Several sustainable energy sources could reduce or eliminate our dependence on fossil fuels and nuclear energy. Some of these sources have been used for centuries but have been neglected since fossil fuel cam into widespread use. Passive solar heat, fuel wood, windmills, and water wheels, for instance, once supplied a major part of the external energy for human activities. With increased concern about the dangers and costs associated with conventional commercial energy, these ancient energy sources are being reexamined as part of a more sustainable future for humankind.Exciting new technologies have been invented to use renewable energy sources. Active solar air, and water heating, for instance, require less material and function more quickly than passive solar collection. Wind is now the cheapest form of new energy in many places. It has potential to supply one third or more of our energy requirement. Parabolic mirrors can produce temperatures high enough to be used as process heat in manufacturing.Fuel cells use catalysts and semi permeable electrolytes to extract energy from fuels such as hydrogen or methanol at high efficiency and with very low emissions. Ocean thermal electric conversion, tidal and wave power stations, and geothermal steam sources can produce useful amounts of energy in some localities. One of the most promising technologies is direct electricity generation by photovoltaic cells. Since solar energy is available everywhere, photovoltaic collectors could provide clean, inexpensive, nonpolluting, renewable energy independent of central power grid or fuel supply system.Biomass also may have some modern application. In addition to direct combustion, biomass can be converted into methane or ethanol, which are clean burning, easily storable, and transportable fuels. These alternative uses of biomass also allow nutrients to be returned to the soil and help reduce our reliance on expensive, energy consuming artificial fertilizer.