The election has been warmed up by climate impact and cost consideration on the previous part. What are another issues to tackle next? Physical properties and how they influence aircraft architectures in general are being the primary discussion. Is it considered as body shaming for an aircraft?
As a reminder (to myself as well), we have 3 alternatives: Sustainable Aviation Fuel (SAF)/synfuel, battery, and hydrogen, alongside with the current widely used fossil fuel.
Physical Properties
Having lean body, thick skin, or six pack. Are those physical properties are we going to discuss? The answer is “YES”, but in a little bit different sense. If we talk about lean body, it means the volume of the hydrogen fuel (or closely related to volumetric density). Thick skin can be interpreted as how thick the insulation needed to store hydrogen fuel in such extreme temperature (i.e. cryogenic temperature). Then, six pack of our muscle is equivalence to how much energy can be generated by liquid hydrogen (i.e. gravimetric energy density and volumetric energy density), for instance. But how the properties are preferred, that’s what we gonna see together…
* A small intermezzo, if you are interested about the literal physical properties, go read Sports Gene: Inside the Science of Extraordinary Athletic Performance by David Epstein, which explain why Usain Bolt becomes 100-m world record holder (one example of many). Mr Epstein elaborated them very thoroughly, so that I can’t survive reading that till the last pages. I suggest reading it, to make the upcoming Summer Olympic 2024 in Paris more meaningful to you.
Let’s jump straight away to the core! Two most important physical properties that distinguish each propulsion are: gravimetric energy density [kWh/kg] and volumetric energy density [kWh/l]. The naming can be broadly varied in some scientific papers, from specific energy density, specific energy to merely energy density. Although, from my point of view, these two are the most obvious and make most sense.
Gravimetric energy density easily means how much energy can be produced in every unit mass. In contrary, volumetric energy density is interpreted as amount of energy can be generated in every unit volume. These terms were proposed by researchers from Kungliga Tekniska Högskolan, Sweden [4]. For sure, I copy-pasted the university name instead of struggling to manually type this ö haha…
Next question is surely, WHY? In order to compare the different propulsion architecture, we need to take identical aircraft’s flight mission, which can be deducted as the same energy consumption. Then, for the exactly same energy required, which propulsion gives less mass/weight and less volume? Thus, less mass and volume for the same energy (means higher gravimetric & volumetric energy density) are preferred because it’d avoid the snowball effect on aircraft design. Snowball effect happens when extra masses are added, it will iteratively increase aircraft’s overall thrust, lift and finally Maximum Take-Off Weight (MTOW). This phenomenon was well-explained by an academician of HAW Hamburg in Germany [5]. I wonder, how would this phenomenon be named if it were found by someone who lives in a tropical country? Please enlighten me ^^
Fortunately, a summary of gravimetric and volumetric energy density were provided by Roland Berger, a consulting company (not Roland Garros which becomes a “training ground” for Rafael Nadal) [6]. You can see in the screen-captured image below.
Liquid hydrogen (without storage, which is impossible for aircraft application) sets the high standard for gravimetric energy density by dinosaur margin (almost 3 times fossil/conventional fuel). Unluckily, this superiority is diminished when we take into account the storage system (or tank for the fuel). This occurs because the liquid phase hydrogen (LH2) must be stored cryogenically and it needs special treatment. So, the LH2 tank should be designed thoroughly, and surely there will be another writing dedicated for this…
When we take a look into the volumetric energy density, this is being the main downfall of hydrogen propulsion. H2 needs approximately 4 times bigger storage than jet fuel (if 3 times is dinosaur, the how 4 times properly be called?). Then, where does the lovely battery propulsion stand? Not such big of a threat, they are trailed way behind in both key factors. They are relegated for the current technology development. But maybe in the future they will get promoted again if Elon Musk somehow finds a way to improve battery state-of-the-art.
To conclude this post, “liquid hydrogen propulsion requires ~4 times larger volume than jet fuel, while having ~3 times lighter weight than its contender for the same energy requirement”. This were said already by several proceedings, such as by researchers at Cranfield University [8], ONERA, SciencesPo and ISAE-SUPAERO [9], and depicted beautifully by Airbus below [10].
The discussion is getting more interesting, isn’t it?
MW
If you wanna explore more:
[3] Amazon.fr
[4] Franzén, K. and Jangelind, F., 2021. States and Prospects of Hydrogen Storage Technologies in Aircraft Applications. Kungliga Tekniska Högskolan, Sweden.
[5] Cheema, J.S., 2020. The Mass Growth Factor–Snowball Effects in Aircraft Design. HAW Hamburg, Germany.
[6] Berger, R., 2020. Hydrogen A future fuel for aviation. Roland Berger GMBH: Munich, Germany.
[7] https://global.jaxa.jp/activity/pr/jaxas/no088/06.html
[8] Sefain, M.J., 2005. Hydrogen aircraft concepts and ground support. Cranfield University, United Kingdom.
[9] Gandon, F., 2022. Hydrogen: a fuel for the next generation of short-range aircraft? ONERA, SciencesPo, ISAE-SUPAERO, France.
[10] Llewellyn, G., ZEROe: Reducing the climate impact of flying. Airbus.
pamotankid.wordpress.com 