# Matching and Control Optimisation of Variable-Geometry Turbochargers for Hydrogen Fuel Cell Systems
## 5 Conclusions, Limitations, and Future Work  
The core aim of this work was to quantifiably compare the performance of VGTs against FGTs for fuel cell applications where potential causes of discrepancies in previous literature findings have been eliminated, namely by the following:  
1. Optimising the mass air flow and pressure to ensure that both the FGT and VGT were able to operate at their maximum system efficiency across the range of current densities. 2. Ensuring that both the FGT and VGT are appropriately sized for the application. 3. Using an identical map, representing the VGT 'flush gap', for the FGT, essentially isolating the 'variable' aspect of the VGT. It was found that the fuel cell was much less sensitive than the air path system to variations in mass flow and pressure, such that optimal system performance was achieved when turbine work recovery was maximised--this corresponds to the 'flush gap' nozzle position, fully open back pressure valve, and fully closed wastegate. This obviously relies on the fact that the turbine (whether FGT or VGT) must be well matched to the fuel cell stack. It was also found that VGTs offer a potential benefit in that they are less sensitive to the operating point, maintaining high efficiency when the cathode air flow, pressure, or both are sub-optimal. Despite our differences in approach, these results correlate well with the findings of [PERSON] et al. [PERSON] et al. (2018), who also found that VGTs were more tolerant when the operating point was compromised. It follows from this that VGTs might offer potential efficiencies for transient loads, which is a suggested area for future work. Certain limitations of this work should be noted. Firstly, the matching exercise in this study was conducted for the VGT only, and the FGT was represented using the 'flush gap' map of the VGT. This was performed to isolate the effect of variable geometry and eliminate other potential differences between the FGT and VGT designs. It ensures the two turbocharger architectures have the same efficiency characteristics and swallowing capacity due to their identical base geometry. However, conducting an independent matching exercise with a catalogue of FGTs may find one with greater power recovery, and in practice, an FGT is likely to exhibit higher efficiency due to its simpler design. Secondly, the models used in this study only partially account for the effect of the high humidity of the exhaust flow from a fuel cell. The differences in the thermodynamic properties of the exhaust gas mixture are included; however, supersaturation of the water vapour leading to condensation forming in the turbine ([PERSON] et al., 2016) is likely to occur in practical applications and is not accounted for. This will likely affect the absolute values of the results but is unlikely to affect the comparison of the FGT and VGT, particularly considering that both turbines were found to operate at almost identical conditions after optimisation. Nevertheless, future work should consider the effects of condensation in the exhaust flow. The approach presented here could be combined with work by [PERSON] et al. [PERSON] et al. (2016), for example, who model turbine inlet flows with 100% humidity without condensation, and [PERSON] et al. [44], who characterise the effects of the nozzle gap on condensation formation, but not its subsequent implications on air system performance. Developing this research area may indicate other benefits for VGTs in managing fuel cell exhaust condensation and is of particular interest to the authors. Finally, this study compares the steady-state performance of both designs. Whilst steady-state conditions represent much of fuel cell operation (which can be less transient than engines by using the onboard battery as a power buffer), future work should assess the performance of the two turbos under transient fuel cell conditions. Air path characteristics during transient loads could be particularly suitable for VGTs, which were found here to have greater flexibility under variable conditions.