Renewable Energy Sources
There is considerable international effort put into the development of renewable energy sources. Many of these energy sources come from the sun, for example wind, waves, tides and, of course, solar energy itself. The average peak solar energy received on the earth's surface is about 600 W/m2, but the actual value, of course, varies considerably with time of day and cloud conditions.
Solar Energy-Thermal Conversion
There is increasing interest in the use of solar energy for generating electricity through thermal energy conversion. In large-scale (central station) installations the sun's rays are concentrated by lenses or mirrors. Both require accurately curved surfaces and steering mechanisms to follow the motion of the sun. Concentrators may
Figure 1.9 Solar thermal generator
be designed to follow the sun's seasonal movement, or additionally to track the sun throughout the day. The former is less expensive and concentration of the sun up to 30 times has been obtained. However, in the French solar furnace in the Pyrenees, two-axis mirrors were used and a concentration of 16000 was achieved. The reflectors concentrated the rays on to a single receiver (boiler), hence raising steam.
An alternative to this scheme (with lower temperatures) is the use of many individual parabolic trough absorbers tracking the sun in one direction only (Figure 1.9), the thermal energy being transferred by a fluid to a central boiler. In the arid regions of the world where direct solar radiation is strong and hence solar thermal generation effective the limited supply of water for the steam cycle and for cooling can be an important consideration. In solar thermal schemes, heat energy storage can be used to mitigate the fluctuating nature of the sun's energy.