Experimental investigation of a CO2 hydrate refrigeration-cold thermal energy storage composite system

Carbon dioxide (R744), as an environmentally benign natural refrigerant, shows strong application potential in cold thermal energy storage (CTES) owing to its zero ozone depletion potential and high latent heat of hydrate formation. However, conventional indirect hydrate-based CTES systems are constrained by built-in coil structures and suffer from large heat transfer resistance, a tendency toward wall blockage, and limited heat transfer efficiency. To address these issues, this study proposes and constructs a direct-contact CO 2 hydrate refrigeration-CTES composite system. An experimental test platform is established to evaluate the temperature evolution characteristics of the system under standalone charging and refrigeration-charging/discharging synergistic operation modes. The results show that when the initial evaporating pressure increases from 34 bar to 40 bar, the average volumetric heat transfer coefficient rises from 22.90 kW/(m 3 ·K) to 57.61 kW/(m 3 ·K), corresponding to a 151.6% improvement in overall heat transfer performance. Meanwhile, increasing the evaporating pressure effectively improves the thermodynamic conditions for hydrate nucleation and growth. Under the 40 bar condition, the charging time is substantially shortened, and the latent cold storage capacity due to phase change increases to 1059.49 kJ, representing a 152% increase, while the latent heat fraction reaches 68.9% of the total stored cooling capacity. In addition, under a benchmark test with a constant cooling load of 2 kW, the refrigeration-discharging synergistic operation mode effectively reduces compressor load by introducing a hydrate-based cooling release process. Under this mode, the average coefficient of performance (COP) reaches 4.08, which is 17.0% higher than that of the conventional standalone refrigeration cycle under the tested 2 kW load. The proposed composite system exhibits potential for peak shaving and provides bench-scale evidence for future scale-up studies.