Solar collector

A solar collector is a device for extracting the energy of the sun directly into a more usable or storable form. The energy in sunlight is in the form of electromagnetic radiation from the infrared (long wavelength) to the ultraviolet (short) wavelengths. The solar energy striking the earth's surface at any one time depends on weather conditions as well as location and orientation of the surface, but overall, it averages about 1000 watts per square meter on a clear day with the surface facing the sun.

A typical solar thermal collector uses water as the storage medium, because it has a high thermal capacity and is convenient to handle. The direct radiation is captured using a black-painted (or suitable coated) surface which absorbs the radiation and conducts it to the storage medium. Metal makes a good conductor, and copper or aluminium is best. By coating the surface with black paint or other suitable coatings, the surface has properties close to that of a black-body radiator, which will readily absorb solar radiation. Special "selective surface" materials can be chosen to reduce heat-loss due to emission in conjunction with using a glass plate cover which also helps reduce losses due to infra-red (long wavelength) emisssions.

As it heats up, the collector will itself start to radiate more heat back into space, which reduces its efficiency. Most of this emitted radiation is of longer wavelength (infra-red).This is countered in two ways. First, a glass plate is placed above the collector plate which will trap the radiated heat within the airspace below it. This exploits the so-called greenhouse effect, which is in this case a property of the glass - it readily transmits solar radiation in the visible and ultraviolet spectrum, but does not transmit the lower frequency infra-red re-radiation very well. The glass plate also traps air in the space, thus reducing heat losses by convection. The plate is also insulated below to prevent losses by radiation and conduction to whatever is below the collector. The second way efficiency is improved is by cooling the collector plate - this is readily done by ensuring that the water is circulated sufficiently quickly through it - the water carries away the absorbed heat so cooling the plate. The warmer water is either directly stored, or else passes through a heat exchanger to warm another tank of water, or used to heat a building directly. The temperature differential across an efficient solar collector is usually only 10 or 20 degrees C - a large differential may seem impressive, but is in fact an indication of a less efficient design.

The two main types of solar collector system are Thermosyphon and pumped. In the thermosyphon system, the storage tank is placed above the collector. As the water in the collector is heated, it will rise and naturally start to circulate to the tank. This draws in colder water from the bottom of the tank. This system is self regulating and requires no moving parts or external energy, so is very attractive. Its main drawback is the need for the tank to be above the collector, which may prove to be physically difficult. A pumped system uses a pump to circulate the water, so the tank can be below the collector. This system requires external energy to run the pump (though this can be solar, since the water should only be circulated when there is incident sunlight). It also requires control electronics to measure the temperature gradient across the collector and modulate the pump accordingly.

Solar collectors can be mounted on a roof but need to face the sun, so a north facing roof in the southern hemisphere, and a south-facing roof in the northern hemisphere are ideal. Collectors are usually also angled to suit the latitude of the location. Where sunshine is readily available, a 6-10 square metre array will provide all the hot water heating required for a typical family house. Such systems are a key feature of sustainable housing, since water and space heating is usually the largest single consumer of energy in households.

Related sites:

• Build your own solar water heater (http://www.ata.org.au/articles/63shw.htm)

• Solar Collector Plans, the Greenhouse Effect and Solar Panels (http://www.jc-solarhomes.com/solar_collector_plans.htm)

• Build a Hot Tub (http://www.hotspringsenthusiast.com/Tub.htm)

• DIY Solar Panel (http://www.bigginhill.co.uk/solar.htm)

• Solar selective coating (http://www.finishing.com/231/42.html)

• Radiant barriers, reflective insulation, low e coatings, selective surfaces, reflective roof coatings (http://www.solec.org/)

• Solar Thermal Selective Coatings (http://www.aetsolar.com/Solar_Products_Services/clear_selective_coatings.htm)

• ClearDome Solar Thermal (http://www.cleardomesolar.com/)

• Solar Energy and Energy Efficiency Information (http://www.epsea.org/)

• ESRU Home Page (http://www.esru.strath.ac.uk/)

• The Home Energy Saver (http://hes.lbl.gov/)

• U.S. DOE Energy Efficiency and Renewable Energy (EERE) Home Page (http://www.eere.energy.gov/)de:Sonnenkollektor

pl:Kolektor słoneczny