Solar energy is not a single energy technology but a term that covers a diverse set of renewable energy technologies. Their common feature is that, unlike oil, gas, coal, and present forms of nuclear power, solar energy is inexhaustible. Solar energy can be divided into three main groups—heating and cooling applications, electricity generation, and fuels from biomass.

Heating and Cooling

     The sun has been used for heating for centuries. The Mesa Verde cliff dwellings in Colorado were constructed with rock projections that provide shade from the high (and hot) summer sun but allow the rays of the lower winter sun to penetrate. Today a design with few or no moving parts that takes advantage of the sun is called passive solar heating. Beginning in the late 1970s, architects increasingly became familiar with passive solar techniques and, in the future, more and more new buildings will be designed to capture the sun’s winter rays and keep out the summer rays.

     Active solar heating and solar hot-water heating are variations on one theme, differing principally in cost and scale. A typical active solar-heating unit consists of tubes installed in panels that are mounted on a roof. Water (or sometimes another fluid) flowing through the tubes is heated by the sun and is then used as a source of hot water and heat for the building. Although the number of active solar-heating installations has grown rapidly since the 1970s, the industry has encountered simple installation and maintenance problems, involving such ordinary occurrences as water leakage and air blockage. Solar cooling requires a higher technology installation in which a fluid is cooled by being heated to an intermediate temperature so that it can be used to drive a refrigeration cycle. To date, relatively few commercial installations have been made.