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solar water heater, A. Jonathan Buhalis
Solar Thermal Energy


Solar power may naturally be thought of as a source of electricity, but solar heating goes back to antiquity. Indeed, solar heating warms the entire earth, and humans only modify this slightly. Here are some of the more common solar heating, or more generally solar thermal, systems:

Passive building heating traps sunlight between glass and a dark surface. A large fraction of the energy used in cities goes toward heating and cooling buildings. A building can mitigate this by using sunlight to warm the interior air directly or warm a water tank on the roof.
A heat absorber behind glass takes advantage of the greenhouse effect. The heat collector will be warmed by the sun's visible light, but heat radiated back bounces off the glass. This arrangement also prevents convection and limits conduction as means of heat loss.

Swimming pool heating works on the same principle. The pool water is directed through dark pipes heated by sunlight. Such a system can also serve as a building's water heater.

Air conditioning uses a solar heat source to drive a heat pump much like a refrigerator.

trough solar concentrator, A. Jonathan BuhalisConcentrated solar power is a system that uses mirrors to focus light from over a large area. A concentrated system can generate very high temperatures, typically to run an electrical generator. A concentrator has to track the sun over the course of the day. A system with reflective troughs focused on fluid pipes (right) can swivel east to west over the course of the day without accounting for seasons. A dish relector (below) has to track the sun's seasonal north-south movement as well as its daily motion.

Suncatcher solar collection dish, A. Jonathan Buhalis

Concentrated systems are typically large installations that generate power for many households.

The many solar power designs vary in size and purpose, but they all share most or all of the following aspects aspects:
  • collection
  • transfer
  • exchange
  • storage
  • transport
  • insulation

Collection is the process of heating a mass exposed to the sun. Water is an excellent choice of collector because it has a high heat capacity. This water may be held in a pool or flow through pipes targeted by concentrating mirrors. Other systems heat building walls or other dark solids.

Transfer takes the collected heat away from the collection point so that it can be heated again. The two common methods are convection and heat exchange. Convection in water or another liquid causes the heated liquid to expand and rise. In a properly designed system, the hot water will flow toward where it is stored and used. Colder water will flow in behind to be heated in turn. Simple air-heating systems are also often designed this way, and the air may circulate without the need of fans.

heat exchanger, A. Jonathan BuhalisA heat exchanger (right) intertwines the heated mass with a similar cool one. This might be a radiator warmed by hot air, but the most typical example is two loops of water pipes that touch. The first loop of pipe brings hot water in from the collector and cool water out. The second loop of water pipe brings cool water and hot water out to where it will be used.

Storage is not needed in all systems. Heat can be stored in a tank of water or a pile of bricks so that the sun's daytime heat can be called on at night.

Transport moves the heat within the system from the transfer point to storage or to living areas. Water pipes and air ducts are common methods.

Insulation makes the system efficient. Good insulation allows storage to last and prevents heat from leaking out of transport pipes before it can be used.

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Content by Jonathan Buhalis
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