Performance Study Of Scroll Compressor/Expander Composite Machine In Compressed Air Energy Storage

Compressed air energy storage is an economic and effective storage technology for developing the intermittent and random wind power. However, the problems of large unit volume and low energy conversion efficiency exist in the traditional compressed air energy storage technology. In order to solve these problems, a new scheme is proposed in which a scroll compressor/expander composite machine is applied to a micro compressed air energy storage-wind power coupling system. In the system, the composite machine combing the function of compression and expansion is connected to the fan impeller and the generator by a mechanical coupling device. Making use of this system, the excessive energy can be converted as the form of compressed air when the wind is strong, and once the wind is weak or there is no wind, the compressed air is controlled to expand and release the saved energy which can be used as an auxiliary driving force for the generator. As the core equipment for energy storage and energy release, the composite machine is very important and its performance lays great impact on the energy utilization and conversion efficiency of the overall system. Therefore, the thesis mainly focuses on the design and the performance of the composite machine and carries out the following researches in detail.According to the functional requirements of energy storage and energy release, a new kind of scroll compressor/expander composite machine containing only a pair of orbiting and fixed scrolls was designed. It can achieve the compression or expansion operation by controlling the working mode switching device, in which a flap valve can rotate by a certain angle in order to seal or diverted from the discharging port at the base of the fixed scroll to meet the requirement of compression or expansion. Then, a mechanical coupling device based on differential planetary gear train and belt drive was designed. Making use of this device, the composite machine and other equipments can be connected to form the micro compressed air energy storage-wind power coupling system. Furthermore, the power decomposition in the case of compression or the power synthesis in the case of expansion can be achieved.A mathematical model concerning the expanding process of the scroll compressor/expander composite machine was established and solved by use of the software Matlab. The simulation results show that the working chamber volume is related to the rotational angle and the structural parameters of the scroll, and that the volume of expansion chamber has a linear relationship with time. Furthermore, the expanding process of the compressed air within the composite machine is not ideally adiabatic but polytropic due to intake loss, discharging loss and leakage. Finally, the output energy of the composite machine is influenced by the inlet pressure and the flow rate. Both the values of the shaft torque and the rotational speed depend on the pressure difference of the gas in the working chamber.The internal flow field of the composite machine was simulated by use of the dynamic grid technology. In addition, the flow field after the modification of the initial scroll profile was also researched and compared with the unmodified case. The gas flow can be described as unsteady, compressible Newton fluid flow. The simulation results show that the pressure decreases gradually along the rotational direction in the expansion chamber, and that the gas density decreases accordingly. It is also indicated that the magnitude of the velocity is high near the outer wall of the orbiting scroll and low near the internal wall of the fixed scroll. Furthermore, the gas flow rate through the intake port increases and the flow stability is improved after profile modification.In order to directly reflect the comprehensive efficiency of the composite machine during the energy conversion and utilization process the experiment, the pneumatic power is put forward to describe the effective energy within the compressed air, and then the energy conversion efficiency is introduced as an evaluation index. A lkW scroll compressor/expander composite machine experimental platform was set up. The experimental results show that the maximum energy conversion efficiency of the prototype under the working condition of compression and expansion mode are36.89%and41.56%, respectively. Combining the simulated and experimental results, it is confirmed that the modification of the initial scroll profile and the change of the size ratio of the discharging port to the charging port are two effective methods for improving the energy conversion efficiency. The experimental results show that the efficiency can reach46.61%after the modification of the profile, and that the ideal ratio range of the discharging port to the charging port is determined as0.6～0.8for the prototype under the experimental conditions.In conclusion, the structural design, thermodynamic and dynamic modeling, flow field simulation and energy conversion efficiency of the scroll compressor/expander composite machine are thoroughly studied in the thesis. The results can provide important theoretic foundation for the application of the scroll composite machine to the compressed air energy storage technology, and are of great significance to further study of compressed air energy storage-wind power coupling technology.