Power is defined as the energy that is consumed or converted in a certain amount of time. In a simple electrical circuit, the power is found by multiplying the voltage and current. An electric current is the movement of charged particles measured in amperes and the voltage of the force driving them. Current that flows in one direction only, such as the current in a battery-powered flashlight, is called direct current. Current that flows back and forth, reversing direction again and again, such as household current, is called alternating current. Household electricity bills are computed on the basis of how many thousand-watt hours (kWh) of energy are consumed over a certain period of time. Today's home consumes, on average, between twelve hundred and two thousand kWh per month.
Most of the world's electric power is generated in steam plants. In a steam turbine generator, fossil fuel, such as coal, oil, natural or synthetic gas are the most common fuels used. Coal-based generation produces about 45 percent of all electricity generated in the United States, and natural or synthetic gas about 35 percent. The remaining, approximately 20 percent of generated electricity derives mostly from nuclear power plants, but includes wind, solar, biomass, diesel, geothermal, hydro, and other sources.
In a power plant, electricity is generated when a loop of conducting wire rotates in a magnetic field. Burning coal or gas produces hot steam that is forced through a turbine, causing it to spin. The spinning motion drives the generating coils within a magnetic field to produce electricity. Modern electricity-generating plants usually have a series of turbines to more effectively utilize the steam heat. The hot water returning to the boiler is used to preheat the fuel, allowing more efficient firing. See the illustration for a diagram of how electricity is generated by burning coal.
An electric power system consists of six main components: the electric power generating plant; a set of transformers at the plant to raise the generated electricity to the high voltages used on the transmission lines; the transmission lines; the substations at which the power is stepped down to the voltage that can be distributed to consumers; the distribution lines; and the transformers that lower the distributed voltage to the level needed by residential, industrial, and commercial users.
New gas turbine generators (analogous to big jet engines) are now being built that burn natural or synthetic gas as it is injected directly into the turbine system. This reduces heat loss and increases the efficiency of the fossil fuel.
Among the most modern systems are coal gasification or biomass gasification, which produce synthetic gases (syngas) by refining coal or biomass in a high-heat, pressurized system (gassifiers). Syngas is a more efficient fuel and contains less pollutant than either biomass or coal. Nitrogen and sulfur products are captured in the conversion process and become industrial and agricultural chemicals. At present, these systems remain expensive to build and much of the technology is still being improved. However, gasification systems are becoming more competitive with coal- or gas-fired steam plants as the costs of pollution abatement continue to rise.
As energy is converted to electricity, it flows to a transmission station where transformers change a large current and low voltage into a small current and high voltage. The electricity flows over high voltage transmission lines to a series of transmission stations where the voltage is stepped down by transformers to levels appropriate for distribution to customers.
Coal has the lowest heat values (British thermal units (BTUs) or BTU per ton) of any of the common fuel sources in the world today. When it is burned to generate steam, the major pollutants are sulfur, nitrogen, very fine ash, and mercury. The amounts of sulfur and nitrogen emitted when coal is burned depend on the kind of coal and where that coal is mined. In the United States, high-sulfur coal is mined in the Appalachian region, New York to Kentucky and the region south of the Great Lakes, Illinois, Iowa, and Kansas. These are the bituminous coal types, with high BTU per ton. Low-sulfur coal is mined in the Midwest and the intermountain regions (Wyoming, Colorado, Utah, and the Dakotas). This coal is mostly bituminous and subbituminous. Subbituminous coal has a lower BTU per ton rating. The nitrogen content of coal varies significantly and does not have the unique geographic distribution of sulfur. Finally, in the Dakotas, there is lignite, which is literally carbon-based earth. It has a very low BTU per ton rating, and is one of the most abundant coal types in the northern Great Plains.
Pollution from electric power generation depends on the type and source of fuel. The emissions, when not captured, produce oxides of nitrogen, commonly referred to as NOx, and sulfate aerosols from sulfur and oxygen, commonly referred to as SOx. Both pollutants are chemically unstable when emitted into the atmosphere and combine with oxygen and moisture to form the SOx and NOx particulates that are recognized as the pollutants. NOx is highly reactive with other pollutants found in urban and industrial areas and, with sunlight, forms smog. SOx is often attributed as the primary source of acid deposits across the landscape, particularly in the northeastern United States, which is downwind from power plants in the Midwest.
Mercury is emitted as elemental mercury vapor. It settles only a short distance from the stacks of power plants. However, it very quickly changes to a methyl mercury form, and when it settles into water, streams, lakes, or cooling ponds, it is absorbed by plants and transferred up the food chain to fish and waterfowl eaten by humans. Although the total annual tonnage is small, science is showing that extremely small amounts of mercury can cause significant harm to humans, particularly the unborn and very young children.
Most ash from burning coal is collected at the bottom of the fire box. However, very fine ash can float out of the smokestack. The particle size that concerns present-day regulators falls in the 10 micron and 2.5 micron range. A micron is one-thousandth of a millimeter. Airborne particulate this small may be contributing to increases in childhood asthma. Electric power generation is not the only source of such particulates in urban and suburban areas. Vehicle emissions from gasoline and diesel engines are also significant contributors.
The ash residual from burning coal is often suitable for the production of road surfaces, some forms of concrete, and lightweight blocks used to reduce erosion along rivers and streams. Once considered a pollutant or waste and dumped into open pit coal mines, coal ash is now becoming a valuable commodity.
Sulfur and nitrogen are captured by passing the hot gases from the combustion chamber through filters and water baths or by selective catalytic converters, thus removing them from the heat passed up the smokestack. The fine ash from the burning process is also filtered by a huge vacuum system with bags able to filter particles as fine as face powder. The concern about emissions of mercury is leading to the design of new systems capable of capturing the mercury vapor before it is released from the smokestack.
Environmental air quality standards are continually changing as new information about potential harm is published. It is a continual struggle between electric power generators and regulators to write and meet pollution standards that protect the environment and human health. Changes to a modern coal-based generator or even a natural gas generator cost thousands of dollars per megawatt of generating capacity. This means that every update, which must be designed onsite, as there are no standardized units, results in millions of dollars in additional costs. A steam generation system is designed to last at least fifty years, with initial investments close to a billion dollars, but because continually shifting requirements for pollution reduction systems cannot be incorporated in its design at the time of construction, the costs of later upgrades are almost inevitably incurred.
Electricity consumption has continued to rise approximately 2 to 5 percent per year as more and more electrical appliances are required to meet daily needs. Paying attention to the efficiency of each appliance, from computers to air conditioners, helps reduce the rate of increase. The higher the efficiency, the less total growth in individual consumer electricity use. More efficient lights, such as compact fluorescent bulbs, can effectively reduce the per capita use of electricity. Most electrical equipment manufacturers now provide comparisons of various appliances, machines, or other power equipment so informed consumers can make efficient choices. The U.S. Environmental Protection Agency (EPA) has a program called Energy Star that rates the efficiency of various appliances, computers, and other equipment. Those manufacturers that are compliant with high-efficiency standards receive an Energy Star stamp of approval.
Deregulation offers opportunities to independent power producers developing green electric power companies, for example, wind, biomass, solar, geothermal, and hydroelectricity generation, that wish to assure consumers their power source will not contribute to the increasing consumption of fossil fuels or emission of greenhouse gases. Such opportunities will, however, continue to come at a slightly increased price over the next decade before technologies to produce green power become more efficient, more generated power of this kind is widely available, and the costs of fossil fuels become more prohibitively expensive.
From 1950 to 1999, the most recent year for which data are available, annual world electric power production and consumption rose from slightly less than 1,000 billion to 14,028 billion kWh. The most commonly used form of power generation also changed. In 1950 about 66 percent of electricity came from thermal (steam-generating) sources and approximately 33 percent from hydroelectric sources. In 1998 thermal sources produced 63 percent of the power, but hydropower had declined to 19 percent, and nuclear power accounted for 17 percent of the total. The growth in nuclear power slowed in some countries, notably the United States, in response to concerns about safety. Nuclear plants generated 20 percent of U.S. electricity in 1999; in France, the world leader, the figure was 76 percent. See the pie chart for 2002 information on the net generation of electricity by fuel source.
see also Abatement; Acid Rain; Air Pollution; Cleanup; Coal; Energy; Energy, Nuclear; Fossil Fuels; Petroleum; Renewable Energy.
Gary R. Evans