Effect of porous material properties on thermal efficiencies of a thermocline storage tank

Abstract

Thermocline energy storage systems can be adapted to store energy in a tank where the hot fluid is pumped in during a charging cycle and cold fluid during a discharging cycle, substituting the need for two storage tanks. Here, a numerical investigation of a hot air flow transferring thermal energy to a solid porous bed is performed to evaluate the influence of the thermal conductivity and capacity ratios on the efficiency of a charging cycle of a thermocline storage tank. The numerical model considers the mixed convection turbulent flow (k-ε model) through clear and porous media using the Local Thermal Non-Equilibrium approach to provide solutions for the solid and fluid phases separately. Macroscopic equations are obtained by the method of volume averaging and numerically processed by finite volume method. The system was modelled as a vented axisymmetric cavity, partially filled with a porous medium, with hot fluid inflow from the top and outflow at the bottom. The investigation included changing Reynolds number (Ret from 8.3 × 103 to 5 × 104), thermal conductivity ratios (ks/kf from 3.5 to 1062) and thermal capacity ratio (ρscps/ρfcpf from 1483 to 7415). It was found that lower ks/kf ratios decrease the heat loss throughout the charging cycle which allow for higher temperatures in the later stages of the cycle and thus improve charging efficiency. However, this effect decreases in importance as the system undergoes higher Ret number flows. On the other hand, increasing the ρscps/ρfcpf ratio affects the entire cycle, increasing the temperature difference between phases, lowering the velocity in which the solid raises its temperature but storing heat more efficiently. Finally, a design consideration is highlighted as the importance of the ks/kf ratio on the thermal efficiency is predominant at lower Ret flows, whereas as Ret increases, ρscps/ρfcpf becomes the dominant parameter providing efficiency gains.