Analysis of Pillar Stability in Thermal Energy Storage Caverns Using Numerical Modeling

Abstract

Underground Thermal Energy Storage (UTES) systems store thermal energy in aquifers, boreholes, or caverns with the purpose of avoiding the sporadic characteristics of renewable-energy resources or recovering industrial waste heat. Cavern TES (CTES) system may be more expensive in construction than the other two storage methods, but its advantages include high injection and extraction powers and the flexibility in selecting a heat storage medium. The first large-scale cavern for CTES system, which is toroidal shaped, was constructed for storing hot water of 40–90 C in Lyckebo, Sweden and 5,500 MWh of heat was stored between seasons (Nordell et al. 2007). Storage cavern in CTES system should be designed to ensure structural stability of storage space and provide good thermal storage performance as well. However, these two objectives conflict with each other, because increasing height-to-width ratio of the cavern improves the storage performance and simultaneously increase the structural instability due to its slender shape (Park et al. 2013). It can be thus suggested to divide a single large cavern into multiple smaller caverns with high aspect ratios (Park et al. 2014), and cavern spacing affecting the temperature and stress distribution of the pillar between caverns becomes one of the main factors taken into account when designing the multiple caverns. Generally, closely spaced caverns are preferred for efficient placement of surface and underground facilities and favorable to reduce the heat loss due to the temperature difference between surrounding rock mass and storage medium. This technical note seeks to determine a width of the pillar that is structurally stable between two rock caverns for thermal energy storage, based on numerical simulations using a thermal–mechanical coupled model. The influence of pillar width on the stability is compared with that of cavern depth and in situ stress condition, and the stress distributions of pillars with different widths are analyzed.