Assessment of a Fuel Cell Powered Air Taxi in Urban Flight Conditions

M. Husemann1, C. Glaser2, E. Stumpf3 Institute of Aerospace Systems (ILR), RWTH Aachen University, 52062 Aachen, Germany

This paper presents a first estimation of potential impacts on the flight operations of small air taxis in urban areas using a fuel cell instead of a battery as an energy resource. The expanding application of electric components is seen as a possibility to reduce operating costs and environmental impacts in the form of noise and pollutant emissions due to lower consumption of fossil fuels. The majority of such designs have so far been based on the use of (not yet) sufficiently efficient batteries. Long charging times, possible overheating or a limited service life in the form of limited charging cycles pose a challenge to the development of such aircraft. Parameter studies are conducted to identify possible advantages of using a fuel cell. In particular, the range and payload capacity ist investigated and first effects on the cost structure will be presented. The evaluation of the studies shows that the use of fuel cells enables significantly longer ranges than the use of batteries. In addition, the range potential gained can be used, for example, to transport more payload over the same distance. Furthermore, the technological maturity in the form of the individual energy density and the weight of the powertrain unit has a significant effect on the cost structure. Fuel cells therefore have a high potential for applications in the mobility sector, but still require extensive research efforts.

I. Introduction Increasing traffic volume due to advanced technologies and growing mobility demand often leads to heavy traffic and circumstantial routing, especially in metropolitan areas. Ambitious goals, for example in the form of a door-to- door journey within Europe of maximum four hours set by "Flightpath 2050" and restrictive environmental requirements, are additionally increasing the pressure to develop efficient mobility technologies and solutions. In passenger and freight transportation, however, air transportation was largely omitted as the third dimension for short distances of up to 500 km. Ground-based transportation infrastructures has been offering economic and organizational advantages in terms of cost and time factors and were clearly superior to air traffic on short routes. Also, air transportation is affected by additional, time-consuming access to and from airports, which are often built outside urban areas for reasons of noise and environmental protection. With the introduction of low-noise and electric powertrains and the continuous technological progress of vertical (VTOL) and short take-off and landing (STOL) aircraft, passenger transportation in the short-haul sector is becoming increasingly important. On-demand air vehicles, also called air taxis, have the potential to overcome previous restrictions in the mobility sector and to fundamentally restructure regional and urban passenger transportation. Depending on the mission and its individual topographical (e.g., bay areas or hills) and architectonic (e.g., high-rise buildings and narrow streets) circumstances, different aircraft types will be appropriate: VTOL concepts are particularly suitable for inner-city operations and feeder traffic to airports, whereas fixed-wing aircraft are largely used for longer distances in regional air traffic. Research and development of such vehicles has been mainly focused on the implementation of high-performance electric motors powered by sufficiently large batteries. Long charging times, possible overheating of the energy storage and propulsion units as well as limited energy density are still an obstacle to the successful implementation of a smooth on-demand air transportation system. The application of fuel cells, which have mainly been used and tested in the automotive industry, offers a serious alternative to battery systems

1 Research Associate and Ph.D. candidate, Institute of Aerospace Systems (ILR), RWTH Aachen University, Wuellnerstrasse 7, 52062 Aachen, Germany, [email protected]. 2 Graduate Student, Institute of Aerospace Systems (ILR), RWTH Aachen University, Wuellnerstrasse 7, 52062 Aachen, Germany, [email protected]. 3 Head of Institute, Institute of Aerospace Systems (ILR), RWTH Aachen University, Wuellnerstrasse 7, 52062 Aachen, Germany, [email protected].

1 due to short refueling times and potentially longer ranges. The use in air transportation not only facilitates the avoidance of many technical complexities as mentioned before, but also allows a more flexible handling and thus the operation of a cheaper on-demand air transportation system. After a short overview of the central properties of fuel cells and their functionalities (see Section II), the reference aircraft Vahana by A³ as well as its design and performance parameters will be presented (see Section III). By carrying out parameter studies, central effects on flight operations will be investigated if a comparable fuel cell is implemented as main energy source instead of a battery, while small batteries are retained for possible power peaks (see Section IV). Conspicuous changes and important influences will be identified and analyzed with regard to associated advantages. Finally, a short assessment of possible effects of economic parameters is made.

II. The Concept of Fuel Cells The first practical concept of a fuel cell was introduced in 1932 in the United Kingdom by F.T. Bacon leading to a 1.5 kW fuel cell version used during the NASA Apollo program that started in 1961. Although current fuel cell concepts differ both in weight and available power, it was the successful application at that time that resulted in a massive growth in fuel cell research. [19] Today’s applications of fuel cells include civil transportation (by sea, land and air), military purposes and space missions. [4]

A. Fundamentals of Fuel Cells The composition and functional principle of fuel cells are similar to those of batteries since chemical energy is converted into electrical energy for further usage. In contrast to conventional technologies such as combustion engines, the chemical energy – stored in hydrogen – is transformed directly into electricity without generating heat first. [11] A fuel cell generally consists of an anode and a cathode which are separated by a layer of electrolyte. Once the cathode is exposed to oxygen and the anode to hydrogen, a circuit is formed externally as depicted in Fig. 1. For this reason, fuel cells are also classified as galvanic cells. Additional information on the general structure can be found in [3]. As the detailed design and structure of the electrodes is not the subject of this paper, it will not be discussed any further. To ensure permeability towards gas or liquid, both electrodes are designed sufficiently porous. Unlike anode and cathode, the electrolyte is meant to be impermeable. The overall chemical reaction occurring in fuel cells can be described as: [4]

1 퐻 + 푂 → 퐻 푂. (2.1) 2 2 2 2

More specifically, the anode oxidizes hydrogen according to the following equation; the protons migrate to the cathode, whereas the electrons enter the external circuit:

+ − 퐻2 → 2퐻 + 2푒 . (2.2)

Oxygen from ambient air is split into double-negatively charged oxygen ions (O2-):

− 2− 푂2 + 4푒 → 2푂 . (2.3)

Finally, oxygen ions and protons form water at the cathode according to:

2− + 푂 + 2퐻 → 퐻2푂. (2.4)

The electrochemical reactions taking place at both electrodes determine their equilibrium potential. Possible differences in potentials result in a fuel cell voltage of 1.23 V at 25 °C conditions. [4] To increase voltage, several individual fuel cells need to be interconnected to provide the total power required for the actual application. Also, since fuel cells are more suitable for a constant output of electrical power and cannot react as quickly as conventional combustion engines or batteries to varying power requirements, it is mostly necessary to use an additional battery to buffer possible power peaks. Other crucial design parameters such as space, vehicle dimensions and weight must be taken into consideration when integrating appropriate fuel cells.

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Fig. 1: Illustration of a hydrogen/ oxygen fuel cell setup (proton exchange membrane fuel cell, PEMFC) [4] The definition of following two characteristics (volumetric and gravimetric power) allows a comparison between fuel cells and other energy conversion systems. Volumetric power density is defined as [12]:

푝표푤푒푟 푝표푤푒푟 푑푒푛푠𝑖푡푦 = , (2.5) 푣표푙푢푚푒 whereas gravimetric power density (also defined as specific power) gives:

푝표푤푒푟 푠푝푒푐𝑖푓𝑖푐 푝표푤푒푟 = . (2.6) 푚푎푠푠

State of the art Proton-Exchange Membrane (PEM) fuel cells in aeronautical applications can achieve about 1.5- 1.6 kWh/kg in the range of 100 kW [20,7] but are expected to increase up to 8 kWh/kg in the near future. [7] For comparison, the Honeywell TPE 331-10 turboprop engine has a specific power of 4.38 kW/kg, the Pratt & Whitney PW545B turbofan operates at 10.86 kWh/kg. [5] The low specific power of fuel cells is one of the main disadvantages for high power applications such as long-range flights. Therefore, hybrid propulsion systems (combination of different propulsion types such as batteries or fuel cells with combustion engines) are prioritized for those applications. [5] However, since only ranges from 10-100 km with an average power demand of several hundreds of kW in urban applications are subject of this paper, hybrid options are not further discussed. Fuel cells provide several advantages making them a promising alternative in propulsion systems. Most notably, they produce very low greenhouse emissions, a characteristic that is becoming more and more important in terms of climate change due to environmental restrictions and regulatory specifications. In fact, only water is emitted as a result of the chemical process, yet a substantial part of the greenhouse effect. The production and distribution of needed hydrogen causes greenhouse emissions which must be considered when a holistic comparison with other propulsion types is made. [12] Fuel cells are not directly dependent on fossil fuels and have a significant higher efficiency compared to conventional jet engines. Especially their high energy density makes them superior to battery cells. [9] Also, energy is produced as long as fuel is supplied, which means that no recharging is required. The refueling process of fuel cells takes significantly less time than recharging a battery. [5] In space applications, produced water is used as additional water supply. [19] Fuel cells can be applied in a wide range of applications from microwatts to megawatts due to their modularity and are highly reliable due to few moving parts and overall simplicity. [12]

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B. Types of Fuel Cells Cell types are generally classified by the utilized electrolyte. The only type not following this classification approach is the Direct Methanol Fuel Cell (DMFC) where methanol as fuel is exposed to the anode. [4] A second classification can be done by sorting the types of fuel cells in terms of their operating temperature. This leads to low- temperature fuel cells with operating temperatures between 30-220 °C and high-temperature cells operating between 500-1000 °C. Currently six classes of fuel cells are considered feasible and most promising for mobility options and vehicle designs. All types of fuel cells and their respective applications are listed in Table 1. [12]

Table 1 Types of fuel cells [12]

Fuel Cell Type Mobile Ion Operating Temperature Applications and Remarks Alkaline (AFC) 푂퐻− 50-200 °C Space vehicles + Proton exchange membrane (PEMFC) 퐻 30-100 °C Vehicles and mobile applications Direct methanol (DMFC) 퐻+ 20-90 °C Low power portable electronic systems Phosphoric acid (PAFC) 퐻+ ~220 °C Small power plants 2− Molten carbonate (MCFC) 퐶푂3 ~650 °C Medium-large power plants 2− Solid oxide (SOFC) 푂 500-1000 °C Multi MW power plants

For mobile applications such as vehicles and small aircraft concepts, PEMFC are considered most suitable due to several reasons. [22,4,23] Those advantages include characteristics such as: [4]  lightweight  simple design  low operating temperature  higher power density  high energy conversion efficiency. The PEMFC is also used in the Toyota MIRAI [23] as well as in different UAV concepts. [16] Consequently, the proton exchange membrane fuel cell is chosen as viable option for the objective of this paper and all other types are not further considered.

C. Energy Storage Methods Although hydrogen shows one of the highest gravimetric densities (energy per weight) of all elements, it is the low volumetric energy density (energy per volume) that calls for special storage technologies since the overall weight and available space is crucial in air vehicles. Hydrogen can be stored either under pressure, liquid or in hybrids. [20] In latter options, the hydrogen is stored in metals such as magnesia. Due to high energetic effort and system weight, metal hybrid storage only has been used in research. [20] To reach a liquid state, hydrogen must be cooled down to 21 Kelvin. In this state, the volumetric energy density is significantly higher, but because of the energy necessary to cool down the hydrogen and the boil-off phenomena (due to imperfect isolation, the liquid hydrogen heats up and goes back into gaseous state), liquid hydrogen storage methods are generally not considered in the field of terrestrial vehicles. [20] As long as hydrogen consumption is comparatively high, however, this problem is not predominant. It should be noted, that in commercial aeronautical applications, liquid hydrogen storage is subject to promising research [21]. In the field of vehicles and small aircraft high pressure storage is considered most viable. [23, 20] Hydrogen is stored at 350-700 bar in carbon fiber tanks and, hence, the volumetric energy density can be significantly improved. [23, 20] An overview of the most common energy storage systems is given in Fig. 2. [6] It must be mentioned, that both liquid and pressure storage of hydrogen is far superior compared to batteries.

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Volume specific energy V*, Wh/liter Volumeenergy V*, specific

Mass specific energy E*, Wh/kg

Fig. 2: Energy characteristics of energy storage systems [6]

D. Equivalent Specific Energy In the previous sections, energy storage and energy conversion characteristics have been considered separately. In reality, the electric motor is directly supplied with energy from the battery, whereas a fuel cell requires several components such as a fuel tank, liquid or gaseous hydrogen as well as the actual fuel cell unit in order to generate electric energy. Such a system is called a fuel cell system. [6] Other components are also included in a fuel cell system (e.g., pressure valves, sensors, inverter), but for the sake of simplicity these additional weights are neglected for further examinations.

H2 gas FC H2 η=55% (5.5 kg + 95.5 kg) (50 kg)

liquid fuel H2 gas Reformer FC Kerosene η=75% η=55% (20.5 kg + 12 kg) (33.3 kg) (50 kg)

liquid fuel shaft I/C engine Generator Kerosene η=35% η=95% (22.5 kg + 15 kg) (25 kg) (25 kg)

Battery (556 kg)

m, kg Fig.3: Specific energy of electric power generation systems at 50 kW [6]

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To be fully capable of comparison, it is necessary to transform the different parameters into one equivalent specific energy as shown in Fig. 3. [6] However, the superiority of the fuel cell over the battery in terms of specific energy is still significant. The fuel cell is only surpassed by a kerosene engine generator, which is not subject to discussion in this paper.

III. Project Vahana Vahana is the first project of A³ (“A-cubed”) founded by Airbus Ventures located in Silicon Valley and is a self- piloted vertical take-off and landing (VTOL) all electric personal air vehicle primary for urban mobility options. [1,18] In the preliminary design phase two configurations – a tilt wing and a helicopter configuration – were considered, albeit only the tilt-wing option has been developed and tested so far. A “productizable demonstrator” is planned to be ready by 2020. [17]

A. The Design of the Vahana Concept Figure 4 shows the design and dimensions of the tilt-wing configuration according to the Vahana team prototyping session presented on June 8th, 2017. [17]

Fig. 4: Dimensions of the tilt-wing Vahana configuration [18] On January 31st, 2018 the first full-scale Vahana tilt wing aircraft (labeled Alpha One) took off in Pendleton, Oregon. The test flight lasted 53 seconds and consisted of hovering at a height of about five meters. Alpha One’s primary battery system used 8% of its total energy while each motor consumed 53 kW. [17] The dimensions of Alpha One are listed in Table 2. [18] Table 2 Alpha One characteristics [18]

Pax capacity 1 Fuselage length 5.7 m Overall height 2.81 m Wingspan 6.25 m Empty weight 475 kg MTOW 815 kg Payload 90 kg

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The evaluation of first studies and simulations show a cost advantage of the tilt-wing-configuration especially on longer distances. This is mainly due to the fact, that the wings and not the rotors are used to generate lift during cruise flights. Since the business model of the prototype initially aimed at ranges above 100 km, the Vahana engineering team decided to further develop this option. [17] The corresponding range vs. cost study is illustrated in Fig. 5, where the size of the dots represents the respective take-off mass. [17] It is obvious that in terms of direct operating costs (DOC), the helicopter is advantageous on shorter ranges, whereas the tilt-wing configuration surpasses at ranges above 80 km. Therefore, only the helicopter configuration is considered for further studies and discussions since the objective of this paper is the analysis of urban ranges below 100 km.

3.5 Electric Helicopter Electric Tilt-Wing Multicopter 3.0

2.5

2.0

DOC, DOC, $/km 1.5

1.0

0.5

0 0 50 100 150 200 Range, km

Fig. 5: Direct operating costs versus range of both Vahana configurations [3]

B. Multidisciplinary Design Optimization Environment Team Vahana applies a multidisciplinary design optimization approach coded in Matlab aiming for minimized direct operating costs (DOC). The underlying DOC model is based on the model used by the Air Transport Association (ATA). [17] The link between all design parameters is shown in Fig. 6. [17] Design variables such as range, payload, cruise speed, rotor radius, battery mass, motor mass and takeoff mass are optimized while DOC are minimized, and a number of constraints are considered. Global assumptions regarding energy densities, power and maintenance costs can easily be adjusted, making the MDO-approach highly customizable. Team Vahana decided to provide the source code as open source making it available to the public. [17] This allows the authors of this paper to choose the Vahana- Code in Matlab as simulation tool.

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Fig. 6: Data flow of the Vahana source code [3]

IV. Parametric Studies All necessary information and preparations for the successful implementation of parameter studies are explained in the following sections. First, the segmentation of the mission study and the basic assumptions of the electrical configuration are described. Parameters affected by the replacement of the powertrain unit are identified. By carrying out parameter studies, initial quantitative conclusions can then be drawn about the positive and negative effects of such modification.

A. Simulation Approach Since the objective of this paper is to compare the technical performance of a battery and fuel cell propulsion system, it is first necessary to identify all of those parameters that have a relevant influence on flight operations. For this purpose, a sensitivity study is carried out to assess the actual influence of all selected parameters on the range capacity of the vehicle (see section B). The range is selected as the output parameter. This decision provides a simple overview of the overall performance. The results of the first sensitivity study are analyzed and conclusions drawn so that an additional study can be carried out on payload capacity (see Section C).

1. Mission Profile First, an appropriate design mission needs to be defined. As the project Vahana is still at an early stage, the mission profile is kept simple to minimize computational effort. It consists of a 90 seconds vertical take-off and hover period including transition followed by an adjustable cruise distance (“X” km). The final segment again is a 90 seconds transition and vertical landing segment. An additional buffer time of 20 minutes (3 min hover + transition time and 17 min additional buffer time) is added to handle any emergency situation and to be able to meet FAR regulations in the long term. The whole mission profile is illustrated in Fig. 7. [17]

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Cruise ‘‘X“ km

90 sec 90 sec hover + transition transition + hover + 20 minute buffer: . 3 min hover + transition . 17 min at min. power

Fig. 7: Design mission [17] 2. Electric Baseline Configuration General assumptions of the battery configuration are based on a forecast predicting values of about three years in the future, that is 2021, when Vahana is planning to enter production. Performance values and efficiencies of the electric powertrain unit (battery and motor) as well as the dimensions of the vehicle are shown in the table on the left. All important information in terms of operating costs due to daily flight operations, such as replacement of individual wearing parts and personnel costs, are listed in table in the middle. Besides, material and production costs can be found in the table on the right (see Table 3). [17]

Table 3 Assumptions in electric helicopter configuration [17]

Parameter Value Unit Cost designation Value Unit Battery specific energy 230 Wh/kg Material 220 $/kg Motor specific power 5 kW/kg Battery 161 $/kg Depth of discharge 95 % Motor 150 $/kg Fuselage width 1 m Servo 800 $/Piece Fuselage length 5 m Avionics 30,000 $ Fuselage height 1.6 m Insurance 6.5 % of aq. Cost Gearbox efficiency 98 % Facility rental 2 $/ft²/month Motor efficiency 85 % Electricity 0.12 $/kWh

Maintenance Value Unit Battery replacement 2000 Cycles Motor replacement 6000 flight hours Servo replacement 6000 flight hours Human cost 60 $/h Periodic maintenance 0.05 MHR/flight hour

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3. Parameters Affected by Fuel Cell Replacement In a next step, each parameter of the Vahana helicopter configuration affected by a propulsion modification is identified and adjusted to simulate the application of a fuel cell. Those parameters can be clustered based on hardware and cost factors (see Table 3). Since the original source code has been kept rather simple, the implementation of an equivalent fuel cell is carried out by adjusting the performance values of the previous battery. Based on the values in Table 3, these are the following parameters:  battery specific energy,  depth of discharge. The latter parameter was chosen since a fuel cell system, unlike a battery system, can use 100% of its available energy. It should be noted that the actual engine remains unchanged, since associated data can be adopted and only those performance values are examined that are influenced by an exchange of the energy unit (fuel cell or battery). The same assumption also applies to all servo components and efficiencies. Besides, it is assumed that avionics, all insurances, leasing and personnel costs are not affected by the implementation of a fuel cell system. It is expected that battery costs per mass unit (kg) will change as the price of the fuel cell system will be different. It is also expected that the replacement cycle of the fuel cell and battery system will change due to a longer life expectancy of fuel cells. As shown in Figure 3, the fuel cell system and battery characteristics can be effectively compared based on specific energy. Therefore, for the sake of simplicity, the specific energy is selected as the most important single parameter for the following sensitivity study. It should be noted that costs and replacement cycles are initially neglected, but will be discussed later in Section E.

B. Density versus Range Sensitivity Study In this study, the payload is kept constant and all default parameters listed in Table 3 are applied with the exception of energy density. The maximum possible range interval is shown, while the specific energy density varies from 150 Wh/kg to 1000 Wh/kg. The results of the sensitivity study are depicted in Fig. 8. It should be noted that the range increases almost linearly with increasing energy density and that both parameters show a strong correlation since there are no real weight effects during the flight due to empty and lighter tanks. In addition to the study results, three major density values are marked in Fig. 8. First, the grey dashed line at 230 Wh/kg characterizes the standard density used for the original Vahana configuration during the preliminary design phase. The orange line marks the 400 Wh/kg limit, which represents the future energy density of batteries expected to be achieved between 2020 and 2030. The blue vertical line shows the specific energy achieved by PEMFC, as shown in Figure 3. [6] Also, this value is expected to increase in the future. The sensitivity study shows the significant range advantage a fuel cell over a battery, even if today’s performance of fuel cells is compared to hypothetic future performances of batteries. This observation is associated with those made in the field of UAV, where it is possible to extend flight time by seven times using fuel cells instead of batteries. [8] 800 Vahana 700

600 battery specific energy till 2020/30 500

400 equivalent 300 specific energy PEMFC maximum range, km range, maximum 200

100

0

energy density, Wh/kg

Fig. 8: Density versus maximum range

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C. Density versus Payload Sensitivity Study As already described in Section B, a fuel cell system has the potential to significantly increase the possible range in flight operations. For the use of air taxis in urban areas only a limited range is required. Therefore, the previously observed range advantage is considered in the next step by using the additional range potential to increase the maximum payload. Based on the previous results, a sensitivity study will be performed to investigate the relationship between available energy density and maximum payload. For this purpose, the maximum range is limited to four different values (60 km, 80 km, 100 km, 120 km), which seem to make sense for applications in urban areas. Similar to the procedure in section B, the results are shown in Figure 9, where relevant density values both for current standard batteries and fuel cells are highlighted. As the capacity of batteries is expected to increase in the near future, a further mark at 400 Wh/kg is shown. In the preliminary design of the battery-operated standard configuration, a payload of approx. 250 lbs was assumed, which corresponds to the equivalent of a single passenger including luggage. Depending on the available energy density, the possible payload in this study varies between 200 lbs and 1500 lbs, the latter corresponds to a vehicle for a total of six people. Since the design of operating concepts is based on a limited number of persons, like the number of persons in ground traffic, the maximum payload is limited to 1500 lbs. The buffer time was increased from 20 min to a total of 40 min (see Fig. 7), as more energy is required for holding patterns or in case of emergencies due to the higher vehicle mass. A positive, almost linear correlation between the possible payload capacity and the required energy density is recognizable (see Fig. 9). Two main aspects can be identified: First, a higher energy density results in a higher maximum payload capacity for a given range. Also, if a longer range is assumed for the study mission, then a higher energy density for the transport of a certain payload becomes even more apparent. Both findings underline the sensitivity and thus the relevance of the design parameters range and payload capacity in the preliminary design process. However, it should be mentioned that the increase in payload and the resulting increase in vehicle dimensions also influences the lift over drag ratio (aerodynamic quality). In the present study, this effect was not taken into account and, hence, should be investigated in detail in further studies. It is assumed that a significant increase in the payload capacity of the vehicle leads to a reduction in operating costs and thus in ticket prices. 1600 1400 1200

1000 battery specific energy 800 till 2020/30 600 equivalent 400 specific energy

PEMFC maximum payload, lb payload, maximum 200 0

energy density, Wh/kg

60km 80km 100km 120km

Fig. 9: Density versus payload sensitivity study

D. Consideration of Replacement Cycles and Costs In the previous sensitivity studies, only the energy density and its effect on other performance parameters have been considered. In order to allow a more comprehensive evaluation of fuel cells and their application to small air vehicles, the durability of components and operating costs are also crucial parameters to be considered. [14] After all, the less frequently the powertrain system needs to be replaced, the lower the repair and maintenance costs for any replacement parts. A separate cost assessment is given in the following section, where the impact of future replacement frequencies of fuel cells and batteries is explained. Due to limited amount of data in terms of component replacements, it is rather difficult to derive the frequencies of exchange processes. This is why following assumptions are based on analogous values taken from the literature (see Figure 10). [2, 13, 17] The lifetime is given in cycles and a complete battery discharge is assumed for each flight. The values from [13, 17] were already given in cycles, while values from

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[2] were given as 8000 h lifetime. The average flight time of the 100 km helicopter configuration was 30 min, thus, resulting in 16,000 cycles. Luo et al. list a possible lifetime of Li-ion batteries of about 20,000 cycles, represented by a dashed line in Fig. 10. All figures may only be considered qualitatively, but as estimates, since the actual service life is subject to both continuous improvement and operating conditions. It is obvious that the individual values differ significantly depending on the actual publication. 2500025000

2000020000

1500015000 ycles

ensity, c ensity, 1000010000

D Durability [Cycles] Durability

50005000

0 BatteryBattery [16][17] BatteryBattery [13] FuelFuel Cell [2][2] FuelFuel Cell [13][13]

Fig. 10: Lifetime of both power systems (30 min flight time) Since the average lifetime of both powertrain concepts varies significantly depending on the publication (see Fig. 10), it is necessary to derive the anticipated replacement costs to fully compare both options in terms of economic benefits. Therefore, a reference study for the helicopter configuration with a range of 100 km is selected and the resulting maximum power and energy demand obtained. The mission results of the study are listed in Table 4. Table 4: Helicopter flight characteristics

Range Max. power Energy Flight time 100 km ~200 kW 112.4 kWh 1843.2 s

Considering these values, it is possible to calculate the costs of both powertrain concepts. For this purpose, both a fuel cell and a battery system are implemented into the vehicle model, each of which can supply the values shown in Table 4. Depending on whether a fuel cell or a battery system is implemented, the following equations are used for the cost calculation (see Eq. 4.1 and Eq. 4.2). In order to take into account possible price fluctuations due to different market developments, a high- and a low-price scenario are assumed in both cases: The high-price scenario indicates that the research effort is low and therefore higher costs for both systems due to immature systems emerge. In the low- price scenario it is assumed that research efforts in both areas are steadily increasing, which is why prices are constantly falling and services increasing. The following approach is used to calculate hypothetical battery costs first: [17]

푏푎푡푡푒푟푦 푐표푠푡푠 = 푒푛푒푟푔푦 푑푒푛푠𝑖푡푦 ∗ 푒푛푒푟푔푦 푝푟𝑖푐푒 ∗ 푏푎푡푡푒푟푦 푤푒𝑖푔ℎ푡 (4.1)

Following that equation, both the costs for the low and high-price scenario are calculated and shown in Table 5. Additional data is taken from the previous 100 km study. Table 5: Calculation of battery costs [10,14,17]

Scenario Energy density Energy price Battery weight Total battery costs High-price 230 Wh/kg 700 $/kWh 753.11 kg 121,250 $ Low-price 400 Wh/kg 350 $/kWh 225.77 kg 31,607 $

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A different approach needs to be selected to calculate the costs when a fuel cell is applied. As shown in Fig. 3, the battery equivalent to a fuel cell system consists of both the fuel cell providing the actual power needed and the hydrogen storage system (tank). The calculation is based on following calculation and results shown in Table 6:

푓푢푒푙 푐푒푙푙 푐표푠푡푠 = 푝푟𝑖푐푒 푝푒푟 푝표푤푒푟 ∗ 푚푎푥 푝표푤푒푟 + ℎ푦푑푟표푔푒푛 푠푡표푟푎푔푒 푐표푠푡푠 ∗ 푒푛푒푟푔푦 푑푒푚푎푛푑 (4.1) Table 6: Calculation of fuel cell costs [15] Hydrogen Energy Total fuel cell Scenario Price per power Max. power storage costs demand system costs High-price 230 $/kW 200 kW 33 $/kWh 112.4 kWh 49,709 $ Low-price 40 $/kW 200 kW 10 $/kWh 112.4 kWh 9,124 $

For both the battery and the fuel cell, the total costs fluctuate considerably – despite the same study mission – due to different price scenarios. For the battery scenario, both scenarios differ by a factor of about 4 (see Table 5), for the fuel cell scenario even by a factor of about 5 (see Table 6). This is not only due to the deviating technological maturity of both concepts and the associated production costs and market prices, but also to disagreement or uncertainty about current and future price developments of the actual energy sources (electricity vs. hydrogen). Accordingly, a transparent cost comparison is made more difficult by taking both factors into account. In addition, the durability of both powertrain concepts must be included in a reasonable comparison. It is assumed that the total number of charging cycles of a battery will be subject to the service life of a fuel cell, which is why higher replacement costs must be added. As clear as the technological advantages of a fuel cell may be in flight operations, it is necessary to carry out more intensive investigations with regard to economic considerations, which emphasize the cost expectations more clearly.

V. Summary and Conclusions A dense, intact infrastructure network is indispensable for efficient traffic management. Often, this network is in a poor condition due to excessive wear and inevitably leads to congestion and longer travel times as a result of maintenance, repair and detours. The steadily increasing volume of traffic, especially in metropolitan areas, intensifies this effect. Out of this problem and by using previously uncommon technologies such as electric powertrains, the idea arose to shift part of the high traffic volume into the third dimension and to establish an intermodal air taxi network. The electrification of aviation opens up a new dimension for the design and operation of aircraft. The extended use of electrical components is seen as an opportunity to reduce operating costs and environmental impact in the form of noise and pollutant emissions. To date, the majority of such designs have relied on the application of batteries that are (not yet) efficient enough. Long charging times, possible overheating or a limited service life in the form of limited charging cycles pose a challenge to the development of such aircraft. The aim of this paper was the simulative implementation of a sufficiently dimensioned fuel cell in a reference aircraft and the investigation of possible effects on flight operations. To this end, changes in affected performance parameters such as range and payload capacities as well as the associated economic effects were to be highlighted and evaluated. After a short introduction of central properties of fuel cells, the reference aircraft Vahana by A³ was presented. By carrying out parameter studies in a Matlab tool provided by A³, the performance values of the standard configuration and fuel cell option introduced here were then compared on the basis of a previously defined study mission. It turned out that already today's fuel cells have a significantly higher energy density, which is why significantly longer ranges could be achieved. Limited range distances are assumed especially for use in urban areas. Accordingly, the advantage of a higher range capacity can instead be used for a larger number of missions without having to refuel in between, which facilitates smooth and faster flight operations. Furthermore, the higher range capacity can also be used to increase the maximum payload, for example to carry more passengers. Sensitivity studies carried out for this purpose have shown that the use of current fuel cells would allow up to six times the payload compared with the standard configuration. The manufacturing costs and purchase prices of both propulsion concepts are currently still very vague and, in some cases, vary massively. A statement about possible effects on cost structures is therefore difficult to estimate. Parameters such as the individual energy density and the weight of the powertrain unit have a significant influence on the final costs and must therefore be optimized. Finally, it should be noted that all study results are based on a comparatively simple design tool from A³ and should therefore be supplemented by further results or findings. Fundamental statements regarding the range capacity and further advantages in the operational procedure remain valid. External factors such as the distribution of hydrogen, i.e. the maturity of the infrastructures, were also not taken into account and must be included in further investigations.

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Acknowledgments

This research was funded by the Ministry of Innovation, Higher Education and Research of North Rhine-Westphalia (Germany) within the interdisciplinary research project “Forschungskolleg ACCESS!”. This support is gratefully acknowledged.

References

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