As we look together from the last few posts, hydrogen (H2) has arisen to be one most potential propulsion for aiming near-zero emission aviation. But you shall ask [“How to harness hydrogen energy to power an aircraft?”; “How is an energy source can produce only zero-emission byproducts?”; “What are the additional system should be implemented to aircrafts to accommodate hydrogen propulsion?”]. This Python-syntaxed array of questions is the thing we’ll scrutinize together….
* Sometimes you even need to debug the questions to obtain the right answers. Btw do you know what syntax error with the Python code above?
SImilar to how you choose installing shower or gayung in your bathroom, now the propulsion varieties are narrowed down to hydrogen combustion engine or hydrogen fuel cells. They use the same power sources: water and hydrogen respectively, but the processes are different. One takes the water directly from a bathtub by your hand, the another need additional system to manage the flow of water before reaching your skin. The analogy goes equivalently to hydrogen combustion engine, which leverages the existing engineering work of the current commercial aircrafts (turbofan, turboprop, etc) versus hydrogen fuel cells who needs brand new systems that don’t exist before [2].
This illustration below show the super simple scheme of hydrogen fuel cells and combustion system applied on aircraft. While the fuel cell type converts the liquid hydrogen (LH2) from the tank to generate electricity and rotate the new engine, the combustion type takes the LH2 right away from the tank to be burned in combustion chamber (like how the aircraft engines work nowadays). You don’t need the scheme of gayung and shower, do you?
Let’s dive deep into the characteristics of each. By the way, the main literature comes from University of Michigan in the US, one of the author is Joaquim R.R.A. Martins [2]. He’s like N’Golo Kante, who covers any spaces in the pitch. Similarly, Martins covers almost all topics and researches of aircraft design and multidisciplinary design and optimization (MDO)-related. Go look at the paper yourself to get more comprehensive view. But don’t ask him about Gayung, ask him fuel cell instead.
Hydrogen Fuel Cells
First thing first, aligned with the primary objective of hydrogen aircraft to reduce emission, the only byproduct of hydrogen fuel cells is water vapor, which gives no emission at all [2]. That’s an apparent advantage, but everything has downfall. Hydrogen fuel cells has constraints of specific power and thermal capability. The recent advancement of specific power is merely 0.75 kW/kg, which isn’t considered suffice enough. The target is 2 kW/kg by 2035 [4]. Therefore, this type is preferred for shorter range/fewer number of passenger’s aircrafts. While hydrogen combustion types are dominant for bigger aircrafts as depicted on this dotted map [2]. To put into perspective, a Jakarta-Surabaya trip (range of 570 nautical miles [5]) utilizes a hydrogen fuel cells airplane. On the other hand, you may take hydrogen combustion airplane to reach Indonesia’s super priority tourism destinations of Labuan Bajo from Danau Toba.
There are two common types of fuel cells widely produced: Polymer Electrolyte Membrane/Proton Exchange Membrane (PEM) and Solid-Oxide Fuel Cells (SOFC). Uniquely, that PEM abbreviation comes from 2 phrases, like ATM from Automated Teller Machine and Anjungan Tunai Mandiri hahaha. Both has their own exhaustive characteristics, but one main differentiation is their operating temperatures. PEM works perfectly in low temperature (30-100 C), while SOFC should be installed in relatively higher temperature (600-1000 C) [6]. With these specified operating conditions, they need a cooling system as thermal management and many other systems, such as humidification and pressurization. The whole fuel cells system mass properties can be quantified as Balance of Plant (BoP). The BoP measures how heavy the peripheral system masses (thermal, pressure, humidity management) in proportion of fuel cells mass. Surely, it’s preferred to have a small BoP to minimize the overall mass.
But then how does fuel cell works? For now, just make it simple as your high school’s chemistry class level. The chemical reactions occur between hydrogen (H2) (enters to anode) and air/oxygen (O2) (get into cathode), then yield electrons movement. Again by basic physics knowledge, the electrons flow generate electrical current that will rotate the motors. That’s it, classic and simple (not sure though!)…
Long enough already? Combustion explanation is sneaking over the corner…
Hydrogen Combustion Engine
As proclaimed before, this type leverages the existing combustion engines. Although, hydrogen and avtur ain’t the same at all. So, some components inside must be adjusted and optimized accordingly. Cranfield’s researchers state in their paper, “The major changes come from combustor due to hydrogen’s wider flammability limit enable burn leaner than kerosene” [7]. Shall we trust them? I’ll follow their lead. Then, the combustor design must be adjusted to avoid flashback phenomenon. FYI, flashback isn’t only element in movies, but for aerospace engineering as well. This is a simple epitome on how aerospace engineering is closely related to our daily life. Flashback here means a phenomena where pre-mixing happens due to hydrogen’s reactivity level, so that the mixing might travel upstream of combustor [2]. That’s why combustor should be re-designed. Meanwhile, other components like compressor, turbine, fan, and nacelle barely require changes.
One disadvantage of combustion over fuel cells: it remains producing an emission byproduct Nitrogen Oxide NOx (but lower than by conventional jet fuel) in addition to water vapor [2]. The target set by 2035 is reducing NOx up to 80% by the H2 combustion engine [4]. Even though that downside, the big four (MU, Liverpool, Arsenal, and Chelsea) (GE Aerospace, Rolls-Royce, Pratt & Whitney, and Safran) have already tested or build the new re-designed hydrogen combustion engines [2][3]. As aforementioned, this type suites the higher range and more passengers’ aircrafts due to its higher specific power than fuel cells [2].
Next question, how hydrogen combustion engine works? More or less is the same as existing jet combustion engines. Do your own research this time and let me know after hehehe…
The more options, the better judgement?
Do you agree with that statement? Or somehow, more options make us more confused? Apparently, choosing hydrogen fuel cells or combustion engine is not as easy as your inner conflict to install shower to your existing gayung in your bathroom. Multi-faceted deeper analysis shall be made…
MW
If you wanna know more…
[2] Eytan J. Adler and Joaquim R.R.A. Martins. “Hydrogen-Powered Aircraft: Fundamental Concepts, Key Technologies, and Environmental Impacts.” Progress in Aerospace Sciences (2023).
[3] https://www.cnbc.com/2022/12/02/rolls-royce-uses-green-hydrogen-in-jet-engine-test.html
[4] Gandon, F., 2022. Hydrogen: a fuel for the next generation of short-range aircraft? ONERA, SciencesPo, ISAE-SUPAERO, France.
[5] https://12go.asia/in/travel/surabaya/jakarta
[6] K. M.Spencer and C. A. Martin. Investigation of Potential Fuel Cell Use in Aircraft. Tech. rep. IDA Document D-5043. Institute for Defense Analyses, Dec. 2013 (cited on page 17).
[7] Bhupendra Khandelwal et al. “Hydrogen powered aircraft: The future of air transport”. In: Progress in Aerospace Sciences 60 (July 2013), pp. 45 59. doi: 10.1016/j.paerosci.2012.12.002 (cited on pages 27, 28).
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