RESUMEN
Thermochemical adsorption heat storage based on gas-solid interaction is an energy storage technology for the effective recovery of industrial waste heat and renewable energy sources such as solar energy. Currently, hygroscopic salts and water are commonly used as the working pairs in thermochemical thermal storage systems. Strontium bromide (SrBr2) is a highly potential material for low-grade thermal energy storage and building applications due to its high sorption capacity and reaction enthalpy. In order to better understand the microscopic mechanism of adsorption, the water adsorption behavior of strontium bromide surfaces on the atomic scale is investigated in this study by using density functional theory (DFT). The effects of doping Ca or Mg into strontium bromide on water adsorption are analyzed by the comparison of the energy barrier, density of states (DOS) and crystal orbital Hamilton population (COHP) obtained before and after metal atom doping. The energy barrier of the SrBr2/H2O reaction system is 5.916 kcal mol-1, and the energy barrier of Ca-doped SrBr2 reduces to 3.432 kcal mol-1, whereas the energy barrier of Mg-doped SrBr2 increases to 13.394 kcal mol-1. The thermodynamic properties of strontium bromide are significantly improved by doping with calcium atoms. The absolute value of the COHP for Ca-doped SrBr2 decreases, which indicates that Ca-doped SrBr2 can adsorb the H2O molecules more easily at the same temperature. The doping of Ca atoms has a positive effect on the adsorption heat storage process. This study provides insight into the adsorption mechanism of water molecules on strontium bromide and could facilitate the design of efficient composite adsorbents.
RESUMEN
The development of energy storage technology is beneficial for the efficient use of energy and sustainable development. As an effectual approach for storing and transporting thermal energy, latent heat storage using phase change materials (PCMs) has attracted tremendous attention. However, low thermal conductivity, poor stability, and leakages are considerable challenges to the widespread application of solid-liquid PCMs. Composite phase change materials (CPCMs) were prepared by combining expanded graphite (EG) and sodium acetate trihydrate (CH3COONa·3H2O, SAT). EG as a supporting material plays a crucial part in both enhancing the thermal conductivity and preventing the melted PCMs from leakage. The chemical structure, micromorphology, thermal stability, thermal conductivity, phase change behavior and heat storage performance of SAT/EG CPCMs have been extensively investigated by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), thermal conductivity analysis, differential scanning calorimetry (DSC), and cycling stability measurement. The results of SEM indicate that EG with a loose and porous layered structure has a good molding effect and can adsorb SAT well. XRD and FTIR results show that only a simple physical combination between EG and SAT exists, and no new substances have been produced. Compared with pure SAT, thermal conductivity and supercooling tests show that the supercooling degree of the CPCMs was decreased and the thermal conductivity was increased by 205.1%. In addition, the addition of 2 wt% of disodium hydrogen phosphate dodecahydrate (Na2HPO4·12H2O, DHPD) as a nucleating agent and 0.5 wt% of gelatin as a thickening agent to SAT could reduce the supercooling degree and inhibit the phase separation well. Based on SAT/EG-8% CPCMs, an oven with phase change energy storage was designed and the heat storage/release performance of the oven was investigated under different operating conditions.
