| The energy consumption per unit of GDP of China is 2.2 times over international average level, which indicates our negative energy situation. Energy conservation and emission reduction is the principal theme nowadays, and there are great amounts of prevalent waste heat and excess pressure in industries as well as renewable resources such as geothermy and solar energy, which attract unparalleled attention and researches on power generation with medium and low temperature heat source. The merits of Screw Expander are its high internal efficiency, good reliability, self-cleaning ability and being friendly to the environment and so on, which make it the ideal power machine for recovery and power generation of medium and low temperature heat source. It is an important content of Screw Expander to study and analyze its performance on off-design condition and rotate speed control methods, being of great significance to raise the efficiency of waste heat recovery. In order to provide theoretical guidance and supply for practical application, this article discusses models of electric and electro-hydraulic control system as well as Screw Expander, afterwards conducts the influence analysis on variation of control system parameters.Firstly, mathematical models of assembly units of electric control system are established, such as stepping motor and transmission mechanism. So are assembly units of electro-hydraulic control system, such as proportional directional valve and hydraulic servo-motor. Then simulation models of the two control system are conducted and simplified for convenient usage in generating set simulation. Meanwhile, performances of the regulating systems are compared and the result shows that electro-hydraulic control system is obvious superior in rapidity and accuracy than electric system.Secondly, simplified models for thermodynamic calculation and indicator diagrams are established based on the operating feature of Screw Expander, and calculating method on variable conditions is presented, in order to conduct case calculation. A Screw Expander system based on organic Rankine cycle(ORC) using R123 as working medium is applied to power generation with associated hot fluid during oil recovery in Huabei Oil Field. After screening main factors, systematic characteristics under variable conditions are attained, by solving new steady state using iteration, analyzing net power, thermal efficiency and energy destructions. The result shows that superheated vapor leads to the decline of output network, although it benefits the output power per unit working medium. The temperature of inlet cooling water affects systematic performance most and the higher temperature in hot summer results in great deviation of systematic output from design condition. Ensuring the cooling capacity is an effective measure to improve the performance. The influence caused by the change of heat source temperature is larger than that caused by mass flow. The conclusion is almost the same in the case of total flow system.Thirdly, with the models of control system and Screw Expander mentioned above, simulation platform is built in Matlab/simulink. And then the impact of electric and electro-hydraulic system on the property of isolated power system is analyzed when load changes suddenly. Furthermore, how different discharge characteristic and response speed of the valve affect the dynamic response curve is also conducted. It comes to the conclusion that it is much better to use electro-hydraulic system to control the generating unit. The positive correlation between the valve respond speed and dynamic respond performance of the generation set is also found. Comparatively, equal percentage trim would be a better choice.Finally, how the disturbance of the heat source affects the performance of the generating set is studied. It turns out that the change of heat source causes max output variation of Screw Expander and the simulation model changes consequently. But the electro-hydraulic system can handle with all these and ensure the quality of auto-control. A generating unit with larger rated power would also decrease the difficulty of auto-control. |
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