Alternative Energy Sources

Solar energy can be utilized either directly as heat or indirectly after conversion to electrical power using photovoltaic cells or steam generators. Greenhouses and solariums are common examples of the direct use of solar energy, having glass surfaces that allow the passage of visible light from the sun but slow the escape of heat and infrared. Another direct method involves flat-plate solar collectors that can be mounted on rooftops to provide energy needed for water heating or space heating. Windows and collectors are considered passive systems for harnessing solar energy. Active solar systems use fans, pumps, or other machinery to transport heat derived from sunlight.

Photovoltaic cells are flat electronic devices that convert some of the light that falls on them directly to electricity. Typical commercial photovoltaic cells convert 10–20% of the sunlight that falls on them to electricity. In the laboratory, the highest efficiency demonstrated so far is over 30%. (Photovoltaic efficiency is important even though sunlight is free; higher-efficiency cells produce more power in a limited space, such as on a rooftop.) Photovoltaics are already economic for use in remote applications, such as highway construction signs, spacecraft, lighthouses, boats, rural villages, and isolated homes, and large-scale initiatives are under way in California Wind generators in Whitewater, California. JLM Visuals. Reproduced by permission. and other places to produce hundreds of megawatts of power from rooftop-mounted photovoltaic systems.

Thermal-electric solar systems have also been developed using tracking circuits that follow the sun and mirrored reflectors that concentrate its energy. These systems develop intense heat that generates steam, which in turn drives a turbine generator to produce electricity.