General Aviation Aircraft : Fuel Cell Hybrids For Electric .

UNCLASSIFIEDGeneral Aviation Aircraft :Fuel cell hybrids for electric propulsion Phil Barnes Power Sources Group QinetiQ Haslar QINETIQ/19/04409UNCLASSIFIED

UNCLASSIFIEDIntroduction to fuel cells “Gas Voltaic Battery” invented by William Grove(Welsh Judge and Physicist) in 1842– Hydrogen fuel, oxygen as oxidant, sulfuric acid electrolyte In 1932 English engineer Francis Bacon developed a5 kW alkaline fuel cell (AFC) for stationary applications AFC used by NASA for space applications since mid1960sGrove’s Gas Voltaic BatteryCredit:narenko (Public Domain)– Apollo missions and Space Shuttle both employed AFCs First Polymer Electrolyte Membrane (PEM) fuel cellinvented in early 1960s by General Electric (USA)– PEM is the most applicable technology for electric aviation Other types of fuel cell include:– Solid Oxide Fuel Cell (SOFC), Molten Carbonate Fuel CellCredit:Tim Evanson (CC BY-SA 2.0)(MCFC), Phosphoric Acid Fuel Cell (PAFC), DirectMethanol Fuel Cell (DMFC)2RAeS Light Aircraft Design Conference 18 Nov 2019 QinetiQApollo Alkaline Fuel cellUNCLASSIFIED

UNCLASSIFIEDOperating principle of a PEM fuel cell A fuel cell is a type of electrochemical cell where reactants aresupplied from an external source Fuel (hydrogen) and oxidant (oxygen) are delivered to the fuel cell– Hydrogen (H2) is reduced at the anode (H2 2H 2e-)– Hydrogen ions (H ) diffuse through the proton-conducting electrolyte to thecathode, where they react with oxygen ions (O2-) reduced at the cathodeto form water– Other than water, the only output from the system is heat– Most fuel cells use oxygen from the air - nitrogen and other major aircomponents pass thought the fuel cell without reaction Typically fuel cells employ a bipolar stack configuration– Cells arranged electrically as a series pile– Bipolar plates act as current collector and flow-field for gases– The area of the cell defines the current capability– Stack voltage is dependent on the number of cells in the stack Energy storage is defined by the amount of hydrogen stored3RAeS Light Aircraft Design Conference 18 Nov 2019 QinetiQUNCLASSIFIED

UNCLASSIFIEDWhy select PEMFC for electric aviation? Highest specific power and power density of all fuel cell types––––Specific power up to 2-3 kW/kg at stack levelPower density up to 2-3 kW/lSystems readily available at 10-100 kW stack ratingStacks can be combined in series or parallel configurations Low operating temperature– Typically 60 - 80 C Rapid start-up capability– Seconds to tens of seconds to full power Rugged and lightweight stack technology––––Metallic bipolar plate technologies now widespreadDisplaced graphite-based bipolar plates for weight critical applicationsAll solid-state constructionOrientation-independent operation Proven in other motive power applications such as electric vehicles Drawbacks?QinetiQ prototype electric vehicle PEM stack system used in LifeCar project– Control of state of hydration of polymer membrane is critical to operation4RAeS Light Aircraft Design Conference 18 Nov 2019 QinetiQUNCLASSIFIED

UNCLASSIFIEDPEMFC system architectures Open cathode (W to low kW systems)– Stack has open air channels for oxidant air and cooling– Air provided by fan/blower mounted on side of stack– Not practical to pre-humidify incoming air– Hydrogen supply is “dead-ended” with a purge valve which periodicallyopens to refresh hydrogen supply to stack– Successfully employed in UAVs to double endurance over batteryequivalent Closed cathode (10s of kW power rating and above)– Compressed air supplied to cathode flow field via manifold– Cooling channels are separate – typically liquid cooling is employed– May employ continuous flow of hydrogen to stack with recirculation– Active humidification system normally required– May be internal or external to stack– Includes recovery of water produced in operation– Most suitable type for general aviation5RAeS Light Aircraft Design Conference 18 Nov 2019 QinetiQUNCLASSIFIED

UNCLASSIFIEDFuel cell operating point trade-off Cell voltage operating point– Typical range 0.55 – 0.65 V/cell– Corresponds to 49% to 57% efficiency at stack level Operating at a higher voltage improves efficiency––––Lower fuel consumptionFor 100 kW gross power, 99% hydrogen utilisationAt 0.60 V H2 consumption 6.33 kg/hAt 0.70 V H2 consumption 5.42 kg/h Operating at a lower voltage reduces the size of stackfor a given power output– Increased fuel consumption– Higher airflow required– More heat to manage Major parasitic load is the air delivery subsystem– 10-20% total parasitic load would be typical6RAeS Light Aircraft Design Conference 18 Nov 2019 QinetiQUNCLASSIFIED

UNCLASSIFIEDFuel cell based hybrid electric propulsion systems Battery provides:– Start-up power (including fuel cell air compressor system) and pre-heating (if required)– Peak power capability for take-off and climb– Emergency power provision in case of fuel cell failure (engine power and avionics) Fuel cell provides:– Main power for cruiseSeries hybrid architectureAll power to motors provided via batteryFuel cell recharges battery– Supplemental power capability for take-off and climbFuel cellFuel cellParallel hybrid architectureFuel cell may supply propulsion lsionMotorRAeS Light Aircraft Design Conference 18 Nov 2019 SSIFIED

UNCLASSIFIEDOther components of fuel cell hybrid power system Hydrogen storage– For long flight endurance, mass fraction of hydrogen as function of total storage system mass is key Hydrogen supply system– Valves, flow and pressure control and pipework Thermal management system– Cooling of fuel cell and other components of system– Heating may be required for fuel cell start-up and/or battery system at low temperatures Control systems– Control key fuel cell operating parameters– Power management to manage load balance between fuel cell and battery– Battery Management System (BMS) Power conversion and electrical distribution– dc-to-dc conversion, inverters and wiring8RAeS Light Aircraft Design Conference 18 Nov 2019 QinetiQUNCLASSIFIED

UNCLASSIFIEDAdvantages of fuel cell hybrid propulsion Fast refuel– Much faster than battery recharging– More comparable with liquid hydrocarbon fuels Clean technology– Only emission in flight is water vapour Low carbon footprint if hydrogen generated using renewable energy sources– e.g. solar-powered electrolysis– but compression and liquefaction processes are energy intensive Fuel cell stack and infrastructure development can benefit from progress onfuel cell electric vehicles9RAeS Light Aircraft Design Conference 18 Nov 2019 QinetiQUNCLASSIFIED

UNCLASSIFIEDHydrogen storage options Compressed gas – lightweight composite cylinders– 300 bar – 700 bar pressure– Diminishing returns with increasing pressure because of nonideality of hydrogen– Fibre-reinforced composite with fiberglass, aramid orcarbon fibre and gas-impervious liner– Type III cylinder uses metal liner (typically aluminium)– Type IV cylinder uses thermoplastic liner Cryogenic storage– Hydrogen is liquid below 252.87 C– Double-walled vessel with vacuum insulation– Low pressure (a few bar)– Need to allow for losses of hydrogen to boil-off– 1 to 3 % per dayToyota Mirai700 bar hydrogen storage system– Benefits most from economies of scaleCredit: whoisjohngalt (CC BY-SA 4.0)– 7.5 wt% for 5 kg H2, 15 wt% for 50 kg10 RAeS Light Aircraft Design Conference 18 Nov 2019 QinetiQUNCLASSIFIED

UNCLASSIFIEDComparison of different hydrogen storage options at 5 kg H2 scaleStorage typeHydrogen(gas)Hydrogen(liquid)350 bar cylinder(Type III)700 bar cylinder(Type III)700 bar cylinder(Type )1.8 (6.48)1.4 (5.04)1.8 (6.48)2.5 (9.0)2.75 x 10-3(9.9 x10-3)2.36 (8.50)0.58 (2.1)0.81 (2.93)1.36 (4.9)1.78 (6.4)Volumetric capacity/ g/L0.0870.8517.724.440.853.3Gravimetric capacity/ wt%1001005.44.25.47.5Weight of 5 kg H2 storage5 kg5 kg92.6 kg119.0 kg92.6 kg66.7 kgVolume of 5 kg H2 storage62.5 m370.57 L282.5 L204.9 L122.5 L93.8 LkWh/kg (MJ/kg)kWh/l (MJ/L)11 RAeS Light Aircraft Design Conference 18 Nov 2019 QinetiQUNCLASSIFIED

UNCLASSIFIEDHydrogen vs aviation fuelsSpecifc energy/ s (fuel only)Avgas (including HDPE tank)Avtur (fuel only)Hydrogen (fuel only)Fuel or SystemHydrogen gas (Type IV 700 bar)Fuel efficiency (including conversionand propulsion losses)Avgas – Piston engine, constant speed3.04 kWh/kgAvtur – Small turboprop constant speed2.75 kWh/kgFuel cell electric propulsion with Type IV 700 bar H2 cylinder0.53 kWh/kgFuel cell electric propulsion with cryogenic H2 cylinder0.73 kWh/kg12 RAeS Light Aircraft Design Conference 18 Nov 2019 QinetiQHydrogen (Cryogenic liquid)UNCLASSIFIED

UNCLASSIFIEDFuel cell aircraft example – Regional transport HY4 - World’s first 4-seat hydrogen fuel cell powered aircraft– Developed by DLR, H2Fly, Pipistrel, the University of Ulm, and Hydrogenics– Maiden flight 29/09/2016 from Stuttgart AirportHY4 Aircraft characteristics (based on Pipistrel Taurus G4)SizeLength 7.4 m, Wingspan 21.36 mEngine Power80 kW (peak), 26 kW at cruiseSpeed200 km/h (peak), 140 km/h cruiseRange750 km to 1,500 km (depending on the speed, load and altitude)MassMTOW 1500 kgEmpty weight without power system 630 kgWeight of power system including tanks 400 kgPEM Fuel cell3 x 15 kW stacksHydrogen storage2 x 300-400 bar composite cylinders (1 per fuselage)Li-ion battery pack21 kWh, 45 kW peak powerProvides peak power for take off and climb 15 minutes emergency powerPicture Credits: DLR (CC-BY 3.0)13 RAeS Light Aircraft Design Conference 18 Nov 2019 QinetiQUNCLASSIFIED

UNCLASSIFIEDFuel cell aircraft development ZeroAvia - HyFlyer project - announced September 2019– 2.7M Funding from UK Government ATI programme, supported by the––––Department for Business, Energy & Industrial Strategy, the AerospaceTechnology Institute and Innovate UKProject to deliver 250-300 NM range for a Piper M-class six-seater in 2022Cranfield Aerospace Solutions (CAeS) provide aircraft integration expertiseFuel cell to be developed by Intelligent EnergyCompressed hydrogen storage H3 Dynamics - Element One – announced September 2018– Distributed propulsion and energy storage system concept– Nacelle contains 5 kW fuel cell, compressed H2 storage and battery– Provides redundancy/safety benefits– Targeting regional transport for 4 passengers– Ground infrastructure at airport could include H2 production via renewables– First prototype planned for demonstration by 202514 RAeS Light Aircraft Design Conference 18 Nov 2019 QinetiQPowered by HESImage of Element one used with permission of H3 DynamicsUNCLASSIFIED

UNCLASSIFIEDBritten-Norman IslanderLilium JetCassutt Special Pipistrel PantheraPipistrel Alpha Trainer15 RAeS Light Aircraft Design Conference 18 Nov 2019 QinetiQUNCLASSIFIED

UNCLASSIFIEDPipistrel Alpha TrainerPipistrel PantheraCassutt SpecialBritten-Norman IslanderLilium Jet16 RAeS Light Aircraft Design Conference 18 Nov 2019 QinetiQUNCLASSIFIED

UNCLASSIFIEDConclusions Fuel cell-battery hybrid electric propulsion offers potential advantages over battery only propulsion More than double the range of battery-only option for smaller GA aircraft May make electric propulsion more feasible for larger GA aircraft types Fuel cells are most effective if sized to provide main propulsion load in cruise phase Batteries are required to provide additional power in take-off and climb and to provide transient load response Batteries are also likely to be required for emergency use Hydrogen refuelling is much better suited to fast turnaround than battery recharging17 RAeS Light Aircraft Design Conference 18 Nov 2019 QinetiQUNCLASSIFIED

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