| The adsorption cooling system(ACS)has attracted more attention as a promising green cooling technology.Metal-organic frameworks(MOFs)exhibit potential application in ACS due to their excellent water adsorption performance.Practically,the cyclic cooling capacity of ACS is mainly determined by the water adsorption kinetics of MOFs.Given the great structure diversity of MOFs,their structural properties may impose various impacts on water adsorption kinetics.However,the correlations are elusive between the structural properties of MOFs and their water adsorption kinetics(e.g.water adsorption rate)as well as the adsorption cooling performance(e.g.the specific cooling power(SCP)and the coefficient of performance for cooling(COP_C)).In this work,representative MOFs were chosen to explore the above correlations of MOFs using experimental and numerical simulation methods.Moreover,the effective strategies of tuning structural properties of MOFs were proposed to improve the water adsorption kinetics and cooling performance.The main conclusions are as follows:(1)The water adsorption rates of representative MOFs were as follows:DUT-67>NU-1000>MIL-101(Cr)>PCN-777>MIL-100(Fe)>Ui O-66.By analyzing the correlations between the structural characteristics of those MOFs and their water adsorption rates as well as SCP and COP_C,it was found that the particle size,pore size,porosity and surface hydrophilicity jointly affect the water adsorption rate.The SCP and COP_C of MOFs were dominated by the water adsorption rate.Morever,it was found that the large particle and pore size,the high porosity and the strong hydrophilicity of MOFs favor the water adsorption rate,which in turn were beneficial for the SCP and COP_C.Thus,the water adsorption kinetics of MOFs may be regulated by modifying the structural properties of MOFs,thereby improving their adsorption cooling performance.(2)MIL-101(Cr)with potential application was selected for further investigations.MIL-101(Cr)/GO composites were prepared by integrating MIL-101(Cr)with graphene oxide(GO)to enlarge pore size for improving water adsorption rate and cooling performance.It was found that the maximum water adsorption rate,SCP and COP_C of MIL-101(Cr)/GO are 37.8%,49.8%and 14.0%higher than MIL-101(Cr),respectively.When GO content was lower than 10 wt%,the water adsorption rate of MIL-101(Cr)/GO increased with the enlarged pore size,thereby SCP and COP_C enhanced.When GO content was higher than 10 wt%,water adsorption rate and SCP were decreased due to the reduced hydrophilicity of MIL-101(Cr)/GO.Nevetherless,COP_Cenhanced owing to the reduced the specific heat capacity and heat of adsorption by GO adding,which favored the decrease of total heat consumption of ACS.(3)MOP/MIL-101(Cr)composites were prepared by integrating MIL-101(Cr)with metal-organic polyhedra(MOP3)to enhance hydrophilicy of MIL-101(Cr)for improving water adsorption rate and cooling performance.It was found that the highest water adsorption rate,SCP and COP_C achieved at 35 wt%MOP3 contents,which are 37.8%,49.8%and 14.0%higher than MIL-101(Cr)respectively.The increased hydrophilicity signicantly improved water adsorption rate,SCP and COP_C of MOP/MIL-101(Cr).Moreover,the enhancd hydrophilicity had a great impact on water adsorption rate and cooling performance of MIL-101(Cr).However,the reduced pore size of MOP/MIL-101(Cr)had an adverse effect on significant improvement of water adsorption rate and cooling performance.(4)Li Cl/MIL-101(Cr)composites were prepared by integrating MIL-101(Cr)with a small size of lithium chloride(Li Cl)to improve water adsorption rate and cooling performance though collaborative regulation of pore size and hydrophilicity.It was found that hydrophilicity of Li Cl/MIL-101(Cr)remarkably enhances with the increase of Li Cl content without significantly reducing the pore size,resulting in a great improvement of water adsorption rate,SCP and COP_C for Li Cl/MIL-101(Cr).Thus,the highest water adsorption rate,SCP and COP_C of Li Cl/MIL-101(Cr)was 17.3,2.2 and 1.5 times that of MIL-101(Cr),even higher than that of MIL-101(Cr)/GO and MOP/MIL-101(Cr)composites.Therefore,Li Cl/MIL-101(Cr)had a great potential application in ACS. |
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