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.

• 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 flatplate 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.

Residential Solar 

C

Heating Collectors 

LEARI

NGHOUSE

ENERGY

EFFICIENCY AND

RENEWABLE

ENERGY

T OF ENERGY

DEPARTMEN

U

E

NITED STAT S OF A

ERICA

M

This document was produced for the U.S. Department of Energy (DOE) by the National Renewable Energy Laboratory (NREL), a DOE national laboratory.

The document was produced by the Information Services Program, under the DOE Office of Energy Efficiency and Renewable Energy. The Energy Efficiency

and Renewable Energy Clearinghouse (EREC) is operated by NCI Information Systems, Inc., for NREL / DOE. The statements contained herein are based on

information known to EREC and NREL at the time of printing. No recommendation or endorsement of any product or service is implied if mentioned by EREC.

Printed with a renewable-source ink on paper containing at least 50% wastepaper, including 20% postconsumer waste

DOE/GO-10096-051

FS 112

March 1996



Flat-Plate Collector

Inlet connection

Enclosure

Flow tubes

Absorber plate

Insulation

Glazing frame

Glazing

Outlet

connection

A typical flat-plate collector is an insulated metal box with a glass

or plastic cover and a dark-colored absorber plate. The flow tubes

can be routed in parallel, as shown here, or in a serpentine pattern.
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 “closedloop” (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 nonmetallic 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 

2

The heart of a solar

energy system is the

solar collector—a

device that absorbs

the sun’s light energy

and changes it into

heat energy. 

Unglazed Solar Collectors

Tube-on-sheet

collector

Serpentine plastic

pipe collector

Pump

flow

Pump flow

Typically

1 1/2" ABS pipe

Metal deck

Unglazed collectors are commonly used to heat 

swimming pool water. No glazing is needed because

the system is used when both outdoor temperatures

and solar radiation levels are high.
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 lessexpensive 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.

3



Solar Air Collector

   





Glazing 

Cool air in

Warm air out

Absorber

Duct

Insulation

An air collector is a simple flat-plate collector used

mainly for space heating. Air flows through the collector

by natural convection or when forced by a fan.

    

Evacuated-Tube Collector

Evacuated tube

Cross section

Inflow

Outflow

Reflector

Glazing

Outer glass tube

Absorbing coating 

Inner glass tube

Fluid tubes

Copper sheet

Evacuated space 

An evacuated-tube collector generally consists of a fluid-filled

absorber tube surrounded by a vacuum.

Because unglazed

transpired air 

collectors require no

glazing or insulation,

they are inexpensive

to manufacture,

resulting in a very

cost-effective source 

of solar heat.
These collectors are more efficient than flatplate 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.

4

Parabolic-Trough Collector

Parabolic reflector

Receiver (absorber) on

focal line of reflector

Tracking mechanism

A parabolic-trough collector can achieve higher temperatures than flat-plate or

evacuated-tube collectors because it focuses sunlight collected over a large area

onto a receiver located along a narrow focal line.

Solar collectors 

can heat air and 

water cost effectively 

for residential 

applications.
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. 

5

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.

   

Batch Solar Collector

Pressure tank with

selective surface

Double or triple

glazing

Cusp reflector

Drain valves

Insulated Pump flow plumbing lines

Insulated collector box
Source List

The following organizations can provide you with more

information about different types of solar collectors.

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 publishes 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 one of the largest and most active alternative energy

centers in the country. 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 rates and certifies the performance of solar equipment.

The SRCC offers a directory of certified solar energy 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 as

well as your local solar energy association for region-specific

information on solar collectors.

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.

6