1 Magnetic Field Energy Harvesting in Railway Asbjørn Engmark Espe , Member, IEEE, Thomas S. Haugan, and Geir Mathisen, Senior Member, IEEE Abstract—Magnetic field energy harvesting (MFEH) is a points, and tunnels [6]. Due to their low cost, low energy method by which a system can harness an ambient, alternating consumption, flexibility, and rapid deployment, wireless sensor magnetic field in order to scavenge energy. Presented in this paper networks (WSNs) have become an attractive solution for real- is a prototype energy harvesting solution aimed at the magnetic fields circulating the rail current in electrified railway. Due to time condition monitoring [7]. its non-invasive nature, the solution may be deployed as part While a wireless sensor node typically requires tens to of a low-cost trackside condition monitoring system, in order hundreds of microwatts in active operation, and several hun- to increase lifetime and reduce maintenance requirements. Two dred milliwatts during wireless transmission, the node may different configurations are assessed in a controlled laboratory substantially decrease average power usage by spending most environment, and it is demonstrated that at a distance of 25 cm from a typical rail current of 120 A, the system can of its time in ultra-low power modes [8]. Traditionally powered 2 harvest 340 µW and 912 µW at 16 ⁄3 Hz and 50 Hz, respectively. by batteries, wireless sensor nodes are naturally limited by A prototype system tested in situ at two points along Norwegian the capacity of their energy reserves. To ensure reliability railway validates the performance in real-world conditions. In the and continuous operation, an additional maintenance step is field, the system is able to harvest 109 mJ from a single freight in many cases stipulated in the form of periodic battery train, rendering an estimated daily energy output of 1.14 J. It is argued that the approach could indeed eliminate the need replacements or charging—which is generally undesirable for for battery replacements, and potentially increase the lifetime of end users [9]. As an alternative to battery replacements, energy an energy-efficient, battery-powered condition monitoring system harvesting is established as a viable solution, provided that the indefinitely. amount of energy that can be harvested is beyond the demand Index Terms—Energy harvesting, magnetic fields, rail trans- of the system [10]. portation maintenance, monitoring, intelligent sensors. Many conventional energy harvesting sources such as solar energy, wind energy, and vibration energy have been employed successfully for railway condition monitoring systems [11]. I. INTRODUCTION Interestingly, techniques such as magnetic and electric field energy harvesting (MFEH and EFEH) appear to not yet have substantial number of both commercial and private been prototyped in a railway context, even though the contact actors routinely rely on the rail networks for the timely A line system of electrified railway closely resembles overhead and secure transportation of goods and passengers as part power lines in many aspects, and several implementations of of their daily endeavours. In 2016, over 440 billion tonne- both MFEH and EFEH for the latter can be found in the kilometres and 470 billion passenger-kilometres were recorded literature [12]. across Europe [1]. Accordingly, the safe and continuous op- Nonetheless, previous work has determined that MFEH may eration of relevant infrastructure are of cardinal significance be a feasible solution in electrified railway [13]. Shown in in our modern society, and hence central priorities for railway Figure 1 is one variant of the concept in which the current administrations. In Norway alone, there are more than 2600 through the rails gives rise to a circulating magnetic field. In bridges and 700 tunnels that must be kept available and AC systems, the time-varying nature of this current allows it properly maintained at all times [2]. And as the frequency to be a source for energy harvesting if an electromagnetic coil of extreme weather appears to increase in response to climate is placed in its vicinity. change, a considerable amount of resources is expended to arXiv:2012.04965v1 [eess.SY] 9 Dec 2020 support traditional periodic maintenance schemes. Enabled by advances in smart maintenance technologies, condition-based maintenance schemes have been introduced in a diverse range of areas as a more resource-conservative approach [3]. In the last few years, railway authorities have × + × × embodied this paradigm shift and taken an increasing interest ir × Φ E × in smart maintenance and its enabling technologies [4], [5]. As − a central element of smart maintenance in railway, trackside condition monitoring systems may be installed to monitor the structural integrity of infrastructure such as bridges, tracks, A. E. Espe and G. Mathisen are with the Department of Engineering Cybernetics, Norwegian University of Science and Technology, Trondheim, Fig. 1: An alternating current ir in the rails will give rise Norway (e-mail: [email protected]; [email protected]) to a magnetic field from which energy can be harvested. By T. S. Haugan is with the Department of Electric Power Engineering, placing a wire coil nearby, the varying flux Φ through the wire Norwegian University of Science and Technology, Trondheim, Norway (e- mail: [email protected]) loops will induce an electromotive force E. This work has been submitted to the IEEE for possible publication. Copyright may be transferred without notice, after which this version may no longer be accessible. 3 km While MFEH has not previously been prototyped in railway, BT CL BT a preliminary feasibility study was conducted as part of this is project. In this study, [13], the theoretical model of such a SS i Rails r system was derived and simulated, and it was shown that the induced potential in a solid-core wire coil can be modelled ig in phasor form as (1), and the power PL dissipated in an impedance-matched load as (2). Fig. 2: System B—the most common electrification configura- tion in Norway. Power is delivered from the nearest substation Nµ µ d r2 V = j! e 0 e I (1) (SS) to the locomotive through the contact line circuit. The 4 r current is through the contact line (CL) is complemented by 2 2 2πr2Nµ µ d fI currents through the rails (ir) and ground (ig). At regular jVj e 0 e r PL = = (2) intervals, booster transformers (BT) are employed to minimise 4R 64R the ground leakage currents. In these expressions, N, R, and r are parameters of the coil, and denote—respectively—the number of windings, the coil’s resistance, and the winding radius. µe represents the core’s While currently undemonstrated within railway, there are effective permeability, and lies in the region 1 ≤ µe ≤ µr. many examples documenting the use of MFEH for power grid Next, f = 2π! and Ir are the frequency and root-mean-square monitoring systems. In particular, the most relevant solutions (RMS) magnitude of the rail current that gives rise to the are free-standing ferrite-core energy harvesting coils to be magnetic field. Lastly, the coil’s distance to the rail current is placed in the proximity of overhead power lines, such as the given by the parameter de. Since there are two current-carrying works by Yuan et al. [14], [15]. Both works being part of the rails, this parameter is a measure of the effective distance, same project, the former presents a ferrite core in the shape defined as of a bow-tie, reporting a power output of 360 µW from a 7 µT 1 1 de = + ; (3) magnetic field, while the latter improves this figure fourfold dn dn + drr introducing a more complex helical design. In [16], a small where dn is the distance to the nearest rail, and drr is the design physically attached to the power line is able to harvest rail-to-rail distance. more than 30 mW. At the expense of an even more involved It is desirable to maximise the power output from the coil. installation procedure, [17] reports a figure of 230 mW using a Since the energy harvesting coil represents a highly inductive flux guide that wraps around the power line itself, essentially circuit component, the load to which it is connected must be operating as a current transformer. capacitive in order to form a resonant circuit, and thereby It has been demonstrated that a ferrite core with a narrower maximise power transfer. As will be further detailed in the diameter at its centre than at its ends is beneficial in terms following sections, Figure 4 shows how the coil’s impedance— of efficiency [14], [18]. In fact, [18] reports that employing a being equivalent to an inductor in series with a resistor— funnel-shaped ferrite core to help guide the magnetic flux may may be matched with a load that has an equal resistance and improve the power density by an order of magnitude compared compensating capacitance. to a similarly-sized rectangular coil. III. HARVESTER COIL DESIGN II. BACKGROUND In this project, two harvester coil designs are tested. Pictured A railway electrification system may employ either direct in Figure 3, their design employs a dumbbell shape inspired by current (DC) or alternating current (AC) for power distribution. both the bow-tie shape introduced in [14] and the funnel core As of 2018, 63 % of the world’s electrified railway uses in [18]. The effective permeability of a given core depends AC [19]. The most common voltage system is 25 kV, 50 Hz, on its geometry [21], and, compared to a constant-diameter while a handful of nations in Central Europe and Scandinavia core, a core with a narrower diameter at the centre has been 2 1 employ 15 kV, 16 ⁄3 Hz .