ABSTRACT
Solar energy can play a significant role in satisfying world energy demands, contributing in the same time to the overall pollution reductions. In particular, solar thermal energy (STE) systems could easily cover significant share of the electricity, heat and hot water demands. STEs refer to the solar energy harnessing systems to generate thermal energy. Combined with energy storage, solar thermal plants can decouple electricity generation from the sources, a significant advantage compared with other renewable energy conversion systems. Various solar thermal technologies and systems are reviewed in this chapter with emphasis on their characteristics, performances and applications. The solar radiation is used to generate heat since ancient times. In 213 B.C., the Greek scientist Archimedes used mirrors to direct sunlight onto the Roman fleet, besieging his city Syracuse, Sicily, setting the ships on fire. Modern solar collectors are not necessarily more sophisticated than that of Archimedes. A solar collector collects and converts the solar radiation into heat. The low- and medium-temperature solar collectors are used primarily for residential heat and hot water applications, while the high-temperature for electricity production or to provide thermal energy to industrial processes. In the last few decades the solar energy conversion into heat has evolved to a well-developed and mature technology. STE basic principle is that part of the solar radiation striking a body is absorbed, increasing its temperature. The solar collector efficiency depends on the absorption efficiency, reducing the thermal and re-radiation losses, and the transfer efficiency of the collected energy. The STE costs can be lowered by using concentrating solar collectors, replacing part of the expensive absorber by cheaper metallic reflectors to concentrate the solar energy onto absorber, while through the smaller absorber surface the heat losses are reduced. Sun-tracking concentrating collectors with a high concentration ratio are capturing significantly more solar radiation per absorber area, generating heat a much higher temperatures. According to the solar radiation concentration type, STE systems are divided into: concentrating (point and line focusing systems), and non-concentrating systems. The former ones are further divided according to the solar radiation receiver type, heat transfer media and heat storage, and if or not energy back-up units are included. Concentrating solar collectors can reach temperature levels similar to that of existing fossil-fuel power stations, while similar thermal-electrical units are used.
