Thermal performance of a thermal-storage unit by using a multichannel flat tube and rectangular fins

Highlights: • A small temperature difference can achieve charge of the thermal storage unit. • Bottom injection mode of heat-transfer fluid is the optimal injection method. • The multichannel flat tube has excellent heat transfer performance. • The thermal storage unit has good charge/discharge performance. -- Abstract: The low thermal conductivity of phase-change materials (PCMs) limits the widespread use of phase-change thermal-storage units (TSUs). This problem can be solved by expanding the heat-exchange area (HEA) in the PCM side. Related studies have shown that expanding HEA can greatly increase the heat-transfer rate of TSU. However, few studies have been able to increase the compactness factor (CF) of TSUs while expanding the HEA of the PCM side to become sufficiently large. In this work, a multichannel flat-tube phase-change TSU was constructed based on the CF and ratio of HEA to PCM volume (δ). The developed TSU uses a multichannel flat tube as the heat-exchange element, water as the heat-transfer flow (HTF), and lauric acid as the PCM. The δ and CF of the multichannel flat tube TSU are 238.9 1/m and 82%, respectively. The temperature distribution, power, and average effectiveness of the TSU at different HTF-injection modes, inlet temperatures, and mass-flow rates are studied experimentally. Results show that the multichannel flat tube exhibits excellent heat-transfer performance, and the convective heat-transfer coefficient under experimental conditions reaches 515 W/(m²⋅k) or more. The maximum effectiveness during charge and discharge is 0.235 and 0.232, respectively. Moreover, the corresponding pressure loss and heat-transfer temperature difference between the inlet temperature and melting point of PCM are 3986 Pa and 22 °C, respectively. Results also show that δ and CF are parameters that need to be fully considered when designing a practical TSU.