUTHOR(S)L.L.Boyd (U. of MN),G. C.Ashley,
. S. Hietala,R. V. Stansfield, and T. R. C. Tonkinson
8. PERFORMING ORGAN 12 AT ION REPORT NO.
PERFORMING ORGANIZATION NAME AND ADDRESS
Northern States Power Company.
414 Nicollet Mall
Minneapolis, Minnesota 55401
10. PROGRAM ELEMENT NO.
EHE624
11. CONTRACT/GRANT NO.
Grant S-803770
2. SPONSORING AGENCY NAME AND ADDRESS
EPA, Office of Research and Development
Industrial Environmental Research Laboratory
Research Triangle Park, NC 27711
13. TYPE OF REPORT AND PERIOD COVERED
Grant Final; 5/75-4/80
14. SPONSORING AGENCY CODE
EPA/600/13
15. SUPPLEMENTARY NOTES IERL_RTpject officer jg Theodore G. Brna, Mail Drop 61,
919/541-2683.
is. ABSTRACT
report gives results of a project to demonstrate that warmed cooling
water from condensers of electric generating plants can effectively and economically
heat greenhouses. The 0.2-hectare demonstration greenhouse, at Northern States
Power Co. 's Sherburne County (Sherco) Generating Plant, used 29.4 C water to heat
Doth air and soil: finned-tube commercial heat exchangers were used to heat the air;
and buried plastic pipes , the soil. Warm water from the Sherco 1 cooling tower was
piped over 0. 8 km to the greenhouse where it was cooled from 2. 7 to 5. 6 C before
returning to the cooling tower basin. Roses and tomatoes were the principal crops
in the 3-year test, although other flowers and vegetables, and conifer seedlings
were also grown. The warm water heating system supplied all the greenhouse heating
requirements, even at ambient temperatures as low as -40 C. Roses, snapdragons,
eraniums , tomatoes, lettuce, and evergreen seedlings were grown successfully.
The demonstration proved the concept to be both technically and economically feas-
ible at Sherco, with an apparent saving of S"4500/hectare in 1978 dollars over fuel oil
heating, plus an annual oil savings of about 500 cu m/hectare. Privately financed
commercial greenhouses heated with warm water were built at Sherco in 1977. The
commercial greenhouses will expand from 0.48 to almost 1 hectare by late 1980.
7.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
Pollution
Electric Power Plants
Cooling Water
Greenhouses
Heating
Vegetables
Flowers
Softwoods
Seeds
b.IDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
Pollution Control
Stationary Sources
Conifers
Seedlings
13B
10B
13A
02 C
13H
02 D
11L
13. DISTRIBUTION STATEMENT
Release to Public
19. SECURITY CLASS (This Report/
Unclassified
21. NO. OF PAGES
73
20. SECURITY CLASS (This page)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
65
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