Optimal design of energy storage systems for hybrid vehicle drivetrains T. Hofman, D. Hoekstra, R.M. van Druten, and M. Steinbuch Technische Universiteit Eindhoven Dept. of Mechanical Engineering, Control Systems Technology Group NL 5600 MB Eindhoven, The Netherlands Email: t.hofman@tue.nl Abstract— Current hybrid drivetrain simulation packages are based on discrete (existing) system components and predefined system structures. Optimization of the performance of the hybrid drivetrain is then based on finding the most efficient control strategy of the primary and secondary power source and finally comparing the performance of the different candidate drive- trains. In this paper, the secondary power source components, part of the energy storage system (S), are modeled continuously, i.e., scalable to power and/ or energy capacity needs. In this way, the size of the components of S can be added as an optimization parameter to a hybrid drivetrain design procedure. Keywords: energy storage, hybrid drivetrain, multi-objective design problem, scalable model, SQP optimization, Penalty and Barrier functions I. I NTRODUCTION In recent years several drivetrain simulation software packages have been developed, these packages can be used to develop hybrid drivetrain configurations and control strategies (e.g. ADVISOR [1], SIMPLEV [2], CarSim [3], HVEC [4], CSM HEV [5], V-Elph [6]) [7]. An optimal hybrid drivetrain design is obtained by optimizing a selected hybrid drivetrain configuration (e.g. series, parallel, series-parallel), to components and control. A drawback of the mentioned software packages is that they are based on a discrete set of (existing) drivetrain components, fixed in size (power, energy capacity). With the software tool QSS-Toolbox [8] it is possible to build drivetrain structures with scalable models for the Electric Machine (EM) and combustion engine. In this paper, in addition to scalable models for the EM, continuously scalable models will be constructed and evaluated for ultra-capacitor, battery, gas-pressurized tank, sub- and supercritical flywheel storage systems. Hybridization of a vehicle drivetrain implies adding a Sec- ondary power source (S, mostly a battery and an electric motor) to a Primary power source (P , usually an internal combustion engine). The objectives of a hybrid drivetrain are to improve the driving functions of a vehicle, i.e., fuel econ- omy, emissions, driveability, comfort and safety. Hybridization allows performing brake energy recovery, downsizing the engine and optimizing the power flows over the different thermal, mechanical and electrical paths between the different power sources. The NWO 1 research programme “Impulse Drive” currently focuses on determining the required design specifications of the systems components for a hybrid vehicle fulfilling the required driving function improvements. The influence of the generic design specifications for the S on fuel economy and Energy Management Strategy (EMS) has been investigated in [9], the required vehicle driving function improvements serve to identify the system component specifications [10] [11]. The storage and conversion components, that provide the hybrid functionality, will be modeled continuously over their power and/or energy range in order to find an optimal design to a given control strategy. This will provide insights into (1) the solutions of the possible set of components necessary for the design of S, (2) quantification of the design trade-offs in achieving the objectives and into (3) the possibilities and limitations of the technologies that are under investigation. In order to be able to compare and evaluate performance of the hybrid drivetrain component designs, models will be generated of the efficiency, mass, volume and cost. II. HYBRID S SYSTEM The system that brings the hybrid functionality, i.e., S is defined as a system that can store and deliver energy to the drivetrain through one rotating mechanical drive shaft. Within this constraint, six different concept S system configurations have been identified, based on current technological possibil- ities, that will be analyzed and modeled further (see Table I). To be able to compare the S i designs (i =1, .., 6), each S i also has the design constraint of providing control over transmitted power flow regardless of the rotating speed or torque. Adding a Continuously Variable Transmission (CVT) to system S 2 is thus necessary to be able to change torque for a required output power at a certain angular shaft speed. Several of these systems have been modeled based on analytical models, a reference design was then constructed to identify the design and size scaling parameters x . Scaling of look-up table based components is done by interpolation of properties (efficiencies, mass, volume) between discrete existing designs. A component cost model was constructed by literature investigation into manufacturing prices [12]. It is assumed that all dynamic effects of the components can be 1 The Netherlands Organization for Scientific Research