Numerical Analysis-Based Shape Design of Underground Rock Caverns for Thermal Energy Storage

Abstract

The efficiency of thermal energy storage (TES) using water can be improved by storing the water in a thermally stratified form. Previous studies on the thermal performance of heat storage tanks, undertaken by Lavan and Thompson (1977), Cotter and Charles (1993), Matrawy et al. (1996), Ismail et al. (1997), Eames and Norton (1998), and Bouhdjar and Harhad (2002), have demonstrated that better thermal stratification can be obtained by increasing the aspect ratio (height-to-width ratio) of heat storage containers. However, a high-aspect-ratio storage design may lead to structural instability of the storage space because of its narrow, tall shape. Therefore, heat storage spaces should be designed to provide good thermal performance but should also consider the stability of the storage. This is an important issue in the design of heat storage, particularly for underground TES using rock caverns, because the stability of rock caverns is greatly influenced by geotechnical factors such as in situ stresses and rock properties. Therefore, a quantitative stability assessment is required to determine the shape of rock caverns used for TES, and to thus ensure the structural stability of the caverns. This technical note describes a numerical approach for the shape design of a rock cavern in which to store hot water for district heating. For reliable evaluation of the stability of the cavern, the approach employs probabilistic methods that can take into account the variability of input parameters using probability distributions. The arch height of the cavern roof is determined through a comparison of excavation-induced ground displacements between caverns with different arch heights.