Electric Flight Potential and Limitations

Transcription

1 Electric Flight Potential and Limitations Energy Efficient Aircraft Configurations, Technologies and Concepts of Operation, Sao José dos Campos, November 2013 Dr. Martin Hepperle DLR Institute of Aerodynamics and Flow Technology Braunschweig, Germany

2 History and Predictions Air Traffic Source: Airbus Global Market Forecast Page 2

3 History and Predictions Oil Production? Page 3

4 Electric Propulsion of Aircraft? Motivation: Air traffic is growing. Availability of fossil fuels is be limited. Electric propulsion systems offer high efficiency. Electric propulsion systems can be zero-emission in situ. Specifics of air transport: Aircraft are already quite efficient (3-4 Liters/PAX/100km). Aircraft usually operate on long distances ( km). Mass is much more important than in ground transportation. Safety standards are very high. Page 4

5 Performance of Aircraft The classical polar apporximation yields a relation between drag and flight speed. Plotted here Two additional graphs are derived from this curve. These three graphs contain the main performance criteria. The minimum of each curve defines the tangent to the next curve. The minima are at different speeds. Page 5

6 Performance of Aircraft Point : Minimum of D/v maximum of v L/D or M L/D. Most cost efficient flight of jet aircraft. Maximum range of jet aircraft. Page 6

7 Performance of Aircraft Point : Minimum of D maximum of L/D Minimum energy consumption per distance. Maximum range of propeller aircraft. Maximum endurance of jet aircraft. Page 7

8 Performance of Aircraft Point : Minimum of D v Minimum of required power. Propulsion system of lowest weight. Maximum endurance of propeller aircraft. Page 8

9 Performance of Aircraft 3.0 Page 9

10 Performance of Aircraft Page 10

11 Performance of Aircraft Page 11

12 There is nothing new under the sun... One of the Pioneers of Electric Flight Fred Militky 1940 first trials, after 1945 chief engineer at Graupner hobby company. Motor glider MB-E1 (HB-3, b=12 m, m = 440 kg) 21. October 1973: worldwide first flight with electric motor, duration 9-14 Min, altitude 360 m, Pilot Heino Brditschka, performance not reached for 10 years, NiCd batteries by Varta, Motor by Bosch ( /min). Also built solar powered R/C model aircraft in Source: Internet Source: Internet Source: Brditschka Silentius Silentius Hi-Fly Hi-Fly MB-E1 MB-E1 Page 12

13 Conventional Propulsion Systems Energy storage: liquid fuel, alternative: Gas (e.g. H 2 ). Conversion to propulsive power: Turbofan, Turboshaft / piston engine and Propeller, RPM adaption as needed by a gearbox. Fuel is burnt, mass reduces with flight time. Page 13

14 Electric Propulsion There are many possibilities. Mainly two types of interest. Fuel cell systems complex and still expensive, usage of conventional energy storage systems (Kerosene, Methanol, H2), variable mass. Batteries simpler systems, constant mass, much recent development. also: hybrid systems Page 14

15 Total Efficiency The Chain from on-board Energy to Propulsion Page 15

16 Characteristics of Energy Storage Systems Specific Energy Content of the pure Energy Provider Page 16

17 Characteristics of Energy Storage Systems Not Fuel Mass but System Mass is important Kerosene, Methanol Tanks (often integral part of the structure), tubing, pumps. Hydrogen Gas: high pressure tanks (typical: bar), tubing,, Liquid: low pressure tanks (-250 C), insulation, tubing,. Option: structurally integrated tanks, e.g. metal-hydrides? Fuel Cell compressors, tubing, water,, Kerosene/Gas/Alcohol: reformer. Battery Casing, heating, ventilation, wiring, Page 17

18 Equivalent Energy Density of Propulsion Systems Page 18

19 What is available today? Mass specific Energy of Batteries Lithium-Polymer Technology E* = Wh / kg, V* = Wh / Liter Lithium Sulfur 800 Wh/kg chemical limit? Lithium-Air 2500 Wh/kg chemical limit? Page 19

20 What is available today? Mass specific Power of Electric Motors Electric motors achieve a similar level as piston engines. Today realistic: 2-3 kw / kg. Desired: lightweight motors with mass specific power 6 kw / kg. Page 20

21 What is available today? Comparison with Turboshaft Engines 2-4 kw / kg at cruise power 2-8 kw / kg at takeoff power Page 21

22 What is available today? Comparison with Turbofan Engines 2-4 kw / kg at cruise power Page 22

23 Range of Aircraft with Energy Storage in Batteries Battery Aircraft E* = Energy per mass [J/kg, Ws/kg] P = power [W] L/D = lift over drag t = time [s] v = flight speed [m/s] m = aircraft mass [kg] R = range [m] g = 9.81 [m/s 2 ] = total efficiency from battery (neglecting fuel reserves as well as takeoff and landing) Page 23

24 Range of Aircraft A) Energy from decomposing / burning fuel (hot or cold): Fuel consumption reduces mass during the flight time. classical range equation ( Breguet-equation ) applies. B) Energy drawn from batteries or solar energy: Mass stays constant. Page 24

25 Impact of variable Mass on Range Aircraft with a small mass fraction m fuel /m of energy storage experience a small effect. Short range aircraft lose about 5-10% in range. Long range aircraft lose about 20-25% of range. This effect must be compensated by additional energy or efficiency. fuel fuel mass mass fraction fraction Page 25

26 Range of Aircraft with Energy Storage in Batteries Range with payload How large is the maximum range with a given technology? payload zero Battery Systems Aerodynamics Structures This limit cannot be exceeded. Limit case, allows for a rapid assessment of weird concepts, realistic ranges are always lower! Page 26

27 Sizing Equation Determine required Aircraft Mass for Range rearranging the range equation yields the aircraft mass for a given range only a small number of parameters needed: number of passengers PAX and mass per PAX m pax, empty mass fraction m empty /m, desired range R, specific energy E* of the battery system, total efficiency of the system from battery to thrust, lift over drag ratio L/D. no direct influence of cruise altitude! for R=0 we obtain the absolute minimum mass of the aircraft. Page 27

28 Empty Mass Fraction of Aircraft Current Technology Page 28

29 Sizing Limits Aircraft mass for given range Constraints for solution (m > 0) Range = 500 km, E* = 180 Wh/kg, = 70% PAX=1 PAX=2 PAX=4 PAX=10 PAX= m [kg] L/D Page 29

30 Sizing Limits Aircraft mass for given range Constraints for solution (m > 0) Range = 500 km, LD = 30, = 70% PAX=1 PAX=2 PAX=4 PAX=10 PAX= m [kg] E* [Wh/kg] Page 30

31 Sizing Limits Aircraft mass for given range Constraints for solution (m > 0) Range = 500 km, E* = 180 Wh/kg, = 70%, L/D = 30 PAX=1 PAX=2 PAX=4 PAX=10 PAX= m [kg] m e /m Page 31

32 Trading Aerodynamics and Structures Page 32

33 Trading Aerodynamics and Structures Page 33

34 Mass Growth with Design Range E* = 200 Wh/kg Do 328 TP R = 1200 km Page 34

35 Mass Growth with Design Range E* = 200 Wh/kg 400 Wh/kg Do 328 TP R = 1200 km Page 35

36 Practical Range Limit Derivative of mass with respect to range yields the mass growth per increase of range [kg/km] Solving for the range produces the maximum range R max which is limited by the acceptable mass growth. The value for the acceptable mass growth depends on aircraft size. Page 36

37 Refined Model Aircraft geometry and structures wing span, wing area, empty mass fraction. Aerodynamics square polar, zero lift drag, k-factor. System Battery: E*, U(t); Motor: P(U), efficiencies. Propeller diameter, speed, number efficiency = f(t, v, H). Energy optimized mission climb with optimum speed (incl. propeller), cruise with optimum speed (incl. propeller), descent with max. L/D (only secondary energy consumption), no reserves! Page 37

38 Refined Model Propeller Model The propeller efficiency drops with reduced flight speed. Large propellers are less critical w.r.t motor-propeller matching Page 38

39 Refined Model Energy in Climb Without a proper efficiency chain model one obtains academic optima. Page 39

40 Results Page 40

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43 Example: Regional Aircraft The range of the aircraft with 32 passengers is about 1200 km. With full tanks and 28 passengers it grows to 2200 km. The lift over drag ratio is about 16. Modification: Replacing fuel system and engines by an electric system of identical mass. With current technology the aircraft would reach a range of 202 km, however without any reserves (with reserves: R=50 km). Page 43

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49 Big Steps in Technology Development are Required. Energy optimized flight: The cruise speed drops due to higher wing span below 300 km/h (The turboprop aircraft flies at 480 km/h.) L/D = The high aspect ratio requires high lift coefficients (climb: 0.9, cruise: 1.2). Consumption with a turboprop would be about 1.5 Liter/PAX/100km Page 49

50 Note on Range Flexibility Trading fuel / batteries for range is more useful for (lightweight) kerosene than for (heavy) batteries. Page 50

51 Battery Powered Aircraft? Conclusions: Electric propulsion systems with batteries are possible for small aircraft, The range is strongly limited, but useable for General Aviation and UAVs, For larger aircraft the battery technology must be drastically improved to at least 1000 Wh/kg (factor 5, better 10), This seems to be unlikely within the next 10 years, but may be within years. Costs are less relevant as they will shrink due to automotive and consumer industry. Still many open questions: What is the total balance including production and recycling? Are the raw materials for automotive and aviation available in the long term? Page 51

52 Solar Powered Aircraft? Power delivered by solar cells depends on wing area and solar irradiation. Power required by the aircraft depends on wing area, mass and flight speed. Page 52

53 Sizing of Solar Powered Aircraft Power delivered by solar cells Power required by the aircraft P solar = P cruise can solve e.g. for wing span b or for mass m Page 53

54 Size of Solar Powered Aircraft Selected requirement: aircraft shall fly for 24 hours. We have to use the time average of the solar irradiation E*. Energy has to be stored for night time usage. Page 54

55 Solar Powered Aircraft? What is possible today? m = 1.6 t, S = 210 m 2, L/D = 35, v = 50 km/h, P = 20 kw R = unlimited, payload: one person (the pilot) Efficiency of solar cells is about 20% ( in the future maybe 40%) Design for minimum power consumption leads to low flight speeds Page 55

56 Solar Powered Aircraft? Aircraft of Airbus A320 size m = 75 t, S = 120 m 2, L/D = 20, v = 750 km/h, R = 4000 km t = 5.3 h, P = 7.66 MW, E = 1.47E5 MJ Kerosene, H = 42.8 MJ/kg, η = 30% m = 9 t, V = 12 m³ Hydrogen, H = MJ/kg, η = 30% m = 3 t, V = 44 m³ Li-Po batteries, H = 0.8 MJ/kg, η = 60% m = 190 t, V = 77 m³ Solar cells, E* = 650 W/m², η = 40% m = 10 t, S = m² 12:00 noon optimistic Page 56

57 Solar Powered Aircraft? Conclusions: Solar-Electric propulsion systems are possible for very special low power aircraft, Available power is very much limited by efficiency of solar cells. If efficiency can be doubled size will be halved. Promising developments in the laboratory, but not yet available for a reasonable price. Are an alternative for low power applications in the hobby and long endurance UAV field. Not suitable for heavy and fast aircraft with high power demands, even if solar cells of 100% efficiency would be available. Page 57

58 There is nothing new under the sun... One of the Pioneers of Electric Flight Fred Militky 1940 first trials, after 1945 chief engineer at Graupner. Motor glider MB-E1 (HB-3, b=12 m, m = 440 kg) 21. October 1973: worldwide first flight with electric motor, duration 9-14 Min, altitude 360 m, Pilot Heino Brditschka, performance not reached for 10 years, NiCd batteries by Varta, Motor by Bosch ( /min). Source: Internet Source: Internet Today, 40 years later, using commercially available battery systems, the flight time could be extended to 2.5 hours. Source: Brditschka Silentius Silentius Hi-Fly Hi-Fly MB-E1 MB-E1 Page 58

59 Return to the Future with Whole Milk? Thank You for Listening! Page 59 CAE-