Overview - Generating Capability/Capacity
Generating Capability/Capacity
More than one-third of the primary energy in the Nation is used to generate electricity.1 Consumers expect electricity to be instantly available; that is, at the flick of a switch. In fact, electricity is so important to the functioning of our society that its unavailability is newsworthy.
The U.S. electric power industry is organized to ensure that an adequate supply of electricity is available to meet all demand requirements at any given instant, both now and in the future. This section provides a discussion on the capability of the electric power industry to meet that demand and the various methods used within the U.S. industry for converting energy into electricity.
The rating of a generator (that is, its nameplate capacity) is a measure of its ability to produce electricity. Nameplate capacity is the full-load continuous rating of the generator under specified conditions, as designated by the manufacturer, and is usually indicated on a metal plate attached to the generator. Net capability is the steady hourly output that the generating unit is expected to supply to the system load, as demonstrated by test procedures. The capability of the generating unit in the summer is generally less than in the winter due to high ambient-air and cooling-water temperatures, which cause generating units to be less efficient. The nameplate capacity of a generator is generally greater than its net capability.
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Figure 9. U.S. Utility and Nonutility Nameplate Capacity, 1994-1998 |
Electric Utilities
The generating units
operated by an electric utility vary by intended usage; that is, by the
three major types of load (generally categorized as base, intermediate,
and peak) requirements the utility must meet.
- A baseload generating unit is normally used to satisfy all or part of the minimum or base load of the system and, as a consequence, produces electricity at an essentially constant rate and runs continuously. Baseload units are generally the newest, largest, and most efficient of the three types of units.
- A peakload generating unit, normally the least efficient of the three unit types, is used to meet requirements during the periods of greatest or peak load on the system.
- Intermediate-load generating units meet system requirements that are greater than base load but less than peak load. Intermediate-load units are used during the transition between baseload and peak load requirements.
| The operating efficiency of a generating unit is a function of the amount of net heat that it can extract from the energy source for use in the production of electricity. |
Utilities also have reserve or standby generating units, which are available to the system in the event of an unexpected increase in load or an unexpected outage within the system. Consequently, an inventory of net capability must account for reserve or standby capability, as well as generating units that are not available to the system for various reasons (such as routine maintenance).
Net capability refers
to that which is operable and includes both active and inactive capability.
Once a new generator has been declared available to generate power to
the electrical grid, it is considered a part of the operable capability
of the utility until it is retired from service. Generating units that
are used for standby service, cold standby, and generators that are out
of service for an extended period (exceeding 1 year) comprise the inactive
operable capability. Active operable capability includes generators that
are generating or available to generate; this includes generators that
may be down for scheduled maintenance, refueling, or forced outages.
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Figure 10. Generating Capability at U.S. Electric Utilities by Energy Source, 1998 |
An electric utility plant (station) contains generating units and auxiliary equipment used to convert various types of energy into electric energy. A fossil-fueled generating unit may be designed to use (burn) one or more fossil fuels to produce electricity. A generating unit capable of burning more than one fossil fuel is referred to as a dual-fired unit. Some dual-fired units can only burn one fuel at a time (that is, the fuels are fired sequentially), while others can burn more than one fuel simultaneously (concurrent firing of different fuels). A sequentially fired unit generally uses one fossil fuel as its primary energy source, but can switch to a second fossil fuel as an alternate energy source.
Prime
Movers
Electric utilities use a variety of prime movers based on the loads, availability
of fuels, and energy requirements of the utility. The most common prime
movers are:
- steam turbine,
- internal-combustion engine,
- gas combustion turbine,
- water turbine, and
- wind turbine.
| A turbine converts the kinetic energy of a moving fluid (liquid or gas) to mechanical energy. Turbines have a series of blades mounted on a shaft against which fluids are forced, thus rotating the shaft connected to the generator. The fluids most commonly used in turbines are steam, hot air, or combustion products, and water. |
Most prime movers used to produce electricity today are turbines. The energy sources most often used with prime movers are the fossil fuels: coal, petroleum, and natural gas.
Steam-Turbine Generating Units.
Most of the electricity in the United States is produced in steam turbines.
In a fossil-fueled steam turbine, the fuel is burned in a boiler
to produce steam. The resulting steam then turns the turbine blades that
turn the shaft of the generator to produce electricity. In a nuclear-powered
steam turbine, the boiler is replaced by a reactor containing a core of
nuclear fuel (primarily enriched uranium). Heat produced in the reactor
by fission of the uranium is used to make steam. The steam is then passed
through the turbine generator to produce electricity, as in the fossil-fueled
steam turbine. Steam-turbine generating units are used primarily to serve
the base load of electric utilities. Fossil-fueled steam-turbine generating
units range in size (nameplate capacity) from 1 megawatt to more than
1,000 megawatts. The size of nuclear-powered steam-turbine generating
units in operation today ranges from 75 megawatts to more than 1,400 megawatts.
Gas Turbine Generating Units.
In a gas turbine (combustion-turbine) unit, hot gases produced from the
combustion of natural gas and distillate oil in a high-pressure combustion
chamber are passed directly through the turbine, which spins the generator
to produce electricity. Gas turbines are commonly used to serve the peak
loads of the electric utility. Gas-turbine units can be installed at a
variety of site locations, because their size is generally less than 100
megawatts. Gas-turbine units also have a quick startup time, compared
with steam-turbine units. As a result, gas-turbine units are suitable
for peaking, emergency, and reserve-power requirements.
The gas turbine, as is typical with peaking units, has a lower efficiency
than the steam turbine used for baseload power. The efficiency of the
gas turbine is increased when coupled with a steam turbine in a combined-cycle
operation. In this operation, hot gases (which have already been used
to spin one turbine generator) are moved to a waste-heat recovery steam
boiler where the water is heated to produce steam that, in turn, produces
electricity by running a second steam-turbine generator. In this way,
two generators produce electricity from one initial fuel input. All or
part of the heat required to produce steam may come from the exhaust of
the gas turbine. Thus, the steam-turbine generator may be supplementarily
fired in addition to the waste heat. Combined-cycle generating units generally
serve intermediate loads.
Internal-Combustion Engines. These prime movers have one or more cylinders in which the combustion of fuel takes place. The engine, which is connected to the shaft of the generator, provides the mechanical energy to drive the generator to produce electricity. Internal-combustion (or diesel) generators can be easily transported, can be installed upon short notice, and can begin producing electricity nearly at the moment they start. Thus, like gas turbines, they are usually operated during periods of high demand for electricity. They are generally about 5 megawatts in size.
Hydroelectric Generating Units.
Hydroelectric power is the result of a process in which flowing water
is used to spin a turbine connected to a generator. The two basic types
of hydroelectric systems are those based on:
- falling water and
- and natural river current.
Another kind of hydroelectric power generation is the pumped storage hydroelectric system. Pumped storage hydroelectric plants use the same principle for generation of power as the conventional hydroelectric operations based on falling water and river current. However, in a pumped storage operation, low-cost off-peak energy is used to pump water to an upper reservoir where it is stored as potential energy. The water is then released to flow back down through the turbine generator to produce electricity during periods of high demand for electricity.
Other Generating Units. Other methods of electric power generation, which presently contribute only small amounts to total power production, have potential for expansion. These include:
- geothermal,
- solar,
- wind, and
- biomass (wood, municipal solid waste, agricultural waste, etc.).
Geothermal power comes from heat energy buried beneath the surface of the earth. Although most of this heat is at depths beyond current drilling methods, in some areas of the country, magma--the molten matter under the earth's crust from which igneous rock is formed by cooling--flows close enough to the surface of the earth to produce steam. That steam can then be harnessed for use in conventional steam-turbine plants. Solar power is derived from the energy (both light and heat) of the sun. Photovoltaic conversion generates electric power directly from the light of the sun; whereas, solar-thermal electric generators use the heat from the sun to produce steam to drive turbines. Wind power is derived from the conversion of the energy contained in wind into electricity. A wind turbine is similar to a typical wind mill. However, because of the intermittent nature of sunlight and wind, high capacity utilization factors cannot be achieved for these plants. Several electric utilities have incorporated wood and waste (for example, municipal waste, corn cobs, and oats) as energy sources for producing electricity at their power plants. These sources replace fossil fuels in the boiler. The combustion of wood and waste creates steam that is typically used in conventional steam-electric plants.
Nonutility Power Producers
Early in the Twentieth Century, more than half of all electricity produced in the United States came from industrial firms. During the first half of the Twentieth Century, however, most industrial plants opted to purchase electricity from their local utilities. By 1950, the electric utility industry was serving virtually all demand for electricity--except for a few industries that generated small amounts for their own use. By the late 1970's, changing economic conditions and legislation made nonutility generation attractive once again. (For a review of legislative activities impacting the electric power industry, access the following: The Changing Structure of the Electric Power Industry: An Update.)
The nonutility power producing industry operates in various sectors of the U.S. economy and is classified according to the North American Industry Classification System (NAICS). The main classifications are:
- Agriculture, Forestry, and Fishing
- Mining
- Construction
- Manufacturing
- Transportation and Public Utilities
- Wholesale and Retail Trade
- Finance, Insurance, and Real Estate
- Services
- Public Administration
- Other.
| Major Characteristics of U.S. Nonutility Power Producers by Type | |
| Cogenerators (QF) |
• Are qualified under PURPA by meeting certain ownership, operating, and efficiency criteria established by FERC • Sequentially produce electric energy and another form of energy, such as heat or steam, using the same fuel source • Are guaranteed that utilities will purchase their output at a price based on the utility's “avoided cost” and will provide backup service at nondiscriminatory rates |
| Small Power Producers (QF) |
• Are qualified under PURPA by meeting certain ownership, operating, and efficiency criteria, established by FERC • Use biomass, waste, water, wind, solar, or geothermal as a primary energy source • Fossil fuels can be used but renewable resources must provide at least 75 percent of the total energy input • Are guaranteed that utilities will purchase their output at a price based on the utility's “avoided cost” and will provide backup service at nondiscriminatory rates |
| Exempt Wholesale Generators |
• Creation authorized by EPACT • Are exempt from PUHCA's corporate and geographic restrictions • Are wholesale producers; do not sell retail • Do not possess significant transmission facilities • Utilities are not required to purchase their electricity • Are regulated but usually may charge market-based rates |
| Cogenerators (Non-QF) |
• Are not qualified under the provisions of PURPA • Are nonutilities, utilizing a cogenerating technology, which may themselves consume part of the electricity they cogenerate |
| Noncogenerators (Non-QF) |
• Are not qualified under the provisions of PURPA • Do not utilize a cogenerating technology |
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QF = Qualifying facility (under PURPA). Source: Energy Information Administration, Electric Power Annual 1995, Volume II, DOE/EIA-0348(95)/2 (Washington, DC, December 1996). |
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Endnote:
1Energy Information Administration, Monthly Energy Review, DOE/EIA-0035(99/03) (Washington, DC, March 1999), Table 2.1, p. 23.
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