"Dirty Detail": Water use and solar energy

[magicgroove]

Discussion of water use for solar plants, from the neighbors in
Nevada. Via the Las Vegas Sun.

http://www.lasvegassun.com/news/2009/sep/18/dirty-detail-solar-panels-need-water/

"Dirty detail: Solar panels need water"

How much is the question, as developers downplay frequency of
cleanings
By Stephanie Tavares (contact)

Friday, Sept. 18, 2009 | 2 a.m.

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.”