The U.S. Department of Energy’s National Energy Technology Laboratory is pursuing a new integrated energy-water R&D program that addresses water management issues relative to coal-fired power generation that takes into account the major impacts of CCS on water use. The goal of this research is to promote more efficient use of water in power plant operations and increase the availability of heretofore unusable waters for power plant use. Those practices can mitigate the impacts of CCS on power plant water use and allow for continued development of energy resources.
The production of energy requires a reliable, abundant, and predictable source of freshwater — a resource that is limited in many parts of the U.S. As we experience increasing demand for energy and water and growing limitations on supply, these resources must be managed together to maintain reliable energy and water supplies that will enable full use of the nation’s energy reserves. This may become a larger issue in a carbon-constrained world, given that current carbon capture technologies may result in increased water usage. Recent studies conducted by the U.S. Department of Energy’s National Energy Technology Laboratory (DOE/NETL) indicate that deployment of carbon dioxide (CO2) capture systems could significantly increase power plant water usage.
A key objective of DOE/NETL research and development (R&D) activities is to mitigate any impact of carbon capture on water resources. DOE/NETL is addressing this issue under two key components of its Existing Plants, Emissions, and Capture (EPEC) Program:
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The development of advanced CO2 capture technologies that require less freshwater.
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The development of advanced water management technologies that reduce freshwater usage throughout the power plant and that encourage the use of waters that heretofore were thought to be unacceptable for use.
Water Usage in Electricity Generation
Thermoelectric power generation requires significant quantities of water. For example, a 500-MW coal-fired power plant uses more than 12 million gallons of water per hour. The largest demand for this water is process cooling (the conversion of steam to liquid water in a condenser).
The two commonly used metrics to measure power plant water use are withdrawal and consumption. "Consumption" is used to describe the loss of withdrawn water — typically through evaporation into the air — that is not returned to the source. Water "use" or "withdrawal" describes drawing on water sources, typically freshwater, for process use and then returning it to the original source. So even though the plant described above may use 12 million gallons per hour, depending on the type of cooling system used, water consumption will be a very small fraction of that.
In thermoelectric power plants, there are basically two types of cooling water system designs: once-through (open loop) and recirculating (closed loop). Dry cooling systems do exist, but they are not widely used because of their negative impact on power plant efficiency and their high first cost. In once-through systems the cooling water is withdrawn from a local water body and passed through the condenser, where heat from the steam is transferred to the cooling water. The used warm cooling water is subsequently discharged back to the same water body.
The most common type of recirculating or closed-loop system uses wet cooling towers to cool condenser water. In these systems the warm cooling water is typically pumped from the condenser to a cooling tower, where the heat is dissipated directly to ambient air by evaporation of a fraction of the water and by heating the air. The remaining cooling water is then recycled back to the condenser. Because of evaporative losses, a portion of the cooling water needs to be periodically discharged from the system — known as blowdown — to prevent the buildup of minerals and sediment in the water that could adversely affect performance. For a wet recirculating system, only makeup water needs to be withdrawn from the local water body to replace water lost through evaporation and blowdown.
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