The influence of nitrogen on an expander in a carbon dioxide transcritical heat pump

In this study, an expander was employed in a heat pump to replace the throttle valve in order to recover the expansion work. The influence of a non-condensable gas, nitrogen (N2), on the performance of the expander in a carbon dioxide transcritical heat pump is investigated. The phase change delay during the expansion process can affect the recovery work and the superheat limit will influence the phase change of the fluid. The superheat limits of carbon dioxide with and without N2 are calculated. It is found that the superheat limit and the critical radius of the bubble could decrease with the presence of nitrogen (N2). Then the required energy of bubbling can be lowered as the phase change occurs. The performance of the expander was investigated when the N2 concentration increased from 1% to 2.9% in a carbon dioxide transcritical cycle. The experimental results show that the recovery work of the expander with the presence of N2 is higher than that without N2. However, the efficiency of the expander decreases with increasing N2 concentration.