Solar thermal energy storage loses its way

Does phase-change material storage have advantages over molten salt thermal energy storage for a concentrated solar power plant? Photo credit: Crescent Dunes Solar Energy Project, SolarReserve

Is phase-change material storage better than molten salt thermal energy storage for a concentrated solar power plant? Photo: Crescent Dunes Solar Energy Project, SolarReserve

Too much salt is not just bad for your health. It could also harm the likelihood of thermal energy storage (TES) cost reduction across the entire concentrated solar power (CSP) industry.

Right now, molten salt TES is seen as critical in justifying the high cost of CSP versus other renewable energy sources, such as solar PV or wind.

TES allows CSP, or solar thermal energy, to deliver stable, round-the-clock power, which is more valuable to grid operators than the intermittent generation provided by renewable alternatives.

But it is just possible that a growing preference for molten salt among CSP developers could hamper the chances of adopting more efficient and cost-effective types of TES.

Terrafore Technologies, for example, is sitting on a TES system based on phase-change materials (PCMs).

Reducing TES costs by 40%

Founder Anoop Mathur is convinced PCMs could reduce the cost of TES by as much as 40% compared to current molten salt systems, mainly because it is a more efficient storage system that requires smaller containment vessels.

Currently, though, commercialisation of the PCM system is stalled due to a lack of takers. Mathur says major CSP companies such as SolarReserve have shown an interest in the technology, but none have stepped forward to try it out yet.

“They all say it’s a good thing, but there’s a shortage of funding,” says Mathur. “Funding is an issue with these companies.”

Part of the problem is that you can’t just swap a molten salt tank for PCM. The two types of TES require different types of CSP plant. Molten salt, for example, is used as both a storage medium and a heat transfer fluid.

That means that the salt is heated directly in the receivers upon which sunlight is focused. The hot salt then flows through insulated pipes to heat up steam in a turbine or, if not immediately needed, goes to a storage tank for later use.

PCM TES designs

With a PCM TES design, however, the heat transfer fluid is the same steam that is used to power the turbine. Part of this is used to heat up the PCM while the solar field is active.

When the sun goes in, the PCM gives up its thermal energy to keep the steam hot and the turbine running. Even without PCM storage, direct steam generation (DSG) plants are favoured by some CSP developers because of their high efficiency.

Abengoa Solar of Spain, for example, is using DSG technology in its landmark Khi Solar One plant in South Africa. There, saturated steam will provide up to two hours of TES.

But at the company’s KaXu Solar One plant, also in South Africa, Abengoa has chosen molten salt to provide 2.5 hours of storage. Other developers are increasingly doing the same.

At the US CSP developer SolarReserve, for instance, chief technology officer Bill Gould is firmly committed to molten salt. PCM could one day overtake molten salt, he says, but “it’s not going to happen in five years. It could happen in 10.”

However, even that will be very dependent on PCM development being able to move forward. And currently there does not seem to be much of an appetite to take on the technology. For a start, the CSP industry has bigger fish to fry.

The current cost-reduction focus in CSP

“The whole phase-change discussion is aimed at reducing the cost of the salt and the volume of the tank,” says Gould. “Right now that’s not large in comparison with the total plant cost.

“It might be in the order of USD$20m to $30m for a large plant like Crescent Dunes. The Crescent Dunes capital cost is $750m, so you’re talking about influencing something that’s less than 5% of the total cost.”

Another problem for PCM is that the solar thermal industry as a whole is struggling to prove CSP’s worth against PV, which means it is hardly time to start trying out new TES technologies, even if they might offer significant cost reductions.

Dr Markus Eck, research area manager for thermal energy storage at the German Aerospace Center, says: “We have done a [PCM] study a few years ago and according to the investigation you can expect a reduction in the levelised cost of energy.

“But it’s a new approach. You have a risk. It’s not sure you can be cost-effective in the first approach, or even the third.”

In addition, he says: “A lot of CSP players are risk-averse. DSG was the focus three to five years ago. Then the interest went down and molten salt came up. I’m not sure if the situation will stay like this. Both options are applicable, but both are challenging.”

Against this backdrop, there could be a very good chance that molten salt will become embedded into CSP thermal energy storage designs as a matter of course. And phase-change material, potentially the better choice from a technology perspective, could be left on the shelf.

Written by Jason Deign