Cycles for CO₂ Working Fluid: Rankine, Brayton, Transcritical and Their Differences

Cycles for CO₂ Working Fluid: Rankine, Brayton, Transcritical Explained

Using carbon dioxide (CO₂) as a working fluid in power cycles has become a very active field, especially due to its favorable thermal properties near its critical point. But what kind of cycle corresponds to which conditions — gas-phase only, phase change, or mixed? Below are the main types of cycles, how they differ, and when each applies for CO₂.

What is a Rankine Cycle

The Rankine cycle is the classic thermodynamic cycle in which a working fluid undergoes a phase-change from liquid to vapor to produce work:

1. The fluid is pumped as liquid to a boiler or heat source,

2. Heated until it vaporizes (and possibly superheated),

3. Expanded through a turbine or expander to produce work,

4. Condensed back to liquid in a condenser,

5. Re-pumped.

If using CO₂ in such a cycle, it means you must condense CO₂ and have phase change from liquid to vapor. CO₂ condenses only below its critical temperature (~31 °C) and at sufficient pressure. Such a cycle is feasible when your heat source and sink allow you to cross into the subcritical region (liquid ↔ vapor).

What is Organic Rankine Cycle (ORC)

The Organic Rankine Cycle is a variation of the Rankine cycle that uses organic fluids (hydrocarbons, refrigerants, etc.) rather than water. The advantage is that organic fluids often boil at lower temperatures or pressures, making ORC well suited for low-temperature heat sources or low grade waste heat.

With CO₂, using ORC means you do not use CO₂ itself as the working organic fluid but rather another fluid chosen to match lower heat source temperatures.

If someone refers to “CO₂ ORC,” they might mean using CO₂ in some parts of the system (for example as coolant or in a topping cycle) but usually not that CO₂ itself is substituting as the organic fluid in ORC.

What is the Brayton Cycle

The Brayton cycle is a gas-phase cycle (no phase change) in which:

1. A working fluid (gas) is compressed,

2. Heated at high pressure,

3. Expanded through a turbine,

4. Cooled back to initial state,

5. Repeated.

When using CO₂ in a Brayton cycle, if all steps remain in the gas or supercritical phase (i.e. you never condense to liquid), then that is essentially a CO₂ Brayton cycle. Often called a supercritical CO₂ Brayton cycle when it operates above the fluid’s critical pressure and temperature. The gas behaves more like a dense fluid, improving efficiency relative to air or simpler gases at high pressure.

What is a Transcritical Cycle

A transcritical cycle is one in which the working fluid operates through both subcritical and supercritical states in different parts of the cycle. For CO₂:

It might be pumped as liquid (subcritical) in some intervals, heated past the critical point (supercritical), then expanded and cooled but not fully condensed in the supercritical region.

Transcritical CO₂ cycles are common in refrigeration and heat pump applications, and are also under study for power generation, especially when heat sources and sinks are in temperature ranges such that the phases cross the critical point. ([transcritical cycle wikipedia](https://en.wikipedia.org/wiki/Transcritical_cycle?utm_source=chatgpt.com))

Comparison: Which Cycle is Which When Using CO₂

| Cycle Type | CO₂ Phase Behavior | Major Components | When it applies best |

| --• | --• | --• | --• |

| Rankine | CO₂ must condense (liquid ↔ vapor) | Pump, boiler/evaporator, turbine/expander, condenser | When you have a heat sink low enough (< critical temperature) to condense CO₂; useful if you have large temperature difference and can safely handle condensation and two-phase flow |

| Organic Rankine Cycle (ORC) | Uses other working fluids, not CO₂ as ORC fluid | Similar components (pump, evaporator, expander, condenser) but with fluids tuned to low boiling points | Low temperature heat sources; low-grade waste heat; geothermal; when using organic fluids gives better efficiency than using CO₂ if CO₂ cannot condense well |

| Brayton (CO₂ or sCO₂ Brayton) | CO₂ remains gas or supercritical (no condensation) | Compressor, heater, turbine, cooler (no condenser or phase change) | When heat source temperature and pressure are high; compact turbomachinery; high power density; when working fluid remains supercritical throughout or ends in gas phase |

| Transcritical CO₂ | Some parts subcritical (e.g. pumping liquid), then heat crosses critical point, ends expansion without condensation (or limited condensation) | Mix of pump, heater, turbine, gas cooler instead of condenser, etc. | For applications like refrigeration, heat pumps, waste heat recovery, when heat sink is above ambient but system can access chamber above critical pressure and somewhat high temperature heat sources |

Examples from Recent CO₂ Cycle Research

Several projects combine supercritical CO₂ Brayton cycles with ORC or other bottoming cycles to extract more work from lower temperature heat sources. ([search0search0 & search0search2])

Comparative studies show transcritical CO₂ cycles outperform ORC or subcritical Rankine cycles when heat source is sufficiently high, and working fluid remains efficient in supercritical state. ([search0search4])

If CO₂ is pumped as a liquid, then expanded through a turbine but never allowed to condense back to liquid, that would align with a gas phase (or supercritical) Brayton-type cycle rather than a Rankine cycle.

That configuration cannot strictly be called a Rankine cycle unless a phase change occurs. If your system remains entirely gas or supercritical, it performs more like a Brayton (or sCO₂ Brayton) cycle. Efficiency will depend heavily on specifics: pressure ratio, turbine inlet temperature, heat source/sink temperature, and component losses.

Summary

• Rankine cycles require phase change (liquid ↔ vapor). If CO₂ is used and you condense it, that qualifies.

Organic Rankine Cycle refers to using organic working fluids (not CO₂) for low temperature sources. Not applicable if you stay in CO₂.

• Brayton cycle, or more precisely supercritical CO₂ Brayton, is gas / supercritical only; no condensation. That matches a setup where you pump and expand CO₂ in gas phase.

• Transcritical cycle sits in between: some parts behave like Rankine (liquid), others like supercritical; useful when crossing critical point gives benefits.

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