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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 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, , Fig. 1: An 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 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 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 = jω 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 |V| e 0 e r PL = = (2) intervals, (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 = 2πω 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 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 . 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 in series with a — 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 . 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 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 . For practical reasons, the in situ determined to be more efficient [14], [18]. This is substantiated system presented in this work targets Norwegian railway, by the expression (2), in which output power scales with the which employs the latter. effective permeability squared. While [14] argues that their A substantial part of electrified Norwegian railway employs bow-tie shape is slightly more efficient than a dumbbell shape the configuration known as System B [20], illustrated in at focussing the magnetic field, the latter represents a simpler Figure 2. Generally lacking a dedicated return conductor, the shape to construct, and will therefore presumably result in a configuration utilises the rails themselves as a return path for more economical solution. the current. New and modernised railway in Norway will now typically feature a dedicated return conductor, however the A. Magnetic characteristics return current is still carried a short distance through the rails to the nearest pole. Both cores feature steel disks at either end connected by a number of bundled cylindrical ferrite rods. The steel disks 1In 2000, the nominal frequency was changed to 16.7 Hz in Germany, have a thickness of 10 mm and a diameter of 50 mm, and Austria, and Switzerland. each ferrite rod a length of 150 mm and a diameter of 8 mm. ir Coil M Current (a) L R source s 1:200 A C (b)

Voltage- R R Current- sense L cs sense

Fig. 4: Equivalent circuit diagram of the laboratory setup. The rail current circuit (a) induces a voltage in the energy harvesting circuit (b), which has voltage- and current-sense ports so that the output power may be calculated in real-time by an oscilloscope.

represents a premium solution, while Coil B may serve as a more economical alternative.

(a) (b) IV. LABORATORY SETUP Fig. 3: Two coils have been constructed: (a) Coil A with In order to validate the design of the energy harvester, parameters L = 1000 H,Rs = 17.2 kΩ, and (b) Coil B with an experiment was conducted in a smart grid laboratory. L = 500 H,Rs = 9.2 kΩ. Both units have a total length of The controlled environment allowed the energy harvesting 170 mm. system to be tested near currents of configurable magnitudes and . As highlighted in the equivalent circuit in Figure 4, the experimental setup consisted of two galvanically Arranged in a hexagonal pattern, Coil A is comprised of seven isolated circuits: (a) a high-current circuit representing the ferrite rods while Coil B only employs three. rail, which is magnetically coupled to (b) the low-power The material of the ferrite rods is Ferroxcube’s 4B1, energy harvesting circuit. While a real-world scenario will which has a relative permeability of 250 and a resistivity of have two current-carrying rails separated by a distance drr, 1 × 105 Ω m [22]. This material was chosen for three reasons. the experimental setup simplified matters by only emulating a Firstly, its resistivity is very high, meaning that any losses singular rail. This simplification manifests itself as a rail-to- stemming from eddy currents will be substantially reduced. rail distance drr of 0 m in (3). The laboratory setup is depicted Secondly, its relative permeability is close to the point at which in Figure 5. the effective permeability becomes saturated. [14] estimates the point of to be around µr = 400 for a similar A. Rail current emulation core, and that any increase in the relative permeability beyond The rail was realised as a single, low-resistance cable which this will lead to diminishing returns. Thirdly, the material is carried a controllable AC current, and thereby gave rise to affordable and available in a diverse range of sizes. the magnetic field from which energy could be harvested. Steel disks are used at either end to further guide the Connected as a loop, the cable acted as the secondary winding magnetic flux lines towards the ferrite cores. While ferrite of a 1:200 current transformer (CT) circuit. The primary side would presumably be a more efficient material, steel was of the CT was driven by a programmable current source. chosen for the flux guides as a compromise; the required shape For fine control of the current magnitude and frequency, an was unavailable in ferrite material—or simply too expensive ITECH IT7600 series programmable AC power supply was to acquire, and would therefore be undesirable for an end- used. It is worth noting that the CT is not an ideal circuit user. However, due to the narrow thickness of the disks, weak element, which means that one cannot entirely rely on the magnetic fields, and low frequencies involved, it is expected 1:200 transformation ratio between its primary and secondary that eddy currents will not severely impact the magnetic currents. Hence, a Fluke 325 clamp meter was employed as a characteristics of the cores. feedback mechanism in order to ensure that the desired current magnitude was attained. B. Electrical characteristics Using enamelled copper wire with a diameter of 0.1 mm, B. Energy harvesting circuit 80 000 windings were applied to Coil A and 62 000 windings The energy harvesting circuit was comprised of the en- to Coil B. The number of windings resulted in Coil A having ergy harvesting coil in series connection with an impedance- an of 1000 H and a resistance of 17.2 kΩ, and Coil matching circuit. The coil was positioned in the vicinity of B approximately halving both figures with an inductance of the emulated rail in order to generate a potential in the 500 H and a resistance of 9.2 kΩ. With a higher number of circuit. Introducing the necessary capacitive reactance and windings and additional ferrite rods, it follows that Coil A resistance to complement that of the inductor, the matching Fig. 6: Measuring output power on the oscilloscope. The voltage is fed to channel 1 (top, yellow), and the current to channel 2 (middle, pink). The MATH function (bottom, red) is used to calculate instantaneous and apparent power.

V. LABORATORY RESULTS AND DISCUSSION Fig. 5: The physical setup used in the lab. The two distances Depicted in Figure 6 is the oscilloscope display during at which measurements were conducted are marked with black testing. The current and voltage seem to be in-phase, meaning tape. that cos φ ≈ 1 and that the apparent power reported by the oscilloscope can be interpreted as active power. It therefore TABLE I: Capacitance values required for the matching cir- follows that resonant circuit was quite well tuned. cuit. The results from the laboratory trials are summarised in 2 Figure 7. The two coils were tested at both 16 ⁄3 Hz and 50 Hz, Coil L [H] f [Hz] C [F] resulting in four separate data sets. For each configuration, 2 −9 A 1000 16 ⁄3 91 × 10 three different current levels were applied, namely 30, 60, and A 1000 50 10 × 10−9 120 A. Quadratic curve-fitting is applied to the data points to 2 −9 B 500 16 ⁄3 182 × 10 highlight the expected relationship. Note that for readability, B 500 50 20 × 10−9 the figure uses logarithmic axes.

A. Power and energy output circuit was built into an enclosure with a number of switches for configuration. These switches were used to select the The power draw of an electric locomotive is highly depen- appropriate capacitance and resistance parameters depending dent on the mass it pulls and the slope of the track. However, on which coil was to be tested, at what frequency. Two test the typical current requirement of a passenger along a flat 2 frequencies were supported—16 ⁄3 Hz and 50 Hz. track section is estimated to be above 100 A [23], and may Employing (4), it is straightforward to derive the capacitance therefore be represented by the measurements gathered at C necessary to form a resonant LC-circuit given a frequency f 120 A. The power output from the two energy harvester coils at 2 and an inductance L. Table I lists the applicable values given a distance of 0.25 m from this current at 16 ⁄3 Hz was 340 µW the specific combinations of inductance and frequency. and 181 µW for Coils A and B, respectively. Increasing the frequency to 50 Hz reveals a power output of 912 µW and 1 324 µW for the coils. C = 2 2 (4) 4π f L The energy output will depend on the number of trains The matching circuit was connected to a Teledyne Lecroy passing by, and the duration for which the locomotives draw WaveJet Touch 354 oscilloscope, providing real-time readings current of this magnitude. However, the given power output of the induced current and closed-loop voltage in the circuit. shows promise in terms of the viability of MFEH in railway. These readings were further used to compute the instantaneous According to [8], the typical power requirements of a node in and apparent power. The voltage was measured as the potential a wireless sensor network is in the range of tens to hundreds across the main load resistor, RL, while the current was of microwatts in active operation, and tens of nanowatts in derived from the potential across a low-value current-sense sleep-mode. By employing energy accumulation and adaptive resistor, Rcs. The latter was specifically chosen to be as close duty-cycling techniques, it is not unreasonable that a WSN to 10 Ω as possible. Its value being around 0.1 % of that of node could be solely powered by MFEH. Due to the scheduled main load resistor, the current-sense resistor did not noticeably nature of railway traffic, prediction models utilising knowledge affect the circuit. of train timetables could be especially beneficial. Further 2 Power output at 16 ⁄3 Hz Power output at 50 Hz 1,000 1,000

100 100 [µW] [µW] L L P 10 P 10

1 1 30 60 120 30 60 120

Ir [A] Ir [A]

Fig. 7: Measured power output PL from Coil A (solid blue) and Coil B (dashed orange) as a function of rail current Ir. The data were recorded with coils placed at two distances: 0.5 m (circle) and 0.25 m (square). examination of the energy output was conducted as part of current magnitude. At 30 A, the effect was about a doubling the in situ testing, discussed in Section VII. for Coil A at both distances. However the increase was only about 1.6 for Coil B at 0.25 m and 1.4 at 0.5 m. At the B. Model evaluation maximum current, 120 A, the picture is further complicated as the frequency change lead to a 2.2 times increase for Coil In the following, the results will be presented and discussed A at 0.5 m, and a factor of slightly less than 2.7 at 0.25 m. For in the context of the theoretical model (2), repeated here for Coil B, the increase at this current level was by factors of 1.4 reference. and 1.8, respectively. 2 2 These results lead into a discussion of the validity of the 2πr Nµeµ0defIr PL = (2 revisited) theoretical model (2). It is apparent that there are other factors 64R at play not accounted for, since the coefficients of reduction or As expected, the power PL dissipated in the load increases increase were impacted by changes in frequency in a manner with an increasing rail current Ir. The relationship seems to not predicted by the model. Apart from differences in the be that a doubling of the rail current, results in slightly less tuning of circuit resonance, there are presumably losses in the than four times the output power. This resonates well with the core that scale by some function of the frequency. According theoretical model, which would dictate a quadratic relationship to [21], losses may be modelled as scaling between rail current and power output. While the relationship with frequency squared, which would explain some of these is not perfectly quadratic, variations can presumably be at- deviations since these losses are disregarded in the theoretical tributed to non-linear losses in the magnetic coupling and the model. Especially considering the choice of steel as material core itself, as well as imperfections in selecting the magnitude for the flux guides, which would amplify the effects of eddy of the rail current. current losses compared to a pure ferrite core. 2 Since the effective distance is defined as de = /d for a single rail, the power output should be inversely proportional VI.INSITUSETUP to the square of the distance at which the cores are placed. Following the laboratory trials, the coils were tested in a However, in view of the measurements, a significant discrep- real-world setting. The field setup was similar to that of the ancy is revealed. The output power changed by a factor of 2 lab tests, albeit employing more rugged solutions so that the around 1/10 at 16 ⁄3 Hz and 1/16 at 50 Hz when the distance was system may endure rougher conditions such as water and dirt doubled from 0.25 m to 0.5 m. The discrepancy is most likely splashing, rain, and wind. The resulting design is shown in attributed to the increasing curvature of the magnetic field, as Figure 8a. model assumes a uniform field. In [13], this assumption was For practical reasons, the coils were only tested along shown to be largely valid from about 0.4 m and outwards when Norwegian railway, for which the electrification frequency is employing two current-carrying rails. Using a single rail will 2 16 ⁄3 Hz. Highlighted on the map in Figure 8b, two different presumably highlight the discrepancy to a greater degree, as test sites were used in the in situ testing: a flat section of the core essentially begins to enclose the current. railway south of Trondheim Central station (Trondheim S), It is anticipated from the model that a change in the and an inclined section with a slope of 18 ‰ south of Heimdal frequency of the rail current should have a quadratic impact on station. the output power in a perfectly resonant circuit. The results 2 show that going from 16 ⁄3 Hz to 50 Hz—an increase by a factor of 3—does not manifest as a power output increase A. Data acquisition of 9. Indeed, it has a varying effect on the output power The power output of the coils was recorded using a custom depending on the coil under test, the distance, and the rail data acquisition solution, albeit similar to the circuit used Fig. 9: The coils were placed at a distance of 0.5 m from the rail, here shown along the sloped railway section.

intervals deemed representative for the recorded data, one from (a) (b) each test site. As illustrated in Figure 8b, the flat section is situated Fig. 8: The more rugged system designed to withstand the just 2.35 km south of Trondheim S, which means that trains conditions of real-world testing is shown in (a). In (b), the running past in either direction will not draw a substantial two test sites are marked. The flat and sloped sections are both amount of current. Northbound trains are arriving at the along the line—550.52 km and 537.64 km, respectively. station and therefore generally do not draw current, while southbound trains are leaving the station and limited to a speed in the laboratory. An Arduino MKR ZERO was employed of 30 km/h, resulting in a modest rail current. According to to sample the instantaneous over the coils’ load the daily schedule, the trains recorded in Figure 10a are both , downscaled to be within the range of the board’s southbound trains leaving the station—one passenger and one ADC inputs. A 100 Hz sampling rate was selected, being freight. sufficient to satisfy the Nyquist-Shannon sampling theorem— Figure 10b shows the output from the energy harvester as 2 a northbound freight train passes by up the 18 ‰ slope. The i.e. higher than twice the signal frequency of 16 ⁄3 Hz—while still conservative in order to save energy. The recorded samples rail current is in the same order of magnitude as that seen were subsequently saved to an SD card for post-processing. for the flat section, albeit maintained at higher levels for a To allow for placement in remote areas, a 20 600 mAh power considerably longer duration of time. Compared to the short bank was used to power the data acquisition solution. bursts of current in Figure 10a, a substantial current is drawn for about 9 minutes. In this location, only northbound trains, i.e. those running up the sloped section, will draw a significant VII.INSITURESULTSANDDISCUSSION current. It is presumably locations like this for which MFEH The installation procedure was completed quickly and ef- will be most effective. fortlessly; only about 10 minutes passed from the point where access to the section of railway was granted until the har- vesting system was fully deployed and traffic could resume. A. Power and energy output This speaks to the ease of installation and non-invasiveness In Figure 10a, the voltage induced in Coil A by the passen- of MFEH along railway. Since access to the electrification ger train peaks at 2 V amplitude, resulting in an instantaneous system itself is not necessary, a condition monitoring system power output of 233 µW. For the freight train passing a few employing MFEH can be quickly installed with minimal minutes later, the peak induced voltage is 4.17 V and the impact on railway traffic. The installed system is pictured in power output 1.01 mW. For Coil B, the power figures are Figure 9. approximately halved—169 µW and 569 µW for the passenger The system was placed in each location for approximately and freight trains, respectively. two days, continuously recording the power output from the Comparing Figures 10a and 10b, it is evident that the peak energy harvesting coils. The envelope of the output voltage voltages induced by the two freight trains are similar. However, indicates whenever a train draws current, and its magnitude is a the power output from the sloped section is sustained for a result of the amount of current drawn by the locomotive, and— substantially longer time. Numerically integrating the power due to ground leakage currents—the locomotive’s distance to outputs as the freight train passes by the sloped section gives a the harvester coil. The signal was matched with the daily train figure of 109 mJ and 80.4 mJ for Coils A and B, respectively. schedules to determine what type of train likely passed at Typically, around nine northbound freight trains pass by which point in time. Shown in Figure 10 are two selected this point every day, which—through extrapolation—renders a 5 Coil A 5 Coil A Coil B Coil B [V] 0 [V] 0 L L V V −5 −5 1.0 1.0 [mW] [mW] 0.5 0.5 L L P P 0.0 0.0 0 200 400 600 800 1000 0 200 400 600 800 1000 Time [s] Time [s] (a) Flat section (b) Sloped section

Fig. 10: The voltage VL over the load resistor RL was recorded as trains passed by. The instantaneous dissipated power was 2 derived as PL = VL /RL. In (a), a passenger train followed a few minutes later by a freight train both depart from Trondheim S, drawing relatively short bursts of current. Subfigure (b) shows how a freight train sustains a larger current draw in order to maintain speed up the 18 ‰ slope. presumed daily energy harvest of around 981 mJ and 724 mJ single freight train passing by up a sloped section. Depending for Coils A and B, respectively. When accounting for the on the number of trains passing by, the estimated daily output four northbound electric passenger trains that also pass by is in excess of 1 J, which is sufficient for a low-power sensor (40 mJ and 26 mJ per train), the estimates become 1.14 J and node. 828 mJ per day for Coils A and B. Both coils therefore It was shown that the power output is dependent on a set seem to be viable solutions for powering condition monitoring of factors, but chief among them, perhaps, the magnitude of systems along railway. [10] reports instances of structural the rail current. The amount of energy that can be harvested is health monitoring systems requiring as little as 132 mJ per therefore highly affected by the location of the system, railway day, a little below one tenth of the amount generated by Coil topology, and the degree to which the railway experiences A on the sloped section. traffic. Other factors include the coil’s distance to the rail and The placement of the energy harvesting system has a sub- the magnetic properties of the core. It is expected that the stantial impact on the amount of energy that can be scavenged. power output can be substantially increased by increasing the It has already been shown that a steeper incline is better in core’s effective permeability, and by employing a design that terms of sustained power output, although only trains running allows placement closer to the rails. up the slope will provide a significant rail current. Apart from The MFEH solution lends itself to a quick, cost-effective, the slope, there are several other factors at play. The current and non-invasive installation procedure, and may therefore be return path will generally be directed towards the closest an attractive approach for railway administrations towards ex- power substation, which means that only when the harvester is tending the lifetime of battery-powered condition monitoring located between the locomotive and the substation will there systems. MFEH is a predictable source of energy, and pre- be a current in the rails. Placing the coil close to a power diction models incorporating knowledge of the train schedule substation will therefore presumably increase average power could enable further optimisation in energy consumption of output. trackside equipment. It should be noted that for both in situ tests, the coils were placed 0.5 m from the rail due to restrictions placed on the height of trackside equipment. If the coils were constructed REFERENCES in a more compact form-factor, they could have been placed [1] Publications Office of the European Union, “EU transport in figures,” much closer to the rail, and—as seen in the laboratory trials— Luxembourg, 2018. provided a substantially larger power output. [2] Norwegian Railway Directorate, “Jernbanestatistikk 2018,” , Nor- way, 2019, (in Norwegian). [3] H. Akkermans, L. Besselink, L. van Dongen, and R. Schouten, “Smart VIII.CONCLUSIONS Moves for Smart Maintenance,” in World Class Maintenance, 2016. In this article, magnetic field energy harvesting (MFEH) in [4] Bane NOR (Norwegian National Rail Administration), “We create the railway has been demonstrated. Following a brief feasibility railway of the future,” Oslo, Norway, 2017. [5] M. Takikawa, “Innovation in railway maintenance utilizing information study, a prototype energy harvesting system was constructed. and communication technology (smart maintenance initiative).” Japan Tests were performed in a controlled laboratory environment Railway & Transport Rev., no. 67, pp. 22–35, 2016. to further demonstrate feasibility, after which a more rugged [6] V. J. Hodge, S. O’Keefe, M. Weeks, and A. Moulds, “Wireless Sensor Networks for Condition Monitoring in the Railway Industry: A Survey,” system was placed in situ. The solution established the viabil- IEEE Trans. Intell. Transp. Syst., vol. 16, no. 3, pp. 1088–1106, Jun. ity of MFEH in railway, harvesting approximately 0.1 J from a 2015. [7] I. F. Aky`ıld`ız, W. Su, Y. Sankarasubramaniam, and E. C¸ayırcı, “Wireless sensor networks: A survey,” Comput. Netw., vol. 38, no. 4, pp. 393–422, Mar. 2002. [8] D. E. Boyle, M. E. Kiziroglou, P. D. Mitcheson, and E. M. Yeatman, “Energy Provision and Storage for Pervasive Computing,” IEEE Perva- sive Comput., vol. 15, no. 4, pp. 28–35, Oct. 2016. [9] A. Gjerstad, Bane NOR (Norwegian National Rail Administration), Aug. 2020, personal communication. [10] K. S. Adu-Manu, N. Adam, C. Tapparello, H. Ayatollahi, and W. Heinzelman, “Energy-Harvesting Wireless Sensor Networks (EH- WSNs): A Review,” ACM Trans. Sensor Netw., vol. 14, no. 2, pp. 1–50, Apr. 2018. [11] C. Ulianov, Z. Hadas,ˇ P. Hyde, and J. Smilek, “Novel Energy Harvesting Solutions for Powering Trackside Electronic Equipment,” in Sustainable Rail Transport, M. Marinov and J. Piip, Eds. Cham, Switzerland: Springer, 2020. [12] F. Yang, L. Du, H. Yu, and P. Huang, “Magnetic and Electric Energy Harvesting Technologies in Power Grids: A Review,” Sensors, vol. 20, no. 5, p. 1496, Apr. 2020. [13] A. E. Espe and G. Mathisen, “Towards Energy Harvest- ing near Electrified Railway Tracks,” in 9th Mediterranean Conf. on Embedded Comput., Budva, Montenegro, 2020, pp. 1–4. [14] S. Yuan, Y. Huang, J. Zhou, Q. Xu, C. Song, and P. Thompson, “Magnetic Field Energy Harvesting under Overhead Power Lines,” IEEE Trans. Power Electron., vol. 30, no. 11, pp. 6191–6202, Nov. 2015. [15] S. Yuan, Y. Huang, J. Zhou, Q. Xu, C. Song, and G. Yuan, “A High- Efficiency Helical Core for Magnetic Field Energy Harvesting,” IEEE Trans. Power Electron., vol. 32, no. 7, pp. 5365–5376, Jul. 2017. [16] W. Jiang, J. Lu, S. Hashimoto, and Z. Lin, “A non-intrusive magnetic energy scavanger for renewable power generation state monitoring,” in IEEE Int. Conf. on Renewable Energy Res. and Appl., Birmingham, UK, 2016, pp. 562–566. [17] Z. Wu, D. S. Nguyen, R. M. White, P. K. Wright, G. O’Toole, and J. R. Stetter, “Electromagnetic energy harvester for atmospheric sensors on overhead power distribution lines,” J. of Phys.: Conf. Ser., vol. 1052, no. 1, pp. 1–4, Jul. 2018. [18] S. W. Wright, M. E. Kiziroglou, S. Spasic, N. Radosevic, and E. M. Yeatman, “Inductive Energy Harvesting From Current-Carrying Struc- tures,” IEEE Sensors Lett., vol. 3, no. 6, pp. 1–4, Jun. 2019. [19] F. Kiessling, R. Puschmann, A. Schmieder, and E. Schneider, Contact Lines for Electric Railways, 1st ed. Munich, Germany: Publicis, 2001. [20] F. Nilsen, “Sporstrømmer og potensialer i kontaktledningsanlegg,” Jern- baneverket, Tech. Rep., 1997, (in Norwegian). [21] D. Jiles, Introduction to Magnetism and Magnetic Materials, 3rd ed. Boca Raton, FL, USA: CRC Press, 2016. [22] Ferroxcube, “4B1 Material Specification,” 2008. [On- line]. Available: https://elnamagnetics.com/wp-content/uploads/library/ Ferroxcube-Materials/4B1 Material Specification.pdf [23] B. I. Olsen, Bane NOR (Norwegian National Rail Administration), Mar. 2020, personal communication.