Thermodynamic Analysis of a CO₂ Hydrate-based Refrigeration System With Cold Energy Storage

A novel CO₂ hydrate-based refrigeration system with cold energy storage is proposed to shift the peak load of buildings. The refrigerant utilized in the system cycle is CO₂, and CO₂ hydrate is used as the cold storage medium. The direct contact heat exchange method is applied to the system, which can greatly improve the performance of the unit. A mathematical model of the system was developed based on the thermodynamic analysis method and theoretical analysis was conducted. Results show that when the consumption of CO₂ increases to 20% of the refrigerant mass flow rate, the coefficient of performance of the system is improved by 1.25 times compared to the original coefficient of performance. Based on the property that CO₂ hydrates are more likely to be generated at lower temperatures, the latent heat storage capacity decreases as the evaporation temperature increases. Additionally, the total storage capacity and compressor power consumption decrease and the coefficient of performance increases as the evaporation temperature increases. The improvement of system performance can be achieved by increasing the evaporation temperature and reducing the gas cooler outlet temperature. The gas cooler outlet temperature has a greater effect on the optimal discharge pressure of the system, while the change in evaporation temperature has a lesser effect on the optimal discharge pressure of the system. When the evaporation temperature increases from 2 ℃ to 8 ℃, the optimal discharge pressure of the system remains at about 8.6 MPa. When the gas cooler outlet temperature increases from 35 ℃ to 45 ℃, the average increase of the optimal discharge pressure is about 0.24 MPa/℃.