Phase equilibrium and formation behaviour of CO₂-TBAB semi-clathrate hydrate at low pressures for cold storage air conditioning applications

Cold thermal storage has been widely used in air conditioning systems. For electric air conditioning, it helps to shift the peak load on electricity grids; for solar cooling, it enables cooling supply during solar outages. CO₂ hydrate has been proposed as a cold storage medium for its suitable phase change temperature and large latent heat. Tetra-n-butyl ammonium bromide (TBAB) has been studied to moderate the formation pressure of CO₂ hydrate. In this study, formation and dissociation of CO₂-TBAB semi-clathrate hydrate were conducted at low pressures (<10 bar) applicable to the operating conditions of air conditioning systems. The hydrates were formed with the TBAB mass fraction of 10, 20 and 32 wt%. Using the T-history method, the formation enthalpy of CO₂-TBAB semi-clathrate hydrate was measured in a self-fabricated reaction tube. In addition, the formation behaviour of CO₂-TBAB semi-clathrate hydrate, namely the CO₂ gas uptake, the induction time and the supercooling degree, were investigated under various feed pressures and heat transfer fluid (HTF) temperatures. Besides, secondary promoters (tetra-n-butylammonium fluoride (TBAF) at the mass fraction of 0.1, 0.5, 2.0 and 3.5 wt%, sodium dodecyl sulphate (SDS) at the mass fraction of 0.1, 0.3 and 0.5 wt%, and TiO₂ nanoparticle in 20 and 80 nm) were used to aid the hydrate formation. The aim of this study is to reduce the capital and operating cost of a CO₂ hydrate cold storage system by increasing the gas uptake, decreasing the supercooling degree and shortening the induction time.