How does electric power production use and consume water?

Cooling Technologies
Thermal electric generating facilities make electricity by converting water into high-pressure steam that drives turbines. Once water has gone through this cycle, it is cooled and condensed back to water and then reheated to drive the turbines again. The process of condensation requires a separate cooling water body to absorb the heat of the steam. These condenser systems typically consist of banks of thousands of one-inch diameter tubes, through which cooling water is run, and over which the hot steam and water is circulated.

Two cooling technologies are in use today:

• Closed-cycle systems discharge heat through evaporation in cooling towers and recycle water within the power plant. The water required to do this is comparatively small since it is limited to the amount lost through the evaporative process. Because of the expense associated with closed-cycle cooling, once-through systems are far more common.
  
  
• Once-through systems require the intake of a continual flow of cooling water. The water demand for the once-through system is 30 to 50 times that of a closed cycle system.

The amount of water used for power plant cooling also varies by each specific power plant's electricity generating technology and size. For example, nuclear reactors require the most water for cooling, and baseload fossil fuel power plants come in second. Steam electric generating plants across the nation draw in more than 200 billion gallons per day. Most renewable energy technologies require little or no water for cooling.

Hydropower Generation
To generate power, hydropower plants divert water from the river through turbines. Water is diverted from the river via an intake at the dam. At some hydropower plants, the turbines are located in the dam and thus the water is released again right below the dam. At other hydropower plants, the turbines are located in a powerhouse significantly downstream from the dam (in order to generate enough height difference, or "head," between where the water is diverted to where the power is generated). This means that the water can be diverted outside of the stream for some distances, sometimes several miles, before being released back in the river.

What do we mean by water use and consumption?

Most electric power plants require water to operate. Nuclear and fossil fuel power plants drink over 185 billion gallons of water per day. Geothermal power plants add another 2 billion or so gallons a day. Hydropower plants use water directly to generate power. These power plants represent the single largest consumer of water among any industrial, governmental or residential activity. Since 98 percent of the water used in power plants is returned to its source, distinctions are made between use and consumption.

Water use is a measure of the amount of water that is withdrawn from an adjacent water body (lakes, streams, rivers, estuaries, etc.), passes through various components of a power plant, and is then ultimately discharged back into the original water body. Environmental concerns surrounding water use center around any chemical or physical alteration of the water body and any impacts these changes may have on the plants, fish and animals who reside in the ecosystem.

Water consumption refers to water sucked up in power plant operations that is lost, typically through evaporation. The primary concerns surrounding water consumption is how best to utilize this essential resource, especially in areas, such as deserts in the West, where water is in short supply.

What are the consequences of water use and consumption?

Withdrawal of large volumes of surface water for either power plant cooling or hydropower generation can kill fish, larvae and other organisms trapped against intake structures (impinged), or swept up (entrained) in the flow through the different sections of a power plant.

Large fossil fuel and nuclear plants require incredible quantities of water for cooling and ongoing maintenance. The Salem Nuclear Generating Station alone takes 3 billion gallons a day from the Delaware Bay. Studies of the environmental consequences of this phenomenal water demand indicates that Salem is responsible for an annual 11 percent reduction in weakfish and 31 percent reduction in bay anchovy. At the Indian Point 2 and 3 reactors on the Hudson River, the number of fish impinged totaled over 1.5 million fish in 1987. The 90 power plants using once-through-cooling (see below) on the Great Lakes kill in excess of 40 million fish per year due to impingement (Pace University, Environmental Costs of Electricity, p. 287).

The use of water to generate power at hydropower facilities imposes unique, and by no means insignificant, ecological impacts. The diversion of water out of the river removes water for healthy in-stream ecosystems. Stretches below dams are often completely de-watered. Fluctuations in water flow from peaking operations create a "tidal effect," disrupting the downstream riparian community that supports its unique ecosystem. A dam's impoundment slows water flows, which hinders natural downstream migration of many fish species. By slowing river flows, dams also allow silt to collect on river and reservoir bottoms and bury fish spawning habitat. Silt trapped above dams accumulates heavy metals and other pollutants. Disrupting the natural flow of sediments in rivers also leads to erosion of riverbeds downstream of the dam and increases risks of floods.

The impoundment of water by hydropower facilities fundamentally reshapes the physical habitat from a riverine to an artificial pond community. This often eliminates native populations of fish and other wildlife. Dams also impede the upstream and downstream movement of fish and other wildlife, and prevent the flow of plants and nutrients. This impact is most significant on migratory fish, which are born in the river and must migrate downstream early in life to the ocean and then migrate upstream again to lay their eggs (or "spawn"). As mentioned above, withdrawal of water into turbines can also impinge or entrain significant numbers of fish.

(See also Hydropower Generation, Water Quality and Land Impacts Issue Papers for more information on hydropower impacts.)