Power Generation from a Hybrid Generator (TENG-EMG) Run by a Thermomagnetic Engine Harnessing Low Temperature Waste Heat

Article Power Generation from a Hybrid Generator (TENG-EMG) Run by a Thermomagnetic Engine Harnessing Low Temperature Waste Heat

Zeeshan 1, Rahate Ahmed 1, Wongee Chun 1 , Seung Jin Oh 2 and Yeongmin Kim 1,*

1 Department of Nuclear and Energy Engineering, Jeju National University, Jeju 63243, Korea; [email protected] (Z.); [email protected] (R.A.); [email protected] (W.C.) 2 Clean Innovation Technology Group, Korea Institute of Industrial Technology, Jeju 63243, Korea; [email protected] * Correspondence: [email protected]; Tel.: +82-064-754-3646

Received: 16 April 2019; Accepted: 6 May 2019; Published: 10 May 2019

Abstract: This work explored the scavenging of low temperature waste heat and conversion of it into electrical energy through the operation of a gadolinium (Gd) based thermomagnetic engine. Gd is one of the unique materials whose magnetic property changes from ferromagnetic to paramagnetic depending on the temperature (“the Curie temperature”), which is around 20 ◦C. In the present work, two different types of generators were designed and applied to the rotating shaft of a Gd-based thermomagnetic engine developed for low temperature differential (LTD) applications. Of these, one is the so-called triboelectric nanogenerator (TENG), and the other is the electromagnetic generator (EMG). These have been designed to produce electricity from the rotating shaft of the thermomagnetic engine, exploiting both the electromagnetic and triboelectric effects. When operated at a rotational speed of 251 rpm with a temperature difference of 45 ◦C between the hot and cold water jets, the hybrid (TENG-EMG) generator produced a combined pulsating DC open circuit voltage of 5 V and a short circuit current of 0.7 mA. The hybrid generator effectively produced a maximum output power of 0.75 mW at a loading resistance of 10 kΩ.

Keywords: thermomagnetic engine; triboelectric nanogenerator (TENG); electromagnetic generator (EMG); triboelectric eﬀect; electromagnetic eﬀect

1. Introduction Energy crises all over the world have drawn serious attention to green energy. Many researchers have been paying attention to nanogenerators (NGs) for their wide applicability as well as studying how to harvest energy from the ambient environment to deal with energy related issues. A large amount of low grade waste heat is produced from various industrial processes and discharged into the atmosphere without being put into practical use. The United Nations reported in the 21st Conference of the Parties (COP21) that around 20% to 50% of the energy given to diﬀerent industries is lost as waste heat. The opportunity to harvest this low grade thermal energy can open a new pathway and contribute to meet our energy needs. At present, many researchers are showing interest in utilizing low grade waste heat for power generation. There are many low temperature diﬀerential (LTD) heat engines which can scavenge low grade thermal energy into electrical energy [1,2]. Haneman [3] reported a research study based on LTD Stirling engines for energy scavenging. Some other types of magnetic engines have been reported to scavenge low grade thermal energy; however, these engines have some drawbacks and limitations and have not been found to be practical.

Energies 2019, 12, 1774; doi:10.3390/en12091774 www.mdpi.com/journal/energies Energies 2019, 12, 1774 2 of 13

The discovery of gadolinium (Gd) [4] was the breakthrough in the development of magnetic engines. Gd is used as a transition material in thermomagnetic engines. Thulium has the same properties as Gd and also can be used as a transition material in magnetic engines. However, Gd is preferred due to cost advantages and greater availability. Gadolinium acts like a ferromagnetic material below its Curie temperature of 20 ◦C, and is strongly attracted to a magnetic field. On the other hand, it shows paramagnetic behavior above this temperature. Gd causes a relative moment due to the distortion of flux lines in a magnetic field by heating and cooling it above and below the Curie point. When the hot stream of a jet impinges on a Gd block, it becomes paramagnetic while the cold stream of a jet makes it ferromagnetic. This change in the magnetic property of a Gd block interacts with a bar magnet and generates rotational mechanical energy. In recent years, triboelectric nanogenerators (TENGs) [5–8] have been introduced, which can convert mechanical energy, such as wind power [9], ocean waves [10], and rain drops, into electrical energy based on the triboelectric effect and electrostatic induction [11,12]. TENG have advantages, such as light weight, low cost, and simple fabrication [13–15]; however, there are some issues related to the output performance of TENG that need further improvement for long term operation. A TENG generates a high output voltage and a very low output current that limit its application to some extent. To overcome these issues, many researchers have hybridized it with other generators, like piezoelectric [16], electromagnetic [17], and thermoelectric generators. In a previous work, Shaislamov et al. [18] designed a TENG which can produce electricity from a rotating disk driven by an LTD heat engine. They were able to produce an effective output voltage of Voc = 70 V and an output current of about Isc = 0.31 µA from the non-contact sliding mode of the TENG. However, an important issue related to the reported TENG is the very low output current, which needs to be addressed. In the existing TENG, electricity is produced by only the triboelectric effect. This work presents a hybrid (TENG-EMG) rotating disk type generator integrated with an LTD thermomagnetic engine for converting the rotational mechanical energy of a thermomagnetic engine into electrical energy. The main idea of the proposed work is how to generate energy at a low temperature difference between the heat source and the sink. Therefore, we introduced a novel design for a Gd-based thermomagnetic generator that can work in conjunction with the disk type (TENG-EMG) generator to scavenge low grade thermal energy into electrical energy. The hybrid (TENG-EMG) generator has a better output performance than that of individual NGs (EMG or TENG). Operated at a rotational speed of 251 rpm, the TENG produces an average open circuit voltage of 9 V and a short circuit current of 0.9 µA while the electromagnetic generator (EMG) produces an average open circuit voltage of 2.3 V and a short circuit current of 1.5 mA under the same rotational speed. The output from both the TENG and EMG were combined for maximum efficiency. The presented thermomagnetic engine is an improved magnetic engine, which produces mechanical work by utilizing the properties of Gd and converts this mechanical energy into electrical energy by using the triboelectric and electromagnetic effect.

2. Experimental Section

2.1. Fabrication of the Electromagnetic Generator Part The EMG rotor and stator parts were fabricated by a 3D printer. The rotor and stator parts are acrylic wheels with an outer diameter of 120 mm and an inner diameter of 110 mm and a thickness of 4 mm. These acrylic wheels were used as a substrate for the magnets and coils. For the rotor of the EMG, 12 cylindrical shaped strong neodymium magnets with the magnetic poles in alternating behavior were assembled with the plastic wheel. The corresponding 12 coils were integrated onto the acrylic substrate in a series connection, which generates an alternating current output when the magnetic ﬂux changes periodically in the coils. Figure1a,b shows the rotor and stator parts of the EMG, respectively. Energies 2019, 12, x 3 of 13

2.2. Fabrication of the Triboelectric Nanogenerator Part The rotor part of the TENG is a segmented structure disk with a diameter of 120 mm. The acrylic disk was fabricated by a 3D printer and used as a substrate for the ﬂuorinated ethylene propylene (FEP) film. This acrylic disk was coated with an FEP film, as shown in Figure 1c. On the other hand, the stator part of the TENG was fabricated from a 1-mm thick aluminum plate by laser cutting. Both of the aluminum electrodes were attached as shown in Figure 1d. Two wires were linked to the Al electrodes to produce the output current/voltage signals. The combined weight Energies 2019, 12, 1774 3 of 13 (EMG and TENG) of the rotor part of this hybrid NG is around 90 g.

(a) (b)

Figure 1.1. Schematic diagram and fabricated design of the hybridized nanogenerator (NG): (a) rotor part ofof the the electromagnetic electromagnetic generator generator (EMG); (EMG); (b) stator (b) stator part of part the EMG;of the (c )EMG; rotor part(c) ofrotor the triboelectricpart of the nanogeneratortriboelectric nanogenerator (TENG); and ((TENG);d) stator partand of(d) the stator TENG. part FEP: of fluorinatedthe TENG. ethyleneFEP: fluorinated propylene. ethylene propylene. 2.2. Fabrication of the Triboelectric Nanogenerator Part 2.3. FabricationThe rotor partof the of Thermomagnetic the TENG is a segmentedGenerator structure disk with a diameter of 120 mm. The acrylic diskThe was thermomagnetic fabricated by a 3D engine printer consists and used of 16 as equa a substratelly spaced for Gd the fluorinatedblocks and a ethylene strong permanent propylene (FEP)bar magnet. film. This Three acrylic water disk jets, was one coated hot and with two an cold, FEP film,120° asapart shown from in each Figure other,1c. On were the otherdesigned hand, to thesequentially stator part heat of theand TENG cool the was Gd fabricated blocks above from and a 1-mm below thick its Curie aluminum point plate as shown by laser in the cutting. Figure Both 2a. ofWhen the aluminumthe hot water electrodes jet on were the attachedbar magnet as shown side indischarges Figure1d. onto Two wiresthe Gd were blocks, linked it tobecomes the Al electrodesparamagnetic to produce while the the cold output water current jets opposite/voltage signals.the bar magnet The combined make it weight ferromagnetic. (EMG and This TENG) change of thein the rotor magnetic part of thisproperty hybrid of NG the is transition around 90 materi g. al (Gd blocks) interacts with the bar magnet and 2.3.makes Fabrication the rotor of turn. the Thermomagnetic Commercially available Generator light weight Nylon beveled gears with a speed ratio of (3:1) and a solid acrylic shaft of 12 mm in diameter were used to integrate the (TENG-EMG) generatorThe thermomagnetic with the thermomagnetic engine consists engine of to 16 conv equallyert the spaced mechanical Gd blocks energy and into a strong electrical permanent energy, baras shown magnet. in Figure Three water2b. The jets, torque one hotof the and thermo two cold,magnetic 120◦ apartengine from depends each other,on how were fast designed a magnetic to sequentiallytransition takes heat place and between cool the Gdthe blocksferromagnetic above and and below paramagnetic its Curie state. point as shown in the Figure2a. When the hot water jet on the bar magnet side discharges onto the Gd blocks, it becomes paramagnetic while the cold water jets opposite the bar magnet make it ferromagnetic. This change in the magnetic property of the transition material (Gd blocks) interacts with the bar magnet and makes the rotor turn. Commercially available light weight Nylon beveled gears with a speed ratio of (3:1) and a solid acrylic shaft of 12 mm in diameter were used to integrate the (TENG-EMG) generator with the thermomagnetic engine to convert the mechanical energy into electrical energy, as shown in Figure2b. The torque of the thermomagnetic engine depends on how fast a magnetic transition takes place between the ferromagnetic and paramagnetic state.

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(a)

(b) (b) Figure 2. Schematic illustrations: (a) a thermomagnetic engine; and (b) a thermomagnetic engine FigureFigure 2. Schematic 2. Schematic illustrations: illustrations: (a) ( aa) thermomagnetic a thermomagnetic engine; engine; and and (b ()b a) a thermomagnetic thermomagnetic engineengine with with gears and shaft for power transmission. gearswith and gears shaft and for shaft power for transmission. power transmission.

3. Working 3. Working Principle Principle The The hybrid hybrid (TENG-EMG) (TENG-EMG) disk disk generator generator produces electricity electricity when when there there is a isrelative a relative motion motion betweenbetween the rotorthe rotor and and stator stator parts parts of of the the TENG TENG andand EMG. EMG. The The thermomagnetic thermomagnetic Gd Gdengine engine provides provides rotation to the NG when the hot and cold water jets impinge on the Gd blocks. The Gd block has a rotation to the NG whenwhen thethe hothot andand coldcold waterwater jetsjets impingeimpinge onon thethe GdGd blocks.blocks. The Gd block has a curie temperature very near to room temperature, and it changes its magnetic properties, i.e., from curie temperature very near to room temperature, and it changes its magnetic properties, i.e., from ferromagnetic to paramagnetic, depending on its temperature controlled by the water jets. ferromagnetic to paramagnetic, depending on its temperature controlled by the water jets. ferromagneticThe rotor to paramagnetic, of the hybrid generator depending consists on its of temperature two layers (1–2) controlled as shown by in the Figure water 3a. jets.Layer 1 is Thethe EMG rotor part of thewith hybridhybrid 12 magnets generator while layer consists 2 is the of twoTENG layers part, consisting (1–2) as shown of a disk in with FigureFigure a diameter3 3a.a. LayerLayer of 11 isis the EMG 120 mm, part which with with 12 12is coveredmagnets magnets by while while the FE layer layerP as 2a 2 istriboelectric is the the TE TENGNG material. part, part, consisting consisting The stator of ofa diskthe a disk hybrid with with aNG diameter a diameteralso of of120 120 mm,consists mm, which which of two is is coveredlayers covered (3–4) by by asthe the shown FE FEPP as in as aFigure atriboelectric triboelectric 3b. Layer material. material. 3 is aluminum The The stator stator electrodes of of the the while hybrid layer NG 4 also consistsconsists ofof two two of layersthe layers corresponding (3–4) (3–4) as as shown shown12 serially in Figure in connectFigure3b. Layered 3b. coils, Layer 3 isfixed aluminum 3 inis alignm aluminum electrodesent with electrodes the while magnets. layer while 4 consists layer 4 ofconsists the corresponding of the corresponding 12 serially 12connected serially connect coils, ﬁxeded coils, in alignmentfixed in alignm withent the with magnets. the magnets.

Figure 3. Schematic diagram of the hybrid NG: (a) schematic diagram of the rotor part of the hybrid NG; and (b) schematic diagram of the stator part of the hybrid NG. Figure 3. Schematic diagram of the hybrid NG: (a) schematic diagram of the rotor part of the hybrid NG; and (b)) schematicschematic diagramdiagram ofof thethe statorstator partpart ofof thethe hybridhybrid NG.NG.

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The total output energy produced from this generator is divided into two parts: (1) The energy produced by the TENG part; and (2) the energy produced by the EMG part. The TENG produces electricity when both triboelectric surfaces (FEP and Al) are first brought into contact to produce charges due to the difference in the electron affinity between the Al and FEP. This is called the initial contact electrification step in which negative charges appears on the FEP layer and positive charges Energieson the 2019aluminum, 12, 1774x electrode [19–21] as shown in Figure 4. In the next step, the two layers (FEP and5 of Al) 13 are brought away from each other, leaving a small air gap between them. The total output energy produced from this generator is divided into two parts: (1) The energy When the FEP portion rotates from Al1 to Al2, the electrons will move from Al2 to Al1 to cover producedThe total by the output TENG energy part; produced and (2) the from energy this generatorproduced isby divided the EMG into part. two The parts: TENG (1) The produces energy the potential difference on the FEP surface produced by the negative charges (step II). When the FEP electricityproduced bywhen the both TENG triboelectric part; and (2)surfaces the energy (FEP producedand Al) are by first the brought EMG part. into The contact TENG to producesproduce reaches the overlapping position of Al2, most of the electrons will move to Al1, which produces the chargeselectricity due when to the both difference triboelectric in the surfaceselectron affinity (FEP and between Al) are the first Al broughtand FEP. into This contact is called to the produce initial majority of the positive ions on Al2 (step III). In the next step, the FEP segment will start its motion contactcharges electrification due to the diff steperence in which in the electronnegative a charffinityges between appears the on Althe and FEP FEP. layer This and is positive called the charges initial back toward the adjacent Al1 segment. As such, the electrons will travel back from Al1 to Al2, oncontact the aluminum electrification electrode step in [19–21] which as negative shown charges in Figure appears 4. In the on next the FEPstep, layer the two and layers positive (FEP charges and Al) on producing an output current in the opposite direction (step IV) until the FEP segment reaches the arethe brought aluminum away electrode from each [19–21 other,] as shown leaving in a Figure small 4air. In gap the between next step, them. the two layers (FEP and Al) are primary position. The working principle of the TENG part is shown in Figure 4. broughtWhen away the fromFEP portion each other, rotates leaving from a smallAl1 to airAl2 gap, the between electrons them. will move from Al2 to Al1 to cover the potential difference on the FEP surface produced by the negative charges (step II). When the FEP reaches the overlapping position of Al2, most of the electrons will move to Al1, which produces the majority of the positive ions on Al2 (step III). In the next step, the FEP segment will start its motion back toward the adjacent Al1 segment. As such, the electrons will travel back from Al1 to Al2, producing an output current in the opposite direction (step IV) until the FEP segment reaches the primary position. The working principle of the TENG part is shown in Figure 4.

Figure 4.4. SchematicSchematic diagramdiagram ofof the the working working mechanism mechanism of of the the TENG TENG based based on theon the rotation rotation of the of FEPthe FEPon the on aluminum the aluminum electrodes. electrodes.

InWhen the theexperiment, FEP portion the rotates rotational from Alparts1 to Alwere2, the driven electrons by willthe movethermomagnetic from Al2 to engine Al1 to coverat a rotationalthe potential speed diﬀ erenceof 251 onrpm, the and FEP the surface stationary produced part bywas the fixed negative just adjacent charges (stepto the II). rotational When the parts. FEP Thereaches schematic the overlapping diagram and position the fabricated of Al2, most model of the of electronsthe whole will integrated move to system Al1, which (thermomagnetic produces the enginemajority and of theNG) positive are shown ions onin Figures Al2 (step 5 III).and In6. the next step, the FEP segment will start its motion back towardFigure the adjacent4. Schematic Al1 diagramsegment. of Asthe such, working the electronsmechanism will of travelthe TENG back based from on Al 1theto rotation Al2, producing of the an outputFEP current on the aluminum in the opposite electrodes. direction (step IV) until the FEP segment reaches the primary position. The working principle of the TENG part is shown in Figure4. In thethe experiment,experiment, the the rotational rotational parts parts were were driven driven by the by thermomagnetic the thermomagnetic engine atengine a rotational at a rotationalspeed of 251 speed rpm, of and 251 the rpm, stationary and the part stationary was ﬁxed part just was adjacent fixed tojust the adjacent rotational to parts.the rotational The schematic parts. Thediagram schematic and the diagram fabricated and model the fabricated of the whole model integrated of the whole system integrated (thermomagnetic system (thermomagnetic engine and NG) engineare shown and inNG) Figures are shown5 and6 .in Figures 5 and 6.

(a)

Figure 5. Cont.

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(b)

Figure 5. Schematic diagram of the integrated thermomagnetic engine with the hybrid (TENG-EMG) generator: (a) cross sectional view of the system; and (b) isometric view of the system.

As shown in Figure 5a, the triboelectric generator (layer 2 and layer 3) is sandwiched between Energiesthe EMG2019 (layer, 12, 1774 1 and layer 4). Layer 2 is a thin FEP disk, which is adhered to layer 1 (magnets disk).6 of 13 Energies 2019, 12, x 6 of 13 Layer 1 and layer 2 make up the rotor part of the hybrid (TENG, EMG) generator. Layer 3 is a 1 mm-thick disk-shaped aluminum electrode mounted on layer 4 (coil disk). Layer 3 and layer 4 make the stator part of the hybrid (TENG, EMG) generator. The gap distance between the rotor and stator parts was selected based on the output performance of the generator. The output of the TENG part relies on the surface charge density, surface area of the FEP and Al, and the rotational angular velocity. The EMG produces an alternating current as directed by Faraday's law of electromagnetic induction based on the periodic change of the magnetic flux in the coils.

Voc = −N dΦ/dt (1)

Isc = Voc/R (2)

The output of the EMG depends on the number(b) of turns (N) of each coil, the internal resistance of the coil (R), and the changing flux, dΦ/dt (rotational speed). The output from both the TENG and Figure 5. Schematic diagram of the integrated thermomagnetic engine with the hybrid (TENG-EMG) EMGFigure were 5.combined Schematic for diagram maximum of the efficiency.integrated thermomagnetic engine with the hybrid (TENG-EMG) generator: ( (aa)) cross cross sectional sectional view view of of the system; and ( b) isometric view of the system.

As shown in Figure 5a, the triboelectric generator (layer 2 and layer 3) is sandwiched between the EMG (layer 1 and layer 4). Layer 2 is a thin FEP disk, which is adhered to layer 1 (magnets disk). Layer 1 and layer 2 make up the rotor part of the hybrid (TENG, EMG) generator. Layer 3 is a 1 mm-thick disk-shaped aluminum electrode mounted on layer 4 (coil disk). Layer 3 and layer 4 make the stator part of the hybrid (TENG, EMG) generator. The gap distance between the rotor and stator parts was selected based on the output performance of the generator. The output of the TENG part relies on the surface charge density, surface area of the FEP and Al, and the rotational angular velocity. The EMG produces an alternating current as directed by Faraday's law of electromagnetic induction based on the periodic change of the magnetic flux in the coils.

Voc = −N dΦ/dt (1)

Isc = Voc/R (2) Figure 6. FabricatedFabricated model model of the integrated thermoma thermomagneticgnetic engine with the hybrid (TENG-EMG). The output of the EMG depends on the number of turns (N) of each coil, the internal resistance of4. ResultstheAs coil shown (andR), andDiscussion in Figure the changing5a, the triboelectric flux, dΦ/dt generator (rotational (layer speed). 2 and The layer output 3) is from sandwiched both the between TENG and the EMG (layerwere combined 1 and layer for 4). maximum Layer 2 is efficiency. a thin FEP disk, which is adhered to layer 1 (magnets disk). Layer The non-contact sliding mode of the TENG and conventional EMG was experimentally tested 1 and layer 2 make up the rotor part of the hybrid (TENG, EMG) generator. Layer 3 is a 1 mm-thick for its application to a Gd based thermomagnetic engine to harness low-temperature heat sources. disk-shaped aluminum electrode mounted on layer 4 (coil disk). Layer 3 and layer 4 make the stator The Gd based thermomagnetic engine was operated at different temperatures between hot and cold part of the hybrid (TENG, EMG) generator. The gap distance between the rotor and stator parts was selected based on the output performance of the generator. The output of the TENG part relies on the surface charge density, surface area of the FEP and Al, and the rotational angular velocity. The EMG produces an alternating current as directed by Faraday’s law of electromagnetic induction based on the periodic change of the magnetic ﬂux in the coils.

Voc = N dΦ/dt (1) −

Isc = Voc/R (2) The output of the EMG depends on the number of turns (N) of each coil, the internal resistance of the coil (R), and the changing ﬂux, dΦ/dt (rotational speed). The output from both the TENG and EMG were combined for maximum eﬃciency.

4. ResultsFigure and 6. Fabricated Discussion model of the integrated thermomagnetic engine with the hybrid (TENG-EMG).

4. ResultsThe non-contact and Discussion sliding mode of the TENG and conventional EMG was experimentally tested for its application to a Gd based thermomagnetic engine to harness low-temperature heat sources. The Gd The non-contact sliding mode of the TENG and conventional EMG was experimentally tested based thermomagnetic engine was operated at diﬀerent temperatures between hot and cold water for its application to a Gd based thermomagnetic engine to harness low-temperature heat sources. The Gd based thermomagnetic engine was operated at different temperatures between hot and cold

Energies 2019, 12, x 7 of 13 Energies 2019, 12, 1774 7 of 13 water jets. The cold water jet was kept constant at 18.5 °C while that of the hot water was changed from 22.5 °C to 90 °C. The minimum rotational speed of 77 rpm was measured at the temperature jets. The cold water jet was kept constant at 18.5 ◦C while that of the hot water was changed from differenceEnergies 2019of 4, 12°C., x Meanwhile, the maximum rotational speed of 251 rpm was observed 7when of 13 the 22.5temperature◦C to 90 ◦differenceC. The minimum between rotational the two speedjets reached of 77 rpm 45 °C, was as measured shown in atFigure the temperature 7. In the experiment, difference ofthe 4 hybrid◦waterC. Meanwhile, jets. (TENG-EMG) The cold the water maximum generator jet was rotationalkept was constant integrated speed at 18.5of with 251°C whilethe rpm rotating wasthat of observed the shaft hot ofwater when the wasthermomagnetic the changed temperature diengine,fferencefrom which 22.5 between °C efficiently to 90 the °C. two harvestedThe jets minimum reached both rotational 45the◦ triboeleC, as sp showneedctric of and77 in rpm Figure electromagnetic was7 .measured In the experiment, effect.at the temperatureThe open the hybrid circuit (TENG-EMG)outputdifference voltage generatorof was 4 °C. measured Meanwhile, was integrated by the10 M maximumΩ with probe the rotationalof rotating a DS07052B speed shaft oscilloscopeof of 251 the rpm thermomagnetic was (Agilent, observed Santa engine,when Clara, the which CA, temperature difference between the two jets reached 45 °C, as shown in Figure 7. In the experiment, eUSA)fficiently while harvested Keysight both B2902A the triboelectric(Keysight, Santa and electromagnetic Rosa, CA, USA) e wasffect. used The to open measure circuit the output short voltage circuit the hybrid (TENG-EMG) generator was integrated with the rotating shaft of the thermomagnetic wascurrent measured of the generator. by 10 MΩ Theprobe continuous of a DS07052B forward oscilloscope and backward (Agilent, sliding Santa motion Clara, of CA,the FEP USA) over while the Keysightengine, B2902A which (Keysight,efficiently harvested Santa Rosa, both CA, the USA)triboele wasctric used and toelectromagnetic measure the effect. short circuitThe open current circuit of the Al electrodesoutput voltage produced was measured an AC byvoltage 10 MΩ signalprobe ofwh a DS07052Bich was converted oscilloscope to (Agilent, a DC signal Santa Clara, with CA,a bridge generator. The continuous forward and backward sliding motion of the FEP over the Al electrodes rectifierUSA) as while shown Keysight in FigureB2902A (Keysi8. Operatedght, Santa at Rosa, a continuous CA, USA) was rotational used to measure speed theof short251 circuitrpm for a producedtemperaturecurrent an of ACdifference the voltage generator. of signal 45 The °C whichcontinuous between was convertedforwardthe hot and to backwardcold a DC water signal sliding jets, with motionthe a bridge TENG of the rectifier produced FEP over as shown thean open in FigurecircuitAl8 voltage.electrodes Operated of produced9 at V aand continuous aan short AC voltagecircuit rotational signalcurrent speed wh ofich of0.9 was 251 μ A, rpmconverted respectively, for a to temperature a DC with signal a distance di withfference a bridge of of1.2 45 mm◦C betweenbetweenrectifier thethe hotastwo shown and triboelectric cold in Figure water surfaces. jets,8. Operated the The TENG shortat produceda continuouscircuit current an openrotational analysis circuit speed voltageof theof 251TENG of 9rpm V is and forshown aa short in circuitFiguretemperature current9. Under of difference 0.9the µsameA, respectively, ofrotational 45 °C between withmotion, athe distance thehot EMGand of cold 1.2produced water mm between jets, an the average theTENG two openproduced triboelectric circuit an voltageopen surfaces. of The2.3 V shortcircuit and circuita voltage short current circuit of 9 V analysiscurrentand a short of of 1.5 thecircuit mA, TENG current respectively, is shown of 0.9 inμ A,as Figure shownrespectively,9. Underin Figures with the a same 10distance and rotational 11. of The1.2 mm magnets motion, between the two triboelectric surfaces. The short circuit current analysis of the TENG is shown in theand EMGcoils were produced 2.5 mm an apart average from open each circuit other voltagewhen the of experiments 2.3 V and a shortwere circuitcarried currentout for ofthe 1.5 output mA, Figure 9. Under the same rotational motion, the EMG produced an average open circuit voltage of respectively,performance asof shownthe EMG. in Figures Figure 1012a,b and displays 11. The magnetsthe dependence and coils of werethe resistance 2.5 mm apart on the from output each other2.3 when V and the a short experiments circuit current were of carried 1.5 mA, out respectively, for the output as shown performance in Figures 10 of and the 11. EMG. The magnets Figure 12 a,b currentand and coils the were corresponding 2.5 mm apart fromoutput each powers other when of bo theth theexperiments TENG and were EMG, carried respectively. out for the outputThe output displays the dependence of the resistance on the output current and the corresponding output powers currentsperformance of the TENGof the EMG.and EMGFigure decreased 12a,b displays with the in creasingdependence load of theresistance, resistance whereas on the outputthe output ofelectrical bothcurrent the power TENGand the of and corresponding both EMG, generators respectively. output initially powers The outputincreasedof both currentsthe andTENG then of and the decreasedEMG, TENG respectively. and under EMG aThe decreased large output loading with increasingresistance.currents loadThe of resistance,thelargest TENG output and whereas EMGpower thedecreased values output ofwith electrical the in TENGcreasing power and load ofEMG bothresistance, were generators approximately whereas initially the output increased0.144 μW andand then0.88electrical mW decreased powerat a loading underof both resistance a generators large loading of initially 1 M resistance.Ω andincreased 1 kΩ The , andrespectively, largest then decreased output as shown power under in valuesa Figure large ofloading 12. the TENG and EMGresistance. were The approximately largest output 0.144powerµ valuesW and of 0.88 the TENG mW at and a loadingEMG were resistance approximately of 1 M 0.144Ω and μW 1 kΩ, respectively,and 0.88 mW as shown at a loading in Figure resistance 12. of 1 MΩ and 1 kΩ, respectively, as shown in Figure 12.

FigureFigureFigure 7.7. TemperatureTemperature 7. Temperature di differenceﬀ differenceerence between between between hot hot and and cold coldcold water wa waterter jetsjets jets vs.vs. vs. the the rotational rotational speed speed speed (rpm) (rpm) (rpm) of of the of thermomagneticthe thermomagneticthe thermomagnetic engine. engine engine. .

Figure 8. Open circuit voltage analysis of the TENG at a speed of 251 rpm and a temperature FigureFiguredifference 8.8.Open Open of circuit 45circuit °C. voltage voltage analysis analysis of the of TENGthe TENG at a speed at a ofspeed 251 rpmof 251 and rpm a temperature and a temperature diﬀerence ofdifference 45 ◦C. of 45 °C.

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Figure 9. Short circuit analysis of the TENG at a speed of 251 rpm, and a temperature difference of 45 Figure 9. ShortShort circuit circuit analysis analysis of of the the TENG TENG at at a aspeed speed of of 251 251 rpm, rpm, and and a temperature a temperature difference diﬀerence of 45 of °C.°C. 45°C.◦ C.

Figure 10.10. Open circuit voltage analysis of the EMG at a speed of 251 rpm and a temperature Figure 10. OpenOpen circuit circuit voltage voltage analysis analysis of the of EMGthe EMG at a speed at a ofspeed 251 rpmof 251 and rpm a temperature and a temperature diﬀerence differenceof 45 C. of 45 °C. difference◦ of 45 °C.

Figure 11. ShortShort circuit circuit analysis analysis of of the the EMG EMG at at a aspeed speed of of 251 251 rpm rpm and and a temperature a temperature difference diﬀerence of 45 of Figure 11. Short circuit analysis of the EMG at a speed of 251 rpm and a temperature difference of 45 °C.45 C. °C.◦

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FigureFigure 12.12. Relationship between the output powerpower//currentcurrent andand thethe loadingloading resistance:resistance: ((aa)) TENG;TENG; andand ((bb)) EMG.EMG.

TheThe optimumoptimum spacing’s for the TENG and EMG were selected to be 1.2 mm and 2.5 mm, respectively,respectively, after after carefully carefully analyzing analyzing the the rotational rotational speed speed and and the outputthe output voltage. voltage. As far As as thefar TENGas the isTENG concerned, is concerned, the various the various gap distances gap distances (ranging (ran fromging 0.5from mm 0.5 to mm 2 mm) to 2 betweenmm) between the FEP the diskFEP disk and aluminumand aluminum electrodes electrodes were were analyzed analyzed before before finalizing finalizing the optimum the optimum value; valu see Figuree; see Figure13a. The 13a. initial The spacinginitial spacing between between the FEP the disk FEP and disk the and aluminum the aluminum electrodes electrodes was selected was selected to be 0.5 to mm, be 0.5 but mm, at this but gap at distance,this gap thedistance, rotor occasionallythe rotor occasionally touched the touched stator because the stator of its because wobbling of motionits wobbling when rotating.motion when This causedrotating. a substantialThis caused decrease a substantial in the rotationaldecrease speedin the androtational hence resultedspeed and in a hence low output resulted voltage. in a Thelow maximumoutput voltage. output The voltage maximum of 9 V wasoutput observed voltage at aof gap 9 V distance was observed of 1.2 mm, at wherea gap thedistance rotor achievedof 1.2 mm, its maximumwhere the rotationalrotor achieved speed its of 251maximum rpm without rotational interacting speed (touching)of 251 rpm the without stator. interacting However, the(touching) voltage startedthe stator. todecline However, when the the voltage gap distance started to exceeded decline 1.2when mm. the This gap isdistance due to theexceeded fact that 1.2 there mm. wasThis no is noticeabledue to the fact increase that there in the was rotational no noticeable speed (almostincrease the in the same rotational as in the speed case of(almost 1.2 mm), the same but the as largerin the gapcase distanceof 1.2 mm), resulted but the in larger a weaker gap electrostatic distance result interactioned in a weaker between electrostatic the electrodes. interaction between the electrodes.In conclusion, a gap distance of less than 1.2 mm caused a decrease in the rotational speed due to unwantedIn conclusion, occasional a gap contactdistance between of less than the rotor1.2 mm and caused stator, a decrease resulting in in the the rotational degradation speed of due the TENG’sto unwanted output occasional performance. contact Similarly, between its the performance rotor and alsostator, degraded resulting when in the the degradation gap distance of was the increasedTENG’s output beyond performance. 1.2 mm. This Similarly, is because its a furtherperformance increase also in thedegraded gap only when weakened the gap the distance electrostatic was interactionincreased beyond between 1.2 the electrodesmm. This withoutis because any increasea further in theincrease rotational in the speed gap of only the shaft weakened driving thethe generators.electrostatic The interaction relation betweenbetween thethe output electrodes performance without ofany the increase TENG and in the the rotational gap distances speed is shownof the inshaft Figure driving 13b. the generators. The relation between the output performance of the TENG and the gap distancesThe gapis shown distance in Figure of the 13b. EMG was dictated by that of the TENG as the latter was designed to be locatedThe gap between distance the of coils the EMG and the was magnets dictated making by that up of the EMG,TENG asas shownthe latter in Figurewas designed5a. In other to be words,locatedthe between optimum the gapcoils for and the the EMG magnets is inherently making dependent up the EMG, on the as spacing shown ofin the Figure TENG. 5a.That In other is, a changewords, inthe the optimum spacing gap of the for TENG the EMG will is result inherently in a change dependent in the gapon the distance spacing between of the TENG. the magnets That andis, a coils.change As in shown the spacing in Figure of the13a, TENG the Al will electrode result andin a FEPchange film in together the gaprequire distance at between least 1.3 mmthe magnets spacing availableand coils. for As installation. shown in Figure Taking 13a, this the into Al consideration, electrode and the FEP performance film together of therequire EMG at was least analyzed 1.3 mm forspacing the gap available distances for betweeninstallation. 1.5 mmTaking and this 3 mm into as consideration, shown in Figure the 13performancec. The optimum of the spacingEMG was of 2.5analyzed mm for for the the EMG gap wasdistances obtained between when 1.5 the mm gap an distanced 3 mm of as the shown TENG in was Figure maintained 13c. The at optimum 1.2 mm. Atspacing a rotational of 2.5 mm speed for ofthe 251 EMG rpm, was the obtained EMG produced when the itsgap maximum distance of output. the TENG As aforementioned,was maintained at a decrease1.2 mm. inAt the a spacingrotational to lessspeed than of 2.5 251 mm rpm, resulted the inEMG undesirable produced interaction its maximum between output. the TENG As components,aforementioned, leading a decrease to disruptions in the spacing in the rotation to less ofthan the 2.5 shaft mm driven resulted by the in thermomagneticundesirable interaction engine andbetween hence the poorer TENG performance components, for leading both generators. to disruptions Similarly, in the the rotation output of performance the shaft driven of the by EMG the decreasedthermomagnetic when the engine gap distanceand hence exceeded poorer 2.5 performa mm. Whennce thefor spacingboth generators. for the TENG Similarly, increased the beyondoutput 1.2performance mm, the gap of the distance EMG decreased between the when rotor the (magnets) gap distance and exceeded the stator 2.5 (coils) mm. ofWhen the EMGthe spacing exceeded for 2.5the mm,TENG causing increased weaker beyond magnetic 1.2 mm, fluxes the through gap distance the coils between of the EMG the androtor hence (magnets) impairing andthe the EMG’s stator performance.(coils) of the EMG This exceeded can be readily 2.5 mm, observed causing in weaker Figure magnetic13c, where fluxes the relationshipthrough the betweencoils of the the EMG gap distanceand hence and impairing the output the voltage EMG’s ofperformance. the EMG is given.This can be readily observed in Figure 13c, where the relationship between the gap distance and the output voltage of the EMG is given.

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Figure 13. ((a)) The dimensions and the gap distance between the rotor and stator; (b) relationship between the gap distance and outputoutput voltage for the TENG; and (c) relationship between the gap distance and output voltage for the EMG.

The EMG produced high current output with low impedance whereas the TENG generated low current outputoutput with with high high impedance. impedance. Due Due to the to huge the dihugeﬀerence difference in the impedance in the impedance of the two of generators, the two itgenerators, was diﬃcult it towas connect difficult them to directlyconnect in them parallel. directly To deal in withparallel. this impedanceTo deal with mismatch this impedance and to get themismatch optimum and electrical to get the output optimum in terms electrical of the output voltage in and terms current, of the transformers voltage and were current, designed transformers for both generators.were designed The for turn both ratio generators. of the transformers The turn ratio was determinedof the transformers by Equation was (3),determined as given below:by Equation (3), as given below: Vp Np = (3) V 𝑉 N𝑁 s = s (3) 𝑉 𝑁 were Vs and Vp are the voltages of the secondary and primary coils, respectively. wereAs Vs and compared Vp are tothe the voltages TENG of (~1 the M secondaryΩ), the EMG and showedprimary muchcoils, respectively. smaller impedance (~1 kΩ). To accomplishAs compared impedance to the matching TENG (~1 of the MΩ generators,), the EMG the showed TENG much was connected smaller impedance to a transformer (~1 kΩ with). To a turnaccomplish ratio of impedance 1.8:1. Similarly, matching the EMG of the was generators, connected the to TENG a transformer was connected with a turn to a ratiotransformer of 1:2:1. with After a connectingturn ratio of the 1.8:1. generators Similarly, (EMG, the EMG TENG) was to connected the transformers, to a transformer the matched with resistance a turn ratio was of modulated 1:2:1. After to aboutconnecting 10 kΩ the. A generators series of measurements (EMG, TENG) demonstrated to the transformers, the capability the ma oftched the hybridresistance NG, was producing modulated the maximumto about 10 electrical kΩ. A series output of measurements power of 0.75mW demonstrated at a loading the resistance capability of of 10 the kΩ hybridas shown NG, in producing Figure 14. Figurethe maximum 15 shows electrical the circuit output diagram power to of measure 0.75 mW the at a performance loading resistance of the hybridof 10 kΩ generator as shown where in Figure the EMG14. Figure and TENG15 shows are the connected circuit diagram in parallel. to Asmeasur showne the in Figuresperformance 16 and of 17 the, the hybrid hybrid generator generator where was capablethe EMG of and producing TENG anare open connected circuit in voltage parallel. of 5 As V andshown a short in Figures circuit current16 and 0.717, mA,the hybrid respectively. generator The mechanicalwas capable power of producing of the system an open (thermomagnetic circuit voltage engine) of 5 wasV and determined a short bycircuit measuring current the 0.7 torque mA, andrespectively. angular velocity,The mechanical which is givenpower as: of the system (thermomagnetic engine) was determined by measuring the torque and angular velocity, whichP = τωis given as: (4)

𝑃 = 𝜏𝜔 (4)

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FigureFigure 14.14. MeasuredMeasured electricalelectrical powerpower outputoutput onon differentdifferent loadload resistancesresistances atat aa rotationalrotational speedspeed ofof 251251 ° rpmrpm andand temperaturetemperature differencedifference ofof 4545 °C.C. FigureFigure 14. Measured 14.Measured Measured electrical electrical electrical power powerpower output output output onon on different diﬀerent load load load resistances resistances resistances at aat rotational a at rotational a rotational speed speed of speed251 of 251 of rpm and temperature difference of 45 °C. rpm251 rpm and andtemperature temperature difference diﬀerence of 45 of ° 45C. ◦C.

FigureFigure 15.15. CircuitCircuit diagramdiagram ofof thethe connectionconnection mechanismmechanism ofof TENGTENG andand EMG.EMG. FigureFigure 15.15. Circuit diagram diagram of of the the connection connection mechanism mechanism of TENG of TENG and EMG. and EMG. Figure 15. Circuit diagram of the connection mechanism of TENG and EMG.

Figure 16. Combined (TENG-EMG) open circuit voltage analysis at the speed of 251 rpm. FigureFigure 16.16. CombinedCombined (TENG-EMG)(TENG-EMG) openopen circuitcircuit voltagevoltvoltageage analysisanalysis atatat thethethe speedspeedspeed ofofof 251251251 rpm. rpm.rpm.

Figure 16. Combined (TENG-EMG) open circuit voltage analysis at the speed of 251 rpm.

Figure 17. Combined (TENG-EMG) short circuit analysis at a rotational speed of 251 rpm.

FigureFigure 17.17. CombinedCombined (TENG-EMG)(TENG-EMG) shortshort circuitcircuit analysisanalanalysisysis atat aa rotationalrotational speed speedspeed of ofof 251 251251 rpm. rpm.rpm.

Figure 17. Combined (TENG-EMG) short circuit analysis at a rotational speed of 251 rpm.

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InIn FigureFigure 18 18,, the the mechanical mechanical power power of theof the system syst deliveredem delivered by the by thermomagnetic the thermomagnetic engine engine is given is atgiven various at various temperature temperature diﬀerences differences between thebetween hot and the cold hot waterand cold jets. Thewater maximum jets. The mechanical maximum mechanical power of 29.5 mW was observed at the temperature difference of 45 °C between the two power of 29.5 mW was observed at the temperature diﬀerence of 45 ◦C between the two jets, when the rotationaljets, when speed the rotational was 251 rpm.speed was 251 rpm.

FigureFigure 18.18. MechanicalMechanical powerpower ofof thethe systemsystem delivereddelivered byby thethe thermomagneticthermomagnetic engineengine atat variousvarious temperaturetemperature di differencesﬀerences between between hot hot and and cold cold water water jets. jets.

5.5. ConclusionsConclusions InIn summary,summary, we we successfully successfully fabricated fabricated a Gd a basedGd based thermomagnetic thermomagnetic engine engine that can that convert can convert waste heatwaste energy heat energy into mechanical into mechanical energy energy and then and provided then provided proof proof of concept of concept as its as final its conversionfinal conversion into electricalinto electrical power power by the by use the of use a (TENG-EMG)of a (TENG-EMG) generator. generator. The conventionalThe conventional EMG EMG and and TENG TENG with with the non-contactthe non-contact sliding sliding mode wasmode experimentally was experimentally tested for itstested application for itsto application a Gd based thermomagneticto a Gd based engine,thermomagnetic to reclaim engine, low-temperature to reclaim wastelow-temperature heat. Under waste a rotational heat. Under speed a ofrotational 251 rpm, speed the TENGof 251 µ producedrpm, the TENG an average produced open an circuit average voltage open ofcircuit 9 V andvoltage a short of 9 V circuit and a current short circuit of 0.9 currentA; meanwhile, of 0.9 μA; themeanwhile, EMG produced the EMG an produced average openan average circuit open voltage circuit of voltage 2.3 V and of 2.3 a short V and circuit a short current circuit of current 1.5 mA of under1.5 mA the under same the rotational same rotational speed. Thespeed. mechanical The mechanic to electricalal to electrical conversion conversion efficiency efficiency of the Gd of the based Gd thermomagneticbased thermomagnetic engine engine in the in present the present work waswork around was around 2.6%. 2.6%. However, However, surface surface treatment treatment of the of triboelectricthe triboelectric material material and coils and withcoils a with higher a crosshigher sectional cross sectional area can area improve can theimprove conversion the conversion efficiency ofefficiency the engine. of Asthe shown engine. in thisAs work, shown hybridized in this generatorswork, hybridized with a Gd-based generators thermomagnetic with a Gd-based engine maythermomagnetic be a significant engine step forwardmay be a towards significant practical step forward applications towards to harness practical low applications grade thermal to harness energy intolow electricalgrade thermal energy energy from diintofferent electrical industries energy to fr fulfillom different energy demands. industries to fulfill energy demands. Author Contributions: Conceptualization, Z. and Y.K.; methodology, Z. and Y.K.; software, R.A. and S.J.O.; validation,Author Contributions: Z., Y.K. and W.C.;Conceptualization, formal analysis, Z. Y.K.and andY.K.; S.J.O.; methodology, investigation, Z. and W.C.; Y.K.; resources, software, Z.; R.A. data and curation, S.J.O.; R.A.validation, and S.J.O.; Z., Y.K. writing—original and W.C.; formal draft analysis, preparation, Y.K. Z.; and writing—review S.J.O.; investigation, and editing, W.C.; Y.K.resources, and W.C.; Z.; data visualization, curation, W.C.;R.A. supervision,and S.J.O.; Y.K.;writing—original project administration, draft preparation, W.C.; funding Z.; acquisition,writing—revi W.C.ew and editing, Y.K. and W.C.; Acknowledgments:visualization, W.C.; supervision,This research Y.K.; project was administration, supported by W. NationalC.; funding Research acquisition, Foundation W.C. of Korea (NRF-2017R1A2A1A05001461). Acknowledgments: This research was supported by National Research Foundation of Korea Conflicts(NRF-2017R1A2A1A05001461). of Interest: The authors declare no conflict of interest.

Conflicts of Interest: The authors declare no conflict of interest. References

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