Optimal insulation of underground spherical tanks for seasonal thermal energy storage applications

Abstract

The literature deals specifically with compressed gas characteristics, solar radiation, storage volume and heat load fluctuation in aboveground storage and thermal energy storage (TES) applications. To prevent their negative effects, the use of underground insulated spherical tanks in the storage process has been overlooked. This study details the physical and economic aspects of using insulation in underground spherical tanks for seasonal TES systems. In determining the storage heat load, the degree-time method is recommended for the heat transfer mechanism between soil and storage temperature degrees. Using life cycle cost analysis, the insulation thickness, energy saving and payback period in the underground spherical tank are discussed in detail for hot and cold storage capacities. The results of the study indicated that the degree-hour method can be used in the design of hot and cold TES systems despite the temperature fluctuation. Insulation can be taken care of in hot fluid storage instead of cold alternative. With insulation in the underground spherical tank, it is observed than about 200 % energy savings is possible with approximately 50 % shorter payback period. For the hot fluid storage with insulation, as the storage fluid temperature, soil thermal conductivity and tank diameter rise and the depth falls, but the optimum insulation thickness value increases. As a result, this study is expected to be a guide for further seasonal TES applications using insulation in underground spherical tanks.