Phase Equilibrium and Formation Behavior of the CO2-TBPB Semiclathrate Hydrate for Cold Storage Applications

Abstract

Cold thermal storage is primarily used for demand management on electricity grids due to air conditioner use. The CO2 hydrate has been widely studied as a cold storage medium for its proper phase equilibrium temperature range and large latent heat. This study presents experimental work on the phase equilibrium behavior of the CO2 hydrate under applicable pressures (<10 bar) for the potential to use in air conditioning systems. The formation of hydrate was aided by tetra-n-butylphosphonium bromide (TBPB) at mass fractions of 10, 20, 30, and 37 wt % to lower the required pressure. Instead of using complex calorimeters, the enthalpy of the CO2–TBPB hydrate was measured using a modified T-history method in a self-fabricated pressure tube. Through this test, the enthalpy of the CO2–TBPB hydrate was found to be much higher than for other normally used phase change materials. However, the hydrate formation was also manifested in a large degree of supercooling and a long induction delay. To reduce them, secondary promoters, namely, sodium dodecyl sulfate and TiO2 nanoparticles, were used. Meanwhile, the effect of feed pressure, heat-transfer fluid temperature, promoter concentration, and the memory effect on CO2–TBPB hydrate formation was investigated. This study provides suggestions for effective and efficient operation of CO2–TBPB hydrate-based cold storage air conditioning systems.