Recycling water
As mentioned earlier, closed-loop cooling systems require less fresh water withdrawals than once-through systems, but they consume more water due to evaporation. In addition, water may be consumed by flue gas scrubbing and be lost to cooling tower blowdown. The development of new technologies could reduce losses from each of these processes, as could the reuse of “gray water” for cooling.
The 480 gal/MWh loss to evaporation from a typical coal-fired power plant represents the greatest opportunity for savings. Evaporative losses can be reduced if water vapor can be condensed and returned to the cooling system. Small-scale tests of one technology, which uses crosscurrents of ambient air for condensation, show potential for capturing 12% to 30% of evaporative losses if engineered to full scale. A 2006 paper by Ken Mortenson argued that this technology could cut losses by 60 to 140 gal/MWh, with the high end applying to hotter climates.
This reduction in water losses can be translated into dollar savings at the plant level by assuming a cost of water and a plant capacity. Using the representative midrange total water cost of $2.82/kgal developed earlier, the savings would range from $0.17 to $0.39/MWh. For a 350-MW baseload plant operating year-round, savings from reducing evaporation from its cooling towers would amount to between $500,000 and $1,200,000, with a midrange value of $870,000. Increasing the towers’ cycles of concentration and reducing blowdown losses (see below) might save the same plant another $300,000 to $1,200,000 annually.
Beware of blowdown
As water evaporates from a cooling tower, the concentrations of dissolved and suspended solids in the remaining water increase. To minimize scaling, fouling, and corrosion of the cooling system, these concentrations are reduced by blowdown. Blowdown is the term for the discharge of water from the cooling system and its replacement by fresh makeup water taken from a river, lake, or well. The term “cycles of concentration” describes the proportion by which evaporation increases constituent concentrations (assuming the typical evaporation rate of 480 gal/MWh). For example, at two cycles of concentration, evaporation doubles constituent concentrations, relative to intake water.
The development of cooling system materials that are resistant to scaling, corrosion, and fouling may make it possible to operate at higher solids concentrations, significantly reducing blowdown losses. A study by EPRI and the California Energy Commission found that doubling cycles of concentration from 4 to 8, which exceeds the usual allowable range, could reduce blowdown by about 100 gal/MWh (Figure 2). (See the POWER articles, “Southern California Public Power Authority’s Magnolia Power Project” in September 2005 and “High Desert Power Plant” in September 2003, for examples of plants running high cycles-of-concentration cooling towers with zero liquid discharge systems.)
2. Blowdown blowup. Typical water losses from cooling towers at various cycles of concentration. Source: EPRI, 2007
As we did for reductions in evaporative losses, we can translate reductions in blowdown water losses into dollar savings at the plant level by assuming a cost of water and a plant capacity. Using $1 to $4/kgal for the total water cost range, savings from reducing blowdown losses would come in at 10 to 40 cents/MWh. As mentioned earlier, for a 350-MW baseload plant operating year-round, the potential savings would be $300,000 to $1,200,000, with a midrange value of $860,000.
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