Consumptive Water Use for U.S. Power Production

Evaporative cooling systems in buildings have been criticized for their water use and acclaimed for their low energy consumption, especially when compared to typical cooling systems. In order to determine the overall effectiveness of cooling systems in buildings, both water and energy need to be considered; however, there must be a metric to compare the amount of energy used at the site to the amount of water used at the power plant.

A study of power plants and their respective water consumption was completed to effectively analyze evaporative cooling systems. Eighty-nine percent of electricity in the United States is produced with thermally driven water-cooled energy conversion cycles. Thermoelectric power plants withdraw a tremendous amount of water, but only a small percentage is evaporated. The evaporative or consumptive use1 is approximately 2.5% or 3,310 million gal per day (MGD) (12,530 x 106 L/d). Moreover, hydroelectric plants produce approximately 9% of the nation’s electricity. Evaporative water loss from the reservoir surfaces also results in water being evaporated for electrical production.

In thermoelectric plants, 0.47 gal (1.8 L) of fresh water is evaporated per kWh of electricity consumed at the point of end use. Hydroelectric plants evaporate an average of 18 gal (68 L) of fresh water per kWh used by the consumer. The national weighted average for thermoelectric and hydroelectric water use is 2.0 gal (7.6 L) of evaporated water per kWh of electricity consumed at the point of end use. From this information, different types of building cooling systems can be compared for relative water consumption. This paper will aid in High Performance Building research by providing a metric in determining water efficiency in building cooling systems. Further analysis is planned to determine the overall water efficiency of evaporative cooling systems compared to conventional direct expansion systems and chiller systems with cooling towers.