Solar thermal is a technology designed to harness sunlight for its thermal energy (heat). This heat is often used for heating water used in homes, businesses, swimming pools, and for heating the insides of buildings (space heating).
In order to heat water using sunlight, a solar thermal collector heats a fluid that is pumped through it. As the fluid is pumped through the collector, the fluid becomes heated. The now heated fluid then is pumped out of the collector and through a heat exchanger.
Heat exchangers are usually comprised of copper, and are normally found inside a solar storage tank. This allows the heat in the fluid to be transmitted – or exchanged, hence the name - into the water in the storage tank.
The storage tank is an essential element of any solar thermal system, as it allows all the heat being generated by the solar thermal collector to be stored for use whenever it is needed.
Solar thermal collectors are classified by the Energy Information Administration (EIA) as high, medium, or low temperature collectors.
High temperature collectors – also referred to as concentrating collectors - uses mirrors and/or lenses to concentrate sunlight to achieve very high temperatures (750F to 1000F). This technique is called Concentrated Solar Power, or CSP. These high temperatures are used in large scale power production, usually to turn steam turbines.
Parabolic Trough - Parabolic trough power plants use a curved mirror to reflect sunlight onto a central focal point – usually a glass tube containing a heat transfer fluid. This tube runs the length of the trough, positioned at the focal point of the mirrors in order to collect large amounts of thermal energy.
This technology is in use and in full operation all over the world. In California, the SEGS system uses this technology in 9 different power plants to generate over 350mW of power. The Nevada Solar One plant also uses this type of collector with a capacity of 64MW.
Power Tower – also known as central tower power plants – generate large amounts of thermal energy by using thousands of mirrors with tracking systems to constantly capture and focus the sun's thermal energy on a central focal tower. Within the tower the concentrated sunlight heats a transfer medium – usually molten salt – to over 1000F. This molten salt then flows into a large storage tank, where the energy is stored, and eventually pumped to a steam generator. The steam generator then produces electricity.
Solar Two was one such power plant to make use of this technology, and many other power plants are currently under construction around the world using this technology.
A solar dish system uses a large, reflective parabolic dish to focus sunlight up to a single focal point. At this focal point, a receiver captures the thermal energy and transforms it into electricity, either by using a steam engine or a Stirling engine.
This system is used due to the high temperatures it can achieve because of the high concentration of light. Higher temperatures allow for a better conversion of electricity.
This technology is currently being used to produce large amounts of electricity in California by both Southern California Edison and San Diego Gas & Electric, for a combined capacity of over 750MW of power.
Medium temperature collectors often refer to solar water heating systems in the form of flat plates or evacuated tube collectors. These collectors are use to collect, store, utilize heat for domestic hot water (such as for showers, laundry, or process applications, among others), for space heating, and for space cooling (using various types of thermally driven cooling systems, such as an absorption chiller.
Evacuated tube collectors (ETC) are very efficient at retaining a large percentage of the heat collected from the sun. Each tube works independently from the others, and is surrounded by a double wall tube of glass. In between the double walls is a deep vacuum, producing sa "thermos" effect which greatly increases it's thermal insulation. This design allows the sunlight to pass through the glass, but allows very little heat to escape.
In many evacuated tube collectors, such as the SPP-30A, heat pipe technology is used. A copper heat pipe is inside the evacuated tube, held in place by thin pieces of metal, called heat transfer fins. The heat pipe is also under a vacuum, which allows the water inside to boil at a much lower temperature.
As the water boils, the vapor rises to the top of the heat pipe, which sits inside the collector's header. Water, or heat transfer fluid (usually a water/glycol mixture) passes through the header, where it comes in contact with the tops of the heat pipes, thus heating up rapidly. It then passes through a heat exchanger, usually part of a storage tank, where the heat is stored for immediate or future use.
Flat plate collectors are generally comprised of a series of copper pipes in a very well insulated glass box. Sunlight strikes the glass, and the heat is trapped and held inside by heavy insulation. As water or a heat transfer fluid is passed through the collector, the heat trapped from the sun is transferred into the fluid.
This fluid is then heated, and circulated back through a heat exchanger, where the heat is stored for immediate or later use in domestic hot water or space heating systems.
Low temperature collectors generally refer to unglazed or uninsulated flat panels for pool heating. These collectors are largely dependent on direct sunlight and warm weather conditions to work efficiently.
Solar thermal collectors used for pool heating are often made of PVC or other plastic composites. Pool water is generally circulated directly through these pool panels using the existing pool filter. Occasionally, an additional "booster pump" may be needed, especially on larger commercial systems.
A differential controller is often used to divert the pool water when conditions are warm and favorable, and to keep pool water from entering the collectors when the temperature falls, such as at night or during unfavorable weather or conditions.