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4.1.1 Power Plant Cooling Water

Electric power generation uses water in several ways and varying amounts depending on the type of generation technology and cooling system employed (CEC, 2002, p. 1-3). Water is used primarily for condensing steam, which is referred to as power plant cooling. The basic process is shown in Exhibit 4-1. A boiler or other heat source is used to produce steam, which is used to turn a turbine. The turbine turns the generator, which produces electricity. After turning the turbine, the steam must be condensed back to water. A condenser is used to transfer the heat from the steam to cooling water. The condensed water is pumped back to the boiler to start the cycle again.

Cooling water systems are configured in two types: once-through and recirculating.

• Once-Through Systems. In cooling systems that only use water once—i.e., once-through systems—the cooling water is drawn from a source such as a river, lake or ocean. The cooling water takes the heat that is transferred from the steam in the condenser and is discharged, typically back to its source. The cooling water typically increases by about 20°F (CEC, 2002, p. 2-4), and can increase by more than 30°F (CEC, 2001, p. 27).

• Recirculating Systems. Some cooling systems recirculate the cooling water. As shown with dashed lines in the exhibit, the recirculated cooling water is typically cooled using a cooling tower. The cooled water can then be pumped back to the condenser to pick up heat. In some cases a cooling pond is used in place of a cooling tower.

Condensing steam in the condenser is a critical component of the power plant. A properly operating condenser, with cooling water at the appropriate temperature, prevents backpressure from building up at the turbine. If backpressure builds up, a 5 to 10 percent reduction in

electricity production can occur (USEPA, 2001c, pp. 3-9 to 3-12). If the backpressure rises to unacceptable levels, the plant must be taken off line. Consequently, care is taken to ensure that the cooling system operates properly.

Exhibit 4-1: Typical Cooling Water Configurations

Once-through cooling systems withdraw very large amounts of water, and discharge virtually all the water back to its original source. Prior to the enactment of the Clean Water Act, once-

through cooling was the dominant choice for power plant design (Micheletti and Burns, undated, p. 3). Following the requirement that best available technologies be used to minimize the

environmental impact of cooling water use, recirculating systems became the standard cooling method for newly constructed power plants (Micheletti and Burns, undated, p. 2). Recirculating systems withdraw much less water, but a portion of the water is evaporated in the cooling tower. Nearly all the power plants in the United States that use steam turbines use either once-through or recirculating cooling water systems (or a combination of both). The steam may be produced

Steam Pump Condenser Boiler or Other Heat Source Generator Pump Cooling Water Intake Cooling Water Discharge

(Once Through) Cooling Tower

Recirculating Cooling Water

Steam Turbine Pump Recirculating Cooling Water Cooling Water Discharge (Recirculating) Steam Pump Condenser Boiler or Other Heat Source Generator Pump Cooling Water Intake Cooling Water Discharge

(Once Through) Cooling Tower

Recirculating Cooling Water

Steam Turbine Pump Recirculating Cooling Water Cooling Water Discharge (Recirculating)

by coal, gas, nuclear energy, or other fuels. In nearly all cases, however, cooling water is used to condense the steam. About two-thirds of the utility electric power plant capacity in the U.S. is steam generation (analysis of data in EIA, 2008) and about 85 percent of recent U.S. electricity production was from steam (EPRI, 2002, p. vii). Consequently, cooling water is commonly used to condense steam at power plants in the U.S.

About 22 percent of electric power generating capacity is combustion turbines in which fuel (typically natural gas) is burned directly in a turbine.11 Because steam is not used in the process, cooling water is not used to condense steam. Nearly one-third of this combustion turbine capacity is configured in combination with a steam turbine to provide a “combined cycle” power plant. In this configuration, fuel is burned directly in the combustion turbine to turn a generator. The exhaust from the turbine is hot, so it can be used to produce steam.

In Exhibit 4-1, the hot exhaust from the combustion turbine would be the heat source for the steam. Once the steam is produced, the process of using the steam to produce electricity is as depicted in the exhibit. Consequently, in combined cycle power plants, cooling water is used on the steam portion of the power production, but not on the combustion portion.12

The remaining electric power capacity in the U.S. is made up of hydroelectric power (about 10 percent) and other miscellaneous sources. Cooling water is not used in these other plants.

4.1.2 Other Uses of Water at Power Plants

Water is used at electric power plants for several purposes in addition to cooling water for condensing steam, including the following (CEC, 2002, p 1-4):

• Steam: The water for producing steam must be replaced periodically. • Emissions Control: Water is used in some NOx control systems.

• Auxiliary Equipment Cooling: Water may be used to cool various pieces of equipment. Chief among the cooling applications can be intake air cooling for combustion turbines. Intake air to the combustion turbine may be cooled to prevent loss of power output of the turbine, particularly during hot weather (TICA, 2008).

• Plant Maintenance and Personnel Needs: Water is used for cleaning and related uses, as well as for toilets, showers, drinking water, and other personnel needs.

Although these water uses are small compared to cooling water withdrawals, they can comprise up to nearly one-third of total water consumption at individual combined cycle power plants with recirculating cooling systems (CEC, 2002, p. 1-4).

One study recently examined whether hydroelectric power is an important consumer of water (Torcellini, et al., 2003). Water flowing through turbines and into the river is not considered a consumptive use of water because the water is immediately available for other uses. However, the authors examined the increased evaporation associated with converting a flowing stream to a reservoir. When viewed in this way, the creation of a reservoir causes a substantial increase in evaporation, which may be considered a consumptive use of water because the water is no longer available for use downstream (Torcellini, et al., 2003, p. 3). The amount of increased

A combustion turbine is similar to a jet engine, in which fuel is combusted to turn the turbine directly without the use of steam.

12

Gas-fired combined cycle power plants are increasingly the design of choice of new plant construction. By virtue of using the waste heat from the combustion cycle, they are more efficient than traditional steam plants. Additionally, by using natural gas, they have lower air emissions than typical coal-fired plants.

evaporation was calculated to be significant, with total water consumption more than double the consumption for power plant cooling water.13

It is important to recognize that dams and reservoirs have wide ranging impacts on the local environment, as well as diverse benefits in terms of water supply, recreation, and flood control. It is inappropriate to assign 100 percent of the evaporative consumption of water from reservoirs solely to electric power production. For purposes of this report, we acknowledge that these evaporative losses are significant, but do not address them further.

As mentioned above, once-through cooling systems increase the temperature of the cooling water prior to its discharge. The increased water temperature can increase the amount of evaporation from the receiving body of water (EPRI, 2002, p. 3-2). These incremental

evaporative losses can be significant, and may be considered a consumptive use of water from these plants even though the consumption does not occur on site.