- Simulation of control strategies for small-scale pumped-storage plants including the use of pumps as turbines- Hydro-B S.A.

Abstract

In a small-scale pumped-storage plant the gross head varies over a large fraction of its mean value during a single cycle, so the machines never settle at one duty point and the way they are controlled decides how much energy comes back. This work quantifies that dependence and compares the machines such a plant can use, in particular a standard pump run in reverse during the discharge. A simulation tool was developed for that purpose. It resolves at each time step the equilibrium between the machines, the hydraulic circuit and the two reservoirs whose levels it updates, the machines being described by characteristic curves transposed by the similarity laws or by a hill chart. Three control strategies are implemented, fixed speed, tracking of the best efficiency point and constant power, and the model is applied to two Belgian projects, Temse and Zutendaal. A standard pump was also characterised in both modes on the test bench of the University of Liège. Tracking the best efficiency point brings the round-trip efficiency of the pump-as-turbine configuration to 65.8 % at Temse and 66.5 % at Zutendaal, against 63.7 % and 62.6 % at fixed speed, but the converter it requires consumes most of that gain. Constant power costs fourteen points at Temse and is unattainable at Zutendaal, a machine without guide vanes holding the setpoint only far from its best efficiency point. A pump used as a turbine gives up between eight and twelve points of recovered energy against a dedicated machine. None of the three strategies dominates the other two, the ranking depending on whether the operator is paid for the energy returned or committed to a firm power. Variable speed pays only where the head varies enough to exceed the cost of the drive, and a separate pump and turbine remains the better arrangement at both sites on performance grounds.