| Energy shortage has become a major issue which seriously affects people’s lives,and hinders the sustainable development of society. Solar thermal power generation,also known as concentrating solar power (short for CSP), has become the mostpromising way of power generation besides wind power. Because the supply of solarenergy is not stable, an energy storage system which absorbs solar energy andconverts it into electrical energy continuously is required to ensure steady running ofsolar energy power generation system. However, the thermal conductivity of phasechange materials (short for PCM) used in such system is very small, which affects therate of heat release and storage. So measures are needed to enhance the effectivetransfer rate of phase change.In this paper, experimental and numerical simulation methods were adopted.Firstly, graphite foam/eutectic salts composite was prepared. Next, the numericalsimulation method was employed to analysis the effective thermal conductivity (ke)of composite PCM in the condition of steady state and transient state. The maincontents and results of the paper were described as follows:1. Firstly, graphite foam/eutectic salts composite was fabricated successfully,then the thermo-physical properties was tested. The results showed that: the meltingpoint of the eutectic salt and latent heat did not change after the impregnation.Composite PCM with high effective thermal conductivity, excellent heat transferperformance and high energy density completely meet the requirements of hightemperature heat storage;2. Two models called body centered cubic and face centered cubic model wereused in the modeling process of composite phase change materials. Since BCC modelis in good agreement with the experimental data, the choice of BCC model was madein the numerical simulation;3. Regardless of the PCM melting process, the impact of liquid PCM’s naturalconvection on the overall ke can be ignored. The ke of composite PCM was obtained through curve fitting. The pore size also had no effect on the overall ke. After that, thedifference of FCC and BCC model in ke was discussed through comparative analysis.It was found that ke was determined by the thermal conductivity of graphite foamskeleton. The ke decreased with characteristic temperature increased in the wholeporosity range with the same trend as the thermal conductivity of graphite foamskeleton. When porosity is0.75, the overall ke can be calculated approximately in thisexpressionke=51.33-0.11×T+(1.072E-4)×T~2-(4.6E-8)×T~34. When PCM was located in the melting process, the impact of molten PCMnatural convection on overall ke can be ignored. The changing curve of ke wasacquired in the condition of porosity0.8. After analysis of seven curves in porosityrange0.6-0.9, the result showed that heat transferred faster in low porosity modelthan other models. Model with small pore size shorten the melting time by21%.Theresult showed that model with small aperture had better heat transfer performancebecause of larger contact area under same porosity.5. In order to balance heat transfer rate and storage capacity, the definition of theequivalent ke was introduced. Finally, the balance porosity0.738was acquired byusing this parameter in combination with total energy. |
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