Hydraulic dissipation analysis in reversible pump with a novel double-bend impeller for small pumped hydro storage based on entropy generation theory

Mass-produced centrifugal pump is defended as a feasible solution to promoting small pumped hydro storage that can utilize the low-head resources of urban buildings and enhance the power grid stability. To address the round-trip efficiency issue, a novel double-bend impeller design method is proposed in this paper based on the Stodola equation. The effects of impeller modification on hydraulic performance in both pump and turbine modes are numerically revealed via the entropy generation theory. Results demonstrate that hydraulic dissipations are concentrated in the impeller and volute, and the high-dissipation areas are linked with flow abnormalities. Specifically, the entropy generation of volute accounts for 53 %-61 %, which is mainly attributed to the wake flow, in pump mode and 24 %-33 %, due to the shock caused by fluid entering the impeller, in turbine mode. The impeller contributes to 33 %-47 % of entropy generation in pump mode and 41 %-53 % in turbine mode. Internal flow characteristics identify that high dissipations in impeller are mainly linked with shock phenomenon, flow separation, vortices and wake flow. The designed double-bend impeller can improve the turbine-mode efficiency while basically keep the pump-mode efficiency, by mitigating the interference of flow abnormalities and therefore improving the local entropy generation distribution in volute and impeller. The maximum efficiency increase reaches 7.0 % under part-load flow rate. Moreover, the double-bend impeller also extends the upper limit of adequate head that matches the required fluid potential to over-load flow rate. This paper provides a fundamental guidance for impeller modification in small pumped hydro storage.