Stirling dish technology delivers top solar-to-grid conversion efficiency, has no water requirement, and is both scalable and cost competitive. So what are we waiting for?

By Emma Clarke in London

First invented in 1816 and used in aerospace and defence applications since the 1970s, Stirling heat engines could finally be on the brink of joining the solar race.

In January earlier this year, Maricopa, a 1.5MW demonstration plant, went into operation in Arizona, using 60 of Stirling Energy Systems’ (SES) SunCatcher dishes.

This was the first commercial deployment of the technology, and paves the way for construction of 1.5GW of Stirling engine solar farms in California and Texas.

The SunCatcher uses a mirrored parabolic concentrator dish (12 metres across) that concentrates sun onto a receiver, or power conversion unit (PCU). This tracks the sunlight and heats hydrogen gas to power a four-cylinder Stirling engine that generates electricity. Each of SunCatcher’s dishes can generate up to 25KW of power.

Infinia, based in Washington USA, is also developing a 3KW Stirling engine solar system, the Infinia Solar System with small demonstration projects in operation.

One of the key benefits of Stirling dishes versus other CSP technologies is that they don’t require water for power production, says Lori Singleton, manager of sustainable initiatives and technologies at utility Salt River Project (SRP) that will purchase the power generated from the Maricopa plant.

“For operations in the middle of the desert where water is a critical resource, this makes this technology very attractive,” she says.

SunCatcher dishes also hold the world record for solar-to-grid conversion efficiency at 31.25%, usually converting around 25-26% of solar energy into electricity, says Sean Gallagher, vice president of market strategy and regulatory affairs at SES. Parabolic trough and power towers have peak efficiencies of around 20% and 19-23% respectively.

SunCatchers also have a modular and scalable design, which means they can come online more quickly than other large-scale CSP technologies and incur minimal land disturbance, says Gallagher. Their modularity also allows the units to be installed on sloping land with up to 5% grade.

Gallagher claims costs are competitive with other CSP and PV technologies, with installed costs of the SunCatcher system set at, or below, US$3000 per kilowatt.

Why the wait?

The Stirling engine has been around for decades. The reason it has taken so long to enter the CSP market comes down to execution and cost rather than the technology itself, says Gallagher.

As he explains, SES has been waiting for manufacturing techniques to develop in order to make the precision technology on a mass scale for bulk power production.

“Advances in manufacturing techniques in the last 15-20 years have allowed us to make Stirling engines at high volume that are as efficient as if they were made by hand,” he says. More than 90 percent of the SunCatcher components will be manufactured in North America by automotive suppliers on an assembly line process.

SES was also able to make greater progress in the last couple of years following the US$100 million windfall from Irish developer NTR in 2008 in return for a controlling stake in the company. This investment enabled SES to overhaul the system to further reduce costs and optimise the system for high-volume manufacturing.

Construction of the demonstration project, that began in September 2009, has allowed SES to further fine-tune designs. Despite some minor changes, the project was finished ahead of schedule in December and started delivering power to the grid in January.

SES plans to start volume manufacturing in Summer 2010, hoping to break ground on three large utility projects later in the year. Long-term purchasing power agreements have been signed with San Diego Gas & Electric for Imperial Valley, a 750MW project near El Centro, California; with Southern California Edison for Calico, a 850MW plant near Barstow, California; and with CPS Energy for Western Ranch, a 27MW project in West Texas.

The Maricopa demonstration plant is a significant milestone for the technology, but as Reese Tisdale, from Emerging Energy Research points out, there’s still a great deal to be proven before the technology takes off. He says: “The O&M needs to be proven out. For example, how do these things handle dust storms?”

Storage not an option…yet

Another challenge is that, unlike trough and solar power towers, Stirling dish technology does not lend itself to thermal storage. The only – and more costly – option is to use electrical storage, for instance by means of batteries.

SES is investigating electrical storage methods, but these will not be included in initial projects. Infinia is also working with the US Department of Energy (DOE) to investigate thermal storage for Stirling machines.

Success also depends on securing finance. “SES has signed contracts in place, but getting the investment for these packages is a real barrier right now. Maybe they will be able to do it, maybe they won’t,” says Thomas Mancini, CSP Program Manager at Sandia National Laboratories.

As Gallagher says, the demonstration project will be critical to win financial support, so investors can see a slice of the system in operation. He adds that the DOE Loan Guarantee Program will also be important for the financing of these projects. SES is also still waiting to attain land-use permits for its plants from the Bureau of Land Management.

If all goes to plan, Gallagher says they will get permits in late summer for Imperial Valley, and early autumn for Calico, with construction expected to start immediately after in both cases.

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