Geothermal

• Geothermal power in Iceland.
  
  Geothermal power uses the heat of the inner Earth to run a conventional thermal power plant. In a thermal power plant, water is heated into steam, and the kinetic energy of the steam is turned into mechanical energy by a turbine, and then into electricity by a generator. In other thermal power plant designs, oil, natural gas or coal are burned to produce the heat, or else the heat is provided by nuclear fission. Geothermal plants tap into the volcanic heat of the interior of the Earth to boil their water into steam. However, it is a minor technology, providing a mere 1 percent of the world's electricity.
  
  

Solar Power

• The mirrors and tower of a solar-fired thermal power plant.
  
  Solar electric cells convert sunlight into electricity through the photovoltaic effect. The photovoltaic effect works by using the characteristic for particular substances to emit electrons when they are hit by electromagnetic radiation, including sunlight. The electrons emitted in a solar cell are turned into electric current. Solar power is also used to harness sunlight as a source of heat, as in solar cookers and solar water heaters. There are even designs of solar-fired power plants, which work by using mirrors to concentrate sunlight as the source of heat in a thermal power plant.
  
  

Hydropower

• Hoover Dam
  
  Hydropower includes a broad group of machines that make electricity by harnessing the kinetic energy of water in motion. It is usually identified with its most common manifestation: the hydroelectric dam. However, this is not the only form of hydropower. There is also the emerging technology of tidal and wave hydropower. Using hydroelectric dams to harness the kinetic energy of rivers is a technology that is decades old, but using the kinetic energy of waves and the tides to do the same is not yet ready for mass employment. Hydropower accounts for 16 percent of the world's electrical output.
  
  

Wind Power

• Wind has been driving machines for thousands of years. Now it produces electricity, and is the new boom market in alternative energy. Wind power works by capturing the kinetic energy present in wind and converting it first into mechanical energy, and then into electricity. The first step is the conversion to mechanical energy, which is done when wind drives the turning of a wind turbine's blades. This turning motion is then transferred by gears to the turbine's rotor, and the turbine runs an electric generator. Although wind power has been around since the 1980s, it is only recently that its costs have dropped enough to be put into use on a large scale. Wind power currently provides 1 percent of the world's total electricity, but the world's maximum output has been estimated to be 5 times its current consumption, and wind power is growing rapidly.
  
  

What About Nuclear Energy?

• Nuclear energy has been promoted as an energy alternative because of its zero carbon emissions and its domestically supplied and plentiful--albeit not renewable--supply of fuel. Nuclear energy is generated inside a plant called a reactor. The power source is the heat produced in a controlled nuclear fission chain reaction, using either uranium or plutonium for fuel. This reaction involves striking and splitting a fuel atom with a neutron. The results of the fission of these large fuel atoms are the creation of new, smaller atoms as byproducts, radiation, heat and more neutrons. Those neutrons rocket out and strike other fuel atoms, causing a chain reaction. This is kept under control by neutron moderators, which vary depending on the design of the reactor. Examples include graphite rods and simple water.
  
  With the heat released, a nuclear reactor then makes electricity in the same manner as the other thermal-based power plant designs previously discussed: the heat boils water into steam, and the steam is used to turn the blades of a turbine, which runs the generator.
  
  While it does not emit carbon dioxide, nuclear energy does produce highly toxic radioactive waste. Therefore, it cannot be considered "green energy," although it could be considered part of the solution for global warming and achieving energy independence.