Photovoltaic devices use semiconducting materials to convert sunlight directly into electricity. Solar radiation, which is nearly constant outside the Earth's atmosphere, varies with changing atmospheric conditions (clouds and dust) and the changing position of the Earth relative to the sun. Nevertheless, almost all U.S. regions have useful solar resources that can be accessed.
We have been harnessing the wind's energy for hundreds of years. From old Holland to farms in the United States, windmills have been used for pumping water or grinding grain. Today, the windmill's modern equivalent - a wind turbine - can use the wind's energy to generate electricity.
Wind turbines, like windmills, are mounted on a tower to capture the most energy. At 100 feet (30 meters) or more aboveground, they can take advantage of the faster and less turbulent wind. Turbines catch the wind's energy with their propeller-like blades. Usually, two or three blades are mounted on a shaft to form a rotor.
A blade acts much like an airplane wing. When the wind blows, a pocket of low-pressure air forms on the downwind side of the blade. The low-pressure air pocket then pulls the blade toward it, causing the rotor to turn. This is called lift. The force of the lift is actually much stronger than the wind's force against the front side of the blade, which is called drag. The combination of lift and drag causes the rotor to spin like a propeller, and the turning shaft spins a generator to make electricity.
Micro-hydro power is the small-scale harnessing of energy from falling water; for example, harnessing enough water from a local river to power a small factory or village. It will typically generate from 5kW up to 100 kW; usually providing power for a small community or rural industry in remote areas away from the grid. Hydropower is a well-proven technology, relying on a non-polluting, renewable and indigenous resource, which can integrate easily with irrigation and water supply projects. Over the last few decades, there has been a growing realization in developing countries that small-scale hydro-electric schemes have an important role to play in the economic development of remote rural areas, especially mountainous ones. Micro-hydro schemes can provide power for industrial, agricultural and domestic uses through direct mechanical power or by the coupling of the turbine to a generator to produce electricity.
Geothermal energy is the heat from the Earth. It's clean and sustainable. Resources of geothermal energy range from the shallow ground to hot water and hot rock found a few miles beneath the Earth's surface, and down even deeper to the extremely high temperatures of molten rock called magma.
Almost everywhere, the shallow ground or upper 10 feet of the Earth's surface maintains a nearly constant temperature between 50° and 60°F (10° and 16°C). Geothermal heat pumps can tap into this resource to heat and cool buildings. A geothermal heat pump system consists of a heat pump, an air delivery system (ductwork), and a heat exchanger-a system of pipes buried in the shallow ground near the building. In the winter, the heat pump removes heat from the heat exchanger and pumps it into the indoor air delivery system. In the summer, the process is reversed, and the heat pump moves heat from the indoor air into the heat exchanger. The heat removed from the indoor air during the summer can also be used to provide a free source of hot water.
Solar water heating systems include storage tanks and solar collectors. Most solar water heaters require a well-insulated storage tank. Solar storage tanks have an additional outlet and inlet connected to and from the collector. In two-tank systems, the solar water heater preheats water before it enters the conventional water heater. In one-tank systems, the back-up heater is combined with the solar storage in one tank.
New construction offers the greatest opportunity for incorporating passive solar design features, as demonstrated by the design of the Solar Energy Research Facility in Golden, Colorado. For retrofit projects, consider daylighting strategies, heat control techniques, and using passive solar heating strategies to allow modification of HVAC systems.
Passive solar systems make use of natural energy flows as the primary means of harvesting solar energy. Passive solar systems can provide space heating, cooling load avoidance, natural ventilation, water heating, and daylighting. This section focuses on passive solar heating, but the other strategies also need to be integrated and coordinated into a whole-building design. Passive solar design is an approach that integrates building components—exterior walls, windows, and building materials—to provide solar collection, heat storage, and heat distribution. Passive solar heating systems are typically categorized as sun-tempered, direct-gain, sunspaces, and thermal storage walls (Trombe walls).
Photovoltaics (PV) is an important energy technology for many reasons. As a solar energy technology, it has numerous environmental benefits. As a domestic source of electricity, it contributes to the nation's energy security. As a relatively young, high-tech industry, it helps to create jobs and strengthen the economy. As it costs increasingly less to produce and use, it becomes more affordable and available.
Few power-generation technologies have as little impact on the environment as photovoltaics. As it quietly generates electricity from light, PV produces no air pollution or hazardous waste. It doesn't require liquid or gaseous fuels to be transported or combusted. And because its energy source - sunlight - is free and abundant, PV systems can guarantee access to electric power.
PV frees us from the cost and uncertainties surrounding energy supplies from politically volatile regions. And in addition to reducing our trade deficit, a robust domestic PV industry creates new jobs and strengthens the U.S. economy.
