Effect of separation distance on the mechanical stability and thermal performance of twin hot-water storage caverns

Large hot-water storage systems can be used for seasonal storage of solar thermal heat as part of district heating systems. Underground thermal energy storage (TES) using rock caverns is an alternative method for large-scale, long-term TES utilization because it is an economical large-scale storage method. Cavern stability is a crucial consideration in the design of TES caverns. Stability issues limit cavern size and spacing because the safe dimensions of underground caverns largely depend on geotechnical conditions. If the maximum available size of a single cavern does not meet the volume requirements for TES, the use of multiple caverns should be considered. When using multiple TES caverns, the separation distance between the caverns is a decisive factor in the design of storage spaces because multiple caverns may interact mechanically and thermally if they are closely spaced. In this paper, a numerical investigation is conducted to understand how the distance between TES caverns affects their mechanical stability and thermal performance. The investigation is based on twin caverns used to store hot water for district heating in residential areas. The mechanical stability of the caverns is assessed in terms of the probability of cavern failure; cavern thermal performance is assessed in terms of the thermal stratification within the caverns and the heat loss to the surroundings. The heating characteristics of the rock around the caverns are also investigated with respect to operating time. The effect of the separation distance on mechanical stability and thermal performance is presented and discussed in detail, and we suggest design considerations and a performance evaluation process for the use of multiple TES caverns.