Low-head pumped hydro energy storage: Maximising the round-trip efficiency to increase competitiveness for providing grid services

According to the European-Green-Deal, the demand for deployment of storage technologies is predetermined to increase the share of renewable energy sources e.g. solar/wind in electricity generation. Pumped hydro energy storage (PHES) systems represent a well-developed storage technology. Traditionally, PHES systems are established in mountainous regions, but lowland countries with huge offshore wind parks lack such regions. Therefore, new developments in low-head turbomachinery, power take-off (PTO) units and operation schemes are needed to shape low-head PHES towards a viable future technology (performance/investment). Hence, a novel contra-rotating, variable-speed, reversible pump-turbine and PTO design for low-head operation is developed with an optimised round-trip efficiency (RTE) to guarantee competitiveness to other energy storage system (ESS) technologies. The investigations demonstrate that high RTE over 72 %, comparable to those of high-head PHES systems, can be reached. Operation scenarios “best efficiency” show a higher RTE than “10 MW constant power output” due to wider operation ranges (fallhead/discharge) at high efficiencies. Additionally, larger dam sizes lead to a higher RTE due to higher operation flexibilities but with higher investment costs. Furthermore, the novel offshore low-head pumped hydro energy storage (LH-PHES) design benefits, besides its highly efficient operation, from being insensitive to climate change impacts, e.g. floods/droughts, extreme temperature changes compared to e.g. alpine PHES or batteries. • LH-PHES enables expansion of RES for electricity generation in lowland countries. • Novel CR-VS-RPT design with optimised bulb (incl. PTO) for wide high efficiency range. • The round trip efficiency of a LH-PHES depends on bulb loss and operation scenario • Increase of round trip efficiency of LH-PHES by expansion of reservoir diameter. • A LH-PHES round trip efficiency above 72% can be reached.