Simultaneous and consecutive charging and discharging of a PCM-based domestic air heater with metal foam

Abstract

• Study simultaneous and consecutive charging and discharging of storage heater. • Study a composite phase change material/metal heat exchanger cooled by air. • Study different modes of system operation for the purpose of domestic space heating. • Lower output air temperature for a higher air flow with a higher discharging rate. • 6.5 h for charging 15.32 kg phase change material with 90% porosity aluminum foam. Numerical investigations of the melting/solidification in a metal foam saturated with phase change material (PCM) were performed for simultaneous and consecutive operational modes. The composite is embedded in a rectangular compound cooled by passing air in a middle channel which is then employed to heat the room as a space heater. The composite is heated by two-rod heating elements to store thermal energy for peak-shaving purposes. The study covered the evaluation of the system in different operational modes for charging and discharging rate, the impacts of the metal foam and the influence of coolant flow rate on the solidification performance. The presence of PCM on one hand due to having almost constant temperature during the phase change process and the use of metal foam on the other hand due to proving high heat transfer rate from the PCM to the coolant, help in providing a uniform output temperature from the system which is a key factor for highly efficient space heaters. Moreover, evaluation of the operational modes can help to understand the behavior of the system in real scenarios when there is a need to charge the storage system and heat the room (discharging) simultaneously. The results show that the melting process is fully achieved due to the faster-charging process rate in modes I (8-hour charging and 8-hour discharging separately) and III (2-hour charging and 14-hour simultaneous charging-discharging), compared with mode II (2-hour charging and 2-hour discharging separately, repeated for 16 h). The temperature distribution in Mode III was more constant, which produced uniform heat exchanged between the PCM and the cooling fluid. The porosity is inversely proportional to the liquid development rate. The PCM melts entirely within 6.5 h for 90% porosity while 78% of the PCM melts in 8 h for the 95% porosity case. The final mean PCM temperature changed from 69.9 °C to 66.4 °C, when the air flow rate increases from 0.01 kg/s to 0.03 kg/s.