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The Water-Energy Connection

 

Nationally, roughly 4% of total electricity use in the United States is for pumping and treating potable water and wastewater. The figure is a lot higher if you include energy use for the things we do with water, such as heating it. For example, 19% of California electricity use is dedicated to water when water heating is included.

The farther we have to pump water, the greater the energy use—especially if we have to pump it over mountain ranges, like in California. For many cities and towns in the U.S., water pumping and sewage treatment use more electricity than anything else. On a per-capita basis, this energy use for water pumping and treatment varies from about 350 kWh/year in the South Atlantic states to over 750 kWh/year in the Mountain states, according to a 2002 Electric Power Research Institute report—about as much annual use as a refrigerator.

Just as it takes energy to provide water, it also takes water to provide energy. Roughly 89% of U.S. electricity is produced in thermoelectric plants—plants that use a heat source such as coal or nuclear fission to produce steam, which spins a turbine that generates electricity. Water is used to create the steam, and then more water is used to cool that steam and condense it back into water. Averaged nationally, thermoelectric plants use 0.47 gallons of water for each kWh of electricity produced, according to a 2003 National Renewable Energy Laboratory (NREL) paper (see “Save Energy to Save Water,” EBN Oct. 2002).

Hydropower, which accounts for 9% of U.S. generation, consumes a lot more water because of evaporation from reservoirs. The same NREL study examined evaporation from 120 of our largest reservoirs and extrapolated that to all 2,300 of our power-generation reservoirs, calculating a national average water-intensity of 18 gallons/kWh for hydropower—with much higher consumption in some states: 65 gallons/kWh in Arizona, and 137 gallons/kWh in Oklahoma, for example. By weighting the thermoelectric and hydropower values, NREL found a national average of 2.0 gallons of water per kWh.

Water intensity is highly variable for fossil fuels. According to a 2006 Department of Energy report to Congress, conventional onshore oil extraction consumes 0.12–0.31 gallons of water per gallon of oil, but some oil extraction techniques can dramatically increase that. Canadian tar sands require 140–360 gallons of water per gallon of oil, according to data from the Pacific Institute. Another 1.0–2.5 gallons of water are required to process and transport each gallon of oil. With natural gas, conventional onshore extraction uses negligible water but processing and transport averages 3 gallons of water per million Btu.

On the renewable fuel side, corn-based ethanol is highly water-intensive. A 2008 paper in Environmental Science and Technology reported that a light-duty vehicle driven on an E85 ethanol mix (85% ethanol) “consumes” 28 gallons of water per mile! By conserving water we save energy, and by conserving energy we save water. It’s a win-win!

October 1, 2010

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