Basket

Liquid hydrogen for aviation

Colaboration between AeroDelft and Teesing.

AeroDelft has one mission: to prove that emission-free aviation is possible by pioneering electric aircraft powered by liquid hydrogen. Dreaming of a future where emission-free flying is possible: that was the dream AeroDelft started with in 2017. They formulated a very concrete goal: to fly from Amsterdam to London with zero emissions. The energy carrier in that case could be hydrogen. But to have sufficient energy density, the hydrogen will have to be massively compressed as a liquid on board. And that presents quite a few challenges. Challenges that AeroDelft is addressing and Teesing is helping with ultra light assemblies in aluminum.

Photo: three generations of hybrid-electric and hydrogen-electric aircraft as planned by Airbus.

The main shortcoming of batteries of the current generation is their low energy density compared to kerosene. For example, the density of lithium-ion (Li-ion) batteries widely used in the automotive sector is about 200 watt-hours per kilogram (Wh/kg). By comparison, the energy density of kerosene is about 50 times higher. Even if Li-Ion has further room for improvement, aircraft electrification needs something much more powerful.

AeroDelft did consider and test batteries, but it results in too high a mass, so they abandoned that. There is a belief within the team that batteries are never going to achieve the necessary density either.
The energy density of hydrogen is also not naturally sufficient. Existing vehicle systems work with gaseous hydrogen (GH2) and a tank pressure of 350 bar. This is sufficient for small vehicles such as city cars, forklifts and stationary power generators. But for heavier truck traffic, systems that work with gaseous hydrogen at 700 bar refueling pressure are being looked at to create sufficient range while not taking up too much space for the hydrogen tanks.

The same thing plays out with an airplane and to an even greater extent - the plane needs more hydrogen than it could carry.
Further compression into liquid is then the only option. That means we are talking about a cryogenic system with liquid hydrogen (LH2) at a low operating pressure (about 5 bar). To maintain the cryogenic state, the hydrogen must remain cool: -250 degrees Celsius. This is the challenge that not only AeroDelft with 50 team members is working on: major manufacturers Airbus are also working on zero emission aviation based on hydrogen.

De zero emission aviation roadmap

Airbus takes electrification of flying seriously: a ZEROe Development Center for hydrogen technologies is set to open in Stade, northern Germany, in 2024. The center will accelerate the development of composite hydrogen system technologies for the storage and distribution of cryogenic liquid hydrogen. They will focus mainly on cryogenic hydrogen tanks based on composite technology to achieve cost-competitive lightweight hydrogen systems.

That is, however, a solution that will only be available in the long term (2035 at the earliest). Therefore, Airbus envisions an interim solution in the form of hybrid-electric propulsion. Only hybrid propulsion gives only a 5% improvement in energy efficiency and reduction in CO2 emissions, they expect.
And future hybrid and all-electric aircraft will need megawatts of power to operate. This implies huge improvements in power electronics in terms of integration, performance, efficiency and component size and weight. The illustration below shows the energy challenge we face.

Photo: AeroDelft test rig with an electrically driven propeller where the electric motor is powered from a hydrogen stack. This test rig works (still) with gaseous hydrogen.

Illustration: to electrify aviation requires a huge amount of on-board energy. Airbus envisions an interim phase where aircraft fly on hybrid electric systems before full hydrogen aircraft are a reality. (Source image: Airbus)

Photo: one of the development rooms at AeroDelft. With a partially assembled hydrogen-powered prototype aircraft in the foreground.

AeroDelft's system

Our hostess, Lucille Guda, explains that AeroDelft initially created a system based on gaseous hydrogen. That was tested with a separate propeller and works. The similarity to vehicle systems as we know them is great: hydrogen is stored under high pressure and brought to the working pressure of the hydrogen stack in 2 steps. The electronic control and auxiliary systems such as dehumidifiers and humidifiers are all integrated in house.

In parallel, they are developing the liquid hydrogen-based tank system. This makes sense because the whole system is almost the same after the liquid hydrogen is converted to gas by a heat exchanger.

For the liquid hydrogen system, AeroDelft chose a double-walled aluminum tank. The insulation to keep the hydrogen at -250 degrees Celsius is based on a vacuum between the 2 tank walls.

Evaporation of liquid hydrogen

Insulating the hydrogen tank is just one of many challenges - another major challenge is to vaporize the liquid hydrogen in a controlled manner so that it can be fed into the fuel cell.

Few measurement and control components can withstand cryogenic temperatures while being small and light. And the expansion and contraction of components when the temperature of hydrogen is increased from -250 degrees to ambient temperature also plays a role.

Therefore, heating is done in 2 steps: the liquid becomes gaseous with heating tapes. Next comes a tubular heat exchanger that takes care of heating from -250 degrees to ambient temperature, which the pressure regulator after it can handle. The output of this flow meter/regulator is used by the control electronics to keep the system pressure at 3 bar.

Photo: the heat exchanger and electronic control unit that turn liquid hydrogen back into gaseous hydrogen for the fuel cell.

Photo: the cryogenic liquid hydrogen system in test setup.

Power management of the hydrogen stack

Good flight requires instantaneous control of the power of the electric motor driving the propeller. But controlling the power of a fuel cell is difficult and too slow. That is why a hydrogen-powered aircraft will always need a battery as a buffer, AeroDelft believes. The goal is to start regulating the fuel cell by consumption.

The batteries are also needed to provide peak power at takeoff and for emergencies. The batteries are charged from the fuel cell during flight. There is a separate workgroup that deals with this: the Flight Profile workgroup. At takeoff the maximum power is needed, at cruise speed and altitude consumption is about 2/3 of the maximum power, giving opportunity to recharge the batteries.

And LOHC? Ammonia versus hydrogen

In various applications with our customers, we see hydrogen bound to other molecules to overcome the disadvantage of high pressures and low energy density of hydrogen gas or low temperatures of liquid hydrogen (called LOHC technology). For example, ammonia which contains hydrogen in bound form. This allows hydrogen to be transported under low pressure. For liquid, green, ammonia, the pressure required is atmospheric when cooled to -33.4 degrees or the pressure is about 7 to 10 bars at room temperature. Moreover, the energy density of ammonia is significantly higher than that of hydrogen. So it can be stored in much thinner, smaller and lighter tanks that are significantly cheaper. There are also safety risks that may be insurmountable for aviation, as ammonia is highly toxic. In addition, the technology is still in its infancy, so that route is not being explored by either AeroDelft or Airbus.

We use cookies just to track visits to our website, we store no personal details. We also us commercial cookies, accept them for a better user experience. Please refer to our Privacy Policy.