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.
“I’d buy that for a dollar”. Great series – thanks.
A quick question regarding materials.
Many materials that are resilient at higher temperatures tend to become brittle at lower temperatures…stainless steel is a good example.
Since hydrogen fuel is very cold, any ideas on possible material modifications required in/near the fuel injection system? Although we’re not necessarily talking about moving parts here, there’s plenty of vibration present…and that tends to be a problem in combination with brittleness.
There are several studies that have looked at this problem and they apply the considerable knowledge of fuel supply to engines in the Space launcher industry.
For the Airbus Cryoplane study, Air Liquide and SNECMA (today SAFRAN) are example companies that were involved, both engaged in space launchers. SNECMA is also an expert on the jet engines we discuss for conversion (50% of CFM).
NASA did quite a bit of research headed by Lockheed and this is summarized in a very well written book by Brewer: Hydrogen Aircraft Technology. Available at Amazon Kindle. It deals with material questions in dept including the fuel systems.
In summary, there are things to consider regarding materials according to these studies but it’s an area where there is adequate knowledge. It’s judged to not present show stoppers.
Of course, engines for use on space rockets are only used for a very short time, as opposed to the very intensive usage (in cycles and years) of an airplane powerplant. But that hopefully won’t present a problem.
SpaceX version of 300 series steel for Starship gets stronger at low temperatures and is weldable.
As for vibration 400 series steels does have high vibration damping that stays high as long as steady state tensile stress is not high.
The space programs has done lots of work on this and normally different Inconel alloys are used, Stationary gas turbines from GE has been runing with high H2 content in the fuel using process gases from steelmills as an example.
So when liquids contianing LH2 are fed the Engines GE/PWA/RR knows how to handle it and optimize the burners for it. The latest Engines NOX emissions are just a fraction of allowed limits.
Using the LH2 to cool the Engine’s compressor and turbines give the design engineers good options, but the EASA/FAA rules have to march in step with it to be able to certify these solutions for better efficiency or running the core Engine colder allowing cheaper materials in the last stages of the compressor and those for the HPT.
The increased volume flow due to liquid to gas phase change (expander cycle) can be used to power electrical alternators, producing some, or possibly all, the electricity needed onboard.
Good series, thanks. The big surprise to me is how easy and economically gas turbine technology can be converted to H2.
Its almost standard for both land based Aeroderivative Turbine to run on natural gas or other gas products. Mostly power generation.
Split frame gas turbines as well but the Aeroderivative would be relevant in this case.
This came out this week:
Yes, this is a low-end solution using prefilled gas bottles (guess 700 bar bottles) that are distributed in bundles like LPG gas bottles are today.
To this you need a fuel cell for energy conversion, and then inverter and motor driving a propeller (most likely) or fan. It will have a short range and questionable economics but offer low emissions.
This is further detail for the Universal Hydrogen proposal
” Its modules, which measure about seven feet long and three feet in diameter, can carry the hydrogen in liquid or compressed gas form to be loaded into the back of the aircraft via standard cargo loading equipment or a forklift.
“According to the company, aircraft of the size of the Dash 8 or the ATR 42 will offer a 400 nm usable range (allowing for reserves) with compressed gas hydrogen or 550 nm with liquid hydrogen. That means they would be able to serve, respectively, about 75 percent and 95 percent of the routes now flown by those aircraft models.”
Liquid hydrogen bottles will be a challenge with the distribution concept shown. You must keep the bottles at 20K for a rather long period or below 73K and above 500 bars if you go for a supercritical bottle type. The first has isolation and gas boil off and therefore venting problems (as you need to vent hydrogen gas to avoid too high pressure), the latter structural weight issues. No aeronautical investigation I have come by have preferred critical tanks because of the structural weight.
Some more detail on the ‘pods’. The intermodal concept is attractive at the moment but time is a factor too when shipping hydrogen.
The information suggests the seating capacity is reduced for balance reasons – 2 rows ?
“So Eremenko’s startup, called Universal Hydrogen, has developed Kevlar-coated, pill-shaped pods — about 7 feet in length and 3 in diameter (2.13m x 0.9m) – filled with hydrogen.
The pods are designed to double as a storage container for transporting the hydrogen, by truck, train or other means, and a gas tank when loaded into a plane. If filled with water, each would hold about 208 gallons, and they can be stacked in racks so that 54 would fit inside a standard freight shipping container. They can even be loaded into a plane with a forklift, he said. The point is that airports wouldn’t need pipelines or underground tanks.
“We want to basically turn hydrogen into dry freight,” said Eremenko. He and other founders have poured about US$3 million into the startup.”
I’d have to add, its only a paper project, I dont think its even made a single ‘pod’ and of course the planes only exist on paper too.
Often these projects are in the fund raising stage for relatively unsophisticated investors who think of ‘getting the next Uber’
The hydrogen handling at airports is a big revenue opportunity for airports and its owners. Storage, cooling, fueling and defueling of aircrafts, some big airports with massive electrical power available can install hydrogen production plants from Linde, Air Liquid or Paxair (now Nippon Gases). The long time desire to tax aviation Jet fuel by many goverments can then be achived thanks to hydrogen and only allowing hydrogen/electrical aircraft to land as they become certified and available in ever bigger sizes.
Why are contrails a problem? Aren’t they basically clouds?
H2O is the biggest greenhouse gas, far exceeds all the rest
This year vs. last year should give some “hands on” information on air traffic climate effects.
Water vapor is a potent greenhouse gas but it mostly works as amplifier to the other green house gases.
While CO2 and the other primary green house gases are “just there” water vapor content is temperature dependent.
A hotter atmosphere intrinsically carries more of it.
The water cycle is also somewhat self-regulating, unlike the other greenhouse gasses. Energy from the sun absorbed at the surface by evaporation, is released by condensation in the atmosphere as clouds. Some of that heat escapes the atmosphere, and the clouds then reduce insolation at the surface.
As Uwe says, the other gasses just hang out in the atmosphere, although there is some buffering effect by the solubility in the oceans, formation of minerals, and photosynthesis. But apart from volcanic release, human activity is the main driver for atmospheric concentrations, up or down, so humans are effectively the regulator. We are doing a poor job of that at present.
Yes, contrails are similar to high cirrus clouds. Research has been done on the effect of cloud cover on global warming, that shows low thick clouds cool the earth, but high cirrus clouds warm it.
The reason is the difference in optical reflection and absorption properties of cirrus clouds, for the wavelengths radiated by the sun, as opposed to those radiated from the earth. There is net gain of energy for cirrus, but a net loss for low clouds.
Thus adding contrails with engine exhaust can add to global warming, but the effect is not large, a few to several percent of the total warming effect from all sources, depending on estimates.
This is a part of the overall focus on aviation as a small but highly visible source of climate change, whereas the much larger terrestrial sources tend to be much less visible.
Adding on to what Rob said, there’s a lot of uncertainty, but contrails and contrail-cirrus clouds may account for 50% of aviation’s climate impact. The magnitude of impact partially depends on the discount rate used, since contrails only last hours while CO2 lasts hundreds of years.
There are almost 200 species of ruminant, in vast quantities on Earth, belching out astronomical quantities of methane every day.
Methane is also being produced in vast quantities by all that organic matter decomposing in bogs/marshes and at the bottom of waterways.
And then there’s the very unpredictable outgassing from metastable methane hydrate on the sea floor.
Against that vast background, it seems a little disproportionate to be fussing about contrails. But, I suppose, if we don’t remove every possible gram of controversy from the possible climate effects of aircraft, we’ll just be giving the Trolltunga ammunition for future attacks.
Thanks Bjorn for your great aanalyses around LH2 topic!
Do you have any roughly idea of how much flight level should be reduced to keep LH2 A/C contrails to today level?
I worry about the consequences on drag and thus range.
A/C (and engine) aerodynamic could be optimised for lower level flights, but not up to match today efficiency.
Alban, you’re right about the trade-off between contrail avoidance and fuel burn. Contrail formation depends on the temperature and humidity profiles in the atmosphere, which vary with location and time. So the strategies are to change the timing, routing, or altitude of flights, with altitude being the most cost-effective method.
The common altitude range for contrails is 29,000 to 41,000 feet. However stepped adjustments within that range (on the order of +/- 2,000 feet) are often sufficient, if the atmospheric profiles are known.
Obviously this is less feasible in crowded airspace with occupied altitude slots, but for long-range cruise it is possible.
To find the optimum, a metric is determined that evaluates the warming contribution from both contrail and fuel burn, then sums them together as the overall contribution of the aircraft. The flight altitude profile (and sometimes route profile) is then selected to minimize the metric.
Numerical studies have shown that the metric can be reduced from 13% to 94%, depending on circumstances, with a common value of around 66%. However the metric begins with a small absolute value for the baseline case of normal flight, so the overall impact is not that large.
Contrails and cirrus clouds are formed from ice crystals, rather than water vapor as in lower clouds. Thus they can “seed” the atmosphere and grow with time, which is why you often see them spread outwards from the initial path. Thus as Albert said, the contribution can be larger in the right conditions.
It’s more about flying different flight levels than lower, as described by others. For an airliner pilot, the weather is a key parameter in deciding your flight plan/flight profile. You already today plan your flight profile based on atmospheric values to minimize fuel and time. This introduces an additional planning parameter, something today’s aviation is used to.
I have also found additional research on the subject. It’s about how the ice crystals from a hydrogen engine are different from today’s engines and this changes the persistence and environmental effect of the contrails. It includes advanced modeling and assumptions, and I would like to understand what’s done and cross-check before describing it, not to put us down the wrong alley.
But if correct, it says that the contrail problem is not increased with a hydrogen engine despite putting out more water, as the formed crystals are larger and by it has a different effect on the atmosphere. But as said, I like to crosscheck that this is valid with today’s knowledge.
Funny thing about tech is when there are a few breakthroughs and old tech gets sexy again and people think its a panacea when its not. We are seeing this with hydrogen, which is getting mature for utility fossil fuel displacement, exciting yes, but not so mature for portable equipment, transport, its really still impinged by other technologies that are holding it back. Not to let that deter serious engineers but usually its myopic ppl jumping on profit bandwagons, this aint for you. People think “hydrogen as a fuel” as monolithic but its really distinct technologies: fixed turbine combustion vs portable fuel cell, totally different. When thinking about contrail reduction or similar rationale, existing totally disparate technology like ground transport, or rewriting rules can alter outcomes as much as trying to crack the hydrogen fuel cell conumdrum. Sure hydrogen fuel cell advances will come someday but im not holding my breathe.
I think most people here understand that hydrogen-powered aviation is not mature, or something that will radically alter climate change. The point of the series is, how might it be done, what are the challenges and possible solutions. It’s an interesting problem and there is a lot we can learn.
Bjorn has pointed out that hydrogen is more feasible than all-electric, so the required technology advances are less daunting to achieve a commercial aircraft. But still something we may not see for a long time.
Also if you’re aware of his interviews, he’s candid about all of this, he’s stated that terrestrial mitigation has the greater benefit and should receive the greater focus. But since these programs are being put forward by governments for aviation, it’s best to be armed with a good understanding of the topics, which he is providing for us here.
So if you receive & participate in the series in that spirit, there is a benefit to be had. If you dismiss it in favor of your own opinion, you deprive yourself of that benefit.
Summarizing that rather evangelical (and strangely North Korean) final paragraph: apparently, there’s only one “officially sanctioned” road to enlightenment, and deviation therefrom is to be frowned upon. In particular, if one finds oneself forming ones own opinion, then one should immediately engage in self-chastisement, for daring to deviate from the centrally approved official reading.
It’s a good job that that attitude doesn’t prevail in astrophysics: if it did, we’d never find out what dark matter is composed of.
To net it out. To fly requires energy. We can generate that by a) burning fossil fuels or b) by using energy that was generated in some other way and stored for using in flight.
The point is to avoid net CO2 so that rules out a). In the case of b) we have several contenders:
2) synthetic hydrocarbons
4) hydrogen fuel cells
5) burn hydrogen
6) some combination of the above
All of b) require a primary source of energy but the idea is to pick a system that results in less CO2 than burning hydrocarbons directly but can still result on aircraft with performance in the same range as todays jetfuel powered ones.
I don’t think anyone is saying there is only one approved way forward. On the contrary the series seems to be exploring the options.
My remark wasn’t directed at the series: it was directed at the last paragraph of the comment prior to mine.
Bryce, please note the issue of contrails has come up before in this series. No one has ever claimed it to be more impactful than it is, or that aviation is more impactful than it is, or that hydrogen is the only solution for reducing carbon, as jkeebo pointed out. The effort was only to provide understanding.
Your posts have implied these things in a derisive way, as you frequently do here, but those assumptions were yours alone.
Would you consider (at some stage) extending this series of articles by reporting on advances in / feasibility of biofuels for use in the aviation industry, such as algal fuels and/or fuels made from recycled cooking fat? KLM is investing heavily in the latter (or was doing so, prior to CoViD).
Obviously, successful implementation of such fuels would be a relatively simple solution, since it would require essentially no modification of existing aircraft design.
Although combustion of such fuels produces CO2, that’s essentially re-absorbed immediately by the production processes concerned; in any case, it’s a much shorter re-absorption cycle than in the case of biomass combustion in power stations.
I will happily do that, though it was dealt with in the series before this.
Please revert to Part 15 (and following weeks) of this series:
Sorry, Bjorn, I wasn’t aware of the previous treatment of this topic by you…I wasn’t following Leeham last Spring.
That having been said, it does pop up in the news from time to time, and there may be new developments to consider:
Seems Airbus is actively looking at Hydrogen powered aircraft:
Airbus shows off hydrogen-fuelled concept aircraft for 2035 service entry
This article appeared this morning on a Dutch-language aviation site.
I just checked Reuters, but found no equivalent (as yet).
The article describes how Airbus has announced an official ambition to be flying hydrogen aircraft by 2035.
Three conceptual aircraft are presented: a replacement for the A320 (120-200 pax, range 3700km), a regional prop plane (max 100 pax, range 1750km), and a “flying wing” concept (200 pax, 3700 km).
In reference to my post above, the same subject matter is now available in English on FlightGlobal:
Shades of early turboprops and small fan engines – large cross-section to get more drag.