-------
3. Agricultural and Industrial Process Heat - A range
of solar collection systems is used to produce hot
air, hot water and steam within three temperature
ranges: low, less than 100 degrees C (212 degrees F) ;
intermediate, 100 to 177 degrees C (212 to 350
degrees F); and high, greater than 177 degrees C
(350 degrees F). Depending on system design and
operation, heat is utilized either directly or
through the application of heat exchangers. The
actual energy use and the range of required temper-
atures are diverse and require specific process de-
signs.
4. Photovoltaic Energy Systems - Sunlight is converted
to electricity by solar cells, which are made from
various semi-conductor materials. Research is under
way to create improved, high efficiency, lower cost
devices.
5. Solar Thermal Power Systems - In these systems, the
sun's heat is concentrated and used to heat water or
some other fluid to provide industrial process heat
or to drive a turbogenerator. Total energy systems
applications which supply both heat and electricity
are included.
During fiscal year 1980, DOE appropriations in these five
areas amounted to approximately $417 million (5).
Solar Applications in POTW's
Based on data from Reference 14 on energy use in POTW's by
source (activated sludge secondary treatment with sludge treat-
ment and disposal), the following table is presented showing
forms of energy used in wastewater treatment facilities.
Form of Energy Used - Percent of Total
Facility Size Electrical Fuel Oil or Gas
3,785 ra3/d (1 mgd)
37,850 m3/d (10 mgd)
378,500 m3/d (100 mgd)
86%
66%
63%
14%
34%
37%
Considering the data presented in Section 1 on energy con-
sumption by POTW's, the majority of the electrical energy is
consumed by electrical motors on pumps, blowers, drives, etc.
Therefore, the greatest potential for solar energy utilization
at a POTW would be for photovoltaic conversion to electrical en-
ergy. Alternatively, solar energy could be utilized to produce
steam to run steam-driven engines.
14
-------
For smaller plants, heating accounts for a substantial por-
tion of the total energy requirements: 19 percent for the 3,785
m3/d (1 mgd) facility versus 5 percent for 378,500 m3/d (100
mgd) facility. Additionally, heating and cooling loads can be
substantial for facilities in extreme northern or southern lo-
cales. Therefore, there is a potential for active and passive
solar heating/cooling at a POTW. The seasonal nature of both
heating and cooling, however, decreases the cost.-ef fectiveness
of these systems because they are not used year-round. Solar
heated hot water with a year-round demand has a greater poten-
tial for being cost-effective.
One often overlooked area for solar energy utilization is
natural lighting. Natural lighting is an attractive alternative
as it is a one-time capital cost expenditure .with minimal O&M
requirements. Although natural lighting will not be considered
further, its utilization in POTW's is recommended.
Additional potential uses of solar energy are for heating
aeration basin mixed liquor to improve either carbonaceous or
nitrogenous BOD removal kinetics and to eliminate winter freez-
ing problems. For a 3,785 m3/d (1 mgd} facility, 12,210 kWh
(41.7 million Btu) would be required to raise the water tempera-
ture 2.8 degrees C (5 degrees F) (neglecting recycle and side-
stream inputs). This is approximately ten times the total ener-
gy required for treatment and, therefore, is uneconomical. How-
ever, the use of a passive device, such as a solar pool heater,
to cover the basin (primary or secondary clarifier, aeration
basin, trickling filter, etc.) to reduce convective heat trans-
port and increase solar heat gain has greater potential.
Because anaerobic digestion is the most popular method of
recovering energy from wastewater treatment facilities, and an-
aerobic digestion requires a heat source to maintain mesophilic
(or possibly thermophilic) conditions within the digester, the
possibility of substituting solar-derived heat for combustion of
digester gas exists. The advantage of a solar heated digester
is that the gas which is conserved (hot combusted) can be used
to either run motors directly, or to run a generator and operate
the process equipment with the generated electricity. The sec-
ond option is advantageous as only one piece of equipment (the
generator) need burn the "dirty" fuel, and the electricity can
be directed to the motors utilizing the existing electrical ca-
-bles. The disadvantage, however, is that the gas must be con-
verted to electricity at a relatively low efficiency, further
reducing the energy available at the point of usage.
15
-------
A final potential use for solar energy is in a sludge drying
operation, in whicn tne heat energy could be used witn either an
active or passive system to evaporate water and dry the sludge
either prior to sale as a fertilizer amendment.or prior to in-
cineration with energy recovery. Currently, two processes for
solar-aided sludge drying are described in the literature. In
the first process (20) , active flat-plate collectors heat air
which is blown into a dryer similar to that used for soybean
drying. This system nas been proposed for a 11,355 m3/d (3 mgd)
facility in Denver Colorado.
The second concept proposes passive sludge drying on a 20-
to 30-degree inclined plane beneath a glazing, thereby creating
a greenhouse effect plus a convective air flow (21). A travel-
ing rake on the inclined plane moves the material to expose wet
portions and also moves the drying sludge down the incline.
Screw conveyors both spread and collect the sludge from the dry-
er. Evaporative cooling should maintain the sludge temperature
below 38 degrees C (100 degrees P) to minimize odors. During
February 1979, a 7.6 m (25 ft) long prototype model was tested,
and the evaporation rate averaged 0.70 kg/m2-hr (0.14 Ib/ft2-hr)
at an incident radiation of 910 kJ/m2 (80 Btu/ft2-hr).
It should be noted that one form of solar-aided sludge dry-
ing has been practiced for many years by enclosing sand drying
beds with glass to increase sludge drying rates.
The first detailed analysis of utilizing solar energy to
heat an anaerobic digester was performed during late 1975 and
early 1976 to assess the feasibility of utilizing an active so-
lar system (flat-plate collector) to preheat primary sludge to
one of two anaerobic digesters at the 17,000 m3/d (4.5 mgd)
Annapolis, Maryland Wastewater Treatment Plant (18). The au-
thors concluded that the system was feasible and economically
justifiable. Furthermore, it was concluded that, where physi-
cally possible, all existing anaerobic digesters should be con-
verted to solar heating and all new treatment facilities should
utilize solar-heated digesters.
Only one solar-heated anaerobic digester is currently (1980)
known to be in operation at a POTvtf in the United States (Wilton,
Maine). However, an anaerobic digester treating dairy manure
has been tested utilizing both a passive solar energy "bread
box" (tank(s) of water painted flat-black, covered by glass and
enclosed in an insulated box which is oriented south) and a so-
lar pond collector (22). Numerous references to the potential
use and the feasibility of solar energy in anaerobic digesters
are available (6,10,11,14,16,23,24).
16
-------
The Wilton, Maine facility, which has been operational since
September 1978, includes both active and passive solar energy
utilization, plus other energy recovery systems including efflu-
ent heat recovery, digester gas utilization, electricity genera-
tion, and air-to-air heat recovery (25). The 1,700 m3/d
(450,000 gpd) Wilton facility consists of preliminary treatment,
primary screening, rotating biological contactors (RBC's), sec-
ondary clarification, chlorination, and either surface discharge
or land application (spray irrigation). Sludge treatment con-
sists of mesophilic anaerobic digestion followed by sludge de-
watering.
The Wilton system is designed as an integrated energy source
and utilization system (25). The sources of heat can work ei-
ther individually or in combination with the basic heat utiliza-
tion systems. The overall philosophy is that tne plant will use
solar energy as the primary energy source, digester gas as the
secondary energy source, and effluent heat recovery as a back-up
and supplementary energy source. A conceptual diagram of the
energy systems is presented in Figure 5.
The active solar collectors consist of 139 m2 (1,500 ft2)
of flat-plate collectors with an ethylene glycol/water collector
loop, a heat exchanger, and a storage loop. The collector array
consists of 54 double-glazed panels with an effective collection.
area of 119 m2 (1,286 ft2) facing, two degrees west of south
at an angle of 60 degrees from the horizontal. The active col-
lectors were designed to collect between 232 and 274 GJ/yr (220
to 260 million Btu/yr), and this energy is exchanged to the
plant's circulating water system. The active collectors supply
heat for domestic hot water, digester heating, and building
heating.
The passive solar system utilizes fiberglass panels which
allow solar heat and light radiation into the clarifier room.
The passive solar array consists of 83 m2 (896 ft2) of pan-
els (75 m2 (812 ft2) of effective collection area) facing
two degrees west of south at an angle of 60 degrees from the
horizontal. A building over-hang provides partial shading in
the summer and full exposure in the winter, thereby aiding sum-
mer cooling and winter heating. Tne panels are constructed of
four layers of fiberglass, with a transmissivity listed as 66
percent by the manufacturer (25). The passive collectors were
sized to collect between 106 and 137 GJ/yr (100 to 130 million
Btu/yr) .
Based on information provided by Fuller, et al.(25) for the
period June 1979 through March 1980, the following preliminary
conclusions were reported:
17
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18
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1. An overall theoretical collector efficiency of 49
percent for the active solaf array was calculated.
Despite insolation values 13 percent in excess of
design, measured efficiency was 23 percent (47 per-
cent of design). The major cause of the discrepancy
appeared to be the response of the collectors to
actual weather conditions (i.e., the collectors did
not efficiently collect the radiation from a short-
time duration event) and the lack of a calculation
procedure to accurately simulate this interaction.
The authors believe that the characteristics of
observed performance place the cost-effectiveness
of solar-aided anaerobic digestion in question.
Based on actual performance, the simple payback
period for the active system was 54 years.
2. The passive solar system transoiissivity ranged from
35 percent in July to 57 percent in January. Part
of the reduction from the 66 percent estimated
transmissivity is due to the overhang, whereas the
remaining reduction is due to dust and sun-panel
angle resulting in surface reflection. The simple
payback period for the passive solar system was
calculated to be 30 years, based on actual system
performance.
At the time of this writing, the design and/or construction
of five additional facilities utilizing solar comfort and proc-
ess heating have been funded under the provisions of the Innova-
tive and Alternative Technology Program as summarized in Table
2.
TABLE 2. PROPOSED SOLAR THERMAL ENERGY APPLICATIONS
IN POTW's (14)
Facility
Capacity
Solar Energy Application
Hillsborough, NH
Gardiner, ME
Jackson, WY
Pine River, MN
Pella, IA
1,800 m3/d
(0.475 mgd)
6,060 m3/d
(1. 6 mgd)
13,250 m3/d
(3.5 mgd)
946
(0.25 mgd)
8,630
(2.28 mgd)
Space Heating, Passive and
Active Anaerobic Digester
Heating, Active
Domestic Hot Water
Space Heating
Space Heating, Passive and
Active
Anaerobic Digester
Heating, Active
T9~
-------
AVAILABLE EQUIPMENT AND HARDWARE
A solar energy system is composed of numerous individual
parts such as collectors, storage, distribution network, con-
trols, heat exchangers, etc. The parts are assembled in a Y3
ety of combinations depending on function, component compatibil-
ity, climatic conditions, required performance, site character-
istics, and architectural requirements. Various types of hard-
ware will be discussed in this subsection.
Flat-Plate Collectors
The flat-plate collector is the most common active solar
collection device for space and hot-water heating in use today
(27). The collector converts the sun's radiation into heat on
a simple surface within an enclosure. The collector is designed
to utilize either gas (generally air) or liquid (water, water
with anti-freeze). Regardless of the thermal transfer medium
used, most flat-plate collectors consist of the same components.
The purpose of these components is as follows: the cover plate
(glazing) is a transparent sheet of glass or plastic, mounted
above the absorber plate. The sun's rays penetrate the glass
and are transformed to heat energy on the absorber plate. The
glazing serves to minimize both convective and radiant heat
losses. The absorber plate has an absorptive coating which im-
proves its ability to absorb and not reflect energy. The ab-
sorber plate also has heat transfer fluid passages which consist
of tubes or fins attached above, below, or integral with the
absorber plate for the purpose of transferring thermal energy
to storage, or end use. The greatest variation in flat-plate
collector design occurs within the heat transfer fluid passage
unit and its combination with the absorber plate. Tube on
plate, integral tube and sheet, open channel flow, corrugated
sheets, deformed sheets, extruded sheet, and finned tubes are
some of the types of techniques used for liquid collectors. Air
collectors utilize configurations such as gauze or screens,
overlapping plates, corrugated sheets, and finned plates and
tubes (27).
Since the absorber plate must have a good thermal bond with
the fluid passages, an absorber integral with the heat transfer
medium is most common and optimum. Insulation is employed to
reduce heat loss through the rear of the collector. The insula-
tion must be suitable for the high temperatures that may occur.
The final component is the collector housing which contains all
of the components and makes the assembly waterproof. Rubber
seals or gaskets are used to fasten the cover glazing to the
housing. Various materials used for flat-plate collectors are
presented in Table 3 (28). The components of typical flat-plate
collectors are illustrated in Figure 6.
20
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TABLE 3. MATERIALS OF
TYPICAL FLAT-
CONSTRUCTION USED FOR
PLATE COLLECTORS
Collector Component
Materials Used
Cover Plate or Glazing
Absorber Plate Coating
Absorber Plate
Fluid Passages
Insulation
Housing
Gasketing
Heat-Transfer Medium
Glass, fiberglass laminates, ther-
moplastic sheeting, and film
Selective metal oxides, nonselec-
tive black paints
Copper, aluminum, stainless or
carbon steel
Aluminum or copper tubes, integral
spaces in absorber plate
Fiberglass, glass foam, foamed
thermoplastics
Metal, honeycombed concrete, fiber-
glass laminates, extruded thermo-
plastics
Silicone, EPDM, butyl, PVC elasto-
mers
Air, water, silicone fluid, hydro-
carbon oils, water/glycol mixtures
Adapted from Reference 28.
Flat-plate collectors are classified according to the type
of heat transfer medium they use. Liquid-type collectors use a
liquid such as water, water with glycol silicone fluid, or other
liquids, whereas air-type collectors use air as the heat-trans-
fer medium. Liquid-type collectors can be used for both space
and water heating, whereas air-type collectors are used pri-
marily for space heating (28) .
The operation of a liquid-type flat-plate collector system
is relatively simple. Solar radiation passes through the glaz-
ing and strikes the absorber plate coating. The absorber plate
and coating then convert the radiation to usable heat. The heat
is then absorbed by the heat-transfer medium in the plate's flu-
id passages. A pump in the collector loop circulates the heated
fluid to a heat exchanger. The heat exchanger is part of a sec-
onday loop which transfers the heat energy from the fluid and
transports the energy to storage or directly to the end use.
The system is shown in Figure 7.
21
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Tubes for
Absorber
Plate
Transparent
Covers
Metal
Frame Box
. Inlet Header
Plumbing Fitting
Insulation
Liquid Flat-Plate Collector
Metal Frame Box
Transparent
Covers
Absorber Plate
Insulation
Air Passages
Air Flat-Plate Collector i
Figure 6. Typical flat-plate collector components.
22
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Pump
Solar
Thermal
Storage
Unit
Auxiliary
Furnace
(Boiler)
Automatic
Valve Pump
Load
Figure 7.
Typical liquid flat-plate solar energy collection
system.
23
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In the air-type, flat-plate collector, the operation is sim-
ilar to the liquid-type collector. However, as air has a lower
heat capacity and density than water, approximately 100 m3
(3,500 ft3) of air are needed to transport the same amount of
heat as 0.028 m3 (1 ft3) of water ('28). As a result, the
air-type collector is usually much larger than the liquid-type
collector of comparable capacity. In the air-type collector,
the fluid passages are replaced by larger air ducts. The under-
side of the absorber is usually roughened and made with fins or
baffles to promote turbulence and heat transfer. In addition,
the pump in the system is replaced by a blower, and the liquid
(heat) storage is a much larger rock-pebble storage bed. An
air-type collector system is shown in Figure 8.
The design of both types of flat-plate collectors is well
known and, unlike other types of collectors, all three types of
radiation (direct, diffuse, and reflected) are collected. Both
air and water systems are especially efficient at collection
temperatures of less than 82 degrees C (180 degrees P) as typi-
cally used for water and space heating (29). They are not as
efficient as other collectors at the higher temperatures needed
for purposes such as industrial uses.
Evacuated-Tube Collectors
In this device, a vacuum is used to insulate and protect the
absorber coating from deterioration. The collector itself con-
sists of a vacuum bottle placed over a U-shaped liquid-filled
tube as shown in Figure 9. The double walled glass bottle has
an absorber coating on its inner glass. During operation, inci-
dent radiation travels through the evacuated area, strikes the
selective coating, and heats the air within the inner bottle.
This heated air in turn heats the liquid in the tube. For the
type of absorber shown, both air and water are used for heat
transfer. Other designs use all air or all water heat transfer
(28) .
The evacuated-tube collector collects .direct solar radiation
very efficiently, and some designs collect both direct and dif-
fuse radiation efficiently. It is most efficient for high tem-
perature applications such as for industrial processing or ab-
sorption cooling. Its efficiency for low-temperature applica-
tions such as water or space heating is lower than flat-plate
collectors (30).
24
-------
3-Way Damper
Blower
Auxiliary
Furnace
Y/
*> <
Storage
Unit
Load
1 1
Figure 8. Typical air flat-plate solar energy collection
system.
25
-------
Selective Surface
Liquid
Passages
Outer Glass
Hard Vacuum
Inner
Glass
Inlet
Manifold
Outer
Glass
Hard
Vacuum
Inner Glass
Selective Surface
Liquid Heat
Transfer Media Tube
Air
Insulated Receiver
Outlet Manifold
Figure 9. Typical evacuated tube collector,
26
-------
Concentrating Collectors
Concentrating collectors, which are also known as focusing
or tracking collectors, work on the principle that the sun's en-
ergy can be concentrated by reflecting it off one or more mir-
rors to concentrate it onto a smaller absorber. There are num-
erous types of concentrating collectors, most of which require a
mechanical device to shift the collector position to track the
sun. In addition, some require special optical lens arrange-
ments to focus the energy. Three of the most promising collec-
tors are the linear concentrating collector; the linear-trough,
fresnel lens collector; and the compound parabolic mirror col-
lector. The first two types of collectors only collect direct
radiation and track the-sun, whereas the parabolic mirror gath-
ers both direct and diffuse solar radiation without tracking the
sun (28) .
Concentrating collectors show the most promise for indus-
trial-type applications, as they can produce extremely high tem-
peratures efficiently. The costs, however, rule them out for
residential space heating. Maintenance of the mirror and track-
ing mechanism further limits their application (30).
Collector Arrangements
When more than one collector module is used, the functional
arrangement is important for effective energy collection and
system operation. Three basic configurations for multiple col-
lectors exist: parallel flow, direct return; parallel flow, re-
verse return; and series flow. Parallel flow-reverse return
systems are preferable to direct-return systems since flow bal-
ancing through the collectors is easier as the pressure drop
(head loss) through each collector is approximately equivalent.
Series flow is often used to either reduce the piping require-
ment or increase temperature output of the collectors. With
series flow, either direct or reverse^return systems can be used
(27). The configurations are presented in Figure 10.
Energy Storage
Because of the periodic and intermittent character of inso-
lation, storage of thermal energy is important. Heat must be
stored when the available solar energy exceeds demand. Storage
can be as simple as a concrete wall or floor which re-radiates
heat when the ambient temperature drops (sensible heat storage),
or as relatively complex as latent heat storage.
27
-------
Balance Valve
or Damper
Header
or Manifold
Balance Valve
or Damper
Parallel FlowDirect Return
Parallel FlowReverse Return
Series Flow
Header or Manifold
Figure 10. Common arrangements for multiple collector systems,
28
-------
Sensible heat storage involves raising the temperature of
inert substances such as rock, water, and masonry for subsequent
release of heat. Various methods are used including room air
and/or exposed surfaces, rock storage, and water storage. Rock
storage is most often associated with flat-plate collectors
which use air as the heat transfer medium (31). The rock is
heated with hot air from the collector, and the sensible heat
is recovered later by blowing air through the rock pile. Water
storage, by comparison, requires only 40 percent of the space
required by rock to store an equivalent amount of energy for the
same temperature range (28). In addition, water is inexpensive;
however, potential disadvantages include leakage, corrosion, and
freezing. Heat is generally transferred to and from storage by
a working fluid, either directly or by a heat exchanger.
The second type of heat storage involves utilizing the heat
of fusion or heat of vaporization associated with changes of
state or with chemical reactions. Numerous physical/chemical
processes have been investigated and numerous advantages of la-
tent heat storage versus sensible heat storage exist. However,
a completely reliable system has yet to be developed. Under
consideration, however, are salt hydrates (such as Glauber salt)
which, when raised to a specific temperature, release water of
crystallization which dissolves the salt. When the temperature
drops below the crystallization temperature, the stored heat is
released from the solution and the salt recrystallizes. The
phase change allows the salt to store a large amount of heat per
unit volume (31). Unfortunately, many phase change cycles tend
to break down the salt hydrates. A similar storage method is
possible using the thermal energy stored by the heat of fusion
of paraffin. Unfortunately, waxes tend to shrink upon solidifi-
cation and lose contact with the heat exchange surfaces (31).
Heat Exchange
By definition, a heat exchanger is a device which transfers
heat from one substance to another without mixing the two. Heat
exchangers applicable to solar energy may transfer heat from
air-to-air, liquid-to-air, and liquid-to-liquid (28). Because
the rate of heat transfer is a function of the temperature dif-
ference, the heat exchanger must be carefully matched to the
system collectors, storage capabilities, and heat load.
There are four basic flow configurations for a liquid-to-
liquid exchanger: coil-in-tank, counter-flow, mixed-flow, and
parallel-flow. For solar energy systems, the counter-flow de-
signs, which include coil-in-tank exchangers, are the most effi-
cient, followed by mixed-flow and parallel-flow exchangers (28).
29
-------
Another consideration for heat exchanger specification is
whether single or double walled exchangers are required. Double
walled heat exchangers are used when non-potable collector fluid
must be separated from potable water such as domestic hot wa-
ter. In a double walled exchanger, the resistance to heat
transfer per unit area is greater; therefore, a larger exchanger
is required to achieve the same efficiency. In addition, the
construction is more complex; both items result in greater
costs. A single walled exchanger is used whenever water used
for heat storage is not used for potable purposes (27).
Heat Transfer Fluids
Four liquids are in general use: water, water/glycol mix-
tures, hydrocarbons, and silicone fluids (28). Water is safe,
available, and inexpensive. However, it is subject to freez-
ing, supports galvanic corrosion, boils at a low temperature,
and promotes scale formation. These limitations require the
use of more expensive materials of construction, more compli-
cated controls, and periodic use of corrosion inhibitors.
A water/glycol mixture will not freeze at temperatures
greater than -37 degrees C (-35 degrees F), and, although addi-
tives can prevent scale and offer some corrosion resistance, it
boils at only a slightly higher temperature than water, and does
support galvanic corrosion. Because it rapidly decomposes at
138 to 149 degrees C (280 to 300 degrees F), forming sludge and
organic acids, the fluid must be replaced frequently. The reli-
ability of a water/glycol system depends on maintenance.
Hydrocarbon heat transfer fluids, typically highly refined
mineral oils, are low cost, nonvolatile, relatively nontoxic,
and not subject to freezing. Unfortunately, they have relative-
ly poor stability at high temperatures that results in sludge
and acid formation. Additional problems include high viscosity
at low temperature, incompatibility with copper, and a harmful
effect on some roofing materials. Because of their low flash-
point, they should be used in only lower efficiency panels with
stagnation temperatures from 121 to 191 degrees C (250 to 375
degrees F).
Silicone fluids have certain advantages in that they do not
freeze or boil under operating temperatures, they do not corrode
metals including aluminum, and they do not cause scale or sludge
build-up. Silicone fluids have disadvantages in lower heat ca-
pacity which results in larger heat exchangers, their high vis-
cosity at low temperature, high initial cost, and a propensity
to seep at pipe joints (27,28).
30
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Solar roof ponds (also known as thermal storage roofs) are
unique for passive solar systems as they are the only passive
solar system that can provide both heating and cooling. The
most widely employed version of the solar roof pond uses a shal-
low pond of water (in bags) in thermal contact with a highly
conductive flat roof and ceiling structure. In the heating mode
the bags are exposed to solar heat gain during the day, and pro-
tected (insulated) against heat loss at night. Cooling is ac-
complished in the reverse manner.
The solar roof pond replaces the ceiling and roof of a con-
ventional structure. The system includes a steel floor deck/
ceiling, plastic water bag thermal storage, wood framed glazing,
reflective wall, double glazed skylight system and movable in-
sulation sub-system with controls. The movable insulation may
consist of a system in which polystyrene beads are blown from a
central storage unit between the glazing, and later drawn from
the space by a vacuum pump. This system is shown in Figure 16.
Solar roof ponds are characterized by low temperature oper-
ation. The daily temperature swing may, in winter, average '2.8
degrees C (5 degrees F), and the average mid-winter temperature
of the pond may only be 5.6 degrees C (10 degrees F) over room
temperature. The average daily heating contribution by a 25 mm
(10 in.) deep pond is in the range of 1,140 to 2,840 kJ/m2-hr
(10 to 20 Btu/ft^-hr), depending on surface temperature of
the pond heated ceiling and the room temperature (17).
PROCESS CAPABILITIES AND LIMITATIONS
In terms of capabilities alone, current solar energy tech-
nology could supply the entire energy needs of a POTW if de-
sired. However, applicability must be based on two factors:
cost and reliability. The high capital cost and the requirement
for and cost of back-up energy make photovoltaic solar energy
unattractive given the development status of current technology.
A similar conclusion regarding utilization of solar energy to
produce process steam is valid.
Aside from the potential use of solar energy systems for
sludge drying, the current utilization of solar energy in POTW
applications is basically limited to typical applications such
as space and water heating.
Space heating can be accomplished utilizing either passive
or active solar collectors and either air or liquid flat-plate
collectors. The choice between the numerous available technolo-
gies is site specific and must be based on a detailed analysis.
The potential exists for significant savings in conventional
46
-------
Radiation
From Storage
1. Exterior Glazing System
2. Concrete Wall
3. Air Vents
4. Foundation Insulation
Figure 15. Typical Trombe wall design (19).
45
-------
The Trombe wall is the most common of the thermal storage
walls, and is a south-facing concrete or masonry wall covered on
the exterior by light-transmitting glazing (17,34). The rec-
ommended design has vents at the top and bottom to permit natur-
al convective air flow as can be seen in Figure 15. The outer
window consists of two layers of translucent or semi-transparent
low-cost plastic rather than glass. Unlike the direct gain
passive system, views out are not possible, and views in showing
rough concrete may be undesirable. Maximum system temperatures
are in the 66 to 82 degree C (150 to 180 degrees F) range, but
more likely at the lower end.
In the Trombe wall, the thermal storage wall is concrete,
either cast in place or constructed with blocks and mortar.
Dampers are used to prevent air circulation in the "wrong" di-
rection; without proper dampers, the performance can be reduced
by as much as 20 percent. On the interior of the Trombe wall,
the finish must be such that it does not prevent heat from
radiating into the room. Therefore, wood or gypsum should not
be used, but rather the concrete should be exposed and finished
off. In a typical design, the concrete wall is 0.25 m (10 in.)
thick, with vents placed 0.61 m (2 ft) on center along the
length. The exterior glazing is mounted 76 to 102 mm (3 to 4
in.) away from the exterior face of the concrete, which is
blackened to increase absorption.
The heated air in the Trombe wall generally does not exceed
66 degrees C (150 degrees F) and air delivered to the room does
not exceed 32 degrees C (90.9 degrees F). The vertical south
wall orientation enables good winter heating, and minimizes sum-
mer overheating.
The maximum delivered air temperature tends to occur eight
hours prior to the maximum interior wall surface temperature.
Total convective and radiant heat transfer from the interior
wall is usually not more than 397 kJ/m2-hr (35 Btu/ft2-hr)
(27). Over a heating season, the Trombe wall will provide
enough solar energy to cancel all thermal losses through the
wall and thus deliver excess heat to the remaining building
load.
A second type of thermal storage wall utilizes water in-
stead of masonry materials for energy storage. Tubes of water,
0.21 m3 (55 gallon) drums and specially fabricated water walls
are typical. Radiant heat rather than natural flow of air is
usually the major design consideration (34).
44
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1. Insulating Glass
2. Wall Framing
3. Metal Absorber Plate
4. Insulating Core
5. Interior Finish
6. Continuous Air Vents
Natural
Convection
(behind collector plate)
Figure 14. Typical thermosiphon air panel collector.
43
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direct gain area. During the heating season, the south-facing,
direct-gain area takes advantage of the sun's low position in
the sky. In the summer when the sun is high, the glass is shad-
ed by overhangs, awnings, or trees.
Direct gain systems also utilize sunlight to heat,opaque
surfaces such as roofs and walls. The color of these surfaces
is important and in warm climates the surface should be light in
color to reflect sunlight, whereas, in cool and cold climates a
dark color should be used. The location of the structure is al-
so important as orientation affects the amount of radiation ab-
sorbed. Due south is, in general, the optimal direction for the
passive collector. However, the designer must also consider:
the type of structure; its method of construction; and the geom-
etry of both the structure and the glass. Sufficient thermal
capacity inside the building must be provided so that excess
thermal energy can be absorbed and stored for later release
(17).
As a fluid increases in temperature, its density decreases
and it becomes more buoyant than the cooler fluid. This is the
theory behind convective loops. Thermal circulation is a natur-
al convective loop that allows a fluid heated by an absorbing
surface to rise and, thereby, draw cooler fluid into the col-
lector area to replace the warm rising fluid.
Figure J.4 illustrates the simplest form of a convective
loop, the thermosiphoning air collector.
In the thermosiphoning air collector, air flow is provided
by the pressure differential created between solar heated air
and the lower room air temperature. The air heater consists of
exterior glazing, a composite wall element consisting of the
thermosiphoning absorber plate, rigid insulation and interior
finish, and air grills and dampers. The low mass of the system
allows it to undergo greater temperature extremes than the Trom-
be wall. Double-pane insulating glass is typically used as
glazing.
Thermosiphoning air heaters are suited to structures where
the heating load is large compared to the panel area. All of
the panel output can be absorbed by the building load, as there
are no thermal storage provisions. Facilities with intermittent
use (schools and office buildings) are well-suited to the ther-
mosiphoning air heater's daily cycle. Thermal performance of the
collectors is dependent on the natural convection in the system,
and air flow is low to non-existent during periods of little or
no sun. A collector, as shown in Figure 14, has an average out-
let temperature of 35 degrees C (95 degrees F) at a maximum flow
rate of 0.057 irP/min (2 ft3/min) resulting in 1,022 kJ/m2-hr
(90 Btu/ft2-hr) as an average heat gain.
42
-------
(U
en
to
QJ
S-l
m
w
ID
QJ
W
(0
m
-G
O
CD
41
-------
L
T/Tn
H
T
is the total monthly load in kJ (Btu)
is the ratio of the average monthly collector
cover transmittance to the transmittance of
normal incidence (0.90 - 0.95)
is the ratio of the collector surface solar
absorptance to the solar absorptance at normal
incidence (0.95)
is the long-term insolation, kJ/m2-hr (Btu/
ft2-hr)
Once monthly values of X and Y are calculated, the fraction
of the monthly energy load supplied by solar energy (f-value) is
obtained from graphs such as shown in Figure 13 for liquid based
systems. Having obtained f-values for each month, the annual
solar fraction is calculated by summation. In summary, the
f-chart method is an empirical method based on computer simula-
tions, experiments, and years of experience which can be used to
predict flat-plate collector performance.
PASSIVE SOLAR SYSTEM CONCEPTUAL DESIGN
As opposed to the active solar system which requires collec-
tors, thermal storage, and a thermal energy transport system,
passive solar energy does hot use any mechanical power to trans-
fer energy into and out of a structure. Controls and comfort-
regulating devices can be incorporated into the design, but are
not required. In certain systems, mechanical energy is utilized
to improve energy transfer, and these systems are designated as
hybrid. This section is intended to give the reader an idea of
the concepts utilized in passive solar design. For a further
discussion of concepts, as well as design guidelines, the reader
is referred to References 34 and 35 or other similar documents.
Passive solar systems are characterized by the linkage of
solar collection, thermal energy storage, the space to be heat-
ed/cooled, and the application of energy conservation concepts
to the design of the desired structure. Passive systems can be
divided into four different types: direct gain, convective
loops, thermal storage (Trombe) walls, and thermal storage roofs
(34).
Direct gain systems use sunlight entering directly through
glass or plastic to the space to be heated, and virtually all
the sunlight entering is converted to heat. A thermal mass for
storing excess heat (concrete floor, brick wall) is utilized to
absorb solar heat. To reduce heat loss at night and, therefore,
increase thermal performance, insulation may be applied on the
40
-------
OJ M-J
10
(0 TD
X! 0)
I N
T3 -H
H rH
3 -H
e
= oj
T3 4-1
S-l W
(0 >i
TD W
C
m en
-p c
to -H
= -P
(0
u
m cu
O 4-1
(0
e s
to en
u TD -H
cr> c w
(0 (0 >(
H H
TD Q) (0
O C
O (0 (0
cu
.c
O O
CO CO
-------
To simplify collector sizing, the Duffie-Beckman-Klein pro-
cedure was developed. This general design procedure was devel-
oped based on numerous simulations of solar heating systems
utilizing a detailed simulation program and a specific system
configuration (19,30). This exercise resulted in the empirical
"f-chart" method to predict the performance of "standard" solar
systems providing space heating and domestic hot water hea'ti'rig.
Figure 12 shows the standard active solar heating system utiliz-
ing liquid flat-plate collectors. A similar air, flat-plate
system is also available. When utilizing the f-chart analysis,
meteorological data in the form of long-term monthly temperature
and insolation data are required.
A detailed discussion of the f-chart procedure can be found
in References 19 and 30, and a brief description is presented
below. The thermal efficiency is predicted through the calcula-
tion of two dimensionless parameters: X, which represents the
ratio of solar collector energy losses at a reference operating
condition to the total system heating demands; and Y, which is
the ratio of solar energy absorbed by the collector to the total
system heating demand. The equations used for calculating X and
Y are as follows:
X ' FR UL
FR I A (Tref - Ta>
AT
R
where:
FR UL and
A
Tref
AT
Y = FR (Ta)n
Ji
n
R
n
a
a
AHL
n
are parameters describing performance of a flat-
plate collector and are the slope and y-inter-
cept, respectively, of the efficiency curve of
Figure 11.
is the collector-tank heat exchanger performance
efficiency (0.90 - 1.0)
is a collector area, m^ (ft^)
100 degrees C (212 degrees F)
is the average ambient monthly temperature
is the number of hours in the month
38
-------
100
. I-
hpool J
Heating |
10.0
15.0
20.0
25.0
T Inlet - T Ambient ฐC - hr - m2
Incident Flux
Figure 11. Collector efficiencies for various liquid
collectors.
37
-------
As opposed to conventional systems, solar systems are typi-
cally not oversized because the cost of the system is directly
related to collector size. In addition, solar systems are not
designed to provide 100 percent of the demand, since the collec-
tion and storage system would have to be_designed for successive
sunless days (30). During sunny days, therefore, the system
would be overdesigned and the energy wasted. For home heating,
sufficient capacity for the evening hours and the few early
hours of the following day are typically provided. Concentrat-
ing collectors are not currently considered practical for resi-
dential usage due to their high capital cost and maintenance re-
quirements. Because flat-plate collectors can provide suffi-
ciently high temperatures for POTW applications, they will be
the only active collector considered.
Solar collector efficiency is defined as the ratio of useful
heat delivered by the collector to insolation over the same time
period. Typical flat-plate efficiencies vary from 20 to 60 per-
cent, depending on fluid and ambient air temperatures. Factors
accounting for efficiency losses include emittance or reradia-
tion and convective losses. Low fluid flow and high collector
temperatures lead to low efficiency. With a large fluid flow,
the fluid .and collector temperatures are low, heat losses from
the collector are lower, and efficiencies high. At the normal
fluid temperatures of from 38 to 60 degrees C (100 to 140 de-
grees F), efficiencies of 35 to 40 percent are typical (19).
For standardization and convenience, the efficiency of a
collector is correlated with the ratio:
(Collector Inlet\ /Atmospheric\
/ V
\
Temperature
^Temperature/
Lin
_.j ^, ป..ป ซ / \ *. >**i*jys*-* wl *ป<
/ Solar Radiation per Time \
yper Collector Surface Area/
H
When efficiency is correlated to this ratio, a straight line
results, as can be seen in Figure 11 for various collector
types. The slope and intercept of the line are used as measures
of collector properties and performance in design calculations.
The performance of an active solar system must be related to
the climatological conditions which prevail at the site and the
specific system design and control strategy. Therefore, only
generalized computer simulations are flexible enough to predict
system performance of any conceivable system. However, these
programs require hour-by-hour meteorological data and the per-
formance predictions only apply to the time period of the input
data.
36
-------
of solar energy is typically required. The solar window will
change with geographical area, as the top and bottom of the win-
dow depend on latitude while the sides are a function of the
longitude (28).
Based on the solar window, the shade caused by interferring
objects can be plotted and the optimum location of the collector
determined. Based on the sun's path, the collector's surface
should be oriented to point due south. Due south should be bas-
ed on geographical south rather than magnetic south, and compass
readings should be corrected by the Isogonic Chart readings
which show magnetic deviations from due north.
Due to the site specific conditions such as shading or local
weather conditions, a shift in collector direction of 15 degrees
east or west of south is acceptable. This shift will reduce,
although not drastically, the energy collected. Beyond 15 de-
grees to 20 degrees east or west of south, energy collection is
significantly reduced (30).
The angle between the collector and a horizontal surface is
called the collector tilt angle. The collector tilt angle is a
function of both the geographical location and the energy use.
For domestic hot water heating, the tilt angle should be the
same as the latitude of the location. Therefore, at a latitude
of 35 degrees N, the collector should be tilted 35 degrees from
horizontal to ensure maximum energy collection throughout the
year (27) .
For space heating applications, however, it is desirable to
collect the maximum amount of energy during the winter months
when the demand is the greatest. As the sun is lower in the sky
daring the winter months, the collector must be tilted to lati-
tude plus 15 degrees. Therefore, at 35 degrees N, a collector
should be oriented 50 degrees from horizontal. Variations in
collector tilt of 10 degrees either side of optimum are accept-
able and will not significantly reduce energy collection. In
some cases, a different angle may be desirable due to architec-
tural or other reasons (27).
4
ACTIVE SOLAR SYSTEM CONCEPTUAL DESIGN
This section is intended to present the methodology for the
process design (sizing) of an active solar heating system, given
a known load. However, it is not intended to give the specific
details of design or various system modifications. A methodol-
ogy for calculating the heat load required for anaerobic digest-
er heating is given in Appendix A. Calculation of building
heating loads is available from numerous sources.
35
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The earth rotates on its axis once every 24 hours; the axis
of rotation is tilted at an angle of 23.5 degrees to the plane
of the earth's orbit. If the earth were not tilted, the equa-
torial regions of the earth, which are closest to the angle of
solar radiation, would always receive the maximum insolation.
However, due to the tilt of the earth's axis, the area receiving
the maximum solar radiation moves north and south, between the
Tropic of Cancer and the Tropic of Capricorn, causing changes in
insolation. One other factor affecting insolation rate is the
length of the daylight period, which is a function of day of
year. Based on these factors, each area of the earth will be
affected differently. The total amount of insolation and the
distribution of direct and diffuse insolation will vary as a re-
sult of these modifying factors.
Numerous models are available to predict the amount of solar
radiation reaching the earth's surface as a function of time of
year and location. Two of those available include the Liu and
Jordan method, and the Klein, Duffie, and Beckman method (30") .
Both methods involve utilizing long-term solar radiation data to
predict the amount of solar radiation which will be converted to
usable energy by a solar collector. Owing to the variations in
insolation, solar collector sizing cannot be determined by sim-
ply choosing the solar radiation data for a particular hour,
day, month, or even year. Therefore, solar system sizing must
be based on long-term averages for insolation and weather condi-
tions. Long-term averages for insolation as well as air temper-
ature are available in tabular form for numerous locations with-
in the United States.
Collector Positioning
Regardless of the type of solar energy collection system to
be used, correct collector positioning is necessary so that the
optimum amount of energy can be collected. Typically, the solar
window concept, in which one assumes the sky as a transparent
dome, is used to pictorally demonstrate the sun's position with
respect to the desired solar collector location (28,29,31). In
this method, the bottom of the window is formed by the sun's
path at the start of winter (December 21) and the top of the
window by the sun's path at the start of summer (June 21). The
sides of the window are 9 A.M. and 3 P.M. This window outlines
the area through which a maximum amount of solar energy could
reach the collectors during the year. By plotting the solar
window, objects such as trees or buildings which might interfere
with solar collection can be identified. Objects which cast a
shadow when the sun is low in the sky (winter) are extremely im-
portant to identify as this is the time when the maximum amount
34
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SECTION 3
TECHNOLOGY EVALUATION
PROCESS -THEORY
Basics of Solar Energy
Sun, wind, temperature, humidity, and various other factors
shape the climates of the earth. As far as solar energy is con-
cerned, there are four elements of the climate which are impor-
tant. These are solar radiation, air temperature, humidity, and
air movement.
The sun provides the earth with essentially all of its ener-
gy. This energy is received in the form of electromagnetic ra-
diation transmitted in wavelengths varying from 0.29 to 3 mi-
crons. By comparison, the human eye can detect visible light at
wavelengths between 0.36 and 0.76 microns.
The intensity of radiation reaching the upper surface of the
earth's atmosphere (solar constant) varies as much as plus or
minus 2 percent due to variation in the sun's energy output and
plus or minus 3.5 percent due to changes in the distance between
the sun and earth. On a plane perpendicular to the sun's rays,
the solar constant is 4,874 kJ/m2-hr (429.2 Btu/ft2-hr) (31).
The radiation which arrives at the earth's surface (insolation)
is less than the solar constant for various reasons. Insolation
values are maximum per unit area when solar radiation impacts on
a surface perpendicular to the incident radiation. Because of
the curvature of the earth, solar radiation strikes the earth at
discrete angles varying up to the maximum of 90 degrees. The
design intent of tilting solar collectors from a horizontal
plane is to compensate for this phenomenon in order to maximize
insolation.
>
Radiation reaching the earth's surface is also affected by
the condition of the atmosphere in terms of its vapor, dust, and
smoke content because radiation is absorbed and scattered by
these elements. In addition, the lower the solar altitude, the
greater the path through the atmosphere the radiation must trav-
el, further reducing the amount reaching the earth's surface.
33
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cells are silicon and cadmium sulfide cells. A great deal of
research is under way aimed at solar cell development; however,
the technology is still in the developmental stage. The primary
use of solar cells is to supply small amounts of electricity in
remote locations where conventional sources are not available.
EQUIPMENT AVAILABILITY
A total of 223 firms were identified as manufacturers of so-
lar thermal collectors during the first half of 1980 (32). These
223 manufacturers shipped 820,000 m2 (8.83 x 106 ft2) of solar
collectors from January through June 1980; this is an increase
of over 20 percent compared to the second half of 1979, and 28
percent over the first half of 1979 (32), indicative of a
growth industry.
Of the collectors shipped during the first half of 1980,
low-temperature collectors (temperatures below 43 degrees C
(110 degrees F), no glazing or insulation, generally plastic or
rubber) accounted for 68 percent of total production, with 97
percent of these collectors for swimming pool heating. Medium-
temperature solar collectors (typical operating temperatures of
60 to 82 degrees C (140 to 180 degrees P), single or double
glazed, metal absorber with integral or attached tubing or duct-
ing, insulated) accounted for 29 percent of the shipments. Of
this 29 percent, 238/000 m2 (91 percent) used liquid heat
transfer. Of the medium-temperature liquid heat transfer col-
lectors, 62 percent were used for domestic hot water and 22 per-
cent for space heating. Special collectors (evacuated tube or
concentrating/focusing collectors) accounted for 3 percent of
the total producer shipments (32).
The residential market accounted for 84 percent of the solar
collectors installed in the first half of 1980; the commercial
sector accounted for 12 percent of the applications; and indus-
try, agriculture, and other uses accounted for 3 percent. The
government sector accounted for 3 percent; however, this sector
overlaps the others (32) .
Twelve firms manufactured photovoltaic solar collector mod-
ules with a capacity of 1,841 peak kilowatts during the first
half of 1980. Industry accounted for 63 percent of this market;
residential 18 percent; and commercial, agricultural, govern-
ment, and other uses 19 percent. Of the total, 41 percent was
for export.
Numerous publications contain lists of solar collector and
system component manufacturers. Publications such as the "Solar
Industry Index" (27) or the "Solar Products Specifications
Guide" (33) list companies involved in and specifications for
solar collector equipment.
32
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Solar Space Cooling
Solar cooling is typically accomplished by using solar heat
to operate a thermal absorption type refrigeration system. The
system differs from electrically operated refrigeration in that
in electrical units a vapor such as ammonia is condensed to a
liquid by a motor-driven pump and the released neat is removed
by blowing air through the condenser. The liquid is then vapor-
ized and the heat is absorbed by the vaporization, resulting in
cooling. In solar refrigeration, the cycle is similar; however,
the ammonia (or lithium bromide) is condensed by heating a con-
centrated solution, thereby causing a high vapor pressure. The
details of the complete cycle can be found in numerous refer-
ences (8,19,29,30).
Solar powered air conditioning is possible also using an or-
ganic Rankine cycle engine. In this process, solar heat vapor-
izes an organic liquid, drives an organic Rankine cycle engine,
which in turn drives a conventional compressor of an air condi-
tioner (8,30) .
Passive Solar Systems
There are three general passive solar collection concepts.
The first concept involves incidental heat traps such as win-
dows, skylights and glass structures. Aside from direct solar
heat gain such as lighting and ventilation, these incidental
heat traps typically serve a variety of purposes. The second
concept is known as thermosiphoning. This approach utilizes the
heat absorbed by a wall or roof structure by drawing if off or
siphoning it to a room or to storage. The third method of pas-
sive solar heating or cooling involves the thermal storage pond
or roof concept. In this method, control of solar heat gain or
heat loss is controlled by the use of movable insulating panels
to expose or conceal the ponds. Solar ponds have found their
greatest use where cooling is the principal design condition,
and when summer nighttime temperatures are substantially lower
than daytime temperatures (31). A further description of pas-
sive systems will be presented in Section 3.
Photovoltaic Conversion
Solar cells offer a means of direct conversion of sunlight
into electricity with high reliability and low maintenance (8).
The present disadvantages are the high capital cost and the
difficulty in storing electricity for later use. The cost of
photovoltaic cells will hopefully be reduced when the cells are
manufactured in large quantities using new production tech-
niques. The two most promising materials for inexpensive solar
31
-------
Solar
Radiation
\A1
Reflective
Surface.
1. Skylight Glazing System
2. Movable Insulation Plumbing
3. Movable Insulation Storage Tank
4. Reflector Wall
5. Water Bags
6. Steel Deck
Radiation
From
Storage
Figure 16. Typical solar roof pond system.
47
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energy at smaller facilities. Space heating accounts for about
19 percent of the total energy requirements at a 3,785 m3/d
(1 mgd) facility, but only 5 percent at a 378,500 m3/d (100
mgd) POTW (see Table 1). In terms of overall plant energy us-
age, the installation of a solar domestic hot water heater would
have minimal impact, although, if installed in conjunction with
an active space heating system, it may prove economical. Solar-
aided anaerobic digestion has the potential for energy savings
of about 9 percent at 3,785 m3/d (1 mgd), and 2 percent at
378,500 m-Vd (100 mgd).
In summary, assuming the "typical" wastewater treatment
plant of Table 1 and 90 percent solar replacement, the maximum
possible energy savings by utilizing solar energy are presented
in Table 4.
TABLE 4. MAXIMUM ENERGY REDUCTIONS POSSIBLE BY THE UTILIZATION
OP SOLAR THERMAL ENERGY ALTERNATIVES1.
3,785 m3/d
(1 mgd)
37,850 m3/d
(10 mgd)
378,500 m3/d
(100 mgd)
Anaerobic Digester
Heating
Building Heating
Sludge Drying
(instead of
incineration)
9.0%
16.9%
4.8%
Total Maximum Savings 30.7%
5.0%
7.8%
3.2%
2.3%
4.4%
3.2%
16.0%
9.9%
Assuming 90 percent replacement of conventional fuel with
solar thermal energy.
OPERATION AND MAINTENANCE CONSIDERATIONS
The installation of an active solar anaerobic digester heat-
ing system, or the installation of either active or passive
space heating, both require installation of additional equipment
at the POTW. However, these systems are not overly complicated,
and should not pose an operation or maintenance problem for the
48
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plant staff. In fact, such systems are typically automatically
controlled, and operation would consist of only occasional moni-
toring. Operating costs for the active systems (exclusive of
labor) are limited to pumping power, whereas the passive systems
have near zero operating cost.
In terms of maintenance of an active solar system, 15 items
have been identified as requiring maintenance: collector glaz-
ing, collector gasketing, collector sealants, absorber plate,
absorber plate coating, insulation, heat-transfer fluid, pumps,
heat exchangers, piping, valves, expansion tank, connectors,
storage tank, and manifolds (28).
Although there is a wide range of items requiring mainte-
nance, the installation of premium grade materials tends to min-
imize maintenance requirements. A high quality solar system is
estimated to last 20 to 25 years. Yearly maintenance costs have
been estimated to be about 1 to 3 percent of the installed cap-
ital cost (29).
A recent article (36) reviews the results of the Department
of Energy's National Solar Heating and Cooling Demonstration
Program through September 1980. Of 12 active solar heating sys-
tems (both air and liquid based), only one provided the expected
solar fraction. Nine problems were reported as causing poor
system performance:
1. Air leakage
2. Water leakage
3. Freezing problems
4. Control problems
5. Storage problems
6. Storage heat loss problems
7. Severe weather
8. Lower energy requirements than design load
9. Supplemental heat problems
As opposed to the opeation and maintenance associated with
either digester or space heating, the potential operations and
maintenance problems with active or passive sludge drying are
difficult to assess. The two schemes discussed previously are
likely to be operator and maintenance intensive owing to the
non-homogeneous properties of and associated difficultues in
handling sludge.
49
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COST CONSIDERATIONS
A basic consideration of any solar energy alternative is the
installed capital cost. As solar energy is not a continuous,.,
dependable source, back-up equipment to provide supplemental en-
ergy is required. In the case of active solar digester heating
and space heating, the back-up systems will be approximately the
same size as the equipment which would be installed without so-
lar energy. Specifically, for both digester heating and space
heating, both a combustion burner and a heat exchanger are re-
quired whether or not solar energy is utilized. Therefore, the
annual energy savings accrued due to solar energy must be bal-
anced against the higher initial capital cost due to the solar
equipment. A similar case for passive solar heating exists.
For the case of solar aided sludge drying, the non-continu-
ous nature of solar energy will cause the required sludge stor-
age facilities to be larger. In addition, the drying facilities
may also have to be enlarged over conventional gas fired dryers
if the solar heated air is used directly, thereby increasing the
capital cost. Once again the energy savings must be compared to
this additional capital cost.
The final consideration involves the increased system opera-
tion and maintenance costs due to solar energy use which also
must be subtracted from the yearly energy savings.
ENERGY CONSIDERATIONS
The energy considerations associated with solar energy are
obvious in that, with the exception of energy consumed by system
operation, all other energy produced represents a net gain.
This does not consider the secondary energy requirements associ-
ated with production of the solar energy collection equipment.
50
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SECTION 4
COMPARISON WITH EQUIVALENT TECHNOLOGY
EQUIVALENT CONVENTIONAL CONCEPT
The equivalent conventional concept used in the analysis of
solar energy applications in POTW's is the process which utiliz-
es conventional fuel sources in lieu of solar energy. For solar
aided anaerobic digestion, an equivalent technology could be di-
gester heating utilizing waste heat from the internal combustion
engine which burns the digester gas. However, as this is not
considered to be conventional practice, it is not discussed fur-
ther, and all comparisons are to anaerobic digestion utilizing
fossil fuels as a supplemental heat source.
The comparative evaluation of the solar aided anaerobic di-
gestion process with the equivalent technology was conducted
primarily with respect to cost and energy requirements. It
should be noted that the original intention was to compare solar
aided mesophilic and thermophilic digestion as well as sludge
drying. However, based on the results and conclusions presented
here, and further detailed in Appendices A and B, only solar
aided mesophilic anaerobic digestion was considered.
COST COMPARISON
Summary of Available Cost Data
Numerous references present the cost of solar collectors,
however, the data are based on estimates rather than on install-
ed capital cost information. Two recent articles (39,40) pre-
sent data on solar system costs in industrial applications. In
Reference 39, it is stated that the widespread use of solar en-
ergy has been predicted (by researchers and manufacturers) to
allow for a 70 percent cost reduction by 1990. However, experi-
ence during the period 1977 to 1980 shows that both cost and
performance have failed to meet expectations, leading to lower
than expected rates of return on solar investments.
"This may be expected of a new technology, however, and is
not necessarily indicative of future potential. Of six projects
reviewed which utilized solar energy for process heat, annual
51
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average efficiencies ranged from 8.1 to 19.7 percent based on
insolation rates which is only 25 to 50 percent of the predict-
ed performance (39). From a cost standpoint, there appeared to
be no tendency for economies of scale. Although there is reason
to be optimistic about future cost reductions, there is no evi-
dence available today to show any reduction in costs, and
$538.21/m2 ($50.00/ft2) of collector area is representative
of current (1980) installed costs of solar energy systems.
Reference 40 presents a detailed review of construction
costs for 14 facilities within the National Solar Heating and
Cooling Demonstration Program, and includes process hot water,
space heating (air and liquid), and space cooling systems.
The cost breakdown among system components is presented in
Table 5. The costs for solar space heating (considered closest
to the digester application), including installation and profit,
but excluding design, instrumentation, or auxiliary equipment in
1977 dollars averaged $527.45/m2 ($49/ft2) (50). System costs
showed only a slight economy of scale. The authors then re-
viewed the data to determine the average and minimum potential
cost for the hot water and space heating systems. The cost
data, by category, is as follows (40):
TABLE 5. SOLAR SYSTEM COST DATA (40)
Average Costs
Minimum,Cost
Collector
Support
Piping, Duct, and
Insulation
Storage
Electrical and
Controls
General
Construction
Total
143.
80.
117.
30.
37.
20.
429.
16
73
33
14
67
45
49
(13.
(7.
(10.
(2.
(3.
(1.
(39.
30)
50)
90)
80)
50)
90)
90)
33.
18.
27.
7.
8.
4.
3
8
3
0
8
8
130.
30.
67.
12.
13.
'10.
265.
25
14
81
92
99
76
88
(12.
(2.
(6.
(1.
(1.
(1ซ
(24.
10)
80)
30)
20)
30)
00)
70)
49.0
11.3
25.5
4.9
5.3
4.0
Note: All costs are in 1977 dollars.
52
-------
Therefore, the actual solar collector accounts for only
about 33 to 50 percent of the total system costs. The authors
also concluded that retrofit- applications could be as much as 15
percent more expensive mainly due to piping, ductwork, and in-
sulation (40) .
The second solar system cost item is the annual operations
and maintenance cost. Although information regarding mainte-
nance requirements is available, few actual O&M cost data are
available. Exclusive of operating costs, a 1 to 3 percent main-
tenance cost has been presented (29).
Methodology for Cost Analysis
In order to obtain regional conclusions, nine cities in the
United States, three in each of the north, central, and southern
regions, were selected as shown in Figure 17. Within each re-
gion, three locations were chosen so that each site had similar
climates in terms of degree-days, yet varying insolation rates
(Table A-l, Appendix A). This selection was made so that, for a
similar climate, the effect of varying insolation rates could be
obtained and the results potentially extrapolated to generaliza-
tions for the entire United States.
The methodology for determining the cost-effectiveness of
solar aided mesophilic anaerobic digestion consists of comparing
the present worth cost of digester gas saved due to solar to the
cost associated with producing the solar energy. Costs to pro-
duce solar energy include the capital cost of the equipment nec-
essary to capture and transfer the thermal energy, and the op-
eration and maintenance costs associated with the equipment.
In sizing a solar energy collector, various "rules of thumb"
exist as to what portion of the total heat load should be sup-
plied by solar energy. For the analysis described within, it
was decided that the solar collector system size be based on the
most cost-effective solution. Since the anaerobic digester fa-
cilities are equivalent whether or not solar energy is used to
preheat the sludge, then the cost-effectiveness analysis should
compare the additional cost associated with implementation of
the solar energy collection system (both initial capital cost
and yearly O&M costs) to the value of the digester gas saved
(i.e., not combusted). Net present worth costs were calculated
and were defined as the total present worth of the solar collec-
tion facilities (including O&M), minus the present worth cost of
the digester gas saved. Thus, only where net present worth val-
ues are negative would there be an economic advantage to utiliz-
ing solar energy for digester heating.
53
-------
0)
N
M
rH W
H C
P O
D -H
4J
W (0
-------
No cost savings are associated with the gas that may be gen-
erated in excess of that required for digester heating, as this
gas would be available regardless of whether or not the solar
energy system is installed.
For this analysis, a solar system cost (including collector,
pumps, piping, insulation, heat exchanger, and appurtenances) of
$538.21/m2 ($50/ft2) was used. In addition, a yearly opera-
tion and maintenance cost of 4 percent of initial installed cap-
ital cost was assumed. The annual maintenance cost (1 to 3 per-
cent of installed capital cost (29)) was revised to 4 percent to
include annual operating costs. As No. 2 fuel oil or natural
gas is typically utilized for supplemental digester heating, the
energy saved by installation of a solar collection system was
assigned a cost value based on these two fuels, after considera-
tion of the various energy conversion efficiencies (see Appendix
B). Regional energy costs were used to more closely simulate
market conditions. Natural gas prices for commercial users for
December 1980 were utilized (nationwide average price, $3.70/GJ;
$3.51/million Btu), as was October 1980 fuel oil costs to indus-
trial customers (nationwide average price, $226/m3 ($0.855/
gallon); $6.45/GJ ($6.11/million Btu) (38)). For comparison
purposes, electricity at $0.05/kWh is equivalent to $15.48/GJ
($14.64/million Btu).
The cost-effectiveness analysis was performed utilizing the
EPA-approved discount rate of 7-3/8 percent. The collectors were
assumed to have a 20-year life, with zero salvage value after 20
years.
Results of Cost Analysis
Preliminary computations for the Rapid City, South Dakota
3,785 m3/d (1 mgd) facility indicated that the net present worth
cost of the solar collection equipped digester increased with
increased size of collector areas. In other words, as the per-
cent utilization of solar energy was increased (collector
area), the difference between the total present worth of the so-
lar collection system and the dollar value of the digester gas
saved became greater. The calculated range in net present worth
costs for various levels of solar energy utilization expressed
as a percent of digester heat load is illustrated in Figure 18.
A similar economic analysis for the 37,850 m3/a (io mgd) ca-
pacity plant revealed identical results, indicating that the
cost of energy derived from solar collectors is significantly
greater than the cost of energy derived from either No. 2 fuel
oil or natural gas. In order to confirm these findings and to
extrapolate to other geographical areas, the City of Yuma, Ari-
zona, having a higher insolation value and a lower degree-day
55
-------
240
Natural Gas at
$4.41/GJ ($4.65/106 Btu).
200
2
"5
Q
ra
CO
JC
ts
5
I
*-
c
0)
Q_
ซ*-*
z
160
120
80
40
No. 2 Fuel Oil
at $8.93/GJ
($9.42/106 Btu)
Range in Net Present
Worth Cost
20
80
100
Figure 18,
40 60
Percent Digester Heat Load
*Net Present Worth = (Capital Cost) Collection + (Present Worth
of O&M) Collector - Present Worth of Fuel Saved
Effect of incremental solar collector area on solar
system net present worth (Rapid City, SD; plant
capacity 3f785 m3/d).
56
-------
total than Rapid City, was evaluated. The results were found to
be similar to the Rapid City site for the 37,850 m3/d (10 mgd)
plant capacity. In fact, the Yuma site was more expensive on
the basis of present worth cost, since the overall system effi-
ciency decreased. This reduction in system efficiency is due to
the lower heat load requirement per unit area of collector re-
sulting in decreased system performance. Based on these prelim-
inary results, solar aided anaerobic digestion did not appear
cost-effective, and a sensitivity analysis was performed to de-
termine the controlling variables. The re.sults of the present
worth analysis are summarized in Tables 6 through 8.
Further investigations assessed the effect of feed solids
concentration on the economic feasibility of solar aided anaero-
bic digestion, as feed sludge at a higher concentration has a
lower sensible heat requirement and the smaller sludge volume
allows for a smaller digester volume. However, as with the
Rapid City versus Yuma comparison, the resulting smaller heat
load/unit area collector caused the actual system efficiency to
decrease, resulting in an increased present worth. The results
at 6 percent and 8 percent feed solids are presented in Tables
9 and 10.
Sensitivity Analysis
The results of the economic evaluation indicated that active
solar aided anaerobic digestion cannot be recommended for any
location or treatment plant size within the United States. Sev-
eral factors contributed to this conclusion including the unit
price for the solar collector, the assumptions relative to the
operation and maintenance cost, unit equivalent fuel price, etc.
In order to evaluate the sensitivity of these various cost ele-
ments on net present worth cost, the cost analysis was repeated
incorporating various ranges for these cost elements as follows:
1. Installed Capital Cost for Solar Collector System
= $161.50 to $538.20/m2 ($15 to $50/ft2).
2. Annual Operation and Maintenance Cost = 1 to 4
percent of capital cost.
3. Escalation in Fuel Cost (geometric series present
worth cost) = 4 to 8 percent.
In general, the cost evaluation results indicated that the
most cost sensitive factor affecting the total net present worth
of the solar collector system is the initial cost of installing
the collector. In order for the technology to be economically
viable at the present fuel costs, assuming a 4 percent annual
57
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fuel cost escalation, the collector system cost would have to be
reduced to between $161.50 and $322.93/m2 ($15 to 30/ft2) .
The results of this analysis are shown in Figures 19 and 20.
The second factor considered was the annual operation and
maintenance cost for the solar collector system. In this analy-
sis, 4 percent of the installed capital cost was used in the in-
itial evaluation. However, with improvements in the design and
construction of these facilities, the O&M costs can be poten-
tially reduced. For this reason, a range of annual O&M -costs
between 1 and 4 percent were evaluated. These results are pre-
sented in Figure 21. In addition, these computations were based
on a unit installation cost of $161.50 m2 ($15/ft2) so that the
effects of the reduced O&M costs could be illustrated. Based on
these results, it can be concluded that the total costs are sen-
sitive to O&M costs, particularly in marginal applications.
However, the capital cost of the collector system is still the
major cost consideration.
Current EPA cost-effectiveness analysis guidelines utilize a
4 percent per annum escalation factor on natural gas only. This
4 percent escalation, however, was also applied to fuel oil in
this analysis. As the Regional Administrators of EPA can dic-
tate a different escalation factor, and owing to future market
uncertainties, escalation rates of 4, 6, and 8 percent at an in-
terest rate of 7-3/8 percent over a 20-year period were consid-
ered. The geometric series present worth factor (GESPWF) was
utilized to compute present worth values of 'equivalent energy
savings realized from solar collector systems. These results
are presented in Figure 22, and indicate that the appreciation
of fuel prices at 4, 6, and 8 percent per annum do not, by them-
selves, make for a cost-effective solution at a solar system
cost of $538.21/m2 ($50/ft2). Therefore, for the range of
4 to 8 percent, the overall analysis is not sensitive to fuel
escalation factors.
It should be noted that a fuel escalation rate of 4 percent
per annum over the 20-year planning period is higher than DOE
projected price increases (41).
Based on the information presented on a net present worth
basis, the solar heating of anaerobic digesters is not cost-ef-
fective given the current prices for the solar collection sys-
tem and conventional fuels. This conclusion was identical for
the 3,785 m3/d (1 mgd) and 37,850 m3/d (10 mgd) facilities.
The facility location did not impact the conclusions.
63
-------
250
Legend:
200
Solar system cost at $161.50/m2 ($15.00/ft2)
Solar system cost at $322.93/m2 ($30.00/ft2)
Solar system cost at $538.21/m2) ($50.00/ft2)
150
100
15
Q
T3
10
CO
O
8
O
1
*->
I
ฃ
Q.
50
-50
-100
20 40 60 80
Percent Digester Heat Load from Solar Collectors
100
Figure 19. Effect of solar system cost on net present worth
cost (Rapid City, SD; plant capacity 3,785 m3/d;
No. 2 fuel oil used as basis for comparison).
64
-------
250
E
200
TJ
(0
in
o
O
O
I
QL
15
150
100
50
Legend:
Solar system cost at $161.50/m2 ($15.00/ft2)
A Solar system cost at $322.93/m2 ($30.00/ft2)
Solar system cost at $538.21/m2 ($50.00/ft2)
40 60 80
Percent Digester Heat Load from Solar Collectors
100
Figure 20.
Effect of solar system cost on net present worth
cost (Rapid City, SD; plant capacity 3,785 m3/d;
natural gas used as basis for comparison).
65
-------
Legend:
ta
~o
a
o
o
O
1
4-*i
ง
a.
0>
40
20
-20
-40
No. 1 Fuel Oil Basis
A Natural Gas Basis
No. 2 Fuel Oil Basis
Natural Gas Basis
$161.50/m2 ($15.00/ft2) Solar
System Cost, O&M Cost 1% of Capital
Cost
$161.50/m2 ($15.00/ft2) Solar
System Cost, O&M Cost 4% of Capital
Cost
-60
20 40 60 80 100
Percent Digester Heat Load from Solar Collectors
Figure 21. Effect of operations and maintenance cost on net
present worth cost (Rapid City, SD; plant capacity
3,785 m
66
-------
250
Legend:
200
Q
4% Escalation for Fuel Cost; GESPWF = 13.9863
6% Escalation for Fuel Cost; GESPWF = 16.5326
8% Escalation for Fuel Cost; GESPWF = 19.7635
Solar System Cost at $538.21 An2 ($50.00/ft2)
O&M Cost 4% of Capital Cost
CO
CO
O
.C
o
I
0)
Q_
-t-ซ
0>
150
100
50
,40 60 80
Percent Digester Heat Load from Solar Collectors
100
Figure 22.
Effect of fuel price escalation factor on net
present worth cost (Rapid City, SD; plant capacity
3,785 m3/d; No. 2 fuel oil used as basis for
comparison).
67
-------
It should be noted that numerous previously referenced arti-
cles (25,36,39,40) all discussed the fact that most design pro-
cedures have over-estimated the amount of energy actually col-
lected by the system. This places additional doubt on the cost-
effectiveness of solar aided anaerobic digestion.
A final comment ,is in order, as previous research (18) indi-
cated that solar aided anaerobic digestion was cost-effective at
every location within the United States. The discrepancy be-
tween the previous and current research is due to the question-
able cost-effectiveness analysis, as the authors used a 12 per-
cent per annum escalation factor in the cost of natural gas. If
the present worth analysis presented in Reference 18 is redone
utilizing the geometric series present worth factor at 4 percent
escalation for 20 years at 7-3/8 percent, the analysis indicates
that the solar system is not cost-effective.
ENERGY CONSIDERATIONS
With the exception of any energy used for pumps or blowers
in the solar energy system, all energy produced is a net energy
gain. As solar aided anaerobic digestion does not appear cost-
effective, there will be minimal use of the technology, and all
potential energy savings are associated with POTW space heating.
68
-------
SECTION 5
NATIONAL IMPACT ASSESSMENT
MARKET POTENTIAL
Because collection of solar energy for heating anaerobic di-
gesters does not appear to be cost-effective given the present
cost of collector systems, resulting in a high cost per unit of
energy, it is not considered as currently having a market poten-
tial. However, if solar collector system costs decrease sub-
stantially, or conventional fuel costs increase dramatically,
solar-heated digesters may prove cost-effective. (See Section 4
for further details).
Active solar-aided sludge drying is an unproven technology
and, given the high costs of solar energy production from active
systems, is more than likely not economically attractive. The
feasibility of a passive sludge dryer should be investigated
further.
Therefore, the only potential market for solar applications
at POTW's would appear to be for space/domestic hot water heat-
ing. The potential for active and passive space heating should
be investigated on a case by case basis. Utilizing data from
the 1978 Needs Survey (42), as of 1978, 14,592 treatment plants
were in operation, with an additional 8,176 facilities planned
by the year 2000. Therefore, a great potential for the applica-
tion of solar energy technology for space heating exists, with
implementation dependent on economic feasibility.
COST AND J3NERGY IMPACT
The implementation of solar space heating for retrofitting
existing facilities and for construction of new facilities could
reduce the conventional energy requirements of POTW's (see Table
1). However, all decisions must be based on firm engineering
judgment and engineering economics.
69
-------
RISK ASSESSMENT
There is a small risk associated with the application of so-
lar space heating in POTW's. The risk involved can be divided
into two areas. First, solar collection systems have not typi-
cally been operating at their design efficiencies. Second, as
the technology is relatively new, there are little data to as-
sess the long-term maintenance requirements and system life.
70
-------
SECTION 6
RECOMMENDATIONS
FURTHER RESEARCH AND DEVELOPMENT EFFORTS
The only area for which further research efforts appear war-
ranted is passive sludge drying. Efforts could include both
conceptual design and pilot testing. There is presently a great
deal of research being conducted on space heating and process
thermal energy conversion.
Once finalized, the results and conclusions of the Wilton,
Maine energy systems monitoring report should be compared to the
analysis contained here. If the final report confirms the find-
ings of this study (as the preliminary report results have indi-
cated) , then no additional research on active solar energy col-
lection and utilization is warranted.
PROCESS/TECHNOLOGY MODIFICATIONS
Aside from any potential modifications to the conceptual de-
sign of the passive sludge dryer, no additional process or tech-
nology modifications are required. Precluding any major ad-
vances in solar energy technology, there appear to be limited
uses of solar thermal energy in POTW's with the exception of
building/space heating.
71
-------
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Department of Energy, DOE/CS-0127/2, Washington, DC, 1980.
36. Spielvogel, L.G. The Solar Bottom Line. ASHRAE Journal,
38-40, November 1980.
37. Water Pollution Control Federation. Manual of Practice No,
8, Wastewater Treatment Plant Design. Lancaster Press,
Inc., Lancaster, Pennsylvania, 1977.
38. Energy User News, Monday, 26 January 1981.
39. Brown, K.C. How to Determine the Cost-Effectiveness of
Solar-Energy Projects. Power, 72-75, March 1981.
40. King, T.A. and J.B. Carlock III. Construction Costs in
Commercial Solar. Energy Engineering, 11-31, December
1979/January 1980.
41. U.S. Department of Commerce. NBS Handbook 135, Life-Cycle
Costing Manual for the Federal Energy Management Pro-
grams. Washington, DC, 1980.
42. U.S. Environmental Protection Agency. 1978 Needs Survey.
EPA-430/9-79-002, Washington, DC, 1979.
75
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APPENDIX A1
DESIGN AND EVALUATION OF SOLAR AIDED
ANAEROBIC DIGESTER HEATING
OBJECTIVES
This appendix describes the methodology utilized for evalua-
ting the economics of digester heating using active solar energy
collection. Since the solar energy recovery potential is a
function of sunlight hours and geographical location (latitude),
three geographical regions were considered for the evaluation,
as follows:
1. Northern United States
2. Middle United States
3. Southern United States
Based on the climatological data within each region, nine
model cities were selected for evaluating the potential for so-
lar anaerobic digester heating. These model cities were select-
ed on the basis of their total annual degree-days (an indication
of the annual heating load) and horizontal incident solar radia-
tion data. The purpose of city selection was to choose three
cities in each of the three areas with an equivalent number of
degree days, yet with varying insolation rates. In this way, it
was hoped that for a given climate (i.e., number of annual
degree-days), a correlation could be made between insolation
rate and economic feasibility of solar digester heating, and
thereby allow for some national or regional conclusions to be
derived. The selected cities and their respective data are
summarized in Table A-l. Furthermore, to assess the effect of
plant size, solar aided anaerobic digestion was investigated
for treatment facilities having a design capacity of 3,785 and
37,850 m.3/d (1 and 10 mgd) .
appendix utilizes both traditional (English) and SI
units, as existing equipment specifications and design pro-
cedures are based on English units.
76
-------
TABLE A-l. SUMMARY OF CLIMATOLOGICAL DATA FOR MODEL U.S. CITIES
City
Apalachiola, FL
New Orleans, LA
Yuma, AZ
Columbia, MO
Dodge City, KS
New York, NY
Albany, NY
Pocatello, ID
Rapid City, SD
Geographical
region
Southern
Southern
Southern
Middle
Middle
Middle
Northern
Northern
Northern
Latitude
29.45
29.59
32.40
38.58
37.46
40.46
42.40
42.55
44.09
Annual
degree
days
1,308
1,385
1,217
5,046
4,986
5,000
6,875
7,033
7,345
Horizontall
radiation
(Btu/sq ft)
1,539
1,316
1,629
1,193
1,399
964
946
1,216
1,156
^Horizontal radiation values for month of October have been
taken as typical values for this evaluation.
DIGESTER AND HEAT LOAD SIZING
Two-stage high rate mesophilic anaerobic digesters for the
3,785 and 37,850 m3/d (1 and 10 mgd) facilities were sized by
assuming primary and secondary sludge generation rates, sludge
solids concentration, and a mass volatile solids loading rate in
the first stage. The assumptions utilized for design are pre-
sented in Appendix B.
The total heat load associated with digester operations con-
sists of two components:
Total Heating Requirement
Sludge Heating Requirement +
Digester Heat Loss
The sludge heating requirement (sensible heat requirement)
to preheat the sludge to 35 degrees C (95 degrees F) was calcu-
lated assuming the sludge to have a specific heat of 1.0
Btu/ฐF-lb:
Sludge Heating Requirement, Btu/yr = (Sludge Feed Rate,
Ib/yr) j; Btu (95ฐF - Sludge Temperature, ฐF) (hr/yr)
77
-------
Sludge temperatures were assumed to vary monthly and differ-
ent influent temperatures were used for each of the three zones
of the United States (Table B-l).
Utilizing typical heat transfer coefficients for digester
components (Appendix B), the heat loss due to heat radiation
from the digester was calculated:
Heat Loss from Digester, Btu/yr = [Heat Loss Through (Roof +
Walls + Floor) Btu/hr] (hr/yr)
The total heat loss from the digester structure was calcu-
lated utilizing monthly average weather data (ambient air tem-
perature) . A summation of the sensible heat requirement plus
the radiant heat requirement yields an equation for the total
heat requirement/ which is then solved on a monthly basis for
the nine different cities.
In order to assess the effect of plant size on the feasibil-
ity of solar-aided anaerobic digester heating, the calculations
were done for both 3,785 and 37,850 m3/d (1 and 10 mgd) facil-
ities. The total digester heat load requirements for a 3,785
m-Vd (1 mgd) facility with a digester feed solids concentra-
tion of 4 percent (dry weight basis) are summarized in Tables
A-2 through A-10 for each of the selected cities. The tables
also include the average ambient temperature and average daily
insolation data. Similarly, the data for the 37,850 mVd (1ฐ
mgd) facilities are summarized in Tables A-ll through A-19.
SOLAR COLLECTOR DESIGN
Based on the digester head load requirements, the solar en-
ergy which can be potentially utilized from various collector
areas was computed from each size and for the two design capa-
bilities. The collector area selected for evaluating the 3,785
m3/<3 (1 mgd) plant include: 46.5, 92.9, 186, 279, and 372 m2
(500, 1,000, 2,000, 3,000, and 4,000 ft2). The collector areas
selected for the 37,850 m3/d (10 mgd) plant include: 697, 1,161,
1,626, 2,090, and 2,555 m2 (7,500, 12,500 17,500, 22,500, and
27,500 ft2). Collector area sizes were selected on the basis
of available solar insolation versus heating load after consid-
eration of collector efficiency.
All computations involving solar collector system efficiency
were done utilizing the f-chart analysis procedure. All calcula-
tions were performed utilizing WESTON's program entitled SOLECO,
a computerized f-chart analysis.
78
-------
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October
November
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96
-------
APPENDIX B
DESIGN AND EVALUATION OF SOLAR AIDED
ANAEROBIC DIGESTER HEATING: ASSUMPTIONS
1. Primary sludge production rate: 0.625 tons (dry weight) of
primary sludge per million gallons of wastewater. treated.
Primary sludge solids concentration at 5 percent by weight.
2. Waste activated sludge production rate: 0.535 tons (dry
weight) per million gallons of wastewater treated. Thick-
ened waste activated sludge concentration at 2.8 percent by
weight.
3. Total undigested sludge production rate: 1.16 tons (dry
weight basis) per million gallons of wastewater treated.
Digester feed sludge concentration at 4 percent by weight.
Volatile fraction in digester feed is approximately 0.68
by weight.
4. Volatile solids loading to the primary digester is assumed
at 0.16 Ib VSS/ft3-day.
5. Digester dimensions for the 1 mgd case were calculated by
assuming equal diameter and sidewater depth. This will
minimize heat loss from exposed digester surfaces. For
10 mgd facility, a maximum digester depth of 40 feet was
used. One digester (including primary and secondary
stages) rather than two digesters was used to minimize heat
losses.
6. Operating temperature for the primary digester is assumed
at 35 degrees C (95 degrees F) (mesophilic conditions).
No heating of the secondary digester is provided.
7. The influent sludge temperatures are assumed to vary at the
rate of 0.56 degrees C (1 degree F) per month with average
temperatures occurring during the months of April and
October of each year. The assumed influent sludge temper-
atures for the three geographical regions are shown summar-
ized in Table B-l.
8. The thermal capacity of digester feed sludge is assumed
equal to that of water (specific heat of sludge =1.0
Btu/ฐF-lb) .
97
-------
9. The following heat transfer coefficients were used (37):
Floating cover with built-up roof - 0.24 Btu/hr-ft2
12-inch thick concrete walls - 0.25 Btu/hr-ft2
Floor and surrounding soil - 0.12 Btu/hr-ft2
The soil temperature was assumed equal to the monthly am-
bient ^air temperature. :
10. The rate of gas production from anaerobic digestion has
been assumed at 15 ft3 per pound of volatile suspended
solids destroyed.
11. For evaluating overall process economics, the heating val-
ues for digester gas, No. 2 fuel oil, and natural gas have
been assumed as 600 Btu/ft3, 140,000 Btu/gallon, and
1,000 Btu/ft3, respectively. In addition, a combustion
efficiency of 65 percent is assumed for the conversion of
either natural gas or fuel oil to energy supplied to the
digester contents when calculating the dollar value of
digester gas.
12. Facilities to store and utilize the excess digester gas are
assumed to be existing and of sufficient capacity to util-
ize any additional gas saved due to the application of so-
lar digester heating.
98
-------
TABLE B-l.
ASSUMED FEED SLUDGE TEMPERATURES FOR THE
THREE GEOGRAPHICAL REGIONSi
Month
Northern U.S.
Middle U.S,
o,..
Southern U.S,
o,.,
January
February
March
April
May
June
July
August
September
October
November
December
55
56
57
58
59
60
61
60
59
58
57
56
60
61
62
63
64
65
66
65
64
63
62
61
65
66
67
68
69
70
71
70
69
68
67
66
Average yearly temperatures of sludge for each region from
Reference 16.
99
US. GOVERNMENT PRINTING OFFICE: 1982-559-092/3375
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