The steam power plant water going through the condensing unit requires cooling so that liquid condensate can be pumped back into the boiler. Cooling for the condenser may be supplied by once through cooling, a wet cooling tower, dry (air) cooling, a hybrid (wet/dry) system, or a cooling pond.
The condensing unit is a crucial part of a steam power plant water circulation system . The condenser must have cooling in order to convert low pressure steam leaving the turbine to liquid condensate that can be pumped into the boiler. The typical options for this cooling are cooling water that is discharged back to the source body of water at an elevated temperature (once through cooling), a wet cooling tower that provides evaporative cooling, an air cooled condenser (dry cooling), a hybrid wet/dry cooling system (primarily air cooled with evaporative cooling use as needed), or a cooling pond.
The Condenser as part of the Power Cycle
The part that the condensing unit plays in a Rankine cycle steam power plant is shown in the diagram at the left. High pressure steam from the boiler passes through the turbine, thus causing it to rotate and generate electricity. The low pressure steam leaving the turbine must be converted to liquid condensate that can be pumped into the boiler to keep the cycle going. The diagram shows the simplest type of cooling system, once through cooling, whereby cooling water is withdrawn, typically from a surface water body, with heated water from the condenser discharged back to the surface water body. This type of condenser cooling withdraws water from the source water body at a very great rate. It returns almost all of that water to the source water body, but at an elevated temperature. This type of steam power plant water cooling system is used in most U.S. plants built before 1970. (Bistline, 2008)
Wet Cooling Tower Cooling
An alternative cooling method that requires much less cooling water withdrawal from the source water body is the wet cooling tower. The diagram at the right shows the general flow patterns. The heated water leaving the condenser passes through a cooling tower and is cooled by evaporative cooling. This type of cooling system withdraws much less water from the source water body, but it consumes much more water than the once through system, due to evaporation into the atmosphere. This type of steam power plant water cooling system is used in many U.S. plants built since 1970. (Bistline, 2008)
Dry Cooling with an Air Cooled Condenser
Dry cooling, utilizing an air cooled condenser, is illustrated in the diagram at the left. The air cooled condenser, often referred to as 'dry cooling,' eliminates the need for steam power plant water use for cooling. There has been increasing interest in dry cooling/air cooled condenser use, because of increased concern about and regulation of water use for electric power production. The dry cooling option, however, comes at an increased initial cost, increased operating cost and increased heat rate (decreased efficiency), leading to increased cost for production of electricity. Recently hybrid wet/dry systems are being developed and put into use, with less cost and less efficiency penalty than strictly dry cooling.
Use of a Cooling Pond
A final option is use of a cooling pond as shown in the diagram at the right. The water consumption is about the same as for a wet cooling tower, because the cooling in the pond is primarily due to evaporation. Based on information from a Department of Energy Report, cooling ponds are used by about 15 % of U.S. electric steam power plant generating capacity as the primary method of cooling for their condensing unit. (DOE/NETL, 2008)
1. Bistline, John, 2008, Impacts of Electric Generation Portfolio on Water Resources, Presented at Carnegie Mellon Electric Industry Seminar Series, March 26, 2008.
2. DOE/NETL, 2008, Estimating Freshwater Needs to Meet Future Thermoelectric Generating Requirements - 2008 Update, DOE/NETL-400/2008/1339, Sept. 30, 2008.
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.