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.     

纳米硅锗热电材料和纳米器件的研究进展

近年来,纳米技术逐渐被用来设计和制备硅锗（Si−Ge）热电材料和新型器件。为了提高Si−Ge热电材料的热电性能,研究学者利用各种纳米结构对Si−Ge热电材料进行了理论研究。其中,利用纳米线、超晶格和量子点等结构中的能带机理与散射机理,从理论上设计了降低Si−Ge纳米结构热导率和提高其功率因子的途径。同时,高效的Si−Ge纳米热电材料被制备出来,包括纳米块体材料的热电性能得到大幅度提高,室温下薄膜和纳米线的热电性能实现了重大突破。在高性能材料的基础上,新型Si−Ge纳米热电器件的研发除了关注于制备工艺优化外,还包括传热结构和原型器件的设计。     

Thermoelectric transport in graphene and 2D layered materials

ABSTRACT

In early 90s, Hicks and Dresselhaus proposed that low dimensional materials are advantages for thermoelectric applications due to the sharp features in their density-of-states, resulting in a high Seebeck coefficient and, potentially, in a high thermoelectric power factor. Two-dimensional (2D) materials are the latest class of low dimensional materials studied for thermoelectric applications. The experimental exfoliation of graphene, a single-layer of carbon atoms in 2004, triggered an avalanche of studies devoted to 2D materials in view of electronic, thermal, and optical applications. One can mix and match and stack 2D layers to form van der Waals hetero-structures. Such structures have extreme anisotropic transport properties. Both in-plane and cross-plane thermoelectric transport in these structures are of interest. In this short review article, we first review the progress achieved so far in the study of thermoelectric transport properties of graphene, the most widely studied 2D material, as a representative of interesting in-plane thermoelectric properties. Then, we turn our attention to the layered materials, in their cross-plane direction, highlighting their role as potential structures for solid-state thermionic power generators and coolers.     

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.     

Effect of heat exchanger material and fouling on thermoelectric exhaust heat recovery

Thermoelectric devices are being investigated as a means of improving fuel economy for diesel and gasoline vehicles through the conversion of wasted fuel energy, in the form of heat, to useable electricity. By capturing a small portion of the energy that is available with thermoelectric devices can reduce engine loads thus decreasing pollutant emissions, fuel consumption, and CO₂ to further reduce green house gas emissions. This study is conducted in an effort to better understand and improve the performance of thermoelectric heat recovery systems for automotive use. For this purpose an experimental investigation of thermoelectrics in contact with clean and fouled heat exchangers of different materials is performed. The thermoelectric devices are tested on a bench-scale thermoelectric heat recovery apparatus that simulates automotive exhaust. It is observed that for higher exhaust gas flowrates, thermoelectric power output increases from 2 to 3.8 W while overall system efficiency decreases from 0.95% to 0.6%. Degradation of the effectiveness of the EGR-type heat exchangers over a period of driving is also simulated by exposing the heat exchangers to diesel engine exhaust under thermophoretic conditions to form a deposit layer. For the fouled EGR-type heat exchangers, power output and system efficiency is observed to be 5-10% lower for all conditions tested.     

Vibration Control of Composite Spacecraft Booms Subjected to Solar Heating

In this paper, piezoelectric feedback control of vibration and instability of spacecraft booms modeled as circular thin-walled cross-section beams and subjected to solar radiant heating is investigated. Having in view that composite material systems are likely to play a great role in the design of these devices, the beam constituent materials encompass non-classical effects such as anisotropy and transverse shear. In addition, in order to induce beneficial elastic couplings, a special ply-angle distribution achieved via the usual helically wounding fiber-reinforced technology, the so called filament winding, is implemented. The dynamic governing equations including the temperature effects and the related boundary conditions are obtained via the application of Hamilton's principle. Toward the end of controlling the oscillations and prevent the occurrence of the thermal dynamic instability, a feedback control capability based on the use of the piezoelectric induced strain actuation is implemented. The performance of its implementation considered in conjunction with that of the structural tailoring are highlighted and pertinent conclusions are derived.     

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.     

A Study on Coupled Thermo-Elasto-Plastic-Damage Dissipative Phenomena: Models and Application to Some Innovative Materials

This is a review paper on some irreversible thermodynamics-based constitutive models capable of capturing complex dissipative physical bahaviour in multifunctional innovative materials. The thermomechanical response of such materials accounts for two basic sources of material nonlinearity, plasticity and damage, which may result in various failure mechanisms. A number of couplings, such as thermo-elastic-damage, elastic-plastic hardening, plastic-damage hardening, thermo-elastic-plastic-damage, etc., are discussed. Various material symmetry classes, including anisotropy, orthotropy, transverse isotropy, are referred to some innovative materials, such as composite materials, functionally graded structures, thermal or wear resistant coatings, etc. Examples of implementation of chosen models for simulation of some initial boundary problems are presented.     

Finite element analysis and material sensitivity of Peltier thermoelectric cells coolers

In this work, a finite element simulation of a commercial thermoelectric cell, working as a cooling heat pump, is presented. The specially developed finite element is three-dimensional, non-linear in its formulation (using quadratic temperature-dependence on material properties) and fully coupled, including the Seebeck, Peltier, Thomson and Joule effects. Another special interface finite element is developed to prescribe the electric intensity, taking advantage of repetitions and symmetries. A thorough study of the distributions of voltage, temperature and the corresponding fluxes is presented, and the performance of the cell is compared with that of the manufacturer and with simplified analytical formulations, showing a good agreement. Combining the finite element model with the Monte Carlo technique, a sensitivity analysis is presented to take into account the performance dependence on the material properties, geometrical parameters and prescribed values. This analysis, which can be considered a first step to optimize these devices, concludes that the temperature-dependence of the material properties of electric conductivity and Seebeck coefficient is very relevant on cell performance.     

Recent trends in MXene/Metal chalcogenides for electro-/photocatalytic hydrogen evolution reactions

Hydrogen due to high energy density and ecologically benign characteristics can become an excellent energy carrier for a sustainable energy economy and to appease the energy demand of humankind. Moreover, cost-effective and long-lasting photocatalysts can make the hydrogen generating process more economical and suitable. Recently, MXene have become one of the most sought-after composite materials for photocatalytic hydrogen generation. However, the photocstalytic performance can be further enhanced by doping with other semiconductor materials. Transition metal chalcogenides (Transition metals = Cu, Co, Ni, Zn, Cd, Mo, W)/MXene composites and mixed transition metal chalcogenide/MXene nanocomposites have been extensively investigated for the photocatalytic hydrogen generation. These materials possess unique two-dimensional layered structure that ameliorates the photocatalytic water splitting performance by increasing the light adsorption even at low photon flux density. The 2D design assists in reducing the distance necessary to transverse charge carriers to the surface. Because the layered structure tends to trap electrons in the ultrathin layers, 2D materials have unusual optoelectronic properties. In-plane covalent bonding assisted the creation of various heterojunctions and heterostructures in these 2D materials. Water splitting and hydrogen production are aided by the high surface area of these 2D materials. Due to its diverse elemental composition, unique 2D structure, good photoelectronic characteristics, large surface area, and many surface terminations. The design and production of many types of materials used as catalysts for the hydrogen evolution process are discussed in this article.     

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.     

Understanding the origin of half‐metallicity and thermophysical properties of ductile La2CuMnO6 double perovskite

For first time, the magneto‐electronic structure with thermoelectric and mechanical properties of lanthanum‐based double perovskite La₂CuMnO₆ are investigated, using first‐principle methods. Generalized gradient approximation and modified Becke‐Jhonson potentials are integrated to figure out exchange‐correlation potential. The alloy stabilizes in cubic structure with ferromagnetic nature and determined structural parameters are consistent with experimental results. The band profile reveals the half‐metallic character, which is further confirmed by calculated electronic conductivities of up and down spin channels. The effect of pressure on the structural and electronic profile is demonstrated here. The analysis of the transport properties portrays that the highest value of 0.39 is achieved for figure of merit at higher temperatures. The mechanical stability of La₂CuMnO₆ is established, by determination of elastic constants. The calculated elastic parameters specify the ductile behavior of alloy with high melting temperature. The efficient thermoelectric parameters with half‐metallic and ductile character suggest the likelihood of applications of alloy to design hard spintronic devices or potential thermoelectric materials.     

Hydrogen storage behavior of Mg/Ni layered nanostructured composite materials produced by accumulative fold-forging

An advanced and newly developed severe plastic deformation (SPD) method called accumulative fold-forging (AFF) was applied to produce layered nanostructured MgNi alloys exhibiting superior hydrogen storage capacity. Microstructural developments and storage properties were characterized in depth to correlate the structure and performance of this advanced material. The enhanced hydrogen storage performance of the magnesium-based layered composite material was investigated in comparison to the pristine state by conducting hydrogenation and dehydrogenation testing. It was also shown that the hydrogen uptake and release characteristics can be controlled by adjusting the layered structure or the Mg: Ni stoichiometry ratio. Refining the grain structure of the magnesium alloy down to the nano-scale range (～400–900 nm) by applying high cycles AFF consolidation to promote creation of multi-million nanometric interfaces led to superior storage performance with a remarkable hydrogen absorption capacity of up to ～1.425 wt%. X-ray diffraction (XRD) analysis of the hydrogenated products revealed the formation of MgH₂ that indicates the dominance of the magnesium matrix for controlling the hydrogen storage behavior of the layered Mg/Ni composite material. Finally, the relationship between the directional hydrogen storage behavior and the induced structural features upon AFF treatment were also established using quantitative characterization and analytical tools.     

High performance solid-state thermoelectric energy conversion via inorganic metal halide perovskites under tailored mechanical deformation

Solid-state thermoelectric energy conversion devices attract broad research interests because of their great promises in waste heat recycling, space power generation, deep water power generation, and temperature control, but the search for essential thermoelectric materials with high performance still remains a great challenge. As an emerging low cost, solution-processed thermoelectric material, inorganic metal halide perovskites CsPb(I_1–xBr_x)₃ under mechanical deformation is systematically investigated using the first-principle calculations and the Boltzmann transport theory. It is demonstrated that halogen mixing and mechanical deformation are efficient methods to tailor electronic structures and charge transport properties in CsPb(I_1–xBr_x)₃ synergistically. Halogen mixing leads to band splitting and anisotropic charge transport due to symmetry-breaking-induced intrinsic strains. Such band splitting reconstructs the band edge and can decrease the charge carrier effective mass, leading to excellent charge transport properties. Mechanical deformation can further push the orbital energies apart from each other in a more controllable manner, surpassing the impact from intrinsic strains. Both anisotropic charge transport properties andZT values are sensitive to the direction and magnitude of strain, showing a wide range of variation from 20% to 400% (with a ZT value of up to 1.85) compared with unstrained cases. The power generation efficiency of the thermoelectric device can reach as high as approximately 12% using mixed halide perovskites under tailored mechanical deformation when the heat-source is at 500 K and the cold side is maintained at 300 K, surpassing the performance of many existing bulk thermoelectric materials.     

Heat Removal from Power Electronics in Two Direction Sets Using Embedded Solid State Cooling Layers a Proposed Non-Numerical Calculation Method

The use of embedded cooling layers consisting of materials with high thermal conductivities can significantly reduce peak temperatures within solid-state heat-generating media. Inversely, such layers can also allow for increases in heat-generating densities for a given maximum peak temperature. This is applicable in, for instance, integrated passive power electronics, where power densities are limited by the low thermal conductivities of materials being used. In this paper, the thermal performance of embedded cooling layers in three-dimensional rectangular heat-generating components is investigated numerically for a boundary condition where heat escapes to the ambient in two orthogonal direction sets (sets of orthogonal positive and/or negative directions). The allowable increase in heat generation density for fixed maximum peak temperatures is described for a wide range of geometric shape conditions and thermal conductivities of materials present in such composite structures. Correlations were developed for conditions with and without significant thermal resistance at the internal interfaces of the material layers and externally between the composite component structure and the environment. Conventional one-dimensional and first-order approximations traditionally used in composite solid conduction problems can accurately account for neither the relative thickness of material layers, nor the impact that internal interfacial resistance has. This paper presents a method with which the peak temperature within a stacked sandwich structure containing embedded cooling layered and where heat is removed in two orthogonal direction sets can be determined without the use of a numerical package. The method was developed for a wide range of material properties, geometric sizes and interfacial resistance values.     

Heat transfer behaviour of thermal energy storage components using composite phase change materials

对基于复合相变材料储热单元的储热性能进行了研究。建立了复合材料和储热单元体内部的二维传热模型,考察了复合材料物性和结构尺寸及传热流体操作条件（流体流速）对单元体储热性能的影响,对比了两种不同结构单元体的储热性能,并搭建实验平台进行了实验对比研究。对比结果表明,模型结果与实验结果趋于一致,验证了模型的准确性。复合材料物性和结构尺寸及传热流体操作条件对单元体储热性能有较大的影响。相比较单管储热单元体,同心管储热单元体有着更优的储热特性,在相同的操作条件下,同心管储热单元体的储热、放热时间较单管储热单元体分别减少10%和15%。     

Computational and experimental analysis of a commercially available Seebeck module

During the last decade thermoelectrics have emerged as a promising alternative amongst other green power production technologies due to the unique advantages they present. In this respect, performance prediction of thermoelectric devices is critical both for evaluating the potential application of new materials and defining the crucial design parameters of thermoelectric generators and systems. This paper investigates, computationally as well as experimentally, the performance of a commercially available Seebeck module under steady-state operating conditions. Computational results, retrieved using ANSYS Workbench (v. 14.0), were compared to performance data available by the manufacturer. Additionally to that, in order to further verify the integrity of the modelling procedure, experimental evaluation using the same commercial module was conducted in laboratory environment. Although a relatively large deviation between computational and manufacturer data was observed when the mean operating temperature of the generator was taken into account, a very good agreement was established in terms of generator efficiency, providing also a rational explanation to the resulting divergence of the first case. Furthermore, the outcomes of the experimental analysis validated the accuracy of the finite element modelling process.     

An analytical mass transfer model for predicting VOC emissions from multi-layered building materials with convective surfaces on both sides

An analytical mass transfer model for predicting emissions of Volatile Organic Compounds (VOCs) from multi-layered building materials and the instantaneous VOC material-phase distribution is developed. Different from the mass transfer models in the literature, it is able to describe the characteristics of VOC emissions from a wall with an arbitrary number of layers of different materials and with convective surfaces on both sides, and it does not neglect the mass transfer resistance through the gas-phase boundary layer. The model is validated with experimental data from the literature. The model provides a powerful tool for predicting VOC emissions from composite materials such as furniture and layered wall structures. Based upon the model and the dimensionless analysis, the applicable condition of Kumar and Little’s double-layer model is discussed.     

A symplectic analytical singular element for steady-state thermal conduction with singularities in composite structures

ABSTRACT

In modern design of composite structures, multiple materials with different properties are bound together. Accurate prediction of the strength of the interface between different materials, especially with the existence of cracks under thermal loading, is demanded in engineering. To this end, detailed knowledge on the distribution of temperature and heat flux is required. This study conducts a systematical investigation on the cracks terminated at material interface under steady-state thermal conduction. A new symplectic analytical singular element is constructed for the numerical modeling. Combining the proposed element with conventional finite elements, the generalized flux intensity factors can be solved accurately.     

Bi-Te基热电材料的能带结构计算

采用基于密度泛函理论的自洽赝势方法，计算了Bi—Te基热电材料不同化学配比下的电子结构。介绍了Bi2Te3材料的能带以及态密度，并计算了不同配比材料的载流子有效质量。计算结果显示：随着碲含量的增加，Bi—Te基热电材料从N型半导体向P型转变，在导电性质确定的情况下，塞贝克系数随着碲含量的增加而升高。