National Handbook of Recommended Methods for Water Data Acquisition

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11.J.1. Description

The thermoelectric power generation category includes water used in the generation of electric power when the following fuel types are used: fossil, nuclear, biomass, solid waste, or geothermal energy. A thermoelectric plant basically works by heating water in a boiler until it turns into steam. The steam is used directly to turn the turbine-generator, which produces electricity (fig. 10). After the steam is used to spin the turbine-generator that produces electricity, it is sent to the condenser to be cooled back into water. Most of the water used in thermoelectric power generation is used in the condenser to cool the steam back into water. Then the condensed water is pumped back to the steam generator to become steam again while the cooling water is discharged as return flow or is recycled through cooling ponds or towers. The SIC code for this category is 4911.

Figure 10.

Figure 10. Generalized diagram of thermoelectric power generating plant.

The thermoelectric power generation category includes water-use activities, such as: withdrawals from ground and surface water; deliveries from water suppliers; consumptive use from cooling towers, cooling ponds, and steam venting; water and wastewater treatment; and return flow (figure 11). The water in the steam cycle usually is treated at the power plant before use to reduce impurities that would cause build-up of mineral residue inside the boiler. The cooling water, which comes from lakes, rivers, or oceans, always is separate from the boiling water/steam (non-contact cooling water) and can be (1) discharged directly to the ocean, lake, or major river (once-through cooling), (2) discharged to a canal, cooling pond, or cooling tower before returning to the river, or (3) sent to a cooling pond or cooling tower before being recycled. Although most of the water used in thermoelectric plants is cooling water for condensing the steam, water is required for makeup water to replace the water lost as steam, blowdown (purging) of boilers, washing of stacks, plant and employee sanitation, water and wastewater treatment, and in nuclear plants, to keep the nuclear fuel from overheating and melting. Water from non-cooling uses goes to either a public wastewater treatment facility or the plant's onsite wastewater treatment facility. Storm water from roof drains and area storm sewers also may be treated at the wastewater facility and may cause discharge values to be higher than expected.

Figure 11.

Figure 11. Diagram of thermoelectric water use.

The volume of water required in the thermoelectric power generation and the rate of consumptive use are dependent on whether cooling towers are used. Some plants built on an ocean or large river simply pump in large amounts of water to cool the steam. This cooling water then is returned back to the ocean or river. The water is somewhat warmer than when it entered the plant. If the cooling water is discharged to a significant water body, such as an ocean, lake, or major river, the water can be discharged directly. However, if the water is discharged to a river, its return to the river may be delayed by routing it through a canal or cooling pond first. Some plants recycle cooling water. Cooling ponds and towers are used to transfer the heat in the cooling water to the air.

A cooling pond is a shallow reservoir having a large surface area for removing heat from water. The surface area exposed to the air may be increased through the use of spray nozzles. Cooling ponds are used where land is relatively inexpensive, cooling water is scarce or expensive, or where there are strict thermal loading restrictions in place. If cooling ponds are used, water in the pond can be reused, thus reducing the overall water-withdrawal requirement.

A cooling tower is designed to remove heat by pumping water up into the tower and allowing it to fall down inside the tower. Air comes in from the sides of the tower and passes by the falling water. As the air passes the water, it exchanges some of the heat and evaporates some of the water. This heat and evaporated water flow out the top of the tower is in the form of a fine cloud-like mist. The cooled water is collected at the bottom of the tower and pumped back into the plant for reuse. Cooling towers are used where land and (or) water are expensive, or regulations prohibit the return of once-through cooling waters.

There are two primary types of thermoelectric plants--fossil fuel plants and nuclear plants. Although there are many similarities between them, there are some important differences that affect how each plant uses water. In fossil fuel plants, coal, natural gas, or oil are burned to provide the heat necessary to turn the water into steam. Biomass or solid waste fuel types are included with fossil fuels. Water is required to take care of the ash waste created during combustion. This includes both maintaining the stacks and carrying the waste ash away from the plant (fig.11). In nuclear plants, water is not required for ash disposal but is needed to keep the nuclear material from overheating and melting. Geothermal plants are only in California, Nevada, North Dakota, and Utah (Solley and others, 1993).

Power generation is measured in watt-hours, which is an electrical energy unit of measure equal to one watt of power supplied to, or taken from, an electrical circuit steadily for one hour. Most power generated from power plants are reported in megawatt-hours, or a million (1,000,000) watt-hours.

The thermoelectric power generation water-use data are the rate of (1) withdrawal by source, (2) deliveries from public water supply, (3) evaporation, (4) return flow, and (5) recycled water. The power-generation values also are collected for quality assurance.

11.J.2. Sources of thermoelectric power generation water-use information

Sources of most of the above thermoelectric-power generation water-use data are (1) the major utility companies (2) State agency responsible for compliance to the USEPA's Clean Water Act Program; (3) the U.S. Department of Energy, Energy Information Administration (USDOE, EIA) or the State agency working with USDOE, EIA, if any; and (4) the State agency regulating water withdrawals. Individual facilities also are an important source of information.

Individual thermoelectric power generating facilities are frequently owned or managed by large utility companies. These major utilities usually handle the required environmental paperwork making them a logical first step in obtaining water-use information. Utilities that are in densely populated areas generally are involved in some sort of power pool, for example, New England Power Pool (NEPOOL), which may be able to provide some data on all the power plants in the pool. Power is bought and sold between power plants in the pool.

The State agency responsible for compliance to the USEPA's Clean Water Act Program is an important source for two primary types of information. The USEPA administers the Permit Compliance System (PCS) database, which was designed to track permit, compliance and enforcement status data for the National Pollutant Discharge Elimination System (NPDES) program under the Clean Water Act. A NPDES permit is required for all point discharges into United States waterways. The PCS database contains descriptive information on major power generating facilities, their location, and monthly return flows. The NPDES permit application and permit itself usually includes a detailed description of the plant which will provide basic information on all the sources of supply for the plant, the different ways in which water is used in the plant, and what water is included in the reported discharge values. Additionally, monthly return-flow data are available from PCS.

USDOE, EIA maintains power-generation statistics such as ownership, location, generation capacity, and power generated for a given calendar year. The information is available as hardcopy EIA Form 759. More extensive information on each facility, such as turbine data, is also available from USDOE, EIA. Some states may also have an energy agency that works with USDOE,EIA from which data can be obtained. If there is a State Agency regulating withdrawals, they probably receive reported withdrawals from the power plants.

Information also can be obtained from the plant engineer or environmental section of the individual power plants, specifically the one who prepares the Discharge Monitoring Reports (DMR) for the NPDES program. DMRs contain volume discharged from all pipes in the facility, and when compared to the permit or permit application will correlate to the source of the water and how it was used.

11.J.3. Measurement, estimation, and data-collection methods for thermoelectric power generation water-use

Thermoelectric power generation water-use data are the rate of (1) withdrawal by source, (2) deliveries from public water supply, (3) evaporation, (4) return flow, and (5) recycled water. Since thermoelectric plants tend to be major water users, data generally are metered and are available from at least two federal agencies, one or more State agencies, from the power utility, and from individual power plants. A reasonable approach to determine water use would be to do (1) an inventory of secondary data, (2) compare collected data with a coefficient, and (3) resolve inconsistencies. It generally is considered unnecessary to measure water withdrawals because of the availability of metered data.

11.J.4. Thermoelectric power generation selected references

These references are supplemental to the ones in the General reference Section.

Aschner, F.S., 1978, Planning fundamentals of thermal power plants: New York, John Wiley, 738 p.

Backus, C.E., and Brown, M.L., 1976, Water requirements for solar energy: American Water Works Association Journal, v. 68, no. 7, p. 366-369.

Cheremisinoff, N.P., and Cheremisinoff, P.N., 1981, Cooling towers selection, design and practice: Ann Arbor, Mich., Ann Arbor Science Publishers, 347 p.

Dorfman, M.H., 1976, Water required to develop geothermal energy: American Water Works Association Journal, v. 68, no. 7, p. 370-375.

Gold, H., and others, 1977, Water requirements for steam-electric power generation and synthetic fuel plants in the Western United States: U.S. Environmental Protection Agency Report no. 600/7-77-037, 259 p.

Graham, F.D., revised by Buffington, Charlie, 1983, Power Plant Engineers Guide: New York, The Bobbs-Merrill Co., Inc.

Nero, A.V., 1979, A guidebook to nuclear reactors: Berkeley, University of California Press, 303 p.

Sonnichsen, J.C., Jr., 1978, Calculations of evaporation loss requirements for thermal power plants: HEDL-TME 78-33.

U.S. Department of Energy, 1994, Electric power annual, 1993: Energy Information Administration.

Weisman, J., and Eckart, R., 1985, Modern power plant engineering: Englewood Cliffs, N.J., Prentice-Hall, 506 p.

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