3. Renewables - hydroelectric, biomass, wind, geothermal and solar. Currently, these power sources account for about 7% of our energy supply. However, they are the most hoped-for solution to our growing need for clean energy.
Hydroelectric is the leading renewable, responsible for 6% of overall electrical generation, and 34% of renewable power. A hydroelectric station is built by damming a river and erecting turbines, which are run by falling water. The energy produced is cheap, clean and constant. But dams are costly to build, may cause flooding and can disrupt ecological plant and animal systems.
Biomass accounts for 53% of renewable power. It is now used mostly for the transportation fuels ethanol and biodiesel. However, biomass fuels, (i.e., fast-growing "energy" crops, agricultural wastes and methane from landfills) may be burned, fermented or "digested" by bacteria to provide an inexpensive, relatively clean electrical power source.
Wind is now producing 7% of renewable energy. That's just .51% of the total energy yield, but wind energy has nearly tripled in production since 2007. It works by using the wind to turn tall propeller-like turbines, whose spinning hubs connect to generators which convert mechanical to electrical energy. Wind is relatively cheap, very clean and a wind farm takes about 2 years to build. But it is intermittent as a power source, hurts birds, and neighbors say it's annoyingly noisy.
Geothermal power stations tap into reservoirs of deep underground water heated by surrounding hotbeds of rock and volcanic magma. Steam from that water is what ultimately powers geothermal generators to produce 5% of renewable energy or .35% of total electrical generation. Small geothemal systems can heat and cool buildings efficiently, though at considerable installation cost.
Solar power currently comes in two forms: concentrated solar power (CSP) and photovoltaic (PV).
- Concentrated solar power (CSP), presently the most practical method for large-scale solar power generation, concentrates the sun's heat on an absorbant surface, which transfers the heat to water. The resulting steam drives a turbine linked to an electric generator. CSP plants employ large reflective surfaces, which tilt with the movement of the sun to focus solar rays on absorber tubes, which contain a fluid that transfers the heat to the power plant's boilers. For smaller applications like heating and cooling buildings or heating water, flat heat-absorbent panels pass heat through to tubes of fluid which transfer it to an insulated water tank. CSP plants now generate a tiny .09% of U.S. electrical power. The world's largest solar plant, built in the 1980's in the Mohave Desert of California, uses the CSP method.
- Photovoltaic (PV) cells, when exposed to the radiant energy of the sun, produce an electrical voltage. At a photovoltaic power station, multiple panels of silicon-based PV cells covered by non-reflective glass collect electromagnetic energy directly from the sun. Electric fields within the cells force the sunlight electrons to move in a given direction, establishing direct current (DC) electricity. Photovoltaic energy must be stored in batteries or other special storage systems for use in cloudy conditions or darkness.