Electricity storage in hot stones validated by $3.4 million investment

By William L. Driscoll

A long-duration thermal battery technology described to the California Energy Commission last December has gained credence from a Danish government investment.

The Danish Energy Agency will invest $3.4 million in a 10 MWh plant in Denmark to demonstrate the technology, which stores electricity as heat in small basalt stones. The system’s round-trip efficiency is projected to be 60%.

A U.S. project team aims to build a second demonstration project, twenty times larger, in the U.S.

Danish firm Stiesdal and its U.S.-based partner Magellan Stortech described the long-duration “GridScale” thermal battery system last December in a public workshop hosted by the California Energy Commission (CEC).

A thermal battery plant coupled with a solar farm could be configured to charge daily for 8 hours and discharge for 16 hours, the firms said. Or, because the hot stones would be stored in insulated tanks, the system could store energy for up to 10 days.

The two firms said they “look forward” to working with teams at the University of California Merced and the consulting firm E3 to “clarify the benefits that long-duration storage can provide,” as a follow-on to the CEC’s initial workshop on long duration storage scenarios.

The thermal battery system, already tested at 1:10 scale in Denmark, uses the principle of an electric heat pump to shift heat from cold pea-sized stones in one tank to hot stones in another tank, heating them up to 600 degrees C. The heat is later extracted at a constant temperature to drive one or more gas turbines to generate electricity. A modular design facilitates scaling, up to 1 GW capacity or larger.

As of December, Stiesdal and Magellan Stortech were exploring options for deploying a second and larger demonstration plant in the U.S., sized at 20 MW of 10-hour storage, or 200 MWh. The firms aimed for subsequent commercial-scale U.S. projects.

Advantages of the thermal battery system include the low cost of crushed stone, and the use of charging and discharging technologies that have been “applied for a century within other industries” and are well-suited for mass production, said Peder Riis Nickelsen, CEO of Stiesdal Storage Technologies.