Publicação
Development of a thermoelectric generator for the exhaust of a vehicle
| Resumo: | The automotive industry is facing increasingly tighter targets regarding the emissions of greenhouse gases and pollutants. Added to this, there is a need to achieve higher energetic efficiencies in automotive applications. One of the ways to achieve this is to implement technologies and devices that allow to recover waste energy. The thermal power released by the exhaust gases has a great potential of recovery due to the high exhaust temperatures. This thermal energy can be converted into electricity by the Seebeck effect using thermoelectric generator modules. The research group of the Internal Combustion Engines Lab of UMinho has been exploring a concept of such a generator which has the ability of controlling the temperature to which the modules are subjected. This is made through the use of a thermosiphon / heat pipe (HP) device placed as a thermal buffer between the heat source (exhaust gases) and the thermoelectric modules. It converts the heat source temperature down to the desired level for the modules (~250 ºC) by means of a phase change process. The temperature control is made by controlling the phase change temperature with the inner pressure of the HP buffer. In the present work several sections of the thermoelectric generator concept have been modelled and assessed. This was made, on one hand, by improving and updating an existing MATLAB program which models the thermal and electric phenomena occurring in the generator through analytical and empirical analyses, and on the other hand, by modelling the exhaust heat exchanger through Computational Fluid Dynamics (CFD) techniques. The main contributions of the present work were: the update of the existing model with an unsteady heat transfer model of the exhaust heat exchanger based on explicit and implicit finite difference methods (a computational time reduction of more than 90% for driving cycle simulations was achieved with the implicit method, and with lower accumulated errors); the modelling of the heat transfer at the water cooling system, in which a comparison in terms of heat transfer and pressure drop was made between series and parallel ducts configuration; the improvement of the thermosiphon /HP model in order to allow inner pressure and boiling temperature variation. This modelling allowed to assess impact of the expansion tank size on the generator’s operation, this component being responsible to stabilize the pressure and operating temperature; and the CFD modelling of an exhaust heat exchanger based on a staggered tube bundle configuration with the assessment of the influence of parameters such as wall vanes and tube fins. The final exhaust heat exchanger design allowed to achieve an average effectiveness around 84% for a highway driving cycle, with a negligible pressure drop for a car engine. |
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| Autores principais: | Oliveira, João Paulo Dourado |
| Assunto: | Engenharia e Tecnologia::Engenharia Mecânica |
| Ano: | 2015 |
| País: | Portugal |
| Tipo de documento: | dissertação de mestrado |
| Tipo de acesso: | acesso aberto |
| Instituição associada: | Universidade do Minho |
| Idioma: | inglês |
| Origem: | RepositóriUM - Universidade do Minho |
| Resumo: | The automotive industry is facing increasingly tighter targets regarding the emissions of greenhouse gases and pollutants. Added to this, there is a need to achieve higher energetic efficiencies in automotive applications. One of the ways to achieve this is to implement technologies and devices that allow to recover waste energy. The thermal power released by the exhaust gases has a great potential of recovery due to the high exhaust temperatures. This thermal energy can be converted into electricity by the Seebeck effect using thermoelectric generator modules. The research group of the Internal Combustion Engines Lab of UMinho has been exploring a concept of such a generator which has the ability of controlling the temperature to which the modules are subjected. This is made through the use of a thermosiphon / heat pipe (HP) device placed as a thermal buffer between the heat source (exhaust gases) and the thermoelectric modules. It converts the heat source temperature down to the desired level for the modules (~250 ºC) by means of a phase change process. The temperature control is made by controlling the phase change temperature with the inner pressure of the HP buffer. In the present work several sections of the thermoelectric generator concept have been modelled and assessed. This was made, on one hand, by improving and updating an existing MATLAB program which models the thermal and electric phenomena occurring in the generator through analytical and empirical analyses, and on the other hand, by modelling the exhaust heat exchanger through Computational Fluid Dynamics (CFD) techniques. The main contributions of the present work were: the update of the existing model with an unsteady heat transfer model of the exhaust heat exchanger based on explicit and implicit finite difference methods (a computational time reduction of more than 90% for driving cycle simulations was achieved with the implicit method, and with lower accumulated errors); the modelling of the heat transfer at the water cooling system, in which a comparison in terms of heat transfer and pressure drop was made between series and parallel ducts configuration; the improvement of the thermosiphon /HP model in order to allow inner pressure and boiling temperature variation. This modelling allowed to assess impact of the expansion tank size on the generator’s operation, this component being responsible to stabilize the pressure and operating temperature; and the CFD modelling of an exhaust heat exchanger based on a staggered tube bundle configuration with the assessment of the influence of parameters such as wall vanes and tube fins. The final exhaust heat exchanger design allowed to achieve an average effectiveness around 84% for a highway driving cycle, with a negligible pressure drop for a car engine. |
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