“Dry” Power Plants Produce Energy Using Less Water

Are air-cooled plants the future in arid areas?

The debate about building and operating power plants in the state has recently taken a new turn, with the merits of “dry” vs. wet cooling now an issue in the controversy. As might be inferred this is essentially a water use issue, with those advocating the use of dry cooling claiming the system uses about 90 percent less water to cool plants than wet systems. Meanwhile others argue that dry cooling technology, which uses air-cooled condensers instead of conventional cooling towers, has limitations that restrict its use.

Although in use for about 40 years, dry cooling technology for power plants has only recently emerged as an issue in Arizona. In the fall of 1999, when a surge in power plant applications hit the state, dry cooling was not a consideration. As a testimony to those times all plants now either under construction or permitted in the state are slated to be wet cooled. Although the surge in power plant applications has since abated, the few applications now submitted still designate wet cooling.

Reliant Energy’s Signal Peak plant has been the only dry power generating facility to be actually proposed for Arizona. After announcing the project in August 2001, however, Reliant withdrew its application, and the project is now on hold.

In some ways, power plant developers’ preference for wet cooling makes good economic sense, partly because water is relatively cheap and seemingly plentiful in the state. An ample supply appears available at reasonable cost, ensuring the maximum amount of megawatts for the number of dollars invested. In such circumstances, developers ask why take on the additional capital costs to construct and operate dry-cooled plants that use less water. Further, they argue that dry-cooled plants are less efficient than wet-cooled ones, especially in desert areas with periods of high air temperatures.

Air-cooled combined-cyclepower plant just south of Ciudad Juarez, Mexico. Photo by GEA Power Cooling Systems.

In response, advocates of dry cooling argue its use will promote Arizona’s environmental and economic self-interest. To those people who believe that water conservation is a good unto itself, dry cooling has an obvious appeal, and they view its pursuit as good public policy.

Others supporting dry cooling take a more practical approach by agreeing that water used to generate power for Arizona citizens may be water well used, but they go on to say that enough plants are now in the works to meet the future energy needs of the state. Any additional plants should therefore be required to be dry-cooled to preserve Arizona’s water resources for other uses. They question the presumption that water used as a cooling medium will find no better use over the next 40 years, the life expectancy of a power plant.

In further building their case, advocates of dry cooling point out that such plants are up and operating in various western states, including California and Nevada. Mexico has taken a lead among nations committed to dry cooling in arid regions.

Advocates of dry cooling also dispute the power industry’s cost comparison figures that favor wet cooling over dry. In questioning some of the assumptions used to compute costs, dry cooling proponents note that cost comparisons between wet and dry systems often are limited to the capital cost and energy demand of the two technologies. Dry cooling fan energy cost is compared to wet circulation pump plus fan energy costs. They say this formula fails to take into account that dry cooling eliminates costs associated with purchasing land for its water rights, well-digging, piping and pumping, water treatment systems, evaporation ponds and aquifer protection permitting costs.

Regardless of how costs are calculated, however, most agree that the bottom line generally shows that dry plants are more expensive to build and operate than wet. It is the extent of the cost differential that is disputed. Advocates argue that the cost differential is not that significant to rule out dry cooling, with its water-saving advantages.

What is generally acknowledged among all interests is that dry cooling has decreased efficiency in high ambient temperatures, a serious limitation in desert areas. Dry cooling is capable of handling the entire cooling load up to an ambient temperature of 85-90 oF. Beyond that point, the air temperature becomes too high for effective cooling. Developers planning to locate a plant in hot climates cite this limitation for not adopting dry-cooled technology.

Some plants located in such an environment have learned to live with the situation. For example, dry-cooled power plants are located in Nevada, parts of Texas and Northern Mexico, in areas without access to water supplies for a cooling system. On a 110 oF day, these plants accept an energy production penalty of approximately five percent overall (including turbine and steam cycles), compared to a plant equipped with a wet-cooling system.

To offset this limitation, a parallel dry-wet system has been developed, with a wet-cooling component to augment dry-cooling to maintain power output on hot days. The parallel or hybrid system has the advantage of achieving essentially the same hot-day performance as a wet cooling system, with an evaporation lost of less than five percent on an annual basis when compared to a wet system. A further advantage is that the parallel system requires a much smaller water transport, treatment and cooling tower/condensing plant infrastructure.

If, as many of its proponents claim, dry-cooled power plants are an idea whose time has come, why have they been so slow in catching on in Arizona? Cost is the main factor. In a competitive market place, dry cooling is not likely to be a first choice, unless the cost of water is high or water is simply not available. Nevada has a number of dry plants because developers confront high water costs. Developers in California have turned to dry cooling since state water policy ranks beneficial uses of potable water, and water for plant cooling is very low priority.

The issue would seem ripe for public policy discussion in Arizona, and some discussion is in fact occurring as two state regulatory bodies review power plant applications. An Arizona Power Plant and Transmission Line Siting Committee evaluates power plant applications to decide whether to issue a certificate of environmental compatibility (CEC). The committee examines a broad range of environmental issues, including present and future availability of water. The application then goes to the Arizona Corporation Commission for formal approval

The Siting Committee has the authority to impose conditions on power plant applications to minimize environmental impact. The topic of dry cooling has come up during committee deliberations and was addressed at some length during a recent consideration of an application submitted by Allegheny Energy and Supply Company to build a power plant in La Paz County. The committee, however, granted Allegheny a CEC without imposing dry-cooling requirements. The application then went to the ACC for its review and approval.

In an effort to encourage the ACC to impose dry-cooling requirements at La Paz , the Arizona Unions for Reliable Energy (AZURE) requested that the commission review the issuance of the CEC and accept briefs and oral arguments from interested parties. AZURE argued that the ACC “should modify the CEC to require the impacts of this project to be fully mitigated.” Among the mitigation measures proposed by AZURE is the requirement that the La Paz plant install dry cooling.

On one other occasion the ACC had required that a plant adopt dry cooling. It had imposed the requirement on Duke Energy when it sought approval of its Arlington Valley I plant, although the Siting Committee had already granted the project a CEC. Duke Energy appealed the ACC decision, and the commission rescinded its dry-cooling requirement when the contractors agreed to recharge a quantity of CAP water equal to the amount the plant used for cooling. Now the La Paz application was again raising the issue of dry cooling before the ACC.

“We are working to have La Paz be the first project for the commission to actually bite the bullet and require dry cooling,” says Mark R. Wolfe, an attorney for AZURE. On April 8, the ACC heard oral arguments on the La Paz plant before voting on the issue.

The ACC voted not to require that the La Paz plant install dry cooling. ACC Chairman Bill Mundell had made several motions that the plant include some aspect of dry-cooling technology but the motions died for lack of a second. The plant was approved with a recharge provision instead.

Although not a victory for dry-cooling supporters the experience did not leave them completely discouraged either. The dry-cool issue is on the table and getting attention from both the Siting Committee and the ACC. As an AZURE member noted after the hearing, “Even if a power company does not want to commit to dry cooling, they now feel they have to explain why not. It is an advance over where we were a couple years ago.”

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