| Latent thermal energy storage(LTES) is one of the effective technologies to eliminate the discrepancy between thermal energy supply and its demand, and ensure a rational and efficient distribution of energy. By utilizing phase change material(PCM), this technology can provide a high heat storage density and moderate temperature variation, which makes it applicable to many fields including thermal management of electronic equipment, building energy conservation, industrial waste heat recovering and utilization of renewable energy. For LTES systems with a working temperature at 200 oC, nitrates and their eutectics are recognized as one kind of the most attractive PCMs. Owning to the great properties such as stable physical and chemical characteristic, flexible phase change temperature, low sub-cooling degree and investment cost, this kind of materials are widely used especially in solar heat settings. However, compared to other PCMs with the same phase change temperature range, the latent heat of fusion and thermal conductivity of eutectic systems of the nitrates are comparatively low. Such drawbacks become the obstacles that block the improvement of heat storage performances of the LTES systems.This work focuses on the development and investigation of a novel, efficient and stable composite phase change material with servicing temperature around 200 oC. According to the application requirements, eutectic system Li NO3-KCl whose phase change enthalpy is high and expanded graphite(EG) which possesses excellent heat transfer ability are chosen as PCM and heat transfer enhancer, respectively, to prepare a Li NO3-KCl/EG composite phase change material. Investigation means of scanning electron microscope(SEM), differential scanning calorimeter(DSC), hot-disk analyzer and heat storage performance tests in a LTES unit are devoted to the thermo-physical property characterization and heat storage/retrieval performance analysis of the Li NO3-KCl/EG composite. The obtained results indicate that the eutectic Li NO3-KCl in molten state can be successfully impregnated into the porous inner skeleton of EG to form the composite material. The variation of EG content induces a slight change in the phase change temperature of the composite, but a decrease in the phase change latent heat. In comparison with the eutectic Li NO3-KCl, the presence of EG brings in a remarkable improvement in the thermal conductivity. The effective thermal conductivity of the studied composite are not only related to the mass ratio of EG but also associated with the apparent density of the composite. The utilization of the Li NO3-KCl/EG composite with higher apparent density and EG mass fraction can further enhance the heat transfer through the effective thermal conductivity improvement. The latent heat storage or retrieval durations are affected by both the heat conductive ability and heat storage density of the composite material.To study the thermal stability of the Li NO3-KCl/EG composite and compatibility of this composite with container materials, experiments by means of thermogravimetric analyzer(TG), energy dispersive spectrometer(EDS) and DSC are conducted, as well as gravimetric analysis of corrosion specimens. Experimental results reveal that the onset temperature of thermal instability of the composite material is 315 oC. The major reason for the weight loss of the composite above this temperature is the decomposition of Li NO3. The discrepancies of the thermo-physical parameters of the composite material including phase change temperature and phase change latent heat after different thermal cycles are minor and irregular, which confirms the recyclability of the Li NO3-KCl/EG composite material in its whole lifetime for thermal energy storage application. The specimens made of stainless steel 304 L and carbon steel 20# present a good compatibility with the composite material when EG mass fraction is 20 %, whereas serious corrosion is observed for copper H68. Moreover, the increase in EG content can alleviate the corrosion reaction between the composite and container materials. All the results given above will play key roles for the practical applications of the Li NO3-KCl/EG composite material.Besides, by introducing geometrical fractal theory, a self-similar fractal model, which consists of multiple fractal units of the type Serpinski carpet with different fractal dimensions, is established to describe the microstructural characters of the Li NO3-KCl/EG composite. With the assistance of the well-known thermal-electrical analogy, the effective thermal conductivity of this composite is calculated. Comparisons between the values obtained by the current model and experimental determinations show a good agreement. This work might be of great help to understand the heat conduction performance of inorganic salt/EG composite and can act as guidance to enhance the heat transfer ability of inorganic salts and their eutectics as phase change material. |
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