Considerations for Fuel Cells in Aviation Applications Flashcards
1
Q
Current Typical Civil Passenger Aircraft
A
- energy carrier: kerosene
- fuel storage in wings -> narrow fuselage
- mechanical power transmission
- onboard voltages: <230V AC, <30V DC
- gas turbine propulsion (10s of MW)
- combustion: large ∆T to ambient facilitates cooling
-> need for decarbonisation
2
Q
Adaptions to current Aircraft for Hydrogen FC aircraft
A
- energy carrier: hydrogen
- fuel storage in fuselage -> increased drag
- electrical power transmission
- onboard voltages: unknown
- FC (hybrid) electric propulsion
- depending on FC type: (very) high cooling efforts
- regulation/certification perspective unclear
- generally high system-level specific power is key
- high fuel efficiency required
3
Q
Different Fits for Different Segments
A
Liquid hydrogen could be fuel option for several segments:
- full hydrogen fuel cell electric for regional applications
- hybridisation with batteries possible for small aircraft & short ranges
- limitations mainly related to system specific power & system scalability
- fuel-cell-turbine-hybrids or fully combustion based approaches for medium to long range
4
Q
Requirements to Propulsion with Fuel Cells
A
- very challenging
- high specific power requirements
- high durability requirements
- large system size
- long peak power duration at takeoff
- very constant load profile during cruise
- drastic change of ambient conditions
5
Q
FC sizing considerations
A
- trade off between system power & efficiency
- choice of operating point impacts mass & volume; may affect hydrogen utilisation
- efficiency directly impacts fuel demand & amount of heat dissipation - high power operating point
- typically higher degradation rates
- requires adequate BoP sizing (parasitic loads) - power management: number of cells per stack & parallel/serial connection of cells impacts power levels ( & weight)
- space constraints: depending on where FC system is located, space limitations/certain dimension requirements may apply
6
Q
FC Characteristics & Sizing Considerations - Aufzählung
A
- Efficiency
- Waste heat & Thermal management
- Balance of Plant Requirements
- Specific Power & Power Density
- Ageing Characteristics
- Transient Load Capability
- Fuel Utilisation & need for Recycling
-> all these depend on operating points/ambient conditions -> relevant to specific application
7
Q
FC as Auxiliary Power Units (APU)
A
- significantly smaller system sizes (compared to road)
- 90kW Airbus 320
- 430kW Boeing 787
- FCs feature high electrical efficiencies as compared to the turbines currently used as APUs
- more electric aircraft (MEA) as specifically interesting application case
- FCs directly provide electrical power for subsystems
- product water could be used onboard (e.g. toilets)
- estimates for power requirements range from 500 to 1500kW
8
Q
Electric Power Onboard
A
- increasing onboard power demand through increased aircraft size/more amenities
- increase in electric sub-systems
- motivation: fuel savings, less maintenance
- drawback: mass increase
- e.g.: air conditioning, flight control, engine controls
9
Q
Power Electronics & Transmission
A
- AC current used -> higher power density & reliability observed
- recently move to more electrified Planes -> growing demand for electric power; high relevance of DC sources
- Challenge: high voltages required (efficiency), But stark limitations (high altitude)
- reduced critical field strength
- Corona discharge
- Arcing
- cosmic radiation
-> Voltage has to be carefully selected - specific power of electro- significant developments required bc different specific power requirements
10
Q
Thermal Management
A
- at high altitude
- lower T facilitate heat rejection to environment
- lower air density makes heat rejection less efficient - heat rejection through ram air -> heat exchangers induce drag
- Thermal management system needs to be functional at ambient conditions, at altitude, & on ground -> sizing
- in case the aircraft is not in operation
- water freezing in Fuel Cell system should be prevented
- before start, operating T of FC has to be reached (fast in PEFC, slow in SOFC)
11
Q
Air Supply
A
- oxygen partial pressure changes throughout the flight
- overall system mass/volume should be kept low
-> providing high oxygen pressures/stoichiometries leads to large compression effort -> trade off -> adapt compressor sizing or tolerate reduced FC output power at high altitude
12
Q
Water Management
A
- system mass is key -> carrying water for humidification is not feasible
- product water is used for reactant humidification
13
Q
Mechanical Force Effects on PEFCs
A
- number of concepts suggest fuel cell stacks located in wings, pods … -> mechanical force on the stacks esp. during landing
- very limited literature on mechanical force impacting fuel cell performance/durability available -> limited understanding for aviation application; focus on road application
- effects of mechanical force depend on direction & magnitude
14
Q
List of effects of mechanical force on PEFC stack
A
- vibration
- aff. water distribution
- sustained vibration can lead to tightness/leakage issues
- electrical contact resistance increase observed
- over-compression
- destruction porous network/impact on mass transport
- degradation effects at material interface (e.g. pinholes, GDL fibre breakage…)
- mechanical shock
- displacements of cell components
15
Q
Mechanical Force Effects on SOFCs
A
- number of concepts suggest fuel cell stacks located in wings, pods … -> mechanical force on the stacks esp. during landing
- SOFC typically used for stationary applications -> limited understanding of mechanical force effects due to transportation application
- generally SOFC ceramics are brittle -> potential degradation introduced by mechanical force more severe
16
Q
general challenges for FC in Aviation
A
- much higher system size than for car application
- power management
- reactant management
- safety
- health monitoring
- system/BoP control
- redundancy considerations: system design requires inherent redundancy
17
Q
Fuel Cell Maintenance - critical parameters for aviation
A
- FC undergo various degradation/ageing mechanisms
- reliability/safety
- certification for air transport strictly enforces high safety requirements
- catastrophic failure must be avoided at all cost
- redundancy is always required - schedule maintenance
- from cost perspective, any unplanned maintenance procedures are to be avoided - health monitoring, maintenance, repair of high importance
- adequate infrastructure at airports has to be introduced - requirements are harder to satisfy for aviation
18
Q
Maintenance: Applicability of Learnings from the road transport
A
- ageing mechanisms for individual components have been studied to a certain extent
- some similarities with aeronautic application
- mechanical force (vibrations, mechanical shock…)
- variety of ambient conditions
- varying power demand - some distinct differences
- more radical & more frequent changes in ambient conditions
- representative power profile drastically different from aviation
- LH2 strage tanks & implications
-> some learnings transferable
-> generally, more in-depth understanding still required
19
Q
BoP components already used in aviation on a regular basis & in relevant scale
A
- compressors
- heat exchangers
- liquid cooling systems incl. pumping
-> learnings can be applied to FC BoP maintenance
-> unprecedented interdependencies may exist
20
Q
Maintenance: Influence of Operating Point
A
- operating points for FC have to be carefully chosen & considered in system design
- for high peak power duration & rapid transients stack degradation is enhanced
- higher maintenance needs (more frequent change of stack)
- one of many factors to consider for fuel cell sizing - many stack-level degradation mechanisms highly depend on operating conditions
- optimisation for each operating point requires adjusting operation of BoP components
- directly correlated with demands on e.g. compressor, humidifier
- requires adequate monitoring, which adds weight & complexity
21
Q
main energy converter options for using hydrogen as an aviation fuel
A
- PEFC
- medium specific power
- good transient performance
- medium/good efficiency
- good NOx emissions - SOFC
- medium/low specific power
- bad transient performance
- good efficiency
- good NOx emissions - Gas turbines
- good specific power
- medium/good transient performance
- medium efficiency
- bad NOx emissions
-> synergistic hybrids of FC & GT heavily investigated
22
Q
SOFC - GT hybrids
A
- motivation for hybridisation
- similar operating T range
- high efficiency of SOFC
- fuel flexibility
- GT can compensate for very limited load following capability of SOFC - investigated previously for stationary applications (e.g. CHP units)
- both technologies can be utilised & optimised for various fuel options
- unlike transport applications: high power density/specific power not major optimisation target - for aeronautic mostly APU applications studied
23
Q
SOFC - GT Hybrids - Improvements over GT
A
- provide water
- improve overall fuel efficiency
- provide (high-quality) heat
- reduce noise
- reduce impact on local air quality
- provide electrical energy
24
Q
SOFC - GT Hybrids - drawbacks
A
- enhanced weight & complexity of power train
- low maturity of SOFC esp. for mobile applications
- lower reliability than well-understood GT
- transient performance/start-up limitations
- limited pressure range of SOFC
25
SOFC - GT Hybrids - Concepts
- serial vs parallel hybrid
- electrical vs mechanical node
- FC topping cycle - energy conversion in the FC occurs upstream of the GT
- FC bottoming cycle - energy conversion in the FC occurs downstream of the GT
- direct hybrid cycle - upstream elements directly provide flow to the downstream elements
- indirect hybrid cycle - GT & FC flows are decoupled; thermal integration via heat exchangers
26
PEFC-Battery Hybrids
- motivation for hybridisation
- hydrogen as more effective way to store energy aboard (compared to battery)
- PEFC can cover most of the power profile
- battery for peak power shaving & transient loads
- increased system redundancy
- overall power train properties
- excellent transient properties
- high efficiency
- limited power & energy density
-> currently only relevant for smaller aircraft
27
PEFC-Battery Hybrids - Concepts
- power train layout & degree of hybridisation - both in terms of power & energy - drastically depends on application
- road transport: system cost & durability may be main optimising criteria
- VTOLs: very high peak power demand & duration during VTOL -> likely partial cover with battery necessary
- regional aircraft: optimised gravimetric & volumetric power density of power train as main drivers -> battery for (small-scale) buffer storage only
- battery can be recharged by fuel cell upon operation
28
Take Away Messages H)
- aircraft span over a wide range of sizes, covered distance, power demand & energy aboard
- application cases differ
- no "one size fits all" when it comes to (partially) FC electric power trains
- compared to road transport, power profiles in aviation differ drastically
- new optimisation
- drastic change of ambient conditions & interactions with other subsystems in aircraft have to be considered when sizing FCs (+BoP)
- electrified power trains bring along new challenges:
- power density & reliability of power electronics
- risks associated with high voltages at high altitude
- reliability & safety are key
- FC durability & maintenance of importance
- various hybrid concepts
- to exploit synergies between energy converters
- to optimise fuel efficiency & power performance
- to provide additional redundancy
