Public Power - July/August 2008 - (Page 23)

tors, crystalline silicon cells, thin film technologies and emerging photovoltaic. Multi-junction concentrators have achieved the highest efficiencies in the lab—up to 40.7 percent. Crystalline cells are at 27.6 percent, while in March, National Renewable Energy Laboratory announced a new thin film record of 19.9 percent using CIGS. Third generation emerging photovoltaic rates are the lowest. For example, dye-sensitized cells stand at 11.1 percent while another organic cell variety recently hit 5.4 percent. The efficiency of laboratory photovoltaic cells is one thing. Ratings of photovoltaic modules and complete photovoltaic systems are another. National Renewable Energy Laboratory’s Margolis quotes the spread of efficiency ratings of commercial crystalline silicon modules as ranging from 13 to 19 percent. Commercial thin film photovoltaic module ratings vary between 6 and 11 percent, but are improving rapidly. It is not as meaningful to talk about the overall efficiency of photovoltaic systems, because there are too many variables, such as how a system is laid out and the efficiency of other components such as wires and inverters, the latter of which averages 95 percent efficiency, said Margolis. One way to improve efficiency is to include additional layers of material that absorb other wavelengths. Third generation photovoltaic is generally low cost and inefficient. However, it has high efficiency potential because it can be assembled with multiple layers of p-n junctions. A second way to improve efficiency is to use mirrors or lenses to focus, or “concentrate,” more sunlight on a unit area of photovoltaic cell surface. Consequently it is not surprising that two- and three-junction concentrators boast the highest recorded lab efficiency ratings of any photovoltaic technology. The idea of assembling photovoltaic modules into arrays in which mirrors or lenses concentrate sunlight to yield greater power has been around for years. This pushes efficiency up and reduces the size or number of cells needed. To work, the mirrors must be on dual-axis tracks so they can follow the sun through the day. The additional light also brings more heat, reducing www.APPAnet.org cell efficiency and straining components, so steps must be taken to keep them cool. There are other reasons experts disagree about the future of concentrating photovoltaic technology. Since it requires tracking, it is not considered practical in the smaller, roof-top applications that dominate the market, but may lend itself to utilityscale use. At least one utility-scale plant now on the drawing board is a 200-MW concentrating photovoltaic project. National Renewable Energy Laboratory reported more than $150 million of private money went into concentrating photovoltaic during 2007. Unlike photovoltaic, concentrating solar power generates AC power. There are four types of concentrating solar power: parabolic troughs, solar power towers, or central receivers, dish/engine and linear fresnel systems. Concentrating photovoltaic systems are sometimes categorized as concentrating solar power. Parabolic troughs feature a pipe running parallel to a row of curved mirrors that focus sunlight on the pipe, which contains a heat transfer fluid that flows to a heat exchanger. EPRI considers parabolic troughs the most mature and least costly concentrating solar power option, at least in the near term. Most concentrating solar power in development are parabolic troughs. There are two commercial parabolic troughs in the United States greater than 1 MW—the 354-MW Solar Energy Generation Station (SEGS) and the 64-MW Nevada Solar One. SEGS is a collection of nine plants, ranging from 13 to 80 MW each. SEGS, most of which is owned by TerraGen Power LLC (160 MW) and FPL Group (148 MW), was built in the Mojave Desert in nine stages ending in 1991 and is still the largest solar power plant in the world. Over the past 22 years, it has generated 11,000 GWh, produced $1.7 billion in revenue, is still profitable and is expected to operate another 50 years. Acciona’s $266 million Nevada Solar One, the world’s third largest concentrating solar power plant, went on line in June 2007 at a site 32 miles south of Las Vegas. The plant’s 182,000 mirrors are set on 300 acres and arranged in 760 parabolic con- centrators that can track the sun and deliver its light to more than 18,000 tubes capable of heating the transfer fluid—a high temperature oil—to 735 F. Solar power towers are composed of an array of flat, computer-controlled mirrors built within a large circle that track the sun and focus its light onto a tower rising from the center of the circle. They allow higher operating temperatures and greater efficiency. There were two tower demonstration projects in Barstow, Calif. BrightSource Energy has announced plans to build three solar towers in Nevada totaling 400 MW. Dish/engine systems use mirrors built into large dishes sitting on pedestals that track the sun, focusing the light on a power conversion unit or engine at the end of a boom rising from the center of the dish. They are modular and have very high efficiency. Although there are no commercial dish/engine systems in the United States, Sandia National Lab has a dish-Stirling engine demonstration in Al- vCentrix makes deploying high-quality, reliable VoIP easy. Our fully hosted solutions provide an immediate path to offering VoIP and realizing big bottom line results without big capital expense. vCentrix www.vcentrix.net 617-904-5000 Sales sales@vCentrix.net Call vCentrix today to learn more about increasing your RPUs and margins. JULY-AUGUST 2008 23 http://www.vcentrix.net http://www.APPAnet.org

Table of Contents Feed for the Digital Edition of Public Power - July/August 2008

Public Power- July/August 2008 Contents Perspective 10 Questions Solar Energy Rising Sacramento's Solar Shares Gainesville Crowns a Conservation Idol By the Numbers Curbing Costs of Outages Reliability Green Energy Hometown Connections Customer Service Parting Shot

Public Power - July/August 2008

For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.