There are many possible ways of tapping solar energy. It may be intercepted in space by a satellite and beamed to earth as an intense beam. More simply. it can be collected as it reaches the earth's surface by collectors that are basically hot water radiators working in reverse. Such collectors can provide space heating in houses or can be used as the basis for air-conditioning systems, solar energy can also be converted directly into electricity, at low efficiency. by solar cells similar to those used in satellites. Alternatively, it can be collected by using it to grow plants that are then used directly as fuel, or converted into a liquid fuel by chemical conver- sion or by the use of micro-organism. Finally, the heat stored in the sea can be used to provide energy by employing the temperature difference between water at the surface exposed to the sun and cold deep water.