sensors
Article Medium and Short Wave RF Energy Harvester for Powering Wireless Sensor Networks
Jesus A. Leon-Gil 1,2, Agustin Cortes-Loredo 1,2, Angel Fabian-Mijangos 1,2, Javier J. Martinez-Flores 1,2, Marco Tovar-Padilla 1,2, M. Antonia Cardona-Castro 1, Alfredo Morales-Sánchez 1 ID and Jaime Alvarez-Quintana 1,2,*
1 Advanced Materials Research Center S. C. -Monterrey, Alianza Norte # 202, Autopista Monterrey-Aeropuerto Km.10., C.P. 66600 Apodaca, Nuevo León, Mexico; [email protected] (J.A.L.-G.); [email protected] (A.C.-L.); [email protected] (A.F.-M.); [email protected] (J.J.M.-F.); [email protected] (M.T.P.); [email protected] (M.A.C.-C.); [email protected] (A.M.-S.) 2 Genes-Group of Embedded Nanomaterials for Energy Scavenging, CIMAV-Unidad Monterrey, C.P. 66600 Apodaca, Nuevo León, Mexico * Correspondence: [email protected]
Received: 10 November 2017; Accepted: 24 January 2018; Published: 3 March 2018
Abstract: Internet of Things (IoT) is an emerging platform in which every day physical objects provided with unique identifiers are connected to the Internet without requiring human interaction. The possibilities of such a connected world enables new forms of automation to make our lives easier and safer. Evidently, in order to keep billions of these communicating devices powered long-term, a self-sustainable operation is a key point for realization of such a complex network. In this sense, energy-harvesting technologies combined with low power consumption ICs eliminate the need for batteries, removing an obstacle to the success of the IoT. In this work, a Radio Frequency (RF) energy harvester tuned at AM broadcast has been developed for low consumption power devices. The AM signals from ambient are detected via a high-performance antenna-free LC circuit with an efficiency of 3.2%. To maximize energy scavenging, the RF-DC conversion stage is based on a full-wave Cockcroft–Walton voltage multiplier (CWVM) with efficiency up to 90%. System performance is evaluated by rating the maximum power delivered into the load via its output impedance, which is around 62 µW, although power level seems to be low, it is able to power up low consumption devices such as Leds, portable calculators and weather monitoring stations.
Keywords: Internet of Things; RF energy scavenging; AM waves detection; full wave Cockcroft–Walton multiplier
1. Introduction Along with “zero-power” self-sustainable standalone electronics, emerging platforms such as Internet of Things (IoT), Smart Skins (SS), and Smart Cities (SC) have been designed as networks to interconnect, sense, quickly monitor and easily pinpoint problem areas without requiring human interaction, as well as present the information in an accessible way. Evidently, such an interacting world will need to keep millions and millions of interfaced devices powered long term. A feasible answer to the above-mentioned failings is to harvest electromagnetic-waves energy from external environmental sources which could be used as power supply for battery less operation or even extended battery life in such low power consumption devices [1,2]. Although there exists a great interest in RF harvesting from ISM and WLAN bands (900–928 MHz and 2.4/5 GHz respectively) [3–10]; TV (54–890 MHz), FM (88–108 MHz) and AM (530–1700 KHz)
Sensors 2018, 18, 768; doi:10.3390/s18030768 www.mdpi.com/journal/sensors Sensors 2018, 18, 768 2 of 13 bandsSens haveors 2018 recently, 18, x FOR beingPEER REVIEW explored because they are characterized by covering both urban2 of 13 and non-urban areas because of their continuous emission as commercial bands [11,12]. Besides, compared Although there exists a great interest in RF harvesting from ISM and WLAN bands (900–928 MHz to commercial bands, ISM and WLAN bands because of their relative high frequency, the distance and 2.4/5 GHz respectively) [3–10]; TV (54–890 MHz), FM (88–108 MHz) and AM (530–1700 KHz) between the RF source and the harvester dispositive becomes fairly short [13]. Hence, AM band bands have recently being explored because they are characterized by covering both urban and is preferrednon-urban due areas to its because long-range of their and continuous low attenuation emission within as commercial building materials bands [11,12]. like concrete Besides, [14]. Likewise,compared AM to electromagnetic commercial bands, wave ISM transmitters and WLAN bands and receiversbecause of do their not relative need tohigh be frequency, in line of the sight of eachdistance other. between the RF source and the harvester dispositive becomes fairly short [13]. Hence, AM Inband this is workpreferred we due propose to its long an optimized-range and low and attenuation portable AM-RFwithin building energy materials harvesting like toconcrete switch [14]. on low energyLikewise, utilization AM electromagnetic devices. The high wave yield transmitters of the current and receivers AM harvester do not need is based to be in on line the of development sight of of a high-performanceeach other. AM resonator which is maximized by optimizing the quality factor Q of the antenna, asIn wellthis wo asrk a stage-optimizedwe propose an optimized Cockcroft–Walton and portable voltage AM-RF multiplier energy harvesting (CWVM) to asswitch AC-DC on low voltage energy utilization devices. The high yield of the current AM harvester is based on the development converter [15]. of a high-performance AM resonator which is maximized by optimizing the quality factor Q of the The conceptual diagram of an AM RF-energy harvester is shown in Figure1. Basically, antenna, as well as a stage-optimized Cockcroft–Walton voltage multiplier (CWVM) as AC-DC electromagnetic-wavevoltage converter [15]. power irradiated by way of AM commercial station is detected by the AM harvesterThe system, conceptual and then diagram it is applied of an toAM switch RF-energy on low harvester energy utilization is shown in devices. Figure 1. Basically, Besides,electromagnetic Figure-1wave shows power a graphical irradiated representation by way of AM of commercial the RF harvester station system, is detected which by consiststhe AM of a high-qualityharvester factor system, AM and resonator then it is whichapplied acts to switch as a high on low performance energy utilization receptor, devices. an RF-DC converter, and the powerBesides, storage Figure and consumption 1 shows a graphical stage. representation The receptor of of the electromagnetic RF harvester system, waves which transduces consists the of AM wavesa intohigh- anquality ACelectrical factor AM signal; resonator hence, which the acts AM as antenna a high performance selects only receptor, one signal an atRF a-DC desired converter, frequency. Then,and an RF-DCthe power converter storage transformsand consumption the tuned stage. AC The signal receptor by theof electromagnetic resonator into waves DC signal, transduces which can the AM waves into an AC electrical signal; hence, the AM antenna selects only one signal at a finally be stored on an electrical energy storage device such a capacitor or immediately used to switch desired frequency. Then, an RF-DC converter transforms the tuned AC signal by the resonator into on a charge.DC signal, Although which can all finally stages be in storedthe AM on harvester an electrical system energy are storage important; device one such of a the capacitor most relevant or onesimmediately is AC-DC conversion. used to switch The power on a charge. harvesting Although device all must stages not in only the AM be capable harvester of system converting are AC signalsimportant; into DC one voltage, of the butmost it relevant also must ones increase is AC-DC the conversion. DC signal The into power an appropriate harvesting device level, must and then supplynot the only maximum be capable powerof convert intoing the AC charge. signals into This DC is notvoltage, an easybut it task also becausemust increase it depends the DC signal mainly on the outputinto an and appropri inputate electrical level, and impedance then supply of the the maximum converter. power Therefore, into the thecharge. key This challenges is not an to easy develop an efficienttask because AM harvester it depends are: mainly (i) efficient on the yet output small and antenna input electrical to pick up impedance the long of wavelengths the converter. of the AM electromagneticTherefore, the key waves challenges (177 to m–566 develop m), an and efficient (ii) AC-DC AM harvester converters are: (i) with efficient proper yet small input antenna and output impedancesto pick up based the long on efficientwavelengths zero of threshold the AM electromagnetic diodes for matching waves (17 to7the m–56 aforementioned6 m), and (ii) AC antennas.-DC converters with proper input and output impedances based on efficient zero threshold diodes for Hence, because of their long electrical wavelengths, design of compact resonators with satisfactory matching to the aforementioned antennas. Hence, because of their long electrical wavelengths, performancedesign of iscompact a very resonators difficult task. with satisfactory performance is a very difficult task.
Power Storage AM Transmission AM AC-DC and Broadcasting Space Resonator Converter Consumption
Figure 1. Conceptual platform of an AM-RF energy harvesting system. Figure 1. Conceptual platform of an AM-RF energy harvesting system. 2. Portable Medium and Short Wave RF Energy Harvester Design 2. Portable Medium and Short Wave RF Energy Harvester Design AM stations have some disadvantages compared to other RF sources; for instance, they are AMlimited stations to transmit have power some well disadvantages below 50 kW, and compared TV stations to othertransmit RF power sources; up to for 1 M instance,W in the U theySA are limitedBesides, to transmit due to power the long well belowwavelengths, 50 kW, tiny and AMTV stations antenna transmits broadly power go through up to 1 a MW very in low the USA Besides,performance. due to the Nevertheless, long wavelengths, despite all tiny those AM constraints, antennas broadly the model go of through a high-sensitivity a very low scavenger performance. Nevertheless,that is abledespite to harvest all very those low constraints, intensity electromagnetic the model ofwaves a high-sensitivity can be reached by scavenger designingthat a high is able efficiency AM antenna, as well as an optimized AC-DC converter, as it will be explained in the next to harvest very low intensity electromagnetic waves can be reached by designing a high efficiency sections. A detailed electrical circuit of the suggested RF harvester is shown in Figure 2. It is made of AM antenna, as well as an optimized AC-DC converter, as it will be explained in the next sections. A detailed electrical circuit of the suggested RF harvester is shown in Figure2. It is made of the high-Q Sensors 2018, 18, 768 3 of 13
Sensors 2018, 18, x FOR PEER REVIEW 3 of 13 AM resonator (LC tank circuit), the boost converter (CWVM), and the load which can be either a the high-Q AM resonator (LC tank circuit), the boost converter (CWVM), and the load which can be capacitor for energy storage or a low power consumption device. either a capacitor for energy storage or a low power consumption device. Sensors 2018, 18, x FOR PEER REVIEW 3 of 13
the high-Q AM resonator (LC tank circuit), the boost converter (CWVM), and the load which can be either a capacitor for energy storage or a low power consumption device.
Power Storage High-Q and Resonator Full-Wave CWVM Load Stage
Figure 2. Illustration of the AM-RF harvesting system circuit.Power Storage Figure 2.HighIllustration-Q of the AM-RF harvesting system circuit. and Resonator Full-Wave CWVM Load Stage 2.1. AM Resonator 2.1. AM Resonator Figure 2. Illustration of the AM-RF harvesting system circuit. As mentioned previously, because of their long electrical wavelengths, development of As mentioned previously, because of their long electrical wavelengths, development of compact compact2.1. AM resonators Resonator with appreciable performance is hard work. The crucial parameter is the value resonatorsof the withquality appreciable factor (Q) of performance the inductor is(L); hard as the work. quality The factor crucial of the parameter inductor increases, is the value it also of the qualityincreases factorAs (Q) the mentioned of performance the inductor previously, of (L); the as because harvester. the quality of Ferrite their factor long based of electrical the antennas inductor wavelengths, are increases, commonly deve it alsolopment used increases in of AM the compact resonators with appreciable performance is hard work. The crucial parameter is the value receivers to enhance the magnetic flux while maintaining a small size; hence, in AM receivers the performanceof the of quality the harvester. factor (Q) Ferriteof the inductor based antennas(L); as the arequality commonly factor of usedthe inductor in AM increases, receivers it toalso enhance detected RF signal is further amplified via an RF amplifier, thus a single ferrite core antenna is the magneticincreases flux the while performance maintaining of the a harvester. small size; Ferrite hence, based in antennasAM receivers are commonly the detected used in RF AM signal is enough to perform the task efficiently. In general, the efficiency of the ferrite resonator can be further amplifiedreceivers to via enhance an RF the amplifier, magnetic thus flux awhile single maintaining ferrite core a small antenna size; hence, is enough in AM to receivers perform the the task improved due to the rise of its quality factor Q well beyond 100. Therefore, ferrite antenna losses efficiently.detected In general, RF signal the is efficiency further amplified of the via ferrite an RF resonator amplifier, can thus be a singleimproved ferrite due core to antenna the rise is of its because of ferrite core, DC wiring and loop’s radiation resistance must be reduced to improve its quality factorenough Q to well perform beyond the 100. task Therefore, efficiently. In ferrite general, antenna the efficiency losses because of the ferrite of ferrite resonator core, can DC be wiring efficiency,improved so duethe tothree the mainrise of parameters its quality factorare the Q Qwell of beyondthe tuned 100. circuit, Therefore, the antennaferrite antenna loss resistance losses and loop’s radiation resistance must be reduced to improve its efficiency, so the three main parameters andbecause the radiation of ferrite resistance.core, DC wiring For clarity,and loop’s tuned radiation ferrite resistance rod antenna must be and reduced its equivalent to improve circuit its are theincluding Qefficiency, of the losses tuned so thedue circuit, three to parasi main thetic antenna parameters resistances loss are resistanceis theshown Q of in the and Figure tuned the 3 radiation acircuit,,b respectively. the resistance. antenna loss For resistance clarity, tuned ferrite rodand antenna the radiation and its resistance. equivalent For circuit clarity, including tuned ferrite losses rod due antenna to parasitic and its resistances equivalent circuit is shown in Figure3a,bincluding respectively. losses due to parasitic resistances is shown in Figure 3a,b respectively.
a) b)
a) b) Figure 3. Illustration of (a) ferrite rod antenna, and (b) equivalent circuit of tuned ferrite rod antenna. Figure 3.FigureIllustration 3. Illustration of (a) of ferrite (a) ferrite rod rod antenna, antenna, and and ( b(b)) equivalentequivalent circuit circuit of oftuned tuned ferrite ferrite rod antenna. rod antenna. The resonator radiation Rr resistance is expressed as [16]
The resonator radiation Rr resistance is expressed as [16]2 2 푁퐴 The resonator radiation Rr resistance푅 is expressed= 31200휇 as( [16) ] (1) 푟 휆 2 2 푁퐴 푅푟 = 31200휇 ( ) (1) where N is the number of windings, μ is the permeability NA휆 of 2the ferrite core, A is the cross section of R = 31200µ2 (1) the wherecore, and N is λ the is numberthe wavelength of windings, of the μr desiredis the permeability AM-RF λwave of the to ferritebe harvested. core, A is the cross section of the core, and λ is the wavelength of the desired AM-RF wave to be harvested. where N is the number of windings, µ is the permeability of the ferrite core, A is the cross section of the core, and λ is the wavelength of the desired AM-RF wave to be harvested. Sensors 2018, 18, 768 4 of 13
For a ferrite rod, the ferrite itself also tends to absorb some of the signal power. This is caused by the requirement that the alternating magnetic field has to “flip” the magnetic alignment of the magnetic domains inside the granular structure of the ferrite. For a ferrite rod, the ferrite loss has an equivalent resistance, µ00 A R = ωµ µ N2 (2) f o µ0 l Here, µ00 and µ0 are the imaginary and real part of the ferrite’s permeability, and l the length of the ferrite rod. The quality factor is defined by Q = µ0/µ00 . On the other hand, the quality factor Q and loss resistance Rls are related by
ωL Q = (3) Rls
Here, √ω is the angular frequency of the AM-RF wave detected by the AM resonator, and it is given by ω = 1/ LC, where L is the inductance of the coil and C the capacitance of the variable condenser. Here, L is given by A L = µ µN2 (4) o l
Hence, the total loss resistance Rls of the ferrite core antenna is defined as [17,18]
Rls = Rdc + Rr + R f (5) where Rdc, Rr y Rf are resistances due to dc wiring, loop’s radiation resistance and for the ferrite rod the extra ‘ferrite’ loss respectively as mentioned previously. Such resistances are related to the loss tangents respectively by Rls = tanδdc + tanδac + tanδf ωL (6)
And the corresponding loss tangents are given by
4ρ l 109 = c w tanδdc 2 (7) ωAL Nnπd
4 KE f Nnd tanδac = (8) AL
tanδf = K (9) where AL is a geometry coefficient determined by the ratio inductance of the coil and the square of the number of the windings, Lw is the perimeter of each winding, ρc is the electrical resistivity of the winding material, n and d are the number of strands and diameter of each strand of the stranded copper wire, KE is a geometry proximity effect coefficient, and K is a parameter dependent on the properties of the ferrite core. AM antennas based on ferrite cores increase significantly the radiation resistance Rr of the antenna. Therefore, placing the ferrite core in the coil has the effect of raising the radiation resistance by a factor of µ2, as you can see in Equation (1). Evidently, then the introduction of the ferrite core not only rises the radiation resistance but also reduces the losses due to the resistance of the coil. Moreover, one of the requirements for an efficient ferrite rod antenna is that it should have a Q bigger than 100 at the frequencies over which it operates. Thus, according to Equation (3), as Q increases Rls decreases, this reduces the loss resistance and hence the higher performance of the AM harvesting system. Besides, according to Equation (3) an alternative way to increase Q is by increasing L, in this sense a single core ferrite antenna could be inefficient because of its small cross section A. Thus, in the present work in order to enhance L via A as shown in Equation (4), a ferrite antenna made of a set of stacked planar Sensors 2018, 18, 768 5 of 13 Sensors 2018, 18, x FOR PEER REVIEW 5 of 13 ferrite cores hasused been for developed.development Figure of the4a ferrite shows antenna the single; the ferrite inset cores shows used the forstacked development cores with of thethe aimferrite of antenna;increasing the the inset L value shows of the stackedcoil. cores with the aim of increasing the L value of the coil.
a) b) Coils
Ferrites core
Figure 4. (a) Ferrite cores used for antenna coil, inset shows the the stacked cores, cores, and and ( b) scheme of the composed ferrite core.
Dimensions of a single ferrite core are 100 mm (l) × 20 mm (w) × 4 mm (t), with a value of Dimensions of a single ferrite core are 100 mm (l) × 20 mm (w) × 4 mm (t), with a value of relative permeability of around 135. By stacking 60 ferrite cores, a composed ferrite core of relative permeability of around 135. By stacking 60 ferrite cores, a composed ferrite core of 100 mm (l) 100 mm (l) × 60 mm (w) × 60 mm (t) has been developed; in this way, the cross section of the coil has × 60 mm (w) × 60 mm (t) has been developed; in this way, the cross section of the coil has been been increased by a factor of 45. Moreover, as you can see in Figure 2, a full wave resonator (double increased by a factor of 45. Moreover, as you can see in Figure2, a full wave resonator (double LC LC tank circuit) has been chosen so as to pick up the total energy present in the amplitude tank circuit) has been chosen so as to pick up the total energy present in the amplitude modulated modulated electromagnetic wave. The two coils needed for the double LC tank circuit have been electromagnetic wave. The two coils needed for the double LC tank circuit have been wounded in the wounded in the same ferrite core by using 22 awg stranded copper wire. Figure 4b shows an same ferrite core by using 22 awg stranded copper wire. Figure4b shows an illustrative scheme of illustrative scheme of the composed ferrite core including windings, the value of a single coil is 110 μH, the composed ferrite core including windings, the value of a single coil is 110 µH, and the value of and the value of the two-section variable capacitor varies from 10–100 pF to 365–485 pF. Therefore, the two-section variable capacitor varies from 10–100 pF to 365–485 pF. Therefore, the detector is able the detector is able to tune up frequencies ranging from 590 KHz to 5 MHz, which covers short and to tune up frequencies ranging from 590 KHz to 5 MHz, which covers short and medium wave RF medium wave RF spectrum. By using the geometrical and physical parameters of the AM spectrum. By using the geometrical and physical parameters of the AM resonator, a value of Q ~700 resonator, a value of Q ~ 700 has been estimated, which ensures the high performance of the AM-RF has been estimated, which ensures the high performance of the AM-RF energy harvesting system. energy harvesting system. 2.2. AC-DC Converter: Cockcroft-Walton Voltage Multiplier 2.2. AC-DC Converter: Cockcroft-Walton Voltage Multiplier Voltage multipliers are a special type of diode rectifier circuit which can potentially produce an Voltage multipliers are a special type of diode rectifier circuit which can potentially produce an output voltage many times greater than that of the applied input voltage [15]. It has become essential output voltage many times greater than that of the applied input voltage [15]. It has become essential in microwave ovens, strong electric field coils for cathode-ray tubes, electrostatic and high voltage test in microwave ovens, strong electric field coils for cathode-ray tubes, electrostatic and high voltage equipment, etc., where it is necessary to have a very high DC voltage generated from a relatively low test equipment, etc., where it is necessary to have a very high DC voltage generated from a relatively AC power supply [19–24]. low AC power supply [19–24]. Besides, voltage multipliers are simple circuits made from diodes and capacitors that can increase Besides, voltage multipliers are simple circuits made from diodes and capacitors that can the input voltage by two, three or n times to supply the desired DC voltage to a given load without increase the input voltage by two, three or n times to supply the desired DC voltage to a given load the need for a heavy step-up transformer. Figure5a shows the circuit for half-wave conventional without the need for a heavy step-up transformer. Figure 5a shows the circuit for half-wave CWVM. Theoretically, any desired amount voltage multiplication can be obtained and a cascade of conventional CWVM. Theoretically, any desired amount voltage multiplication can be obtained and “N” doublers, would produce an output voltage of V0 = 2 NVip, where N is the total number of stages a cascade of “N” doublers, would produce an output voltage of V0 = 2 NVip, where N is the total and Vp the input peak voltage. Nevertheless, with many stages, stray shunt capacitances due to number of stages and Vp the input peak voltage. Nevertheless, with many stages, stray shunt diode junction capacitance permit altering currents to flow in the series capacitances of each stage. capacitances due to diode junction capacitance permit altering currents to flow in the series In this sense, by increasing the number of stages, the output voltage also is increased. However, it has capacitances of each stage. In this sense, by increasing the number of stages, the output voltage also been previously demonstrated that diode junction capacitance conducts altering currents into stage is increased. However, it has been previously demonstrated that diode junction capacitance capacitances, such as current lower output voltage; so there is an optimal number of stages which conducts altering currents into stage capacitances, such as current lower output voltage; so there is results in a maximum output power, and this number is around six stages as previously reported [5,12]. an optimal number of stages which results in a maximum output power, and this number is around six stages as previously reported [5,12].
Sensors 2018, 18, 768 6 of 13 Sensors 2018, 18, x FOR PEER REVIEW 6 of 13
Cs Cs Cs SensorsCs 2018, 18, x FORCs PEER REVIEWCs Vi 6 of 13 Vi Cs Cs Cs Cs Cs Cs Vi o Gnd Vi Gnd V Vo Cs Cs Cs
Cs Cs Cs o Gnd Gnd V Vo Vi Cs Cs Cs Cs Cs Cs Cs Cs Cs a) Vi b) Cs Cs Cs Figure 5. 5.( a()a circuit) circuit for a for) six-stage a six-stage conventional conventional CWVM, CWVM, and (b) circuit and (b for) acircuit six-stageb) for full a sixwave-CWVM.-stage full waveHere,-CWVM. Cs stands Here, for the Cs seriesstands capacitances. for the series capacitances. Figure 5. (a) circuit for a six-stage conventional CWVM, and (b) circuit for a six-stage full On the wave other -CWVM. hand, Here, in CsAM AM-RF stands-RF waves,for the series it can capacitances. be seen that the envelope of the signal follows the contours of the modulating signal. In In fact, fact, there there are are two enveloping signals being modulated by by the the On the other hand, in AM-RF waves, it can be seen that the envelope of the signal follows the modulating signal; this feature leads to an increase in the maximum power which can be obtained modulatingcontours signal; of the this modulating featureleads signal. to In an fact, increase there are in thetwo maximum enveloping power signals which being canmodulated be obtained by the from from that type of wave. However, in order to get the total power, it is necessary to use a full that typemodulating of wave. signal However,; this feature in order leads to to get an the incr totalease power,in the maximum it is necessary power to which use a can full be wave-CWVM obtained waveinstead-fromCWVM of a that conventional instead type of of wave. a CWVM.conventional However, Figure inCWVM.5 orderb shows to Figure get the the circuit 5b total shows for power athe full, circuitit wave-CWVM. is necessary for a full to wave By use using a-CWVM. full such Bya circuit, usingwave notsuch-CWVM only a circ theinsteaduit, output not of aonly powerconventional the is output increased, CWVM. power butFigure is also increased, 5b the shows number the but circuit ofalso charging forthe a fullnumber cycleswave -ofCWVM. per charging second cyclesis doubled By per using second which such cuts isa circ doubled downuit, not the which only voltage the cuts output drop down power and the dramatically voltageis increased, drop reducebut and also dramatically ripplethe number factor of reduce in charging the output ripple factorvoltage. cycles in Accordingthe per output second to voltage. is this, doubled a According 6-stage which full cuts to wave-CWVM this, down a the 6-stage voltage has full been drop wave andused-CWVM dramatically in this has research been reduce use in rippled order in this to researchincreasefactor in the order in power the to output increase of detected voltage. the power electromagnetic According of detected to this, AM aelectromagnetic 6-stage wave. full The wave full-wave-CWVM AM-CWVM wave. has The been full has use-waved been in- CWVM this based research in order to increase the power of detected electromagnetic AM wave. The full-wave-CWVM hason diodesbeen based type on BAT85 diodes which type presentBAT85 which a very present low dc a forward very low bias dc voltageforward aroundbias voltage 200 mV around at 25 20◦C0 has been based on diodes type BAT85 which present a very low dc forward bias voltage around 200 maccordingV at 25 °C to according datasheet; to this datasheet striking; this characteristic striking char ensuresacteristic low ensures power consumptionlow power consumption by the CWVM. by mV at 25 °C according to datasheet; this striking characteristic ensures low power consumption by the CWVM. Figure 6 shows an illustrative scheme of the implemented AM-RF energy harvesting Figurethe6 shows CWVM. an Figure illustrative 6 shows scheme an illustrative of the implemented scheme of the AM-RF implemented energy harvestingAM-RF energy system; harvesting the main system; the main parts are also indicated. parts aresystem also; the indicated. main parts are also indicated.
CoilsCoils
CockcroftCockcroft-Walton-Walton MultiplerMultipler
Ferrite core Ferrite core
Variable Capacitor Output
Variable Capacitor Output Figure 6. Pictorial image of the AM-RF energy harvesting system. Figure 6. Pictorial image of the AM-RF energy harvesting system. Figure 6. Pictorial image of the AM-RF energy harvesting system.
Sensors 2018, 18, 768 7 of 13
2.3. Power Storage and Load Stage Finally, the output energy from the full wave-CWVM can be stored in an energy storage element, Sensors 2018, 18, x FOR PEER REVIEW 7 of 13 which could be a capacitor or a rechargeable battery. Afterwards, the battery energy storage system can be configured2.3. Power Storage in one and of Load two Stage ways: a power configuration or an energy configuration, depending on their intendedFinally, application. the output energy In our from case, the the full energy wave-CWVM was used can directly be stored to in supply an energy a steady storage amount of powerelement, into the which load. could Inthis be a case, capacitor the AM-RF or a rechargeable harvester battery. works Afterwards,as a power the source; battery thus, energy it behaves as an energystorage source system in can series be configured with an internal in one resistance. of two ways: Therefore, a power configurationthis implies that or an if energy we connect a variableconfiguration, load resistance depending on the on power their intended source then application. the power In our dissipated case, the energy by the was load used will directly depend to on the internalsupply resistance a steady of theamount power of power source. into Therefore, the load. In the this load case, resistancethe AM-RF mustharvester be matchedworks as a to power the internal source; thus, it behaves as an energy source in series with an internal resistance. Therefore, this resistanceimplies of the that harvester if we connect in order a variable to access load resistance to the maximum on the power power source given then bythe thepower AM dissipated harvester; this, in accordanceby the to load the will maximum depend onpower the internaltransfer resistance theorem. of In the this power sense, source. previously Therefore, reported the load works [12] indicateresistance that frequency must be of matched the input to RF the wave, internal the resistance number of of the stages harvester N, the in stage order capacitances to access to Cs,the as well as the rectifiermaximum junction power capacitancegiven by the AM and harvester; junction this, resistance in accordance affect theto the output maximum impedance power transfer of the CWVM. For thistheorem. reason, inIn this the sense, present previously work the reported maximum works power [12] indicate transferred that frequency into load of the has input also RF been wave, evaluated the number of stages N, the stage capacitances Cs, as well as the rectifier junction capacitance and by implementing the full wave-CWVM using different stage capacitances and different load resistances. junction resistance affect the output impedance of the CWVM. For this reason, in the present work In this way,the maximum it has been power optimized transferred the performance into load has ofalso the been AM-RF evaluated energy by harvesting implementing system. the full wave-CWVM using different stage capacitances and different load resistances. In this way, it has 3. Performancebeen optimized of the the RF performance Energy Harvesting of the AM-RF System energy forharvesting AM Broadcasting system. In order to analyze the functionality of the AM-RF energy harvester, a proof-of-concept device 3. Performance of the RF Energy Harvesting System for AM Broadcasting has been implemented according to the design presented in Section2. The resonator circuit stage plays an importantIn roleorder in to the analyze performance the functionality of AM of harvester, the AM-RF because energy harvester, the amount a proof of the-of-concept harvested device RF energy has been implemented according to the design presented in Section 2. The resonator circuit stage dependsplays strongly an important on it. role Hence, in the a performance resonant circuit of AM with harvester, a high because Q is preferredthe amount inof orderthe harvested to get a lot of energy.RF The energy advantage depends of strongly this topology on it. Hence, is that a itresonant can be circuit easily with modified a high toQ detectis preferred RF energy in order at to different frequenciesget a and lot of power energy. levels The advantage in the AM of band this topology by simply is that tuning it can a be variable easily modified capacitor. to detect RF Figureenergy7a at shows different the frequencies detected and RF power spectrum; levels clearly,in the AM in band the centralby simply part tuning of thea variable spectrum capacitor. appears the 1 MHz signal.Figure Based 7a shows on this, the the detected AM harvesterRF spectrum; has clearly, been tunedin the central to 1 MHz. part of Figure the spectrum7b presents appears the signal the 1 MHz signal. Based on this, the AM harvester has been tuned to 1 MHz. Figure 7b presents the detected by the resonant circuit; it can be observed a value of 1.5 Vpp at 1 MHz. signal detected by the resonant circuit; it can be observed a value of 1.5 Vpp at 1 MHz.
a) b)
Figure 7. (a) RF spectrum of the AM band detected, and (b) AM signal detected by the LC circuit is Figure 7. a b 1.5( V) at RF 1 MHz. spectrum of the AM band detected, and ( ) AM signal detected by the LC circuit is 1.5 V at 1 MHz. Figure 8a shows the output voltage vs the load resistance for a conventional CWVMs based on Figure10 μF8 astage shows capacitors, the output and with voltage different vs number the load of resistancestages, N = 3, for 6 and a conventional 10. For all CWVMs CWVMs it can basedbe on observed that as the number of stages increases no necessarily the output voltage increases; for 10 µF stage capacitors, and with different number of stages, N = 3, 6 and 10. For all CWVMs it can instance, Vout (N = 6) ˃ Vout (N = 10). Moreover, Figure 8b shows the power delivered into the load be observedresistance. that Evidently, as the number when Z ofout stages≈ Rload, increases and N = 6, no a maximum necessarily power the is output dissipated voltage on the increases; load for instance,resistance. Vout (N Therefore,= 6) >Vout as (theN =number 10). Moreover, of stages increases, Figure 8theb showsoutput impedance the power increases. delivered Hence, into by the load resistance.modifying Evidently, N the output when impedance Zout ≈ R loadof the, and systemN can= 6, be a controlled maximum according power to is the dissipated load. on the load resistance. Therefore, as the number of stages increases, the output impedance increases. Hence, by modifying N the output impedance of the system can be controlled according to the load. Sensors 2018, 18, 768 8 of 13 Sensors 2018, 18, x FOR PEER REVIEW 8 of 13
Multipliers with 10F Capacitors Multipliers with 10F Capacitors 14 a) 1000 b) Sensors 2018, 18, x FOR PEER REVIEWNumber of stages Number of stages 8 of 13 3 3 12 6 6 100 Multipliers with 10F Capacitors Multipliers with 10F 10 Capacitors 14 a) 10 1000 b)
10 Number of stages Number of stages W)
3 3 N=6 12 10 6 8 6 N=6 100 10 10
10 6 W) N=10 N=6 1 10 N=3
Output Voltage(V) Output 8 N=6
4 N=10 Power ( Output
0.1 N=3 6 N=10 2 N=3 1 N=6 N=3
Output Voltage(V) Output
4 N=10 Power ( Output 0.01 0 0.1 N=3N=10 2 N=3 0 2 4 6 8 10 12 1E-4N=6 1E-3 0.01 0.1 1 10 100 Load Resistance() 0.01 Load Resistance (M) 0 N=10
FigureFigure 8.0 (a )8. Output (a2) Output voltage,4 voltage6 and, and (8b ()b output) output10 powerpower12 vs vs the1E-4 the load load 1E-3resistance resistance0.01 for for a CWVM0.1 a CWVM with1 with different10 different 100 number of stages.Load Detected Resistance( input) signal in LC resonator is 1.5 V at 1 MHz.Load Resistance (M) number of stages. Detected input signal in LC resonator is 1.5 V at 1 MHz. InFigure addition, 8. (a) Output the output voltage power, and (ofb) theoutput device power has vs been the load evaluated resistance vs forthe a load CWVM resistance with different and stage number of stages. Detected input signal in LC resonator is 1.5 V at 1 MHz. Incapac addition,itors but the by fixing output the power number of of the stages device to N has = 6. been Thus, evaluated three CWVMs vs the with load identical resistance diodes and but stage different stage capacitors, 1 nF, 10 nF, and 10 μF, has been characterized. Results are shown in Figure 9a. capacitorsIn but addition, by fixing the output the number power of of the stages device to hasN =been 6. Thus,evaluated three vs CWVMsthe load resistance with identical and stage diodes It can be seen that output voltage depends also on the stage capacitors; in fact, as the stage but differentcapacitors stage but by capacitors, fixing the number 1 nF, 10 ofnF, stages and to 10 N =µ F,6. Thus, has been three characterized. CWVMs with identical Results diodes are shown but in capacitance increases the output voltage increases. Besides, by plotting the output power of the Figuredifferent9a. It canstage becapacitors, seen that 1 nF, output 10 nF, and voltage 10 μF, depends has been characterized. also on the stageResults capacitors; are shown in in Figure fact, 9 asa. the harvester as a function of the load resistance as shown in Figure 9b, it can be observed that there is stageIt capacitance can be seen increases that output the output voltage voltage depends increases. also on the Besides, stage by capacitors; plotting in the fact, output as the power stage of the an optimal capacitance for maximum output power. In this case, a value of 10 μF for stage capacitors harvestercapacitance as a function increases of the the output load resistance voltage increases. as shown Besides, in Figure by plotting9b, it can the be output observed power that of therethe is delivers the maximum power into the load; thus, capacitance values below and above this value harvester as a function of the load resistance as shown in Figure 9b, it can be observedµ that there is an optimalresult in capacitance a lower out forput maximumpower. Clearly, output the power.experimental In this output case, aimpedance value of 10is aroundF for stage1.5 MΩ capacitors for an optimal capacitance for maximum output power. In this case, a value of 10 μF for stage capacitors deliversthe CWVM the maximum with N power= 6 and intoC = 10 the μ load;F, it means thus, that capacitance loads with values impedances below around and above 1.5 MΩ this will value get result delivers the maximum power into the load; thus, capacitance values below and above this value in a lowerthe maximum output power. power from Clearly, the AM the experimentalharvester. output impedance is around 1.5 MΩ for the CWVM result in a lower output power. Clearly, the experimental output impedance is around 1.5 MΩ for with N = 6 and C = 10 µF, it means that loads with impedances around 1.5 M Ω will get the maximum the CWVM with N = 6 and C = 10 μF, it means that loads with impedances around 1.5 MΩ will get 1000 powerthe from14 maximuma) the AM power harvester. from theMultipliers AM harvester. with 6 stages b) Multipliers with 6 stages 1nF 1nF 12 10nF 10nF 100 10F 14 a) Multipliers with 10 6F stages 1000 b) Multipliers with 6 stages 10 W) 1nF 1nF 10F 12 10nF 8 10nF 10nF 10 10F 100 10F
10F 10nF W)
10 6 10F 1 1nF 8 10nF Power ( Output 10 10F Output Voltage(V) Output 4 1nF 10nF
6 2 0.1
1 1nF Output Power ( Output Output Voltage(V) Output 4 0 1nF 0.01 2 0.1 0 2 4 6 8 10 12 1E-4 1E-3 0.01 0.1 1 10 100 Load Resistance (M) Load Resistance (M) 0 0.01 Figure0 9. (a2) Output4 voltage,6 and 8(b) output10 power12 vs the1E-4 load resistance1E-3 0.01 for a CWVM0.1 with1 N = 610 and 100 Load Resistance (M) with different stage capacitors. Detected input signal in LC resonator isLoad 1.5 Resistance V at 1 MHz. (M )
FigureInFigure 9.order(a )9. Output (toa) Outputclarify voltage, voltage,the effect and and (ofb () bthe output) output number power power of vsstages the the load load and resistance resistancethe output for for a impedance CWVM a CWVM with withon N =the 6N and =output 6 and with different stage capacitors. Detected input signal in LC resonator is 1.5 V at 1 MHz. withpower different of the harvester, stage capacitors. the output Detected power input of the signal RF harvester in LC resonator has been is measured 1.5 V at 1 MHz.as function of such variables. Results are shown in Figure 10a. Evidently, there exists an optimum number of stages In order to clarify the effect of the number of stages and the output impedance on the output which defines a maximum output impedance; as a result, such an optimal point delivers a maximum Inpower order of the to clarifyharvester, the the effect output of thepower number of the RF of stagesharvester and has the been output measured impedance as function on of the such output output power into load. Therefore, an increment in the number of stages does not necessarily powervariables. of the harvester, Results are the shown output in powerFigure 1 of0a. the Evidently, RF harvester there exists has been an optimum measured number as function of stages of such increase the output power; however, an increment in stage capacitances not only increases the which defines a maximum output impedance; as a result, such an optimal point delivers a maximum variables.output Results impedance, are shown but also in the Figure output 10 a.powe Evidently,r, as youthere can see exists in Figure an optimum 10b. number of stages which definesoutput a maximum power into output load. impedance; Therefore, an as a increment result, such in the an optimal number pointof stages delivers does a not maximum necessarily output power increase into load. the output Therefore, power; an however, increment an in increment the number in stage of stages capacitances does not not necessarily only increases increase the the output impedance, but also the output power, as you can see in Figure 10b. output power; however, an increment in stage capacitances not only increases the output impedance, but also the output power, as you can see in Figure 10b. Sensors 2018, 18, 768 9 of 13 Sensors 2018, 18, x FOR PEER REVIEW 9 of 13
30 30 a) b)
25 Sensors 2018, 18, x FOR PEER REVIEW 25 9 of 13
W)
W) 20 20 30 30 a) b) 15 15
25 25
W)
W)
10 20 2010 Maximun Output Power( Output Maximun
Maximun Output Power( Output Maximun 5 15 15 5
0 0 1.6 10 1.6 ) 10 1.4 ) 0 1.4 0 2 1.2 1.2 1.0 Power( Output Maximun 2 1.0
Maximun Output Power( Output Maximun 5 5 Multiplier Capacitors( Number4 of Stages 0.8 4 0.8 6 0.6 6 0.6 8 0.4 0.4 1.6 0 1.6 ) 0 8 ) 0 0.2 1.4 0.2 1.4 10 1.2 0 F) 10 1.2 2 0.0 2 0.0 1.0 1.0 Multiplier Capacitors( Number4 of Stages Optimal Output0.8 Impedance(M 4 Optimal0.8 Output Impedance(M 6 0.6 6 0.6 8 0.4 8 0.4 0.2 0.2 10 FigureFigure 10.10. ((aa)) MaximumMaximum outputoutput0.0 power vs the numbernumber of stages of thetheF) CWVMCWVM10 0.0 and load resistance,resistance, Optimal Output Impedance(M Optimal Output Impedance(M andand ((bb)) MaximumMaximum outputoutput powerpower vsvs ofof thethe stagestage capacitorscapacitors ofof thethe CWVMCWVM andand loadload resistance.resistance. Figure 10. (a) Maximum output power vs the number of stages of the CWVM and load resistance, Figureand 11a (b) Maximumshows an output image power of the vs ofimplemented the stage capacitors AM -ofRF the harvester CWVM and system load resistance. without CWVM. Such Figure 11a shows an image of the implemented AM-RF harvester system without CWVM. Such a a prototype detects RF signals ranging from 540 KHz to 1700 KHz via the tuning capacitor, the signal prototype detects RF signals ranging from 540 KHz to 1700 KHz via the tuning capacitor, the signal detected isFigure shown 11a in shows Figure an 7 imageb, and ofit thecorresponds implemented to a AM commercial-RF harvester AM systemstation without transmitting CWVM. at 100Such0 KHz. detecteda prototype is shown detects in Figure RF signals7b, and ranging it corresponds from 540 K toH azcommercial to 1700 KHz AMvia the station tuning transmitting capacitor, the at signal 1000 KHz. The CWVM used in the present prototype is shown in Figure 11b, and it corresponds to a full-wave The CWVMdetected usedis shown in thein Figure present 7b, and prototype it corresponds is shown to a commercial in Figure 11 Ab,M station and it transmitting corresponds at to100 a0 full-waveKHz. CWVM. As explained previously, the aim for using a full-wave CWVM is to get the double of the CWVM.The CWVM As explained used in previously,the present prototype the aim foris shown using in a Figure full-wave 11b, and CWVM it corresponds is to get to the a full double-wave of the power from the RF signal. Based on the above discussion, in order to obtain the maximum power powerCWVM. from the As RFexplained signal. previously, Based on the the above aim for discussion, using a full in-wave order CWVM to obtain is to the get maximum the double power of the from from the harvester a 6-stages/10 μF dc-dc CWVM with BAT85 rectifiers has been selected. Figure 11c,d the harvesterpower from a 6-stages/10the RF signal.µ FBas dc-dced on CWVM the above with discussion, BAT85 in rectifiers order to has obtain been the selected. maximum Figure power 11 c,d showfrom a top the andharvester lateral a 6 -viewstages/10 respectively μF dc-dc CWVM of the withRF detectorBAT85 rectifiers (AM resonator) has been selected. and the Figure CWVM 11c,d once show a top and lateral view respectively of the RF detector (AM resonator) and the CWVM once they they showhave abeen top integrated.and lateral view respectively of the RF detector (AM resonator) and the CWVM once have been integrated. they have been integrated. a) b) a) b)
d) d) c)
Figure 11. (a) AM resonator implemented, (b) Full-wave CWVM implemented, (c) top view of the Figure 11. (a) AM resonator implemented, (b) Full-wave CWVM implemented, (c) top view of the AM FigureAM 11. resonator (a) AM andresonator full-wave implemented, CWVM, and ( b()d )Full lateral-wave view CWVM of the implemented, AM resonator ( andc) top full view-wave of the resonator and full-wave CWVM, and (d) lateral view of the AM resonator and full-wave CWVM. AM CWVM. resonator and full-wave CWVM, and (d) lateral view of the AM resonator and full-wave CWVM. The performanceThe performance of theof the AM-RF AM-RF harvester harvester based on on a a full full-wave-wave CWVM CWVM is evaluated is evaluated by way by wayof of the maximum output power transferred into the load. To compare the performance, such system is the maximumThe performance output powerof the AM transferred-RF harvester into the based load. on To a comparefull-wave the CWVM performance, is evaluated such by system way of is contrasted against a conventional CWVM (half-wave), results are shown in Figure 12a. As expected, contrasted against a conventional CWVM (half-wave), results are shown in Figure 12a. As expected, the maximumthe maximum output power power obtained transferred from a full into-wave the CWVM load. Tois twofold compare that the of theperformance, conventional such one. system is thecontrasted maximum against power a conventional obtained from CWVM a full-wave (half-wave), CWVM results is twofold are shown that of in the Figure conventional 12a. As expected, one. the maximum power obtained from a full-wave CWVM is twofold that of the conventional one.
Sensors 2018, 18, 768 10 of 13 Sensors 2018, 18, x FOR PEER REVIEW 10 of 13
1000 CWVMs with 6 stages and C=10F a) Half-wave CWVM b) 100 Full-wave CWVM P=32W
W)
10
1
Output Power( Output 0.1
0.01
1E-4 1E-3 0.01 0.1 1 10 Load Resistance (M)
Figure 12. 12.( a()a) Output Output power power for fora half-wave a half-wave and full-wave and full- CWVM,wave CWVM, and (b) portableand (b) electronic portable electroniccalculator runningcalculator with running the AM with energy the AM harvester. energy harveste Detectedr. Detected input signal input in signal LC resonator in LC resonator is 1.5 V is at 1.5 1 MHz.V at 1 MHz.
Figure 12b shows an image of the AM-RF energy harvester connected to a portable electronic Figure 12b shows an image of the AM-RF energy harvester connected to a portable electronic calculator. Evidently, the device works properly, it is worth mentioning that prior to this action, the calculator. Evidently, the device works properly, it is worth mentioning that prior to this action, the battery was removed from the calculator, thus the device is working only with the RF energy battery was removed from the calculator, thus the device is working only with the RF energy harvested harvested from a commercial AM station located at 2.5 Km. away from the harvester. AM station from a commercial AM station located at 2.5 Km. away from the harvester. AM station broadcasting broadcasting frequency and power is 1 MHz and 10 KW respectively. frequency and power is 1 MHz and 10 KW respectively. On the other hand, it is well known that total system efficiency depends on the efficiency of On the other hand, it is well known that total system efficiency depends on the efficiency of each each of the stages that are part of the system. Thus, each component introduces its own inefficiency of the stages that are part of the system. Thus, each component introduces its own inefficiency to to the entire system. Each efficiency is multiplied together to obtain an overall efficiency for the the entire system. Each efficiency is multiplied together to obtain an overall efficiency for the system. system. In this case, the AM harvester system is mainly composed of three stages; the LC resonator In this case, the AM harvester system is mainly composed of three stages; the LC resonator (LC tuned (LC tuned antenna), the Dc-Dc CWVM (boost converter), and the load. So, the overall efficiency of antenna), the Dc-Dc CWVM (boost converter), and the load. So, the overall efficiency of the system is the system is
휂푡표푡푎푙ηtotal==((휂η푎푛푡ant)()η(휂CWVM퐶푊푉푀)()η(load휂푙표푎푑) ) (10)(10)
The antenna efficiencyefficiency is defineddefined as the portion of power that is not absorbed in the antenna structure as ohmic losses. This is characterized by the radiation resistance, Rr, and the ferrite loss structure as ohmic losses. This is characterized by the radiation resistance, Rr, and the ferrite loss resistance, Rf, with which the antenna efficiency, ηant, is resistance, Rf, with which the antenna efficiency, ηant, is 푅푟 휂푎푛푡 = Rr (11) ηant = 푅푟 + 푅푓 (11) Rr + R f Hence, the antenna efficiency can be derived by combining Equations (1) and (2) into Equation (9). Hence, the antenna efficiency can be derived by combining Equations (1) and (2) into Equation (9). Therefore, the estimated antenna efficiency is around 3.2% taking into consideration that we are Therefore, the estimated antenna efficiency is around 3.2% taking into consideration that we are using using a double LC resonator configuration. This value is very superior to the value of 휂푎푛푡 = 0.04% a double LC resonator configuration. This value is very superior to the value of η = 0.04% reported reported for a single ferrite core LC resonator used as AM-RF energy harvester [25,26].ant for a single ferrite core LC resonator used as AM-RF energy harvester [25,26]. Besides, the efficiency of a Dc-Dc CWVM can be estimated from Besides, the efficiency of a Dc-Dc CWVM can be estimated from 푃퐿 휂퐶푊푉푀 = P 2 (12) η = 푃 + 2퐶 푓L (푉 ⁄푁2) (12) CWVM 퐿 푗 2퐿 2 PL + 2Cj f VL /N where PL and VL stands for power and voltage on the load respectively. N is the number of stages whereand Cj PandL and f theVL diodestands junction for power capacitance and voltage and onoperating the load frequency respectively. of theN isCWVM. the number Evidently, of stages the andefficiencyCj and dependsf the diode on the junction value of capacitance the load. In andcase operating of the load frequency matching, of Equation the CWVM. (12) transforms Evidently, into the efficiency depends on the value of the load. In case of the load matching, Equation (12) transforms into 푃퐿푚푎푥 휂 = 퐶푊푉푀 P 2 2 (13) η =푃퐿푚푎푥 + 2퐶Lmax푗푓(푉퐿푚푎푥⁄푁 ) (13) CWVM 2 2 PLmax + 2Cj f VLmax/N By taking the experimental data of PLmax and RLmax from Figure 12a, VLmax can be calculated. Moreover, junction capacitance for BAT85 diode is around 5 pF, operating frequency at 1 MHz, and
N = 6, and 12 for the half-wave and full-wave CWVMs respectively. Hence, efficiencies 휂ℎ푤 = 70.6% and 휂푓푤 = 90.6% are estimated for the half-wave and full-wave boost converters respectively. Such
Sensors 2018, 18, 768 11 of 13
By taking the experimental data of PLmax and RLmax from Figure 12a, VLmax can be calculated. Moreover, junction capacitance for BAT85 diode is around 5 pF, operating frequency at 1 MHz, and N = 6, and 12 for the half-wave and full-wave CWVMs respectively. Hence, efficiencies ηhw = 70.6% and η f w = 90.6% are estimated for the half-wave and full-wave boost converters respectively. Such efficiency values are competitive with previously reported boost converters based on the discontinuous mode architecture which present an efficiency of 60% [25,26]. Finally, the efficiency of resistive loads is 100% because the total power supplied to the resistive element is dissipated; therefore, based on Equation (10) the overall efficiency of the AM-RF harvester system is 1.13% and 2.9% for the half-wave and full-wave boost converter respectively. Such value could seem to be a low efficiency; however, such efficiencies are 47 and 120 times larger than that of 0.024% estimated for AM-RF energy harvester based on a single ferrite core LC resonator with ηant = 0.04%, and a discontinuous mode converter with ηConverter = 60% [25,26]. To compare, Table1 shows the amount of RF energy harvested from different RF power sources at different distances. In general, the power harvested in the present work is very significant, especially if we consider the broadcasting distance as well as the power. For instance, the maximum power harvested by a similar harvester previously reported by Wang et al. [25,26] at a distance of 2.5 km is 0.6 µW, whereas that of the system here developed is 62 µW. Besides, for a harvester with similar power harvested of 60 µW at 4.1 km, the broadcasting power must be as high as 960 KW.
Table 1. Comparison among several RF energy harvesters.
Frequency Broadcasting Power Distance Energy Harvested 1 MHz [This work] 10 KW 2.5 km 62 µW 1.27 MHz [25,26] 50 KW 2.5 km 0.6 µW 902–928 MHz [27] 4 W 15 m 5.5 µW 868 MHZ [28] 1.78 W 25 m 2.3 µW 915 MHz [29] 3 W 11 m 1 µW 674–680 MHz [30] 960 KW 4.1 km 60 µW
4. Conclusions An AM-RF energy harvester has been developed as a proof-of-concept device so as to power up low power consumption devices such as portable electronic calculators located at 2.5 km away from the AM broadcasting. The maximum RF power harvested is achieved via a full-wave six-stage CWVM based on BAT85 diodes and 10 µF capacitors, and an efficiency η f w = 90.6%. In this sense, experimental data show that the output impedance of the device depends strongly on the stage capacitors. In fact, as the stage capacitance increases, the output impedance of the harvester also increases. This striking characteristic allows for the control of the output impedance of the harvester system according to the input impedance of the load with the aim of transferring the maximum power into it. In fact, a system with output impedance of 1.5 MΩ delivers a maximum power of 62 µW into an electronic calculator. Hence, it is demonstrated that energy-harvesting technologies combined with low power consumption ICs eliminate the need for batteries, removing an obstacle to the success of the emerging networks such as IoT, SS, and SC. Therefore, it is demonstrated that AM-RF signals can be used as an alternative source of “free energy” to power the next generation of wireless networks.
Acknowledgments: J. A. Leon-Gil, A. Fabian-Mijangos, J. J. Martinez-Flores, M. Tovar-Padilla acknowledge Conacyt Mexico for fellowship. This work was supported by the Mexican Council for Science and Technology-Conacyt Mexico, through the Grants for fundamental research No. 241597 and national issues No.1358. Author Contributions: J. Alvarez and A. Morales conceived and designed the experiments; A. Cortes and J. Leon performed the experiments; A. Fabian and J. Martinez analyzed the data; A. Cardona and M. Tovar retrieved the literature as well as wrote the paper; Besides J. Alvarez was responsible for the revision and discussion of the final version. Conflicts of Interest: The authors declare no conflict of interest. Sensors 2018, 18, 768 12 of 13
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