Large heat storages play a key role for the implementation of renewable energy and industrial waste heat in heat distribution networks. One option is to use underground space for large heat storages. In this paper a case study for large underground heat storages in the heat distribution network of Salzburg is presented. Possible locations for large underground heat storages along the existing heat distribution network were identified, also based on the geological conditions. Two locations were selected and the temperature distribution in the underground around the heat storage cavern was numerically simulated using COMSOL with the aim to determine the energy losses and the temperature change at the ground surface. The results showed that the energy losses can be reduced considerably by >70% compared to insulated steel tank storages and that the surface temperatures are not much influenced by the heat storage below.
For the incorporation of additional large heat storages into existing heat distribution networks, certain criteria shall be fulfilled:
• The storages need to be located close to the existing heat distribution network to minimize (a) construction costs for new pipelines and (b) heat losses along pipelines during operation.
• The storages need to be located close to the heat generators and/or large diameter distribution pipelines to allow for high load capacities.
• To avoid heat exchangers between the network and the storage, which cause energy losses during operation and which are costly and frequently to refurbish, the heat storages need to be at a defined elevation to allow for the appropriate pressure and back-pressure in the heat distribution network. For the case of Salzburg the "zero" pressure elevation is at around 412 m asl. (depending on the location in the network) and the maximum excess pressure in in the range of 35 m. Hence, the top of the storage cavern has to be at a level of app. 447 m asl.
• The cavern should have a competent rock overburden of at least 100 m to (1) reduce heat losses and temperature effects on the surface and (2) to minimize support by activating rock strength.
• The access tunnels to the caverns shall be as short as possible to reduce construction costs and the access should be located along an existing road in order to minimize costs and effects to the surrounding nature.
• Furthermore, nature deserving protection such as springs and other protective nature should not be located above and close by the location of heat storages.
