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.