An analytical technique for the optimal designs of tube-in-tank thermal energy storage systems using PCM

Latent heat thermal energy storage (LHTES) using phase change material (PCM) has many application potentials, in view of the fact that it has higher thermal energy storage capacity than the sensible heat thermal energy storage such as stratified water storage (SWS). However, the stored thermal energy in an LHTES system is not all effective in discharging due to the additional heat transfer resistance between PCM and heat transfer fluid (HTF). An LHTES system could have a higher effective storage capacity than an SWS system only if it is well designed. The optimal design of an LHTES system could be realized only after the effective energy storage ratio, Est, is analyzed in the design stage. In comparison to the laborious computational analysis method, in this work, we developed an analytical technique from the effectiveness-NTU theory to calculate the required heat transfer length and predict Est of the basic unit in any tube-in-tank LHTES system. The technique was first compared with the numerical simulation method we reported earlier, and then used to find the optimal design under various constraints. It was shown that heat transfer enhancement in PCM could effectively improve the effective storage capacity of an optimal design. However, the maximum Est was limited by the thermal resistance in HTF when the enhancement in PCM was over a certain threshold. It was demonstrated that the use of HTF in a low-Re turbulent region could enhance heat transfer in HTF and achieve an Est as high as that in a laminar region while obtaining a higher discharging rate if heat transfer in PCM was well enhanced. This analysis provided quantitative guidelines on designing optimal tube-in-tank LHTES systems.