Miguel Ángel Castillo – Director of Technological Development Aernnova Group
8 March 2023
The use of hydrogen as an energy vector in aircraft to achieve zero CO2 emissions has came to light with the announcement of the launch of the Airbus ZEROe programme, which aims to bring a 100-passenger short-range commercial transport aircraft into service in 2035. This context is leading to the launch of major research and technological development projects for the use of hydrogen in aeronautic, as it is the opportunity for us to fly without burning fossil fuels.
Aernnova is a founding member of Clean Aviation, a project in which we have a relevant participation, financed with 1800 M€ of public aid and part of Horizon Europe, the European Union’s Framework Program for Research and Innovation. The project is divided into three pillars, one of which is exclusively dedicated to hydrogen.
The European commitment has been made and, together with the aeronautical technological objective, assumes that by the middle of the next decade there will be a green hydrogen supply chain, generated by renewable energies and a safe distribution system at the airports where the new aircraft operate.
Liquid hydrogen, LH2, as an energy vector in aircraft, can be translated into direct combustion in engines, a priori more suitable for large transport aircraft, or it can also feed fuel cells that generate electricity for electric or hybrid-electric engines. This second line is of particular interest for general and regional aviation.
Hydrogen combustion has its advantages over kerosene fuel: no CO2 is emitted although three times as much water is emitted which, depending on flight altitude, generates condensation clouds that contribute to the greenhouse effect. This is currently being studied and is the subject of scientific controversy because although contrails last less time in the atmosphere – maximum 14 hours – compared to CO2 – decades – they can have a greater greenhouse effect.
Hydrogen has the advantage of having a higher specific energy than kerosene. In fact, space launch vehicles use liquid hydrogen (LH2) as fuel. But its energy density is only 25% that of paraffin, which means larger fuel tanks. Increasing the size of the tanks implies major challenges in the design of the aircraft by having to lighten both the tanks and the supporting structure. Added to this is the requirement to design thermally insulated tanks to maintain a temperature of -253°C inside the tank to preserve the liquid phase, which implies an integral thermo-mechanical design. Other relevant requirements are impermeability to avoid leakage, a venting system and that the tanks have a service life comparable to that of the aircraft.
These challenges are great and for structure and system suppliers, such as Aernnova, they translate into developing, designing and integrating new aerostructures, such as liquid hydrogen tanks, in a safe and competitive way. Competitiveness will come from the lightness of the solution, its design, and the industrial means to manufacture it. Hence the interest in the use of composite material as a design basis. The need for lightness in aeronautical structures and systems offers a possibility for new developments in composite materials as opposed to metallic solutions. And a starting condition for developments in aeronautics: there have been no flights with LH2 tanks made of composite material.
Aernnova, together with important partners in the European aeronautics sector, has been working on a feasibility project for the development of liquid hydrogen tanks for aircraft. Our partners were the aeronautical manufacturers IAI from Israel and HAI from Greece, the technology centres DLR (Germany), INTA (Spain) and RISE (Sweden), the University of Patras in Greece, and the SMEs Invent (Germany), Oxeon (Sweden) and Cryospain (Spain). As a result of these studies that have kept us busy beyond the first part of last year, we concluded that it was possible but that we needed the support of Clean Aviation. Our proposal was called HYTALIA, and addressed the development of a composite LH2 tank to achieve a gravimetric index above 35%. It included very significant scopes and although it was not selected, it has helped us to characterise the most relevant aspects for our future challenges as we configured an LH2 tank composed of three key elements: the inner tank, the outer tank and the thermal insulation system as can be seen in the figure below along with other key subsystems.
