Investigation of the impact of the thermodynamic property method on the performance, preliminary component sizing and maximum efficiency configuration of the NET power cycle

Abstract

This paper investigates the effect of thermodynamic property methods on the NET Power cycle, which is a novel supercritical CO2 power cycle based on the oxy-combustion technology. A numerical model of the most advanced configuration of NET Power cycle and air separation unit was developed in Aspen Plus to characterize the thermodynamic performance, key components presizing, and maximum efficiency operating configuration. The Peng-Robinson cubic Equation of State (EoS) has traditionally been adopted as the reference EoS (REF EoS) in previous thermodynamic studies on the NET Power cycle. However, its elevated predictive uncertainty, espe- cially in phase modeling, may have led to inconsistent results. For that reason, and as a novelty, in present work, different EoS such as cubic, viral, SAFT and multiparametric Helmholtz free energy-based methods were considered, to evaluate the effect of the EoS on the cycle components and to optimize the operating conditions of the cycle. REFPROP + LKP was also included as the most reliable method. The results reveal that REFPROP + LKP estimates a fluid density in the liquid-like phase pumping stages 25 % higher than the cubic EoSs at nominal conditions. Thus, the compression work is 11.57 % lower and the net cycle efficiency 1.48 % higher. The higher relative deviations in cycle efficiency were obtained with PC-SAFT and GERG-2008 models. REF EoS estimates a recirculation pump impeller diameter 7.49 % larger than REFPROP + LKP. An oversized pump would operate outside the design point with low efficiency, flow control difficulties, and potential vibration and overpressure issues. For REFPROP + LKP, the heat exchange area required by the recuperator is 6.46 % lower than that estimated by REF EoS. This suggests that the manufacturing costs are significantly lower and transient response faster than expected. The maximum cycle efficiency resulted in 55.94 %, for a combustor outlet temperature of 1103.93 ◦C, turbine inlet and outlet pressures of 273.99 bar and 44.83 bar, and bypass split fraction of 11.37 %