A renewable energy resource is one that can constantly be tapped because it replenishes itself. The search for viable renewable energy sources is a matter of growing urgency as non-renewable sources of energy become more difficult and expensive to discover and exploit. The major sources of renewable energy include solar energy, wind energy, bioenergy, geothermal energy, hydropower, and ocean energy. However, these sources have limitations that make them more expensive or less efficient in situations worldwide.

Solar energy, or energy derived from the Sun, works well in areas that have a good supply of direct sun light. Certain types of elements, such as cadmium sulfide or gallium arsenide, separated by layers of crystalline silicon, when exposed to direct sunlight, will produce an electric current. Light striking these substances knocks electrons off the element's surface. If the substances are formed into photovoltaic cells, the electrons...