Avaliação de incertezas para ciclos de potência operando com CO2 supercrítico

Abstract

Supercritical CO2 power cycles promise high thermal efficiency, simplicity, and compactness, especially when the fluid is compressed near its critical point. Research involving sCO2 power cycles have exponentially grown in the last two decades, but it has not been possible yet to reproduce these expected benefits in a commercial product. A hypothesis to explain the dissonance between theory and practice is that it is very difficult to produce reliable experimental results because the properties of the fluid vary sharply in the vicinity of its critical point and the researchers have generally neglected this aspect. This study was composed of two parts, one to demonstrate the fragility of some published experimental results, and the other to prove the hypothesis. For the first part, experimental data of the turbomachinery inlet and outlet pressure and temperature of the experimental test benches from the six major projects around the world were selected. Using the experimental data, the isentropic efficiency and the uncertainty associated with the result were calculated, and the values were compared with the design nominal efficiencies of the turbomachines. The results show that the calculated values for the isentropic efficiency are very close to their nominal design value, but the uncertainty associated with the value is high when the fluid is near its critical point and can be even greater than the efficiency value itself, demonstrating that many theoretical results presented may not be experimentally reproduced. The second part of the study aims to prove the hypothesis that highly accurate instruments are required for measurements of the thermodynamic properties of supercritical CO2 near its critical point to yield reliable thermodynamic results. To achieve this, simulations of adiabatic compressions were conducted over a range of temperature and pressure, followed by the subsequent analysis of uncertainty in the fluid’s outlet temperature. The simulation results indicate that in regions near the fluid's critical point, particularly along the pseudocritical line, sharp variations in thermodynamic properties require extremely high accuracy in temperature and pressure measurements. This level of accuracy is sometimes impossible to achieve due to limitations in measurement instruments and the inherent oscillations in compressor operation. The results of the second part were discussed in terms of uncertainty analysis method, compression pressure ratio, the unstable nature of the compression process, and the influence of the Span and Wagner Equation.