ÜDS-Fen 2001-2 Soru=68

The passage contains a warning that .

To obtain power from the sun's rays is to use nuclear
power developed at no expense in a laboratory 93 million
miles away, for the radiant energy of the sun is
maintained by nuclear transformation of chemical
elements occurring in the sun's interior at temperatures
of many million degrees, and at pressures of many
million atmospheres. The resources of solar power are
enormous. If 100 per cent efficiency could be secured in
the transformation of radiant solar energy into
mechanical work, a horsepower per square yard of
ground surface would be available under cloudless skies.
The expense of collecting solar energy still prevents its
competition with the usual power sources .Yet, unless
the vague promise of safe thermonuclear power from
oceans becomes realized, solar power must supply the
enormous and growing requirements of posterity within
two centuries. Because the ground sources (coal, oil and
uranium) as they near exhaustion will become more
costly than solar power.