When the well’s dry, we know the worth of water,” wrote Benjamin Franklin in Poor Richard’s Almanac (1746). Power plant owners are becoming very familiar with that economic lesson.
The electric power industry requires reliable supplies of water in large quantities for cooling and—to a lesser extent—for flue gas desulfurization and ash handling. Water use remains a contentious issue for the U.S. industry, whose plants account for 40% of freshwater withdrawals nationwide but only 3% of freshwater consumption, according to a 2004 U.S. Geological Survey (USGS) report.
As America’s population and electricity use continue to grow, power plants are increasingly competing with farms, factories, businesses, and households for limited supplies of water. Because the growth of fresh water supplies is limited, growth in electricity demand can be met only by developing technologies that reduce the volume of fresh water required per kilowatt-hour of power generated.
Power generators have a vested interest in conserving water to make local and regional supplies last longer. Doing so helps guarantee not only future plant operations but also a growing economy with greater electricity demand.
In a 2006 report, the National Energy Technology Laboratory (NETL) projected that the lion’s share of the new capacity installed between 2005 and 2030 will be in arid regions, including southeast, southwest, and western states. Those are the areas where adopting new water-conserving technologies will likely be most cost-effective for plant operators, due to the shrinking availability and the rising cost of water there (Figure 1).
1. Supply vs. demand. The Thermoelectric Cooling Constraint Index is based on the Water Supply Sustainability Index (WSSI), which takes into account the amount of available renewable water and sustainable groundwater use, limits on freshwater withdrawals needed to protect endangered species, an area’s susceptibility to drought, and its expected growth in water use and power production. An area is considered highly constrained if its WSSI is 3 or greater and moderately constrained if its WSSI is between 2 and 3. Source: NETL, 2006
The purpose of this article is not to review all the available conservation technologies but rather to introduce their potential cost savings to power developers. Another aim is to challenge the original equipment manufacturing community to produce engineered products that minimize water consumption and/or use.
Though existing plants can benefit from retrofitting new technologies, the greatest potential cost savings lies in integrating new technologies into new plant designs. The longer amortization period of investment in new plants makes new technologies more attractive for those plants.
Open- vs. closed-loop cooling
Not all water withdrawals result in consumptive use, and this distinction is especially important for the electric power industry. Many older plants use once-through cooling, which heats large volumes of water and then returns that water, with little volume loss, to a river, a lake, or an ocean.
As a result of Clean Water Act Section 316(b) provisions and public pressures, most jurisdictions now discourage or prohibit construction of new once-through cooling systems. A 2002 EPRI report found that a typical system at a plant burning a fossil fuel, biomass, or waste requires withdrawals of 20,000 to 50,000 gallons/MWh, although it only consumes (loses) 300 gal/MWh. However, the large volume of water withdrawn by a once-through system can entrain and impinge aquatic organisms, and the discharge of heat to surface waters may have adverse ecological effects. Once-through systems may be retrofitted with helper towers or use groundwater to dilute discharge and mitigate temperature problems.
For new installations, closed-loop (recirculating) cooling systems are increasingly required. Because recirculating systems cool by evaporation from towers or cooling ponds, they consume more water than once-through systems, but they withdraw a lot less. The actual rates of water withdrawal and consumption depend on the plant’s generation technology and environmental conditions. But for a typical plant, as described in the previous paragraph, a closed-loop system would require withdrawals of just 500 to 600 gal/MWh and lose 480 gal/MWh to evaporation, according to the 2002 EPRI report.
The changing mix of once-through and recirculating cooling systems—as well as water-conserving improvements to them—enabled the electric power industry to reduce its water withdrawals per unit of power generated by a factor of three over a 50-year period: from 63,000 gal/MWh in 1950 to 21,000 gal/MWh in 2000 (Table 1). Over the same period, power generation increased by a factor of 15.
Table 1. Water use efficiency. These were the historical generation and water withdrawal and efficiency values for closed-loop cooling systems serving U.S. coal-, biomass-, and waste-fired power plants. Sources: USGS and EIA
Table 2. Water costs. Here are recent representative costs of acquiring, transporting, and treating/disposing of 1,000 gallons of water. A reasonable range for the overall cost of water is $1/kgal to $4/kgal. Source: EPRI, 2004
The cost of delivering water depends on distance and terrain but varies over a narrower range than acquisition cost. Research shows this component of water cost can be as little as 13 cents/kgal or as much as $1.20/kgal.
The cost to treat and dispose of cooling water varies much more widely, depending on the characteristics of the raw water. Surface water may be suitable for cooling with minimal treatment or may require removal of suspended solids. Because effluent from wastewater treatment plants is typically treated to make it suitable for discharge, it is usually of fairly high quality. However, nutrients and bacteria may restrict wastewater’s use for cooling unless the power plant treats it further (see POWER, May 2006, “Recycling, reuse define future plant designs”).
Fresh groundwater has higher concentrations of dissolved solids that can become scale unless they are removed by pretreatment in a closed-loop cooling system. Saline water from the ocean or coastal areas also requires treatment and/or the use of special corrosion-resistant materials to make it suitable for plant use. Degraded waters from coal and oil production may be available, but they have much greater pretreatment requirements. For example, low pH is an issue for water pumped from spent coal mines, and the effluent of oil and gas well operations can have high levels of salts, silica, and hardness. And because recirculating cooling water also concentrates dissolved constituents in cooling tower blowdown, it may need to be post-treated if it is discharged to surface waters.
EPRI’s “Comparison of Alternate Cooling Technologies for U.S. Power Plants” (2004) determined that the cost of pre- and post-treating available water can range from as low as 22 cents/kgal (where treatment requirements are minimal) to as much as $4.28/kgal (if the water left over from oil and gas exploration is used).
As Table 2 shows, the sum of the medium estimates of component costs is $2.82/kgal. It is unlikely that a water source would be used if the costs of acquiring, delivering, and treating/disposing of it were all at the high or low end of their ranges; a reasonable range for the overall cost of water is $1/kgal to $4/kgal.
This wide range of water costs has important implications for the sustainability of supplies. Because costs vary widely from one location to another, so does the attractiveness of water conservation technologies across locations and regions. The development of new technologies increases the options for plant developers and decision-makers, enabling them to reduce water-related costs and plant profitability.
Better cooling options can even make it easier to site a plant near its market and fuel supplies, potentially boosting profits. Ideally, water availability and cost should not be second-tier considerations during the planning of a power project; they should be as important as electricity demand and fuel availability. When more technological options—plus more-reliable information about water supplies and costs and the economic benefits of new technologies—are available, planners can do a better job of planning and siting new capacity to use water supplies wisely.
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