Researchers in China have developed a novel free-piston linear generator (FPLG) to recover exhaust waste heat efficiently from a vehicle engine. The FPLG can be used in a small-scale organic Rankine cycle (ORC) system and can directly convert the thermodynamic energy of working fluid into electricity.
In a paper published in the journal Applied Thermal Engineering, the team from Beijing University of Technology, Collaborative Innovation Center of Electric Vehicles in Beijing, and Datong North Tianli Turbocharging Technology Co., Ltd. reports that the energy conversion efficiency of the FPLG can reach up to 45.82% with an intake pressure of 2.6 bar.
… the internal combustion engine is the power system of most vehicles. Therefore, it is necessary to recover exhaust energy of vehicle engine, and many strategies and methods have been adopted to reduce the energy waste and improve the fuel utilization efficiency.
In recent years, more and more researches focus on the Organic Rankine Cycle (ORC) system, which is an effective technical solution and a promising means for industrialization of energy savings among all the ways to recover waste heat. As a key component of the ORC system, the performance of the expander has a significant impact on the efficiency of the ORC system.
… In this study, a novel free piston expander coupling with a linear generator integrated unit, which can be used in a small-scale ORC system to recover exhaust waste heat from vehicle engine, is presented.—Tian et al.
The FPLG in the study consists of a dual-opposed piston type free piston expander with a linear generator placed in the middle of two cylinders and a variety of auxiliary components. The piston is connected with linear generator mover by a connecting rod. The piston can move freely between its top dead center and bottom dead center. As the piston is not restricted by the crankshaft, its motion is influenced by in-cylinder gas pressure, electromagnetic force and mechanical friction. The researchers used a compressed air test rig for the experiments.
|Prototype FPLG. Click to enlarge.|
The two cylinders are alternately in the intake-expansion stroke and exhaust stroke. The compressed air flows alternately into each cylinder to drive the piston assembly back and forth, and the linear generator converts parts of the kinetic energy of the piston assembly into electricity.
Because the piston is not restricted by the crankshaft, top dead center and bottom dead center may be varied during the working process.
Among their findings:
The piston displacement profile is similar to a sinusoidal wave; the piston amplitude increases significantly with the increase of the intake pressure.
An increase in the operation frequency brings about a decrease in both piston amplitude and motion symmetry.
The peak piston velocity and power output show a nearly linear relation with the intake pressure, indicating that the power output is sensitive to the piston velocity. When the operation frequency is 1.0Hz and the external load resistance is 20Ω, the maximum peak velocity is about 0.69m/s and the highest power output 96W is obtained.
The conversion efficiency of the FPLG increases with the increase of the intake pressure.
When the operation frequency is 2.0Hz, energy conversion efficiency of the FPLG can reach up to 45.82% with an intake pressure of 2.6 bar.
Having determined the potential energy conversion efficiency of the system, the researchers will next investigate the effects of higher pressure and operation frequency on the FPLG and test FPLG in a small-scale ORC system.
Yaming Tian, Hongguang Zhang, Gaosheng Li, Xiaochen Hou, Fei Yu, Fubin Yang, Yuxin Yang, Yi Liu (2017) “Experimental study on free piston linear generator (FPLG) used for waste heat recovery of vehicle engine,” Applied Thermal Engineering, Volume 127, Pages 184-193 doi: 10.1016/j.applthermaleng.2017.08.031