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J. Phys.: Energy 1 (2019) 042001 https://doi.org/10.1088/2515-7655/ab2f1e

TOPICAL REVIEW

Energy harvesting using thermoelectricity for IoT (Internet of Things)and E-skin sensors

Hwanjoo Park,Dongkeon Lee,Gimin Park, Sungjin Park, SalmanKhan, JiyongKim andWoochul KimSchool ofMechanical Engineering, Yonsei University, Seoul 120-749, Republic of Korea

E-mail: woochul@yonsei.ac.kr

Keywords: thermoelectricity, IoT, energy harvesting, body-heat harvesting

AbstractWith the increasing demand for Internet of Things (IoT)with integratedwireless sensor networks(WSNs), sustainable power supply andmanagement have become important issues to be addressed.Thermal energy in forms of waste heat ormetabolic heat is a promising source for reliably supplyingpower to electronic devices; for instance, thermoelectric power generators arewidely being researchedas they are able to convert thermal energy into electricity. This paper specifically looks over theapplication of thermoelectricity as a sustainable power source for IoT includingWSNs. Also, wediscuss a few thermoelectric systems capable of operating electronic skin (e-skin) sensors despite theirlow output power frombody heat. For amore accurate analysis on body heat harvesting,models of thehuman thermoregulatory systemhave been investigated. In addition, some clever designs of heat sinksthat can be integratedwith thermoelectric systems have also been introduced. For their powermanagement, the integrationwith aDC–DCconverter is addressed to boost its low output voltage to amore usable level.

1. Introduction

In 1999, Kelvin Ashtonfirst introduced the concept of ‘Internet of Things (IoT)’which emphasized humandependency on internet and computers to gather information [1]. IoT integratesmany objects with theirsurroundings through networks affecting the users’ quality of life [2–4]. Typical IoT systems include sensors,processors, a transmission component and a power source to allow interaction between the user and theelectronic system [4].Moreover, the demand forwireless sensor networks (WSNs) has increasedwhere efficientsustainable energy harvesting and energymanagement still remain key issues [5]. Since the past decade, themarket for internet-connected smart devices has grown to reach over a trillion dollars [6]. IoT systems consist ofseveral components: sensors to acquire information fromobjects, communication and processing devices todeliver information, and a cloud service to collect and transfer information. IoT systems using smart devicesallow awide range of applications including, but not limited to, home appliances, health care devices,transportation, and industry. For example, amonitored terrestrial ecosystemwhich collects and transfers theconsequential information to a data storage has been proposed [7]. IoT systems are also applied in homes andindustries in various forms such as transceivers, receivers, and sensors to detect any changes inmotion, ambientconditions, smoke/fire, and security [8]. IoT systems are alsowidely used infields of airbornewireless networksto communicate with a ground station or a satellite system [9]. Inmeans of their continuous operation, energyharvesting from the surrounding environment can act as the sustainable power source forWSNs [10]. Commonforms of harvestable energy from the ambient environment include solar energy [11–15], wind energy [16–20],and thermal energy [21–23]. Of these forms, wemainly focus on thermal energy and its conversion to electricity(widely known as thermoelectricity). The thermoelectric effect is the energy conversion process between thermaland electrical energy [24]. An advantage of thermoelectric power generation is its sustainable and reliableconversion of thermal energy into electricity with nomoving parts.With the increasing demand ofWSNs, thethermoelectric generators (TEG) have received the spotlight as a source of sustainable power supply. The

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19 June 2019

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3 July 2019

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22August 2019

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researches about the application of thermoelectricity forWSNs in various environments will be introduced insection 2.While the demand for smart devices related tomonitoring the health of a human has been increasing,the human body heat has been focused as the power source for sustainable operation. At the beginning ofsection 3.1, body heat harvesting using flexible TEGwill be introduced. Amore detailed system including bodyheat harvesting, powermanagement and operation of sensors will also be addressed. To reproduce the humanbody behaviormore accurately, a detailed analysis on body heat harvesting is conducted by examining varioushuman thermoregulatorymodels in section 3.2 In addition, various heat sinkmodels with the potential of beingintegratedwith thermoelectric systems are also explored in section 3.3 for further enhancement of the system. Asthe output voltage frombody heat harvesting-based thermoelectric systems stands only at about several tens ofmV [25, 26], a voltage converter or a voltage booster is required to operate sensors. Researches on voltageconverters will be explored in detail, section 3.4, as their integrationwith thermoelectric systems is yet anunfamiliar field of research.

2. Sustainable energy harvesting for IoT system

Asmentionedbefore, for continuous use of IoT systems, especially forWSNs, a sustainable power source basedonenergyharvesting is required. To convert diverse formsof energy from the ambient environment such as solar,wind, thermal, andmechanical energy into electrical energy, variousmethods of energy harvestinghave beenresearched as followed. Piezoelectric effects have beenwidely researched toharvest energy fromfluidmotion[27, 28], humanmotion [29–34], andwind [16, 19, 20]. Although theyharvest notable amounts of energy fromvarious conditions asmentioned above, they donot guarantee sustainability as energy is not harvestedwith anymotion. Energyharvesting using the triboelectric effect through contact electrificationhas also been researched for abroader usage [35–40]. Such effect has beendemonstrated [41]byharvesting energy fromwater drops [42, 43] andalso inmicro-scales-waterwaves [44, 45].Despite its possibility to be used as an energy source forWSNs, it stillrequires an actuating part, similar to piezoelectric energyharvesting. Solar energy, awidely used source of energy, is asuitable candidate for sustainablepower supply ofWSNs [11–15]. Although solar energyharvesting advances inhigh sustainability, its biggest drawback lies in the short durationof its operation time.

Unlike solar andwind energy, an imponderable amount of thermal energy from the industrial field isreleased as waste heat [46]. Thus, energy harvesting from this waste heat has also beenwidely researched infieldsof transportation [47–50] and industry [51, 52].Where there is thermal energy, TEG can be a sustainable powersupply forWSNs.Nakagawa et al [53] demonstrate a thermoelectric generator systemwith an internal heatstoragemade of paraffin.When installed on a bridge, it successfully harvests 58.5 J of heat energy per day,demonstrating its possible outdoor usewithWSNs. Samson et al [54] demonstrate the installation of athermoelectric generator with a heat storage, voltage converter, and awireless sensor unit on an aircraft fuselage,as shown infigure 1(a). The heat storage unit containswater as the phase changematerial (PCM) tomaintain atemperature difference along the thermoelectric generator. The thermoelectric system successfully operates thewireless sensor unit with a power consumption of 189μWunder the converted output voltage of 3.3 V. Zhu et al[55] demonstrate the thin-film based thermoelectric device integratedwith solar energy source, as infigure 1(b).To enhance the concentration of illumination intensities on hot side of thermoelectric device, a Fresnel lens wasincorporated into the system.With increasing illumination intensities, the harvested energy fromthermoelectric device is also enhanced, inwhich output voltage represents light intensity acting as light sensor.This work demonstrates the thermoelectric device for harvesting solar energywhich also acts as a light sensor.

For a broader application of thermoelectric systems, the demand forflexible TEGhas grown incomparably.Conventional thermoelectricmodules are in rigid formswith ceramic plates on top and bottom sides whichlimit their application toflat surfaces. To overcome such limitation, thermoelectric devices based on a flexiblesubstrate using a printingmethod [56–62] or structural designwith inorganic bulk thermoelectricmaterials[25, 26, 47, 63] have been researched. Thermoelectricmaterials used near room temperature aremainly Bi-Tebased compounds, Bi2–xSbxTe3 for p-type andBi2Te3–xSex for n-type, due to its high thermoelectricperformance [64, 65].Materials for both printing and inorganic bulk thermoelectricmaterials are based onBi-Te compounds. For thematerial for printing, Bi-Te powdersmixedwith solvents are adopted for themanufacturing [58, 61]. Inorganic bulk thermoelectricmaterials used near room temperature are normallyBi-Te compounds in polycrystalline structure. Kim et al [66] demonstrate aflexible thermoelectric generatorthat powers awireless sensor nodewhile attached to a curved heat pipe, as infigure 2(a). For flexibility, the deviceconsists of inorganic bulk thermoelectricmaterials embedded in aflexible polymer [25].With the temperatureof the heat pipe and ambient air set constant at 70 °Cand 20 °C, respectively, in an experimental setup, amaximumpower density of about 2.2 mW cm−2 is harvested. The harvested power transferred to the powermanagement IC is then boosted to 4.2 Vwith a voltage converter and down to 3.3 V for operationwhile thesurplus power is used to charge a battery.WSNs integratedwith TEG successfully collect and transfer

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information on the temperature of ambient air and the circular pipe, CO2 concentration, and open circuitvoltage. Iezzi et al [67] demonstrate aflexible thermoelectric generator integratedwithwireless sensors that aremanufactured from a printing process. To fabricate the thermoelectric device, bulkmetallicmaterials, Ag andNi, are used as the p- and n-type, respectively. The devices, as shown infigure 2(b), are installed vertically on aheat pipe in an experimental setup. The output voltage is then boostedwith a commercial DC–DCconverter(LTC 3108) to charge a super capacitor. The charged capacitor successfully operatesWSNs to transmitinformation on temperature to the user.

Researches on operatingWSNs by harvesting energy from thermoelectricity have beenwidely conducted invarious environments asmentioned above.With the existence of a reliable heat source,WSNs poweredwith

Figure 1. (a)Wireless sensor systempowered by aircraft specific thermoelectric energy harvesting [54]. (b) Schematic of the thin-filmthermoelectric device integratedwith an optical concentrator [55]. (a)Reprinted from [54], Copyright 2011, with permission fromElsevier B.V. All rights reserved. (b)Reprinted from [55], Copyright 2017, with permission fromElsevier. All rights reserved.

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thermoelectric devices can be applied. For efficient energy conversion, however, other components such as heatsink should also be considered.More details about heat sinkwill be addressed in section 3.3.

3. IoT systemusing body heat harvesting

3.1. Body heat harvestingRecently, there has been a great amount of interest in health care andmedical applications; thus, sensors capableofmonitoring the patient’s health conditions for a long period of time are longed for. There are numerous kindsofmedical sensors thatmeasure the conditions of a patient. For example, blood pressure sensors,

Figure 2. (a) Systemofwireless sensor nodes powered by aflexible thermoelectric generator [66]. (b) System of printedmetal-basedthermoelectric generator for poweringwireless sensor [67]. (a)Reprinted from [66], Copyright 2018, with permission fromElsevier.All rights reserved. (b)Reprinted from [67], Copyright 2017, with permission fromElsevier. All rights reserved.

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electromyography sensors, electrocardiography (ECG) sensors, and SpO2 pulse oximetry sensors arewidelyused tomonitor patients with health issues. Themost important requirement of the aforementionedmedicalsensors is a stable and continuous power supply. To supply sustainable power formedical sensors,thermoelectric devices capable of generating power by harvesting heat from a human body are used. The humanbody can be used as a heat source as it generates heat continuously thoughmetabolism; however, to do so, it isimportant to understand the human thermoregulatory system. The thermoregulatory systemof the humanbody controls its skin temperature and therefore certain parts of the body aremore suitable to operate certaindevices than the rest as shown infigure 3.However, the conventional TED is hard to adapt on to the human skindue to its rigidity. Tomake thermoelectric devicesmore applicable to the human skin, researches onflexibleTEG are being actively conducted.Majority offlexible TEG for body heat harvesting are based on inorganic bulkthermoelectricmaterials instead of printedmaterials. Since thermal energy from a human body is so called the‘low-grade heat’, it is hard to yield a sufficient amount of temperature difference in a film-type thermoelectricdevice due to its small thickness forflexibility. Kim et al [59] demonstrates the body heat harvesting using thin-film basedflexible thermoelectric device while generating only an output power of 3 μW.Most researchesrecently published onflexible thermoelectric devices for body heat harvesting adopt inorganic bulkthermoelectricmaterials which have sufficient height to retain enough temperature difference.

Table 1 shows the experimental results of body heat harvesting using TEG. Researches referenced in table 1have used inorganic bulk thermoelectricmaterials based onBi-Te compounds. Suarez et al [69] use inorganicbulk BiTe compounds and EGaIn as theflexiblemetal liquid electrodes which are placed on both top andbottom sides of their FTED, as infigure 4(a). The liquidmetal is encapsulated in polydimethylsiloxane (PDMS)to prevent leakage. The device was able to generate a voltage ranging from1.47 to 2.96 mV and power from

Table 1.Experimental results of thermoelectric generators on human body.

References Number of TE couples Output power density (μW cm−2) Open circuit voltage (mV) Deviceflexibility

Kim et al [25] 72 2.28 21.2 Flexible

Suarez et al [69] 32 0.37 3.5 Flexible

Park et al [26] 15 5.60 8.8 Flexible

Eom et al [63] 20 3.20 6.6 Flexible

Kim et al [59] 11 0.70 2.9 Flexible

Hyland et al [70] 25 4.60 12.0 Rigid

Wang et al [71] 52 0.30 6.6 Flexible

Kim et al [72] 50 13.00 60.0 Flexible

Figure 3. Schematic of body heat powered E-skins by using thermoelectric generator andwireless sensor system [68]. Reprinted from[94], Copyright 2019, with permission fromElsevier.

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1.48 to 6.0 μWunder varying air velocity conditions. Eom et al [63] demonstrate a bracelet-shapedthermoelectric device based on rigid inorganic bulk thermoelectricmaterials which has a hinge structure, so itcan be structurally flexible as infigure 4(b). The bracelet-shaped thermoelectric device attains an output powerranging from40 to 80 μWwith open circuit voltage from6.7 to 9.1 mV. Park et al [26] propose amat-shapedflexible thermoelectric device. It is specialized to be applied to a curved surface. Cubic-shaped inorganic bulkthermoelectricmaterials are inserted into holders that can be connected to each other with awire. Considering

Figure 4. Flexible thermoelectric generator based on inorganic bulk thermoelectricmaterials for body heat harvesting: (a) deviceintegratedwith liquid-metal electrode [69]. (b)Watch-shaped device to be applied on human’s wrist [63]. (c)The device with largearea and highflexibility [25]. (a)Reprinted from [69], Copyright 2017, with permission fromElsevier. (b)Reprinted from [63],Copyright 2017, with permission fromElsevier. All rights reserved. (c)Reprinted from [25], Copyright (2018), with permission fromElsevier. All rights reserved.

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that the radius of curvature of an adult human arm ismore than 30 mm, the radius of curvature of themat-shaped FTED is less than 12 mm; thus, it is suitable for human body heat harvesting. Theflexible thermoelectricdevice harvests energy frombody heat andwas able to obtain an output power of 138.67 μWwith an 8.8 mVopen circuit voltage. Kim et al [25]demonstrate a flexible thermoelectric device inwhich inorganic bulkthermoelectricmaterials are embedded in aflexible polymer, as shown infigure 4(c). Body heat harvesting isconducted under various conditions: varying height of thermoelectricmaterials, fill factor, and air velocity,while themaximumoutput power and voltage are 1.46 mWand 38.24 mV, respectively.

Asmentioned above,many studies on body heat harvesting using aflexible thermoelectric generator arereported and are still ongoing. Further applications on self-poweredWSNs driven by body heat harvesting arebeing carried out widely. Thielen et al [73] propose an application of TEGon a human’s wrist integratedwith aDC–DCconverter as infigure 5(a). Two different types of TEG are demonstrated:μTEGwhich has a lowthermal resistance with a high output voltage andmTEGwhich has a high thermal resistancewith a lowelectrical resistance. Proposed TEGs showhigh output power densities, which range from13 to 14 μWcm−2,with amaximumof 24% converting efficiency.Magno et al [74] propose amulti-source harvesting circuit whichconsists of a solar harvester and aTEG for body heat harvesting.While harvesting an average power up to550 μWbyphotovoltaic in indoor situations, the TEGharvests 98 μWwith a 3 K temperature difference fromahumanbody. Themulti-source harvesting system successfully operates several devices such as a camera,microphone, accelerometer and temperature sensor.Myers et al [75] present a thermoelectric body heatgenerator integratedwith components that transmit powermanagement data and various sensors, as shown in

Figure 5. (a)Abody heat poweredwearable thermoelectric system and its conversion performance [73, 76, 77]. (b)Thermoelectricgenerating systemoperating sensors powered by body heat [75]. (a)Reprinted from [73], Copyright 2017, with permission fromElsevier. All rights reserved. (b)Reprinted from [75], Copyright 2017, with permission fromElsevier. All rights reserved.

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figure 5(b). The proposed systemdemonstrates a successful operation of sensors under varying temperature andhumidity conditions while the subject is either running, walking or resting.Necessity for consideration of wetbulb temperaturewhile body heat harvesting is also demonstrated.Wang et al [71] demonstrate the operation ofaminiaturized accelerometer powered by body heat harvesting on a human’s wrist, as infigure 6(a). Theproposedflexible TEG ismade of inorganic bulk thermoelectricmaterials which are embedded in PDMS. Itsuccessfully generates an open circuit voltage of 6.6 mV from ahumanbody under indoor conditions. Tooperate aminiaturized accelerometer which requires 2.4 μWof powerwith a 1.6 mVvoltage, a DC–DCconverter with an efficiency of 50% is adopted to boost the voltage up to 2.2 V. A demonstrated thermoelectricsystem successfully operates theminiaturized accelerometer powered frombody heat, as infigure 6(a). Kim et al[72] represent a self-poweredwearable ECG systemusing aDC–DCconverter, as infigure 6(b). The systemconsists of aDC–DCconverter, VDD shifter and an ECGmodule. The ECG sensor is fabricated on a flexibleprinted circuit boardwhile the power is supplied through the flexible TEGusing human body heat. TheflexibleTEGharvests 1 mWof powerwith 40–100 mVof open circuit voltage. To operate the ECG sensor, voltage isboosted up to 3.3 Vusing a commercial DC–DCconverter (LTC3108) and dropped down to 1.0 V through aVDD shifter. Due to the existence of efficiency for voltage converting, 70 μWof power is supplied, inwhich itsuccessfully operates the ECG sensor.

Figure 6. Flexible thermoelectric systemwith a voltage converter andwireless sensor powered by body heat: (a) the systemoperatingan accelerometer fromhumanmotion [71]. (b)Operation of self-poweredwearable electrocardiography [72]. (a)Reprinted from[71], Copyright 2018, with permission fromElsevier. All rights reserved. (b)Reprintedwith permission from [72]. Copyright 2018AmericanChemical Society.

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In designing the thermoelectric system, the thermal contact resistance between the thermoelectric deviceand the heat source or sink should be also considered. Especially when applied to a human body inwhich thetemperature difference between the top and bottom side of thermoelectric device is relatively small compared toindustry applications, the reduction of thermal contact resistance is crucial in achieving high power generation.

3.2.Human thermoregulatorymodelFor accurate prediction or analysis of body heat harvesting by aflexible TEG, understanding of the humanthermoregulatory systemmust be preceded. The human body has an intricate built-in system to regulate itstemperature tomaintain a balance betweenmetabolic heat production and heat loss to the environment. Thisbalance between heat production and loss is important inmaintaining a constant core temperature of thehuman body [78]. This thermoregulatory system is divided further into two parts consisting of active and passivesystems [79, 80]. The active systemdirectly controls the temperature of the body throughmeans of sweating,shivering, vasoconstriction, and vasodilation [80]. This leads to the rise and fall of the skin temperatures tomaintain a balance between heat production and heat loss. The passive system represents the geometry of thehuman body,metabolic heat production, and the thermal interaction of the human bodywith the environment(convection, radiation, and evaporation) [81]. Throughout the years,manymodels have been developed tosimulate the thermoregulation of the human body. The thermoregulatorymodels can be divided into twomajorcategories as infigure 7(a). Thefirst simplermodel is the two-nodemodel. The key feature of the two-nodemodel is the representation of the human body as cylinders (single-segment) consisting of its core and skin [82].This simplification allows for a quick analysis of the human thermal system. Themulti-nodemodel ismorecomplicated as it represents the human body as cylinders subdivided into usually the core,muscle, fat, and skinlayers [78].

Themost well-known two-nodemodel was developed byGagge in 1971 [82]. The human body inGagge’smodel is treated as two coaxial cylinders each representing the core and the skin. Themetabolic heat generatedfrom the core is transferred to the skin through conduction and blood flow. The boundary condition is set on thesurface of the skin surface and given as evaporation, convection, and radiation heat losses. The active systemhereismodeled using vasodilation, vasoconstriction, and sweating functions.

The Stolwijkmodel is the basis ofmanymulti-node thermoregulatorymodels [84]. The human bodygeometry consists of six segments (head, trunk, arms, hands, legs, and feet), eachwith four nodes (core,muscle,

Figure 7. (a) Schematic of twomajor categories for human thermoregulatorymodel: two- andmulti-nodemodel [78]. (b)Multi-nodethermoregulatorymodel [83]. (a)Reproduced from [78]. CCBY4.0.© 2016TheAuthors. Published byElsevier Ltd. (b)Reprinted bypermission fromSpringerNatureCustomer ServiceCentreGmbH:Nature, International Journal of Biometeorology, [83]©ISB 2006.

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fat, and skin), and one central blood pool. Each node exchanges heat with each other through conduction andwith the blood pool through convection. The active system in Stolwijk’smodel is divided into three parts. Infirstpart, the thermoreceptors sense the hot or cold state. Then the second part sends this information to the thirdpart which controls the action of the active system according to the information received to induce actions suchas vasodilation and constriction [84]. Thismodel was developed forNASA to simulate the thermoregulatorysystemof astronauts and is capable of predicting the skin temperature in low-activity conditions [85]. Anotherwell-knownmulti-node thermoregulatorymodel is the Fialamodel [79, 80, 86, 83]. The human body isconstructed as 15 cylindrical or spherical segments each containingmultiple layers of tissue accordingly(figure 7(b)). The skin layer in thismodel is divided into the inner and outer layer. The inner layer is responsiblefor blood perfusionwhile the outer layer is responsible for heat exchangewith the environment throughevaporation [86]. Penne’s so called bio-heat equation [87], is expressed in equation (1)

kT

x

T

y

T

zQ c m T T c

T

t, 1m b bl bl

2

2

2

2

2

2r

¶¶

+¶¶

+¶¶

+ + - =¶¶

⎛⎝⎜

⎞⎠⎟ ( ) ( )

where k, ρ and c are the thermal conductivity, density and specific heat of each layer respectively. Also,Qm is themetabolic heat generation andmbl is the volumetric bloodflow rate. Equation (1)was applied to each tissue layerwith appropriatematerial properties of an averageman.Heat is transferred between the tissue layers throughconduction and at the surface of the skin through convection, radiation, and evaporation to the environment.The Fialamodel also takes into account countercurrent heat exchange to simulate amore accurate arterial bloodtemperature [79, 86]. The active systemwasmodeled based on a regression analysis of physiological responsesfrommultiple subjects which resulted in a temperature-based systemdriven by the body core temperature and

Figure 8. (a)Analysis offinned heat sinkwith PCM [90]. (b) Schematic and performance of heat sink composed of coppermetal foamand paraffin composites [91]. (c)Heat sinkwith a double layermicrochannel using various fluids [92]. (a)Reprinted from [90],Copyright 2017, with permission fromElsevier. All rights reserved. (b)Reprinted from [91], Copyright 2012, with permission fromElsevier. All rights reserved. (c)Reprinted from [92], Copyright 2015, with permission fromElsevier. All rights reserved.

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themean skin temperature [80]. The Fialamodel was later adapted by theUTCI-Fialamodel which is the basis oftheUniversal Thermal Climate Index [86]. A simplified thermoregulatorymodel was proposed byWijethungeet al [88] that focuses on the forearm segment. The forearmwas themain focus of thismodel as the purposewasto provide a simplifiedmodel for designingwearable thermoelectric devices. The boundary conditions at theskin surface were given as convection, radiation, and evaporation heat fluxes. Assuming the body coretemperature as constant, shiveringwas neglected as it occurs when the body core temperature is considerablylow. In addition, thismodel used thermal resistance of the skin to express the temperature difference across thethermoelectric elements. This temperature difference was then used to calculate the voltage and in return thepower generated by a TEG.

3.3.Heat sink for TEGAswidely known, the temperature difference across the length of a thermoelectricmaterial is a dominant factorin determining the performance of a thermoelectric device. A larger temperature difference leads to a higherpower output and thus, an appropriate heat sink is necessary to achieve this large temperature difference. In thatpoint of view, integration of heat sinkswith a thermoelectric device is critical in improving its performance but isvery poorly investigated. In this section, some clever designs of heat sinks, not specifically designed for TEGs, butpossible to be integratedwith thermoelectric systems are demonstrated.

Among the various designs of heat sinks that can be integratedwith thermoelectric devices, ones with PCMshave shown great premise in reducing the temperature of a heat source. PCMs are known to possess high latentheat and thus, such property has been researchedwidely for its potential use in heat sinks. Themechanismbehind PCMs is utilizing its high latent heat when it changes phase from solid to liquid to absorb the heat fromits heat source. To enhance the performance of the heat sink based on PCMs, researches to increase thermalconductivity have beenwidely conducted.Wang et al [89] investigate the effect of using a porousmetalfibersintered felt (PMFSF) to increase the thermal conductivity of PCMs. They use a high-power LEDwith a powerconsumption ranging from1 to 5Was the heat source and used red copper as themetal foam to prepare theparaffin/PMFSF composite. The thermal conductivity of the composite was enhanced from0.2Wm−1 K−1

(pure paraffin) to 26.76Wm−1 K−1. Bayat et al [90] also attempt to increase the thermal conductivity of a PCMheat sink by adding a lowpercentage of nanoparticles. They only numerically simulate the effects ofnanoparticles due to complexities in nanoparticle fabrication andwere able to obtain enhanced thermalconductivities as shown infigure 8(a). Unlike their expectations, adding only 2%of nanoparticles shows the bestperformance and the performance gradually decreases with further addition of nanoparticles. Qu et al [91] alsodesign a passive thermalmanagement systemby utilizing ametal foam saturatedwith PCM in a heat sink toreduce the temperature of a heat source. Similar to thework done byWang et al [89], copper is used as themetalfoam and compared to a pure PCMand pure copper basement, the foam-PCMcomposite was themost efficientat preventing the increase in temperature of the heat source (figure 8(b)). Despite all these efforts, the biggestproblemof the PCMheat sink still stands evenwith an alteration- passive thermalmanagement.Without doubt,as it changes phase, it can absorbmore heat than in its solid formdue to its high latent heat; however, when thePCMreaches itsmaximumheat capacity after some time, the surface temperature of the heat source starts toincrease rapidly. Thus, thermallymanaging an electronic device with a PCMheat sink for a long period of time isnot pragmatic and is only appropriate for a short duration of time.

Another well-known type of heat sinks is a system composed ofmicro-channels with a circulating fluid. Firstproposed by Tuckerman and Pease [93], a liquid-cooled heat-exchanger design by using fluid channels inmicroscopic dimensions allows the operation of circuits with power densities higher than 1000W cm−2. Theidea behindmicro-channels is to use numerous separate channels, rather than a single channel toflowover thesurface of the heat source and to increase the surface area of the heat sink that the fluidmakes contact with.Rajabifar [92] adds an additional channel on top of the bottom channel, ultimately designing a double layeredcounter-flowmicro-channel with nanofluids and PCMslurries as the coolants as shown infigure 8(c).Withdifferent types of fluid employed in the upper and the lower channel, the heat flux absorbed by the heat sink isthe greatest when the nanofluids and the PCMslurries are deployed in the upper and lower channel, respectively.

Different fromheat sinks with PCMs ormicro-channelsmentioned above, notmuch research has been doneonflexible heat sinks as their demand has been low. There have been several works [26, 72, 94]which adopt aflexible heat sink on a thermoelectric generator for body heat harvesting. Although the performance of body heatharvesting has been enhancedwith the existence of aflexible heat sink, those research have aimedmore on theflexible thermoelectric system rather than flexible heat sink. The demand forflexible heat sinkswithsustainability and high performance is expected to be increased for broader applications.

3.4.DC–DCconverter for powermanagementTo operate an IoT system integratedwithWSNs,management of the harvested energy is as important as energyharvesting. Themanagement ismainly focused on the design of aDC–DCvoltage converter which boosts up

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harvested voltage, normally ranging from a fewmV in body heat harvesting to several hundredmV-level inindustry application, to a usable level for operatingwith high efficiency. Conventional DC–DCconverters have atwo-stage circuit [66], boost converter and a buck converter. Chen et al [95] demonstrate a one-stageDC–DCconverter inwhich the input voltage is 100 mV and the output voltage ranges from500 to 600 mV.Owing to thecapacitive bootstrapping technique, the proposedDC–DCconverter shows amaximum conversion efficiency of76.4%over various loads ranging from1 to 500 μW.Luo et al [96] demonstrate a power converter for athermoelectric generator with low input voltage integratedwith a self-start circuit. Once the chip self-starts at210 mV, only 7 mV input voltage is required to continuously operate themain converter. The proposed powerconverter shows amaximumoutput power of 229 μWwith 73.5% end-to-end efficiency. V et al [97] alsodemonstrate a voltage converter with a single-stage regulator. Since the proposed converter is aimed to be drivenby human body heat, it possesses low input voltage ranging from25 to 210 mV. The converting system showsmaximumefficiency of 65%with a peak delivering power of 1.03 mW.

4. Conclusion

With the rapid increase in demand for smart devices, themarket for IoT systems is also consequently growing.Concurrently, research on sustainable power supplies is beingwidely conducted. This paper explores the recentprogress in TEG as a power supply for IoT systems integratedwithWSNs. AlthoughTEG are able to harvestenough energy, the output voltage is not enough to operate commercial ormicro-fabricated sensors. In thispaper, not only the energy harvesting by thermoelectricity but energymanagement using aDC–DCconverter toboost the output voltage is also discussed. Considering the increase in application of TEGon a human body, anaccurate analysis of the human thermoregulatory system is required. For further enhancement of thermoelectricsystems, candidates for integrable heat sinks with TEGhave been presented aswell. Not forget tomention,thermoelectric systems forwearable applications also show the potential for commercializationwithwell-designedflexible TEG and powermanagement systems.

Acknowledgments

Thisworkwas supported by aNational Research Foundation of Korea (NRF) grant funded by theKoreanGovernment (MSIP) (NRF-2015R1A5A1036133&NRF-2018K1A3A1A20026439).

ORCID iDs

Woochul Kim https://orcid.org/0000-0003-3966-8414

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