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Impact of Hybrid-Electric Aircraft on Contrail Coverage

Impact of Hybrid-Electric Aircraft on Contrail Coverage aerospace Article Impact of Hybrid-Electric Aircraft on Contrail Coverage 1 , 1 , 2 2 Feijia Yin * , Volker Grewe and Klaus Gierens Faculty of Aerospace Engineering, Delft University of Technology, 2629HS Delft, The Netherlands; Volker.Grewe@dlr.de Deutsches Zentrum für Luft- und Raumfahrt, Institut für Physik der Atmosphäre, Oberpfa enhofen, D-82234 Weßling, Germany; klaus.gierens@dlr.de * Correspondence: f.yin@tudelft.nl Received: 31 July 2020; Accepted: 9 October 2020; Published: 12 October 2020 Abstract: Aviation is responsible for approximately 5% of global warming and is expected to increase substantially in the future. Given the continuing expansion of air trac, mitigation of aviation’s climate impact becomes challenging but imperative. Among various mitigation options, hybrid-electric aircraft (HEA) have drawn intensive attention due to their considerable potential in reducing greenhouse gas emissions (e.g., CO ). However, the non-CO e ects (especially contrails) 2 2 of HEA on climate change are more challenging to assess. As the first step to understanding the climate impact of HEA, this research investigates the e ects on the formation of persistent contrails when flying with HEA. The simulation is performed using an Earth System Model (EMAC) coupled with a submodel (CONTRAIL), where the contrail formation criterion, the Schmidt–Appleman criterion (SAC), is adapted to globally estimate changes in the potential contrail coverage (PCC). We compared the HEA to conventional (reference) aircraft with the same characteristics, except for the propulsion system. The analysis showed that the temperature threshold of contrail formation for HEA is lower; therefore, conventional reference aircraft can form contrails at lower flight altitudes, whereas the HEA does not. For a given flight altitude, with a small fraction of electric power in use (less than 30%), the potential contrail coverage remained nearly unchanged. As the electric power fraction increased, the reduction in contrail formation was mainly observed in the mid-latitudes (30 N and 40 S) or tropical regions and was very much localized with a maximum value of about 40% locally. The analysis of seasonal e ects showed that in non-summer, the reduction in contrail formation using electric power was more pronounced at lower flight altitudes, whereas in summer the changes in PCC were nearly constant with respect to altitude. Keywords: hybrid-electric aircraft; potential contrail coverage; Schmidt–Appleman criterion; degrees of hybridization 1. Introduction Civil aviation satisfies modern society’s needs for mobility and is an essential economic driver. Air transportation demand increases at around 4.4% per annum and is forecast to maintain that growth rate for the next few decades [1]. Although the global COVID-19 pandemic has put a great challenge on the aviation industry, we expect that aviation will eventually recover, as aviation has become a fundamental part of the modern world, providing long-range mobility. Aviation is responsible for approximately 5% of the anthropogenic causes of global warming [2], and it is expected to increase substantially in the future. Given the continuing expansion of air trac, mitigation of aviation’s climate impact becomes challenging but imperative. Aerospace 2020, 7, 147; doi:10.3390/aerospace7100147 www.mdpi.com/journal/aerospace Aerospace 2020, 7, 147 2 of 18 Aerospace 2020, 7, x FOR PEER REVIEW 2 of 18 An overview of the climate impact of aviation associated with various species/e ects is presented An overview of the climate impact of aviation associated with various species/effects is presented in Figure 1in Fig [3]. Both ure 1 CO [3]. Both and non- CO2 CO and non-CO e ects from 2 effect NOx s from N (ozone O formation x (ozone fo and rmat methane ion and met depletion), hane 2 2 water vapor, contrails, and direct aerosols are included. One can see that CO emissions share less depletion), water vapor, contrails, and direct aerosols are included. One can see that CO2 emissions sha than re l half ess than hal of the total f of the t aviation otal radiative aviation ra for dia cing tive f (RF), orcing (R and the F), a rest nd the rest is from i non-CO s from non- e CO ects. 2 eff It e is ctalso s. It noticeable that contrail cirrus is the largest contributor to the total aviation RF, with some uncertainties is also noticeable that contrail cirrus is the largest contributor to the total aviation RF, with some uncert in the a curr inties ent in t level he c of urrent understanding. level of understanding. Figure 1. Figure 1.Aviat Aviation-induced ion-induced radiative fo radiative for rcing cingfrom different from di erent components fo components for r the year 2005 the year 2005 [3–10]. [3–10]. Because of its long lifetime, aviation’s climate impact from CO is mainly determined by Because of its long lifetime, aviation’s climate impact from CO2 is mainly determined by the the amount of CO emissions. The induced perturbation of the atmospheric CO concentration is 2 2 amount of CO2 emissions. The induced perturbation of the atmospheric CO2 concentration is determined by several lifetimes that are associated with individual processes, such as land-uptake determined by several lifetimes that are associated with individual processes, such as land-uptake (biosphere; 1–100 years) and sediment formation (1000 to 10,000 years) [11]. Due to the long lifetimes, (biosphere; 1–100 years) and sediment formation (1000 to 10,000 years) [11]. Due to the long lifetimes, the concentration change can be estimated to first-order by accumulating CO emissions over aviation the concentration change can be estimated to first-order by accumulating CO2 2 emissions over aviation history. On the contrary, the short-lived non-CO e ects depend not only on the emission quantity history. On the contrary, the short-lived non-CO2 effects depend not only on the emission quantity but also on the geographical location, altitude, time, and the local weather conditions. It is possible but also on the geographical location, altitude, time, and the local weather conditions. It is possible to mitigate aviation’s climate impact via operational measures to avoid climate-sensitive regions to mitigate aviation’s climate impact via operational measures to avoid climate-sensitive regions associated with non-CO e ects, e.g., to contrail avoidance [12–17]. associated with non-CO2 2 effects, e.g., to contrail avoidance [12–17]. The formation of persistent contrails depends on environmental conditions and aircraft/engine The formation of persistent contrails depends on environmental conditions and aircraft/engine technologies. The well-known Schmidt–Appleman criterion (SAC) [18,19] suggests that the possible technologies. The well-known Schmidt–Appleman criterion (SAC) [18,19] suggests that the possible measures to reduce aviation’s contrail formation are reducing the H O emission index, increasing the measures to reduce aviation’s contrail formation are reducing the H2O emission index, increasing the fuel lower heating value, or decreasing the overall propulsion eciency. Various options can a ect fuel lower heating value, or decreasing the overall propulsion efficiency. Various options can affect these three measures, e.g., new aircraft design [20] and alternative fuels [21–24]. these three measures, e.g., new aircraft design [20] and alternative fuels [21–24]. Electric aircraft is one of the possible options as well, since using a battery will eliminate exhaust Electric aircraft is one of the possible options as well, since using a battery will eliminate exhaust emissions. Due to technology limitations, a purely electric aircraft would not be feasible. Instead, emissions. Due to technology limitations, a purely electric aircraft would not be feasible. Instead, hybrid-electric aircraft is proposed for regional/narrow-body airliners [25,26]. For long-range flights, hybrid-electric aircraft is proposed for regional/narrow-body airliners [25,26]. For long-range flights, the electric propulsion system’s additional weight makes it dicult to achieve substantial fuel saving. the electric propulsion system’s additional weight makes it difficult to achieve substantial fuel saving. A hybrid-electric propulsion system consists of a gas turbine engine combined with an electric motor A hybrid-electric propulsion system consists of a gas turbine engine combined with an electric motor (EM) and a battery pack. There are several possible ways to connect these components in a power (EM) and a battery pack. There are several possible ways to connect these components in a power train. The most frequently proposed configurations are the series and parallel type, as demonstrated train. The most frequently proposed configurations are the series and parallel type, as demonstrated in Figure 2. In a series configuration, the gas turbine generates electricity via a generator. EM then in Figure 2. In a series configuration, the gas turbine generates electricity via a generator. EM then uses the electricity to drive a fan/propeller. Batteries can assist in supplying power to the EM. In the uses the electricity to drive a fan/propeller. Batteries can assist in supplying power to the EM. In the parallel type, a gas turbine and EM drive the fan/propeller simultaneously, with di erent hybridization parallel type, a gas turbine and EM drive the fan/propeller simultaneously, with different degrees. In addition, the operational flexibility of the parallel hybrid-electric system (HEPS) may allow hybridization degrees. In addition, the operational flexibility of the parallel hybrid-electric system Aerospace 2020, 7, x FOR PEER REVIEW 3 of 18 Aerospace 2020, 7, 147 3 of 18 Aerospace 2020, 7, x FOR PEER REVIEW 3 of 18 (HEPS) may allow for full electrical power through the climate-sensitive regions, such as contrail- vulnerable parts of a flight, hence eliminating contrail formation. for full electrical power through the climate-sensitive regions, such as contrail-vulnerable parts of a (HEPS) may allow for full electrical power through the climate-sensitive regions, such as contrail- flight, hence eliminating contrail formation. vulnerable parts of a flight, hence eliminating contrail formation. (a) (b) Figure 2. Schematic of a hybrid-electric propulsion system: (a) series configuration, (b) parallel (a) (b) configuration [27]. Figure Figure 2. 2. Schematic Scheof ma atic of hybrid-electric a hybrid-ele propulsion ctric propu system: lsion sy (a) series stem: ( configuration, a) series configu (b) parallel ration, ( configuration b) parallel [27]. configuration [27]. As the first step towards understanding the climate impact of hybrid-electric aircraft, this As the first step towards understanding the climate impact of hybrid-electric aircraft, this research research investigates the effects on the formation of contrails when flying with hybrid-electric aircraft investigates the e ects on the formation of contrails when flying with hybrid-electric aircraft (HEA). As the first step towards understanding the climate impact of hybrid-electric aircraft, this (HEA). A parallel hybrid configuration, as described in Figure 2b, is considered. In terms of the A parallel hybrid configuration, as described in Figure 2b, is considered. In terms of the baseline aircraft, research investigates the effects on the formation of contrails when flying with hybrid-electric aircraft baseline aircraft, an earlier study suggested a possible mission range of 1000 km for a parallel hybrid- an earlier study suggested a possible mission range of 1000 km for a parallel hybrid-electric plane to (HEA). A parallel hybrid configuration, as described in Figure 2b, is considered. In terms of the electric plane to allow reasonable benefits of fuel savings [26]. The A320 type aircraft is then our allow reasonable benefits of fuel savings [26]. The A320 type aircraft is then our potential baseline. baseline aircraft, an earlier study suggested a possible mission range of 1000 km for a parallel hybrid- potential baseline. Figure 3 shows an example of a power management strategy for a parallel Figure 3 shows an example of a power management strategy for a parallel configuration. The degrees electric plane to allow reasonable benefits of fuel savings [26]. The A320 type aircraft is then our configuration. The degrees of hybridization between fuel and electricity can be varied to achieve of hybridization between fuel and electricity can be varied to achieve di erent mission performance. potential baseline. Figure 3 shows an example of a power management strategy for a parallel different mission performance. For contrail avoidance, it would be better to have a 100% electric flight For contrail avoidance, it would be better to have a 100% electric flight at the contrail-sensitive regions configuration. The degrees of hybridization between fuel and electricity can be varied to achieve at the contrail-sensitive regions of the cruise flight only, which is analyzed in detail in the current of the cruise flight only, which is analyzed in detail in the current paper. different mission performance. For contrail avoidance, it would be better to have a 100% electric flight paper. at the contrail-sensitive regions of the cruise flight only, which is analyzed in detail in the current paper. Figure 3. An example power management strategy adapted from [26]. During taxi-out/taxi-in phases, Figure 3. An example power management strategy adapted from [26]. During taxi-out/taxi-in phases, 100% electrical power is used, whereas during take-off, climb, and cruise conditions, battery and 100% electrical power is used, whereas during take-o , climb, and cruise conditions, battery and Figure 3. An example power management strategy adapted from [26]. During taxi-out/taxi-in phases, turbofan engine are used in parallel. The arrow on the mission profile indicates different flight stages. turbofan engine are used in parallel. The arrow on the mission profile indicates di erent flight stages. 100% electrical power is used, whereas during take-off, climb, and cruise conditions, battery and turbofan engine are used in parallel. The arrow on the mission profile indicates different flight stages. The paper The paper is is o or rgan ganized ized as as fo follows. llows. In In Sect Section ion 2 2,, tthe he met methods hods use used d tto o predict predict tthe he po pot te ential ntial cont contrail rail coverage coverage (PC (PCC), C), de defined finedas asthe the at atmospheric mospheric ability ability t to for o form m contrails contrail for s for a given a given a aircraft ircrand aft and fu fuel type el ty [28 pe ], The paper is organized as follows. In Section 2, the methods used to predict the potential contrail [2 ar8] e , elaborated. are elaborat The ed. The co conditions nditions of contrail of contformation rail formatfor ion for p parallel arahybrid-electric llel hybrid-elect configuration ric configuratar ion e also are coverage (PCC), defined as the atmospheric ability to form contrails for a given aircraft and fuel type also discus discussed. Section sed. Section 3 analyses 3 analys the ees th ectse effects o of the variation f the variation of cruise altitude, of cruise degraltitude, de ees of hybridization, grees of [28], are elaborated. The conditions of contrail formation for parallel hybrid-electric configuration are hyb andrseasons idization on , an PCC d sechanges. asons on PC Finally C chan , conclusions ges. Finally ar , c e o drawn nclusions in Section are drawn in 4. Section 4. also discussed. Section 3 analyses the effects of the variation of cruise altitude, degrees of hybridization, and seasons on PCC changes. Finally, conclusions are drawn in Section 4. 2. Methodology 2. Methodology We have envisaged a research roadmap (as presented in Figure 4) to investigate the potential We have envisaged a research roadmap (as presented in Figure 4) to investigate the potential of 2. Methodology of flying a hybrid-electric plane for contrail avoidance. The complete research chain contains the flying a hybrid-electric plane for contrail avoidance. The complete research chain contains the We have envisaged a research roadmap (as presented in Figure 4) to investigate the potential of relevant technical aspects of HEA for contrail formation (the green box), the PCC calculation procedure, relevant technical aspects of HEA for contrail formation (the green box), the PCC calculation flying a hybrid-electric plane for contrail avoidance. The complete research chain contains the the operational strategy of HEA to avoid contrails, and the actual mitigation potential when flying procedure, the operational strategy of HEA to avoid contrails, and the actual mitigation potential relevant technical aspects of HEA for contrail formation (the green box), the PCC calculation HEA. In this paper, we present the first steps of the roadmap (marked in blue). Subsequent research when flying HEA. In this paper, we present the first steps of the roadmap (marked in blue). procedure, the operational strategy of HEA to avoid contrails, and the actual mitigation potential will cover the later steps (marked in orange). This section elaborates on the details of the methodology Subsequent research will cover the later steps (marked in orange). This section elaborates on the when flying HEA. In this paper, we present the first steps of the roadmap (marked in blue). of the first steps (those in blue). details of the methodology of the first steps (those in blue). Subsequent research will cover the later steps (marked in orange). This section elaborates on the details of the methodology of the first steps (those in blue). Aerospace 2020, 7, 147 4 of 18 Aerospace 2020, 7, x FOR PEER REVIEW 4 of 18 Hybrid electric aircraft (HEA): • Turbofan engine • Battery technology Change of propulsion efficiency • Power management strategy Derivation of Schmidt Appleman Criterion for HEA Calculate the potential contrail coverage (PCC) Calculate the changes of PCC w.r.t. various hybridizations • A single day analysis to illustrate A given fleet size and routing the local changes of PCC network • Climatological analysis on the changes of contrail formation The changes of PCC for a given fleet and network The operational strategy for HEA to avoid contrail sensitive regionsn Contrail avoidance by HEA + operations Figure 4. A schematic of a research methodology map to investigate the potentials of flying a Figure 4. A schematic of a research methodology map to investigate the potentials of flying a hybrid- hybrid-electric aircraft (HEA) for contrail avoidance. electric aircraft (HEA) for contrail avoidance. An Earth System Model (EMAC) coupled with a CONTRAIL submodel was used to predict the An Earth System Model (EMAC) coupled with a CONTRAIL submodel was used to predict the hybrid-electric aircraft’s PCC. The CONTRAIL submodel includes a revision of the contrail formation hybrid-electric aircraft’s PCC. The CONTRAIL submodel includes a revision of the contrail formation criterion for HEA. Please note that in this analysis, we focus on the contrails formed at the engine criterion for HEA. Please note that in this analysis, we focus on the contrails formed at the engine exhaust, whereas aerodynamic contrails are not considered. The formation of aerodynamic contrails exhaust, whereas aerodynamic contrails are not considered. The formation of aerodynamic contrails mainly depends on the aerodynamic design of aircraft, especially the airfoil [29], which is assumed mainly depends on the aerodynamic design of aircraft, especially the airfoil [29], which is assumed to be unchanged in this analysis and of minor importance concerning their climate impact [30]. to be unchanged in this analysis and of minor importance concerning their climate impact [30]. 2.1. The Base Model EMAC 2.1. The Base Model EMAC The ECHAM/MESSy Atmospheric Chemistry (EMAC) model is a numerical chemistry and The ECHAM/MESSy Atmospheric Chemistry (EMAC) model is a numerical chemistry and climate simulation system that includes submodels describing tropospheric and middle atmosphere climate simulation system that includes submodels describing tropospheric and middle atmosphere processes and their interaction with oceans, land, and human influences [31]. For the present study, processes and their interaction with oceans, land, and human influences [31]. For the present study, we applied EMAC (ECHAM5 version 5.3.02, MESSy version 2.52.0) in T42L31ECMWF-resolution, we applied EMAC (ECHAM5 version 5.3.02, MESSy version 2.52.0) in T42L31ECMWF-resolution, corresponding to a horizontal grid of about 310 km and a vertical resolution of roughly 1 km up to an corresponding to a horizontal grid of about 310 km and a vertical resolution of roughly 1 km up to altitude of approximately 30 km. The simulation time step is 12 min. Such a model resolution will an altitude of approximately 30 km. The simulation time step is 12 min. Such a model resolution will provide us with reasonable weather data to calculate the potential contrail coverage. provide us with reasonable weather data to calculate the potential contrail coverage. EMAC has been extensively validated with other models, for instance, ACCMIP presented EMAC has been extensively validated with other models, for instance, ACCMIP presented in in [32], concerning atmospheric dynamics, cloud occurrence, chemistry, etc. An overview is given [32], concerning atmospheric dynamics, cloud occurrence, chemistry, etc. An overview is given in in [33]. In this paper, we use the submodel CONTRAIL V1.0. Section 2.2 discusses details on the [33]. In this paper, we use the submodel CONTRAIL V1.0. Section 2.2 discusses details on the CONTRAIL submodel. CONTRAIL submodel. 2.2. The CONTRAIL Submodel 2.2. The CONTRAIL Submodel CONTRAIL is one of the submodels in EMAC, developed by Frömming et al. (supplement of [12]) CONTRAIL is one of the submodels in EMAC, developed by Frömming et al. (supplement of to calculate the potential coverage of persistent contrails instantaneously, with the EMAC resolution [12]) to calculate the potential coverage of persistent contrails instantaneously, with the EMAC specified in the previous section. The thermodynamic condition of contrail formation is given by the resolution specified in the previous section. The thermodynamic condition of contrail formation is given by the Schmidt–Appleman criterion [18,19], the derivation of which is presented in Section 2.3 of this paper. In the CONTRAIL submodel, the PCC is calculated as the difference between the Aerospace 2020, 7, 147 5 of 18 Schmidt–Appleman criterion [18,19], the derivation of which is presented in Section 2.3 of this paper. In the CONTRAIL submodel, the PCC is calculated as the di erence between the maximum possible coverage of both contrails and cirrus and the natural cirrus coverage alone. Supersaturation with respect to ice is used to determine if the contrails are persistent or not. 2.3. The HEA Extended Schmidt–Appleman Criterion Contrails form when the mixture of engine exhaust and ambient air reaches water saturation at a suciently low temperature, and they persist when the ambient air is ice-supersaturated. The mixing process is assumed to be isobaric; therefore, the mixing trajectory is represented as a straight line on the T-e diagram (see Figure 1 of [34]; e is the partial pressure of water vapor in the mixture; T is the static temperature of the mixture). The slope of this straight line (G) determining the temperature threshold of contrail formation is given by the SAC and calculated using Equation (1). pc EIH2O G = (1) "Q(1  ) where p is the ambient pressure in Pa; " is the ratio of the molar mass of water vapor and dry air (0.622 constant); c is the isobaric heat capacity of air (1004 J/kg/K); Q is the lower heating value of fuel in MJ/kg; EIH2O is the water vapor emission index in kg/kg(fuel); and the notation ( ) is the overall propulsion eciency of the pure kerosene aircraft (reference), for which we assumed a value of 0.4 in the current study. This eciency value was in line with the value computed using the aircraft/engine performance model presented in [27]. For HEA, the thrust power is provided by two energy sources, as defined by Equation (2). FV =  m Q +  P (2) K f E E where F is the thrust requirement in N, V is the flight speed in m/s, m is the fuel flow rate of kerosene in kg/s,  is the overall eciency of the aircraft when electric power is working alone, the notation (P ) is the electric power in watts. Accordingly, the original SAC was adapted using the assumption that kerosene plus electric power are combined to the necessary power FV results (details are discussed in Appendix A). The revised calculation procedure for slope G leads to Equation (3), which considers the e ects of various degrees of hybridization. c p p a REIH2O G = (3) R(1  )Q + (1 R)(1  )Q K E . . . where Rm /m . For the same thrust power, the maximum fuel flow rate (m ) corresponds to the f f f max max fuel consumption of the reference aircraft, where only kerosene is used. Hence, at pure liquid-fuel operation, R = 1, and pure electric operation, R = 0. The notation (Q ) is a quasi-electric energy content, defined in Equation (4): Q := Q( / ) (4) K E Note that the SAC derivation in this paper only applies to the situation where batteries provide electric power. A battery has no water vapor emission. Suppose the electric power is provided by a fuel cell or another gas turbine via a generator. In that case, water vapor is emitted by these components. A di erent form of the Schmidt–Appleman criterion must be derived to consider the e ects of water vapor emitted by, e.g., the fuel cell. Aerospace 2020, 7, 147 6 of 18 3. Results 3.1. The Threshold of Contrail Formation Calculated by SAC On the basis of the SAC derivation in Section 2.3, we studied the threshold of contrail formation for HEA. Figure 5 shows how the contrail factor G varies with the share of electric propulsion. It is Aerospace 2020, 7, x FOR PEER REVIEW 6 of 18 zero for pure electric propulsion (R = 0) and increases non-linearly with the fraction of liquid-fuel Aerospace 2020, 7, x FOR PEER REVIEW 6 of 18 propulsion until it reaches its maximum at full liquid-fuel propulsion. The shape of the curve depends depends on the efficiency 𝜂 of the electric powertrain. The higher is 𝜂 , the flatter the relation close on the eciency  of the electric powertrain. The higher is  , the flatter the relation close to R = 1. E E depends on the efficiency 𝜂 of the electric powertrain. The higher is 𝜂 , the flatter the relation close to 𝑅= 1 . to 𝑅= 1 . Figure 5. Dependence of the contrail factor G on the ratio R for parallel-hybrid-electric propulsion. Figure 5. Dependence of the contrail factor G on the ratio R for parallel-hybrid-electric propulsion. Figure 5. 𝑅= 1 is full fuel propulsion, Dependence of the contrail factor 𝑅= 0 is full ele G on the ra ctric propu tio R for parallel- lsion. Parameters for the hybrid-electri calcu c propu lation have lsion. R = 1 is full fuel propulsion, R = 0 is full electric propulsion. Parameters for the calculation have been 𝑅= been 1 𝜂 is full fuel propulsion, =0.4, Q=43.2 MJ/kg, 𝑅= 𝑝 = 0 250 is f hPa, ull ele and ctric 𝑄 propu =(𝜂 lsion. ⁄𝜂 )𝑄Param . eters for the calculation have = 0.4, Q = 43.2 MJ/kg, p = 250 hPa, and Q = ( / )Q. K a K E been 𝜂 =0.4, Q=43.2 MJ/kg, 𝑝 = 250 hPa, and 𝑄 =(𝜂 ⁄𝜂 )𝑄 . For the maximum temperature ( 𝑇 ,) ([35], Equation (5)), at which contrail formation is For the maximum temperature (T ,) ([35], Equation (5)), at which contrail formation is possible, max For the maximum temperature ( 𝑇 ,) ([35], Equation (5)), at which contrail formation is possible, we studied the effects of various 𝜂 with respect to the fraction of electric power (see Figure we studied the e ects of various  with respect to the fraction of electric power (see Figure 6). For a possible, we studied the effects of various 𝜂 with respect to the fraction of electric power (see Figure 6). For a more efficient electric system with a high 𝜂 , it needs a relatively high electric share (R is more ecient electric system with a high  , it needs a relatively high electric share (R is very small) to 6). For a more efficient electric system with a high 𝜂 , it needs a relatively high electric share (R is very small) to achieve a substantial lowering of 𝑇 . In our example, the maximum temperature at achieve a substantial lowering of T . In our example, the maximum temperature at which contrails very small) to achieve a substantia max l lowering of 𝑇 . In our example, the maximum temperature at which contrails can be formed when only liquid fuel is used (𝑅= 1 ) is −40 °C. To reduce this by 5 K, can be formed when only liquid fuel is used (R = 1) is 40 C. To reduce this by 5 K, to achieve a which cont to achieve a ramaximum ils can be fo temperature o rmed when on f − ly 45 liqu °C, id it nee fuel d is s rou used g ( hly 𝑅= 1𝑅=) is 0.3 −40 , or 70 °C. T % of o re the thrust power duce this by 5 K, maximum to achieve needs to be provided by th temperatur a maximum e of temperature o 45 e electric mot C, it needs f −45 or when it r °C, oughly it nees e dRs rou f= ficienc 0.3 ghly , y or is 𝑅= 70% 0.0 8. For t .3 of , or 70 the he sm thr % of ust al the thrust power ler e power fficienc needs y, the to be needs to be provided by the electric motor when its efficiency is 0.8. For the smaller efficiency, the reduction in 𝑇 is larger. This may sound paradoxical, but it is the consequence of the simple provided by the electric motor when its eciency is 0.8. For the smaller eciency, the reduction reduction in 𝑇 is larger. This may sound paradoxical, but it is the consequence of the simple physical fact that contrail formation is easier when the exhaust gas is colder, for what is higher in T is larger. This may sound paradoxical, but it is the consequence of the simple physical fact max physical fact that contrail formation is easier when the exhaust gas is colder, for what is higher combined overall propulsion efficiencies. that contrail formation is easier when the exhaust gas is colder, for what is higher combined overall combined overall propulsion efficiencies. propulsion eciencies. Figure 6. Dependence of the maximum temperature at which contrails are possible, 𝑇 , on the ratio Figure 6. R for paralle Depend l-hybrid-electri ence of the c p maxi ropu mu lsion m tem . Parameters are as in Figure 4. perature at which contrails are possible, 𝑇 , on the ratio Figure 6. Dependence of the maximum temperature at which contrails are possible, T , on the ratio max R for parallel-hybrid-electric propulsion. Parameters are as in Figure 4. R for parallel-hybrid-electric propulsion. Parameters are as in Figure 4. This paper considered the technology level of 2030 for electric motor, inverter, and battery, as This paper considered the technology level of 2030 for electric motor, inverter, and battery, as summarized in [27,36]. Furthermore, a typical value of 0.9 for fan efficiency was used. Eventually, summarized in [27,36]. Furthermore, a typical value of 0.9 for fan efficiency was used. Eventually, the value of 𝜂 was about 0.8. This value was then used in the further analysis of PCC. the value of 𝜂 was about 0.8. This value was then used in the further analysis of PCC. Aerospace 2020, 7, 147 7 of 18 Aerospace 2020, 7, x FOR PEER REVIEW 7 of 18 This paper considered the technology level of 2030 for electric motor, inverter, and battery, as summarized in [27,36]. Furthermore, a typical value of 0.9 for fan eciency was used. Eventually, A further example of the parameters for SAC on different aircraft is presented in Table 1. The the value of  was about 0.8. This value was then used in the further analysis of PCC. numbers wereE calculated for an altitude of 11 km. Figure 7 shows three critical mixing lines: one for A further example of the parameters for SAC on di erent aircraft is presented in Table 1. conventional aircraft with an overall efficiency of 0.4, one for HEA with 40% electric power, and one The numbers were calculated for an altitude of 11 km. Figure 7 shows three critical mixing lines: one for for HEA with 80% electric power. These three mixing lines ran from the engine exhaust conditions conventional aircraft with an overall eciency of 0.4, one for HEA with 40% electric power, and one and were tangential to the water saturation pressure line. At the same flight altitude, the temperature for HEA with 80% electric power. These three mixing lines ran from the engine exhaust conditions threshold and the slope G decreased with the increase of electric power in use. Therefore, the chance and were tangential to the water saturation pressure line. At the same flight altitude, the temperature for HEA to form contrails reduced as the electric power fraction increased. threshold and the slope G decreased with the increase of electric power in use. Therefore, the chance for HEA Table 1. to form Calculated parameters for th contrails reduced as the e electric Schmidpower t–Appleman fraction criterion on hy increased. brid-electric aircraft. Hybrid- Conven Table 1. Calculated parameters for the Schmidt–Appleman criterion on hybrid-electric aircraft. Parameters Descriptions Electric tional Units Conventional Aircraft Aircraft Parameters Descriptions Hybrid-Electric Aircraft Units Aircraft Electric power 40% 80% 0 [−] Electric power fraction 40% 80% 0 [] fraction EIH2O Water emission index 1.25 kg/kg(fuel) EIH2O Water emission index 1.25 kg/kg(fuel) η Overall efficiency 0.8 0.4 [−] Overall eciency 0.8 0.4 [] The lower heating value of the The lower heating value Q Q 43.243.2 MJ/ MJ/kgkg of the fuel fuel G Slope at 11 km altitude 1.6 1.1 1.8 Pa/K G Slope at 11 km altitude 1.6 1.1 1.8 Pa/K Figure 7. Mixing line for threshold conditions. Water vapor pressure vs. temperature phase diagram Figure 7. Mixing line for threshold conditions. Water vapor pressure vs. temperature phase diagram representing thermodynamics of contrail formation for a conventional aircraft with overall propulsion representing thermodynamics of contrail formation for a conventional aircraft with overall eciency of 0.4 (blue line) and HEA with two di erent degrees of hybridization: 40% electric power propulsion efficiency of 0.4 (blue line) and HEA with two different degrees of hybridization: 40% (red line) and 80% electric power (green line). The two black curves are the saturation vapor pressure electric power (red line) and 80% electric power (green line). The two black curves are the saturation curves for water (solid) and with respect to ice (dashed). The vertical dashed lines represent the vapor pressure curves for water (solid) and with respect to ice (dashed). The vertical dashed lines temperature threshold for the conventional aircraft (blue), HEA with 40% electric power (red), and HEA represent the temperature threshold for the conventional aircraft (blue), HEA with 40% electric power with 80% electric power (green). (red), and HEA with 80% electric power (green). 3.2. Changes in Potential Contrail Coverage 3.2. Changes in Potential Contrail Coverage We studied the variations of potential contrail coverage caused by di erent e ects, for instance, We studied the variations of potential contrail coverage caused by different effects, for instance, altitude, degree of hybridization, and seasons. In this section, we present the results. The data are altitude, degree of hybridization, and seasons. In this section, we present the results. The data are based on a one-year simulation with EMAC, including the updated CONTRAIL submodel (see above). based on a one-year simulation with EMAC, including the updated CONTRAIL submodel (see For illustration purposes, we first present results for an arbitrary day in winter to give an impression above). For illustration purposes, we first present results for an arbitrary day in winter to give an of an actual weather situation and then present climatological values. All the results were obtained impression of an actual weather situation and then present climatological values. All the results were with 0.8 electric power eciency and 0.4 kerosene system eciency. obtained with 0.8 electric power efficiency and 0.4 kerosene system efficiency. Aerospace 2020, 7, 147 8 of 18 Aerospace 2020, 7, x FOR PEER REVIEW 8 of 18 3.2.1. One-Day Case Study 3.2.1. One-Day Case Study Figure 8a shows the PCC of conventional aircraft at 300 hPa (FL300) on a specific day. In contrast, Figure 8a shows the PCC of conventional aircraft at 300 hPa (FL300) on a specific day. In contrast, the changes in PCC caused by 50% electric hybridization of HEA (i.e., R = 0.5 in Equation (3)) at the the changes in PCC caused by 50% electric hybridization of HEA (i.e., R = 0.5 in Equation (3)) at the same altitude is given in Figure 8b. For the pure kerosene case, the contrails were mainly formed at the same altitude is given in Figure 8b. For the pure kerosene case, the contrails were mainly formed at mid-latitudes and polar regions, where the local temperature was suciently low to form contrails. the mid-latitudes and polar regions, where the local temperature was sufficiently low to form With 50% thrust power supplied by the battery, the reduction in contrail formation was observed at contrails. With 50% thrust power supplied by the battery, the reduction in contrail formation was about 30 N and 40 S, where the tropopause climbed to the higher altitudes. The local temperature in observed at about 30° N and 40° S, where the tropopause climbed to the higher altitudes. The local this region was close to the temperature threshold for the pure kerosene aircraft. A transition to HEA temperature in this region was close to the temperature threshold for the pure kerosene aircraft. A lowered the temperature threshold by a few degrees Celsius. It was also noticeable that the contrail transition to HEA lowered the temperature threshold by a few degrees Celsius. It was also noticeable formation’s reduction was very much localized with a maximum value of around 0.4, which might that the contrail formation’s reduction was very much localized with a maximum value of around have been related to the local temperature and humidity. 0.4, which might have been related to the local temperature and humidity. 2 2 2 −2 Figure 8. The potential contrail coverage (PCC) (contour) and geopotential (black contour; m s ) Figure 8. The potential contrail coverage (PCC) (contour) and geopotential (black contour; m s ) at at 300 hPa (FL300) on a specific day: (a) conventional aircraft; (b) absolute changes caused by HEA 300 hPa (FL300) on a specific day: (a) conventional aircraft; (b) absolute changes caused by HEA with with 50% electric power. 50% electric power. When increasing the flight altitude from 300 hPa to 250 hPa (FL340), for the conventional reference When increasing the flight altitude from 300 hPa to 250 hPa (FL340), for the conventional aircraft, we found the areas of potential contrail coverage in the tropical regions as well (Figure 9a). reference aircraft, we found the areas of potential contrail coverage in the tropical regions as well For HEA, with 50% of electric hybridization, the reduction of contrail formation was more pronounced (Figure 9a). For HEA, with 50% of electric hybridization, the reduction of contrail formation was more in the tropical regions, as seen in Figure 9b. pronounced in the tropical regions, as seen in Figure 9b. Aerospace 2020, 7, 147 9 of 18 Aerospace 2020, 7, x FOR PEER REVIEW 9 of 18 2 2 Figure 9. The PCC (contour) and geopotential (black contour; m 2 s−2 ) at 250 hPa (FL340) on a specific Figure 9. The PCC (contour) and geopotential (black contour; m s ) at 250 hPa (FL340) on a specific day: (a) conventional aircraft; (b) absolute changes caused by HEA with 50% electric power. day: (a) conventional aircraft; (b) absolute changes caused by HEA with 50% electric power. When we increased the flight altitude further to 200 hPa (FL390), HEA with 50% of electric When we increased the flight altitude further to 200 hPa (FL390), HEA with 50% of electric hybridization had no significant change in contrail formation. At this level, the ambient temperature hybridization had no significant change in contrail formation. At this level, the ambient temperature was far below the threshold for contrail formation for conventional aircraft. HEA did not lead to was far below the threshold for contrail formation for conventional aircraft. HEA did not lead to a a sucient lowering of that temperature threshold with 50% electric power. However, increasing sufficient lowering of that temperature threshold with 50% electric power. However, increasing the the hybridization to 90% allowed for a further reduction of the temperature threshold for contrail hybridization to 90% allowed for a further reduction of the temperature threshold for contrail formation. Accordingly, a reduction in contrails was observed in several locations of the tropical region, formation. Accordingly, a reduction in contrails was observed in several locations of the tropical as shown in Figure 10b. region, as shown in Figure 10b. 3.2.2. E ect of Degree of Hybridization In Figure 11, we present a statistical analysis of the local reduction of contrail coverage when di erent hybridization degrees were considered. The annual mean PCC was used. We observed that the contrail coverage remained nearly unchanged with a smaller fraction of electric power in use (less than 30%). As the hybridization rate increased from 10–90%, an exponential reduction trend was found, which was already indicated by the theoretical analysis in Figures 5 and 6. Aerospace 2020, 7, 147 10 of 18 Aerospace 2020, 7, x FOR PEER REVIEW 10 of 18 2 2 2 −2 Figure 10. Figure 10. The The PCC (contour) PCC (contour) and geopotenti and geopotential al (b (black lack contour; contour; m m s s )) at at 20 200 0 hPa hPa (FL390) (FL390) o on n a a specific specific day day: : ((a a)) conven conventional tional air airc craft; raft; ((b b) abso ) absolute lute chang changes es cau caused sed by by HEA HEA with 90 with 90% % ele electric ctric pow power er. . Aerospace 2020, 7, x FOR PEER REVIEW 11 of 18 3.2.2. Effect of Degree of Hybridization In Figure 11, we present a statistical analysis of the local reduction of contrail coverage when different hybridization degrees were considered. The annual mean PCC was used. We observed that the contrail coverage remained nearly unchanged with a smaller fraction of electric power in use (less than 30%). As the hybridization rate increased from 10–90%, an exponential reduction trend was found, which was already indicated by the theoretical analysis in Figures 5 and 6. Figure 11. Distribution of the reduction in PCC with respect to di erent degrees of electric hybridization. Figure 11. Distribution of the reduction in PCC with respect to different degrees of electric hybridization. 3.2.3. Climatology of Contrail Formation The zonal annual mean values of PCC for the reference aircraft and various hybridization degrees are presented in Figure 12. Figure 12a shows the potential contrail coverage for the conventional aircraft, which was also used as a baseline to evaluate different degrees of hybridization effects. From Figure 12b–d, the degree of hybridization increased from 30% to 50% and up to 90%. Again, corresponding to the SAC theory, HEA did not form contrails at the higher temperature and could not form contrails in the lower region of the potential contrail coverage of conventional contrails. Therefore, the reduction of HEA in contrail formation occurred mostly at the lower flight altitudes. However, as the hybridization increased, the altitude range, where contrail formation was reduced, grew. Aerospace 2020, 7, 147 11 of 18 3.2.3. Climatology of Contrail Formation The zonal annual mean values of PCC for the reference aircraft and various hybridization degrees are presented in Figure 12. Figure 12a shows the potential contrail coverage for the conventional aircraft, which was also used as a baseline to evaluate di erent degrees of hybridization e ects. From Figure 12b–d, the degree of hybridization increased from 30% to 50% and up to 90%. Again, corresponding to the SAC theory, HEA did not form contrails at the higher temperature and could not form contrails in the lower region of the potential contrail coverage of conventional contrails. Therefore, the reduction of HEA in contrail formation occurred mostly at the lower flight altitudes. However, Aerospace 2020, 7, x FOR PEER REVIEW 12 of 18 as the hybridization increased, the altitude range, where contrail formation was reduced, grew. Figure 12. Annual zonal mean potential contrail coverage and changes due to electric hybridization: Figure 12. Annual zonal mean potential contrail coverage and changes due to electric hybridization: (a) conventional aircraft; (b) 30% electric hybridization; (c) 50% electric hybridization; (d) 90% (a) conventional aircraft; (b) 30% electric hybridization; (c) 50% electric hybridization; (d) 90% electric electric hybridization. hybridization. 3.2.4. Seasonal Effects 3.2.4. Seasonal E ects On the ba On the si basis s of the a of then annual nual ssimulation imulation res results, ults, we we studied studied the tseasonal he seaso en al ects. effect Figur s. Figure e 13 presents 13 pres the ents the vari variations ations in in PCC PCC for for 90% 90of % o hybridization. f hybridizat Tion he r . The esults rwer esueltgr s were ouped g into rouped four seasons into foat ur s four eas pr ons essur ate four altitudes. Generally, we expected the largest e ect (i.e., the greatest reduction in PCC) on the lowest pressure altitudes. Generally, we expected the largest effect (i.e., the greatest reduction in PCC) on pressure level, since the temperatures on that level were closest to the SAC-temperature threshold. the lowest pressure level, since the temperatures on that level were closest to the SAC-temperature threshold. Similarly, we expected the PCC reduction to become smaller towards higher pressure altitudes with their lower temperatures. This pattern was evident in the figure, except for summer. There were more substantial reductions on 150 hPa (FL450) and 200 hPa (FL390) levels in summer than in the other seasons. This behavior was partly due to higher temperatures, closer to the SAC- threshold, but lower relative humidity than those in different seasons may contribute to PCC reduction. The reduction was found to be more or less constant in summer at all flight levels (differences were insignificant), although the difference between the actual and the SAC-threshold temperature increased with flight level. It is likely that a decreasing frequency of high humidity (including ice-supersaturation) cases balanced the temperature effect such that the PCC reduction in summer depended little on the flight altitude. Aerospace 2020, 7, 147 12 of 18 Similarly, we expected the PCC reduction to become smaller towards higher pressure altitudes with their lower temperatures. This pattern was evident in the figure, except for summer. There were more substantial reductions on 150 hPa (FL450) and 200 hPa (FL390) levels in summer than in the other seasons. This behavior was partly due to higher temperatures, closer to the SAC-threshold, but lower relative humidity than those in di erent seasons may contribute to PCC reduction. The reduction was found to be more or less constant in summer at all flight levels (di erences were insignificant), although the di erence between the actual and the SAC-threshold temperature increased with flight level. It is likely that a decreasing frequency of high humidity (including ice-supersaturation) cases balanced the Aerospace 2020, 7, x FOR PEER REVIEW 13 of 18 temperature e ect such that the PCC reduction in summer depended little on the flight altitude. th 90 percentile th 75 percentile: Q3 Median th 25 percentile: Q1 th 10 percentile (a) (b) Figure 13. Seasonal e ects on the changes of potential contrail coverage at various pressure altitudes Figure 13. Seasonal effects on the changes of potential contrail coverage at various pressure altitudes for 90% electric hybridization (a) and the explanation of the boxplot (b). DJF: December, January, for 90% electric hybridization (a) and the explanation of the boxplot (b). DJF: December, January, and and February; MAM: March, April, and May; JJA: June, July, and August; SON: September, October, February; MAM: March, April, and May; JJA: June, July, and August; SON: September, October, and and November. For the boxplot: the 10th and 90th percentiles of the data are plotted. Triangle indicates November. For the boxplot: the 10th and 90th percentiles of the data are plotted. Triangle indicates the mean value. As the altitude increases, e.g., at 150 hPa (FL450), the values at 90th percentile and the mean value. As the altitude increases, e.g., at 150 hPa (FL450), the values at 90th percentile and 75th percentile are equal to the minimum of the datasets as zero, therefore not visible in Figure (a). 75th percentile are equal to the minimum of the datasets as zero, therefore not visible in Figure (a). 4. Discussion 4. Discussion This study demonstrated the e ects of hybrid-electric aircraft on potential contrail coverage. This study demonstrated the effects of hybrid-electric aircraft on potential contrail coverage. The The actual contrail properties—the number of soot particles related to using a hybrid-electric propulsion actual contrail properties—the number of soot particles related to using a hybrid-electric propulsion system, the lifetime of contrails, etc.—were not considered. Earlier research shows that the number of system, the lifetime of contrails, etc.—were not considered. Earlier research shows that the number soot particles produced by a turbofan engine is strongly dependent on the thrust setting [37]. A higher of soot particles produced by a turbofan engine is strongly dependent on the thrust setting [37]. A thrust setting would lead to an increase in the combustion temperature, hence, increasing the rate of higher thrust setting would lead to an increase in the combustion temperature, hence, increasing the soot formation, and vice versa. As di erent degrees of electric hybridization were considered, the gas rate of soot formation, and vice versa. As different degrees of electric hybridization were considered, turbine engine’s thrust setting and the number of soot particles varied accordingly. The change in soot the gas turbine engine’s thrust setting and the number of soot particles varied accordingly. The particles will a ect the contrails’ actual optical properties, thus the resulting contrail radiative forcing. change in soot particles will affect the contrails’ actual optical properties, thus the resulting contrail Furthermore, the Schmidt–Appleman criterion’s derivation in this paper is applicable for a radiative forcing. hybrid-electric propulsion system, for which the electric power is generated by battery. If the choice goes Furth for erm a fuel ore, cell the hybrid-electric Schmidt–Appl prema opulsion n criter system, ion’s one derwould ivationr equir in th eis a di pa pe erent r is consideration. applicable for a The SAC for the fuel cell type of hybrid system depends on how the fuel cell’s exhaust is handled. If a hybrid-electric propulsion system, for which the electric power is generated by battery. If the choice heat exchanger is used to collect the fuel cell’s exhaust heat, the water vapor at the fuel cell’s exhaust goes for a fuel cell hybrid-electric propulsion system, one would require a different consideration. is then condensed into a liquid. In this case, contrails are only formed from the gas turbine exhaust. The SAC for the fuel cell type of hybrid system depends on how the fuel cell’s exhaust is handle d. If The SAC derivation in the current paper can still be valid. However, if no condensation process is a heat exchanger is used to collect the fuel cell’s exhaust heat, the water vapor at the fuel cell’s exhaust is then condensed into a liquid. In this case, contrails are only formed from the gas turbine exhaust. The SAC derivation in the current paper can still be valid. However, if no condensation process is involved, the fuel cell then also produces water vapor in the exhaust, which must be considered to derivate an alternative SAC. As observed from our analysis, the reduction in contrail coverage by hybrid-electric aircraft is mostly localized. The consequences are twofold. In a case where the flight route does not cross a region of reduction (that is, the temperature is significantly lower than the contrail formation threshold even for a high portion of electric power), flying with a hybrid-electric aircraft does not affect the contrail formation. Still, the contrail properties (lifetime, optical thickness, and eventually individual radiative forcing) may change with the degree of hybridization. In another case where the contrail reduction is possible along the flight route (that is, the temperature is close to the contrail formation threshold), using a large fraction of electric power at that specific location would effectively reduce contrail formation. For the second case, a specific power management strategy can be developed to design the hybrid-electric system. Eventually, we expect the actual effect to be a IQR=Q3-Q1 Aerospace 2020, 7, 147 13 of 18 involved, the fuel cell then also produces water vapor in the exhaust, which must be considered to derivate an alternative SAC. As observed from our analysis, the reduction in contrail coverage by hybrid-electric aircraft is mostly localized. The consequences are twofold. In a case where the flight route does not cross a region of reduction (that is, the temperature is significantly lower than the contrail formation threshold even for a high portion of electric power), flying with a hybrid-electric aircraft does not a ect the contrail formation. Still, the contrail properties (lifetime, optical thickness, and eventually individual radiative forcing) may change with the degree of hybridization. In another case where the contrail reduction is possible along the flight route (that is, the temperature is close to the contrail formation threshold), using a large fraction of electric power at that specific location would e ectively reduce contrail formation. For the second case, a specific power management strategy can be developed to design the hybrid-electric system. Eventually, we expect the actual e ect to be a convolution of the existing air trac patterns with the PCC-reduction patterns. In further analysis, we will investigate the e ectiveness of contrail avoidance by hybrid-electric aircraft, considering the routing e ects. In addition, in our analysis, we used a typical value of 0.4 as the overall propulsion eciency, and we left it constant regardless of the amount of electrical power used. As more electrical energy is used, the gas turbine engine’s thrust setting is reduced; correspondingly, the propulsion eciency would decrease slightly. If we consider such eciency deterioration, the temperature threshold of contrail formation for the hybrid-electric system is reduced further (see, e.g., Equation (1) and Figure 6 of this paper). The lower temperature threshold implies that contrail formation is more likely to decrease, but it requires a thorough analysis in future research. 5. Conclusions This paper presents the changes in potential contrail coverage when flying with hybrid-electric aircraft. On the basis of the analysis, we have drawn the following conclusions: The atmospheric areas of contrail formation of hybrid-electric aircraft are smaller than those of conventional aircraft and require lower atmospheric temperatures. The reduction in contrail formation by hybrid-electric aircraft is more pronounced in a tropical region where the temperatures are higher. With a small degree of hybridization (below 30% in the current study), the contrail coverage remains nearly unchanged. A maximum reduction of about 40% in contrail coverage was observed locally, with 90% electric power in use. In non-summer, the reduction in potential contrail coverage by hybrid-electric aircraft was more noticeable at lower flight altitudes. In contrast, the changes in potential contrail coverage were nearly constant (about 20%) for all flight altitudes studied in summer. Author Contributions: Conceptualization of this study, V.G. and F.Y.; derivation of the Schmidt–Appleman criterion for hybrid-electric aircraft, V.G. and K.G.; formal analysis and writing the original draft, F.Y.; writing—review, V.G. and K.G.; funding acquisition, V.G. and F.Y. All authors have read and agreed to the published version of the manuscript. Funding: Initial work received funding from the SESAR Joint Undertaking under grant agreement no. 699395 under the European Union’s Horizon 2020 research and innovation program within the Exploratory Research project ATM4E. The individual author of this work receives funding from the Dutch Research Council (NWO) under the talent scheme VENI and in-kind contribution from DLR-Oberpfa enhofen. The project number is 17367. Conflicts of Interest: The authors declare no conflict of interest. Aerospace 2020, 7, 147 14 of 18 Nomenclature Abbreviations DJF December, January, and February EIH2O Water vapor emission index kg/kg(fuel) EM Electric motor HEA Hybrid-electric aircraft JJA June, July, and August MAM March, April, and May PCC Potential contrail coverage PDF Probability density function RF Radiative forcing SAC Schmidt–Appleman criterion SON September, October, and November Symbols c Isobaric heat capacity of the air J/kg/K F Thrust N G The slope of the mixing line pa/K m Fuel mass flow rate kg/s p Ambient pressure pa P Electric power W Q The lower heating value of fuel MJ/kg R Degrees of hybridization [] The maximum temperature at which T C max contrail formation is possible V Velocity m/s The ratio of the molar mass of water " [] vapor and dry air The overall eciency of the electric [] powertrain The overall eciency of the pure [] kerosene aircraft Appendix A. Derivation of Schmidt–Appleman Criterion for Hybrid-Electric Aircraft The thermodynamic theory of contrail formation was developed for traditional jet engines many years ago [18,19]. A modern derivation is provided by [35] using the conservation principles of mass, momentum and energy. The traditional engine type has only one source of energy, namely, the fuel with its specific energy content Q. For each kilogram of fuel burnt, a mass of EIH2O kilogram water vapor is produced and emitted. For a . . fuel flow rate of m , the rate of water vapor emission is m EIH2O. f uel f uel The condition for contrail formation is the Schmidt–Appleman criterion (SAC). The most important factor in the theory is the so-called contrail factor G = de /dT , that is, the change of partial pressure of water vapor in the p p exhaust plume, e , with plume temperature, T . This change occurs when the plume is expanding and mixing with p p ambient air. This mixing is isobaric at ambient pressure, p , and the mixing trajectory of the exhaust gases in a thermodynamic, e T, diagram is thus a straight line with slope G. The endpoint of that trajectory at infinite mixing is represented by the ambient conditions: water vapor partial pressure, e , and temperature, T . Thus we find a a e e p a G = (A1) T T p a It is practical to use mass mixing ratios q = "e /p , where " = 0.622 is the ratio of molar masses of H O x x a 2 and air. The partial pressure of water vapor at the engine exit is . . m q + m EIH2O p a a f e =  (A2) . . m + m where the notation (m ) is the mass flow rate of air through the engine. In this derivation, we do not consider the separation of the core flow and bypass flow, as the two air streams mix anyway at the engine exit within a few milliseconds. Such a consideration will avoid an unnecessary complication of the equations. The equation states Aerospace 2020, 7, 147 15 of 18 that the vapor partial pressure at the engine exit is composed of that carried by the air needed to burn the fuel plus the contribution from the fuel itself. Since an electric motor does not emit water vapor, there is hence no contribution. Thus, the numerator in the formula for G is . . m (EIH2O q ) m EIH2O p f a p f a a e e =    (A3) p a . . . . " " m + m m + m a a f f where the approximation is possible since EIH2O  q . The aircraft needs a thrust F to overcome drag and friction. When it flies with a velocity V, the engines must produce a power FV =  m Q +  P (A4) K E E That is, two sources of energy, from the liquid fuel and the electric motor, add their powers with their respective thermodynamic eciencies,  . The notation (P ) is the (variable) power of the electric engine. P varies E E in response to variable fuel flow, such that the above sum equals FV. Thus, P is a function of m . E f 0 0 P = P am with P = FV/ and a = Q( / ) (A5) E f E K E E E Here, P is the electric power when it is driving the aircraft alone. The higher the fuel flow, the lower P is. The maximum fuel flow to achieve a thrust power of FV is m = FV/ Q. It turns out to be useful also to define max a quasi-electric energy content, namely, 0 0 Q := P /m = a = Q( / ) (A6) f K E max E E As the eciencies are never 1.0, the remaining part of the produced power is wasted for heating and expelling the exhaust gases (from the burnt fuel and the air flowing through the engine). These are thermal and kinetic energies. For the present derivation, we neglected the kinetic energy since it is much smaller than the thermal energy. Thus, we have h   i . . . (1  )m Q + (1  )P = c m T T + m T (A7) p a p a p K f E E f That is, the engine air is heated from its ambient temperature to the plume temperature, and the gas added by burning the liquid fuel is heated to T as well. (A few other energy sources and sinks are neglected here: the enthalpy of the liquid fuel, the heating of the engine parts, for instance). The symbol (c ) is the heat capacity (at constant pressure) of air. After a few steps, we find . . . ( ) ( ) ( ) ( ) 1  m Q + 1  P c m T 1  m Q + 1  P K f E E p f a K f E E T T =      (A8) p a . . . . c m + m c m + m p a f p a f This approximation is possible since the enthalpy of the ambient air is much smaller than the energy content of the fuel or the energy produced by the electric motor. Dividing Equation (A3) by Equation (A8) gives an expression for the contrail factor m EIH2O c p p a f G =    (A9) . . (1  )m Q + (1  )P m K f E E f where the dependence of P on the fuel flow is made explicit for clarity. Now it appears convenient to normalize the fuel flow rate by its maximum, . . R := m /m (A10) f f max At pure liquid-fuel operation, R = 1, and at pure electric operation, R = 0. Having this and the other definitions from above, we arrive after a few steps at a favorable expression for G: c p p a EIH2O G = R (A11) R(1  )Q + (1 R)(1  )Q K E Equation (A11) is the desired expression. It has the correct limiting properties. For R = 1 we retain the form of the traditional SAC, but for pure electric propulsion, R = 0, hence, G = 0, which implies that contrail formation is impossible. Aerospace 2020, 7, 147 16 of 18 From here, it is relatively straightforward to formulate a generalization to more than two energy sources. Let FV =  m Q +  P (A12) K f i i where apart from the liquid fuel, we have a number of energy sources (index i) that do not produce water in the exhaust. Then we introduce in analogy to the derivation above: 0 0 0 P := FV/ , Q := P /m = ( / )Q (A13) i f K i max i i i Furthermore, we define weights w P /P . 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In Proceedings of the 2019 International Conference on Electrotechnical Complexes and Systems (ICOECS), Ufa, Russia, 21–25 October 2019; pp. 1–3. [CrossRef] Aerospace 2020, 7, 147 18 of 18 37. Boies, A.M.; Stettler, M.E.J.; Swanson, J.J.; Johnson, T.J.; Olfert, J.S.; Johnson, M.; Eggersdorfer, M.L.; Rindlisbacher, T.; Wang, J.; Thomson, K.; et al. Particle Emission Characteristics of a Gas Turbine with a Double Annular Combustor. Aerosol Sci. Technol. 2015, 49, 842–855. [CrossRef] © 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Aerospace Multidisciplinary Digital Publishing Institute

Impact of Hybrid-Electric Aircraft on Contrail Coverage

Yin, Feijia; Grewe, Volker; Gierens, Klaus
Aerospace , Volume 7 (10) – Oct 12, 2020
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aerospace Article Impact of Hybrid-Electric Aircraft on Contrail Coverage 1 , 1 , 2 2 Feijia Yin * , Volker Grewe and Klaus Gierens Faculty of Aerospace Engineering, Delft University of Technology, 2629HS Delft, The Netherlands; Volker.Grewe@dlr.de Deutsches Zentrum für Luft- und Raumfahrt, Institut für Physik der Atmosphäre, Oberpfa enhofen, D-82234 Weßling, Germany; klaus.gierens@dlr.de * Correspondence: f.yin@tudelft.nl Received: 31 July 2020; Accepted: 9 October 2020; Published: 12 October 2020 Abstract: Aviation is responsible for approximately 5% of global warming and is expected to increase substantially in the future. Given the continuing expansion of air trac, mitigation of aviation’s climate impact becomes challenging but imperative. Among various mitigation options, hybrid-electric aircraft (HEA) have drawn intensive attention due to their considerable potential in reducing greenhouse gas emissions (e.g., CO ). However, the non-CO e ects (especially contrails) 2 2 of HEA on climate change are more challenging to assess. As the first step to understanding the climate impact of HEA, this research investigates the e ects on the formation of persistent contrails when flying with HEA. The simulation is performed using an Earth System Model (EMAC) coupled with a submodel (CONTRAIL), where the contrail formation criterion, the Schmidt–Appleman criterion (SAC), is adapted to globally estimate changes in the potential contrail coverage (PCC). We compared the HEA to conventional (reference) aircraft with the same characteristics, except for the propulsion system. The analysis showed that the temperature threshold of contrail formation for HEA is lower; therefore, conventional reference aircraft can form contrails at lower flight altitudes, whereas the HEA does not. For a given flight altitude, with a small fraction of electric power in use (less than 30%), the potential contrail coverage remained nearly unchanged. As the electric power fraction increased, the reduction in contrail formation was mainly observed in the mid-latitudes (30 N and 40 S) or tropical regions and was very much localized with a maximum value of about 40% locally. The analysis of seasonal e ects showed that in non-summer, the reduction in contrail formation using electric power was more pronounced at lower flight altitudes, whereas in summer the changes in PCC were nearly constant with respect to altitude. Keywords: hybrid-electric aircraft; potential contrail coverage; Schmidt–Appleman criterion; degrees of hybridization 1. Introduction Civil aviation satisfies modern society’s needs for mobility and is an essential economic driver. Air transportation demand increases at around 4.4% per annum and is forecast to maintain that growth rate for the next few decades [1]. Although the global COVID-19 pandemic has put a great challenge on the aviation industry, we expect that aviation will eventually recover, as aviation has become a fundamental part of the modern world, providing long-range mobility. Aviation is responsible for approximately 5% of the anthropogenic causes of global warming [2], and it is expected to increase substantially in the future. Given the continuing expansion of air trac, mitigation of aviation’s climate impact becomes challenging but imperative. Aerospace 2020, 7, 147; doi:10.3390/aerospace7100147 www.mdpi.com/journal/aerospace Aerospace 2020, 7, 147 2 of 18 Aerospace 2020, 7, x FOR PEER REVIEW 2 of 18 An overview of the climate impact of aviation associated with various species/e ects is presented An overview of the climate impact of aviation associated with various species/effects is presented in Figure 1in Fig [3]. Both ure 1 CO [3]. Both and non- CO2 CO and non-CO e ects from 2 effect NOx s from N (ozone O formation x (ozone fo and rmat methane ion and met depletion), hane 2 2 water vapor, contrails, and direct aerosols are included. One can see that CO emissions share less depletion), water vapor, contrails, and direct aerosols are included. One can see that CO2 emissions sha than re l half ess than hal of the total f of the t aviation otal radiative aviation ra for dia cing tive f (RF), orcing (R and the F), a rest nd the rest is from i non-CO s from non- e CO ects. 2 eff It e is ctalso s. It noticeable that contrail cirrus is the largest contributor to the total aviation RF, with some uncertainties is also noticeable that contrail cirrus is the largest contributor to the total aviation RF, with some uncert in the a curr inties ent in t level he c of urrent understanding. level of understanding. Figure 1. Figure 1.Aviat Aviation-induced ion-induced radiative fo radiative for rcing cingfrom different from di erent components fo components for r the year 2005 the year 2005 [3–10]. [3–10]. Because of its long lifetime, aviation’s climate impact from CO is mainly determined by Because of its long lifetime, aviation’s climate impact from CO2 is mainly determined by the the amount of CO emissions. The induced perturbation of the atmospheric CO concentration is 2 2 amount of CO2 emissions. The induced perturbation of the atmospheric CO2 concentration is determined by several lifetimes that are associated with individual processes, such as land-uptake determined by several lifetimes that are associated with individual processes, such as land-uptake (biosphere; 1–100 years) and sediment formation (1000 to 10,000 years) [11]. Due to the long lifetimes, (biosphere; 1–100 years) and sediment formation (1000 to 10,000 years) [11]. Due to the long lifetimes, the concentration change can be estimated to first-order by accumulating CO emissions over aviation the concentration change can be estimated to first-order by accumulating CO2 2 emissions over aviation history. On the contrary, the short-lived non-CO e ects depend not only on the emission quantity history. On the contrary, the short-lived non-CO2 effects depend not only on the emission quantity but also on the geographical location, altitude, time, and the local weather conditions. It is possible but also on the geographical location, altitude, time, and the local weather conditions. It is possible to mitigate aviation’s climate impact via operational measures to avoid climate-sensitive regions to mitigate aviation’s climate impact via operational measures to avoid climate-sensitive regions associated with non-CO e ects, e.g., to contrail avoidance [12–17]. associated with non-CO2 2 effects, e.g., to contrail avoidance [12–17]. The formation of persistent contrails depends on environmental conditions and aircraft/engine The formation of persistent contrails depends on environmental conditions and aircraft/engine technologies. The well-known Schmidt–Appleman criterion (SAC) [18,19] suggests that the possible technologies. The well-known Schmidt–Appleman criterion (SAC) [18,19] suggests that the possible measures to reduce aviation’s contrail formation are reducing the H O emission index, increasing the measures to reduce aviation’s contrail formation are reducing the H2O emission index, increasing the fuel lower heating value, or decreasing the overall propulsion eciency. Various options can a ect fuel lower heating value, or decreasing the overall propulsion efficiency. Various options can affect these three measures, e.g., new aircraft design [20] and alternative fuels [21–24]. these three measures, e.g., new aircraft design [20] and alternative fuels [21–24]. Electric aircraft is one of the possible options as well, since using a battery will eliminate exhaust Electric aircraft is one of the possible options as well, since using a battery will eliminate exhaust emissions. Due to technology limitations, a purely electric aircraft would not be feasible. Instead, emissions. Due to technology limitations, a purely electric aircraft would not be feasible. Instead, hybrid-electric aircraft is proposed for regional/narrow-body airliners [25,26]. For long-range flights, hybrid-electric aircraft is proposed for regional/narrow-body airliners [25,26]. For long-range flights, the electric propulsion system’s additional weight makes it dicult to achieve substantial fuel saving. the electric propulsion system’s additional weight makes it difficult to achieve substantial fuel saving. A hybrid-electric propulsion system consists of a gas turbine engine combined with an electric motor A hybrid-electric propulsion system consists of a gas turbine engine combined with an electric motor (EM) and a battery pack. There are several possible ways to connect these components in a power (EM) and a battery pack. There are several possible ways to connect these components in a power train. The most frequently proposed configurations are the series and parallel type, as demonstrated train. The most frequently proposed configurations are the series and parallel type, as demonstrated in Figure 2. In a series configuration, the gas turbine generates electricity via a generator. EM then in Figure 2. In a series configuration, the gas turbine generates electricity via a generator. EM then uses the electricity to drive a fan/propeller. Batteries can assist in supplying power to the EM. In the uses the electricity to drive a fan/propeller. Batteries can assist in supplying power to the EM. In the parallel type, a gas turbine and EM drive the fan/propeller simultaneously, with di erent hybridization parallel type, a gas turbine and EM drive the fan/propeller simultaneously, with different degrees. In addition, the operational flexibility of the parallel hybrid-electric system (HEPS) may allow hybridization degrees. In addition, the operational flexibility of the parallel hybrid-electric system Aerospace 2020, 7, x FOR PEER REVIEW 3 of 18 Aerospace 2020, 7, 147 3 of 18 Aerospace 2020, 7, x FOR PEER REVIEW 3 of 18 (HEPS) may allow for full electrical power through the climate-sensitive regions, such as contrail- vulnerable parts of a flight, hence eliminating contrail formation. for full electrical power through the climate-sensitive regions, such as contrail-vulnerable parts of a (HEPS) may allow for full electrical power through the climate-sensitive regions, such as contrail- flight, hence eliminating contrail formation. vulnerable parts of a flight, hence eliminating contrail formation. (a) (b) Figure 2. Schematic of a hybrid-electric propulsion system: (a) series configuration, (b) parallel (a) (b) configuration [27]. Figure Figure 2. 2. Schematic Scheof ma atic of hybrid-electric a hybrid-ele propulsion ctric propu system: lsion sy (a) series stem: ( configuration, a) series configu (b) parallel ration, ( configuration b) parallel [27]. configuration [27]. As the first step towards understanding the climate impact of hybrid-electric aircraft, this As the first step towards understanding the climate impact of hybrid-electric aircraft, this research research investigates the effects on the formation of contrails when flying with hybrid-electric aircraft investigates the e ects on the formation of contrails when flying with hybrid-electric aircraft (HEA). As the first step towards understanding the climate impact of hybrid-electric aircraft, this (HEA). A parallel hybrid configuration, as described in Figure 2b, is considered. In terms of the A parallel hybrid configuration, as described in Figure 2b, is considered. In terms of the baseline aircraft, research investigates the effects on the formation of contrails when flying with hybrid-electric aircraft baseline aircraft, an earlier study suggested a possible mission range of 1000 km for a parallel hybrid- an earlier study suggested a possible mission range of 1000 km for a parallel hybrid-electric plane to (HEA). A parallel hybrid configuration, as described in Figure 2b, is considered. In terms of the electric plane to allow reasonable benefits of fuel savings [26]. The A320 type aircraft is then our allow reasonable benefits of fuel savings [26]. The A320 type aircraft is then our potential baseline. baseline aircraft, an earlier study suggested a possible mission range of 1000 km for a parallel hybrid- potential baseline. Figure 3 shows an example of a power management strategy for a parallel Figure 3 shows an example of a power management strategy for a parallel configuration. The degrees electric plane to allow reasonable benefits of fuel savings [26]. The A320 type aircraft is then our configuration. The degrees of hybridization between fuel and electricity can be varied to achieve of hybridization between fuel and electricity can be varied to achieve di erent mission performance. potential baseline. Figure 3 shows an example of a power management strategy for a parallel different mission performance. For contrail avoidance, it would be better to have a 100% electric flight For contrail avoidance, it would be better to have a 100% electric flight at the contrail-sensitive regions configuration. The degrees of hybridization between fuel and electricity can be varied to achieve at the contrail-sensitive regions of the cruise flight only, which is analyzed in detail in the current of the cruise flight only, which is analyzed in detail in the current paper. different mission performance. For contrail avoidance, it would be better to have a 100% electric flight paper. at the contrail-sensitive regions of the cruise flight only, which is analyzed in detail in the current paper. Figure 3. An example power management strategy adapted from [26]. During taxi-out/taxi-in phases, Figure 3. An example power management strategy adapted from [26]. During taxi-out/taxi-in phases, 100% electrical power is used, whereas during take-off, climb, and cruise conditions, battery and 100% electrical power is used, whereas during take-o , climb, and cruise conditions, battery and Figure 3. An example power management strategy adapted from [26]. During taxi-out/taxi-in phases, turbofan engine are used in parallel. The arrow on the mission profile indicates different flight stages. turbofan engine are used in parallel. The arrow on the mission profile indicates di erent flight stages. 100% electrical power is used, whereas during take-off, climb, and cruise conditions, battery and turbofan engine are used in parallel. The arrow on the mission profile indicates different flight stages. The paper The paper is is o or rgan ganized ized as as fo follows. llows. In In Sect Section ion 2 2,, tthe he met methods hods use used d tto o predict predict tthe he po pot te ential ntial cont contrail rail coverage coverage (PC (PCC), C), de defined finedas asthe the at atmospheric mospheric ability ability t to for o form m contrails contrail for s for a given a given a aircraft ircrand aft and fu fuel type el ty [28 pe ], The paper is organized as follows. In Section 2, the methods used to predict the potential contrail [2 ar8] e , elaborated. are elaborat The ed. The co conditions nditions of contrail of contformation rail formatfor ion for p parallel arahybrid-electric llel hybrid-elect configuration ric configuratar ion e also are coverage (PCC), defined as the atmospheric ability to form contrails for a given aircraft and fuel type also discus discussed. Section sed. Section 3 analyses 3 analys the ees th ectse effects o of the variation f the variation of cruise altitude, of cruise degraltitude, de ees of hybridization, grees of [28], are elaborated. The conditions of contrail formation for parallel hybrid-electric configuration are hyb andrseasons idization on , an PCC d sechanges. asons on PC Finally C chan , conclusions ges. Finally ar , c e o drawn nclusions in Section are drawn in 4. Section 4. also discussed. Section 3 analyses the effects of the variation of cruise altitude, degrees of hybridization, and seasons on PCC changes. Finally, conclusions are drawn in Section 4. 2. Methodology 2. Methodology We have envisaged a research roadmap (as presented in Figure 4) to investigate the potential We have envisaged a research roadmap (as presented in Figure 4) to investigate the potential of 2. Methodology of flying a hybrid-electric plane for contrail avoidance. The complete research chain contains the flying a hybrid-electric plane for contrail avoidance. The complete research chain contains the We have envisaged a research roadmap (as presented in Figure 4) to investigate the potential of relevant technical aspects of HEA for contrail formation (the green box), the PCC calculation procedure, relevant technical aspects of HEA for contrail formation (the green box), the PCC calculation flying a hybrid-electric plane for contrail avoidance. The complete research chain contains the the operational strategy of HEA to avoid contrails, and the actual mitigation potential when flying procedure, the operational strategy of HEA to avoid contrails, and the actual mitigation potential relevant technical aspects of HEA for contrail formation (the green box), the PCC calculation HEA. In this paper, we present the first steps of the roadmap (marked in blue). Subsequent research when flying HEA. In this paper, we present the first steps of the roadmap (marked in blue). procedure, the operational strategy of HEA to avoid contrails, and the actual mitigation potential will cover the later steps (marked in orange). This section elaborates on the details of the methodology Subsequent research will cover the later steps (marked in orange). This section elaborates on the when flying HEA. In this paper, we present the first steps of the roadmap (marked in blue). of the first steps (those in blue). details of the methodology of the first steps (those in blue). Subsequent research will cover the later steps (marked in orange). This section elaborates on the details of the methodology of the first steps (those in blue). Aerospace 2020, 7, 147 4 of 18 Aerospace 2020, 7, x FOR PEER REVIEW 4 of 18 Hybrid electric aircraft (HEA): • Turbofan engine • Battery technology Change of propulsion efficiency • Power management strategy Derivation of Schmidt Appleman Criterion for HEA Calculate the potential contrail coverage (PCC) Calculate the changes of PCC w.r.t. various hybridizations • A single day analysis to illustrate A given fleet size and routing the local changes of PCC network • Climatological analysis on the changes of contrail formation The changes of PCC for a given fleet and network The operational strategy for HEA to avoid contrail sensitive regionsn Contrail avoidance by HEA + operations Figure 4. A schematic of a research methodology map to investigate the potentials of flying a Figure 4. A schematic of a research methodology map to investigate the potentials of flying a hybrid- hybrid-electric aircraft (HEA) for contrail avoidance. electric aircraft (HEA) for contrail avoidance. An Earth System Model (EMAC) coupled with a CONTRAIL submodel was used to predict the An Earth System Model (EMAC) coupled with a CONTRAIL submodel was used to predict the hybrid-electric aircraft’s PCC. The CONTRAIL submodel includes a revision of the contrail formation hybrid-electric aircraft’s PCC. The CONTRAIL submodel includes a revision of the contrail formation criterion for HEA. Please note that in this analysis, we focus on the contrails formed at the engine criterion for HEA. Please note that in this analysis, we focus on the contrails formed at the engine exhaust, whereas aerodynamic contrails are not considered. The formation of aerodynamic contrails exhaust, whereas aerodynamic contrails are not considered. The formation of aerodynamic contrails mainly depends on the aerodynamic design of aircraft, especially the airfoil [29], which is assumed mainly depends on the aerodynamic design of aircraft, especially the airfoil [29], which is assumed to be unchanged in this analysis and of minor importance concerning their climate impact [30]. to be unchanged in this analysis and of minor importance concerning their climate impact [30]. 2.1. The Base Model EMAC 2.1. The Base Model EMAC The ECHAM/MESSy Atmospheric Chemistry (EMAC) model is a numerical chemistry and The ECHAM/MESSy Atmospheric Chemistry (EMAC) model is a numerical chemistry and climate simulation system that includes submodels describing tropospheric and middle atmosphere climate simulation system that includes submodels describing tropospheric and middle atmosphere processes and their interaction with oceans, land, and human influences [31]. For the present study, processes and their interaction with oceans, land, and human influences [31]. For the present study, we applied EMAC (ECHAM5 version 5.3.02, MESSy version 2.52.0) in T42L31ECMWF-resolution, we applied EMAC (ECHAM5 version 5.3.02, MESSy version 2.52.0) in T42L31ECMWF-resolution, corresponding to a horizontal grid of about 310 km and a vertical resolution of roughly 1 km up to an corresponding to a horizontal grid of about 310 km and a vertical resolution of roughly 1 km up to altitude of approximately 30 km. The simulation time step is 12 min. Such a model resolution will an altitude of approximately 30 km. The simulation time step is 12 min. Such a model resolution will provide us with reasonable weather data to calculate the potential contrail coverage. provide us with reasonable weather data to calculate the potential contrail coverage. EMAC has been extensively validated with other models, for instance, ACCMIP presented EMAC has been extensively validated with other models, for instance, ACCMIP presented in in [32], concerning atmospheric dynamics, cloud occurrence, chemistry, etc. An overview is given [32], concerning atmospheric dynamics, cloud occurrence, chemistry, etc. An overview is given in in [33]. In this paper, we use the submodel CONTRAIL V1.0. Section 2.2 discusses details on the [33]. In this paper, we use the submodel CONTRAIL V1.0. Section 2.2 discusses details on the CONTRAIL submodel. CONTRAIL submodel. 2.2. The CONTRAIL Submodel 2.2. The CONTRAIL Submodel CONTRAIL is one of the submodels in EMAC, developed by Frömming et al. (supplement of [12]) CONTRAIL is one of the submodels in EMAC, developed by Frömming et al. (supplement of to calculate the potential coverage of persistent contrails instantaneously, with the EMAC resolution [12]) to calculate the potential coverage of persistent contrails instantaneously, with the EMAC specified in the previous section. The thermodynamic condition of contrail formation is given by the resolution specified in the previous section. The thermodynamic condition of contrail formation is given by the Schmidt–Appleman criterion [18,19], the derivation of which is presented in Section 2.3 of this paper. In the CONTRAIL submodel, the PCC is calculated as the difference between the Aerospace 2020, 7, 147 5 of 18 Schmidt–Appleman criterion [18,19], the derivation of which is presented in Section 2.3 of this paper. In the CONTRAIL submodel, the PCC is calculated as the di erence between the maximum possible coverage of both contrails and cirrus and the natural cirrus coverage alone. Supersaturation with respect to ice is used to determine if the contrails are persistent or not. 2.3. The HEA Extended Schmidt–Appleman Criterion Contrails form when the mixture of engine exhaust and ambient air reaches water saturation at a suciently low temperature, and they persist when the ambient air is ice-supersaturated. The mixing process is assumed to be isobaric; therefore, the mixing trajectory is represented as a straight line on the T-e diagram (see Figure 1 of [34]; e is the partial pressure of water vapor in the mixture; T is the static temperature of the mixture). The slope of this straight line (G) determining the temperature threshold of contrail formation is given by the SAC and calculated using Equation (1). pc EIH2O G = (1) "Q(1  ) where p is the ambient pressure in Pa; " is the ratio of the molar mass of water vapor and dry air (0.622 constant); c is the isobaric heat capacity of air (1004 J/kg/K); Q is the lower heating value of fuel in MJ/kg; EIH2O is the water vapor emission index in kg/kg(fuel); and the notation ( ) is the overall propulsion eciency of the pure kerosene aircraft (reference), for which we assumed a value of 0.4 in the current study. This eciency value was in line with the value computed using the aircraft/engine performance model presented in [27]. For HEA, the thrust power is provided by two energy sources, as defined by Equation (2). FV =  m Q +  P (2) K f E E where F is the thrust requirement in N, V is the flight speed in m/s, m is the fuel flow rate of kerosene in kg/s,  is the overall eciency of the aircraft when electric power is working alone, the notation (P ) is the electric power in watts. Accordingly, the original SAC was adapted using the assumption that kerosene plus electric power are combined to the necessary power FV results (details are discussed in Appendix A). The revised calculation procedure for slope G leads to Equation (3), which considers the e ects of various degrees of hybridization. c p p a REIH2O G = (3) R(1  )Q + (1 R)(1  )Q K E . . . where Rm /m . For the same thrust power, the maximum fuel flow rate (m ) corresponds to the f f f max max fuel consumption of the reference aircraft, where only kerosene is used. Hence, at pure liquid-fuel operation, R = 1, and pure electric operation, R = 0. The notation (Q ) is a quasi-electric energy content, defined in Equation (4): Q := Q( / ) (4) K E Note that the SAC derivation in this paper only applies to the situation where batteries provide electric power. A battery has no water vapor emission. Suppose the electric power is provided by a fuel cell or another gas turbine via a generator. In that case, water vapor is emitted by these components. A di erent form of the Schmidt–Appleman criterion must be derived to consider the e ects of water vapor emitted by, e.g., the fuel cell. Aerospace 2020, 7, 147 6 of 18 3. Results 3.1. The Threshold of Contrail Formation Calculated by SAC On the basis of the SAC derivation in Section 2.3, we studied the threshold of contrail formation for HEA. Figure 5 shows how the contrail factor G varies with the share of electric propulsion. It is Aerospace 2020, 7, x FOR PEER REVIEW 6 of 18 zero for pure electric propulsion (R = 0) and increases non-linearly with the fraction of liquid-fuel Aerospace 2020, 7, x FOR PEER REVIEW 6 of 18 propulsion until it reaches its maximum at full liquid-fuel propulsion. The shape of the curve depends depends on the efficiency 𝜂 of the electric powertrain. The higher is 𝜂 , the flatter the relation close on the eciency  of the electric powertrain. The higher is  , the flatter the relation close to R = 1. E E depends on the efficiency 𝜂 of the electric powertrain. The higher is 𝜂 , the flatter the relation close to 𝑅= 1 . to 𝑅= 1 . Figure 5. Dependence of the contrail factor G on the ratio R for parallel-hybrid-electric propulsion. Figure 5. Dependence of the contrail factor G on the ratio R for parallel-hybrid-electric propulsion. Figure 5. 𝑅= 1 is full fuel propulsion, Dependence of the contrail factor 𝑅= 0 is full ele G on the ra ctric propu tio R for parallel- lsion. Parameters for the hybrid-electri calcu c propu lation have lsion. R = 1 is full fuel propulsion, R = 0 is full electric propulsion. Parameters for the calculation have been 𝑅= been 1 𝜂 is full fuel propulsion, =0.4, Q=43.2 MJ/kg, 𝑅= 𝑝 = 0 250 is f hPa, ull ele and ctric 𝑄 propu =(𝜂 lsion. ⁄𝜂 )𝑄Param . eters for the calculation have = 0.4, Q = 43.2 MJ/kg, p = 250 hPa, and Q = ( / )Q. K a K E been 𝜂 =0.4, Q=43.2 MJ/kg, 𝑝 = 250 hPa, and 𝑄 =(𝜂 ⁄𝜂 )𝑄 . For the maximum temperature ( 𝑇 ,) ([35], Equation (5)), at which contrail formation is For the maximum temperature (T ,) ([35], Equation (5)), at which contrail formation is possible, max For the maximum temperature ( 𝑇 ,) ([35], Equation (5)), at which contrail formation is possible, we studied the effects of various 𝜂 with respect to the fraction of electric power (see Figure we studied the e ects of various  with respect to the fraction of electric power (see Figure 6). For a possible, we studied the effects of various 𝜂 with respect to the fraction of electric power (see Figure 6). For a more efficient electric system with a high 𝜂 , it needs a relatively high electric share (R is more ecient electric system with a high  , it needs a relatively high electric share (R is very small) to 6). For a more efficient electric system with a high 𝜂 , it needs a relatively high electric share (R is very small) to achieve a substantial lowering of 𝑇 . In our example, the maximum temperature at achieve a substantial lowering of T . In our example, the maximum temperature at which contrails very small) to achieve a substantia max l lowering of 𝑇 . In our example, the maximum temperature at which contrails can be formed when only liquid fuel is used (𝑅= 1 ) is −40 °C. To reduce this by 5 K, can be formed when only liquid fuel is used (R = 1) is 40 C. To reduce this by 5 K, to achieve a which cont to achieve a ramaximum ils can be fo temperature o rmed when on f − ly 45 liqu °C, id it nee fuel d is s rou used g ( hly 𝑅= 1𝑅=) is 0.3 −40 , or 70 °C. T % of o re the thrust power duce this by 5 K, maximum to achieve needs to be provided by th temperatur a maximum e of temperature o 45 e electric mot C, it needs f −45 or when it r °C, oughly it nees e dRs rou f= ficienc 0.3 ghly , y or is 𝑅= 70% 0.0 8. For t .3 of , or 70 the he sm thr % of ust al the thrust power ler e power fficienc needs y, the to be needs to be provided by the electric motor when its efficiency is 0.8. For the smaller efficiency, the reduction in 𝑇 is larger. This may sound paradoxical, but it is the consequence of the simple provided by the electric motor when its eciency is 0.8. For the smaller eciency, the reduction reduction in 𝑇 is larger. This may sound paradoxical, but it is the consequence of the simple physical fact that contrail formation is easier when the exhaust gas is colder, for what is higher in T is larger. This may sound paradoxical, but it is the consequence of the simple physical fact max physical fact that contrail formation is easier when the exhaust gas is colder, for what is higher combined overall propulsion efficiencies. that contrail formation is easier when the exhaust gas is colder, for what is higher combined overall combined overall propulsion efficiencies. propulsion eciencies. Figure 6. Dependence of the maximum temperature at which contrails are possible, 𝑇 , on the ratio Figure 6. R for paralle Depend l-hybrid-electri ence of the c p maxi ropu mu lsion m tem . Parameters are as in Figure 4. perature at which contrails are possible, 𝑇 , on the ratio Figure 6. Dependence of the maximum temperature at which contrails are possible, T , on the ratio max R for parallel-hybrid-electric propulsion. Parameters are as in Figure 4. R for parallel-hybrid-electric propulsion. Parameters are as in Figure 4. This paper considered the technology level of 2030 for electric motor, inverter, and battery, as This paper considered the technology level of 2030 for electric motor, inverter, and battery, as summarized in [27,36]. Furthermore, a typical value of 0.9 for fan efficiency was used. Eventually, summarized in [27,36]. Furthermore, a typical value of 0.9 for fan efficiency was used. Eventually, the value of 𝜂 was about 0.8. This value was then used in the further analysis of PCC. the value of 𝜂 was about 0.8. This value was then used in the further analysis of PCC. Aerospace 2020, 7, 147 7 of 18 Aerospace 2020, 7, x FOR PEER REVIEW 7 of 18 This paper considered the technology level of 2030 for electric motor, inverter, and battery, as summarized in [27,36]. Furthermore, a typical value of 0.9 for fan eciency was used. Eventually, A further example of the parameters for SAC on different aircraft is presented in Table 1. The the value of  was about 0.8. This value was then used in the further analysis of PCC. numbers wereE calculated for an altitude of 11 km. Figure 7 shows three critical mixing lines: one for A further example of the parameters for SAC on di erent aircraft is presented in Table 1. conventional aircraft with an overall efficiency of 0.4, one for HEA with 40% electric power, and one The numbers were calculated for an altitude of 11 km. Figure 7 shows three critical mixing lines: one for for HEA with 80% electric power. These three mixing lines ran from the engine exhaust conditions conventional aircraft with an overall eciency of 0.4, one for HEA with 40% electric power, and one and were tangential to the water saturation pressure line. At the same flight altitude, the temperature for HEA with 80% electric power. These three mixing lines ran from the engine exhaust conditions threshold and the slope G decreased with the increase of electric power in use. Therefore, the chance and were tangential to the water saturation pressure line. At the same flight altitude, the temperature for HEA to form contrails reduced as the electric power fraction increased. threshold and the slope G decreased with the increase of electric power in use. Therefore, the chance for HEA Table 1. to form Calculated parameters for th contrails reduced as the e electric Schmidpower t–Appleman fraction criterion on hy increased. brid-electric aircraft. Hybrid- Conven Table 1. Calculated parameters for the Schmidt–Appleman criterion on hybrid-electric aircraft. Parameters Descriptions Electric tional Units Conventional Aircraft Aircraft Parameters Descriptions Hybrid-Electric Aircraft Units Aircraft Electric power 40% 80% 0 [−] Electric power fraction 40% 80% 0 [] fraction EIH2O Water emission index 1.25 kg/kg(fuel) EIH2O Water emission index 1.25 kg/kg(fuel) η Overall efficiency 0.8 0.4 [−] Overall eciency 0.8 0.4 [] The lower heating value of the The lower heating value Q Q 43.243.2 MJ/ MJ/kgkg of the fuel fuel G Slope at 11 km altitude 1.6 1.1 1.8 Pa/K G Slope at 11 km altitude 1.6 1.1 1.8 Pa/K Figure 7. Mixing line for threshold conditions. Water vapor pressure vs. temperature phase diagram Figure 7. Mixing line for threshold conditions. Water vapor pressure vs. temperature phase diagram representing thermodynamics of contrail formation for a conventional aircraft with overall propulsion representing thermodynamics of contrail formation for a conventional aircraft with overall eciency of 0.4 (blue line) and HEA with two di erent degrees of hybridization: 40% electric power propulsion efficiency of 0.4 (blue line) and HEA with two different degrees of hybridization: 40% (red line) and 80% electric power (green line). The two black curves are the saturation vapor pressure electric power (red line) and 80% electric power (green line). The two black curves are the saturation curves for water (solid) and with respect to ice (dashed). The vertical dashed lines represent the vapor pressure curves for water (solid) and with respect to ice (dashed). The vertical dashed lines temperature threshold for the conventional aircraft (blue), HEA with 40% electric power (red), and HEA represent the temperature threshold for the conventional aircraft (blue), HEA with 40% electric power with 80% electric power (green). (red), and HEA with 80% electric power (green). 3.2. Changes in Potential Contrail Coverage 3.2. Changes in Potential Contrail Coverage We studied the variations of potential contrail coverage caused by di erent e ects, for instance, We studied the variations of potential contrail coverage caused by different effects, for instance, altitude, degree of hybridization, and seasons. In this section, we present the results. The data are altitude, degree of hybridization, and seasons. In this section, we present the results. The data are based on a one-year simulation with EMAC, including the updated CONTRAIL submodel (see above). based on a one-year simulation with EMAC, including the updated CONTRAIL submodel (see For illustration purposes, we first present results for an arbitrary day in winter to give an impression above). For illustration purposes, we first present results for an arbitrary day in winter to give an of an actual weather situation and then present climatological values. All the results were obtained impression of an actual weather situation and then present climatological values. All the results were with 0.8 electric power eciency and 0.4 kerosene system eciency. obtained with 0.8 electric power efficiency and 0.4 kerosene system efficiency. Aerospace 2020, 7, 147 8 of 18 Aerospace 2020, 7, x FOR PEER REVIEW 8 of 18 3.2.1. One-Day Case Study 3.2.1. One-Day Case Study Figure 8a shows the PCC of conventional aircraft at 300 hPa (FL300) on a specific day. In contrast, Figure 8a shows the PCC of conventional aircraft at 300 hPa (FL300) on a specific day. In contrast, the changes in PCC caused by 50% electric hybridization of HEA (i.e., R = 0.5 in Equation (3)) at the the changes in PCC caused by 50% electric hybridization of HEA (i.e., R = 0.5 in Equation (3)) at the same altitude is given in Figure 8b. For the pure kerosene case, the contrails were mainly formed at the same altitude is given in Figure 8b. For the pure kerosene case, the contrails were mainly formed at mid-latitudes and polar regions, where the local temperature was suciently low to form contrails. the mid-latitudes and polar regions, where the local temperature was sufficiently low to form With 50% thrust power supplied by the battery, the reduction in contrail formation was observed at contrails. With 50% thrust power supplied by the battery, the reduction in contrail formation was about 30 N and 40 S, where the tropopause climbed to the higher altitudes. The local temperature in observed at about 30° N and 40° S, where the tropopause climbed to the higher altitudes. The local this region was close to the temperature threshold for the pure kerosene aircraft. A transition to HEA temperature in this region was close to the temperature threshold for the pure kerosene aircraft. A lowered the temperature threshold by a few degrees Celsius. It was also noticeable that the contrail transition to HEA lowered the temperature threshold by a few degrees Celsius. It was also noticeable formation’s reduction was very much localized with a maximum value of around 0.4, which might that the contrail formation’s reduction was very much localized with a maximum value of around have been related to the local temperature and humidity. 0.4, which might have been related to the local temperature and humidity. 2 2 2 −2 Figure 8. The potential contrail coverage (PCC) (contour) and geopotential (black contour; m s ) Figure 8. The potential contrail coverage (PCC) (contour) and geopotential (black contour; m s ) at at 300 hPa (FL300) on a specific day: (a) conventional aircraft; (b) absolute changes caused by HEA 300 hPa (FL300) on a specific day: (a) conventional aircraft; (b) absolute changes caused by HEA with with 50% electric power. 50% electric power. When increasing the flight altitude from 300 hPa to 250 hPa (FL340), for the conventional reference When increasing the flight altitude from 300 hPa to 250 hPa (FL340), for the conventional aircraft, we found the areas of potential contrail coverage in the tropical regions as well (Figure 9a). reference aircraft, we found the areas of potential contrail coverage in the tropical regions as well For HEA, with 50% of electric hybridization, the reduction of contrail formation was more pronounced (Figure 9a). For HEA, with 50% of electric hybridization, the reduction of contrail formation was more in the tropical regions, as seen in Figure 9b. pronounced in the tropical regions, as seen in Figure 9b. Aerospace 2020, 7, 147 9 of 18 Aerospace 2020, 7, x FOR PEER REVIEW 9 of 18 2 2 Figure 9. The PCC (contour) and geopotential (black contour; m 2 s−2 ) at 250 hPa (FL340) on a specific Figure 9. The PCC (contour) and geopotential (black contour; m s ) at 250 hPa (FL340) on a specific day: (a) conventional aircraft; (b) absolute changes caused by HEA with 50% electric power. day: (a) conventional aircraft; (b) absolute changes caused by HEA with 50% electric power. When we increased the flight altitude further to 200 hPa (FL390), HEA with 50% of electric When we increased the flight altitude further to 200 hPa (FL390), HEA with 50% of electric hybridization had no significant change in contrail formation. At this level, the ambient temperature hybridization had no significant change in contrail formation. At this level, the ambient temperature was far below the threshold for contrail formation for conventional aircraft. HEA did not lead to was far below the threshold for contrail formation for conventional aircraft. HEA did not lead to a a sucient lowering of that temperature threshold with 50% electric power. However, increasing sufficient lowering of that temperature threshold with 50% electric power. However, increasing the the hybridization to 90% allowed for a further reduction of the temperature threshold for contrail hybridization to 90% allowed for a further reduction of the temperature threshold for contrail formation. Accordingly, a reduction in contrails was observed in several locations of the tropical region, formation. Accordingly, a reduction in contrails was observed in several locations of the tropical as shown in Figure 10b. region, as shown in Figure 10b. 3.2.2. E ect of Degree of Hybridization In Figure 11, we present a statistical analysis of the local reduction of contrail coverage when di erent hybridization degrees were considered. The annual mean PCC was used. We observed that the contrail coverage remained nearly unchanged with a smaller fraction of electric power in use (less than 30%). As the hybridization rate increased from 10–90%, an exponential reduction trend was found, which was already indicated by the theoretical analysis in Figures 5 and 6. Aerospace 2020, 7, 147 10 of 18 Aerospace 2020, 7, x FOR PEER REVIEW 10 of 18 2 2 2 −2 Figure 10. Figure 10. The The PCC (contour) PCC (contour) and geopotenti and geopotential al (b (black lack contour; contour; m m s s )) at at 20 200 0 hPa hPa (FL390) (FL390) o on n a a specific specific day day: : ((a a)) conven conventional tional air airc craft; raft; ((b b) abso ) absolute lute chang changes es cau caused sed by by HEA HEA with 90 with 90% % ele electric ctric pow power er. . Aerospace 2020, 7, x FOR PEER REVIEW 11 of 18 3.2.2. Effect of Degree of Hybridization In Figure 11, we present a statistical analysis of the local reduction of contrail coverage when different hybridization degrees were considered. The annual mean PCC was used. We observed that the contrail coverage remained nearly unchanged with a smaller fraction of electric power in use (less than 30%). As the hybridization rate increased from 10–90%, an exponential reduction trend was found, which was already indicated by the theoretical analysis in Figures 5 and 6. Figure 11. Distribution of the reduction in PCC with respect to di erent degrees of electric hybridization. Figure 11. Distribution of the reduction in PCC with respect to different degrees of electric hybridization. 3.2.3. Climatology of Contrail Formation The zonal annual mean values of PCC for the reference aircraft and various hybridization degrees are presented in Figure 12. Figure 12a shows the potential contrail coverage for the conventional aircraft, which was also used as a baseline to evaluate different degrees of hybridization effects. From Figure 12b–d, the degree of hybridization increased from 30% to 50% and up to 90%. Again, corresponding to the SAC theory, HEA did not form contrails at the higher temperature and could not form contrails in the lower region of the potential contrail coverage of conventional contrails. Therefore, the reduction of HEA in contrail formation occurred mostly at the lower flight altitudes. However, as the hybridization increased, the altitude range, where contrail formation was reduced, grew. Aerospace 2020, 7, 147 11 of 18 3.2.3. Climatology of Contrail Formation The zonal annual mean values of PCC for the reference aircraft and various hybridization degrees are presented in Figure 12. Figure 12a shows the potential contrail coverage for the conventional aircraft, which was also used as a baseline to evaluate di erent degrees of hybridization e ects. From Figure 12b–d, the degree of hybridization increased from 30% to 50% and up to 90%. Again, corresponding to the SAC theory, HEA did not form contrails at the higher temperature and could not form contrails in the lower region of the potential contrail coverage of conventional contrails. Therefore, the reduction of HEA in contrail formation occurred mostly at the lower flight altitudes. However, Aerospace 2020, 7, x FOR PEER REVIEW 12 of 18 as the hybridization increased, the altitude range, where contrail formation was reduced, grew. Figure 12. Annual zonal mean potential contrail coverage and changes due to electric hybridization: Figure 12. Annual zonal mean potential contrail coverage and changes due to electric hybridization: (a) conventional aircraft; (b) 30% electric hybridization; (c) 50% electric hybridization; (d) 90% (a) conventional aircraft; (b) 30% electric hybridization; (c) 50% electric hybridization; (d) 90% electric electric hybridization. hybridization. 3.2.4. Seasonal Effects 3.2.4. Seasonal E ects On the ba On the si basis s of the a of then annual nual ssimulation imulation res results, ults, we we studied studied the tseasonal he seaso en al ects. effect Figur s. Figure e 13 presents 13 pres the ents the vari variations ations in in PCC PCC for for 90% 90of % o hybridization. f hybridizat Tion he r . The esults rwer esueltgr s were ouped g into rouped four seasons into foat ur s four eas pr ons essur ate four altitudes. Generally, we expected the largest e ect (i.e., the greatest reduction in PCC) on the lowest pressure altitudes. Generally, we expected the largest effect (i.e., the greatest reduction in PCC) on pressure level, since the temperatures on that level were closest to the SAC-temperature threshold. the lowest pressure level, since the temperatures on that level were closest to the SAC-temperature threshold. Similarly, we expected the PCC reduction to become smaller towards higher pressure altitudes with their lower temperatures. This pattern was evident in the figure, except for summer. There were more substantial reductions on 150 hPa (FL450) and 200 hPa (FL390) levels in summer than in the other seasons. This behavior was partly due to higher temperatures, closer to the SAC- threshold, but lower relative humidity than those in different seasons may contribute to PCC reduction. The reduction was found to be more or less constant in summer at all flight levels (differences were insignificant), although the difference between the actual and the SAC-threshold temperature increased with flight level. It is likely that a decreasing frequency of high humidity (including ice-supersaturation) cases balanced the temperature effect such that the PCC reduction in summer depended little on the flight altitude. Aerospace 2020, 7, 147 12 of 18 Similarly, we expected the PCC reduction to become smaller towards higher pressure altitudes with their lower temperatures. This pattern was evident in the figure, except for summer. There were more substantial reductions on 150 hPa (FL450) and 200 hPa (FL390) levels in summer than in the other seasons. This behavior was partly due to higher temperatures, closer to the SAC-threshold, but lower relative humidity than those in di erent seasons may contribute to PCC reduction. The reduction was found to be more or less constant in summer at all flight levels (di erences were insignificant), although the di erence between the actual and the SAC-threshold temperature increased with flight level. It is likely that a decreasing frequency of high humidity (including ice-supersaturation) cases balanced the Aerospace 2020, 7, x FOR PEER REVIEW 13 of 18 temperature e ect such that the PCC reduction in summer depended little on the flight altitude. th 90 percentile th 75 percentile: Q3 Median th 25 percentile: Q1 th 10 percentile (a) (b) Figure 13. Seasonal e ects on the changes of potential contrail coverage at various pressure altitudes Figure 13. Seasonal effects on the changes of potential contrail coverage at various pressure altitudes for 90% electric hybridization (a) and the explanation of the boxplot (b). DJF: December, January, for 90% electric hybridization (a) and the explanation of the boxplot (b). DJF: December, January, and and February; MAM: March, April, and May; JJA: June, July, and August; SON: September, October, February; MAM: March, April, and May; JJA: June, July, and August; SON: September, October, and and November. For the boxplot: the 10th and 90th percentiles of the data are plotted. Triangle indicates November. For the boxplot: the 10th and 90th percentiles of the data are plotted. Triangle indicates the mean value. As the altitude increases, e.g., at 150 hPa (FL450), the values at 90th percentile and the mean value. As the altitude increases, e.g., at 150 hPa (FL450), the values at 90th percentile and 75th percentile are equal to the minimum of the datasets as zero, therefore not visible in Figure (a). 75th percentile are equal to the minimum of the datasets as zero, therefore not visible in Figure (a). 4. Discussion 4. Discussion This study demonstrated the e ects of hybrid-electric aircraft on potential contrail coverage. This study demonstrated the effects of hybrid-electric aircraft on potential contrail coverage. The The actual contrail properties—the number of soot particles related to using a hybrid-electric propulsion actual contrail properties—the number of soot particles related to using a hybrid-electric propulsion system, the lifetime of contrails, etc.—were not considered. Earlier research shows that the number of system, the lifetime of contrails, etc.—were not considered. Earlier research shows that the number soot particles produced by a turbofan engine is strongly dependent on the thrust setting [37]. A higher of soot particles produced by a turbofan engine is strongly dependent on the thrust setting [37]. A thrust setting would lead to an increase in the combustion temperature, hence, increasing the rate of higher thrust setting would lead to an increase in the combustion temperature, hence, increasing the soot formation, and vice versa. As di erent degrees of electric hybridization were considered, the gas rate of soot formation, and vice versa. As different degrees of electric hybridization were considered, turbine engine’s thrust setting and the number of soot particles varied accordingly. The change in soot the gas turbine engine’s thrust setting and the number of soot particles varied accordingly. The particles will a ect the contrails’ actual optical properties, thus the resulting contrail radiative forcing. change in soot particles will affect the contrails’ actual optical properties, thus the resulting contrail Furthermore, the Schmidt–Appleman criterion’s derivation in this paper is applicable for a radiative forcing. hybrid-electric propulsion system, for which the electric power is generated by battery. If the choice goes Furth for erm a fuel ore, cell the hybrid-electric Schmidt–Appl prema opulsion n criter system, ion’s one derwould ivationr equir in th eis a di pa pe erent r is consideration. applicable for a The SAC for the fuel cell type of hybrid system depends on how the fuel cell’s exhaust is handled. If a hybrid-electric propulsion system, for which the electric power is generated by battery. If the choice heat exchanger is used to collect the fuel cell’s exhaust heat, the water vapor at the fuel cell’s exhaust goes for a fuel cell hybrid-electric propulsion system, one would require a different consideration. is then condensed into a liquid. In this case, contrails are only formed from the gas turbine exhaust. The SAC for the fuel cell type of hybrid system depends on how the fuel cell’s exhaust is handle d. If The SAC derivation in the current paper can still be valid. However, if no condensation process is a heat exchanger is used to collect the fuel cell’s exhaust heat, the water vapor at the fuel cell’s exhaust is then condensed into a liquid. In this case, contrails are only formed from the gas turbine exhaust. The SAC derivation in the current paper can still be valid. However, if no condensation process is involved, the fuel cell then also produces water vapor in the exhaust, which must be considered to derivate an alternative SAC. As observed from our analysis, the reduction in contrail coverage by hybrid-electric aircraft is mostly localized. The consequences are twofold. In a case where the flight route does not cross a region of reduction (that is, the temperature is significantly lower than the contrail formation threshold even for a high portion of electric power), flying with a hybrid-electric aircraft does not affect the contrail formation. Still, the contrail properties (lifetime, optical thickness, and eventually individual radiative forcing) may change with the degree of hybridization. In another case where the contrail reduction is possible along the flight route (that is, the temperature is close to the contrail formation threshold), using a large fraction of electric power at that specific location would effectively reduce contrail formation. For the second case, a specific power management strategy can be developed to design the hybrid-electric system. Eventually, we expect the actual effect to be a IQR=Q3-Q1 Aerospace 2020, 7, 147 13 of 18 involved, the fuel cell then also produces water vapor in the exhaust, which must be considered to derivate an alternative SAC. As observed from our analysis, the reduction in contrail coverage by hybrid-electric aircraft is mostly localized. The consequences are twofold. In a case where the flight route does not cross a region of reduction (that is, the temperature is significantly lower than the contrail formation threshold even for a high portion of electric power), flying with a hybrid-electric aircraft does not a ect the contrail formation. Still, the contrail properties (lifetime, optical thickness, and eventually individual radiative forcing) may change with the degree of hybridization. In another case where the contrail reduction is possible along the flight route (that is, the temperature is close to the contrail formation threshold), using a large fraction of electric power at that specific location would e ectively reduce contrail formation. For the second case, a specific power management strategy can be developed to design the hybrid-electric system. Eventually, we expect the actual e ect to be a convolution of the existing air trac patterns with the PCC-reduction patterns. In further analysis, we will investigate the e ectiveness of contrail avoidance by hybrid-electric aircraft, considering the routing e ects. In addition, in our analysis, we used a typical value of 0.4 as the overall propulsion eciency, and we left it constant regardless of the amount of electrical power used. As more electrical energy is used, the gas turbine engine’s thrust setting is reduced; correspondingly, the propulsion eciency would decrease slightly. If we consider such eciency deterioration, the temperature threshold of contrail formation for the hybrid-electric system is reduced further (see, e.g., Equation (1) and Figure 6 of this paper). The lower temperature threshold implies that contrail formation is more likely to decrease, but it requires a thorough analysis in future research. 5. Conclusions This paper presents the changes in potential contrail coverage when flying with hybrid-electric aircraft. On the basis of the analysis, we have drawn the following conclusions: The atmospheric areas of contrail formation of hybrid-electric aircraft are smaller than those of conventional aircraft and require lower atmospheric temperatures. The reduction in contrail formation by hybrid-electric aircraft is more pronounced in a tropical region where the temperatures are higher. With a small degree of hybridization (below 30% in the current study), the contrail coverage remains nearly unchanged. A maximum reduction of about 40% in contrail coverage was observed locally, with 90% electric power in use. In non-summer, the reduction in potential contrail coverage by hybrid-electric aircraft was more noticeable at lower flight altitudes. In contrast, the changes in potential contrail coverage were nearly constant (about 20%) for all flight altitudes studied in summer. Author Contributions: Conceptualization of this study, V.G. and F.Y.; derivation of the Schmidt–Appleman criterion for hybrid-electric aircraft, V.G. and K.G.; formal analysis and writing the original draft, F.Y.; writing—review, V.G. and K.G.; funding acquisition, V.G. and F.Y. All authors have read and agreed to the published version of the manuscript. Funding: Initial work received funding from the SESAR Joint Undertaking under grant agreement no. 699395 under the European Union’s Horizon 2020 research and innovation program within the Exploratory Research project ATM4E. The individual author of this work receives funding from the Dutch Research Council (NWO) under the talent scheme VENI and in-kind contribution from DLR-Oberpfa enhofen. The project number is 17367. Conflicts of Interest: The authors declare no conflict of interest. Aerospace 2020, 7, 147 14 of 18 Nomenclature Abbreviations DJF December, January, and February EIH2O Water vapor emission index kg/kg(fuel) EM Electric motor HEA Hybrid-electric aircraft JJA June, July, and August MAM March, April, and May PCC Potential contrail coverage PDF Probability density function RF Radiative forcing SAC Schmidt–Appleman criterion SON September, October, and November Symbols c Isobaric heat capacity of the air J/kg/K F Thrust N G The slope of the mixing line pa/K m Fuel mass flow rate kg/s p Ambient pressure pa P Electric power W Q The lower heating value of fuel MJ/kg R Degrees of hybridization [] The maximum temperature at which T C max contrail formation is possible V Velocity m/s The ratio of the molar mass of water " [] vapor and dry air The overall eciency of the electric [] powertrain The overall eciency of the pure [] kerosene aircraft Appendix A. Derivation of Schmidt–Appleman Criterion for Hybrid-Electric Aircraft The thermodynamic theory of contrail formation was developed for traditional jet engines many years ago [18,19]. A modern derivation is provided by [35] using the conservation principles of mass, momentum and energy. The traditional engine type has only one source of energy, namely, the fuel with its specific energy content Q. For each kilogram of fuel burnt, a mass of EIH2O kilogram water vapor is produced and emitted. For a . . fuel flow rate of m , the rate of water vapor emission is m EIH2O. f uel f uel The condition for contrail formation is the Schmidt–Appleman criterion (SAC). The most important factor in the theory is the so-called contrail factor G = de /dT , that is, the change of partial pressure of water vapor in the p p exhaust plume, e , with plume temperature, T . This change occurs when the plume is expanding and mixing with p p ambient air. This mixing is isobaric at ambient pressure, p , and the mixing trajectory of the exhaust gases in a thermodynamic, e T, diagram is thus a straight line with slope G. The endpoint of that trajectory at infinite mixing is represented by the ambient conditions: water vapor partial pressure, e , and temperature, T . Thus we find a a e e p a G = (A1) T T p a It is practical to use mass mixing ratios q = "e /p , where " = 0.622 is the ratio of molar masses of H O x x a 2 and air. The partial pressure of water vapor at the engine exit is . . m q + m EIH2O p a a f e =  (A2) . . m + m where the notation (m ) is the mass flow rate of air through the engine. In this derivation, we do not consider the separation of the core flow and bypass flow, as the two air streams mix anyway at the engine exit within a few milliseconds. Such a consideration will avoid an unnecessary complication of the equations. The equation states Aerospace 2020, 7, 147 15 of 18 that the vapor partial pressure at the engine exit is composed of that carried by the air needed to burn the fuel plus the contribution from the fuel itself. Since an electric motor does not emit water vapor, there is hence no contribution. Thus, the numerator in the formula for G is . . m (EIH2O q ) m EIH2O p f a p f a a e e =    (A3) p a . . . . " " m + m m + m a a f f where the approximation is possible since EIH2O  q . The aircraft needs a thrust F to overcome drag and friction. When it flies with a velocity V, the engines must produce a power FV =  m Q +  P (A4) K E E That is, two sources of energy, from the liquid fuel and the electric motor, add their powers with their respective thermodynamic eciencies,  . The notation (P ) is the (variable) power of the electric engine. P varies E E in response to variable fuel flow, such that the above sum equals FV. Thus, P is a function of m . E f 0 0 P = P am with P = FV/ and a = Q( / ) (A5) E f E K E E E Here, P is the electric power when it is driving the aircraft alone. The higher the fuel flow, the lower P is. The maximum fuel flow to achieve a thrust power of FV is m = FV/ Q. It turns out to be useful also to define max a quasi-electric energy content, namely, 0 0 Q := P /m = a = Q( / ) (A6) f K E max E E As the eciencies are never 1.0, the remaining part of the produced power is wasted for heating and expelling the exhaust gases (from the burnt fuel and the air flowing through the engine). These are thermal and kinetic energies. For the present derivation, we neglected the kinetic energy since it is much smaller than the thermal energy. Thus, we have h   i . . . (1  )m Q + (1  )P = c m T T + m T (A7) p a p a p K f E E f That is, the engine air is heated from its ambient temperature to the plume temperature, and the gas added by burning the liquid fuel is heated to T as well. (A few other energy sources and sinks are neglected here: the enthalpy of the liquid fuel, the heating of the engine parts, for instance). The symbol (c ) is the heat capacity (at constant pressure) of air. After a few steps, we find . . . ( ) ( ) ( ) ( ) 1  m Q + 1  P c m T 1  m Q + 1  P K f E E p f a K f E E T T =      (A8) p a . . . . c m + m c m + m p a f p a f This approximation is possible since the enthalpy of the ambient air is much smaller than the energy content of the fuel or the energy produced by the electric motor. Dividing Equation (A3) by Equation (A8) gives an expression for the contrail factor m EIH2O c p p a f G =    (A9) . . (1  )m Q + (1  )P m K f E E f where the dependence of P on the fuel flow is made explicit for clarity. Now it appears convenient to normalize the fuel flow rate by its maximum, . . R := m /m (A10) f f max At pure liquid-fuel operation, R = 1, and at pure electric operation, R = 0. Having this and the other definitions from above, we arrive after a few steps at a favorable expression for G: c p p a EIH2O G = R (A11) R(1  )Q + (1 R)(1  )Q K E Equation (A11) is the desired expression. It has the correct limiting properties. For R = 1 we retain the form of the traditional SAC, but for pure electric propulsion, R = 0, hence, G = 0, which implies that contrail formation is impossible. Aerospace 2020, 7, 147 16 of 18 From here, it is relatively straightforward to formulate a generalization to more than two energy sources. Let FV =  m Q +  P (A12) K f i i where apart from the liquid fuel, we have a number of energy sources (index i) that do not produce water in the exhaust. Then we introduce in analogy to the derivation above: 0 0 0 P := FV/ , Q := P /m = ( / )Q (A13) i f K i max i i i Furthermore, we define weights w P /P . 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Published: Oct 12, 2020

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