Storage tank at Gemasolar, a 20 MWe molten salt tower plant in Spain.

By Jason Deign 

One of the big losses in Areva’s departure from CSP is that the French company seemed on the verge of an industry cost-reduction breakthrough.

Shortly before abandoning the sector, Areva confirmed it had found a way to add molten salt thermal energy storage to its compact linear Fresnel technology.

The combination, developed in association with the US Department of Energy’s Sandia National Laboratories, promised a new lease of life for linear Fresnel by giving it a way of delivering base-load power.

Significantly, too, it represented a welcome new application for molten salt, which has emerged as the thermal energy storage material of choice for the industry but which faces a number of challenges in terms of cost reduction.

Make no mistake, though: at present, molten salt is highly cost effective.

“If you have a need for storage, I’m not sure there are cheaper alternatives,” says Bill Gould, chief technology officer at SolarReserve, which is at the forefront of developing plants that store energy using molten salt.

“When most people are thinking about energy storage they think about batteries. And the price of batteries right now is between five and 20 times more costly than storing heat. It’s just very expensive to store electricity and much cheaper to store thermal energy.”

In addition, he says, batteries based on lithium-ion, the current darling of battery chemistries, are close to their theoretical limits in terms of performance and efficiency. As a result, the US government is pouring funds into battery research, but electrical storage remains costly today.

Less bankable

Other options, such as flywheels or compressed air energy storage, are similarly expensive and, in many cases, less bankable. The cost for adding molten salt thermal energy storage to a CSP plant, meanwhile, depends on whether it is used as a collection fluid or not.

“If you have two heat transfer fluids it is going to be a much larger added cost,” Gould says.

“In the case of a molten salt power tower, which has salt for collection and for storage, we’ve done cost comparisons which say what a plant costs with no storage and what a plant costs with storage, and the answer is in the noise. They are almost indistinguishable.”

This happens because if storage is removed then the size of the turbine has to be increased for a given solar field. In parabolic trough plants, meanwhile, integrating molten salt can be more of a challenge because of the need to keep the material fluid across kilometres of horizontal tubing.

The problem with power towers, though, is that they are currently one of the most expensive technologies not just in CSP but in power generation generally. So the question for developers is whether this cost can be reduced.

The answer is almost certainly ‘yes’, but what is more doubtful is whether the molten salt storage part of the equation will be of much help.

Although molten salt manufacturers are working on cheaper production methods, insiders point out that prices are strongly dependent on the supply and demand dynamics of other markets that consume a vastly greater amount of the raw materials.

Giuseppe Casubolo, sales and marketing director for solar salts at SQM, one the industry’s main suppliers, says: “A more stable CSP market and a clearer view of its evolution would definitely help to more stable prices levels. However, the main drivers today are still other traditional markets.”

Cost reduction

Moreover, within the CSP industry there is general awareness that the potential for cost reduction is greatest in parts of plant design that have little to do with storage. “They are largely in the area of the heliostat field,” comments Gould.

“We have some new designs that will be far cheaper than the existing legacy design for a heliostat.”

With regards to thermal energy storage specifically, a fair amount of research is being carried out to find molten salt alternatives with a better energy density.

Much of this is focused on phase-change materials, where there are already companies such as Terrafore Technologies with pre-commercial offerings. “So far, they have not been successful,” says Gould.

“The state of the art today is the same molten salt technology that has been around for 20 years.”

Nevertheless, says Dr. Markus Eck, research area manager for thermal energy storages at the German Aerospace Center Institute of Engineering Thermodynamics, it is also true that the industry only has limited operating experience of molten salt storage in power towers.

“If it really turns out that molten salt in towers is an applicable option then it will be a very charming option,” he says. “But this has to be proven. If it were that easy I could imagine we would see more molten salt towers coming into operation”, he adds.