THE COMBI HEAT STORE - A COMBINED ROCK CAVERN BOREHOLE HEAT STORE

Since 1988, Sweden and Finland have collaborated bilaterally on therma l energy storage with respect to information exchange and collaborativ e R&D projects. The two countries have both investigated underground t hermal energy storage for nearly two decades, and have similar bedrock -the Fenno-Scandian granitic rocks. This paper reviews the work perfor med in the field of combined rock cavern and borehole heat stores, con cerned with construction technology, costs and design principles. One example is an asymmetric store, in the form of 40- to 60-m-long horizo ntal boreholes between two rock caverns, with the caverns themselves c omprising only about 10% of the total storage volume. This design has a specific cost of $US0.40 million/GWh and $US0.24 million/GWh for sto rage capacities of 6 GWh and 36 GWh, respectively. Half of the total c onstruction cost relates to the rock cavern part of the store.