Heating with the sun's energy

Image of a house with solar cells 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 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). Flat-plate collectors are the most common type of nonconcentrating collectors and are used when temperatures lower than 200°F are sufficient. Solar systems for heating water or air usually have nonconcentrating collectors.

Flat-plate solar collectors have four main components:

  • A flat-plate absorber that intercepts and absorbs the solar energy
  • A transparent cover that allows solar energy to pass through the cover but reduces heat loss from the absorber
  • A heat-transport fluid (air or liquid) flowing through tubes to remove heat from the absorber
  • A layer of insulation on the back of the absorber

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.