Optimization of supercritical carbon dioxide recompression Brayton cycle considering anti-condensation design of centrifugal compressor

Abstract

Supercritical carbon dioxide Brayton cycle has the advantages of high-efficiency and compactness, and is one of the most promising propulsion and power system options in shipboard application. The high-efficiency design of supercritical carbon dioxide Brayton cycle requires the compressor to operate near the critical point of carbon dioxide where the carbon dioxide changes dramatically in thermophysical properties resulting in inaccurate performance prediction for compressor and condensation tends to occur especially at the inlet of compressor impeller. So far, most of the studies on supercritical carbon dioxide cycle optimization have taken thermal efficiency and exergy performance into account while the anti-condensation performance is rarely considered. In this context, a design optimization strategy is proposed for supercritical carbon dioxide Brayton cycle, in which both the thermal efficiency and condensation margin are considered as objective functions in an attempt to improve the performance and anti-condensation. In addition, an aerodynamic design method is developed for supercritical carbon dioxide centrifugal compressor by using real gas two-zone model to alleviate the reliance on model empiricisms. The proposed strategy and the developed method are validated and then applied to a 50 MW shipboard supercritical carbon dioxide Brayton recompression cycle optimization and the main compressor design. The analysis results show that condensation margin and cycle thermal efficiency are conflictive with each other. The higher temperature and lower pressure at the compressor inlet can improve cycle anti-condensation while the opposite is true for the cycle thermal efficiency. As the condensation margin is 0.1, the thermal efficiency of optimized cycle is 48.16%. The computational fluid dynamics simulations confirm that the designed main compressor meets the anti-condensation requirements as condensation margin is above 0.1. This work is of certain significance to promote the application of supercritical carbon dioxide cycle in shipboard usage.