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(a) Cross sectional view of thermoelectric unit couple. Unit couple consist of p and n type thermoelectric materials, filler materials, electrodes and heat sink as indicated. (b) Thermal resistance network for the thermoelectric device. Here Tcore indicates core body temperature, Rt,skin indicates skin thermal resistance, Rt,contanct indicates skin and device contact thermal resistance, Rt,C indicates heat sink thermal resistance, Rt,fill indicates gap filler thermal resistance, Rt,TEG indicates thermoelectric material thermal resistance, TTEG,H indicates hot side thermoelectric element temperature, TTEG,c indicates cold side thermoelectric element temperature and Ta indicates ambient temperature.
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Research on wearable and implantable devices have become popular with the strong need in market. A precise understanding of the thermal properties of human skin, which are not constant values but vary depending on ambient condition, is required for the development of such devices. In this paper, we present simplified human thermoregulatory model fo...
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Citations
... . v b is the rate of blood flow through a particular region of tissue, v b is the volume of the tissue through which the blood is flowing, ∆τ is the time duration over which the blood flow is being considered. When the blood flow through the skin increases, it is called vasodilation, and when it decreases it, is referred to as vasoconstriction [77]). Moreover, . ...
Thermoelectricity can assist in creating comfortable thermal environments through wearable solutions and local applications that keep the temperature comfortable around individuals. In the analysis of an indoor environment, thermal comfort depends on the global characteristics of the indoor volume and on the local thermal environment where the individuals develop their activity. This paper addresses the heat transfer mechanisms that refer to individuals, which operate in their working ambient when wearable thermoelectric solutions are used for enhancing heating or cooling within the local environment. After recalling the characteristics of the thermoelectric generators and illustrating the heat transfer mechanisms between the human body and the environment, the interactions between wearable thermoelectric generators and the human skin are discussed, considering the analytical representations of the thermal phenomena. The wearable solutions with thermoelectric generators for personal thermal management are then categorized by considering active and passive thermal management methods, natural and assisted heat exchange, autonomous and nonautonomous devices, and direct or indirect contact with the human body.
... 8 For precise prediction of WTEG performance and regulation of their geometrical configuration, accurate models that consider the effect of human skin are critical. 9 Wijethunge et al. 10 proposed a simplified human thermoregulatory model for the design of WTEGs, which estimated the thermal properties of human skin without rigorous calculations. The authors found that failing to consider physiological processes could result in deviations of 10-60% in the prediction of WTEG performance. ...
... Upon an examination of references, [9][10][11][12][13][14] it was observed that they exclusively focused on flat WTEGs with thermoelectric legs featuring a flat cross-sectional configuration. However, for practical applications, such as recovering body heat from the wrist, neck, lower leg, and ankle, the heat source may be considered cylindrical in shape. ...
This paper presents a theoretical model for a human skin-wearable annular thermoelectric generator (WATEG) system and provides analytical solutions for its energy conversion performance. The Pennes equation is used to model the heat transfer of human skin, which is assumed to be a cylindrical multilayer structure composed of subcutis, dermis, and epidermis. The heat exchanges induced by blood perfusion and metabolic heat generation within the skin tissue are taken into account. It is found that the influence of skin effect and contact thermal resistance between the human skin and flexible substrate plays a significant role in the energy conversion performance of the WATEG and should be considered. The matched load resistance, optimal fill factor, and height of thermoelectric legs are determined through numerical analysis. The findings of this study can be applied to the practical design of WATEG devices and are expected to contribute to their optimization.
... However, Huang et al. [27] discovered that some of these oversimplified energy equations did not satisfactorily match the test results, because some equations were only theoretically based on specific simplifications and assumptions that might not hold in actual cases. Accordingly, a further generic one-dimensional transient model has been investigated by numerous studies [28][29][30][31][32][33], and its governing equation is as follows: ...
... Recently, using COMSOL commercial FEM software, some studies [33,43] built the overall human thermoregulatory models for estimating the thermal properties of TE with the bio-heat transfer, which considers the thermoregulatory mechanics such as blood flow, sweat evaporation, metabolic rate, etc. More in-depth, optimizations for TE geometrical parameters [44], heat dissipation methods [45,46], and package forms [47] were adopted, despite those studies were aimed at analyzing TEGs, but the modeling and analysis concepts can be adapted to TECs as well. ...
In recent decades, personalized and customized thermal management technologies are emerging along with the flourishing of diverse personal thermoregulation requirements. Among all potential technologies that may replace conventional bulky air conditioning system, thermoelectric (TE) technology has several advantages over conventional cooling systems, including low driving energy, compact in size, eco-friendly and low-maintenance requirement, no direct mechanical moving parts and working fluid, and having it simple to switch between cooling and heating modes. Recent researches on thermoelectric technology for personal thermal management (PTM) are discussed in this Chapter, including modeling and design methodologies, novel system/device structures, and innovative TE materials. Current representative studies are summarized, and prospective future trends in terms of modeling, structures, and materials are discussed.KeywordsThermoelectric technologyPersonal thermal managementWearables
... In this study, we used a simplified human thermoregulation model for finite element simulation and introduced virtual thermal resistance to represent heat transfer properties in the skin. When the ambient temperature is lower than 25 • C, the thermal resistance of skin remains constant; however, when the ambient temperature exceeds 25 • C, thermal resistance suddenly decreases [51][52][53]. ...
Wearable thermoelectric generators (w-TEGs) convert thermal energy into electrical energy to realize self-powering of intelligent electronic devices, thus reducing the burden of battery replacement and charging, and improving the usage time and efficiency of electronic devices. Through finite element simulation, this study successfully designed high-performance thermoelectric generator and made it into wearable thermoelectric module by adopting “rigid device—flexible connection” method. It was found that higher convective heat transfer coefficient (h) on cold-end leads to larger effective temperature difference (ΔTeff) and better power generation performance of device in typical wearable scenario. Meanwhile, at same h on the cold-end, longer TE leg length leads to larger ΔTeff established at both ends of device, larger device output power (Pout) and open-circuit voltage (Uoc). However, when the h increases to a certain level, optimization effect of increasing TE leg length on device power generation performance will gradually diminish. For devices with fixed temperature difference between two ends, longer TE leg length leads to higher resistance of TEGs, resulting in lower device Pout but slight increase in Uoc. Finally, sixteen 16 × 4 × 2 mm2 TEGs (L = 1.38 mm, W = 0.6 mm) and two modules were fabricated and tested. At hot end temperature Th = 33 °C and cold end temperature Tc = 30 °C, the actual maximum Pout of the TEG was about 0.2 mW, and the actual maximum Pout of the TEG module was about 1.602 mW, which is highly consistent with the simulated value. This work brings great convenience to research and development of wearable thermoelectric modules and provides new, environmentally friendly and efficient power solution for wearable devices.
... In comparison, the dynamic human thermoregulatory model has shown great potential for WTEGs. By building blood flow and sweat evaporation in skin multilayers, Kim et al. [27] used COMSOL to build a human thermoregulatory model for WTEGs, in which the feedback regulations for body heat could be achieved based on external changes. However, the physical activities of the human body were not well embodied. ...
... However, as demonstrated in the literature [20], where a structure only 1 mm high is employed for adapting to the irregular surface of the human body, a higher rigid structure is not advantageous for flexibility. The fill factor is the key value to power matching given the capability to adjust the area of heating and cooling, and the thermal resistance composition as well, which can be explained by Eq. (27). A significant peak at around 0.1 ~ 0.15 for the fill factor is observed for all three cold side structures. ...
Wearable thermoelectric generators (WTEGs) have shown great potential for harvesting low-grade body heat. However, inappropriate design of cold side heat sinks leads to unsatisfactory power generation efficiency. To overcome this, radiative cooling cold side has been introduced by some earlier researchers. Nonetheless, a reliable performance evaluation model lacks for WTEGs with radiative cooling at outdoors. In this paper, we propose a novel analytic model, which considers comprehensive thermoregulatory mechanisms and detailed radiative heat transfer process. The model demonstrates good accuracy with less than 9% error in a variety of conditions. Furthermore, the proposed dynamic model greatly outperforms the constant model and static model in terms of WTEG output performance prediction at outdoors, which can reduce the deviations by up to 196.4% and 71.6%, respectively. Among the three types of cold side structures (bare, finned, and radiative cooling), we found that the radiative cooling cold side performed best. Accordingly, the output performance of the WTEG with radiative cooling was further investigated, and the influences of climatic conditions and spectral properties of radiative cooling on WTEG were obtained. Different human body segments were also considered in outdoor simulations, with the shoulder giving the best performance. Finally, based on the proposed model, the thermal resistance improvement indicates that 220% output performance enhancement is possible, which could be beneficial for device design and utilization in many other applications.
... In this study, we used a simplified human thermoregulation model for finite element simulation and introduced virtual thermal resistance to represent heat transfer properties in the skin. When the ambient temperature is lower than 25 °C, the thermal resistance of skin remains constant; however, when the ambient temperature exceeds 25 °C, thermal resistance suddenly decreases [51][52][53]. ...
Wearable thermoelectric generators (w-TEGs) convert thermal energy into electrical energy to realize self-powering of intelligent electronic devices, thus reducing the burden of battery replacement and charging, and improving the usage time and efficiency of electronic devices. Through finite element simulation, this study successfully designed high-performance thermoelectric generator and made it into wearable thermoelectric module by adopting “rigid device - flexible connection” method. It was found that higher convective heat transfer coefficient on cold-end leads to larger effective temperature difference and better power generation performance of device in typical wearable scenario. Meanwhile, at same convective heat transfer coefficient on the cold-end, longer TE leg length leads to larger temperature difference established at both ends of device, larger device output power and open-circuit voltage. However, when the convective heat transfer coefficient increases to a certain level, optimization effect of increasing TE leg length on device power generation performance will gradually diminish. For devices with fixed temperature difference between two ends, longer TE leg length leads to higher resistance of TEG, resulting in lower device output power but slight increase in open-circuit voltage. Finally, sixteen 16×4×2 mm2 TEGs (L=1.38 mm, W=0.6 mm) and two modules were fabricated and tested. At hot end temperature Th=33 ℃ and cold end temperature Tc=30 ℃, the actual maximum output power Pout of TEG is about 0.2 mW, and the actual maximum output power Pout of TEG module is about 1.602 mW, which is highly consistent with the simulated value. This work brings great convenience to research and development of wearable thermoelectric modules and provides new, environmentally friendly and efficient power solution for wearable devices.
... Wijethunge et al. [102], investigated the importance of assuming a constant or variable value for the thermal resistance of the skin in the temperature distribution on the skin surface and, as a result, the effect on the performance of a WTEG under different environmental conditions. Since blood perfusion rate plays a vital role in heat transfer inside the body, they used Pennes's bio-heat equation to find the temperature distribution inside the body tissue by assuming the body core temperature to be constant. ...
Thermoelectric generators are devices that directly convert heat flux or the temperature difference between two hot and cold surfaces into electricity. With the advancement of the Internet of Things (IoT) technology and the development of wearable and portable devices, the issue of providing a sustainable power source is one of the main challenges in the development path of these tools. Creating electric power by harvesting the waste heat from the human body is one of the effective solutions in this way. For this reason, the development and improvement of the technology of wearable thermoelectric generators have received much attention recently. Due to the low-temperature difference between the two sides of wearable thermoelectric generators and the high thermal resistance between the skin and the heated surface of these modules, the performance of these systems is highly dependent on their structural parameters and environmental factors. In this paper, it has tried to review all the previous studies regarding the impact of structural factors (such as the matching of internal and external thermal resistances, geometrical parameters of the module, design of heat source and sink, and flexibility of thermoelectric module) and environmental parameters (including the effect of ambient air temperature and humidity, skin temperature, and the interaction of power consumers with thermoelectric modules). Based on the studies, it seems that in optimizing the performance of wearable thermoelectric generators (WTEGs), it is necessary to consider the effect of the human body's thermoregulatory responses, such as skin temperature and sweating rate. The change in skin temperature directly affects the performance of WTEGs, and the change in sweating rate can also affect the thermal resistance between the skin and the hot plate and overshadow the matching of thermal resistances during operation.
... arm, wrist, chest or forehead) [23][24][25], contact thermal resistance between skin and WTEG [14,15,26] and physiological characters of human skin (e.g. metabolism and blood circulation) [15,27]. ...
... A one-dimensional theoretical model for estimating the power output of WTEGs was established by Zhang et al. [16], and the closed-form optimal solutions such as loads resistance, fill factor and leg's height are obtained, and it is concluded that the heat transfer by blood perfusion in skin tissue should be considered. The performance of WTEGs was studied by Wijethunge et al. based on a simplified human thermoregulatory model, and it is found that the deviations of 10-60% are generated by neglecting the thermal resistance of human skin [27]. A WTEG with a flexible phase-change-material (PCM) heatsink was proposed by Lee et al. [43], the temperature difference across WTEG can be hold for a long time by absorbing a large amount of heat energy at critical temperature of PCM, which the dynamic model is obviously required to evaluate the energy conversion performance of WTEG accurately. ...
... The variations of temperature difference across thermoelectric couple and power density of WTEG with material properties and height of thermoelectric semiconductors are shown in Fig. 5, the temperatureindependent material properties are obtained by Eq. (27) and T is set as the ambient temperature 288 K. We can see that there is almost no difference for numerical results between two methods. ...
A thermodynamic model for skin and wearable thermoelectric generators (WTEGs) system is developed based on dual-phase-lag (DPL) bioheat transfer. Human skin is regarded as a multi-layer structure consisted of subcutis, dermis and epidermis. Analytical solutions for temperature profile inside skin-WTEG system and energy conversion performance of WTEGs are obtained. Numerical results show that very small deviations for power output of WTEGs are caused by using the room-temperature physical properties of Bi2Te3-based thermoelectric semiconductors. However, the effect of heat convection by blood perfusion inside skin tissue should be considered to estimate energy conversion performance of WTEGs accurately. The influence of the contact thermal resistance between the skin and WTEG can be neglected when the thermal conductance ratio of the skin-WTEG interface to the flexible substrate is larger than 0.1. It takes more than 10 min to attain the steady-state again when a thermal perturbation is applied to the skin-WTEG system. The classic Fourier bioheat transfer model may also provide acceptable accuracy for the energy conversion analysis of WTEG compared with the non-Fourier bioheat transfer model if the response time exceeds 1 min. This paper provides a useful theoretical model for designing WTEG devices.
... The FF is defined as the ratio between the area of the thermoelectric material and the area of the device. In a fixed heat flux system, such as in wearable technologies [82], it can be shown that there is an optimum FF for a maximum power density, especially ...
Thermoelectric generators have long been seen as a possible renewable energy source for both small scale and large scale applications. These devices use no direct fuel and therefore fossil fuels to produce power and are solid state so require little maintenance. However, efficiencies of these devices are currently insufficient to be seriously considered as primary power sources and are currently only considered for small scale applications, or where this is the only option such as in radioisotope thermoelectric generators for deep space probes. To improve these devices, two main approaches can be considered, one is to improve the thermal and electrical performance of devices by carefully optimised design, and the other is to improve the materials electrical conductivity, thermal conductivity and Seebeck coefficient. A new corrugated thin film thermoelectric generator design is considered and an analytical model for this is verified using finite element method simulations showing a maximum discrepancy of 15% over a wide range of parameters. The result of simulation and modelling shows that increasing the interconnect electrical conductivity and reducing the pitch of the device increases the power density. The power density is also increased by increasing the fill factor, and this thin film design can achieve higher fill factors compared to that of a conventional device at a specific minimum feature size. To evaluate thin film thermoelectric materials, methods for the measurement of thermoelectric properties are developed. For the measurement of the Seebeck coefficient and electrical conductivity, a Joule Yacht MRS-3L is used allowing for measurements from 100 - 600 K. The capabilities of this tool have been extended to allow for the more precise measurement of highly resistive films. A ω − 3 ω system is developed for the measurement of thermal conductivity of films. This system is verified by the measurement of bulk silicon and thin films of bismuth telluride and show good agreement with literature values for both materials. Thin films of low pressure chemical vapour deposited (LPCVD) Bi<sub>2</sub>Te<sub>3</sub> are optimised by the alloying of Bi<sub>2</sub>Te<sub>3</sub> and Bi<sub>2</sub>Se<sub>3</sub> to deposit ternary Bi<sub>2</sub>Te<sub>3</sub>-<sub>x</sub>Se<sub>x</sub>. The composition of the ternary films are tuned to optimise the combination of carrier concentration and mobility to give a three-fold enhancement of the thermoelectric power factor at 300 K, and six-fold enhancement at 500 K, with respect to Bi<sub>2</sub>Te<sub>3</sub> . This improvement from the substitution of Te with Se is believed to be due to donor effects, as well as point defects caused by substitution. Pre-patterned substrates with open SiO<sub>2</sub> holes on TiN were used for selective deposition of Bi<sub>2</sub>Te<sub>3</sub>-<sub>x</sub>Se<sub>x</sub> on to the conductive TiN. This selective deposition behaviour allows for a reduction in fabrication steps for a thermoelectric micro-generator, and a reduction in wasted material. Deposition of Sb<sub>2</sub>Te<sub>3</sub> by LPCVD is optimised by varying deposition temperature. The carrier concentration and mobility of the films can be optimised by reducing the deposition temperature to 364 ◦C, resulting in a power factor of 16.5 µW cm−1 K −2 at 350 K. The Sb<sub>2</sub>Te<sub>3</sub> films also shows selective behaviour on the conductive TiN surface, which enabled the fabrication of a single-type thermoelectric micro-generator. The fabricated generator had a pitch of 400 µm, a fill factor of 25%, and 72 Sb<sub>2</sub>Te<sub>3</sub> thermoelements. This prototype device was measured using a custom system and a maximum temperature difference of 0.11 K was achieved across the 500 nm thick thermoelements, leading to a voltage of 0.4 mV and current of 0.7 µA giving a power output of 280 pW. It is then shown by simulation that this power output is significantly limited by the interconnect resistance, and that by reducing the pitch down to 10 µm the power output could reach 500 nW. The thermoelectric properties of tin chalcogenides are investigated by comparing SnS, SnSe, and SnTe. It is found that the SnS and SnSe films deposited by LPCVD are much more resistive than the SnTe. The high resistivity is caused by low carrier concentrations, which also lead to high Seebeck coefficients of 650 and 790 µV K<sup>−1</sup> at 300 K for SnS and SnSe, respectively. Comparatively, the SnTe films show a resistivity of 4 orders of magnitude lower, due to a high carrier concentration and comparable mobility. Overall the SnTe films show the highest power factor of 8.3 µW cm<sup>−1</sup> K<sup>−2</sup> at 615 K. The SnTe films also show selective behaviour, but the SnS and SnSe do not.
... The main reason is that the novel design of porous sandwich-structure substrate and direct-soldering Cu-foam leg-shaped heat sink is designed to improve thermal conditions significantly. It is noteworthy that the thermal resistance of body heat would affect the WTEG performance [43,44]. Thus, the experimental results have indicated that the WTEG performance for the simulated heat sources is better than that for harvesting body heat. ...
This paper develops a superhigh-performance wearable thermoelectric generator (WTEG) for harvesting body heat, achieving an output power density of 15.8 μW/cm² in windless and moveless conditions, and 97.6 μW/cm² for walking of 0.8 m/s. A novel WTEG configuration integrated with the porous sandwich substrate and direct-soldering Cu-foam heat sink is designed to significantly improve its flexibility and considerably reduce the thermal resistance at the cold/hot sides. A new compact low-voltage boosting converter is optimized to obtain a high conversion efficiency (such as >50%@100 mV) and allow low self-startup input voltage (20 mV) and stable output voltage. A semi-automatization manufacturing process of the self-powered wearable sensor system is also designed to integrate the WTEG module, energy management module, multi-sensor (including acceleration, temperature, humidity, heart rate, and blood oxygen) module, and Bluetooth module onto the flexible substrate. The experimental results indicate that the sandwich substrate increases WTEG performance by 25% and achieves about 40 mW/cm² for the constant temperatures at the cold/hot sides with the temperature difference of 60 °C. When the cold side of WTEG is exposed to the ambient air, the Cu-foam heat sink enables a performance increase of 73.6% compared to that without a heat sink for the ambient temperature of 18 °C, which even reaches 302% under the wind speed of 2 m/s (obtaining 457.97 μW/cm²). It is interesting to find that the bending WTEG could significantly increase its performance by 45.6% (for the curvature radius of 20 mm) compared to the plane case due to the thermal condition improvement. For harvesting body heat, the performance of WTEG fixed on the forehead is better than that on the arm or shank. It achieves a superhigh average output power of 3.12 mW for walking at the ambient temperature of 18 °C, which could fully power the wearable multi-sensor health monitoring system continuously.
