By Jason Deign in Barcelona

Brayton cycle turbines, used commercially in aircraft jet engines and other areas, have significant advantages over the turbine models found in most CSP developments.

They are a proven technology that does not need water and can operate at almost twice the 18 or 19% efficiency typical of steam turbines, says Luis Crespo Rodríguez, secretary general of Protermosolar, the Spanish CSP industry body.

He should know. In the 1980s Crespo was part of a Hispano-German team that carried out one the first tests of a receiver for a solar-powered Brayton cycle turbine generator, at what is now the Spanish Department of Energy’s Almería Solar Platform research facility.

The GAST pilot indicated a heliostat field could heat air pressurised to nine atmospheres to a temperature of around 800°C. This could then be increased to between 1,200°C and 1,300°C in a gas-fired process before injecting it into the turbine.

Using the waste heat generated by the turbine in a combined cycle would allow the overall efficiency of the plant to come close to 40%, Crespo says. “We were proud of what we achieved with the pilot. We showed this process had significant potential."

Too risky?

“Then along came the long dark night of the soul for the solar industry, between the 1980s and the emergence a new generation of plants from 2007. This was a riskier option than steam turbines, which is what the industry opted for even though their performance is lower.”

However, Crespo notes, now that CSP is beginning to mature, “these concepts are being revisited. And they have a lot of potential.”

Certainly, interest in Brayton cycle-powered CSP seems to be at an all-time high, with a number of pilots being announced in recent months.

Wilson Solarpower Corporation is planning to use the technology in the United States and Spain’s Abengoa Solar is part of a consortium backing a project called Solugas, to ‘demonstrate the performance and cost reduction potential of a solar-hybrid driven gas turbine system.’

In Australia, the Commonwealth Scientific and Industrial Research Organisation (CSIRO) is building what it claims is the largest Brayton cycle solar tower development in the world, at the National Solar Energy Centre in Newcastle.

Costing AUD$5m and due to become operational in March 2011, the two-year pilot will cover 4,000m2 and use 450 heliostats to heat air up to more than 900°C to power a 200kW commercially available micro gas turbine in a 30m tower.

Water availability

“There are a number of reasons for going with Brayton cycle,” says CSIRO Energy Technology energy systems manager James McGregor. “One of the challenges of CSP is that it traditionally uses steam-based cycles, so water availability becomes a key site selection criterion.

“The other thing with CSP is it is advantageous to reduce cost if you can capitalise on economies of scale with a mature technology.”

But for Australians, in particular, one of the biggest bonuses of Brayton is that, unlike steam cycles, it can work efficiently in small-scale plants. “With Brayton cycle you get good economies of scale at the 10MW scale,” McGregor says.

“In Australia it’s principally likely to be used for our resource sector. We have a lot of mining facilities in some of the best locations for solar power, but they don’t have water. In these locations we are competing with diesel generators, so there’s no point having massive plants.”

McGregor sees Brayton’s ability to compete at the distributed power generation level as opening up “an important niche market.” But all experts agree that the technology has to overcome a significant challenge before it becomes commercially viable.

High temperatures

“It is difficult to obtain the operational conditions you need for Brayton with the sun,” says Dr Pinchas Doron, chief technology officer at Aora in Israel, which has been operating a nominal 100kW pilot of the technology since last summer.

“Steam cycles operate at temperatures which are a few hundred degrees lower, which is a major difference. When you need to go to these high temperatures you have a problem.”

As a result, most of the research in current pilots is in how to achieve and manage sufficiently high temperatures. Doron says the Aora pilot faced “many challenges”, including getting sufficient focusing power from the heliostats, but insists “we now have the solutions.”

The company is now planning a larger pilot in Spain, in association with an unspecified partner, and “next year you’ll see a few of our units going beyond the pilot stage,” says Doron.

McGregor, too, thinks the current development effort will help crack the operating temperature issue imminently. “Within five years there will be a commercial offering available to the market,” he predicts. So it looks like traditional CSP could soon have a hot new competitor.

Correction:

In the 1970’s and 1980’s Sanders Associates (Nashua NH, USA) working under the US DoE and JPL contracts built and tested several generations of solar-Brayton systems, several years before the GAST pilot.

This article was updated August 31, 2010.

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Jason Deign: jdeign@csptoday.com

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Rikki Stancich: rstancich@csptoday.com

Image: World's first solar-dish Brayton (1982-1984), comprising an Allied Signal gas turbine, a Sanders silicon carbide receiver and a LaJet dish. Image Courtesy of Brayton Energy LLC.