An official website of the United States government.

We've made some changes to EPA.gov. If the information you are looking for is not here, you may be able to find it on the EPA Web Archive or the January 19, 2017 Web Snapshot.

Renewable Heating And Cooling

Solar Heating and Cooling Technologies

Can I use solar thermal technology where I live?

Solar thermal technologies can be used anywhere in the United States. However, some regions naturally receive more intense and more reliable solar energy than others, depending on latitude, typical weather patterns, and other factors. The National Renewable Energy Laboratory provides maps that show the solar energy potential where you live.

Solar thermal technologies absorb the heat of the sun and transfer it to useful applications, such as heating buildings or water. There are several major types of solar thermal technologies in use:

In addition to the solar thermal technologies above, technologies such as solar photovoltaic modules can produce electricity, and buildings can be designed to capture passive solar heat.

Solar energy is considered a renewable resource because it is continuously supplied to the Earth by the sun. Visit EPA’s Clean Energy website to learn more about non-thermal solar technologies and the environmental benefits and impacts of solar energy.


Unglazed Solar Collectors

photo of building
An unglazed solar collector on the roof of a pool and fitness center.
Credit: Albert Nunez, NREL 10651

An unglazed solar collector is one of the simplest forms of solar thermal technology. A heat-conducting material, usually a dark metal or plastic, absorbs sunlight and transfers the energy to a fluid passing through or behind the heat-conducting surface. The process is similar to how a garden hose, laying out in the open, will absorb the sun’s energy and heat the water inside the hose.

These collectors are described as “unglazed” because they do not have a glass covering or “glazing” on the collector box to trap heat. The lack of glazing creates a trade-off. Unglazed solar collectors are simple and inexpensive, but without a way to trap heat, they lose heat back to the environment and they operate at relatively low temperatures. Thus, unglazed collectors typically work best with small to moderate heating applications or as a complement to traditional heating systems, where they can reduce fuel burdens by pre-heating water or air.

Solar pool heating collectors are the most commonly used unglazed solar technology in the United States. These devices often use black plastic tubular panels mounted on a roof or other support structure. A water pump circulates pool water directly through the tubular panels, then returns the water to the pool at a higher temperature. Although used primarily for pool heating, these collectors can also pre-heat large volumes of water for other commercial and industrial applications.

How It Works

Diagram showing an unglazed solar collector. Components are labeled with numbers that match the text.
  1. Sunlight: Sunlight hits the dark material in the collector, which heats up.
  2. Circulation: Cool fluid (water) or air circulates through the collector, absorbing heat.
  3. Use: The warmer fluid is used for applications such as pool heating.

Learn More About Unglazed Solar Collectors

Top of Page

Transpired Solar Air Collectors

 photo of warehouse
The south-facing wall of this warehouse is a transpired solar collector.
Credit: DOE Office of Energy Efficiency and Renewable Energy

Transpired solar air collectors typically consist of a dark-colored, perforated metal cladding material mounted on an existing wall on the south side of a building. A fan pulls outside air through the perforations and into the space behind the metal cladding, where the air heats to as much as 30°F-100°F above the ambient air temperature. The fan then pulls the air into the building, where it is distributed through the building’s ventilation system.

The transpired solar collector is a proven but still emerging solar heating technology. This type of technology is best for heating air and ventilating indoor spaces. It can also be applied to several manufacturing and agricultural applications, such as crop drying.

How it works

Diagram showing a transpired solar collector. Components are labeled with numbers that match the text.
  1. Sunlight: Sunlight hits the dark perforated metal cladding, which heats up.
  2. Circulation: A circulation fan pulls air through the perforations behind the metal cladding, heating the air, which is then pulled into the building for distribution.

Learn More About Transpired Solar Air Collectors

Potential Applications

Technical Resources

Top of Page

Flat-Plate Solar Collectors

photo of solar panels
An array of flat-plate solar collectors on the roof of a school.
Credit: Joe Ryan, NREL 19690

Most flat-plate collectors consist of copper tubing and other heat-absorbing materials inside an insulated frame or housing, covered with clear glazing (glass). The-heat absorbing materials may have a special coating that absorbs heat more effectively than an uncoated surface.

Glazed flat-plate collectors can operate efficiently at a wider temperature range than unglazed collectors. Flat-plate collectors are often used to complement traditional water boilers, pre-heating water to reduce fuel demand. They can also be effective for space heating. Using a heat exchange system, they can reliably produce hot air for large buildings during daylight hours.

How It Works

Diagram showing a flat-plate solar collector. Components are labeled with numbers that match the text.
  1. Sunlight: Sunlight travels through the glass and hits the dark material inside the collector, which heats up.
  2. Heat reflection: A clear glass or plastic casing traps heat that would otherwise radiate out. This is similar to the way a greenhouse traps heat inside.
  3. Circulation: Cold water or another fluid circulates through the collector, absorbing heat.

Learn More About Flat-Plate Solar Collectors

Top of Page

Evacuated Tube Solar Collectors

 photo of collector
An evacuated tube solar collector on a roof.
Credit: NREL PIX 09501

Evacuated tube collectors feature thin, copper tubes filled with a fluid, such as water, housed inside larger vacuum-sealed clear glass or plastic tubes.

Evacuated tubes use the sun’s energy more efficiently and can produce higher temperatures than flat-plate collectors for a few reasons. First, the tube design increases the surface area available to the sun, efficiently absorbing direct sunlight from many different angles. Second, the tubes also have a partial vacuum within the clear glass enclosure, which significantly reduces heat loss to the outside environment.

How It Works

Diagram showing a parabolic trough, which is one example of a concentrating solar collector. Components are labeled with numbers that match the text.
  1. Sunlight: Sunlight hits a dark cylinder, efficiently heating it from any angle.
  2. Heat reflection: A clear glass or plastic casing traps heat that would otherwise radiate out. This is similar to the way a greenhouse traps heat inside.
  3. Convection: A copper tube running through each cylinder absorbs the cylinder’s stored heat, causing fluid inside the tube to heat up and rise to the top of the cylinder.
  4. Circulation: Cold water circulates through the tops of the cylinders, absorbing heat.

Evacuated tube systems are typically more expensive than flat-plate collectors, but they are more efficient and can produce higher temperatures. Evacuated tubes can reliably produce very hot water for batch or on-demand water heating and for many industrial processes, and they can produce enough heat to handle almost any space heating or space cooling application.

Learn More About Evacuated Tube Solar Collectors

Top of Page

Concentrating Solar Systems

 photo of solar collectors
This rooftop array of parabolic trough concentrating solar collectors provides process heat for a winery. These collectors have a unique design that allows them to produce electricity in addition to heat.
Credit: SunWater Solar

Concentrating solar systems work by reflecting and directing solar energy from a large area onto a small one. Smaller, reflective bowl-shaped arrays can produce water at a few hundred degrees for industrial or agricultural processes or for heating large volumes of water, such as resort swimming pools. Some arrays work with long parabolic troughs that concentrate sunlight onto a pipe running the length of the trough, which carries a heat transfer fluid. Even larger systems use fields of mirrors to reflect sunlight onto a central tower. These types of arrays produce high-pressure steam or other superheated fluids for a range of activities, from heat-intensive chemical processing to electric power generation.

How It Works

Diagram showing a parabolic trough, which is one example of a concentrating solar collector. Components are labeled with numbers that match the text.
  1. Sunlight: Sunlight hits a reflective material (i.e., a mirrored surface), usually in the shape of a trough (shown here) or a dish.
  2. Solar reflection: The reflective material redirects the sunlight onto to a single point (for a dish) or a pipe (for a trough).
  3. Circulation: Cold water or a special heat transfer fluid circulates through the pipe, absorbing heat.

Concentrating systems are capable of producing enormously hot fluids for a variety of processes, and they can produce a relatively large amount of energy for each dollar invested. However, these systems tend to be much larger and more complex than the other types of solar collectors described above, with a higher total price tag. Thus, concentrating solar technology tends to be most effective for large-scale, high-temperature uses, although lower-temperature uses may still be cost-effective under certain circumstances.

Learn More About Concentrating Solar Systems

Potential Applications

Key End Use Sectors

Technical Resources

Top of Page