# The Water-Enhanced Turbofan as Enabler for Climate-Neutral Aviation
## 5 Climate Impact Evaluation  
The WET climate impact was evaluated for cruise conditions based on the cycle presented in Section 4.2 and the methodology presented in Section 4 for CO\({}_{2}\), NOx, and contrails. WET reduced SFC by 13% compared to a turbofan with an equal technology level, based on the cycle presented in Section 4.2. The resulting engine weight is of the order of 40% higher, mainly driven by heat exchanger weights. The weight increase incorporates heat exchanger designs using advanced fin geometries and surface treatments, as well as geometry optimization, on the engine level. Nacelle length increases by 40% to incorporate condensers. The resulting fuel burn improvement on the design mission is estimated, on the aircraft level, to be up to 10%. Hence, mission CO\({}_{2}\) emissions are 10% lower.  
The influence on contrail formation was quantified as a reduction of more than 50%, based on the discussion in Section 4.5.2. The \(APGWP_{100}\) was calculated for a cruise segment at 35,000 ft and ISA conditions, using the LEEA methodology described in Section 4.5.4. To represent the higher engine weight of the WET concept, as a first order estimate, the cruise thrust was increased such that the fuel flow was matched to the 10% fuel flow improvement compared to the TF2030+.  
The fuel burn reduces by up to 10%, and, thus, CO\({}_{2}\) emissions and APGWP of CO\({}_{2}\) reduce by the same percentage. Depending on the amount of WAR that can be effectively used to suppress NOx formation and combined with this fuel burn reduction, NOx emissions can be reduced by more than 90% in cruise.  
The results for the relevant climate effects, and the total climate impact reduction in terms of \(APGWP_{100}\), are shown in Figure 7. The NOx emissions are divided into three separate effects: a short-term ozone (O\({}_{3}\)) increase, a reduction in methane (CH\({}_{4}\)) lifetime and abundance, and a long-term ozone decrease resulting from methane reduction (O\({}_{3}\leftarrow\) CH\({}_{4}\)). The overall achievable climate impact reduction with the WET concept is more than 40%, composed of a 10% fuel burn reduction (CO\({}_{2}\)), a 90% NOx reduction through the steam injection, and a 50% reduction in contrail RF. The share of the different emissions results from the methodology described in Section 4.5.4. It can be seen that the uncertainty regarding contrail reduction has a strong influence on the overall result. Only a cruise segment at 35,000 ft is considered here for simplicity. In a complete flight, CO\({}_{2}\) emissions would have a higher relative share, while the share of contrails and NOx would be reduced. Therefore, the overall climate impact reduction is over-predicted here compared to a full mission assessment. Furthermore, the climate impact of contrails and NOx emissions is influenced by flight altitude, such that using higher or lower cruise flight levels results in different shares of the different emissions.  
Further reductions in climate impact can be expected using SAF, which has the potential to remove the CO\({}_{2}\) impact and reduces particles created during combustion. When burning hydrogen, in-flight CO\({}_{2}\) and non-volatile particle emissions would be eliminated, while NOx emissions would persist at a low level. Contrails would form, instead, on volatile particles. To date, this process is not well understood. Applying WET technology would still be beneficial, considering the Schmidt-Appleman criterion, by condensing significant parts of water from the core exhaust.