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A renewable energy project is one that aims to generate energy in a sustainable way by not exploiting exhaustible natural resources such as fossil fuels. Nuclear power is not considered sustainable because it requires the mining of non-renewable uranium deposits. Renewable energy projects use a wide variety of energy sources that can be broken down into four main types: light, heat, kinetic and chemical. Solar energy is an example of light as an energy source; solar and geothermal energy are examples of heat as an energy source; hydroelectric power, wind, wave and tidal power are examples of kinetic energy, or the energy of movement; and biomass and biofuel energy are examples of chemical energy, or the energy locked in chemical compounds that can be released by combustion.
Although heat can be used directly to heat buildings, the usual aim of a renewable energy project is to generate electricity. Solar energy can generate electricity through the photoelectric effect, whereby certain substances produce an electric current on exposure to light, but most electricity is generated using turbines that might be driven by steam produced by natural sources of heat or directly by sources of kinetic energy. Biofuels might be used directly to power vehicles, as an alternative to fossil fuels.
Concerns about the exhaustion of non-renewable energy sources have led to widespread implementation of renewable energy projects. Climate change, pollution and the safety of nuclear power plants also are concerns that have contributed to the search for renewable energy. A great deal of research has gone into finding efficient and cost-effective ways of exploiting sources of renewable energy.
Solar power can be harnessed in two main ways. Photovoltaic cells, often called solar cells, convert light directly into electricity and can be grouped into large solar panels that generate significant amounts of energy. These can be employed on a small scale to provide electricity to homes or in extensive arrays for large-scale electricity production.
Another method of harnessing solar power is by using mirrors to focus the sun’s energy onto a target. This generates heat to drive a power plant that might produce steam to drive a turbine or use the expansion of heated gas to drive a piston. Alternatively, on a smaller scale, the heat can be used directly to provide hot water or for cooking.
A renewable energy project based on geothermal energy uses heat originating in the Earth’s core as an energy source. It will not run out for billions of years, so it is regarded as renewable. Alternative energy projects might exploit geothermal energy by using natural hot water near the surface to heat buildings or by pumping water into heat sources deep underground, converting it to steam to power turbines.
Large-scale hydroelectric projects exploit the potential energy of elevated bodies of water, using the gravity-powered flow of water to drive turbines. These projects require suitable terrain and significant amounts of surface water, and most hydroelectric schemes have involved the construction of dams and the flooding of areas of land. Smaller-scale projects involving running water from rivers have also been implemented.
Wind power is harnessed using wind turbines. These normally consist of three blades mounted on a metal column. The turning of the blades drives a turbine for electricity generation. Wind turbines can be installed singly to provide energy for buildings, but for large-scale electricity production, they must be installed in large numbers, creating “wind farms” covering extensive areas of land.
Renewable energy projects based on wave and tidal power generate electricity using the movement of ocean water. Wave power generators exploit the motion of ocean waves and rely on converting wave motion into rotational motion. This might be achieved by shore-based plants that use incoming waves to force air through a chamber into a turbine or by offshore structures that capture the kinetic energy of waves and use it to pump fluid to an onshore plant. There are many designs, some involving long, flexible tubes that float on the surface and some involving hinged barriers that move backward and forward.
Tidal energy can be captured using a barrage that acts like a dam across a tidal river estuary. Water from incoming or outgoing tides is forced through tunnels to drive turbines. Although the energy-generating potential is considerable, tidal barrages generate electricity only when the tides are flowing and might disrupt delicate estuary ecosystems.
A biomass renewable energy project uses chemical energy locked in organic materials. These projects might use biomass directly by burning agricultural waste or use it to produce alternative fuels, such as ethanol from the fermentation of sugar cane or methane from sewage and animal waste. Although these resources are renewable, biofuels produce carbon dioxide just like fossil fuels, so they are believed by many people to contribute to climate change.