Reclaimed cooling water's impact on surface condensers and heat exchangers

Dennis J. Schumerth, Valtimet Inc.

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Two hundred years after Samuel Taylor Coleridge wrote, "Water, water, everywhere, Nor any drop to drink" in "The Rime of the Ancient Mariner," power plants in many regions of the U.S. face the same challenge. Developers are increasingly finding that water may be available, even abundant, but not for power plant cooling. Particularly in the Southwest, regulators have earmarked dwindling water sources for public use only and forbidden future plants from tapping existing aquifers for cooling water makeup see box).

Power plants and water must mix

The U.S. Department of Energy's National Energy Technology Laboratory (NETL) is actively developing technologies for reducing water consumption by the next generation of power plants. The U.S. Geological Survey reports that 345 billion gallons of fresh water were withdrawn from American public supply and other sources daily in 2000. According to NETL, it takes an average of 25 gallons of water to produce 1 kilowatt-hour of electricity, or almost 100 billion gallons of fresh water a day to cool the entire U.S. thermal power plant fleet (Figure 3) The only sector that uses more water is agriculture (for irrigation). But plentiful supplies in the past are not a valid indicator of future availability in many regions.

3. Percentage of U.S. fresh water withdrawal by user category (2000). Source: NETL

NETL, in its latest research report, "Freshwater Needs Project for Future Fleet" (available online at www.netl.doe.gov/technologies/coalpower/ewr/water/index.html), notes the distinction between water use and water consumption. Water use is a temporary "borrowing" of the water, as in a once-through cooling system. Consumption, by contrast, represents actual water loss to a process or evaporation.

The report notes that in 1995 (the last year for which NETL says data are available), fossil-fueled power plants "were second only to agriculture in total freshwater withdrawals . . . [with] more than 34 trillion gallons from fresh surface and ground waters." The report then notes that "more than 97% of all withdrawals were returned to the source. Of the 36.5 trillion gallons of U.S. fresh water consumed in 1995, thermoelectric generators were responsible for less than 3.5% of the total, making it the smallest consumer category of U.S. fresh water resources. In comparison, 81% of the water used in irrigation is consumed [Figure 4]."

4. Percentage of U.S. fresh water consumption by user category (1995). Source: NETL

The challenge for the industry is to reduce withdrawal and consumption of fresh water at the same time that Clean Water Act Section 316(b) regulations and a concerted push to reduce river and ocean once-through cooling will inevitably require more makeup water for closed-loop cooling water systems.

The U.S. enjoys a much lower cost of raw water than other developed nations. For example, the price in Southern California is about $2.25/1,000 gallons, and it's no more than $3/1,000 gallons in Phoenix, even during the summer. Europeans pay a lot more, for instance, over $6/1,000 gallons in Germany. Water is a great bargain, compared to that cup of Starbucks brew you paid $32/gallon for this morning and that $21/gallon bottle of Evian you drank at lunch.

Surprisingly, although technology for making the "gray water" effluent of municipal wastewater treatment plants suitable for industrial or agricultural use is tried and true, relatively little of it is recycled. Of the 24,000 plants of that sort in the U.S., only about 1,500 are equipped with water reuse facilities. Indeed, only 6% of total U.S. municipal wastewater volume is "repurposed," rather than dumped into waterways. Each day, U.S. industrial plants consume 25 billion gallons of water and generate about 20 billion gallons of wastewater. American thermal power plants alone use 186 billion gallons of water daily (Figure 1).

1. Thirsty processes. Levels of water input, output, and internal use in a typical 500-MW coal-fired power plant cooled by recirculation. Source: NETL

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