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Carbon Dioxide (R744) — The Natural Working Fluid

Carbon Dioxide (R744) — The Natural Working Fluid

Carbon Dioxide (R744) — The Natural Working Fluid

CO₂ (refrigerant designation R744) is a natural, low-cost, non-flammable, near-zero-GWP working fluid that shows up in this KB in two distinct cooling roles — as a transcritical mechanical-cycle refrigerant and as the adsorbate in adsorption cooling. It is the fluid the field keeps returning to as synthetic high-GWP refrigerants are phased down; its penalty is high operating pressure and a low critical temperature (31 °C), which pushes practical cycles transcritical.

CO₂ (refrigerant designation R744) is a natural, low-cost, non-flammable, near-zero-GWP working fluid that shows up in this KB in two distinct cooling roles. It is the refrigerant the field keeps returning to as synthetic high-GWP refrigerants are phased down — its penalty is high operating pressure and a low critical temperature (31 °C), which pushes practical cycles transcritical.

Role 1 — CO₂ as a mechanical-cycle working fluid (transcritical)

The collection’s anchor source (Comprehensive performance assessment and multi-objective optimization of a new combined cooling, heating and power system with CO₂ working fluid, Yang et al., Applied Thermal Engineering 2023) is a CCHP (combined cooling, heating, power) system that couples a supercritical CO₂ Brayton power cycle with a transcritical CO₂ refrigeration cycle, arranged so heat exchangers recover overall waste heat into useful heating. Reported at a 170 °C heat source: ~115% thermal efficiency (heat-recovery accounting), ~41% exergy efficiency. The point for a joule-heist strategy: one CO₂ loop family can deliver power, heating, and cooling off a single medium-/low-grade heat source — the trigeneration ideal for a site with waste heat.

Role 2 — CO₂ as the adsorbate in adsorption cooling

CO₂ is also a major adsorption working pair partner — the activated-carbon/CO₂ pair runs through the adsorption collection (coconut-shell and biomass-derived carbons, subcritical and transcritical adsorption cycles, CO₂ onto activated-carbon–graphene composites). Here CO₂ is attractive precisely where water can’t go: sub-zero evaporator temperatures and pressurized, compact systems, without water’s freeze and sub-atmospheric-vacuum problems. See Adsorption Cooling for the working-pairs context.

Why it matters

CO₂ is the through-line between the mechanical and sorption sides of this KB: the same benign molecule serves as a transcritical refrigerant in a compressor cycle and as the adsorbate in a heat-driven adsorption cycle. For a waste-heat-rich site, the CCHP framing is the ambition — trigeneration from one heat source — while the activated-carbon/CO₂ adsorption route is the no-compressor, no-water variant.

Source caveat: this theme rests on a single journal article (plus the activated-carbon/CO₂ thread inside adsorption). Treat the CCHP performance numbers as one modeled case at a 170 °C source, not a general result.

See also