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'Sunshade' global-cooling plan would ruin solar power

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Attempts to deal with global warming by putting a particulate "sunshade" into the atmosphere would have adverse effects on solar power generation, according to a US federal boffin.

Dan Murphy of the National Oceanic and Atmospheric Administration (NOAA) investigates a major inspiration for "sunshade" schemes, the volcanic eruption of Mount Pinatubo in 1991. Millions of tons of material ejected from the volcano hovered in the atmosphere for years, cooling the entire planet by half a degree as it reflected sunlight back into space. With severe-case global warming set to warm the planet by four degrees over a century, this is quite significant.

But Murphy says that for every watt of power reflected away by aerosol particles, a further three watts of inbound sunbeam are turned "diffuse" by the airborne particulates. Diffuse or scattered sunlight is of no use to "concentrating" solar plants, he says.

Most solar power generation today uses flat photovoltaic cells or hot-water panels, which have no problems with diffuse light. But, especially in California, some large solar plants use parabolic mirrors or other concentrating devices to focus sunbeams on a smaller area. This technique is commonly used in commercial scale solar-thermal generating plants, which can generate power more cheaply than photovoltaics in sunny areas.

The NOAA headquarters press release says:

After the eruption of Mt Pinatubo, peak power output of Solar Electric Generating Stations in California, the largest collective of solar power plants in the world, fell by up to 20 percent, even though the stratospheric particles from the eruption reduced total sunlight that year by less than three percent.

"The sensitivity of concentrating solar systems to stratospheric particles may seem surprising," said Murphy. "But because these systems use only direct sunlight, increasing stratospheric particles has a disproportionately large effect on them."

Murphy's actual article (here: subscription required) reveals that the annual energy output of solar-thermal concentrating power stations in California - as opposed to the largely irrelevant peak figure - fell by 16 per cent the year after Mt Pinatubo.

A primary reason to shift to any kind of solar energy - the minority concentrating-optics kind or more mainstream forms - is to avoid global warming. In some ways it seems odd to say "we mustn't halt global warming using sunshade aerosols, because then we can't halt it using solar-thermal".

On the other hand, even assuming that some kind of ongoing Mt Pinatubo event could be reliably conjured up - without the various negative consequences such as ozone depletion, towns destroyed, rains of ash etc - there would still be security of supplies to worry about on the fossil-fuels, not to mention their eventual depletion. ®

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