DESIGN AND PERFORMANCE ANALYSIS OF A SUPERCRITICAL CO_2 CENTRIFUGAL COMPRESSOR WITH VARIABLE GEOMETRY

摘要

The supercritical carbon dioxide (SCO2) Brayton cycle is one of the most promising power cycles due to its high efficiency, compactness and environmentally friendliness. The centrifugal compressor is a key component of small and medium SCO_2 Brayton cycles, and its efficiency has a significant impact on the cycle efficiency. Since the required electric load of power cycles always fluctuates over the year, the SCO2 compressor will operate away from its design point and the narrow stable operating range of a compressor is always a restriction. In this paper, the variable-geometry method, which refers to the combination of a variable inlet-guide-vanes and variable diffuser vanes is proposed for the operating range extension of SCO_2 compressors. A set of one-dimensional (1D) loss correlations has been found to accurately predict various losses of the SCO; compressor components. Based on the ID thermodynamic model, two programs with internal MATLAB codes coupled with the NIST REFPROP database have been developed for preliminary optimization design and off-design performance predictions of the variable geometry SCO_2 compressor. The contributions from the variable-inlet prewhirl and variable diffuser vanes to the shifts of the surge line and choke line are discussed in this paper. The results show the variable-geometry SCO_2 compressor has a superior performance at off-design conditions and a wider operating range.