CO2 Power Cycles Compared: Rankine/ORC vs. sCO₂ Brayton Across 45 °C to 700 °C
CO2 Power Cycles Compared: Rankine/ORC vs. sCO₂ Brayton Across 45 °C to 700 °C
Brayton Cycle: a gas-phase cycle (often supercritical CO₂ Brayton, sCO₂) using a compressor → heater → turbine → cooler → compressor loop, typically with recuperation (and often recompression) to lift efficiency. It stays single-phase; no condenser. ([OSTI][1])Rankine / ORC: a phase-change cycle that pumps liquid, boils/evaporates in a heater, expands through a turbine, then condenses. With CO₂ specifically, true subcritical Rankine needs a sink below the 31 °C critical temperature; otherwise CO₂ cycles are transcritical. ORC usually means non-CO₂ organic fluids at low temperatures. ([ScienceDirect][2])sCO₂ Brayton shows very high thermal efficiency once turbine-inlet temperature (TIT) reaches ~450–600 °C, with ~40% at ~500 °C reported for optimized layouts, and ~50% near ~710–720 °C (recompression variants). ([ScienceDirect][3])At low-to-moderate heat-source temperatures, ORC and transcritical CO₂ cycles are favored; reviews report single-digit to low-teens percent at ~100 °C for ORC and transcritical CO₂, and ≤~20–24% for high-temperature ORC around a few hundred °C. ([MDPI][4])Multiple DOE/Sandia/NETL sources position sCO₂ Brayton as compact and efficient above ~300–450 °C, with recompression cycles excelling as TIT climbs. ([OSTI][5])What to pair with the turbine: compressor (Brayton) vs. condenser+pump (Rankine/ORC)?Rule of thumb:Below ~200–300 °C heat sources → condense & pump (ORC or transcritical CO₂) generally beats gas-only Brayton because compressor work dominates at low TIT and recuperation cannot recover enough. ([MDPI][4])≥~450–600 °C heat sources → sCO₂ Brayton (with recompression/recuperation) surpasses ORC/Rankine on efficiency and power density. ([ScienceDirect][3])Temperature-by-temperature recommendations45 °C heat source (very low grade)Best fit: ORC with a very low-boiling organic fluid, or transcritical CO₂ in niche WHR layouts.Typical efficiency band: ~3–8% depending on temperature lift and pinch constraints; 45 °C is on the edge of practicality for power. ([MDPI][4])Brayton? Not advisable. Gas compression losses overwhelm at such low TIT.Verdict: Condense & pump (ORC/transcritical) wins on net output at this temperature.100 °C heat sourceBest fit: ORC or transcritical CO₂ power cycle; literature reports single-digit to low-teens thermal efficiencies at ~100 °C when well optimized. ([MDPI][4])Brayton? Still poor; insufficient TIT for competitive Brayton efficiency.Verdict: Condense & pump (ORC/transcritical) is more efficient and uses less input work than gas-only Brayton.300 °C heat sourceBorderline region.ORC: High-temperature ORC can approach ~20–24% in the best commercial/academic reports. ([ScienceDirect][2])sCO₂ Brayton: With recuperation (possibly recompression), sCO₂ at 300 °C can be competitive to superior depending on pressure ratio and heat exchanger effectiveness; Sandia/DOE work positions sCO₂ as “high efficiency >300 °C” but not yet at its sweet spot. ([OSTI][5])Verdict: Tie/lean ORC at 300 °C if your HX pinch and recuperator performance are modest; lean Brayton if you can run an optimized recompression layout with high-effectiveness recuperators.500 °C heat sourcesCO₂ Brayton: Now in the strong zone. Optimized designs report ~40% cycle efficiency at ~500 °C (recompression variants). ([ScienceDirect][6])ORC: Typically below this, even with advanced fluids. ([ScienceDirect][2])Verdict: Pair the turbine with a compressor (i.e., sCO₂ Brayton)—more net output and less specific input work than condense-and-pump routes at the same source conditions.700 °C heat sourcesCO₂ Brayton: Multiple studies and program overviews show ~45–50% with recompression near ~700–720 °C, depending on cooling conditions and HX effectiveness. ([sco2symposium.com][7])ORC: Well below sCO₂ Brayton at this temperature range. ([ScienceDirect][2])Verdict: Brayton by a wide margin for efficiency and net output.Why these crossovers happenBrayton (sCO₂) needs TIT. Compressor work is a large penalty at low TIT; only with high recuperation and high TIT does Brayton recover enough enthalpy to outpace ORC. DOE/Sandia show this inflection as temperature rises above ~300–450 °C. ([OSTI][5])Rankine/ORC leverage pumps. Liquid pumps raise pressure at tiny specific work versus gas compressors; at low source temperatures, this dominates, making condense+pump pathways superior. ([ScienceDirect][2])Bottom-line comparison table| Heat-source temp | Better turbine pairing | Expected efficiency band (indicative) | Rationale || • | • | • | --• || 45 °C | Condense & pump (ORC or transcritical CO₂) | ~3–8% | Only ORC/transcritical are practical; Brayton compressor work dominates at such low TIT. ([MDPI][4]) || 100 °C | Condense & pump (ORC/transcritical CO₂) | ~6–12% | Reviews show single-digit to low-teens at ~100 °C; Brayton still not competitive. ([MDPI][4]) || 300 °C | Borderline (site-specific) | ~20–30% class (layout-dependent) | High-temp ORC ~20–24%; sCO₂ Brayton can rival with strong recuperation and low pinch losses. ([ScienceDirect][2]) || 500 °C | sCO₂ Brayton (compressor-turbine) | ~40% (optimized recompression) | Published analyses show ~40% at ~500 °C for recompression sCO₂. ([ScienceDirect][6]) || 700 °C | sCO₂ Brayton (compressor-turbine) | ~45–50% (best cases) | Multiple studies/papers target ~50% near 710–720 °C. ([sco2symposium.com][7]) |Practical takeaways for design1. If your source is ≤100 °C: plan ORC or transcritical CO₂; the condense+pump paradigm uses far less input work than gas compression. ([MDPI][4])2. Around 300 °C: run a trade study; if you can afford high-effectiveness recuperators and a recompression layout, sCO₂ Brayton can compete; otherwise high-temp ORC is solid. ([ScienceDirect][2])3. ≥500 °C: go sCO₂ Brayton with recompression; expect a step-change in cycle efficiency and power density. ([ScienceDirect][6])[1]: https://www.osti.gov/servlets/purl/1378068?utm_source=chatgpt.com Supercritical CO2-Brayton Cycle[2]: https://www.sciencedirect.com/science/article/pii/S1364032113000592?utm_source=chatgpt.com Techno-economic survey of Organic Rankine Cycle (ORC) ...[3]: https://www.sciencedirect.com/science/article/pii/S0029549320302612?utm_source=chatgpt.com A review of research and development of supercritical ...[4]: https://www.mdpi.com/2227-9717/11/7/1982?utm_source=chatgpt.com A Comprehensive Review of Organic Rankine Cycles[5]: https://www.osti.gov/servlets/purl/1108068?utm_source=chatgpt.com Summary of the Sandia Supercritical CO2 Development ...[6]: https://www.sciencedirect.com/science/article/pii/S0306261920301562?utm_source=chatgpt.com A novel supercritical CO2 recompression Brayton power ...[7]: https://sco2symposium.com/papers2018/power-plants-applications/093_Paper.pdf?utm_source=chatgpt.com Optimizing the Supercritical CO2 Brayton Cycle for ...
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