energies

Article Liquefied Natural Gas for Civil Aviation

Pavlos Rompokos * , Sajal Kissoon , Ioannis Roumeliotis * , Devaiah Nalianda, Theoklis Nikolaidis and Andrew Rolt Propulsion Engineering Center, School of Aerospace, Transport and Manufacturing, Cranfield University, Building 52, College Rd, Cranfield, Wharley End, Bedford MK43 0AL, UK; S.Kissoon@cranfield.ac.uk (S.K.); devaiah.nalianda@cranfield.ac.uk (D.N.); t.nikolaidis@cranfield.ac.uk (T.N.); a.rolt@cranfield.ac.uk (A.R.) * Correspondence: Pavlos.Rompokos@cranfield.ac.uk (P.R.); I.Roumeliotis@cranfield.ac.uk (I.R.)



Received: 23 October 2020; Accepted: 9 November 2020; Published: 13 November 2020


Abstract: The growth in air transport and the ambitious targets in emission reductions set by advisory agencies are some of the driving factors behind research towards new fuels for aviation. Liquefied Natural Gas (LNG) could be both environmentally and economically beneficial. However, its implementation in aviation has technical challenges that needs to be quantified. This paper assesses the application of LNG in civil aviation using an integrated simulation and design framework, including Cranfield University’s aircraft performance tool, Orion, and engine performance simulation tool Turbomatch, integrated with an LNG tank sizing module and an aircraft weight estimation module. Changes in tank design, natural gas composition, airframe changes, and propulsion system performance are assessed. The performance benefits are quantified against a Boeing 737–800 aircraft. Overall, LNG conversion leads to a slightly heavier aircraft in terms of the operating weight empty (OWE) and maximum take-off weight (MTOW). The converted aircraft has a slightly reduced range compared to the conventional aircraft when the maximum payload is considered. Compared to a conventional aircraft, the results indicate that although the energy consumption is increased in the case of LNG, the mission fuel mass is decreased and CO2 emissions are reduced by more than 15%. These benefits come with a significant reduction in fuel cost per passenger, highlighting the potential benefits of adopting LNG for aviation.

Keywords: liquefied natural gas; LNG; civil aviation; engine performance; mission analysis; short-range aircraft; CO2 emission reduction

1. Introduction The Earth is reported to have lost 28 trillion tons of ice since 1994 [1]. This finding coincides with a worst-case scenario predicted by the United Nation’s Intergovernmental Panel on Climate Change (IPCC). This massive response to the rise in global temperatures only accentuates the immediate need for action. According to the Air Transport Action Group [2], the global aviation industry produces around 2% of all human-induced CO2 emissions, which constitutes approximately 12% of CO2 emissions within the transport sector. These figures are bound to grow with projected increases in passenger demand for air travel. In approximately 10 years, the global fleet is expected to grow by 42% [3]. In an attempt to curb greenhouse gas emissions and protect the environment, the Advisory Council for Aviation Research and Innovation in Europe (ACARE), through FlightPath2050 [4], set up ambitious goals for the aviation industry for the year 2050. Specifically, a 75% reduction in CO2 emissions per passenger km, 90% reduction in NOx emissions, and a 65% reduction in perceived noise compared to a typical new aircraft in 2000 are targeted. To achieve these goals, improvements at the system level are required, encompassing improvements to the airframe, engine, and operations in a synergistic manner.

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Engine development is estimated to be capable of achieving a 20% reduction in fuel burn [5]. For example, the UltraFan® engine (Rolls Royce, West Sussex, UK), with an ultra-high bypass ratio design, should achieve a 25% improvement in fuel burn compared to Trent 772B engine, which is a representative year 2000 engine [6]. However, the fan diameter is limited by the space available under the wing, installed drag, and weight. In terms of airframe development, a radically new configuration deviating away from the traditional tube-and-wing design is required to achieve a significant reduction in fuel consumption. The blended wing body is one such example developed by NASA, which achieves improved fuel benefits due to the superior lift-to-drag ratio, improved payload capability, and lower noise level. Additionally, superior air traffic management and operations, which could optimize flight trajectories and reduce ground idling times, could further decrease unnecessary fuel burn [7]. However, these improvements can only cut fuel consumption to a certain extent, and only reduce the amount of CO2 generated pro rata to the fuel burned. In order to achieve up to a 75% reduction in emission of carbon, alternative fuels may be required, as discussed in [4]. The choice of an aviation fuel is dictated by various criteria, which are mainly dependent on also availability,required to emissions,produce drop-ins price, energy in sufficient density, and quantity safety. for Kerosene the aviation has matched sector. all Additionally, those requirements, the productionbut the of growing drop-in need fuels to can reduce still greenhousecontribute to gas net (GHG) life-cycle emissions CO2 and makes pollutant the continued emissions. use Cryogenic of this fossil fuelsfuel such less as viableLNG, liquefied for aviation. biomethane and liquid hydrogen (LH2) are attractive alternatives due to their highVarious specificalternatives energy density, areavailable—drop-in but they are at a disadvantage replacement compared fuels from to vegetable other fuel oils in andterms Syn-Jet of volumetriccan be energy blended density, with kerosene meaning and that used they completely require more interchangeably storage volume, with a conventionalcritical aspect fuel for [8]. aviation.They Figure have generated 2 summarizes significant the differences interest from betw aviationeen potential stakeholders, alternative but there fuels are for issues aviation that needwith to regardbe to resolved. energy density, For instance, both specific although energy vegetable density oils (SED) are available and volumetric in large energy quantities, density their (VED) use as [16]. a fuel couldKerosene lead is to found conflicts at the with top foodof the production. chart, which Syn-jet, makes havingit an ideal nearly candidate identical for propertiesaviation. Just to kerosene,below are otherwould fuels not with solve high environmental carbon content concerns and naturall due to itsy in high liquid carbon states. content. With Significanta decrease in investments carbon to in hydrogenmanufacturing ratio, there are is also a reduction required toin produceCO2 emission drop-ins after in sucombustion.fficient quantities This makes for the the aviation fuel more sector. attractive from an environmental point of view. However, it is accompanied by a decrease in volumetric Additionally, the production of drop-in fuels can still contribute to net lifecycle CO2 and pollutant energy density, which implies that for a fixed specific energy density, more storage space is required emissions. Cryogenic fuels such as LNG, liquefied biomethane, and liquid hydrogen (LH2) are fuel tanksattractive to store alternatives the same due amount to their ofhigh energy. specific Even energy in its cryogenically density, but they cooled are at liquid a disadvantage state, natural compared gas volumetricto other energy fuels indensity terms is of 35% their less volumetric than kerosene, energy while density, hydrogen meaning demonstrates that they requirea volumetric greater energy storage densityvolume, reduced a critical by 75% aspect compared for aviation. to kerosene. Figure 1Although summarizes LH2 the has di approximatelyfferences between 170% potential higher alternativespecific energyfuels density for aviation compared with to regard kerosene, to energy and has density, the po includingtential for both zero specific carbon energy emission density if produced (SED) and cleanly,volumetric its implementation energy density would (VED) require [9]. a complete redesign of the aircraft and propulsion system. Furthermore, scaling-up green hydrogen production is still a significant challenge. 40 Kerosene 35 Gasolene Vegetable oil 30 Avgas Ethanol 25 LNG 20

15 Liquid Hydrogen 10

5 Nickel battery Natural Gas Volumetric Energy Density Energy Density (MJ/L) Volumetric Hydrogen Gas 0 0 20 40 60 80 100 120 140 Specific Energy Density (MJ/kg)

Figure 1. Fuel energy density comparison chart. Figure 2. Energy Density Comparison Chart [16]

Natural gas, which is mostly methane, has been suggested as an alternative fuel for aviation, and liquefying it minimizes fuel tank weight and volume. Several prior studies have been conducted to assess LNG use for subsonic aircraft. In 1980, the Beech Sundowner, a two-seater turboprop, was the first aircraft to operate on liquid methane [8]. Around the same time, Lockheed performed a thorough analysis on the prospects of LNG in civil aviation. They concluded that the modifications required to implement the use of a cryogenic fuel were too extensive to make sense economically, given the low price of kerosene at that time [8]. In 1988 a modified Tupolev TU-154, the TU-155, had its first flight with one of its three engines operating on LNG. The aircraft performed several demonstration flights to international airports of Moscow, Bratislava, Nice, Berlin and Hannover, which totalled more than 100 flight hours [8], the project was discontinued after the fall of the Soviet Union. NASA came to the conclusion that the use of methane had the potential to be competitive in all major areas namely direct operating cost, gross weight, initial cost and energy utilization if appropriate standards of handling and safety were maintained [9]. Following this, a similar study was performed

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Kerosene is at the top of the chart, which makes it an ideal candidate for aviation. Just below are other fuels with high carbon contents and which are naturally in liquid states. With a decrease in the carbon-to-hydrogen ratio, there is a reduction in CO2 emissions after combustion. This makes the fuel more attractive from an environmental point of view. However, this is accompanied by a decrease in the volumetric energy density, which implies that for a fixed specific energy density, more storage space is required in the fuel tanks to store the same amount of energy. Even in its cryogenically cooled liquid state, the volumetric energy density of natural gas is 35% less than for kerosene, while the volumetric energy density of hydrogen is reduced by 75% compared to kerosene. Although LH2 has approximately 170% higher specific energy density compared to kerosene and has the potential for zero carbon emissions if produced cleanly, its implementation would require a complete redesign of the aircraft and propulsion systems. Furthermore, scaling-up green hydrogen production is still a significant challenge. Natural gas, which is mostly methane, has been suggested as an alternative fuel for aviation, and liquefying it minimizes the fuel tank weight and volume. Several prior studies have been conducted to assess LNG use for subsonic aircrafts. In 1980, the Beech Sundowner, a two-seater turboprop, was the first aircraft to operate on liquid methane [10]. Around the same time, Lockheed performed a thorough analysis on the prospects of LNG in civil aviation. They concluded that the modifications required to implement the use of a cryogenic fuel were too extensive to make sense economically, given the low price of kerosene at that time [8]. In 1988, a modified Tupolev TU-154, the TU-155, had its first flight, with one of its three engines operating on LNG. The aircraft performed several demonstration flights to international airports in Moscow, Bratislava, Nice, Berlin, and Hannover, which totaled more than 100 flight hours [10]; the project was discontinued after the fall of the Soviet Union. NASA came to the conclusion that the use of methane had the potential to be competitive in all major areas, reducing the direct operating cost, gross weight, initial cost, and energy utilization if appropriate standards of handling and safety were maintained [11]. Following this, a similar study was performed by AIR-LNG, who instead inserted the tanks in LD6 containers within the cargo bay, whereby the only modification to the aircraft fuel system was the addition of LNG equipment. Through the partial or full replacement of kerosene, they estimated the possibility of implementing LNG for air transport within a timeframe of 3–6 years [12]. Although this configuration can be implemented with little redesign, which makes its implementation feasible in such a short timeframe, less space is available for the storage of LNG which reduces the range capability of LNG-flown aircraft. More recently, the start-up company Savion Aerospace proposed a business turboprop aircraft design cruising at Mach 0.55 and powered using LNG. Specifically, the founder, Jonathan Gibbs, claims that by using existing engines and retrofitting the combustor to burn natural gas, 75% of the development cost can be reduced, with the added benefit of a 15% reduction in maintenance costs due to the lower sulfur content [13]. However, the implementation of LNG in aviation has some underlying complexities that need to be addressed first. For instance, airports will need to accommodate large LNG storage tanks or liquefaction facilities in addition to pipelines to ensure safe and effective refueling. In addition, extra safety measures need to be taken due to the properties of this fuel. For instance, in case of a crash in water, water must not be allowed to enter the tanks due to the violent reaction of LNG with water. Nevertheless, the world could significantly benefit both economically and environmentally from using LNG in aviation if those hurdles were addressed. Following the discovery and expansion of vast reserves of natural gas using shale fracking, the price of natural gas dropped significantly, and in 2020 reached its lowest level since 1995 [14]. In addition to it having a 16% higher heating value and a higher hydrogen-to-carbon ratio molecule than Jet A, LNG presents itself as a competitive alternative aviation fuel. In this context, LNG could prove itself to be a clean alternative and act as a stepping stone towards the development of net-zero carbon emission alternatives, such as biomethane or hydrogen. This paper aims to assess the performance benefits achieved in terms of CO2 emissions and energy-saving potential by replacing kerosene with LNG, in addition to discussing the challenges Energies 2020, 13, 5925 4 of 20 presented by the use of a cryogenic fuel for aircraft propulsion. In order to achieve this, an integrated design and simulation framework are developed and applied for the case of a single-aisle, short-range aircraft that is extensively used in civil aviation. The assessment is performed at the mission level, taking into consideration the effects of fuel synthesis on engine performance and accounting for the added weight and drag due to the necessary changes to the airframe. To the authors’ knowledge, such a holistic assessment addressing the multitude of factors affecting aircraft and engine performance and discussing the implications of the required modifications had not previously been published within the research community.

2. LNG Challenges LNG has the potential to act as a cleaner alternative to jet fuel. However, several technical challenges have to be addressed prior to considering it as a solution for aviation. To begin with, the varying composition of natural gas, depending on its source, affects both the lower heating value (LHV) and the density of the fuel, potentially affecting the aircraft performance. Additionally, both airframe and engine modifications are required before being able to use this cryogenic fuel. Another factor that needs to be taken into consideration is the environmental implications of using LNG in terms of CO2 emissions. For instance, the combustion of LNG compared to jet fuel leads to a reduction in CO2 emissions. However, methane, which is its major constituent, is a higher contributor to global warming. The escape or non-combustion of just 1% of the fuel would more than wipe out its global warming benefits in terms of CO2 emissions reductions. This section aims to give a brief description of the underlying implications and challenges that come with the use of LNG.

2.1. LNG Compositions Natural gas is available in several countries and its composition varies significantly depending on the field. The differences in composition mean that the fuel properties vary, for example the higher heating values (HHV) can vary from 27.5 to 48.7 MJ/m3 between different European fields [15]. LNG synthesis and properties also vary depending on the source, albeit the heating value (HV) fluctuations are smaller due to CO2 and N2 removal to meet specifications for pipeline transport [16,17]. For this study, the lower heating values for different LNG sources are considered in order to assess the effect that this diversity may have. The LHV is calculated based on the composition reported in [18] utilizing the ISO method [19]. The values considered range from 48,339 to 49,877 kJ/kg, as seen in Figure2. Additionally, the composition a ffects the density. As seen in Figure3, for the case of LNG liquefied at 160 C, the density values vary from 421.4 to 470.2 kg/m3 for the cases examined herein; − ◦ the density is calculated according to ISO 6578 [20]. These results indicate that for a specific aircraft and tankEnergies volume, 2020,the 13, x aircraft FOR PEER onboard REVIEW energy will depend on the LNG composition available at the5 of airport. 20

50

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48.5

48 LHV (MJ/kg) at 288.15 K 288.15 at (MJ/kg) LHV

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Figure 2. Variations in LHV per Region. Figure 2. Variations in LHV per Region.

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410 Density [kg/m Density 400 Australia Australia Algeria Algeria Egypt Norway USA Russia Indonisia Peru Qatar NWS Darwin Skikda Bethioua Idku Alaska Sakhalin Badak

Figure 3. Variations in density per region at 113.15 K.

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Figure 4. Variations in density per region at 113.15 K

From these data, three cases are selected in order to assess the effects of composition variation on the application of LNG for aviation. The selection is made with respect to the LHV and density. The highest LHV LNG from the data is the Alaskan source (type 1), the lowest LHV and highest 5 Energies 2020, 13, x FOR PEER REVIEW 5 of 20

50 Energies 2020, 13, x FOR PEER REVIEW 5 of 20 49.5 50 49 49.5 48.5 49 48 LHV (MJ/kg) at 288.15 K 288.15 at (MJ/kg) LHV 48.5 47.5 48 AustraliaAustralia Algeria Algeria Egypt Norway USA Russia Indonisia Peru Qatar LHV (MJ/kg) at 288.15 K 288.15 at (MJ/kg) LHV NWS Darwin Skikda Bethioua Idku Alaska Sakhalin Badak

Energies47.52020, 13, 5925 5 of 20 AustraliaAustralia AlgeriaFigureAlgeria 2. VariationEgypts inNorway LHV per USARegion.Russia Indonisia Peru Qatar NWS Darwin Skikda Bethioua Idku Alaska Sakhalin Badak

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430450 ] at 113.15 K 113.15 at ] 3 420440 410 Density [kg/m Density 430 400420 Australia Australia Algeria Algeria Egypt Norway USA Russia Indonisia Peru Qatar 410 NWS Darwin Skikda Bethioua Idku Alaska Sakhalin Badak Density [kg/m Density 400 Figure 3. Variations in density per region at 113.15 K. Australia AustraliaFigureAlgeria 3. VariationAlgeria s inEgypt densityNorway per regionUSA at 113.15Russia K. Indonisia Peru Qatar AnotherNWS parameterDarwin of interestSkikda Bethioua is the COIdkupotential ofAlaska the LNG,Sakhalin whichBadak is depicted in Figure4, 2 expressed2.760 in the form of a CO2 emission index. However, it is apparent that the environmental footprint KeroseneFigure EICO2: 3. Variation3.15 kg s in/kg density per region at 113.15 K.

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2.720 AustraliaAustraliaFigureAlgeria 4. VariationAlgerias inEgypt densityNorway per regionUSA at 113.15Russia K Indonisia Peru Qatar NWS Darwin Skikda Bethioua Idku Alaska Sakhalin Badak From these data, three cases are selected in order to assess the effects of composition variation Figure 4. Variations of CO emission index per region. on the application of LNGFigure for aviation. 4. Variation Thes sinelection density2 isper made region with at 113.15 respect K to the LHV and density. The higheFromst these LHV data, LNG three from cases the data are selectedis the Alaskan in order source to assess (type the 1), eff theects lowest of composition LHV and variation highest on theFrom application these data, of LNGthree forcases aviation. are selected The selectionin order isto madeassess withthe effect respects of to composition the LHV and variation density.5 onThe the highest application LHV LNGof LNG from for theaviation. data isThe the selection Alaskan is source made (typewith respect 1), the lowestto the LHV LHV and and density. highest Thedensity highe isst the LHV North LNG West from Shelf the (NWS) data sourceis the Alaskan (type 3), whilesource the (type Sakhalin 1), the source lowest (type LHV 2) isand selected highest as a representative source. 5

2.2. Airframe Chalenges Modifications are required on various fronts to enable the use of LNG. From an airframe perspective, the conventional tube-and-wing architecture requires extra storage space to accommodate the fuel. Given that cryogenic tanks will be required, their placement within the wings is less feasible. Some possible alternatives are shown in Figure5. The conventional tube-and-wing configuration will increase in length if the tanks are placed at the front and at the back of the fuselage, and fuel must be drawn from both tanks together to prevent the center of gravity of the aircraft from moving beyond acceptable limits. Alternatively, the tanks could be placed at the top of the fuselage, which would lead to a larger frontal area. External tanks beneath the wings might also be used. Although this Energies 2020, 13, 5925 6 of 20 would provide wing bending moment relief, because of the larger surface area exposed to the external air flow, the boil-off rate would be higher compared to the other two configurations. Other airframe configurations could also provide greater storage capacity without impinging too much on the lift-drag ratio, which is inevitably reduced in the conventional tube-and-wing aircraft case. The double-bubble configuration is a promising candidate, which does not look too different from the tube-and-wing configuration. Alternatively, the blended wing body (BWB) configuration could provide more storage space in addition to a significant improvement in lift-to-drag ratio. However, a BWB design would probably require significantly more time to be implemented.

Modifications to the tube and wing configuration Front and Aft Tanks Top Tanks External Tanks under wings

Novel Airframe Architectures Double bubble fuselage Blended wing body

FigureFigure 5.7. ModificationsModifications to to the the Tube tube-and-wing and Wing Configuration configuration.

2.3.3.3 Engine System Modifications Modifications InIn termsterms ofof engineengine modifications, modifications, changes changes are are required required to tothe the combustor combustor and andto the the fuel fuel system, system, includingincluding the fuel fuel pumps pumps and and injectors. injectors. Due Due to tothe the high higherer stoichiometric stoichiometric flame flame temperatures temperatures potentially potentially obtainedobtained duringduring thethe combustioncombustion of LNG, combustion chambers chambers need need to to be be redesigned redesigned to to modify modify the the raterate at at which which air air enters enters, in to order provide to provide rapid mixing rapid mixing to prevent to prevent pockets pockets of very of high very temperature high temperature gas gasforming, forming, which which would would favour favor the the formation formation of NOx. of NOx. Additionally, Additionally, LNG LNG needs needs to be toin begaseous in a gaseous state before entering the combustion chamber. Fuel injectors for handling liquid fuel need to be completely state before entering the combustion chamber. The fuel injectors required to handle liquid fuel need to redesigned. be completely redesigned. The cooling potential of LNG could be utilized by using a heat exchanger to reduce the The cooling potential of LNG could be utilized by using a heat exchanger to reduce the temperature temperature of air bled from the compressor to cool the turbine blades, hence reducing the amount of of air bled from the compressor to cool the turbine blades, hence reducing the amount of air needed air needed from the compressor to improve the thermal efficiency of the core. The fuel distribution frompipes the also compressor need to accommodate to improve LNG the thermal in both eliquifficiencyd and of gaseous the core. states. The Boil-off fuel distribution gas from the pipes tanks also needcould to be accommodate mixed with the LNG gassified in both LNG liquid after andthe heat gaseous exchanger states. and Boil-o so suppliedff gas from to the the engine. tanks could be mixed with the gasified LNG after the heat exchanger and supplied to the engine. 3.4. Global Warming Potential of LNG 2.4. Global Warming Potential of LNG The GHG emission potential of using LNG is not always apparent. For instance, leakage of LNG at anyThe point GHG emissionduring its potential transport of usingand distribution LNG is not alwayscould have apparent. a higher For instance,negative leakageimpact ofon LNG the at anyenvironment point during due its to transport its higher andglobal distribution warming couldpotential have (GWP) a higher compared negative to impactCO2. Methane on the environmenthas a GWP dueof 28-36 to its while higher CO global2 (which warming is used potential as the reference) (GWP) compared has a GWP to COof 1.2. MethaneAlthough hasnatural a GWP gas ofin 28–36, the whileatmosphere CO2 (which has a isshorter used aslifespan, the reference) which is has reflected a GWP in of the 1. AlthoughGWP index, natural it could gas lead in the to the atmosphere generation has of ozone which is another GHG. Due to methane’s higher hydrogen to carbon ratio, more water will be produced during the combustion process, which could lead to the formation of more contrails which may contribute to global warming. However, the reduction in soot and sulphur-containing particulate emissions may help reduce the optical intensity of the contrails. Flight trajectories could be modified in the future to take account of weather conditions to minimize the formation of contrails [28].

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Energies 2020, 13, x FOR PEER REVIEW 7 of 20 in a gaseous state before entering the combustion chamber. The fuel injectors required to handle liquid fuel need to be completely redesigned. The cooling potential of LNG could be utilized by using a heat exchanger to reduce the temperature of air bled from the compressor to cool the turbine blades, hence reducing the amount of air needed from the compressor to improve the thermal efficiency of the core. The fuel distribution pipes also need to accommodate LNG in both liquid and gaseous states. Boil-off gas from the tanks could be mixed with the gasified LNG after the heat exchanger and supplied to the engine.

2.4. Global Warming Potential of LNG The GHG emission potential of using LNG is not always apparent. For instance, leakage of LNG at any point during its transport and distribution could have a higher negative impact on the environmentEnergies 2020, 13 ,due 5925 to its higher global warming potential (GWP) compared to CO2. Methane has7 of 20a GWP of 28–36, while CO2 (which is used as the reference) has a GWP of 1. Although natural gas in the atmosphere has a shorter lifespan, which is reflected in the GWP index, it could lead to the generationa shorter lifespan, of ozone which, which is reflected is another in theGHG. GWP Due index, to methane’s it could lead higher to the hydrogen generation to carbon of ozone, ratio, which more is wateranother will GHG. be produced Due to methane’s during the higher combustion hydrogen process, to carbon which ratio, could more lead water to will the beformation produced of during more contrailsthe combustion, which process, may contribute which could to global lead to warming. the formation However, of more the contrails, reduction which in soot may and contribute sulfur- containingto global warming. particulate However, emissions the reduction may help in reduce soot and the sulfur-containing optical intensity particulate of the contrails. emissions Flight may trajectorieshelp reduce could the optical be modified intensity in of the the future contrails. to account Flight trajectoriesfor weather could conditions be modified in order in theto minimize future to theaccount formation for weather of contrails conditions [21]. in order to minimize the formation of contrails [21].

3. Methodology Methodology To assessassess the performance of LNG-fueledLNG-fueled aircraft, a design and simulation framework waswas developed, integratingintegrating CranfieldCranfield University’s University’s aircraft aircraft performance performance tool tool Orion Orion [22] [22] with with a detailed a detailed LNG LNGtank sizingtank sizing module module and an and aircraft an aircraft weight weight estimation estimation module. module. The di ffTheerent diff moduleserent modules require require certain certaincodependent codependent inputs to inputs size the to aircraft.size the Foraircraft. a given For propulsion a given propulsion system, a system, loop is required a loop is for required the design for themission, design combining mission, combining aircraft performance, aircraft performance, tank sizing, tank and sizing weight, and estimation.weight estimation. For example, For example for an, forinitial an estimationinitial estimation of the OWE of the of OWE the aircraft, of the aircraft, the onboard the onboard fuel is calculated fuel is calculated and is used and to is estimateused to estimatethe weight the of weight the tanks. of the The tanks. weight The of weight the tanks of the is thentanks fed is then back fed to the back weight to the estimation, weight estimation together, withtogether the calculated with the calculated take-off weight take-off (TOW) weight and (TOW) landing and weight landing (LW). weight The OWE (LW). is The then OWE recalculated is then recalculatedand introduced and back introduced into the back aircraft into performance the aircraft performance assessment. assessment This loop ends. This when loopthe ends OWE when of the OWEprevious of the step previous matches step that matches of the next. that Theof the loop next. is schematicallyThe loop is schematically presented in presented Figure6. in Figure 6.

Figure 6 6.. Design mission sizing loop loop..

3.1. Aircr Aircraftaft and Engine Performance Modeling InIn-house-house tools were utilized to simulate the aircraft and engine performance. For For the the aircraft, aircraft, Orion was used to predict the aerodynamic characteristics given given the the required required inputs inputs for for an an aircraft, aircraft, combined with with Turbomatch, Turbomatc theh, theengine engine performance performance tool, in tool, order in to calculate order to the calculate overall performance the overall performanceas an integrated as an aircraft–engine integrated aircraft system.–engine The point system. mass The method point was mass utilized, method which was equatesutilized, the which rate of work done by forces acting on the aircraft to the rate of increase in potential and kinetic energy (total energy model) [23,24]. 7 Turbomatch is an in-house 0-D performance simulation code, also featuring off-design (OD) and transient simulation [25]. A library of several pre-programmed modules corresponding to models of individual gas turbine components was used to set up the engine model. Due to its modular structure, a number of optimizers and simulators may use the capabilities of Turbomatch. It has been used extensively to perform aircraft–engine studies, emissions studies, and techno-economic and environmental risk analyses (TERA) [26].

3.2. Aircraft Weight Estimation The weight estimation module is based on the method developed by NASA [27] as part of the flight optimization system (FLOPS). It includes different estimation modes according to aircraft type (i.e., civil aviation, fighter, or general aviation) for all the components contributing to the OWE, and is Energies 2020, 13, 5925 8 of 20 preferred instead of the methodologies of Raymer [28] and Roskam [29] due to the capability to also estimate the weight of conceptual BWB airframes. Wells [27] provided a detailed description of the equations and inputs required and Horvath [30] validated the tool and discussed some differences between the weight estimation methods. The tool was also validated in this study using six tube-and-wing aircrafts with different sizes and range capabilities. The airframe data and geometry were derived from the corresponding aircraft characteristics for airport planning documents [31,32] and each engine weight was used directly as an input. The validation results are presented in Table1 in terms of the predicted OWE and its component breakdown. In all cases, due to a lack of detailed information, only the estimated OWE could be compared against the manufacturer’s quoted values. The tool presents only small prediction errors.

Table 1. Weight estimation tool validation.

Aircraft 737–200 737–800 767–200 777–200LR A320–200 A340–200 Structure total 14.1 20.2 41.6 91.2 20.8 70.8 Propulsion total 3.6 6.0 11.3 22.4 6.1 13.8 Systems and Equipment total 8.6 12.0 16.7 25.0 11.9 24.4 Operating items total 1.5 2.5 4.9 7.1 3.1 7.2 Predicted OWE 27.9 40.7 74.6 145.7 41.9 116.2 Actual OWE 28.4 41.4 79.0 146.0 42.6 117.9 Prediction Error 1.90% 1.74% 5.54% 0.26% 1.62% 1.48% − − − − − −

3.3. LNG Tank Sizing

3.3.1. Tank Shape

Similarly to liquid hydrogen (LH2), LNG needs to be stored in an isolated environment to avoid contact with air, therefore pressurized vessels are used for storage and transportation. Integrating such storage on aircrafts poses a big challenge. Currently, in civil aviation, kerosene is mostly stored inside the wings and occasionally in the fuselage, however the available internal space in the wings would lead to heavy cryogenic fuel tanks, which are unlikely to hold all the fuel required to match the range characteristics. Therefore, spherical tanks can be used, as demonstrated by the Boeing Phantom eye [33] or cylindrical tanks with hemispherical or ellipsoidal cap ends, as suggested in [34,35].

3.3.2. Tank Walls The tank comprises walls and insulation. The walls have to withstand the pressure loads caused by the pressure difference between the fuel and the ambient air. A NASA study [36] on an LNG-fueled subsonic aircraft, and afterwards Brewer [37] and Winnefeld [38] suggested the use of Al 2219 due to its ease of manufacture, performance under cryogenic temperatures, and its ability to maintain the structural integrity of the tank. The wall thickness is determined by Equation (1) for the cylindrical part and by Equation (2) for the hemispherical cap ends [39] according to the internal tank diameter (D0), the maximum allowable pressure difference (∆p), a pressure related factor of safety (FoS), the maximum allowable strength of the material (σa), and the weld efficiency (ew). In this study, a ∆p value is considered that would not exceed 200 kPa for short-range missions and the SF is set to 2.2, as discussed by Brewer [37]. The wall material properties and sizing parameters are shown in Table2.

D0 ∆p FoS tw = × × (1) 2σaew 1.2 ∆p FoS − × ×

D0 ∆p FoS tw = × × (2) 4σaew 0.4 ∆p FoS − × × Energies 2020, 13, 5925 9 of 20

Table 2. Al 2219 tank wall design properties.

Parameter Units Value

Maximum allowable strength, σa MPa 172.4 Density, ρ kg/m3 2840 Factor of Safety, FoS - 2.2 Maximum allowable pressure difference, ∆p kPa 200 Weld efficiency, ew - 0.8

3.3.3. Insulation The second component of the tank wall, the insulation, prevents excessive heat flux between the ambient air and the fuel. As heat is transferred through the tank walls, the fuel is heated and evaporates. The higher the insulation thickness, the lower the heat flux, and therefore the lower the total fuel evaporated; however, increased insulation thickness also leads to heavier tanks. For LH2 flight weight tank applications, there are several types of insulation proposed, and the outstanding ones, as discussed by Kandelwa [40], are vacuum, multilayer, and rigid foam. Vacuum insulation offers the lowest thermal conductivity with close to zero boil-off rates, but with the disadvantage of having heavy outer walls to avoid buckling due to external pressure. Multilayer insulation has higher thermal conductivity than vacuum insulation but also is a heavy option. Rigid polymer foam insulation has higher thermal conductivity than the previous options but offers a good combination of low density, ease of implementation and manufacture, and low cost. Given that the LNG is stored at higher temperatures than LH2, and hence the levels of heat transfer are significantly lower, foam insulation is selected. A list of potential materials used by Colozza [41] is presented in Table3.

Table 3. Polymer foam insulation materials and properties [41].

Insulation Type Density (kg/m3) Thermal Conductivity (W/m K) Polymethacrylimide, Rigid, Closed-Cell 35.3 0.0096 Polyurethane, Rigid, Open-Cell 32.1 0.0112 Polyvinylchloride, Rigid, Closed-Cell 49.8 0.0046 Polyurethane +10% Chopped Glass Fibres, 64.2 0.0064 Rigid, Closed-Cell

3.3.4. Sizing and Heat Transfer Model The calculation of the tank dimensions is a straightforward process using cylindrical and spherical shell geometries. For a given insulation thickness, the dimensions are then defined according to any two of the following parameters:

1. Total fuel stored, which according to fuel density defines the internal volume, with an extra ullage of 7.2% considered, as discussed by Brewer [37]; 2. The length of the cylinder; 3. The total tank length; 4. The external diameter.

The heat transfer model was adapted from previous LH2 tank sizing studies from Reynolds [42] and Colozza [41] and was adapted to the LNG application. The heat flux into the tank is estimated by balancing the heat transfer via convection (Qconv) and radiation (Qrad) from the ambient air to the outer surface of the tank and the heat transfer via conduction (Qcond) through the insulation, as shown in Equation (3). A detailed description of the estimation of these terms is provided in [41] and is based on the tank cylinder dimensions (diameter and length), insulation thickness and properties, and the temperature of the ambient air and the LNG stored. It is also assumed that: Energies 2020, 13, x FOR PEER REVIEW 10 of 20

The heat transfer model was adapted from previous LH2 tank sizing studies from Reynolds [42] and Colozza [41] and was adapted to the LNG application. The heat flux into the tank is estimated by balancing the heat transfer via convection (Qconv) and radiation (Qrad) from the ambient air to the outer surface of the tank and the heat transfer via conduction (Qcond) through the insulation, as shown in Equation (3). A detailed description of the estimation of these terms is provided in [41] and is based on the tank cylinder dimensions (diameter and length), insulation thickness and properties, and the Energies 2020, 13, 5925 10 of 20 temperature of the ambient air and the LNG stored. It is also assumed that:  The heat transfer occurs naturally without any additional heat source; The heat transfer occurs naturally without any additional heat source; • The tank has a constant volume (neglecting thermal expansion of the walls); The tank has a constant volume (neglecting thermal expansion of the walls); • The thermal resistance of the aluminium walls is discounted, as it is orders of magnitude lower The thermal resistance of the aluminium walls is discounted, as it is orders of magnitude lower • than that of the insulation;  Thethan temperature that of the insulation; of the inner surface of the tank is the same as that of the fuel. The temperature of the inner surface of the tank is the same as that of the fuel. • The rate at which the fuel is evaporated, the boil-off rate (mboiloff), is calculated with Equation (4) accordingThe rate to the at whichtotal heat the fueltransferred is evaporated, inside thethe boil-otank andff rate the (m latentboiloff), heat is calculated from vaporization with Equation of the (4) fuelaccording (hlg). Integrating to the total th heatis at the transferred mission level, inside different the tank boil and-off the rates latent can heat be estimated from vaporization for the varying of the ambientfuel (hlg). temperatures Integrating this of every at the flight mission segment. level, diSubsequentlyfferent boil-o,ff multiplyingrates can be this estimated by the corresponding for the varying timeambient and temperaturesthen summing of the every result flight will segment. provide Subsequently, the total fuel multiplying evaporated thisthroughout by the corresponding the mission, showtimen and as thenEquation summing (5). If thethe resultevaporated will provide fuel is unusable the total fuelby the evaporated engines, several throughout iterations the mission, will be requiredshown as to Equation re-size the (5). tanks If the and evaporated compensate fuel for is the unusable unusable by fuel. the engines, several iterations will be required to re-size the tanks and compensate for the unusable fuel. QQQ (3) cond conv rad Q = Qconv + Q (3) cond Q rad m  cond . boiloff Qcond (4) mboilo f f = hlg (4) hlg

nsegseg Xh . i MM=m m tt (5)(5) evapevap,mission,,, missionboilo boiloff f f i,i × segmentsegment i,i 1 ForFor the the selecti selectionon of of the insulation material, material, a a parametric parametric study study wa wass carried carried out out based based on on the the propertiesproperties presented in Table 3 3,, withwith varyingvarying insulationinsulation thicknesses.thicknesses. TheThe tanktank waswas sizedsized forfor aa defineddefined missionmission profile, profile, amount of fuel stored, stored, and external diameter diameterss in all case cases.s. The The effect effect of of increasing increasing the the insulationinsulation thickness thickness on on the the total total tank tank weight weight is is presented presented in in Figure Figure 77.. InIn thethe samesame graph,graph, designsdesigns areare alsoalso marked marked for for each each insulation insulation material, material such, such that all that of them all of allow them for allow the same for theamount same of amountevaporated of evaporatedfuel throughout fuel throughout the mission. the Evidently, mission. rigidEvidently, polyvinylchloride rigid polyvinylchloride closed-cell foamclosed provides-cell foam the provides lightest thedesign lightest and design occupies and the occupies least external the least space. external space.

2.2

2 Polyurethane and Chopped Glass Fiber 1.8 V + 3% Polyvinychloride

(t) 1.6 V + 15% V + 11% Polyurethane 1.4

1.2 Polymethacrylimide

Total tank mass added to OWE OWE to added mass tank Total 0.02 0.04 0.06 0.08 0.1 Insulation thickness (m)

Figure 7. Total tank weight to insulation thickness. Figure 7. Total tank weight to insulation thickness. 4. Case Study The aircraft model selected to demonstrate the LNG application was based on a Boeing 737–800, with a capacity of 185 economy passengers. The following section discusses the modeling of the baseline10 Jet A-1 model, the modifications considered for the LNG version, and the results and comparison of the different applications. Energies 2020, 13, 5925 11 of 20

4.1. Turbomatch Engine Model The aircraft was equipped with two turbofan engines based on the CFM56-7B26, namely two-spool enginecapable of 117.4 kN of thrust at sea level static (SLS) with +30 ◦C ambient temperature [43]. A two-spool engine model was created in Turbomatch by matching available performance characteristics from [43] and assuming parameters such as component efficiencies, cooling bleeds, duct pressure losses, and power-off takes. In Table4, the performance of the model is presented as simulated for three characteristic points—SLS, mid-cruise, and top of climb. Multipoint matching was used to input the underlined values in Table4, while the remaining values are the results.

Table 4. CFM56-7B26 Turbomatch model.

Parameters Units SLS Cruise Top of Climb Altitude ft 0 35,000 35,000 Mach Number - 0 0.8 0.8 dTisa K 15 0 0 Net Thrust kN 117.4 24.4 26.5 Bypass Ration - 5.1 5.2 5.1 Overall Pressure Ratio - 27.9 29.9 31.6 Inlet Air Mass Flow Rate kg/s 355 150 153 Specific Fuel Consumption g/kN 10.54 17.76 18.02

4.2. Jet A-1 Model The baseline aircraft model was then created by utilizing the engine model and the geometry of the aircraft found in [44]. Characteristic weights such as the maximum take-off weight (MTOW), operating weight empty (OWE), maximum payload, and fuel were also included. The flight profile used throughout the mission analyses that will follow is presented in Table5, partially based on information from [44,45].

Table 5. Flight profile.

Flight Profile Parameters Inputs Calibrated Airspeed (CAS) Initial climb up to 10,000 ft 250 Calibrated Airspeed (CAS) Climb after 10,000 ft 310 Calibrated Airspeed (CAS) Descent 300 Contingency 5% of theTrip Fuel Diversion 130 nmi at 25,000 ft Hold 30 min 1500 ft

Based on the aforementioned inputs, the baseline model was compared to the actual performance of the aircraft, as quoted in [44], for three key missions—the maximum payload, maximum fuel, and ferry range. Standard day conditions with zero wind and normal power extraction from the engines were considered. It was assumed that for the maximum payload and fuel missions, the aircraft would cruise at 35,000 ft (design cruising altitude) and at 0.78 and 0.76 Mach respectively; while for the ferry range mission the aircraft would cruise at 37,000 ft and 0.75 Mach. The resulting payload-to-range diagram against the reference Boeing data is depicted in Figure8. The results indicate that the engine aircraft performance is predicted with adequate accuracy for assessing system performance changes due to drag, weight, and fuel properties. Energies 2020, 13, 5925 12 of 20 Energies 2020, 13, x FOR PEER REVIEW 12 of 20

25

20 Max payload Max fuel 0.37% 15 1.52%

10 Payload (t) Payload

5 Ferry -2.77%

0 0 500 1000 1500 2000 2500 3000 3500 4000 Range (nmi)

Figure 8. Payload-to-range performance for the baseline aircraft. Baseline Jet-A1 737 800 4.3. LNG Model Converting theFigure baseline 8. Payload aircraft-to to-range use LNGperformance is not afor straightforward the baseline aircraft process.. Due to the size and shape of the tanks that need to be integrated, modifications are required. A solution would be to 4.3.mount LNG them Model inside the fuselage and reduce the passenger capacity; however, the resulting aircraft wouldConverting not be comparable the baseline to aircraft the baseline. to use LNG Therefore, is not a it straightforward was considered process. that the Due two to models the size would and shapehave aof common the tanks mission, that need which to wouldbe integrated, set the requirementsmodifications for are designing required. the A LNGsolution version. would For be this, to mountthe maximum them inside fuel mission the fuselage was selected, and reduce since the the passenger fuel tanks needcapacity to be; however, sized for thethe maximum resulting capacity.aircraft wouldIn the designnot be mission,comparable the LNGto the aircraft baseline. is capable Therefore of carrying, it was considered a payload oftha 16.8t the tons two at models a range would of 2855 havenmi, a with common the same mission, flight which profile would as the set baseline the requirements model using for the design typeing 1 (Alaskan) the LNG LNG version. composition. For this, theAs discussed, maximum the fuel performance mission was of selected, the aircraft since will the also fuel be tanks examined need for to the be sizedtype 2 for (Australian the maximum NSW) capacity.and type In 3 (Sakhalin) the design compositions. mission, the TheLNG properties aircraft is of capable the selected of carrying fuels used a payload for sizing of and 16.8 analysis tons at are a rangepresented of 2855 in Tablenmi, 6with and the were same calculated flight profile based as on the 150 baseline kPa initial model storage using pressure. the type Ideally,1 (Alaskan) the lowerLNG composition.the pressure, theAs higher discussed, the density; the performance however, considering of the aircraft the potentialwill also di beffi examinedculties related for tothe refueling type 2 (theAustralian tanks at NSW) close toand ambient type 3 pressure,(Sakhalin) a composition slightly highers. The value properties was chosen. of the Of selected the three fuels fuels, used type for 1, sizingthe design and analysis choice, hasare thepresented highest in LHV, Table and 6 thereforeand were wouldcalculated require based less on LNG 150 masskPa initial than the storage other pressure.two. However, Ideally, for the the lower same the volume, pressure type, the 3higher stores the greater density energy; however, and could considering potentially the potential allow for difficultiesgreater range related capabilities. to refuel Finally,ing the the tanks CO2 atemission close to index ambient (EICO pressure2) greatly, a dependsslightly onhigher the composition value was chosen.of the fuel. Of the three fuels, type 1, the design choice, has the highest LHV, and therefore would require less LNG mass than the other two. However, for the same volume, type 3 stores greater energy and Table 6. LNG fuels considered. could potentially allow for greater range capabilities. Finally, the CO2 emission index (EICO2) greatly depends on the compositionProperties of the fuel. Units Type 1 Type 2 Type 3 Pressure kPa 150 Temperature (K)Table 6. KLNG fuels cons 116.3idered. 116.9 109.8 Liquid density (kg/m3) kg/m3 416.0 427.9 449.6 LHVProperties MJ/kgUnits 49.9Type 1 49.5Type 2 Type 48.3 3 Volumetric EnergyPressure density MJ/m3 kPa 20,751 21,185150 21,733 Temperature (K) K 116.3 116.9 109.8 EICO2 kgCO2/kgfuel 2.74 2.76 2.73 Liquid density (kg/m3) kg/m3 416.0 427.9 449.6 LHV MJ/kg 49.9 49.5 48.3 Tank Position, Weight, and Aircraft Modifications Volumetric Energy density MJ/m3 20,751 21,185 21,733 For tube-and-wingEICO aircraft,2 the cryogenickgCO2 tanks/kgfuel can be2.74 positioned 2.76 in several2.73 locations. In a long-range application study, Brewer [38] suggested placing them forward and aft of the passenger Tankcabin Position to maintain, Weight stability., and InAircraft this case, Modifications however, these positions would require significant elongation of theFor fuselage, tube-and which-wing would aircraft, in the turn cryogenic require a tanks longer can undercarriage be positioned to in avoid several issues locations. with the In rotationa long- rangeangle application of the aircraft. study The, Brewer Cryoplane [38] suggested [46] and ENABLEH2placing them (ENABLing forward and cryogEnic aft of the Hydrogen-basedpassenger cabin toCO2-free maintain air stability. transport) In [ 47this] studies case, however, suggested these that placingpositions the would tanks onrequire top of significant the fuselage elongation would avoid of thesuch fuselage problems, which and would lead to in a moreturn require efficient a design; longer therefore,undercarriage such to an avoid arrangement issues with was the adopted rotation for angle of the aircraft. The Cryoplane [46] and ENABLEH2 (ENABLing cryogEnic Hydrogen-based

12 Energies 2020, 13, x FOR PEER REVIEW 13 of 20

CO2Energies-free2020 air, 13 transport, 5925 ) [47] studies suggested that placing the tanks on top of the fuselage would13 of 20 avoid such problems and lead to a more efficient design; therefore, such an arrangement was adopted forthis this application application as a well.s well. The The tanks tanks werewere coveredcovered byby fairing to protect protect them them from from external external damage, damage, reducereduce drag drag,, and and prevent prevent higher higher heat heat transfer transfer by by forced forced convection convection from from the the free free-stream-stream air. air. ConsideringConsidering the the available available space space above above the the passenger passenger cabin, cabin, several several restrictions restrictions and and assumptions assumptions wewerere set. set. The The fairing fairing wa wass assumed assumed to to have have a athickness thickness of of 5 5cm, cm, half half of of that that of of the the main main fuselage fuselage,, becausebecause it it did did not not contain contain a apressurized pressurized compartment. compartment. An An additional additional spacing spacing of of 5 5cm cm wa wass considered considered betweenbetween the the inner inner fairing fairing surface surface and and the the oute outerr surface surface of of the the tanks. tanks. The The weight weight per per unit unit of of wetted wetted surfacesurface area area wa wass 10 10 kg/m kg/m2,2 also, also half half of of that that of of the the fuselage fuselage skin, skin, as as calculated calculated by by the the weight weight estimation estimation module.module. The The center center of of gravity gravity (CoG) (CoG) position position wa wass assumed assumed to to be be approximately approximately 90% 90% of of the the distance distance fromfrom the the front toto thethe rear rear undercarriage. undercarriage. The The frontal frontal area area of the of fairingthe fairing matched matche thed slope the ofslope the cockpit,of the cockpitand by, mirroringand by mirroring the area the around area around the CoG the for CoG balancing, for balancing a total internal, a total internal length of length 24.2 m of was 24.2 available. m was available.In order toIn avoidorder catastrophicto avoid catastrophic damage damage if a rotor if disc a rotor burst disc should burst occur,should a occur, spacing a spacing between between area the area the tanks was considered. According to [48], the space situated 15° from the fan face and 5° from tanks was considered. According to [48], the space situated 15◦ from the fan face and 5◦ from the theturbine turbine exit exit was wa lefts left empty. empty. The Th remaininge remaining available available length length was wa divideds divided into into two; two; the front the front was 7.3wa ms 7.3and m theand rear the wasrear 13.05was 13.05 m. These m. These considerations considerations are shown are shown schematically schematically in Figure in Figure9. 9.

Figure 9. Schematic representation of the 737–800 type adopted from [44]. Figure 9. Schematic representation of the 737–800 type adopted from [44]. Multiple tanks can be accommodated in the available space, however only two were selected, sinceM theyultiple would tanks add can thebe accommodated least surface area in tothe the available fairing, space, overall however were a lighteronly two option, were andselected since, sitheynce they offered would a lower add the surface-to-volume least surface area ratio to the in fairing, order tooverall minimize were a heat lighter transfer. option Both, and tankssince they were offerconsidereded a lower to have surface the same-to-volume external ratio diameter, in order and in to order minimize to minimize heat this transfer. the front Both tank tanks took we upre all consideredof the available to have length. the same The selectedexternal externaldiameter diameter, and in order allowed to minimize for a fuel splitthis the between front tank the tanks took thatup allbalanced of the available the moment length. around The theselected CoG developedexternal diameter by the added allow massesed for a (tank fuel split and fuel).between The the insulation tanks thatthickness balance ofd each the momenttank was around chosen to the provide CoG developed similar evaporation by the added rates. masses Finally, (tank the tank and mountingsfuel). The insulationwere assumed thickness to be o 20%f each of thetank weight was chosen of tank. to provide similar evaporation rates. Finally, the tank mountings were assumed to be 20% of the weight of tank. 4.4. Results and Comparison 4.4. Results and Comparison The LNG aircraft was sized by utilizing the simulation framework, which combines all of the above-mentionedThe LNG aircraft modules. was sized Using by theutili samezing propulsionthe simulation system framework but with, which the type combines 1 LNG properties,all of the athebove baseline-mentioned aircraft modules. was used Using as the a starting same propulsion point and system was converted but with to the the type LNG 1 LNG application properties, once thethe baseline sizing loopaircraft converged was used (i.e., as a when starting the point codependent and was converted inputs of allto the the LNG individual application modules once were the sizingmatched). loop In converge the followingd (i.e. sections,, when the the tankcodependent sizing results, inputs the of aircraft all the characteristic individual weights, modules and we there matched).payload-to-range In the following characteristics sections for, the the tank three sizing types results, of fuels the are aircraft presented characteristic for the sized weights, aircraft. and the payload-to-range characteristics for the three types of fuels are presented for the sized aircraft. 4.4.1. Cryogenic Storage Properties 4.4.1. Cryogenic Storage Properties To begin with, the sized tanks are presented in Table7. Based on the above considerations, both tanksTo had begin a diameter with, the of sized 2 m, thetanks rear are tank presented had an overallin Table length 7. Based of 12.2 on mthe (within above theconsiderations set limits), and, both the tanksfuel splithad a between diameter the of front 2 m, andthe rear the afttank tank had was an 36–64%,overall length respectively. of 12.2 m The (within thickness the set selected limits) allows, and thefor fuel a mission-evaporated split between the fuelfront to and total t fuelhe aft stored tank ratiowas of36 approximately–64%, respectively. 0.3% forThe both thickness tanks. selected The total allowsadded for fuel a storagemission weight-evaporated for the fuel aircraft to total is 3.9 fuel tons stored and the ratio added of approximately fairing increases 0.3% the for fuselage both tanks. wetted area by 22.8%.

13 Energies 2020, 13, 5925 14 of 20

Table 7. Sized LNG aircraft tanks.

Tank Characteristics Units Fore Tank Aft Tank External diameter (m) 2.0 Overall length (m) 7.30 12.18 Insulation thickness (m) 0.055 0.05 Design mission evaporated fuel (kg) 19.7 36.3 Useful fuel mass (kg) 7118 12,536 Total tank mass (kg) 450 929 Fairing weight (kg) 2216 Total added weight (kg) 3928

4.4.2. Weight Characteristics and Payload–Range Performance The weight characteristics are presented in Table8 for the baseline model and the LNG variant. The sized aircraft is heavier than the baseline due to the addition of the fuel storage system, and in combination with increased wetted area, requires more energy. However, due to the higher LHV of the LNG compared to Jet A-1, less mass of fuel is required for the same mission. Overall, the combined effect leads to a slightly heavier maximum take-off weight (MTOW) and maximum landing weight (MLW). Figure 10 presents the payload-to-range diagram for the baseline aircraft and the LNG variant with the design choice LNG. Since the maximum fuel mission is the same, only the maximum payload and ferry range missions differ, where the sized aircraft shows a slight decrease in range in comparison to the baseline. Figure 10 also includes the performance of the LNG aircraft for the other two examined fuels. For the maximum payload mission, the fuel that can be carried is restricted by the MTOW and the payload, therefore the fuels with less LHV have up to 5.7% lower range compared to the baseline. − For the maximum fuel mission, the fuel that can be carried is dictated by the volume of the sized tanks, and due to the alternative fuel densities more fuel mass (and energy) can be stored, as shown in Table8. This leads to increased range but with a reduced payload, since the latter is restricted by the MTOW. The same applies for the ferry mission, where the alternative options provide longer range capabilities Energiesof up to2020 5%, 13 compared, x FOR PEER to REVIEW the baseline. 15 of 20

25 Maximum Fuel Payload Range 20 Type 1 0.0% 0.0% Type 2 -3.4% 2.6% Type 3 -9.5% 6.3% 15 Maximum Payload Range Type 1 -1.7% Ferry Range

Payload (t) Payload 10 Type 2 -2.8% Range Type 3 -5.7% Type 1 0.9% 5 Type 2 2.7% Type 3 4.8%

0 1000 1500 2000 2500 3000 3500 4000 Range (nmi)

Baseline Jet-A1 Type 1 Type 2 Type 3

Figure 10. Payload-to-range diagram for the baseline model and the LNG variant with three fuel options. Figure 10. Payload-to-range diagram for the baseline model and the LNG variant with three fuel options.

4.4.3. Typical Mission Performance and Comparison For this assessment, the LNG aircraft with the three examined fuels carries a typical payload of 18.12 tons (85% of the maximum) for a range that varies from 250 to 2250 nmi, which is compared against the baseline. The flight profile for all the missions is the one proposed in Table 5, with a cruising speed of 0.78 Mach at 35,000 ft. The results are presented in terms of the energy, fuel, and CO2 emissions per passenger nmi in Figures 11a, 12a and 13a, respectively and in Figures 11b, 12b and 13b, the corresponding differences of the LNG performance against the baseline are shown. Figure 11a shows that the LNG aircraft requires more energy due to the increased OWE and surface area than the baseline, while Figure 11b shows that less energy is consumed when the aircraft is using a LNG composition with higher LHVs (lighter aircraft). From Figure 12a, it can be observed that due to the fuel properties, less fuel mass is consumed by the LNG variant compared to the Jet-A1 baseline. Finally, in Figure 13a the CO2 emissions are quantified, which do not present the same trend as the fuel burn, which is attributed to the differences in the EICO2. In Figure 13b, it is evident that by replacing LNG with Jet A-1, a reduction of at least 15% in CO2 emissions can be achieved for the particular application, and by using LNG with a higher methane composition the reduction can be maximized to 17–18%. Finally, based on the fuel costs considered by Withers [49], the average fuel cost difference compared with Jet A-1 is presented in Figure 14. For this application, switching to LNG would potentially save about 16% on the cost of fuel.

15 Energies 2020, 13, 5925 15 of 20

Table 8. Sized aircraft weight breakdown.

Characteristic Weights Units Jet-A1 Baseline Type 1 Type 2 Type 3 MTOW (kg) 79,016 81,544 OWE (kg) 41,413 45,070 MLW (kg) 65,217 68,708 Maximum Fuel Capacity (kg) 20,894 19,654 20,214 21,239 Maximum Payload (kg) 21,319

4.4.3. Typical Mission Performance and Comparison For this assessment, the LNG aircraft with the three examined fuels carries a typical payload of 18.12 tons (85% of the maximum) for a range that varies from 250 to 2250 nmi, which is compared against the baseline. The flight profile for all the missions is the one proposed in Table5, with a cruising speed of 0.78 Mach at 35,000 ft. The results are presented in terms of the energy, fuel, and CO2 emissions per passenger nmi in Figures 11a, 12a and 13a, respectively and in Figures 11b, 12b and 13b, the corresponding differences of the LNG performance against the baseline are shown. Figure 11a shows that the LNG aircraft requires more energy due to the increased OWE and surface area than the baseline, while Figure 11b shows that less energy is consumed when the aircraft is using a LNG composition with higher LHVs (lighter aircraft). From Figure 12a, it can be observed that due to the fuel properties, less fuel mass is consumed by the LNG variant compared to the Jet-A1 baseline. Finally, in Figure 13a the CO2 emissions are quantified, which do not present the same trend as the fuel burn, which is attributed to the differences in the EICO2. In Figure 13b, it is evident that by replacing LNG with Jet A-1, a reduction of at least 15% in CO2 emissions can be achieved for the particular application, and by using LNG with a higher methane composition the reduction can be maximized to 17–18%. Finally, based on the fuel costs considered by Withers [49], the average fuel cost difference compared with Jet A-1 is presented in EnergiesFigure 20 1420., For13, x thisFOR application,PEER REVIEW switching to LNG would potentially save about 16% on the cost16 of of fuel. 20

2.4 11%

2.2 10%

2.0 9%

1.8 8% (MJ/pax nmi) (MJ/pax

1.6 7%

Energy per pax nmi pax per Energy

Energy consumption Energy difference to baseline to difference 1.4 6% 250 750 1250 1750 2250 250 750 1250 1750 2250 Range (nmi) Range (nmi) (a) (b)

Figure 11. (a) Energy per pax nmi against range and (b) corresponding difference from the baseline. Figure 11. (a) Energy per pax nmi against range and (b) corresponding difference from the baseline.

0.050 0% -1% 0.046 -2% 0.042 -3% -4% 0.038 -5%

(kg/pax nmi) (kg/pax -6% 0.034

difference to baseline to difference -7%

Fuel burn per pax nmi pax per burn Fuel Fuel burn consumption burn Fuel 0.030 -8% 250 750 1250 1750 2250 250 750 1250 1750 2250 Range (nmi) Range (nmi) (a) (b)

Figure 12. (a) Fuel burn per pax nmi against range and (b) the corresponding difference from the baseline.

0.16 -14% 0.15 -15% 0.14 -16% 0.13

/ pax nmi) pax / 0.12 -17%

2

emissions per pax nmi pax per 0.11 2

2 -18%

0.10 CO CO

(kgCO -19%

0.09 baseline to difference 0.08 -20% 250 750 1250 1750 2250 250 1250 2250 Range (nmi) Range (nmi) (a) (b)

16 Energies 2020, 13, x FOR PEER REVIEW 16 of 20 Energies 2020, 13, x FOR PEER REVIEW 16 of 20 2.4 11% 2.4 11% 2.2 10% 2.2 10% 2.0 9% 2.0 9% 1.8 8%

1.8 8% (MJ/pax nmi) (MJ/pax

1.6 7%

(MJ/pax nmi) (MJ/pax

Energy per pax nmi pax per Energy Energy consumption Energy

1.6 baseline to difference 7%

Energy per pax nmi pax per Energy Energy consumption Energy 1.4 baseline to difference 6% 1.4 250 750 1250 1750 2250 6% 250 750 1250 1750 2250 250 750 1250 1750 2250 250 750 1250 1750 2250 Range (nmi) Range (nmi) Range( a(nmi)) Range( b(nmi)) (a) (b)

EnergiesFigure2020, 13 11, 5925. (a) Energy per pax nmi against range and (b) corresponding difference from the baseline16. of 20 Figure 11. (a) Energy per pax nmi against range and (b) corresponding difference from the baseline. 0.050 0% 0.050 0%-1% 0.046 -1%-2% 0.046 -2% 0.042 -3% 0.042 -3%-4% 0.038 -4%-5% 0.038 (kg/pax nmi) (kg/pax -5%-6% 0.034 (kg/pax nmi) (kg/pax -6%

0.034 baseline to difference -7%

Fuel burn per pax nmi pax per burn Fuel Fuel burn consumption burn Fuel

difference to baseline to difference -7%

Fuel burn per pax nmi pax per burn Fuel 0.030 -8% Fuel burn consumption burn Fuel 0.030 250 750 1250 1750 2250 -8% 250 750 1250 1750 2250 250 750 1250 1750 2250 250 750 1250 1750 2250 Range (nmi) Range (nmi) Range( a(nmi)) Range( b(nmi)) (a) (b)

Figure 12. (a) Fuel burn per pax nmi against range and (b) the corresponding difference from the Figure 12. (a) Fuel burn per pax nmi against range and (b) the corresponding difference from Figurebaseline 12.. (a) Fuel burn per pax nmi against range and (b) the corresponding difference from the baselinethe baseline.. 0.16 -14% 0.160.15 -14% -15% 0.15 0.14 -15% -16% 0.140.13 -16% 0.13

/ pax nmi) pax / 0.12 -17%

2 emissions

/ pax nmi) pax / 0.12 -17% per pax nmi pax per

0.11 2

2 emissions

2 -18% per pax nmi pax per

0.11 2 CO

2 0.10 -18% CO

(kgCO -19% 0.10 CO

0.09 baseline to difference CO

(kgCO -19%

0.090.08 baseline to difference -20% 0.08 250 750 1250 1750 2250 -20% 250 1250 2250 250 750 1250 1750 2250 250 1250 2250 Range (nmi) Range (nmi) Range(a) (nmi) Range(b) (nmi) Energies 2020, 13, x FOR PEER REVIEW(a) (b) 17 of 20

Figure 13. (a) kg of CO2 emissions per pax nmi against range and (b) the corresponding difference fromFigure the 13. baseline(a) kg. of CO2 emissions per pax nmi against range and (b) the corresponding difference from the baseline. 16

-14% 16

1 - -15%

-16%

-17%

-18%

-19% Fuel cost difference to Jet A to difference cost Fuel -20% 250 500 750 1000 1250 1500 1750 2000 2250 Range (nmi)

Figure 14. Average fuel cost difference to baseline Jet A-1. Figure 14. Average fuel cost difference to baseline Jet A-1.

5. Conclusions The potential benefits of introducing LNG in civil aviation were quantified by utilizing a simulation framework integrating aircraft engine performance models, a tank sizing module, and an aircraft weight estimation module. A case study based on the popular Boeing 737–800 aircraft was performed and the changes on the airframe weight and volume due to conversion to LNG were considered. The “above-the-passenger-cabin” tank configuration was selected, utilizing two cylindrical tanks with a frontal diameter of 2 m, leading to an increased fuselage wetted area and higher drag. The front tank has a length of 7.3 m and the rear tank has a length of 12.2 m, designed considering both safety issues and minimum impact on the aircraft’s center of gravity. The added weight of the fuel storage system is 4 tons, resulting in an increase in the aircraft maximum take-off weight of 4% (2.5 tons). The aircraft performance was assessed against the baseline case (Jet A-1) for three different LNG sources in terms of the payload range performance, mission energy and fuel consumption, CO2 emissions, and fuel cost. The LNG-converted aircraft had a slightly reduced range for the case of maximum payload. The decrease in range varied from 1.7% to 5.7% for the different LNG sources. For the ferry mission, LNG with increased energy density provided a benefit in terms of range of 0.9–4.8% for the different sources. For the maximum payload, the added weight resulted in a decrease of range when LNG was considered, while for the ferry mission it was the fuel specific energy that provided the benefit, since there was no limitation on fuel mass due to payload. These results indicate that when assessing LNG for aviation, it is important to consider the source and synthesis, since these factors affects both LHV and density, and hence the mission-available energy. These are also operational parameters that have to be assessed if LNG is used in aviation, since aircraft performance will change depending on the take-off airport. It is apparent that the increases of aircraft weight and frontal area for the LNG cases will result in higher energy consumption per passenger. The results indicate that the increase in energy will be in the range of 10% for a typical flight (1000–2000 nmi). Of course considering the difference between the LHV of kerosene and LNG, this is translated to a fuel reduction of 4% (1.6–5.0%, depending on the source) for typical flights, and up to 6.7% (4.3–7.6%) reduction for the shortest flights that this aircraft operates (in the range of 250 nmi). This fuel reduction in conjunction with the lower CO2 index of LNG compared to kerosene offers a substantial CO2 emissions reduction in the range of 16% for typical missions and up to 18% for very short missions. The lower cost of LNG compared to kerosene offers a significant fuel cost per passenger reduction of approximately 16%. In this context, it is apparent that LNG has significant environmental and economic benefits compared to kerosene,

17 Energies 2020, 13, 5925 17 of 20

5. Conclusions The potential benefits of introducing LNG in civil aviation were quantified by utilizing a simulation framework integrating aircraft engine performance models, a tank sizing module, and an aircraft weight estimation module. A case study based on the popular Boeing 737–800 aircraft was performed and the changes on the airframe weight and volume due to conversion to LNG were considered. The “above-the-passenger-cabin” tank configuration was selected, utilizing two cylindrical tanks with a frontal diameter of 2 m, leading to an increased fuselage wetted area and higher drag. The front tank has a length of 7.3 m and the rear tank has a length of 12.2 m, designed considering both safety issues and minimum impact on the aircraft’s center of gravity. The added weight of the fuel storage system is 4 tons, resulting in an increase in the aircraft maximum take-off weight of 4% (2.5 tons). The aircraft performance was assessed against the baseline case (Jet A-1) for three different LNG sources in terms of the payload range performance, mission energy and fuel consumption, CO2 emissions, and fuel cost. The LNG-converted aircraft had a slightly reduced range for the case of maximum payload. The decrease in range varied from 1.7% to 5.7% for the different LNG sources. For the ferry mission, LNG with increased energy density provided a benefit in terms of range of 0.9–4.8% for the different sources. For the maximum payload, the added weight resulted in a decrease of range when LNG was considered, while for the ferry mission it was the fuel specific energy that provided the benefit, since there was no limitation on fuel mass due to payload. These results indicate that when assessing LNG for aviation, it is important to consider the source and synthesis, since these factors affects both LHV and density, and hence the mission-available energy. These are also operational parameters that have to be assessed if LNG is used in aviation, since aircraft performance will change depending on the take-off airport. It is apparent that the increases of aircraft weight and frontal area for the LNG cases will result in higher energy consumption per passenger. The results indicate that the increase in energy will be in the range of 10% for a typical flight (1000–2000 nmi). Of course considering the difference between the LHV of kerosene and LNG, this is translated to a fuel reduction of 4% (1.6–5.0%, depending on the source) for typical flights, and up to 6.7% (4.3–7.6%) reduction for the shortest flights that this aircraft operates (in the range of 250 nmi). This fuel reduction in conjunction with the lower CO2 index of LNG compared to kerosene offers a substantial CO2 emissions reduction in the range of 16% for typical missions and up to 18% for very short missions. The lower cost of LNG compared to kerosene offers a significant fuel cost per passenger reduction of approximately 16%. In this context, it is apparent that LNG has significant environmental and economic benefits compared to kerosene, even when accounting for weight and drag penalties. It is also important to consider that gas turbine engines operate without the need for a major redesign or retrofitting with NG, while if we consider future propulsion configurations (e.g., hybrid-electric, supersonic), LNG thermal management capabilities may act as technology enablers. Of course, the adoption of LNG in aviation will have to address the current lack of infrastructure and the need for a new logistic approach considering aspects such as the source and synthesis, fuel delivery, and fuel stability. The prospects for LNG utilization in other transport industries (e.g., marine) may offer experience that can support its application for aviation.

Author Contributions: Conceptualization, P.R., I.R., S.K., A.R.; data and software contributions, P.R., D.N., A.R., T.N., S.K., I.R.; analysis, P.R., I.R., D.N., S.K., T.N.; writing—original draft preparation, P.R., S.K., I.R., T.N.; writing—review and editing, P.R., S.K., I.R., A.R. 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. Energies 2020, 13, 5925 18 of 20

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