Thermodynamic analysis of an underground sensible energy storage system

Abstract

Electricity production from concentrated solar power (CSP) plants has been more commonplace in the last decade since the sun is one of the most abundant, renewable energy sources. The heat transfer fluid temperature in a CSP plant may go up to 1000 C; however, most of the current power plants operate on temperature ranges between 220 C and 565 C due to decomposition of molten salts in high temperatures. Since the sun is not available at nights and cloudy days, an important consideration is how to store the energy received by the sun to use at other times. In this study, a three-dimensional borehole heat exchanger model is developed to store solar energy underground using concrete and molten salt as a storage medium and heat transfer fluid, respectively.While molten salt is circulating through a pipe, which is placed into the concrete, heat is transferred from the molten salt to the concrete or vice versa during the charging and discharging processes. Numerous simulations are conducted using ANSYS Fluent, with varying borehole diameters, mass flow rates, and thermal resistances of the borehole wall. Average concrete temperature, outlet heat transfer fluid temperature, and energy and exergy efficiencies are investigated for each case. It was found here that while concrete temperature increases with increasing mass flow rate, the increasing trend is minimal after the mass flow rate increases beyond 6 kg/s. There exists a negative relation between the borehole diameter and average concrete temperature during the charging process, and vice versa in discharging. Energy and exergy efficiencies varied from 0.2% to 98.1% and 0.1% to 77.9%, respectively. While the most efficient system was found at a borehole diameter of 550 mm for adiabatic cases, it was found to be 750 mm when heat leakage is taken into consideration. Borehole diameters of 2000 mm performed the worst among all cases due to low heat transfer rates. Heat leakage was found to have a significant impact on energy and exergy efficiencies, especially in energy efficiencies for higher borehole diameters and low mass flow rates in the discharging process.

Review of borehole thermal energy storage systems in unsaturated soils: Materials, mechanism, and future development

“…Sensible heat storage requires that the medium has a higher volume specific heat capacity, and the cost of the medium also needs to be considered. Therefore, water, soil, concrete, or rock are usually used as heat storage mediums in large‐scale TES systems 12,13 …”