energies

Article Development of a Tree-Shaped Hybrid Nanogenerator Using Flexible Sheets of Photovoltaic and Piezoelectric Films

Rahate Ahmed, Yeongmin Kim, Zeeshan and Wongee Chun * Department of Nuclear and Energy Engineering, Jeju National University, Jeju 63243, Korea; [email protected] (R.A.); [email protected] (Y.K.); [email protected] (Z.) * Correspondence: [email protected]; Tel.: +82-064-754-3646

 Received: 29 October 2018; Accepted: 4 January 2019; Published: 12 January 2019 

Abstract: This paper reports on the feasibility of a tree-shaped hybrid nanogenerator (TSHG) made of flexible sheets of photovoltaic (PV) and piezoelectric (piezo) films for harnessing both wind and solar energy. The proposed system has been designed to produce if there is any light, wind or strong rainfall. It shows how the power developed by each piezo film sheet was integrated in conjunction with its limited power output which is produced by the sporadic movement of the sheets. Regardless of its magnitude, the AC power output of each piezo film sheet was converted with a full wave bridge rectifier and then passed to a capacitor. The TSHG has an excellent performance with an open circuit voltage of 5.071 V, a short-circuit current of 1.282 mA, and a maximum power output of 3.42 mW at a loading resistance of 5 kΩ. Moreover, a wind driven TSHG was capable of charging a 1000 µF capacitor, which was subsequently discharged through LED lighting.

Keywords: hybrid nanogenerator; sheets of piezoelectric and photovoltaic films; full wave bridge rectifier; wind energy; solar energy

1. Introduction Nowadays, the importance of renewable energy production has increased significantly with the increase in global warming [1–3]. The Sun is the most abundant renewable energy source in the world. Solar energy can be exploited in different ways; for example, photovoltaic (PV) power generation is one of the most reliable and widely applied technologies around the world. Solar cells, the basic elements of PVs, are designed to convert photons to electrical energy using the photovoltaic effect [4–6]. The first modern photovoltaic based on silicon was invented by Russel [7]. First generation solar cells were thin silicon wafers that directly convert light energy into electrical energy. Until the development of a thin film solar cell, silicon wafer-based technology was the most popular. Recent advancement in nanotechnology, however, has given birth to the second generation of solar cells [8,9] with improved performance and flexibility [10,11]. Especially, flexible PV films have significantly expanded the area of solar energy application because they are bendable, easy to handle and adaptable to various operations [12,13]. They enable flexibility in harvesting solar energy for power generation integrated with other means of energy conversion for maximum energy efficiency. The piezoelectric effect has attracted much attention these days in harnessing small , especially from the wind. Many studies have been done that used PV technology in conjunction with piezoelectric (piezo) materials for the generation of electric power from both solar and wind energy [14–16]. Recently, a single hybrid cell was developed to harvest both solar and mechanical power at the same time [17–19]. A micro-cable structured solar and mechanical energy harvester was proposed by Chen and et al. [20]; they demonstrated its performance by charging a 2 µF capacitor up to 2V in 1 minute.

Energies 2019, 12, 229; doi:10.3390/en12020229 www.mdpi.com/journal/energies Energies 2019, 12, 229 2 of 10

During the last decade, technological breakthroughs in the synthesis of piezoelectric materials have expedited the development new designs of piezoelectric nanogenerators [21,22]. Wang and Song successfully introduced an innovative piezoelectric nanogenerator in 2006, which was able to produce several millivolts by bending a ZnO with the tip of an atomic force microscope (AFM) [23]. This attracted many researchers because it opened a new field in nanoscale applications of piezoelectric materials. Since then, researchers around the world have come up with different designs of nanogenerators for diverse applications focusing on practicability [24–27]. However, in diversifying the electricity generation system, many researchers came up with the idea of simultaneously harnessing wind and solar energy. Kekkogg et al. [28] proposed the optimal unit sizing for a hybrid wind/photovoltaic generating system, in which they investigated a stand-alone and network connected system. In addition, to scavenge wind power by using nanomaterials, Oh et al. [29] proposed a tree shaped generating system. They compared output power by utilizing lead-zirconium-titanium (PZT) and polyvinylidene fluoride (PVDF) films. An r-shaped hybrid nanogenerator was designed and fabricated by Han et al. [30] to enhance in the presence of triboelectric charges. In their experiment, a power density of 2.04 mW/cm3 was produced by the triboelectric device while 10.95 mW/cm3 was produced by the piezoelectric device. For ambient energy scavenging, a flat panel-shaped hybrid piezo/triboelectric nanogenerator was proposed by Hassan et al. [31]. Their hybrid device produced a combined peak voltage of 300 V in an open circuit and a maximum short-circuit current density of 16 mA/cm3 at 50 Ω load. The present work introduces a high-performance system, which is a tree- shaped hybrid nanogenerator (TSHG) made of flexible solar cells (films) and piezoelectric film sheets. To experimentally fabricate a TSHG, fluorinated ethylene propylene (FEP) films were used to make the structure of leafs. A solar cell was attached to the top surface of each leaf while a sheet of piezo film was attached to its other (bottom) side. To demonstrate its operational characteristics and functional robustness of the TSHG, a capacitor was used to store electricity and then applied to an external load.

2. Operation of the TSHG for Motion and Solar Energy Harvesting

2.1. Fabrication of the (PVDF) Polymer-Based Leaf Some single crystal materials show the following phenomenon which describes the operating principle of piezoelectric materials to produce electric potential. Due to the deformation of a crystal by application of an external force, electric charges appear on certain faces of the crystal. When the exerted force is released, the polarity of the electric charge changes [32]. This property of the crystal is called the piezoelectric effect, a typical structure of piezoelectric crystals as shown in Figure1a. Currently, different types of PVDF based piezoelectric films are available. In the present study, a nanogenerator made of a LDT4-028K/L piezo film was used in conjunction with the reclamation of wind energy. When a sheet of LDT4-028K/L film is induced to bend, it generates voltage proportionate to the degree of bending. Table1 and Figure1b give a detailed description of PVDF used in this work. To facilitate the harvesting of wind energy, an artificial tree was made with many leaves, where each leaf had sheets of piezo films attached, as shown in Figure2b.

Table 1. Dimensions of a piezo film (LDT4-028K/L) [26]

Film Thickness A Film B Electrode C Film D Electrode Total Thickness Capacitor (µm) (mm) (mm) (mm) (mm) (µm) (nF) 28 21 18 170 155.7 205 11 Energies 2019, 12, 229 3 of 10 Energies 2018, 11, x FOR PEER REVIEW 3 of 10 Energies 2018, 11, x FOR PEER REVIEW 3 of 10

FigureFigure 1. 1. Piezoelectricity:Piezoelectricity: ( (aa)) piezoelectric piezoelectric effect effect generated generated by by an an external external force force [33] [32]( (bb)) dimensions dimensions of of Figure 1. Piezoelectricity: (a) piezoelectric effect generated by an external force [33] (b) dimensions of aa piezo piezo film film (with (with reference to Table 1 1))[ [26].26]. a piezo film (with reference to Table 1) [26]. 2.2.2.2. Fabrication Fabrication of of the the Photovoltaic Photovoltaic Cell Cell Based Based Leaf Leaf 2.2. Fabrication of the Photovoltaic Cell Based Leaf AA solar cell converts light energy into electricity by the photovoltaic effect. Hundreds Hundreds of of solar solar A solar cell converts light energy into electricity by the photovoltaic effect. Hundreds of solar cellscells make make up up a photovoltaic arra array.y. Solar cells contain materials with semiconducting properties properties in in cells make up a photovoltaic array. Solar cells contain materials with semiconducting properties in which their their electrons electrons become become excited excited and and turn turn into into an an electrical electrical current current when when struck struck by by light light energy. energy. which their electrons become excited and turn into an electrical current when struck by light energy. Among the the many many kinds kinds of of solar solar cells, cells, the the present present study study used used a a thin thin film film solar solar cell cell (infinityPV, (infinityPV, Ltd., Ltd., Among the many kinds of solar cells, the present study used a thin film solar cell (infinityPV, Ltd., Jyllinge,Jyllinge, Denmark) Denmark) which was fixed fixed onto the surface of a leaf.leaf. Figure Figure 22aa describesdescribes thethe fabricationfabrication Jyllinge, Denmark) which was fixed onto the surface of a leaf. Figure 2a describes the fabrication processprocess involved in conjunction with exploiting solar energy. Our TSHG hashas threethree piecespieces (110(110 mmmm ×× process involved in conjunction with exploiting solar energy. Our TSHG has three pieces (110 mm × 5050 mm) mm) of of leaves leaves with a PV film. film. 50 mm) of leaves with a PV film.

Figure 2. Structure of the generator (a) flexible photovoltaic film affixed onto the surface of a leaf (b) a Figure 2. Structure of the generator (a) flexible photovoltaic film affixed onto the surface of a leaf (b) FigurePVDF film2. Structure attached of with the bottomgenerator of the(a) flexible leaf (c) photovoltaicphotovoltaic filmfilm andaffixed PVDF onto film-based the surface leaf. of a leaf (b) a PVDF film attached with bottom of the leaf (c) photovoltaic film and PVDF film-based leaf. 3. Manufacturinga PVDF film attached of the wi TSHGth bottom of the leaf (c) photovoltaic film and PVDF film-based leaf. 3. Manufacturing of the TSHG 3. ManufacturingThe main objective of the of TSHG this work was to design a hybrid system that simultaneously converts wind and solarThe main (light) objective energy intoof this electricity. work was To to implement design a this,hybrid an system artificial that tree simultaneously was designed shownconverts in The main objective of this work was to design a hybrid system that simultaneously converts windFigure and3. There solar were (light) ten energy leaves oninto the electricity. TSHG. Of To these, implement three had this, both an PV artificial and piezo tree films was fixed designed onto wind and solar (light) energy into electricity. To implement this, an artificial tree was designed showneither sidein Figure of a leaf 3. There while were each ten of the leaves remaining on the leavesTSHG. wasOf these, affixed three with had a sheet both ofPV piezo and filmpiezo shown films shown in Figure 3. There were ten leaves on the TSHG. Of these, three had both PV and piezo films fixedin Figure onto2 b.either By doingside of this, a leaf the while TSHG each was of capable the remaining of capturing leaves small was affixed disturbances with a causedsheet of by piezo the fixed onto either side of a leaf while each of the remaining leaves was affixed with a sheet of piezo filmwind shown and producing in Figure a high2b. By voltage doing for this, generating the TSHG electricity. was capable The fine of wirescapturing to transmit small thedisturbances electricity film shown in Figure 2b. By doing this, the TSHG was capable of capturing small disturbances causedgenerated by bythe the wind sheets and of piezoproducing and PV a filmshigh werevoltage placed forin generating the hollow electricity. space available The atfine the wires center to of caused by the wind and producing a high voltage for generating electricity. The fine wires to transmitthe trunk the (hollow electricity pipe). generated An actuator by circuit the sheets was installed of piezo at and the PV bottom films of were the tree, placed where in eachthe hollow output transmit the electricity generated by the sheets of piezo and PV films were placed in the hollow space available at the center of the trunk (hollow pipe). An actuator circuit was installed at the space available at the center of the trunk (hollow pipe). An actuator circuit was installed at the bottom of the tree, where each output of the generators was integrated. The alternating output bottom of the tree, where each output of the generators was integrated. The alternating output

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Energies 2018, 11, x FOR PEER REVIEW 4 of 10 of the generators was integrated. The alternating output power (i.e., AC current) from each sheet of powerEnergies (i.e., 2018 AC, 11 , current)x FOR PEER from REVIEW each sheet of piezo film was transmitted to a bridged before4 of 10 it piezo film was transmitted to a bridged rectifier before it was stored in a capacitor. was stored in a capacitor. power (i.e., AC current) from each sheet of piezo film was transmitted to a bridged rectifier before it was stored in a capacitor.

FigureFigure 3. Tree 3. Tree shaped shaped hybrid hybrid nanogenerator nanogenerator (TSHG) (TSHG) (a) (conceptuala) conceptual design design (b) ( bactually) actually fabrication. fabrication. Figure 3. Tree shaped hybrid nanogenerator (TSHG) (a) conceptual design (b) actually fabrication. Meanwhile,Meanwhile, the the DC DC output output (DC (DC current) current) by bythe the PV PV films films was was directly directly used used to tocharge charge the the same same capacitorcapacitorMeanwhile, by by passing passing the any DC any rectifier. output rectifier. (DC When When current) sheets sheets by of ofthe both PV thefilms PV was andand directly piezopiezo filmsusedfilms were towere charge affixed affixed the onto same onto a leaf,a leaf,thecapacitor the leaf leaf was by was apassing bit a stifferbit anystiffer than rectifier. than the leavesthe When leaves with sheets with only of sheetsonlyboth thesheets of PV piezo ofand piezo films. piezo films. It, films however, It,were however, affixed was still wasonto bendable stilla bendablebyleaf, a moderatethe by leaf a moderatewas wind. a bit A stifferwind. series than A of series tests the leaves were of tests performed with we onlyre performed sheets to assess of piezo theto assess practicability films. the It, practicabilityhowever, of the TSHGwas ofstill underthe TSHGvariousbendable under wind by various a speeds moderate wind using wind.speeds an electricA using series fanan of electric showntests we infanre Figure performedshown3. in Figure to assess 3. the practicability of the TSHG under various wind speeds using an electric fan shown in Figure 3.

Figure 4. Circuit for the integration of the power output of the TSHG (a) logic diagram (b) power FigureFigure 4. 4.Circuit Circuit for for the the integration integration of of the the powerpower outputoutput ofof thethe TSHGTSHG ( a(a) )logic logic diagram diagram (b ()b power) power management circuit. circuit. management circuit. 4.4. Circuit Construction for for the the TSHG TSHG 4. Circuit Construction for the TSHG The power output output by by the the sheets sheets of of the the PV PV films films was was enough enough to readily to readily run runa small a small electronic electronic The power output by the sheets of the PV films was enough to readily run a small electronic devicedevice while that that by by the the sheets sheets of of the the piezo piezo films films was was insufficient insufficient to directly to directly drive drive a device. a device. This Thisis is devicesomewhatsomewhat while expectedthat expected by the because because sheets each of each the sheet sheetpiezo of of piezofilms piezo wasfilm film insufficientis only is only capable capable to directly of producing of producing drive a device.small a small power This power is somewhatoutputoutput due expected to its its inherent inherentbecause technical technicaleach sheet limit. limit. of Topiezo To resolve resolve film this, is this, only a capacitor a capacitorcapable was of was placedproducing placed before beforea smallany anypower power power outputwas applieddue to itsto inherentthe load. technicalFigures 4a,b limit. show To resolvethe logic this, and a actual capacitor design was details placed of beforethe constructed any power wascircuit applied for theto theintegration load. Figures of the PV 4a,b and show piezo the power logic output. and actual For a moderatedesign details level ofof wind the constructedspeed (7– circuit for the integration of the PV and piezo power output. For a moderate level of wind speed (7–

Energies 2019, 12, 229 5 of 10 was applied to the load. Figure4a,b show the logic and actual design details of the constructed circuit for the integration of the PV and piezo power output. For a moderate level of wind speed (7–8 m/s), aEnergies single 2018 piezo, 11, x filmFOR PEER generates REVIEW a very small amount of electricity with a very high spike of5 of voltage. 10 To deal with this situation, a full wave bridge rectifier was used to handle the electrical output from 8 m/s), a single piezo film generates a very small amount of electricity with a very high spike of a single piezo film; the electrical output was rectified to charge a capacitor. That is, the electrical voltage. To deal with this situation, a full wave bridge rectifier was used to handle the electrical outputoutput from of each a single piezo piezo film wasfilm; processedthe electrical with output a rectifier was rectified before to it wascharge integrated a capacitor. with That the is, others the to chargeelectrical a capacitor.output of Beforeeach piezo proposing film was this processed power accumulating with a rectifier circuit, before attempts it was integrated were made with to integrate the theothers generated to charge AC a capacitor. output power Before by proposing piezo films this withoutpower accumulating using a separate circuit, rectifier attempts circuit. were made However, thisto integrate was not the feasible generated for due AC to output sporadic power movements by piezo films of the without piezo films using and a sepa producedrate rectifier very circuit. low output power.However, To this overcome was not this feasible technical for due problem, to sporadic the proposedmovements circuit of the was piezo designed films and for produced maximum very power accumulationlow output power. by exploiting To overcome the piezo this films.technical problem, the proposed circuit was designed for maximum power accumulation by exploiting the piezo films. 5. Performance Measurement 5. Performance Measurement 5.1. Basic ElectricalOoutput of the Piezoelectric Nanogenerator 5.1. BasicBefore ElectricalOoutput conducting full of scalethe Piezoelectric measurements, Nanogenerator the basic electrical output from a single piezo film was measured.Before Evenconducting under full small scale wind measurements, speeds, a sheet the ba ofsic piezo electrical film was output capable from ofa single generating piezo electricalfilm signals.was measured. A positive Even potential under small generates wind in speeds, the circuit a sheet when of apiezo force f appliedilm was to capable a piezo of film. generating Conversely, releasingelectrical signals. the applied A positive force potential created agenerates negative in potential the circuit in when the circuit. a force Figureapplied5 toshows a piezo the film. output characteristicsConversely, releasing of a piezoelectric the applied force nanogenerator. created a negative Figure 5potentiala schematically in the circuit. shows Figure the operation 5 shows the of the nanogenerator,output characteristics where of a a force piezoelectric is applied nanogenerator. onto and released Figure 5a from schematically a simple piezo shows film. the operation Subject to the externalof the nanogenerator, force applied where on the a crystalforce is structure, applied onto an electricaland released signal from is generateda simple piezo in the film. connected Subject to circuit. Thethe external power output force applied of a piezoelectric on the crystal nanogenerator structure, an depends electrical on signal the magnitudeis generated of in oscillations the connected induced oncircuit. the filmThe power surface. output A short-circuit of a piezoelectric current nanogenerator generated during depends the on process the magnitude of applying of oscillations and releasing ainduced force on on a the single film piezo surface. film A is short-circuit shown in Figure current5b. generated At wind during speeds the of 7–8process m/s, of a applying single piezo and film generatedreleasing a aforce short-circuit on a single current piezo film of 2.72 is shownµA and in anFigure open 5b. circuit At wind voltage speeds of of 1.2 7–8 V. m/s, A sample a single of the piezo film generated a short-circuit current of 2.72 µA and an open circuit voltage of 1.2 V. A sample rectified electrical output during such a process is shown in Figure5d. of the rectified electrical output during such a process is shown in Figure 5d.

Figure 5. PerformancePerformance of of a asimple simple piezoelectric piezoelectric nanogenerator nanogenerator by a by compressive a compressive force: force: (a) a (forcea) a force applied and and released released on on a simple a simple piezo piezo film film(b) short-circuit (b) short-circuit current current generated generated by a single by piezo a single film piezo film(c) open (c) opencircuit circuit voltage voltage generated generated during the during bending the bendingand releasing and releasingmodes (d) modesrectified (d open) rectified circuit open circuitvoltage. voltage.

Measurements were made for an open circuit with 10 sheets of piezo films connected separately with the rectifier. To simulate mechanical forces on the piezo film in real situations, an electric fan

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Measurements were made for an open circuit with 10 sheets of piezo films connected separately with the rectifier. To simulate mechanical forces on the piezo film in real situations, an electric fan was usedEnergies generating 2018, 11 a, x steady FOR PEER flow REVIEW of wind. Different wind speeds were tested to investigate the6 movement of 10 of the leaves as they deform constantly and develop electric power by air flow. At the start of the air was used generating a steady flow of wind. Different wind speeds were tested to investigate the flow, large spikes of AC voltage were developed in the piezo films, which were subsequently rectified movement of the leaves as they deform constantly and develop electric power by air flow. At the µ and integratedstart of the toair produce flow, large an spikes open circuitof AC voltagevoltage were of 4.89 developed V and a in short-circuit the piezo films, current which of 9.32were A at windsubsequently speeds of 7–8 rectified m/s as and shown integrated in Figure to produce6a,b. Furtheran open testscircuit were voltage conducted of 4.89 V atand different a short-circuit operating conditionscurrent (windof 9.32 speeds).µA at wind At speeds wind of speeds 7–8 m/s ranging as shown from in Figures 5 m/s 6a,b. to 6 Further m/s, the tests respective were conducted integrated openat circuit different voltage operating and conditions short-circuit (wind current speeds). were At recordedwind speeds to beranging 2.65 Vfrom and 5 5.94m/s toµA, 6 m/s, as shown the in Figurerespective6c,d. For integrated the case ofopen wind circuit speeds voltage between and shor 3 m/st-circuit and current 4 m/s, were the recorded to values be 2.65 were V and 1.68 V and 4.835.94 µA,µA, as respectively, shown in Figures as shown 6c,d. inFor Figure the case6e,f. of The wind output speeds parameters between 3 ofm/s the and piezo 4 m/s, films the were affectedrecorded by deviations values were in wind 1.68 speed.V and To4.83 investigate µA, respectively, the performance as shown characteristicsin Figures 6e,f. of The each output piezo film parameters of the piezo films were affected by deviations in wind speed. To investigate the in conjunction with wind directions, a series of tests were conducted repeatedly to find the conditions performance characteristics of each piezo film in conjunction with wind directions, a series of tests that give the highest output. These conditions were then adopted to get the maximum power output were conducted repeatedly to find the conditions that give the highest output. These conditions fromwere the TSHG.then adopted to get the maximum power output from the TSHG.

FigureFigure 6. Electrical 6. Electrical output output of of thethe piezo films: films: (a)( arectified) rectified open open circuit circuit voltage voltage at wind at speeds wind of speeds 7–8 of 7–8 m/sm/s (b) short-circuit current current at at wind wind speeds speeds of of7–8 7–8 m/s m/s (c) (rectifiedc) rectified open open circuit circuit voltage voltage at wind at wind speedsspeeds of 5–6 of m/s5–6 m/s (d )(d short-circuit) short-circuit current current atat windwind speeds speeds of of 5–6 5–6 m/s m/s (e) (rectifiede) rectified open open circuit circuit voltage voltage at windat wind speeds speeds of 3–4 of 3–4 m/s m/s (f )(f short-circuit) short-circuit currentcurrent at at wind wind speeds speeds of of3–4 3–4 m/s. m/s.

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5.2.5.2. Basic Basic Electrical Electrical Output Output of the of thePhotovoltaic Photovoltaic Solar Solar Cell Cell TheThe bending bending flexibility flexibility suffered suffered when when a aleaf leaf was was affixed affixed withwith bothboth PVPV and and piezo piezo films. films. This This was wasmainly mainly due due to theto the inherent inherent stiffness stiffness of the of solarthe solar cell film,cell film, and underminedand undermined the performance the performance of the piezoof thefilms. piezo Consequently, films. Consequently, the power theoutput power byoutput the piezo by the films piezo on films these on leaves these was leaves smaller was thansmaller that than of the 2 thatleaves of the only leaves affixed only with affixed piezo with flims. piezo When flims. illuminated When illuminated with an with intensity an intensity of 478 lumen/m of 478 lumen/m2, the solar, thecell solar film cell (110 film mm (110× mm50 mm) × 50 produced mm) produced 1.29 mW 1.29 of mW power of power output output at a loading at a loading resistance resistance of 2 kΩ ofand 2 a kΩshort-circuit and a short-circuit current current of 1.28 mAof 1.28 with mA an with open an circuit open voltage circuit ofvoltage 1.93 V, of as 1.93 shown V, as in shown Figure in7a. Figure 7a.

Figure 7. Combined output parameters of the TSHG: (a) combined open circuit voltage (b) I-V Figure 7. Combined output parameters of the TSHG: (a) combined open circuit voltage (b) I-V characteristics curve of a single PV film (c) combined short-circuit current (d) power output at different characteristics curve of a single PV film (c) combined short-circuit current (d) power output at loading conditions. different loading conditions. 6. Output Measurements of the Tree-Shaped Hybrid Nanogenerator (TSHG) 6. Output Measurements of the Tree-Shaped Hybrid Nanogenerator (TSHG) As the power output of the piezo films were varied with the wind speeds, the power output ofAs the the PV power films changed output of in the a manner piezo films proportionate were varied to with the light the wind intensity. speeds, Before the combingpower output the power of theoutput PV films of the changed piezo andin a PVmanner films, proportionate measurements to were the madelight intensity. separately Before on each combing for comparison. the power In a output of the piezo and PV films, measurements were made separately on each for comparison. In a luminous environment of 700 lx on an indoor task plane made by fluorescent lamps, the Voc and Isc luminouswere 5.06 environment V and 1.264 of mA, 700 respectively. lx on an indoor As aforementioned task plane made above, by fluorescent the power outputlamps, ofthe the Voc piezo and filmsIsc werewas 5.06 measured V and at1.264 moderate mA, respectively. wind speeds As of 7–8aforem m/s.entioned Figure7 above,gives an the example power ofoutput the combined of the piezo power filmsoutput was bymeasured the TSHG, at moderate where a wind maximum speeds power of 7–8 output m/s. Figure of 3.42 7 gives mW atan a example loading of resistance the combined of 5 k Ω powerwas observedoutput by as the shown TSHG, in where Figure a7 b.maximum Figure7c,d power are the output results of of3.42 the mW combined at a loading output resistance for the TSHG of 5 kΩconsisting was observed of 10 as piezo shown films in and Figure three 7b. photovoltaic Figures 7c,d films, are the which results gave of the the maximum combined Voc output of 5.071 for Vthe and TSHGmaximum consisting Isc ofof 1.28110 piezo mA, films respectively. and three The phot electricovoltaic energy films, saved which in gave the capacitor the maximum was discharged Voc of 5.071through V and an maximum LED. Isc of 1.281 mA, respectively. The electric energy saved in the capacitor was dischargedFigure through8 shows an threeLED. different cases of charging the capacitor. Figure8a shows a fast charging process delivered by the PV films. It took only 15 seconds to charge the capacitor to its saturated electric potential of 5.045 V. Figure8b shows the case when it was charged only by the generated currents from 10 sheets of piezo films. As shown, the capacitor was charged very slowly. It took 35 min to charge the capacitor up to 4.38 V. As compared to the case of the piezo films, Figure8c shows

Energies 2019, 12, 229 8 of 10 the charging of the capacitor when both outputs are used from the sheets of the piezo and PV films. As shown, no appreciable difference is observed compared to Figure8a because most of the power is Energiesgenerated 2018,by 11, thex FOR PV PEER films. REVIEW 8 of 10

FigureFigure 8. 8. ChargingCharging of of the the capacitor capacitor by by ( (aa)) PV PV films films ( (b)) piezoelectric piezoelectric films films ( c) combined (TSHG). 7. Conclusions Figure 8 shows three different cases of charging the capacitor. Figure 8a shows a fast charging processA small-scaledelivered by tree-shaped the PV films. hybrid It took nanogenerator only 15 seconds (TSHG) to was charge designed, the capacitor fabricated, to andits saturated tested to electricanalyze potential its feasibility of 5.045 in harvesting V. Figure energy8b shows from the wind case andwhen the it Sun was at charged the same only time. by The the testgenerated results currentshave demonstrated from 10 sheets its potential of piezo offilms. generating As shown, appreciable the capacitor amount was of charged electricity very if sufficient slowly. It numbers took 35 minutesof piezoelectric to charge film the sheets capacitor are used up to along 4.38 with V. As flexible compared photovoltaic to the case (PV) of cells.the piezo The power films, outputFigure by8c showsthese two the differentcharging distinct of the capacitor types of electricwhen both generators outputs were are used combined from tothe charge sheets a of capacitor the piezo separately and PV films.or simultaneously As shown, no depending appreciable on difference the availability is observed of energy compared sources. to From Figure a series8a because of measurements most of the powerconducted, is generated it was proven by the PV that films. the TSHG can generate a maximum output power of 3.42 mW with the application of a load resistance of 5 kΩ. The error analysis carried out under different conditions 7.revealed Conclusions a deviation of ±0.24 V, which accounts for an error of 4.73%. AAlthough small-scale the tree-shaped power developed hybrid bynanogenerator the 10 piezo (TSHG) film sheets was was designed, not comparable fabricated, to and that tested of a PV to analyzecell, the its present feasibility work in clearly harvesting demonstrated energy from that appreciablewind and the power Sun at can the be same produced time. byThe the test former results if havemore demonstrated sheets are used. its Itspotential total combined of generating output appreciable could be sufficientamount of to electricity drive an electronicif sufficient device numbers and ofeven piezoelectric is comparable film tosheets a PV are cell. used Hybrid along operation with flex byible implementing photovoltaic two(PV) different cells. The types power of generatorsoutput by theseenables two a ratherdifferent steady distinct flow oftypes electricity of electric to the ge capacitornerators (storagewere combined unit) most to ofcharge the time. a capacitor During separatelycloudy days or with simultaneously little sunshine depending and some wind,on the power availability will be mostlyof energy developed sources. by From piezo a film series sheets. of measurementsConversely, on conducted, a clear and it calmwas proven day with that little the wind, TSHG the can power generate will a bemaximum mostly developedoutput power by PVof 3.42cells. mW This with evidently the application justifies the of hybrida load operationresistance ofof two 5 kΩ different. The error types analysis of generators carriedin out charging under differenta storage conditions unit as done revealed by the a presentdeviation work, of ±0.24 compared V, which to accounts other options for an of error using of separate4.73%. units for differentAlthough generators. the power developed by the 10 piezo film sheets was not comparable to that of a PV cell, the present work clearly demonstrated that appreciable power can be produced by the former if Author Contributions: Conceptualization, R.A. and W.C.; methodology, R.A. and Y.K.; software, Z.; validation, moreR.A., Y.K.sheets and are Z.; formalused. Its analysis, total combined Y.K.; investigation, output could W.C.; resources,be sufficient R.A.; to data drive curation, an electronic Z.; writing—original device and evendraft is preparation, comparable R.A.; to a writing—reviewPV cell. Hybrid andoperation editing, by W.C.; implementing visualization, two Y.K.; differ supervision,ent types of W.C.; generators project enablesadministration, a rather W.C.; steady funding flow acquisition, of electricity W.C. to the capacitor (storage unit) most of the time. During cloudy days with little sunshine and some wind, power will be mostly developed by piezo film sheets. Conversely, on a clear and calm day with little wind, the power will be mostly developed by PV cells. This evidently justifies the hybrid operation of two different types of generators in charging a storage unit as done by the present work, compared to other options of using separate units for different generators.

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Funding: The authors would like to acknowledge the support from the National Research Foundation of Korea through the Ministry of Science, ICT & Future Planning (Grant Number 2017R1A2A1A05001461). Conflicts of Interest: The authors declare no conflict of interest.

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