Renewable Energy Perspectives for Aviation H. Kuhn, C. Falter, A. Sizmann Bauhaus Luftfahrt e.V., Lyonel-Feininger Straße 28, 80807 Munich Keywords: alternative renewable energies, solar drop-in fuels, solar hydrogen, feasibility of electric flying

Table 1 Main benefits and limitations of alternative fuels for aviation. The color code indicates a qualitative aptitude of a fuel option in a 2050 time frame.

Scalability

The substitution of fossil kerosene by suitable, sustainable and scalable alternative energy carriers is a key requirement for which strategies in three categories, i.e. the drop-in fuel, nondrop-in fuel and fully electric energy path, are presented. For long-term renewable energy perspectives, drop-in fuels from solar thermal reactors, the use of renewable hydrogen for either synthetic paraffinic kerosene or fuel cells, and the material advancements in electric energy storage promise substantial innovation potential. Based on fundamental principles for future energy technology assessment it is shown that the fully electric aircraft may be closer to realization that generally assumed.

Sustainability

The transformation of the energy supply from fossil to renewable energy is the single most important challenge for the aviation industry’s long-term future.

Suitability

Abstract

most dependent on fossil fuels which are ideal energy carriers due to their high energy density and convenient handling and storage properties. The most significant challenge for the future of aviation is therefore the substitution of fossil fuels or the aircraft power system to implement entirely renewable carbon-neutral energy carriers and power systems in aviation. Selected alternative fuels are summarised in Table 1.

Drop-in fuels CTL, GTL BTL HEFA STL Non-drop-in fuels

1

Introduction

The future of aviation will be shaped by a growing mobility demand, limited fossil energy resources and by the need for climate protection. Today, individual transportation in general and aviation in particular are sectors that are

LNG LH2 Alcohols CTL/GTL/BTL: coal/gas/biomass-to-liquid; HEFA: hydroprocessed esthers and fatty acids; STL: sunlight-toliquid; LNG: liquid natural gas; LH2: liquid hydrogen;

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H2O

CO2

H2O

Inverse combustion

Water splitting

PV/CSP

CO/H2 FT

O2

H2

Electric energy

O2

CxHy Electrochemical conversion

Combustion

Heat

Motive power

Battery

Electric energy

Heat

Electric energy Electric motor

Electric motor

Motive power

Motive power

Renewable drop-in fuel path

Three paths for renewable energy perspectives in aviation. PV: photovoltaics; CSP: concentrated solar power.

Suitability, sustainability and scalability serve as major criteria in order to evaluate different alternative fuels. A low suitability score indicates that a re-design of the aircraft fuel system and changes in the infrastructure are necessary for implementation. The scalability criterion addresses the technical feedstock potential, the processing potential and challenges in high volume logistics. Examples of three alternative energy strategies are shown in Fig. 1, (1) the drop-in-capable solar fuel option, using novel solar chemical reactors as discussed below or the well-known photosynthesis path, both of which present a renewable energy perspective with the advantage of no changes to infrastructure or aircraft fuel systems, (2) the non-drop-in solar fuel option which opens the perspective of highly efficient fuel cell power systems and electric propulsion in aviation, and (3) the all-electric option using solar, wind and other renewable power, the latter two require a complete re-design of the aircraft but offer the ultimate flexibility in the choice of primary energy. Figure 2 shows the energy and exergy densities of various energy carriers and the large “gap” between hydrocarbon fuels and batteries. For a fair comparison, the net exergy, i.e. the net useful work, and the physical potential have to be assessed (see also Section 4.1). This reduces the gap of 56 to a factor of approximately 8 and greatly enhances the feasibility of fully electric aviation.

In this paper the feasibility of strategies ranging from solar drop-in fuels to step-change design of the all-electrically powered aircraft are discussed. 100

Specific Exergy/Energy / kWh/kg

Fig. 1

All-electric motive power path

Renewable non-drop-in fuel path

10

gas. H2

liq. H2 Kerosene ~8x

~56x

1 + o Hydrogen + o Kerosene + o Comm. Li Battery + o Exp. Li Battery

0.1 0.001

Exp. Li Battery Comm. Li Battery

0.01 0.1 1 10 Exergy/Energy Density / kWh/l

100

Fig. 2 Specific exergy vs. exergy density (+) as well as specific energy and energy density (o) of selected energy carriers.

2

Drop-in Fuels

The use of drop-in fuels which have equal characteristics as conventional kerosene requires in general no adaptation of the fuel distribution infrastructure, on-board fuel systems or combustion engines. One way to reduce the carbon footprint of aviation is to introduce sustainable, carbon-neutral drop-in fuels that can be used to substitute conventional kerosene in present motive power systems and could therefore

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represent an elegant way to fulfill the International Air Transport Associations (IATA) target of carbon-neutral growth starting in the year 2020. Certified drop-in fuels based on different resources exist and enter long-term testing phases (Lufthansa, KLM).

2.1

H2O

CO2

E Electrochemical lectrochemical • Electrolysis

Biofuels

The biomass-to-liquid (BTL) and the hydroprocessed-ester-and-fatty-acids (HEFA) processes convert biomass to liquid fuels by gasification/FT and hydrogenation, respectively, and provide a potential sustainable path. Their capa-

Photochemical P hotochemical • Photosynthesis •Biomass gasification/pyrolysis + Water gas shift •Algae • Photocatalysis

Thermochemical T hermochemical • One-step •Direct Thermolysis • Two-step •Metal-oxide redox reactions (CoFe, CeO2) • Three-step •Sulfur-iodine cycle, UT-3 cycle

CO H2 Syngas (H2/CO) Fig. 3

Three paths to solar syngas (hydrogen and carbon monoxide) production, a precursor for solar fuels.

Different approaches exist for the production of sustainable fuels based on synthetic gas (syngas), see Fig. 3, all of which satisfy the requirement of suitability which is an important feature for quick implementation of renewable aviation fuel options. Gas-to-liquid (GTL) and coal-to-liquid (CTL) processes require fossil energy carriers that are gasified to produce syngas, a mixture of hydrogen and carbon monoxide, which is converted to liquid hydrocarbons in the Fischer-Tropsch (FT) process, invented in Germany in the 1920s. The use of nonsustainable coal and natural gas reduces finite reserves of energy carriers and adds significantly to CO2 emissions [1].

bility to supply an appreciable amount of liquid fuels is a topic of scientific research and remains to be quantified. The growing of biomass requires the allocation and tilling of farmland and therefore competes in general with the production of food plants which has already lead once to rising corn prices in Mexico, for example, and which might lead to a growing conflict of food versus fuel production. An analysis addressing land availability is published elsewhere [2]. A possible remedy for the problem of common farmland is the use of solar energy which is not confined to arable land but is, in varying concentrations, incident on every part of

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the earth. Regions most interesting for the use of solar energy, due to their relatively high amount of accumulated irradiation, are found mainly in deserts near the equator where no replacement of food crops or settlements is to be expected. 2.2

Solar Drop-in Fuels

2.2.1 Paths to Solar Drop-in Fuels Solar fuels are produced from syngas which is derived electrochemically, photochemically or thermochemically, see Fig. 2, and converted via FT-synthesis to liquid hydrocarbons. In electrochemistry, hydrogen is produced via solar electrolysis, a well-known process, the efficiency of which is limited today to roughly 15%1, partly due to the twofold energy conversion. Photochemistry converts photoenergy into chemical energy via photosynthesis or photocatalysis. Gasification of biomass allows for the direct production of syngas, making use of the natural photosynthetical process which includes CO2 capture from the atmosphere. The conversion efficiency of solar energy to chemical energy stored in the liquid fuel is limited to less than two percent2, mainly due to a rather low photosynthetical conversion efficiency. Concentrated solar energy incident on a solar reactor can be used to drive thermochemical reverse combustion cycle. 2.2.2 Thermochemical Solar Fuels In a two-step solar thermochemical cycle a metal oxide is first reduced at an elevated temperature which causes an oxygen deficiency in the material. This chemical potential can be used in the second step at a lower temperature of about 1000 K to split water and carbon dio-

1

Electrolysis efficiency is assumed to be 70%, solar-toelectrical energy efficiency 20%. The latter value could represent the limit for industrial scale multi-cristalline silicon cells where further improvement is not precluded due to e.g. the use of mono-cristalline material or tandem cells. 2 Photosynthetical efficiency (not accounting for plant growth) is assumed to be 5% [6], gasification efficiency 70% and FT-synthesis efficiency 50%.

xide into syngas. Implementations of this cycle comprise the use of the redox-pairs ZnO/Zn, Fe3O4/FeO [3,4] or CeO2 (non-stoichiometric oxygen deficiency) [5]. The theoretical thermodynamic potential of the latter cycle is 19% without considering heat recuperation [5]. Provided the technical challenges, e.g. scale-up or CO2 capture from air can be solved, the ceria cycle combines suitability, sustainability and scalability with a relatively high efficiency and is therefore a very promising way to produce liquid fuels from solar energy, carbon dioxide and water. The combustion of a solar fuel will produce water and CO2, thereby completing the material balance. The conversion efficiency of the thermochemical path can be subdivided in absorption efficiency ηabsoprtion of the solar reactor and conversion efficiency of heat to work ηheat-to-work. (1) ηabsoprtion is the rate of useful heat absorbed divided by the incident solar power: (2) wherein σ is the Stefan-Boltzmann constant, T the nominal reactor temperature, I the solar irradiation and C the concentration ratio. The maximum amount of work that can be gained from a heat engine working between a high (TH) and a low (TL) temperature is defined by the Carnot efficiency: (3) In Fig. 4 the solar-to-chemical energy conversion efficiency is shown for different concentration ratios of the optical system and in comparison to the ideal Carnot efficiency. For increasing concentration ratios the attainable system efficiency rises, showing peak values at an optimum temperature that shifts towards higher values as the concentration ratio increases. This is due to adverse effects of heat radia-

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tion losses off the reactor and increased efficiency of heat conversion. The overall level of efficiency that can be reached in principle with thermochemical conversion is relatively high and well above 50% for concentration ratios of 1000 or more. Recently, a lab-scale experiment has successfully proven the concept of solar thermochemical syngas production [5], whereas the conversion of syngas to liquid hydrocarbons is a well established process. While still in development and therefore showing low efficiency today, its potential makes this process highly interesting for a mid-term alternative fuel option. 1

Carnot

0.9

C=40000

0.8

C=10000

solar-to-w ork

0.7

C=1000

0.6 0.5

C=100

0.4 0.3

3.2 C=10

0.2 0.1 0

500

1000

1500 2000 Temperature [K]

2500

3000

Fig. 4 Efficiency of thermochemical energy conversion for different solar concentration ratios [7,8].

3 3.1

from air is being investigated and has been shown to work at lab-scale. It is, however, not yet ready to be used in an industrial scale. Alcohols are produced by fermentation of biomass (e.g. sugar cane, corn or cellulosic material) and require in large concentrations modifications of the engine and storage system, because sealings are negatively affected. As mentioned before, the growing of energy crops may lead to a conflict with food production. Electrical energy gained from renewable primary energy sources is also considered to be a sustainable non-dropin energy carrier which is stored in batteries. Benefits arise from the high overall efficiency of fully electric systems. However, the accommodation of batteries in an aircraft requires large redesign efforts. Hydrogen can in principle be produced from any renewable primary energy by electrolytical splitting of water, where solar energy additionally offers the use of photochemical and thermochemical paths.

Renewable Non-Drop-in Fuels Alternative Non-Drop-in Fuels

As opposed to fossil or solar kerosene, nondrop-in fuels are not compatible with today’s transportation fuel systems and therefore cannot be used without major adaptations in infrastructure including fuel production, distribution and storage, and motive power system of the aircraft. Renewably produced hydrogen and carbon dioxide captured from air can be methanized to produce methane that can be fed to motive power systems. Methane can be either stored at high pressure in its gaseous state or at low temperature in its liquid state. CO2 capture

Solar Hydrogen

A much discussed alternative to fossil kerosene is the implementation of hydrogen as a fuel for use in motive power systems such as aircrafts. Hydrogen can either be burned with oxygen from air within an internal combustion engine (ICE, i.e. turbine, engine, etc.) to produce shaft power or electrochemically converted to electricity within a fuel cell to produce shaft power by the use of an electric motor. Several advantages promote the implementation of hydrogen as a fuel for aviation: given a sustainable production through by the use of renewable energy sources, the hydrogen cycle can be CO2-free since water is the only emission. On the other hand, the combustion of fossil fuels releases CO, CO2, SOx, NOx and soot, besides others, some of which have adverse effects on human health. As no carbon is used in the cycle carbon dioxide capture is not required which could be a considerable simplification of the overall process. Hydrogen production and its utilisation in fuel cells is a quite mature technology that has been demonstrated in stationary and mobile applications from mobile computers

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to small aeroplanes (e.g. Boeing, DLR, ENFICAS). The foreseeable risk in its implementation is therefore rather low. Although having been demonstrated as a working technology, the use of hydrogen as an energy carrier in the transportation sector faces technological challenges that need to be overcome for its widespread implementation. Primarily, storage of hydrogen still seems to present an issue regarding its use in mobile systems. In airplanes, a high volumetric energy density is needed to reduce the impact of fuel volume on the whole system. Hydrogen has its highest volumetric energy density in its liquid state at 10.1 MJ/l which is about one third of the kerosene value of about 33 MJ/l. The reduced energy density in connection with the given size of fuel storage tanks causes a reduction in mission length. A major safety issue may be a leakage of the hydrogen tank and hence the formation of an explosive gas mixture. Hydrogen being a non-drop-in fuel requires adaptations of on-board fuel infrastructure and motive power systems of an aircraft which demands financial efforts as well as time for its implementation. Hydrogen replacing a considerable share of fossil kerosene in aviation will therefore most certainly be a long-time perspective due to its implied structural changes in the current transportation system. Paths to produce solar hydrogen are shown in Fig. 3 and are identical with the paths mentioned in Section 2 since hydrogen is a constituent of syngas. 4

Electric Motive Power Systems

The all-electric aircraft is the most radical approach in developing a new long-term energy perspective for aviation. Battery-based power systems provide in-flight zero-emission performance and the ultimate flexibility in the choice of primary energy (solar, wind, hydro, etc.) and potentially minimises the environmental footprint of aviation. Hybrid power systems that generate electricity by fuel cell or turbo-engine-generator systems may significantly reduce harmful in-flight emissions as well.

However, the feasibility of electric flying requires a careful analysis of future electric technologies because of the obvious energy “gap” as shown in Fig. 2. 4.1

Feasibility of Electric Flying

The feasibility perspectives and scaling properties of fully or hybrid electric motive power systems are governed by scientific developments outside the field of aviation and the following three fundamental principles as proposed by Bauhaus Luftfahrt [9,10]: 1. Exergy concept: the usefulness of an energy carrier for aviation is determined by its gravimetric and volumetric exergy density and not by its energy content 2. Specific power and Ragone metrics: the usefulness of an energy carrier in combination with a power converter is determined by its combined power density and exergy density (Ragone metrics) 3. Hybridisation degree of freedom: two or more energy storage and/or conversion devices that each are inadequate for electric flying may constitute an enabling power system when combined to a hybrid system. Exergy is the part of the energy that can be entirely transformed into useful work. The net exergy of combustion engines is fundamentally limited in contrast to fully electric power systems. As shown in Fig. 2 the exergy gap between kerosene and batteries is greatly reduces compared to the energy gap. The next principle indicates that the feasibility of electric flying is also determined by the specific power requirements for take-off and climb, and not just by the exergy density. The hybridisation degree of freedom enables feasible solutions for electric flying by combining e.g. high power batteries and a fuel cell system. At first, the system boundaries need to be defined for a meaningful comparison of different motive power systems, based on thermal and non-thermal energy conversion. The green boxes shown in Fig. 5 describe a combined enrgy storage and conversion system that delivers electric energy. The red dashed boxes describe

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Fuel

System complexity

ICE

Gen.

sufficient power however with insufficient range. 10

Relative Power Density

energy conversion subsystems with fuel input and electric power output, i.e. energy conversion devices such as fuel cells or turbo-engines. These devices are inherently characterised by specific power densities, whereas the combined systems are characterised by specific power and specific exergy densities as required for the feasibility analysis by the second principle. The performance scaling of these subsystems is one key to proper power system design.

C1

C2

D

1

A

0.1

System boundary of fuel-to-electricity conversion module

0.01

System boundary of conversion module

B 0.1 1 Relative Exergy Density

10

Fuel Fuel Cell BoP

fuel flow mechanical power electric power

Fig. 5 System boundaries for comparison of fundamentally different energy storage and power conversion systems. (ICE: internal combustion engine; Gen.: el. generator; BoP: balance of plant)

Figure 5 shows that the ICE and fuel cell systems allow for an independent scaling of power level and energy content. Batteries incorporate both characteristics in one device. Hence either characteristic defines the necessary battery size. The system complexity increases with the number of components and may have negative impact on maintenance and overhaul. As shown in Fig. 5 and discussed in Section 4.3, the usefulness of the battery concept strongly depends on advances in material science and development. The feasibility assessment of electric flying therefore requires an exergy and power metric that puts all energy carriers into a proper perspective with respect to conventional aircraft fuel, i.e. kerosene. Different motive power systems can be easily characterised and compared within the Ragone diagram, Fig. 6. Power systems in the quadrants A and B provide insufficient power for take-off. Only power systems in the quadrants D and C2 enable electric flying over reasonable ranges. Subquadrant C1 offers

Fig. 6 Visualisation of the second principle for the feasibility assessment of electric flying. A hydrogen fuel cell system in the performance quadrants A and B provides insufficient power, whereas a future battery system provides the required power in quadrant C2 with a reduced range due to lower exergy density.

Figure 7 shows a Ragone diagram for several battery technologies and representative data points for a typical conventional, an improved conventional long range business jet. Here the energy gap of Fig. 2 translates into a gap in operating range: the reduction in range is in first approximation proportional to the reduced exergy density of the batteries. Applying the Breguet range equation for the battery-powered (Rel), Eq. 4, and the conventional powered aircraft (Rc), Eq. 5, L

Rel

L

Rc

P

g

D

g

P

B E,m

D

TE

el

K E,m

ln

m Battery MTOM M

1 mFuel 1 MTOM

(4)

(5)

and replacing the fuel mass of the conventional aircraft by batteries in this preliminary assessment, the range of the battery-powered aircraft settles at below 555 km (300 NM) for an exergy density of 200 Wh/kg. This is roughly 5% of the original range according to a battery technology level of 2010.

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105

Specific Power [W/kg]

104 High Power

555 km

20% efficiency gain

103

11390 km

Pb

Lithium

ATR72 is given as well. As expected, the range increases with increasing specific energy of the battery. Interestingly to attain a range of 1000 km3 a specific energy of 800 Wh/kg, i.e. four times the state-of-the-art commercial battery capacity, is required.

102

10

High Energy NiMH NaCl NiCd

4500

102

103 Specific Exergy [Wh/kg]

3500

104

Breguet Range [km]

1 10

Kerosene 200 Wh/kg 400 Wh/kg 800 Wh/kg 1000 Wh/kg 1500 Wh/kg

4000

New materials and electrode structures

Fig. 7 Ragone diagram of selected battery technologies, prospects of lithium batteries and performance characteristics of a conventional (magenta dot; range of 11390 km (6150 NM)) and battery powered (green dot; range of less than 555 km (300 NM); technology level 2010) long-range business aircraft.

3000 2500 2000 1500 1000 500 0

In the equations above, L/D is the lift-over-drag ratio, ηP is the propulsor efficiency, ηel is the efficiency of the electric system comprising the battery, the power management and distribution system (PMAD) and the electric motor, ηTE is the efficiency of the turbo-engine and E,B m and K E,m

are the specific energies of the batteries and kerosene, respectively. mBattery and mFuel is the mass of the batteries and kerosene and MTOM is the maximum take-off mass. Depicted is also an assumed efficiency gain of 20% for the conventional aircraft efficiency through improved aerodynamics. Due to the reduced demand of fuel for the fixed range of 11390 km (6150 NM) and the higher overall efficiency of 20%, the exergy density requirement for the motive power system of the conventional jet is slightly relaxed. However the most significant contribution to closing the range gap is expected to comes from advances in battery technology as shown by the light blue area in Fig. 7.

ATR 72

0

0.05

0.1

0.15 0.2 0.25 mFuel,Battery /MTOW

0.3

0.35

0.4

Fig. 8 Breguet range of a conventional turbo-engine powered ATR (black) and fully electric battery powered range estimates.

4.2

Motive Power System Architectures

The integration of fully or hybrid electric motive power systems allows for the optimisation of e.g. aerodynamic and propulsive efficiency, control dynamics, service accessibility as well as mass and load distribution. The new degrees of freedom arise due to the separation of power generation and power consumption compared to the traditional arrangement with turbo-engines under the wings. From a wide combinatorial variety of energy storage and power generation subsystems, Figure 9 gives an overview of a few selected motive power systems.

The Breguet range of a conventional and a battery-powered ATR72 is plotted versus the fuel mass-to-MTOM in Fig. 8. Different specific energies for the batteries are assumed and the corresponding ratio for the conventional 3

99% of all ATR72 flights are at great circles distances below 1050 km, whereas 90% are below 600 km. Most flights are in the great circle distances of 251-300 km (16% of all ATR72 flights) Source: Official Airline Guide (OAG).

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4.3

Key Technologies

The key technologies for electric flying are batteries, fuel cells and electric motors. a

Gen. PMAD

b

Motor

ICE Motor

PMAD

c

Fuel Cell PMAD

d

Motor

E

PMAD

Motor

P

Fuel flow

Mech. power

Electr. power

Fig. 9 Schematic depiction of selected hybrid motive power systems out of a combinatorial variety of power generator subsystems.

4.3.1 Lithium Batteries Lithium batteries offer the highest available specific energy among all battery technologies known today and are therefore at the focus of current research. Crucial battery components with significant innovation potential are the electrodes, separators and electrolytes. In the following we discuss selected positive and negative electrode materials, see Tables 2 and 3, an elaborated evaluation of electrode materials will be published in a separate article as it would exceed the scope of this article [11]. LiCoO2, LiMn2O4 and LiFePO4 are commonly used as positive electrode materials for commercial lithium batteries and in combination with the primary negative electrode material, LiC6, commercial batteries with a specific energy of up to 250-280 Wh/kg can be realised today. Li2S and LiTiS2 are proposed as new positive electrode materials as they offer very high capacities (Li2S) [12,13,14] and discharg-

ing/charging rates (LiTiS2) [15]. But these electrode materials especially have to overcome lifetime and mass production issues. Due to safety and economical reasons LiC6, basic material is graphite, is the primary negative electrode material for lithium batteries instead of metallic lithium although it exhibits a significant reduced capacity. LiC6 offers reasonable lifetimes and acceptable capacities at low costs. Nevertheless, in order to increase a battery’s capacity LiSi alloys are in the focus of research due to the multi-fold higher capacity prospects of this electrode material. Step-change advancements in nano-technology yielded in ten-fold higher capacities compared to graphite (LiC6) and have been already demonstrated at lab-scale [16,17]. For a quick recharge of the batteries or temporary high power demands LiFePO4 seems to be a proper positive electrode material. Charging rates of up to 400C have been demonstrated with this electrode material [18] allowing for a complete charge of the electrode in 400-1 hour. These developments on the electrode materials level significantly reduce the exergy difference between today’s battery technologies and kerosene fuelled propulsion systems, thus bringing electric flying closer to realization than generally assumed. Table 2 Positive electrode materials for lithium batteries. Capacities refer to the charged state. Material LiCoO2 LiMn2O4 LiFePO4 Li2S LiTiS2

Eq. Voltage vs. Li/Li+ 3.9 3.9 3.5 2.2 1.96

Rev. Range 0.5 1 0.95 2 1

Capacity Ah/kg 147 154 169 1671 239

Table 3 Negative electrode materials for lithium batteries. Capacities refer to the charged state. Material LiC6 Lisolid Li4.4Si

Eq. Voltage vs. Li/Li+ 0.1 0.1 0.3

Rev. Range 1 1 4.4

Capacity Ah/kg 339 3861 2011

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4.3.2 Electric Motors Conventional high torque or high speed electric motors have typical specific powers around 3-8 kW/kg today. Hybrid or high-temperature superconducting electric motors and generators exhibit specific powers around 9 kW/kg and show prospects for specific powers of well above 15 kW/kg including the cryo-coolers, hence outperforming state-of-the-art turboengines [19]. Thus, these devices could theoretically replace turbo-engines without mass penalties. 4.3.3 Fuel Cells Renewable hydrogen fed to polymer electrolyte fuel cells (PEFC) provide a sustainable way of supplying electric power and heat for onboard systems. PEFC exhibit the highest specific powers among fuel cells at 1.2 kW/kg on stack level today. The advantage of fuel cell systems over batteries is the independent scalability of the amount of energy stored in form of fuel, independent from the level of power generation in the stack. New catalysts still allow for performance enhancements, decreased costs as well as for higher impurity tolerances, which for example increases durability and stability [20,21]. Alternative membrane materials allow for altered operating conditions compared to state-of-the-art membrane materials [22,23], i.e. lower gas humidification needs which in turn reduces additional subsystems and power demand. Although solid oxide fuel cells (SOFC) may operate on kerosene, their low specific power implies significant mass penalty which is one of the main drawbacks of this technology. 5

Conclusion

The analysis of the energy options shows that (1) solar fuels are able to overcome known sustainability and/or scalability limitations of bioto-liquid and other drop-in bio-fuels, that (2) the (non-drop-in) hydrogen fuel perspective has inherent limitations that are either solved by feeding into the solar hydrocarbon fuel process or by serving the electric propulsion paradigm shift via PEM fuel cells, and that (3) the all-electric regional aircraft is well within the physical regime of feasibility and potentially closer to re-

alization than generally assumed, mainly driven by a few key advancements in material science and nanotechnology. References [1] Group on Int. Aviation and Climate Change, “Agende Item 2: Review of Aviation Emissions Related Activities Within ICAO and Internationally: U.S. Fuel Trend Analysis and Comparison to GIACC/4-IP/1”, GIACC/4-IP/12, 4th Meeting, Montreal, Canada, May 25-28, 2009. [2] Riegel F. and Steinsdörfer J., “Bioenergy in Aviation: The Question of Land Availability, Yields and True Sustainability”, 3rd CEAS Air&Space Conference, Venice, 2011. [3] Stamatiou A., Loutzenheiser P.G. and Steinfeld A., “Solar syngas production via H2O/CO2-splitting thermochemical cycles with Zn/ZnO and FeO/Fe3O4 redox reactions”, Chem. Mat., Vol. 22, 2010, pp. 851-859. [4] Loutzenheiser P.G., Galvez M.E., Hischier I., Graf A. and Steinfeld A., “CO2 splitting in an aerosol flow reactor via the two-step Zn/ZnO solar thermochemical cycle”, Chem. Eng. Sci., Vol. 65, No. 5, 2010, pp. 1855-1864. [5] Chueh W.C. et al., “High-Flux Solar-Driven Thermochemical Dissociation of CO2 and H2O Using Nonstoichiometric Ceria”, Science, Vol. 330, No. 6012, 2010, pp. 1797-1801. [6] Hall D.O. and Rao K.K., “Photosynthesis”, 6th ed., Cambridge University Press, Cambridge, 1999. [7] Meier A. and Sattler C., “Solar fuels from concentrated sunlight”, IEA SolarPACES, 2009. [8] Fletcher E.A. and Moen R.L., “Hydrogen and Oxygen from Water”, Science, Vol. 197, No. 4308, 1977, pp. 1050-1056. [9] Sizmann A., “Neue Energieperspektiven der Luftfahrt”, Fuelling the Climate 2010, Hamburg, Germany, 2010, http://www.hawhamburg.de/klimaschutz_in_der_luftfahrt.html [10] Kuhn H., Sizmann A., “Progress and Perspectives of Electric Air Transport”, ICAS 2012, Brisbane, Australia, 2012, accepted [11] Kuhn H., to be submitted [12] Yang Y. et al., “New Nanostructured Li2S/Silicon Rechargeable Battery with High Specific Energy”, Nano Lett., Vol. 10, No. 4, 2010, pp. 1486-1491. [13] Hassoun J. and Scrosati B., “A High-Performance Polymer Tin Sulfur Lithium Ion Battery”, Angew. Chem. Int. Edit., Vol. 122, No. 13, 2010, pp. 2421-2424. [14] Hassoun J., Panero S., Reale P. and Scrosati B., “A New Type of Lithium-ion Battery Based on Tin Electroplated Negative Electrodes”, Int. J. Electrochem. Sci., Vol. 1, No. 3, 2006, pp. 110-121. [15] Zhou S., Liu X. and Wang D., “Si/TiSi2 Heteronanostructures as High-Capacity Anode

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