Study On The Properties Of Solid-gas-liquid Sorbents And Phase-change Materials And The Application In Heat Storage Air-to-water System

The supply of fresh water has always been a serious problem for mankind.As an extended application of sorption refrigeration technology,sorption air-to-water has attracted extensive attentions recently.Compared with conventional air-to-water,sorption air-to-water offers advantages of simple structure,small volume,high efficiency,environmental protection and energy saving,i.e.The research on sorption air-to-water is generally concentrated in the study of sorbents and circulatory system.The system requires that the heat and mass?water?transfer performance of the composite sorbents is high,but it is difficult to satisfy simultaneously for general sorbents.In addition,to further increase the sorption performance of sorbents,deliquescence phenomenon will occur in the sorbents,and it is to be avoided in traditional sorption refrigeration.The shape and filling method of sorbents and design of air flow passage in the sorption bed have great influence on the performance of the system.In allusion to these issues,the three-phase sorption cycle is firstly analyzed theoretically in this article.Then,the composite sorbents are prepared using LiCl and mixed salts of LiCl and CaCl₂ as hygroscopic salts,expanded natural graphite treated by the sulphuric acid?ENG-TSA?and activated carbon fiber?ACF?felt as host materials respectively,and there thermal conductivity,sorption performance and stability,etc are studied.The solid-gas two-phase and solid-gas-liquid three-phase heat storage sorption air-to-water devices are designed and the advantages of three-phase sorption cycle in air-to-water are verified experimentally.The main contents and conclusions of this paper are as follows:?1?The reaction of hygroscopic salts and water vapor and complex interaction mechanism are described.The mechanism of deliquescence process is analyzed comprehensively from the perspective of chemical potential,especially.It includes solid/gas chemical sorption?hydration reaction?,solid-gas-liquid three-phase deliquescence and gas-liquid absorption process.The complete water uptake process of hygroscopic salts can be considered as the three-phase process of all the above processes,namely the three-phase sorption process.?2?The hygroscopic salts of composite sorbents are selected,and the deliquescence relative humidity of LiBr,LiCl and CaCl₂ are all less than 30%.The sorption performance and economic performance of these three kinds of hygroscopic salts were compared,and the sorption performance of LiCl is the best?1.34 g/g?;CaCl₂has the cost advantage.The matrixes of composite sorbents are selected,and it is concluded that the thermal conductivity and malleability of ENG-TSA are good,but the mechanical properties are poor;The ACF has the advantages of large surface area,uniformly micro-pore and strong stability,but the thermal conductivity is relatively weak.So,the composite sorbents are prepared using LiCl and mixed salts of LiCl and CaCl₂ as hygroscopic salts,ENG-TSA and ACF felt as host materials respectively.?3?The thermal conductivity,sorption properties and stability,i.e of the prepared composite sorbents are studied.Results reveal that:the axial and radial thermal conductivity of the composite sorbents ENG-TSA-LiCl increase with the increase of the density and the best thermal conductivity of them is 5.67 W/?m·K?.The sorption performance of composite sorbents decreases with the increase of density.The biggest water uptake of sorbents is 1.54 g/g at 20?and 80%RH,but their mechanical properties will be significantly reduced after the sorption.The axial and radial thermal conductivity of ACF-LiCl are increased with the increase of LiCl solution concentration.The sorption performance of composite sorbents by vacuum impregnation method is better than atmospheric impregnation method.The water uptake of consolidated sorbent of ASLi40 is 1.59 g/g and the desorption quantity of it is1.22 g/g at 90?.Moreover,its mechanical properties and regeneration capacity are relatively strong.The water uptake of composite sorbent ALiCa30?3:1?is 2.98 g/g at25?and 90%RH and the advantages of its performance are explained in terms of microstructure.The sorption dynamics model of it is studied and the sorption rate coefficient is higher than ACF-CaCl₂30.The thermal characteristic of it is tested,and its mass energy storage density is 0.41 kWh/kg.It is found that a large quantity of energy is stored in the three-phase crystallization/deliquescence process of sorbents.?4?The heat storage materials are prepared and studied.The composite phase-change thermal energy storage material ENG-TSA-SA is suitable for solar energy and other low-grade heat energy recovery with high radial thermal conductivity?22.2 W/?m·K??and proper internal of phase transition temperature?65.9⁷7.1??.The addition of the heat reservoir significantly slows down the decrease to desorption temperature.?5?In order to verify the advantages of the three-phase sorption cycle in the air-to-water system,two sorption air-to-water devices including a solid-gas two-phase and a solid-gas-liquid three-phase heat storage are designed.The performance of four kinds of composite sorbents is tested in solid-gas two-phase air-to-water device.The performance of composite sorbents whose host material is ACF is better than sorbents whose host material is silica gel.The sorption and desorption performance of ACF-LiCl is the best.The freshwater yield is 0.80 kg after 6 hours desorption when the outlet temperature of sorbent bed is 90^oC,but far below its design objective.Two-phase sorption air-to-water device has the limitations of not reaching the sorption equilibrium and weak regeneration capacity of sorbents,partial water vapor uncondensing and desorption temperature instability,i.e after analysis.In allusion to these issues,a three-phase heat storage sorption air-to-water device is designed.The system uses the consolidated composite sorbent ASLi40,adopts open sorption and closed desorption cycle,adds the heat reservoir and extends the sorption time.The device achieves 62.5kg of water withdrawal under the condition of sorption temperature 32?,sorption relative humidity 75%and desorption temperature about 92?.The water quality meets the drinking water sanitation standard after filtration.The amount of heat needed to collect 1kg of water is 1.96 kW·h.The advantages of three-phase sorption in air-to-water are verified.