A Finite Volume Analysis of Thermoelectric Generators

ABSTRACT

The electric power produced by a thermoelectric generator (TEG) is strongly influenced by the applied heat sink. While a TEG is aimed at harvesting waste heat, the optimization of the efficiency of the heat sink is a key task for the design of waste heat recovery systems implementing TEG. A TEG model is proposed and implemented in an open source toolbox for field operation and manipulation (OpenFOAM) for the purpose of performing optimizations of the heat sink, using a commercially available TEG as basis. This model includes the multi-physics thermoelectric coupled effects. Conservation principles of energy and current are considered simultaneously. This includes the thermal and electric conduction, Seebeck effect, Peltier effect, Thomson effect, and Joule heating. Particular attention is given to a proper modeling of the boundary conditions. The thermoelectric model is implemented in such a way that it can readily be combined with other physical models in OpenFOAM. The model is validated by comparing the predictions to analytical results, measurements as well as the simulation data of other authors.     

A cascaded thermoelectric generation system for low-grade heat harvesting

Thermoelectric generation (TEG) has its unique advantages in terms of distributed energy supply, low-grade heat recovery, and clean energy technology, but its low efficiency has constrained its application and promotion seriously. In order to obtain satisfactory efficiency and realize the adaptability to different heat sources, a novel cascaded TEG system (CTEG) is proposed. According to the temperature of heat source, three types of thermoelectric modules are used in the generator system. The module arrangement, cooling medium, and mode directly affect the performance of this whole system, since that the detailed model and CFD simulation were conducted to obtain the optimal collocation. Based on the optimization modeling, a CTEG prototype was constructed. From the experimental results, the efficiency of the CTEG achieved 5.92%, which significantly improved about 21.56% compared with the TEG which only contains one stage. Moreover, the key parameters of the system were identified. The stability and the improvement of the system were discussed comprehensively. For TEG system, CTEG increase the utilization of high-temperature heat sources, but also provides a structure for a TEG adapting to multiple temperature heat sources.     

Back‐analyses of in‐situ thermal response test (TRT) for evaluating ground thermal conductivity

In this study, a series of computational fluid dynamics (CFD) numerical analyses was performed in order to evaluate the performance of six full‐scale closed‐loop vertical ground heat exchangers constructed in a test bed located in Wonju, South Korea. The high‐density polyethylene pipe, borehole grouting and surrounding ground formation were modeled using FLUENT, a finite‐volume method program, for analyzing the heat transfer process of the system. Two user‐defined functions accounting for the difference in the temperatures of the circulating inflow and outflow fluid and the variation of the surrounding ground temperature with depth were adopted in the FLUENT model. The relevant thermal properties of materials measured in laboratory were used in the numerical analyses to compare the thermal efficiency of various types of the heat exchangers installed in the test bed. The numerical simulations provide verification for the in‐situ thermal response test (TRT) results. The numerical analysis with the ground thermal conductivity of 4.0 W/m?K yielded by the back‐analysis was in better agreement with the in‐situ TRT result than with the ground thermal conductivity of 3.0 W/m?K. From the results of CFD back‐analyses, the effective thermal conductivities estimated from both the in‐situ TRT and numerical analysis are smaller than the ground thermal conductivity (=4.0 W/m?K) that is input in the numerical model because of the intrinsic limitation of the line source model that simplifies a borehole assemblage as an infinitely long line source in the homogeneous material. However, the discrepancy between the ground thermal conductivity and the effective thermal conductivity from the in‐situ TRT decreases when borehole resistance decreases with a new three pipe‐type heat exchanger leads to less thermal interference between the inlet and outlet pipes than the conventional U‐loop type heat exchanger. Copyright © 2012 John Wiley & Sons, Ltd.     

In-cylinder fluid flow,turbulence and spray models—A review

This article is a literature review of use of computational fluid dynamics (CFD) codes to model the in-cylinder fluid flow, turbulence and spray characteristics. This study is based on the reports of about 60 scientists, who published their results between 1978 and 2008. Most of the scientists and researchers used CFD codes to analyze the models under simulation conditions and compared these simulated results with experimental results. Some scientists reported that different engines exhibit different behaviors with similar fuel sprays and Re-Normalized Group (RNG) k-? model is the best applicable turbulence model for engine simulation. The KIVA code is widely used for model development in academia due to the availability of the source. However, its capability for resolving complex geometries is limited. On the other hand, other commercial CFD codes such as STAR-CD, FIRE, VECTIS and FLUENT are frequently used by the industry due to their superior mesh generation interfaces and because of their available user support. Some scientists combined STAR-CD and KIVA code for the engine simulations but they concluded that, it would be preferable to implement the advanced submodels directly into one commercial code for engine simulations.     

CFD simulation of a Gifford–McMahon type pulse tube refrigerator

Pulse tube refrigerator has the advantages of long life and low vibration over the conventional cryocoolers, such as Gifford–McMahon (GM) and Stirling coolers because of the absence of moving parts in low temperature. This paper performs a two-dimensional computational fluid dynamic (CFD) simulation of a Gifford–McMahon type double inlet pulse tube refrigerator (DIPTR), operating under a variety of thermal boundary conditions. A commercial Computational Fluid Dynamics (CFD) software package Fluent 6.1 is used to model the oscillating flow inside a pulse tube refrigerator. Helium is used as working fluid for the entire simulation. The simulated DIPTR consists of a transfer line, an after cooler, a regenerator, a pulse tube, a pair of heat exchangers for cold and hot end, an orifice valve with connecting pipe, a double inlet valve with connecting pipe and a reservoir. The simulation represents fully coupled systems operating in steady-periodic mode. The externally imposed boundary condition is sinusoidal pressure inlet by user defined function at one end of the tube and constant temperature or heat flux boundaries at the external walls of the hot end and cold-end heat exchangers. The general results, such as the cool down behaviors of the system, phase relation between mass flow rate and pressure at pulse tube section and the temperature profile along the wall of the cooler are presented.The simulation shows the minimum decrease in temperature at cold-end heat exchanger for a particular combination of cryocooler assembly. The CFD simulation results are compared with available experimental data. Comparisons show that there is a reasonable agreement between CFD simulation and experimental results.     

Flexibly using results of CFD and simplified heat transfer model for pulverized coal‐fired boilers

The efficient use of pulverized coal is crucial to the utility industries. The use of computational fluid dynamics (CFD)‐based numerical models has an important role in the design of new boiler furnaces or in retrofitting situations. The results of CFD simulations can be used to better understand the complex processes occurring within the boiler furnace. The use of these results to support boiler operation and training of operators requires that the CFD models can be easily accessed and the results are easily analysed. This paper discusses two ways to simulate the heat transfer process in boiler furnaces. The method directly applying CFD results is employed, in which the grid for solving the energy equation is the same as the flow grid in the CFD simulation while radiation heat transfer is solved in another relatively coarse grid. Comparison of the prediction results between CFD and Heat Transfer code (Simple model) is performed under boiler full load (100%) with one side wall fouling, as well as for different boiler loads (100, 98 and 95 per cent boiler full load, respectively). Finally, the flexible use of the results of CFD and the simple model for pulverized coal‐fired boilers is presented. To facilitate the use of the system, a user‐friendly interface was developed which enables the user to manipulate new calculations and to view results, namely performing ‘what–if’ analysis. Copyright © 2000 John Wiley & Sons, Ltd.     

Optimal configuration of discrete heat sources in a vertical duct under conjugate mixed convection using artificial neural networks

This paper reports the results of a numerical investigation of the problem of finding the optimum configuration for five discrete heat sources, mounted on a wall of a three-dimensional vertical duct under mixed convection heat transfer, using artificial neural networks (ANN). The objective is to locate the positions for the five heat sources in such a way that the maximum temperature of any of the heat sources in a given configuration is a minimum. The three-dimensional governing equations of mass, momentum and energy equations for the fluid flow and the energy equation for the solid regime have been solved by using FLUENT 6.3 and a database of temperature versus configuration was generated. The temperature database developed from CFD simulations is used to train the neural network. The trained neural network predicts the temperature of the heat sources very accurately and much faster than the CFD software. With the use of this network, an exhaustive search for all possible configurations was done that resulted in a global optimum for the problem.     

Validation of a CFD model for the simulation of heat transfer in a tubes-in-tank PCM storage unit

A computational fluid dynamics (CFD) model was developed for the simulation of a phase change thermal energy storage process in a 100 l cylindrical tank, horizontally placed. The model is validated with experimental data obtained for the same configuration. The cold storage unit was charged using water as the heat transfer medium, flowing inside a horizontal tube bundle, and the selected phase change material (PCM) was microencapsulated slurry in 45% w/w concentration. The mathematical model is based on the three-dimensional transient Navier–Stokes equations with nonlinear temperature dependent thermo-physical properties of the PCM during the phase change range. These properties were experimentally determined using analytical methods. The governing equations were solved using the ANSYS/FLUENT commercial software package. The mathematical model is validated with experimental data for three different flow rates of the heat transfer fluid during the charging process. Bulk temperature, heat transfer rate and amount of energy stored were used as performance indicators. It was found that the PCM bulk temperatures were predicted within 5% of the experimental data. The results have also shown that the total accumulated energy was within 10% of the observed value, and thus it can be concluded that the model predicts the heat transfer inside the storage system with good accuracy.     

Experimental study of a domestic thermoelectric cogeneration system

Thermoelectric application for power generation does not appear to be appealing due to the low conversion efficiency given by the current commercially available thermoelectric module. This drawback inhibits its wide application because of the overall low thermal efficiency delivered by typical thermoelectric applications. This paper presents an innovative domestic thermoelectric cogeneration system (TCS) which overcomes this barrier by using available heat sources in domestic environment to generate electricity and produce preheated water for home use. This system design integrates the thermoelectric cogeneration to the existing domestic boiler using a thermal cycle and enables the system to utilise the unconverted heat, which represents over 95% of the total absorbed heat, to preheat feed water for domestic boiler. The experimental study, based on a model scale prototype which consists of oriented designs of heat exchangers and system construction configurations. An introduction to the design and performance of heat exchangers has been given. A theoretical modelling for analysing the system performance has been established for a good understanding of the system performance at both the practical and theoretical level. Insight has also been shed onto the measurements of the parameters that characterise the system performance under steady heat input. Finally, the system performance including electric performance, thermal energy performance, hydraulic performance and dynamic thermal response are introduced.     

Theoretical Investigation of Waste Heat Recovery from an IC Engine Using Vapor Absorption Refrigeration System and Thermoelectric Converter

This study proposes the preliminary simulation of a single cylinder spark ignition engine with waste heat recovery system. To harvest waste heat energy from the engine exhaust a thermoelectric generator coupled to a vapor absorption refrigeration (VAR) system was proposed in this simulation work. Parametric simulation of engine, thermoelectric generator and VAR using thermodynamic relations was carried out in MATLAB – Simulink software. An attempt has been made mathematically to integrate engine, thermoelectric generator and VAR system to study the effect of engine load, speed, equivalence ratio on thermoelectric output and coefficient of performance (COP) of a VAR system. In this study, the VAR system runs by taking heat energy from the exhaust gas and the electric power produced by a thermoelectric generator was utilized to run the pump of the refrigeration system. It was found that COP of the absorption refrigeration system depends on engine load, speed and air fuel equivalence ratio. The study also reveals that about 10% to 15% of the total exhaust energy can be harvested using this system.     

Numerical Modeling of Thermoelectric Generators With Varing Material Properties in a Circuit Simulator

When a thermoelectric generator (TEG) and its external load circuitry are considered together as a system, the codesign and cooptimization of the electronics and the device are crucial in maximizing the system efficiency. In this paper, an accurate TEG model is proposed and implemented in a SPICE-compatible environment. This model of thermoelectric battery accounts for all temperature-dependent characteristics of the thermoelectric materials to include the nonlinear voltage, current, and electrothermal coupled effects. It is validated with simulation data from the recognized program ANSYS and experimental data from a real thermoelectric device, respectively. Within a common circuit simulator, the model can be easily connected to various electrical models of applied loads to predict and optimize the system performance.      

Performance study of one-dimensional models for stratified thermal storage tanks

Two basic approaches are used to model the temperature distribution in thermal storage tanks for solar domestic hot water (SDHW) systems. In the multinode approach, the tank is divided into N nodes, with an energy balance written for each node. This approach results in a set of N differential equations that can be solved for the temperatures of the nodes as a function of time. In the plug flow approach, segments of liquid of different temperatures and sizes are assumed to move through the tank in a plug flow manner. The sizes of the fluid elements are determined mainly by the simulation time step and the flow rates. Whenever the incoming fluid from the heat source is colder than the fluid at the top of the tank, “plume entrainment” occurs. A model describing plume entrainment has been incorporated into both the multinode and the plug flow models in the TRNSYS program[1]. A performance study of the TRNSYS tank models has been carried out with experimental data from two different sources. Three performance numbers have been defined for quantifying the accuracy of the models compared with experimental data. Recommendations are given as to which tank model should be used under which conditions.     

Investigation on no-vent filling process of liquid hydrogen tank under microgravity condition

In order to investigate the no-vent filling performance under microgravity, the computational fluid dynamic (CFD) method is introduced to the study, where a model aiming at filling a liquid hydrogen (LH₂) receiver tank is especially established. In this model, the solid and fluid regions are considered together to predict the coupled heat transfer process. The phase change effect during the filling process is also taken into account by embedding a pair of mass and heat transfer models into the CFD software FLUENT, one of which involves liquid flash driven by pressure difference between the fluid saturated pressure and the tank pressure, and the other one indicates and calculates the evaporation–condensation process driven by temperature difference between fluid and its saturated state. This CFD model, verified by experimental data, could accurately simulate the no-vent filling process with good flexibility. Moreover, no-vent filling processes under different gravities are comparatively analyzed and the effects of four factors including inlet configuration, inlet liquid temperature, initial wall temperature and inlet flow rate, are discussed, respectively. Main conclusions could be made as follows: 1) Compared to the situations in normal gravity, the no-vent filling in microgravity experiences a more adequate liquid–vapor mix, which results in a more steady pressure response and better filling performance. 2) Inlet configuration seems to have negligible effect on the no-vent filling performance under microgravity since liquid could easily reach the tank wall and then cause a sufficient fluid-wall contact under any inlet condition. 3) Higher initial tank wall temperature may directly cause a higher pressure rise in the beginning, while this effect on the final pressure is not significant. Sufficient precooling and reasonable inlet liquid subcooled degree are suggested to guarantee the reliability and efficiency of the no-vent fill under microgravity.     

某中速柴油机冷却液流动及流固耦合传热计算分析

为获得直观的流场和温度场数据,对某16缸中速柴油机建立了完整的冷却水三维模型,采用FLUENT流体计算软件对其进行了绝热CFD计算分析,得到了冷却水流速、压力等数据。考虑到各缸冷却水套为并列排布方式,各缸之间相对独立,建立了单缸的缸盖-缸套-冷却水耦合传热模型,对其进行了缸盖-缸套-冷却水耦合传热仿真,获得了各部件比较精确直观的温度场分布。结果表明:仿真结果与实测数据吻合较好,从而为柴油机冷却水套的优化设计提供了依据。     

Optimization of high-pressure shell-and-tube heat exchanger for syngas cooling in an IGCC

This work investigates the flow field and the heat transfer characteristics of a shell-and-tube heat exchanger for the cooling of syngas. Finite volume method based on FLUENT software was used and the RNG k–ε turbulence model was adopted for modeling turbulent flow. The porosity rate, the distribution of the resistance and the distribution of the heat source are introduced to FLUENT by coupling the user defined function. The pressure drop, the temperature distribution and the variation of local heat transfer are studied under the effects of the syngas components and the operating pressure, and the effect of the arrangement of the baffles on the heat transfer is studied. The results show that higher operation pressure can improve the heat transfer, however brings bigger pressure drop. The components of the syngas significantly affect the pressure drop and the heat transfer. The arrangement of the baffles influences the fluid flow.     

Three-dimensional computational fluid dynamics modeling of anode-supported planar SOFC

Modeling plays a very important role in the development of fuel cells and fuel cell systems. The aim of this work is to investigate the electrochemical processes of a Solid Oxide Fuel Cell (SOFC) and to evaluate the performance of the proposed SOFC design. For this aim a three-dimensional Computational Fluid Dynamics (CFD) model has been developed for an anode-supported planar SOFC with corrugated bipolar plates serving as gas channels and current collector. The conservation of mass, momentum, energy and species is solved by using the commercial CFD code FLUENT in the developed model. The add-on FLUENT SOFC module is implemented for modeling the electrochemical reactions, loss mechanisms and related electric parameters throughout the cell. The distributions of temperature, flow velocity, pressure and gaseous (fuel and air) concentrations through the cell structure and gas channels is investigated. The relevant fuel cell variables such as the potential and current distribution over the cell and fuel utilization are calculated and studied. The modeling results indicate that, for the proposed SOFC design, reasonably uniform distributions of current density over the active cell area can be achieved. The geometry of the cathode gas channel has a substantial effect on the oxygen distribution and thus the overall cell performance. Methods for arriving at improved cell designs are discussed.     

舰载燃气轮机间冷器内部流动的三维数值模拟

板翅式换热器是舰载燃气轮机首选换热器形式。针对平直型翅片的矩形通道的结构特点,建立了流动换热分析的耦合计算模型,采用计算流体动力学（CFD）方法对间冷器的通道流场进行了数值模拟。给出并分析了计算区域内多个截面的温度、压力、速度、局部传热系数等参数的分布图形和变化趋势,并考察了不同工况下间冷器的工作能力。     

Simulation of velocity and temperature distributions of displacement ventilation system with single or double heat sources

In order to obtain a better understanding of flow characteristics of displacement ventilation,thethree-dimensional numerical models are developed using the CFD technology.The numericalsimulation results are verified by experiments,based on this,the velocity and temperature distributionof three-dimensional displacement ventilation system with single and double heat sources are studied.Velocity and temperature fields under two different cases of heat source are analyzed and compared.The numerical results show that there are three layers in vertical temperature fields of displacementventilation system with single or double heat sources,and the vertical temperature distribution ofsingle heat source is different from that of double heat sources.When indoor load is large,the comfortrequirement of people indoor can't be satisfied with displacement ventilation system only,thus anadditional refrigeration system is necessary.Furthermore,under the condition of two heat sources,thedisplacement ventilation parameters can't be computed simply according to single heat source inletparameters,therefore the interaction between heat sources should be considered.     

椭圆横管外降膜流动和换热特性分析

本研究基于VOF算法编写用户UDF(自定义函数),采用FLUENT软件建立了椭圆横管外降膜流动和换热的计算模型。根据CFD(计算流体力学)模型计算和分析了在不同长短轴比下管外降膜速度分布、压力分布、液膜内温度分布和管外换热分布的变化规律。研究结果表明:长短轴比的变化影响了管外液膜速度分布、压力分布和膜内温度分布;相比圆管,椭圆管的管外膜内液体流速更快。壁面压力沿周向逐渐减少并在X=0.9附近快速回升;随长短轴比e的增加,周向压力最小值位置逐渐向后推移。局部Nu数分布与压力分布在趋势上存在一致性。当e=1.65附近时,椭圆的换热性能最优;最后,通过对管形的研究分析,提出横管的传热分区模型。     

Soil temperature distribution around a U-tube heat exchanger in a multi-function ground source heat pump system

The imbalance of heat extracted from the earth by the underground heat exchangers in winter and ejected into it in summer is expected to affect the long term performance of conventional ground source heat pump (GSHP) in territories with a cold winter and a warm summer such as the middle and downstream areas of the Yangtze River in China. This paper presents a new multi-function ground source heat pump (MFGSHP) system which supplies hot water as well as space cooling/heating to mitigate the soil imbalance of the extracted and ejected heat by a ground source heat pump system. The heat transfer characteristic is studied and the soil temperature around the underground heat exchangers are simulated under a typical climatic condition of the Yangtze River. A three-dimensional model was constructed with the commercial computational fluid dynamics software FLUENT based on the inner heat source theory. Temperature distribution and variation trend of a tube cluster of the underground heat exchanger are simulated for the long term performance. The results show that the soil temperature around the underground tube keeps increasing due to the surplus heat ejected into the earth in summer, which deteriorates the system performance and may lead to the eventual system deterioration. The simulation shows that MFGSHP can effectively alleviate the temperature rise by balancing the heat ejected to/extracted from underground by the conventional ground source heat pump system. The new system also improves the energy efficiency.