Guess how much water it takes to burn just one 60-watt incandescent light bulb 12 hours a day for one year. Between 3,000 and 6,000 gallons of water — depending on how the electricity that powers the bulb is produced.
That's just one in a whole bunch of startling findings in a study by researchers at Virginia Tech who focused on a problem that has probably eluded most folks. It takes lots of water to produce energy, and rising energy demands will almost certainly tap into a rapidly dwindling supply of fresh water.
Some of the thirstiest energy sources are the same ones that are now getting the strongest push, such as ethanol and biodiesel.
"Basically, all the conventional energy sources are water dependent," research professor of water resources Tamin Younos said in a telephone interview.
That won't come as a surprise to scientists at the federal departments of agriculture and energy, who have produced numerous reports tallying up the enormous quantities of water used to produce fuel and electricity. But Younos and an undergraduate assistant, Rachelle Hill, took those same reports and added a new twist.
Younos wanted to determine the water-use efficiency of various energy sources, but that's not easy because energy comes in so many different packages, from watts to joules and so forth. The researchers needed some unit of measurement that all energy sources have in common. So they came up with the British thermal unit (BTU).
"We selected BTU as a standard unit because it indicates pure energy as heat and is applicable to all energy production and power generation methods," Younos said. A BTU is the amount of energy required to raise the temperature of one pound of water from 58.5 degrees Fahrenheit to 59.5 degrees.
The researchers looked at 11 different fuels used to produce energy, ranging from natural gas (a frugal water user) to biodiesel (a real water pig), and five different types of power plants, from hydroelectric (frugal) to nuclear (a pig, but some plants use water from the ocean, not fresh water.)
The numbers they came up with should not be etched in stone, because there are many variables that affect the use of water, but all sources use some. Even oil fields use some water to pump back into the ground to maintain down-hole pressure. Power plants use various fuels to produce heat to convert water into steam to turn a turbine and produce electricity. And the new rage, renewal resources like ethanol and biodiesel, use water to grow crops and convert waste into fuel.
In the United States, power plants consume 136 billion gallons of water a day, the researchers found, but they did not distinguish between fresh water and ocean water. Nearly all inland plants, however, rely on fresh water.
The findings also show a wide range in efficiency for nearly all resources because of different ways that the resources are produced. Coal, for example, ranges from a low of 41 gallons of water to a high of 164 gallons to produce a million British thermal units.
But the real bad actors among fuel sources are ethanol, ranging from 2,510 to 29,100 gallons to produce a million units, and biodiesel, ranging from 14,000 to a whopping 75,000 gallons.