Introduction & Motivation

Question 1

Hydrogen production

Answer 1

• Needs to be sourced from low-emission intense sources to be favourable in life cycle balance -> Green Hydrogen
• has low volumetric density -> needs to be liquified (-253°C)
• Electrolysis
• Produced electrochemically from water & renewable electricity; expected to be dominant source
• through nuclear energy
  
  • low CO2 Emissions
  • medium environmental impact
  • high cleanliness level of H2
• through mixture of sources (FF & RE)
  
  • low CO2 Emissions
  • medium environmental impact
  • high cleanliness level of H2
• through renewable sources
  
  • low CO2 Emissions
  • low environmental impact
  • high cleanliness level of H2
• Thermochemical water splitting
• produced thermochemical with help of a redox material directly employing concentrated sunlight
  
  • produced by concentrated solar energy
  • low CO2 Emissions
  • low environmental impact
  • high cleanliness level of H2
• -> both energy intensive processes

Question 2

location dependency of H2 production

Answer 2

• renewable electricity generation at scale required
  
  • highest potential for cost-effectiveness in high wind/solar regions
  • capacity needs to be drastically increased -> rising demand
• location of hydrogen use may be characterised by non-ideal production conditions -> logistical efforts of supply & distribution
• fundamentally different properties, production pathways & supply chains -> radical developments required

Question 3

benefits of Hydrogen fuels

Answer 3

• short & efficient process chain -> high production efficiency
• lower minimum fuel selling price
• lower global warming potential -> improved local air quality through reduced high altitude climate impact
• opportunity for reduced high altitude climate impact -> improved local air quality

Question 4

Fuel Cell basics

Answer 4

• type of energy converter, allows direct harvest of chemical energy stored in a fuel through an electrochemical process
• works through Redox reaction with fuel & Oxygen:
  
  • Oxidation: H2 -> 2H+ + 2e-
    
    • Hydrogen Oxidation Reaction (HOR)
  • Reduction: 0.5 O2 + 2H+ -> H2O
    
    • Oxygen Reduction Reaction (ORR)
• electrons travel through the external circuit & power electrical devices

Question 5

Electrochemical conversion in fuel cells

Answer 5

• chemical Energy -> electrical energy -> mechanical energy
• high exergy content in electrical energy
• practically through limited mass transport, sluggish kinetics lead to a significant inner resistance of the fuel cell

Question 6

Fuel cells in Aviation

Answer 6

• very challenging application
• High specific power requirements
• High durability requirements
• large system size
• drastic change of ambient conditions
• -> interdisciplinary skills required for optimisation

Question 7

Kinds of Fuel Cells

Answer 7

• Polymeric types
  
  • Polymer Electrolyte Fuel Cell PEFC
    
    • state of the art for mobile applications (esp cars)
  • High Temperature Polymer Electrolyte Fuel Cell HT-PEFC
• Ceramic type
  
  • Proton Ceramic Fuel Cell PCFC
  • Solid Oxide Fuel Cell SOFC
    
    • state of the art for stationary application

Question 8

Fuel Cell efficiency

Answer 8

𝜂 = output electrical energy / theoretical energy released upon full fuel combustion (= Heating value of fuel)

Question 9

Polymer Electrolyte Fuel Cell

Answer 9

• 𝜂 ≈ 80%, with HHV
• operating temperature 60 - 95 °C, typically 80°C
• load response < 1s
• operating pressure range: 1-3 bar
• no fuel flexibility
• high technological maturity
• membrane made of perfluorosulfonic acid PFSA
• Challenges
  
  • requires humidification/water management
  • significant amount of heat dissipated for low ΔT to ambient
  • use of expensive Nobel-metal catalysts bc of sluggish kinetics at 80°C

Question 10

High-Temperature Polymer Electrolyte Fuel Cell

Answer 10

• 𝜂 ≈ 77-79%, with HHV
• operating temperature: 100-220°C
• intermediate technological maturity
• rest like PEFC
• Challenges
  
  • durability limited due to high corrosion rates at the higher temperatures
  • limited power densities observed
  • limitations of membranes (made of Phosphoric Acid-Doped Polybenzimidazole (PA-PBI))

Question 11

Proton Conducting Ceramic Fuel Cell

Answer 11

• 𝜂 ≈ 68-74%, with HHV
• operating temperature 450 - 700 °C
• load response ≈ 10s to minutes
• operating pressure range: 1-3 bar
• fuel flexibility
• low technological maturity
• membrane of Protonic ceramic conductors
• Challenges
  
  • high mass/ion-transport associated resistances within the cell -> limited performance

Question 12

Solide Oxide Fuel Cell

Answer 12

• 𝜂 ≈ 60-70%, with HHV
• operating temperature 600 - 1200 °C
• load response ≈ 10s to minutes
• operating pressure range: 1-8 bar
• fuel flexibility
• intermediate technological maturity
• membrane of Protonic ceramic conductors
• Challenges
  
  • sealing at high temperatures
  • thermo-mechanical strain due to different thermal expansion behaviours components -> degradation

Question 13

Trade offs of Ceramic FC in Mobile applications

Answer 13

• comparatively high fuel efficiency
  
  • less heat to be dissipated & at higher ΔT to ambient
• thermal management facilitated
• comparatively low stack specific power
• less developed for mobile applications, due to
  
  • slow load response
  • issues with thermo-mechanical degradation
  • harsher conditions for corrosion & sealing materials

Question 14

Fuel cell system

Answer 14

• consists of a large number of individual cells
• connected in series to create stacks -> I = I1; U = ∑Un (balance required)
• thermal management
• water management
• reactant supply

Question 15

stack performance assessment & enhancement

Answer 15

• properties to govern FC performance
  
  • electrochemical reactions
  • mass transfer
  • ionic transport
  • electronic transport
  • water management
  • thermal management
• optimising
  
  • research to understand processes
  • research & engineering the materials involved

Question 16

Batteries in Aeronautic applications

Answer 16

• achievable power & energy stored coupled (decoupled for FC)
• energy stored in battery limits range (FC only converts)
• active material amount is range limiting factor (for FC hydrogen limits range)
• aircraft carries both redox partners (only one for FC)
• aircraft mass does not change during flight (gets lighter for FC)
• fast response to transient loads
• achievable efficiency ≈ 95% (≈40-65% for FC)

Question 17

Disadvantages of batteries

Answer 17

• heavy
• large
• extensive thermal management needed for them not to over-/underheat
• not suitable for large aircraft of long transports (only with hybrid-electric approaches investigated)

Question 18

Disadvantages of Hybrid-Electric approaches

Answer 18

• stark range constraints for hybridisation bc of limitations in specific energy of batteries (true even for mild degrees of hybridisaiton)
• final impact will largely depend on battery development
• currently, adequate battery performance not in reach

Question 19

Take away message A)

Answer 19

• aviation requires many measures to achieve net-zero carbon emissions
• green hydrogen as alt. energy carrier
  - high emission reduction
  - used in aviation & other sectors
  - hydrogen as “energy vector”
• LH2 & kerosene properties differ fundamentally
  - challenges in aircraft design
  - infrastructure adaptions required
• FC covers energy stored in H2 electrochemically with comp. high efficiency
  - av. FC tech differ in tech. maturity, suitable operating conditions & performance
  - FC fundamentally differ from Batteries
• aviation represents a challenging field of application for FC
  - innovation & development on cell, system & aircraft level required