Shafts and spiral tunnels for large heat storages – an economic study on ideal geometries

Large heat storages are playing a key role in district heating networks in the future. They allow for a broad integration of renewable energy sources and industrial waste heat by providing producers and consumers with the necessary flexibility. Currently, insulated steel tanks with volumes of around 30,000 m³ and heights of around 60 meters are frequently used by district heating providers in Austria for this purpose. However, due to the limited storage volume and the significant heat loss of around 23 kWh/m³ to 73 kWh/m³ storage volume, particularly during the cold season, further developments are required. The use of large rock caverns allow for large storage volumes of more than 100,000 m³ and a reasonably steady temperature profile of the boundary conditions over the year with significantly reduced heat losses during winter seasons as compared with conventional strorages. Heat storages for this purpose need a minimum height of 60 m to provide the pressures needed in directly connected district heating systems. In this paper, varying geometries of rock caverns with a vertical dimension of in minimum 60 meter are analyzed with respect to efficient and economic construction methods for typical rock conditions in Alpine regions. The costs per cubic meter storage volume are estimated considering in particular (i) construction costs of caverns and access structures; (ii) costs for annual heat losses; (iii) operating costs. The results of this analysis show that the storage cost per cubic meter in rock caverns built in sound rock are in the range of 120 €/m³ to 170 €/m³ approximately for storage volumes of > 100,000 m³ to 500,000 m³. This is lower than the current costs per m³ for conventional insulated steel tanks. Concurrently large storage volumes have the additional advantages of reduced heat losses and a long service life with limited maintenance costs. This is of particular relevance for seasonal storages and for the application of the storage caverns for a service life of more than 100 years.