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Real Gas Effects in Carbon Dioxide Cycles
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1969
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Fuel CycleCarbon DioxideEngineeringHeat RecoveryEnergy EfficiencyEnergy RecoveryMechanical EngineeringGreenhouse Gas EmissionGas Turbine EngineCarbon Dioxide CyclesEngineering ThermodynamicsRefrigerationChemical EngineeringGreenhouse GasesAndrews CurvePotential PerformanceCarbon CycleCarbon SequestrationHeat TransferFluid MachineryThermal EngineeringEmissions
CO₂ can outperform other gases in thermodynamic cycles because its real‑gas behavior near the Andrews curve reduces compression work, though simple cycles suffer from heat‑transfer losses that limit efficiency. The study proposes six advanced CO₂ cycle designs that lower heat‑transfer losses, preserve low compression work, and achieve efficiencies comparable to top steam cycles, with some tailored for moderate‑pressure nuclear applications. The authors analyze how pressure‑dependent mechanical, thermal, and transport properties of CO₂ improve heat‑transfer coefficients, and evaluate turbo‑machine sizing and regeneration surface requirements. CO₂ cycles demonstrate superior economic performance over ideal‑gas cycles, with cooling‑water demands comparable to those of steam plants.
The potential performance of carbon dioxide as working fluid is recognized to be similar to that of steam, which justifies thorough thermodynamic analysis of possible cycles. The substantially better results achievable with CO2 with respect to other gases are due to the real gas behaviour in the vicinity of the Andrews curve. Simple cycles benefit from the reduced compression work, but their efficiency is compromised by significant losses caused by irreversible heat transfer. Their economy, however, is appreciably better than that of perfect gas cycles. More complex cycle arrangements, six of which are proposed and analyzed in detail, reduce heat transfer losses while maintaining the advantage of low compression work and raise cycle efficiency to values attained only by the best steam practice. Some of the cycles presented were conceived to give a good efficiency at moderate pressure which is of particular value in direct-cycle nuclear applications. The favourable influence on heat transfer coefficients of the combined variation with pressure of mechanical, thermal and transport properties, due to real gas effects, is illustrated. Technical aspects as turbo-machines dimensions and heat transfer surfaces needed for regeneration are also considered. Cooling water requirements are found to be not much more stringent than in steam stations.