| Due to the features of lower energy flow density,intermittent solar radiation,periodicity and instability in solar radiation,thermal energy storage of the medium-temperature solar thermal system is one of the key technologies for stable utilization of solar thermal systems.As an important energy storage way,latent heat thermal energy storage has obtained a great progress in phase transition mechanism of phase change materials,materials preparation and selection,determination of thermal physical properties and performance parameters,the solution of heat transfer problems during phase transition,performance enhancement,and design and optimization of heat exchange devices.However,more attention is paid to heat transfer enhancement of heat storage and heat release,including increasing the thermal conductivity of the material itself,or increasing the heat transfer area of the device,and ignoring the relationship between the energy adjustment of the thermal heat storage unit and the energy transfer of the solar collection unit.Based on combined consideration of solar collection and thermal heat storage units,this paper conducts the study on the mechanism of the influence of the coupling properties of the phase change materials on the performance of the heat storage/release process,and the correction calculation model of the heat transfer characteristics of the solar heat utilization system under the typical meteorological conditions.Under specific operating modes,the synergy mechanism between the energy efficient utilization and energy accommodation characteristics in the solar energy system are studied.The research lays a theoretical foundation for the efficient use of medium-temperature solar heating systems and provids technical support for solving the large-scale application of medium-temperature solar industrial process heating system.The main research contents and relevant results are as follows:(1)A heat transfer thermal resistance network theoretical model is established for the core component absorber in the solar collector unit and then verified by experimental heat loss when heated heat transfer fluid flow throught the absorber.Based on it,the sensitivity analysis of the heat loss characteristics is performed,and it is found that the calculated trend is consistent with the experimental results and calculation model has high accuracy under the condition of large wind velocity.To achieve a high agreement between the theoretical model and the actual situation,the heat sink dissipation term,which is linear with the temperature of heat transfer fluid,and the heat loss correction term expressed by the power function of wind velocity,should be added in the heat balance equation.Based on the theoretical model,the effects of solar radiation,flow rate of heat transfer fluid,environmental temperature and wind velocity on the heating rate and heat collecting efficiency were compared.The solar radiation has the greatest influence on the temperature change of heat transfer fluid,followed by the ambient wind velocity.Both of them have equal weight on the heat collecting efficiency.(2)As for a phase change thermal storage unit with good heat transfer effect and several available medium temperature phase change materials,the influence of thermal conductivity on the characteristics of heat storage and heat release was explored,taking into account other multiple physical parameters impact.Additionally,the actual heat storage charateristics under different solar collection conditions are studied to find the influence of different thermal boundary categories on the phase change heat transfer process.Therefore,an analytical method for the combined effect of multi-physical properties of phase change materials on heat storage/heat release properties was proposed and the combined effect of phase transition temperature and thermal conductivity on the overall performance was obtained.This work breaks through the limitations of single enhancing thermal conductivity.And the phase transition temperature has a greater impact on short-term heat storage and heat release.In the longer evaluation period,the thermal conductivity has a greater impact on overall performance.(3)Since the heat transfer analytical solution model of phase change thermal storage subsystem is difficult to be directly known,empirical formulas for calculating and calibrating the PCM temperature and mass fraction of liquid phase under two types of weather conditions were established based on numerical simulated and experimental data driving.Therefore,this paper obtained the empirical heat transfer model of solar thermal energy coupled phase change thermal storage under sunny and cloudy weather conditions.And then a complete heat transfer model for heat collection and heat storage based on the typical weather was proposed,which provides a theoretical basis for the performance analysis of solar phase change heat storage.In addition,for different weather conditions,different operating strategies and modes need to be adopted to ensure the best performance of the heat collction/storage coupling system to achieve dynamic parameter matching.At the same time,the matching regulatory analysis methods and deployment directions of operating thermal storage devices under typical climatic conditions were proposed.The equivalence heat utilization process can be regarded as the fluctuation of radiation intensity,just like cloud cover.To obtain better energy storage effect,there are critical moments or critical temperatures for switching the operation mode of the thermal storage system under different weather conditions.(4)The heat transfer performance of the thermal storage system is not only related to the material properties and thermal boundary properties but also directly affected by the structure of the phase change thermal storage device.Therefore,the optimization direction of the internal phase change heat storage structure is guided by analyzing the typical heat conduction and natural convection heat transfer mechanism.This paper proposed the structure pattern of dispersal heat source and a heat storage optimized structure with a fin-type multi-tube bundle based on a dispersive heat source to optimize the performance.The numerical simulation method is used to verify the effectiveness of the design.The results show that the dispersal heat source distribution method is basically consistent with the original structure in heat transfer efficiency,but it has a higher uniformity of the temperature field,and the fin-type multi-tube bundle based on a dispersive heat source has the much larger effect than the original structure's.The percentage of the N3-D13.5-3C70 in the region greater than 223°C at 3 hours is 94.99%,while that of the original structure is 56.56%. |
PDF Full Text Request