Chapter 8 - New paradigm for efficient thermoelectrics

It is demonstrated that spacer fabrics made of polyester can be coated with Al-doped ZnO (AZO) as thermoelectric (TE) material and conductive silver as contact material to enable smart textiles. An atomic layer deposition process was used for the AZO coating and the conductive silver paste was manually applied. A TE generator and cooling based on the Seebeck and Peltier effect can be observed if a temperature difference or direct current is applied, respectively. Both effects were proven to exist and evaluated. The Seebeck coefficient was determined of up to 50 µV · K−1 and a TE power of about 0.2 µW was generated. Without additional active heat dissipation or cooling, a temperature difference between both spacer fabric surfaces of up to 12 K was achieved. By changing the polarity of the electrical contacts it can significantly be shown that the cold-warm side is changed, as expected for a Peltier element. We observed experimentally the Peltier effect using a single element as well as electrically interconnected sample pairs, the latter could be important for the cooling application. Additionally, the heat transport through the spacer fabrics was tested and showed that there is no heat equalization between the surfaces via air and filaments at temperatures up to 50 °C. The Peltier effect as a fundamental TE effect, that enables site-specific and on-demand cooling applications, has not yet been comprehensively investigated using spacer fabrics and AZO as TE material. These investigations predict that AZO on spacer fabrics hold the special potential to enable flexible and textile solid-state cooling applications.

The U.S. military uses large amounts of fuel during deployments and battlefield operations. Consequently, the U.S. military has a strong need to develop technologies that increase fuel efficiency and minimize fuel requirements all along the logistics trail and in all battlefield operations. There are additional requirements to reduce and minimize the environmental footprint of various military equipment and operations and reduce the need for batteries (non-rechargeable) in battlefield operations. The tri-agency SERDP (Strategic Environmental Research and Development Program) office is sponsoring a challenging, high-payoff project to develop a lightweight, small form-factor, soldier-portable advanced thermoelectric generator (TEG) system prototype to recover and convert waste heat from a variety of deployed equipment with the ultimate purpose of obtaining additional power for soldier battery charging, advanced capacitor charging, and other battlefield power applications. The project seeks to achieve power conversion efficiencies of 10% (double current commercial TE conversion efficiencies) in a system with ˜1.6-kW power output for a spectrum of battlefield power applications. In order to meet this objective, the project is taking on the multi-faceted challenges of tailoring LAST/LASTT-based thermoelectric (TE) materials for the proper temperature ranges (300 K – 700 K), fabricating these materials with cost-effective hot-pressed and sintered processes while maintaining their TE properties, measuring and characterizing their thermal fatigue and structural properties, developing the proper manufacturing processes for the TE materials and modules, designing and fabricating the necessary microtechnology heat exchangers, and fabricating and testing the final TEG system. The ultimate goal is to provide an opportunity to deploy these TEG systems in a wide variety of current military equipment. This would help the Army in achieving one of the Office of Secretary of Defense’s major strategic objectives to maintain and enhance operational effectiveness while reducing total force energy demands. The presentation will review the progress made on 1) the performance of LAST / LASTT TE materials and tailoring their temperature dependency; 2) evaluating the structural (Elastic modulus, Poisson’s ratio and mechanical strength) properties of these materials, 3) development of the necessary LAST/LASTT-based TE modules, 4) development of the required hot- and cold-side microtechnology heat exchangers, and 5) the overall system designs for 30 kW and 60 kW TQG applications and potential performance pathways/differences for these two TQG cases. This work leverages critical fundamental research performed by the Office of Naval Research in developing LAST/LASTT materials.

Patternable and flexible thermoelectric generators based on Bi2Te3/silk fibroin composites for temperature sensing and wearable applications

Abstract

Advancements in thermoelectric materials and their performance for thermoelectric generators (TEGs): A short review

New and more efficient thermoelectric (TE) materials that make use of nanotechnology have been developed. These new materials, called quantum wells (QW), are composed of alternating layers of 10 nm thick silicon and SiGe films. They can be deposited by various techniques and magnetron sputtering was used to obtain uniform layered structures that exhibited no degradation of the TE properties or microstructure after thermal aging. For QW thin films, the heat and current flow are “in plane” and in this orientation all of the thermoelectric properties (the Seebeck coefficient, electrical resistivty, and the thermal conductivity) are improved to increase the TE Figure of Merit, ZT (see equation 3, p. 4, for the definition of Z). From the most recent QW test data, ZTs greater than 3 at room temperature have been obtained which constitutes a significant improvement over the state-of-the-art (SOTA) bulk thermoelectrics which have ZTs less than 1. QW materials have the best measured TE power factor (Seebeck coefficient squared divided by electrical resistivity) and, combined with low thermal conductivity substrates, should provide very high efficiency TE modules. The QW TE materials with ZTs greater than 3 lead to conversion efficiencies greater than 20 percent, which allows for much wider commercial applications, particularly in the applications such as the waste-heat recovery from truck engines, refrigeration, and air conditioning, where the SOTA bulk TE modules were shown to be technically feasible but economically unjustified due to low conversion efficiencies. With higher efficiency QW materials, these applications become economically attractive. For the refrigeration and air conditioning applications, the QW TE materials are predicted to have higher coefficients of performance (COP) than the SOTA vapor compression systems, with the additional advantages of having no compressors, no moving parts, no refrigerants, no vibrations, no noise, and practically no maintenance. With such significant advantages, it is very important to have independent confirmation of the QW TE properties that lead to such improved performance. Three independent researchers have confirmed the previously measured QW TE properties using conventional test techniques, and a totally new test technique was developed to measure the TE properties and performance and the results provided yet another confirmation of the superior TE performance of the QW materials versus the SOTA bulk thermoelectrics. The temperature range for the applications is anticipated to be as low as −150C to the upper temperature of 1000C, with the power generation capacity ranging from milliwatts to kilowatts and cooling capacity ranging from watts to several tons of refrigeration.

Flexible thermoelectrics that enable conformal contact with heat sources of arbitrary shape are indispensable for self‐powered wearable electronics. Scalable integration of flexible thermoelectric (TE) materials into functional devices has improved over the past few years, however, the practical applications of flexible TE materials are still hindered by low performance. Herein, highly aligned carbon‐nanotube yarns (CNTYs) are proposed, combined with selective doping via picoliter scale inkjet printing. Coagulation assisted by van der Waals forces ensures a highly aligned structure of the CNTY, thus achieving the ultrahigh power factors of 4091 and 4739 µW m −1 K −2 for the p ‐ and n ‐type, respectively. The proposed TE materials can be effortlessly up‐scaled into highly integrated modules via inkjet printing. A highly integrated, flexible CNTY‐based TE generator (TEG) with 600 PN pairs generates unparalleled milliwatt‐scale power at Δ T = 25 K, which is a few orders of magnitude higher than those of previously reported flexible material‐based TEGs. This TEG successfully powers a red light‐emitting diode using body heat alone, requiring no external power sources. For the seamless operation of practical applications requiring high power, this work explores the key design parameters for flexible TEGs with high performance and manufacturability and presents new platforms for self‐powered wearable electronics.

A series of Ba0.4In0.4Co4Sb12/Bi2Te2.7Se0.3 (FS/BT) thermoelectric (TE) materials were fabricated by a two-step spark plasma sintering method. The samples contained various numbers of gradient layers between FS and BT as follows: one gradient layer (1GL) with FS/BT volume ratio of 5:5, three GLs with ratios of 3:7, 5:5, and 7:3 (denoted as 3GLs-I with 3:7–5:5–7:3), 3GLs-II with 7:3–5:5–3:7, 5GLs-I with 3:7–4:6–5:5–6:4–7:3, and 5GLs-II with 2:8–3:7–5:5–7:3–8:2. The interfacial structure and mechanical properties of the FS/BT TE materials were investigated in this work. In FS/BT TE materials with no GLs, a large number of macroscopic cracks occurred on the FS bulk material side. It was discovered that designing and optimizing GLs between the FS and BT bulk materials could effectively relax the thermal stress induced by the large difference in coefficient of thermal expansion, eliminating the macroscopic cracks and resulting in a remarkable enhancement of the interfacial mechanical properties of the FS/BT TE materials. The flexural strength of the FS/BT TE material with 1GL reached 9.67 MPa, increased by 85% compared with that of the FS/BT TE material with no GLs. The present work indicates that increasing the BT content in the GL near to the FS bulk material side is an effective method to completely eliminate macroscopic cracking. The optimized gradient interface of the FS/BT TE material was 3GLs-II with FS/BT volume ratios of 3:7–5:5–7:3. The highest flexural strength reached 12.76 MPa, representing a 144% increase.

Thermoelectric materials based on porous hybrid and MOFs constitute a very recent development, with many new and undiscovered phenomena for the research field. These porous Hybrid materials and MOF (Metal-Organic-Framework) films represent modern designer materials that exhibit many requirements of a near ideal and tunable future thermoelectric (TE) material. It is anticipated that especially the inherent porosity can be used advantageously in the design and simulation predictions of future thermoelectric materials. In principle, porous Hybrid and MOFs materials can comply with most of the requirements of an ideal and tunable future thermoelectric (TE) material. In contrast to traditional semiconducting bulk thermoelectric materials, porous hybrid MOF templates can be used to overcome some of the constraints of physics in bulk thermoelectric materials. These porous hybrid systems are amenable for simulation and modeling to design novel optimized electron-crystal phonon-glass materials with potentially very high Figure of Merit (ZT) numbers. Porous MOF and Hybrid materials possess an ultra-low thermal conductivity, which can be further modulated by phonon engineering within their complex porous and hierarchical architecture to advance the thermoelectric figure of merit ZT. This class of novel materials offers new approaches to the design of complex and hierarchical porous structures, possessing ultra-low thermal conductivity, which are able to trap, and localize phonons, thus resulting in a new generation of thermoelectric materials with a high ZT number. This work is demonstrating recent results of MOF thermoelectric Materials and provides a future outlook to the search for the next generation thermoelectric porous Hybrid and MOF materials, which could be part of a green renewable energy revolution with novel materials of sustainable high ZT values.

This paper describes the design, modeling, initial build and testing of a novel thermoelectric power generator (TPG), incorporating state of the art material technology with optimized thermal management. A numerical model simulates the operation of the device and facilitates its design. Advanced multi-parameter, gradient-based optimization techniques are used to better understand the interactions between various design variables and parameters in order to progress towards an optimal TPG design. The device, made up of a series of segmented elements each comprised of up to three different materials, combines thermal isolation in the direction of flow with high power density thermoelectric (TE) material integrated directly into the heat exchanger device. Electrical current runs parallel to the heat source and sink surfaces in the device, allowing the integration of the TE material with multiple geometric degrees of freedom. This design attribute coupled with the thermal isolation thermodynamic cycle, allows each element of the TE device to be optimized semi-independently. Each p- and n-type element can have different aspect ratios (cross-sectional area divided by thickness) so that each material layer of each element has the highest possible ZT for each temperature range. The increased design flexibility helps address TE material compatibility issues associated with segmented elements and fluid flow that ordinarily degrade performance. Eliminating the impact of thermal expansion mismatch while still maintaining excellent thermal and electrical contacts is also a design goal. Additional design considerations are also discussed, including electrical and thermal connector design and minimizing interfacial resistances. The device described is suitable for both waste heat recovery and primary power applications. Initial test results from prototype builds are discussed

This project presents the design, fabrication, and performance study of a solar thermoelectric generator. Solar energy is considered one of the most effective types and sources of renewable energy. Among the available options, solar energy has predominantly been harnessed through the use of solar PV panels. However, solar PV panels are quite expensive, an alternative system is designed to utilize solar energy through thermoelectric generators. This new design is a cheaper system because it replaces solar PV panels with thermoelectric generators that can absorb heat and convert it directly into electricity through a concept known as the Seebeck effect. The Seebeck effect is based on the temperature difference between two sides. By heating the hot side of a thermoelectric material, electrons migrate from the hot side to the cooler side, thereby generating an electrical current. Since the Seebeck effect depends on higher temperature differences for more efficiency, a cooling system positioned below which will cool the cold side of the STEG, while the sun above will heat the hot side, creating a higher temperature difference and improved efficiency. This system focuses on a two-stage thermoelectric generator comprised of two modules: a lower-temperature generator and a medium-temperature generator. In accordance with the optimal operating temperature of thermoelectric materials, the medium- temperature generator is positioned on the hot side, while the lower-temperature generator is situated on the cold side. Ultimately, the performance of the two-stage STEG is assessed across various operational conditions. The primary advantage of employing a two-stage STEG is that the heat-to-electricity conversion process occurs twice as the heat passes through the two-stage STEG, resulting in higher efficiency compared to a single-stage thermoelectric generator.

Thermoelectric materials and devices fabricated by additive manufacturing

Thermoelectric generators produce electricity by directly converting temperature differences into electrical energy through Seebeck effect. Thermoelectric generators possess many unique features, such as having no moving part and being very reliable with a relatively long lifespan once built. They are solid-state waste heat harvesters. With various waste heat available throughout many industries, thermoelectric generators could offer economically competitive renewable energy. The current state of the art thermoelectric materials includes materials with relatively toxic elements and must work under vacuum, making them quite expensive to produce. Among the various thermoelectric materials, ceramic oxides have been proved to be more chemically stable at high temperatures. Nevertheless, the thermoelectric ceramic oxides do not yet reach the efficiency of the current state of the art materials. Recent efforts have been made to improve the efficiency of ceramic oxide for their potential commercial applications in the waste heat harvester. The present work encompasses the synthesis, characterization, and analysis of p-type thermoelectric Ca3-xCo4O9+δ doped with rare earth elements. The sol-gel chemical solution was applied to synthesize the precursor materials for ceramic oxide with homogeneous chemistry. The results show that to a certain substitution level, doping rare-earth element decreases the thermal conductivity and improves the thermoelectric properties of the oxide thermoelectric materials. Meanwhile, doping also introduces the formation of nano-inclusions that is highly desirable for thermoelectric materials. The formation mechanism of the nano-inclusions and their resultant impact on the electrical and thermal properties of the misfit layered oxide is systematically investigated through this thesis work.

Lateral comparison of the coupling parameters on the novel hexagonal shaped cross flow thermoelectric generator

A large-sized thermoelectric module composed of cement-based composite blocks for pavement energy harvesting and surface temperature reducing