Design and Performance Assessment of an R-744 Refrigeration System with Phase Change Material Thermal Storage for a Fishing Vessel

Refrigeration systems on board fishing vessels place substantial energy demands on the vessel through the freezing and storage of fish, which makes low-environmental-impact refrigerants and thermal energy storage particularly attractive. This study designs and theoretically assesses a carbon dioxide (R-744) refrigeration system for a fishing vessel, integrating the phase change material (PCM) HS23N as a thermal store. A research vessel was taken as the case study: the heat loads associated with freezing and storing the fish were determined, the refrigeration systems were sized, and their thermodynamic and environmental performance was assessed. The loading configuration considered gave a storage density of 417 kg/m3 and required refrigeration capacities of 3.32 kW for the refrigerated hold and 19.32 kW for the plate freezer. For the equipment selected, the thermodynamic coefficient of performance (COPth) was 2.48 and 1.87, while the coefficient of performance based on electrical power input (COPel) was 1.99 and 1.49 for the refrigerated hold and the plate freezer, respectively. The Total Equivalent Warming Impact (TEWI) assessment, without PCM and over a 20-year service life, gave estimated cumulative indirect emissions of 57.415 and 527.748 t CO2 for the refrigerated hold and the plate freezer, respectively, with direct emissions contributing little given that R-744 has a GWP of 1. For the PCM integration, a charge of 50 kg of HS23N per freezing cycle was considered. Taking 90% of the theoretical thermal capacity as useful energy gives a useful thermal energy of approximately 3.26 kWh per cycle and a theoretical minimum charging time for complete PCM solidification of 20.4 min. With four cycles a day, this corresponds to a potential capacity to store and shift the thermal load of approximately 13.04 kWh of thermal energy per day. The results demonstrate the theoretical potential of HS23N as a complementary means of storing thermal energy and shifting the refrigeration load in time in R-744 marine refrigeration systems. Quantifying the net energy and environmental benefit, however, requires experimental validation and a full energy balance of the charging and discharging of the PCM.