general aviation aircraft : fuel cell hybrids for electric

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General Aviation Aircraft :Fuel cell hybrids for electric propulsion

• Phil Barnes

• Power Sources Group

• QinetiQ Haslar

• QINETIQ/19/04409

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• “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 a

5 kW alkaline fuel cell (AFC) for stationary applications

• AFC used by NASA for space applications since mid-

1960s

– Apollo missions and Space Shuttle both employed AFCs

• First Polymer Electrolyte Membrane (PEM) fuel cell

invented 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 Cell

(MCFC), Phosphoric Acid Fuel Cell (PAFC), Direct

Methanol Fuel Cell (DMFC)

RAeS Light Aircraft Design Conference | 18 Nov 2019| © QinetiQ2

Introduction to fuel cells

Apollo Alkaline Fuel cellCredit:Tim Evanson (CC BY-SA 2.0)

Grove’s Gas Voltaic BatteryCredit:narenko (Public Domain)

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• A fuel cell is a type of electrochemical cell where reactants are supplied 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 the cathode, where they react with oxygen ions (O2

-) reduced at the cathode to 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 air components 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 stored


Operating principle of a PEM fuel cell

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• Highest specific power and power density of all fuel cell types– Specific power up to 2-3 kW/kg at stack level– Power density up to 2-3 kW/l– Systems readily available at 10-100 kW stack rating– Stacks 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 widespread– Displaced graphite-based bipolar plates for weight critical applications– All solid-state construction– Orientation-independent operation

• Proven in other motive power applications such as electric vehicles

• Drawbacks?– Control of state of hydration of polymer membrane is critical to operation


Why select PEMFC for electric aviation?

QinetiQ prototype electric vehicle PEM stack system used in LifeCar project

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• 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 periodically

opens to refresh hydrogen supply to stack

– Successfully employed in UAVs to double endurance over battery

equivalent

• 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 aviation


PEMFC system architectures

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• 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 consumption

– For 100 kW gross power, 99% hydrogen utilisation

– At 0.60 V H2 consumption = 6.33 kg/h

– At 0.70 V H2 consumption = 5.42 kg/h

• Operating at a lower voltage reduces the size of stack for 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 typical


Fuel cell operating point trade-off

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• 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 cruise

– Supplemental power capability for take-off and climb


Fuel cell based hybrid electric propulsion systems

Series hybrid architecture

All power to motors provided via battery

Fuel cell recharges battery

Fuel cell

Battery

DC-DC

Inverter

Propulsion

Motor

Power

management

Fuel cell

Battery

DC-DC

InverterPropulsion

Motor

Parallel hybrid architecture

Fuel cell may supply propulsion load

directly

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UNCLASSIFIEDRAeS Light Aircraft Design Conference | 18 Nov 2019| © QinetiQ8

Other 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 wiring

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• 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 on

fuel cell electric vehicles


Advantages of fuel cell hybrid propulsion

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• Compressed gas – lightweight composite cylinders

– 300 bar – 700 bar pressure

– Diminishing returns with increasing pressure because of non-

ideality of hydrogen

– Fibre-reinforced composite with fiberglass, aramid or

carbon 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 day

– Benefits most from economies of scale

– 7.5 wt% for 5 kg H2, ~15 wt% for 50 kg


Hydrogen storage options

Toyota Mirai

700 bar hydrogen storage systemCredit: whoisjohngalt (CC BY-SA 4.0)

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Comparison of different hydrogen storage options at 5 kg H2 scale

Storage type Hydrogen

(gas)

Hydrogen

(liquid)

350 bar cylinder

(Type III)

700 bar cylinder

(Type III)

700 bar cylinder

(Type IV)

Cryogenic

storage

system

kWh/kg (MJ/kg) 33.29

(119.93)

33.29

(119.93)

1.8 (6.48) 1.4 (5.04) 1.8 (6.48) 2.5 (9.0)

kWh/l (MJ/L) 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/L 0.08 70.85 17.7 24.4 40.8 53.3

Gravimetric capacity/ wt% 100 100 5.4 4.2 5.4 7.5

Weight of 5 kg H2 storage 5 kg 5 kg 92.6 kg 119.0 kg 92.6 kg 66.7 kg

Volume of 5 kg H2 storage 62.5 m3 70.57 L 282.5 L 204.9 L 122.5 L 93.8 L

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Hydrogen vs aviation fuels

Fuel or System Fuel efficiency (including conversion

and propulsion losses)

Avgas – Piston engine, constant speed 3.04 kWh/kg

Avtur – Small turboprop constant speed 2.75 kWh/kg

Fuel cell electric propulsion with Type IV 700 bar H2 cylinder 0.53 kWh/kg

Fuel cell electric propulsion with cryogenic H2 cylinder 0.73 kWh/kg

12.1410.48

11.94

33.29

1.8 2.5

0

5

10

15

20

25

30

35

Sp

ecifc e

ne

rgy/

kW

h/k

g

Avgas (fuel only) Avgas (including HDPE tank) Avtur (fuel only) Hydrogen (fuel only) Hydrogen gas (Type IV 700 bar) Hydrogen (Cryogenic liquid)

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• 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 Airport


Fuel cell aircraft example – Regional transport

Picture Credits: DLR (CC-BY 3.0)

HY4 Aircraft characteristics (based on Pipistrel Taurus G4)

Size Length 7.4 m, Wingspan 21.36 m

Engine Power 80 kW (peak), 26 kW at cruise

Speed 200 km/h (peak), 140 km/h cruise

Range 750 km to 1,500 km (depending on the speed, load and altitude)

Mass MTOW 1500 kg

Empty weight without power system ~630 kg

Weight of power system including tanks 400 kg

PEM Fuel cell 3 x 15 kW stacks

Hydrogen storage 2 x 300-400 bar composite cylinders (1 per fuselage)

Li-ion battery pack 21 kWh, 45 kW peak power

Provides peak power for take off and climb + 15 minutes emergency power

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• ZeroAvia - HyFlyer project - announced September 2019

– £2.7M Funding from UK Government ATI programme, supported by the

Department for Business, Energy & Industrial Strategy, the Aerospace

Technology Institute and Innovate UK

– Project to deliver 250-300 NM range for a Piper M-class six-seater in 2022

– Cranfield Aerospace Solutions (CAeS) provide aircraft integration expertise

– Fuel cell to be developed by Intelligent Energy

– Compressed 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 2025


Fuel cell aircraft development

Powered by HESImage of Element one used with permission of H3 Dynamics

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Pipistrel Alpha Trainer

Lilium Jet

Cassutt Special

Britten-Norman Islander

Pipistrel Panthera

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• 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 recharging

RAeS Light Aircraft Design Conference | 18 Nov 2019 | © QinetiQ17

Conclusions

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• Reproduction of Grove’s Gaseous Voltaic Battery

– This work has been released into the public domain by its author, Noraneko. Noraneko grants anyone the right to use this work for

any purpose, without any conditions, unless such conditions are required by law.

– https://commons.wikimedia.org/wiki/File:Grove%27s_Gaseous_Voltaic_Battery.png

• Apollo Fuel Cell

– Credit: Tim Evanson (Attribution-ShareAlike 2.0 Generic (CC BY-SA 2.0)) https://www.flickr.com/photos/timevanson/9338135397

• Toyota Mirai compressed hydrogen storage

– Credit: whoisjohngalt (CC BY-SA 4.0) https://commons.wikimedia.org/wiki/File:Hydrogen_tanks_for_Toyota_Mirai.png

• First flight of four-passenger fuel cell aircraft (HY4)

– Credit: DLR (CC-BY 3.0) https://www.dlr.de/content/en/images/aeronautics/first-flight-of-four-passenger-fuel-cell-aircraft.html

• The HY4 fuel cell propulsion system

– Credit: DLR (CC-BY 3.0) https://www.dlr.de/content/en/images/2017/4/the-hy4-fuel-cell-propulsion-system_28734.html

• Image of Element one copyright of H3 Dynamics, used with permission.

• All other photos and graphics are QinetiQ copyright


Photo credits and licensing

https://commons.wikimedia.org/wiki/File:Grove%27s_Gaseous_Voltaic_Battery.png

https://www.flickr.com/photos/timevanson/9338135397

https://commons.wikimedia.org/wiki/File:Hydrogen_tanks_for_Toyota_Mirai.png

https://www.dlr.de/content/en/images/aeronautics/first-flight-of-four-passenger-fuel-cell-aircraft.html

https://www.dlr.de/content/en/images/2017/4/the-hy4-fuel-cell-propulsion-system_28734.html

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related to it, please visit our contact page:

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general aviation aircraft : fuel cell hybrids for electric

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