| Thermal energy storage (TES) is an effective approach to moderate energy peak-valley load and improve the energy output stability. It is essential to use the intermittency and instability of energy, namely the thermal use of solar energy, the intermitted industrial waste heat and the cross-seasonal heating and cooling in energy-saving building. Compared with other method of thermal energy storage, Latent heat thermal energy storage (LHTES) has several advantages, including that of high energy density, suitable phase change temperature, chemical stability and a reasonable price. So the LHTES has a great range of potential utilities and a prospect of development. However, as is generally known, most phase change materials (PCMs) suffer from low thermal conductivity leading to low energy charging and release rate, which is a great defect in the application. Based on the issue, metal foam is used to enhance the conductivity of PCM. The metal foam is a kind of porous media with a high porosity and open-cell rate. The phase change process of PCMs is a typical process under the multi-scale, multi-phase and multi-field condition, hence the flow and heat transfer process of this problem is very complicated and need to do a systematic study with experimental and theoretical methods before the large-scale application for using metal foam to enhance the heat transfer performance.An experimental system is designed and set up to prepare PCMs-metal foam composite and test dynamic thermal behavior. The method of vacuum impregnation was used to prepare for the paraffin-copper foam composite. The calculated impregnation ratio of the test sample can reach to 96.7%. Based on the shell-and-tube type of LHTES, three samples for pure paraffin, paraffin-copper foam composite and composite with bottom fin are tested by the experimental system. The temperature variations of the selected detected points inside PCM, and the solid-liquid interface evaluation in axial plane of symmetry for all three samples under different heating temperatures and flow rates of heat transfer fluid (HTF). The experimental results indicate that the interface is funnel-shaped for pure paraffin and the melting process is from top to bottom because of the dominated effect of natural convection. The interface for composite with a bottom fin is the inverse of that under the pure paraffin. For the composite without fin, it is a cone type solid-liquid interface and the interface is developed from inside to outside. The charging time of the paraffin-copper foam composite can be shortened by more than 1/3 than that of pure paraffin under the same operating conditions. The temperature of the heat transfer fluid has a significant influence on the charging process of all three samples. Compared with that, the effect of HTF flow rate is weak.The models of effective thermal conductivity, permeability, inertial resistance factor and interstitial Nusselt number are summarized and reviewed. Many correlations are chosen and compared with each other to calculate the effective thermal conductivity, permeability, inertial resistance factor and interstitial heat transfer coefficient respectively after the comparison among lots of models in the resent literature. Then the Brinkman-Forchheimer extension to Darcy law is employed to model the porous resistance and the local thermal equilibrium model and thermal non-equilibrium model are developed for simulate the heat transfer of phase change process in the metal foam. The models are validated against the experimental results. In contrast, the local thermal non-equilibrium model is more accurate, while the local thermal equilibrium owns higher calculation efficiency.This paper proposes a new thermal energy storage system (TESS) that metal foam and fins are used to enhance the conductivity of PCM and analyzed the heat transfer performance of this system by the validated simulation method. Three adjacent energy storage cells are extracted from TESS as representative domain, and the heat transfer performance of middle cell is used to predict that of whole TESS with the influence of other two cells. The position of solid/liquid interface is explored and the effects of design parameters, including configuration of metal foam and fin and operating parameters, on melting and solidifying rate and energy stored in each time step are documented and discussed. The reasonable technical and economic comparison to determine the optimum configuration parameters for the thick and short style and thin and long style in application. The calculated results shows that metal foam and fins can effectively improve the performance of heat transfer for TES and decrease charging and discharging time. The unbalanced heat flow rate distribution from upper and lower surface of fins narrowed the melting and solidifying process of PCM in both sides of fins. The porosity and heating temperature has a significant influence on the charging process. Compared with that, the effect of pore density and fin thickness are weaker.The pore structure and distribution of metal foam are very important parameters to heat transfer performance of phase change process in metal foam, especially the porosity distribution. A transient numerical model for simulate the melting and solidification of phase change material in copper foams with gradient change porosity was developed, trying to find out the effect of linearly changed porosity on the heat transfer process. The simulated results depicts that metal foam with gradient change porosity can change the heat transfer performance during melting process contrast to constant porosity. The enhancement effect is weak at the beginning of melting. When lots of solid PCM changes to liquid, the effect is stronger and stronger. The synergy of velocity field and pressure field is better for composite with linearly increased porosity from bottom to top. This type of metal foam can promote the flow of molten PCM and shorten the completely melted time of PCM more than 13.8% and as the gradient increasing, the effect is more and more significant. |
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