Attractive though it is, direct solar energy is diffuse - spread thinly over the surface of the Earth - and needs to be concentrated before we can use it. (50 microgrammes of deuterium contain as much energy as a whole day's solar rays on a square metre of Earth's surface.) Except in space it is not always available where or when we need it. However, solar energy is responsible for a number of indirect forms of energy which can be harnessed, so that in effect we capture solar energy which arrived elsewhere. The Sun powers Earth's great weather engine, providing over 3,000 Q per year for the purpose. Of this, one third is absorbed on passing through the atmosphere, while the rest - short wave radiation - heats the surface and is then returned to te atmosphere as long wave radiation and by the evaporation of water. Ultimately this heat is lost back into space at just about the same rate as it is received. As an energy source we are generally concerned here not so much with the heat as with the mechanical energy into which it is transformed; there is however at least one further use for the heating power of the Sun.

Principle of OTEC. One scientist has calculated that if all the world's energy were produced by this method it would reduce the temperature of the tropical oceans by 1 degree Centigrade resulting in a reduction of tropical rainfall...

The idea of "solar sea power", making use of the temprature differences between the warm surface layer and the colder layers further down (especially in tropical waters) was first put forward by a French physicist, Jacques d'Arsonval, in Paris in 1881. Now known as OTEC (ocean thermal energy conversion), the theory was pout into practice in 1930 in a small way by Georges Claude, a student of d'Arsonval. His plant off the coast of Peru generated only 22kW and was soon destroyed by heavy waves, and interest in the idea waned until 1964 when an American, J Hilbert Anderson, became enthusiastic about the scheme - even though fossil fuels then seemed plentiful and cheap. In 1972 the National Science Foundattion alotted $84,000 to his project. In 1975 this was increased to $3 million. Solar energy R & D then passed to ERDA, and in 1976 OTEC received $8 million. (If this sounds a lot, it should be compared with the $10 billion spent in 1975 on th fast breeder.)

There are various designs for OTEC, most having design features in common with that illustrated here. The "working fluid", seperate from the sea water and usually ammonia, is vaporized by the temperature differences - about 20 degrees Centigrade - between surface "heat reservoir", and cold water brought up by a pipe from the "heat sink' 1,200m below, driving generators. The cold water, as low as 5 degrees Centigrade, then condences the working fluid back to liquid for re-use. In addition to elecricity, transmitted ashore by wire, such plants could make products such as hydrogen, oxygen and fresh water from the sea, or even ammonia and methanol from sea and surrounding air. all have energy applications.

GRAVITY POWER