Centrifugal compressor design analysis for large-scale transcritical carbon dioxide heat pumps

Implementation of heat pumps can be an efficient solution to de-carbonize heating systems. In this study, centrifugal compressor designs for large-scale transcritical carbon dioxide heat pumps were investigated by connecting a cycle analysis with a compressor analysis. The heating capacity was varied from 1 MW to 20 MW and 1 to 4 compressor stages were used. The effect of compressor specific speed and heat pump capacity on compressor main dimensions, rotational speed, and isentropic efficiency were investigated. Generally, centrifugal compressors can be considered as an efficient and compact compressor technology for large-scale carbon dioxide heat pumps. Stage efficiencies from 70 % to over 90 % were predicted and compressor sizes were compact with respect to the compressor capacity, having impeller dimensions ranging from 40 mm to above 350 mm depending on the heat pump capacity. The use of multistage compressors reduced the shaft rotational speeds, that were generally observed to be high with respect to the compressor power level. In addition, the use of multistage configurations resulted in slightly higher impeller dimensions when compared to single-stage designs. Pressure losses between the compressor stages, tip clearance heights, and stage pressure ratios were observed to have notable effects on the compressor design and efficiency.