DOE Report to Congress: Reducing Water Consumption of Concentrating Solar Power

The US Department of Energy has issued a report to congress that explores a number of strategies to reduce the water consumption of concentrating solar power (CSP) electricity generation, a promising renewable energy option that may substantially increase water demands in some of the country's most arid regions.

The report, dryly titled (no pun intended) Concentrating Solar Power Commercial Application Study: Reducing Water Consumption of Concentrating Solar Power Electricity Generation (PDF), was published by the DOE’s Office of Energy Efficiency and Renewable Energy this year. It looks at a range of CSP technologies – including parabolic troughs, linear Fresnel reflectors, power towers and dish/engine systems – and compares the water consumption, energy loss and capital costs of available cooling technologies (cooling is the most water-intensive process of CSP). As the report describes there are a number of tradeoffs associated with more water-efficient cooling processes:

As with fossil and nuclear power plants, water cooling is generally more economical than air cooling for CSP plants because water cooling has a lower capital cost and higher thermal efficiency, and it maintains its consistent efficiency levels year-round. In contrast, air cooling has reduced effectiveness when the air temperature is high. In the current commercial CSP plants, water is required to condense steam, provide make-up water for the steam cycle, and for mirror washing. The regions where CSP is most effective are those that have many hours of direct sunlight; these places often have relatively little water. Supplying water from more distant sources or purifying low quality water for CSP systems that use conventional water cooling can then increase costs. This report discusses various options by which CSP systems can operate efficiently with significantly less water consumption than they consume today.

The majority of new fossil power plants use evaporative water cooling to reject the steamcycle heat. A typical coal plant or nuclear plant consumes 500 gallons of water per M(gal/MWh) of electricity generated. This is similar to the water consumption by a power tower. A combined-cycle natural gas plant consumes about 200 gal/MWh. A water-cooled parabolic trough plant consumes about 800 gal/MWh. Of this, 2% is usefor mirror washing. Dish/engine systems only require water for mirror washing (approximately 20 gal/MWh)

To address water limitations and environmental regulations, air cooling can be used for new thermoelectric power plants, which eliminates over 90% of water usage. The typical dry-cooled power plant routes turbine exhaust steam directly to finned tubes on air-cooled condensors. A study of a dry-cooled parabolic trough plant located in the Mojave Desert concluded that dry cooling would provide 5% less electric energy on an annual basis and increase the cost of produced electricity by 7 to 9%. However, the results are location specific. For example, air cooling at a site in New Mexico would increase the cost of electricity by only 2% because maximum daytime temperatures are considerable lower there than in the Mojave Desert.

To download the full report visit: Concentrating Solar Power Commercial Application Study: Reducing Water Consumption of Concentrating Solar Power Electricity Generation (PDF)