Off-design performance comparison of single-stage axial turbines using CO₂ and zeotropic mixture for low-temperature heat source

Organic Rankine cycle (ORC) and transcritical CO₂ power cycle are two effective methods for low-grade energy utilization. Turbo-expanders of these power cycles need to maintain a high efficiency under both design and off-design conditions. In this study, the performances of two single-stage axial turbines are compared using a zeotropic mixture and CO₂ as working fluids for a low-temperature geothermal source. First, a preliminary design model is established based on the mean line method. Subsequently, the input conditions for each turbine are determined and the performances of the two turbines are analyzed under the design condition. Then, the operation characteristics of the two turbines are investigated under various off-design conditions and the extent of the turbine efficiency reduction is evaluated. In addition, the effects of composition variation for the ORC turbine and the inlet temperature of the CO₂ turbine are estimated respectively. The results indicate that the efficiency of the CO₂ turbine is 0.875 at the design point, which is slightly higher than that of the ORC turbine as 0.864. The geometric size of the CO₂ turbine is smaller than the ORC turbine apparently. The CO₂ turbine has a higher efficiency over the entire operation range compared with the ORC turbine. Under the off-design conditions, the variation magnitude of the efficiency for the CO₂ turbine is apparently less than that of the ORC turbine due to a lower pressure ratio and a larger flow rate.