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Design methodology for waste heat recovery systems in vehicles considering optimal control
Philipp Petr, Wilhelm Tegethoff, Jürgen Köhler
Session: Session 1B: Dynamic Simulation
Session starts: Wednesday 13 September, 10:30
Presentation starts: 12:10
Room: Building 27 - Lecture room 02

Philipp Petr (TLK-Thermo GmbH)
Wilhelm Tegethoff (TLK-Thermo GmbH)
Jürgen Köhler (TU Braunschweig, Institut für Thermodynamik)

In a motor vehicle, the internal combustion engine satisfies the prior demands such as mechanic energy for propulsion and auxiliaries as well as thermal energy for compartment heating. An applied waste heat recovery system based on the Organic Rankine Cycle (ORC) is a subordinate process. Thus, it underlies varying exhaust gas temperatures and mass flow rates as well as limitations given by superior systems like the cooling system or the after-treatment of exhaust gases. Inadequate system design and controls challenge the cost effectiveness of these small-scale recovery systems. In order to minimize the fuel consumption of a vehicle over the total drive, the design of the ORC system and particularly the control strategy have to be optimized with respect to dynamically changing boundary values. Consequently, the optimum expander inlet state is not static but depends on the actual exhaust gas inlet temperature and mass flow rate, the actual losses and time constants of the ORC components. The resulting (dynamic) volume flow rates and pressure ratios have to be considered when designing the ORC components. In this article, a methodology and framework for the design of waste heat recovery systems especially for transient operation is presented. In a first step, in total 32 working fluids are evaluated under various component parametrizations for a broad range of exhaust temperatures. Beside optimum expander inlet states, impacts on heat exchanger and expander design were assessed in quasi-static cycle simulations. For the design process, (dynamic) Modelica models of the ORC system implying working fluid models, controls and other involved systems are enhanced regarding simulation speed and robustness. The system models are exported as FMU for further investigations and optimization in the developed framework. In order to optimize component parameters (i.e. heat exchanger geometry), controller parameters are computed for the tested parametrization over a broad range of operating conditions to ensure accurate control and comparable conditions. In the next step, the cycle performance for the parametrization is evaluated. Therefore, an optimizer calculates optimal control variables in small time steps. By means of the developed framework, the design optimization process of the evaporator geometry for a waste heat recovery system in an omnibus is presented. Finally, the potential reduction of the fuel consumption of the presented methodology is evaluated in virtual test drives.