Fact Check: How Much Water Does Solar Power Really Use?

There's an infographic going around lately that claims to show the relative amounts of water used by four different sources of electrical power: coal, nuclear, natural gas and solar. The graphic claims that solar comes out the clear winner in terms of water conservation, using no water at all to generate power. But is the claim correct? Not quite.

The graphic, produced by the "Climate Reality Project," is making the rounds of social media. It's pretty straightforward, at first glance. Coal-fired power plants use up 1,100 gallons of water for each megawatt-hour of power produced. (A megawatt-hour is about what a typical California household would consume in six or seven weeks.) Nuclear and natural-gas-fired power plants use water 800 and 300 gallons for the same amount of power, respectively. And solar, according to the Climate Reality Project, is the least water-wasteful of all four sources of energy, at zero gallons of water per megawatt-hour.

On Facebook, the graphic's creators share the news breathlessly, saying "Whoa - you probably know that solar power plants produce electricity without producing carbon pollution, but did you all realize they also save so much water? 'Share' to let your friends know, too!" But is the graphic accurate? That depends what you mean by "accurate."

Here's the infographic in full:

A few things jump out at us in looking at this graphic. First, there's no indication where the authors got their water use figures. The only link is to the Climate Reality Project's website, which offers no illumination on the topic that we could find. No source for the material? That's almost always a bad sign, one that should trip your inner skeptic. It's bad for two reasons.

First, we have no idea who actually did the work of compiling the figures. Did the authors come up with it themselves? Or did they borrow someone else's work without crediting it?

Second, with no indication as to what went in to making the water use calculations in the graphic, we have no clue what the authors consider "water use" for each kind of energy generation. Does the figure of 300 gallons per megawatt-hour of natural gas power include water used in fracking? Does the coal figure include water used to wash the raw coal? Or to transport it from the mine to the power plant, whether by train or by slurry pipeline?

Without any indication of what assumptions went into calculating those figures in the graphic, it's hard to gauge their accuracy. If the authors intended only to count water used directly in generating electricity, then nuclear's total would be a whole lot closer to coal's.

In fact, according to a report on energy production's water use published in 2012 by the River Network, entitled "Burning Our Rivers," nuclear power's water use is very close to coal's, and both are well above the figures quoted in the Climate Reality Project graphic. On average, and assuming 2009 cooling technology, coal-fired plants use 15,514 gallons for every megawatt-hour they produce, and 506 gallons of that is lost as steam or evaporated. Nuclear uses 14,732 gallons per megawatt hour, 94 percent of what coal plants use, and they lose even more to evaporation: 532 gallons per megawatt hour.

But how about the main claim of the Climate Reality Project graphic: that solar uses no water at all to create power? That depends again on what you define as "creating power," whether direct use in generation or in the other activities that support making power.

It also depends on what you define as solar. Climate Reality Project's graphic uses an icon to represent solar that looks very much like a photovoltaic panel, and it's true: if you restrict your considerations only to things like making steam and turning turbines, photovoltaic panels use no water to generate electricity.

But photovoltaic panels do require some water, even though they don't have turbines to turn. In the desert and in semi-arid coastal California, where rain may not fall for many months at a time, dust accumulates on those panels, and dust cuts into power output. Some PV technologies keep pumping out power even in low light conditions caused by a film of dust. (The photographer's original title on Flickr for the above photo is "Annual 'cleaning the solar panels' day.") Still, unless you wash that dust off, you eventually stop generating power.

Most utility-scale photovoltaic power plants plan to wash their panels, especially in the desert. The 550-megawatt Desert Sunlight project in Riverside County estimated during the course of its environmental assessment that it would be using one fifth of an acre-foot a year to wash its panels once construction was complete. The project will likely produce around 1,600,000 megawatt-hours of power each year, and using about .2 acre-feet or around 65,200 gallons to clean the panels, that works out to a very thrifty
2/3 cup of water per megawatt hour.

Close enough to zero? On paper, perhaps. Those water use estimates for the project have been criticized, even ridiculed, as supremely optimistic. The River Network's 2012 paper estimates water used directly in photovoltaic power generation (read: washing panels) at around two gallons per megawatt-hour, which is on one hand far better than any of the fossil fuel equivalents and on the other hand, not zero.

But there's another kind of solar power: concentrating solar thermal. That's the technology in which the sun's heat is concentrated by mirrors to heat either water or another fluid that then heats water, creating steam to drive a turbine. These projects usually use parabolic trough mirrors to focus sunlight, though a project recently went online using mirrors that aim sunlight at boilers atop towers, and a few more of that general type are in various stages of planning and construction. Concentrating solar thermal projects can use a fair amount of water per megawatt-hour, especially if they're so-called "wet-cooled" projects that use escaping water vapor to keep operating temperatures from getting too high. In California, state policy requires that solar thermal projects use "dry-cooling" technology, which uses air flow instead of water to cool things down.

The Ivanpah Solar Electric Generating System is formally permitted to use 100 acre-feet of water each year to run its turbines and to keep its thousands of mirrored heliostats moderately clean. That's 32,585,143 gallons per year. A 377-megawatt plant, Ivanpah is -- using its designers' optimistic figures -- capable of putting out about 1,040,300 megawatt-hour per year. That's an optimistic 32 gallons per megawatt-hour.

Ivanpah's water use estimates have also been challenged, and the plant so far isn't performing at peak output, so the real gallons-per-megawatt-hour figure may prove to be significantly higher for Ivanpah. And of course, the Climate Reality Project doesn't specify that its graphic's figures apply only to California: its vague "solar" category must be assumed to include facilities in other states where dry cooling isn't mandated.

For what it's worth, the River Network estimates solar thermal water use at around 800 gallons per megawatt-hour (the same as nuclear in the infographic), all of it lost to evaporation, which implies wet-cooling. In 2012 there were no functioning solar power tower projects in the United States, so those figures apply only to parabolic trough installations.

That's a lot more complexity than you generally find in infographics, and a designer can be excused for not wanting to clutter up an image with a long list of footnotes and caveats.

Still: solar tech requires water, and it's doing no one a service to imply otherwise.

There are two interesting side notes drawn from the River Network's report worth mentioning. First, solar isn't the most water-efficient form of energy generation, according to those 2012 figures. Wind handily beats out even solar PV at less than a gallon per megawatt hour.

And second, the most widely used and generally reliable form of renewable energy we use is absolutely the worst in terms of water wastage. Hydroelectric power uses 440,000 gallons of water per megawatt-hour and wastes 9,000 gallons of that total, generally by way of evaporation from the surfaces of reservoirs backed up behind dams. That's about 18 times as much wasted water from clean hydro as from dirty coal.

That doesn't mean coal's better than hydro: it just means that sometimes "Reality" doesn't fit in an infographic.