Advanced cooling technologies for water savings at coal-fired power plants

By Sean Bushart
Senior Program Manager, Electric Power Research Institute

Electric power generation requires reliable access to large volumes of water. This need persists at a time of declining supply, when regions of the world are experiencing water constraints due to population growth, precipitation fluctuations, and changing demand patterns. Water constraints could affect future electricity generation technology selection, plant siting, and plant operation.

Although water needs are plant specific, for most pulverized coal-fired power plants over 90% of water demand is drawn for cooling. As a result, the Electric Power Research Institute (EPRI) and other research organizations worldwide are seeking to optimize power plant water utilization by developing technologies to reduce the largest single use—cooling. Thermal power plants cannot operate without adequate cooling; steam from the electricity generation turbine must be cooled to minimize back pressure on the turbine. Improved cooling allows for power plants to operate at an overall higher efficiency.

Most existing power plant cooling systems in the U.S. are based on wet cooling technologies. Alternative dry cooling technologies that reduce water consumption are available and becoming more prevalent. However, these technologies often come with economic tradeoffs (higher capital expenses, increased operational/maintenance costs) and steam-condensing performance penalties.

Advanced cooling technology development, therefore, is focused on research to improve the efficiency of existing cooling technologies and to discover techniques, designs, and applications that reduce the economic disadvantages and improve the efficiency associated with alternative technologies. Other important considerations for improvements include reducing the size or footprint of cooling systems, utilizing alternative coolants instead of potable water, and enhancing condensation, evaporation, and sensible heat transfer mechanisms.

Cooling Systems

Power plant cooling technologies generally include four different types: once-through cooling, recirculated wet cooling, dry cooling, and hybrid cooling.

Once-Through Cooling

Once-through cooling (OTC) systems withdraw water from a natural water body (such as a lake, river, ocean, or manmade reservoir). The water is pumped through the tubes of a steam condenser (see Figure 1 for a schematic) where it is warmed about 10-30°F (8-17°C) depending on system design, after which it is returned to the original source. The amount withdrawn varies from 25,000-50,000 gallons/MWh (95-190 m3/MWh). Although none of the water is consumed within the plant, some consumptive loss results due to evaporation from the receiving water body because of the increased temperature of the discharge. The amount of water lost due to evaporation is difficult to accurately calculate because of site-specific factors (e.g., temperature differential, wind speed, ambient humidity), but it has been variously estimated as 0.5-2% of the withdrawn amount, or 100-400 gallons/MWh (0.38-1.5 m3/MWh).

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Taxonomy

• Clean Tech
• Cooling Systems