Low head pumped hydro storage with contra-rotating pump-turbines

Licentiate thesis, 2022

The increasing share of electrical energy production by intermittent sources pushes the demand for energy storage. Pumped hydro storage (PHS) has a long history of providing a cost-efficient energy storage solution. However, for PHS to be a viable option, a large head is typically required. This makes energy storage via PHS difficult in countries that lack high mountain regions. To address this problem, and thus allow for PHS in flat countries, the EU project ALPHEUS was formed. In ALPHEUS, new pump-turbine technologies intended for low head PHS are evaluated. One of the investigated designs is a shaft-driven contra-rotating pump-turbine (CRPT). In this thesis, CRPTs are numerically simulated with computational fluid dynamics at stationary and transient operating conditions. The stationary operation is studied through both steady-state and unsteady simulations. The steady-state computations are made to get an understanding of the operating range of the CRPT. The unsteady simulations are carried out on selected operating points with the aim to identify the complex flow behaviour. The transient operations cover startup and shutdown procedures in both pump and turbine modes. The pump mode startup procedure is the major focus since it was found when evaluating preliminary startup and shutdown sequences that the pump mode startup was exposed to the largest loads. Hence, three parallel studies are presented in this thesis to determine how to startup the CRPT in pump mode and limit high-amplitude load variations. This contributes to the machine's lifetime and flexible operation. The outcome of this research shows the potential of using CRPTs in a low head PHS situation and may also help with solving the inherent problems correlated to energy production from intermittent sources.