Summary: This fact sheet describes the different types of solar collectors used for residences. It also briefly covers the solar heating systems for which they are best suited. To view illustrations, you can download the PDF version (PDF 160 KB) of this fact sheet (Download Acrobat Reader). See Related Links below for more publications and resources on related topics, which aren't included in the PDF version.
Solar collectors are the heart of most solar energy systems. The collector absorbs the sun's light energy and changes it into heat energy. Solar collectors heat a fluid, either air or liquid. This fluid then is used to heat—directly or indirectly—the following.
- Water for household use
- Indoor spaces
- Water for swimming pools
- Water or air for commercial use
- Air to regenerate desiccant (drying) material in a desiccant cooling system.
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 more information, see Solar Water Heating.
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 them (see "Source List" below).
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.
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)
E-mail: ases@ases.org
ASES is a nonprofit educational organization founded in 1954 to
encourage the use of solar energy technologies. ASES publishes
a bimonthly magazine, Solar Today, and offers a variety of solar
publications through its catalogue.
Florida Solar Energy Center (FSEC)
E-mail: info@fsec.ucf.edu
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)
E-mail: Solarsklar@aol.com
SEIA provides lists of solar-equipment manufacturers and dealers.
Solar Rating & Certification Corporation (SRCC)
E-mail: srcc@fsec.ucf.edu
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.
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 Home Energy Magazine is a source of information on reducing energy consumption.
Solar Today Solar Today covers all the solar technologies, both mature and emerging, in a general-interest format. Each issue includes a solar building case study.
Related Links
Here's more information on solar energy systems:
Solar Space Heating and Cooling Fact Sheets
Solar Space Heating and Cooling Resource and Reading Lists
Solar Water Heating Fact Sheets
Solar Water Heating Resource and Reading Lists
This fact sheet was produced for the U.S. Department of Energy (DOE) by the National Renewable Energy Laboratory (NREL), a DOE national laboratory.
DOE/GO-10096-051
FS 112
March 1996
Updated February 2000
NOTICE
This report was prepared as an account of work sponsored by an agency of the United States government. Neither the United States government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States government or any agency thereof.
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