Variable speed operation of centrifugal pumps running as turbines. Experimental investigation

Pumps running as turbines are pointed out as a cost-effective solution for energy recovery in pressurised water supply systems. However, these hydraulic machines feature low efficiency under variable discharge operation due to the lack of an inlet flow control component. Variable speed operation is an approach for controlling the discharge at the pump as turbine inlet aiming at increasing the operational efficiency. This research work presents the experimental investigation for measuring the variable speed characteristic curves of pumps running as turbines, focusing on the turbine and on the extended operation modes. Three single-stage end-suction closed-impeller centrifugal pumps with different unit specific speed values are tested. Turbine mode test results show that the discharge-specific energy operating range is broadened with increasing efficiency if the machines are operated with variable speed. Extended operation results show that these hydraulic machines do not feature the instability region near the runaway conditions, the so-called the “s-curve”. Outcomes of this experimental investigation provide the required insights for establishing the design technical specifications of micro hydropower plants with variable speed pumps running as turbines, aiming at maximizing the energy recovered in pressurised water supply systems.