Hydrogen as an Aviation Fuel

Question 1

Properties of Hydrogen

Answer 1

• smallest & lightest known molecule
  - molecular weight: 2g/mol
  - energy density of gaseous hydrogen atm: 0.003 kWh/l
  - for liquid hydrogen: 2.35 kWh/l -> pretty low
  - 1dm^3 of gaseous hydrogen atm weighs 0.09g-> 14 times lighter than air
  - density of liquid hydrogen 70 g/dm^3
• odorless
• colorless
• tasteless
• flammable between 4-74%
• comparatively high concentrations needed to form explosive mixture
• non-toxic
• boiling point: -252.76°C
• melting point: -259.19°C
• abundance
  - very low natural atm abundance of hydrogen molecule
  - increases with altitude - highly abundant in space
  - element H is highly abundant at earth, but bound to other elements -> needs to be extracted

Question 2

Hydrogen - diffusivity

Answer 2

• physical diffusion: gas diffusivity is proportional to its molar weight -> diffuses fast bc of low weight
• chemical diffusion: can diffuse through most materials including metals
  - makes materials brittle
  - poses challenges to (leak-tight) hydrogen storage design

Question 3

Safety Aspects

Answer 3

• in a ventilated/open space GH2 leakages dissipate very fast
• accumulation of H2 can lead to sparks -> flames forming
• flame in daylight hot, high, fast burning, but invisible -> flame isn’t deteceable
• due to odorlessness & colorlessness leaks need to be detected via sensors
• safety standards for high amounts of LH2 in aviation not available yet

Question 4

types of LH2 storages

Answer 4

• vacuum insulated
• foam insulated

Question 5

LH2 Storage Integration in Fuselage

Answer 5

• spherical tanks pose challenge for integration but have ideal surface-to-volume-ratio -> need to be integrated in fuselage
• integral tank structure
  - integrated within airframe
  - bears structural loads -> allows weight savings
  - tank diameter = fuselage diameter
• non-integral tank structure
  - self-standing structure within fuselage
  - cannot bear any additional loads
  - tank diameter < fuselage diameter
• redundancy requirements
  - sectioned tank structures
  - entirely separate tanks

Question 6

drawbacks of tank integration in fuselage

Answer 6

• larger fuselage cross-section
• additional drag
• as no fuel is in the wing, wing needs more structural support
• less cabin space
• intrinsic challenge of flying LH2 tank -> flight dynamics of LH2

Question 7

LH2 fuel system

Answer 7

• LH2 taken from tank
  - additional valves, sensors, venting system required
  - typically not included when tank gravimetric index is reported
  - esp. for smaller tanks
• hydrogen needs to be distributed onboard
  - considerable temperature & pressure gradients involved
  - puts strain on materials
  - cooling & heating potentials need to be effectively used

Question 8

Green Hydrogen Production - Scaling Aspects

Answer 8

• electrolysis covers &laquo_space;1% of global hydrogen demand
• studies report that targets can only be met assuming high growth rates of ~100%/a
• currently major H2 production path with natural gas (vast majority FF)
• dominant processes
  - Steam methane reforming: CH4 + H2O <-> CO + 3 H2; CO + H2O <-> CO2 + H2
  - partial oxidation: CnHm + nH2O -> nCO + (n+m/2) H2; CnHm + n/2 O2 <-> nCO + m/2 H2

Question 9

Liquid Green Hydrogen Logistic & Supply - Off Site production & Liquefaction

Answer 9

Green Hydrogen Production -> Liquefaction -> Transportation -> Airport Storage -> Airport Distribution -> Refuelling

Question 10

Liquid Green Hydrogen Logistic & Supply - Off Site Production & On Site Liquefaction

Answer 10

Green Hydrogen Production -> Transportation -> Liquefaction -> Airport Storage -> Airport Distribution -> Refuelling

Question 11

Liquid Green Hydrogen Logistic & Supply - On Site Production & Liquefaction

Answer 11

Green Hydrogen Production -> Liquefaction -> Airport Storage -> Airport Distribution -> Refuelling

Question 12

LH2 Transportation

Answer 12

• transport vehicle availability very limited
• developments & scale up necessary
• cost-optimised fuel supply scenario may include transport over long distances to exploit cheap electricity
• unavoidable boil-off: reuse e.g. for propulsion for optimised efficiency

Question 13

Refuelling & Distribution at the Airport

Answer 13

• major infrastructural adaptions
• key components in (early) development
• Hydrogen of high purity needed as impurities would solidify
• due to lower volumetric flow longer turnover times may be necessary
• bendable refuelling hoses not useable anymore
• most suitable distribution & refuelling concept depends on hydrogen demand at respective airport
• tanker concept: Mobile refuelling bowser with automated refuelling arm
• hydrant concept: Transfer tank & pipe go into Mobile refuelling equipment
• washing of equipment necessary

Question 14

Green Hydrogen for Electricity Storage - round trip efficiency

Answer 14

• battery: very high; ca. 96%
• Hydrogen: medium; ca. 45%
• Methane: medium to low; ca. 20%
• liquid hydrocarbons: low; ca. 16%

Question 15

Green Hydrogen for Electricity Storage - ease of storage

Answer 15

• battery: charging -> discharging
• Hydrogen: Electrolysis -> FC
• Methane: CO-Electrolysis/Electrolysis & Methanation -> FC/Combustion
• liquid hydrocarbons: CO-Electrolysis/Electrolysis - Fischer Tropsch -> Combustion

Question 16

Green Hydrogen for Electricity Storage - duration & cost of storage

Answer 16

• battery: minutes - hours; low cost
• Hydrogen: days - months; medium to high cost
• Methane & liquid hydrocarbons: low; ca. 16%stored easily over long periods but are expensive

Question 17

Hydrogen - Climate impact - Well-to-tank

Answer 17

• comparatively high production efficiencies
• low global warming potential
• comparatively low water use
• comparatively low land use

Question 18

Hydrogen - Climate Impact - In-Flight

Answer 18

• Until now: aviation contributes significantly to anthropogenic climate change
• no carbon-based in-flight emissions
• water as main product
• NOx also forms within the hydrogen combustion process (dep. on flame T, equivalence ratio, residence time…)
• large uncertainties with climate impact of non-CO2
• large uncertainties with effect of hydrogen as fuel for overall climate impact

Question 19

Effects of Emissions - CO2

Answer 19

• long-lived greenhouse gas
• not directly emitted by a H2 aircraft
• can be emitted during H2 production

Question 20

Effects of Emissions - NOx

Answer 20

• indirectly affects the climate by
  - ↓CH4 leading to cooling effect
  - ↑ozone leading to a warming effect -> net warming
  . formed during combustion (& to a way lesser extent in HT FCs)
• strongly dependent on combustion process design & parameters as well as fuel properties

Question 21

Effects of Emissions - Hydrogen

Answer 21

• may need to be vented
• indirectly affects the climate by increasing in stratosphere
  - ↑CH4
  - ↑ozone
  - ↑H2O -> net warming
• significant uncertainties

Question 22

Effects of Emissions - contrails/contrail cirrus

Answer 22

• short-lived
• affects climate by “trapping” thermal irradiation -> net warming
• potentially most significant contributor to aviation’s climate impact
• formation affected by soot & other particle emissions -> act as nucleation sites -> no soot in H2 aircraft
• affected by ambient humidity & temperature
  - formation in low T ice-supersaturated air
  - rerouting/intermediate water storage may allow reducing contrails
• high uncertainties associated

Question 23

Effects of Emissions - soot & sulfur

Answer 23

• affect local air quality
• affect contrail formation

Question 24

In-Flight Climate Impact - Research Effort

Answer 24

• better understanding of aviation’s overall climate impact required
• investigations of influence of soot on contrail formation using chase aircraft
• further studies on contrail mitigation options
• emphasis on low-box combustion designs
• tool development towards adjusting aircraft design objectives towards least climate impact

Question 25

Take Away Messages I)

Answer 25

- H2 fundamentally different properties than kerosene - energy density - volatility - diffusivity - mostly used in liquid form, affecting - supply chain - handling & refuelling - storage (&its interaction in aircraft) - H2 needs sustainable production to decarbonise production - production via electrolysis - location-dependence of favourable renewable sites - liquefaction as a considerable electricity consumer needs to be taken into account - green H2 also considered in other sectors - many uncertainties when it comes to overall climate impact of aviation in general & H2 aviation specifically - H2 aircraft can reduce/avoid in-flight emission of CO2, soot, sulfur - NOx emission strongly depends on detailed layout of energy conversion process - effect of contrails & mitigation as a major uncertainty & research effort