| Phase change material (PCM) can store or release a large amount of latent heat in a narrow temperature range during phase change. So the size of storage divice can be reduced which makes latent heat thermal storage high valuable. Latent heat thermal storage can be widely used in applications such as building heating, waste heat utilization, compressed air storage system and solar thermal power system. In recent years, thermophysical property measurement of PCM, phase change heat transfer process and phse change heat storage device have been the research spots.Firstly, in this study, erythritol is chosen as storage media according to phase change material slection principles in suitable temperature range. And then comprehensive measurements of erythritol on thermophysical properties including phase change temperature, melting enthalpy, thermal conductivity, viscosity and decomposition temperature have been carried out.The conculsions are as follows:Erythritol has a melting temperature of 120.39 ℃ and the melting enthalpy is 319.5 kJ/kg. Erythritol shows strong subcooling phenomena and the subcooling degree is between 93.64 ℃ and 113.19 ℃ according to the measurement. Erythritol can be stable at temperature below 180 ℃, and decomposes completely at temperature above 250 ℃. The thermal conductivity of solid erythritol decreases linearly with an increasing temperature, but that of the liquid erythritol increases linearly with an increasing temperature. The viscosity of erythritol decreases with an increasing temperature.Secondly, an experimental setup is built. Air is used as the heat transfer fluid (HTF) and erythritol is used as the storage media. Charging and discharging experiments of different working conditions have been conducted to study the melting and solidification behavior and the heat transfer mechanism of erythritol in a shell-and-tube latent heat storage unit. In addition, the effects of operation parameters such as inlet temperature, pressure and mass flow rate on the heat transfer characteristics have been studied. The results are as follows:(1) During melting, the liquid erythritol first occupies the top region of the unit and then spreads from top to bottom. The heat transfer in erythritol is dominated by condction at the beginning and then replaced by natural convection. (2) During solidification, obvious subcooling happens, and the heat transfer in erythritol is initially goverened by natural convection and then replaced by condction. (3) Increasing the inlet temperature and the mass flow rate of air during charging can enhance the heat transfer rate and shorten the charging time, and increasing the pressure of air shows little effect on the heat transfer rate and the heat transfer coefficient. (4) Incrasing the mass flow rate of air during diacharging can enhance the heat transfer. The change of mass flow rate of air has little effect on the subcooling degree duo to the cold finger effect.Thirdly, based on the enthalpy method to simulate phase change process, using effective thermal conductivity to consider the natural convetion in liquid PCM during melting, a numerical simulation method of double pipe latent heat storage is developed. Previous models could not simulate the process that liquid PCM first occupies the top region and then spreads from top to bottom. Consequently, a new model is proposed in the study. Appropriate coefficients of the model are obtained through calculation, and the numerical simulation shows good agreement with the experimental data.Based on the simulation method, a numerical investigation on a cascaded 3PCM latent heat storage unit filled with three types PCM namely, erythritol, adonitol, and xylitol is carried out. The heat transfer of subparts of the cascaded 3PCM unit is studied. Moreover, the influence of HTF flow rate and inlet temperature on the system performance is studied by energy and exergy analysis methods.The conculsions are as follows:(1) Cascaded 3PCM unit shows higher heat transfer rate than 1PCM unit using erythritol during charging. (2) The charging and discharging process can be devided into three stages according phase change:solid sensible heat dominant period, latent heat dominant period, and liquid sensible heat dominant period. (3) Increasing the inlet temperature and flow rate during charging will speed the energy and exergy storage velocity. And increasing the flow rate and decreasing the inlet temperature during discharging will speed the energy and exergy release velocity. (4) Higher cyclic energy efficiency and exergy efficiency will be obtained at a shorter charging period and a longer discharging period. |
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