Thermoelectric Legs Research Articles

Effects of leg geometry and multistaging of thermoelectric modules on the performance of a photovoltaic‐thermoelectric system using different photovoltaic cells

Research into improving the performance of photovoltaic (PV) systems using thermoelectric generators (TEGs) has gained increased interest in recent times. Few studies have separately investigated the effects of multistaging, thermoelectric geometry, and type of PV cell on overall performance of PV-thermoelectric (PV-TE) systems. Thermoelectric leg geometries considered in these studies were either rectangular or trapezoidal legs. The research work related to examine the combined consequences of PV cell type, TEG leg geometry and multistaging on the overall performance of PV-TEs has not been carried out till date. Therefore, a three-dimensional model of this hybrid system considering two different types of PV cells and the rectangular and circular TEG leg geometries in single and multistage arrangements has been investigated using ANSYS 2020 R2 software. A parametric study considering the effects of concentrated solar irradiation, TEG cold junction convective film coefficient and load resistance on the overall performance of the hybrid system has been carried out. Results show that, for both types of PV cell, the circular leg geometry of TEG provides the highest PV-TE performance. Also, the PV cell type significantly affects the optimum number of TEG stages in the PV-TE system. Finally, multistaging increases the thermal stresses developed in the PV-TE system, although the circular leg TEG provides the least thermal stresses as compared with the traditional rectangular leg TEG. Quantitatively, the PV-TE system with single and two-stage circular leg TEGs reduced the thermal stress levels of their rectangular leg counterparts by 3.31% and 0.78%, respectively. The findings of this study can be used as a reference for all types of PV cell in the PV-TE systems.

Theoretically comparative study of spectrally selective solar absorbers in concentrated solar-thermoelectric generators working at high temperature

A spectrally selective solar absorber (SSA) made from alternating tungsten and SiO2 layers is an important component in a concentrated solar-thermoelectric generator (CSTEG). However, the highly efficient SSA consisting of a high number of tungsten and SiO2 layers may require complicated fabrication procedures, thereby raising the overall cost of the CSTEG. In this study, we have theoretically optimized SSAs consisting of a different number of layers, and the performances of CSTEGs installed with different optimized SSAs and thermoelectric materials (Si80Ge20 alloys, SnSe crystals) are theoretically investigated. It is found that two SSAs, one with only one layer of SiO2 glass (88 nm) coated on a tungsten reflective plane, and the other with three alternating layers of SiO2(92nm)/tungsten(7nm)/SiO2(78nm) coated on a tungsten reflective plane, have the highest average solar absorptance equal to 0.7050 and 0.9999, respectively. To reach the same steady-state temperature (Th), the one-layer SSA requires higher optical concentrations (Copt) than the three-layer SSA. The CSTEG with Si80Ge20 alloys requires much higher optical Copt than the CSTEG with SnSe crystals in order to reach the same Th because of much a higher rate of heat flux. At Th=800deg⁡C, the CSTEG with Si80Ge20 alloys have the output electrical power per unit cross-sectional area of TEG about 6 times higher than that of the CSTEG with SnSe crystals. The net efficiency of CSTEG with SnSe crystals could reach 25.6% if the electrical conductivity of its p-type thermoelectric leg could be enhanced to be comparable to the electrical conductivity of its n-type thermoelectric leg.

Geometric optimization of segmented thermoelectric generators for waste heat recovery systems using genetic algorithm

Recently, kinds of thermoelectric (TE) materials have been applied in the waste heat recovery system. This paper further presents a geometric optimization for segmented thermoelectric generator (STEG) modules to improve the energy harvesting performance. Sixteen length ratios of two TE materials are optimized to achieve the maximum output power. In the optimization procedure, the genetic algorithm is employed to find the optimal solution, while thermal, flow, and electric fields are fully coupled and solved by the finite element method. The optimization results indicate that the maximum output power of optimal STEGs is 24.39% (parallel-flow) and 24.33% (counter-flow) higher than that of non-segmented TEGs. Besides, the STEG under parallel-flow has better electrical performances due to the increase of temperature difference and the variation of temperature distribution. The interface temperatures are just fluctuating around the turning point of ZT values, despite of the significant variations of end temperatures. The performances of each TE legs are also investigated, which shows that some TE materials have little contribution to the whole TE module. Subsequently, hybrid TEG modules are proposed to reduce the manufacturing difficulty, which are further ranked by a multiple criteria decision-making technique. Compared with optimal STEGs, two best compromise solutions decrease the maximum output power only about 1.5%, but reduce the proportion of segmented TE legs by 56.25% and 50%, respectively.

Tubular ring thermoelectric module for exhaust pipes: From Skutterudite nanopowders to the final device

There is an important number of thermoelectric applications in the range of medium temperatures (between 200 and 600 °C), such as heat recovery in automotive applications, where these temperatures are easily reached in the exhaust or the motor. Among the various materials which exhibit good thermoelectric properties in this range, Skutterudites (with the general formula CoSb3) stand out as some of the most used. In recent years, different improvements have been done to decrease their thermal conductivity, from using rattler atoms inside their structure to nanostructuring by sintering nanoparticles and increasing the number of grain boundaries. In this work, we present the subsequent development of the nanopowder ball-milling fabrication method, to obtain a scalable production of n-type and p-type nanostructured Skutterudites. The sintering of these powders into nano-composites was done by different pressing and annealing treatments, which not only could substitute other sintering processes such as Spark Plasma Sintering, giving competitive values of the thermoelectric properties of the final nanostructured material, but can also be used to directly obtain different geometries to be used in certain implementations. This procedure has been tested by fabricating thermoelectric legs of the thermoelectric nanostructured material in the shape of rings, in contrast with the more generally available thermoelectric modules, which have a planar architecture. The ring-shaped architecture was obtained by pressing the nano-powders between copper rings, and from them, cylindrical thermoelectric generators were implemented, which present several of advantages for their use in automotive waste heat recovery from the exhaust, for instance. The optimal parameters, as far as geometry and the materials used in the final device, can be extracted from simulations based on the actual measurements obtained in the fabricated thermoelectric rings. Taking all these into account, in this work we present a route to fabricate low-cost nano-structured Skutterudite in an easily scalable way, suitable for the implementation of different architectures of thermoelectric generators for mid-range temperature applications.

Cu2Se-based thermoelectric cellular architectures for efficient and durable power generation

Thermoelectric power generation offers a promising way to recover waste heat. The geometrical design of thermoelectric legs in modules is important to ensure sustainable power generation but cannot be easily achieved by traditional fabrication processes. Herein, we propose the design of cellular thermoelectric architectures for efficient and durable power generation, realized by the extrusion-based 3D printing process of Cu2Se thermoelectric materials. We design the optimum aspect ratio of a cuboid thermoelectric leg to maximize the power output and extend this design to the mechanically stiff cellular architectures of hollow hexagonal column- and honeycomb-based thermoelectric legs. Moreover, we develop organic binder-free Cu2Se-based 3D-printing inks with desirable viscoelasticity, tailored with an additive of inorganic Se82− polyanion, fabricating the designed topologies. The computational simulation and experimental measurement demonstrate the superior power output and mechanical stiffness of the proposed cellular thermoelectric architectures to other designs, unveiling the importance of topological designs of thermoelectric legs toward higher power and longer durability.

Manufacturing and performances of silicide-based thermoelectric modules

Silicide-based materials are among the most promising candidates for a mass manufacturing of thermoelectric devices allowing converting waste heat into electricity in the medium temperature range (250–500 °C), as they are formed from abundant, low cost and non-toxic elements while exhibiting good thermoelectric properties. In order to manage the detrimental mismatch of thermal expansion coefficients between the n and p-type materials constituting the thermoelectric legs, inducing thus thermomechanical stresses, we propose in this paper a new design of modules having a 'half-skeleton' structure. Twenty-two modules consisting of two couples of thermoelectric legs combining n-type magnesium silicide Mg2(Si, Sn) and p-type higher manganese silicide have been fabricated according to this design, the thermoelectric materials being manufactured by kilograms. It is clearly shown that all the interfaces present in the modules are free from cracks, oxygen, and diffusion. The remarkable repeatability of the measured thermoelectric performance attests the robustness of our manufacturing process. An average power output of 0.37 W has been achieved, i.e. a power density of 0.95 W/cm2, for a temperature difference of 400 °C (hot side temperature of 450 °C), placing our modules at the state of the art level while using simple production tools and materials mass production. Comparing this performance to finite elements modelling, the performance could be even enhanced. Long term stability tests at a given temperature and under cycling conditions in different working atmospheres are underway.

Recyclable, Healable, and Stretchable High‐Power Thermoelectric Generator

AbstractDirect energy conversion based on thermoelectric (TE) materials is a long‐term and maintenance‐free energy harvesting technique, and therefore is very promising for self‐powered wearable electronics. Yet, it is challenging to achieve high‐performance stretchable, healable, and even recyclable thermoelectric generators (TEGs) without compromising TE conversion performance due to the intrinsic mechanical rigidity and brittleness of the inorganic TE materials. Herein, recyclable, healable, and stretchable TEGs (RHS‐TEGs) are reported that are assembled from commercial Bi2Te3 and Sb2Te3 TE legs generating superior power density via the use of liquid metal as interconnects and dynamic covalent thermoset polyimine as encapsulation. The TEGs fabricated using this strategy are endowed with excellent TE performance, mechanical compliance, and healing and recycling capabilities. The normalized output power density and mechanical stretchability can reach up to 1.08 µW cm−2·K2 and 50%, respectively. After healing and recycling, the TEGs show output performance comparable to the original devices. The TEGs also exhibit high reliability and stability under cyclic deformation. This study paves the way for sustainable application of TEGs as energy harvesters to power wearable electronics using body heat.

Fabrication of wooden thermoelectric legs to construct a generator

Herein, wooden-stick-based thermoelectric legs are introduced as a new type of thermoelectric legs. As a proof of concept, graphite- and bismuth-coated wooden sticks are fabricated with a green method, and their use as thermoelectric elements is successfully demonstrated by constructing a test thermoelectric generator. The generator based on 20 pairs of p–n legs shows a maximum power output of ∼18 nW on heating one side of the device to 70°C. The strategy can suitably be extended to other good thermoelectric materials. The approach can be promising due to the ultralow thermal conductivity of the wooden substrate pillars in contrast to other typical substrates such as glass, and hence, better thermal gradients across the device can be achieved. These environmentally friendly, non-toxic and low-cost wooden-stick-based thermoelectric devices with good thermoelectric material coatings can be conveniently used for low-grade industrial waste heat conversion applications.

Overall performance optimisation of tapered leg geometry based solar thermoelectric generators under isoflux conditions

The performance of thermoelectric devices improves either by changing the thermoelectric material properties or the geometry of the thermoelectric leg. Many research papers show that the tapered leg geometry improves the performance of thermoelectric generators (TEGs). However, they assume isothermal conditions across the hot and cold junctions of both variable area and rectangular area leg-based TEGs. This is rather inaccurate and unrealistic, because when exposed to same heat flux, temperatures vary across the hot and cold junctions of these devices. Scarcely has anything been said pertaining the performance evaluation and optimisation of variable area leg TEGs using realistic isoflux conditions. Therefore, this study presents a thermo-mechanical analysis of variable area and rectangular area leg TEGs, using ANSYS 2020 R2 software, with realistic isoflux boundary conditions instead of previous unrealistic isothermal ones. The leg height, area, external load resistance and optical concentration ratio are optimised. Results indicate that short legs are crucial for maximum power while long legs improve the device efficiencies. Also, although tapered thermoelectric leg TEGs improve the electrical performance of conventional rectangular leg TEGs, they decrease the device mechanical performance and thermodynamic stability. Finally, the optimum ranges in which tapered leg TEG outperforms the conventional device are obtained.

Thermoelectric Generators: High Power Density Thermoelectric Generators with Skutterudites (Adv. Energy Mater. 19/2021)

In article number 2100580, Bertrand Lenoir and co-workers demonstrate the fabrication of high-power-density thermoelectric generators in which skutterudites act as the active materials. Using a nonconventional design of the thermoelectric legs that enables a strong reduction of the volume of skutterudites integrated, record-breaking power densities ranging between 2.8 and 7.6 W cm−2 for temperature differences varying from 450 to 630 K are achieved.

Performance analysis and design optimization of a compact thermoelectric generator with T-Shaped configuration

In this study, a compact thermoelectric generator (TEG) configuration composed of T-shaped electrodes and connected P–N thermoelectric legs was developed to improve its performance. Based on the 1D self-consistent numerical model, the electrical impedance matching relations and optimal thermoelectric leg sizes for T-shaped TEG were semi-analytically derived, and demonstrated with a high accurate prediction. The comparison between the developed T-shaped and the conventional π-shaped TEGs was conducted, which indicated that the specific output power was greatly increased by 90.1% under the same given thermal conditions. Furthermore, impacts of the coupled thermal-electrical conditions, thermal conditions and geometric parameters on the performance of T-shaped TEG have been studied in detail. Single-parameter and two-parameter geometric optimizations were also discussed. Then, the optimal geometric size could be obtained under the given heat transfer conditions and external electrical loading. Finally, the optimization of geometric parameters for T-shaped TEG was conducted in different heat transfer conditions. Under the constant heat transfer conditions, the maximum power has improved by 21.9%. The novel structure of the T-shaped TEG proposed could be applied in many application scenarios.

Large thermoelectric power factor in wafer-scale free-standing single-walled carbon nanotube films

Single-walled carbon nanotubes (SWCNTs) have the potential for application in thermoelectric energy generators owing to their advantages, such as good charge-carrier transport properties, mechanical flexibility and robustness, and tunability of polarity. However, the fabrication of SWCNTs still remains a problem due to its complexity and high cost. In this paper, we propose an approach for the direct formation of free-standing SWCNT films from as-grown SWCNT mats without any dispersion or separation processes. We used this approach to develop high-performance SWCNT-based thermoelectric leg materials. The as-grown SWCNT mats were synthesized by an enhanced direct injection pyrolytic synthesis (eDIPS) method. The selectivity of the tube diameter for the eDIPS method clarified the dependence of the thermoelectric performance of the free-standing SWCNT films on the tube diameter. The Seebeck coefficients and thermal conductivities were found to correlate with the tube diameter and agreed with the theoretical predictions. Owing to the dispersion-free film formation, our SWCNT films afforded large thermoelectric power factors. In particular, a power factor of 350 μW/(m K2) was obtained for the mean tube diameter of 1.7 nm without any semiconductor extraction or doping treatments. Our approach allowed the fabrication of thermoelectric legs with an arbitrary size; thus, it offers a useful strategy for the simpler, cheaper, and low-waste manufacturing of high-performance organic thermoelectric devices.

Generic Approach for Contacting Thermoelectric Solid Solutions: Case Study in n- and p-Type Mg2Si0.3Sn0.7.

Metallization (known as contacting) of thermoelectric (TE) legs is vital to the long-term performance of a TE device. It is often observed that the compositional changes in a TE solid solution may render a given contact material unsuitable due to a mismatch in the thermal expansion coefficient values. Finding suitable contact materials for TE solid solutions (which often are the best TE materials) remains a challenge. In this work, we propose a multilayer single-step approach in which the same combination of contact materials can be used for a wide compositional range in a solid solution. The outer layer is a metal foil, which helps in creating an Ohmic contact with the interconnects. The intermediate layer is a mixture of the TE material and a metal powder, which results in the formation of the diffusion barrier. The innermost layer is the TE material, which is the active component of the device. The strategy was applied on n- and p-doped Mg2Si0.3Sn0.7 with elemental Cu and Ni providing the desired interface functionalities. Single-step compaction was carried out using the monoblock sintering technique. Microscopic investigation reveals the formation of a well-bonded crack-free interface. Various intermetallic phases were identified at the interface, and the formation of the MgNi2Sn phase was found to be critical to prevent any interdiffusion of elements. Electrical contact resistance (rc) measurements were conducted, and low values of 3 and 19 μΩ cm2 were measured in n- and p-type legs, respectively. The contacted TE legs were further annealed at 400 °C for 7 days to check their stability. Microstructural and electrical resistance measurements reveal minimal changes in the interface layer and rc values, indicating the workability of the multilayer technique.

Characteristics analysis and parameter optimization of a micro-combustion based thermoelectric generator

Micro combustion based thermoelectric generator (micro-CTEG) has great potential to provide reliable electricity for portable electronic devices. Previous works on micro-CTEG has focused on the micro combustion issue, here we explore the heat transfer and thermoelectric (TE) coupling mechanism and give the guidelines for optimizing the performance of micro-CTEG. An analytical model was derived considering heat exchange inside the combustor, TE generator structure and temperature dependent material properties. Based on the optimization results, the temperature drop across the TE leg remains nearly constant for filling fraction approaching 1.0 and decreases remarkably with decreasing input fuel power for filling fraction approaching 0.1 due to the excessive gap heat loss. High filling fraction and high input fuel power is preferred to balance the power density and total efficiency. With identical thermoelectric figure of merit (ZT), increasing power factor under low filling fraction and decreasing thermal conductivity under high filling fraction are preferred respectively. The optimal load resistance factor, which represents the ratio of load resistance to internal resistance of TE generator, shows a decreasing trend with increasing temperature drop across TE leg. The optimal leg length remains nearly unchanged as thermal resistance of the heat sink is lower than 1 K/W.

Structural Design Optimization of Micro-Thermoelectric Generator for Wearable Biomedical Devices

Wearable sensors to monitor vital health are becoming increasingly popular both in our daily lives and in medical diagnostics. The human body being a huge source of thermal energy makes it interesting to harvest this energy to power such wearables. Thermoelectric devices are capable of converting the abundantly available body heat into useful electrical energy using the Seebeck effect. However, high thermal resistance between the skin and the device leads to low-temperature gradients (2–10 K), making it difficult to generate useful power by this device. This study focuses on the design optimization of the micro-thermoelectric generator for such low-temperature applications and investigates the role of structural geometries in enhancing the overall power output. Electroplated p-type bismuth antimony telluride (BiSbTe) and n-type copper telluride (CuTe) materials’ properties are used in this study. All the simulations and design optimizations were completed following microfabrication constraints along with realistic temperature gradient scenarios. A series of structural optimizations were performed including the thermoelectric pillar geometries, interconnect contact material layers and fill factor of the overall device. The optimized structural design of the micro-thermoelectric device footprint of 4.5 × 3.5 mm2, with 240 thermoelectric leg pairs, showcased a maximum power output of 0.796 mW and 3.18 mW when subjected to the low-temperature gradient of 5 K and 10 K, respectively. These output power values have high potential to pave the way of realizing future wearable devices.

Influence of structural factors on thermal stress in skutterudite-based thermoelectric module

Thermoelectric (TE) modules for power generation usually operate under high temperatures and large temperature differences, which inevitably introduce thermal stress in the modules. Suppressing the thermal stress is then one of the important issues for improving the service reliability of TE modules. In the past decades, various approaches have been developed for the design optimization of TE module primarily being focused on the enhancement of conversion efficiency, while the influence of structural factors on the module’s mechanical reliability is often overlooked. In this work, we proposed a multi-objective optimization strategy by using the finite element method to evaluate the structural reliability of a TE module, which integrates the module mounting mode, configurational structure of ceramic substrates, thickness of ceramic substrates and electrodes, cross-sectional shape of TE legs, and gaps between [Formula: see text]- and [Formula: see text]-type legs. As a typical sample, the thermal stress of an 8-pair skutterudite (SKD)-based TE module with the framework dimensions of 20 × 22 × 12 mm3 was well studied under the service conditions. The results reveal that a split structure of ceramic substrates and a pressing mounting mode with 2 MPa pressure on the module’s hot-side can significantly reduce the thermal stress in the module. Meanwhile, increasing the gap distance between [Formula: see text]- and [Formula: see text]-type legs, rounding the square column shaped legs, using thin ceramic substrate and thick electrode also can relieve the thermal stress somewhat. The simulation gives a comprehensive solution to reduce the thermal stress and enhance the module’s service reliability for the practical applications through structure optimization.

Versatile four-leg thermoelectric module test setup adapted to a commercial sample holder system for high temperatures and controlled atmospheres.

A high temperature thermoelectric test setup for the NORECS ProboStat™ sample holder cell has been designed, constructed, and tested. It holds four thermoelectric legs of up to 5 × 5mm2 area each and flexible height, allows various interconnects to be tested, and utilizes the spring-load system of the ProboStat for fixation and contact. A custom stainless steel support tube flushed with water provides the cold sink, enabling large temperature gradients. Thermocouples and electrodes as well as the gas supply and outer tube use standard ProboStat base unit feedthroughs and dimensions. The setup allows for testing in controlled atmospheres with the hot side temperature of up to around 1000 °C and a temperature gradient of up to 600 °C. We demonstrate the test setup on a four-leg Li-NiO/Al-ZnO module with gold interconnects. The comparison between the predicted performance based on individual material parameters and the experimentally obtained module performance underlines the necessity for testing materials in combination, including interconnects. The four-leg setup allows versatile match-screening, performance evaluation, and long-term stability studies of thermoelectric materials in combination with hot and cold side interconnects under realistic operational conditions.

High Power Density Thermoelectric Generators with Skutterudites

AbstractThermoelectric generators (TEGs) offer a versatile solution to convert low‐grade heat into useful electrical power. While reducing the length of the active thermoelectric legs provides an efficient strategy to increase the maximum output power density pmax, both the high electrical contact resistances and thermomechanical stresses are two central issues that have so far prevented a strong reduction in the volume of thermoelectric materials integrated. Here, it is demonstrated that these barriers can be lifted by using a nonconventional architecture of the legs which involves inserting thick metallic layers. Using skutterudites as a proof‐of‐principle, several single‐couple and multi‐couple TEGs with skutterudite layers of only 1 mm are fabricated, yielding record pmax ranging from 3.4 up to 7.6 W cm−2 under temperature differences varying between 450 and 630 K. The highest pmax achieved corresponds to a 60‐fold increase per unit volume of skutterudites compared to 1 cm long legs. This work establishes thick metallic layers as a robust strategy through which high power density TEGs may be developed.

Development of a solid-state multi-stage thermoelectric cooler

In this work, a unique thermoelectric solid-state multi-stage cooler for operating temperatures up to ~140 K was developed. The extrusion technology was applied to fabricate thermoelectric legs with sufficient mechanical properties for the device construction. A cooling prototype was designed and fabricated using a four-stage commercial module and a two-stage ultra-low temperature module. The ultra-low temperature module, operating in the temperature range of 180–140 K, was fabricated using Bi0·91Sb0.09 alloys as an n-type leg and an optimized composition of Bi1·6Sb0·4Te3 as a p-type leg. The four-stage commercial thermoelectric module based on n- and p-type Bi2Te3 based alloys was utilized for operation in the temperature range of 180–300 K. The solid-state cooler was tested at a hot temperature Th = 300 K in a vacuum. At the cold temperature of the fourth stage 180 K, the maximum temperature difference of the ultra-low temperature module achieves high values of maximum temperature difference ΔT = 45 K and maximum cooling capacity Qc = 85 mW. Furthermore, we demonstrated the applicability of the ultra-low cooler for refrigeration up to 150 K for a mid-infrared sensor based on a PbTe photodiode with a cut-off wavelength λc ≈ 4.2 μm and a detectivity D ≈ 1010 cm Hz1/2/W revealing the potential of the developed device for practical applications.

A GPU-Accelerated ray-tracing method for determining radiation view factors in multi-junction thermoelectric generators

A robust computational framework was developed and implemented to numerically resolve the radiation view factor, Fij, within three-dimensional geometries, and, in particular, thermoelectric generators (TEGs). The proposed numerical methodology utilizes a graphics processing unit-accelerated ray-tracing algorithm to capitalize on the parallel nature of the view factor formulation. The shadow effect, resulting from interference with the TEGs conductive interconnectors and thermoelectric legs, was accounted for via the Möller-Trumbore ray-triangle intersection algorithm with back-face culling enabled. The effect of interconnector thickness, thermoelectric leg height-to-width ratios, TEG packing density, and the number of junctions on Fij is explored for various TEG configurations. Validation is performed against analytical values for planar and non-planar geometries, in addition to a point-in-polygon intersection algorithm for single-junction TEGs. Results indicate that for a constant packing density, Fij asymptotically decreases with increasing distance across the TEG’s hot- and cold-sides. For an increasing packing density and constant distance across the TEG’s junction, Fij decreases. In a multi-junction device, Fij was found to asymptotically increase with junction number, implying that for large multi-junction TEG designs, a simpler model may serve to accurately predict the view factor. The code developed herein is open-source and can be found at https://github.com/AasherH/GPU-Accelerated-View-Factor-Calculator.