By Susan Kraemer

The deployment of molten salt storage on a wider range of generation types would increase the bankability of CSP with storage, Raade told CSP Today.

Halotechnics is proposing to use thermal energy storage as a standalone storage technology to shift cheap night-time wind power from off-peak to on-peak grid availability.

Instead of integrating the storage within a CSP facility, electricity would heat molten salts and a steam turbine would turn it back to electricity, creating an application with a wide potential as pure grid storage.

“If we can build more projects that use thermal energy storage technology for this electrical storage application then people will get more familiar with it. Those designs will improve, the costs will come down, and that will benefit future CSP plants with molten salt storage,” Raade said.

“As demonstrated by the learning curve in other manufacturing industries, production costs of components tend to decrease by a certain amount when production increases,” said a spokesman for Abengoa, which has deployed the most CSP globally.

“We expect these curves to be applicable for storage components and other solar thermal energy components too,” the spokesman said.

The idea of using thermal energy storage with molten salts as a standalone storage concept has already been suggested by Terrafore's Anoop Mathur.

"You could convert electricity to heat and then back to electricity using a turbine or other heat-to-electrical conversion means," Mathur suggested in 2013.

"The efficiency of going this route is low; you get back 40% of what you collect. But economically it may make sense because battery cost today is very high…and the life cycle is perhaps under 10 years."

"Steam turbines are the bottleneck." Raade said, but added the economics are much better than batteries at current prices.

The technology

A 50 MW traditional hot tank/cold tank thermal energy storage system, comprising two 70-foot diameter tanks, about 30 feet tall, would use electricity to heat readily available off-the-shelf industrial electric filaments within 5,000 tons of molten salt. A steam turbine would extract the energy at the other end.

The system would bolt-on to displace the duct burners in a natural gas combined-cycle plant, utilizing the steam turbine already in place. Natural Gas Combined Cycle (NGCC) plants in California are around 600 MW on average, suiting Halotechnics' 50 MW storage system, and it could be adjusted for plants down to 150 MW.

Power plants typically consume some grid power for parasitic load. Halotechnics could install additional electrical equipment allowing for more power consumption while charging.

The electricity input would come from excess wind after midnight in California, at off-peak rates.
"In principle we could build the storage system and a steam turbine at a wind farm. But the cost would be much higher because of the steam turbine cost," said Raade.

For this reason, the idea is to attach storage to an NGCC plant because it already has the steam turbine needed to extract the electricity from the storage system.
California's after-midnight wind would be stored all day to supply the following evening's ramp-up in load. Molten salts lose about 0.5% of stored heat over 12 hours.

Figure 1: Diagram illustrating the use of molten salt as a heat transfer fluid and storage medium

Source: CSP Solar Tower Report 2014: Cost, Performance and Thermal Storage

Competitive in California’s storage market

California utilities are obligated to buy 1.3 GW of energy storage by 2020.

"The plant owner can bolt on storage that qualifies for a new storage payment in California and they can make more money," Raade said.

California utilities have begun testing batteries at grid scale for storage to solve intermittency on its increasingly renewable-rich grid.

Renewable capacity rose to 21 GW in 2014, excluding hydroelectric power, representing over 24% of total capacity and on track for 33% by 2020.

Batteries amount to more than ten times the cost of CSP-integrated storage. A recent estimate by SolarReserve CEO Kevin Smith put the cost of the molten salt storage component at the 110 MW Crescent Dunes facility - for 10 hours of storage - at around $80 million for 1,100 megawatt hours.

SCE's 8 MW battery pilot at Tecachapi has four hours of storage and costs $50 million for 32 MWh.

Halotechnics’s thermal battery would be 50 MW with four hours of storage and it would cost $40 million for 200 MWh.

“We’ve done base case design of our technology,” said Raade. “And at that scale they would have an estimated cost of approximately $200 per kilowatt hour. That is comparable to batteries.”

Stand-alone electricity-in thermal storage in molten salts is currently lower cost than grid-scale batteries, Raade noted.

"Per kilowatt hour, molten salt is already competitive," he said.