By Richard Forsyth

Despite being relatively young, the CSP parabolic trough collector market is highly competitive and is fast advancing. With collector troughs accounting for roughly half the total plant cost, focusing R&D on troughs can reduce costs and also improve efficiency.

A recent report from CSP Today highlights a number of innovations that are making parabolic troughs better performers for harnessing the sun's phenomenal power. The latest second generation designs of collector troughs have wider apertures, advanced solar tracking devices, alternative mirror surfaces, innovative Heat Transfer Fluids (HTFs) and thermal storage methods.

Reflecting on cost

A good example of the latest in parabolic trough technology can be seen in the SkyTrough by the concentrated solar power company, SkyFuel. Advantages of the second-generation trough include an advanced sun tracking system, a lightweight space-frame assembly. But the 'jewel in the crown' for SkyFuel is their unique mirror film that replaces glass mirror facets.

The innovation is a low-cost, high-reflectance silverized polymer film called ReflecTech that was developed collaboratively with National Renewable Energy Lab (NREL) and which reflects 94% of sunlight, creating highly accurate, large monolithic mirror panels. Alison Mason, spokesperson for SkyFuel said, “To see the tangible benefits of advancing trough technology, the SkyTrough costs twenty to twenty five percent less than competing trough technologies today.”

The shatterproof mirrors only weigh half as much as traditional glass mirrors and weathering tests at NREL have shown them to be durable for 25 years.

“The lightweight components are easily sourced from local fabricators in the project country, reducing shipping and import costs, and giving a boost to the local economy,” adds Mason. “Market pressures to lower costs and stimulate job creation are forcing CSP technology companies to innovate.”

A demonstration plant using these troughs has been operating at SEGS II in Daggett, California for the last 18 months and efficiency tests undertaken by NREL confirm the thermal efficiency is over 73%, which means they compete with glass mirror alternatives but with reduced costs of installing the collector of around 25-30% (compared with Acciona's Nevada Solar One concentrators).

Second generation parabolic troughs can also be seen more as an evolutionary development and a matter of modifying design rather than radical reinvention. When it comes to cost reduction and increased efficiency it is the tweaking that can make all the difference.

Susanne Krebs, spokesperson for Solar Millenium AG explains “Our research and development work has now brought a second generation of collectors to the point of industrial maturity. The expansion of the module width of the new HelioTrough collectors by around twenty percent compared to its predecessor the Skal-ET, improved optical efficiency and a more efficient mirror configuration, [enables] the ratio of kilowatt-hours generated per surface unit to have further improved. This reduces the building area needed to construct a plant to meet the same energy demand.”

Alternative heat transfer fluids

The Archimede pilot plant built by ENEL uses molten salt as the heat transfer fluid in a parabolic solar field (technology licensed from Italian research Centre ENEA), which went into operation in the summer of 2010. According to Massimo Falchetta, a senior scientist at ENEA who worked closely on the project, the pressure in a molten salt field is lower than in an equivalent oil operated solar field.  "Archimede design pressure is 15 bar, its normal operating pressure is in the range of 7-8 bar", he said.

However, he notes that piping and components materials; piping electric pre-heating equipment; quality control on piping heat insulation execution; and the need for high temperature receivers, increase the overall cost.  "Managing such a large piping network filled with salts, remains a challenge" he says. "It is still open to improvement and needs R&D effort."

He points out that factors that reduce costs include a system that bypasses the need for an oil to salt heat exchanger; that it requires less than half volume of storage for the same energy; and that molten salt it is a non-polluting HTF. 

Another promising method that is still in early development stages is Direct Steam Generation (DSG) – which similarly can achieve higher temperatures. DSG also needs lower investment and O&M costs, can achieve a higher plant efficiency (with a relatively simple plant configuration) and there is lower environmental risk as water is used instead of oil.

As Chuck Kutscher, Principal Engineer at National Renewable Energy Laboratory explains, the maximum operating temperature of current parabolic trough collector fields is about three hundred and ninety degrees centigrade, which corresponds to the maximum continuous temperature allowable for the heat transfer oil.

“By using molten salt or direct steam generation in the collector field, higher operating temperatures are possible”, says Kutscher. This is because elimination of oil in the collector field can also eliminate the need for heat exchangers, along with their associated cost and temperature drop penalty.

“Molten salt is currently the fluid of choice for thermal storage because of its low vapour pressure and higher operating temperature. By using molten salt in the collector field the heat exchangers between the collector field and storage can be eliminated,” he adds.

Taking its place in the energy mix

Solar field technology is developing at pace in a drive to prove a sustainable and competitive method of harvesting thermal power for mass consumption in countries like Spain, United States, Greece, Mexico, Egypt, India and Morocco. Although many technical barriers remain, the goal for the industry is to adopt a range of improvements to reduce the levelised electricity cost by approximately 50%.

“In the near-term and mid-term it is expected that troughs, power towers, dishes, and PV technologies will all co-exist. Recent analyses suggest that power towers offer the greatest long-term potential for cost reduction compared to other CSP technologies, but this approach is not yet proven,” says Kutscher.

“It’s not clear that one solar plant type will win out over all others in the end, just as today’s electric grid consists of coal, nuclear, hydropower, and different types of natural gas plants.”

To respond to this article, please write to the editor:

Rikki Stancich: rstancich@csptoday.com

Related Event:

5th Concentrated Solar Thermal Power Summit, 29-30 November, Sevilla (Spain)

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