Optimization on the melting performance of triplex-layer PCMs in a horizontal finned shell and tube thermal energy storage unit

This paper numerically investigates the melting performance of triplex-layer phase change materials (PCMs) in a horizontal shell and tube latent thermal energy storage (LTES) unit. The paraffin wax with different melting temperatures (i.e., RT42, RT50, and RT60) are selected as PCMs. To accelerate the heat transfer, metal fins are adopted in the unit. Considering the heat transfer enhancement and thermal energy storage capacity loss by metal fins, a comprehensive storage density evaluation (CSDE) criterion was innovatively proposed to assess the melting performance of the LTES unit with triplex-layer PCMs. Based on CSDE, the effects of partition walls and fin arrangements were firstly discussed in a transient two-dimensional model. It was found that the natural convection of liquid PCMs was significantly suppressed by partition walls. Compared with Case 1 (no fins), gradual fin additions in triplex-layer PCMs significantly saved the melting time by 55.3% (Case 2), 66.1% (Case 3), and 71.0% (Case 4). The uniform radial fin arrangement in each PCM layers (Case 4) presents the maximum value of CSDE. Subsequently, the uniformity of PCM melting rates was optimized by different arrangements of metal fins, PCM sequences and space ratios, respectively. Finally, the optimum case was achieved and the value of CSDE further increased 36.2% with the comparison of Case 4. The criterion of CSDE can be used to optimize the design of the horizontal shell and tube LTES unit with metal fins for heat transfer enhancement, and provides a guidance for the further experimental study.