| Solar energy has the advantages of large reserves,wide distribution,clean and pollution-free,and high cost performance.It is the most potential renewable energy.With the continuous advancement of science and technology,more and more solar technologies are being developed and utilized,photovoltaic and solar thermal are the two main aspects of solar energy utilization,among them,the use of solar heat collection to meet low-grade energy consumption such as heating and hot water supply demand has great energy-saving benefits,and has always been the focus of building energy-saving research and development.However,the utilization of solar energy is restricted by factors such as intermittent and seasonal effects of sunlight and low heat utilization efficiency.Its low guarantee rate is one of the problems exposed during the current use.How to efficiently collect,store and transfer solar energy is an important issues we are facing.Due to the advantages of high latent heat,high heat storage density and slow heat absorption and exothermic process,the phase change heat storage materials have become the most important heat storage methods at present.Therefore,this paper studies the combination of focused solar technology and phase change heat storage technology to solve the problem of insufficient solar heating guarantee rate and achieve the purpose of improving solar thermal utilization.In this study,through the analysis of different types of phase change materials and packaging technology,low melting point alloys with high thermal conductivity,high energy storage density,low supercooling and stable physical properties were selected as phase change materials,expanded graphite was selected as the supporting material,and a granular alloy(140°C)/expanded graphite(mass ratio of 9:1)composite phase change energy storage material was prepared,which was pressed into a fixed block composite phase change material by mechanical pressing method,filled in the focused solar thermal storage system,and combined with experimental testing and numerical simulation to study the changes of the material during the system heat storage and heat release process.First,the particulate alloy/expanded graphite composite phase change material was characterized by scanning electron microscope(SEM),x-ray powder diffraction(XRD)and differential scanning calorimetry(DSC).The results showed that the material did not leakage and no new substances is generated during the compounding process,the enthalpy value is45.05k J/kg.Then the thermophysical properties of the compacted block-shaped composite phase-change material after pressing were compared and analyzed,and the thermal conductivity of the shaped block-shaped composite phase-change material with a density of5000kg/m~3can reach 32.64 W/(m·K).The energy density can reach 2.25×10~5k J/m~3.After repeated high-temperature heating,the shape remains unchanged,no alloy leaks,and the surface absorbance can reach 90%after optimization.Secondly,a test system for the solar thermal utilization of fixed block composite PCMs was built,its working principle and operating mode were introduced,and the effects of different light intensity(530W/m~2,560W/m~2,590W/m~2)and water flow velocity(0.4m/s,0.5m/s and 0.6 m/s)on the heat storage and heat release process of the system were studied.The experimental results show that during the heat storage process,the temperature distribution of the composite PCM in the energy storage device is relatively uniform,which can well complete the phase change heat storage process and reach 180°C.With the increase of light intensity,the heat storage rate Significantly improved;during the heat release process,the phase change energy storage device can instantly release a large amount of heat,so that the fluid is quickly heated to the required temperature,and as the water flow speed increases,the heat release time can be effectively shortened.In addition,the heat storage efficiency and volume advantage of the system are analyzed by theoretical calculation.The heat storage efficiency increases with the increase of light intensity,when the light intensity is 590W/?,its heat storage efficiency can reach 86.3%;Compared with the common solar thermal storage media(paraffin or high temperature water(water temperature 90?)),the composite PCM can greatly reduce the volume of the energy storage device.Finally,in terms of simulation,this study simplified the physical model of the experiment,and used the computational fluid dynamics software Fluent to simulate the effects of different solar light intensity and water flow velocity on the system's heat storage and heat release process.The results of the data were consistent with the experimental data,which verifies the correctness and reliability of the mathematical model and program,and also shows that the system has good application feasibility. |
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