Study On Ejectors And A Solar Ejector-refrigeration System With Dual Energy-storage Tanks And Multiple Ejectors

Energy shortage and environmental degradation have become a major problem which restricts the development of society and economy. Since solar energy is a renewable and clean energy, more and more researchers focuses on the air conditioning and refrigeration systems powered by solar energy. The ejector performance and performance prediction model were studied. Based on these works, a new type of solar ejector-refrigeration system with dual energy-storage tank and multi-ejector is proposed. The performance of the system was studied with the methods of theoretical analysis, experimental verification and performance simulation. The main research work includes the following aspects.1. An ejector model and method for the performance prediction at the whole mode were put forward. Based on the theory of constant pressure mixing, models of ejector performance prediction for dry-vapor and wet-vapor refrigerant at critical mode were developed. The accuracy of the dry-vapor ejector model was improved by introducing ejector component efficiencies into the model. In comparison with the experimental data from literature at critical point using R245 fa as work fluid, the errors of the predicting entrainment ratio and condensation pressure are 4.0% and-2.1%, respectively. The model of sound velocity in two-phase flow was introduced in the wet-vapor ejector model to calculate the velocity at throttle. Hence, the prediction accuracy of the model was improved. In comparison with the experimental data at critical point using R134 a as work fluid from this thesis, the errors of the predicting entrainment ratio and condensation pressure are 7.05% and 7.13%, respectively. The concept of ultimate points was introduced to put forward the method for the ejector performance prediction at subcritical mode. An ejector model for the performance prediction at the whole mode were developed by combining the critical model and regression model of ultimate points based on the methods liner performance of the ejector at subcritical mode.2. The bottleneck problem of determining ejector component efficiencies is solved by a set of methods, i.e. the method based on the effect of the change(EOC) of efficiencies and the sparsity-enhanced optimization(SEO) method. EOC method is used to find out which efficiency need to optimize, SEO method is used to determine the efficiency. The ejector model for dry-vapor refrigerant presented in this thesis is taken as an example to illustrate the methods. The maximum prediction error of entrainment ratio is-7.66% by using the methods, and the errors of predicting condensation pressure is in 5%. The maximum error can be reduced by 15% for the model using fixed efficiencies in literature.3. An ejector cooling system using R134 a as refrigerant was built for experiment investigation. High and low level reservoirs were set up in the system and the branch to evaporator is set before the reservoir. Hence, more stable states are obtained. Experiment data obtained by experiment using R134 a as refrigerant were more than those in literature and the range of generation temperature and evaporation temperature is larger than those in literature. The experiment results are derived at 24 parameter points and 12 parameter points for the critical point and limit point, respectively. The range of generation temperature and evaporation temperature are 75~85 ℃ and 5~15 ℃, respectively.4. The ejector performance was analyzed based on the experiment results. Analyses on the performance of ejector, which works at critical and sub-critical mode alternately, were carried out. The variation characteristics of ejector performance with condensation temperature, evaporation temperature, generation temperature and area ratio are studied. The COP of the system can be increased 52.8% when generation temperature decreased from 85 to 75℃ at the conditions that area ratio is 3.96 and evaporation temperature is 10℃. However, the critical condensation temperature is reduced by 6℃. The COP of the system is improved by 67.6% when the area ratio of ejector is enlarged from 3.32 to 3.96 at the condition that generation temperature is 75℃ and evaporation temperature is 10℃. But the critical condensation temperature is decreased by 3℃. The conclusion is that the condensation temperature is one of the main factors which restrict the ejector performance. The performance of ejector can be optimized by adjusting the generation temperature and area ratio according to the condensation temperature.5. The characteristics of building cooling load and cooling supply of an ejector refrigeration system were analyzed. Then, the solar ejector-refrigeration system with dual energy-storage tank and multiple ejectors is proposed in this thesis. The calculation models of generator, condenser, evaporator, ejector, solar collector, cooling tower and energy storage tank are developed. The optimum volume factor for heat storage tank is 60L/m2(collector area). The method for seeking the optimum switching temperature based on meteorological parameter of a typical day and for determining the number of ejectors and the throttle areas of ejectors. The principle for selecting the typical day was also presented. It is found that the performance of the solar ejector system with three ejectors, whose throttle areas are different, is optimum. The COP of the system is 0.45, whereas it is 0.35 for the system with three ejectors whose area are the same. The COP of system in June, July and August are 0.53, 0.42 and 0.45, respectively. The system can operate for 87 days when is used to supply cooling for a small office building from June to August.