Comfortable wearable thermoelectric generator with high output power

Wearable thermoelectric generators (w-TEGs) offer a promising solution for powering wearable electronics, harnessing body heat via the Seebeck effect. While advancements have been made in increasing w-TEG output power, comfort remains a significant, yet often overlooked, challenge. Wearable comfort necessitates appropriate skin temperature, minimal pressure, and mechanical flexibility to accommodate body movement. Optimizing both output power and wearability is complex, as these factors are interconnected through device structure, thermoelectric (TE) material properties, and the external environment. Existing research lacks a comprehensive analytical framework for simultaneously optimizing these aspects. This study addresses this gap by developing a model for w-TEGs, enabling quantitative analysis of multidimensional parameters for both comfort and power output.

The existing literature highlights significant progress in enhancing the output power of w-TEGs under extreme conditions. However, there's a consistent lack of focus on the comfort aspect, which is crucial for practical applications. While some studies address the importance of flexibility and minimal skin pressure, a general analytical formula for optimizing both system efficiency and wearability remains absent. Previous research often employs computationally expensive finite element analysis (FEA), lacking concise guidance for macroscopic parameter optimization. One-dimensional (1D) numerical analysis offers an efficient alternative for macroscopic mechanism quantification, but its application to comprehensive w-TEG design requires further development.

This study introduces a complete design strategy based on a 1D coupled field analysis integrating mechanical and thermoelectric models. The model considers environmental temperature (Ta), bending radius (r), body temperature (Tb), and parameters of TE and encapsulation materials (Young’s modulus (E) and thermal conductivity (κ)). The model links wearability parameters (skin pressure (Pr) and skin temperature (Ts)) to structural parameters (TE leg length (L), width (w), fill factor (F), p-/n-type material area ratio (Fp/n), ambient and total heat transfer coefficient (h, hs)). The model considers a sandwiched w-TEG structure, deriving formulas to quantify the optimization of multidimensional parameters. Initially, the model determines the range of structural parameters for acceptable wearability (Pr and Ts). Then, using these constraints, the model optimizes TE leg geometry to maximize output efficiency. Material properties of the TE and encapsulation materials are considered, acknowledging the trade-off between flexibility (achieved through low-E fillers or absence of fillers) and device reliability. Finally, FEA is used for precise adjustment to maximize power output while maintaining wearability. The model is validated using w-TEGs fabricated with n-type Mg3Bi2 and p-type MgAgSb-based TE materials, known for cost-effectiveness and high performance near room temperature. The mathematical model incorporates thermal-electric and mechanical models, represented by Equations (1) through (14), along with the detailed derivations provided in the supplementary materials. These equations relate the bending stiffness (EITEG), skin pressure (Pr), temperature distribution, heat flux, and power density (Pd) to the device structure and material properties. The models simplify the human skin as a multi-layered system with equivalent heat transfer coefficients, accounting for various heat transfer pathways.

The study's findings demonstrate a successful strategy for optimizing both the output power and wearing comfort of w-TEGs. The researchers derived analytical expressions relating the device's structural parameters to its output power and comfort parameters. The optimized design resulted in a power density of 18.4 µWcm² at a comfortable skin temperature of 33 °C and a pressure of 0.8 kPa. The model predicts that the maximum achievable power density for a Mg-based w-TEG with ZT of approximately 0.75 is 67 µWcm² on body parts with low equivalent skin thermal resistance (like the forehead), and 33 µWcm² on body parts with higher resistance (like the foot). The experimental results closely matched the model's predictions, validating the design strategy. The fabricated Mg-based w-TEGs showed excellent flexibility, withstanding multiple bending and stretching cycles without significant performance degradation. The device maintained its output even when bent to a radius of 5 mm or stretched by up to 30%. The study demonstrates the effectiveness of the optimized w-TEG in harvesting body heat for powering microwatt-scale wearable electronic devices, achieving performance comparable to state-of-the-art Bi2Te3-based devices while offering advantages in cost-effectiveness and material toxicity. Further, the design strategy was successfully adapted for collecting heat from hot water, powering a Bluetooth thermometer, demonstrating its versatility.

This study successfully addresses the critical challenge of designing comfortable and high-performance w-TEGs. The developed analytical model provides a powerful tool for optimizing w-TEG design, enabling a systematic approach to balancing power generation with wearer comfort. The use of cost-effective and less toxic Mg-based TE materials demonstrates a significant advancement in the field, making w-TEGs more practical for widespread adoption. The close agreement between experimental results and model predictions validates the accuracy and reliability of the proposed design strategy. The high flexibility and durability of the fabricated w-TEGs highlight their suitability for real-world applications. The findings underscore the potential of w-TEGs to power a wide range of wearable electronics, contributing significantly to the development of self-powered wearable health monitoring and other personal electronic devices.

This research presents a novel design strategy for w-TEGs that successfully integrates power generation optimization with wearer comfort considerations. The developed analytical model and experimental validation demonstrate the feasibility of creating high-performance, comfortable w-TEGs using cost-effective Mg-based thermoelectric materials. Future research could explore the application of this model to other TE materials and explore different device configurations to further optimize performance and comfort.

The model makes some simplifying assumptions, such as modeling human skin as a multi-layered system with equivalent heat transfer coefficients, neglecting the complex thermal properties of the skin and individual variations in body temperature and heat transfer. The analysis also simplifies the heat transfer within the device by neglecting certain secondary thermal effects, such as the thermal diffusion resistance within the substrate and the contact thermal resistance between the filler and the TE leg. While the experimental results closely matched the model's predictions, further investigation into these limitations is needed to improve the model's accuracy and predictive capabilities in more complex real-world scenarios.