Two-Dimensional Thermal Resistance Analysis of a Waste Heat Recovery System with Thermoelectric Generators

In this study, it is shown that two-dimensional (2D) thermal resistance analysis is a rapid and simple method to predict the power generated from a waste heat recovery system with thermoelectric generators (TEGs). Performance prediction is an important part of system design, generally being simulated by numerical methods with high accuracy but long computational duration. Use of the presented analysis saves much time relative to such numerical methods. The simple 2D model of the waste heat recovery system comprises three parts: a recovery chamber, the TEGs, and a cooling system. A fin-structured duct serves as a heat recovery chamber, to which were attached the hot sides of two TEGs; the cold sides were attached to a cooling system. The TEG module and duct had the same width. In the 2D analysis, unknown temperatures are located at the centroid of each cell into which the system is divided. The relations among the unknown temperatures of the cells are based on the principle of energy conservation and the definition of thermal resistance. The temperatures of the waste hot gas at the inlet and of the ambient fluid are known. With these boundary conditions, the unknown temperatures in the system become solvable, and the power generated by the TEGs can be predicted. Meanwhile, a three-dimensional (3D) model of the system was simulated in FloTHERM 9.2. The 3D numerical solution matched the solution of the 2D analysis within 10%.     

Detection of Thermal Radiation,Sensing of Heat Flux,and Recovery of Waste Heat by the Transverse Thermoelectric Effect

The transverse thermoelectric effect is unique in that an output voltage can be extracted in the direction perpendicular to the input temperature gradient. This paper describes how this transverse feature can be exploited to realize simple and promising configurations of thermoelectric devices. For detection of thermal radiation, two-dimensional imaging has been demonstrated by a fabricated sensor array of tilt-oriented Ca _x CoO₂ epitaxial thin film. We have also developed a serpentine heat flux sensor made of multilayered Bi/Cu, and Bi_0.5Sb_1.5Te₃/Ni tubular thermoelectric devices for power generation. The fabrication processes and test results are presented.     

Waste Heat Recovery from a Marine Waste IncineratorUsing a Thermoelectric Generator

A marine waste incinerator has been evaluated for waste heat harvesting using thermoelectric generators (TEG). The application has been evaluated using mathematical modeling to optimize the heat exchanger and some vital design parameters of the TEG. The calculation shows that it is possible to extract 58?kW_el at a price of 6.6?US$/W from an 850-kW_th incinerator when optimizing for maximum power. However, minimizing the cost, it is possible to get 25?kW_el at a price of 2.5?US$/W. A trade-off between the two targets leads to a combination that gives 38?kW_el at a price of 2.7?US$/W.     

烧结余热回收控制系统设计

针对余热锅炉的特点,文章设计的余热回收控制系统完成了除氧器和锅炉给水泵变频控制及电动调节阀的自动控制,完成除氧器水位、温度的显示及控制,3个蒸汽发生器的入口废气温度,除氧器给水压力、流量,汽包水位、压力,过热蒸气出口压力、流量、温度的显示,及汽包水位的控制。     

Thermal Conductivity and ZT in Disordered Organic Thermoelectrics

For decades, continuous attempts have been made to improve the figure of merit (ZT) of thermoelectrics. The theory behind the Seebeck effect itself is well researched, but the problem with ZT is related to materials properties that offset one another. This work analyzed the link between the site energy distributions and thermal conductivity of oxidized poly(3,4-ethylenedioxythiophene-tosylate) (PEDOT:Tos), which was reported to be a good organic thermoelectric. To understand how heat flow was affected by “disorder” in PEDOT:Tos and the associated electron–phonon interactions, we computed the values of the thermal conductivity κ and ZT using materials parameters extracted from the open literature. By varying the values of the parameters separately, we were able to identify their individual influence on κ and ZT. Our results suggest that ZT is most sensitive to changes in σ, the bandwidth of the density of states (DOS) of the transport sites, and less so to changes in n _eff, the effective carrier density. Our simulations also suggested that ZT could become exceptionally large (approaching a value of ~20) if σ were lowered to 1 meV to 2 meV. This would be a tremendous approach to increase ZT in oxidized PEDOT:Tos.     

Transport Properties of Bulk Thermoelectrics: An International Round-Robin Study, Part II: Thermal Diffusivity, Specific Heat, and Thermal Conductivity

For bulk thermoelectrics, improvement of the figure of merit ZT to above 2 from the current values of 1.0 to 1.5 would enhance their competitiveness with alternative technologies. In recent years, the most significant improvements in ZT have mainly been due to successful reduction of thermal conductivity. However, thermal conductivity is difficult to measure directly at high temperatures. Combined measurements of thermal diffusivity, specific heat, and mass density are a widely used alternative to direct measurement of thermal conductivity. In this work, thermal conductivity is shown to be the factor in the calculation of ZT with the greatest measurement uncertainty. The International Energy Agency (IEA) group, under the implementing agreement for Advanced Materials for Transportation (AMT), has conducted two international round-robins since 2009. This paper, part II of our report on the international round-robin testing of transport properties of bulk bismuth telluride, focuses on thermal diffusivity, specific heat, and thermal conductivity measurements.     

Thermoelectric Properties of Cold-Pressed Higher Manganese Silicides for Waste Heat Recovery

Polycrystalline higher manganese silicide (HMS) samples with different grain sizes have been obtained by cold-pressing HMS powder under high pressure of about 3?GPa and postprocessing annealing. It was found that the cold-pressing process can reduce the grain size of HMS to 120?nm. The cold-pressed pellets were then annealed at different temperatures to obtain a series of samples with different grain sizes. For comparison, an additional sample was prepared in a regular die under low pressure of 300?MPa, which resulted in lower density and higher porosity than the high-pressure process. For these samples, the effect of grain size and porosity on Seebeck coefficient was not as apparent as that on electrical conductivity and thermal conductivity. The electrical conductivity of the cold-pressed samples increases as the grains grow, and the grain boundary connection is improved during the postprocessing annealing. A significant reduction in the thermal conductivity of the cold-pressed samples was observed. The sample prepared with the low-pressure pressing shows the lowest thermal conductivity of 1.2?W?m^?1?K^?1 at 300?K, which can be attributed to its high porosity of 34% and low phonon transmission coefficient through the grain boundaries. The low-temperature thermal conductivity data of all samples were analyzed to obtain the phonon transmission coefficient and the Kapitza resistance at the grain boundaries.     

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.     

Parallel Dislocation Networks and Cottrell Atmospheres Reduce Thermal Conductivity of PbTe Thermoelectrics

Dislocations play an important role in thermal transport by scattering phonons. Nevertheless, for materials with intrinsically low thermal conductivity, such as thermoelectrics, classical models require exceedingly high numbers of dislocations (>10¹² cm^–2) to further impede thermal transport. In this work, a significant reduction in thermal conductivity of Na_0.025Eu_0.03Pb_0.945Te is demonstrated at a moderate dislocation density of 1 × 10¹⁰ cm^–2. Further characteristics of dislocations, including their arrangement, orientation, and local chemistry are shown to be crucial to their phonon-scattering effect and are characterized by correlative microscopy techniques. Electron channeling contrast imaging reveals a uniform distribution of dislocations within individual grains, with parallel lines along four <111> directions. Transmission electron microscopy (TEM) shows the parallel networks are edge-type and share the same Burgers vectors within each group. Atom probe tomography reveals the enrichment of dopant Na at dislocation cores, forming Cottrell atmospheres. The dislocation network is demonstrated to be stable during in situ heating in the TEM. Using the Callaway transport model, it is demonstrated that both parallel arrangement of dislocations and Cottrell atmospheres make dislocations more efficient in phonon scattering. These two mechanisms provide new avenues to lower the thermal conductivity in materials for thermal-insulating applications.     

Progress Report on BSST-Led US Department of Energy Automotive Waste Heat Recovery Program

As global consumption of energy continues to increase at an exponential rate, the need to find technologies that can help reduce this rate of consumption, particularly in passenger vehicles, is imperative. This paper provides a progress report on the BSST-led US Department of Energy-sponsored automotive thermoelectric waste heat recovery project, which has transitioned from phase 3 and is completing phase 4. Thermoelectric generator (TEG) development will be discussed, including modeling and thermal cycling of subassemblies. The design includes the division of the TEG into different temperature zones, where the subassembly materials and aspect ratios are optimized to match the temperature gradients for the particular zone. Test results for a phase 3 quarter-scale device of the phase 4 high-temperature TEG will be discussed, where power outputs of up to 125 W were achieved on a 600°C hot-air test bench. The design of the TEG, which uses high-power-density segmented thermoelectric elements, has evolved from a planar design in phase 3 to a cylindrical design in phase 4. The culmination of phase 4 includes testing of the generator on a dynamometer at the National Renewable Energy Laboratory with a high-performance production engine.     

Thermoelectric Generators for Automotive Waste Heat Recovery Systems Part II: Parametric Evaluation and Topological Studies

A comprehensive numerical model has been proposed to model thermoelectric generators (TEGs) for automotive waste heat recovery. Details of the model and results from the analysis of General Motors’ prototype TEG were described in part I of the study. In part II of this study, parametric evaluations are considered to assess the influence of heat exchanger, geometry, and thermoelectric module configurations to achieve optimization of the baseline model. The computational tool is also adapted to model other topologies such as transverse and circular configurations (hexagonal and cylindrical) maintaining the same volume as the baseline TEG. Performance analysis of these different topologies and parameters is presented and compared with the baseline design.     

Comparison of Waste Heat Recovery from the Exhaust of a Spark Ignition and a Diesel Engine

We present herein a design for and performance measurements of a prototype thermoelectric generator (TEG) mounted on both a spark ignition engine (0.9 dm³) and a self-ignition engine (1.3 dm³). Using the prototype TEG as a tool, benchmark studies were performed in order to compare its parameters in terms of heat recovery from exhaust gases of both engine types. The test bed study was performed with an Automex AMX-210/100 eddy-current brake dynamometer. To provide a comprehensive overview of the TEG operating conditions, characterization of its parameters such as temperature distribution, heat flux density, and efficiency was done at engine speeds and loads similar to those within the range of operation of real road conditions.     

Thermoelectric Generators for Automotive Waste Heat Recovery Systems Part I: Numerical Modeling and Baseline Model Analysis

A numerical model has been developed to simulate coupled thermal and electrical energy transfer processes in a thermoelectric generator (TEG) designed for automotive waste heat recovery systems. This model is capable of computing the overall heat transferred, the electrical power output, and the associated pressure drop for given inlet conditions of the exhaust gas and the available TEG volume. Multiple-filled skutterudites and conventional bismuth telluride are considered for thermoelectric modules (TEMs) for conversion of waste heat from exhaust into usable electrical power. Heat transfer between the hot exhaust gas and the hot side of the TEMs is enhanced with the use of a plate-fin heat exchanger integrated within the TEG and using liquid coolant on the cold side. The TEG is discretized along the exhaust flow direction using a finite-volume method. Each control volume is modeled as a thermal resistance network which consists of integrated submodels including a heat exchanger and a thermoelectric device. The pressure drop along the TEG is calculated using standard pressure loss correlations and viscous drag models. The model is validated to preserve global energy balances and is applied to analyze a prototype TEG with data provided by General Motors. Detailed results are provided for local and global heat transfer and electric power generation. In the companion paper, the model is then applied to consider various TEG topologies using skutterudite and bismuth telluride TEMs.     

Self-Assembled Germanium Quantum-Dot Supercrystals in Silicon with Extremely Low Thermal Conductivities for Thermoelectrics

Superlattices with one-dimensional (1D) phonon confinement were studied to obtain a low thermal conductivity for thermoelectrics. Since they are composed of materials with a lattice mismatch, they often show dislocations. Like 1D nanowires, they also decrease heat transport in only one main propagation direction. It is therefore challenging to design superlattices with a thermoelectric figure of merit ZT higher than unity. Epitaxial self-assembly is a major technology to fabricate three-dimensional (3D) Ge quantum-dot (QD) arrays in Si. They have been used for quantum and solar-energy devices. Using the atomic-scale phononic crystal model, 3D Ge QD supercrystals in Si also present an extreme reduction of the thermal conductivity to a value that can be under 0.04 W/m/K. Owing to incoherent phonon scattering, the same conclusion holds for 3D supercrystals with moderate QD disordering. As a result, they might be considered for the design of highly efficient complementary metal–oxide–semiconductor (CMOS)-compatible thermoelectric devices with ZT possibly much higher than unity. Such a small thermal conductivity was only obtained for two-dimensional layered WSe₂ crystals in an experimental study. However, electronic conduction in the Si/Ge compounds is significantly enhanced. The 0.04 W/m/K value can be computed for different Ge QD filling ratios of the Si/Ge supercrystal with size parameters in the range of current fabrication technologies.     

热管热泵低温热能回收机组的经济性分析

随着人民生活水平的提高,空调普及率逐年提高其能耗也在不断增长,解决此问题,提出了将热管热泵低温热能回收机组应用于空调新风机组,充分利用建筑物的排风预热新风,从而提高新风机组入口的新风温度,并在其初投资和成本回收、节能性等方面进行分析,得出了热管热泵低温热能回收机组运行的经济合理性。     

热管热泵低温热能回收机组的可行性分析

随着经济快速发展及人民生活水平的提高,空调普及率逐年提高其能耗也在不断增长,空调余热量的回收显得格外重要。为解决此问题,提出了将热管热泵低温热能回收机组应用于空调新风机组,充分利用建筑物的排风预热新风,从而提高新风机组入口的新风温度,并从经济性、节能性及健康性等方面进行分析,得出了热管热泵低温热能回收机组运行的可行性。