Preparation And Performance Study Of Phase Change Material With Hydrated Salt For Cold Storage In Air Conditioning

As a new type of energy storage technology,air conditioning storage technology has a very promising prospect to realize the economic operation of air conditioning system by using the peak-to-valley price difference of power grid,and to realize the high proportion of renewable energy by storing and transforming the unstable renewable energy,which is one of the important measures to achieve carbon neutrality.The application of phase change materials（PCMs）in air conditioning storage can achieve a win-win situation in terms of operational energy efficiency and initial investment.The low price,high latent heat of phase change and non-flammability of hydrated salt materials have a broad application prospect in the field of air conditioning storage,but their problems of subcooling,phase separation and poor stability have hindered their promotion.To address the above shortcomings,this work proposes a new formulation of phase change material based on the application of air conditioning storage system with sodium sulfate decahydrate（SSD）as the main material and optimizes the performance of the material in all aspects.The main works are as follows.Firstly,the ratio of nucleating agent borax was determined by step-cooling curve method using sodium sulfate decahydrate as the main phase change material to eliminate the phenomenon of material overcooling.On this basis,the effects of different types and contents of thickeners（sodium polyacrylate（PAAS）,polyacrylamide（PAM）,carboxymethyl cellulose（CMC）,polyanionic cellulose（PAC）and xanthan gum（XG））on the phase separation and latent heat of phase change of the material were investigated and analyzed,and the experimental results showed that the addition of borax with a mass fraction of 3%had the best effect on reducing the material supercooling;the addition of PAAS with a mass fraction of 1.0%-2.0%could eliminate the phase separation phenomenon of the material,and PAAS with a mass fraction of 2%had less effect on the latent heat of phase change material.Next,the phase change temperature of sodium decahydrate sulfate was changed by adding temperature regulators ammonium chloride（NH₄Cl）and potassium chloride（KCl）to the phase change system without subcooling and phase separation to investigate the regulation of inorganic salt regulators on the phase change temperature of sodium decahydrate sulfate,and the regulator ratios to meet the air-conditioning temperature zone were screened by step-cooling curve experiments,and then SSD-BPA low-temperature phase change materials were prepared.The results showed that ammonium chloride with a mass fraction of 14%could regulate the phase change temperature of sodium decahydrate sulfate to 7.5℃and remained stable after several cycles;ammonium chloride and potassium chloride could control the phase change temperature of sodium decahydrate sulfate between6℃-21℃,and ammonium chloride had a stronger effect on the regulation of phase change temperature of sodium decahydrate sulfate compared with potassium chloride.Finally,SSD-BPA/EG composite phase change material was prepared by using SSD-BPA as the main phase change material and adding high thermal conductivity matrix expanded graphite（EG）to further improve the thermal conductivity and stability of the material,and SEM morphological observation,thermal conductivity test,and thermal cycling test were performed.The results showed that the material had high thermal conductivity and shape stability when the mass ratio of SSD-BPA to EG was 93:7.The phase change temperature of the optimized composite phase change cold storage material was 7.4℃,the latent heat of phase change was 117.4J/g,the thermal conductivity was1.876W/（m·K）,and the phase change temperature of the material remained stable after 200 cycles,and the latent heat decay rate was 14.05%.Compared with the existing storage materials,the thermal conductivity of SSD-BPA/EG₇is increased by 3-4 times,the subcooling is effectively reduced,the phase separation problem is alleviated by adding PAAS and EG,and the number of stable cycles is increased from 50,100 to 200,which successfully solves the key problems of hydrated salt phase change materials in current practical applications and provides an effective demonstration for the preparation of phase change media for cold storage in air conditioning.