Although articles on solar power appear often in public media, it is not always made clear that there are many ways to capture that energy, and that these methods are quite different from one another. First, there is local solar vs. central solar. Locally, sun falls on every rooftop, and there is no excuse not to use this free energy. Centralized solar power plants are another matter. These take up large areas and have to transmit the energy from sparsely populated to densely populated regions. The plants also have to compete with coal and nuclear plants on cost.

There is also a big difference between solar thermal and solar electric. In solar thermal, sunlight is used to heat a liquid, typically water, and that heat is either used directly for heating or is used to generate electricity. Local use of solar thermal is very simple: water heated on the roof can directly reduce one’s gas or oil bill. Centralized solar thermal is literally done with mirrors. Acres of mirrors motorized to follow the sun focus the sunlight into a boiler on top of a tower. There a liquid such as water or liquid salt is rapidly heated and stored in tanks on the ground. Since heat is hard to transport long distances, the hot liquid is used in a steam generator to produce electricity. Most of the energy is then lost in the thermal cycle, as was explained in Chap. 2.

Solar electric is commonly called photovoltaic or PV. There are two main kinds of PV: silicon and thin film. Solar cells made of silicon are expensive, and there are several kinds of these: polycrystalline, amorphous, and microcrystalline. Polycrystalline silicon solar cells can be very efficient, but these are so expensive that they are used where cost does not matter, as in space satellites. Amorphous sili­con is less efficient but much less expensive, and they could be competitive in the market. The new microcrystalline silicon cells under development may turn out to be a good compromise. The fastest growing segment, however, is in thin-film solar cells. These are much cheaper than silicon ones, use very little material, and can be used for both local and central power. Although thin-film cells are the most ineffi­cient of all, the possibilities for their deployment are tremendous. For instance, windows could conceivably be coated with thin-film cells. The following sections will tell how these various solar energy methods work.