September 18, 2020, ©. Leeham News: In our series on hydrogen as an energy store for airliners we analyze the conversion of the present Turbofan and Turboprop airliner engines to hydrogen as fuel instead of carbon-based fuels.
We know it’s possible as the world’s first jet engine from 1937 ran on hydrogen, Figure 1. But what are the problems and how good are the hydrogen-fueled engines in efficiency and emissions?
There are no big changes needed to convert a gas turbine to run on hydrogen instead of carbon-based Jet fuel.
It has been analyzed in detail in several studies and converted research engines have confirmed the research findings.
The major changes are in the combustor. It needs a different design to burn the gaseous hydrogen which enters at about 200K (-73°C). To take the LH2 from 20K liquid to 200K gas just before entering the engine a heat exchanger is used.
The exchanger can be placed around the exhaust of the gas turbine, but more sophisticated approaches can be used that also enhances the efficiency of the engine.
A straight converted engine with a simple heat exchanger has the same efficiency as the carbon fueled engine. As hydrogen has three times the energy content the fuel consumption will be one third.
The hydrogen burns a bit differently which means the turbine will run about 40K cooler for the same performance level. This will increase the turbine life for the engine.
An important difference is the emission level. As no carbon is burned the emissions are H2O and NOx. So the CO2 emission problem is solved.
The increased level of water in the exhaust has to be managed by flying slightly different flight levels. With these precautions, the level of contrails in the sky can be kept at the same levels as today. The NOx level can be lowered to about 20% of today’s engines by careful design of the combustor and its processes.
The heat needed to convert 20K LH2 to 200K gas can be used to increase the engine’s efficiency. Different alternatives have been looked at.
One is an intercooled compressor. The principle is the same as for an intercooled turbo engine for a car.
Another idea is the use the hydrogen to cool the cooling air for the turbines. The first turbine stages are cooled by air from the last compressor stages to tap air at a pressure that is above the turbine pressure. This air is hot, around 500-600°C. If the hydrogen heat exchanger cools this air, the turbine runs cooler and can develop more power for the same size turbine.
These smarter use of the heat exchanger can save up to 5% in fuel consumption compared with a straight converted engine.
In addition to a redesigned combustor with injectors, the fuel system needs adaptations. But in all, the conversion of today’s engines is straight forward and they are gamer changers in emissions.
The CO2 problem is gone and NOx emission levels are improved. The contrails need work but there are clear schemes on how this shall not be a new problem.
With further developments, the hydrogen gas turbine can surpass carbon fueled engines in efficiency.
With the performance and reliability we have achieved with today’s gas turbine-based propulsion systems, I see little cause for focusing the first steps to hydrogen on fuel cell hybrids.
Fuel cell research shall be focused on replacing the APU and that the fuel cell energy source can be powerful and reliable enough to replace today’s engine-driven generators.
This energy is then used in a more electric aircraft for de-icing, environmental control systems, and electricity consumers in the aircraft (non-active generators are kept on the engines as backup).
This requires efficient and reliable fuel cells of several MW and is a big enough challenge. We can also use these fuel cells to start work on small (20 to 50 seat) electrical hybrid turboprops to learn this trade in steps.
With well-converted hydrogen engines and a more electric aircraft driven by a fuel cell replacing the APU, we not only solve the emission problems but can also drive further efficiency into air transport.