Use of constructed wetlands or a cooling pond is a good option for thermal power plant condenser cooling. Power plant water use for cooling has become an issue of concern for thermal power plant siting and new construction. A cooling pond or constructed wetlands are lower cost than dry cooling.
Thermal power plant condenser cooling can be accomplished with a cooling pond or with constructed wetlands. This is a low profile, relatively low cost power plant water cooling option that isn't getting much publicity. Once through cooling and evaporative cooling with a cooling tower have been the mainstays for thermal power plant condenser cooling, as needed to convert low pressure steam from the turbine to liquid water that can be pumped into the boiler to keep the Rankine cycle going. More recently dry cooling with an air cooled condenser and hybrid wet and dry cooling are coming into use, but at an increased capital and operating cost. Meanwhile, quietly, behind the scenes, the cooling pond option has come into use for 15% of U.S. non-nuclear electric generating capacity.
How Would a Cooling Pond Work?
The flow pattern for a cooling pond used for condenser cooling in a thermal power plant may be 'once through' or 'recirculating' as shown in the two flow diagrams in this section.
For the once through cooling pond system the pond is simply used to eliminate or reduce the thermal effect on the river or lake serving as the power plant water source. There will be water loss from the cooling pond by evaporation, which is the primary cooling mechanism.
For a recirculating cooling pond system, cooling water is drawn from and discharged to the cooling pond. Only enough makeup water to compensate for the evaporation loss is needed from the river.
For either type of cooling pond system, rainwater harvesting will bring direct precipitation and stormwater runoff to the pond, to compensate for part of the evaporation losses, as indicated in the diagrams. If the site hydrology is favorable, the evaporation losses from the cooling pond could be balanced by rainwater harvesting input, making the cooling pond power plant water system a “closed system," needing no water from the river.
Constructed Wetlands for Thermal Power Plant Condenser Cooling
Much of the United States was wetlands (swamps, marshes, bogs, etc.) 200 years ago. A lot of the wetlands were drained to allow the land to be put to productive use, but there are still many sites suitable as locations for restored or constructed wetlands. The general flow patterns for constructed wetlands used for thermal power plant cooling would be similar to those discussed in the last section.
Constructed wetlands would typically not be as deep as a cooling pond, so more land area would be needed to give the same volume of water. Also, constructed wetlands would be designed to accommodate wetland plants and to serve as a wildlife habitat. A few power plants around the country use constructed wetlands in some way. One of the references at the end of this article provides information about one of them.
Comparison with Other Thermal Power Plant Condenser Cooling Options
The diagram at the left gives information about the number of each of the common types of thermal power plant condenser cooling systems, and about the average cost for each type in $/kW. The graph is adapted from one in the DOE/NETL 2009 reference given below, which is based on data from the U.S. Department of Energy EIA reference. This graph show the cost of a cooling pond for thermal power plant condenser cooling to be similar to that for once through cooling or recirculating cooling with a cooling tower. As discussed above constructed wetlands or a cooling pond system has the potential to perform better than a standard once through or recirculating system with regard to power plant water withdrawal and consumption rates.
DOE/NETL, Water Requirements for Existing and Emerging Thermoelectric Plant Technologies, DOE/NETL Report 402/080108. August 2008 (April 2009 revision).
U.S. Department of Energy, Energy Information Administration (EIA). Form EIA-767: Annual Steam Electric Power Plant Operation and Design Data. 2005 data. http://www.eia.doe.gov/cneaf/electricity/page/eia767.html.
Sparks, S, Dougherty, T, and Whitford, J, 2009, "Habitat Creation Using Constructed Wetlands," Land and Water, Jan/Feb 2009. Available at http://www.nawe-pa.com/docs/51%20Habitat%20Creation%20Using%20Const%20Wetlands.pdf.
About the Author
Dr. Harlan Bengtson is a registered professional engineer with 30 years of university teaching experience in engineering science and civil engineering. He holds a PhD in Chemical Engineering.