Renewable Energy Resources
Hydropower comes from moving water and ocean waves. Hydropower systems use the energy in flowing water for mechanical purposes or to produce electricity. Hydroelectric plants use the kinetic energy of moving water to spin the turbine generator. Most sites suitable for large-scale hydroelectric plants in the U.S. have already been developed. New developments are small-scale, which can efficiently supply local needs. The U.S. and Canada have the greatest number of hydroelectric plants. Virtually every other country in the world has some plants—hydroelectricity is the power source of choice for many developing countries. There are concerns over the detrimental environmental effects of hydroelectric power, which include siltation and erosion, soil and water salinity, the breaking up of the free passage between oceans and rivers, weed growth, floods due to dam failures, and disease spread by small organisms that live in stagnant water.
Biomass materials such as wood, agricultural crop wastes, fast-growing willow and switchgrass crops, animal wastes, and even garbage can be used as renewable sources of energy to generate heat and power. They also can be used as alternatives to petrochemicals in making plastics and other products. Today, biomass energy systems are very small-scale; some examples include ethanol in gasoline, and use of municipal waste to produce methane gas. The biggest user of biomass energy is the forest products industry, which burns much of its waste to make heat and electricity. Biomass fuels contain a lot of carbon; using them requires high-tech burners that reduce smoke. Still, biomass energy can productively use wastes that would otherwise go to landfills or incinerators.
Passive solar heating
Passive solar heating for buildings is a common application of renewable energy. A passive solar heating system collects energy from the sun. It uses this energy to heat a space directly, or to heat a fluid that later radiates heat to a space. A sunroom, an example of a passive solar approach, uses double-layered windows trap heat in the room and reduce the amount of heat loss by convection. Solar systems are dependent on the weather conditions and number of daylight hours. Their effectiveness is greatly affected by climate, season, building orientation, and site conditions.
Active solar systems
Active solar systems—or photovoltaic systems—are another way of capturing the sun’s energy. These systems use solar cells to directly produce electricity from solar radiation. The solar cell is made of two semiconducting materials—generally silicon-based—with a boundary between them. When a photon of electromagnetic energy from the sun strikes an electron near the boundary between the semiconductors, it starts a series of reactions that separates electrons and "holes"—the unoccupied spaces left behind when the electrons leave. The electrons move in one direction through the conductor, and the holes move in the other direction. This creates an electrical current when the object being powered is connected in a circuit to the semiconductor. Some homes and businesses use solar cells to reduce the amount of power they buy from the electric utility. Some experimental cars also use photovoltaic cells to power electric motors.
At this time, photovoltaic systems are relatively expensive to build and maintain. They also require a back-up source of power, or batteries, to provide power when sunshine is inadequate. Because semiconductors contain toxic materials, the environmental impacts of manufacturing and disposing of solar cells and their batteries are also a concern.
Solar thermal electricity
Solar thermal electricity is another variation on solar energy. These plants use a highly curved mirror to focus sunlight onto a pipe, concentrating the heat to boil water and create steam. That steam is then used to turn a turbine generator to make electricity.
Wind power uses energy from the moving air to turn large blades on windmills. In the past the motion of the blades was used to grind flour or pump water, but now the blades turn turbines, which rotate generators in order to produce electricity. Very large, wide open windy spaces are needed for this system to be efficient. Although wind energy makes noise and can harm birds, it produces no air or water pollution. The major constraint to wind power is the limited availability of sites with steady wind. Today, costs of electricity from wind are generally higher than costs of power from other sources. These costs have been declining as wind turbines are made more efficient and long-lived.
Geothermal energy systems
Geothermal energy systems use hot water from below ground to heat homes and buildings during the winter. The hot water circulates through insulated pipes placed hundreds of feet into the ground. Some regions, such as areas of Iceland, use active springs and geysers to heat buildings. A few places have so much steam and hot water that it can be used to generate electricity. Even though vast amounts of energy are available within the earth, our ability to use it is limited by site considerations. Today’s geothermal heat pumps are also more costly than conventional heating systems. A variation on geothermal power systems uses ocean thermal energy. Ocean thermal energy conversion is currently being used in Japan and Hawaii in some demonstration projects.