Recent developments in thermoelectric materials

The increased activity in attempts to develop improved thermoelectric semiconductors for use in the direct conversion of heat into electrical energy results mostly from research sponsorship by the US Military and NASA. Thermoelectric generators have no moving parts and are difficult to detect by visual, aural or thermal infrared means. Fossil multifuelled thermoelectric generators are the leading candidates for replacing standard US Military engine generator sets up to 1·5 kW under the SLEEP programme (Signature Suppressed Lightweight Electric Energy Plants). When coupled to an isotopic heat source, thermoelectric generators are able to operate reliably and unattended for long periods of time and have a proven performance record in supplying electrical power to the Lunar Experimental Package (Apollo Program) and in providing onboard electrical power to the Voyager spacecrafts.In both military and space applications any improvement in the thermoelectric generators' conversion efficiency would result in a saving in fuel—an important consideration. One way of improving the conversion efficiency is by increasing the so called ‘Figure of merit’ of the semiconductor material employed in the fabrication of the generators' thermocouples. In this paper an assessment is made of current thermoelectric materials; recent attempts to improve the figure of merit of existing materials are discussed and a number of new thermoelectric materials described.Significant headway has been made in reducing the lattice thermal conductivity of thermoelectric materials through the use of additives, small grain sizes or combinations of both. This development will result in substantial improvements in the thermoelectric figure of merit, provided the electrical properties can be maintained close to single crystal values. It is concluded that, because in the past the development of new thermoelectric materials has occupied long periods of time, even during periods of intense research activity, it is likely that established or ‘modified’ established materials will remain the mainstay of military and space applications at least for the forseeable future.     

非均质温差发电器的性能分析

建立非均质温差发电器(TEG)理论模型,考虑热电材料的非均质导热系数以及温差发电器与热源间的传热热阻的影响,分析非均质温差发电器的一般性能.讨论热电元件对数、热导率、高温热源温度对非均质温差发电器性能特性的影响.结果表明,相较于均质温差发电器,导热系数不均匀强度越大,非均质温差发电器的最大输出功率和最大效率越高;热电元...     

Optimization of Heat Sink–Limited Thermoelectric Generators

Many recent advances in thermoelectrics have focused on the nanoscale engineering of materials for higher figure of merit (Z). A thermoelectric generator using these thin-film materials can present new challenges due to its inherently large temperature gradient, but also correspondingly larger generated power if the heat can be managed. In such cases performance is expected to be limited as much by the heat sink as by intrinsic material properties. New criteria for optimizing the generated power density of devices in this regime are discussed here The effects of future material improvements on performance are studied, with the surprising result that optimizing material Z is not the best strategy for optimizing efficiency or power in this regime. The theory is tested with a numerical solution of the Onsager relations.     

Modeling and optimization of hybrid solar thermoelectric systems with thermosyphons

We present the modeling and optimization of a new hybrid solar thermoelectric (HSTE) system which uses a thermosyphon to passively transfer heat to a bottoming cycle for various applications. A parabolic trough mirror concentrates solar energy onto a selective surface coated thermoelectric to produce electrical power. Meanwhile, a thermosyphon adjacent to the back side of the thermoelectric maintains the temperature of the cold junction and carries the remaining thermal energy to a bottoming cycle. Bismuth telluride, lead telluride, and silicon germanium thermoelectrics were studied with copper–water, stainless steel–mercury, and nickel–liquid potassium thermosyphon-working fluid combinations. An energy-based model of the HSTE system with a thermal resistance network was developed to determine overall performance. In addition, the HSTE system efficiency was investigated for temperatures of 300–1200 K, solar concentrations of 1–100 suns, and different thermosyphon and thermoelectric materials with a geometry resembling an evacuated tube solar collector. Optimizations of the HSTE show ideal system efficiencies as high as 52.6% can be achieved at solar concentrations of 100 suns and bottoming cycle temperatures of 776 K. For solar concentrations less than 4 suns, systems with thermosyphon wall thermal conductivities as low as 1.2 W/mK have comparable efficiencies to that of high conductivity material thermosyphons, i.e. copper, which suggests that lower cost materials including glass can be used. This work provides guidelines for the design, as well as the optimization and selection of thermoelectric and thermosyphon components for future high performance HSTE systems.     

Segmented thermoelectric generator: exponential area variation in leg

The innovative design of segmented thermoelectric generator with exponential area variation is introduced. Thermal efficiency and power output are assessed for various values of the design parameter (a = (L/x) ln[A_a/A(x)], where A_a is constant, and a is the dimensionless geometric parameter, L is the pin length, and A(x) is the pin cross‐sectional area), external load parameter (R_L/R₀, ratio of external electrical resistance to reference electrical resistance), and temperature parameter (θ = T_low/T_high, ratio of cold junction temperature to high junction temperature). The device efficiency obtained is validated through the previous experimental data for various hot and cold junction temperature differences. The findings reveal that thermal efficiency resulted from the current study agrees well with the experimental data. The innovative design of the segmented thermoelectric generator with exponentially decaying pin configuration enhances the thermal efficiency and output power as compared with the device having a single material pin configuration. Increasing temperature ratio results in the reduction in the thermal efficiency and the output power of thermoelectric generator. In addition, lowering the external load parameter improves the thermal efficiency and the output power of the thermoelectric device. The design parameter that maximizes the thermal efficiency of the thermoelectric generator does not maximize the device output power.     

氧化钛基半导体热电材料的研究进展

热电材料—即实现热能和电能之间直接相互转换的一类功能材料,提供了一种制冷或发电的新方法—在解决能源和环境危机问题上正在扮演越来越重要的角色。传统的三维材料中,由于几个决定热电性能的关键物理参数相互关联,使得现有热电材料很难获得较高热电优值（ZT）。金属氧化物热电材料由于其良好的耐高温性能,是中高温区使用的理想候选者。如果能提高氧化钛基化合物的热电优值,那么氧化钛基化合物将是一类非常优秀的热电材料,因为其不仅具有优良的化学稳定性和热稳定性,而且原材料丰富、不含有毒元素以及制备工艺简单。纳米化能显著降低材料的热导率,是最近二十年提高热电性能的一条主要途径。同时,通过界面和化学组成调控增加与电学性能相关的功率因子也是一种继续提高热电性能的重要方法。本文综述了我们近期对氧化钛基热电材料的研究成果,包括对钛酸盐纳米管较大赛贝克（Seebeck）系数的实验发现,提出利用一维纳米材料独特的空心结构和纳米管层状特殊构造,将两个相关联的物理参数（热导率和电导率）分别调控;通过合成氧化钛基纳米复合材料,研究界面对载流子和声子散射的作用,提出通过载流子能量过滤效应提高其热电性能;采用尿素燃烧法和高温烧结等方法合成具有纳米结构和化学组成调控的氧化钛基化合物,认识化学组成以及界面对声电输运的作用规律;最后介绍能显著提高热电材料功率因子的载流子非对称迁移的理论。     

The influence of operating and device parameters on the maximum efficiency and the maximum output power of thermoelectric generator

Thermoelectric power generators are one of the promising green energy sources. The operating and the generator parameters influence the generator output performance. In the present study, the influence of the operating and the generator parameters on the maximum output power and the efficiency of the thermoelectric power generator are examined. The output power corresponding to the maximum efficiency and the maximum attainable output power of the generator are compared. It is found that the maximum power of the thermoelectric generator corresponding to the high Figure of Merit is very sensitive to the operating temperature. The maximum power attainable is larger than that its counterpart corresponding to the maximum generator efficiency. Copyright © 2010 John Wiley & Sons, Ltd.     

Developments in semiconductor thermoelectric materials

A surge in interest in developing alternative renewable energy technologies has been observed in recent years. In particular, thermoelectrics has drawn attention because thermoelectric effects enable direct conversion between thermal and electrical energy, and provide power generation and refrigeration alternatives. During the past decade, the performance of thermoelectric materials has been considerably improved; however, many challenges continue to exist. Developing thermoelectric materials with superior performance means tailoring interconnected thermoelectric physical parameters-electrical conductivities, Seebeck coefficients, and thermal conductivities for a crystalline system. The objectives of this paper are to introduce the recent developments in semiconductor thermoelectric materials, and briefly summarize the applications of such materials.     

Evaluation of efficiency factors and internal resistance of thermoelectric materials

It is well known that the figure of merit (ZT) is unreliable in calculating the efficiency (?) of micro thermoelectric generators system level and unrealistic when comparing the performance of thermoelectric (TE) materials in the same metric units. To solve this problem, we have used COMSOL multiphysics to design a single leg of micro thermoelectric generators model for computing efficiency factors (? ) and internal resistance using TE materials' constants, such as electrical conductivity (σ ), TE conductivity (K ), and Seebeck coefficient (α ). The TE materials were placed between two copper electrodes, and the first data analyzed were the voltages per meter and electric currents per meter. The internal resistances were calculated by taking the ration of voltages to electric currents, and at the same time, the electric powers were calculated from the products of electric currents and voltages yielding power per unit area in μW cm^?2. The ? were calculated using changes in power (ΔP ), temperature gradient (ΔT ), and the surface area (A ). The obtained results showed that the TE materials with highest ? when the temperatures are between 375 and 550 K are n‐type SiGe and p‐type SiGe. When the temperatures are between 550 and 780 K, the TE materials with the highest ? are PbTe‐Pbl₂, PbTe‐CdTe, and PbTe‐SrTe‐Na. We noted that the ? obtained from eight TE materials in this work are within the range as those reported in the literature between 0.001 and 0.091 μW cm^?2 K^?2. The TE materials with high internal resistances such as PbS, PbTe, and PbSe have ? that is <0.0001 μW cm^?2 K^?2, and those with low internal resistances have ? in the range between 0.002 and 0.0091 μW cm^?2 K^?2. This work has shown that COMSOL multiphysics is a powerful computational tool that can be used to analyze internal resistances and ? of TE materials in the same temperature ranges. Copyright © 2016 John Wiley & Sons, Ltd.     

Comparative performance and thermoeconomic analyses of high temperature polymer electrolyte membrane based two hybrid systems

The main objective of this study is to compare the two systems in terms of the thermoeconomic and the performance. The first one is called hybrid I and consists of high temperature polymer electrolyte membrane and thermocapacitive cycle. The second one is named hybrid II, which is composed of high temperature polymer electrolyte membrane and thermoelectric generator. Thermocapacitive cycle and thermoelectric generator have various advantages, such as generally lower cost and higher power density. So, they have good potential to utilize waste heat. The performance parameters of the considered hybrid systems include power density, energy efficiency, exergy efficiency and exergy destruction rate. The results have shown that hybrid I is more advantageous than hybrid II. The maximum power density values for hybrid I and hybrid II are obtained to be 2536.91W and 2049.62W while their energy efficiencies are 77.4% and 76.8%, respectively.     

High temperature solar thermoelectric generator

The performance of a sintered SiGe thermoelectric generator is assessed by computing power output per unit area and the efficiency of thermoelectric conversion. Previously developed high temperature solar energy absorbers were used. The power output and efficiency are considerably improved by using a water- or vapour-cooled heat sink in place of a radiative heat sink. The power density in the thermoelectric generator compares well with those in photovoltaic heterojunction cells using concentrated solar energy.     

Parametric study of asymmetric thermoelectric devices for power generation

Thermoelectric devices are considered a promising technique for recycling waste heat. In the present work, a three-dimensional numerical model is developed to study the output performance of thermoelectric devices. A comprehensive analysis is performed based on a conventional π-type thermoelectric couple. The results indicate that the maximum power of thermoelectric devices generally increases with a decrease in height and an increase in cross-sectional area; the maximum efficiency exhibits the opposite trends. The best way to reduce heat losses is by using ceramic plates with higher thermal conductivity. Moreover, the parasitic internal resistance exists in the thermoelements, and its influencing factors are studied. To minimize electric losses, an asymmetric structure is proposed for thermoelectric devices. The results exhibit that the optimal cross-sectional area ratio of the p-type and n-type legs (S_p/S_n) is mainly contingent upon the thermoelectric material parameters; the greater the differences in the parameters of p-type and n-type thermoelectric materials, the greater the gains provided by the asymmetric structure. Furthermore, the experimental data present great consistency with the numerical results. The research results may help guide the design of thermoelectric devices with relatively lower power losses.     

High-temperature thermoelectric energy conversion—I. Theory

The theory of high-temperature thermoelectric energy conversion is reviewed. Semiconductors are shown to be the preferred class of materials for this application. Optimization of the figure of merit of both broad- and narrow-band semiconductors is discussed as a function of temperature. Phonon scattering mechanisms are discussed, and basic material guidelines are given for reduction of thermal conductivity. Radiative transfer of thermal energy does not appear to be a problem in thermoelectric materials operating below 2000°C. There appears to be no theoretical limit to the achievement of high ZT values.     

Configuration of segmented leg for the enhanced performance of segmented thermoelectric generator

Thermal analysis of a segmented thermoelectric generator is performed, and the segmented leg configurations maximizing the efficiency and the output power are formulated. The effect of operating conditions such as external load resistance, the temperatures of hot and cold junctions, on the device performance is studied. The segmented thermoelectric generator has the leg configuration consisting of the combination of modified lead telluride and modified bismuth telluride. The segmented thermoelectric generator performance, such as device efficiency and output power, is compared with those corresponding to a single material leg configuration (modified lead telluride or modified bismuth telluride) for various operating conditions. It is found that a unique value of the segmented leg combination maximizes the efficiency and the output power for each operating condition. The variation in the operating conditions changed the locus points of the maximum efficiency and the maximum output power. The segmented thermoelectric generator gives rise to the higher device efficiency and the output power than those of the single material leg configuration, especially for the low external load resistance. Copyright © 2016 John Wiley & Sons, Ltd.     

Thermal transport in organic/inorganic composites

Composite materials, which consist of organic and inorganic components, are widely used in various fields because of their excellent mechanical properties, resistance to corrosion, low-cost fabrication, etc. Thermal properties of organic/inorganic composites play a crucial role in some applications such as thermal interface materials for micro-electronic packaging, nano-porous materials for sensor development, thermal insulators for aerospace, and high-performance thermoelectric materials for power generation and refrigeration. In the past few years, many studies have been conducted to reveal the physical mechanism of thermal transport in organic/inorganic composite materials in order to stimulate their practical applications. In this paper, the theoretical and experimental progresses in this field are reviewed. Besides, main factors affecting the thermal conductivity of organic/inorganic composites are discussed, including the intrinsic properties of organic matrix and inorganic fillers, topological structure of composites, loading volume fraction, and the interfacial thermal resistance between fillers and organic matrix.     

Experiments and simulations on low-temperature waste heat harvesting system by thermoelectric power generators

In this case study, a system to recover waste heat comprised 24 thermoelectric generators (TEG) to convert heat from the exhaust pipe of an automobile to electrical energy has been constructed. Simulations and experiments for the thermoelectric module in this system are undertaken to assess the feasibility of these applications. A slopping block is designed on the basis of simulation results to uniform the interior thermal field that improves the performance of TEG modules. Besides simulations, the system is designed and assembled. Measurements followed the connection of the system to the middle of an exhaust pipe. Open circuit voltage and maximum power output of the system are characterized as a function of temperature difference. Through these simulations and experiments, the power generated with a commercial TEG module is presented. Overview this case study and our previous work, the results establish the fundamental development of low-temperature waste heat thermoelectric generator system that enhances the TEG efficiency for vehicles.     

Methodological reviews and analyses on the emerging research trends and progresses of thermoelectric generators

Thermoelectric generator is among the earliest initiated electricity‐harvesting methods. It is a very potential power harvester that can convert wasteful thermal energy into electricity. However, it often suffers from low energy conversion rate due to its inconsistent heat source, inefficient thermoelectric material (or thermoelement) performance, and incompetent structural issues. Progressively for the first time, detailed methodological surveys and analyses are made for bulk, thick, and thin films in this review. This is in order to accommodate better insights and comprehensions on the emerging trends and progresses of thermoelectric generators from 1989 to 2017. The research interests in thermoelectric generators have started back in 1989, and have continuously experienced emerging progresses in the number of studies over the last years. The methodological reviews and analyses of thermoelectric generator showed that almost 46.6% of bulk and 46.1% of thick and thin film research works, respectively, are actively progressed in 2014 to 2017. Nearly 86.2% of bulk and 44.1% of thick and thin film thermoelectric generators are realizing in between 0.001 and 4 μW cm^?2 K^?2, while 43.1% of thick and thin films are earning among 10^?6 to 0.001 μW cm^?2 K^?2. The highest achievement made until now is 2.5 W cm^?2 at a temperature difference of 140 K and thermoelectric efficiency factor of 127.55 μW cm^?2 K^?2. This achievement remarked positive elevation for the field and interest in thermoelectric power generation. Consecutively, the research trends of fundamental devices' structure, thermoelement, fabrication, substrate, and heat source characteristics are analyzed too, along with the desired improvement highlights for the applications of thermoelectric generators.     

Emerging solar cells energy trade‐off: Interface engineering materials impact on stability and efficiency progress

Emerging solar cell (ESC) carrier's dynamic transfer difficulties and instability at dissimilar material's interface seems to be potential distress. Electrical permittivity influences on photo carrier dynamics of nonpolarized organic and polarized organic‐inorganic perovskite solar cell. ESC electrical efficiency and stability are consider to be managed broadly by carrier accumulation, extraction, and conduction techniques. Active materials optoelectric interaction and carrier accumulation are petite known. Its energetic dealings with dissimilar interface materials how support to faster carrier extraction and conduction are still in haze. In this drill, focus on active to interface materials key properties are imperative to find a way to promote electrical energy conversion and its steadiness. Selected advanced interface engineering materials (IEMs) architectures, conduction, and multifarious relations to active materials for efficient energy transfer ability and stability are reported precisely by the model. The interface structure consistency supports to systematize energy transfer, otherwise, disorder or energy loss is anticipated. Therefore, active material linkage to IEM photophysical, quantum, and choice of materials technologies are focused comprehensively to explore the pathway of ESC stability and efficiency progression.     

Performance analysis of a hybrid solar thermoelectric generator

In this paper, a theoretical model is developed to investigate the performance of the hybrid solar thermoelectric generator (HSTEG) system, which is designed without (B-HSTEG) and with an evacuated glass tube (V-HSTEG). The heat loss, power output, thermal efficiency, and electrical efficiency of the B-HSTEG/V-HSTEG system are evaluated by analyzing the design parameters such as geometric solar concentration ratio, thermoelectric figure of merit, and cold-side inlet fluid temperature. The performance of the B-HSTEG is compared with the V-HSTEG system using two heat transfer fluids: water and Therminol VP-1. The maximum electrical efficiency of the B-HSTEG and V-HSTEG is estimated to be 12.2 and 15.6% (ZT = 3) with a corresponding thermal efficiency of about 61.9 and 60.3%, respectively. Overall, this paper provides a systematic performance analysis of HSTEG systems.     

A review on the performance of photovoltaic/thermoelectric hybrid generators

Utilization of a broad range of solar spectrum has the potential for high power output from solar cells. However, solar photovoltaics (PVs) can convert only part of the solar electromagnetic spectrum into electricity efficiently. The remaining of the solar radiation is often dissipated in the form of heat, which causes performance reduction and reduces the life expectancy of the solar PV cell. Thermoelectric generators (TEGs) are devices that operate like a heat engine by converting thermal energy into electricity through thermoelectric effect. Integrating a TEG into a PV converter will enhance its efficiency and reduce the amount of heat dissipated. Different studies have been carried out and are still taking place to increase the total efficiency of a coupled photovoltaic thermoelectric generator (PV-TEG) system. This review discusses the concept of PV converters and thermoelectric devices and presents the various models and numerical and experimental investigations on performance enhancement of integrated PV-TEGs. The influence of key parameters on the performance of PV-TEG were also discussed. The review is expected to serve as a reference to recent work on research and development of integrated PV-TEG systems.