There is a great understandable hunger for energy in the world. As Fig. 2.1 illustrates, increased uses of energy are strongly correlated with the gross domestic product, a measure of the average standard of living of the inhabitants of a country. But if the principal sources of energy are fossil fuels, we are faced with a dilemma: burning these fuels contributes to the greenhouse effects and thus to the warming of the Earth, causing serious environmental consequences.
While there are many forms of energy with which we are familiar--mechanical, chemical, nuclear, light, thermal energy, heat, to name a few--and there are many sources for all forms of energy, the inhabitants of our planet have found it convenient to exploit relatively few. The burning of the so-called fossil fuels (coal, oil, and natural gas) currently provide us with about 90 percent of the energy we use. These are finite resources and their use is environmentally harmful. We intend to explore in this book what alternatives are available to us. The purpose of pressing for the ultimate replacement of fossil fuels in our economy is not only to improve our environment but to prepare ourselves for the time when these fuels become scarcer, and therefore more expensive, or have been depleted altogether.
Virtually all of the energy available to us on Earth comes from the sun. (The only exceptions are the relatively small amounts we use from geothermal sources, nuclear energy, and the tides.) Of all of the energy the sun radiates, only a small fraction is directed toward