August 15, 2007

Global Monitor (August 2007)

Nevada bets on solar thermal

A parabolic solar thermal power plant in the Nevada desert began generating electricity for the state's grid this June. The 64-MW project, say its German and Spanish developers, "is the first large solar thermal plant to be built in 15 years."

The developers' claim is technically accurate. But the project, as the developers acknowledge, is smaller than the Luz parabolic concentrator projects of the 1980s that sold power to Southern California Edison under contracts designed to implement the 1978 Public Utilities Regulatory Policies Act.

"Nevada Solar One" (Figure 7) covers 250 acres of the Nevada desert with arrays of solar parabolic concentrating troughs (Figure 7) that focus the heat energy of the sun on power-producing turbines. The plant's output will be sold to Nevada Power and Sierra Pacific Power under long-term power-purchase agreements.

7. Dishing up the sun. These parabolic concentrators in the Nevada desert have begun producing 64 MW of sun power for Sierra Pacific Resources' two Nevada utilities. They are similar to those used by earlier Luz solar thermal generation projects in California. Courtesy: Schott North America

Udo Ungeheuer, head of the solar power promoter Schott AG—a German glass maker that provided the concentrator technology—said at the end of June in Dusseldorf that the EU "should work towards establishing the necessary basic parameters so Europe can one day be supplied with power from solar thermal power plants based along the Mediterranean."

According to Ungeheuer, the parabolic trough technology "has already proven itself, and the costs of generating electricity will soon be competitive." Acciona Solar Power Inc., a subsidiary of Spain's Acciona group, built the plant and will operate it.

Similar to the Luz projects in California, the Nevada project has trough-shaped mirrors that concentrate sunlight onto absorber tubes along the focal length of the mirrors. The working fluid in the tubes is first heated to nearly 750F and then is pumped to a heat exchanger plant that transfers the energy to water used to make steam. The steam turns a steam turbine-generator. On cloudy days, a gas-fired generator is used to maintain the plant's output.

Climate models questioned

Current global climate circulation models, used to predict climate change, don't accurately reflect the roles of the Northern and Southern Hemispheres, according to a recent research article in Science magazine, the journal of the American Association for the Advancement of Science.

Conventional wisdom, according to an article in the June 22 edition of the magazine, holds that the Northern Hemisphere and the tropics are a strong sink and source, respectively, of CO₂.

Not necessarily so, says a study by a group of climate scientists led by Britton Stephens of the National Center for Atmospheric Research in Boulder, Colo. Their report in Science finds that measurements of the vertical distribution of CO₂ in the atmosphere (Figure 8) are "inconsistent with atmospheric models that estimate a large transfer of terrestrial carbon from tropical to northern latitudes." In other words, the tropics are sequestering more carbon than the models predict.

8. Layering the atmosphere. New research, using vertical distributions of carbon dioxide in the atmosphere measured by aircraft at 12 global sites, suggests that global circulation models do not do a good job of assessing the "sources and sinks" relationship of the Northern and Southern Hemispheres. Courtesy: Britton Stephens

The implication, that models of very complex nonlinear systems have limitations, is a point that climate change skeptics have been raising for more than a decade. The article states, "Our ability to diagnose the fate of anthropogenic carbon emissions depends critically on interpreting spatial and temporal gradients of CO₂ concentrations."

An earlier questioning of the climate models' accuracy on sources and sinks came in a May 17 article in Science's online service; it suggested that the uptake of CO₂ by the Southern Ocean has slowed more than the models predict. Lead author Dr. Corrine Le Quéré of the Max Planck Institute in Germany noted that carbon sinks "are highly variable and sensitive to climate, yet they are poorly constrained by observations." In other words, the models make assumptions that aren't necessary based on empirical data.

What does all this mean? Nothing definitive, apparently, but just a cautionary tale about the limitations of computer modeling when it comes to very complex, nonlinear systems.