Small-scale supercritical CO2 gas turbine cogeneration systems

A closed regenerative supercritical CO2 Brayton cycle indirectly heated by methane combustion products is analyzed. The combustion-air preheater is explicitly excluded. Real-fluid CO2 properties are evaluated with REFPROP, while the compressor and turbine are represented by prescribed isentropic efficiencies. The primary heat exchanger (PHX), recuperator and cooler are sized preliminarily by the LMTD method with fixed overall heat transfer coefficients. A separate methane-combustion balance determines the excess-air ratio, fuel and air flow rates, stack temperature and LHV-based fuel-to-electric efficiency. One-dimensional parameter sweeps are complemented by two-dimensional maps in compressor pressure ratio and recuperator effectiveness. At the nominal point of 8/24 MPa, 35/800°C, 1.8 kg s‒1 CO2 and a recuperator recovery fraction of 0.80, the cycle produces 276.7 kW of net shaft power with a PHX-based thermal efficiency of 44.8%. The corresponding methane flow is 0.02694 kg s‒1, the calculated stack temperature is 848.4°C, and the net electrical output is 257.7 kW, giving an LHV-based fuel-to-electric efficiency of 19.1%. The maps show that high pressure ratio and high recuperator effectiveness increase the closed-loop efficiency, but the feasible domain contracts because of high-side pressure and decreasing recuperator temperature approach, while the required recuperator area rises sharply.