A hybrid analytical model for thermodynamic calculations of supercritical carbon dioxide centrifugal compressors

Abstract

Thermodynamic calculations are important procedures for preliminary design or performances assessment of supercritical carbon dioxide (SCO₂) centrifugal compressors. However, the thermodynamic calculations are usually based on the discrete database (NIST REFPROP). No analytical models with enough accuracies are alternative for the thermodynamic calculations of SCO₂ compressors. For this, a hybrid analytical model is proposed for their thermodynamic calculations. Firstly, a BWRS-based model for thermodynamic properties calculation of CO₂ is derived by combining the BWRS equation of state (EOS) and the fundamental thermodynamic relations. Validations show that the accuracy of the model is acceptable except for the regions near the critical point as well as the relatively low temperature range (300~360 K). Then, a polynomial model is introduced for the regions with large errors. Eventually, a hybrid model is established by combining the BWRS-based model and the polynomial model. The hybrid model is used to calculate the thermodynamic properties of the Sandia SCO₂ compressor, and applied to the preliminary design of a high-pressure-ratio SCO₂ compressor. The maximum errors of the two test cases are about 1.5% and 2.6% respectively compared to the reference values. The hybrid model has provided an alternative approach for the thermodynamic calculations of SCO₂ compressors.