Heating with the sun's energy

Image of a house with solar collectors  on the roof.

Source: Adapted from National Energy Education Development Project (public domain)

People use solar thermal energy to heat water and air. The two general types of solar heating systems are passive systems and active systems.

Passive solar space heating happens when the sun shines through the windows of a building and warms the interior. Building designs that optimize passive solar heating usually have south-facing windows that allow the sun to shine on solar heat-absorbing walls or floors during the winter. The solar energy heats the building by natural radiation and convection. Window overhangs or shades block the sun from entering the windows during the summer to keep the building cool.

Active solar heating systems use a collector and a fluid that absorbs solar radiation. Fans or pumps circulate air or heat-absorbing liquids through collectors and then transfer the heated fluid directly to a room or to a heat storage system. Active solar water heating systems usually have a tank for storing solar heated water.

Solar collectors are either nonconcentrating or concentrating


Nonconcentrating collectors—The collector area (the area that intercepts the solar radiation) is the same as the absorber area (the area absorbing the radiation). Solar systems for heating water or air usually have nonconcentrating collectors. Flat-plate collectors are the most common type of nonconcentrating collectors for water and space heating in buildings and are used when temperatures lower than 200°F are sufficient.

Flat-plate solar collectors usually have three main components:

  • A flat metal plate that intercepts and absorbs solar energy
  • A transparent cover that allows solar energy to pass through the cover and reduces heat loss from the absorber
  • A layer of insulation on the back of the absorber to reduce heat loss

Solar water heating collectors have metal tubes attached to the absorber. A heat-transfer fluid is pumped through the absorber tubes to remove heat from the absorber and transfer the heat to water in a storage tank. Solar systems for heating swimming pool water in warm climates usually do not have covers or insulation for the absorber, and pool water is circulated from the pool through the collectors and back to the pool.

Solar air heating systems use fans to move air through flat-plate collectors and into the interior of buildings.

Concentrating collectors—The area intercepting the solar radiation is greater, sometimes hundreds of times greater, than the absorber area. The collector focuses, or concentrates, solar energy onto an absorber. The collector usually moves so that it maintains a high degree of concentration on the absorber. Solar thermal power plants use concentrating solar collector systems because they can produce high temperature heat.