A comparative performance analysis of thermoelectric generators with a novel leg geometries

Thermoelectric (TE) devices utilize the Seebeck effect to produce electricity from waste heat. The optimization of TE leg geometry and its structure has lately drawn more attention to improving TE power generators. This study uses the finite element method to examine thermoelectric generators' thermal reliability and performance (TEGs) for novel TE leg designs. New leg shapes, such as a cross-vertical and butterfly, are introduced and contrasted with the squared (conventional) leg and other leg shapes, such as trapezoidal, cross-horizontal, I-shaped, X-shaped, and Y-shaped. The design of the main geometric parameters, including structure, height, volume, and surface area of the thermoelectric leg, is discussed in detail. As the TE leg has a larger heated surface area, the results demonstrate that the cross-vertical and butterfly legs perform 39.2% and 30.4%, respectively, higher than the squared (conventional) shape. However, the square-shaped leg shows the least thermal stress compared to other designs, especially at high thermal gradients. For low-temperature gradients (Th < 100 °C), the new TE leg designs of the cross-vertical and butterfly structures exhibit thermal stresses below the yield stress of bismuth telluride, indicating that their use is suitable for upcoming low-temperature gradient thermoelectric system applications

Simulation Guided Design of Thermoelectric Generators for Waste Heat Recovery Applications

Thermoelectricity (TE) is an attractive technology dealing with the conversion of unused heat energy into electrical power and vice versa. Thermoelectric conversion systems share the advantages of being reliable, silent in operation, environmentally friendly and compact systems without moving parts. The areas of research that need to be taken into consideration to develop superior TEG devices are efficiency, manufacturing, and modeling of these systems. The low energy conversion efficiency in thermoelectric materials hampers the applications of thermoelectric generators (TEGs). Besides the efficiency of thermoelectric materials and the construction shape of TE modules are other crucial factors that can also directly affect TEG reliability and performance. The conventional methods used for TEG fabrication are generally limited to flat or rectangular geometries. Another drawback of TEGs is the lack of accurate numerical modeling of these systems. Simulation and optimization of TE systems are necessary to prevent over or under design which leads to saving a lot of time and money. This thesis addresses the modeling limitation of TEGs by introducing numerical simulation guided of TEGs for waste heat recovery applications. In this thesis, the background of TE modeling is addressed by defining the governing equations of TE energy conversion process. A numerical technique of TEG simulation using ANSYS software is introduced to obtain the TE performance and the reliability assessment of these systems. Different TE leg and module configurations (flat and annular TEGs) which can significantly benefit the energy conversion field in terms of developing future thermoelectric systems with improved efficiency and reliability are presented. Energy conversion efficiency in TE systems is enhanced when the temperature gradient between the hot and cold surfaces of TEGs is maximized. This, however, dramatically increases the thermal stresses built-up within the thermoelectric modules which lowers the TEG reliability. A promising method to reduce thermal stresses in TEG systems is considering unileg TEGs system instead of traditional unicouple configuration. The concept of flat and annular unileg TE systems is introduced and the thermoelectric efficiency and reliability of these systems are quantified using finite element simulations. Implemented TE modeling for waste heat recovery applications such as exhaust pipe system and IC engine oil pan are also introduced in this work. The feasibility of replacing flat TE systems with the annular unicouple and the annular unileg systems is explored for exhaust gas heat recovery applications in automobiles to assess the real-life usage of the proposed systems. Another TE application is the (IC) engine oil pan, unlike the radiator and exhaust system applications of thermoelectricity described above, in this thesis, a novel implementation of TE systems within the oil pans of internal combustion (IC) engines, where a higher amount of heat is generated and lost to the environment is presented. For future work, the manufacturability of the TE system and the comparison of existing TE applications by performing additive manufacturing (3D printing) are also discussed for present and future work. By using binder jetting additive manufacturing method, the accuracy of fabricating smaller TE element sizes can be examined, furthermore, the binder jetting method can be used to fabricate different and complex configurations of TE modules.</p

Numerical analysis of energy conversion efficiency and thermal reliability of novel, unileg segmented thermoelectric generation systems

Summary Despite their great potential to recover waste heat, thermoelectric generators (TEGs) find limited usage since thermoelectric materials are only efficient within a limited temperature range. Using multiple materials in segmented TEGs can significantly enhance the overall energy conversion efficiency. However, thermal reliability is questionable in these systems especially at elevated temperatures and in annular configurations. This study explores the feasibility of utilizing unileg (single material) segmented TEG configuration as a remedy for the thermal stress problem. This study introduces the concept of unileg, segmented thermoelectric configurations (flat and annular) for the first time, and three‐dimensional finite element simulations are conducted to investigate the thermoelectric performance and thermal reliability analysis in comparison with the conventional unicouple (dual material) systems. Results indicate that thermal stresses are significantly lowered in unileg systems compared to the unicouple configuration. In addition to the enhanced thermal reliability, power generation and thermoelectric conversion efficiency are higher in unileg systems since the material with higher performance is used solely eliminating the need of poorly performing, second thermoelectric leg material. Thermal reliability is a concern in segmented thermoelectric systems especially at elevated temperatures. In this study, unileg‐segmented systems are investigated numerically for the first time and it is found out that thermal stress is reduced significantly in unileg systems compared to unicouple configurations. Segment configuration, thermoelectric device shape, contact topology and leg length simultaneously affect the thermoelectric performance and reliability of the segmented design

Design of cascade thermoelectric generation systems with improved thermal reliability

This paper presents a comprehensive analysis of novel, unileg cascade thermoelectric systems. Unileg systems offer the flexibility of selecting a single (p- or n-) thermoelectric material in a thermoelectric system compared to two (n- and p-) materials. Finite element analysis simulations were performed to design and analyze two and three stages of cascade systems. Simulation results show that unileg cascade systems perform significantly (∼75%) better than their unicouple counterparts in both two and three stage configurations due to the elimination of the poorly performing thermoelectric leg. In addition to the enhanced thermoelectric power generation, thermal stress is lowered in unileg systems since the mismatch of the thermal expansion coefficient originating between different TE legs material can be minimized. Overall, the presented unileg systems show their promise for the future thermoelectric energy generation or cooling applications for electronics. •Comparison between unicouple and unileg cascade TEG systems is introduced.•Thermal reliability and TE performance are investigated for cascade TEG systems.•Unileg cascade TEG generates higher TE power and displays lower Von Mises stresses.•Unicouple cascade TEG shows higher heat losses compared to unileg cascade TEG.•Thomson effect is higher in Unileg TEG compared to unicouple TEG

Increasing Energy Efficiency in Vehicles by Harvesting Wasted Engine Heat

Nearly 75% of energy produced by fuel is eventually rejected to the environment and ultimately goes unused in terms of waste heat in motor vehicles. A promising method of reclaiming energy waste is to use thermoelectric (TE) energy harvesters which are multi-material solid-state devices that convert a thermal gradient directly into electric potential. In current automotive applications, waste heat recovery systems using TE are only limited to integration on exhaust pipes to convert hot exhaust gases into electricity. In this study, we explored the use of TE materials in the shape of a car oil pan to utilize the temperature difference of hot engine oil and cool outside air and convert this temperature gradient into electricity. In this study, we performed finite element simulations to optimize the geometry and the quantity of thermoelectric modules. This optimization was performed to achieve maximum thermoelectric power under the constraints of manufacturability. Using these optimum design parameters, we determined that 2.3 kW output power can be recovered from the flat plate oil pan and 2.6 kW from the oil pan with a single step due to the enhanced surface area. These power amounts were found to be higher than those previously obtained from thermoelectric systems integrated to exhaust pipes

Energy conversion and thermal reliability of thermoelectric materials in unileg annular configuration

•Annular thermoelectric materials implemented on exhaust pipe enhances output power.•Unileg annular thermoelectric system is introduced for the first time.•Unileg system reduces thermal stresses without lowering conversion efficiency. Increasing temperature gradients are desired in thermoelectric (TE) systems to produce higher amounts of power. However, high temperatures lead to increased thermal stresses impacting the reliability of TE materials. Stresses are more pronounced in annular systems despite their space saving advantages compared to flat configurations. One possible solution to reduce thermal stress problem is utilizing single material (unileg) TE module instead of dual material (unicouple) system. This study introduces unileg, annular TE configuration for the first time. The results indicate that replacing flat TE system in exhaust pipe with annular configuration can increase TE performance and thermal stresses can be reduced significantly

Design optimization of oil pan thermoelectric generator to recover waste heat from internal combustion engines

Nearly 75% of fuel energy is rejected to the environment and ultimately becomes waste heat in motor vehicles. To recover some of this waste heat and enhance fuel efficiency, thermoelectric energy generators (TEGs) possess high potential. We investigated the feasibility of utilizing TEGs in terms of oil pans to recover waste heat generated in internal combustion engines. Hot oil at the top surface of TEG and air cooling at the bottom create a high thermal gradient for the thermoelectric conversion. An extensive multi-physics simulation framework was introduced to accurately simulate conversion of heat into electricity taking into account thermoelectricity, joule heating, heat conduction and turbulent air cooling. To maximize the thermoelectric power, dimensions and the total number of thermoelectric modules were optimized under different oil pan geometries and driving conditions. Our simulations show that the maximum power density of 5.77 kW m−2 is achieved with multi-step oil pan geometry under a 76 °C temperature difference between the hot and cold sides. This power density surpassed those reported for the previous, conventional (exhaust and radiator) thermoelectric applications and indicated that harvesting thermal energy from combustion engines using oil pans is a feasible energy recovery methodology to enhance fuel efficiency in automotive vehicles. •Waste heat recovery is limited to exhaust systems in automotive applications.•Thermoelectric modules can be used in engine oil pans as robust energy harvesters.•Multiphysics numerical simulations can predict thermoelectric energy harvesting.•Kilowatt range power is generated at high efficiency by multi-step engine oil pan.•Modular, thermoelectric oil pan can save significant fuel without any water-cooling system

Effect of Leg Topologies on Thermal Reliability of Thermoelectric Generators Systems

Increasing the temperature gradient across thermoelectric (TE) devices improves the TE power output significantly. However, enhanced thermal gradients lead to high thermal stresses and these stresses must be taken into consideration in the design process to develop TE systems with enhanced thermal reliability. The shape of TE leg plays a major role on the performance of these systems. Conventional prismatic TE leg structure has been the preferred leg topology commonly utilized in TE devices due to its simplicity and high thermoelectric performance. Other leg topologies have also been considered in the literature to quantify their effects on TE performance. This current study investigates the thermal reliability of a several TE leg topologies such as trapezoidal leg, butterfly leg, vertical and horizontal cross-shaped leg, X-leg, Y-leg, I-leg according to the construction of each shape. 3D numerical model is presented to quantify the Von Mises stresses of these configurations and compared to conventional rectangular TE leg shape. The vertical cross-shaped and butterfly configurations generate 36% and 27% TE power higher than the conventional rectangular TE leg, respectively. On the other hand, the conventional TE leg has the lowest thermal stress compared to other TE leg topologies

Prismatic Spreading-Constriction Expression for the Improvement of Impedance Spectroscopy Models and a More Accurate Determination of the Internal Thermal Contact Resistances of Thermoelectric Modules

Thermoelectric (TE) devices can convert heat to electrical power or use electrical power to generate a temperature difference. Their characterization is essential to develop devices with higher efficiency. Impedance spectroscopy models have been developed in the last few years, and it has become a highly advantageous method for TE system characterization. Recently, it has been shown that this technique can also be used to determine internal thermal contacts (between the TE legs and the metallic strips that connect them and between the metallic strips and the outer layers). Here, we developed for the first time a spreading-constriction expression which does not assume cylindrical geometry. The enhanced model is also used to characterize four TE devices from different manufacturers, highlighting overestimations up to 13% when the previous cylindrical approximation is used. A code is provided in the Supporting Information ready to fit the experimental data. This study positions impedance spectroscopy as a powerful tool to detect and monitor issues during manufacturing or operation of TE devices, which typically occur at the contacts

Numerical simulations of cell flow and trapping within microfluidic channels for stiffness based cell isolation

Analysis of rare cells in heterogenous mixtures is proven to be beneficial for regenerative medicine, cancer treatment and prenatal diagnostics. Scarcity of these cells, however, makes the isolation process extremely challenging. Efficiency in cell isolation is still low and therefore, novel cell isolation strategies with new biomarkers need exploration. In this study, we investigated the feasibility of using the mechanical stiffness difference to detect and isolate the rare cells from the surrounding cells without labelling them. Fluid and solid mechanics simulations have shown that cell isolation can be performed at high efficiency using stiffness-based isolation. Accuracy of the numerical simulations is established using microfluidic flow chamber experiments