Enhanced power density during energy charging of a shell-and-tube thermal storage unit: Comparison between the inclusion of metal fins and foams

Abstract

Introducing metal fins or foams can both enhance the performance of shell-and-tube phase change thermal energy storage (TES) devices, but the heat transfer mechanisms are different, i.e., heat transfer through a micro-liquid film, named close-contact melting (CCM) mode, brought by fins and reinforced-heat-conduction is triggered by foams. In order to find a method with maximum power density boost for TES devices, an experimental study was conducted to reveal the effect of parallel fins (N = 5, 10) and foams (20 ppi, 40 ppi) attachments on the power density of a shell-and-tube TES device with a heating condition of the 30 °C superheat. Additionally, the finned cases were studied numerically to observe their melting behavior. It was shown that under the working conditions set in this work, the effective use of CCM mode allows addition of fins to win the competition with metal foams for higher power density. At a penalty of energy density loss of only 7 %, the ten-fin-attached TES device achieves a clear boost of power density over 70 % (up to 0.503 W/cm3) and a reduction in melting time of ~47 %. because the CCM mechanism dominates comparing with the two basic heat transfer modes of heat conduction and convection. Introducing metal foams causes both conduction enhancement and convection suppression, thus resulting in high initial power density at the beginning of the melting process and a gradual decrease in power density as the melting process progresses. Properly increasing pore density of metal foams is effective for thermal conductivity enhancement, but too dense structures may lead to deteriorating heat transfer. On balance, taking advantage of CCM mechanism with rational fin design in phase change TES may be a promising and cost-effective way to increase power density, which should be widely used in practice.