27 March 2009
Increasing the number of particles in Earth's stratosphere could significantly reduce the output from large solar electric power generating plants, according to a study by CSD scientist Dan Murphy. The study was published on March 11 in Environmental Science and Technology journal.
Murphy showed that stratospheric particles change both the amount and the nature of the sunlight that strikes the Earth. Though a fraction of the incoming sunlight bounces back to space (the cooling effect), a much larger amount becomes "diffuse" or scattered light that cannot be used by concentrating solar collectors. The study showed that on average, for every 1 watt of sunlight the particles reflect away from the Earth, another 3 watts of direct sunlight are converted to diffuse sunlight. Solar power generation systems using parabolic or other concentrating methods can only use direct sunlight and would be affected most. Flat photovoltaic or heating panels would be affected much less.
Murphy verified his calculations using long-term NOAA observations of direct and diffuse sunlight before and after the 1991 eruption of Mt. Pinatubo, which released gases and particles that led to a large increase in stratospheric particles. After the eruption, the power output of Solar Electric Generating Systems plants in California fell by 10 to 20%, depending on the measure used for electrical output. The amount of particles added by the Mt. Pinatubo eruption was similar to amounts in some deliberate enhancement proposals.
Murphy's calculations and the Pinatubo eruption reveal a perhaps surprising sensitivity. Murphy explains, "Stratospheric particles have a disproportionately large effect on concentrating solar systems."
Although Murphy considered only stratospheric particles in the study, scattering by clouds or particles in the lower atmosphere would also reduce direct sunlight. Other consequences of the changes from direct to diffuse sunlight, such as effects on ecosystem productivity, were not examined in the study.
Background: "Direct" sunlight comes straight from the sun and casts sharp shadows. "Diffuse" sunlight comes from the sky in all directions. Particles preferentially scatter light in the forward direction. This causes more of the light that hits a tiny particle to reach the Earth as diffuse light than to bounce away from the Earth.
Significance: The result means that so-called "geoengineering" proposals that would deliberately add particles to the stratosphere to help cool the planet would have unintended consequences for one of the alternative energy sources that is expected to be a part of the transition away from fossil fuel-based sources in the coming decades.
Murphy D. M., Effect of stratospheric aerosols on direct sunlight and implications for concentrating solar power, Environmental Science and Technology, doi:10.1021/es802206b, 2009.