Charging performance of a CO₂ semi-clathrate hydrate based PCM in a lab-scale cold storage system

Cold thermal energy storage aids in the efficient deployment of thermal energy whenever there is a mismatch between energy generation and energy use. In this study, a lab-scale demonstration of cold storage system was built to investigate the performance of CO₂ semi-clathrate hydrate as a cold storage material in the charge of a realistic cold thermal storage. The experimental rig was basically a vessel equipped with an external loop and an ultrasonic crystallizer to boost the hydrate formation. The hydrate was formed from a salt solution composed of tetra-n-butyl ammonium bromide 20 wt%, tetra-n-butyl ammonium fluoride 0.25 wt% and sodium decyl sulfate 0.15 wt%. At a constant pressure condition of 5.0 bar, the hydrate formation was triggered by the heat transfer fluid at 7.5 °C. The charged cooling capacity of two control strategies, namely ‘constant pressure’ and ‘constant mass’, were compared. The repeatability and stability of hydrate formation conditions were studied. It was indicated that CO₂ semi-clathrate hydrate could serve in cold storage systems effectively. However, it was also found that under the experimental conditions, hydrate formation was hard to thoroughly complete due to the lack of sufficient driving force or heat/mass transfer.