They may not be gas-guzzlers, but electric cars have a raging thirst for water.
A comparison of the volume of coolant water used in the thermoelectric power plants that provide most of our electricity and that used in extracting and refining petroleum suggests that electric vehicles require significantly more water per mile than those powered by gasoline.
The findings could bode ill for drought stricken areas in the event of a large scale switch to plug-in vehicles.
"I wouldn't sound the alarm that this is going to ruin the day," says Carey King from the University of Texas, Austin, US, noting that no mass-market electric vehicle is currently available. "But looking into the future, this is something we should take into account."
King and colleagues found that cars, light trucks, and SUVs running off the electric grid consume three times more water and withdraw 17 times more water per mile than their equivalent gasoline-powered vehicles.
For electricity generation, "consumed" water is the amount of water lost to evaporation whereas "withdrawn" water is the amount of surface water a power plant uses and later returns to its source, typically a nearby lake or river.
King says one way to mitigate water-use impacts of electric vehicles is by switching to dry cooling – using forced air instead of water to cool steam in power plants. The technology has been around for years but remains more expensive than water cooling; something King says could change as available surface water becomes more scarce.
Another alternative is to move away from thermoelectric energy sources such as coal, nuclear, and natural gas, to renewable sources. "If we use only wind or solar energy, water use would be essentially zero," King says.
Paul Denholm of the US Department of Energy's National Renewable Energy Laboratory in Golden, Colorado agrees that water scarcity will become an increasing problem for utilities, but he doesn't think electric vehicle usage will have much of an impact.
"As electricity demand increases in general, water requirements – especially in drought prone areas – will become increasingly important," Denholm says. But "the overall impacts of plug-in vehicles are modest in the larger scheme of things".
Denholm co-authored a 2007 study based on data from Colorado showing that if 30% of gasoline-powered vehicles were replaced with plug-in hybrid electric vehicles getting 39% of their energy from the grid, the region would experience only a 3% increase in total energy demand.
The current infrastructure could easily handle this increase because most vehicles would be charged overnight during off peak hours.
"It's going to be several decades before we see enough plug-in vehicles to have any kind of impact," Denholm says. "It's hard to say if the grid we have now will be the same grid we have when we begin to see a large number of plug-ins."