UQ eSpace

Abstract

Driven by recent changes to carbon emissions legislation car manufacturers are required to come up with innovative ways of reducing fuel consumption and CO2 emissions on their vehicles. It is estimated that roughly 25% of the fossil fuel energy being burnt in the combustion process is utilized for mobility, 5% is lost in auxiliary systems, 30% lost in the coolant and 40% is lost in the form of waste heat through the exhaust and other heat radiating components [1]. A thermoelectric generator (TEG), as a waste heat recovery system, relies on the Seebeck and Peltier effects which describe how a temperature difference across two dissimilar semiconductors, metals or other materials can create an electric potential across the module. This electric potential allows current to flow which may be harvested for further use as useful power. TEGs are solid-state devices which makes for a robust and reliable system. A TEG consists out of thermoelectric modules sandwiched between a heat sink and a heat source. Heat exchangers (HXR) are used to enhance heat transfer between the heat sink and heat source. The idea for an automotive TEG is to extract heat from the exhaust flow of an internal combustion engine with the hot side HXR and rejecting heat through diverting the coolant line of the vehicle to flow through the cold side HXR. The temperature gradient that this imposes on the thermoelectric modules enables a current to flow which can be converted it into power. In this work open-cell metal foam heat exchangers are considered for the exhaust gas flow HXR due to their superior thermal capabilities, light weight, high surface area and high permeability. Additionally, this work proposes to position the TEG in the exhaust gas recirculation (EGR) path of diesel vehicle. In most modern diesel vehicles, the EGR systems are equipped with both a hot and cold side HXR, positioned on a bypass duct and controlled through an EGR valve. This is most of the infrastructure that a TEG requires for safe operation, therefor integrating these systems minimises cost and improves the financial feasibility of the system. The energy recovered by the thermoelectric generator may be used to reduce the electrical load on the alternator and thereby reduce the mechanical load from the alternator on the engine. Reducing the load on the engine is directly proportional to a reduction in fuel consumption and reduced CO2 emissions. The feasibility of implementing TEGs is limited by the thermoelectric materials' energy conversion efficiency. From an economic standpoint the implementation of TEGs is only feasible for commercialization in medium to heavy-duty diesel engines at an average reduction in overall fuel consumption of ~ 4% from implementing a TEG.