ABSTRACT
A simple flat-plate collector is quite suitable for producing small quantities of hot water, but a water pump may be driven either electrically by solar cells, or mechanically or electrically by a heat engine. When comparing solar energy with other resources, it may be a criterion that more energy is delivered by the solar collector than was consumed during its production. High grade energy may be derived from solar radiation by a "quantum converter", which uses photovoltaic or photochemical action to convert high energy photons directly to electrical or chemical energy without an intermediate thermal process. Water pumping in many countries has traditionally been powered by windmills, which use a form of solar energy, but it is equally possible to use radiant solar energy. A major contribution to an arid region's requirements may be made by solar energy in supplying and purifying water.
