Molecular simulation of energy storage of R1234yf, R1234ze(z), R32, and their mixtures in Co-MOF-74 materials

Abstract

The heat capacity of working fluid can be enhanced through the mutual transformation between thermal energy and surface energy during the absorption and separation process of fluid molecules in porous materials. In this paper, molecular simulation (molecular dynamics and grand canonical Monte Carlo) methods were used to study the absorption and energy storage properties of R1234yf, R1234ze(z), R32 and their mixtures in Co-MOF-74. In order to evaluate the properties of thermal energy storage of metal organic heat carriers (MOHCs), the enthalpy difference (Δ h MOHCs) of MOHCs was calculated. Δ h MOHCs consists of the enthalpy of pure organic fluid (∆ h Fluid), the energy change of metal organic framework (MOF) nanoparticles (( ∫c p d T )MOFs) and the enthalpy of desorption (∆ h desorption). It can be inferred that MOHCs can strengthen the energy storage capacity of basic fluid when the sum of ( ∫c p d T )MOFs and ∆ h desorption is larger than ∆ h Fluid, and vice versa. For the purpose of calculating the energy change of metal organic framework nanoparticles (( ∫c p d T )MOFs), molecular dynamics simulation was adopted. Furthermore, grand canonical Monte Carlo methods were used to calculate the enthalpy of desorption (∆ h desorption). The results suggested that the adsorption amount of R32 in Co-MOF-74 is higher than that of R1234yf and R1234ze(z). The saturated adsorption pressure of R1234yf and R1234ze(z) is lower than that of R32. Besides, the saturated absorption amount of R1234ze(z) is higher than that of R1234yf. And the desorption heat of R32 is lower than that of R1234yf and R1234ze(z). As a result of phase transition, the enthalpy of organic fluid significantly increases at the temperature of phase transition. However, the enthalpy of phase transition of R32 is higher than that of R1234yf and R1234ze(z). The addition of Co-MOF-74 nanoparticles could enhance the energy storage capacity of the studied pure refrigerants. R1234yf and R1234ze(z) nanofluids have superior enhancement effect to that of R32 nanofluid. Nonetheless, the energy storage capacity of R32 nanofluid is depressed at the temperature of phase transition. This can be attributed to that the enthalpy of phase transition of R32 (∆ h Fluid) is higher than the sum of the energy of Co-MOF-74 nanoparticles (( ∫c p d T )MOFs) and the enthalpy of desorption (∆ h desorption) of R32 in Co-MOF-74. Moreover, due to the temperature of phase transition increases with the pressure increasing, the lowest point of energy storage enhancement appears at higher temperature. Besides, the absorption amount of R1234ze(z) and R1234yf is lower than that of R32 in the refrigerant mixture adsorption. What’s more, the absorption amount of R1234ze(z) in R1234ze(z)/R32 mixture is lower than that of R1234yf in R1234yf/R32 mixture. And this result is different from the adsorption of pure working fluid. Also, with the increase in temperature, the adsorption amount of R1234ze(z) and R1234yf shows a gradually increasing trend, while that of R32 gradually decreases. The desorption heat of R1234ze(z) and R1234yf is higher than that of R32 in the mixture adsorption. In general, this study is of great significance for theoretical analysis of adsorption characteristics of refrigerant molecules in porous media. And it can provide reliable basis for selecting appropriate refrigerant working mediums and MOFs to form optimal MOHCs in engineering.