SOLAR THERMAL CONVERSION
Energy is not a good unto itself; it is valued rather as a means of satisfying important
needs of a society. In classical thermodynamics, energy is defined as the capacity to do
work; but from a more practical point of view, energy is the main stay of any industrial
society. In the United States, energy is currently provided by seven primary sources:
petroleum, natural gas, coal, hydro-power, nuclear fission, geothermal, and wood and
waste. The first three of these sources are fossil fuels. They are stored forms of solar
energy that received their solar input eons ago, have changed their characteristics over
time, and now are in a highly concentrated and convenient form. It is apparent, however,
that these stored forms of solar energy are being used so rapidly that they soon will be
depleted. To maintain our present social structure, it is desirable, therefore, that we
supply an increasing portion of our energy needs from renewable sources.