Master thesis of Lucas de Vilder (2017), Technical University Delft in the Netherlands, exploring the possibility of a large scale inverse pumped hydro storage facility in the middle of the North Sea at the Dutch part of the Doggerbank. The idea is to build a ring dike of typically 40 meter high for pumped hydro energy storage purpose. The facility would be “inverse”, meaning that the dike would hardly be elevated above sea level. The required altitude difference between sea level and reservoir level is achieved by digging a deep basin rather than building a high dike.
Spoiler: For a preferred 25GWh storage capacity and 2.65GW of installed power the LCOS may vary from 68.2€/MWh to 19.0€/MWh of which 40€/MWh is found the most realistic. At an average Dutch power consumption of 13 GW, the storage facility would contain the equivalent of 2 hours of Dutch electricity consumption and could be released in 10 hours. The cost would be 3-18 times lower per unit of energy compared to batteries and power-to-gas.
A US study has shown that with 4-8 hours of storage you can cover 55% renewable energy. Let’s assume 6 hours. 13 GW average Dutch power means that 6 hours correspond to 6 hours x 13 GW = 78 GWh. In a graph below it is claimed that the cost of a 80 GWh storage is ca. 4 billion euro. For a country like Holland with an expected GDP of 888 billion euro in 2019 and public debt below 60%, that is absolutely doable. With such a storage in place, the Netherlands can roll out its intended 2 x 7 GW monster offshore wind projects “with two fingers in the nose” and buy a decade or so time to wait for longer duration storage solutions to emerge, most likely hydrogen or derivative ammonia. 80% or more efficiency at electrolysis of steam, rather than water, has already been realized.