Experimental characterisation of a novel thermal energy storage based on open-cell copper foams immersed in organic phase change material

Abstract

In this paper, a novel tubes-in-tank thermal energy storage (TIT-TES) based on open-cell copper foams immersed in organic paraffin is presented and experimentally studied. The system consists of a rectangular external case which encloses 16 U-tubes and phase change material (PCM) embedded in copper foams matrix, which guarantee an average power during both heat charge and discharge phases close to 1 kW and high storing/releasing energy efficiency. With the aim of measuring internal temperature, six thermocouples have been installed at two different sections, S1 and S2, located at 1/3 and at 2/3 of the total height, respectively. In order to thermally characterize the system during the heat charge and discharge phases, a series of tests have carried out under different inlet temperatures and flow rate of the heat transfer fluid (HTF). Temperature evolutions have obtained during the tests, and directional temperature derivative profiles, time-durations, average powers and energy efficiencies have derived so to evaluate and characterize the TIT-TES performance. The results have shown how both HTF volumetric flow rate and inlet temperature play a relevant role in heat charge and discharge phases. A larger temperature difference between the HTF and the PCM and a higher HTF input flow rate improve heat transfer and consequently increase the average exchanged power and energy efficiency as well. In particular, during the thermal charge phase, when the HTF temperature increases from 80 °C to 90 °C, the energy efficiencies increase from 73.6% to 91.4% and from 78.7% to 93.8% for the minimum and maximum HTF flow conditions respectively.The analysis of directional temperature derivative profiles have allowed to define some important characteristics of the apparatus, highlighting the preferential directions of the liquefaction/solidification front paths, and also confirming the importance of the HTF inlet temperature and flow rate.