Heat Transfer Behavior Of Supercritical CO₂in Solar Collector And Rankine Cycle

As a new and renewable energy, solar energy has many advantages, such as rich-reserved, clean, safety and so on. At present, many countries in the world have strengthened the research and development of utilization of the solar energy. As supercritical fluid has some unique chemical and physical properties, more and more attention has been paid to the supercritical fluid in recent years. Solar energy is combined with eco-friendly CO2in this paper, and the heat transfer behavior and law of supercritical CO2in the solar collector are researched by numerical simulation and experiment. A supercritical CO2solar energy system which is based on Rankine cycle theory is built, the cycle performance of the system is discussed. The model and experiment result will provide the design reference and theory basis for the industrial applications of solar energy Rankine cycle system in the future.The main research contents of this paper are as follows:(1) Heat transfer process of supercritical CO2in the serpentine hose collector is studied by simulation method, and the influence of mass flow, heat flux, pressure parameters on the heat transfer performance is investigated. The simulation results are compared with part of the heat transfer correlation. The results of the study show that, the increase of the mass flow rate and heat flux can improve the heat transfer performance of supercritical CO2effectively, while pressure does not have significant effect on the temperature difference between the inlet and outlet of the solar collector and the average heat transfer coefficient. The simulation results agree with the heat transfer correlation proposed by Gnielinski well.(2) Heat transfer process of supercritical CO2in the serpentine hose collector is studied by experimental method, and the influences of mass flow, solar radiation intensity and pressure on heat transfer performance is investigated. The results of the experiment and numerical simulation are contrasted and analyzed. The research results show that the mass flow, solar radiation intensity, and pressure have essential influence on the local heat transfer coefficient. The heat transfer performance of supercritical CO2in the collector can be increased by increasing the mass flux and radiation intensity, but the effect of pressure is lower. In the experimental and simulation research, the local heat transfer coefficient increases firstly in the pipe length direction of the collector, and then decreases, with the maximum around the pseudo-critical temperature. Under close working condition, the local heat transfer coefficient obtained in the experiment is obviously lower than the value of simulation.(3) A solar Rankine cycle system, using supercritical CO2as the working substance, is established to make the theoretical analysis for cycling. The experiment data for one specific day is chosen to analyze the operating characteristics of the cycle system. The result shows that the temperature of CO2at the outlet of the collector increases with the solar radiation intensity increasing. However, the change of inlet temperature of the collector is small. To the heat exchanger, the inlet temperature has more fluctuation relatively, but its outlet temperature is more stable. The pressure is stable before and after the expansion valve. When the mass flow rate keeps constant, the efficiency of the collector remains stable. The efficiency of the Rankine cycle system has the same changing tendency to the solar radiation intensity. The average cycle efficiency throughout the experimental period is about14%.