The Cooling Performance Of Metal-organic Frameworks-water Working Pairs In Cascaded Adsorption Heat Pumps

The coal-combustion power plants increase carbon emissions,leading to a series of environmental problems including global warming.Adsorption heat pumps(AHPs)powered by low-grade thermal energy or renewable solar energy for cooling have gained growing research interests,which may replace traditional compression refrigeration systems due to their environmental friendliness and electricity saving.However,the coefficient of performance for cooling(COP_C)of the AHPs is lower than the compression refrigeration system,which limits its large-scale commercial application.Conventional adsorbents(e.g.zeolites and activated carbon)with lower uptakes or too high heat of adsorption are unfavorable for COP_C.Besides,the cascaded AHPs consisting of high-temperature stage(HS)and low-temperature stage(LS)exhibit the higher COP_Cthan single cycle AHPs.Therefore,searching high-performance working pairs in advanced cascaded AHPs is essential to improve the cooling performance.In recent years,metal-organic frameworks(MOFs)are recognized as potential adsorbents for AHPs because of their outstanding adsorption performance.Therefore,in this work,three MOFs including Ui O-66,NU-1000,DUT-67 are chosen as adsorbents with water as working fluid.The cooling performance of nine different working pairs for low-temperature stage(LS)and high-temperature stage(HS)of four-bed cascaded adsorption refrigeration cycle were tested by a dynamic adsorption cooling model under different heat source temperature(363 K and 423 K).It is demonstrated that the working pair DUT-67/water for LS and DUT-67/water for HS in cascaded AHPs achieved the highest COP_C of 1.43 and circulating cooling capacity of2.6×10^4 k J among nine working pairs.Moreover,the working conditions including heat source temperature,hot water mass flow,cycle time and adsorbents mass ratio in LS and HS are optimized to improve the cooling performance.It is revealed that the hot water inlet temperature of 373 K,the hot and heat recovery water mass flow of 0.5 kg/s,the chilled water mass flow of 1 kg/s,the adsorbent of LS and HS mass ratio of 1,and the cycle time of1600～2000 s are beneficial for COP_C.This study reports the strategy to improve the cooling performance of cascaded AHPs by choosing suitable working pairs based on MOFs,which provides insights into the exploration of high-performing MOF adsorbents for improved cooling performance in cascaded AHPs.