A solar collector is a device that captures solar radiation, converts it to heat, then transfer that heat to water, solar fluid, or air. The solar thermal energy can be used in solar water heating systems, solar pool heaters, solar space heating systems and other end uses.
The absorber is a dark color surface that absorbs radiation converts it to heat. Absorbers are normally black because dark surfaces exhibit a particularly high degree of light absorption. As the absorber warms up to a temperature higher than the ambient temperature, it releases some of the accumulated solar energy in form of long-wave heat radiation. The ratio of absorbed energy to emitted heat is indicated by the degree of emission. Energy loss through heat emission is reduced by a selective surface coating on the absorber that converts a high proportion of the solar radiation into heat, while simultaneously reducing the emission of heat.
Flat plate and box-type collectors are typically used in domestic and light industry applications. Parabolic troughs, dishes and towers are used almost exclusively in solar power generating stations or for research purposes.
Flat plate collectors
A flat plate is the most common type of solar thermal collector, and is usually used as a solar hot water panel to produce hot water. A weatherproofed, insulated box containing a black metal absorber sheet with built in pipes is placed in the path of sunlight. Solar energy heats up water in the pipes causing it to circulate through the system by natural convection (thermosyphon). The water is usually passed to a storage tank located above the collector. This passive solar water heating system is generally used in hotels and homes in sunny climates such as those found in southern Europe and the Middle East.
For these purposes, the general practice is to use flat-plate solar energy or evacuated tube collectors with a fixed orientation (position). The highest efficiency with a fixed flat-plate collector or evacuated tube collector is obtained if it faces toward the sun and slopes at an angle to the horizon equal to the latitude plus about 10 degrees. Solar collectors fall into two general categories: non-concentrating and concentrating.
In the non-concentrating type, the collector area (i.e. the area that intercepts the solar radiation) is the same as the absorber area (i.e., the area absorbing the radiation).
There are many flat-plate collector designs but generally all consist of (1) a flat-plate absorber, which intercepts and absorbs the solar energy, (2) a transparent cover(s) that allows solar energy to pass through but reduces heat loss from the absorber, (3) a heat-transport fluid (air, antifreeze or water) flowing through tubes to remove heat from the absorber, and (4) a heat insulating backing. One flat plate collector is designed to be evacuated, to prevent heat loss.
Air flat-plate collectors are used primarily for solar space heating. The absorber plates in air collectors can be metal sheets, layers of screen, or non-metallic materials. The air flows past the absorber by using natural convection or a fan. Because air conducts heat much less readily than liquid does, less heat is transferred from an air collector's absorber than from a liquid collector's absorber, and air collectors are typically less efficient than liquid collectors.
Evacuated-tube collectors can achieve extremely high temperatures (170°F to 350°F), making them more appropriate for cooling applications and commercial and industrial application. However, evacuated-tube collectors are more expensive than flat-plate collectors, with unit area costs about twice that of flat-plate collectors. The collectors are usually made of parallel rows of transparent glass tubes. Each tube contains a glass outer tube and metal absorber tube attached to a fin. The fin is covered with a coating that absorbs solar energy well, but which inhibits radiative heat loss. Air is removed, or evacuated, from the space between the two glass tubes to form a vacuum, which eliminates conductive and convective heat loss.
Integral collector-storage systems
Integral collector-storage systems, also known as ICS or "batch" systems, are made of one or more black tanks or tubes in an insulated glazed box. Cold water first passes through the solar collector, which preheats the water, and then continues to the conventional backup water heater.
ICS systems are simple, reliable solar water heaters. However, they should be installed only in climates with mild freezing because the collector itself or the outdoor pipes could freeze in severely cold weather. Some recent work indicates that the problem with freezing pipes can be overcome in some cases by using freeze-tolerant piping in conjunction with a freeze-protection method.
Concentrating solar collectors
Concentrating solar collectors use reflective surfaces to concentrate solar radiation onto a small area, where it is absorbed and converted to heat, thereby providing higher temperatures that flat plate collectors.
A parabolic collector or "trough" is constructed as a long parabolic mirror (usually coated silver or polished aluminum) with a linear receiver, also called a heat collection element, running along its length at the focal point. Sunlight is reflected by the mirror and concentrated on the receiver. The trough is usually aligned on a north-south axis, and rotated to track the sun. Heat transfer fluid (usually oil) runs through the tube to absorb the concentrated sunlight. The heat transfer fluid is then used to heat steam in a standard turbine generator. parabolic troughs can deliver heat at temperatures ranging from 40 to 290 °C (100 to 560 °F ) for applications such as hot water, space heating, air-conditioning, steam generation, industrial process heating, desalination and power generation.
Solar dish/engine systems convert the thermal energy in solar radiation to mechanical energy and then to electrical energy in much the same way that conventional power plants convert thermal energy from combustion of a fossil fuel to electricity. Dish/engine systems use a mirror array to reflect and concentrate incoming direct normal insolation to a receiver, in order to achieve the temperatures required to efficiently convert heat to work. This requires that the dish track the sun in two axes. The concentrated solar radiation is absorbed by the receiver and transferred to an engine.
Solar power tower
A solar power tower, also know as a solar central receiver system, generates electric power from sunlight by focusing concentrated solar radiation on a tower-mounted heat exchanger (receiver), which is converted into thermal energy which, in turn, is converted to electricity. The system uses hundreds to thousands of sun-tracking mirrors called heliostats to reflect the incident sunlight onto the receiver. Each heliostat can rotate in two axes (elevation and azimuth), enabling it to accurately track the sun and maintain the reflecting focus on the receiver.
- U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Solar technologies program, Accessed 14 October 2008.
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