By Jason Deign in Barcelona

Michael Magdich, general manager of Global CSP & CPV at Guardian Industries, has a disaster scenario for anyone operating parabolic troughs with tempered glass reflectors.

If a mirror breaks and prevailing winds are strong enough to propel a two-to-four-gram fragment of glass towards another sheet at 90mph or more, he says, the force of the impact will shatter the second pane, sending out more fragments in a potentially devastating domino effect.

A tightly packed parabolic trough field could be wiped out in seconds. “Has it happened yet? No,” says Magdich. “Could it happen in the 30-year life of a field? We are concerned.”

That is why Guardian recommends building parabolic trough fields—or at least their more exposed outer edges—with laminated glass, of the kind used in car windshields, which cracks but does not shatter and therefore remains operational even when broken.

But then again, when it comes to parabolic trough components the recommendations you get can be as varied as the manufacturers you talk to.

Although the fundamental principles of the technology have not changed much since the early versions installed by Luz Industries at the Solar Energy Generating Systems facility in the 80s, recent efforts by manufacturers have led to a bewildering array of options for trough operators. 

Automotive technology

Glass-bending equipment manufacturer Glasstech, for example, agrees on the suitability of glass for mirrors but leaves the decision on how to strengthen it to downstream partners.

“For a long time glass was being used in its annealed form, which has issues in terms of breakage,” says Mike Ondrus, the company’s director of Solar Energy Systems. “If the glass breaks it might come down on some other expensive equipment.”

Ondrus says adapting automotive technology to CSP can produce mirrors four or five times stronger than traditional glass. “Glass has a significant track record,” he adds, and is currently in use in fields across Spain and the United States.

Yet speak to Kelly Beninga, chief commercialisation officer at SkyFuel, and aluminium with a ReflecTech mirror film is the way to go because there is practically no danger of breakage. “It can be shipped flat and slides into slots in a parabolic frame,” he adds.

“It is inexpensive and fast in the field, in the range of 20% to 30% cheaper than other options, including labour costs.” 

While SkyFuel cites major cost reductions, unbreakable mirrors, rapid field assembly, compact transportation and low maintenance, glassmakers assert non-glass surfaces such as metal and plastic films have a shakier track record, lower performance and less resistance to weathering.

“You can scrape them with your fingernail,” Magdich says.

Solar reflectance

Both sides defend the performance of their surfaces with empirical data. SkyFuel, for instance, claims 94% solar-weighted reflectance for its ReflecTech mirrors.

Guardian guarantees 94.5% for its laminated glass, and quotes a National Renewable Energy Laboratory measurement of 96.75%.

But the problem in some instances is that measurement systems are still being developed alongside the technologies they are supposed to measure, says Dr Christoph Richter, Solar Research Almería site manager for the German Aerospace Center (DLR).

Regarding durability, for example, “We are just starting to look at these new materials and starting to develop ways of measuring aging. The aging of glass is very low and difficult to measure. It is difficult to assess if aluminium has the same lifespan. You have to guess a little.”

The debate on the best reflector materials looks set to run for some time, and the competition between manufacturers appears to have caused problems for some.

Patriot Solar Group, for one, previously commercialised a plastic-metal alloy reflector but, says operations manager Ken Sexton: “We did not have enough revenue to pursue troughs. Right now they will not be in our portfolio; they could be in the distant future.”

Size matters

At least in other areas, though, there is broad agreement on trough technology trends.

The troughs are getting larger, for example, with seven-metre-aperture versions in development as part of a shift towards a theoretical maximum of about 10m, at which point wind loads may limit further increases. “A potential optimum would be slightly smaller than 10m,” Richter muses.

Another point most sources agree on is that trough technology will not harbour any major surprises in years to come.

Current research and development is around enhancements to the concepts perfected by Luz, for example by increasing the operating temperature of heat transfer fluids or perfecting drive and control systems.

“I think there could be important incremental improvements but nothing radically new,” Richter says. “But a number of small improvements can still make a significant contribution to cost reduction, which is the overall goal.

In addition, he says, there are important potential improvements to be made in areas such as manufacturing and supply chain logistics. In future, the big deal for parabolic trough manufacturers might not be what they put into solar fields, but how and where they make it.

To respond to this article, please write to:

Jason Deign: jdeign@csptoday.com

Or write to the editor:

Rikki Stancich: rstancich@csptoday.com

Image credit: Guardian EcoGuard Solar Boost-MP monolithic parabolic mirrors for CSP parabolic trough systems