Thermoelectric-Generator-Based DC- DC Conversion Network for Automotive Applications
MOLAN LI
KTH Information and Communication Technology
Master of Science Thesis Stockholm, Sweden 2011
TRITA-ICT-EX-2011:58
Abstract As waste heat recovering techniques, especially thermoelectric generator (TEG) technologies, develop during recent years,its utilization in automotive industry is attempted from many aspects. Previous research shows that TEG as a waste heat harvesting method is feasible. Even though efficiencies for TEGs are as low as 3-5% with existing technology, useful electricity generation is possible due to the great amount of waste heat emitted from the internal combustion engine operation.
This thesis proposes the innovative concept of thermoelectric-generator-based DC-DC conversion network. The proposed structure is a distributed multi-section multi-stage network. The target is to tackle problems facing the traditional single-stage system and to advance TEG application in automotive settings. The objectives of the project consists of providing optimal solution for the DC-DC converter utilized in the network, as well as developing a systematic and bottom-up design approach for the proposed network.
The main problems of the DC-DC converters utilized in the TEG system are presented and analyzed, with solution to dynamic impedance matching suggested. First, theoretically-possible approaches to balance the large TEG internal resistance and small converter input resistance are discussed, and their limitations are presented. Then, a maximum power point tracking (MPPT) regulation model is developed to address the temperature-sensitive issue of converters. The model is integrated into a TEG-converter system and simulated under Simulink/Simscape environment, verifying the merits of MPPT regulation mechanism. With the developed model, MPPT matching efficiency over 99% is achieved within the hot side temperature range of 200°C ~300°C.
A design flow is suggested for the proposed network. Analysis is conducted regarding aspects of the design flow. Several state-of-the-art thermoelectric materials are analyzed for the purpose of power generation at each waste heat harvesting location on a vehicle. Optimal materials and TE couple configurations are suggested. Besides, a comparison of prevailing DC-DC conversion techniques was made with respect to applications at each conversion level within the network. Furthermore, higher level design considerations are discussed according to system specifications. Finally, a case study is performed comparing the performances of the proposed network and traditional single-stage system. The results show that the proposed network enhances the system conversion efficiency by up to 400% in the context of the studied case.
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Outline of the Report This thesis proposes the innovative concept of TEG-based DC-DC conversion network, develops a systematic and bottom-up design approach for the proposed network. At the same time, this thesis also provides optimal solution for DC-DC converter utilized in this network. This thesis report is written according to the structure of the related research work. Hence, the report is divided into four separate parts: Part I: Theory, Part II: DC-DC Converter Optimal Solution, Part III: TEG-Based DC-DC Conversion Network, and Part IV: Conclusions and Future Work.
Part I starts with a general overview of the waste heat recovering technology and its automotive applications. Following it is a brief description of recent automotive thermoelectric projects. Then, significant results of previous research which are related to this thesis project will be reviewed, leading to the purpose and target of this project. In order for a broader range of readers to understand the work presented in this report, the mechanism of TEG is briefly explained (in Chapter 2), together with the principle of DC-DC converters (in Chapter 3).
In Part II, Chapter 4 analyzes the issue of impedance matching between TEG internal resistance and DC-DC converter input resistance. Theoretically-possible solutions for steady-state and dynamic matching are investigated regarding their feasibility. Then, Chapter 5 is dedicated to describe the development and evaluation of a MPPT Simulink/Simscape model targeting at achieving dynamic impedance matching.
Part III first introduce the concept of the proposed TEG-based DC-DC converter network for automotive applications, and the topology of the proposed network is compared with that of the traditional single-stage topology. Then in Chapter 7, a design flow is suggested for the proposed network, with details of aspects in the flow analyzed. Most importantly, at the end of Chapter 7, a case study is presented, demonstrating the advantage of the proposed network over the traditional single-stage system. The result shows significant enhancement of system conversion efficiency.
This report is concluded with Part IV, which is supposed to summarize major results achieved in this thesis work, and to give hints on possible tracks of future research.
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Acknowledgment It is said that gratitude is a virtue. This part is dedicated to special thanks that I would like to deliver to the people who help make possible the fulfillment of this thesis project.
First of all, I would like to thank Professor Li-Rong Zheng for introducing me to this intriguing research field. Thank you for the enlightening insights you provided during our first discussion about this thesis.
Also inspiring is the guidance from Dr. Qiang Chen, my supervisor in this thesis project. I am especially grateful that you treat me like a friend, respect my decisions and plans, and encourage me with every progress, no matter how humble it is, along the way.
I would like to thank Dr. Shaohui Xu for providing me with the collection of data from his investigation on thermoelectric materials. Without these data, many parts of this thesis would be less convincing.
I would also like to thank Dr. Sascha Populoh from Swiss Federal Laboratories for Materials Testing and Research, whom I met during the 2010 International Conference on Thermoelectrics in Shanghai. Thank you for providing me with friendly reminds on related new regulations and unsolicited suggestions on the selection of thermoelectric materials utilized in automobiles.
I would like to say thank you to my parents and my boyfriend. With your support in times of difficulties and your understanding during my mood-swings, I feel like I could conquer every hardship and reach even higher.
Last but definitely not the least, I thank all my dear friends around the world, and also my fellows in KTH. You guys stimulate me and show up helping hands in times of need. It is true that friend in need is friend indeed!
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Nomenclature