Pumped hydro storage plants with improved operational flexibility using constant speed Francis runners

Pumped hydro storage (PHS) is a crucial technology for balancing large steam power plants, and may become increasingly important for storing renewable energies. Hence, capacity ranges of PHS, as well as its dynamic response to renewable power variability, will become progressively relevant. In this paper, we focus on determining capacity ranges and efficiencies of PHS plants using conventional constant speed Francis runners, adopting unconventional runner sets, arranged in innovative fashion. In the pumping mode, it is assumed that the impellers run at a single speed, but that they can have, depending on the plant, either the same or different design capacities. In the turbine mode, it is assumed that the runners can access the well-established range from 60% to 100% of design capacity via wicket gate adjustment. In order to extend the capacity ranges with constant speed runners, bypass loops to balance the plant are considered. Because bypass operation implies losses, the possible efficiencies are studied. The results show that (a) bypass is an effective means of extending capacity ranges, but high by-pass ratios decrease efficiencies. (b) One of the impeller sets postulated in this work offers the possibility of almost continuous capacity at high efficiencies, with relatively small capacity variation within the set.