Alternative aviation fuels and energy sources are intended to form the basis for climate-friendly flying in the future. The aviation sector is focusing on hydrogen propulsion systems, novel battery technologies, and synthetic fuels for the future.

Due to its high energy density, hydrogen is considered a future fuel for aircraft. Hydrogen also has a higher energy density than lithium-ion batteries for electric cars, as it can store more energy per unit of weight and consequently achieve greater ranges than an electric motor. Therefore, hydrogen can store more energy than batteries of the same weight. Hydrogen also holds an advantage over batteries in terms of refueling time. A further benefit of hydrogen is its capability for longer-term energy storage compared to batteries.

Batteries, on the other hand, offer high efficiency in converting electricity into propulsion and can generally be designed to be lighter, as they do not require heavy pressure tanks. The development of new battery technologies is focused on increasing energy density and reducing weight.

Hydrogen requires special tanks and conditions for storage and transport. Hydrogen must be stored either under high pressure or in cryogenic liquefied form. Both methods require robust tanks and specialized materials, which increases weight and complexity.

Additional components required:

In the case of a purely electric-powered aircraft, a hydrogen system requires not only a tank but also a fuel cell to convert the hydrogen into electricity, as well as other components that also contribute weight.

Nonetheless, hydrogen is considered a crucial building block for a climate-friendly energy supply, as its utilization produces no greenhouse gases. However, its production requires splitting water into hydrogen and oxygen via electrolysis using electrical current, a process that demands significant energy input.

Challenges for Industry in Hydrogen Storage:

The storage and transport of hydrogen present a challenge due to its low volumetric energy density and high volatility. Consequently, hydrogen is stored either as a compressed gas under high pressure or in a deeply cooled, liquefied state.

Objectives:

- Increase storage density while accounting for the weight of the tank and the energy loss during compression
- Store as much hydrogen as possible within a given volume

Conclusion: Although hydrogen offers a higher energy density, the necessity for heavy pressure tanks and other components results in an overall hydrogen storage system having a higher total weight than a comparable battery. For applications where weight is a critical factor, such as in motor vehicles, this can be a disadvantage. Due to its low storage efficiency under ambient conditions, storing a significant amount of hydrogen requires large volumes. The tanks are designed for high strength to maintain the extreme pressure, making them heavy. Consequently, the specific energy of the entire compressed hydrogen storage system is reduced.

- Colour: Colourless
- Odour: Odourless
- Lightest Element: Hydrogen is approximately 14 times lighter than air
- State of Matter: Gaseous at room temperature
- Energy Content: Possesses the highest energy content per unit of mass of all chemical fuels:
- Lower Heating Value (LHV): 120 MJ/kg / For comparison: Kerosene: 43 MJ/kg
- Density: Very low density: At 273 K and 1 bar, the density of gaseous hydrogen is only 0.0899 kg/m³ / For comparison: Air under the same conditions: 1.225 kg/m³; Kerosene: 800 kg/m³ / Hydrogen density increases under high pressures and at very low temperatures, close to absolute zero
- Usability: Can be utilised in a cooled liquid state (cryogenic hydrogen storage) or in a gaseous state under high pressure (compressed hydrogen storage)
- Specific Gravity: Liquid hydrogen has a specific gravity of 70.99 g/l / For comparison: Kerosene has a specific gravity of 730 g/l
- Melting Point: -259.125 °C
- Boiling Point: -252.882 °C
- Flammability: Pure hydrogen is not flammable, but is highly combustible when mixed with oxygen
- Magnetism: Hydrogen is diamagnetic

- Reactivity: Hydrogen is highly reactive and readily enters into chemical reactions with other elements
- Flammability: When combined with oxygen, hydrogen is highly flammable and forms explosive mixtures. The combustion of hydrogen primarily produces water. However, at high temperatures, nitrogen oxides (NOx) can also form due to the reaction of nitrogen from the air with oxygen
- Redox Properties: Hydrogen can act as both a reducing agent (electron donor) and an oxidizing agent (electron acceptor), depending on the reaction partner
- Formation of Hydrogen Bonds: Hydrogen can form hydrogen bonds with highly electronegative elements such as oxygen and fluorine, which has significant implications for the properties of molecules in which these bonds occur
- Formation of Compounds: Hydrogen is a key component of many organic and inorganic compounds
- Energy Carrier: Hydrogen can be used as an energy carrier, as it releases a significant amount of energy when combusted with oxygen
- High Volatility: Hydrogen is a very light gas and diffuses easily through porous materials and even through some metals
- Diamagnetism: Hydrogen is diamagnetic, meaning it is repelled by a magnetic field
- Isotopy: Hydrogen has three isotopes: Protium (¹H), Deuterium (²H), and Tritium (³H). The isotopes differ in their number of neutrons and thus have slightly different properties

In hydrogen storage methods, a distinction is made between cryogenic hydrogen storage (liquid hydrogen; LH2) and compressed hydrogen storage (gaseous hydrogen; GH2). The pressure requirements for gaseous hydrogen (GH2) and the cryogenic requirements for liquid hydrogen (LH2) result in heavy storage tanks.

Liquid hydrogen (LH2) is hydrogen gas that has been cooled to a very low temperature of -253 degrees Celsius (20 Kelvin), causing it to liquefy. In this state, it has a higher energy density than gaseous hydrogen, making it easier to store and transport. Its primary application lies in the storage and transportation of hydrogen, particularly for applications requiring high energy density, such as in aerospace or for specific vehicle applications.

Liquid hydrogen is colorless, not self-ignitable, and has a slight bluish shimmer.

Since the energy content of liquid hydrogen is very high and hydrogen as a fuel is much lighter than kerosene, liquid hydrogen offers future potential for aircraft manufacturers. Cryogenic tanks made of aluminum are used for the storage of liquid hydrogen. Hydrogen storage methods that are not yet technologically mature include metal and chemical hydrides. Liquid storage significantly increases the tank's storage capacity, as liquid hydrogen has a considerably higher density, approximately three times higher than 350-bar gaseous hydrogen.

Gaseous hydrogen is a colorless, odorless, and tasteless gas. It is the lightest gas. Gaseous hydrogen itself is not explosive, not self-igniting, and can only be ignited in mixture ratios starting from 4% H₂ in air.

Gaseous hydrogen occupies a very large volume relative to its weight under high pressure (23 grams per liter. For comparison: liquid water is 1000 grams per liter). Under standard conditions: while 1 liter of liquid water weighs approximately 1000 grams (1 kg), 1 kilogram of gaseous hydrogen occupies a volume of about 11 cubic meters (11,000 liters). This means that gaseous hydrogen has a very low volumetric density compared to water, even though it possesses a very high energy density for its weight.

Density Comparison:

-Liquid water: approx. 1000 g per liter
-Gaseous hydrogen: approx. 0.09 kg per cubic meter or 0.09 g per liter under standard conditions

Energy Losses in Hydrogen Compression:

-Practical energy requirement for hydrogen compression at 700 bar: 5-20% of the Lower Heating Value (LHV)
-For comparison: the theoretical energy requirement is only 4%

Advantages of liquid hydrogen

Higher Energy Density:

- Gravimetric energy density: 33.3 kWh/kg
- Average volumetric energy density: 2.4 kWh/l (vs. ~0.003 kWh/l for 700 bar GH2)

Less Weight:

- The tanks can be designed with smaller dimensions. Liquid hydrogen is therefore significantly more space-efficient than gaseous hydrogen

More Suitable for Long-Haul Flights:

- Due to its superior volume-to-weight ratio, liquid hydrogen is ideal for long-haul flights

Disadvantages of liquid hydrogen

- Extremely low temperatures required (-253°C): Necessitates specialized insulation and cryogenic technology

- Boil-off losses: A portion of the hydrogen evaporates over time, even with effective insulation, leading to efficiency losses

- Complex handling: Refueling and storing liquid hydrogen are more demanding processes compared to GH2

Gravimetric Energy Density: The amount of energy that can be stored per unit of weight (mass) of a substance (stored energy per mass). It describes a measurement of the stored energy per mass of any given substance (e.g., fuel, battery, or other energy sources).
Volumetric Energy Density: The amount of energy that can be stored per unit of volume (liter or cubic meter) of a specific substance (stored energy per volume).
Calorific Value (or Heating Value): The amount of energy that can be obtained from the complete combustion of a fuel.

Compared to kerosene, hydrogen requires four times the volume to achieve the same amount of energy. It burns significantly hotter than kerosene and approximately ten times faster. The mass of hydrogen required to provide an equivalent amount of energy for a jet powered by kerosene is approximately one-third; that is, the mass of hydrogen needed for an equivalent energy output is about one-third of the mass of kerosene required.

Mass Fraction: The mass of the fuel is used to quantify the efficiency of fuel tanks. The Mass Fraction (MF) is defined as the fuel mass divided by the mass of the tank system including the fuel.

Gravimetric Storage Density: The gravimetric index is a measure of the gravimetric storage density, indicating how much hydrogen (in weight percent) can be stored in a specific storage medium. The gravimetric index is a measure of a medium's storage efficiency (in this case, hydrogen), as it indicates how much hydrogen can be stored relative to the weight of the storage system.

Significance: A high gravimetric index is desirable because it means the system can store more hydrogen per unit weight. This is particularly important for applications like fuel cell vehicles, where weight plays a crucial role in determining range. Hydrogen inherently has a high gravimetric energy density, but its low density makes storage challenging. Therefore, it is important to develop efficient storage methods that enable a high gravimetric index.

Comparison: A gravimetric index of 15% means that 15% of the total weight of the storage system consists of hydrogen.
Example: If a hydrogen storage system weighs 100 kg and 15 kg of that is hydrogen, then the gravimetric index is 15%.

A gravimetric index of 15% is not achievable with gaseous storage due to the tank weight and the low density of hydrogen.

$MF := \frac{m_{\text{Fuel}}}{m_{\text{Fuel}} + {m_{\text{Tank}}}}$ = Mass fraction

${m_{\text{Fuel}}}$ := Fuel mass (Kerosin; JET A)
${m_{\text{Tank}}}$ := Mass of the empty tank

Aircraft powered by LH2 require a modified fuel delivery system to manage the increased volumetric flow and cryogenic temperatures. This system includes a heat exchanger to convert LH2 into GH2 prior to combustion, as well as redesigned fuel lines, pumps, seals, and valves. Since there is no standardized method yet for calculating the weight of these systems, their weight is combined with the tank mass and expressed as the gravimetric index GI of the fuel system.

$GI := \frac{m_{\text{Fuel}}}{m_{\text{Fuel}} + {m_{\text{Fuel}}}_{\text{System}}}$ = Gravimetric index
(System Weight as Gravimetric Index (GI) of the Fuel System)

${m_{\text{Fuel}}}$ := Fuel mass, LH2 (Mass of stored fuel)
${m_{\text{Fuel}}}_{\text{System}}$ : Fuel system mass (Mass of the entire fuel system: Mass of the entire H2 fuel system: Tare weight of the storage tanks, heat exchangers, and fuel delivery components such as pipes, pumps, valves, and seals)

The following applies: GI <= MF

Kerosene can achieve very high mass fractions (effectively 1.0) using integral fuel tanks built into the wing structure. To achieve specific energy parity between hydrogen and Jet A at the system level, a GI of 0.34 for the hydrogen fuel system would be required.

In aircraft powered by hydrogen fuel cells, new components are introduced. The sizing of these new hydrogen-based components influences the aircraft's geometry and necessitates a modification of the aforementioned formula. The newly considered components include:

-Hydrogen fuel cell powertrain: hydrogen tank, fuel cell
-Fuselage geometry and mass resulting from the implementation of a hydrogen fuel cell propulsion system
-Other relevant powertrain and auxiliary components: novel pipe system

Hamburg is becoming a research hub for future generations of hydrogen-powered aircraft. The Hanseatic city has announced trials for maintenance and ground handling processes involving liquid hydrogen. Streamlining these procedures is intended to facilitate the transition to sustainable fuel for future aircraft generations. This development platform is being implemented in collaboration with Lufthansa Technik in Hamburg, the German Aerospace Center (DLR), the Center for Applied Aeronautical Research (ZAL), and Hamburg Airport. An Airbus A320 family aircraft is being converted into a stationary laboratory.

Is a higher volumetric flow rate required for the combustion of LH2 or GH2 compared to the combustion of kerosene?

Yes, the combustion of gaseous hydrogen (GH2) or liquid hydrogen (LH2) generally requires a higher volumetric flow rate than the combustion of kerosene. This is because hydrogen has a lower volumetric energy density than kerosene, necessitating a larger quantity of hydrogen to be burned to release the same amount of energy.

Energy Density: Kerosene has a higher volumetric energy density than hydrogen, both in its liquid and gaseous forms. This means that a given volume of kerosene contains more energy than the same volume of hydrogen.

Volumetric Flow Rate: To generate an equivalent amount of energy, a larger volume of hydrogen must be combusted resulting in a higher volumetric flow rate.

Liquid vs. Gaseous Hydrogen: Liquid hydrogen has a higher volumetric energy density than gaseous hydrogen as its molecules are packed more densely. Consequently the volumetric flow rate for LH2 is somewhat lower than for GH2, but it is still higher than that for kerosene.