| Energy consumption has continued to grow,and China's energy structure has been continuously upgraded.Energy storage technology can effectively improve the utilization rate of solar energy,and the development of energy storage technology and improvement of energy storage efficiency have become the focus of current research.This topic has designed a new energy storage solar heat pump heating water system,based on the seasonal changes in solar radiation intensity,through the composite phase change material(PCM)to improve the utilization of solar energy,through the gravity heat pipe for efficient heat transfer or heat pump cycle for energy-saving heating to improve energy efficiency for the whole year,through reducing the multistage intermediate heat loss to improve the overall performance,implementing round-the-clock operation of the system.However,a single PCM can not meet the all-weather energy storage requirements,so a novel composite PCM with two phase change points is proposed to prepare,which corresponds to the energy storage requirements of summer and winter conditions respectively,and can be used in the system to store energy in different seasons to maximize the utilization of solar energy system characteristics.In this paper,numerical simulation and experimental research were combined to analyze the phase change heat transfer process of the heat transfer unit inside the collector,to explore the factors affecting the heat storage efficiency of the PCM,and to study the thermal performance of the composite PCM which can be divided into seasonal energy storage,which has important guiding significance to realize the efficient utilization of energy.First of all,the heat transfer characteristics of energy storage heating solar collector was analyzed.A three-dimensional model of the unit inside the solar vacuum tube collector was built via commercial software GAMBIT?.Based on Solidification/Melting model of commercial software FLUENT?,the simulation study of the heat transfer unit was carried out.The influences of the arrangement mode,natural convection and the thermal physical properties of the PCM on the phase change process in the heat transfer unit were compared and analyzed,and the methods to improve the heat storage efficiency were explored.The results show that the horizontal arrangement,natural convection and the selection of appropriate thermal properties can improve the heat storage efficiency of phase change materials in the vacuum tube.Second,according to the theory phase diagrams by MATLAB?,a series of binary and ternary composite PCMs were prepared by melting blending of several organic PCMs which meet the requirements of system energy storage.Experimental study on the thermal and physical properties of the composite PCMs was carried out to explore the change rule of key parameters with non-eutectic ratio,and the optimal composite PCM for seasonal energy storage were selected.The results show that the two phase transformation points of binary non-eutectic composite PCMs are lower than the corresponding phase change temperature of the single component,and the influence of different component composition on the change range of phase change temperature is different.The change of total enthalpy of phase change does not have a fixed rule,but the higher the latent heat of phase change of a single component is,the higher the upper limit of latent heat range is,and the thermal physical properties of different components in the composite PCM are mutually affected.The change trend of phase transition temperature of ternary non-eutectic composite system with the ratio is consistent with that of binary composite system.Compared with 62/LA/SA(4:3:1),the supercooling degree of 62/LA/C18(4:3:1)is smaller and the total latent heat of phase transition is higher.The two phase transition points of 62/LA/C18(4:3:1)are 33.77?and53.46?,respectively,with a high latent heat value of 223.46 J·g-1.After 300 thermal cycle tests,the phase transition temperature and the degree of undercooling have little change,and the total latent heat of phase transition is only reduced by 2.2%.FT-IR analysis shows that the components of the composite PCM were only physically recombined,and there was no attenuation of chemical substances before and after thermal cycling.Therefore,the ternary composite PCM 62/LA/C18(4:3:1)meets the energy storage requirements of the system and has excellent performance,which can be applied to the energy storage solar heat pump system.Finally,UDF programming was used to simulate the heat storage process of 62/LA/C18(4:3:1),capric acid(CA)and paraffin in the vacuum tube heat transfer unit at 9:00?18:00 in the actual working condition,and the phase transformation characteristics of the composite PCM were compared and analyzed.The results show that the temperature gradient in the vacuum tube accelerates the whole liquefaction rate of the composite PCM,and its complete melting time is the shortest,followed by CA,and the paraffin does not complete the phase change process in the daytime.The accumulated heat of 1m3 unit of the composite PCM,CA and paraffin is 250.5MJ,201.5MJ and 137.7MJ,respectively.Therefore,the composite PCM 62/LA/C18(4:3:1)has the highest heat storage efficiency and the strongest heat storage capacity,which can realize the maximum utilization of solar energy and has obvious advantages compared with two single phase change materials. |
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