The study results indicated that, for Rankine-cycle power plants, dry cooling has the potential to reduce water consumption by more than 90%.
The National Renewable Energy Laboratory (NREL) and consulting firm WorleyParsons Group, Inc., recently teamed up to study how to reduce water use in concentrating solar power (CSP) plants—usually sited in hot, dry regions where sunlight is abundant but water is precious.
Results have been published in the NREL report, "Water Use in Parabolic Trough Power Plants: Summary Results from WorleyParson’s Analyses." Such knowledge is valuable, considering the huge potential for CSP plants to generate electricity in the southwest United States and around the world.
Currently, most CSP plants, whether of the parabolic-trough or power-tower type, use the Rankine thermal power cycle. In general terms, this cycle consists of concentrating the sun’s thermal energy into a heat-transfer fluid (typical a synthetic oil for today’s parabolic-trough plants), which converts liquid water to steam, which drives a steam turbine, which generates electricity. In the Rankine cycle, the steam is then condensed using external cooling water, and the condensed cycle water is directed back to be converted to steam, and the closed cycle continues. It is the water used to condense the steam that is being targeted for reduction.
In 2009 and 2010, NREL contracted with WorleyParsons to estimate the impacts of switching from "wet" to "dry" cooling systems. The company also examined the use of hybrid cooling systems, that is, using both dry and wet systems concurrently. The study compared the water use, performance, and cost of numerous scenarios. WorleyParsons analyzed 13 different cases in three general locations (Daggett, California; Las Vegas, Nevada; and Alamosa, Colorado). NREL subsequently used its Solar Advisor Model (SAM) software tool to provide a comparison for each scenario studied by WorleyParsons. The NREL effort was led by Craig Turchi and Mike Wagner, two engineers in the laboratory’s Thermal Systems Group.
The study results indicated that, for Rankine-cycle power plants, dry cooling has the potential to reduce water consumption by more than 90%. In the case of parabolic trough plants, the levelized cost of electricity (LCOE) was estimated to increase approximately 2.5% to 8%, depending on the location in the Southwest. Hybrid cooling was found to reduce the LCOE increase—but at a higher capital cost and operational complexity. (A more detailed study of hybrid cooling is the subject of a pending NREL publication.)