Wearable Thermoelectric Generator Research Articles

Wearable multi-sensing double-chain thermoelectric generator

Wearable electronics play a crucial role in advancing the rapid development of artificial intelligence, and as an attractive future vision, all-in-one wearable microsystems integrating powering, sensing, actuating and other functional components on a single chip have become an appealing tendency. Herein, we propose a wearable thermoelectric generator (ThEG) with a novel double-chain configuration to simultaneously realize sustainable energy harvesting and multi-functional sensing. In contrast to traditional single-chain ThEGs with the sole function of thermal energy harvesting, each individual chain of the developed double-chain thermoelectric generator (DC-ThEG) can be utilized to scavenge heat energy, and moreover, the combination of the two chains can be employed as functional sensing electrodes at the same time. The mature mass-fabrication technology of screen printing was successfully introduced to print n-type and p-type thermoelectric inks atop a polymeric substrate to form thermocouples to construct two independent chains, which makes this DC-ThEG flexible, high-performance and cost-efficient. The emerging material of silk fibroin was employed to cover the gap of the fabricated two chains to serve as a functional layer for sensing the existence of liquid water molecules in the air and the temperature. The powering and sensing functions of the developed DC-ThEG and their interactions were systematically studied via experimental measurements, which proved the DC-ThEG to be a robust multi-functional power source with a 151 mV open-circuit voltage. In addition, it was successfully demonstrated that this DC-ThEG can convert heat energy to achieve a 3.3 V output, matching common power demands of wearable electronics, and harvest biothermal energy to drive commercial electronics (i.e., a calculator). The integration approach of powering and multi-functional sensing based on this new double-chain configuration might open a new chapter in advanced thermoelectric generators, especially in the applications of all-in-one self-powered microsystems.

Comprehensive performance prediction and power promotion for wearable thermoelectric generator with flexible encapsulation in practical application

With ability of converting the heat harvested from human body into electricity, the wearable thermoelectric generators (WTEGs) are attracting lots of interests for self-powered electronic devices. However, the output power density of the WTEG is limited for supplying the electronic devices. Therefore, the performance enhancement by optimal design of the developed WTEGs is needed. In this paper, a method of comprehensive modeling of a WTEG is proposed by considering all essential components and realistic boundary conditions. By solving the model with finite element modeling (FEM) method, the calculation performances agree well with experimental results at various temperature difference and load resistance. Through the temperature and voltage distribution of the WTEG obtained, the effects of side heat loss and heat conduction of the encapsulation are revealed. Besides, the impact of the thermal conductivity of encapsulation and the geometric dimensions of thermoelectric legs on the output performance are explored. Finally, the performance prediction results indicate that at the temperature difference of 15 K, the output power of the WTEG with optimized design can be improved about 598.1% with matched loading resistance compared to the device with original design.

Design of heat sinks for wearable thermoelectric generators to power personal heating garments: A numerical study

To mitigate climate change attributed to the built environments, there have been tremendous efforts to improve air conditioning systems in the buildings. The possibility of harvesting body heat as a renewable energy source to power a wearable personal heating system is investigated. The aim of this study is to integrate a wearable personal heating system with a thermoelectric generator (TEG) that harvests the body heat which is used to convert it into electricity. Moreover, the interaction between the TEG configuration and power output is studied. The power generation of TEG system is obtained by COMSOL Multiphysics software. The simulation results concluded that all the four proposed heat sink configurations can improve the power output of the wearable TEG at 1.4 m/s and 3m/s compared to that of the reference model. Furthermore, the perforated and trapezium shapes of heat sinks have a significantly better performance in comparison to conventional heat sinks.

High-Performance, Wearable Thermoelectric Generator Based on a Highly Aligned Carbon Nanotube Sheet

A high-performance, wearable thermoelectric generator (TEG) was fabricated with a highly aligned carbon nanotube (CNT) sheet. The aligned CNT sheet exhibits extraordinary electrical conductivity co...

Free-standing flexible multiwalled carbon nanotubes paper for wearable thermoelectric power generator

Most of the thermoelectric research, these days, is being focussed towards development of flexible thermoelectric power generators (TEG) for wearable applications. Organic thermoelectric materials being flexible and solution processable, though, seem promising cannot render themselves for designing of conventional thermoelectric devices which require both p- and n-type of legs; due to lack of air-stable and flexible n-type materials. This work shows the possibility of conversion of a p-type flexible free-standing multiwalled carbon nanotubes (MWCNTs) paper into n-type on treatment with polyethylenimine (PEI). X-ray photoelectron spectroscopy results suggest that ‘as-grown’ MWCNTs attained n-type nature due to electron donation by imine group of PEI. Power factors of ~0.2 and ~0.06 μW/mK2 observed respectively for p- and n-type MWCNTs papers, owing to extremely low thermal conductivity (~0.05W/mK), resulted in figure-of-merit (ZT) of 1.27 × 10−3 and 3.02 × 10−4 at 68 °C. A prototype TEG designed using ‘as-grown’ and ‘PEI-modified’ MWCNTs as p- and n-type thermoelements respectively exhibited output of 227 μV/7.6 μA for a temperature difference of 40 °C. In short, facile scalability of MWCNTs when collaborated with such a low cost, environment friendly method that can easily modify its conduction to n-type can certainly open opportunities for scalable production of flexible roll-to-roll type wearable thermoelectric modules.

A review on recent developments of thermoelectric materials for room-temperature applications

Wearable thermoelectric generators (TEGs) emerge as a viable renewable energy source, which directly convert the heat dissipated from human skin into electricity. Extensive reviews have been conducted on the efficiency of thermoelectric materials (TE) as the dominant element of TEGs. TE materials are categorised as inorganic, organic, and hybrid. Each of these reviews focused on either a specific type of TE materials, or on a certain specification (i.e. flexibility) of them. However, less attention has been paid to comprehensively review all these types without taking into account a certain specification. Therefore, the purpose of this paper is to summarize the progress and current state-of-the-art research on the three types of TE materials respecting their TE properties and efficiency at 300 K, which is the operating temperature of wearable TEGs. Concerning the inorganic TE materials, the results show that Bi0.4−xSb1.6+xTe3 and Bi2Te2.7Se0.3 are the most optimal TE materials, which exhibit the greatest efficiencies at room temperature. In addition, it is remarkably more efficient to replace polymer based TE composites with carbon based TE composites in the organic and the hybrid types. In total, this comprehensive review paves the way for researchers to find out the most suitable TE materials at room temperatures.

Thermoelectric generators for wearable body heat harvesting: Material and device concurrent optimization

Body heat harvesting systems based on thermoelectric generators (TEGs) can play a significant role in wearable electronics intended for continuous, long-term health monitoring. However, to date, the harvested power density from the body using TEGs is limited to a few micro-watts per square centimeter, which is not sufficient to turn on many wearables. The thermoelectric materials research has been mainly focused on enhancing the single parameter zT, which is insufficient to meet the requirements for wearable applications. To develop TEGs that work effectively in wearable devices, one has to consider the material, device, and system requirements concurrently. Due to the lack of an efficient heatsink and the skin thermal resistance, a key challenge to achieving this goal is to design systems that maximize the temperature differential across the TEG while not compromising the body comfort. This requires favoring approaches that deliver the largest possible device thermal resistance relative to the external parasitic resistances. Therefore, materials with low thermal conductivity are critically important to maximize the temperature gradient. Also, to achieve a high boost converter efficiency, wearable TEGs need to have the highest possible output voltage, which calls for a high Seebeck coefficient. At the device level, dimensions of the legs (length versus the base area) and fill factor are both critical parameters to ensure that the parasitic thermal resistances are again negligible compared to the resistance of the module itself. In this study, the concurrent impact of material and device parameters on the efficiency of wearable TEGs is considered. Nanocomposite thermoelectric materials based on bismuth telluride alloys were synthesized using microwave processing and optimized to meet the requirements of wearable TEGs. Microwave energy decrystallized the material leading to a strong reduction of the thermal conductivity while maintaining a high zT at the body temperature. A comprehensive quasi-3D analytical model was developed and used to optimize the material and device parameters. The nanocomposite TEG produced 44 μW/cm2 under no air flow condition, and 156.5 μW/cm2 under airflow. In comparison to commercial TEGs tested under similar conditions, the nanocomposite based TEGs exhibited 4–7 times higher power density on the human body depending on the convective cooling conditions.

Fully Integrated Autonomous Interface With Maximum Power Point Tracking for Energy Harvesting TEGs With High Power Capacity

In this article, a novel fully autonomous and integrated power management interface circuit is introduced for energy harvesting using thermoelectric generators (TEGs) to supply power to Internet of Thing nodes. The circuit consists of a self-starting dc-dc converter based on a dual-phase charge pump with LC-tank oscillator, a digital MPPT unit, and a 1-V LDO regulator. The novel maximum power point tracking (MPPT) algorithm avoids open-circuit state, and accommodates varying input power and ultra-low voltage conditions. Validation data from the fabricated test-chip in 180 nm standard CMOS technology indicates the circuit start-up voltage is as low as 170 mV. The maximum output power capacity is 0.5 mW, which is the highest noted in the literature for a fully integrated solution. The high output power at low cost is achieved with a peak system efficiency of 30%. The relatively low efficiency is expected, since the focus of the design is high power capacity at low cost. The MPPT algorithm reaches 98% maximum accuracy for a source output resistance of 40 Ω, which is typical for wearable TEG modules.

Textile‐Based Thermoelectric Generators and Their Applications

With the rapid development of Internet of Things and miniaturized electronics, the demand for wearable power sources with high reliability and long duty cycle promotes the exploration of wearable thermoelectric generators (TEGs). In particular, textile‐based TEGs that can perpetually convert the ubiquitous temperature gradient between human body and ambience into electrical energy have attracted intensive attention to date. These lightweight and three‐dimensional deformable TEGs comprised of fibers, filaments, yarns, or fabrics offer unique merits as wearable power source in comparison with conventional TEGs. In this review, we systematically summarize the state‐of‐the‐art strategies for textile‐based TEGs, including the structure design, fabrication, device performance, and application. Existing critical issues and future research emphasis are also discussed.

Human Body Micro-power plant

This paper explores the heat generated by the human body and the capabilities of wearable thermoelectric generators in harvesting the thermal energy for practical uses. The wearable thermoelectric generator (WTEG) has shown great potential for use in this field, as such small-scale devices consume little power. While batteries are cheap and reliable sources of power, they have the unavoidable limitation of needing to be changed or charged periodically. WTEGs could significantly cut down the size of the batteries required for many devices and in some cases eliminate them altogether. WTEGs also have the distinct advantage of being solid state and maintenance-free. These factors have also made them lucrative options for implantable biomedical devices, which must have surgical procedures performed to replace batteries or conduct maintenance. From these possibilities, an enormous amount of research has surfaced in a short period of time. When analyzing thermoelectric generators, there are several limitations that have to be examined in order to understand maximum power outputs, maximum temperature differences, and thermal matching. This paper provides an in-depth explanation of heat recovery from the human body, including the thermoelectric capabilities of some thermoelectric materials, as well as the true “wearability” of a TEG for everyday use.

Optimisation of Wearable Thermoelectric Generators

This meta-study explores some factors that can potentially affect the efficiency of a wearable thermoelectric generator. These include, but are not limited to; doping percentage, manufacturing technology, thermocouple length, area, use of heat spreaders, material, airflow and specific position on the human body. These specific designs and materials have been reviewed in this paper and specific variables have been proposed to ensure greater efficiency. In this meta- study, Bi0.5Sb1.5Te3 and Ag2Se are found to be the most effective materials, with PVD as the most effective manufacturing method. A broad temperature differential generates greater power output. Practically, a condition where there is a difference in temperature of more than 40K between the body and its environment in the application of wearable thermoelectric devices is unlikely. Despite this, a temperature difference below 40K, although small, is extremely feasible and would be able to enough power to keep intended wearable thermoelectric devices running at a constant.
 Keywords: Thermoelectric; Seebeck Effect; Peltier; TEG; ZT; Wearable

Semi-transparent copper iodide thin films on flexible substrates as p-type thermolegs for a wearable thermoelectric generator

The article presents crystal structure, composition, optical, electrical and thermoelectric properties of semi-transparent CuI films produced in different modes of the cost-effective and large scalable method Successive Ionic Layer Adsorption and Reaction (SILAR) on flexible poly(ethylene terephthalate) (PET) substrates. The obtained data together with the analysis of output thermoelectric characteristics of p-type thermolegs based on these CuI films are used to select the SILAR mode to create semi-transparent p-type thermolegs for a new flexible wearable thin film thermoelectric generator (TEG). The best obtained flexible p-CuI thin film single thermoleg demonstrates output power 17.1 μW/m2 at the temperature gradient ∆T = 35 K at near-room temperatures. Therefore, CuI films produced in such mode of the SILAR method on the flexible PET substrates can be used as a part of a new lightweight low cost flexible wearable thin film TEG for a production of electrical energy for low-power devices due to absorption of low-potential heat.

Flexible heatsink based on a phase-change material for a wearable thermoelectric generator

Thermoelectric generators (TEGs) represent a promising technology for self-powered wearable systems. Since the conventional TEGs have limitation to be used on the human body due to its structural rigidity, flexible TEGs have been studied and developed so that TEGs can be attached to an arbitrarily shaped surface on the human body. However, flexible TEGs require a good heatsink with good flexibility to allow them to produce enough power for wearable devices. In this study, a high-performance flexible heatsink based on a phase-change material (PCM) was proposed. PCM blocks were arranged in an array and good flexibility was realized with an elastomer that filled the space between the PCM blocks. Given that PCM can absorb a large amount of heat at the phase-change temperature, the heatsink can hold the temperature difference across the TEG constant for a relatively long period of time. Thus, the generated power from the flexible TEG was maintained at around 20 μW/cm2 for 33 min. The flexible heatsink can be reused because the PCM solidifies at room temperature. Furthermore, the PCM-based flexible heatsink is smaller and lighter than the conventional metal heatsink. The proposed heatsink is expected to contribute to the commercialization of self-powered wearable devices.

(Invited) Wearable Thermoelectric Generator and Refrigerator Integrated with Flexible Heat Sink

In this talk, we are going to present thermoelectric devices for wearable power generation and refrigeration purpose. We present two types of wearable thermoelectric devices based on bulk inorganic materials; one is bracelet type and the other is mat style. Utilizing high performance that can be extracted from the bulk inorganic materials, the devices based on the materials are bendable and flexible. Performance of the device attached on a human body is compared with theoretical analysis based on a simplified human thermoregulatory model we developed. In thermoelectric application, it is necessary for the device to be integrated with heat sink. Therefore, we also show our recent work on flexible heat sink to be integrated with the devices. The flexible heat sink we demonstrate can be categorized either ‘transient type’ or ‘steady-state type.’ Also, we also discuss the possibility of using the wearable thermoelectric refrigerator as ‘personal heating and/or refrigeration.’

High-performance flexible Bi2Te3 films based wearable thermoelectric generator for energy harvesting

The development of wearable thermoelectric generator for human energy harvesting has attracted significant interests in recent years. However, the thermoelectric properties and flexibility of inorganic thermoelectric films still need to be improved. In this study, high-performance flexible Bi2Te3 films with controlled microstructure are prepared on the flexible polyimide substrate by regulating the sputtering pressures, and the power factor can reach 21.7 μW cm−1 K−2. By coating poly (dimethylsiloxane) on the Bi2Te3 surface, the flexibility of the films is improved significantly, so that the change of resistance is kept below 5% after bending 2000 cycles at the radius of 7 mm. Then an in-plane thermoelectric generator containing 13 pairs of thermocouples is fabricated with the open circuit voltage of 48.9 mV and the output power of 693.5 nW at ΔT = 24 K. Meanwhile, the transient open circuit voltage of 12.99 mV is achieved when using the temperature difference between the ambient air and the body, indicating promising application for human energy harvesting.

Study of Fiber-Based Wearable Energy Systems.

Fiber-based electronic and photonic devices have the most desired human-friendly features, such as being soft, ubiquitous, flexible, stretchable, light, and permeable, and thus are ideal to be employed as the interface platform between humans, the environment, and machines. Today, these smart wearable devices are normally powered by rechargeable batteries. It is extremely desirable to have an undisrupted power supply for us to go anywhere at any time. Harvesting energy from the ambient environment or our body can potentially fulfill such a goal. Explorations of high performance, flexible functional materials and energy conversion devices have led many exiting discoveries. In order to realize their applications, equally important are fundamental studies of the physical phenomena and mechanisms that will provide scientific guidance for the direction of exploration and development of devices and systems. Hence, this Account provides a brief review of our recent progress in this topic area. Based upon materials science, mechanics and device physics, we have succeeded in establishment of several new theoretical models for fiber-based piezoelectric, triboelectric, and hybrid generators. These models have been verified experimentally. Excellent results were obtained for fiber-based triboelectric generators without any adjustable parameters. Reasonable agreement was demonstrated for the piezoelectric generators because of some uncertainty in the material properties and deformation modes. From both simulated and experimental results, we did not detect any synergic effect in the hybrid generator consisting of cascaded piezoelectric and triboelectric units. The verified models can be used to predict the output voltage, current, and power of the devices in terms of material properties, parameters of device structure, harvesting circuits, and operating conditions. Furthermore, by considering the electric breakdown due to field-induced-emission and gas-ionization, we have identified the theoretical upper limits of charge density and output power from contact-mode fiber-based triboelectric nanogenerators. The analysis sheds new light on the scope and focus for further exploration and provides guidance on engineering design of such devices. In addition, we have setup an experimental platform for reliable triboelectric charge measurement of highly deformable and porous materials like fabrics. An extended triboelectric series has been reported by us including 21 types of commercial and new fibers. Based upon the findings, we have made significant improvements of the performance of these energy harvesting devices. Finally, we have explored a class of new flexible thermoelectric materials exhibiting high performance for fiber-based thermoelectric generators, which can be fabricated by low-temperature and cost-effective processes, such as in situ reduction coating and three-dimensional printing. The resultant large-area, flexible, and wearable fiber-base thermoelectric generators are key devices for great potential applications such as powering wearable microelectronic systems, active microclimate regulating systems, and waste thermal energy harvesting.

An efficient PEDOT-coated textile for wearable thermoelectric generators and strain sensors

Highly flexible PEDOT-based electronic textiles were successfully fabricated for wearable thermoelectric generators and strain sensors with high sensitivity and superior water durability.

Defect-engineered reduced graphene oxide sheets with high electric conductivity and controlled thermal conductivity for soft and flexible wearable thermoelectric generators

The direct use of graphene for potential thermoelectric material requires the opening of its bandgap without loss of its high electric conductivity. We herein demonstrate a synchronous reduction and assembly strategy to fabricate large-area reduced graphene oxide films with high electric conductivity and optimized low thermal conductivity assembly. The reduced graphene oxide films have a high electric conductivity and low thermal conductivity, which results from high longitudinal carrier mobility of the lattice domains as well as the enhanced scattering of phonons in the defects and their boundary that substantially reduces the mean phonon free path and the thermal conductivity. Flexible thermoelectric generators were prepared by assembling reduced graphene oxide film on 3D printed polydimethylsiloxane grids, demonstrating a remarkable output voltage of 57.33 mV/g at a temperature difference of 50 K. A wristband-type flexible thermoelectric generator with 7 repeating units generated a maximum power density of 4.19 µW/g at ambient temperature of 15 °C. The 3D printed generator is promising in providing power autonomy to wearable microwatt electronic devices. In addition, we believe that this work can be easily scaled up and can offer the pathway to produce large-scale manufacturing of graphene based materials for future microelectronics and large-scaled flexible and wearable energy harvesting systems.

Thermal Energy Harvesting on the Bodily Surfaces of Arms and Legs through a Wearable Thermo-Electric Generator.

This work analyzes the results of measurements on thermal energy harvesting through a wearable Thermo-electric Generator (TEG) placed on the arms and legs. Four large skin areas were chosen as locations for the placement of the TEGs. In order to place the generator on the body, a special manufactured band guaranteed the proper contact between the skin and TEG. Preliminary measurements were performed to find out the value of the resistor load which maximizes the power output. Then, an experimental investigation was conducted for the measurement of harvested energy while users were performing daily activities, such as sitting, walking, jogging, and riding a bike. The generated power values were in the range from 5 to 50 μW. Moreover, a preliminary hypothesis based on the obtained results indicates the possibility to use TEGs on leg for the recognition of locomotion activities. It is due to the rather high and different biomechanical work, produced by the gastrocnemius muscle, while the user is walking rather than jogging or riding a bike. This result reflects a difference between temperatures associated with the performance of different activities.

Transferable and flexible thermoelectric thin films based on elemental tellurium with a large power factor

The development of high performance flexible thermoelectric materials is important for fabricating wearable thermoelectric generators that can directly convert waste body heat into electricity. In this work, flexible thermoelectric thin films based on elemental tellurium have been fabricated via electrodeposition followed by a transfer process using adhesive substrates, which show not only high flexibility but also a large power factor of 3.21 μW cm−1 K−2 at room temperature. The flexible thermoelectric thin films can be activated by the heat of fingertips, and a large output voltage of 15 mV can be generated thanks to the large room temperature Seebeck coefficient of the films. It is anticipated that this work may pave the way for fabricating low cost and high performance flexible thermoelectric thin films.