Waste-heat Recovery Using Thermoelectricity and Silicon Carbide Power Electronics
Abstract: Energy consumption in the world has increased continuously due to a growing population and increased energy consumption per capita. Moreover, the largest part of consumed energy still comes from fossil sources which in 2016 was more than 130 PWh. In order to minimize the greenhouse effect and meet the climate targets, the world's energy consumption must be greatly reduced and the energy also has to be used more efficiently. Due to the low efficiency of internal combustion engines in vehicles, the transport sector is the second largest source of greenhouse gas emissions, responsible for 20 % of the total CO2 emissions in the EU. In this work the electrical arrangement and power conditioning system suitable for waste heat recovery, using thermoelectric energy conversion in heavy duty vehicles, are investigated. Without a proper power conditioning system, the recovered power from a thermoelectric generator (TEG) disappears in form of Joule-losses. High-efficiency inter-leaved step-down converter with 98 % efficiency was developed and tested on a real-scale prototype truck, equipped with two TEGs. In addition, a strategy was required for the connection of thermoelectric modules (TEM) in the TEGs. A TEG may consist of several hundred TEMs and without a suitable connection, the thermal losses can be so high that the net power, recovered by the TEG is insignificant. In the worst case this can lead to an even higher fuel consumption. Moreover, the possibility to employ silicon carbide (SiC) metal oxide semiconductor field-effect transistor (MOSFET), which is a voltage-controlled and normally-OFF device, with high electric field strength, in such a low-voltage application (100-200 V), was investigated. Due to the high blocking voltage and power density, SiC MOSFETs are expected to replace silicon (Si) insulated gate bipolar transistors (IGBTs) in power converters. However, in low-voltage applications where Si MOSFETs are usually used, there have not been any obvious advantages to use SiC MOSFETs as a substitute for Si MOSFETs. Here, it is shown that SiC MOSFETs can advantageously be used in low-voltage applications. SiC MOSFETs have exceptional properties that nevertheless are fully utilized today. The packages of currently available SiC devices are the same as those previously used for Si devices, with moderate electrical and thermal characteristics. This results in slow switching speed, unnecessary losses. A half-bridge planar module using SiC MOSFET bare dies were designed, manufactured and tested. It was shown that a module with the same structure and 8 SiC MOSFETs can be manufactured with ultra-low parasitic inductances. The total switching energy was found to be 4.4 mJ which is approx. 63 % lower than commercially available modules.This thesis can be divided into three parts. In the first part, thermoelectricity is introduced and an introduction of SiC MOSFETs and its applications are given. In the second part, the results of waste heat recovery using TEG and its electrical arrangement in a Scania truck are presented. In this part, also the output power and the efficiency of the converter, using Si and SiC MOSFETs, are discussed. In the final part, the proposed planar power module with SiC MOSFET bare dies, its benefits such as reduced switching losses and double-sided cooling, are presented.
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