Generation
From Thermal-FluidsPedia
The primary method of electricity generation is by power plants. No matter which energy source is used, the energy produced is transformed into the rotational energy of a turbine, which drives a generator that produces electricity. Wind farms utilize the kinetic energy of air streams to power wind turbines. Hydroelectric and tidal plants use the potential energy of falling water to run water turbines. The potential energy of ocean waves can be used either to rotate a water wheel directly or to compress a column of air and drive a gas turbine.
The generation of electricity by thermal power plants is accomplished in three steps: the conversion of chemical or nuclear energy of the fuel into thermal energy (heat) inside a combustion chamber, the conversion of thermal energy into mechanical (rotational) energy, and the conversion of mechanical energy into electrical energy by an electric generator. In coal, oil, and natural gas power plants, the source of energy is the energy trapped in the chemical bonds of the fossil fuel. In nuclear power plants, the binding energy of the nucleus provides the energy. In solar-thermal and geothermal power plants, thermal energy is directly available.
Besides mechanical and thermal power plants, electricity can be generated directly. Fuel cells produce electricity from electrochemical reactions between hydrogen and oxygen. Photovoltaic cells convert the energy in sunlight directly to a flow of electrons through an external circuit. Thermophotovoltaics work in a similar fashion except that, instead of using the visible light from the sun, they convert infrared radiation or heat from furnaces into electricity.
Magneto hydrodynamic (MHD) power generators omit the intermediate step by converting the thermal energy of fossil fuels directly into electrical energy, without first converting it to mechanical energy. Hot gas from coal combustors are seeded with some seeding agent (usually potassium) to turn into a hot plasma gas. A strong magnet separates out the charges, which drift toward and are collected by electrodes on opposite channel walls. An external circuit can be setup to take advantage of the charge gradient and produce electricity. Because of the low level of heat rejection, efficiency is higher than that of conventional thermal power plants and the demand for cooling water is lower. The hot exhaust gas is used to boil water to steam and produce additional electricity by conventional methods.
Example: According to the data published by the Energy Information Agency (EIA) (1), the world’s total energy production in 2001 was estimated at 403 quads. The same reference gives total electricity generation at 14 trillion kilowatt-hours. What fraction of the world’s total energy is converted to electricity? (1 quad = 1.055 EJ = 2.93x1011 kWh.)
Solution: Assuming a thermal power plant efficiency of 33%, the total energy needed to produce 14 trillion kWh of electricity is:
(14x1012 / 0.33 kWh) (1 quad / 2.93x1011 kWh) = 145 quads
This is 36% of the world’s total energy production.
In the United States, 52% of power plants use coal, 16% use gas, 3% use oil, 20% are nuclear, and the remaining 10% are hydroelectric, solar, wind, or geothermal (Figure 1). Worldwide, about 64% of all electrical power generation is from fossil fuels, 18% is hydroelectric, 16% is nuclear, and only 2% is from geothermal and renewable energy sources. As of yet, no commercial MHD plants are built and the volume of electricity generated by fuel cells is insignificant.
References
(1) EIA, Monthly Energy Review, March 2005.
(2) Toossi Reza, "Energy and the Environment:Sources, technologies, and impacts", Verve Publishers, 2005
Further Reading
Bureau of Naval Personnel, Basic Electricity, Dover Publishing Company.
The Environmental Effects of Electricity Generation, IEEE, 1995.
The Electricity Journal, Direct Science Elsevier Publishing Company, This journal addresses issues related to generating power from natural gas-fired cogeneration and renewable energy plants (wind power, biomass, hydro and solar).
International Journal of Electrical Power and Energy Systems, Direct Science Elsevier Publishing Company.
Home Power Magazine (http://www.homepower.com).
External Links
Federal Energy Regulatory Commission (http://www.ferc.gov).
Energy Information Agency, Department of Energy (http://www.eia.doe.gov/fuelelectric.htm).
California Energy Commission (http://www.energy.ca.gov/electricity).
National Council on Electricity Policy (http://www.ncouncil.org).
Southern California Edison (http://www.sce.com).
Pacific Gas and Electric (http://www.pge.com).