May 6, 2022, ©. Leeham News: Last week, we looked at how we create the shaft power for the thrust device we discussed before. We described the basics of a hydrogen-burning gas turbine alternative.
When we have liquid hydrogen as fuel, several advanced developments are possible. It’s what we look at now.
In previous articles, I described several concepts for how the cryogenic (-253°C) liquid hydrogen (LH2) could be used to cool different parts of a gas turbine. Such concepts were studied in the EU ENABLEH2 project, Figure 2.
Not only did the engine get parts cooled so it could run at higher efficiency, but the LH2 was transformed to H2 gas of high temperature (from 24K to 700K), which increases its energy content (heating value).
A problem with H2 gas turbines is the water vapor in the exhaust. Advanced research is now underway to gain from the water vapor rather than see it as a drawback.
Water injection in gas turbines has been used for 70 years to increase their thrust (the VTOL Harrier Pegasus engine needs water injection for the hovering flight).
Water injection and water vapor recovery from exhausts to form a closed-loop system has been used for stationary gas turbines for decades. It was also studied for airborne engines in MTU’s WET studies.
An engine that recovers the water vapor in the exhaust and uses it to enhance its efficiency is more effective when fueled with LH2 (the WET studies were for kerosene cores).
First, we have more H2O in the exhaust to recover and circle back into the engine; secondly, the LH2 at -253°C is the perfect coolant for the core exhaust heat exchanger that condensates and recovers the water.
Pratt & Whitney’s HySIITE (Hydrogen Steam Injected, Inter‐Cooled Turbine Engine) explores these techniques. The project was awarded $3.8m in February from the US Department of Energy and its Advanced Research Projects Agency-Energy (ARPA-E) to develop base technologies for such an engine.
The two-year program will develop critical parts of such a system, such as the LH2 cooled heat exchanger for water vapor condensation. The focus will be on weight, volume, and H2 combustion’s long-term compatibility.
The next phase for HySIITE would be a ground demonstrator core where techniques such as water vapor injection in the compressor, combustor, and turbines will be studied. The potential for efficiency gains over a non-injected engine is considerable.
Today’s best cores are at 55% efficiency. With HySIITE, this could pass 60%. The concept focuses on a semi-closed loop system that is essentially a bottoming cycle (bottoming cycle = use of a waste product from the process to increase its efficiency) designed to recover water in the exhaust using a condenser. An engine for airline use could be available latter half next decade.
The MTU WET studies foresaw a possibility of capturing the water vapor and dumping it as liquid water into the atmosphere. Should the flight trials with Airbus A380 show we have a real problem with the water vapor causing troublesome contrails, vapor capture technology such as foreseen in HySIITE could be a way to get to this problem.
Gas turbines are highly efficient powerplants for our airliners. When they burn hydrocarbons, they emit CO2, CO, NOx gases, etc. Sustainable Aviation Fuels (SAF) can compensate for the CO2 emissions in their creation phase to create a net-zero CO2 emission. Still, as it’s further processing of a hydrogen derivative and involves CO2 capture, it costs more than hydrogen to produce.
Hydrogen for airliners is only practical in its liquid form, LH2. Complaints are that the additional energy to liquefy hydrogen to LH2 is wasted energy. With concepts such as HySIITE and ENABLEH2, the -253°C of LH2 is aggressively used to increase the LH2 gas turbine core to higher efficiencies than our kerosene cores. We make use of the extra investment in LH2.
We shall add the advantage of a fuel cell APU instead of today’s inefficient and noisy Jet -A1 APUs to the above.
In summary, we will see a lot of development on how to smartest use the LH2 if we get it onboard our airliners.
It will be interesting to see what hot parts of a LH2 engine get cooled and how.
There are lots to choose from. Further along if FAA/EASA develops certification standards and tests for these components containing LH2 in and around the engines. Would be nice for a LH2 engine competition with a $1 bn cash price for the highest efficiency achieved in test cell for a 30k engine.
It will be interesting to see how engine inlet icing is prevented on the precooler. One way is to regulate it so never gets to below the freezing point of water. I believe the SABRE air breathing compound rocket planed to use methanol anti freeze spray.
I apologize if this question is stupid, but does water injection increase thrust because it increases the mass of the flow (i.e. the air-water mixture is denser than pure air)? – Thanks
Both, you can burn more fuel before reaching TGT limit and you increase massflow in the turbines both increase engine power to compressors and fan.
OT: water vapor is actually less dense than dry air.
Air ( 28,96), N2 (28), O2 (32), vs H2O ( 18 )
You add mass when you add the water that vaporizes into steam. You can input it onto hot structures in the burner casing that has the mass and heat to vaporize the water flow or like the old P&W JT9D-7AW have special injectors. Even more effective as mist injection into the compressor. It came out of fashion on aircrafts when somebody put fuel instead of water into one…
“you increase mass flow” TRUE 🙂
Water injection adds procedural complexity,
and stress on engines that were maxed out already.
“It came out of fashion on aircrafts when somebody put fuel instead of water into one…”
Happened just around the corner where I grew up. Bit of a heartbreaking story.
Use ended with progress in engine performance.
It also ended because you needed a separate water tank on the plane and the hassle of filling and maintaining the extra system. With HiSIITE you have an engine-contained continuous system, quite a difference.
By replacing Carbon (in fossil fuels) with Hydrogen there may not be any COx but the NOx .. a more lethal pollutant is still produced (yes.. Air is 80% Nitrogen)…
However, if burnt with pure Oxygen… this could well become a Zero Pollution engine/craft… and then there is fuel cells with electric motors… but could be quite heavy…
This is just a first small step in a long journey to a Zero Pollution Earth… but as mankind faces challenges… it WILL come up with answers… but more work needed here to create a truly ZERO POLLUTION aircraft…
Dear Bjorn, how is power efficiency calculated? A 55% efficiency is already extremely impressive. I assume the Power is calculated from P=F.v where F is thrust and v is some benchmark reference speed near cruise speed (presumably around 200m/sec or 440mph). The efficiency n would thus be n=(energy in 1 second of fuel flow)/P.
Also is there a standard reference speed for ehp?
the 55% is the core or thermal efficiency, so there is no simple formula to calculate it. I use GasTurb with state of the art turbine machinery efficiencies, pressures, and temperatures to produce these values. But the numbers are also mentioned by engine designers in our discussions with them around these engine technologies.
Turbo compounding has been done in both aircrat (piston) and land diesel engines.
Its never proven to be a net winner due to the higher complexity.
I continue to believe you treat emissions as a whole solution, focus on the best bang for the buck (getting rid of Coal) and see where that gets you (and whatever solar you can get, I don’t believe wind is ever going to be a large scale solution that solar can be)
Yes, kind of surprised there are no LNG “Liquid Natural Gas” aircraft plans, less CO2 emissions than JET A-1 and a natural step towards LH2. LNG is nowdays shipped everywhere and the technology is mature. Qatar a major LNG producer and aircraft buyer.
I agree. It is a reasonable first step and it takes up much less volume for the same amount of energy compared to LH2. This would make it reasonable to put cylindrical tanks below the cabin floor and not take up valuable cabin space.