Last year as part of the 2022.1 release of the VehicleDemos library, Claytex added a Hydrogen Fuel Cell Electric (FCEV) bus model. Created out of the existing EV bus example, it showcased integrating models from the Hydrogen Library with VeSyMA vehicle models. Ambitiously, a high level of detail was included. Physical air and hydrogen supply models were built, coupling with a fuel cell stack model where current production was dependent upon the pressure and temperature of the reactants.
For the recent Asian Modelica conference in Tokyo, Japan, Claytex published an academic paper detailing the bus model in detail. A drive cycle test was chosen to exercise the model, with the results presented and discussed at length. It is important to note that with a high fidelity model, the level of complexity can be scaled; areas of interest are prioritised. In this case, a 1D chassis model, suspension and electric motor were used, coupled to the complex physical 1D fluid based Hydrogen Fuel Cell model. But what can we do with a model of such fidelity?
Component selection and specification
The most obvious benefit of a detailed model such as this is the ability to utilise it as a digital twin. As the full system dynamics are captured, judgements regarding the size and types of components can be made. Representative conditions enable a picture of the working environment to be created, so the requirements the component needs to address can be understood.
In terms of a FCEV, the stack is sensitive to the temperature of the Hydrogen Fuel Supply. Therefore, being able to include the ambient temperature effecting the fuel supply pipes, as well as the freezing effect as the Hydrogen fuel expands out of the tank, enables an understanding of what countermeasures need to be taken. If the operating conditions are cold, some form of positive heat exchange with the fuel will be needed.
Figure 1: The Hydrogen supply system for the FCEV bus was complex, with model representing the components found on a typical system. Note: Hydrogen tank (1), fuel pressure valve (2), humidifier (3), recirculation pump (4) and purge valve (5).
System management / Control system design
Using detailed models as a plant model for control system development is common practice. As the model can recreate the phenomena associated with the real system, it makes sense to virtualise the development of the control system.
Reviewing the FCEV model, complex phenomena which will affect the control system are captured. A good example is that of the internal pressure management of the stack, can be observed when analysing the results of the model. Hydrogen supply is increased due to demand; the purge valve (regulating the mass fraction of oxygen in the anode) also opens more during demand events, indicative of a greater oxygen concentration in the anode as more reaction takes place. The action of the cathode exhaust valve corroborates this, as it closes slightly during demand events. Accruing to maintain pressure across the cathode, this indicates a pressure drop of some kind. Therefore, it can be theorised that there is gaseous loss towards the anode. A temperature gradient identified from the cathode inlet to the anode exhaust supports a theory of gaseous loss towards the anode in this manner.
Figure 2: Reviewing the control valve positions of the FCEV bus model can reveal a lot about the dynamics of the system.
System optimisation
Optimising a total system is also something a detailed model can be used for. Studies can be undertaken to determine the control conditions required for the system. This can be used to promote efficiency or extend the life of a system.
Small, detailed phenomena can be captured with such a model. A FCEV with electro-chemical modelling within the stack of the fuel cell, the mass fractions of the gases within are calculated. As this is the case, important dynamics, such as the mass fraction of oxygen in the anode are captured. This can then be used to develop the control system for the purge valve on the recirculation system, to keep the fuel cell operating in the desired performance window.
Closing Remarks
Not all simulation models are required to be hyper detailed. However, for various applications, a high amount of detail is useful.
Written by: Theodor Ensbury – Project Engineer
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