Residential Solar
Heating Collectors
Solar collectors are the heart of most
solar energy systems. The collector absorbs the sun's light energy and changes
it into heat energy. This publication describes the different types of solar
collectors used for residences. It also briefly covers the solar heating systems
for which they are best suited. For more detailed information on these different
solar heating systems, contact The Energy Efficiency and Renewable Energy
Clearinghouse (EREC—see the Source List at the end of this publication).
Solar collectors heat a fluid, either air
or liquid. This fluid then is used to heat—directly or indirectly—the
following.
There are several types of solar
collectors used for residences. These are flat-plate, evacuated-tube, and
concentrating collectors.
Flat-Plate Collectors
Flat-plate collectors are the most
common collector for residential water-heating and space-heating
installations. A typical flat-plate collector is an insulated metal box with a
glass or plastic cover—called the glazing—and a dark-colored absorber
plate. The glazing can be transparent or translucent. Translucent
(transmitting light only), low-iron glass is a common glazing material for
flat-plate collectors because low-iron glass transmits a high percentage of
the total available solar energy. The glazing allows the light to strike the
absorber plate but reduces the amount of heat that can escape. The sides and
bottom of the collector are usually insulated, further minimizing heat loss.
The absorber plate is usually black
because dark colors absorb more solar energy than light colors. Sunlight
passes through the glazing and strikes the absorber plate, which heats up,
changing solar radiation into heat energy. The heat is transferred to the air
or liquid passing through the collector. Absorber plates are commonly covered
with "selective coatings," which retain the absorbed sunlight better
and are more durable than ordinary black paint.
Absorber plates are often made of metal-
usually copper or aluminum—because they are both good heat conductors.
Copper is more expensive, but is a better conductor and is less prone to
corrosion than aluminum.
Flat-plate collectors fall into two
basic categories: liquid and air. And both types can be either glazed or
unglazed.
Liquid Collectors
In a liquid collector, solar energy
heats a liquid as it flows through tubes in or adjacent to the absorber plate.
For this type of collector, the flow tubes are attached to the absorber plate
so the heat absorbed by the absorber plate is readily conducted to the liquid.
The flow tubes can be routed in
parallel, using inlet and outlet headers, or in a serpentine pattern. A
serpentine pattern eliminates the possibility of header leaks and ensures
uniform flow. A serpentine pattern is not appropriate, however, for systems
that must drain for freeze protection because the curved flow passages will
not drain completely.
The simplest liquid systems use potable
household water, which is heated as it passes directly through the collector
and then flows to the house to be used for bathing, laundry, etc. This design
is known as an "open-loop" (or "direct") system. In areas
where freezing temperatures are common, however, liquid collectors must either
drain the water when the temperature drops or use an antifreeze type of
heat-transfer fluid.
In systems with heat-transfer fluids,
the transfer fluid absorbs heat from the collector and then passes through a
heat exchanger. The heat exchanger, which generally is in the water storage
tank inside the house, transfers heat to the water. Such designs are called
"closed-loop" (or "indirect") systems. For information on
solar water-heating systems, contact EREC.
Glazed liquid collectors are used for
heating household water and sometimes for space heating. Unglazed liquid
collectors are commonly used to heat water for swimming pools. Because these
collectors need not withstand high temperatures, they can use less expensive
materials such as plastic or rubber. They also do not require freeze-proofing
because swimming pools are generally used only in warm weather.
Air Collectors
Air collectors are simple, flat-plate
collectors used primarily for space heating. The absorber plates in air
collectors can be metal sheets, layers of screen, or non-metallic materials.
The air flows past the absorber by natural convection or when forced by a fan.
Because air conducts heat much less readily than liquid does, less heat is
transferred between the air and the absorber than in a liquid collector.
In some solar air-heating systems, fins
or corrugations on the absorber are used to increase air turbulence and
improve heat transfer. The disadvantage of this strategy is that it can also
increase the amount of power needed for fans and, thus, increase the costs of
operating the system. In colder climates, the air is routed between the
absorber plate and the back insulation to reduce heat loss through the
glazing. However, if the air will not be heated more than 30°F (17°C) above
the outdoor temperature, the air can flow on both sides of the absorber plate
without sacrificing efficiency.
Air systems have the advantage of
eliminating the freezing and boiling problems associated with liquid systems.
Although leaks are harder to detect and plug in an air system, they are also
less troublesome than leaks in a liquid system. Air systems can often use
less-expensive materials, such as plastic glazing, because their operating
temperatures are usually lower than those of liquid collectors.
Evacuated-Tube Collectors
Evacuated-tube collectors heat water in
residential applications that require higher temperatures. In an
evacuated-tube collector, sunlight enters through the outer glass tube,
strikes the absorber tube, and changes to heat. The heat is transferred to the
liquid flowing through the absorber tube. The collector consists of rows of
parallel transparent glass tubes, each of which contains an absorber tube (in
place of the absorber plate in a flat-plate collector) covered with a
selective coating. Evacuated-tube collectors are modular—tubes can be added
or removed as hot-water needs change.
When evacuated tubes are manufactured,
air is evacuated from the space between the two tubes, forming a vacuum.
Conductive and convective heat losses are eliminated because there is no air
to conduct heat or to circulate and cause convective losses. There can still
be some radiant heat loss (heat energy will move through space from a warmer
to a cooler surface, even across a vacuum). However, this loss is small and of
little consequence compared with the amount of heat transferred to the liquid
in the absorber tube.
Evacuated-tube collectors are available
in a number of designs. Some use a third glass tube inside the absorber tube
or other configurations of heat-transfer fins and fluid tubes. One
commercially available evacuated-tube collector stores 5 gallons (19 liters)
of water in each tube, eliminating the need for a separate solar storage tank.
Reflectors placed behind the evacuated tubes can help to focus additional
sunlight on the collector.
These collectors are more efficient than
flat-plate collectors for a couple of reasons. First, they perform well in
both direct and diffuse solar radiation. This characteristic, combined with
the fact that the vacuum minimizes heat losses to the outdoors, makes these
collectors particularly useful in areas with cold, cloudy winters. Second,
because of the circular shape of the evacuated tube, sunlight is perpendicular
to the absorber for most of the day. For comparison, in a flat-plate collector
that is in a fixed position, the sun is only perpendicular to the collector at
noon. While evacuated-tube collectors achieve both higher temperatures and
higher efficiencies than flat-plate collectors, they are also more expensive.
Concentrating Collectors
Concentrating collectors use mirrored
surfaces to concentrate the sun's energy on an absorber called a receiver.
Concentrating collectors also achieve high temperatures, but unlike
evacuated-tube collectors, they can do so only when direct sunlight is
available. The mirrored surface focuses sunlight collected over a large area
onto a smaller absorber area to achieve high temperatures. Some designs
concentrate solar energy onto a focal point, while others concentrate the
sun's rays along a thin line called the focal line. The receiver is located at
the focal point or along the focal line. A heat-transfer fluid flows through
the receiver and absorbs heat.
These collectors reach much higher
temperatures than flat-plate collectors. However, concentrators can only focus
direct solar radiation, with the result being that their performance is poor
on hazy or cloudy days. Concentrators are most practical in areas of high
insolation (exposure to the sun's rays), such as those close to the equator
and in the desert southwest United States.
Concentrators perform best when pointed
directly at the sun. To do this, these systems use tracking mechanisms to move
the collectors during the day to keep them focused on the sun. Single-axis
trackers move east to west; dual-axis trackers move east and west and north
and south (to follow the sun throughout the year). In addition to these
mechanical trackers, there are passive trackers that use freon to supply the
movement. While not widely used, they do provide a low-maintenance alternative
to mechanical systems.
Concentrators are used mostly in
commercial applications because they are expensive and because the trackers
need frequent maintenance. Some residential solar energy systems use
parabolic-trough concentrating systems. These installations can provide hot
water, space heating, and water purification. Most residential systems use
single-axis trackers, which are less expensive and simpler than dual-axis
trackers.
Technological Improvements
The efficiency of solar heating systems
and collectors has improved from the early 1970s and costs have dropped
somewhat. The efficiencies can be attributed to the use of low-iron, tempered
glass for glazing (low-iron glass allows the transmission of more solar energy
than conventional glass), improved insulation, and the development of durable
selective coatings.
Also, a new solar air collector,
formerly used primarily for commercial buildings, is now available for homes.
Called a transpired collector, it eliminates the cost of the glazing, the
metal box, and the insulation. This collector is made of black, perforated
metal. The sun heats the metal, and a fan pulls air through the holes in the
metal, which heats the air. For residential installations, these collectors
are available in 8-foot by 2.5-foot (2.4-meter by 0.8-meter) panels capable of
heating 40 cubic feet per minute (0.002 cubic meters per second) of outside
air. On a sunny winter day, the panel can produce temperatures up to 50°F
(28°C) higher than the outdoor air temperature. Transpired air collectors not
only heat air, but also improve indoor air quality by directly preheating
fresh outdoor air.
These collectors have achieved very high
efficiencies—more than 70% in some commercial applications. Plus, because
the collectors require no glazing or insulation, they are inexpensive to
manufacture. All these factors make transpired air collectors a very
cost-effective source of solar heat.
There are other prototype cooling
systems operating today. Some use heat from solar collectors for absorption
cooling. Others are being used to renew the desiccant material in desiccant
cooling systems. Desiccants, such as silica gel, naturally attract moisture.
They are used to reduce humidity and the resulting cooling loads in hot, humid
climates.
Collector Performance Ratings
When you are shopping for solar
collectors, you can compare their performance. Look for a Solar Rating &
Certification Corporation (SRCC) or Florida Solar Energy Center (FSEC) sticker
on the equipment you are considering to check their comparative performance
ratings. For more information on SRCC's or FSEC's performance standards,
contact EREC.
A Bright Future
Solar collectors can be used for nearly
any process that requires heat. As environmental laws become stricter and the
price of conventional power increases, it is likely that solar collectors will
be integrated into many applications.
Low-Tech Solar Collectors
Several inexpensive,
"low-tech" solar collectors with specific functions are also
available commercially. Batch heaters are simple, effective solar water
heaters; solar box cookers are used for cooking and for purifying water; and
solar stills produce inexpensive distilled water from virtually any water
source.
Batch heaters, also known as
"breadbox" or integrated collector systems, use one or more black
tanks filled with water and placed in an insulated, glazed box. Some boxes
include reflectors to increase the solar radiation. Solar energy passes
through the glazing and heats the water in the tanks. These devices are
inexpensive solar water heaters but must be drained or protected from freezing
when temperatures drop below freezing.
A batch heater is a simple solar water
heater that uses one or more black tanks filled with water and placed in an
insulated, glazed box.
Solar box cookers are inexpensive to buy
and easy to build and use. They consist of a roomy, insulated box lined with
reflective material, covered with glazing, and fitted with an external
reflector. Black cooking pots serve as absorbers, heating up more quickly than
shiny aluminum or stainless steel cookware. Box cookers can also be used to
kill bacteria in water if the temperature can reach the boiling point.
Solar stills provide inexpensive
distilled water from even salty or badly contaminated water. They work on the
principle that water in an open container will evaporate. A solar still uses
solar energy to speed up the evaporation process. The stills consist of an
insulated, dark-colored container covered with glazing that is tilted so the
condensing fresh water can trickle into a collection trough. A small solar
still, which is about the size of your kitchen stove, can produce two gallons
of distilled water on a sunny day.
Source List
The following organizations can provide
you with information to help you find the solar water heater that is right for
you.
American Solar Energy Society (ASES)
2400 Central Avenue, Unit G-1
Boulder, CO 80301
(303) 443-3130
Fax: (303) 443-3212
ASES is a nonprofit educational
organization founded in 1954 to encourage the use of solar energy technologies.
ASES pub-lishes a bimonthly magazine, Solar Today, and offers a variety of solar
publications through its catalogue.
Florida Solar Energy Center (FSEC)
1679 Clearlake Road
Cocoa, FL 32922-5703
(407) 638-1000
Fax: (407) 638-1010
FSEC is an alternative energy center. The
FSEC staff conducts research on a range of solar technologies, offers solar
energy workshops, and distributes many free publications to the public.
Solar Energy Industries Association (SEIA)
122 C Street, NW, 4th Floor
Washington, DC 20001
(202) 383-2600
Fax: (202) 383-2670
SEIA provides lists of solar-equipment
manufacturers and dealers and publishes a magazine called the Solar Industry
Journal.
Solar Rating & Certification
Corporation (SRCC)
122 C Street, NW, 4th Floor
Washington, DC 20001
(202) 383-2570
SRCC publishes the thermal-performance
ratings of solar energy equipment. The SRCC offers a directory of certified
solar systems and collectors as well as a document (OG-300-91) that details the
operating guidelines and minimum standards for certifying solar hot-water
systems.
For information about many kinds of
energy efficiency and renewable energy topics, contact:
The Energy Efficiency and Renewable
Energy Clearinghouse (EREC)
P.O. Box 3048
Merrifield, VA 22116
(800) DOE-EREC (363-3732)
Fax: (703) 893-0400
Email: doe.erec@nciinc.com
EREC provides free general and technical
information to the public on the many topics and technologies pertaining to
energy efficiency and renewable energy.
You may also contact your state and
local energy offices for region-specific information on solar water heaters.
Reading List
The following publications provide further
information about solar collectors. The list is not exhaustive, nor does the
mention of any publication constitute a recommendation or endorsement.
Books, Pamphlets, and Reports
Consumer Guide to Solar Energy, S.
Sklar and K. Sheinkopf, Bonus Books, Inc., 160 East Illinois Street, Chicago, IL
60611, 1991.
The Fuel Savers, B. Anderson,
Morning Sun Press, Lafayette, CA, 1991.
The New Solar Home Book, B.
Anderson and M. Riorden, Brick House, Amherst, NH, 1987.
Periodicals
Home Energy Magazine, 2124
Kittredge Street, No. 95, Berkeley, CA 94704-9942, (510) 524-5405. Home Energy
Magazine is a source of information on reducing energy consumption.
Solar Industry Journal, Solar
Energy Industries Association, 122 C Street, NW, 4th Floor, Washington, DC
20001. Solar Industry Journal has information on commercializing new
technologies, case studies of commercially available technologies, and articles
on government policies and regulations that affect businesses in the renewable-
energy industry.
Solar Today, 2400 Central Avenue,
Unit G-1, Boulder, CO 80301. Solar Today covers all the solar technologies, both
mature and emerging, in a general-interest format. Each issue includes a solar
building case study.
U.S. Department of Energy
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