Analysis on heat transfer and heat loss characteristics of rock cavern thermal energy storage

• We examine the feasibility of rock cavern for large-scale thermal energy storage.
• The benefit of using rock cavern TES versus using above-ground TES is presented.
• Heat loss rate of rock cavern is remarkably lower than that of above-ground tank.
• Heat loss from rock cavern decreases over time because of surrounding rock heating.
• Rock cavern shows less sensitive heat loss characteristics to insulator performance.

The present study is aimed at demonstrating the feasibility of the rock cavern, compared with the above-ground tank, for the storage of large-scale high-temperature thermal energy by quantitatively evaluating the heat transfer inside the storage tank and the heat loss characteristics of the surrounding environment. As a conceptual model, we consider a thermal energy storage (TES) system coupled with an adiabatic compressed air energy storage plant (A-CAES), which utilizes loosely packed bed of rocks as heat storage medium and stores heat of up to 685 °C. The specifications of the TES model, such as the mass flow rate of the heat transfer material and the storage volume, were determined through the analysis of the heat transfer in the packed bed, using a quasi-one-dimensional two-phase numerical model developed in this study. In this procedure, the inlet and outlet fluid temperatures and the thermal energy rates to be stored or extracted were examined over 200 consecutive daily cycles to ensure the TES met the requirements for the power generation of the A-CAES plant. Then, with the determined specifications of the TES, a comparative study on the heat loss characteristics of the rock cavern-type TES and above-ground-type TES systems was performed by simulating the operations on a daily basis for a period of 10 years using a three-dimensional numerical model. The comparison results indicated that the amount of cumulative heat loss in the rock cavern-type TES system over the operation period was far smaller than that in the above-ground-type TES system because of the surrounding rock heating and the consequent reduction in the thermal gradient between the surrounding rock and the storage medium. In terms of long-term operation, the rate of heat loss from the rock cavern-type TES system exhibited less-sensitive and less-dependent behaviors with respect to the insulator performance than that of the above-ground-type TES system.