Phase-change materials promise to significantly reduce the cost of thermal energy storage. So how come we are still using molten salt?

By Jason Deign

The case for phase-change material (PCM) thermal energy storage (TES) looks pretty good on paper. By locking energy up in one state and releasing it again in another, PCMs should in theory achieve higher levels of efficiency than molten salt TES.

This should equate to more compact TES setups and smaller containment vessels, cutting the cost of storage and making CSP plants cheaper overall. Just what the industry is looking for. And it is not like PCM storage has never been piloted before.

As it happens, at least one company is already waiting to scale PCM storage up into a commercial setting with CSP. Terrafore Technologies, of Minnesota, USA, just needs to find the right partner for its PCM concept, which involves encapsulating sodium nitrate.

Terrafore believes its encapsulated PCM system could reduce the cost of TES to just USD$17.76 per kWh, well below the $28.76 level of current two-tank molten salt systems and not far off the US Department of Energy (DoE) SunShot objective of $15 per kWh.

With results like this, you would expect CSP developers to be beating a path to Terrafore's door, particularly since the encapsulation process relies on tried-and-tested industrial methods. But no: further development of Terrafore's technology is stalled because of a lack of investors.

"I had DoE funding for three years," says Terrafore's founder and chief technology officer, Anoop Mathur. "We made the capsules in the lab and were able to show this could last a lifetime. Scaling up is still where we are at. I'm looking for people to help me take the next step."

Unfortunately for Mathur, who has been investigating PCM processes since the 1970s, there are a few reasons why a suitable backer might be some time in coming forward. The most obvious one is that the CSP industry is hardly flush with cash right now.

Gasping for work

Potential new markets such as India and the Middle East have not yet taken off in the way observers have hoped.

For those projects that do come through the pipeline, in markets such as Chile or South Africa, risk-averse investors are just as likely to be concerned with bankability as with cost. And when it comes to storage, molten salt is the most bankable option there is.

Another problem for PCM is that while it undoubtedly offers the potential to reduce costs, if developers really want to make CSP cheaper then they are perhaps better off focusing on other areas.

According to DoE SunShot figures, for example, the portion of the per-kilowatt-hour CSP cost attributable to the solar field for a plant in 2010 was $0.09, nearly twice the $0.05 cost of TES. The power plant was costing $0.04 and the receiver and heat transfer fluid (HTF) was $0.03.

SunShot's aim is to bring down all these costs, so that by 2020 the solar field and power plant would both come in at $0.02 while TES and the receiver plus HTF would each hit $0.01.

At this point, though, it clearly makes sense for developers to focus on the biggest cost, which is the solar field, rather than worry too much about storage.
"The investment for the storage system is not that big," confirms Dr Markus Eck, research area manager for thermal energy storages at the German Aerospace Center Institute of Engineering Thermodynamics. "The cost of the solar field is the decider."

Direct steam generation

A final hurdle for PCM is that it works best with direct steam generation (DSG), which researchers believe could cut parabolic trough costs by 10% but is currently only used in a minority of CSP plants.
Abengoa, of Spain, is arguably at the forefront of DSG technology in a commercial setting, having selected it for the landmark Khi Solar One power tower in Upington, South Africa. But not even Khi Solar One will use PCM storage. Instead, some steam is saved for TES.

Despite these challenges, it would be a mistake to write off PCM too hastily. There is still plenty of interest in the subject, from a research perspective at least.

At the US National Renewable Energy Laboratory, for example, Craig Turchi of the Concentrating Solar Power Program says: "The push toward higher CSP operating temperatures has opened up a new range of potential PCMs. These offer large heats of fusion and low cost, but we have not performed any system testing with them."

Clearly, when it comes to TES then PCM could still be holding all the aces. It just needs a chance to play.

To respond to this article, please write to Jason Deign