Abolfazl Simorgh: “Hydrogen propulsion would need additional studies on emissions and their impact”

Abolfazl Simorgh, PHD candidate at Universidad Carlos III of Madrid and member of the Hydrogenating CM-UC3M team, is also the main writer of the article “A Comprehensive Survey on Climate Optimal Aircraft Trajectory Planning”.

The paper, a joint work with the FlyATM4E project team, has been published by MDPI in the open access journal Aerospace and aims to study different ways to mitigate the impact of aviation on climate change. In order to know more about it, we have interviewed Abolfazl Simorgh:

What does current data say about the contribution of aviation to climate change?

Currently, aviation is responsible for about 3-5% of total global warming; however, due to the strong growth rate of the aviation industry, the associated climate impact is expected to grow critically. These climate effects are due to CO₂ emission and other non-CO₂ effects. According to the latest estimate by Lee et al. (2021), the effective radiative forcing from non-CO₂ emissions, such as contrails and NOx, is twice as large as CO₂ emission alone.

Fuel is the main responsible for the CO₂ emissions. Is there any realistic alternative to traditional fuel? Is it feasible in the medium/short-term?

Current and near-term aviation propulsion systems are mainly based on kerosene-fuelled jet engines. However, new entrants are expected in the medium term. This is the case for electric propulsion, H₂ propulsion, hybrid aircraft that can combine electric/H₂ engines with paraffin jet, and the use of sustainable aviation fuels. In all cases, additional studies on emissions and their impact would be needed, as well as new dynamic models to adequately capture the dynamic behaviour of such systems.

Your studies suggest the importance of taking climate impact into account in the planning of aircraft trajectories. What examples could you give us of this?

The climate impact of non-CO₂ emissions is highly dependent on geographical location, altitude and timing of emission and can be mitigated by climate-aware aircraft trajectories. For example, the formation of persistent condensation trails can be considered, where sufficiently low temperatures and sufficiently humid environments are required for their formation. Thus, flying in areas that meet these criteria can lead to the formation of condensation trails, which then require ice supersaturation for their persistence.

Which are the operational strategies to reduce climate impact presented in the paper?

As mentioned, the climate impacts of non-CO₂ emissions can potentially be mitigated through climate-aware trajectory planning. In this regard, the operational strategies proposed in the literature can be classified into two categories: non-trajectory-based approach and trajectory optimisation-based approach. Recent studies have identified climate-aware trajectories using optimisation-based strategies to obtain a better balance between reducing climate impacts and the corresponding increase in operational costs. This study reviews the above approaches in detail, looking primarily at the climate metrics used and the optimisation of trajectories.

From this analysis, can you tell which will be your next lines of research?

From the survey, we have identified some scientific gaps. First, since the actual mitigation potential depends largely on the reliability of the quantified climate impacts, the uncertainty associated with the estimated climate impacts -such as weather conditions- needs to be identified and quantified.

Once the different sources of uncertainty have been quantified, trajectory optimisation methods capable of providing optimal solutions while accounting for uncertainties are required to plan robust trajectories. In addition, robust aircraft trajectory optimisation methods must provide results in computational times compatible with operations.

Another line of research is to move, in the medium term, towards electric propulsion, H₂ propulsion, hybrid aircraft that can combine electric/H₂ engines with paraffin jet and the use of sustainable aviation fuels. In all cases, additional studies on emissions and their impact would be needed, together with new dynamic models to adequately capture the dynamic behaviour of such systems.

One last open scientific gap relates to the consideration of climate optimal trajectories within the network scale. Climate hotspots, their incorporation into network-wide models and solution approaches for problems such as demand and capacity balancing, network complexity and resilience have not been studied so far.