Secondary refrigeration application by CO2 hydrate slurry: from hydrate crystallization to their integration in cold storage and transportation processes – Crisalhyd

Cold production is a basic need of development in many fields (food preservation, chemistry, electronics, transportation, air-conditioning), but it represents an energy cost equivalent to 15% of the electricity consumed in industrialized countries. Including the direct impact of current hydrofluorocarbon (HFC) refrigerants, with a high global warming potential (GWP), the refrigeration industry accounts for 8% of greenhouse gas (GHG) emissions. Moreover, regulation on refrigerants imposes constraints (low GWP, taxation) which require technology adaptation oriented towards the implementation of alternative refrigerants or processes limiting refrigerant amount, such as secondary refrigeration. Due to refrigeration and air-conditioning (RAC) market expansion in the coming years, any innovation to improve the energy efficiency of cooling systems and reduce their environmental impact will therefore contribute significantly to the reduction of GHG emissions.
In this context, Irstea is interested for 10 years in hydrates applied to RAC processes. Hydrates are ice-like crystals composed of water and guest molecules (gas/salts), which are stable over a wide range of temperature (253-313 K) and can thus be used as phase change materials (PCM) in cold storage applications. CO2 hydrate is particularly interesting since it is PCM with the highest latent heat of melting (500 kJ.kg-1, at P > 1 MPa) at normal temperature for air conditioning. The possibility of forming mixed CO2-salt hydrates at low pressure (P < 1 MPa) while maintaining a high latent heat of melting (330-450 kJ.kg-1) was demonstrated by Irstea and ENSTA. By dispersing hydrate crystals in liquid phase, it is possible to form environment-friendly CO2-hydrate slurries (low GWP of CO2) capable of carrying cold in secondary loops, then reducing refrigerant amount in the system. Thanks to the high energy density of hydrates, facility design is also improved (reduced pipe diameters and pump powers). The most common slurries are currently ice slurries (> 100 facilities worldwide) and on a smaller scale salt hydrate slurries. Slurry loops are usually coupled to storage tanks, which optimizes facility design, flexibility and energy efficiency. However, slurry generators are often power limited (scraped/brushed surface exchangers). CO2-hydrate-slurry production process is based on gas injection in solution, which can prevent these technological limitations.
Today, the thermodynamic properties (equilibrium, latent heat) of many hydrates (CO2, salts) are known and various studies exist on hydrate slurry rheology in various media (aqueous, organic). Irstea has studied the feasibility of such transport and highlighted in aqueous solution two limitations related to the slow kinetics of formation and rapid agglomeration of hydrates in flow, well known in the oil and gas industry. Thus, the control of hydrate crystallization kinetics and size distribution is still a scientific obstacle for industrial application. Indeed, these kinetic parameters are linked to hydrate slurry flow and rheology as well as mass and heat transfers, properties crucial for the reliability and efficiency of the cooling process.
The Crisalhyd project aims to validate the concept of RAC by CO2 hydrate slurry based on a multi-scale approach ranging from enhancement of microscopic crystallization processes, through the study of their impact on thermophysical properties of slurries and dynamic simulation of secondary loops, until the evaluation and design of pilot-scale facility.