Conventional Thermoelectric Generators Research Articles

Thermoelectric generator modules based on warp knitted glass fiber-metal hybrid composites

Thermoelectric generators (TEG) offer the potential to convert waste heat into electricity and thus contribute to reduce CO2 emissions. The conversion of electrical energy is based on the Seebeck effect of two electrically conductive materials without any mechanical conversion and therefore without wear. The application of conventional TEG modules is limited due to cost-intensive materials and production technology of TEG, and a limited structure design for the integration of Thermoelectric Elements (TE). To address this research challenge, this work presents the development of thermoelectric composite modules based on glass fiber reinforced warp knitted spacer fabrics. In a double needle bed warp knitting machine, glass fibers in warp, weft and pile direction are integrated. The contacting of TE in the form of wires with 45 TE cm−2 were implemented. A TEG module with 20.25 cm2 in size showed a maximum output power of 2.7 μW at a temperature difference of 60 K. The Seebeck factor of S = 142 μV K−1 was determined using this composite TEG with 10 TE strands and nearly 400 thermocouples. A thermoelectric model was developed for the calculation and the modules were characterized. For the first time, thermoelectric composite modules with sufficient structural-mechanical properties in terms of compressive and bending stiffness were realized based on spacer warp knitted fabrics, which can be used for the operation of sensors or small devices.

Photothermal conversion-enhanced thermoelectric generators combined with supercapacitors: An efficacious approach to integrated power generation and storage

Thermoelectric generators (TEGs), which harness and convert solar-thermal energy into electrical energy, possess immense potential within the field of photothermal conversion (PTC). However, the widespread adoption of conventional TEGs in PTC domain has encountered some obstacles, which represent that TEGs have poor inherent PTC performance, resulting in low solar-thermoelectric conversion efficiency, and this solar-thermal-electric conversion method lacks the capability of electrical energy storage. Hence, an urgent imperative exists to develop integrated and synergistic energy technologies that can respond to these underlying challenges. Herein, we reported a novel strategy for employing spinel-type Cu1.5Mn1.5O4 PTC coating to adorn conventional TEG to construct the solar-thermoelectric generator (STEG) which exhibited excellent solar-thermoelectric conversion capability, and then integrating the STEG in series with a supercapacitor composed of synthetic ionic liquid electrolytes and carbon electrodes to fulfill the conversion and storage of solar-thermal energy into electrical energy. The results indicated that, when compared to a conventional TEG, the STEG attained a drastically elevated internal temperature difference due to the excellent PTC capability of spinel-type coating. As a result, remarkable enhancements were observed in both the steady-state voltages (Voc) and maximum output power density (Pmax) of the STEG, exceeding a 6-fold and 40-fold increase, respectively. Furthermore, we conducted experimental validation to ascertain the viability of the solar-driven STEG to charge a supercapacitor, thereby achieving the conversion and storage of solar-thermal energy into electrical energy, and the working mechanism of this integrated system has been revealed. This study offers invaluable insights into the development of highly efficient solar-thermal energy conversion and storage methods.

Impacts of thermal and electric contact resistance on the material design in segmented thermoelectric generators

Segmented thermoelectric generators (STEGs) can exhibit present superior performance than those of the conventional thermoelectric generators. Thermal and electrical contact resistances exist between the thermoelectric material interfaces in each thermoelectric leg. This may significantly hinder performance improvement. In this study, a five-layer STEG with three pairs of thermoelectric (TE) materials was investigated considering the thermal and electrical contact resistances on the material contact surface. The STEG performance under different contact resistances with various combinations of TE materials were analyzed. The relationship between the material sequence and performance indicators under different contact resistances is established by machine learning. Based on the genetic algorithm, for each contact resistance combination, the optimal material sequences were identified by maximizing the electric power and energy conversion efficiency. To reveal the underlying mechanism that determines the heat-to-electrical performance, the total electrical resistance, output voltage, ZT value, and temperature distribution under each optimized scenario were analyzed. The STEG can augment the heat-to-electricity performance only at small contact resistances. A large contact resistance significantly reduces the performance. At an electrical contact resistance of RE = 10–3 K·m2·W−1 and thermal contact resistance of RT = 10–8 Ω·m2, the maximum electric power was reduced to 5.71 mW (90.86 mW without considering the contact resistance). And the maximum energy conversion efficiency is lowered to 2.54% (12.59% without considering the contact resistance).

Thermoelectric Generator Through Dual-Direction Thermal Regulation by Thermal Diodes for Waste Heat Harvesting.

Thermal energy harvesting provides an opportunity for multi-node systems to achieve self-power autonomy. Thermoelectric generators (TEGs), either by thermocouple arrangement with higher-aspect-ratios or thermoelectric films overlay, are limited by the small temperature difference and its short-duration (less than dozens of minutes), hindering the harvesting efficiency. Here, by introducing thermal diodes with dual-direction thermal regulation ability to optimize the heat flux path, the proposed TEGs exhibit enhanced power-supply capability with unprecedented long-duration (more than hours). In contrast with conventional TEGs with fixed-leg dimensions enabled single output, these compact-TEGs can supply up to fourteen output-channels for selection, the produced power ranges from 1.11 to 921.99µW, open circuit voltage ranges from 8.07 to 51.32mV, when the natural temperature difference is 53.84°C. Compared to the most recent TEGs, the proposed TEGs in this study indicate higher power (more than hundreds times) and much longer output duration (2.4-120 times) in a compact manner.

A comprehensive comparison of micro heterojunction thermoelectric generators based on a carrier transport model

To be a crucial aspect in the performance of thermoelectric generators, carrier transport in thermoelectric materials cannot be captured by existing multiphysics models. In this work, a carrier transport model for heterojunction thermoelectric generators (TEG) is developed with the drift-diffusion equation comprehensively considering the effect of carrier mobility on the current density. Subsequently, based on the concept of inverse- and homo-heterojunction, three micro TEG are designed considering Si and Si0.7Ge0.3 bases with the doped boron and arsenic and compared with the original design for revealing the effects of load resistance and temperature difference on the thermoelectric performance. It has been demonstrated that for homo-heterojunction, the thermoelectric power and conversion efficiency are significantly improved at hot temperatures from 600 to 1000 K, with a maximum improvement of 18.09% compared to the conventional TEG. For inverse-heterojunction, the optimal temperature of the thermoelectric performance is 1100–1300 K, and its efficiency is 23.50% higher than that of the conventional TEG. Both the doping impurity ionization and intrinsic carrier excitation contribute to a high density of majority carriers and are responsible for the better performance of homo-heterojunction at low working temperatures. For inverse-heterojunction, a high working temperature can significantly promote the excitation of minority carriers because of the built-in electric field and leads to better performance. Finally, we propose a novel heterojunction by combining both homo- and inverse-heterojunction which can improve thermoelectric efficiency by up to 27.72%.

Effects of material doping on the performance of thermoelectric generator with/without equal segments

Thermoelectricity is a clean energy source that has garnered attention worldwide. Improving the performance of thermoelectric devices to convert waste heat into electricity efficiently is currently a priority for many researchers. One way to do this is by improving the thermoelectric performance of the materials used in thermoelectric generators (TEGs). In recent years, doping different materials into thermoelectric materials to enhance TEG's performance has attracted attention. This study analyzes the improvement of TEGs by performing 3D numerical simulations of thermoelectric devices using a series of SnTe with different doping levels. The results show that the undoped SnTe-AgSbSe2 composite has the highest output power compared with the doped case. In terms of conversion efficiency, the composite AgSbSe2 doped with 3% SnTe achieves the highest conversion efficiency. Furthermore, the performance comparison with conventional TEG shows that the equal segmented thermoelectric generator (STEG) has higher output power and conversion efficiency at all doping ratios. In addition, the SnTe-AgSbSe2 composite with 3% doping concentration has the best thermoelectric performance with an output power of 64.49 mW and an efficiency of 14.17% at a temperature difference of 500 K (a hot side temperature of 800 K and a cold side temperature of 300 K). Accordingly, it is summarized that the combination of doped SnTe thermoelectric material and equal-STEG design can efficiently enhance the thermoelectric module's (TEM) performance.

Innovative Design of Bismuth-Telluride-Based Thermoelectric Transistors.

Conventional thermoelectric generators, predominantly based on the π-type structure, are severely limited in their applications due to the relatively low conversion efficiency. In response to the challenge, in this work, a Bi2Te3-based thermoelectric transistor driven by laser illumination is demonstrated. Under laser illumination, a temperature difference of 46.7 °C is produced between the two ends of the transistor structure. Further, the hole concentrations in each region redistribute and the built-in voltages decrease due to the temperature difference, leading to the formation of the transistor circuit. Additionally, the operation condition of the thermoelectric transistor is presented. The calculation results demonstrate that the maximum output power of such a designed thermoelectric transistor is 0.7093 μW.

Optimizing GeTe-based thermoelectric generator for low-grade heat recovery

GeTe-based thermoelectric (TE) materials have been of great interest in the field of low-grade heat recovery due to their increasing performance near room temperature. To promote the application of GeTe-based materials, we analyze the GeTe-based thermoelectric generator (TEG) from materials to structures through theoretical and experimental methods. Since a TEG generally contains both p-type and n-type materials, we calculate the n-type material parameters suitable for the p-type GeTe, and improve the compatibility between p-type and n-type materials by adjusting the ratio of cross-sectional areas. Meanwhile, the height of TE legs is designed to minimize the impact of interfacial resistivity, while the suitable temperature loading is determined to achieve higher output power and conversion efficiency. Based on theoretical guidance, the conversion efficiency is improved by more than 23% by adjusting the ratio of cross-sectional areas; a conversion efficiency of 6.24% under the temperature gradient of 300 K is realized, which has up to 80% improvement from the conventional TEGs with the advantages of low cost, high reliability and stable service performance. This study provides the theoretical basis and guidance for optimizing GeTe-based TEG in the application of low-grade heat recovery.

Nanoscale homojunction thermoelectric generator built in defect-engineered MoS2

The relatively low efficiency and costly composition of conventional thermoelectric generators (TEGs) hinder pragmatic usage despite their high Carnot efficiency. A PN-junction-based TEG is one candidate to resolve the noted drawbacks, but electron-hole recombination at the junction severely reduces the efficiency and limits the compactness. Here, we propose the nanoscale homojunction thermoelectric generator by engineering the ${\mathrm{MoS}}_{2}$ grain boundary. Ab initio scanning Seebeck microscopy revealed that the influence of deep-level wave functions of atomic defects broadens out to a nanoscale, leading to the doping-tunable Seebeck domain with a different magnitude or even inverted sign. This thermopower utilization using vestiges of localized defect states enables exceptional integration density in a single material, while the simple oxygen substitution inflates the efficiency by barricading the recombination at the junction. Our finding suggests that inevitable structural defects could pave the way for actualizing productive thermoelectric energy recycling at the nanoscale.

Zero-cooling energy thermoelectric system by phase change material heat sink integrated with porous copper foam

Active cooling of thermoelectric generators (TEGs) by fans or water pumps imposes cooling energy reducing net power generation in TEG systems. This study aims to make zero-cooling energy TEG systems by integration of low temperature phase change material (PCM) and porous copper foam on cold side of the TEG. In this design, air fan is eliminated. To study the proposed system under transient heat input, the TEG system was exposed to continues and dynamic heat flows. Results of this study show that, the proposed fanless heat sink is an effective and alternative cooling solution for TEG systems. Utilization of the copper foam in the PCM not only reduced cold side temperature of the TEG by 11.3 %, but it also prevents hot side of the TEG from overheating. With this cooling technique, output power of the TEG increased 53.5 % compared to conventional TEG systems operating with fan. The results of this study provide a guideline for design of self-sufficient and autonomous TEG systems with zero-cooling energy used for energy harvesting for sensors and actuators under dynamic heat sources.

Power generation and thermal stress characterization of thermoelectric modules with different unileg couples by recovering vehicle waste heat

Unileg thermoelectric generators (TEGs) possess a simple structure, high mechanical strength, and lower thermal stress and cost than conventional TEGs. So it is a promising clean and green energy device that directly converts waste heat into power. This study focuses on the performance and thermal stress of unileg thermoelectric modules (TEM) with different leg heights and numbers of legs by harvesting the vehicle's waste heat. This work also combines TEM and computational fluid dynamics (CFD) to analyze the influence of fluid flow. When the hot-side and cold-side temperatures are fixed, the results reveal that the shorter the leg height, the higher the output power, as a consequence of smaller internal resistance. Alternatively, the maximum efficiency is merely reduced by 1.45% when the height decreases from 12 mm to 8 mm. The output power linearly increases with the number of legs, showing no interaction between the unilegs. Compared to the CFD predictions, the results without considering the fluid flows overestimate the maximum output power and efficiency by 80% and 32%, respectively. Because the thermal expansion coefficients of different materials in the TEM are not matched, thermal stress is generated. The thermal stress increases from the center of the TEM outward along the x- and y-axes, yielding maximum thermal stress of 219.48 MPa. Compared with conventional TEM thermal stress (348 MPa) at moderate and high-temperature applications, the maximum thermal stress is reduced by 36.9% from the unileg TEG.

Adiabatic and isothermal configurations for Re4Si7 transverse thermoelectric power generators

Transverse thermoelectric generators (TTEGs) have the potential to overcome two challenging problems that limit the commercial application of classical thermoelectric generators (TEGs): the significant irreversible efficiency losses in the contacts and the thermal degradation of the contacts at the hot end. TTEGs are built from a single material that can generate a thermoelectric current in a direction that is orthogonal to a temperature gradient. They only require a single set of contacts that are displaced away from the hot end. Here, we experimentally explore the device design considerations for optimizing the performance of this far less explored class of thermal-to-electric generation devices. We fabricate Re4Si7 TTEGs and show that there is no measurable loss in efficiency with a single set of contacts that are midway between the hot and cold ends. We show that a new effect must be considered: the appearance of transverse thermal gradients that lead to significant differences between the performance of TTEGs implemented in isothermal and adiabatic geometries. The isothermal TTEG configuration leads to much higher device efficiencies. Overall, this work highlights how different the design of TTEGs is from that of conventional TEGs and the importance of controlling the device geometry for optimum waste heat recovery.

Generalized regression neural network and fitness dependent optimization: Application to energy harvesting of centralized TEG systems

The thermoelectric generator (TEG) system has attracted extensive attention because of its applications in centralized solar heat utilization and recoverable heat energy. The operating efficiency of the TEG system is highly affected by operating conditions. In a series-parallel structure, due to diverse temperature differences, the TEG modules show non-linear performance. Due to the non-uniform temperature distribution (NUTD) condition, several maximum power points (MPPs) appear on the P/V curve. In multiple MPPs, the true global maximum power points (GMPP) are very important for optimum action. The existing conventional technologies have slow tracking speed, low productivity, and unwanted fluctuations in voltage curves. To overcome the TEG system behavior and shortcomings, A novel control technology for the TEG system is proposed, which utilizes the improved generalized regression neural network and fitness dependent optimization (GRNNFDO) to track the GMPP under dynamic operating conditions. Conventional TEG system control techniques are not likely to trace true GMPP. Our novel GRNNFDO can trace the true GMPP for NUTD and under varying temperature conditions In this article, some major contributions in the area of the TEG systems are investigated by solving the issues such as NUTD global maxima tracking, low efficiency of TEG module due to mismatch, and oscillations around optimum point. The results of GRNNFDO are compared with the Cuckoo-search algorithm (CSA), and grasshopper optimization (GHO) algorithm and particle swarm optimization (PSO) algorithm. Results of GRNNFDO are verified with experiments and authenticated with MATLAB/SIMULINK. The proposed GRNNFDO control technique generates up to 7% more energy than PSO and 60% fast-tracking than meta-heuristic algorithms.

Concept of an efficient self-startup voltage converter with dynamic maximum power point tracking for microscale thermoelectric generators

Microscale Thermoelectric Generators (microTEGs) have a high application potential for energy harvesting for autonomous microsystems. In contrast to conventional thermoelectric generators, microTEGs can only supply small output-voltages. Therefore, voltage converters are required to provide supply-voltages that are sufficiently high to power microelectronics. However, for high conversion efficiency, voltage converters need to be optimized for the limited input voltage range and the typically high internal resistance of microTEGs. To overcome the limitations of conventional voltage converters we present an optimized self-startup voltage converter with dynamic maximum power point tracking. The performance potential of our concept is theoretically and experimentally analyzed. The voltage conversion interface demonstrates energy harvesting from open-circuit voltages as low as 30.7 mV, and enables independent and full start-up from 131 mV. No additional external power supply is required at any time during operation. It can be operated with a wide range of internal resistances from 20.6 to − 4 kΩ with a conversation efficiency between η = 68–79%.

Power generation by integrating a thermally regenerative electrochemical cycle (TREC) with a biofuel stove

This paper proposes a system that aims to extract the wasted energy from biofuel stoves using a thermally regenerative electrochemical cycle (TREC). While the biofuel stove heats the space, the wasted heat will be extracted to heat the TREC device. A metal heat sink is used to cool down the TREC device. A theoretical model was developed and validated for the first time for the integrated system to test three types of fuel (Wood, peat, and manure) and three different cooling types (natural convection, forced convection, and evaporative cooling). Based on the parametric study, the net electrical power produced and the efficiency of the system depend on fuel type and mass flow rate, water inlet temperature and mass flow rate, the cooling type used in the heat sink, and the ambient temperature. The efficiency of the TREC varies 0.13%–7.48% based on the operating conditions. A comparison between the new TREC device and the conventional thermoelectric generator (TEG) to extract the wasted heat from the stove is carried out under the same operating conditions. Using the TREC is three times more efficient than TEG. The system represents a simple and low-cost suitable solution for the energy problem in remote areas.

All-in-one single-piece flexible solar thermoelectric generator with scissored heat rectifying p-n modules

Junctions existing in conventional thermoelectric generators (TEGs) may lower their output power by increasing the internal resistances and reduce their flexibility, especially for organic TEGs. In this paper, we reported a new junction free all-in-one single-piece (SP) solar thermoelectric generator which was scissored from a free-standing carbon nanotube thin film with patterned p/n modules. In addition, a heat-rectifying structure has been designed to increase the temperature gradient between the hot/cold side of the TEG. A high sunlight-to-electricity output power of 1709.2 nW was obtained for the SP device, which is about 4.5 times higher than that of the conventional-structure TEG (Con-TEG). The highest output power density of this SP device is about 83 times higher than that of the Con-TEG because this fully organic SP device does not contain heavy metal moieties as connectors that typically appear in the Con-TEGs. Moreover, the junction-free SP device exhibited excellent flexibility which can be bent for 1000 times without noticeable decay and fluctuations in its conductance. While the Con-TEG cracked after being bent for only 100 times. The results indicate a promising way for developing high performance flexible organic thermoelectric devices.

Effect of exhaust parameters on performance of intermediate fluid thermoelectric generator

A thermoelectric generator generates electricity by effectively recovering the waste heat from the exhaust of automobiles. The intermediate fluid thermoelectric generator transfer exhaust heat by the gasification and condensation of the medium, which can improve the power output and reduce the optimal module area, thus significantly increasing the power density. To study the influence of exhaust parameters on the performance of new generator, a mathematical model of the thermoelectric generator is used to analyze the effects of exhaust parameters on the system performance. Compared with the conventional thermoelectric generator, the novel thermoelectric generator becomes more advantageous with a lower exhaust heat transfer coefficient. And it is found that the key parameter affecting the performance is the thermal conduction of the exhaust heat exchanger. The exhaust temperature has no effect on the optimal module area, but the maximum output power increases almost linearly with the increase of exhaust temperature. With the increase of exhaust flow, the optimal module area increases linearly under the condition of sufficient heat exchange of exhaust. The results of this study have certain significance for fully understanding the operation principle and guiding the optimal design of IFTEG.

Comprehensive modelling framework for a low temperature gradient thermoelectric generator

Thermoelectric energy harvesting (TEH) is a promising technology enabling the self-sustainable operation of ultra low-power sensing devices performing monitoring tasks. Conventional TEH research mainly focuses on the improvements of the material properties (i.e thermoelectric figure of merit ZT) and thermoelectric module (TEM) geometry. However, enhanced performance of the thermoelectric generator (TEG) can not be practically achieved without optimal integration of the novel TEM with the DC-DC converter. The power converter, having its own input impedance requirements and conversion efficiency curve, may keep the operation point of TEG far from the maximum power point of TEM, leading to power loss. Furthermore, since the physical processes in the thermoelectric transducer are coupled with the electrical processes in the DCDC converter, both components should be analyzed together to achieve the best harvesting performance. Given the system complexity, an accurate simulation model is required to estimate the output power and make an optimal system design. We report on a simulation approach for low temperature gradient TEG that accounts for both the thermoelectric and the power conversion processes. The proposed approach was validated on the TEG testbed by imitating the realistic ambient conditions for thermoelectric transducer connected to a commercial off-the-shelf DC-DC converter. Experimental results are characterized by 9% mean percentage error of power estimation which outperforms the methods based on conventional TEG representations.

On the concept of an effective temperature Seebeck ratchet

In disordered semiconductors, finite electric fields can heat up the charge carrier distribution to effective temperatures that can significantly exceed the lattice temperature. Here, we demonstrate that such effective temperatures can be utilized to drive a thermoelectric generator (TEG). We use this concept in a variant of the Seebeck ratchet, which was originally introduced by Büttiker, in which out-of-phase spatial modulations of the electrostatic potential and the (effective) temperature cause a DC current. In contrast to conventional TEGs, this design utilizes only a single doped semiconductor and works in a thin film geometry. We demonstrate the concept using kinetic Monte Carlo simulations and show that the proposed concept can outperform conventional TEGs of the same material. We provide an analytical model that semiquantitatively reproduces the numerical results. Finally, we propose how such a device might be realized.

Numerical Study on Optimizing the Geometry of Segmented Asymmetrical Thermoelectric Generator

Quite large amount of heat generated in various industrial processes was wasted. Recovering the waste heat can save cost and benefit the environment protection. The device that convert heat energy to electricity is called thermoelectric generator (TEG), and therefore it can be used for waste heat recovery. Improving the efficiency is the main direction of present study. The configuration and geometry of the thermoelectric legs are proved can influence the thermoelectric performance of the thermoelectric generator. Compared with the conventional TEG which is assembled with symmetrical (rectangular) legs, asymmetrical thermoelectric generator (ATEG) has a greater temperature gradient in a leg due to the convergent geometry which can reduce overall thermal conduction of the device and Thomson effect is also harnessed. In this study, three-dimensional finite element analysis (COMSOL) is employed to investigate the numerical simulations of a segmented pyramidal thermoelectric generator (SPTEG) and a segmented cone thermoelectric generator (SCTEG) to optimize the leg length ratio of two materials, and it’s effect on the electrical and mechanical performances of SPTEG and SCTEG was also studied. Results obtained shows that the height ratio of SATEG can influence electrical and mechanical performances and the optimum height ratio provided a better electrical performance while thermal stress developed in leg was less. In addition, the open-circuit voltage of two types SATEG is similar because they have the same cross sectional area and the same volume, however, SCTEG has a dispersive thermal stress distribution while the thermal stress of SPTEG concentrates in four corners. Compared to the maximum von Mises stress in SPTEG, the maximum von Mises stress in SCTEG reduced about 10%. Results obtained from this study would provide references in producing and design of segmented asymmetrical thermoelectric generators.