Heat Transfer Analysis And Geometry Optimization Of The PCM-based Thermal Energy Storage Unit For Concentrated Solar Plant

As a kind of renewable energy, solar energy is viewed as one effective solution to solve the energy problems. Thermal energy storage(TES) technology is applied into the solar system to deal with the instability problem of energy supply and balance the supply and demand requirement between the energy generator and energy consumer. This paper mainly conducts the heat transfer analysis and geometry optimization of the PCM-based TES unit in concentrated solar plant, which provides theoretical basis to its practical application.Copper is used as metallic fin, and Li2CO3 is selected as high temperature PCM and RT27 is selected as low temperature PCM to develop a 2D model. Constructal method is used to mathematically analyze the heat transfer process and it achieves some equations to calculate the optimal size of the TES models.11 models with different size parameters are selected for the low temperature PCM-based TES unit to conduct the numerical simulation. Through comparison of the PCM melting time and latent heat flux when the liquid fraction is 0.95 among 11 models, it is found that the model whose shape factor is 0.18 has the optimal thermal performance. In addition, some thermal characters are analyzed, like temperature variation, liquid fraction variation, heat flux variation and the influence of phase change parameter β. It is found that the heat transfer boundary makes a big difference on the temperature distribution as well as the solid/liquid phase distribution of RT27, while β doesn’t impact the geometry optimization but impacts the thermal performance of the TES model.A test bench of the low temperature PCM-based TES unit is introduced and four models with different configurations are selected for experiments. The result indicates that the experimental model whose shape factor is 0.15 has the optimal thermal performance. Through comparison of the simulation and experiment results, many problems appearing in experiment are discussed, like heat losses and PCM leakage and so on, which provides a reference for the practical application of the low temperature PCM-based TES unit.11 models with different size for the high temperature PCM-based TES unit are selected to conduct some numerical simulation on the models’ elementary unit, and the change regularity of the temperature, liquid fraction and heat flux is analyzed. It is found that the transverse and longitudinal heat transfer boundaries have a huge influence on the model temperature, liquid fraction and heat flux. It is also found that in the heat transfer process the solid/liquid phase boundary line moves according to the isotherm with the melting temperature of Li2CO3(996K). The result of geometry optimization turns out that the optimal shape factor is 0.57 and the optimal number of the elementary unit is 4.