By Jason Deign

Take two solar plants. Both produce energy only while the sun shines. But one relies on a system that has never been used on a large scale anywhere before, while the other uses a technology you can pick up at Walmart. Which would you invest in?

This, essentially, is the challenge facing parabolic dish CSP. Like other solar thermal technologies, it typically vies for investor dollars with PV, which is more established and currently much cheaper, thanks to economies of scale.

Unlike other solar thermal technologies, however, parabolic dish lacks the one thing that sets CSP apart from PV and makes it worth backing in the eyes of international finance organisations, public administrations and private investors. That thing is storage.

Technologies such as dish Stirling work by focusing solar power on directly on a small turbine instead of using it to heat a thermal energy source such as molten salt. This allows parabolic dish arrays to double the 15% or 16% peak efficiency achieved by other types of CSP.

But it does so by cutting out the need for a heat sink, thereby doing away with what is arguably CSP’s greatest asset.

“Adding molten salt storage systems typically reduces the levelised cost of energy over the cost of a similar system without storage,” observes Bill Gould, chief technology officer at SolarReserve.

The company has followed a growing trend among CSP developers by selecting a molten salt thermal storage system for its 110MW Crescent Dunes power tower project near Tonopah, Nevada, USA, which is due to start operating late next year.

“The addition of energy storage enables longer daily run times and an increased equipment utilisation and plant capacity factor,” Gould says. “Molten salt has been adopted by most of the major CSP technology suppliers for energy storage and/or the collection of energy.”

Molten salt

Granted that the design of most parabolic dishes does not immediately lend itself to handling molten salt. But can developers afford to ignore the benefits of adding storage to their systems?

Ex-financier Joseph Johnson, an analyst with the City of Surprise Public Works Department at the City of Surprise in Arizona, USA, says: “One of the best ways to make CSP more competitive is to find ways to maximise efficient use of all of the thermal energy collected.

“Storing otherwise wasted heat seems like a no-brainer; it may also move the location of the electrical generation from the focal point of the dish to the ground.”

Furthermore, there is no requirement for parabolic systems to use current molten salt mixes as a storage medium; other heat-storing materials currently under development may prove better suited to dish designs.

In August, the US Energy Department provided a boost for alternative heat storage research with two $5 million, five-year funding packages through its SunShot initiative to make solar energy competitive with other power sources by 2020.

One grant went to a team led from the University of California, Los Angeles for research into liquid metals as potential heat transfer fluids, while the other was awarded to the University of Arizona for a programme looking into new molten salt-based compounds.

Another SunShot project, with the Pacific Northwest National Laboratory, is specifically looking to develop and commercialise thermo-chemical energy storage technologies for parabolic dish and Stirling cycle heat engines.

Meanwhile Professor Seth Sanders, of the University of California Berkeley, is overseeing research into heat transfer and storage materials with funding by the US National Science Foundation.

Soybean oil

“We’ve experimented briefly with soybean oil, which is very cheap and seems to meet our temperature objectives,” he says. “We’ve also had a detailed study of salts used as phase-change thermal storage media. Some of these are quite promising.”

Among other approaches to maximising heat transfer efficiency, says Johnson: “Diethelene glycol with carbon nano-fibres seems to be the most promising nano-fluid to date. Zeolite has many exciting properties that lend themselves to inexpensive heat storage.”

Finally, a completely different tack could involve storing the power from dish Stirling engines as electricity rather than heat.

Michael Chang, visiting professor at China’s Yunnan University, says: “Batteries in China are really cheap, so I’ve been exploring the option of stored energy using lithium-ion banks.”

Lithium-ion batteries are increasingly used to provide energy storage for PV, but their manufacture involves pollutants and, says Chang: “Li-ion has a historical risk of spontaneous combustion, and we’re trying to avoid anything resembling fireworks.”

For the time being, then, the hunt is still on for a suitable way to store energy from parabolic troughs. And it has to be said that storage may not necessarily be a panacea for the technology.

As the senior executive of one dish Stirling company says: “A storage technology would strengthen the case, but I would say solving the chicken-and-egg situation with cost-versus-volume is more critical at this point.”

To respond to this article, please write to Jason Deign

Or contact the editor, Jennifer Muirhead