Power Plant Water Management

The availability of clean and reliable sources of water is a critical issue across the United States and throughout the world. Under the Innovations for Existing Plants Program (IEP), the National Energy Technology Laboratory (NETL) has pursued an integrated water-energy R&D program that addresses water management issues relative to coal-based power generation. This initiative intended to clarify the link between energy and water, deepen the understanding of this link and its implications, and integrate current water-related R&D activities into a national water-energy R&D program. Please click on each research area for additional information.

Non-Traditional Sources of Process and Cooling Water   Non-Traditional Sources of Process and Cooling Water
Research in this area typically involves waters that have previously been considered unsuitable for cooling water purposes due to some form of organic or inorganic contamination, such as the presence of high dissolved solids concentrations. These non-traditional sources can range from mine drainage waters to produced waters from mineral extraction processes to municipal wastewaters.
     
Innovative Water Reuse and Recovery   Innovative Water Reuse and Recovery
Research in this area involves capturing water that historically has been discharged in either aqueous or vapor form and reusing the water in the power plant. Applications here range from ash pond waters to water captured from flue gases.
     
Advanced Cooling Technology   Advanced Cooling Technology
Research in advanced cooling involves innovative ways to cool power plant waters while minimizing water consumption. Systems being evaluated range from advanced mechanical systems (i.e., cooling towers) to constructed wetlands that can help cool power plant waters and provide wildlife habitat.
     
Advanced Water Treatment and Detection Technology   Advanced Water Treatment and Detection Technology
Proposed controls on the emission of mercury and other trace elements have raised concerns about the ultimate fate of these contaminants once they are removed from power plant combustion flue gas. Preventing these air pollutants from being transferred to surface or ground waters will be critical. In addition, ammonia from selective catalytic reduction systems used to control nitrogen oxide emissions can appear in a power plant’s wastewater streams.

Thermoelectrical Power Generation
Thermoelectric power generation includes power plants that utilize coal, nuclear, oil, natural gas, and the steam portion of gas-fired combined cycles. Thermoelectric generation represents the largest segment of U.S. electricity production, with coal-based power plants alone generating about half of the nation’s electric supply. According to water use survey data from the U.S. Geological Survey (USGS), thermoelectric generation accounted for 41 percent of all freshwater withdrawals in the nation in 2005, slightly ahead of irrigation. Each kilowatt-hour (kWh) of thermoelectric generation requires the withdrawal of approximately 25 gallons of water (weighted-average for all thermoelectric power generation), which is primarily used for cooling purposes. Other power plant water uses include water for operation of pollution control devices such as flue gas desulfurization (FGD) technology as well as for ash handling, wastewater treatment, and wash water.

The following figure illustrates the percentage of total U.S. freshwater withdrawals by source category for 2005.

U.S. Freshwater Withdrawal 2005

U.S. Freshwater Withdrawl 2005

Source: United States Geological Survey. 
Estimated Use of Water in the United States in 2005.

It is important to distinguish between water withdrawal and water consumption. Water withdrawal represents the total water taken from a source while water consumption represents the amount of that water withdrawal that is not returned to the source, generally lost to evaporation. USGS freshwater consumption data for the year 1995 (the most recent year for which this data is available) is presented in the figure below. Freshwater consumption for thermoelectric uses appears low when compared to other use categories. However, even at 3 percent consumption, over 3 billion gallons a day (BGD) were consumed. It is expected that the quantity of freshwater consumed by thermoelectric generation is higher today than in 1995.

U.S. Freshwater Consumption 1995

U.S. Freshwater Consuption (1995)

Source: United States Geological Survey. 
Estimated Use of Water in the United States in 1995.

Three general types of cooling system designs are used for thermoelectric power plants: once-through, wet recirculating, and dry. The cooling water is drawn from a local body of water for once-through cooling systems and returned to the same body of water after use. As a result, the withdrawal rate is relatively high, while water consumption remains low.

Two primary technologies are used for wet recirculation cooling systems:wet cooling towers and cooling ponds. Warm water from the power plant steam condenser is pumped into the cooling tower where the ambient air transfers the heat from the water, evaporating some of the water, and creating a water vapor plume. The remaining water is returned to the condenser for reuse. This system has low withdrawal but high consumption due to evaporative losses and the replacement water used for “blowdown”: a process to prevent the buildup of minerals and sediments in the water. A cooling pond relies on natural conduction/convection heat transfer from the water to the atmosphere to cool recirculating water.

Dry recirculating cooling systems use either direct or indirect air-cooled steam condensers. In a direct air-cooled steam condenser, the turbine exhaust steam flows through air condenser tubes that are cooled directly by conductive heat transfer using a high flow rate of ambient air that is blown by fans across the outside surface of the tubes. Because air is used to cool the outside of the tubes, cooling water is not used. In an indirect air-cooled steam condenser system, a conventional water-cooled surface condenser is used to condense the steam, but an air-cooled closed heat exchanger is used to conductively transfer the heat from the water to the ambient air. As a result, cooling water is not lost to evaporation with an indirect-air dry recirculating cooling system, and both water withdrawal and consumption are minimal. Dry recirculating cooling systems are not as prevalent as the wet recirculating cooling systems due to relatively higher capital and operating costs and lower performance.


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