Optimization And Application Of Desiccant Materials In Desiccant Coated Heat Exchanger

Conventional vapor compression air conditioning systems usually employ condensation dehumidification to handle latent and sensible heats simultaneously,leading to waste of energy and inferior indoor air quality.Desiccant cooling systems possess good dehumidification performance,but subject to poor sensible heat capability.Recently,temperature and humidity independent control?THIC?systems have been proposed,which combine the merits of heat pump and desiccant cooling systems.However,THIC systems have disadvantages of large volumes and high initial investment,and adsorption heat released during dehumidification process may result in higher regeneration temperature.To solve these problems,a novel desiccant coated heat exchanger based heat pump cycle is put forward.By coating solid desiccant materials onto the surface of a fin-tube heat exchanger,water vapor in the process air can be absorbed by coated desiccant,and heat loads including adsorption heat can be carried away by cooling fluid inside the tube.Meanwhile,desiccant can be regenerated with condensation heat recovery.In this case,sensible and latent loads can be simultaneously handled within one single component.Desiccant coated heat exchanger plays an important role on the whole system,and its performance is intimately connected with coated desiccant and refrigerant inside.Researches on desiccant materials are mainly focused on the improvement of unit adsorption quantity,and often utilized in adsorption cooling systems or desiccant wheels.Little research has been done on thermal dynamics studies of desiccant materials in the novel cycle with different operation conditions.Besides,isothermal adsorption-desorption properties of metal matrix desiccant sheets are scarcely studied.In allusion to these issues,this work starts with the analysis of the novel system and comes out with screening principles between desiccants,refrigerants and application fields.Various composite desiccants are fabricated and thoroughly investigated.Mesoporous silica gel-LiCl composite and novel zeolite like molecular sieves were perceived as promising choices,and coated to metal matrixes and heat exchangers.Simulations and experiments of desiccant coated heat exchangers are finally carried out.The main study of the paper can be described as follows:?1?Through the analysis of air side,desiccant side and refrigerant side,this novel cycle is investigated.R32 and R410A are recommended refrigerants for the cycle.By proper selection of desiccants and refrigerants,energy consumption of this system can be less than that of a traditional vapor compression system.Based on application analysis of the proposed system in different climate zones and architectures,it is concluded that composite desiccants possess flexible dehumidification performance and can be applied in most climate zones,while zeolite-like molecular sieves are especially suitable for ARI summer and ARI humid climates,or humid environments with low requirement for fresh air.Besides,screening indexes of ideal desiccants for this novel system are put forward.?2?To find suitable hygroscopic salts for porous matrix,preliminary screening of 16 conventional salts was carried out.In view of safety and hygroscopicity,LiBr,LiCl and CaCl₂ were further considered,and impregnating into pores of mesoprous,type B and micropostous silica gels.LiCl is perceived as the most promising salt according to experimental results of adsorption and desorption tests.Study on LiCl solutions with varied mass concentrations in the range of 10-40 wt.%showed that salt content in omposite samples has a positive influence on both the thermal conductivity and hygroscopicity of composite desiccants.?3?To find suitable porous matrix for LiCl,seven substrates were preselected from silicagel,mesoporous silicate,activated carbon and natural rock based porous materials.Intensive studies on microstructure,equilibrium adsorption property,heat of dsorption,adsorption kinetics and desorption property of fabricated composite esiccants were conducted.Compared to porous substrates,surface areas and pore olumes of all the composite samples decreased.Adsorption isotherms on water vapor howed that adsorption quantities of composite desiccants were obviously improved, nd adsorption equations were obtained based on the Polanyi adsorption potential theory.According to isosteric heat of adsorption analysis on composite desiccants,adsorption heat was observed to be higher than that of porous hosts when adsorption capacity is low due to chemical sorption,but with the increase of adsorption capacity,it decreased as physical adsorption dominates.Dynamic adsorption tests pointed out that in comparison with pure substrates,adsorption rate coefficients of composites reduced.Results of thermal gravimetric analysis found that composite materials can be regenerated at low temperature.Based on optimal criterion,SGB/LiCl is determined to be the optimal composite desiccant for this novel system.?4?Based on the result of desiccant selection,a SGB/LiCl coated aluminum-fin was fabricated and compared to pure silica gel coated one.The composite sample exhibited higher sorption quantities and larger desorption quantities under heating temperature of 45^oC.Though the composite coated fin showed good adsorption and desorption properties,carryover problem may occur under humid conditions.Thus,zeolite-like molecular sieves,i.e.SAPO-34 and FAPO-34 coated aluminum-fins were prepared.Fabrication procedures are carefully studied to maximize water uptake.SAPO-34 coated fin had the worst dehumidification performance,while performance of FAPO-34 coated one is between pure and composite silica gel coated fins.?5?Silica gel,composite silica gel and FAPO-34 coated heat exchangers were developed.Simulational and experimental research on desiccant coated heat exchanger systems was carried out,in which water was adopted as refrigerant.Simulation results showed that with the benefit of LiCl,dehumidification capacity of composite silica gel coated heat exchanger was greatly enhanced.Both simulational and experimental researches present that dehumidification performance of FAPO-34 coated heat exchanger?FAHE? was better than silica gel coated one?SGHE?,but inferior than composite silica gel ?CSGHE?.For outdoor environment with moderate humidity ratio,FAHE was capable enough to provide content supply air.In addition,FAHE was free of the lyolysis. Mathematical model was proved to be reliable as the overall discrepancy between simulated dehumidification capacity and experimental data was no more than 20%.