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Renewable Energy
Geothermal plants usually drill deep into the Earth to reach the steam produced by the heat of the Earth. Here, steam pipes lead from a geothermal field toward a power-generation plant in New Zealand

Renewable energy is the energy produced by using natural resources that will regenerate in time. Primary sources of renewable energy are water, geothermal heat, wind, and sunlight. The energy produced by using different organic materials, such as wood, agricultural rejects, forestry residues, and urban solid waste (called biomass) is also regarded as “renewable.” According to authoritative international agencies like the United Nations Environment Programme and the Intergovernmental Panel on Climate Change, renewable energies represent the mo st realistic alternative to fossil fuels. At present, renewable sources are employed chiefly in the production of electricity, but also to produce biofuels and to provide heat for industrial and private users. Over the last 10 years, the development of renewable energies has been stimulated by concern over global warming. At this time, it is believed that renewable energy sources will play a key role in limiting greenhouse gases emissions.
Water
Hydroelectric energy has represented, and still represents today, the principal source of renewable energy: With reference to the global production of energy, hydroelectric represents 2.15 percent of the total. Concerning the construction of new large-scale hydroelectric plants today, development seems stalled because they require infrastructure (dams, reservoirs, catchment drains) that causes a considerable environmental impact. Although they produce no polluting emissions, they considerably alter the landscape, impair the balance of local ecosystems, and diminish the volume of water available for uses other than energy production. For these reasons, it seems more appropriate to develop hydroelectric plants with less than 10 megawatts (MW) of power production: These are small plants with a low environmental impact that produce energy by utilizing minor streams and river waterfalls. The realization of such small hydroelectric plants creates only modest environmental and technical problems, particularly where the construction of dams is unnecessary, and the cost of the electricity produced more or less equals that of larger plants.
Some countries today are experimenting with new technologies capable of producing power by using energy flows from seawater. Experiments are being conducted to try to exploit tidal power (in France), the energy potential of waves (in the United Kingdom, Norway, and Japan), and the temperature gradient—the difference in temperature that exists between deep and surface waters (in the United States).
Geothermal
Geothermic energy is generated by physical processes that occur in deepest layers of the Earth's crust (underground, the temperature rises by 30 degrees Celsius every 1,000 meters of depth). Geothermal heat rises to the Earth's surface, where it can generate a thermal current measuring about 0.065 watts per square meter. In general, geothermal plants use the following operational procedure: Through drilling, the steam produced by the heat of the Earth is brought to the surface and conveyed to pipes that carry it to a turbine, where, by means of a driving shaft, it is turned into mechanical energy, and into electrical power by an alternator. In cases where the geothermal fluid does not reach high temperatures, the required temperatures are reached by using hot water. The resulting heat is used, for example, in district heating plants or by agriculture for heating greenhouses.
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