Economic factors
A familiar criticism of wind and solar projects is that they don’t generate power during peak consumption hours. That’s not true with Solar One, where peak generation hours are a close match with its customers’ peak demand patterns. Air conditioning loads dominate in the desert, and those loads are coincident with periods of the highest solar energy production.
The cost of generation from Solar One is estimated to be around $0.15 to $0.17/kWh. With greater efficiencies and economies of scale, costs can be expected to fall in coming years. The DOE/Energy Efficiency and Renewable Energy goal is to reduce the cost for concentrating solar thermal systems to $0.035 to $0.043 per kWh by 2020. A National Energy Technology Laboratory rep says the SEGS projects are producing power in the $0.12 to $0.14/kWh range today.
System improvements
Solargenix has made significant improvements to support and tracking systems since the last SEGS plant went into service. The support structure for the concentrating solar collector is reminiscent of the Luz design, but Solar One uses aluminum structural components that can be machined to close tolerances and then easily assembled in the field while maintaining those tolerances. This improves the concentrator’s ability to precisely focus on the receiver pipes, even under high wind loads.
Additional research and development work has also reduced the thickness of the mirror and its surface coatings to improve performance. The technology to produce lightweight, composite concentrator modules was not available when the SEGS plants were designed.
The Schott receivers employ a number of other improvements that weren’t available to the early SEGS receivers. Among them are a new antireflective glass coating that resists abrasion while allowing more than 96% of the solar radiation to penetrate the receiver and the use of new absorptive steel coatings that improve radiation absorption rates to 95%.
Solar One can also be differentiated from the SEGS projects by the design use of backup natural gas during periods of low-incident solar energy. SEGS is designed for up to 25% natural gas use, whereas Solar One is allowed to only use 2% natural gas to steady temperature fluctuations in the heat-transfer oil and for freeze protection. Solar One is a solar-only system that uses just 30 minutes’ worth of thermal storage to minimize the effects of transients.
One major technical challenge for Shott was to maintain the seal between the glass tubing that contains the vacuum-packed steel receiver that circulates 750F synthetic oil. Schott responded by developing a special borosilicate glass that actually has the same thermal coefficient as steel. This eliminates any differential movement as well as thermal stress in the system caused by daily swings in desert temperatures, which can dip to freezing at night and then bounce back into the triple digits the following day.
What’s next?
Don’t expect to wait another 15 years for the next concentrating solar facility to be built. Acciona is already hard at work developing two similar 50-MW plants in Spain and is in discussions with many other developers around the world to replicate the design of Nevada Solar One. Expect to see similar systems coming to a desert near you.
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