Heat transfer and thermal resistance analysis under various heat transfer fluid flow rates based on a medium-temperature pilot-scale latent heat storage system

Abstract

Assessments often rely on experimentally observed factors when examining heat transfer variations due to different heat transfer fluid (HTF) flow rates. To better comprehend limiting factors and guide operational parameter design for shell-and-tube latent heat system (LHS) systems, a quantitative analysis of thermal resistance hindering heat transfer is crucial. Hence, this article conducted experimental evaluations at various flow rates to critically analyze its thermal performance based on a large-capacity (1000 MJ) medium-temperature LHS system loaded with 4130 kg commercial nitrate mixture (peak melting temperature: 219.90 °C). The heat transfer coefficient and thermal resistance during charging and discharging were quantitatively investigated to identify the main factors influencing heat transfer and dominant mechanisms. The results suggested that augmenting the charging flow rate from 5 to 15 L/min resulted in 2.65 times increase in average power (from 24.01 to 63.64 kW). Besides, during charging, the overall thermal resistance primarily came from the phase change material (PCM) side initially. As PCM melted, the thermal resistance on the PCM side became equivalent to that on the HTF side. Increasing the charging flow rate enhanced heat transfer through two mechanisms: reducing the thermal resistance on the HTF side and increasing the driving force of heat transfer by elevating the average HTF temperature. During discharging, the overall thermal resistance was predominantly derived from the PCM side after solidification. Increasing the discharging flow rate caused a limited influence on decreasing thermal resistance. Enhancing the discharging flow rate only bolstered the driving force for heat transfer by elevating the average HTF temperature. This study provides valuable insights into the factors influencing heat transfer in LHS systems and contributes to optimizing LHS technology.