Approximate analytical characterization and multi-parametric optimization design of single-tank thermocline heat storage system

Abstract

Compared with two tank thermal energy storage (TES) system, the single-tank thermocline (STTC) TES system provides a more cost-effective option for TES systems with low-cost solid filler, and it is progressively becoming an important research topic. In this study, analytical approach solutions derived from the one-dimensional two-phase (1D-2P) without heat loss model and one-dimensional one-phase (1D-1P) with heat loss model in the STTC system are compared with experimental data, it is found that local thermal equilibrium assumption is inappropriate in the thermocline tank because the internal heat transfer in fluid and solid is dominant. Besides, the analytical approach results of 1D-2P model are more exact than numerical results compared with experimental data. The algebraic equations of non-dimensional discharging time and discharging efficiency are derived from analytical modeling results. Based on these important correlations, a multi-parametric optimization design procedure of the thermocline tank considering of stress safety requirement is proposed. Then different design cases are studied and the effects of some critical non-dimensional parameters are investigated. The results show that optimal Pe number exists for designing a thermocline TES system, with which convection heat transfer is dominant and final discharging time is not that short. For the PROMES experimental case, not only the output power increases 145.9% but also the final discharging efficiency improves 0.75% compared with the experimental boundary condition with the optimum Pe = 960.63. Larger value of fluid volumtric heat capacity ratio and higher Bi number increases efficient discharging time and discharging efficiency while decreases thermocline thickness, the improvement is highly significant especially for Bi < 1 × 106.