because of the energy crisis, scientists in the oil-consuming nations have become increasingly interested in the potential of solar energy. Some experts estimate that the present supply of fossil fuel will not last until the end of the twenty-first century. The problem that solar energy researchers face is how to collect the sun's energy effectively and in expansively. One of the most popular methods currently being tested uses rooftop solar collectors and underground storage tanks. An advantage of a properly working system of this type is that