Experimental study of single CO2 and mixed CO2 + TBAB hydrate formation and dissociation in oil-in-water emulsion

• Hydrates and paraffin emulsions were combined to form secondary refrigerants.
• Using hydrates aims at improving system energy density.
• Using emulsion aims at improving fluid flow conditions.
• CO2-TBAB hydrates were studied due to high energy density at pressure below 1 MPa.
• Optimized hydrate-paraffin systems can reach total heat capacity of 142 kJ kg−1.

In secondary refrigeration applications, phase change material (PCM) slurry could be used as secondary refrigerant in order to combine a more efficient secondary loop and fluids with higher energy storage capacity. In such systems, energy is stored mainly during a phase change process of the storage material and a carrier fluid should be used as a continuous phase. The obtained system is called Phase Change Slurry (PCS). It has a higher energy density than single-phase secondary refrigerants due to both sensible and latent heat capacities of the materials.The purpose of this paper is to seek an appropriate secondary refrigerant for cold production and refrigeration applications between 273 and 293 K. Such PCS should have a high melting enthalpy and good flowing capacities. Hence, two types of PCS systems were combined: oil in water emulsion and CO2/CO2 + tetra-n-butylammonium bromide (TBAB) hydrate slurry. A DTA device was used in which the phase change temperatures and the corresponding enthalpies for many PCS systems were measured. A new system was thus developed that consists of oil in water emulsion/tetra-n-butylammonium bromide/carbon dioxide. The total heat capacity of the system was high, up to 142 kJ kg−1. It was higher than many other oil-in-water emulsions in the literature. Furthermore, the final system fits the requirements: It is stable, has a small supercooling degree and the phase changes occur under moderate thermodynamic conditions.

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