BY TRACY BOYER
Source: USGS “Estimated Use of Water in the United States County-Level Data for 2005” (latest data available)
Wet cooling is the most popular technique used in thermal power plants across the country. There are two types of wet cooling: once-through and closed-loop. Closed-loop can also be referred to as cooling towers or recirculated cooling. Kent Zammit from the Electric Power Research Institute summarized:
“The most efficient way to condense steam is with water, since it can absorb much more heat than other coolants, like air. Cooling water can be heated and returned to the environment (once-through cooling) or circulated to a cooling tower to be cooled and reused to condense more steam. A wet cooling tower evaporates part of the water to cool the remaining water. This is the effect you feel when you hold your wet finger to the wind or get out of a swimming pool on a hot day – as the water on your skin evaporates, it provides a cooling effect.”
Under section 316(b) of the Clean Water Act, EPA enforced a performance standard for new thermal plants that limits the thermal discharge impacts to those seen in closed cycle cooling plants, where significantly less water is withdrawn, but approximately 60 to 70 percent more water is consumed in the process.
Degraded water sources (such as reclaimed water, salt water, sewage treatment effluent, agricultural runoff, mine water, saline groundwater and stormwater) can be integrated into the wet cooling process to minimize the use of freshwater as long as the storage and quality of the resource is consistent and the water source is located near the plant.
According to Scott Sutton, environmental communications specialist at Progress Energy, less than 1 percent of total water withdrawn at the company is reclaimed, whereas approximately 25 to 30 percent is salt water.
Dry cooling is an alternative to wet cooling, but is significantly less efficient, since dry cooling can only reduce the cooling water to the ambient air temperature. Other downfalls are the loud fans, increased land requirement, and higher upfront capital and operating costs.
Zammit noted that there is not a perfect solution for cooling that can conserve water and energy within the plant and also preserve wildlife around the plant.
“There are new once-through cooling designs that have proven lower impacts to aquatic communities than cooling towers would have, and the gains in efficiency serve to reduce air and carbon emissions, conserve water and keep costs lower. There are cases where dry cooling is the only solution because water is just not available. In other cases a degraded water source can replace freshwater and can solve two problems at once – finding a use for wastewater that could otherwise cause environmental issues, while providing efficient cooling of a power plant,” he said.
“By having as many options available to plant developers as possible, all of the factors mentioned can be considered when deciding which cooling system is optimal.”