| Solar energy is one kind of clean energy which is inexhaustible. With the increasingly tense of energy supply and prominent of environmental problems, the use of solar energy has received more and more attention. The integration of solar energy and buildings is considered to be one of the main streams of development direction now. This paper presents us a new type of double-glazed vacuum tube with metal fins filled with phase change materials (PCM). And Compared with traditional solar water heating system, this new type vacuum tube does not need the hot water storage tank. During the sunshine hours, the PCM collects energy in the form of latent heat and melts. During off sunshine hours, the hot water is withdrawn and is substituted by cold water, which gains energy from the PCM. With the method of numerical simulation and experiment, the melting/solidifying process of double-glazed vacuum tube with metal fins filled with phase change materials was studied and the structure of mental fins was optimized.Based on the basic principle of solar radiation, the intensity of solar radiation in every period of time of Nanjing was calculated which provides the basic data for the study of heat transfer in the solar vacuum tube below. The length of model vacuum tube is 1800 millimeter and the inner diameter is 105 millimeter. Paraffin58 was chosen as the phase change materials while the U-tube with extended surfaces in form of radial metal fins was chosen as heat transfer channel of fluid. The heat transfer of solar vacuum tube was analyzed, and the effective heat in unit time was obtained by calculating solar radiation heat absorption and heat loss to the surrounding environment. An enthalpy-porosity formulation was used by comparing the phase change energy storage solution of heat transfer process and the operation control equations were given.A three-dimensional model of double-glazed vacuum tube with metal fins filled with phase change was developed by CFD. The effective radiative heat flux obtained by the vacuum tube from 8:00 to 17:00 was programmed using the user-defined function and imported into the simulation software as a boundary condition. The result shows that the paraffin at the top of the vacuum tube began to melt at 10:00 when the temperature of paraffin wax didn’t differ much. Then the paraffin experience the phase change stage from 10:00-14:20 in which the average temperature rose slowly. However, the temperature gap between the top and bottom of the Vacuum tube of the paraffin wax significantly increased and reached 90K at 13:00. When the paraffin wax completely melted, the average temperature rose faster because of the effect of natural convection which accelerated the heat transfer process and further reduce the temperature difference in the tube.To optimize the structure parameters of the metal fins, the models which thickness of fins is chosen from lmm,2mm,4mm,8mm and the spacing from 10mm, 20mm,40mm,80mm were developed and simulated. The result shows that the melting time of the paraffin increases with the increasing spacing of metal fins and decreases with the fins becoming thicker. When the spacing reaches 80mm, the melting time of different thickness is very close, which means that further increasing fin thickness has little effect on the melting time. And when the spacing reduces to less than 10mm, the effect of fin spacing on the melting time is reduced gradually.A double-glazed vacuum tube collector with metal fins filled with paraffin58 was designed and the experiment on the collector was carried out. The experimental results are in good agreement with the simulation. When the paraffin was solidifying and the inlet temperature of water was 17℃, the outlet temperature of water were 41.5℃-55.96℃,32.89℃-42.4℃,29.5℃-33.7℃ while the flow rate changed between 20L/h,40 L/h and70 L/h. |
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