Effects of a Thermoelectric Waste Heat Recovery System on the Engine Performance and the Cooling System of a Natural Gas Powered Heavy-Duty Vehicle

Kurzfassung

The European Commission proposes to reduce CO2-emissions from heavy-duty vehicles by 35% by 2030 compared to 2019. Several technological approaches are required to achieve this ambitious goal. While electrification of short-distance distribution transport seems possible during this period, internal combustion engines continue to dominate long-distance freight transport and must therefore be further developed in terms of their efficiency and emissions. In this context, the market for low-emission natural gas-powered engines is growing. Although these engines produce less CO2-emissions, they are still unable to comply with the upcoming regulations, so that further measures such as waste heat recovery systems are necessary. One technology for this purpose is the thermoelectric generator. It is based on a solid state effect, the so-called Seebeck effect, and uses heat exchangers to convert the heat flow from the exhaust gas to the cooling system directly into electricity using semiconductor materials. In addition to the positive effect of the converted electrical power, negative effects influence the overall system. In particular, the additional pressure loss through the heat exchanger in the exhaust system and the heat transfer to the cooling system are decisive influencing factors. As an example, the additional pressure drop in the exhaust system must be overcome by the engine and reduces its efficiency. The additional heat input into the cooling system causes the cooling fan to be switched on earlier, which leads to a higher electrical consumption. This study aims to quantify these negative effects of waste heat recovery systems, in particular thermoelectric generators, on the overall vehicle system of natural gas powered heavy-duty vehicles. For this purpose, a one-dimensional model of a diesel engine is modified in order to adapt it to a natural gas engine. The CURSOR 13 NG engine serves as a reference. The modifications include the combustion model, the engine control, the turbocharger, the exhaust aftertreatment system and the cooling system. The influences of the thermoelectric generator are simulated for the operating points of the World Harmonized Stationary Cycle. For the pressure drop, various throttle valves are inserted into the exhaust system and the reactions of the engine are simulated. A heat exchanger is modelled for the cooling system, which transfers the heat either to the high or low temperature cooling circuit. The cooling capacity of the radiator is based on the vehicle speed of the World Harmonized Transient Cycle under suitable conditions. The results are summarized for both variables into characteristic maps. The efficiency of the motor decreases almost linearly with the pressure drop and is strongly dependent on the current operating point. At high loads, the pressure drop has a proportionally lower influence on the efficiency of the motor. The remaining capacity of the cooling system depends on the cooling circuit used, the maximum permissible coolant temperature and the vehicle speed. It ranges from around 40 kW to 120 kW. Both data sets show a high potential for the integration of a thermoelectric generator and are used for the further development and optimization of the system. Keywords: Thermoelectric Generator, Heavy-duty Vehicle, Waste Heat Recovery System