Thermoelectric Property Dependence on Performance of Peltier Current Leads Under Overcurrent Conditions

Superconductivity can potentially provide a solution to the world??s energy needs because superconducting transmission and distribution (T&D) systems can decrease losses and are also capable of integrating renewables into the power grid. At Chubu University we have built a 200-m-class superconducting direct-current T&D system (CASER-2). To minimize heat leakage from the current leads, we investigated thermoelectric materials. The Peltier current lead (PCL) is one of the key technologies that will enhance the performance of superconducting systems: as direct current (DC) flows through the current lead, thermoelectric elements on opposite terminations of the superconducting line can be used to decrease the heat ingress to the cryogenic environment (n-type on one end, p-type on the opposite end). The heat leakage to the cryogenic environment depends on the properties of the thermoelectric materials. In this paper, we estimate the performance of PCLs in cryogenic operations, including the potential for overcurrent operation, through both modeling and experiments at CASER-2.     

On the optimal design of gas-cooled Peltier current leads

We perform the optimal design of gas-cooled Peltier current leads (PCLs), such that their resulting heat leaking into superconducting magnets can be minimized. Superior to previous investigations, the effect of gas cooling on both the Cu lead and the thermoelectric element in a PCL is taken into consideration. Analytical temperature distribution is derived as well as the concerned heat leaking into superconducting magnets. Numerically iterative calculations are, therefore, avoided. Moreover, temperature-entropy diagrams are constructed to distinguish conventional all-Cu leads from PCLs with and without gas cooling. Both the heat leak and the input power of current leads can be easily identified from the areas subtended by the isotherms on the simple two-dimensional temperature-entropy plane.     

Electric Current Dependence of a Self-Cooling Device Consisting of Silicon Wafers Connected to a Power MOSFET

A self-cooling device has been developed by combining a commercial n-channel power metal–oxide–semiconductor field-effect transistor (MOSFET) and single-crystalline Sb-doped n-type or B-doped p-type silicon wafers in order to improve the heat removal or cooling quantitatively. The electric current dependence of the temperature distribution in the self-cooling device and the voltage between the source and drain electrodes have been measured to estimate the Peltier heat flux. We found that the average temperature is decreased for a power MOSFET in which an electric current of 50 A flows. In particular, the average temperature of the power MOSFET was decreased by 2.7°C with the n-type Si wafer and by 3.5°C with the p-type Si wafer, although an electric current of 40 A makes little difference. This certainly warrants further work with improved measurement conditions. Nonetheless, the results strongly indicate that such n-type or p-type silicon wafers are candidate materials for use in self-cooling devices.     

Silicon-Rich Higher Manganese Silicides for Thermoelectric Applications

Thermoelectrics are widely explored as a renewable and environmentally friendly method for converting waste heat to electrical power. Higher manganese silicides (HMS) have been identified as promising, nontoxic, inexpensive, p-type materials for thermoelectric applications. To mass-produce practical thermoelectric converters, an inexpensive, effective, and simple production method should be applied for these materials. In the frame of the current research, HMS have been synthesized using furnace melting followed by powdering and spark plasma sintering. Highly dense samples were obtained and measured for their thermoelectric properties. The samples were also characterized using x-ray diffraction, scanning electron microscopy, and Hall-effect measurements. Homogeneous samples were obtained with small inclusions of Si, reaching a figure of merit of about 0.6 at 450°C.     

Thermoelectric Performance Optimization in p-Type Ce y Fe3CoSb12 Skutterudites

In this study, we investigated the impact of the Ce filling fraction on the thermoelectric properties of p-type filled skutterudites Ce _y Fe₃CoSb₁₂ (y = 0.6 to 1.0). The electrical conductivity decreases gradually with increasing y, while the Seebeck coefficient displays an opposite variation tendency, consistent with the expected electron donor role of the Ce filler in this compound. The overall power factors are invariable among all the samples. Alteration of the Ce filling fraction exerts little influence on the phonon transport, but the total thermal conductivity markedly declined with increasing y due to the reduced contribution to heat transfer from carriers. As a consequence, the maximum thermoelectric figure of merit ZT reaches ～0.8 for the sample with y = 0.9, comparable to that of pure Fe-based skutterudite CeFe₄Sb₁₂; more importantly, the former possesses a much larger average ZT between 300 K and 800 K than the latter, showing superior potential for use in intermediate-temperature thermoelectric power generation applications. Further enhancement of ZT in p-type Fe₃CoSb₁₂-based skutterudites could be realized via nanostructuring or a multiple-filling approach.     

Thermoelectric Modules Based on Half-Heusler Materials Produced in Large Quantities

Half-Heusler (HH) compounds are some of the most promising candidates among the medium-temperature thermoelectric materials being investigated for automotive and industrial waste heat recovery applications. For n- as well as p-type material, peak ZT values larger than one have been published recently, and first modules have been built. The next step to facilitate the industrialization of thermoelectric module production is upscaling of material synthesis. In this paper, the latest results of the thermoelectric properties of HH compounds produced in kg batches are presented and compared with values published in the literature. The performance of modules built from these materials is analyzed with respect to power output and long-term stability of the material and electrical contacts.     

Model of Heat Exchangers for Waste Heat Recovery from Diesel Engine Exhaust for Thermoelectric Power Generation

The performance and operating characteristics of a hypothetical thermoelectric generator system designed to extract waste heat from the exhaust of a medium-duty turbocharged diesel engine were modeled. The finite-difference model consisted of two integrated submodels: a heat exchanger model and a thermoelectric device model. The heat exchanger model specified a rectangular cross-sectional geometry with liquid coolant on the cold side, and accounted for the difference between the heat transfer rate from the exhaust and that to the coolant. With the spatial variation of the thermoelectric properties accounted for, the thermoelectric device model calculated the hot-side and cold-side heat flux for the temperature boundary conditions given for the thermoelectric elements, iterating until temperature and heat flux boundary conditions satisfied the convection conditions for both exhaust and coolant, and heat transfer in the thermoelectric device. A downhill simplex method was used to optimize the parameters that affected the electrical power output, including the thermoelectric leg height, thermoelectric n-type to p-type leg area ratio, thermoelectric leg area to void area ratio, load electrical resistance, exhaust duct height, coolant duct height, fin spacing in the exhaust duct, location in the engine exhaust system, and number of flow paths within the constrained package volume. The calculation results showed that the configuration with 32 straight fins was optimal across the 30-cm-wide duct for the case of a single duct with total height of 5.5?cm. In addition, three counterflow parallel ducts or flow paths were found to be an optimum number for the given size constraint of 5.5?cm total height, and parallel ducts with counterflow were a better configuration than serpentine flow. Based on the reported thermoelectric properties of MnSi_1.75 and Mg₂Si_0.5Sn_0.5, the maximum net electrical power achieved for the three parallel flow paths in a counterflow arrangement was 1.06?kW for package volume of 16.5?L and exhaust flow enthalpy flux of 122?kW.     

Physical,Mechanical, and Structural Properties of Highly Efficient Nanostructured n- and p-Silicides for Practical Thermoelectric Applications

Cost-effective highly efficient nanostructured n-type Mg₂Si_1?x Sn _x and p-type higher manganese silicide (HMS) compositions were prepared for the development of practical waste heat generators for automotive and marine thermoelectric applications, in the frame of the European Commission (EC)-funded PowerDriver project. The physical, mechanical, and structural properties were fully characterized as part of a database-generation exercise required for the thermoelectric converter design. A combination of high maximal ZT values of ～0.6 and ～1.1 for the HMS and Mg₂Si_1?x Sn _x compositions, respectively, and adequate mechanical properties was obtained.     

Optimization of termination for a high-temperature superconductingcable with a room temperature dielectric design

This paper describes the design considerations of a termination for a superconducting cable, based on tapes produced with the powder-in-tube method, with a room temperature dielectric design. Most important is the optimization of the current lead that leads the current from room temperature to cryogenic temperature. The current lead is optimized, using analytical as well as numerical methods. The paper proposes a current lead made of copper, with a constant cross-section area. With an optimized length-to-cross-section area ratio, the heat flow to the cold region is 43 W/kA for an uncooled current lead and 20 W/kA for a cooled current lead. The minimum loss in the entire termination is approximately 60 W/kA for a termination optimized for 2 kA. The paper describes why a gas-cooled current lead only reduces the total losses when used in connection with a multistep cooling machine     

Transport-Coefficient Dependence of Current-Induced Cooling Effect in a Two-Dimensional Electron Gas

The dependence of the current-induced cooling effect on the electron mobility??? _e is explored for a two-dimensional electron gas (2DEG) subjected to a perpendicular magnetic field. We calculate the distributions of the electrochemical potentials and the temperatures under a magnetic field, fully taking account of thermoelectric and thermomagnetic phenomena. Whereas the electrochemical potential and the electric current remain qualitatively unchanged, the temperature distribution exhibits drastic mobility dependence. The lower-mobility system has cold and hot areas at opposite corners, which results from the heat current brought about by the Ettingshausen effect in the vicinity of the adiabatic boundaries. The cooling effect is intensified by an increase in??? _e. Intriguingly, the cold and hot areas change places with each other as the mobility??? _e is further increased. This is because the heating current on the adiabatic edges due to the Righi?CLeduc effect exceeds that due to the Ettingshausen effect in the opposite direction.     

Device-Level Optimization of n-Type Mg3(Sb,Bi)2-Based Thermoelectric Modules toward Applications: A Perspective

In recent decades, improvements in thermoelectric material performance have made it more practical to generate electricity from waste heat and to use solid-state devices for refrigeration. However, despite the development of successful strategies to enhance the figure-of-merit zT, optimizing devices for large-scale applications remains challenging. High zT values do not guarantee excellent device performance, and maintaining high zT over a wide temperature range is difficult. Thus, device-level structural optimization is crucial for maximizing overall energy conversion efficiency. Proper interfacial and structure design strategies, including contact layer selection, multi-stage optimization, and size matching for the n- and p-type thermoelectric legs, are necessary for advancing device performance. Additionally, thermal stability issues, device assembly techniques, mechanical properties, and manufacturing costs are crucial considerations for large-scale applications. To achieve actual applications, the thermoelectric community must look beyond simply aiming for high zT values. This article focuses on modules based on n-type Mg₃(Sb, Bi)₂, one of the most promising commercially available thermoelectric materials, and discusses the influence of various parameters on the modules and on the corresponding device-level optimization strategies.     

Scavenging Elemental Sb Through Addition of NbSb2 to Mm0.9Fe3.5Co0.5Sb12 Skutterudites

Our previous work identified thermal instability of p-type skutterudite as a primary cause of degradation during thermoelectric generator operation at 650°C. Residual Sb in the microstructure was believed to be the cause of diminished thermal stability, especially above the melting point of Sb (631°C). This work investigated addition of Nb to p-type skutterudite to from NbSb₂, thus scavenging elemental Sb. The results of this work are reported, along with comprehensive thermoelectric property characterization of the p-type skutterudite?+?NbSb₂ composites.     

The Effects of Polysilastyrene and Au Additions on the Thermoelectric Properties of <Emphasis Type="Italic">β</Emphasis>-SiC/Si Composites

Recently, Yamaguchi et al. proposed a self-cooling device that does not require additional power circuits for cooling because it is Peltier-cooled using its own current in conjunction with a thermoelectric material. Silicon carbide is a promising thermoelectric material for this technology since its electrical conductivity, thermal conductivity, and Seebeck coefficient are higher than those of conventional thermoelectric materials. This study investigates the effects of polysilastyrene and Au additions on the thermoelectric properties of p-type β-SiC/Si polycrystalline semiconductor composites in order to assess whether their addition improves the performance of self-cooling devices.     

Thin-Film Thermoelectric Modules for Power Generation Using Focused Solar Light

We demonstrated the fabrication of thin-film thermoelectric generators and evaluated their generation properties using solar light as a thermal source. Thin-film elements of Bi_0.5Sb_1.5Te₃ (p-type) and Bi₂Te_2.7Se_0.3 (n-type), which were patterned using the lift-off technique, were deposited on glass substrates using radiofrequency magnetron sputtering. After annealing at 300°C, the average Seebeck coefficients of p- and n-type films were 150???V/K and ?104???V/K, respectively, at 50°C to 75°C. A cylindrical lens was used to focus solar light to a line shape onto the hot side of the thin-film thermoelectric module with 15 p?Cn junctions. The minimum width of line-shaped solar light was 0.8?mm with solar concentration of 12.5 suns. We studied the properties of thermoelectric modules with different-sized p?Cn junctions on the hot side, and obtained maximum open voltage and power values of 140?mV and 0.7???W, respectively, for a module with 0.5-mm p?Cn junctions. The conversion efficiency was 8.75?×?10^?4%, which was approximately equal to the value estimated by the finite-element method.     

Micro- and Nano-Technology: A Critical Design Key in Advanced Thermoelectric Cooling Systems

Advanced thermoelectric (TE) cooling technologies are now receiving more research attention, to provide cooling in advanced vehicles and residential systems to assist in increasing overall system energy efficiency and reduce the impact of greenhouse gases from leakage by current R-134a systems. This work explores the systems-related impacts, barriers, and challenges of using micro-technology solutions integrated with advances in nano-scale thermoelectric materials in advanced TE cooling systems. Integrated system-level analyses that simultaneously account for thermal energy transport into and dissipation out of the TE device, environmental effects, temperature- dependent TE and thermo-physical properties, thermal losses, and thermal and electrical contact resistances are presented, to establish accurate optimum system designs using both p-type nanocrystalline-powder-based (NPB) Bi _x Sb_2−x Te₃/n-type Bi₂Te₃-Bi₂Se₃ TE systems and conventional p-type Bi₂Te₃-Sb₂Te₃/n-type Bi₂Te₃-Bi₂Se₃ TE systems. This work established the design trends and identified optimum design regimes and metrics for these types of systems that will minimize system mass, volume, and cost to maximize their commercialization potential in vehicular and residential applications. The relationships between important design metrics, such as coefficient of performance, specific cooling capacity, and cooling heat flux requirements, upper limits, and critical differences in these metrics in p-type NPB Bi _x Sb_2−x Te₃/ n-type Bi₂Te₃-Bi₂Se₃ TE systems and p-type Bi₂Te₃-Sb₂Te₃/n-type Bi₂Te₃-Bi₂Se₃ TE systems, are explored and quantified. Finally, the work discusses the critical role that micro-technologies and nano-technologies can play in enabling miniature TE cooling systems in advanced vehicle and residential applications and gives some key relevant examples. Pacific Northwest National Laboratory—operated for the U.S. Department of Energy by Battelle Memorial Institute under contract DE-AC05-76RLO1830.     

Preparation of Ring-Shaped Thermoelectric Legs from PbTe Powders for Tubular Thermoelectric Modules

Waste heat recovery—for example, in automotive applications—is a major field for thermoelectric research and future application. Commercially available thermoelectric modules are based on planar structures, whereas tubular modules may have advantages for integration and performance in the field of automotive waste heat recovery. One major drawback of tubular generator designs is the necessity for ring-shaped legs made from thermoelectric material. Cutting these geometries from sintered tablets leads to considerable loss of thermoelectric material and therefore high cost. Direct sintering of ring-shaped legs or tubes of thermoelectric material is a solution to this problem. However, sintering such rings with high homogeneity and density faces some difficulties related to the mechanical properties of typical thermoelectric materials such as lead telluride (PbTe)—particularly brittleness and high coefficient of thermal expansion. This work shows a process for production of thermoelectric rings made of p- and n-doped PbTe. Long tubes of PbTe have been sintered in a current-assisted sintering process with specially designed sintering molds, coated with a diffusion barrier, and finally cut into ring-shaped slices. To demonstrate the technology, a tubular thermoelectric module has been assembled using these PbTe rings.     

Resistance and thermoelectric power of carbon fibers upon changing the conductivity type

Upon being subjected to pulsed current, carbon fiber changes the sign of its thermoelectric power from negative to positive. The conductivity activation energy for the initial n-type fiber is 60 meV, and that for a fiber modified to p-type is 16 meV. Fabric constituted by carbon fibers with a set of p–n junctions does not require switching plates. In vacuum or in an inert atmosphere, carbon fibers withstand heating up to 3500 K, which opens up the possibility for developing thermoelectric generators that operate at high temperature gradients.     

Thermoelectric Property Dependence and Geometry Optimization of Peltier Current Leads Using Highly Electrically Conductive Thermoelectric Materials

Thermoelectric materials are promising candidates for use in energy-saving devices in many fields. They are also useful in superconducting applications such as those using Peltier current leads (PCLs) to reduce system heat loss. In the case of PCLs, consideration must be given to Joule heating. Furthermore, the performance of PCLs is intricately dependent on their thermoelectric properties. In addition to the figure of merit Z, consideration of the electrical conductivity is also important for the design of high-performance PCLs. In this paper, we discuss the resistivity dependence of the performance of PCLs using model parameters obtained from real devices.     

Thermoelectric and other phenomena in structures with nonequilibrium charge carriers and nanoparticles

The results of investigations of thermoelectric and other physical phenomena in semiconductor structures with n-p junctions are presented for the conducting direction. The direction of the heat transport is opposite to that observed for the “conventional” Peltier effect, and its magnitude is much higher. The possible parameters of thermoelectric cooling devices based on this effect are estimated. The sizes of elementary structural units in condensed phases are estimated. The thermoelectric, electrogravitational, and other phenomena in substances, the charge carriers in which are multimolecular nanocomplexes—the other-phase nuclei, are described.     

Fabrication of Bismuth Telluride-Based Alloy Thin Film Thermoelectric Devices Grown by Metal Organic Chemical Vapor Deposition

Bismuth–antimony–telluride based thin film materials were grown by metal organic vapor phase deposition (MOCVD). A planar-type thermoelectric device was fabricated with p-type Bi_0.4Sb_1.6Te₃ and n-type Bi₂Te₃ thin films. The generator consisted of 20 pairs of p-type and n-type legs. We demonstrated complex structures of different conduction types of thermoelectric elements on the same substrate using two separate deposition runs of p-type and n-type thermoelectric materials. To demonstrate power generation, we heated one side of the sample with a heating block and measured the voltage output. An estimated power of 1.3 μW was obtained for the temperature difference of 45 K. We provide a promising procedure for fabricating thin film thermoelectric generators by using MOCVD grown thermoelectric materials that may have a nanostructure with high thermoelectric properties.