Thermophysical property analysis of semi-clathrate hydrates of tetrabutyl-(ammonium/phosphonium) salts as phase change materials for cold storage

Abstract

Hydrate has attracted extensive attention because of its adjustable phase change temperature and high cold storage density. In this paper, the thermophysical properties of semi-clathrate hydrates as the phase change materials (PCMs) formed by aqueous solutions of tetrabutyl-(ammonium/phosphonium) salts (TBAF, TBAC, TBAHSO4, and TBPB) with different concentrations (0.05∼0.44, mass fraction) were systematically studied, including formation/dissociation temperature, phase change enthalpy, cycle performance and hydrate number. The results showed that semi-clathrate hydrate had good cycle performance during cold storage and release. The randomness of hydrate formation temperature was much less than that of ice, which suggested the hydrate formation conditions were more controllable. With the increase of solution concentration, the apparent driving force for hydrate formation decreased, and the proportion of hydrate in total enthalpy increased from 5% to 100%. In addition, the adjustable range of phase change temperature of semi-clathrate hydrate formed by the solution above is 0.61∼27.9 °C, where ranged from high to low is TBAF, TBAC, TBPB, and TBAHSO4. The total phase change enthalpy was about 122∼302.6 kJ/kg. The range of the maximum, intermediate and minimum hydrate number of TBAHSO4 hydrate crystal were estimated to be 35∼38.7, 32∼35, and 24∼28.3, respectively, by dissociation temperature and enthalpy analysis using differential scanning calorimeter (DSC). This study could form the basis for energy saving in water-based cold storage process. According to the needs of different cold storage temperatures, different salt solutions can be selected as cold storage media, so as to achieve the purpose of energy saving.