Dynamic-energetic optimization of light FC commercial vehicles

The challenge in designing a fuel cell electric drive lies in the vehicle- and vehicle application-specific dimensioning of the drivetrain components. The essential parameters to be considered for an optimization are the fuel cell output, the dynamics of the fuel cell, the mass of hydrogen in the tank, the capacity and maximum charging power of the HV battery, the output of the drive machine in motor and generator mode and also the dynamic behavior of the converters.

Virtual and real test procedures should be used to verify and validate model-based developed fuel cell drives. For this. a development platform for the dynamic-energetic optimization of these drives was realized at the university for applied sciences Hochschule Kempten (HKE).

In order to also be able to investigate upscaling effects, at a scaling of 1:10, a model- and a hardware-in-the-loop (HiL) performance or system test stand was put into operation, whose mutual digital twin was realized as a model-in-the-loop simulation (MiL simulation).

As an example, an optimized prototype FC drivetrain was implemented in a test vehicle, to be able to compare the results obtained in the road test with those of the simulations and test bench measurements. The use of iterative and recursive procedures ensured the reproducibility of the results and demonstrated the functionality of the methods developed.

The innovation now lies in the fact that small and medium-sized companies can significantly reduce development costs and considerably shorten development times by applying these methods.

Diagram of the development platform realized by the method coupling

The MiL simulations optimally describe the behavior of the drives on the HiL test bench. The HiL test bench measurements optimally predict the behavior of the drives in the test vehicle. Through an iterative and recursive approach, it was possible to achieve the fact that the simulations already provide very good information about the use of the fuel cell drive in the vehicle.

The CleanEngine test bench (HSRM)
The specially developed test bench of the university Hochschule RheinMain (HSRM) enables the detailed investigation of fuel cell systems (FC systems) with a stack output of 3 to 10 kW. This development includes the control of the test bench and the FC system as well as precise monitoring of all relevant parameters of the fuel cell stack and its peripheral components required for operation.

The aim of the project CleanEngine is to achieve performance or energy requirements of real driving situations (WLTP, etc.), as taken from “real trips” of suitable vehicles, to “downscale” them on a test stand and develop a “driving program.” This driving program should allow optimized operation in terms of dynamics and avoidance of critical states of the fuel cell in accordance with the requested road performance. By closely monitoring the system parameters, it is possible to control the FC system in such a way as to minimize the energy consumption of the vehicle’s operation – for example the auxiliary units, which today use up to 15 percent energy and always leave the fuel cell system in its comfort zone. For this purpose, three PEM FC stacks of power classes 3, 6 and 9 kW were procured as part of the project. When designing the FC systems, emphasis was placed on a vehicle-oriented design, in close consultation with the team of HS Kempten regarding its experimental setup.

In addition to the variability of the operating temperature and pressure, the test setup is characterized by passively adjustable humidification and active recirculation of the hydrogen. Initial experience has confirmed that these are important levers for flexible adaptation to different operating conditions and for increasing the efficiency and service life of the systems.

Fig. 2: Test bench of Hochschule RheinMain

A comprehensive sensor system records all mass and energy flows within the FC system. This includes simultaneous single-cell voltage measurement and determination of the power requirements of all system components. Additionally, temperatures, pressures and humidity values are continuously monitored, which enables a precise analysis of the operating states.

The test bench offers the possibility of determining the polarization characteristics of the FC stack as well as carrying out electrochemical impedance spectroscopy on individual cells or optionally on the entire stack. These processes are crucial for understanding the electrochemical properties and performance capability of the fuel cells. In addition to these analytical methods, driving cycle tests and endurance tests can be carried out on the test bench in order to investigate the aging and failure mechanisms of the fuel cells.

The open test system and the flexible control of the FC system allow a wide range of system components to be tested. These include compressors, refrigerants, humidification concepts, valves and a variety of sensors. They can also be used to further develop fuel cell technology. They provide new insights into the performance and efficiency of the FC systems investigated and enable the identification of optimization potential in terms of operating temperature, operating pressure, humidification and recirculation.

They also support the development of improved control and monitoring systems for fuel cell systems, particularly with regard to humidity and temperature curves. The results provide a basis for the further development of analytical methods such as electrochemical impedance spectroscopy in order to better understand the electrochemical properties and performance capabilities of fuel cells. In addition, they show the influence of different operating conditions on aging and failure mechanisms of fuel cells in order to improve the longevity and reliability of FC systems.

Schematic structure of the test stand of Hochschule RheinMain

Configuration of the HiL system test stand (HKE)

While all components are represented by physical models in an MiL simulation, all essential components of the drive can be integrated and characterized as hardware on an HiL test bench. Non-available components, such as the vehicle itself or the environment, etc., are in turn represented by physical models in the form of a residual bus simulation.

The following essential components are currently integrated into the test stand:

• Toyota fuel cell system, 80 kW, dynamics ± 30 kW/s
• Synchronous drive machine, 85 kW
• HV traction battery 36 kWh, lower capacities can be simulated on the software side
• Asynchronous load motor P_max = 250 kW for applying the load cycles
• External storage battery (222 kWh) for storing electrical energy and for grid independence

Features of the HiL system test stand

• Complete drive trains and all individual components can be examined and characterized
• The test stand is currently designed for drives with a max. drive power of 250 kW
• Possible test cycles are WLTC, NEDC, in particular also freely configurable scenarios
• The realization was carried out entirely in-house, from the idea to the start-up of operation

Figure 4 schematically shows the structure of the HiL system test bench. In the lower left block is shown the real integrated vehicle hardware, consisting of fuel cell system, prime mover, cooling system, traction battery, electrical converters and the power distribution unit (PDU). As the central control device, the MicroAutoBox 3 from DSpace is employed. For the complex regulation of energy flows between the fuel cell, drive unit and traction battery, an “intelligent energy flow regulator” was developed as software for the control unit.

Measurements on the system test bench very quickly showed that the electrical energy storage device (namely traction battery or HV battery) is the limiting factor for vehicle applications. It is not only the capacity of the electrical storage system that is decisive, but rather the maximum charging capacity of the battery during recuperation and simultaneous fuel cell lag that limits the storage of the recovered energy, so additional mechanical braking is often required. This results in the need to accelerate the development of battery systems for hydrogen-electric drives.

Topology of the HiL system test stand

Comparison of the results from MiL simulations with HiL test bench measurements

Figure 5 shows the comparison of the simulation results (in the left column) with the test stand measurements (in the right column). Basis of the comparison is the WLTC Class 3 cycle. The first row shows the speed profile in blue and the SoC of the battery in red. The motor torques are compared in the second row, and the motor speeds in the third row.

The fourth row shows the performance curves for the fuel cell system, the motor and the battery. The charging power limit of the battery and the maximum charging power set by the intelligent energy flow regulator are also shown.

Overall, it can be stated that the results of the MiL simulation correspond very well with the results of the HiL test bench measurements.

Comparison of the results from simulation and test stand measurements based on WLTC class 3

H₂ research facility of Hochschule Kempten

The H₂ research facility (test stand and infrastructure) of Hochschule Kempten was installed on the campus of the wastewater association Abwasserverband Kempten (AVKE, see Fig. 1). There, the hydrogen center Wasserstoffzentrum Kempten will appear.

The cooperation of Hochschule Kempten with the AVKE is a result of the project HyAllgäu, which was funded as a feasibility study as part of the program HyLand under the subprogram HyExperts. The subject of the project was the question of the extent to which the Allgäu’s future hydrogen requirements can be covered by H₂ production in the Allgäu region (see H2-international, May 2021).

Next steps and summary
The driver testing by the company ABT e-Line GmbH is currently happening, and then the vehicle measurement data will be compared with the measurement data from the system test bench. That the simulation results agree very well with the results of the test stand measurements has already been mentioned. We are currently working on the dynamic-energetic optimization of the hydrogen-electric drive mentioned. The key question here is: How or by what means can the efficiency of the fuel cell in conjunction with the HV traction battery be raised in order to, for example, minimize the H₂ consumption?

In addition, it was shown that, to meet the requirements of hydrogen-electric drives, further development of electrical storage systems towards hybrid systems consisting of high-performance and high-energy batteries and supercapacitors is urgently recommended.

In the project CleanEngine, we have learned to understand the relevant parameters of energy management and to draw conclusions from them, that is, to analyze the energy flows between the FC system, traction battery and drive motor and to optimize them based on the vehicle type and application using a specially developed intelligent energy flow controller. This is currently in testing. Prerequisite is the optimized dimensioning of the components H₂ tank (H₂ quantity), battery capacity, power of the FC system and the drive unit.

In conclusion, in the project CleanEngine, procedures, methods and tools were developed whose practical application make it possible to answer comprehensive technical and scientific questions in the context of hydrogen-electric drives for stationary and mobile applications.

The results from the support project (Förderprojekt) CleanEngine show the importance of a holistic view of fuel cell systems including the BoP (balance of plant) components. The unique structure of the project enables the zoom from the level of the finished FC hybrid vehicle to a prototype hybrid drivetrain system to the individual components that are needed to operate an FC stack, and thus the representation of the interactions of these system levels and components.

The project CleanEngine is funded by the German ministry for digital infrastructure and transport (BMDV). Administrative responsibility lies with the Nationale Organisation Wasserstoff GmbH (NOW), and the responsibility as project organizer Projektträger Jülich (PTJ). Project partner in addition to Hochschule Kempten (HKE – Yue Ni, André Giesbrecht, Moritz Gegenbauer, Christoph Zettler) and Hochschule RheinMain (HSRM – Max Kleber, Georg Derscheid, Matthias Werner) is the industrial company ABT eLine GmbH. The project duration, after an extension by twelve months, spans from December 1, 2020 to November 30, 2024.

Authors:
Prof. Dr. Birgit Scheppat
Hochschule RheinMain
Birgit.Scheppat@hs-rm.de

Prof. Dr. Werner E. Mehr
Hochschule für angewandte Wissenschaften Kempten
werner.mehr@hs-kempten.de