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Description
Contrails are trails of ice crystals formed by the exhaust of aircraft engines, and form in sufficiently cold and humid air when water vapour in the engine exhaust freezes into ice crystals. Although the physics of contrail formation and how they affect the climate are ambiguous, long-lived contrails, also referred to as aircraft-induced cloudiness (AIC), dominate the non-CO2 impacts of aviation in terms of net radiative forcing (RF) and may account for over half (57%) of the total climate impact of aviation. This has led to the ideation of various technical and operational mitigation strategies to lessen the climate impact of contrails, such as changing contrail properties or avoiding contrail formation.
This project aimed to devise technical solutions for contrail suppression through jet engine redesign, with an emphasis on options that are relatively short-term and that require minimal modifications to existing engines. Three solutions were explored: using liquid hydrogen fuel instead of kerosene, temporarily reducing the engine overall efficiency, and temporarily raising the engine exhaust temperature via reheat.
An extended version of the widely accepted Schmidt-Appleman criterion was used to model contrail formation, and the gas turbine performance software GasTurb 14 was used to model the Rolls-Royce Trent 700 engine as a case study. This engine is commonly used on the Airbus A330 wide-body airliner, and was chosen due to the ease of implementing reheat with its mixed exhaust.
The results for the first solution indicate that using hydrogen fuel leads to contrail formation over a larger range of altitudes than when kerosene is used; however, this does not necessarily imply greater climate impact, as the absence of soot and sulphur emissions results in reduced optical thickness and hence a net climate impact that may be comparable or even more favourable than that of kerosene. Replacing the current fleet of commercial aircraft to hydrogen-fuelled cryoplanes is a long-term solution, and so was not pursued any further.
The second solution investigated the effects of changing the engine overall efficiency; analysis indicated an increase in the contrail-forming altitude range for more efficient engines due to the higher threshold temperatures for contrail formation. Consequently, the engine overall efficiency can be temporarily lowered as a contrail suppression measure when persistent contrails are likely to form, but this measure cannot be applied at altitudes higher than around 9.75 km, implying a severe lack of flexibility, and emphasising the need for coupling with other measures such as rerouted flights.
The most promising solution was found to be the reheated engine with a hotter exhaust. Reheat gives rise to an inevitable fuel burn penalty, but since it is envisaged to be used only when persistent contrails are predicted to form (which is a small portion of a typical flight), the increase in total fuel consumption was found to be manageable. Upon assessing the trade-offs between the increased CO2 impact and reduced contrail impact on the climate with this feature, it was found that using reheat to suppress persistent contrail formation is highly likely to induce less warming than if persistent contrails are allowed to form.
References
[1] B. Kärcher, “Formation and radiative forcing of contrail cirrus,” Nat Commun, vol. 9, no. 1, Dec. 2018, doi: 10.1038/s41467-018-04068-0.
[2] U. Schumann, “On conditions for contrail formation from aircraft exhausts,” Meteorologische Zeitschrift, vol. 5, pp. 4–23, 1996, [Online]. Available: https://elib.dlr.de/32128/1/mz-96.pdf
[3] U. Schumann, “Influence of propulsion efficiency on contrail formation,” Aerosp Sci Technol, vol. 4, pp. 391–401, 2000.
| Keywords | Contrails, Hydrogen, Engine overall efficiency, Reheat |
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