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September 2, 2014

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Dirty detail: Solar panels need water

How much is the question, as developers downplay frequency of cleanings

Beyond the Sun

  • This Las Vegas Sun story about the water needs of solar power plants caught the attention of Mother Jones blogger Osha Gray Davidson, a nationally recognized environmental journalist. Davidson praised the story for making "a compelling case for considering all the environmental impacts of generating energy, no matter what the source." Mother Jones is a nonprofit news Web site and bimonthly magazine specializing in investigative, political, and social justice reporting. Read the blog entry.

Southern Nevada may pose more of a dirty little problem for some solar plant developers than they realize or are letting on.

Solar photovoltaic developers say not to worry about how much water their plants will use because they need only enough water to run the office bathrooms and wash the arrays of panels a couple of times a year.

But people who live near proposed plants or maintain solar panels in the desert guffaw at that last bit and are willing to bet the panels will need to be hosed down more frequently.

Dust on solar panels can decrease their efficiency by about 3 percent, solar photovoltaic experts said. The larger the solar array, the more electricity lost.

“On a home that doesn’t mean much of anything, but on a huge solar power plant that could mean real money,” said Nevada solar panel installer Chris Brooks, director of renewable energy for Bombard Electric.

Most photovoltaic arrays are cleaned with tap water sprayed with a hose or from a water truck. So solar array managers have to add in the cost of labor, truck rental and gasoline. In a water-starved desert, the additional consideration is how much of the region’s most critical natural resource will wind up evaporating or dripping into the desert.

Solar photovoltaic developers say their plants don’t use much water, but “much” is relative. True, they use a fraction of what a water-cooled solar thermal power plant consumes annually — about a 16,689 gallons per megawatt for photovoltaics compared with 2.61 million gallons per megawatt for wet-cooled solar thermal — but a large photovoltaic array can still easily use more water in a year than an entire residential block.

The array planned for Primm, for example, is expected to annually require at least as much water as 10.5 average Las Vegas households. NexLight North and NexLight South, which have been combined in the first industrial-scale solar photovoltaic array planned the Bureau of Land Management land in Nevada, would need to truck in about 6.8 million gallons of water a year, developers reported in planning documents. That’s enough, they say, to clean the thousands of acres of solar panels about twice a year.

Although that is the industry standard for washing large arrays of solar panels, few large solar arrays in the Mojave get away with so few cleanings.

UNLV’s photovoltaic arrays are washed about monthly. NV Energy washes the panels at the Clark Generating Station about four times a year. Other NV Energy owned solar panels are washed three times a year.

When NexLight disclosed plans for biannual cleanings at BLM scoping meetings, locals scoffed. If the dust on the cars in the parking lot was any indication, the developers would be cleaning those panels a lot more than twice a year. The dust in the Ivanpah Valley can be brutal under normal circumstances, residents said. But the area is also a popular spot for large multiday off-road races that can stir up even more dust.

The NexLight plants are planned smack dab in the middle of a popular off-road raceway, which the company proposes rerouting around the solar plant.

Just washing the panels more often is not the easy solution it sounds like. If the increase in electrical output won’t generate more money than it costs to wash the panels, they can just stay dirty.

“Efficiency does drop off with time,” said Bob Boehm, director of UNLV’s Center for Energy Research. “But you really have to balance the loss in efficiency from the dust with the cost of the water and labor.”

So solar array managers try to keep the panels cleanest when the solar panels are operating at maximum efficiency in the long days of spring and summer. Unfortunately, that’s when demand for water is the highest, putting even more strain on a scarce resource.

When they can, operators of solar arrays let Mother Nature do the work for them. Though Southern Nevada gets only about 4 inches of rain in a good year, the weather is relatively predictable. That gives solar array managers time to get the panels ready for cloudy weather and, they hope, a free cleaning.

That preparation is a must. Cold water on a very hot solar panel usually means shattered glass, so managers have to power down arrays well before either a cleaning or rainfall. If the storm produces rain that falls in a torrent, they’ve hit the jackpot.

“A really good rainstorm means you don’t need to worry about washing your panels for a while,” Boehm said. “But if you get this typical Las Vegas rainstorm with tons of wind and dust and forty-five drops of rain, that’s the worst kind of thing. It just plasters the dirt to the panel.”

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