By Jason Deign

While dish Stirling has clear challenges as a technology, it is not clear that sticking any other kind of engine on parabolic dishes could solve the problems.

What is really the problem with dish Stirling? As previously reported in CSP Today, dish Stirling is a technology that offers plenty of promise, particularly in terms of characteristics such as high solar power conversion efficiency and a modular design.

Despite this, though, it has failed to make its mark in commercial terms, raising questions over its effectiveness.

Experts such as Dr Eduardo Zarza Moya of the Spanish Centre for Energy, Environmental and Technology Investigation (Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas or CIEMAT in Spanish) believe this may be due to the nature of the engine.

“This has to work at temperatures above 600ºC and 100 to 200 bars of pressure,” he says. “Furthermore you are working with a gas that has to move through a closed cycle. That means you cannot have any leaks and there is a lot of friction and wear.

“This translates to a high cost of maintenance. The reliability and longevity of the motor are not good, which is why it is difficult to make it competitive.”

However, if the trouble with parabolic dish designs were just to do with the engine, then presumably swapping a Stirling motor for something different would solve the issue. After all, it is perfectly feasible to attach a Brayton cycle motor, for example, to a parabolic dish.

And Jim Kesseli, president of Brayton Energy, a renewable energy research and development firm based in Hampton, New Hampshire, USA, has done just that. He agrees Stirling will not work, but remains unconvinced about other options.

“I have worked quite a lot in both areas,” he says. “The little Stirling engines, like Infinia and SunPower used to make, or those that go into space power, have pretty good reliability. But the problem is when the system is so small it is hard to be cost-competitive.

Brayton cycle

“Braytons are similar in cost to Stirlings at the very small size,” he continues. “But the Brayton cycle would be much more economical at a bigger size, say 1MW scale. The reliability of the Brayton cycle is much more quantified and proven, unlike Stirling engines.

“And you can do the internal combustion with natural gas with a very small capital cost addition.”

However, when Brayton Energy tried to make the economics work by hooking a Brayton cycle engine up to a 320m2 dish in Phoenix, USA, “it was not easily done,” says Kesseli.

“The economy of scale of the dish was better at bigger scale and certainly the economy of the engine is better at bigger scale. However, it’s only incrementally better. It’s not good enough, I’m afraid to say.”

Could other approaches work better? Yanir Blumenthal, chief technology officer of HelioFocus, believes so. “A parabolic dish is a concentrator,” he says. “In its focal, several different engines can be mounted.

“In our case it is a pressurized volumetric receiver utilising air as its heat transfer fluid.”

The HelioFocus system has a closed-loop, pressurised air cycle that can be heated up to 1000ºC for use in a gas turbine or 650ºC to boost existing power stations and for standalone steam turbines.

“We are not producing direct electricity with our parabolic dishes,” Blumenthal says. “We are a manufacturer of thermal energy, and therefore unlike Stirling engine [designs] can directly convert and store thermal energy.

Heat exchange

“Due to the fact that we are using air, we don’t have to deal with molten salt or other heat transfer fluids. We are also able to store energy and utilize it with our heat exchange system.”

The Australian developer Wizard Power is also working on a parabolic dish variant where the dishes all feed superheated steam into a central power block, similar to what happens in trough designs. This could also potentially offer the potential of energy storage.

Other avenues worthy of parabolic dish investigation include supercritical carbon dioxide Brayton cycle engines, which Kesseli says offers “a glimmer of hope”, and off-grid, easily deployable applications that could supplement traditional power, for example for military use.

However, the consensus is very much that unless parabolic dish technologies can somehow be married to storage, they are going to have a hard time justifying their existence no matter what engine they use, purely on a cost basis.

“I think we’re very challenged for a dish system of any power capacity to be competitive in the USD$2-$2.50 a watt range, and that’s barely good enough unless you’re offering other things, like hybrid,” says Kesseli. “You’re not going to beat a PV cell.”

And Jenny Chase, solar insight manager at Bloomberg New Energy Finance, says: “The ones I have looked at have not got storage or thermal inertia, which puts them in competition with PV: not a good place to be.”

Kesseli’s outlook is even starker. “The current state-of-the-art Brayton and Stirling systems are not going to make it,” he says.

To respond to this article, please write to Jason Deign

Or write contact the editor, Jennifer Muirhead