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)

Solar thermal energy can be used to heat water or air. It is most often used for heating water in buildings and in swimming pools. Solar thermal energy is also used to heat the insides of buildings. Solar heating systems can be classified as passive or active.

Passive solar space heating happens in a car on a hot summer day. The sun's rays pass through the windows and heat up the inside of the car. In passive solar heated buildings, air is circulated past a solar heat-absorbing surface and through the building by natural convection. No mechanical equipment is used for passive solar heating.

Active solar heating systems use a collector and a fluid to collect and absorb 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 include a tank for storing water heated by the system.

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. Nonconcentrating collectors are often used for heating water or air for space heating in buildings and in swimming pools.

There are many flat-plate collector designs, but they generally have four specific components:

  • A flat-plate absorber that intercepts and absorbs the solar energy
  • A transparent cover that allows solar energy to pass through 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.