Survey of Energy Resources 2007
Solar thermal collectors are used to heat air, water or other liquids, depending on the applications, while solar photovoltaic (PV) collectors are used to convert sunlight to electricity directly. High-temperature solar thermal collectors are also used to produce electricity indirectly via thermodynamic cycles. Non-concentrating (or flat-plate) types of solar collectors can produce temperatures of about 100?C or less, which is applicable for many uses such as building heating and cooling, domestic hot water and industrial process heat. Medium-temperature concentrating collectors such as parabolic troughs or parabolic dishes may be used to provide temperatures from about 100?C to about 400?C. Such collectors may be used for various applications from refrigeration to industrial process heat and electricity generation. Central-receiver types of solar concentrating collectors are able to produce temperatures as much as 2 000?C or even higher. Therefore, they are used to produce electrical power and as high-temperature furnaces in industrial processes.
PV panels are solid-state and are therefore very rugged, with a long life. At present, panels based on crystalline and polycrystalline silicon solar cells are the most common. Their efficiencies have gradually increased, while costs have decreased over the last three decades. For example, the efficiencies of multijunction cells and concentrating PV have been reported to be as high as 40%, and most panels available in the market have efficiencies of the order of 15%. The retail price of PV panels came down from about US$ 30/W about 30 years ago to about US$ 3/W some 3 years ago. The price went up recently, owing to a very rapid increase in demand which created a temporary shortage of silicon wafers. While this situation has created opportunities for investments in silicon wafer production which will bring supply into line with demand, it has also created opportunities for companies to start production of thin-film solar cells based on cadmium telluride (CdTe) and copper indium diselenide (CIS), which use much less material and therefore have the potential to ultimately bring the cost of PV panels down to about US$ 1/W.
To evaluate the efficiency of solar energy systems, a standard flux of about 1 000 W/m2 is used, which is approximately the solar radiation incident on a surface directly facing the sun on a clear day around noon. Consequently, solar systems are rated in terms of peak watts (output under a 1 kW/m2 illumination).