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Breaking the world’s reliance on fossil fuels requires more than generating renewable electricity. Safe, economically viable storage will be needed for the dark, still days when solar and wind farms do not operate.
Many companies are working on long-duration storage technologies but crucially they must work at grid scale. Thermal energy storage using “hot rocks” is an attractive option, with the added potential to harness heat from industrial processes that is otherwise wasted. Rival tech includes pumped hydro, compressed air, gravity and other battery types. Costs and efficiency will decide which ones win out.
Hot rock tech works by transferring heat energy, either direct from source or generated by electric heaters, into an insulated vessel containing the storage medium.
Denmark’s Stiesdal uses basalt volcanic rock. Charging involves heating the rocks up to about 600C. Heat is stored until needed. It can then be transferred into a separate cold container and drive mechanical dynamos to create electricity en route. Germany’s EnergyNest uses a special thermal concrete for a similar process. Molten salts are already used for thermal storage of solar energy.
Hot rocks offer good potential storage times. The bulk of the 234 gigawatt hours of current estimated installed capacity is used for heating purposes, according to the International Renewable Energy Agency. Its use in power generation is expected to treble capacity by 2030.
Other advantages of solid-state storage materials such as rock or concrete are low costs and abundant availability. For large scale and long duration, thermal’s levelised cost of storage at $0.40 per kilowatt hour is about half to a third the cost of traditional batteries, according to the US Department of Energy. Only hydrogen, pumped hydro and compressed air storage might be marginally cheaper by 2030, the DoE estimates. That gives hot rocks a solid position for a place in the green grids of tomorrow.
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