Alternative working fluids to reduce size of turbomachinery for VHTR plants

The binary mixtures of He-N2 and He-Xe with a molecular weight of 15 g/mol have 5 -8% higher heat transfer coefficient than helium and effectively reduce the number of stages in the turbine and compressor of the Closed Brayton Cycle (CBC) turbomachinery for energy conversion. However, the higher pressure losses decrease the thermal efficiency of the direct CBC He-N2 plant by 1 percentage point, and the thermal efficiency of the indirect CBC He-Xe plant by 3 percentage points compared to helium, when operating at the same reactor exit temperature of 1123 K. Design models of the compressor and turbine are developed to investigate the effect of using He-Xe and He-N2 (15 g/mol) on the size and number of stages in the CBC turbomachinery. The compressors optimized for He-N2 and He-Xe working fluids have 9 and 6 stages, compared to 22 for He. Similarly, the He-N2 and He-Xe turbines have only 3 and 2 stages, compared to 7 for He. In addition, the rotor shaft for the He-N2 and He-Xe turbomachinery is a factor 2 and 3 shorter compared to helium. The shaft rotation speed has little effect on overall plant performance, but the number of stages in the optimized turbines and compressors for 3000 rpm (50 Hz) is about 10 -20% higher than for 3600 rpm (60 Hz). (authors)