Direct-Fired Supercritical CO₂ Combined Cycle: A Closed-Loop Alternative to Gas Turbines
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Direct-Fired Supercritical CO₂ Combined Cycle: A Closed-Loop Alternative to Gas Turbines
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By eliminating the air-breathing gas turbine and directly coupling natural gas combustion to supercritical CO₂ Brayton cycles, a new class of closed-loop combined cycle power systems can deliver higher efficiency, simpler heat recovery, and greater scalability than legacy microturbine-based architectures.Replacing the Gas Turbine with a Direct-Fired sCO₂ Combined CycleConventional natural-gas power systems rely on air-breathing gas turbines as the primary Brayton cycle, followed by steam or organic Rankine bottoming cycles to recover exhaust heat. While mature, this architecture carries inherent efficiency penalties associated with air compression, large exhaust mass flows, and limited temperature matching between the turbine exhaust and bottoming cycles.An alternative approach is to eliminate the air-breathing turbine entirely and instead convert natural gas heat directly into power using a closed-loop supercritical CO₂ (sCO₂) combined cycle. In this configuration, natural gas is burned in a high-efficiency burner, and the heat is transferred through compact, high-temperature heat exchangers directly into one or more closed sCO₂ Brayton cycles.The result is a system that preserves the thermodynamic advantages of Brayton operation while avoiding the inefficiencies of open-air compression and exhaust dilution.System Architecture OverviewThe proposed system consists of two closed CO₂ power loops arranged in a temperature cascade:1. Topping sCO₂ Brayton Cycle Natural gas is combusted in a controlled burner. Hot flue gas passes through a high-temperature CO₂ heater, raising the primary sCO₂ loop to turbine inlet temperatures in the 650–715 °C range. This loop uses an advanced Brayton configuration such as partial-cooling or recompression, with high-effectiveness recuperation to minimize compression work and maximize cycle efficiency.2. Bottoming sCO₂ Brayton Cycle Flue gas exiting the topping heater still contains substantial sensible heat. A second, independent sCO₂ loop captures this mid-temperature energy (typically in the 250–450 °C range) and converts it into additional electrical power. This bottoming loop is optimized for heat recovery rather than peak cycle efficiency.Both loops are fully closed, using CO₂ as the sole working fluid, and reject heat through air coolers operating at ambient conditions (70 °F in the design case).Why Direct-Fired sCO₂ Outperforms Legacy MicroturbinesThis architecture fundamentally changes where efficiency is gained and lost:• Elimination of Air Compression Losses Traditional gas turbines devote a large fraction of shaft work to compressing atmospheric air. Near-critical CO₂ compression requires far less work, particularly when well-managed around the critical region.• Higher Effective Turbine Inlet Temperatures Microturbine exhaust temperatures typically limit bottoming cycle performance. Direct firing allows the primary sCO₂ turbine to operate at temperatures comparable to large utility-scale gas turbines, but in a compact, closed-loop system.• Superior Heat Utilization The dual sCO₂ configuration addresses a known limitation of single-loop sCO₂ cycles: the narrow temperature window of heat addition. By splitting heat recovery across two Brayton loops, the system extracts work from a much larger portion of the flue-gas temperature glide.• Working Fluid Simplicity CO₂ is non-flammable, inexpensive, globally available, and free from regulatory phase-down risk, unlike many organic working fluids.Expected Efficiency PerformanceWith modern combustor and heat-exchanger design, realistic performance expectations for a direct-fired dual sCO₂ system are:• Topping sCO₂ loop net efficiency: approximately 43–49 percent (LHV), after accounting for heater losses, pressure drops, and auxiliaries.• Bottoming sCO₂ loop contribution: an additional 3–7 percentage points (LHV), depending on how aggressively residual flue-gas heat is recovered.This yields a total plant efficiency of approximately 46–56 percent on a lower heating value basis, with corresponding HHV efficiencies in the low-to-mid 40 percent range. These values are competitive with, and in some cases exceed, conventional microturbine-plus-bottoming-cycle systems, especially at small to mid scales.Engineering ConsiderationsThe dominant design challenges are not in turbomachinery, but in heat transfer:• High-temperature CO₂ heaters must withstand large thermal gradients, high internal pressures, and cyclic operation while maintaining low pressure drop.• Recuperator effectiveness strongly influences net efficiency; small degradations can materially impact performance.• Control strategy must coordinate two Brayton loops and the firing system while maintaining stable compressor inlet conditions.When these elements are executed correctly, the architecture offers a powerful combination of efficiency, compactness, and modular scalability.ConclusionA direct-fired, closed-loop supercritical CO₂ combined cycle represents a logical evolution beyond legacy microturbines. By removing the air-breathing gas turbine and cascading heat through dual sCO₂ Brayton cycles, the system converts more of the fuel’s chemical energy into useful electrical work, with fewer moving parts, lower parasitic losses, and a cleaner integration path for future high-temperature heat sources.For applications focused on efficiency, modularity, and advanced waste-heat utilization, the dual-loop sCO₂ combined cycle is not merely an alternative—it is a next-generation platform.
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