add new subclasses of hydrogen

[AhlemRhayem]

Proposed New Ontology Entities

1. Proposed Label Turquoise hydrogen: add and update hydrogen subclasses #2274 #2277 ✅
   • Proposed Superclass: Hydrogen
   • Proposed Definition: "Hydrogen produced by methane pyrolysis."
2. Proposed Label Liquid Hydrogen: add and update hydrogen subclasses #2274 #2277 ✅
   • Proposed Superclass Hydrogen:
   • Proposed Definition: "hydrogen that has been [...] brought to liquid state" at -253°C"
   • Definition Sources/References (links to any reference material that can help others better understand the entity):
3. Proposed Label : Low carbon hydrogen ❌
   • Proposed Superclass Hydrogen:
   • Proposed Definition: "Low-carbon hydrogen comprises hydrogen produced from non-renewable energy sources that achieve greenhouse gas emissions savings of 70% compared to unabated fossil fuels. One approach of producing low-carbon hydrogen includes reforming natural gas if applying carbon capture, utilisation, and storage (CCUS) to prevent the CO2 stemming from the process from being released into the
     atmosphere. This is achieved by either chemically binding the captured CO2 in long-lasting products or by permanently storing it in deep underground geological formations."

I am aware that

• every entry in the ontology should have a definition
• classes should arise from concepts rather than from words
• any ontology content that is proposed in this issue may be altered as needed to fit the structure and conventions of the OEO

[stap-m]

from oeo-dev 125:

1. instead of turquoise hydrogen:
   • primary label: methane pyrolytic hydrogen
   • alternative label: turquoise hydrogen (source: IEA?)
   • definition sources:
     • https://www.weforum.org/stories/2021/07/clean-energy-green-hydrogen/
     • See section 2.1 of this IRENA report for definitions of grey, blue, turquoise, green hydrogen: https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2020/Nov/IRENA_Green_Hydrogen_breakthrough_2021.pdf
2. liquid hydrogen:
   • similar to "liquid air": EquivalentTo hydrogen and (has state of matter value liquid)
3. low carbon hydrogen:
   • tricky to define
   • maybe add a construction via Information Content Entiy (ICE) where different definitions exist
   • needs more research / discussion

[stap-m]

I'll also update the definition source for green, blue and grey hydrogen according to IRENA.

[AhlemRhayem]

Regarding the low carbon hydrogen it is about how the hydrogen is produced: Fossil based, renewable hydrogen and low carbon hydrogen.

https://www.lhyfe.com/more-about-hydrogen/from-renewable-hydrogen-to-fossil-hydrogen/
https://www.europarl.europa.eu/RegData/etudes/BRIE/2025/767227/EPRS_BRI(2025)767227_EN.pdf