Energy Transition in Aviation: the Role of Cryogenic Fuels

aerospace

Review Energy Transition in Aviation: The Role of Cryogenic Fuels

Arvind Gangoli Rao * , Feijia Yin and Henri G.C. Werij Faculty of Aerospace Engineering, Delft University of Technology, 2628 HS Delft, The Netherlands; [email protected] (F.Y.); [email protected] (H.G.C.W.) * Correspondence: [email protected]; Tel.: +31-152-783-833

Received: 15 November 2020; Accepted: 14 December 2020; Published: 18 December 2020

Abstract: Aviation is the backbone of our modern society. In 2019, around 4.5 billion passengers travelled through the air. However, at the same time, aviation was also responsible for around 5% of anthropogenic causes of global warming. The impact of the COVID-19 pandemic on the aviation sector in the short term is clearly very high, but the long-term eﬀects are still unknown. However, with the increase in global GDP, the number of travelers is expected to increase between three- to four-fold by the middle of this century. While other sectors of transportation are making steady progress in decarbonizing, aviation is falling behind. This paper explores some of the various options for energy carriers in aviation and particularly highlights the possibilities and challenges of using cryogenic fuels/energy carriers such as liquid hydrogen (LH2) and liqueﬁed natural gas (LNG).

Keywords: aviation; cryogenic fuels; energy transition; hybrid aircraft; liquid hydrogen; liqueﬁed natural gas; sustainable aviation

1. Introduction Aviation is the backbone of our modern society. In 2019, around 4.5 billion passengers travelled through the air. However, at the same time, aviation was also responsible for around 5% of anthropogenic causes of global warming [1,2]. Even though the number of people travelling by aircraft has reduced drastically in 2020 due to the outbreak of COVID-19, the aviation sector is expected to recover after the invention of a reliable vaccine [3,4]. From then on, air traffic in terms of passenger kilometers is expected to continue to grow at a rate of approximately between 4–5% per year for the next couple of decades [5], implying a doubling in the number of passenger-kilometers every 15–18 years. Figure1 shows the observed growth in passenger transport for di ﬀerent modalities over the past decades and the expected trend in the future. This indicates the ever-increasing number of passenger-kilometers by air travel (in yellow color-coding) [6]. It should be noted that these predictions are lower than the predictions of some of the commercial aircraft manufacturers, but are in line with the predictions of DLR carried out within the WeCare project [7]. As a result, the environmental impact of aviation will continue to increase significantly unless we act now. Moreover, whereas modes of surface transportation systems can reduce CO2 and emissions significantly by means of technological solutions, which are available or in reach (e.g., use of electric/hybrid vehicles), the aviation sector is facing severe technological challenges regarding the energy source, for instance, dictated by the stringent mass and volume constraints. This implies that the contribution to global warming originating from aviation will not only grow in absolute but also in relative terms. Figure2 shows the worldwide consumption of aviation fuel since 1990. Of course, this is proportional to the increasing amount of aviation-related CO2 emission, which in 2019 amounted to around 2.5% of the total global anthropogenic CO2 emission. In developed countries, the share of aviation in their total anthropogenic CO2 footprint can vary substantially from the global average.

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For instance, Figure3 depicts the percentage of aviation CO 2 in the Netherlands. It can be seen that aviation was responsible for almost 8% of CO2 emission in the Netherlands (based on the amount of jetAerospace fuel used2020, 7 on, x FOR all Dutch PEER REVIEW commercial airports) before the COVID-19 crisis [8,9]. 2 of 27

Figure 1. Historical values and future prediction of transport volumes in passenger-kilometers [6].

As a result, the environmental impact of aviation will continue to increase significantly unless we act now. Moreover, whereas modes of surface transportation systems can reduce CO2 and emissions significantly by means of technological solutions, which are available or in reach (e.g., use of electric/hybrid vehicles), the aviation sector is facing severe technological challenges regarding the energy source, for instance, dictated by the stringent mass and volume constraints. This implies that the contribution to global warming originating from aviation will not only grow in absolute but also in relative terms. Figure 2 shows the worldwide consumption of aviation fuel since 1990. Of course, this is proportional to the increasing amount of aviation-related CO2 emission, which in 2019 amounted to around 2.5% of the total global anthropogenic CO2 emission. In developed countries, the share of aviation in their total anthropogenic CO2 footprint can vary substantially from the global average. For instance, Figure 3 depicts the percentage of aviation CO2 in the Netherlands. It can be seen that aviation was responsible for almost 8% of CO2 emission in the Netherlands (based on the amount of Figure 1. jetHistoricalHistorical fuel used on values all Dutch and commercial future prediction airports) before of transport the COVID -volumes 19 crisis [8,9 in]. passenger-kilometerspassenger-kilometers [[6]6]..

Annual fuel usage in aviation As a result, the environmental400 impact of aviation will continue to increase significantly unless we act now. Moreover, whereas modes of surface transportation systems can reduce CO2 and emissions significantly350 by means of technological solutions, which are available or in reach (e.g., use of electric/hybrid vehicles), the aviation sector is facing severe technological challenges regarding the energy source, for instanc300 e, dictated by the stringent mass and volume constraints. This implies that the contribution to global warming originating from aviation will not only grow in absolute but also

in relative terms. L Billion 250 Figure 2 shows the worldwide consumption of aviation fuel since 1990. Of course, this is proportional to the increasing amount of aviation-related CO2 emission, which in 2019 amounted to 200 around 2.5% of the total global anthropogenic CO2 emission. In developed countries, the share of aviation in their total anthropogenic CO2 footprint can vary substantially from the global average. 150

For instance, Figure 3 depicts the percentage of aviation CO2 in the Netherlands. It can be seen that

1991 1992 1990 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 aviation was responsible for almost 8% of CO2 emissionYear in the Netherlands (based on the amount of jet fuel used on all Dutch commercial airports) before the COVID-19 crisis [8,9]. Figure 2. Annual fuel usage of Jet-A fuel (sources: Statistica, (IATA), DoE). AerospaceFigure 2020, 7 2., x FORAnnual PEER REVIEW fuelusage of Jet-A fuel (sources: Statistica, (IATA), DoE).3 of 27

AnnualAviation fuel related usage CO2 emission in aviation in NL 400 9 8

7 350 6

300 4

3 % of the total emission total the of %

2 Billion L Billion 250 1

1994 2011 1990 1991 1992 1993 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2012 2013 2014 2015 2016 2017 200 Year

Figure 3. AnnualFigure CO 3. Annualemission CO2 emission from from aviation aviation as a a percentage percentage of the of total the CO total2 emission CO fromemission the from the Netherlands2 (source: CBS, RHK). 2 Netherlands150 (source: CBS, RHK).

Even though the total kerosene consumption in aviation has increased, the aviation industry has

1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 done1990 a remarkable job in improving its efficiency by using a combination of technological and operational measures. As depicted in Figure 4, theYear system-wide fuel consumption per revenue passenger-kilometer (RPK) has been reduced by more than 50% in the last 30 years. However, this is outpaced by the increase in RPK by around 440%. To shape the aviation industry’s future, the Figure 2. Annual fuel usage of Jet-A fuel (sources: Statistica, (IATA), DoE). Advisory Council for Aeronautical Research and innovation in Europe (ACARE) has set challenging goals to reduce aviation’s environmental impact and to make aviation sustainable in the future. The ACARE flight path 2050 sets targets for the year 2050 of a 75% reduction of CO2 emissions per passenger-kilometer and a 90% reduction in NOx emissions. These targets are relative to the capabilities of typical new aircraft in 2000 [10]. Whereas the goals set by some of the other organizations (comprising Airports Council International (ACI), Civil Air Navigation Services Organization (CANSO), International Air Transport Association (IATA), International Coordinating Council of Aerospace Industries Associations (ICCAIA), and International Business Aviation Council IBAC) [11] state that from 2020 onwards, the net carbon emissions from aviation will be capped through carbon-neutral growth. The goal set by Air Transport Action Group (ATAG) is to reduce the net aviation carbon emissions in 2050 to half of what they were in 2005 [12]. In order to achieve the latter goal, the reduction of CO2 emission per passenger-kilometer should decrease by more than 90%. In 2016 the International Civil Aviation Organization (ICAO) assembly decided that a global market-based measure should be put in place to offset CO2 emissions from international aviation. The scheme has been named CORSIA (Carbon Offsetting and Reduction Scheme for International Aviation) and aims at limiting the CO2 emissions from aviation to the levels of 2019 with the help of offsetting schemes. There are two phases of this scheme, the first one is voluntary until 2027 and the second phase is mandatory after 2027. There are several emission unit programs approved by the ICAO Council. The emissions can be offset by an airline by purchasing emission units equivalent to its offsetting requirements in other sectors such as wind energy, clean cook stoves, methane capture, afforestation and reforestation, carbon capture and storage (CCS), etc. It should be noted that CORSIA is applicable to only international flights, which account for around 60% of all flights (https://www.icao.int/environmental-protection/pages/a39_corsia_faq2.aspx).

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Even though the total kerosene consumption in aviation has increased, the aviation industry has done a remarkable job in improving its efficiency by using a combination of technological and operational measures. As depicted in Figure4, the system-wide fuel consumption per revenue passenger-kilometer (RPK) has been reduced by more than 50% in the last 30 years. However, this is outpaced by the increase in RPK by around 440%. To shape the aviation industry’s future, the Advisory Council for Aeronautical Research and innovation in Europe (ACARE) has set challenging goals to reduce aviation’s environmental impact and to make aviation sustainable in the future. The ACARE flight path 2050 sets targets for the year 2050 of a 75% reduction of CO2 emissions per passenger-kilometer and a 90% reduction in NOx emissions. These targets are relative to the capabilities of typical new aircraft in 2000 [10]. Whereas the goals set by some of the other organizations (comprising Airports Council International (ACI), Civil Air Navigation Services Organization (CANSO), International Air Transport Association (IATA), International Coordinating Council of Aerospace Industries Associations (ICCAIA), and International Business Aviation Council IBAC) [11] state that from 2020 onwards, the net carbon emissions from aviation will be capped through carbon-neutral growth. The goal set by Air Transport Action Group (ATAG) is to reduce the net aviation carbon emissions in 2050 to half of what they were in 2005 [12]. In order to achieve the latter goal, the reduction of CO2 emission per passenger-kilometer should decrease by more than 90%. In 2016 the International Civil Aviation Organization (ICAO) assembly decided that a global market-based measure should be put in place to offset CO2 emissions from international aviation. The scheme has been named CORSIA (Carbon Offsetting and Reduction Scheme for International Aviation) and aims at limiting the CO2 emissions from aviation to the levels of 2019 with the help of offsetting schemes. There are two phases of this scheme, the first one is voluntary until 2027 and the second phase is mandatory after 2027. There are several emission unit programs approved by the ICAO Council. The emissions can be offset by an airline by purchasing emission units equivalent to its offsetting requirements in other sectors such as wind energy, clean cook stoves, methane capture, afforestation and reforestation, carbon capture and storage (CCS), etc. It should be noted that CORSIA is applicable to only international flights, which account for around 60% of all

ﬂights (Aerospacehttps:// 20www.icao.int20, 7, x FOR PEER/ REVIEWenvironmental-protection /pages/a39_corsia_faq2.aspx). 4 of 27

Gain in system wide efficiency

Figure 4.FigureIncrease 4. Increase in global in global revenue revenue passengers-km passengers-km (RPK) (RPK) and and fuel fuel consumption consumption for civil for aviation civil aviation (sources:(source Statistica,s: Statistica, International International Civil Civil Aviation Aviation Organization Organization (ICAO (ICAO),), IATA) IATA)..

AmongstAmong thest various the various possibilities possibilities for for reducing reducing aviation’s environmental environmental footprint, footprint, the aviation the aviation industry is looking at alternative fuels as one of the cornerstones in making aviation sustainable. Fuel industry is looking at alternative fuels as one of the cornerstones in making aviation sustainable. selection is affected by several factors, including fuel price, fuel reserve/availability, and emissions characteristics, as summarized in Figure 5. In 2019, aviation consumed around 1 billion liters of jet fuel every day (corresponding to 300 Mton/year). It is anticipated that this would increase by 3% every year despite the improvements in aircraft and engine efficiency. Decreasing oil reserves and increasing extraction costs, over time, will lead to an increase in the fuel cost. This increase in fuel cost has already increased the fuel share in airlines’ total operating costs to around 30% [13]. Furthermore, the urgency of tackling the climate issue forces us to look for alternative sustainable fuels long before the scarcity of fossil fuels becomes the driving factor. Sole dependence on kerosene also implies that there are significant fluctuations in the price of the fuel, discussed later in Figure 10.

Figure 5. Motivation of alternative fuels for aviation.

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Gain in system wide efficiency

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Figure 4. Increase in global revenue passengers-km (RPK) and fuel consumption for civil aviation Fuel selection is affected(sources by: Statistica, several International factors, Civil including Aviation Organization fuel price, (ICAO), fuelIATA).reserve /availability, and emissions characteristics, as summarized in Figure5. In 2019, aviation consumed around 1 billion liters of jet fuel Amongst the various possibilities for reducing aviation’s environmental footprint, the aviation every day (correspondingindustry is looking to at 300 alternative Mton fuels/year). as one Itof the is anticipatedcornerstones in making that aviation this would sustainable. increase Fuel by 3% every year despite theselection improvements is affected by in several aircraft factors, and including engine fuel e price,fficiency. fuel reserve Decreasing/availability, oil and reserves emissions and increasing characteristics, as summarized in Figure 5. In 2019, aviation consumed around 1 billion liters of jet extraction costs,f overuel every time, day will(corresponding lead to to an 300 increase Mton/year) in. I thet is anticipated fuel cost. that This this would increase increase in fuelby 3%cost has already increased the fuelevery share year despite in airlines’ the improvements total operating in aircraft costsand engine to aroundefficiency. 30%Decreasing [13]. oil Furthermore, reserves and the urgency of tackling the climateincreasingissue extraction forces costs, usover to time look, will forlead alternativeto an increase in sustainable the fuel cost. This fuels increase long in beforefuel the scarcity cost has already increased the fuel share in airlines’ total operating costs to around 30% [13]. of fossil fuels becomesFurthermore the, thedriving urgency of factor.tackling the Sole climate dependence issue forces us onto look kerosene for alternative also sustainable implies that there are significant fluctuationsfuels long before in the the price scarcity of of fossil the fuel,fuels becomes discussed the driving later factor. in FigureSole dependence 10. on kerosene also implies that there are significant fluctuations in the price of the fuel, discussed later in Figure 10.

Aerospace 2020, 7, x FOR PEER REVIEWFigure 5.FigureMotivation 5. Motivation of of alternative alternative fuels fuels for aviation for aviation.. 5 of 27

2.2. AlternativeAlternative EnergyEnergy Carriers

BeforeBefore wewe dwelldwell onon thethe details of didifferentfferent energy carriers carriers for for aviation, aviation, it it is is good good to to know know the the totaltotal energyenergy required for for various various passenger passenger transport transport modes. modes. Figure Figure 6 displays6 displays the theenergy energy carried carried onon-boardboard of some of some commonly commonly used used passenger passenger vehicles. vehicles. It shows It shows that the that energy the energy carried carried on-board on-board of a ofvehicle a vehicle typically typically increases increases with with the thetotal total amount amount of passenger of passenger-km.-km. From From a passenger a passenger-km-km point point of ofview, view, an an e-bicycle, e-bicycle, small small electric electric car car,, and and a passenger a passenger bus busare aremore more efficient efficient than than other other vehicles vehicles (as (asthey they are are below below the the thin thin 45 45° line)◦ line).. An An A320 A320 on on a long a long-range-range mission mission (e.g., (e.g., 6000 6000 km) km) reaches reaches a amillion million passenger-km,passenger-km, moremore thanthan twice the distance between between earth earth and and moon moon if if translated translated to to one one passenger passenger.. ItIt isis alsoalso clearclear fromfrom thethe figurefigure thatthat aa small-mediumsmall-medium range aircraft aircraft (e.g., (e.g., 1000 1000 km km range) range) carries carries more more thanthan 100100 GJGJ ofof energy,energy,about about threethree ordersorders ofof magnitudemagnitude moremore thanthan a small electric car.

Figure 6. Typical values of the total energy carried on-board for diﬀerent types of passenger vehicles. Figure 6. Typical values of the total energy carried on-board for different types of passenger vehicles.

Figure 6, therefore, provides us an insight that the total energy on-board increases linearly with the passenger-km. For aircraft, the range is largely dictated by the amount of fuel and therefore the energy density of fuels/energy carriers becomes vital as it significantly influences the vehicle’s total weight. The weight breakdown of a short-to-medium range single aisle aircraft is shown in Figure 7. If we increase the weight of the energy source/carrier, it will reduce the total number of passengers carried onboard significantly, thereby reducing the passenger-km (shifting the aircraft to the left of the dotted line on the graph), and hence significantly reducing its efficiency.

Figure 7. Typical weight breakdown of a short-to-medium range aircraft (data source [14]).

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2. Alternative Energy Carriers Before we dwell on the details of different energy carriers for aviation, it is good to know the total energy required for various passenger transport modes. Figure 6 displays the energy carried on- board of some commonly used passenger vehicles. It shows that the energy carried on-board of a vehicle typically increases with the total amount of passenger-km. From a passenger-km point of view, an e-bicycle, small electric car, and a passenger bus are more efficient than other vehicles (as they are below the thin 45° line). An A320 on a long-range mission (e.g., 6000 km) reaches a million passenger-km, more than twice the distance between earth and moon if translated to one passenger. It is also clear from the figure that a small-medium range aircraft (e.g., 1000 km range) carries more than 100 GJ of energy, about three orders of magnitude more than a small electric car.

Aerospace 2020Figure, 7, 1816. Typical values of the total energy carried on-board for different types of passenger 5 of 24 vehicles.

FigureFigure 66,, therefore, provides us an insight that the total energy on-boardon-board increases linearly with thethe passenger passenger-km.-km. For For aircraft, aircraft, the the range range is is largely largely dictated dictated by by the the amount amount of of fuel fuel and and therefore therefore the the energyenergy density density of of fuel fuelss/energy/energy carriers carriers becomes becomes vital vital as as it it significantly significantly influences influences the the vehicle vehicle’s’s total total weight.weight. The The weight weight breakdown breakdown of of a a short short-to-medium-to-medium range range single single aisle aisle aircraft aircraft is is shown shown in in Fig Figureure 7.7. IfIf we we increase increase the the weight weight of of the the energy energy source/carrier, source/carrier, it it will will reduce reduce the the total total number number of of passengers passengers carriedcarried onboard onboard significantly,significantly, therebythereby reducing reducing the the passenger-km passenger-km (shifting (shifting the the aircraft aircraft to theto the left left of theof thedotted dotted line line on theon the graph), graph) and, and hence hence significantly significantly reducing reducing its eitsffi ciency.efficiency.

Aerospace 20Figure20, 7, x FOR 7. Typical PEER REVIEW weightbreakdown of a short-to-medium range aircraft (data source [14]). 6 of 27 Figure 7. Typical weight breakdown of a short-to-medium range aircraft (data source [14]). Figure8 8 depicts depicts some some essential essential criteria criteria to to select select fuel fuel for for aviation. aviation Please. Please note note that that these these criteria criteria are are by no means exhaustive. They are used here to provide the reader with an overview and highlight by no means exhaustive. They are used here to provide the reader with an overview and highlight some of the considerations that should be taken into account when discussing options for alternative energy carriers.carriers. The following subsectionssubsections will elaborate onon thethe mostmost representativerepresentative criteria.criteria.

Volumetric Energy Density Specific TRL Energy Density

Climate Emissio Impact ns

Policy Cost

Compatibility Availability

Safety Infrastructure

Figure 8. Some of the important considerations during the selection of alternative fuel/energy fuel/energy carriers inin aviation.aviation. 2.1. Speciﬁc and Volumetric Energy Density 2.1. Specific and Volumetric Energy Density As already highlighted, one of the main criteria is the energy density, as reducing weight and As already highlighted, one of the main criteria is the energy density, as reducing weight and volume is of paramount importance for aviation. Both speciﬁc energy density (SED, amount of energy volume is of paramount importance for aviation. Both specific energy density (SED, amount of energy per unit mass of the fuel) and volumetric energy density (VED, amount of energy per unit volume) are important in this regard. Figure 9 shows several fuels/energy sources in terms of their SED and VED [15]. It is noticeable that Jet-A/kerosene has good SED and VED, and therefore is suitable for aviation. While biofuels and synthetic kerosene also have a similar energy density and therefore are ideal from an energy carrier point of view, their availability and costs are the main constraints, highlighted in subsequent subsections.

Aerospace 2020, 7, 181 6 of 24 per unit mass of the fuel) and volumetric energy density (VED, amount of energy per unit volume) are important in this regard. Figure9 shows several fuels /energy sources in terms of their SED and VED [15]. It is noticeable that Jet-A/kerosene has good SED and VED, and therefore is suitable for aviation. While biofuels and synthetic kerosene also have a similar energy density and therefore are ideal from an energy carrier point of view, their availability and costs are the main constraints, highlightedAerospace 20 in20 subsequent, 7, x FOR PEER subsections.REVIEW 7 of 27

Li-ion

A -

12 Li-ion Future

H2@700 bars

Zinc Air Batt Relative Volume w.r.t. Jet w.r.t. Volume Relative 4 Methanol LH2 LNG Liq. Ammonia Fat Jet-A Ethanol 0 0 1 10 100 Relative weight w.r.t. Jet-A

Figure 9. Comparison of various energy sources for aviation with respect to Jet-A [15]. LH2—cryogenic Figure 9. Comparison of various energy sources for aviation with respect to Jet-A [15]. LH2— hydrogen;cryogenic LNG—liquefied hydrogen; LNG natural—liquefied gas. natural gas. Furthermore, cryogenic hydrogen (LH ) has high SED but a poor VED, implying that we would Furthermore, cryogenic hydrogen (LH22) has high SED but a poor VED, implying that we would requirerequire a large a large volume volume (about (about four four times times largerlarger than for for kerosene, kerosene, not not taking taking into into account account insulated insulated tanks)tanks) to carry to carry any any reasonable reasonable amount amount of of LH LH2.2. In contrast contrast,, the the LH LH2 fuel2 fuel weight weight would would be only be 1/3rd only of 1/ 3rd of kerosenekerosene for the for same the same energy energy content. content Figure. Figure9 also 9 showsalso shows that that liquefied liquefied natural natural gas g (LNG)as (LNG lies) lies halfway betweenhalfway kerosene between and kerosene LH2, in and terms LH of2, in VED, terms while of VED its, SEDwhile is its slightly SED is betterslightly than better that than of kerosene.that of kerosene. 2.2. Cost of the Energy Source 2.2. Cost of the Energy Source Kerosene derived from the distillation of crude oil is a cheap fuel, as aviation fuel used on Kerosene derived from the distillation of crude oil is a cheap fuel, as aviation fuel used on international flights are usually not taxed. This is primarily due to the ICAO Chicago convention international flights are usually not taxed. This is primarily due to the ICAO Chicago convention articlearticle 24 (which 24 (which bans bans taxing taxing fuel fuel and and other other fluids fluids already already onboard onboard an an international international flight flight for for its its usage) and itsusage) subsequent and its subsequent revision inrevisio 1993n in 1993 which in which member member countries countries are are encouraged encouraged not not to to levylevy taxes on fueltaxes for on international fuel for international flights [16flights]. This [16]. is T onehis is of one the of reasons the reasons why why air air travel, travel in, in general, general, is is cheap. However,cheap. it However should be, it notedshould that be noted kerosene’s that kerosene’s price is subjectprice is tosubject crude to oilcrude prices oil prices and therefore and therefore fluctuates significantly.fluctuates Figure significantly. 10 shows Figure the 10 variation shows the in kerosene’s variation in price kerosene over’s the price last over several the decades;last several we can observedecades that; the we price can observeis volatile that and the can price vary is significantly volatile and candepending vary significantly on the economic depending and on geopolitical the situation.economic Such and variation geopolitical is a significant situation. Such risk forvariation airlines, is a and significant therefore risk some for airlines , and hedge therefore on the price some airlines hedge on the price of kerosene in order to reduce the effect of the volatility in fuel price of kerosene in order to reduce the effect of the volatility in fuel price on their economics of operations. on their economics of operations. Although biofuels and synthetic kerosene are similar to kerosene in their chemical and physical properties (except for their low aromatic contents), their cost is currently significantly higher. Figure 11 gives an overview of various fuels in terms of their costs and their greenhouse gas reduction potential [17]. As can be seen from the figure, most of the biofuels and synthetic fuels are significantly more expensive than kerosene, which reduces their likelihood of being used as an energy source in aviation unless enforced by governments through stringent regulations.

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Variability of  10 times Aerospace 2020, 7, 181 7 of 24 Aerospace 2020, 7, x FOR PEER REVIEW 8 of 27

Variability of  10 times

Figure 10. Variation in the price of kerosene over the last few decades (US Energy Information Administration, U.S. Gulf Coast Kerosene-Type Jet Fuel Spot Price FOB, USD per gallon).

Although biofuels and synthetic kerosene are similar to kerosene in their chemical and physical properties (except for their low aromatic contents), their cost is currently significantly higher. Figure 11 gives an overview of various fuels in terms of their costs and their greenhouse gas reduction potential [17]. As can be seen from the figure, most of the biofuels and synthetic fuels are significantly more expensive than kerosene, which reduces their likelihood of being used as an energy source in aviationFigureFigure unless 10. Variation Variationenforced in inby the thegovernments price price of of kerosene kerosene through over over stringent th thee last last regulations. few decades ( (USUS Energy Information Administration, U.S. Gulf Coast Kerosene Kerosene-Type-Type Jet Fuel SpotSpot PricePrice FOB,FOB, USDUSD perper gallon).gallon).

Figure 11. Comparison of cost estimates of various possible fuel/energy sources for aviation [17]. Figure 11. Comparison of cost estimates of various possible fuel/energy sources for aviation [17].

At present LH2 production is expensive and not yet environmentally friendly; however, as we At present LH2 production is expensive and not yet environmentally friendly; however, as we move towards a hydrogen-based economy by utilizing renewable energy sources, the price of LH2 ismove expected towards to reducea hydrogen substantially-based economy while meeting by utilizing sustainability renewable targets energy at sources, the same the time. price Theof LH future2 is expected to reduce substantially while meeting sustainability targets at the same time. The future energy scenario from Shell (Sky Energy Scenario [18]) lays out a roadmap for using H2 in various sectorsenergy toscenario meet the from Paris Shell climate (Sky goalsEnergy [19 Scenario]. Hydrogen [18]) canlays beout produced a roadmap from for renewable using H2 in energy various and serves as a long-term fuel for aviation [20]. LNG, on the other hand, is currently one of the cheapest fuels available. The gas reserves in the worldFigure are enormous 11. Comparison thus implying of cost estimates that LNG of prices various would possible be fuel/energy stable. However, sources when for aviation we have [17] to. move away from fossil fuels and start producing methane either synthetically using renewable energy or usingAt biomass, present theLH price2 production development is expensive in this scenarioand not yet will environmentally be diﬀerent. friendly; however, as we move towards a hydrogen-based economy by utilizing renewable energy sources, the price of LH2 is expected to reduce substantially while meeting sustainability targets at the same time. The future energy scenario from Shell (Sky Energy Scenario [18]) lays out a roadmap for using H2 in various

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2.3. Availability of Energy Source As already mentioned, currently aviation consumes around a billion liters of jet fuel every day (about 300 Mton/year), which would increase by approximately 3% each year, despite the increase in aircraft efficiency. A clear alternative to fossil fuel is biofuel, which is produced from fatty acids and triglycerides obtained from plants or animal processing. Several biofuels have already been certified to be blended up to 50% along with kerosene [21]. The aviation sector has committed to developing bio-jet fuel through voluntary initiatives, and various stakeholders are actively involved. However, biofuels (as well as synthetic kerosene) projections so far have shown to be too optimistic, as their current usage remains very low. Currently, only a tiny fraction of the total fuel consumption comprises of biofuels [22]. There are several challenges in scaling up biofuel production, such as limited availability, ethical issues regarding the sourcing of feedstock, complicated logistics, certification costs, competition from other sectors, low price of kerosene, etc. Therefore, biofuel can be a part of the solution but not the solution itself. Regarding LNG (mainly consisting of methane, CH4), the world’s gas reserves are enormous. LNG is a traded global fuel and has a well-established logistic and industrial network to support scaling of operations if required. Obviously, at present LNG is still a fossil energy source, which, when being used, contributes to global warming, even though the CO2 emission is approximately 20% lower for the same energy content. During the last decade, a lot of progress has been made regarding the production of methane from green hydrogen (produced using sustainable electricity) and CO2 (either captured from flue gasses or the air) [23]. In other words, for LNG/methane, a sustainable route can be a feasible option, where issues regarding the scalability of production plants and costs have to be considered. Regarding H2, around 70 Mton H2/yr is used today in pure form, mostly for oil refining and ammonia manufacture for fertilizers; a further 45 Mton H2 is used in the industry without prior separation from other gases [24]. Thus, the amount of H2 produced today in terms of the energy content is similar to the total amount of jet fuel used in civil aviation (mass energy density of H2 is nearly three times higher than that of jet fuel). However, currently, most of this H2 is grey H2 and therefore still has a large CO2 footprint. There has been an enormous surge in the research related to H2. It is seen as a valuable contributor to a low carbon society and can support energy security in several ways. With the increase in the deployment of renewable energy sources like wind turbines and PV cells, the electricity grid faces many fluctuations. H2 is seen as a buffer, as electricity can be converted to H2 and back or further converted to other fuels, making end-users less dependent on specific energy resources and increasing the resilience of energy supplies. H2, produced from fossil fuels with CCUS (carbon capture and usage) or from biomass, can also increase the diversity of energy sources, especially in a low-carbon economy [24].

2.4. Infrastructure The infrastructure for production, storage, and distribution of biofuels and LNG is well established and has reached good technical maturity. Although H2 is used in several industrial processes, the infrastructure for storing and transporting LH2 is not as mature as for LNG, partly because LH2 has to be stored at very low temperatures, LH2 at 20 K and LNG at 111 K. The current infrastructure of airports is not suited for cryogenic fuels (both LH2 and LNG), especially at big airports. Smaller airports typically supply fuel to the aircraft with tankers, and that can be changed to accommodate LH2/LNG tankers, but still requires substantial changes to the refueling protocols and requires investments. All in all, using cryogenic fuels in aviation has several signiﬁcant challenges, including the need for cryogenic storage where LH2 by its nature requires a larger volume than kerosene, and boil-oﬀ losses come into play. Other aspects, such as safety, logistics, passenger perception, etc., as investigated in the Cryoplane Project [17], will be discussed in Section4. Aerospace 2020, 7, x FOR PEER REVIEW 10 of 27 can be changed to accommodate LH2/LNG tankers, but still requires substantial changes to the refueling protocols and requires investments. All in all, using cryogenic fuels in aviation has several significant challenges, including the need for cryogenic storage where LH2 by its nature requires a larger volume than kerosene, and boil-off losses come into play. Other aspects, such as safety, logistics, passenger perception, etc., as investigated in the Cryoplane Project [17], will be discussed in Section 4. Aerospace 2020, 7, 181 9 of 24 2.5. Sustainability

2.5.The Sustainability combustion of conventional kerosene emits species like CO2, NOx, water vapour, and particulate matters. The interaction of these species with atmosphere cause air pollutants or global The combustion of conventional kerosene emits species like CO2, NOx, water vapour, and particulate warmingmatters. (directly The interaction or indirectly). of these The species actual with effects atmosphere depend cause on where air pollutants and when or globalthey are warming emitted. Figure(directly 12 is ora recent indirectly). study The by actualLee et effects al. [1] dependshowing on aviation’s where and climate when theyimpact are from emitted. different Figure sources 12 is up a to recent 2018 study. Contrails by Lee for etmed al. [1 ] behind showing the aviation’s engine climateexhaust impact play the from biggest different role sources followed up to by 2018. CO2 2 x emissions.Contrails The formed total behind non-CO the effects engine exhaustare more play than the 60% biggest of the role overall followed aviation’s by CO2 emissions. climate impact. The total NO emissionsnon-CO 2ineffects the upper are more troposphere than 60% ofand the lower overall stratosphere aviation’s climate cause impact. formation NOx ofemissions ozone and in the depletion upper of methane.troposphere The and total lower effects stratosphere of NOx causeare warming. formation Water of ozone vapour and depletion itself is of a methane.greenhouse The g totalas. Aerosols effects haveof NOdirectx are or warming. indirect effects. Water vapour The direct itself effects is a greenhouse of soot are gas. warming Aerosols and have from direct sulphur or indirect are effects.cooling. TheThe number direct effectsof soot of particles soot are warmingcan affect and the from microphysics sulphur are of cooling. contrails The and number hence of the soot contrails’ particles actual can climateaffect impact.the microphysics Burning of alternative contrails and fuels hence will the contrails’ affect the actual aviation’s climate impact. emissions, Burning and alternative thereby its environmentalfuels will affect impact. the aviation’s emissions, and thereby its environmental impact.

FigureFigure 12. 12. AnAn overview overview of of aviation’s aviation’s climateclimate impact impact from from global global aviation aviation from from 1940 to1940 2008 to [ 120].0 The8 [1]. bars The barsand and whiskers whiskers show show the bestthe estimatesbest estimates and the and 5–95% the confidence5–95% confidence intervals. intervals. Red bars indicate Red bars warming indicate warmingterms andterms blue and bars blue indicate bars indicate cooling terms. cooling terms. The assessment in Figure 12 is performed using both effective radiative forcing (ERF) (change in The assessment in Figure 12 is performed using both effective radiative forcing (ERF) (change in net downward radiative flux at the trapopause after allowing for stratospheric temperatures to readjust net downward radiative flux at the trapopause after allowing for stratospheric temperatures to to radiative equilibrium, while holding surface and tropospheric temperatures and state variables fixed readjustat the tounperturbed radiative equilibrium, values [25]) and while radiative holding forcing surface (RF) and (change tropospheric in net downward temperatures radiative and flux state variablesat the topfixed of theat the atmosphere unperturbed after val allowingues [25] for) atmosphericand radiative temperatures, forcing (RF)water (change vapour, in net clouds downward and radiativeland albedo flux at to adjust,the top but of with the atmosphere global mean after surface allowing temperature for atmospheric or ocean and temperatures, sea ice conditions water vapour,unchanged clouds [25 and]). Theland ERF albedo allows to theadjust, rapid but adjustments with global of mean components surface of temperature the atmosphere or ocean and surface, and sea ice which conditions are assumed unchanged to be constant [25]). The in RF, ERF therefore, allows the the ERF rapid is considered adjustments as a betterof components indicator of of the the final temperature change, especially associated to short-lived climate forcers, e.g., contrails. We can observe from Figure 12 that the ERF of contrail cirrus is nearly half of the RF. However, the contrail cirrus ERF is still the largest. The contrails cirrus ERF/RF ratio of 0.42 indicates that contrail cirrus is less effective in surface warming than other effects. Aerospace 2020, 7, 181 10 of 24

Biofuels have similar characteristics to kerosene and are therefore exchangeable with kerosene. The CO2 emissions of biofuels depend on the source of the biomass, the type of biomass, the process adopted for converting the biomass, the resources used to grow the biomass, etc. The Renewable Energy Directive (RED) established by the European Commission (Directive (EU) 2018/2001 of 11 December 2018) is a good step in this direction as it lays down comprehensive methodology to calculate the greenhouse gas emissions from biofuels (with road diesel used as proxy). Directive 2009/28/EC introduced sustainability criteria including land usage with high biodiversity value and land with high-carbon stock. For aviation jet fuels, chapter 6 of the CORSIA agreement gives an overview of a set of criteria that have to be adhered to in claiming any biofuel as Corsia Eligible Fuels (ENVReport2019_pg228-231.pdf (icao.int) (https://www.icao.int/environmental-protection/Documents/ EnvironmentalReports/2019/ENVReport2019_pg228-231.pdf). Apart from the CO2 reduction, biofuels, in general, contains substantially fewer aromatic contents than kerosene, which helps reduce the number of soot particles. Currently, certified biofuels can be used up to 50% with kerosene. Regarding LNG, from a combustion technique point of view, using natural gas as a fuel is not a problem for the engine as natural gas is a clean fuel and can be burnt in a premixed or partially premixed mode. This substantially reduces the NOx formation within the combustor when compared to kerosene. However, an additional heat exchanger has to be used for evaporating the LNG to natural gas. The CO2 emission from LNG for the same energy content is around 25% lower than kerosene. The main advantage of using LH2 is that there is no CO2 emission from the combustion of fuel. The engine will emit water vapor and some amount of NOx as exhaust. The combustion properties of H2 are quite different than other fuels as the laminar flame speed is almost an order of magnitude more than that of CH4. Moreover, due to the high flammability limits, there is a huge risk of flashback in combustors designed to operate on H2. The usage of H2 also puts some severe limits on the metal pipes and fuel injectors due to H2 embrittlement. These considerations have to be taken into account when designing turbofan engines using H2. The high water emission index of burning hydrogen/natural gas increases the chance of contrail formation, according to the well-known Schmidt Appleman Criterion [26]. However, earlier research suggests that hydrogen combustion produces no soot particles and methane combustion produces significantly lower soot than kerosene. The decrease in soot number density is expected to reduce initial ice crystal concentrations on average, the contrails’ optical depth and contrails radiative forcing as soot particles form the nucleation sites which initiate ice crystal formation [27].

2.6. Fuel Selection Table1 summarizes the essential properties of several representative fuels. We can see that the energy density of LH is only one-fourth of Jet A and the atmospheric boiling point is 253 C. LNG has a slightly 2 − ◦ lower energy density than Jet A, and the boiling temperature at the atmosphere condition is 162 C. − ◦ Table 1. Fuel properties.

Vol. Energy Density Speciﬁc Energy Density Boiling Point @ 1 Atm Density @ 1 Atm (MJ/L) (MJ/kg) (◦C) (kg/m3) Jet A 35.3 43.02 176 802 LNG 24 53.6 162 450 − LH 8.5 120 253 70.8 2 −

Based on the criteria mentioned in Figure8, Table2 gives a simplistic comparison of di fferent energy sources. It can be seen that apart from emissions/climate, kerosene is favorable in most other aspects, and that is why it is widely used. One can note that LNG has several advantages compared to other fuels/energy sources (apart from kerosene). This table can by no means be seen as a complete basis for a thorough trade-off between the different options. It is a start and will require a follow-on study which considers issues like overall Aerospace 2020, 7, 181 11 of 24 energy requirement for worldwide production, scarcity of materials, modifications to the propulsion systems, logistics, life cycle analysis, etc.

Table 2. Simplistic comparison of diﬀerent energy sources.

Parameter Kerosene Biofuel Syn-Kerosene Batteries LNG * LH2 ** Energy + + + + + + Density − − Volume + + + + + + +/ Density − − − − Emissions + + + + + + − − Cost + + + + + − − − − Availability + + + +/ − − − − − − − Infrastructure + + +/ + − − − − − Safety + + + +/ − − − − − Compatibility + + + + + + +/ − − − Policy + + + +/ + − − Climate + + + + + + Impact − − TRL 9 8 4 5 4 3

* refers to fossil-based LNG. ** refers to green LH2.

3. Hydrogen as an Energy Carrier for Aviation If aviation has to maintain its sustainable growth, the aviation sector must solve energy source problems in the ﬁrst place. This section discusses the potential of hydrogen as an energy carrier for future aviation.

3.1. Features of Liquid Hydrogen for Aviation

The advantages and disadvantages of using LH2 as an energy carrier in aircraft are summarized in this section: As compared to the conventional kerosene, the benefits of using LH2 are: The high energy density (almost three times of kerosene). • Lower fuel weight than kerosene (see Figure8). • No CO emission during the flight. • 2 No secondary emissions such as soot, CO, unburnt hydrocarbon (UHC), and volatile • organic compounds. Large potential to reduce NOx emission from combustion [26,27]. • Usage of the cryogenic heat sink can increase turbofan engine thermal efficiency substantially. • Can also be used in conjunction with fuel cells and electrical motors. • Wide combustion range and flammability limit [28]. • Less prone to combustion instabilities when compared to other fuels [29]. • It can be made by renewable energy sources. • Disadvantages of using LH2 are: The poor volumetric energy density (70.8 kg/m3 for LH vs. 750 kg/m3 for kerosene), approximately • 2 4 times lower than kerosene for the same energy content. Increased storage space compared to conventional jet fuels. • The fuel cannot be stored in the wings but only in the fuselage or in underwing pods. • LH storage requires cryogenic or pressurized tanks. • 2 LH has an extremely low boiling temperature (20.3 K); therefore, it requires very effective • 2 insulation to keep the fuel cool. The fuel cost is higher than the conventional kerosene. • The production capacity for ”green” hydrogen is still inadequate. • The airport logistics are quite difficult. • Aerospace 2020, 7, x FOR PEER REVIEW 13 of 27

 LH2 storage requires cryogenic or pressurized tanks.  LH2 has an extremely low boiling temperature (20.3 K); therefore, it requires very effective insulation to keep the fuel cool.  The fuel cost is higher than the conventional kerosene.  AerospaceThe production2020, 7, 181 capacity for ”green” hydrogen is still inadequate. 12 of 24  The airport logistics are quite difficult.  The water emission increases substantially, water being a greenhouse gas at higher altitudes (>10 The water emission increases substantially, water being a greenhouse gas at higher altitudes (>10 km). •km). Hydrogen has a propensity to leak.  •Hydrogen has a propensity to leak. Hydrogen has a tendency to flashback during the combustion process in a gas turbine.  •Hydrogen has a tendency to flashback during the combustion process in a gas turbine. Safety of operations and usage in an airport environment is challenging.  •Safety of operations and usage in an airport environment is challenging. The energy efficiency for electrolysis and liquefaction is around 50%.  •The energy efficiency for electrolysis and liquefaction is around 50%. 3.2. Hydrogen Storage 3.2. Hydrogen Storage Gaseous hydrogen suffers from a vastly inferior volumetric energy density when compared to kerosene;Gaseous 0.01hydrogen MJ/L for suffers room from temperature a vastly hydrogen inferior volumetric versus 35 MJ energy/L for kerosene.density when Given compared the space to keroseneconstraints; 0.01 on-boardMJ/L for currentroom temperature airliners, matching hydrogen the current versus performance 35 MJ/L andfor kerosene. range of kerosene Given enginesthe space constraintsis not easy. on-board Liquid current hydrogen airliners, boasts of matching a much higher the current volumetric performance density at 8.5 and MJ range/L. The of National kerosene enginesAeronautics is not easy. and Liquid Space Administration hydrogen boasts (NASA) of a much relies onhigher LH2 volumetricfor its rockets density and claims at 8.5 it MJ/L. “is the The signature fuel of the American Space Program”. Liquid hydrogen must be stored at 253 C and National Aeronautics and Space Administration (NASA) relies on LH2 for its rockets and− claims◦ it “is the signaturehandled with fuel extreme of the American care. The tanks Space must Program” be heavily. Liquid insulated hydrogen to prevent must the be LH stored2 from at evaporating −253 °C and as it absorbs heat and expands rapidly; thus, venting is necessary to prevent the tank from exploding. handled with extreme care. The tanks must be heavily insulated to prevent the LH2 from evaporating as it Metalsabsorbs exposed heat and to theexpands extreme rapidly; cold become thus, brittle.venting Moreover, is necessary hydrogen to prevent can leak the through tank from minute exploding. pores in welded seams. Metals exposed to the extreme cold become brittle. Moreover, hydrogen can leak through minute Figure 13 gives a schematic overview of some of the common options available for storing/carrying pores in welded seams. hydrogen. They are normally classified as physical-based or material-based. For aviation, due to the Figure 13 gives a schematic overview of some of the common options available for stringent mass and volume constraints, physically storing LH2 in insulated tanks yields the highest storing/carrying hydrogen. They are normally classified as physical-based or material-based. For volumetric and specific energy density. Other physical means such as storing H2 at high pressure and 2 aviation,cryo-compressed due to the stringent H2 have amass higher and weight volume penalty, constraints, as shown physically in Figure 14 storing from Bauhaus LH in insulated Luftfahrt e.V.tanks yieldsWhat the canhighest be seen volumetric from the figure and specific is that the energy weight density. of hydrogen Other with physical respect means to the weight such as of thestoring tank H2 at highimproves pressure when and large cryo quantities-compressed of H2 Hare2 have stored. a higher For small weight amounts penalty, of H2 as(like shown in drones), in Fig iture is better14 from Bauhausto store Luftfahrt H2 in compressed e.V. What formcan be rather seen than from in the liquid figure form, is butthat for the large weight quantities, of hydrogen it is better with to storerespect to theH 2weightin the formof the of tank cryogenic improves LH2. when It also large indicates quantities that it isof perhaps H2 are betterstored. to For use small larger amount tanks ins theof H2 (likefuselage in drones), than it smaller is better tanks. to store However, H2 in thecompressed tank design form and rather placement than will in liquid become form an integral, but for part large quantities,of the aircraftit is better design to store process H2 asin thethe cryogenicform of cryogenic tank will LH change2. It also not onlyindicates the weight that it and is perhaps drag of thebetter to useaircraft larger but tanks also in stability, the fuselage passenger than evacuation, smaller tanks. operations, However, etc. the tank design and placement will Cryogenic LH tanks are complex storage systems with several subsystems built into it. A schematic become an integral part2 of the aircraft design process as the cryogenic tank will change not only the of a cryogenic tank from Linde® is shown in Figure 15. The following factors are taken into account weight and drag of the aircraft but also stability, passenger evacuation, operations, etc. while selecting the insulating material and type.

Figure 13. An overview of diﬀerent hydrogen storage options (https://www.energy.gov/eere/fuelcells/ hydrogen-storage). Aerospace 2020, 7, x FOR PEER REVIEW 14 of 27

AerospaceFigure2020 , 7, 181 13. An overview of different hydrogen storage options13 of 24 (https://www.energy.gov/eere/fuelcells/hydrogen-storage).

FigureFigure 14. 14. ComparisonComparison of of various various physical physical hydrogen hydrogen storage storage options options (source (source:: Bauhaus Bauhaus Luftfahrt Luftfahrt e.V. e.V. ((https://www.bauhaushttps://www.bauhaus-luftfahrt.net-luftfahrt.net/en/topthema/hy/en/topthema/hy-shair-shair//)).)).

Cryogenic LH2 tanks are complex storage systems with several subsystems built into it. A schematic of a cryogenic tank from Linde® is shown in Figure 15. The following factors are taken into account while selecting the insulating material and type.  The insulation system must minimize boil-off while keeping additional mass to a minimum.  The insulation system must prevent atmospheric gasses from condensing and solidifying onto the tank.  The insulation system must not fail due to the cyclic loading of LH2.  The insulation system must have low thermal conductivity and low density.

Figure 15. Schematic of a cryogenic hydrogen tank (source: Linde®).

The insulation system must minimize boil-off while keeping additional mass to a minimum. • The insulation system must prevent atmospheric gasses from condensing and solidifying onto the tank. • The insulation system must not fail due to the cyclic loading of LH . • 2 The insulation system must have low thermal conductivity and low density. • Multilayer insulation (MLI) systems can achieve lower thermal conductivities compared to flexible polymer foam or Aerogel [30]. MLI systems consist of a spacer and a reflector stacked upon each other. The spacer is often made of thin fiberglass, polyester, silk, or plastic layer. On the other hand, the reflector is made of Mylar or aluminum foil with a thickness of 6–7 µm. Furthermore, MLI systems perform best in high vacuum (HV) conditions but can also operate in a soft vacuum with higher ® thermal conductivityFigure [31]. 15. Schematic of a cryogenic hydrogen tank (source: Linde ).

Aerospace 2020, 7, x FOR PEER REVIEW 15 of 27

Multilayer insulation (MLI) systems can achieve lower thermal conductivities compared to flexible polymer foam or Aerogel [30]. MLI systems consist of a spacer and a reflector stacked upon each other. The spacer is often made of thin fiberglass, polyester, silk, or plastic layer. On the other hand, the reflector is made of Mylar or aluminum foil with a thickness of 6–7 μm. Furthermore, MLI systems perform best in high vacuum (HV) conditions but can also operate in a soft vacuum with higher thermal conductivity [31]. Aerospace 2020, 7, 181 14 of 24

3.3. History of Aircraft Designs with Hydrogen 3.3. History of Aircraft Designs with Hydrogen Even though H2 is not currently used in aviation, it is not a new fuel for aviation. There have

beenEven several though attempts H2 is in not the currently past to use used hydrogen. in aviation, One it isof notthe afirst new attempts fuel for aviation.was by Sir There Han haveOhain been to severaluse hydrogen attempts in inhis the newly past invented to use hydrogen. gas turbine One back of thein 1937. first During attempts the was Cold by War Sir Han, both Ohain the United to use hydrogenStates and in histhe newly Soviet invented Union ( gasUSSR turbine) tried back to inuse 1937. hydrogen During in the aircraft Cold War,s, with both limited the United success. States The and theUnited Soviet States Union built (USSR) two triedaircraft tos use, one hydrogen a reconnaissance in aircrafts, aircraft with limitedcalled Suntan success. (shown The United in Figure States 1 built6a), twoand aircrafts, the other one was a reconnaissance in the NACA- aircraftLewis LH called2 flight Suntan test (shown program in Figureusing 16a modifieda), and the B other-57B Canberra was in the military aircraft (Figure 16b). NACA-Lewis LH2 flight test program using a modified B-57B Canberra military aircraft (Figure 16b).

(a)

Aerospace 2020, 7, x FOR PEER REVIEW 16 of 27 (b)

(c)

FigureFigure 16. 16.Examples Examples ofof hydrogen-poweredhydrogen-powered aircraft. ( a) The Lockheed Lockheed-Martin-Martin Suntan, Suntan, taken taken from from the the symposiumsymposium on on hydrogen-fueledhydrogen-fueled aircraft,aircraft, NASANASA Langley Research Center, Center, 15 15–16–16 May May 1973 1973 ( (bb)) B B-57B-57B aircraftaircraft with with oneone engine capable of of running running on on hydrogen, hydrogen, image image is taken is taken from from NACA NACA RM RM E57D23 E57D23 (c) (cTu) Tu-155-155 aircraft aircraft with with one one engine engine capable capable of of running running on on hydrogen, hydrogen, image image is is taken taken from from Tupolev Tupolev history history - http:- http://www.tupolev.ru/en/about/#history_en.//www.tupolev.ru/en/about/#history_en.

The Russians had an ambitious program to modify TU154 aircraft to run on hydrogen (Figure 16c). The same aircraft was later used with LNG. Cryogenic fuels are stored in a fuel tank with a capacity of 17.5 m3 installed in a special compartment aft of the passenger cabin. On 5 April 1988, the aircraft performed its maiden flight using LH2. Upon flight testing and development, TU-155 performed its first flight on LNG in January 1989. Subsequently, some international flight demonstrations were made. The test flights showed that H2 propulsion was feasible and had various advantages, but not enough to overcome its disadvantages, most prominently the logistics problem of needing LH2 to be available on all airports in addition to kerosene (Hydrogen Aircraft Technology, Brewer, 1991). These trade-offs did not take environmental impact into account. However, recent analysis shows the significant environmental benefits of using H2 compared to kerosene and synthetic fuel generated with sustainably generated energy.

3.4. Proposed Aircraft Designs with Liquid Hydrogen

There has been a resurgence in the interest of designing aircraft using LH2 and several designs utilizing LH2 have been proposed. This section covers the prominent ones. The great challenge for LH2 aircraft is to have a suitable configuration to store cylindrical spherical insulated and pressurized fuel tanks that occupy almost five times the volume compared to kerosene. Since the pressurized vessels do not fit in the wings, the fuselage seems a suitable option. However, the challenge of storing the fuel tanks in the fuselage is the enormous increase in the fuselage volume and the presence of the passengers next to hydrogen fuel tanks. While the former will lead to an increase in fuselage drag and a subsequent increase in aircraft fuel consumption, the latter poses some serious safety challenges. Cryoplane: In the year 2004, the EC-sponsored CRYOPLANE project was completed. The project’s main goal was to design aircraft using LH2, covering various subjects from H2 production methods to environmental compatibility. In total, six categories of aircraft were designed in the project ranging from a small regional aircraft, a business jet to a long-range passenger aircraft. The baseline short–medium range (SMR) aircraft was designed to carry 185 passengers over 4000 nm (similar to the A321 but with more range) and a stretch version to carry 218 passengers over 3300 nm. The initial and final configuration is shown in Figure 17. For the initial design, the aerodynamic penalty due to the oversized inner wing turned out to be a 17.5% loss in L/D. The final configuration was an adaptation of the baseline kerosene aircraft with a low-set wing with a giant fuselage to accommodate the top and aft tank (Figure 17). This increased the trim drag. The horizontal tail was decreased in size compared to the kerosene baseline due to the longer fuselage, but the vertical tail

Aerospace 2020, 7, 181 15 of 24

The Russians had an ambitious program to modify TU154 aircraft to run on hydrogen (Figure 16c). The same aircraft was later used with LNG. Cryogenic fuels are stored in a fuel tank with a capacity of 17.5 m3 installed in a special compartment aft of the passenger cabin. On 5 April 1988, the aircraft performed its maiden flight using LH2. Upon flight testing and development, TU-155 performed its first flight on LNG in January 1989. Subsequently, some international flight demonstrations were made. The test flights showed that H2 propulsion was feasible and had various advantages, but not enough to overcome its disadvantages, most prominently the logistics problem of needing LH2 to be available on all airports in addition to kerosene (Hydrogen Aircraft Technology, Brewer, 1991). These trade-offs did not take environmental impact into account. However, recent analysis shows the significant environmental benefits of using H2 compared to kerosene and synthetic fuel generated with sustainably generated energy.

3.4. Proposed Aircraft Designs with Liquid Hydrogen

There has been a resurgence in the interest of designing aircraft using LH2 and several designs utilizing LH2 have been proposed. This section covers the prominent ones. The great challenge for LH2 aircraft is to have a suitable configuration to store cylindrical spherical insulated and pressurized fuel tanks that occupy almost five times the volume compared to kerosene. Since the pressurized vessels do not fit in the wings, the fuselage seems a suitable option. However, the challenge of storing the fuel tanks in the fuselage is the enormous increase in the fuselage volume and the presence of the passengers next to hydrogen fuel tanks. While the former will lead to an increase in fuselage drag and a subsequent increase in aircraft fuel consumption, the latter poses some serious safety challenges. Cryoplane: In the year 2004, the EC-sponsored CRYOPLANE project was completed. The project’s main goal was to design aircraft using LH2, covering various subjects from H2 production methods to environmental compatibility. In total, six categories of aircraft were designed in the project ranging from a small regional aircraft, a business jet to a long-range passenger aircraft. The baseline short–medium range (SMR) aircraft was designed to carry 185 passengers over 4000 nm (similar to the A321 but with more range) and a stretch version to carry 218 passengers over 3300 nm. The initial and final configuration is shown in Figure 17. For the initial design, the aerodynamic penalty due to the oversized inner wing turned out to be a 17.5% loss in L/D. The final configuration was an adaptation of the baseline kerosene aircraft with a low-set wing with a giant fuselage to accommodate the top and aft tank (Figure 17). This increased the trim drag. The horizontal Aerospace 2020, 7, x FOR PEER REVIEW 17 of 27 tail was decreased in size compared to the kerosene baseline due to the longer fuselage, but the vertical tailneeded needed more more area area due dueto the to increase the increase inside area inside caused area by causedthe overhead by the fuselage overhead tanks. fuselage The tanks. The relativelyrelatively higher higher maximum maximum landing weight weight is required is required for a more for sophisticated a more sophisticated flap system. The flap system. operating empty weight (OEW) increased by around 30% compared to their kerosene baselines, and The operatingthe m emptyaximum weight take-off w (OEW)eight (MTOW increased) decreased byaround by about 3%. 30% compared to their kerosene baselines, and the maximum take-off weight (MTOW) decreased by about 3%.

Figure 17. Schematic of the initial small–medium range (SMR) aircraft conﬁguration and the revised Figure 17. Schematic of the initial small–medium range (SMR) aircraft configuration and the revised aircraft conﬁguration with cryogenic tanks. aircraft configuration with cryogenic tanks.

In a recentIn EU a recent project EU project called called EnableH2, EnableH2, the the aircraft aircraft designed (shown (shown in Figure in Figure 18) looks 18 )similar looks similar to the one designedto the one in designed the Cryoplane in the Cryoplane project. project. However, However the, the technical technical details of of the the aircr aircraftaft have havenot not been published yetbeen to published make a yet thorough to make a comparisonthorough comparison with thewith Cryoplanethe Cryoplane aircraft. aircraft.

Figure 18. Artistic view of the aircraft designed in the EnableH2 project [32].

An interesting characteristic of aircraft designed with LH2 is that their payload range diagram, in general, is different from an equivalent kerosene aircraft, as shown below (Figure 19, from the Cryoplane Project). The low fuel weight of LH2 allows less payload to be traded for fuel, thereby making hydrogen aircraft extremely sensitive to payload variations. The ratio of operating empty weight (OEW) to MTOW for all designs was found to be 0.68, i.e., independent of aircraft category or size. This is an interesting feature since, as for conventional aircraft, this ratio decreases with increasing range and thus fuel fraction. Apparently, for LH2 aircraft, the increase in tank weight and the resulting OEW keeps pace with the lesser growth in MTOW due to the relatively light fuel [33]. Figure 20 shows the variation of the OEW and the MTOW for various aircraft categories with respect to the equivalent kerosene-fuelled aircraft. Overall, the OEW increases, whereas the MTOW reduces in most cases except for the regional propeller aircraft. The largest reduction of about 15% in the MTOW is observed for the long-range aircraft. Verstraete [34] also looked at the design of three types of aircraft fuelled with LH2 and compared them with the kerosene version of the aircraft. He reported that the OEW/MTOW ratio for LH2 aircraft is much higher than a corresponding kerosene fuelled aircraft, however for LH2 aircraft the ratio did decrease slightly when increasing the aircraft design range, from around 0.75 for an SMR aircraft to 0.65 for a long-range aircraft.

Aerospace 2020, 7, x FOR PEER REVIEW 17 of 27

needed more area due to the increase inside area caused by the overhead fuselage tanks. The relatively higher maximum landing weight is required for a more sophisticated flap system. The operating empty weight (OEW) increased by around 30% compared to their kerosene baselines, and the maximum take-off weight (MTOW) decreased by about 3%.

Figure 17. Schematic of the initial small–medium range (SMR) aircraft configuration and the revised aircraft configuration with cryogenic tanks.

In a recent EU project called EnableH2, the aircraft designed (shown in Figure 18) looks similar to the one designed in the Cryoplane project. However, the technical details of the aircraft have not Aerospace 2020, 7, 181 16 of 24 been published yet to make a thorough comparison with the Cryoplane aircraft.

Figure 18. Artistic view of the aircraft designed in the EnableH2 project [[32]32]..

An interesting characteristic of aircraft designed with LH is that their payload range diagram, An interesting characteristic of aircraft designed with LH2 is that their payload range diagram, Aerospace 2020, 7, x FOR PEER REVIEW 18 of 27 in general,general, is didifferentﬀerent from anan equivalentequivalent kerosene aircraft, as shown below (Figure 2019, from thethe Cryoplane Project). The low fuel weight of LH allows less payload to be traded for fuel, thereby making Cryoplane Project). The low fuel weight of 2LH2 allows less payload to be traded for fuel, thereby hydrogenmaking hydrogen aircraft extremelyaircraft extremely sensitive sensitive to payload to payload variations. variations. The ratio of operating empty weight (OEW) to MTOW for all designs was found to be 0.68, i.e., independentThe ratio of o ofperating aircraft e categorympty weight or size. (OEW) This to is MTOW an interesting for all designs feature since,was found as for to conventional be 0.68, i.e., independent of aircraft category or size. This is an interesting feature since, as for conventional aircraft, this ratio decreases with increasing range and thus fuel fraction. Apparently, for LH2 aircraft, theaircraft, increase this inratio tank decreases weight andwith the increasing resulting range OEW and keeps thus pace fuel withfraction. the lesserApparently, growth for in LH MTOW2 aircraft, due tothe the increase relatively in tank light weight fuel [33 and]. Figure the resulting 19 shows OEW the variationkeeps pace of with the OEW the lesser and thegrowth MTOW in MTOW for various due aircraftto the relatively categories light with fuel respect [33]. toFigure the equivalent 20 shows the kerosene-fuelled variation of the aircraft. OEW Overall,and the MTOW the OEW for increases, various whereasaircraft thecategories MTOW reduces with respect in most to cases the except equivalent for the keroseneregional propeller-fuelled aircraft. aircraft. The Overall, largest the reduction OEW ofincreases, about 15% whereas in the the MTOW MTOW is observed reduces forin most the long-range cases except aircraft. for the Verstraete regional propeller [34] alsolooked aircraft. at The the largest reduction of about 15% in the MTOW is observed for the long-range aircraft. Verstraete [34] design of three types of aircraft fuelled with LH2 and compared them with the kerosene version of the also looked at the design of three types of aircraft fuelled with LH2 and compared them with the aircraft. He reported that the OEW/MTOW ratio for LH2 aircraft is much higher than a corresponding kerosene version of the aircraft. He reported that the OEW/MTOW ratio for LH2 aircraft is much kerosene fuelled aircraft, however for LH2 aircraft the ratio did decrease slightly when increasing the aircrafthigher than design a corresponding range, from around kerosene 0.75 fuelled for an SMRaircraft, aircraft however to 0.65 for for LH a2 long-range aircraft the aircraft. ratio did decrease slightly when increasing the aircraft design range, from around 0.75 for an SMR aircraft to 0.65 for a The increase in the LH2 powered aircraft’s energy consumption compared to a conventional aircraft long-range aircraft. for different aircraft types is shown in Figure 21. It can be seen that, in general, LH2 powered aircraft consume around 10–30% more energy per passenger-km depending on the specific designs and mission profile.Figure Using 19. LH Comparison2 for business of the jets payload and very range long-range diagram aircraftfor a reference is the most kerosene detrimental aircraft and in energy a cryogenic increase. This ismedium mainly- duerange to aircraft the slender designed fuselage in the in Cryoplane business jetsproject and [35] the. requirement(RK: reference to kerosene; store a huge CMR1 amount-200: of Cryogenic Medium Range aircraft). LH2 for a very long-range aircraft. Thus, the SMR aircraft is a good category to utilize LH2.

FigureFigure 19. 20.Changes Changes in in the the operating operating empty empty weight weight and and maximum maximum take-o take-ﬀoffweight weight for for an an LH LH2 2powered powered aircraftaircraft as as compared compared to to equivalent equivalent kerosene-fueled kerosene-fueled aircraft. aircraft. [36 [36]]. .

The increase in the LH2 powered aircraft’s energy consumption compared to a conventional aircraft for different aircraft types is shown in Figure 21. It can be seen that, in general, LH2 powered aircraft consume around 10–30% more energy per passenger-km depending on the specific designs and mission profile. Using LH2 for business jets and very long-range aircraft is the most detrimental in energy increase. This is mainly due to the slender fuselage in business jets and the requirement to store a huge amount of LH2 for a very long-range aircraft. Thus, the SMR aircraft is a good category to utilize LH2.

AerospaceAerospace 20202020,, 7,, x 181 FOR PEER REVIEW 1817 of of 27 24

Figure 20. Comparison of the payload range diagram for a reference kerosene aircraft and a cryogenic Figure 19. Comparison of the payload range diagram for a reference kerosene aircraft and a cryogenic Aerospacemedium-range 2020, 7, x FOR aircraft PEER designed REVIEW in the Cryoplane project [35]. (RK: reference kerosene; CMR1-200:19 of 27 mediumCryogenic-range Medium aircraft Range designed aircraft). in the Cryoplane project [35]. (RK: reference kerosene; CMR1-200: Cryogenic Medium Range aircraft).

Figure 21. Change of energy consumption for H2-fueled aircrafts as compared to equivalent Figure 21. Change of energy consumption for H2-fueled aircrafts as compared to equivalent kerosenekerosene-fueled aircrafts [36]. fueled aircrafts [36]. Airbus Zero-e: Recently, Airbus unveiled its concepts for the future with the ZEROe aircraft FigureAirbus 20. Changes Zero-e: in Recently the operating, Airbus empty unveiled weight its and concepts maximum for take the- offfuture weight with for thean LH ZEROe2 powered aircraft shown in Figure 22. All three aircrafts are based on LH2 produced by renewable electricity as an aircraft as compared to equivalent kerosene-fueled aircraft. [36]. energyshown carrier in Figure and use 22. gas Allturbines three aircraft as a powers are based source. on EvenLH2 produced though the by aircraft’s renewable technical electricity details as an are energy carrier and use gas turbines as a power source. Even though the aircraft’s technical details are not published, their performance can be expected to be similar to the ones discussed earlier within notThe published, increase theirin the performance LH2 powered can aircraftbe expected’s energy to be consumptionsimilar to the onescompared discussed to a earli conventionaler within the Cryoplane project, except for the blended-wing body (BWB) aircraft, which has the advantage of aircraftthe Cryoplanefor different proje aircraftct, except type fors is the shown blended in Figure-wing body21. It (canBWB be) aircraft seen that, which, in general has the, advantageLH2 powered of having extra volume for storage of cryogenic tanks. aircrafthaving consume extra volume around for 10 storage–30% more of cryogeni energyc tanks.per passenger -km depending on the specific designs AHEAD multi-fuel concept: Aircraft designs based on an energy mix concept are promising and mission profile. Using LH2 for business jets and very long-range aircraft is the most detrimental ways to resolve cryogenic fuels’ storage issues. The energy mix concept in this context refers to in energy increase. This is mainly due to the slender fuselage in business jets and the requirement to aircraft that use two types of fuels simultaneously. An example is the multi-fuel blended-wing body store a huge amount of LH2 for a very long-range aircraft. Thus, the SMR aircraft is a good category (MF-BWB) aircraft proposed in the AHEAD project [37] for a long-range mission of about 14,000 km. to utilize LH2. The MF-BWB uses a cryogenic fuel (LH2 or LNG) and a liquid fuel (biofuel/kerosene) as energy sources. Figure 23 shows a schematic of the MF-BWB, where cryogenic fuel is stored in cylindrical insulated tanks within the fuselage (grey color-coded area). The biofuel is stored in the wings (blue color-coded area). The energy ratio between the two fuels is around 75%/25%. To power the MF-BWB, a multi-fuel hybrid engine (MFHE) which can simultaneously burn two diﬀerent types of fuels has been studied in the AHEAD project. Figure 24 presents a schematic of

Figure 22. The three aircraft concepts from Airbus using LH2 as an energy carrier to reduce the environmental footprint of aviation.

AHEAD multi-fuel concept: Aircraft designs based on an energy mix concept are promising ways to resolve cryogenic fuels’ storage issues. The energy mix concept in this context refers to aircraft that use two types of fuels simultaneously. An example is the multi-fuel blended-wing body (MF-BWB) aircraft proposed in the AHEAD project [37] for a long-range mission of about 14,000 km. The MF-BWB uses a cryogenic fuel (LH2 or LNG) and a liquid fuel (biofuel/kerosene) as energy sources. Figure 23 shows a schematic of the MF-BWB, where cryogenic fuel is stored in cylindrical insulated tanks within the fuselage (grey color-coded area). The biofuel is stored in the wings (blue color-coded area). The energy ratio between the two fuels is around 75%/25%.

Aerospace 2020, 7, x FOR PEER REVIEW 19 of 27

Figure 21. Change of energy consumption for H2-fueled aircrafts as compared to equivalent kerosene- fueled aircrafts [36].

Airbus Zero-e: Recently, Airbus unveiled its concepts for the future with the ZEROe aircraft Aerospaceshown2020 ,in7, 181Figure 22. All three aircrafts are based on LH2 produced by renewable electricity as an18 of 24 energy carrier and use gas turbines as a power source. Even though the aircraft’s technical details are not published, their performance can be expected to be similar to the ones discussed earlier within the MFHE,the Cryoplane and Figure proje 25ct, illustratesexcept for the the blended design- detailswing body of the (BWB dual) aircraft combustion, which chambers.has the advantage Details of of the enginehaving performance extra volume can for be storage found inof [cryogeni37]. c tanks.

Aerospace 2020, 7, x FOR PEER REVIEW 20 of 27

FigureFigure 22. The 22. The three three aircraft aircraft concepts concepts from from Airbus using using LH LH2 as2 as an an energy energy carrier carrier to reduce to reduce the the environmentalenvironmentalAerospace footprint 2020 footprint, 7, x FOR of PEER aviation.of aviation.REVIEW 20 of 27

AHEAD multi-fuel concept: Aircraft designs based on an energy mix concept are promising

ways to resolveFigure cryogenic 23. The multi fuels’-fuel storageblended - issues.wing body The investigated energy mix in the concept AHEAD in project this context [37]. refers to aircraft that useFigure two 23. types The ofmulti fuels-fuel simultaneously. blended-wing body An investigated example isin the AHEADmulti-f uelproject blended [37]. -wing body (MF-ToBWB) power aircraft the MFproposed-BWB, ain multi the AHEAD-fuel hybrid project engine [37] (MFHE)for a long which-range can mission simultaneously of about 14 burn,000 twokm. differentThe MFTo- BWBtypes power usesof thefuels a MF cryogenic has-BWB, been a multistudied fuel - (fuelLH in 2hybrid theor LNGAHEAD engine) and project.(MFHE) a liquid Figurewhich fuel can24 (b iofuelpresents simultaneously/kerosene) a schematic burn as energy twoof the MFHE,sources.different and Figure typesFigure 23 of shows 2fuels5 illustrates has a schematic been the studied design of thein thedetails MF AHEAD-BWB, of the whereproject. dual cry Figurecombustionogenic 24 presentsfuel chambers. is stored a schematic Detailsin cylindrical of of the the engineinsulatedMFHE, performance tanks and Figure within can 2 5the illustratesbe fuselagefound inthe (grey [37] design. color details-coded of thearea). dual The combustion biofuel is chambers.stored in theDetails wings of the(blue engine performance can be found in [37]. color-codedFigure area). 23. TheTheFigure multi-fuelenergy 23. The mratioulti blended-wing-fuel between blended-w theingbody btwoody investigated fuels investigated is around in the AHEAD in 75%/25%. the AHEADproject [37] . project [37]. To power the MF-BWB, a multi-fuel hybrid engine (MFHE) which can simultaneously burn two different types of fuels has been studied in the AHEAD project. Figure 24 presents a schematic of the MFHE, and Figure 25 illustrates the design details of the dual combustion chambers. Details of the engine performance can be found in [37].

FigureFigure 24.FigureSchematic 24. 24. Schematic Schematic of the of of multi-fuelthe the multi multi-fuel- hybridfuel h hybridybrid engine engineengine investigated investigated in inin the thethe AHEAD AHEAD AHEAD project. project. Adapted AdaptedAdapted from Figurefrom 3from of Figure [ 37Figure]. 3 of 3 of[37] [37]. .

Figure 24. Schematic of the multi-fuel hybrid engine investigated in the AHEAD project. Adapted from Figure 3 of [37].

Figure Figure25. Cross 25. Cross-sectional-sectional view view of ofthe the h ybridhybrid eengine with with two two combustion combustion chambers chambers designed designed in the in the FigureFigure 25.AHEADCross-sectional 25.AHEAD Cross project- sectionalproject [38]. [38] view .view of of the the hybrid hybrid engineengine with with two two combustion combustion chambers chambers designed designed in the in the AHEADAHEAD project project [38 ].[38]. The mainThe featuresmain features of this of this engine engine are are: :  Multi-fuel capability: The MFHE comprises dual combustion chambers. The first combustion  TheMulti main-fuel features capability: of this The engine MFHE are comprises: dual combustion chambers. The first combustion chamber (located between the high-pressure compressor (HPC) and high-pressure turbine  Multichamber-fuel(HPT c apability: (located)) burns cryogenicbetween The MFHE fuel the (e.g., highcomprises LH-pressure2 or LNG) dual compressor in combustion a vaporized (HPC state. chambers.) Inand contrast, high The - thepressure first second combustion turbine chamber(HPT) )combustor (located burns cryogenic is between an inter - fuelstage the (e.g.,t urbinehigh LH -bpressureurner2 or (ITB) LNG) compressorand in uses a vaporized kerosene/biofuels (HPC state.) and [15,37 In high contrast,]. -pressure the second turbine combustor Low eismissions: an inter The-stage combination turbine b ofurner cryogenic (ITB) fuels and anduses biofuels kerosene/biofuels reduces CO2 emission. [15,37]. The (HPT)) burns cryogenic fuel (e.g., LH2 or LNG) in a vaporized state. In contrast, the second previous analysis shows that the LNG version of the MF-BWB aircraft can reduce CO2 emission  Low emissions: The combination of cryogenic fuels and biofuels reduces CO2 emission. The combustorby isaround an inter 50%- stageas compared turbine to theburner baseline (ITB) B777 and-200ER uses for kerosene/biofuels the design mission [38] [15,37. The ]CO. 2 previous analysis shows that the LNG version of the MF-BWB aircraft can reduce CO2 emission  Low emissions: The combination of cryogenic fuels and biofuels reduces CO2 emission. The by around 50% as compared to the baseline B777-200ER for the design mission [38]. The CO2 previous analysis shows that the LNG version of the MF-BWB aircraft can reduce CO2 emission by around 50% as compared to the baseline B777-200ER for the design mission [38]. The CO2

Aerospace 2020, 7, 181 19 of 24

The main features of this engine are:

Multi-fuel capability: The MFHE comprises dual combustion chambers. The first combustion • chamber (located between the high-pressure compressor (HPC) and high-pressure turbine (HPT)) burns cryogenic fuel (e.g., LH2 or LNG) in a vaporized state. In contrast, the second combustor is an inter-stage turbine burner (ITB) and uses kerosene/biofuels [15,37]. Low emissions: The combination of cryogenic fuels and biofuels reduces CO emission. • Aerospace 2020, 7, x FOR PEER REVIEW 2 21 of 27 The previous analysis shows that the LNG version of the MF-BWB aircraft can reduce CO2 emissionemission by would around be 50%much as higher compared for LH to2 theversion baseline of the B777-200ER MF-BWB. The for flammability the design mission limit for [38 ]. Thehydrogen/ CO2 emissionmethane would is wider be much than higherfor kerosene. for LH 2Therefore,version of combustion the MF-BWB. in the The first flammability combustion limit forchamber hydrogen can/methane take place is wider at lean than conditions, for kerosene. hence Therefore, reducing combustion NOx emissions. in the first The combustion vitiated chambercombustion can take products place atfrom lean the conditions, first combustor hence reducingenable flameless NOx emissions. combustion The technology vitiated combustion in the productssecond fromcombustion the first chamber, combustor reducing enable flamelessthe NOx emission combustion fur technologyther [39]. A incombustor the second capable combustion of working on H2 was designed within the AHEAD project and was demonstrated at atmospheric chamber, reducing the NOx emission further [39]. A combustor capable of working on H2 was designedconditions within by the the group AHEAD of Prof projectessor and Paschereit was demonstrated [40,41]. A combustor at atmospheric capable conditions of sustaining by the groupflameless of Professor combustion Paschereit was demonstrated [40,41]. A combustor by the group capable of Prof of sustainingessor Y. Levy flameless at atmospheric combustion wasconditions demonstrated [42]. by the group of Professor Y. Levy at atmospheric conditions [42].  Bleed cooling: The cryogenic fuel first cools the turbine bleed cooling air through a cryogenic Bleed cooling: The cryogenic fuel first cools the turbine bleed cooling air through a cryogenic heat • heat exchanger. The colder bleed air is then used to cool the high-pressure turbine blades. This exchanger. The colder bleed air is then used to cool the high-pressure turbine blades. This process process reduces the amount of air required for turbine cooling air substantially and increases reduces the amount of air required for turbine cooling air substantially and increases engine engine performance. Meanwhile, LH2/LNG vaporizes into the gas phase for the combustion performance. Meanwhile, LH /LNG vaporizes into the gas phase for the combustion process. [37]. process. [37]. 2 Grewe et al. [43] also performed a climate assessment for the MF-BWB aircraft using either LH Grewe et al. [43] also performed a climate assessment for the MF-BWB aircraft using either LH2 2 (AHEAD-LH(AHEAD-LH2)2) or or LNG LNG (AHEAD-LNG)(AHEAD-LNG) combined with with biofuels. biofuels. A AFUT FUT-B787-B787 aircraft aircraft based based on onan an assumedassumed futuristic futuristic B787 B787 available available inin the year 2050 2050 was was used used as as a abaseline baseline to toenable enable a fair a fair comparison. comparison. TheThe B787-FUT B787-FUT burns burns kerosene kerosene but but withwith improvedimproved fuel efficiency. efficiency. The The AHEAD AHEAD-LH-LH2 uses2 uses 70% 70% LH LH2 and2 and 30%30% kerosene kerosene and and uses uses low low NOx NOx combustion combustion techniques; techniques therefore,; therefore, the NOx the eff NOxects (from effects O (from3 formation, O3 methaneformation, depletion) methane are de lesspletion) than are the less B787-FUT than the aircraft.B787-FUT As aircraft. burning As hydrogen burning hydrogen produces produces more water vapor,more thewater water vapor vapor, thee waterffects ofvapor AHEAD-LH effects of 2AHEADfleet are-LH larger2 fleet than are thelarger B787-FUT. than the TheB787 AHEAD-LH-FUT. The 2 contrailAHEAD eff-ectsLH2 arecontrail slightly effects lower are than slightly the B787-FUT. lower than The the water B787 vapor-FUT. e ffTheects water of AHEAD-LNG vapor effec fleetts of are betweenAHEAD the-LNG B787-FUT fleet are andbetween AHEAD-LH the B787-2FUTand and the AHEAD NOx eff-ectsLH2 and of AHEAD-LNG the NOx effects fleetof AHEAD are the-LNG lowest, fleet are the lowest, but the CO2 effects of AHEAD-LNG are slightly higher. An overview of the but the CO2 effects of AHEAD-LNG are slightly higher. An overview of the analysis in Figure 26 analysis in Figure 26 confirmed that MF-BWB using LH2 could reduce the climate impact by about confirmed that MF-BWB using LH2 could reduce the climate impact by about 30% per passenger-km compared30% per passenger to B787-FUT.-km compared to B787-FUT.

Figure 26. Change in average temperature response from 2051 to 2150 (ATR100) for fleets of different Figure 26. Change in average temperature response from 2051 to 2150 (ATR100) for fleets of different aircraft configurations relative to the reference configuration B787-FUT [43]. aircraft configurations relative to the reference configuration B787-FUT [43]. There are a few points that should be taken into consideration when extrapolating these results to There are a few points that should be taken into consideration when extrapolating these results single-aisle aircraft. Firstly, the AHEAD BWB aircraft was designed for cruising at a higher altitude to single-aisle aircraft. Firstly, the AHEAD BWB aircraft was designed for cruising at a higher altitude than conventional tube and wing aircraft due to its lower wing loading, and therefore the effects of water vapor and contrails are amplified. Secondly, the aircraft uses 70% of its energy as LH2 and 30% as kerosene; a tube and wing aircraft on LH2 will therefore not have any soot emission, which acts as nucleation particles during ice crystal formation and therefore might have lower contrail formation. However, there is still a lot of investigation needed in this area.

Aerospace 2020, 7, 181 20 of 24 than conventional tube and wing aircraft due to its lower wing loading, and therefore the eﬀects of water vapor and contrails are ampliﬁed. Secondly, the aircraft uses 70% of its energy as LH2 and 30% as kerosene; a tube and wing aircraft on LH2 will therefore not have any soot emission, which acts as nucleation particles during ice crystal formation and therefore might have lower contrail formation. However, there is still a lot of investigation needed in this area.

4. Liqueﬁed Natural Gas as an Energy Source for Aviation

Apart from LH2, LNG is another potential fuel for future aviation. This section discusses the features of LNG and example designs in using LNG as fuel.

4.1. Features of LNG for Aviation The advantages and disadvantages of using LNG are summarized based on the previous discussions on energy selection criteria and are listed below. Compared to conventional kerosene, advantages of LNG are:

LNG has a lower fuel weight and better combustion properties than conventional kerosene. • Burning LNG reduces CO emissions by about 25%. NOx, and particulate emissions are reduced • 2 substantially while eliminating sulphate emissions. LNG is a cryogenic fuel and is a good heat sink. It can be used beneficially for intercooling, • bleed cooling, air-conditioning, etc., to enhance the thermodynamic efficiency of the engine. Using the LNG for cooling the bleed air used for turbine cooling was found to be most beneficial with SFC reductions in the order of 5% [44]. The world gas reserves are substantial and therefore LNG is substantially cheaper. • Disadvantages of LNG are:

LNG storage requires pressurized tanks and good insulation to keep the fuel cool, resulting in • increased aircraft operating empty weight (OEW). LNG storage requires a larger space than conventional jet fuels. • Airport facilities and logistics for tanking LNG are required. • Methane slip during operations can lead to global warming as CH has a higher greenhouse potential • 4 than CO2, approximately 34 times compared to CO2 over a 100-year period (Table 8.7 of [45]).

4.2. Proposed Aircraft Designs for LNG The MF-BWB concept described in Section 3.3 also had an LNG version, which allows using LNG mixed with kerosene/biofuels for a long-range mission. Figure 26 shows that the MF-BWB LNG version (AHEAD-LNG) can reduce the climate impact by 40% compared to a contemporary B787-FUT aircraft and the biggest reduction is from the reduced NOx and contrails effects. Next to MF-BWB for long-range missions, a group of students at Delft University of Technology (TU Delft) designed a multi-fuel A320 class of aircraft for short- and medium-range missions (shown in Figure 27)[46]. Figure 28 shows the corresponding flight mission of multi-fuel A320. In this design, LNG is used in the landing take-off (LTO) cycle and the climb and descent phase to reduce the near-airport air pollution (soot, CO2, NOx, UHC, VOC, etc.). In the cruise phase, kerosene is used to limit the emission of water vapor. Water vapor is a greenhouse gas with stronger effects at a higher altitude due to longer residence time. Water vapor also forms contrails, as described earlier, which generally enhances global warming. Another advantage of using such a multi-fuel configuration is the flexibility to use the aircraft in places where LNG is not available. The results showed the multi-fuel A320 aircraft reduced emissions substantially when compared to a conventional A320 aircraft. The operating cost was 10% lower due to lower emissions and cheaper fuel [46]. Aerospace 2020, 7, x FOR PEER REVIEW 23 of 27

Aerospace 7 Aerospace2020, 20,20 181, 7, x FOR PEER REVIEW 23 of 2721 of 24

Figure 27. A multi-fuel A320 class of aircraft with podded LNG tanks and open rotors designed by Figure 27. A multi-fuel A320 class of aircraft with podded LNG tanks and open rotors designed by students at Delft University of Technology (TU Delft). studentsFigure at 27. Delft A m Universityulti-fuel A320 of Technologyclass of aircraft (TU with Delft). podded LNG tanks and open rotors designed by students at Delft University of Technology (TU Delft).

2 FigureFigureFigure 28. 28. Mission Mission28. Mission and and and CO CO CO22 emissionemission emission reduction reductionreduction fromfrom from thethe the hybridhybrid hybrid A320 A320 A320 class class class aircraft aircraft aircraft using using using LNG LNG LNGand and and kerosenekerosenekerosene on on a aon typical typical a typical mission mission mission [46] [46 [46]].. .

5. ConclusionsAnother advantage of using such a multi-fuel configuration is the flexibility to use the aircraft Another advantage of using such a multi-fuel configuration is the flexibility to use the aircraft in places where LNG is not available. The results showed the multi-fuel A320 aircraft reduced in placesemissionsA transition where substantially LNG away is from notwhen fossil available. compared fuels Thedemandsto a conventional results some showed radicalA320 theaircraft. changes, multi The-fuel whichoperating A320 certainly cost aircraft was is 10% reduced the case emissionsin aviation,lower substantiallydue where to lower severe emissions when weight compared and and cheaper volume to a f uelconventional constraints [46]. A320 play anaircraft. important The operating role. Liquid cost hydrogen, was 10% lowerwhen due produced to lower using emissions renewable and energy, cheaper is widelyfuel [46] considered. to be a viable candidate to pave the way towards5. Conclusions carbon-free aviation. Here we argue that also methane (the main constituent of natural gas), 5.when Conclusions madeA transition sustainably, away could from fossil be a favorablefuels demands candidate some radical for aviation. changes, which certainly is the case in aviation,The challenges where severe of LH weight2 storage and (at volume253 ◦constrainC) andt thes play associated an important logistical role. Liquidissues arehydrogen, significant, A transition away from fossil fuels demands− some radical changes, which certainly is the case in aviation,whichwhen will where produced increase severe using the weight operating renewable and costsenergy, volume dramatically. is widelyconstrain consideredts The play current aton beimportant a aircraft viable candidate design role. Liquid and to pave architecture hydrogen, the way towards carbon-free aviation. Here we argue that also methane (the main constituent of natural whenwill have produced to be using changed renewable significantly energy, to is accommodate widely considered cryogenic to be fuels. a viable Both candidate LH2 and to LNGpave willthe gas), when made sustainably, could be a favorable candidate for aviation. wayrequire towards significant carbon changes-free aviation. to the aircraftHere we propulsion argue that system. also methane From previous(the main studies, constituent it seems of natural that a The challenges of LH2 storage (at −253 °C) and the associated logistical issues are significant, gasshort-to-medium), when made sustainably range aircraft, could will be be a afavorable suitable candidatecandidate for carryingaviation. cryogenic fuels. However, therewhich is a lack will ofincrease studies the that operating have takencosts dramatically a detailed look. The atcurrent these aircraft designs. design and architecture will Thehave challenges to be changed of LHsignificantly2 storage to(at accommodate −253 °C) and cryogenic the associated fuels. Both logistical LH2 and issues LNG willare requiresignificant , Apart from the storage of cryogenic fuels, there are several important challenges, even beyond whichsignificant will increase changes the to operating the aircraft costs propulsion dramatically system.. TheFrom current previous aircraft studies, de itsi seemsgn and that architecture a short-to- will current cost issues, which have to be overcome before they can be widely used as a sustainable energy have mediumto be changed range aircraft significantly will be a to suitable accommodate candidate cryogenicfor carrying fuels. cryogenic Both fuels. LH2 However,and LNG there will isrequire a significantcarrierlack for of aircraft.changes studies that Theto thehave required aircraft taken ramp-upa propulsion detailed oflook sustainable system. at these From designs. production previous will studies, require it seems a massive that worldwidea short-to- mediuminvestment, rangeApart not fromaircraft only the in willstorage facilities be aof suitable butcryogenic also candidate in fuels, R&D. there The for are finalcarrying several trade-o importantcryogenicff between challenges fuels. diff However,erent, even alternatives beyond there is toa lackfossil ofcurrent aviation studies cost fuelthat issues, (indicativehave which taken have preliminarya d toetailed be overcome look start at before of these such they designs. a trade-ocan be widelyff is shown used as in a Table sustainable2) will energy have to be carrier for aircraft. The required ramp-up of sustainable production will require a massive worldwide performedApart from taking the into storage account of thecryogenic climate fuels, effects there of non-CO are several2 emissions important and challenges the limited, availabilityeven beyond of investment, not only in facilities but also in R&D. The final trade-off between different alternatives currentrenewable cost energy issues, sincewhich aviation have to isbe by overcome far not thebefore only they sector can demanding be widely used sustainable as a sustainable energy carriers.energy For this trade-off, it is also recommended to take a closer look into dual-fuel hybrid aircraft, combining carrier for aircraft. The required ramp-up of sustainable production will require a massive worldwide investment,the advantages not ofonly both in fuelsfacilities while but reducing also in R&D. the associated The final drawbacks trade-off between and risks. different alternatives Furthermore, we need more thorough research on the climate effect from non-CO2 emissions, especially on contrail formation and persistence as LH2 or LNG will increase the emission of water Aerospace 2020, 7, 181 22 of 24

vapor during flight. As shown by several studies, the non-CO2 effects can be more significant than those of CO2. Therefore, looking into the future, carbon-neutral aviation is certainly possible, but climate-neutral aviation is very difficult to achieve.

Author Contributions: Conceptualization, A.G.R.; writing—original draft preparation, A.G.R., F.Y., and H.G.C.W.; writing—review and editing, A.G.R., F.Y., and H.G.C.W. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Conﬂicts of Interest: The authors declare no conﬂict of interest.

Nomenclature

A/C Aircraft ACARE Advisory Council for Aeronautical Research and innovation in Europe ACI Airports Council International AHEAD Advanced Hybrid Engine for Aircraft Development ATAG Air Transport Action Group BWB blended-wing body CBS Central Bureau of Statistics CCS carbon capture and storage CANSO Civil Air Navigation Services Organization CCU carbon capture and usage CORSIA Carbon Offsetting and Reduction Scheme for International Aviation CROR Contra Rotating Open Rotor DoE Department of Energy EC European Commission ERF effective radiative forcing (mW/m2) FOB Freight on Board HPC High-pressure compressor HPT High-pressure turbine IATA International Air Transport Association ICAO International Civil Aviation Organization ICCAIA International Coordinating Council of Aerospace Industries Associations ITB inter-stage turbine burner L/D Lift to Drag Ratio [-] LH2 liquefied hydrogen LNG liquefied natural gas LTO landing take-off MFBWB multi-fuel blended-wing body MFHE multi-fuel hybrid engine MTOW maximum take-off weight (kg) NACA National Advisory Committee for Aeronautics NASA National Aeronautics and Space Administration OEW operating empty weight (kg) PV Photo Voltaic RHK Royal Haskoning DHV RF radiative forcing (mW/m2) SED specific energy density (MJ/kg) SFC Specific Fuel Consumption (kN/gm/s) SMR short–medium range TRL Technology Readiness Level UHC unburnt hydro-carbon VED volumetric energy density (MJ/L) VOC Volatile Organic Compounds Aerospace 2020, 7, 181 23 of 24

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