6___________________

Flat-Plate
Collectors
Flat-plate collectors are in wide use for domestic household hot-water
heating and for space heating, where the demand temperature is low.
Many excellent models of flat-plate collectors are available commercially to
the solar designer. A discussion of flat-plate collectors is included here
because of their use in industrial systems either to supply low-temperature
demands or to preheat the heat transfer fluid before entering a field of
higher-temperature concentrating, collectors. Detailed descriptions of flat-
plate collector design, performance and system design using these
collectors may be found in the following sources: Duffie and Beckman
(1980), Lunde (1980), and Kreider and Kreith (1982). The interested reader
is referred to these for further information.

The emphasis of this chapter is to develop an understanding of both the

attributes of different design features and the performance characteristics
of this class of collector. This understanding will permit the system designer
to evaluate whether flat-plate collectors should be considered in a system
design. Also, the information required to select an appropriate commercial
unit is presented here.

This development will follow the outline below:

o Collector description
 Absorber plate
 Cover sheets
o Advantages
 Non-tracking option
 Diffuse solar radiation utilization
 Cost
o Collector Performance
 Orientation
 Efficiency measurement
 Typical performance
 Comparison with parabolic troughs
o Applications
o Other types
 Evacuated tube collectors
 Solar ponds

o References and bibliography

6.1 Collector Description

The construction of a flat-plate collector is shown in Figure 6. 1. The basic

parts noted are a full-aperture absorber, transparent or translucent cover
sheets, and an insulated box. The absorber is usually a sheet of high-
thermal-conductivity metal with tubes or ducts either integral or attached.
Its surface is painted or coated to maximize radiant energy absorption and
in some cases to minimize radiant emission. The cover sheets,
called glazing, let sunlight pass through to the absorber but insulate the
space above the absorber to prohibit cool air from flowing into this space.
The insulated box provides structure and sealing and reduces heat loss
from the back or sides of the collector.

Figure 6.1 A typical liquid flat-plate collector.

6.1.1 Absorber Plate

The main element of a flat-plate collector is the absorber plate. It covers the full
aperture area of the collector and must perform three functions: absorb the
maximum possible amount of solar irradiance, conduct this heat into the working
fluid at a minimum temperature difference, and lose a minimum amount of heat back
to the surroundings.
Absorption. Solar irradiance passing through the glazing is absorbed
directly on the absorber plate without intermediate reflection as in
concentrating collectors. Surface coatings that have a high absorptance for
short-wavelength (visible) light, are used on the absorber. Usually these
coatings appear dull or "flat," indicating that they will absorb radiation
coming from all directions equally well. Either paint or plating is used, and
the resulting black surface will typically absorb over 95 percent of the
incident solar radiation.

Fin Heat Removal. The second function of the absorber plate is to transfer
the absorbed energy into a heat-transfer fluid at a minimum temperature
difference. This is normally done by conducting the absorbed heat to tubes
or ducts that contain the heat-transfer fluid. The heat-transfer fluid may
either be a liquid (water or water with antifreeze) or gas (air). The important
design criterion here is to provide sufficient heat transfer capability that the
difference between the temperature of the absorber surface and the
working fluid is not excessive; otherwise, the heat loss from the absorber
would be excessive. High heat-transfer rates are usually accomplished at
the expense of pumping power and absorber plate material.

When a liquid is used as the heat-transfer fluid as is most often the case,
special problems occur in transferring the heat absorbed on the absorber
surface into the fluid. Liquid collector absorber plates often consist of a flat
sheet of metal with tubes spaced 10-25 cm (4-10 in.) apart and attached to
it in some fashion (integral, brazed or press fitted). The sheet of metal
absorbs most of the solar irradiance and acts as a fin to bring the absorbed
heat into the fluid. The following are important points in designing a good
‘tube and sheet’ absorber:

1. The fin (absorber sheet) must he made of a material with high

thermal conductivity.
2. The fin should be thick to minimize the temperature difference
required to transfer heat to its base (tube).
3. Tubes should not be spaced too far apart; otherwise, a higher
temperature difference between the tip of the fin (midway between
the tubes) and the base will result.
4. Tubes should be thin-walled and of a high-thermal -conductivity
material.
5. The tube should be brazed or welded to the absorber sheet to
minimize thermal contact resistance.
6. The tube and absorber sheet should be of similar material to prevent
galvanic corrosion between them.

When air is the heat-transfer fluid, often the back side of the absorber plate
usually forms one surface of a duct and heat is transferred through the
absorber sheet to the air over the entire back surface of the absorber. A
thin, rather than thick, absorber sheet of high-thermal-conductivity material
will enhance this heat-transfer process. The internal air passage must be
designed to provide a sufficiently high airflow velocity past the back of the
absorber to give adequate heat transfer without producing a high pressure
drop across the collector. Low heat-transfer rates cause the absorber plate
to become significantly hotter than the heat-transfer fluid, which increases
heat loss. On the other hand, a large pressure drop across the collector
causes high pumping power consumption by the fans supplying the air.

Emittance. Because the temperature of the absorber surface is above

ambient temperature, the surface re-radiates some of the heat it has
absorbed back to the surroundings. This loss mechanism is a function of
the emittance of the surface for low-temperature, long-wavelength
(infrared) radiation. The dilemma is that many coatings that enhance the
absorption of sunlight (short-wavelength radiation) also enhance the long
wavelength radiation loss from the surface. This is true for most dull black
paints.

A class of coatings, mostly produced by metallic plating processes, will

produce an absorber surface that is a good absorber of short-wavelength
solar irradiance but a poor emitter of long-wavelength radiant energy. The
function of these coatings, called selective surfaces, was described in detail
in Chapter 5. Flat-plate absorbers that have selective surfaces typically
lose less heat when operating at high temperature, as will be seen in
Section 6.3.3. However, the absorptance of selective coatings is seldom as
high as for non-selective coatings, and a tradeoff must be made based on
whether the increased high-temperature performance overshadows the
reduced low-temperature performance and expense of the selective
coating.

6.1.2 Cover Sheets

The absorber is usually covered with one or more transparent or

translucent cover sheets to reduce convective heat loss. In the absence of
a cover sheet, heat is lost from the absorber as a result of not only forced
convection caused by local wind, but also natural convective air currents
created because the absorber is hotter than ambient air. The cover sheet
forms a trapped air space above the absorber, thereby reducing these
losses. However, convective loss is not completely eliminated because a
convective current is set up between the absorber and the cover sheet,
transferring heat from the absorber to the cover sheet. External convection
then cools the cover sheet, producing a net heat loss from the absorber. In
addition, heat loss is reduced because of the thermal resistance of the
added air space.
Number of Covers. The number of cover sheets on commercial flat-plate
collectors varies from none to three or more. Collectors with no cover sheet
have high efficiencies when operated at temperatures very near ambient
temperature. This is because incoming energy is not lost by absorption or
reflection by the cover sheet. When no cover sheet is used, however, a
considerable amount of the incident energy is lost during operation at
temperatures much above ambient or at low solar irradiance levels. A
typical application for an uncovered flat-plate collector is for swimming pool
heating, where temperatures less than 10ºC (18ºF) above ambient are
required.

Increases in the number of cover-sheets increases the temperature at

which the collector can operate (or permits a given temperature to be
reached at lower solar irradiance). One or two cover sheets are common,
but triple glazed collectors have been designed for extreme climates. In
addition to the added expense, each added cover sheet increases the
collection efficiency at high temperature by reducing convection loss but
decreases the efficiency at low temperatures because of the added
absorption and reflectance of the cover.

In regions of average mid-latitude temperatures and solar radiation,

collectors with no glazing are generally used for applications to 32ºC
(90ºF), single-glazed collectors are used for applications to 70ºC (158º F),
and double-glazing is used in applications above 70ºC (158ºF). As
discussed in Chapter 5, collector efficiency increases with increasing solar
irradiance level but decreases with increasing operating temperature. In
regions of low average solar irradiance or extremely low temperatures,
therefore, double-glazed collectors are used in applications where single-
glazed collectors should be used normally and single-glazed collectors for
unglazed applications. Also, selective absorber surfaces become more
worthwhile.

Materials. Because of its superior resistance to the environment, glass is

used as the outer cover sheet on most commercial collectors. Usually the
glass is tempered, with a low iron content and 3.2-6.4 mm (0.12-0.25 in.)
thick. The surface may be either smooth, making the glass transparent, or
with a surface pattern, making it translucent. Both types have a
transmittance of around 90 per cent.

Plastic cover sheets are sometimes used for the second cover sheet when
two sheets are required. Installation of the plastic sheet beneath the glass
protects the plastic from the environment. Glass also does not transmit UV
radiation and thus protects the plastic, which is usually sensitive to this
portion of the solar spectrum. Rigid sheets of acrylic-or fiberglass-
reinforced polymers are in use, as are stretched films of polyvinyl fluoride.
Some of these plastic cover sheets have a transmittance approaching that
of low iron glass. A major draw back of this scheme is the potential for
overheating the plastic sheet at collector stagnation (no-flow) temperatures.

6.2 Advantages

Flat-plate collectors will absorb energy coming from all directions above the
absorber (both beam and diffuse solar irradiance). Because of this
characteristic, flat-plate collectors do not need to track the sun. They
receive more solar energy than a similarly oriented concentrating collector,
but when not tracked, have greater cosine losses.

6.2.1 Non-tracking Option

Since tracking is not required, flat-plate collectors may be firmly fixed to a

mounting structure, and rigid plumbing may be used to connect the
collectors to the remainder of the system. Moving structure, motors, and
tracking control systems are eliminated, thereby reducing the complexity of
the system. As discussed in Chapter 5, however, because of the cosine
effect, less total energy falls on a fixed surface over the period of a day
than on a surface that tracks the sun about one or two axes.

In order to increase their output, flat-plate collectors may be repositioned at

intervals or placed on a single- or two-axis tracking mechanism. Either of
these options increases the output of the collector but eliminates the
advantage of fixed piping and mounting structure.

6.2.2 Diffuse Solar Radiation Utilization

A flat-plate collector absorbs both the direct and the diffuse components of
solar radiation. This partially compensates for the fact that fixed surfaces
receive less energy because of the cosine effect. Although the diffuse solar
irradiance is only about 10 percent of the direct normal solar irradiance on
a clear day, on a cloudy day almost all of the available solar irradiance is
diffuse.

A comparison between the energy falling on a fixed and a fully tracking flat-
plate collector and on a fully tracking concentrating collector is shown in
Table 6.1. The data are yearly average values taken from SERI (1981b).
The comparison is location dependent because of the latitude effects on
the incidence angle and the difference in cloud cover.

Table 6.1. Collectible Solar Radiation Comparison Between

Flat-Plate and Concentrating Collectors
 Annual Average Daily

Solar Radiation (MJ/m2)

Collector Albuquerque Madison

Two-axis tracking 31 19.5

flat-plate collector
(direct plus
diffuse)

Fixed, latitude-tilt 23 15
flat-plate collector
(direct plus
diffuse)

Two-axis tracking 26.5 14

concentrator
(direct only)

For both locations, the two-axis tracking flat-plate collector receives more
energy. However, in the dry, high desert climate of Albuquerque, a fixed
flat-plate collector loses more energy from the cosine effect than it gains by
being able to collect diffuse energy. In the Midwestern climate of Madison,
Wisconsin, the energy lost by fixing the flat-plate collector approximately
equals that gained over a concentrator by its ability to collect the diffuse
component of solar irradiance. It should be noted that Table 6.1 only
indicates the relative amounts of energy available for collection. The choice
of collector type or whether to track a flat-plate collector is generally based
on system cost and energy output rather than energy input.

6.2.3 Cost

Currently, flat-plate collectors cost less than concentrating collectors. Part

of reason is the lack of need for a complex tracking system. However, part
of the reason is because many more flat-plate than concentrating collectors
are being produced today. At large production rates, however, it is still not
clear whether it is less expensive to cover an aperture area with an
absorber plate or with reflective material.

Because of their potential to produce more low-temperature energy for a

given cost, flat-plate collectors have been considered for use as pre-
heaters for concentrating collectors in high-temperature industrial process
heat systems. The major prototype of this system configuration is at
Campbell Soup Company in Sacramento, California and is shown in Figure
6.2. The decision of whether to include flat -plate collectors as pre-heaters
ultimately is based on the balance between performance and cost.
Figure 6.2 The Campbell Soup Company industrial process heat installation in Sacramento, CA. Flat-
plate collectors are used to preheat water before entering the parabolic trough concentrators. Courtesy
of Sandia National Laboratories.

6.3 Collector Performance

6.3.1 Orientation

The orientation of a flat-plate collector is a concern in system design. The

designer must decide on both the collector azimuth and tilt angles or to
install the collectors horizontally.

Azimuth. The most obvious azimuth for a fixed surface in the northern
hemisphere is south facing. This will give equal amounts of energy before
and after noon and usually the maximum daily total energy collected. There
are a number of reasons why the system designer may not select this
azimuth. It may be simply that the building or land orientation makes it
desirable to rotate the azimuth axis to fit the installation conditions. Other
performance related factors can affect the collector field orientation.

If the industrial demand is greater in the morning and there is no overnight

thermal storage in the system , the azimuth may be rotated to the east.
This would make the peak energy collection occur earlier in the morning
but give a slightly lower daily total. A similar westerly rotation would provide
more after noon energy if desired.

Another factor causing the collectors not to be oriented toward the south is
the presence of a blockage (mountain or building) that shades morning or
after noon sunlight. In this case the optimum orientation may call for
rotation away from the blockage. Likewise, either persistent morning or
afternoon cloud cover may cause the designer to orient the field azimuth for
optimum energy collection.
It is generally accepted that the azimuth of a fixed field may be rotated up
to 15 degrees from south and not make a significant difference in .the
overall energy collection. With the information presented in Chapters 2
through 4, however, a simple average cloudy-day model can be easily
coded and a comparison made between design alternatives.

Tilt. The most logical tilt angle for the fixed flat-plate collector is to tilt the
surface from horizontal by an angle equal to the latitude angle. At this tilt, if
the collector is facing south, the sun will be normal to the collector at noon
twice a year (at the equinoxes). Also, the noontime sun will only vary above
and below this position by a maximum angle of 23.5 degrees.

However, there are some system design considerations that may

encourage the designer to tilt the collector’s above or below this angle. For
instance, if the slope of the roof where the collectors are to be installed is
within 15 degrees of the latitude tilt, one would probably choose a flush roof
mounting. If the demand is greater in the winter months as with space
heating, the designer may choose to tilt the collectors toward the horizon
more and accept the summer energy loss. Similarly, the reverse may be
true for a heavy summer demand (i.e., cooling).

As with collector azimuth orientation, shadowing objects may be

considered and the collector tilted less because of blockage of solar
irradiance when the altitude of the sun is low. Another factor that may affect
the tilt orientation is the climate. If the particular region has considerable
cloud cover during the winter, the collectors would be tilted to maximize
summer energy collection. One final factor that could influence the
approximate tilting of the collector is the systems operating threshold. If the
system needs a high value of solar irradiance to begin operation, tilting the
collectors closer to vertical may provide more energy to the system during
start-up.

Again, in order to predict these effects, an analysis should be made using

solar radiation data and sun angle computations to predict the exact effects
of a particular set of circumstances

The yearly average data for two sites are shown on Figure 6.3. Plots for the
other sites would be similar. Note that for both sites shown, the maximum
yearly irradiation is obtained by surfaces tilted slightly less than the latitude
angle. This is because of the greater cloud cover in the winter for both
sites. Also note that a wide variation in tilt angle makes little difference in
the irradiation received. This implies that the collector tilt optimization is not
critical and that even horizontal surfaces may be an appropriate design
choice if the cost of installation is considerably less for this orientation.
Figure 6.3 Total (global) irradiation on a south-facing tilted surface. Average ground reflectance was
assumed to be 0.20

6.3.2 Efficiency Measurement

The energy collection efficiency is normally determined by testing. As discussed in

Chapter 5, collector performance test data are correlated with a parameter
comprised of the collector temperature rise above ambient divided by the solar
irradiance. For flat-plate collector performance, the collector temperature and the
solar irradiance used in this correlation are different from those used for
concentrating collectors.

The collector temperature used for flat-plate collector performance

correlation is normally the temperature of the heat-transfer fluid entering
the collector, not the average fluid temperature, as is common for
concentrating collectors.

Although the use of fluid inlet temperature makes application of the

performance correlation easier in design studies, it also makes the
correlation considerably more dependent on the flow rate of the heat-
transfer fluid. Therefore, every correlation using fluid inlet temperature must
specify the fluid flow rate at which the measurements were made. Deviation
from this flow in a particular design requires that a different correlation be
used. The recommended test flow rate (ASHRAE, 1977) for a liquid
collector is 0.02 kg/hr (14.7 lb/hr ft 2) and for an air collector, 0.01 m 3/s
m2 (1.97 cfm/ft2) at atmospheric pressure.

Because a flat-plate collector can collect both beam (direct) and diffuse
solar radiation, the global (total) solar irradiance is used as the basis for
flat-plate collector performance correlations. In testing the collector, the
aperture irradiance is the global (total) solar irradiance measured in the
plane of the collector, which includes the cosine loss of the beam
component and some ground reflection if the collector is tilted from the
horizontal as is usually the case.

A dilemma arises with the use of this performance correlation because the global
(total) solar irradiance on a tilted aperture It,a is used as input into the correlation. This
value must include ground reflection. Equation (4.6) gives the designer a means of
predicting the global (total) aperture irradiance when the aperture is tilted from the
horizontal by an angle .

The appropriate collector performance correlation for a flat-plate collector is

Equation (5.7) modified for a non-concentrating collector:

(6.1)

where the optical efficiency and the overall heat loss coefficient UL were
discussed in Chapter 5, and the total aperture irradiance It,a in Chapter 4, Equation
4.29. The term FR is the collector heat removal efficiency factor and is dependent on
the absorber plate tube spacing and the flow rate of heat transfer fluid through these
tubes. The analytical development of this factor is described in Duffie and Beckman
(1980).

6.3.3 Typical Performance

Figure 6.4 gives typical performance for a number of different types of flat-
plate collectors. These range from an unglazed absorber as is used for
very low temperature applications to double-glazed, selective-surface
collectors. Also included on this plot is an evacuated tube-type collector,
which is discussed in the following paragraphs.
Figure 6.4 Performance of typical commercial flat-plate solar collectors.

A second abscissa scale has been included on this plot to aid in rapid
interpretation. Since the abscissa is the temperature difference divided by
the total solar irradiance, these parameters may be separated in nomogram
style with a separate abscissa.

Although flat-plate collectors may be operated at fairly high temperatures

when solar irradiance or ambient temperature is high, as solar irradiance or
ambient temperature decreases or the angle of incidence increases, the
efficiency falls. An example is shown in Figure 6.4 for a temperature
difference of 50ºC (90ºF), where the solar irradiance has decreased to 600
W/m2 (190 Btu/hr ft2), and the efficiencies of the first three collectors have
dropped below usable levels. This curve supports the conclusion made
previously that flat-plate collectors are adequate for applications where the
temperature rise above ambient is less than about 50ºC (90ºF). An
exception to this conclusion is the evacuated-tube collector, which appears
to be operable at twice that temperature rise. However, there are few high-
temperature applications of the evacuated-tube collector to confirm their
reliability at these temperatures.
6.3.4 Comparison with Parabolic Troughs

Treadwell (1979) used TMY (Typical Meteorological Year) weather data to compare
the long-term performance of flat-plate collectors with parabolic trough collectors to
determine at what collector operating temperature the parabolic trough collects more
energy than the flat-plate collector on an annual basis.

A field of single glazed flat-plate collectors with selective absorber surfaces

was compared with a field of commercial parabolic trough concentrators.
Both horizontal and latitude-tilt south-facing orientations for the flat-plate
collectors were considered, and both north-south and east-west tracking
axis orientations were considered for the parabolic trough collectors.
Typical Meteorological year (TMY) solar radiation data were used from the
26 original SOLMET sites.

It was found that the amount of diffuse energy collected by flat-plate

collectors in most regions of the contiguous United States was not sufficient
to compensate for the tracking capability of the troughs. Also that the
typically higher optical efficiency of the flat-plate collector compensates
only partially for the higher thermal efficiency of the concentrators. Over a
full year’s operation, the north-south trough orientation and the latitude-tilt
flat-plate orientations provided the most energy. It was also found that for a
71ºC (168ºF) demand, flat-plate collectors outperform troughs only in
Florida and southern Louisiana.

The break-even temperatures for the most efficient configurations (north

south troughs and latitude-tilt flat-plates) are shown in Figure 6.5. This
figure shows that troughs and flat-plate collectors have equivalent
performance at about 49ºC (120ºF) in the southwestern region, whereas
flat -plates and troughs may be considered equivalent in terms of
performance, for applications at 66ºC (150ºF) in most of the southeastern
region.
Figure 6.5 Break-even temperature for comparison between flat-plate collectors tilted at the latitude
angle and parabolic troughs with the tracking axis oriented in the north-south direction. From
Treadwell (1979).

6.4 Applications

In addition to the numerous applications of flat-plate collectors for domestic hot-water

heating and space heating for both homes and business, there are a number of
industrial process heat applications of flat-plate collectors. Table 6.2 summarizes
many of these. The information was extracted from Kutscher et al. (1982) and shows
a wide variety of applications. Note that all the demand temperatures are under
100ºC (212ºF).

The Campbell Soup Company process heat system in Sacramento,

California is the singular example of the use of flat-plate collectors to
preheat water before it enters a field of parabolic trough concentrators. The
field, shown in Figure 6.2, incorporates 371.6 m 2 (4000 ft2) of flat-plate
collectors connected in series to 278.7 m2 (3000 ft2) of parabolic trough
concentrators. The industrial demand for this system is for 91ºC (195ºF) hot
water, which is used in a can washing process.

Table 6.2. Industrial Process Heat Systems in the United States Using
Flat-Plate Collectors
Company Location Process Application Temperature Area

(ºC) (m2)

Hot- Water Systems

Anhauser-Busch, Jacksonville, FL Beer pasteurization 60ª 427

Inc.

Aratex Services, Fresno, CA Heat process water 50 -70 624

Inc.

Berkeley Meat Co. S. Lake Tahoe, Sanitation 82 232

CA

Campbell Soup Co. Sacramento, CA Preheat can wash 91 372

water

Coca-Cola Bottling Jackson, TN Bottle washing NAª 881

Co.

Easco Photo Richmond, VA Film processing 46 NA

General Extrusion, Youngstown, OH Solution heating 71-82ª 409

Inc.

Iris Images Mill Valley, CA Film processing 24-38 59

Jhirmack Redding, CA Preheat boiler water 71-93 622

Enterprises, Inc.

Mary Kay Dallas, TX Sanitizing 60 305

Cosmetics

Riegel Textile Corp. LaFrance, SC Heat dye-beck water 88ª 621

Spicer Clutch Auburn, IN Parts washing 54 87

(Dana)

Hot-Air Systems

Gilroy Foods, Inc. Gilroy, CA Preheat drier air/ 90 553

boiler feedwater

Gold Kist, Inc. Decatur, AL Preheat drier air b 82 1217

LaCour Kiln Canton, MS Lumber drying 82 234

Services

Lamanuzzi & Fresno, CA Raisin drying 62 1951

Pantaleo

ªEvacuated tube type

bWater collector fluid/hot-air demand

6.5 Other Types

6.5.1 Evacuated-Tube Collectors

In an attempt to design higher-performance flat-plate collectors with mass

production capability, collectors are being manufactured that are made
from glass tubes with the enclosed space sealed and evacuated. Called
evacuated-tube collectors, these have very low overall heat loss when
operated at high temperatures. This is because they are essentially single
glazed collectors with the space between the glazing and absorber
evacuated, thereby eliminating convective loss. This leaves re-radiation as
the only major loss mechanism. The results of this low heat loss can be
seen in the exceptional high-temperature, low-solar irradiance performance
of this collector design (Figure 6.4).

Figure 6.6 shows two evacuated-tube designs. In Corning Glass

Company’s design, a flat absorber plate with an attached water tube is
sealed into a glass tube that is then evacuated. Solar energy is absorbed
on the absorber plate and is extracted by water or other heat-transfer fluid
flowing through the attached tube.
Figure 6.6 Designs of evacuated-tube collectors. Tubes are normally 5-10 cm (2-4 in.) in diameter.

In the Owens-Illinois design, concentric glass tubes are used with the inner
tube surface becoming the absorber by coating it with a selective absorbing
coating. The space between these tubes is evacuated. Heat-transfer fluid
flows in through a third, inner, concentric feeder tube and flows out in the
annulus outside the feeder tube in contact with the absorber tube surface.

The evacuated tube collector has the potential for developing high efficiencies at
high temperatures. However, as a result of degradation of elastomeric seals,
differential thermal expansion, and coating limitations, these collectors are limited to
temperatures considerably below their maximum potential at noontime isolation
levels. This means not only that applications cannot be planned for temperatures
beyond these temperature limits, but also that under stagnation conditions (no heat
removal i.e., the zero-efficiency conditions) the collectors can exceed these
limits. System designs must incorporate provisions to circumvent this possibility. An
extensive review of the status of evacuated tube collector technology may be found
in Window and Harding (1984).

6.5.2 Solar Ponds

It appears that the least expensive type of solar collector is a large solar
pond. However, they have been considered primarily for large industrial
applications because their cost decreases considerably with increases in
size. There are two basic types of solar ponds: the shallow pond and the
salt-gradient pond. Both are equivalent to horizontal flat-plate collectors,
since they are non-concentrating, accept both direct and diffuse solar
energy, and are limited to low temperature applications.

Shallow Ponds. A shallow solar pond consists of a group of collectors

made of black plastic liners lying on top of insulation that has been laid on
flat graded ground. Above the water bag is at least one translucent cover
sheet, supported by side curbs. When there is adequate sunshine, water is
pumped into the collectors from an underground storage tank. The water
inside the collectors is heated by solar radiation absorbed by the pond liner
and can attain temperatures of up to 60ºC (140ºF). Once heated, the water
in the collectors may be pumped to an industrial demand or a hot storage
tank for later use. At night or during periods of low solar irradiance, the
water in the collectors may be drained back into the underground tank,
thereby conserving the heat collected. A design guide for shallow ponds
has been prepared by Casamajor and Parsons (1979).

Figure 6.7 shows a shallow pond collector design proposed by Dickenson

et al. (1976) for use in a low-temperature solar thermal power system. The
collector modules are 4 m (14 ft) wide and 200 m (656 ft) long. The width
was determined by the width of commercially available unseamed,
weatherable plastic sheets.

Figure 6.7 A shallow solar pond collector. Modules are 4 m ×200 m (13 ft × 656 ft). From Dickenson
et al. (1976).
Salt-Gradient Ponds. When an open body of water absorbs solar energy,
convection currents are created. As the sunrays that pass through the
surface layer are absorbed in lower layers, this water is heated and rises to
the surface, where heat is transferred away by convection to ambient air.
Once the water cools, the density increases and the surface water moves
downward. This movement of water equalizes the temperature throughout
the body of water.

A salt-gradient solar pond employs a salt concentration gradient to

suppress natural convection. Heated water holds more dissolved salt than
does cooler water. The salty, heated water is also heavier and thus
remains at the bottom of the solar pond. Sunlight penetrating through the
top layers of the pond is absorbed at the bottom and trapped by the non-
convecting gradient layer, which acts as an effective thermal insulator
against convection.

In practice, salt-gradient solar ponds consist of three layers, or zones: (1) a

surface convecting zone of low-salinity water, typically 0.2-0.4 m (8-16 in.)
thick; (2) a non-convecting or salinity-gradient zone beneath the surface
zone, in which salt concentration increases with depth, typically 1.0-1.5 m
(3.3-5 ft) thick; and (3) a storage zone at the bottom of the pond of
uniformly high salt concentration that stores heat and is typically 1-3 m
(3.3-10 ft) thick..

For utilization of the heat stored at the bottom of the pond, hot brine is
drawn from the storage zone (bottom layer) of the pond and pumped
through a heat exchanger and back to the bottom of the storage zone. For
power production applications where a Rankine cycle is used, condenser
cooling water is drawn off the top of the pond and passed through the
condenser and back to the surface, where it cools.

Construction of economical solar ponds requires the availability of

inexpensive, flat land; accessibility to water; and an inexpensive source of
salt or brine. A typical arrangement for this process is shown in Figure 6.8.
Figure 6.8 Electrical power production concept using salt-gradient ponds.

A salt-gradient solar pond can provide heat at temperatures in excess of

90ºC (194ºF). Such a pond provides built-in thermal storage of such large
volume that heat can be collected in the summer and stored for use during
the winter. Pioneering work on salt-gradient solar ponds has been done in
Israel. Tabor (1981) gives an extensive review of the status and problems
in this field [ see also Tabor (1983)]. An extensive bibliography on the
subject has been published in SERI (1981a).

The basic physical principles of salt-gradient ponds were defined by

Weinberger (1964). Rabl and Nielsen (1975) subsequently presented a
model that included a convective bottom layer. A computational procedure
for determination of the thermal efficiency of a pond with a diffusely
reflecting bottom may be found in Hull (1982) and a parametric analysis of
various design considerations in Wang and Akbarzadeh (1983).

References and Bibliography

ASHRAE (1977), "Methods of Testing to Determine the thermal

Performance of Solar Collectors," ASHRAE Standard 93-77, American
Society for Heating, Refrigeration, and Air -Conditioning Engineering, New
York.
Casamajor, A. B., and R. E. Parsons (1979), "Design Guide for Shallow
Solar Ponds," Lawrence Livermore Labs Report UCRL 52385 (Rev. 1),
January.

Dickenson, W. C., A. F. Clark, J. A. Day, and L. F. Wouters (1 976), "The

Shallow Solar Pond Energy Conversion System," Solar Energy 18 (1), 3.

Duffie, J. A., and W. A. Beckman (1980), Solar Engineering of Thermal

Processes, John Wiley & Sons, New York, 1980.

Huil, J. R. (1982), "Calculation of Solar Pond Thermal Efficiency with a

Diffusely Reflecting Bottom," Solar Energy 29 (5), 385.

Kreider, J. F., and F. Kreith (1982), Solar Heating and Cooling, 2nd ed.,
McGraw-Hill, New York.

Kutscher, C. F., R. L. Davenport, D. A. Dougherety, R. C. Gee, P. M.

Masterson, and E. K. May (1982), "Design Approaches for Solar Industrial
Process Heat Systems," SERI Report SERI/ TR-253-1356, August.

Lunde, P. J. (1980), Solar Thermal Engineering, John Wiley & Sons, New
York.

Rabl, A., and C. E. Nielsen (1975), "Solar Ponds for Space Heating," Solar
Energy 17 (1), 1.

SERI (198la), "Solar Ponds: A Selected Bibliography," SERI Report

SERI/TR-752-711, November.

SERI (1981b), "Solar Radiation Energy Resource Atlas of the United

States," SERI Report SERI/SP-642-1037, October.

Smith, J. H. (1980), "Handbook of Solar Energy Data for South-Facing

Surfaces in the United States," Jet Propulsion Laboratory Report DOE/JPL-
1012-25, Vol. 1, January.

Tabor, H. (1981), "Solar Ponds," Solar Energy 27(3), 181.

Tabor, H. (1983), "Solar Ponds-Corrections," letter to the editor, Solar

Energy 30(1).

Treadwell, G. W. (1979), "Low-Temperature Performance Comparisons of

Parabolic -Trough and Flat -Plate Collectors Based on Typical
Meteorological Year Data," Sandia National Labs, Report SAND78 -0965,
February.
Wang, Y. F., and A. Akbarzadeh (1983), "A Parametric Study on Solar
Ponds," Solar Energy 30(6), 555.

Weinberger, H. (1964), "The Physics of the Solar Ponds," Solar Energy

8(2), 45.

Window, B., and G. L. Harding (1984), "Progress in the Materials Science

of All-Glass Evacuated Collectors," Solar Energy 32 (5), 609.