Study And Development Of CO₂ Transcritical Expander

This study principally probes into the key technology to improve the efficiency of the CO2 transcritical cycle system from the viewpoint of reducing throttling loss and recovering the expansion power of the system. The purpose of the study on expansion process of the CO2 transcritical cycle and the development of CO2 expander is to make CO2 transcritical cycle system has the advantage of efficiency besides environmentally beneficial and to provide both academic instruction and practical experience in order to put CO2 transcritical cycle system into practice. The major accomplishment are summarized as follows: 1. Theoretical analysis of expansion process and the study on expansion mechanismAccording to non-equilibrium thermodynamics and phase change theory, the three possible phase change expansion processes are analyzed and the optimum process, CO2 from supercritical region goes through sub-cooled region to vapor and liquid two phase region, is chosen as the gist of expander design. In addition, in the light of the phase change mechanism analysis, the characteristics of the design expansion process are obtained. Furthermore, the source of expansion power is discussed both in the subcritical region and in two-phase region, which will provide academic instruction for the expander design.2. The theoretical analysis and design of CO2 rolling piston expanderAccording to the comparison and analysis of different types of expander, CO2 rolling piston expander is chosen as the development object. The working process of rolling piston expander is analyzed thermodynamically and the main construction parameters of the expander are determined. The inlet valve open time, the mass flow rate, the changing regulation of the physical parameters of CO2 fluid in the cylinder and the power characteristics of the expander are calculated. More over, the expander inlet valve control methods are discussed. As a result, the mechanical control method is chosen to be adapted in the expander prototype. The control methods of the CO2 transcritical cycle system with expander at changing conditions are also discussed. On the basis of the movement and force calculations of the slide, the rolling piston and the shaft, the leakage losses, the friction losses and flow resistance losses areevaluated and the improving measures are put forward. 3. The experimental study of CO2 rolling piston expanderA CO2 expander test rig is set up for the experimental study. The experimental result validates the feasibility of the CO2 rolling piston expander prototype. The practical expander operation efficiency and performance are tested. The performance changing regulation at different mass flow rate and different revolution are illustrated. In addition, the feasibility of control method design at changing conditions is tested and the result illustrates that the CO2 rolling piston expander is suitable working at relatively stable conditions. According to the comparison results of throttling valve cycle and the expander cycle, the expander effect on the efficiency is demonstrated.