Toward High Conversion Efficiency of Thermoelectric Modules through Synergistical Optimization of Layered Materials.

Waste-heat electricity generation using high-efficiency solid-state conversion technology can significantly decrease dependence on fossil fuels. Here, a synergistical optimization of layered half-Heusler (hH) materials and module to improve thermoelectric conversion efficiency is reported. This is realized by manufacturing multiple thermoelectric materials with major compositional variations and temperature-gradient-coupled carrier distribution by one-step spark plasma sintering. This strategy provides a solution to overcome the intrinsic concomitants of the conventional segmented architecture that only considers the matching of the figure of merit (zT) with the temperature gradient. The current design is dedicated to temperature-gradient-coupled resistivity and compatibility matching, optimum zT matching, and reducing contact resistance sources. By enhancing the quality factor of the materials by Sb-vapor-pressure-induced annealing, a superior zT of 1.47 at 973 K is achieved for (Nb, Hf)FeSb hH alloys. Along with the low-temperature high-zT hH alloys of (Nb, Ta, Ti, V)FeSb, the single stage layered hH modules are developed with efficiencies of ≈15.2% and ≈13.5% for the single-leg and unicouple thermoelectric modules, respectively, under ΔT of 670 K. Therefore, this work has a transformative impact on the design and development of next-generation thermoelectric generators for any thermoelectric material families.

Conformal High-Power-Density Half-Heusler Thermoelectric Modules: A Pathway toward Practical Power Generators.

Thermoelectric generators (TEGs) exploiting the Seebeck effect provide a promising solution for waste heat recovery. Among the large number of thermoelectric (TE) materials, half-Heusler (hH) alloys are leading candidates for medium- to high-temperature power generation applications. However, the fundamental challenge in this field has been inhomogeneous material properties at large wafer diameters, insufficient power output from the modules, and rigid form factors of TE modules. This has restricted the transition of TEGs in practical applications for over three decades. Here, we successfully demonstrate large diameter wafers with uniform TE properties, high-power conformal hH TE modules for high-temperature application, and their direct integration on flue gas platforms, such as cylindrical tubes, to form large area flexible TEGs. This new conformal architecture design provides a breakthrough toward medium-/high-temperature TEGs over the conventional BiTe- and polymer-based flexible TEG design. A variable fill factor and greater flexibility due to the conformal design result in higher device performance as compared to conventional rigid TEG devices. Modules with 72-couple hH legs exhibit a device high-power-density of 3.13 W cm-2 and a total output power of 56.6 W under a temperature difference of 570 °C. These results provide a promising pathway toward widespread utilization of thermoelectric technology into the waste heat recovery application and will have a significant impact on the development of practical thermal to electrical converters.

3D printed graphene-based self-powered strain sensors for smart tires in autonomous vehicles.

The transition of autonomous vehicles into fleets requires an advanced control system design that relies on continuous feedback from the tires. Smart tires enable continuous monitoring of dynamic parameters by combining strain sensing with traditional tire functions. Here, we provide breakthrough in this direction by demonstrating tire-integrated system that combines direct mask-less 3D printed strain gauges, flexible piezoelectric energy harvester for powering the sensors and secure wireless data transfer electronics, and machine learning for predictive data analysis. Ink of graphene based material was designed to directly print strain sensor for measuring tire-road interactions under varying driving speeds, normal load, and tire pressure. A secure wireless data transfer hardware powered by a piezoelectric patch is implemented to demonstrate self-powered sensing and wireless communication capability. Combined, this study significantly advances the design and fabrication of cost-effective smart tires by demonstrating practical self-powered wireless strain sensing capability.

High-Performance Thermoelectric Generators for Field Deployments.

Thermoelectric power generation is a reliable energy harvesting technique for directly converting heat into electricity. Recent studies have reported the thermal-to-electrical energy conversion efficiency of thermoelectric generators (TEGs) up to 11% under laboratory settings. However, the practical efficiency of TEGs deployed under real environments is still not more than a few percent. In this study, we provide fundamental insight on the operation of TEGs in realistic environments by illustrating the combinatory effect of thermoelectric material properties, device boundary conditions, and environmental thermal resistivity on TEG performance in conjunction with the module parameters. Using numerical and experimental studies, we demonstrate the existence of a critical heat transfer coefficient that dramatically affects the design and performance of TEGs. Results provide a set of concrete design criteria for developing efficient TEGs that meet the metrics for field deployments. High-performance TEGs demonstrated in this study generated up to 28% higher power and 162% higher power per unit mass of thermoelectric materials as compared to the commercial module deployed for low-grade waste heat recovery. This advancement in understanding the TEG operation will have a transformative impact on the development of scalable thermal energy harvesters and in realizing their practical targets for efficiency, power density, and total output power.

High Power Density Body Heat Energy Harvesting.

Thermoelectric generators (TEGs) can convert body heat into electricity, thereby providing a continuous power source for wearable and implantable devices. For wearables, the low fill factor (area occupied by legs over the TEG base area) TEG modules are relevant as they provide large thermal gradient across the legs and require less material, which reduces the cost and weight. However, TEGs with a fill factor below 15% suffer from reduced mechanical robustness; consequently, commercial modules are usually fabricated with a fill factor in the range of 25-50%. In this study, TEG modules with a low and high fill factor are demonstrated and their performance is compared in harvesting body heat. Fabricated modules demonstrate ∼80% output power enhancement as compared to commercially available designs, resulting in high power density of up to 35 µW/cm2 in a steady state. This enhanced power is achieved by using two-third less thermoelectric materials in comparison to commercial modules. These results will advance the ongoing development of wearable devices by providing a consistent high specific power density source.

Ultra-high performance wearable thermoelectric coolers with less materials.

Thermoelectric coolers are attracting significant attention for replacing age-old cooling and refrigeration devices. Localized cooling by wearable thermoelectric coolers will decrease the usage of traditional systems, thereby reducing global warming and providing savings on energy costs. Since human skin as well as ambient air is a poor conductor of heat, wearable thermoelectric coolers operate under huge thermally resistive environment. The external thermal resistances greatly influence thermoelectric material behavior, device design, and device performance, which presents a fundamental challenge in achieving high efficiency for on-body applications. Here, we examine the combined effect of heat source/sink thermal resistances and thermoelectric material properties on thermoelectric cooler performance. Efficient thermoelectric coolers demonstrated here can cool the human skin up to 8.2 °C below the ambient temperature (170% higher cooling than commercial modules). Cost-benefit analysis shows that cooling over material volume for our optimized thermoelectric cooler is 500% higher than that of the commercial modules.

A Review on Low-Grade Thermal Energy Harvesting: Materials, Methods and Devices.

Combined rejected and naturally available heat constitute an enormous energy resource that remains mostly untapped. Thermal energy harvesting can provide a cost-effective and reliable way to convert available heat into mechanical motion or electricity. This extensive review analyzes the literature covering broad topical areas under solid-state low temperature thermal energy harvesting. These topics include thermoelectricity, pyroelectricity, thermomagneticity, and thermoelasticity. For each topical area, a detailed discussion is provided comprising of basic physics, working principle, performance characteristics, state-of-the-art materials, and current generation devices. Technical advancements reported in the literature are utilized to analyze the performance, identify the challenges, and provide guidance for material and mechanism selection. The review provides a detailed analysis of advantages and disadvantages of each energy harvesting mechanism, which will provide guidance towards designing a hybrid thermal energy harvester that can overcome various limitations of the individual mechanism.

Self-Powered Temperature-Mapping Sensors Based on Thermo-Magneto-Electric Generator.

We demonstrate a thermo-magneto-electric generator (TMEG) based on second-order phase transition of soft magnetic materials that provides a promising pathway for scavenging low-grade heat. It takes advantage of the cyclic magnetic forces of attraction and repulsion arising through ferromagnetic-to-paramagnetic phase transition to create mechanical vibrations that are converted into electricity through piezoelectric benders. To enhance the mechanical vibration frequency and thereby the output power of the TMEG, we utilize the nonlinear behavior of piezoelectric cantilevers and enhanced thermal transport through silver (Ag) nanoparticles (NPs) applied on the surface of a soft magnet. This results in large enhancement of the oscillation frequency reaching up to 9 Hz (300% higher compared with that of the prior literature). Optimization of the piezoelectric beam and Ag NP distribution resulted in the realization of nonlinear TMEGs that can generate a high output power of 80 µW across the load resistance of 0.91 MΩ, which is 2200% higher compared with that of the linear TMEG. Using a nonlinear TMEG, we fabricated and evaluated self-powered temperature-mapping sensors for monitoring the thermal variations across the surface. Combined, our results demonstrate that nonlinear TMEGs can provide additional functionality including temperature monitoring, thermal mapping, and powering sensor nodes.

Optimization of segmented thermoelectric generator using Taguchi and ANOVA techniques.

Recent studies have demonstrated that segmented thermoelectric generators (TEGs) can operate over large thermal gradient and thus provide better performance (reported efficiency up to 11%) as compared to traditional TEGs, comprising of single thermoelectric (TE) material. However, segmented TEGs are still in early stages of development due to the inherent complexity in their design optimization and manufacturability. In this study, we demonstrate physics based numerical techniques along with Analysis of variance (ANOVA) and Taguchi optimization method for optimizing the performance of segmented TEGs. We have considered comprehensive set of design parameters, such as geometrical dimensions of p-n legs, height of segmentation, hot-side temperature, and load resistance, in order to optimize output power and efficiency of segmented TEGs. Using the state-of-the-art TE material properties and appropriate statistical tools, we provide near-optimum TEG configuration with only 25 experiments as compared to 3125 experiments needed by the conventional optimization methods. The effect of environmental factors on the optimization of segmented TEGs is also studied. Taguchi results are validated against the results obtained using traditional full factorial optimization technique and a TEG configuration for simultaneous optimization of power and efficiency is obtained.

Thermo-Magneto-Electric Generator Arrays for Active Heat Recovery System.

Continued emphasis on development of thermal cooling systems is being placed that can cycle low grade heat. Examples include solar powered unmanned aerial vehicles (UAVs) and data storage servers. The power efficiency of solar module degrades at elevated temperature, thereby, necessitating the need for heat extraction system. Similarly, data centres in wireless computing system are facing increasing efficiency challenges due to high power consumption associated with managing the waste heat. We provide breakthrough in addressing these problems by developing thermo-magneto-electric generator (TMEG) arrays, composed of soft magnet and piezoelectric polyvinylidene difluoride (PVDF) cantilever. TMEG can serve dual role of extracting the waste heat and converting it into useable electricity. Near room temperature second-order magnetic phase transition in soft magnetic material, gadolinium, was employed to obtain mechanical vibrations on the PVDF cantilever under small thermal gradient. TMEGs were shown to achieve high vibration frequency at small temperature gradients, thereby, demonstrating effective heat transfer.