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Hydrogen's Role in Aviation's Clean Future


We preface this by stating we aim to provide an objective outlook - engineers' perspectives. Kapshur Labs was founded on an unquenching desire to put public good above all else. This writeup is geared toward medium/long-haul subsonic air transport where our work is currently focused on. We attempt to explain to the public as best we can and to the extent we are permitted.

Most recent forecasts suggest aviation alone may account for 24% of global carbon dioxide emissions by 2050 if the current industry pace of improvements continue while other sectors transition away from carbon.

Here's a look at various paths:

  1. Incremental Evolution: technological and operational efficiency improvements
  2. Carbon-Neutral: offset carbon emissions, sustainable aviation fuels (SAFs)
  3. Hybrid: hybrid electric propulsion with use of SAFs
  4. Zero-Carbon: hydrogen combustion, renewable hydrogen production
  5. True Zero: hydrogen fuel cell, renewable hydrogen production, battery-electric, renewable charging

Below we explain these options.

1. Incremental Evolution

This is for the most part the current industry trajectory at large.

Over each generation of aircraft, airframe manufacturers take steps to improve aerodynamic design to lower drag, and implement composite materials and structures to reduce weight. 

Engine manufacturers make incremental strides in fuel efficiency with each generation by use of composite materials, adaptive airflow management, size, etc.

Another area is the optimization of air traffic control networks and clustered flight formations.

2. Carbon-Neutral

Some of this is already taking place: offsetting emissions with greater investments in renewable energy implementation and reforestation efforts. 

Development of SAFs, such as biofuels, synthetic fuels, waste-to-fuel can reduce the carbon impact of traditional fossil fuels. 

3. Hybrid

Similar to some ground vehicles, a hybrid electric/combustion powertrain can reduce fuel burn, absorbing and utilizing waste energy. 

4. Zero-Carbon

Kerosene is replaced with hydrogen. Burning hydrogen produces water vapor, and nitrogen oxide emissions (no carbon). This requires modifications to traditional combustion engines and optimized storage methods. Renewable hydrogen production infrastructure rivaling the cost advantage of traditional fossil fuels is critical in a purely market-based landscape.

5. True Zero

One option is hydrogen fuel cell, which may reduce fuel capacity needs by 20-40% over hydrogen combustion, producing water.

Another option is to pursue battery-electric systems. Battery technologies need significant improvements in energy density to even begin to compete with other options. This is by far the cleanest method, but is far from a reality in the near future. 

Value Proposition of Hydrogen

Compared to SAFs and traditional jet fuel, hydrogen considerably reduces green house gas (GHG) emissions. While hydrogen combustion produces nitrogen oxides and water, both of which have radiative forcing effects (sunlight absorbed by earth and energy radiated back into space), it eliminates harmful carbon emissions. Fuel cells also emit water vapor, which although is a GHG can be mitigated with careful operating parameters. 

Hydrogen has a gravimetric energy density three times that of kerosene (33kWh/kg). Hydrogen refueling is likely much faster than charging batteries, enabling quick turnaround times. 

Hydrogen refueling may help facilitate the transition from kerosene, requiring new piping and (lower) fluid temperatures, as opposed to ultra-fast charging, hot swapping battery replacement options, and localized energy infrastructures. 

Challenges: Hydrogen Combustion

The heavy storage requirements may significantly reduce hydrogen's advantage, as well as its low volumetric density. However, even after accounting for this, hydrogen beats current battery energy densities (0.3 kWh/kg) by a significant margin.

While burning hydrogen eliminates carbon dioxide, carbon monoxide, and sulfur oxides, water vapor and nitrogen oxides are released - which accounts for some GHG atmospheric levels. To mitigate nitrogen oxides, industry promising options include Lean Direct Injection and Micro-Mix Combustors. The latter could produce lower nitrogen oxide levels than kerosene engines. 

Water vapor released from combustion poses significant challenges: contrails and Aviation Induced Cloudiness (AIC). The scientific and engineering community are debating on how this effect compares to a conventional gas turbine. Conventional gas turbines produce soot that act as nucleation points for water vapor; the result is persistent contrails and potential high altitude clouds. Eliminating hydrogen fuel impurities can significantly decrease contrails, however contrail duration could increase from higher water vapor concentrations. 

Hydrogen conversion will require design changes to the propulsion systems, fuel storage, and delivery. Regardless, a much smoother, rapid transition can be expected over the sweeping redesigns required for fuel cell and battery-based propulsion systems. 

Challenges: Hydrogen Fuel Cell (HFC)

HFC could allow a true zero with regards to GHG emissions. The only byproduct is water. However, each kilogram of fuel cell reacted hydrogen produces roughly nine kilograms of water. As with hydrogen combustion, this poses contrail and AIC challenges. Research shows the clean electrolysis process minimizes impurities, and hence the likelihood of contrail and AIC. Lowering operating altitudes and the ability to store the water for release in lower altitudes are possibilities (though its effects are yet to be studied).

Key Barriers

Overall, there are a few obstacles toward hydrogen's role in aviation's green transition. 

  1. Airframe and propulsion redesign. Propulsions, fuselage, fuel storage will need modifications and overhauls. HFC will require complete redesign compared to the partial changes for hydrogen combustion.
  2. Hydrogen storage. Liquid hydrogen offers high volumetric density over a gaseous state. However, this requires cryogenic storage below -253 degrees Celsius and heavy tanks - reducing tank-to-wing efficiency significantly. Additional advances in cooling and light-weight tanks are required.
  3. Clean hydrogen production. Significant strides need to be made toward low-cost, renewable methods of hydrogen conversion/extraction. "Gray" hydrogen produced with GHG emissions currently costs 65% less then "green" (zero-emission) hydrogen production. However, promising new research show lower-cost methods to produce emission-free hydrogen. The transition to peak load renewables will likely involve hydrogen as an energy capture option, increasing supply channels and reducing cost.
  4. Infrastructure. Investments in hydrogen infrastructure is necessary to facilitate fuel delivery to airports and airport refueling systems. Existing natural gas transportation networks and systems can be leveraged to  transport hydrogen gas. Long haul transport mechanisms options may need considerations, as well as localized liquification systems at airports. 
  5. Cost. Hydrogen costs significantly more than kerosene to produce presently on an emission-free basis. Hydrogen production is often met with skepticism due to its many steps in energy conversion. But in the event improvements in battery systems fall short of the unforgiving and rather steep requirements of medium to long-haul flight, hydrogen may be the only clean option.

Hydrogen's success may depend on the demands of multiple energy sectors converging. With a push for a renewable future, this is very likely. Increasing the cost of carbon can help accelerate this process. Policy makers must understand the intricate complexities of working toward a clean future, and that involves looking at the entire picture; the details matter.

Our Work

Here at Kapshur Labs, we lean toward hydrogen combustion as the near future for aviation's green transition. Why?

One year into our hybrid-composite cycle turbine research projected to burn at least 75% less fuel over next-gen engines, we see the path to clean aviation. Our early-stage tests on modified legacy systems running on standard Jet-A fuel are exceeding expectations. The added complexities of our design will require meticulously rethinking design for hydrogen combustion compatibility. But we don't foresee major changes in system architecture, which will help us move quickly within various technical and logistical constraints. With such high efficiency gains, our platform will require just a fraction of the hydrogen storage and cooling capability of conventional propulsion systems. That's a huge advantage. Our in-depth investigation into this future has started.

We're excited. Are you?

Comments are welcome below, especially questions - we may address them in future posts as best we can. We will likely follow up with more, regardless.

Written collectively by Kapshur Labs engineers