| With an annual energy consumption increase by an average value of 1.7% and a projected consumption of 815 quadrillion Btu in 2040,improving energy efficiency and decreasing fossil energy consumption has become an important issue around the world.In general,the effective efficiency of the internal combustion engine(ICE)is only about 20%-45%,and rest of the energy is lost in various forms.Therefore,recovering the waste heat from ICE is an effective method to improve the ICE efficiency.Among all kinds of waste heat recovery technology,organic Rankine cycle(ORC)is a positive energy conversion technology because of its simple structure,low friction loss and great reliability.Therefore,the free piston expander is of great significance to improve the performance of the internal combustion engine ORC waste heat recovery system.The paper aims to improve the thermal efficiency and reduce emissions of internal combustion engines.Firstly,in this study,an air driven free piston expander-linear generator(FPE-LG)test rig and the FPE-LG simulation model based on GTSuite/Simulink have been established.Since the simulation results are in good agreement with the experiment results,this proves the rationality of the simulation model.Afterwards,the performance of FPE-LG under different conditions and the influence of intake temperature,intake pressure and external load resistance on FPELG are conducted.When intake pressure and temperature rise,the in-cylinder volume increases.Furthermore,the displacement,velocity and acceleration increase with intake temperature.The peak voltage output of FPE-LG linearly increases when increasing the intake temperature and pressure.The peak voltage output is more sensitive to intake pressure than temperature.The higher the intake pressure is,the more obvious the intake temperature effects the peak voltage output.The maximum voltage output can reach up to 23.6 V.The peak power output demonstrates an initial increasing and then reducing trend with the external load resistance.And the maximum peak power output can reach up to 22.23 W when the external load resistance is 9 Ω and intake temperature is 373 K.Based on the FPE-LG simulation model established by GT-Suite/Simulink software,the sensitivity analysis of key structural parameters of the system is performed.The net power output of the combined system is used as the optimization target.Then three key structural parameters are optimized by using genetic algorithm.The research shows that the cylinder diameter has the highest main effect value for the FPE-LG system,followed by the cylinder length,and finally clearance volume.The latter two main effect values are equivalent and the impact on the peak output power of the system is relatively small.When the operating frequency is 2 Hz,the peak output power of the system is the largest,and its value is 25.57 W.When the operating frequency is 4 Hz,the system has the maximum net peak power increase,with a value of 1.62 W.Therefore,after optimizing the key structural parameters of the system,the system performance can be significantly improved.In order to investigate the impact of FPE-LG cylinder length,cylinder diameter,operating frequency and external load resistance on the performance of the ORC system,a simulation model of the ORC system with FPE-LG as the expander is established.The results show that under the conditions of a cylinder length of 600 mm and a cylinder diameter of 80 mm,when the operating frequency is increased from 10 Hz to 22 Hz,the piston displacement and velocity show a trend of increasing first and then decreasing.When the operating frequency is 18 Hz,the piston velocity and electromagnetic force reach the maximum values of 36.78 m/s and 1311.92 N,respectively,and the trends of the electromagnetic force and velocity are consistent.When the operating frequency is 18 Hz,the system thermal efficiency and waste heat recovery efficiency have reached the maximum,with values of 3.81% and 3.48%,respectively. |
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