Non-Glass Mirror-Film Promises to Boost Efficiency, Reduce Costs of Concentrating Solar Power

A new “silver-metalized” mirror film invented by the National Renewable Energy Laboratory and ReflecTech is a key element in testing an innovative concentrating solar power system that holds the promise of significantly lower costs and significantly higher sunlight-to-electricity conversion efficiencies, making them more attractive to utilities and their customers.
ReflecTech – the name of the highly reflective film as well as the company that co-invented it – has been applied on a precisely manufactured parabolic base of sheet metal, replacing the more costly, heavier and less durable parabolic glass mirrors typically used in concentrating solar power systems. An inner layer of pure silver protected by multiple layers of polymer films makes the mirror film highly reflective while also protecting the silver from the elements and oxidation.
Albuquerque-based SkyFuel and the NREL in Golden, Colorado have incorporated the “glass-free” mirror film into SkyTrough, a new type of parabolic trough that has been mounted on NREL’s Large Payload Solar Tracker.
* Photo credits: Pat Corkery, NREL

SkyTrough & CSP
Tracking the sun’s path across the sky, the ReflecTech-coated parabolic trough focuses and concentrates the sun’s rays on an oil-filled vacuum tube, heating the oil to temperatures as high as 400 degrees Fahrenheit. The heated oil is used as a heat storage and transfer agent to produce steam, then electricity, by channeling it into a conventional steam generator.
NREL and SkyFuel will be testing and validating the system’s performance at a Colorado research facility for at least three seasons in order to gauge its performance under a variety of weather conditions and angles as the sun’s course varies during the year.
A parabolic trough typically operates at nearly 80% optical efficiency, according to NREL. The SkyTrough team expects their system to perform at least as well while being less expensive to manufacture, transport and maintain. “There is a cascade of opportunities to lose some light at every step in the process,” NREL senior engineer Keith Gawlik explained.
“That’s why we field test the whole unit and get solid data over a number of months. It removes the uncertainty in the final efficiency result… Lots of things come into play when focusing light. “We have to consider the reflectivity of the surface, the accuracy of the surface and then aiming all of the light into the narrow focal line of the receiver tube.”
NREL and SkyFuel will be testing a smaller scale version of SkyTrough that also does not include generating electricity via a steam turbine, though it’s that combination of new renewable and proven, well-established conventional technologies that the partners believe make it particularly attractive to utilities as a source of clean, bulk power during peak and intermediate load periods. A commercial scale version of SkyTrough might be as long as 375 feet and 20 feet high and could supply enough hfor 125 homes, according to NREL.
As they require relatively large tracts of flat land, western states are seen as prime candidates for concentrating solar power systems. The Western Governor’s Association has estimated that CSP plants could generate 4 gigawatts of electricity in the US by 2015.

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