Optimization of Heat Transfer Process With Variable Specific Heat Working Fluids Through Phase Change Latent Heat Adjustment

ABSTRACT

Heat exchange between circulating working fluid and thermal storage medium is an essential process in thermodynamic cycles of energy storage systems. Thermophysical properties of the working fluids therefore play an important role in such systems. Conventional heat‐transfer fluids, such as water and thermal oil, have relatively constant specific heat capacities, whereas high pressure CO ₂ varies significantly with temperature near the critical region. Such a difference causes a specific heat capacity mismatch between these fluids, leading to temperature difference increase along the heat exchanger. The large temperature difference also increases entropy generation. Here we propose a new method to reduce the irreversibility through the adjustment of equivalent specific heat capacity based on phase‐change latent heat. To validate such an approach, a steady‐state model counterflow heat exchanger was used for high‐pressure CO ₂ at 10–15 MPa. Eight paraffins with phase‐transition temperatures from 40°C to 58°C were combined to give a phase‐change material mixture. A genetic algorithm optimized the paraffin proportions, proportional coefficient, and temperature‐shift parameter. The optimized phase change mixture closely matched the temperature‐dependent specific heat capacity profile of CO ₂ within the investigated range. The best agreement occurred at 11 MPa. Under the same pinch‐point temperature difference, the mixed phase‐change material mixture showed a lower entropy‐generation rate than water. This trend persisted throughout the investigated pressure range. The optimal temperature‐shift parameters were 2.8°C, 4.6°C, and 5.2°C, corresponding to pinch‐point temperature differences of 3°C, 4°C, and 5°C, respectively. These results show that tailoring the effective specific heat capacity of the phase‐change mixture can reduce thermophysical mismatch and heat‐transfer irreversibility.