Enclathration of HFO-1234yf within structure II hydrates and H2 within structure H hydrates for energy storage

Abstract

This study focuses on two topics related to energy storage. Refrigerant hydrates are viewed as potential materials to store thermal energy in cold systems because of their operating temperature range and high fusion heat. This study was conducted to examine the encapsulation of 2,3,3,3- tetrafluoropropene (HFO-1234yf), an alternative to conventional refrigerant 1,1,1,2-tetrafluoroethane (HFC-134a), with CO₂. This investigation focused on the phase equilibria, structural, and compositional features of HFO-1234yf + CO₂ hydrates. The findings showed that the phase equilibria curve of HFO- 1234yf + CO₂ hydrates occurred in milder conditions than pure CO₂ hydrate. These differences in the equilibrium curves became more pronounced as the HFO-1234yf concentration increased (from 10% to 30%). The Powder X-ray diffraction (PXRD) analysis illustrated the HFO-1234yf + CO₂ + water mixtures formed as structure II (sII) hydrates with the Fd3̅m space group. Rietveld refinement of the PXRD analysis and Raman spectra demonstrate that HFO-1234yf were encapsulated exclusively in sII large (51264) cages, while CO₂ molecules are predominantly encapsulated in sII small (512) cages. Furthermore, the dissociation enthalpy of the HFO-1234yf (10%) + CO₂ (90%) hydrate was determined by utilizing a high-pressure micro-differential scanning calorimeter, which was 473.4 ± 7.4 J/g-water (58.4 ± 0.9 kJ/mol-guest). These findings provide in-depth insights into the possibility of utilization of HFO-1234yf + CO₂ hydrates in air conditioning systems as cold energy storage materials. To investigate the possibility of efficiently storing hydrogen blended with natural gas in clathrate hydrate, thermodynamic and structural analyses of hydrates composed of H2 (30%) + CH4 (70%) gas and structure-H (sH) formers, methylcyclohexane (MCH) and 1-methylpiperidine (1-MPD), were performed. The three-phase (H-LW-V) equilibria of H2 (30%) + CH4 (70%) + 1-methylpiperidine (1- MPD (2.9 mol%)) and the four-phase (H-LW-LM-V) equilibria of H2 (30%) + CH4 (70%) + methylcyclohexane (MCH (2.9 mol%)) + water systems are under much milder conditions than the H₂ (30%) + CH₄ (70%) + water system. In particular, the system including 1-MPD is slightly milder than MCH. Powder X-ray diffraction (PXRD) analysis indicated that the H2 (30%) + CH4 (70%) hydrate structure is sI, while H2 (30%) + CH4 (70%) + MCH (2.9 mol%) and H2 (30%) + CH4 (70%) + 1-MPD (2.9 mol%) form sH. Raman and 13C Nuclear Magnetic Resonance (13C NMR) spectroscopy were employed to examine the guest molecule encapsulation. Gas chromatography (GC, 8890B, Agilent Technology, USA) was utilized to analyze the composition of gas in the hydrate phase, and gas uptake was also measured. These findings provide the potential of storing hydrogen in gas hydrates effectively by using structure H formers.