11.   Solar energy is an idea whose time has come.

12.   A few years ago a car fuelled by solar energy would have been inconceivable.

13.   How did they avoid coating themselves in iron oxides, which would quickly shut off the bacteria from the solar energy they demand?

14.   A radical new approach to solar energy may come from biotechnology -- a broad class of systems that have at their core photosynthetic and biological energy conversion processes.

15.   For example, chemical energy relates to chemical systems and solar energy relates to the energy of the Sun.

16.   They include biomass, geothermal energy, hydroelectric power, solar energy, tidal energy.

17.   Such energy sources include nuclear fission, nuclear fusion, solar energy together with biomass, controlled thermonuclear fusion and, finally, geothermal energy.

18.   The most important are nuclear fission, wind, wave and tidal energy sources and solar energy by direct conversion and biomass.

19.   In the two sections below we shall consider the direct conversion of solar energy and biomass.

20.   All these provide indirect means of converting solar energy to forms of energy which are useful to us.