Study On Enhancement Of Energy Efficiency For Adsorption Refrigeration System Based On In-Situ Regulated Activated Carbon Composites

As an energy-saving green refrigeration technology,activated carbon-methanol adsorption refrigeration is based on environment-friendly adsorption working pairs,which can effectively use low-grade heat energy(60～150℃).However,the low efficiency of adsorption refrigeration has always been a key scientific and technological problem restricting the wide application of activated carbon-methanol adsorption refrigeration technology.Research and engineering practice at home and abroad have proved that the characteristics of adsorption materials with activated carbon as the core are the key factors affecting the energy efficiency of adsorption refrigeration technology.It is very important to improve the efficiency and stability of adsorption/desorption,enhance the heat transfer performance of carbon materials,improve the mass transfer rate of refrigerant in the adsorption bed and pore structure of carbon materials,and improve the thermal efficiency of adsorption refrigeration cycle.Studies have confirmed that the microporous activated carbon adsorption material,which is mainly commercial activated carbon,has the problems of low adsorption efficiency and stability,although the post modification of activated carbon materials and composite chemical adsorbents can improve the adsorption efficiency to a certain extent,modification and chemical adsorbent coupling often leads to the key bottleneck problems such as the significant reduction of surface area and pore structure volume level of carbon materials,pore blockage and adsorption performance degradation.Based on this,this paper proposes "Research on in-situ directional cutting technolo gy of high-performance activated carbon composites suitable for adsorption refrigeration technology",aiming to explore the influence of in-situ control process of pore structur e of carbon materials on adsorption/desorption cycle characteristics,heat and mass trans fer performance,physicochemical adsorbent coupling performance and energy efficiency improvement of adsorption refrigeration system.The main results are as follows:(1)In-situ synchronous regulate of pore structure of activated carbon materials:The insitu regulated activated carbon GHUM with well-developed mesopores and micropores was successfully prepared by optimizing the in-situ regulated process of carbon precursor compounding,combined catalytic oxidation and phosphoric acid catalytic activation.The results of pore structure distribution and thermal conductivity characterization show that:Compared with the mesoporous carbon SX-100(prepared in this paper)and microporous carbon GDWK-02,the surface area,pore volume,thermal conductivity and thermal stability of GHUM were significantly improved.The in-situ regulated activated carbon GHUM has a larger specific surface area(1243 m2/g)and a well-developed microporous(0.2616 cm3/g)and mesoporous(0.8158 cm3/g)structure;the thermal conductivity increases to 0.417 W/(m·K);the thermal stability is significantly better than GDWK-02 and SX-100.(2)In-situ regulated carbon-methanol adsorption/desorption characteristics:In this paper,the influence of pore structure regulation on the adsorption/desorption capacity of refrigerant pair was compared and analyzed by constructing an in-situ regulated activated carbon GHUMmethanol adsorption/desorption test system.The results of adsorption kinetics and isotherm fitting showed that:The adsorption capacity of GHUM was 368.04±4.64 mg/g,which was significantly higher than that of SX-100(333.5 ± 4.38 mg/g)and GDWK-02(290.83±3.74 mg/g).The diffusion coefficient of methanol on GHUM surface(15Ds0/Rp2)is 299.32 ± 123.16 s-1,which is better than SX-100(242.18±149.33 s-1)and GDWK-02(0.3387±0.125 s-1).(3)Characteristics of composite adsorption refrigeration materials based on in-situ regulated carbon:Carbon based densification composite adsorption refrigeration materials GHCA(CaCl2 composite adsorbent),GHSM(graphene composite adsorbent)and GHSC(CaCl2 and graphene coupled densification adsorbent)were prepared using in-situ regulated activated carbon GHUM as matrix and graphene or CaCl2 as admixture.Among them,the heat transfer and cyclic adsorption capacity of the optimized proportion of CaCl2 and graphene coupling densification adsorbent GHSC were evaluated.The results show that the densified adsorbent GHSC has good structure distribution of large,mesoporous(0.8258 cm3/g)and micropore(0.2916 cm3/g);its cyclic adsorption capacity increases to 487.9±17.09 mg/g;its thermal conductivity reaches 1.9±0.1 W/(m·K).(4)Energy efficiency improvement of adsorption refrigeration system based on carbon matrix composites:In this paper,a double bed assisted mass transfer continuous adsorption refrigeration/air conditioning system was constructed and optimized,and the adsorption refrigeration cycle characteristics of activated carbon GHUM and carbon based densified composite adsorption refrigeration material GHSC were verified.The results show that:The refrigerating capacity(546.74 kJ/kg)and refrigerating power(607.49 kJ/(kg·h))of GHSC-methanol working pair were significantly higher than the carbon-based working pairs compared with it.The heat exchange temperature difference of GHSC-methanol can reach 70℃,and the heat transfer effect is good;the return temperature difference at the end of the air conditioner can reach 10℃,which can reduce the fresh air volume of the end device and save the energy consumption of the air conditioner.In summary,the in-situ regulated and composite densification process of carbon based materials are proposed to realize the synergistic enhancement of heat and mass transfer in the adsorption bed,and solve the problem of pore structure blockage of carbon materials in the process of physicochemical coupling,and has important scientific significance and practical value for effectively improving the refrigeration efficiency of adsorption refrigeration technology and efficiently utilizing low-grade heat energy.