Atmospheric 'sunshade' could reduce solar power generation

Atmospheric 'sunshade' could reduce solar power generation:
March 11th, 2009

The world's largest solar power facility, located near Kramer Junction, Calif., consists of five Solar Electric Generating Stations and covers more than 1,000 acres. (Credit: Department of Energy/National Renewable Energy Laboratory)

The concept of delaying global warming by adding particles into the upper atmosphere to cool the climate could unintentionally reduce peak electricity generated by large solar power plants by as much as one-fifth, according to a new NOAA study. The findings appear in this week's issue of Environmental Science and Technology.

'Injecting particles into the stratosphere could have unintended consequences for one alternative energy source expected to play a role in the transition away from fossil fuels,' said author Daniel Murphy, a scientist at NOAA's Earth System Research Laboratory in Boulder, Colo.

The Earth is heating up as fossil-fuel burning produces carbon dioxide, the primary heat-trapping gas responsible for man-made climate change. To counteract the effect, some geoengineering proposals are designed to slow global warming by shading the Earth from sunlight.

Among the ideas being explored is injecting small particles into the upper atmosphere to produce a climate cooling similar to that of large volcanic eruptions, such as Mt. Pinatubo's in 1991. Airborne sulfur hovering in the stratosphere cooled the Earth for about two years following that eruption.

Murphy found that particles in the stratosphere reduce the amount and change 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.

On average, for every watt of sunlight the particles reflect away from the Earth, another three watts of direct sunlight are converted to diffuse sunlight. Large power-generating solar plants that concentrate sunlight for maximum efficiency depend solely on direct sunlight and cannot use diffuse light.

Murphy verified his calculations using long-term NOAA observations of direct and diffuse sunlight before and after the 1991 eruption.

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 3 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."

Nine Solar Electric Generating Stations operate in California and more are running or are under construction elsewhere in the world. In sunny locations such systems, which use curved mirrors or other concentrating devices, generate electricity at a lower cost than conventional photovoltaic, or solar, cells.

Flat photovoltaic and hot water panels, commonly seen on household roofs, use both diffuse and direct sunlight. Their energy output would decline much less than that from concentrating systems.

Even low-tech measures to balance a home's energy, such as south-facing windows for winter heat and overhangs for summer shade, would be less effective if direct sunlight is reduced.

Source: NOAA

Read more...

Quantum Dots Could Boost Solar Cell Efficiency

Quantum Dots Could Boost Solar Cell Efficiency: "Quantum Dots Could Boost Solar Cell Efficiency
March 11th, 2009 By Kelen Tuttle

(PhysOrg.com) -- The transition to environmentally benign energy sources is one of the most significant challenges of the 21st century. Solar power, which uses sunlight to generate electricity, is one promising source. It has many benefits: sunlight is free; operating solar cells emits no greenhouse gasses; and solar power can be generated almost anywhere in the world. Unfortunately, today's solar energy technologies are inefficient, and thus significantly more expensive than traditional power sources. But hope is on the horizon. Recent results from the joint SLAC-Stanford PULSE Institute for Ultrafast Energy Science may help increase efficiency more than previously thought possible.

'This research is one step toward making solar cells more efficient,' said PULSE researcher Kelly Gaffney. The finding, he continued, shows there is a significant difference between what's on the market now and what's possible.

In their recent experiment, PULSE researchers sought to confirm results of a Los Alamos National Laboratory study in which researchers observed one photon of light generating more than one electron of electricity. Scientists previously assumed that one photon could excite exactly one electron, limiting the efficiency of solar cells.

Both experiments used so-called "quantum dots." At a few billionths of a meter across, these spheres are made of only a few thousand atoms. On this scale, matter acts very differently from matter in bulk form; forcing all of the atoms' electrons into a very small area causes the electrons in a quantum dot to interact more and increases the strength of those interactions.

Over the past five years, several research groups used quantum dots in their attempts to recreate the Los Alamos findings, but without success. "There's been a lot of controversy as to whether this [multiple excitation] actually occurs," Gaffney said. "Not everyone agreed that it's even real."

Working with researchers at PULSE, Stanford University and Lawrence Berkeley National Laboratory, Gaffney used a slightly different experimental method to confirm that a single photon can indeed excite more than one electron in a quantum dot.

The researchers found that the solar cell process could be as much as one third more efficient than previously thought if solar cells used quantum dots instead of solid bulk materials. For bulk materials, the one-to-one ratio still holds; one photon excites only one electron, with any additional energy radiating away as heat. In a quantum dot, this ratio could range from one-to-one to one-to-three, depending on the color of the sunlight.

The next step in the quest for efficient solar power is to build a solar cell that uses quantum dots to realize this efficiency.

"No one has done that yet; that research is just getting started," Gaffney said. "It's a very difficult science and engineering problem, but the opportunity is significant."

Provided by SLAC

Read more...