State-of-the-art of cold energy storage, release and transport using CO2 double hydrate slurry

CO2 hydrate slurry is a promising cold storage and transport medium due to the large latent heat, favorable fluidity and environmental friendliness, and the CO2 utilization can also be simultaneously achieved. However, the phase change pressure of CO2 hydrate is too high for applications in refrigeration system, thus the thermodynamic promoters are used to moderate the phase change conditions by forming CO2 double hydrate. In this review, a comprehensive summarization of research progresses of CO2 double hydrate is presented, encompassing the aspects of thermodynamic and kinetic characteristics, cold storage and release, cold transport and system operation. Quaternary salts, tetrahydrofuran, and cyclopentane are concluded to be the main thermodynamic promoters used to alleviate hydrate formation conditions. The cold storage capacity of CO2 double hydrate slurry highly depends on CO2 absorptivity which can be enhanced by increasing pressure, adding kinetic promoters and external disturbance. The non-Newtonian behaviors of most CO2 double hydrate slurries exhibit shear-thinning characteristics and addition of anti-agglomerates can effectively reduce their apparent viscosity. There is an optimal hydrate mass fraction and phase change temperature that make the system energy efficiency the highest due to the reduced pumping power consumption and improved primary refrigeration system efficiency. On the way to practical applications, the research challenges remain in elucidation of underlying mechanism of CO2 absorption and release with the presence of kinetic promoters, stable control of cold storage and release processes, and the detailed heat and mass transfer characteristics. In addition, operation strategy, optimization techniques, economic analysis and CO2 management method of the entire system are potential research topics.