| Thermal energy storage is considered as one of the most perspective technologies for improving the efficiency of energy conversion and energy utilization processes.Phase change thermal energy storage materials have many advantages in terms of solar thermal utilization,peak-shift of electrical demand and medium-low temperature waste heat recovery and utilization.Among these materials the salt hydrates deserve a special attention due to their advantages such as low price,wide material source and high thermal storage density.However,there are two significant weaknesses of salt hydrates:supercooling and phase separation,and solving these problems is the key to realizing the application of salt hydrates as phase change latent heat materials.In this paper,the magnesium nitrate hexahydrate(MNH)is chosen to study,and the problems of supercooling and phase separation of MNH are studied,and the effective methods to improve the defects are proposed.Then,a latent heat thermal energy storage device filling with the modified MNH is designed,and the application of the device in actual heating system is studied.The main study results are as follows:(1)According to the theory of latent heat thermal energy storage and the selected MNH,the key problems of salt hydrates are summarized:supercooling,phase separation,and corrosivity,and related solutions are proposed.The analysis methods of thermal storage performance of salt hydrates including step cooling curves analysis,cycle stability analysis,phase change latent heat analysis,density and expansion ratio analysis and thermal conductivity analysis are summarized.(2)The MNH phase change material is studied in detail and modified to meet the application requirements.Firstly,a composite phase change materials of magnesium nitrate hexahydrate/calcium sulphate dihydrate(MNH/CSD)and magnesium nitrate hexahydrate/calcium sulphate dihydrate/expanded graphite(MNH/CSD/EG)was prepared respectively.Then,thermal storage performance of the two composite phase change materials is studied.Finally,the relevant conclusions are as follows:the optimum addition amount of the nucleating agent CSD is 2wt.%.The phase transition temperature of the modified MNH is 87.3?,the degree of subcooling is less than 0.5?,the latent heat is 248.7kJ/kg,the solid density is 1.61g/cm~3,the liquid density is 1.52g/cm~3,the volume expansion ratio is6.45%,and the thermal conductivity is 0.61W/(m·K).The modified MNH keeps good thermal storage performance after 50 cycles.The addition of EG can significantly increase the thermal conductivity of the composite phase change materials.The above research results show that the modified MNH can be used as a phase change thermal energy storage material in practice.(3)In order to verify the effect of the modified MNH in practical application,a finned tube latent heat thermal energy storage device is designed,and a heating system combining with the device suitable peak-valley electricity scene is established.Then,charging and discharging performance of the device is studied.Experimental studies show that the latent heat thermal energy storage device performs a heat storage with an average charging power of 24.74 kW and a storage capacity of 151.49 kWh during the valley load period.During the peak load period,the latent heat thermal energy storage device can provide a 16-hour continuous discharging process with a total heat output of 139.47 kWh with an average discharging power of 8.4 kW at a stable exothermic temperature and platform.The charging-discharging efficiency of the storage device is 92.3%.The present study can provide theoretical and experimental basis for the practical application of MNH as phase change materials. |
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