Numerical simulation on scroll expander–compressor unit for CO2 trans-critical cycles

In a two-stage compression CO2 trans-critical cycle, application of a combined scroll expander–compressor unit has been considered in order to improve the cycle COP. For the combined scroll expander–compressor unit, power was recovered from a scroll expander which replaced the expansion valve and transmitted to the first stage auxiliary compressor which was directly coupled to the expander through a common crankshaft. Numerical simulation on its performance showed that for the suction pressure of 3.5 MPa, discharge pressure of 10 MPa with expander inlet temperature of 35 °C, the main compressor input could be reduced by 12.1% by the expander output. Increase in the cooling capacity by the expander was 8.6%. As a consequence, COP improvement of the cycle was estimated to be 23.5% by the application of this expander–compressor unit. COP improvement increased with lowering the expander inlet temperature, but decreased with increasing the suction pressure. Performance of the scroll expander was relatively insensitive to changes of operating pressure ratio, whereas that of the scroll compressor decreased rapidly as the operating pressure ratio became far from the design pressure ratio.