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Article A Novel Single-Wire Power Transfer Method for Sensor Networks

Yang Li, Rui Wang * , Yu-Jie Zhai , Yao Li, Xin Ni, Jingnan Ma and Jiaming Liu

Tianjin Key Laboratory of Advanced and Technology, Tiangong University, Tianjin 300387, China; [email protected] (Y.L.); [email protected] (Y.-J.Z.); [email protected] (Y.L.); [email protected] (X.N.); [email protected] (J.M.); [email protected] (J.L.) * Correspondence: [email protected]; Tel.: +86-152-0222-1822

 Received: 8 September 2020; Accepted: 1 October 2020; Published: 5 October 2020 

Abstract: Wireless sensor networks (WSNs) have broad application prospects due to having the characteristics of low power, low cost, wide distribution and self-organization. At present, most the WSNs are battery powered, but batteries must be changed frequently in this method. If the changes are not on time, the energy of sensors will be insufficient, leading to faults or even networks interruptions. In order to solve the problem of poor reliability in WSNs, a novel power supply method, the single-wire power transfer method, is utilized in this paper. This method uses only one wire to connect source and load. According to the characteristics of WSNs, a single-wire power transfer system for WSNs was designed. The characteristics of and multi-loads were analyzed by and experiments to verify the feasibility of this method. The results show that the total efficiency of the multi-load system can reach more than 70% and there is no directivity. Additionally, the efficiencies are higher than (WPT) systems under the same conductions. The single-wire power transfer method could satisfy the characteristics of WSNs and effectively solve the problem of poor power supply reliability.

Keywords: wireless sensor networks; single wire power transfer; power supply method

1. Introduction Wireless sensor networks (WSNs) are special networks consisting of many micro-nodes with the ability of perception, calculation, storage and wireless communication, and perform long-range, long-term overall perception and accurate control for monitoring areas [1–4]. WSNs have broad application prospects in many areas, such as national security, environmental monitoring and industrial production due to the characteristics of low power, low cost, wide distribution and self-organization [5–7]. At present, most of the WSNs are battery powered, but batteries must be changed frequently in this method. If the changes are not on time, the energy of sensors will be insufficient, which will to node faults or even network interruptions [8]. To solve the abovementioned problems, some researchers tried to use natural energies such as energy, photovoltaic energy, wind energy, droplet impact energy and to power WSNs [9–13]. However, these solutions all harvest energy from nature and they are easily affected by environmental and weather factors. In addition, the size and cost of installations cannot satisfy the actual demands. Additionally, wireless power transfer (WPT) technology is also an effective method. In [14], Peng et al. employed a mobile robot carrying a wireless charger to charge a sensor network based on wireless power transfer via electromagnetic radiation. The results showed that electromagnetic radiation could prolong the network lifetime. However, the low efficiency was a bottleneck. In [15], Xie et al. considered the scenario of a mobile wireless charging periodically traveling inside

Energies 2020, 13, 5182; doi:10.3390/en13195182 www.mdpi.com/journal/energies Energies 2020, 13, 5182 2 of 15

WSNs and charging each sensor node through magnetic resonant wireless power transfer. However, this method was limited to charging one node at a time and was not scalable as node density increased. Compared with other wireless power supply devices, WSNs have special features such as low power, wide distribution and a great number of sensors. Other WPT technologies, such as , , and ultrasonic, cannot also satisfy the features of WSNs because of short transmission distance, low transmission efficiency, poor directivity and low safety [16–19]. Mobile are used to carry chargers to charge sensors in the abovementioned solutions. However, mobile vehicles are limited to certain terrains and cannot work normally in some complex environments. Compared with mobile vehicles, unmanned aerial vehicles can work in any environment and they are good carriers of chargers. However, oversized coils will increase the load of unmanned aerial vehicles, which causes low efficiency and affects working time [20,21]. Despite these intensive efforts, the energy of WSNs remains a performance bottleneck and is perhaps the key factor that hinders wide-scale developments. In order to solve the abovementioned problems, a novel power supply method for WSNs, the single-wire power transfer method, is utilized. It only uses one wire to connect source and load. In terms of power transfer, the electromagnetic field generated by the source is guided and bound by the single wire to the load. The single-wire power transfer system will not be affected easily by the environment compared with the wireless power transfer system. When the single wire is replaced by an existing medium, a generalized WPT can be realized. In terms of system applications, single-wire power transfer can power WSNs reliably without reducing the flexibility of nodes and be applied to various complex environments, which contributes to the further development of WSNs. Besides, it can supply power for some other areas where it is difficult to erect wires, such as isolated islands and remote mountain areas. Additionally, it can complement other power transfer methods to make power application scenarios abundant and improve the use efficiency of power. In this paper, the single-wire power transfer method is used as a solution to the problem of power supply for WSNs. The rest of this paper is organized as follows. The system structure of single-wire power transfer for WSNs is demonstrated in Section2. The characteristics of directivity and multi-loads of single-wire power transfer systems are demonstrated to verify the feasibility of this method from simulations and experiments in Sections3 and4, respectively. Finally, conclusions are drawn in Section5.

2. Structures of Single-Wire Power Transfer Systems Figure1a shows the structure of a single-wire power transfer system evolved from ’s WPT system [22]. The system contains a source, , receiver and load. The transmitter includes a step-up transfer and metal conductor Q1. The receiver comprises a step-down transfer and metal conductor Q2. The source supplies power for the whole system. When the transmitter obtains the power from the source, there will be time-varying electromagnetic field at Q1. The power is transferred from the transmitter to the receiver through coupling the electric field between Q1 and Q2, which is the first path of this system. The receiver provides power for the load. The single wire is the second path. The first and second path form a closed loop for power transfer [23,24]. The coupled electric field strength decreases as distance increases between transmitter and receiver. As a result, power cannot be transferred effectively through Q1 and Q2. The numbers of turns of transfers are very large and there are two metal conductors causing a large volume of the transmitter and receiver. Besides, the obviously increases at the metal conductors because of the large numbers of turns. On the one hand, such a high voltage breaks peripheral air down, leading to the discharge phenomenon and, on the other hand, the high-frequency time-varying electromagnetic field generated by the high voltage affects the normal operation of the surrounding equipment. Energies 2020, 13, 5182 3 of 15 Energies 2020, 13, x FOR PEER REVIEW 3 of 15

E Q1 Q2 Load ~ ~

Source Single wire Step-up Step-down transfer transfer Transmitter Receiver (a) Transmitting coil Receiving coil Load Single wire ~ ~ Source Source coil Load coil

Transmitter Receiver (b)

Figure 1. StructuresStructures of of single-wire single-wire power power transfer transfer systems systems ( (aa)) evolved evolved from from Tesla’s Tesla’s wireless power transfertransfer (WPT) system and ( b) improved in this paper.

The powercoupled supply electric requirements field strength for WSNsdecreases cannot as bedistance satisfied increases by the systembetween shown transmitter in Figure and1a receiver.because ofAs the a result, large volume power andcannot high be voltage. transferred Therefore, effectively the single-wirethrough Q1 power and Q transfer2. The numbers system forof turnsWSNs of is transfers improved are on very the large basis and of Figure there1 area and tw iso metal shown conductors in Figure1 causingb. The metala large conductors volume of are the transmitterremoved and and the receiver. coils are Besides, spiral planes, the voltage which obvi canously realize increases miniaturization. at the me Thetal sourceconductors coil andbecause load ofcoil the are large connected numbers to source of turns. and On load, the respectively. one hand, such One enda high of the voltage transmitting breaks coilperipheral and receiving air down, coil leadingare connected to the todischarge the single phenomenon wire. The other and, ends on ofthe the other transmitting hand, the coil high-frequency and receiving time-varying coil are open electromagneticended. The parameters field generated of the transmitter by the high and voltag receivere affects are completely the normal identical, operation including of the surrounding the number equipment.of turns, inside radii, pitches, wire diameters and radians of coils. The power path generated supply requirements by metal conductors for WSNs does cannot not exit be satisfied when the by metal the system conductors shown are in removed. Figure 1aFigure because2 is theof the equivalent large volume circuit and of the high single-wire voltage. Therefore, power transfer the single-wire system, which power is transfer a transmission system forline WSNs terminated is improved in an open on circuit.the basisSeq ofis Figure the equivalent 1a and is source shown including in Figure the 1b. source The andmetal transmitter conductors in areFigure removed1b. ZL andis the the equivalent coils are loadspiral including planes, which the load can and realize receiver miniaturization. in Figure1b. ∆Thez is source per-unit-length coil and loadquantities. coil areR andconnectedL are series to source resistance and and load, respectively. per unit One length end ofof thethe single transmitting wire. G andcoil Candare receivingshunt conductance coil are connected and to the single per unit wire. length The generatedother ends by of thethe single transmitting wire and coil the and surrounding receiving coilenvironment. are open Accordingended. The to theparameters transmission of the line tran theory,smitter the and voltage receiver and current are completely on the line identical, are: including the number of turns, inside radii, pitches, wire diameters and radians of coils. The path generated by metal conductorsV(z) does = 2V no0 cost exit(βz )when the metal conductors are removed.(1) Figure 2 is the equivalent circuit of the single-wire power transfer system, which is a transmission 2jV0 line terminated in an open circuit. Seq isI (thez) =equivalent− cos source(βz) including the source and transmitter(2) Z0 in Figure 1b. ZL is the equivalent load including the load and receiver in Figure 1b. ∆z is per-unit-length quantities. R and L are series resistance and inductance per unit length of the single

Energies 2020, 13, x FOR PEER REVIEW 4 of 15 wire. G and C are conductance and capacitance per unit length generated by the single wire and the surrounding environment. According to the transmission line theory, the voltage and current on the line are: = Vz() 2 V0 cos()βz (1) − 2jV0 Iz()= cos()βz (2) Z Energies 2020, 13, 5182 0 4 of 15 where β = ω(LC)1/2. The voltage coefficient, Γ, is: 1/2 where β = ω(LC) . The voltage reflection coefficient,Z − ZΓ, is: Γ= L 0 + (3) ZL Z0 Γ = − 0 (3) ZL + Z0 where Z0 is the . There is only one wire in the single-wire power transfer system.where So,Z0 isthe the single-wire characteristic power impedance. transfer There system is only is a one transmission wire in the single-wireline terminated power in transfer an open circuitsystem. and So, ZL the = ∞ single-wire because of power the open transfer circuit system at terminal is a transmission. Dividing line the terminated numerator in and anopen denominator circuit of and(3) byZL Z=L andbecause allowing of the ZL open→∞ show circuit that at terminal. the voltage Dividing reflection the numerator coefficient and for denominator this case is ofΓ = (3) 1. by The ∞ Z and allowing Z show that the voltage reflection coefficient for this case is Γ = 1. The single-wire single-wireL power transferL→∞ system in Figure 1b uses the standing principles to transfer power. Thepower coils transfer are used system to increase in Figure 1voltageb uses thewhich standing can wavedecrease principles the losses to transfer of the power. single The wire. coils The frequencyare used toof increasethe system voltage is determined which can decrease by the theequation losses off = the 1/(2 singleπ(L1C wire.1)1/2), Thewhere frequency L1 is the of thetotal system is determined by the equation f = 1/(2π(L C )1/2), where L is the total inductance including the inductance including the series inductance of single1 1 wire S and1 the inductance of coils, C1, is the totalseries capacitance inductance including of single shunt wire S capacitanceand the inductance of single of wire coils, CC, 1the, is distributed the total capacitance capacitance including between shunt capacitance of single wire C, the distributed capacitance between the coils and single wire and the coils and single wire and between each turn of the coils. Frequency and impedance are between each turn of the coils. Frequency and impedance are determined by L1 and C1. This is the determined by L1 and C1. This is the circuit analysis of the single-wire power transfer system. From circuit analysis of the single-wire power transfer system. From the perspective of electromagnetic the perspective of propagation, the electromagnetic field generated by the field propagation, the electromagnetic field generated by the transmitter propagates to the receiver transmitter propagates to the receiver by the single wire. It is an oriented power transfer and the by the single wire. It is an oriented power transfer and the single wire plays an important role in single wire plays an important role in guiding and binding the electromagnetic field. The guiding and binding the electromagnetic field. The electromagnetic field propagates in all directions electromagneticand covers a large field far-field propagates range in if thereall directions is no single and wire, covers which a large far-field to the low range efficiency if there of theis no singlereceivers. wire, Thiswhich is theleads reason to the why low the efficiency efficiency ofof WPTthe receivers. via the coupled This is electromagnetic the reason why field the decreases efficiency of asWPT the via transmission the coupled distance electromagnetic increases. field decreases as the transmission distance increases.

I(z) I =0 L∆z R∆z L∆z R∆z L Seq C∆z G∆z V(z) C∆z G∆z ZL

∆z ∆z

0 z FigureFigure 2. 2. EquivalentEquivalent circuit circuit of of the single-wire powerpower transfer transfer system. system.

3. Analysis 3. Simulation Analysis The feasibility of the single-wire power transfer method for WSNs is verified by simulations in The feasibility of the single-wire power transfer method for WSNs is verified by simulations in this section. Figure3 shows the simulation model of the single-wire power transfer system, where this section. Figure 3 shows the simulation model of the single-wire power transfer system, where there is a transmitter on the left and a receiver on the right and they are connected to the single wire. thereThe is number a transmitter of turns on of the sourceleft and coil a receiver and load on coil the is oneright and and the they transmitting are connected coil and to receivingthe single coil wire. Thehave number 10 turns. of turns The insideof the radii,source pitches coil and of the load coils coil and is one the wireand diametersthe transmitting are 60 mm,coil and 10 mm receiving and coil1.06 have mm, 10 respectively. turns. The Theinside radians radii, of pitches the source of the and coils load and coil andthe wire transmitting diameters and are receiving 60 mm, coil 10 are mm andthe 1.06 same mm, size. respectively. The feasibility The is verifiedradians throughof the source simulating and load directivity coil and and transmitting multi-loads basedand receiving on the coilcharacteristics are the same of WSNs.size. The feasibility is verified through simulating directivity and multi-loads based on the characteristics of WSNs.

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Figure 3. Simulation model of the single-wire power transfer system. Figure 3. Simulation model of the single-wire power transfer system. Figure 3. Simulation model of the single-wire power transfer system. 3.1. Directivity Simulations 3.1. Directivity Simulations The transmission distance and frequency of the directivity simulation model are 5 m and 10.55 The transmissiontransmission distance distance and and frequency frequency of of the the directivity directivity simulation simulation model model are 5 are m and 5 m 10.55 and MHz,10.55 MHz, respectively. The structure of the single-wire power transfer system is similar to the wireless respectively.MHz, respectively. The structure The structure of the single-wireof the single-wire power power transfer transfer system system is similar is similar to the wirelessto the wireless power power transfer system. The only difference is the single wire. So, the frequency of the single-wire transferpower transfer system. system. The only The di onlyfference difference is the single is the wire.single So, wire. the So, frequency the frequency of the single-wireof the single-wire power power transfer system is close to wireless power transfer system. The rotating directions of the transferpower transfer system issystem close tois wirelessclose to powerwireless transfer power system. transfer The system. rotating The directions rotating ofdirections the receivers of the in receivers in the single-wire power transfer system are shown in Figure 4. The rotating centers are thereceivers single-wire in the power single-wire transfer power system transfer are shown system in Figure are shown4. The in rotating Figure centers 4. The are rotating the centers centers of theare the centers of the coils and the rotating axes are the axes of the three-dimensional coordinate system. coilsthe centers and the of rotating the coils axes and arethe the rotating axesof axes the are three-dimensional the axes of the three-dimensional coordinate system. coordinate The transmitting system. The transmitting power, receiving power and efficiency before rotating are 94.83 W, 87.40 W and power,The transmitting receiving powerpower, and receiving efficiency power before and rotating efficiency are 94.83before W, rotating 87.40 W are and 94.83 92.16%, W, 87.40 respectively. W and 92.16%, respectively. The rotating step and angle are 10° and 360°. The simulation results are shown The92.16%, rotating respectively. step and The angle rotating are 10◦ stepand and 360 ◦angle. The are simulation 10° and results360°. The are simulation shown in Figure results5. are shown inin FigureFigure 5.5.

((aa)) ((bb)) ((cc)) Figure 4. Rotating axes of receivers for (a) x-axis, (b) y-axis and (c) z-axis. Figure 4. Rotating axes of receivers for (a) x-axis, (b) y-axis and (c) z-axis.

x WhenWhen thethe rotatingrotating axisaxis isis thethe xx-axis,-axis, asas shownshown inin FigureFigure5 5a,5a,a, the thethe di differencedifferencefference inin valuesvaluesvalues betweenbetween maximummaximum andandand minimum minimumminimum transmitting transmittingtransmitting power, power,power, receiving receivingreceiving power powerpower and andand effi efficiencyefficiencyciency are areare 3.49 3.493.49 W, 4.67W,W, 4.674.67 W and WW 4.02%,andand 4.02%,4.02%, respectively. respectively.respectively. The di TheThefference differdiffer inenceence values inin values ofvalues the transmitting ofof thethe transmittingtransmitting power, receiving power,power, receivingreceiving power and powerpower efficiency andand areefficiencyefficiency 11.27 W, areare 11.88 11.2711.27 W W,W, and 11.8811.88 4.48% WW andand when 4.48%4.48% the rotating whenwhen thethe axis rotatingrotating is the yaxisaxis-axis, isis the shownthe yy-axis,-axis, in Figure shownshown5 b. inin WhenFigureFigure the 5b.5b. rotatingWhenWhen thethe axis rotatingrotating is the zaxisaxis-axis, isis thethe dizz-axis,-axis,fference thethe in differencedifference values of inin the valuesvalues transmitting ofof thethe transmittingtransmitting power, receiving power,power, power receivingreceiving and epowerpowerfficiency andand in efficiencyefficiency Figure5c in arein FigureFigure 15.30 W, 5c5c are 11.29are 15.3015.30 W and W,W, 11.29 5.3%.11.29 WW andand 5.3%.5.3%.

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Figure 5.5. SimulationSimulation resultsresults ofof the the single-wire single-wire power power transfer transfer system system with with rotating rotating receivers receivers when when the (thea) rotating (a) rotating axis axis is the is xthe-axis, x-axis, (b) rotating(b) rotating axis axis is the isy the-axis y-axis and and (c) rotating (c) rotating axis axis is the isz the-axis. z-axis.

The transmitting power, receiving power and efficiency all change slightly from the original The transmitting power, receiving power and efficiency all change slightly from the original values. Besides, the frequency of system also changes slightly. The frequency is determined by L1 and values. Besides, the frequency of system also changes slightly. The frequency is determined by L1 C1 from Section2. L1 and C1 change slightly when routing receivers lead to the changes in frequency. and C1 from Section 2. L1 and C1 change slightly when routing receivers lead to the changes in Considering the errors in simulation systems, it can be assumed that there is no directivity in the frequency. Considering the errors in simulation systems, it can be assumed that there is no single-wiredirectivity in power the single-wire transfer system. power In transfer addition, system the power. In addition, and efficiency the power can also and satisfy efficiency WSNs. can It also is a favorablesatisfy WSNs. condition It is a for favorable powering condition WSNs withfor poweri the characteristicsng WSNs with of distributionthe characteristics and self-organization. of distribution and self-organization.There is no directivity in the single-wire power transfer system when receivers rotate over each axis independentlyThere is no in directivity the above in simulations. the single-wire Rotating power over transfer three axessystem is exploredwhen receivers for further rotate verification. over each Angleaxis independently of 45◦ and 135 ◦inare the chosen above as thesimulations. rotating angles Rota overting eachover axisthree because axes theis explored rotatingcombination for further isverification. too much overAngle three of 45° axes and and 135° there are are chosen eight combinations.as the rotating The angles simulation over each results axis are because shown the in Tablerotating1. The combination results show is too that much the transmitting over three axes power, and receiving there are power eight combinations. and e fficiency The allstill simulation change slightlyresults are when shown the receivers in Table rotate 1. The over results three show axes. Thethat simulationthe transmitting results power, further receiving verify that power there isand no directivityefficiency all in thestill single-wire change slightly power when transfer the system.receivers rotate over three axes. The simulation results further verify that there is no directivity in the single-wire power transfer system. Table 1. Simulation results of directivity when receivers rotate over three axes.

Table 1. Simulation results of directivityTransmitting when receiversReceiving rotate over three axes. X-Axis Y-Axis Z-Axis Efficiencies Power (W) Power (W) X-Axis Y-Axis Z-Axis Transmitting Power (W) Receiving Power (W) Efficiencies 45◦ 86.51 78.51 90.75% 4545°◦ 86.51 78.51 90.75% 45° 135◦ 87.39 78.38 89.69% 45 ◦ 135° 45 87.39 93.67 85.77 78.38 91.57%89.69% 45° 135 ◦ 45°◦ 135 93.67 97.57 88.49 85.77 90.69%91.57% 135° ◦ 135° 97.57 88.49 90.69% 45◦ 98.32 91.39 92.95% 45◦ 45° 135◦ 98.32 94.83 87.40 91.39 92.16%92.95% 135◦ 45° 135° 45◦ 94.83 82.75 77.48 87.40 93.63%92.16% 135° 135◦ 45° 135◦ 82.75 85.71 80.18 77.48 93.55%93.63% 135° 135° 85.71 80.18 93.55%

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3.2. Multi-Load Simulations 3.2. Multi-Load Simulations The single-wire power transfer system includes a chained system and emanant system The single-wire power transfer system includes a chained system and emanant system according according to the multi-load distribution forms. Identical coils are connected to the single wire in to the multi-load distribution forms. Identical coils are connected to the single wire in turn in chained turn in chained systems (Figure 6). The coil positions are numbered in turn in chained systems. The systems (Figure6). The coil positions are numbered in turn in chained systems. The receiving power receiving power of each receiver can be observed and compared by changing the positions of of each receiver can be observed and compared by changing the positions of to analyze transmitters to analyze power distribution. Taking a two-load chained system as an example, the power distribution. Taking a two-load chained system as an example, the coil positions are numbered coil positions are numbered 1, 2 and 3 and the transmitter can be placed in position 1 and 2 because 1, 2 and 3 and the transmitter can be placed in position 1 and 2 because of symmetry, as shown in of symmetry, as shown in Figure 6a. Other chained systems are consistent with this. Figure6a. Other chained systems are consistent with this.

Figure 6. Electromagnetic field distribution of chained systems with (a) two loads and (b) four loads. Figure 6. Electromagnetic field distribution of chained systems with (a) two loads and (b) four Figure6 shows the electromagnetic field distributionloads. of chained systems, where the transmitters are in the middle. There are electromagnetic fields in the receivers and they are symmetric, as seen fromFigure the color 6 distribution.shows the Itelectromagnetic can be considered field preliminarily distribution that theof receiverschained cansystems, receive where power andthe thetransmitters power may are be in symmetric. the middle. For chainedThere are systems electromagnetic at transmission fields distances in the ofreceivers 2 m, 5 m and and 10they m, theare specificsymmetric, values as seen of transmitting from the color and receivingdistribution. power It can are be shown considered in Figure preliminarily7. State A and that B arethe two-loadreceivers chainedcan receive systems. power State and C,the D power and E aremay four-load be symmetric. chained For systems. chained The systems red boxes at transmission represent transmitting distances powerof 2 m, and 5 m other and 10 colored m, the boxes specific represent values receiving of transmitting power. and The receiving power of diff areerent shown chained in Figure systems 7. areState shown A and in B the are figures. two-load PX chained in the figures systems. represents State C, the D positionsand E are of four-load coils, such chained as P1 insystems. state A ofThe a 2red m chainedboxes represent system, whichtransmitting means power the transmitter and othe isr incolored position boxes 1. The represent total effi receivingciencies ofpower. states A-EThe infrequencies 2 m systems of different are 93.72%, chained 95.04%, systems 90.65%, are 94.32%shown andin the 89.94%, figures. respectively. PX in the figures The total represents efficiencies the inpositions 5 m systems of coils, are such 90.65%, as P1 94.93%, in state 86.94%, A of a 2 90.53% m chained and 91.83%,system, respectively.which means The the totaltransmitter efficiencies is in inposition 10 m systems1. The total are 79.04%,efficiencies 89.45%, of states 80.37%, A-E 87.12%in 2 m systems and 81.73%, are 93.72%, respectively. 95.04%, The 90.65%, receiving 94.32% power and of89.94%, receivers respectively. is obviously The di totalfferent efficiencies when the in transmitters 5 m systems are are placed 90.65%, in di 94.93%,fferent positions.86.94%, 90.53% However, and when91.83%, the respectively. transmitters The are intotal the efficiencies middle of thein 10 system, m systems the receiving are 79.04%, power 89.45%, is symmetric. 80.37%, 87.12% This result and is81.73%, consistent respectively. with the The above receiving electromagnetic power of fieldreceiv distribution.ers is obviously Therefore, different from when the the perspective transmitters of averageare placed power in different distribution, positions. the transmitters However, when should the be transmitters placed in the are middle in the as middle much asof possiblethe system, for the chainedreceiving multi-load power is single-wire symmetric. power This transferresult is systemsconsistent for WSNs.with the above electromagnetic field distribution. Therefore, from the perspective of average power distribution, the transmitters should In emanant systems, the transmitters are in the center and the receivers surround the transmitters be placed in the middle as much as possible for the chained multi-load single-wire power transfer (Figure8). The receivers are numbered counter-clockwise. Taking a five-load emanant system as an systems for WSNs. example, the structure of the system and the numbers of coils are shown in Figure8a. The receiving power of each receiver can be observed and compared by changing the numbers of receivers and transmission distances to analyze power distribution. The electromagnetic field distributions of emanant systems are shown in Figure8. There are electromagnetic field in the receivers and they are almost identical, as seen from the color distribution. The electromagnetic field strength of transmitters

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is obviously higher than that of receivers. It can be considered preliminarily that the receivers can Energiesreceive 2020 power, 13, x FOR and PEER the receiving REVIEW power may be identical. 8 of 15

Figure 7. Specific values of transmitting and receiving power of chained systems, transmission distances are (a) 2 m, (b) 5 m and (c) 10 m.

In emanant systems, the transmitters are in the center and the receivers surround the transmitters (Figure 8). The receivers are numbered counter-clockwise. Taking a five-load emanant system as an example, the structure of the system and the numbers of coils are shown in Figure 8a. The receiving power of each receiver can be observed and compared by changing the numbers of receivers and transmission distances to analyze power distribution. The electromagnetic field distributions of emanant systems are shown in Figure 8. There are electromagnetic field in the receivers and they are almost identical, as seen from the color distribution. The electromagnetic Figure 7. Specific values of transmitting and receiving power of chained systems, transmission field Figurestrength 7. Specific of transmitters values of transmitting is obviously and receiving higher powerthan ofthat chained of receivers. systems, transmission It can be distances considered distancesare (a) 2 m,are ( b(a)) 52 mm, and (b) (5c )m 10 and m. (c) 10 m. preliminarily that the receivers can receive power and the receiving power may be identical. In emanant systems, the transmitters are in the center and the receivers surround the transmitters (Figure 8). The receivers are numbered counter-clockwise. Taking a five-load emanant system as an example, the structure of the system and the numbers of coils are shown in Figure 8a. The receiving power of each receiver can be observed and compared by changing the numbers of receivers and transmission distances to analyze power distribution. The electromagnetic field distributions of emanant systems are shown in Figure 8. There are electromagnetic field in the receivers and they are almost identical, as seen from the color distribution. The electromagnetic field strength of transmitters is obviously higher than that of receivers. It can be considered preliminarily that the receivers can receive power and the receiving power may be identical.

Figure 8. Electromagnetic field field distribution of emanant systems with ( a) five five loads, (b) six loads and (c) seven loads. The specific values of transmitting and receiving power with different numbers of receivers and different transmission distances are shown in Figure9. State A, B, C, D and E are four-, five-, six-, seven- and eight-load emanant systems, respectively. The red boxes represent transmitting power and other colored boxes represent receiving power. The frequencies of different emanant systems are shown

Figure 8. Electromagnetic field distribution of emanant systems with (a) five loads, (b) six loads and (c) seven loads.

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The specific values of transmitting and receiving power with different numbers of receivers and different transmission distances are shown in Figure 9. State A, B, C, D and E are four-, five-, six-, seven- and eight-load emanant systems, respectively. The red boxes represent transmitting powerEnergies and2020 ,other13, 5182 colored boxes represent receiving power. The frequencies of different emanant9 of 15 systems are shown in the figures. The total efficiencies of state A-E in 2 m systems are 92.54%, 94.95%,in the figures.93.78%, The89.00% total and effi ciencies87.75%, ofrespectively. state A-E in The 2 m total systems efficiencies are 92.54%, in 5 94.95%, m systems 93.78%, are 89.00%91.77%, 82.22%,and 87.75%, 73.53%, respectively. 66.14% and The 56.21%, total respectively. efficiencies in The 5 m total systems efficiencies are 91.77%, in 10 82.22%, m systems 73.53%, are 66.14%81.83%, 61.81%,and 56.21%, 53.57%, respectively. 34.66% and The 26.03%, total effi cienciesrespectively. in 10 mThe systems receiving are 81.83%,power 61.81%,of receivers 53.57%, is 34.66%almost identicaland 26.03%, when respectively. the transmission The receiving distances power are fixed. of receivers The receiving is almost power identical of receivers when the is transmission average and thisdistances result is are consistent fixed. The with receiving the abovementioned power of receivers electric is average field distribution. and this result When is consistent the transmission with the distancesabovementioned are set electricas variables, field distribution. the receiving When power the transmission of receivers distances decreases are as set the as variables, transmission the distancesreceiving increase. power of Comprehensively, receivers decreases the as the chaine transmissiond systems distances and emanant increase. systems Comprehensively, can be used the in WSNs,chained but systems the emanant and emanant systems systems are better can than be used chained in WSNs, systems but from the emanant the perspective systems areof average better powerthan chaineddistribution. systems The from node the distributions perspective ofare average complex power in practical distribution. WSNs. The The node combination distributions forms are ofcomplex chained insystems practical and WSNs. emanant The combinationsystems can formsbe used of chainedto make systems complex and WSNs emanant which systems can give can befull playused to to their make characteristics complex WSNs and which make can givethe single full play-wire to their power characteristics transfer method and make more the single-wiresuitable for poweringpower transfer WSNs. method more suitable for powering WSNs.

FigureFigure 9. 9. SpecificSpecific values values of of transmitting transmitting and and receiv receivinging power ofof emanantemanant systems,systems, transmissiontransmission distances are (a) 2 m, (b) 5 m and (c) 10 m. distances are (a) 2 m, (b) 5 m and (c) 10 m. 4. Experiment Results 4. Experiment Results The characteristics of single-wire power transfer were analyzed from the perspective of simulations in SectionThe characteristics3. In this section, of wesingle-wire describe an power experimental transfer platform were thatanalyzed was built from to verifythe perspective the feasibility of simulationsof the single-wire in Section power 3. transferIn this methodsection, forwe powering describe WSNs.an experimental The experimental platform platform that was is shown built in to verifyFigure the 10. Thefeasibility generatorof the single-wire was connected po tower the transfer RF amplifier method to provide for powering high-frequency WSNs. power The experimentalfor the single-wire platform power is transfershown system.in Figure The 10. coils The in thesignal experiments generator were was connected connected to adjustableto the RF amplifiercapacitors to forprovide tuning. high-frequency The transmitting power and receiving for the single-wire powers were power measured transfer by the system. RF power The meters.coils in

Energies 2020, 13, x FOR PEER REVIEW 10 of 15 the experimentsEnergies 2020, 13 ,were 5182 connected to adjustable for tuning. The transmitting and10 receiving of 15 powers were measured by the RF power meters.

Signal Load generator Transmitter Receiver

Single wire Tuning RF power meter Tuning RF power capacitor RF meter

FigureFigure 10. 10.ExperimentalExperimental platform platform of of the single-wiresingle-wire power power transfer transfer system. system.

4.1. Comparative4.1. Comparative Experiments Experiments In orderIn order to toverify verify that that single-wiresingle-wire power power transfer transfer systems systems are more are suitable more forsuitable powering for WSNspowering compared with WPT systems, comparative experiments were first conducted. In the comparative WSNs compared with WPT systems, comparative experiments were first conducted. In the experiments, the coils were the same as in the simulations. The number of turns of the source coil and comparative experiments, the coils were the same as in the simulations. The number of turns of the load coil was one and the transmitting coil and receiving coil had 10 turns. The inside radii, pitches sourceof thecoil coils and and load the coil wire was diameters one and were the 60 mm,transmitting 10 mm and coil 1.06 and mm, receiving respectively. coil The had transmitting 10 turns. The insidepower radii, was pitches fixed andof the transmissioncoils and the distances wire diameters were set from were 5 cm 60 to mm, 50 cm. 10 Themm transmission and 1.06 mm, respectively.efficiencies The for transmitting different transmission power was distances fixed are and shown the transmission in Table2. distances were set from 5 cm to 50 cm. The transmission efficiencies for different transmission distances are shown in Table 2. Table 2. Results of comparative experiments.

Transmission Table 2.Effi Resultsciencies of of comparative Single-Wire experiments.Efficiencies of WPT Distances (cm) Power Transfer System System Transmission Distances Efficiencies of Single-Wire Power Efficiencies of WPT 5 84.65% 78.05% (cm) 10Transfer 83.16% System 74.16% System 5 15 84.47%84.65% 69.40% 78.05% 10 20 80.54%83.16% 41.58% 74.16% 25 82.23% 30.63% 15 30 83.79%84.47% 18.28% 69.40% 20 35 84.87%80.54% 10.25% 41.58% 25 40 85.01%82.23% 6.17% 30.63% 45 84.36% 3.53% 30 50 83.79%83.79% 1.86% 18.28% 35 84.87% 10.25% As the40 transmission distance increased, the85.01% efficiencies of the single-wire power transfer6.17% system were maintained45 above 80%. The transmission84.36% efficiencies of the WPT system gradually3.53% decreased as the transmission50 distance increased. When83.79% the transmission distance was more than1.86% 20 cm, the transmission efficiencies decreased to below 50%. Single-wire power transfer systems can maintain highAs the and stabletransmission transmission distance efficiency increased, within a certainthe ef distance.ficiencies Meanwhile of the single-wire the transmission power distance transfer systemhas were great influencemaintained on theabove transmission 80%. The effi transmisciency insion WPT efficiencies systems. So, of single-wire the WPT powersystem transfer gradually decreasedsystems as are the more transmission suitable for distance powering increased. WSNs. When the transmission distance was more than 20 cm, the transmission efficiencies decreased to below 50%. Single-wire power transfer systems can 4.2. Directivity Experiments maintain high and stable transmission efficiency within a certain distance. Meanwhile the transmissionThe transmissiondistance has distance great was influence set to 5 m on in thethe directivity transmission experiments. efficiency The coilsin WPT were consistentsystems. So, with those of the comparative experiments and simulations. There were dial plates under the receivers, single-wire power transfer systems are more suitable for powering WSNs. which were used to determine rotating angles. The transmitting and receiving powers were measured by power meters. 4.2. Directivity Experiments The transmission distance was set to 5 m in the directivity experiments. The coils were consistent with those of the comparative experiments and simulations. There were dial plates under

Energies 2020, 13, x FOR PEER REVIEW 11 of 15 theEnergies receivers,2020, 13 which, 5182 were used to determine rotating angles. The transmitting and receiving 11powers of 15 were measured by power meters. The transmitting power and receiving power were first recorded when the rotating angle was The transmitting power and receiving power were first recorded when the rotating angle was 0◦. 0°. Then the receivers were rotated and the transmitting and receiving powers of the systems were Then the receivers were rotated and the transmitting and receiving powers of the systems were recorded. recorded. The routing angles were from 0° to 360° and the routing step was 30°. The transmitting The routing angles were from 0◦ to 360◦ and the routing step was 30◦. The transmitting power, receiving power,power andreceiving efficiencies power of and different efficien rotatingcies of axes different of simulations rotating and axes experiments of simulations are shown and inexperiments Figure 11. areThe shown dashed in linesFigure are 11. experimental The dashed results lines andare experi the solidmental lines results are simulation and the results. lines The earefficiencies simulation of results.the experiments The efficiencies were lower of the than experiments those of the simulationswere lower and than the those efficiencies of the were simulations constant duringand the efficienciesrotating. Thewere results constant show during thatthe rotating. receiving The power results was show constant that the during receiving rotating. power There was wasconstant no duringdirectivity rotating. over eachThere axis was in theno single-wiredirectivity powerover each transfer axis system. in the Tosingle-wire further verify power directivity, transfer rotating system. Toexperiments further verify over three directivity, axes were rotating conducted. expe Theriments experimental over resultsthree andaxes simulation were conducted. efficiencies areThe experimentalshown in Table results3. The and e ffi cienciessimulati ofon the efficiencies experiments are were shown also lowerin Table than 3. those The ofefficiencies the simulations. of the experimentsThe efficiencies were were also still lower constant than and those there of wasthe nosimu directivity.lations. The The efficiencies experimental were results still are constant consistent and therewith was those no of directivity. the simulations. The experimental results are consistent with those of the simulations.

FigureFigure 11. 11. ExperimentalExperimental results results of directivity whenwhen the the (a ()a rotating) rotating axis axis is theis thex-axis, x-axis, (b) ( rotatingb) rotating axis axis is the y-axis and (c) rotating axis is the z-axis. is the y-axis and (c) rotating axis is the z-axis.

Energies 2020, 13, x FOR PEER REVIEW 12 of 15

Energies 2020, 13, 5182 12 of 15 Table 3. Experimental results of directivity when receivers rotate over three axes.

Receiving Table 3. ExperimentalTransmitting results of directivity when receivers rotate over three axes. Power in Efficiencies in Efficiencies in X-Axis Y-Axis Z-Axis Power in Transmitting ReceivingExperiments Power Experiments Simulations Experiments (W) Efficiencies in Efficiencies in X-Axis Y-Axis Z-Axis Power in in Experiments (W) Experiments Simulations 45° Experiments90.12 (W) 64.58(W) 71.66% 90.75% 45° 135°45◦ 87.9890.12 62.53 64.58 71.07% 71.66% 90.75% 89.69% 45◦ 45° 135◦ 87.98 62.53 71.07% 89.69% 45◦ 45° 90.45 64.26 71.04% 91.57% 135° 45◦ 90.45 64.26 71.04% 91.57% 135◦ 135° 89.26 63.78 71.45% 90.69% 135◦ 89.26 63.78 71.45% 90.69% 45° 92.56 66.68 72.04% 92.95% 45° 45◦ 92.56 66.68 72.04% 92.95% 45◦ 135° 86.29 61.29 71.03% 92.16% 135° 135◦ 86.29 61.29 71.03% 92.16% 135◦ 45° 90.17 63.17 70.06% 93.63% 135° 45◦ 90.17 63.17 70.06% 93.63% 135◦ 135° 90.56 65.28 72.08% 93.55% 135◦ 90.56 65.28 72.08% 93.55%

4.3. Multi-Load Experiments Two-load andand four-loadfour-load chainedchained systemssystems andand four-load,four-load, five-load,five-load, six-load,six-load, seven-load and eight-load emanantemanant systems systems were were built built based based on the on simulations the simulations in Section in 3Section. The coils 3. wereThe coils consistent were withconsistent those with of the those comparative of the comparative experiments experime and simulations.nts and simulations. The transmission The transmission distance was distance 5 m. In thiswas section,5 m. In thethis influence section, the on theinfluence system on caused the sy bystem the caused positions by ofthe transmitters positions of and transmitters receivers, and the numbersreceivers, ofthe the numbers loads and of the loads power and distribution the power among distribution loads wereamong explored. loads were explored. The experimental results of multi-load chained systems are shown inin FigureFigure 1212a.a. The positions of transmitters and the numbers of receivers were changed synchronously in the multi-load chained experiments. WhenWhen the the transmitters transmitters were were in thein centerthe center positions positions of the chainedof the chained systems, systems, the receiving the powerreceiving was power basically was identical.basically Foridentical. example, For theexample, receiving the powerreceiving was power 31.66 was W and 31.66 35.33 W and W when 35.33 the W transmitterwhen the transmitter was in position was in 2 position of the two-load 2 of the two-load chained systemchained and system the receiving and the receiving power was power 15.32 was W, 16.1915.32 W,W, 14.77 16.19 W W, and 14.77 15.91 W W and when 15.91 the W transmitter when the was transmitter in position was 3 of in the position four-load 3 of chained the four-load system. Aschained a result, system. the receiving As a result, power the receiving can be distributed power ca evenlyn be distributed in chained evenly systems in chained when the systems transmitters when arethe intransmitters the center are positions. in the center When positions. the transmitters When arethe nottransmitters in the center are positionsnot in theof center chained positions systems, of thechained total systems, receiving the power total on receiving the left sidepower of on the the transmitters left side of is the equal transmitters to that on is the equal right to side. that Takingon the theright four-load side. Taking chained the four-load system as chained an example, system the as receiving an example, power the on receiving the left waspower 32.03 on Wthe when left was the transmitter32.03 W when was the in positiontransmitter 2, the was receiving in position power 2, th one thereceiving right was power 11.56 on W, the 10.67 right W was and 11.56 8.9 W W, and 10.67 the totalW and receiving 8.9 W and power the total was 31.13receiving W. power was 31.13 W.

Figure 12. Experimental results of multi-load systems for (a) 5 m chained system and (b) 5 m Figure 12. Experimental results of multi-load systems for (a) 5 m chained system and (b) 5 m emanant system. emanant system. Besides, the positions of each coil (including transmitters and receivers) were moved freely. The coils were not in a straight line in this case. The transmitting power, receiving power and

Energies 2020, 13, 5182 13 of 15 efficiencies all changed slightly from the original values. The receiving power decreases after a few nodes because the transmission distance increases after a few nodes, leading to an increase in system losses. Generally, the power distribution laws of the chained systems in the experiments were consistent with the simulations. Figure 12b shows the experimental results of multi-load emanant systems. The numbers of loads are the main variables in the emanant systems. The numbers of loads were changed from four to eight. The receiving power of each receiver was equal because each receiver was connected to the transmitter. Meanwhile, the angles between receivers were changed in the experiments. For example, the angles were 90◦ when the receivers were symmetric in the four-load emanant system, but the angles were random in this experiment. The results show that the receiving power changed slightly from the original values with the changes of angles between receivers. The transmission efficiencies decreased with the increase in loads. The experimental results of the emanant systems were also consistent with the simulation results. Table4 shows the comparison of the simulation and experimental e fficiencies of 5 m multi-load systems. The transmission efficiencies of the experiments were lower than those of the simulations. The material of the coils and the single wire is the perfect conductor, as conductivity is infinite and there is no insulating layer of the conductor for the convenience of modeling in simulations. There are no conductor losses or losses in simulations. However, the material is copper, whose conductivity is very large, but still finite, and there is an insulating layer of copper, making the coils and single wire experience conductor losses and dielectric losses in experiments. Besides, the simulation models are perfect. However, there are many metal objects, such as experimental equipment, in a laboratory. These metal objects are easily exposed to the electromagnetic field generated by single-wire power transfer systems, leading to eddy current losses in metal objects. As a result, the transmission efficiencies of experiments are lower than those of the simulations.

Table 4. Comparison of simulation efficiencies and experimental efficiencies of 5 m multi-load systems.

Multi-Load Numbers of Simulation Experimental Distribution Forms Loads Efficiencies Efficiencies 2 (P1) 90.65% 70.00% 2 (P2) 94.93% 73.01% Chained multi-load 4 (P1) 86.94% 65.99% systems 4 (P2) 90.53% 71.00% 4 (P3) 91.83% 69.01% 4 91.77% 71.06% 5 82.22% 62.37% Emanant multi-load 6 73.53% 52.59% systems 7 66.14% 45.33% 8 56.21% 32.50%

5. Conclusions In this paper, the single-wire power transfer method for WSNs is studied and the feasibility of this method is verified. The conclusions are summarized as follows:

A novel single wire power transfer system structure is utilized because of the large volume and • high voltage of the single-wire power transfer system that evolved from Tesla’s WPT system. Metal conductors are removed and coils are miniaturized in the novel structure. This structure can improve system flexibility and reduce system complexity under the premise of ensuring transmitting power and efficiency. The directivity of the system is studied based on the characteristics of WSNs. The transmitting • power, receiving power and efficiency all change slightly from the original values with the rotating Energies 2020, 13, 5182 14 of 15

of receivers. The rotating of receivers does not affect the systems. There is no directivity in single-wire power transfer systems. The characteristics of multi-loads are also studied. The multi-load distribution forms can be • chained and emanant. The distribution forms are flexible. These two forms all can distribute power evenly. This feature lays the foundation for powering WSNs. Harvesting energy from nature and wireless power transfer technology are the main solutions • to powering WSNs. However, these two methods have the problem of low efficiency. The load of mobile devices carrying transmitters is limited in the wireless power transfer method. This method is limited to charging one node at a time. However, the single-wire power transfer method still could maintain high efficiencies in a certain range and charge multiple nodes at a time. As a result, the single-wire power transfer method is more suitable for WSNs compared with other solutions. The feasibility of the single-wire power transfer method for WSNs is proved in this paper. • The structure of the single-wire power transfer system is not genuine wireless power transfer because of the existence of the single wire. Besides, the system performance can still be improved, such as efficiency. Medium replacement of the single wire can make this method more suitable for WSNs. The system performance can be improved from the perspective of system parameters and system structure. The electromagnetic losses of the single wire can be decreased by changing system parameters, such as the frequency and radius of the single wire. The coupling losses between the coils and single wire can be decreased by novel structures, such as a conical horn. Medium replacement and the improvement of system performance are the two most significant research directions of the single-wire power transfer method in the future.

Author Contributions: The paper was written by Y.L. (Yang Li), R.W., Y.-J.Z. and Y.L. ( Yao Li) and revised by X.N., J.L. and J.M. The project was conceived, planned and supervised by Y.L.(Yang Li) All authors have contributed to the editing and proofreading of this paper. All authors have read and agree to the published version of the manuscript. Funding: This research was funded by the National Natural Science Foundation of China under Grant 51877151, Grant 51577133, Grant 51677132 and in part by the Program for Innovative Research Teams at the University of Tianjin under Grant TD13-5040. Conflicts of Interest: The authors declare no conflict of interest.

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