Multiphysics CFD Simulation for Design and Analysis of Thermoelectric Power Generation

The multiphysics simulation methodology presented in this paper permits extension of computational fluid dynamics (CFD) simulations to account for electric power generation and its effect on the energy transport, the Seebeck voltage, the electrical currents in thermoelectric systems. The energy transport through Fourier, Peltier, Thomson and Joule mechanisms as a function of temperature and electrical current, and the electrical connection between thermoelectric modules, is modeled using subgrid CFD models which make the approach computational efficient and generic. This also provides a solution to the scale separation problem that arise in CFD analysis of thermoelectric heat exchangers and allows the thermoelectric models to be fully coupled with the energy transport in the CFD analysis. Model validation includes measurement of the relevant fluid dynamic properties (pressure and temperature distribution) and electric properties (current and voltage) for a turbulent flow inside a thermoelectric heat exchanger designed for automotive applications. Predictions of pressure and temperature drop in the system are accurate and the error in predicted current and voltage is less than 1.5% at all exhaust gas flow rates and temperatures studied which is considered very good. Simulation results confirm high computational efficiency and stable simulations with low increase in computational time compared to standard CFD heat-transfer simulations. Analysis of the results also reveals that even at the lowest heat transfer rate studied it is required to use a full two way coupling in the energy transport to accurately predict the electric power generation

Design of Thermoelectric Generators for Automotive EGR Applications

Rising energy prices and greater environmental awareness along with stringent emissions legislation in the automotive industry make it possible to introduce techniques in the aftertreatment system that have previously been unprofitable. One such technique, studied in this work, is heat recovery in the exhaust gas recirculation (EGR) cooler using thermoelectric generators (TEG). CFD and thermoelectric simulations are supported with measurements in order to build models that describe the phenomena in a correct manner.Models describing thermoelectric effects, i.e., the Seebeck, Peltier and Thomson effects, together with Joule heating and Fourier conduction, are well known and describe the phenomena satisfactorily when using temperature-dependent material data. A thermoelectric module contains interfaces between different materials and these are never in perfect contact. Consequently, at material junctions, non-ideal contacts will lower the thermoelectric performance. How large these thermal and electrical contact resistances are is not possible to determine analytically since they depend on several parameters that are dependent on the different materials and process parameters, as well as on contact pressure over the junctions. A method for determining contact resistances that combines measurements of commercial modules and simulations is developed and is shown to predict resistances with excellent results, even for geometrically different modules. Furthermore, measurements and simulations on different levels are performed, from detailed studies of a single thermocouple to a full scale study of a thermoelectric heat exchanger prototype. It is shown that the greatest heat transfer resistance is located on the gas side, and it is of great importance to improve the heat transfer in the gas to achieve good overall efficiency. At the same time, it is also of great importance to maintain a low pressure drop in the exhaust gas flow. Consequently, when integrating TEG in EGR coolers, for heat recuperation, it is also important to focus on the design of the heat exchanger to achieve high efficiency, and not only focus on developing new thermoelectric materials and reducing contact resistances. It is concluded that thermoelectric simulations combined with CFD allow fast and inexpensive concept evaluations, and different potential TEG designs can be evaluated and compared with high accuracy

Multiscale Simulation Methods for Thermoelectric Generators

Rising energy prices and greater environmental awareness, along with stringent emissions legislation, in the automotive industry make it possible to introduce techniques in the aftertreatment system that have previously been unprofitable. One such technique, studied here, is heat recovery from exhaust gases using thermoelectric generators. The design of thermoelectric modules and heat exchangers for thermoelectric generation relies, to a large extent, on simulation tools. Thermoelectric phenomena are well known, and several researchers have used first principle simulation to solve for thermoelectric generation in thermoelectric pairs and single modules. In order to obtain predictions that agree with measurements, knowledge of not only temperature-dependent material but also internal thermal and electrical contact resistances is required. A method that enables accurate quantification of contact resistances inside thermoelectric generators and which gives detailed insight into how these reduce module performance has been developed within the scope of this research. When implementing these resistances in first principle simulations, excellent agreement between measured and simulated performance has been achieved.First principle simulations allow great insight into thermoelectric performance and provide details, such as local current distribution, that are hard to measure or obtain with other methods and are great, for example, when designing modules. First principle models, on the other hand, are computationally too demanding when used to design heat exchangers that contain a large system of modules. Therefore, a novel framework for characterization and simulation of thermoelectric generator systems that allows for accurate and efficient prediction of electric and thermal performance has been developed in this research. When used in conjunction with CFD analysis, this framework allows for efficient modelling of electrical and thermal performance without relaxing the important two-way coupling of energy transport. This efficiency comes from the fact that the modelling does not require full resolution as first principle simulations do. Therefore it solves the scale separation problem and allows for multiphysics simulation with just a minor increase in computational power. All simulations were validated with experiments on different levels, both for individual modules, small systems of modules, and, finally, engine bench tests were used to validate a full-scale heat exchanger prototype containing a large number of modules and a complex fluid flow

Analysis of Thermoelectric Generator Performance by Use of Simulations and Experiments

A method that enables accurate determination of contact resistances in thermoelectric generators and which gives detailed insight into how these reduce module performance is presented in this paper. To understand the importance taking thermal and electrical contact resistances into account in analysis of thermoelectric generators, full-scale modules were studied. Contact resistances were determined by means of non-linear regression analysis on the basis of results from 3D finite element simulations and experiments in a setup in which heat flow, voltage, and current were measured. Statistical evaluation showed that the model and the identified contact resistances enabled excellent prediction of performance over the entire range of operating conditions. It was shown that if contact resistances were not included in the analysis the simulations significantly over-predicted both heat flow and electric power output, and it was concluded that contact resistance should always be included in module simulations. The method presented in this paper gives detailed insight into how thermoelectric modules perform in general, and also enables prediction of potential improvement in module performance by reduction of contact resistances