A portable high-efficiency electromagnetic energy harvesting system using supercapacitors for renewable energy applications in railroads
Graphical abstract
In this study, we develop a portable high-efficiency electromagnetic energy harvesting system with supercapacitors that converts the energy of track vibrations into electricity. The generated electricity is stored in the supercapacitors and used in remote areas for safety facilities or in standby power supplies for rail-side equipment. The proposed system consists of a mechanical transmission and a rectifier. Acting as the energy input and transmission, Gears and a rack amplify the small vibrations of the track, and one-way bearings enhance efficiency by transforming bidirectional motion to unidirectional rotation. Supercapacitors are used in the energy harvesting system for the first time. The supercapacitors permit the storage of energy from rapidly changing transient currents and a steady power supply for external loads. The proposed system is demonstrated through dynamic simulations, which show the rapid response of the system. An efficiency of 55.5% is demonstrated in bench tests, verifying that the proposed electromagnetic energy harvesting system is effective and practical in renewable energy applications for railroads.
Introduction
Rail transportation plays an influential role in the economy and everyday life in China. The railroad infrastructure has been improved dramatically over the last two decades, mainly because of large investments in construction by the Chinese government. The total length of the rail lines in China is greater than 100,000 km [1]. Rail transportation has been improved in many aspects, particularly the total length of railway lines, the electrification rate and the operating speed, which requires greater safety. To ensure the safety of the railway system, it is essential that rail-side electrical equipment such as signal lights, wireless communications, monitoring devices and positive train control are well maintained and operating correctly. Unfortunately, a considerable proportion of the railroad tracks lie in remote areas, where electric power is in short supply [2]. In these regions, equipment such as warning signal lights and wireless sensors for track monitoring often cannot be installed because of the lack of a reliable power supply or low-maintenance batteries. Therefore, it is important to develop a portable, highly efficient and low-cost power source for rail-side equipment in remote areas. Energy can be harvested from the vertical movements of railway tracks caused by transient loads from the train wheels when a train passes over the track. These movements are a potential renewable energy source that can completely or at least partly replace batteries as the source of power in trackside equipment.
Harvesting ambient energy from the environment by transforming mechanical energy into effective electrical power is a desirable solution for remote areas. In railroad applications such as safety facilities or standby power supplies, energy harvesting from the tracks would be particularly useful in remote areas. Existing vibration energy harvesting systems based on the rail tracks can be classified into two categories: piezoelectric and electromagnetic.
Practical and reliable piezoelectric designs have been developed [3], [4], [5], [6], [7], [8]. However, because of their low power density and average power (milliwatts), piezoelectric energy harvesting systems are suitable mainly for microelectronics and micro-electromechanical systems (MEMS) [9]. For example, Hu et al. presented a nano-generator constructed of piezoelectric materials for tire pressure monitors in which the maximum output power density approached 120 μW/cm3 [10]. Guan et al. investigated a piezoelectric energy harvester using a rotating system in which a piezoelectric component was repeatedly compressed by rotors and produced an output power of 83.5–825 μW [11]. The main problem with piezoelectric systems is that the power output of piezoelectric materials is very low; thus, they are restricted to small applications and are not suitable for railway safety facilities.
An alternative to piezoelectric energy harvesting is electromagnetic harvesting [12], [13], [14], [15], [16], [17]. These types of systems have been designed to tap inconspicuous motions in the environment such as road surface vibrations and building vibrations, transforming them into relative motion between permanent magnets and coils. For example, Zuo et al. developed a regenerative vehicle shock absorber with an electromagnetic energy harvesting device consisting of gears, racks and a rectifier to replace conventional shock absorbers. These shock absorbers convert the vibrations of vehicles into energy, delivering a peak power of 68 W [18]. Cassidy et al. developed a transducer for energy harvesting in large-scale structures, utilizing ball screws to convert mechanical energy into electricity. This transducer was coupled to a base-excited, tuned mass-damper system and achieved a power level of 100 W [19]. Lu et al. developed an energy harvester based on a single-degree-of-freedom transmission for use on bridges to scavenge vibrational energy [20]. Zhang et al. developed an energy harvesting system for use in speed bumps, using electromagnetic linear alternators to store the energy of friction and compression between vehicles and the road surface [21]. The device achieved a relatively high output voltage of 194 V, indicating the potential for higher outputs. Electromagnetic energy harvesters appear to have favorable characteristics for applications in ancillary railroad equipment.
Several energy harvesting systems have been used in railroad safety devices. Yuan et al. developed a drum transducer with piezoelectric materials to be placed underneath railroad tracks that produced a peak voltage of 70 V [22]. Pourghodrat et al. designed an energy harvesting system for railroads by converting the kinetic energy of the track into electricity to provide an alternative power source for remote areas [23]. Hansen conducted field tests of an electromagnetic energy harvester for railroads that performed both energy harvesting and track safety monitoring [24]. Nagode proposed two types of energy harvesting devices for railroad applications using linear generators and rotary generators to produce up to 54 W from motions of 0.75 in. at 1 Hz [25]. These systems for railroad energy harvesting can supply the necessary amount of power but are cumbersome; thus, there is a trade-off between output and usability.
To increase the efficiency of energy harvesting systems and to effectively use the electricity produced, several studies have focused on energy management [26], [27], [28]. Hendijianizadeh proposed the efficiency of linear and rotational electromagnetic energy harvesting systems, showing that a rotational system delivers more power than a linear system [29]. Phillips developed a control system for an electromagnetic railroad energy harvester to regulate power and adjust to the electric loads [30]. These results suggest that energy harvesting systems for railroad applications have been developed to a considerable level but are nevertheless inadequate in many respects such as portability, speed and durability.
Although the existing systems have been intended for railroad safety facilities, certain aspects of railroad energy harvesting have not been addressed. Two main challenges remain with this technology: (i) capturing and rectifying the highly variable vibrational energy efficiently and responsively and (ii) increasing the output power so that the system is suitable for trackside equipment that requires high power.
In this study, a novel, portable high-efficiency energy harvesting system with supercapacitors for railroad applications is investigated. A mechanism for converting bidirectional motion to unidirectional rotation enables this system to efficiently perform high power energy harvesting for railroad safety devices. Supercapacitors are included to rapidly capture and rectify the input energy, which is important for energy harvesting. In addition, this design considers portability and ease of installation, which are important for railroad emergency power supplies.
The remainder of this paper is organized as follows. In Section 2, the system architecture is described. The design of the system is described in Section 3, including the mechanical components and the electric circuit, and the operating principle of the system is illustrated. A mathematical model of the system is derived in Section 4. In Section 5, the setup for bench tests performed to validate the system is presented. The test results and a discussion are provided in Section 6. Finally, some conclusions are drawn in Section 7.
Section snippets
Architecture
The goal of this study is to develop an electromagnetic energy harvesting system for use in railroad systems that is highly efficient, portable, reliable and simple. A mechanical transmission consisting of gears and a rack was designed to convert the energy of rail vibrations into electricity. The general architecture of our portable high-efficiency electromagnetic energy harvesting system, shown in Fig. 1, consists mainly of the mechanical transmission and the electrical regulator.
When a train
Mechanical design
Vibrations in the rail produce a small amount of motion. Therefore, a gear set is included to magnify these small but strong vibrations. The rack is connected to the rail by a fixture and transmits the vibration to a pinion gear. Torque is applied to the generator by the gear set, causing the generator to rotate at a high speed to generate power.
The input assembly includes a fixture placed under the track, a vertical rack that meshes with a small pinion gear, a gear set with a high ratio and a
Modeling and analysis of the system
The rails of the track undergo elastic deformation when a train passes. The vertical deflection depends on the weight and the speed of the train and the track properties and conditions. A heavily loaded, fast-moving freight train causes the maximum amount of vibration. Assuming that each train wheel bears an equal amount of the weight, the average track vibration is between 7 mm and 12 mm and the maximum displacement is 25 mm. Based on the length of a train and the wheel arrangements, the
Experimental setup
A full-size prototype of the energy harvesting system was built. Bench tests were conducted using a servo-hydraulic Mechanical Testing and Sensing (MTS) system. The response of the energy harvesting system was tested, and the input force and displacement were recorded by the MTS system. The output electromotive force (EMF) was recorded by a data acquisition (DAQ) card and LabView software shown in Fig. 7.
The MTS testing system was used to move the rack up and down and drive the transmission, as
Results and discussion
Experiments were conducted with the harvester using inputs with an amplitude of 6 mm and frequencies of 1 Hz and 2 Hz. The results for the EMF, the displacement and the force are shown in Fig. 9. The input mechanical power and the output electrical power and the efficiency were calculated.
The experimental results for the input force were consistent with those of the simulations, as can be observed in Fig. 9a and b. The peak force in the experiments was higher than that in the simulations because
Conclusion
This study investigated a portable high-efficiency electromagnetic energy harvesting system with supercapacitors that converts the energy of track vibrations into electricity for use with safety devices, emergency repairs or temporary power supplies in remote areas. The proposed system consisted of a mechanical transmission and a rectifier. A rack and gear set amplified the small vibrations in the rails, and one-way bearings improved the efficiency by transforming bidirectional motion to
Acknowledgments
This work was supported by the Science and Technology Projects of Sichuan under 2015RZ0017, 2016GZ0026 and 2013GZX0138. The asterisk indicates the corresponding author, and the first three authors contributed equally to this work.
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