Assessment of a Fuel Cell Powered Air Taxi in Urban Flight Conditions
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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. 2 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 푑푒푛푠푡푦 = , (2.5) 푣표푙푢푚푒 whereas gravimetric power density (also defined as specific power) gives: 푝표푤푒푟 푠푝푒푐푓푐 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