The direct collection of solar energy involves artificial devices, called solar collectors, that are designed to collect the energy, sometimes through prior focusing of the sun's rays. The energy, once collected, is used in a thermal process or a photoelectric, or photovoltaic, process. In thermal processes, solar energy is used to heat a gas or liquid, which is then stored or distributed. In the photovoltaic process, solar energy is converted directly to electrical energy without intermediate mechanical devices . Solar collectors are of two fundamental types: flat plate collectors and concentrating collectors.





Passive Solar Energy
Active solar heating systems involve installing special equipment that uses energy from the sun to heat or cool existing structures. Passive solar energy systems involve designing the structures themselves in ways that use solar energy for heating and cooling. For example, in this home, a "sun space" serves as a collector in winter when the solar shades are open and as a cooler in summer when the solar shades are closed. Thick concrete walls modulate wide swings in temperature by absorbing heat in winter and insulating in summer. Water compartments provide a thermal mass for storing heat during the day and releasing heat at night.

Flat Plate Collectors
In thermal processes, flat plate collectors intercept solar radiation on an absorber plate in which passages for so-called carrier fluid are integral or to which they are attached. The carrier fluid (liquid or air) passing through these flow channels has its temperature increased by heat transfer from the absorber plate. The energy transferred to the carrier fluid, when divided by the solar energy incident on the collector and expressed as a percentage, is called the instantaneous collector efficiency. Flat plate collectors generally have one or more optically transparent cover plates intended to minimize heat losses from the absorber plate, in an effort to achieve maximum efficiency. Typically, they are capable of heating carrier fluids up to 82° C (180° F) with efficiencies between 40 and 80 percent.


Solar Heating
Flate plate collectors utilize the sun's energy to warm a carrier fluid, which in turn provides usable heat to a household. The carrier fluid, which in this case is water, flows through copper tubing in the solar collector, and in the process absorbs some of the sun's energy. Next, the carrier fluid moves to the heat exchange, where the carrier fluid warms water that is used by the household. Finally, a pump moves the carrier fluid back to the solar collector to repeat the cycle.

Flat plate collectors have been used efficiently for water and comfort heating. Typical residential applications employ roof-mounted, fixed collectors. In the northern hemisphere, they are oriented in a southerly direction; in the southern hemisphere, they are oriented to face north. The optimum angle at which to mount collectors relative to the horizontal plane depends on the latitude of the installation. Generally, for year-round applications such as providing hot water, collectors are tilted (relative to the horizontal plane) at an angle equal to the latitude angle ± 15°, and are oriented to face true south (or north) within ± 20°.

In addition to the flat plate collectors, typical hot-water and comfort heating systems include circulating pumps, temperature sensors, automatic controllers to activate the circulating pump, and a storage device. Either air or a liquid (water or a water-antifreeze mixture) can be used as the fluid in the solar heating system, and a rock bed or a well-insulated water storage tank typically serves as an energy storage medium.

Concentrating Collectors
For applications such as air conditioning, central power generation, and numerous industrial heat requirements, flat plate collectors generally cannot provide carrier fluids at temperatures sufficiently elevated to be effective. They may be used as first-stage heat input devices; the temperature of the carrier fluid is then boosted by other conventional heating means. Alternatively, more complex and expensive concentrating collectors can be used. These are devices that optically reflect and focus incident solar energy onto a small receiving area. As a result of this concentration, the intensity of the solar energy is magnified, and the temperatures that can be achieved at the receiver (called the "target") can approach several hundred or even several thousand degrees Celsius. The concentrators must move to track the sun if they are to perform effectively, and the devices used to achieve this are called heliostats.

Solar Furnaces
One important high-temperature application of concentrators is for solar furnaces. The largest of these, located at Odeillo in the Pyrenees Mountains of France, uses 9600 reflectors with a total area of approximately 1860 sq m (about 20,000 sq ft) to produce temperatures as high as 4000° C (7200° F). Such furnaces are ideal for research requiring high temperatures and contaminant-free environments-for example, materials research.

Central Receivers
Central electric power generation from solar energy is under development. In the central receiver, or "power tower," concept, an array of reflectors mounted on computer-controlled heliostats reflect and focus the sun's rays onto a water boiler mounted on a tower. The steam thus generated can be used in a conventional power-plant cycle to produce electricity.

Solar Cooling
Solar cooling can be achieved through the use of solar energy as a heat source in an absorption cooling cycle. One component of standard absorption cooling systems, called the generator, requires a heat source. Because temperatures in excess of 150° C (300° F) are generally required for the absorption device to perform effectively, concentrating collectors are more suitable than flat plate collectors for cooling applications.