R1234yf,R1234ze(z),R32及其混合工质在Co-MOF-74中吸附储能的分子模拟

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

利用流体分子在纳米多孔材料固体表面吸附分离过程中热能与表面能的相互转化,可以提高工质循环吸热量进行储能.采用分子模拟(分子动力学和巨正则蒙特卡罗)方法开展了制冷剂R1234yf,R1234ze(z),R32及其混合工质在金属有机骨架材料Co-MOF-74中的吸附储能特性研究.研究发现,纯工质吸附时,受分子尺寸影响,R32在MOF中的吸附量高于R1234yf和R1234ze(z).而饱和吸附时,R32的解吸附热低于R1234yf和R1234ze(z).在制冷剂中添加Co-MOF-74纳米颗粒形成纳米流体,可以改良纯工质的储能特性,且R1234yf和R1234ze(z)纳米流体的改良效果强于R32纳米流体.在混合工质吸附中,R1234ze(z)和R1234yf的吸附量低于R32,但随着温度上升,由于不同种类工质竞争吸附,R1234ze(z)和R1234yf的吸附量呈现逐步上升的趋势,而R32的吸附量则逐渐减少.

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 R1234 yf, R1234 ze(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(ΔhMOHCs) of MOHCs was calculated. ΔhMOHCsconsists of the enthalpy of pure organic fluid(ΔhFluid), the energy change of metal organic framework(MOF) nanoparticles((∫cpdT)MOFs) and the enthalpy of desorption(Δhdesorption). It can be inferred that MOHCs can strengthen the energy storage capacity of basic fluid when the sum of(∫cpd T)MOFsand Δhdesorptionis larger than ΔhFluid, and vice versa. For the purpose of calculating the energy change of metal organic framework nanoparticles((∫cpd T)MOFs),molecular dynamics simulation was adopted. Furthermore, grand canonical Monte Carlo methods were used to calculate the enthalpy of desorption(Δhdesorption). The results suggested that the adsorption amount of R32 in Co-MOF-74 is higher than that of R1234 yf and R1234 ze(z). The saturated adsorption pressure of R1234 yf and R1234 ze(z) is lower than that of R32. Besides, the saturated absorption amount of R1234 ze(z) is higher than that of R1234 yf. And the desorption heat of R32 is lower than that of R1234 yf and R1234 ze(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 R1234 yf and R1234 ze(z). The addition of Co-MOF-74 nanoparticles could enhance the energy storage capacity of the studied pure refrigerants. R1234 yf and R1234 ze(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(ΔhFluid) is higher than the sum of the energy of Co-MOF-74 nanoparticles((∫cpd T)MOFs) and the enthalpy of desorption(Δhdesorption) 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 R1234 ze(z) and R1234 yf is lower than that of R32 in the refrigerant mixture adsorption. What’s more, the absorption amount of R1234 ze(z) in R1234 ze(z)/R32 mixture is lower than that of R1234 yf in R1234 yf/R32 mixture. And this result is different from the adsorption of pure working fluid. Also, with the increase in temperature, the adsorption amount of R1234 ze(z) and R1234 yf shows a gradually increasing trend, while that of R32 gradually decreases. The desorption heat of R1234 ze(z) and R1234 yf 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.