Evaluating Jet Pump Integration in CO₂ Power Cycles: Brayton vs Rankine Performance

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Evaluating Jet Pump Integration in CO₂ Power Cycles: Brayton vs Rankine Performance

Introduction

Using CO₂ as a working fluid in power generation or waste heat recovery brings strong potential, especially when employing supercritical or transcritical states. A question arises: can a jet pump (ejector) that uses high-pressure CO₂ feed (before the turbine) serve as the feed pump or partial compressor, reducing the mechanical compression work? How would that affect net cycle output and cost, particularly in gas-only (Brayton) cycles vs Rankine/ORC type cycles with phase change?

Below is a practical assessment of that concept, the benefits and limitations, and how it might perform under various temperature regimes.

What is a Jet Pump / Ejector in this Context

A jet pump or ejector uses a high-pressure motive fluid to entrain a lower-pressure fluid, raising its pressure without a mechanical compressor. In CO₂ cycles, you might imagine:

Having a portion of CO₂ at high pressure (pre-turbine or recycled from turbine exhaust) act as the motive stream;

It entrains lower-pressure CO₂ (from feed or recirculated loop) and raises its pressure to some intermediate value;

This reduces the required work from a conventional compressor or pump.

If properly designed, jet pumps can reduce mechanical energy input, reduce moving parts, and simplify maintenance. However, jet pumps are typically less efficient than mechanical compressors, particularly at high compression ratios, or when motive fluid pressure available is not much higher than the target pressure.

Comparison: Gas-Only (Brayton) vs Phase-Change (Rankine/ORC) Cycles

Below are the two cycle types, followed by evaluation of how jet pump integration might help, and where it runs into limits.

Brayton / Supercritical CO₂ Brayton Cycle

Characteristics:

Working fluid remains gas or supercritical throughout (no condensation to liquid).

Components: compressor(s), heater (heat source), turbine (expansion), cooler (reject heat), often with recuperator(s) or recompression loops to improve efficiency.

Efficiency strongly depends on turbine inlet temperature, pressure ratio, compressor work, heat exchanger effectiveness. (Very sensitive to losses.) ([turn0search11] & [turn0search16])

How a jet pump might be used:

Use motive high-pressure CO₂ (after heater or before turbine) to entrain lower-pressure CO₂ (e.g. from cooler or feed) to reduce compressor load.

Could serve as pre-compression or booster stage: the jet pump lifts pressure from feed to a moderate level, then a smaller mechanical compressor finishes the job.

Advantages: fewer moving parts for part of the pressure rise; possibly less mechanical energy; simpler maintenance for that stage.

Practical limitations:

Jet pump pressure lift (difference between motive inlet pressure and mixed outlet pressure) is limited: motive must be significantly above target to generate effective entrainment. Efficiency of jet pump falls off with higher required lift or low entrainment ratio.

Losses: mixing losses, irreversibilities in ejector, losses in motive fluid expansion. These reduce net gain.

Requires high-quality motive supply: high pressure, high temperature, good flow. May reduce net turbine output if motive fluid is drawn from the turbine output or needs recirculation.

Rankine / ORC (Phase-Change) Cycle

Characteristics:

Working fluid (could be water, but in CO₂ applications, often transcritical CO₂ or in some cases phases crossing critical point) undergoes liquid → vapor (or gas) → expansion → condensation → pumping of liquid.

Compressor (“pump”) work in the liquid domain is very small compared with gas compressors; phase change requires latent heat; efficiency depends on the temperature difference between heat source and sink, quality of heat exchangers, condensation sink temperature. ([turn0search8] & [turn0search11])

How a jet pump might be used:

A jet pump could “boost” the feed liquid side (or saturated fluid after condensation) instead of using a mechanical compressor/pump, or reduce the work on the compression side by giving initial lift; but pumping liquid has lower work already, so marginal benefit may be less dramatic.

In phase-change cycles, much of the work savings come from keeping the compression in the liquid domain, so using high-pressure motive fluid to raise feed pressure may offer incremental gains.

Practical limitations:

Mixing losses in ejector for fluids maybe partially vaporized or have quality issues.

Jet pumps do poorly when fluid is liquid or two-phase, or when motive and entrained flows are very different pressures or flows. They may require high motive pressure overhead.

For cycles with condensers below critical point (meaning CO₂ must condense), heat rejection is required; if sink temperature is high, phases may not properly condense, reducing performance.

Practical Assessment for CO₂ Cycles with Jet Pump vs Compressor + Pump vs Mechanical Compressor

Here is how the two cycles stack up when a jet pump is introduced, given different turbine inlet / heat source temperatures. We assume:

Recuperated / recompression Brayton setups where relevant.

ORC/transcritical Rankine setups for phase-change where feasible (i.e. condensation possible).

Good heat exchanger effectiveness, moderate losses in turbine, compressor, condenser.

| -• | -• | -• | -• |

| ~100 °C | Phase-change (Rankine/ORC) with condensation + pump wins; jet pump offers small benefit if feed pressures close; Brayton would be very inefficient. | Jet pump might reduce a small fraction of feed pump work, but pump work in liquid domain is small anyway; net efficiency for OrC ~5-12%. Brayton likely very poor (~<5%). | At low temperatures, latent heat dominates; gas compression work too large. |

| ~300 °C | Mixed: for moderate size, ORC or transcritical CO₂ likely still better unless Brayton cycle with high pressure ratio, good recuperation and jet pump absorbs compressor load. | Jet pump might reduce compressor size/work; but overall Brayton might start to approach ORC if TIT & pressure ratio high; efficiencies maybe ~20-30% for both depending on design. | Performance tradeoffs rely on cost vs complexity; jet pump adds complexity in ejector design. |

| ~500 °C | Gas-phase (sCO₂ Brayton) with compressor (aided by jet pump) likely outperforms ORC; condensation becomes harder; condensers or coolers need low sink temperature. | With jet pump helping compressor or feed boost, net compression work drops; cycle efficiency perhaps 35-45%. ORC likely much lower at such TIT. | Also depends on sink temperature, material/turbine limits. |

| ~700 °C | Strongly favors supercritical CO₂ Brayton; jet pump boosting feed or pre-compression could help; ORC not competitive. | Jet pump can provide useful lift so compressor sees lower pressure ratio; net output higher. Efficiency perhaps ~45-50% in good designs. | High TIT improves turbine work; compressor work still significant but jet pump helps reduce it. |

Conclusion

For low heat source temperatures (≈ 100 °C or below), phase-change cycles (Rankine or ORC) with mechanical pumping are superior; jet pump offers limited gains.

As temperatures rise (≈ 300-500 °C), Brayton / supercritical CO₂ cycles become competitive. Jet pump integration can help by reducing effective compression work, especially if the motive fluid pressure is already high and available.

At high enough temperatures (≥500-700 °C), a Brayton cycle with compressor + turbine + jet pump booster (if well engineered) typically yields more net work output than a Rankine cycle, given similar heat source and sink conditions.