portable personal cooling, PCM heat exchanger, latent heat storage, thermal performance, COP
Portable personal cooling systems with a vapor compression cycle (VCC), thermal battery, and electrical battery are an attractive option for localized cooling as compared with stationary air-conditioning systems. A phase change material (PCM) can be used for storing waste heat from the VCC condenser, thereby acting as a thermal battery. In the PCM condenser, copper tubes are submerged in the PCM tank. Due to the low thermal conductivity of pure PCM, the overall heat transfer coefficient of the PCM condenser is limited, which results in high condensing VCC pressure and, thus, low system COP. By applying heat transfer augmentation, the effective heat transfer coefficient can be increased. A thermal energy storage (TES) system that stores condenser heat in the PCM during VCC operation and is regenerated through solidification by a thermosyphon operation is proposed. A receiver is utilized to balance refrigerant charge in the VCC and compensate liquid refrigerant in the thermosyphon loop. In the cooling cycle, PCM stores thermal energy from the condenser pipes and changes from a subcooled solid state to a liquid state, which results in the change of condenser pressure and temperature and, thus, subcooling of the refrigerant. In this study, the system performance of the VCC and PCM condenser was experimentally investigated. Effects of the following three types of enhanced PCM condensers on system performance were studied and analyzed: copper sponge enhanced PCM with helically coiled copper tubes, graphite matrix enhanced PCM with straight copper tubes and small tube inner diameter refrigerant distribution header, and graphite matrix enhanced PCM with straight copper tubes and large tube inner diameter refrigerant distribution header. Results show that these TES systems with both PCM discharging and PCM regeneration cycles operate properly and effectively. The entire cooling system was able to provide four hours of continuous cooling, and the PCM was fully regenerated after six hours of thermosyphon operation. The comparison of three PCM condensers indicates that the system with the large header graphite matrix enhanced PCM condenser performed the best with a cooling COP of 4.7 when the PCM is in two-phase region. The copper sponge enhanced system had a COP of 4.2, and the graphite matrix with the small distributor header system had a COP of 3.1. Compared with copper sponge, graphite matrix has better enhancement of PCM heat transfer. Moreover, the condenser with the large distributor header can achieve more uniform refrigerant flow and PCM temperature distribution than one with the small header. This leads to the higher utilization ratio of PCM latent heat. The copper sponge enhanced PCM condenser with helical tube is the second best, but needs higher heat transfer surface area and condenser volume as well as more refrigerant charge because of more liquid refrigerant needed in the thermosyphon loop. In conclusion, the system design with graphite matrix enhanced PCM and condenser having a larger refrigerant distribution header and straight copper tubes is recommended due to its low refrigerant charge, higher system COP, and use of lower cost enhancement materials.