THE future is looking cloudy for solar energy鈥攍iterally. Pollutants in
the air produced by burning fossil fuels could be making solar cells up to 60
per cent less efficient.
快猫短视频s at the University of Tehran in Iran were trying to find the best
tilt angle for solar arrays to maximise their output. But Ebrahim Asl-Soleimani
and his team found that the lowest energy output coincided with days when air
pollution was highest. When air pollution in the capital was low, each solar
array churned out 179 watt-hours per day. But on one particularly polluted day,
they report, the same array could only manage a measly 92 watt-hours. 鈥淭his day
was one of the most polluted in 1999,鈥� Asl-Soleimani says.
Asl-Soleimani blames reduced power on exhaust gases and particulates
suspended in the air, produced mainly by car exhausts and to a lesser extent
local industry. Tehran is a large city of more than 10 million people and is
almost entirely surrounded by mountains. When there is no wind, pollution can
get pretty bad, he reports in the latest edition of the journal Renewable
Energy. Concentrations of these pollutants can increase dramatically,
especially during winter. Over the year the reduction in energy yield due to air
pollution is significant.
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But Daryl Myers, a research scientist for the National Renewable Energy
Laboratory in Boulder, Colorado, is sceptical of the Iranian work. The
relationship between power output and solar radiation is generally linear, so to
create this sort of reduction would be tantamount to a 60 per cent reduction in
solar radiation鈥攁 very large effect, says Myers. This would be 鈥渇ar
greater than the maximum impact of the smoke on Bahrain from over 400 oil-well
fires burning in Kuwait at the conclusion of the 1990 Gulf War鈥�, he says.
Even if you take into account the dirt building up on the cells over time,
says Myers, previous studies in the Middle East have shown that this will only
reduce output by a maximum of 20 per cent. Asl-Soleimani says he did find that
鈥渁 residue of bird droppings covered the surface of some modules and affected
their energy output鈥�. But debris should also reduce output during days when
pollution is low, he points out.