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Solar explained Solar thermal collectors

Heating with the sun's energy

People use solar thermal energy for many purposes, including heating water, air, and the interior of buildings and generating electricity. There are two general types of solar heating systems: 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 (in the northern hemisphere) usually have south-facing windows that allow the sun to shine on solar heat-absorbing walls or floors in the building during the winter. The solar energy is absorbed by the building materials and heats the interior of buildings 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 have collectors for heating a fluid (air or a liquid) and fans or pumps to move the fluid through the collectors, where it is heated, to the interior of a building or to a heat storage system, where the heat is released, and back to the collector to be reheated. Active solar water heating systems usually have a tank for storing solar heated water.

Basic components of a solar water heating system

Solar collectors are either nonconcentrating or concentrating

Nonconcentrating collectors—The collector area (the area that intercepts solar radiation) is the same as the absorber area (the area absorbing the solar energy/radiation). Solar energy 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 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 throughout the day 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 the high temperature heat needed to generate electricity.

Last updated: December 3, 2019