Abstract
The thermodynamic minimum work of separating oxygen from air at standard conditions is approximately 51.3 kWh/t O₂ (192 MJ/t). This is the Carnot-limit floor: it cannot be reduced by any conceivable process improvement. Modern cryogenic air-separation units operate at ~200 kWh/t O₂ for state-of-the-art large plants, with the practical range 150-800 kWh/t depending on configuration. The gap between practical and theoretical is roughly 4× — substantial headroom for efficiency improvement, but not the limiting factor. The fundamental question for q1: LOX supply is not fundamentally constrained at any scale below the Carnot floor. What is constrained is electricity at that scale and capital to build out the ASU fleet — both willingness-to-scale, not physics.
Key claims
- carnot-minimum-air-separation: 51.3 kWh/t O₂ (192 MJ/t) theoretical minimum work for separating O₂ from atmospheric air.
- practical-state-of-the-art: ~200 kWh/t O₂ at modern large-scale cryogenic ASU plants — about 4× the Carnot minimum.
- range-of-current-plants: 150-800 kWh/t depending on size and configuration (Thunder Said Energy literature compile).
- room-for-improvement: 4× gap between current practical and theoretical implies substantial room for efficiency gains as demand scales (driven by capital cost of electricity at scale).
- feedstock-unlimited: Atmospheric oxygen is essentially unlimited; air is 21% O₂ at ~1.2 kg/m³, so ~5 km³ of air contains 1 Gt of O₂ — air feedstock is never the binding constraint.
Reviewer notes
This is the critical reframing finding for q1. The previous draft characterised LOX as "binding at 50-500× current global supply" — but current supply is sized to current demand, not to a hard production ceiling. The Carnot floor establishes that LOX production can scale to any demand level, with the binding constraint being electricity supply (which at Tt/yr scale becomes substantial — see separate extract). LOX itself is NOT fundamentally binding. This is the q1.c12 / q1.c13 reframing.