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Why Not the Sun? Advantages of and Problems with Solar Energy
(Released December 2008)

 
  by Ethan Goffman  

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Solar Thermal

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Although photovoltaic panels have become the prototypical image of solar power, solar thermal is actually older. Leonardo Da Vinci had solar energy designs in his notebooks, while "in the 1870s and 1880s, at the height of the Industrial Revolution, French and U.S. scientists developed an array of solar cookers, steam engines, and electricity generators, all based on a simple concept, a parabolic-shaped solar collector that is coated with a mirrored surface to reflect light coming from different angles onto a single point or line" (Flavin). That design is the basis of many of today's large-scale solar plants, which rely on parabolic troughs.

Bush looks at solar mirrors
US President George W. Bush (R) walks past a parabolic dish during a tour of the Department of Energy's National Solar Thermal Test Facility at Sandia National Laboratories in Albuquerque, NM.

Solar thermal, unlike solar cells, employs glass or other material to concentrate light and convert it to a source of electricity. In general, solar cells are used as a decentralized source of power, often on individual houses. Solar thermal, by contrast, is being developed in large, centralized power plants: "The technology which is generally proposed for harvesting solar energy at large scales in the Sunbelt is Concentrated Solar Power (CSP). This method concentrates the sun's rays to heat water, producing steam which drives turbines to generate electricity in an otherwise conventional way" (Al Bawaba).

Parabolic troughs are currently the standard form of solar thermal used in large power plants. They consist of "long parabolic-shaped rows of mirrors focus sunlight on fluid-filled metal tubes encased in glass. The heat collected drives steam generators similar to those that run coal-fired power plants to make electricity" (Woodside). Explains CSP entrepreneur Randy Gee, "there is a great deal of interest from the investment community because parabolic trough is a proven commodity" (Power Engineering). Research continues apace, not only in parabolic troughs but in other ways of providing solar thermal.

CSP has the huge advantage of storing energy, via molten salt (NREL, Trough), greatly alleviating the intermittency problem and allowing it to be used as a baseline energy source. Explains Gee, "CSP is positioned to be the largest means of generating renewable electricity in the 10-15 year timeframe. . . . CSP systems can provide despatchable [sic] electricity, meaning they can deliver the electricity to the utility or customers wherever it is needed by virtue of the fact that CSP is a thermal technology and can therefore can [sic] incorporate thermal storage. This is what separates it from wind and photovoltaics" (Power Engineering).

plant surrounded by solar arrays
Spanish solar-power-plant developer Abengoa Solar plans to build and begin operating this 280-megawatt solar thermal power plant in Gila Bend, AZ, by 2011.

Solar thermal plants are particularly apt in sunny climates such as the American Southwest. According to one source, "at least 250,000 square miles of land in the Southwest alone are suitable for constructing solar power plants . . . . Converting only 2.5 percent of that radiation into electricity would match the nation's total energy consumption in 2006" (Zweibel). The Nevada Solar One solar thermal plant, which came online in 2007, is a notable example. Its "64 megawatts (MW) capacity makes it the largest solar plant to be built in the world in the last 16 years" (Energy Information Administration). Still, in the American Southwest plans for solar thermal are slowing due to a 22 month moratorium on solar collector applications from the federal Bureau of Land Management to study environmental impact (Economist, Freezing). This conflicting environmental claim illustrates one of the many obstacles and diversions in renewable energy's path becoming a dominant power source.

Critics claim that solar takes too much land to be viable as a large-scale energy source (Hiserodt). Clearly, moving to solar as a major energy producer would mean an enormous reallocation of land and resource use. Yet, according to one source, "installations already in place indicate that the land required for each gigawatt-hour of solar energy produced in the Southwest is less than that needed for a coal-powered plant when factoring in land for coal mining" (Zweibel). Estimates are tricky, because they need to account for all the land and installations needed for any given power source, as well as environmental problems and the cost of transportation and storage.

One problem with concentrated solar in the southwest is distance from where energy is produced to where it is most needed. One expert explains that "major obstacles such as long-life storage and long distance transportation remain to be overcome before solar power becomes a major contributor to the world energy grid" (Al Bawaba). Major infrastructure investment would be needed. According to one advocate, "a new high-voltage, direct-current (HVDC) power transmission backbone would have to be built" using Direct Current (Zweibel). Certainly the initial price would be high, although once energy did begin flowing it would be close to free.

man with solar heating array
Mingde Zhou, who manages a hostel, shows solar panels that supply hot water on the roof of his building in Shanghai, March 1 2006.
If the obstacles can be overcome, according to one report, "concentrating solar power, which always works on a utility scale, could provide seven times of all of America's power needs. This expands on a prediction green advocates have made for years, arguing that, potentially, all of the country's power needs could be met in a 100-square-mile plot of land in a sunny region like Nevada" (Woodside).

One decentralized form of solar thermal that has been around a long time is solar water heaters. Currently, these are a better investment for individual houses than are solar cells; they pay for themselves in about 15 years (City of Columbia) as opposed to 20 or more for photovoltaic cells. Factoring in any government incentives, the payback time could be far less.

Direct solar power for automobiles seems extremely unlikely. However, if the current development of plug-in electric automobiles continues apace, solar energy on the grid may end up powering these vehicles.

Go To The Economics of Solar Power

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