A partially underground rock bed thermal energy storage with a novel air flow configuration

Low-cost, grid-level energy storage is key to maximizing the utilization of renewable energy production and decarbonizing the electricity sector. The design and testing of a high-temperature thermal energy storage based on rocks is presented. Important design features are the three electric heaters mounted on top of the storage and the inner pipe inside the rock bed, allowing for the first time a reversible vertical air flow configuration of a system which is partially underground.

The rock bed storage is highly scalable and based on diabase which is abundant as well as low-cost in Northern Europe and other locations. A pilot plant with a storage capacity of 1 MWhth has been operated up to 675 °C. The first law round-trip efficiency of 70.7% in an initial operation exceeds all efficiencies observed in a previously built thermal storage with horizontal flow configuration.

By improving the operation with charge and discharge air flow rates of 140 and 300 Nm/h3, respectively, the first law round-trip efficiency increases to 80.7%. A maximum thermal output of 58.06 kW is achieved, of which 90% can already be provided within 6 min of start-up. This paper demonstrates that the novel medium-scale storage can operate with a satisfactory performance for several years with no failures to date.