| As a latent heat energy storage technology,the latent heat thermal energy storage tank has many advantages such as high energy storage density,stable and controllable temperature in the heat storage/release process,and low water consumption compared with the traditional sensible heat energy storage water tank,which makes it have a unique broad market prospect and practical application value in the field of energy utilization.However,due to the inherent low thermal conductivity of ordinary phase change materials,the heat storage and release performance of conventional latent heat energy storage systems is poor,which greatly limits the application and development of latent heat thermal energy storage systems.At the same time,in the current research,there is a lack of in-depth research on the influence of the shape size of the phase change module and different layout inclination angles on the formation and development of natural convection and melting performance of the phase change material.And the numerical calculation model of the porous-based phase change water tank is still incomplete.The energy efficiency of conventional solar energy combined with air source heat pump heating system is not ideal,the utilization rate of solar energy is low,and there is a lack of relevant data on the operation of solar energy-porous-based phase change water tank combined with subcooled air source heat pump hot water system.Based on this,this paper firstly takes the plate latent heat thermal energy storage tank as the main research object,and designs four groups of rectangular phase change modules with different dimensions.The temperature distribution inside the module is visualized and analyzed.From the comparison of the temperature contour and the Nusselt number,it can be found that the natural convection of the phase change module with a higher height is stronger,and more hot fluid flows to the top of the container,resulting in the accumulation of sensible heat,and the heating surface and interior of the container form obvious thermal stratification At the same time,the heat transfer enhancement caused by natural convection lifting has little effect on the heat transfer at the bottom of the large aspect ratio container,and the melting time is further prolonged.When the height of the container was increased from12 cm to 48 cm,the melting time of the phase change material was prolonged by 50%,and the average temperature increased by more than 30°C.The scaling theory analysis shows that the melting process of the phase change module with large aspect ratio can be divided into pure heat conduction regime,mixed heat transfer regime and solid shrinkage regime.Different regimes have different dominant modes of heat transfer,natural convection occurs in the initial stage of melting,and the solid shrinkage regime occupies most of the melting process.Secondly,a numerical simulation study was carried out on the heat transfer and melting performance of the porous-based phase change material in a rectangular container at different inclination angles.The results showed that compared with pure phase change materials,the addition of porous metal significantly improved the overall heat transfer performance of phase change materials.Although the porous structure has a strong inhibitory effect on the flow of liquid phase change materials,the porous-based phase change modules still show different phase change thermal performances due to the influence of natural convection under different angles,where the vertical case(θ=90°)with the strongest natural convection,followed by the cases with a inclination angle of45°and 135°.In the horizontal arrangement case(θ=0°,θ=180°),natural convection is completely suppressed,and heat conduction is always the absolute dominant in the heat transfer process.The complete melting time of the vertical case is 6.67%and 12.5%shorter than that of the 45°and 135°cases,respectively,but it is 7.69%longer than that of the horizontal case.Porous-based phase change materials with lower pore densities exhibit stronger natural convection phenomena and larger temperature profile curvature during melting.When the Rayleigh number increased from 2.05×10~7 to 4.11×10~7 and 8.21×10~7,the complete melting time was shortened by 37.5%and 66.7%,respectively.The boundary heat flow distribution shows that the heat transfer intensity of the vertical case is stronger in the early stage of melting,but it is overtaken by the horizontal case in the later stage.Then,based on the simulation results,a flat-plate porous-based phase-change water tank with horizontally arranged phase change modules was selected and designed,and a two-dimensional phase-change heat transfer transient mathematical model of the flat-plate porous-based phase change hot water storage tank was established using the single temperature method as the theoretical basis,and use MATLAB to compile and solve the program.The effect of porous materials,system structures(plate thickness,phase change material volume rate),heat transfer fluid parameters(inlet temperature,flow rate),porous material physical properties(material type,pore size)on the performance parameters(such as phase change ratio,heat storage and discharge intensity,outlet fluid temperatures,etc)during the heat storage/release process of the flat-plate porous-based phase change unit were discussed.Based on the heat transfer and energy storage performance of the porous-based phase change water tank,the best recommended values of system-related design parameters are given.Subsequently,the annual operation of the direct heating air source heat pump hot water system in a university of Chongqing was measured and analyzed,and the energy efficiency performance of the system and the detailed energy consumption composition of each month were obtained.It is found that the increase of system energy consumption and hot water cost is mainly affected by the seasonal heat production of the direct heating unit and the frequent turn-on of the circulating heating mode.According to the actual operation of conventional direct heating air source heat pump hot water system,a high-efficiency air source heat pump system with subcooling waste heat recovery is proposed,and the theoretical calculation model and exergy analysis model of the system cycle are established.The research results show that the optimal subcooling degree of the heat pump system is the maximum subcooling degree that can be obtained under a certain condensing temperature,which is mainly related to the inlet and outlet water temperatures of the heat exchange fluid.At this time,increasing the degree of subcooling of the system through the subcooling heat exchanger can effectively improve the heating capacity and the thermal perfection of the system cycle,the maximum increase of cycle COP under different ambient temperatures can reach more than 30%,and the exergy efficiency in the condensing side and the expansion valve also increases accordingly,and the proportion of exergy loss on the condensing side increases from about 60%to more than 85%.Finally,the TRNSYS simulation platform is used to simulate the annual operating characteristics and energy consumption performance of the solar-porous-based phase change water tank combined with the subcooled air source heat pump hot water system.Compared with the original ordinary heat pump hot water system,the annual energy consumption of the subcooled heat pump system,the solar-porous-based phase change water tank system,and the subcooled heat pump combined with the solar-porous-based phase change water tank system has been reduced by 20.3%(35,178 k Wh),19.1%(33,147 k Wh)and 34%(58,946 k Wh).The use of porous-based phase change water tanks and subcooled heat pumps have different energy-saving characteristics for the system.The water supply ratio of the porous-based phase change water tank increases month by month,reaching a maximum value of 68.9%in July.The temperature of the water supply through the porous-based phase change water tank fluctuates from 46.3 to 52.6°C throughout the year.Except for winter conditions,the phase change water tank has good heat storage and release performance throughout the year.Combined with the annual water temperature and water level of the terminal water tank,the research results show that the solar-porous-based phase change water tank combined with the subcooled high-efficiency heat pump hot water system can have good energy saving potential on the basis of ensuring the terminal water use experience. |
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