Introduction While the world moves towards modern renewable energy sources, these sources are not always available which makes them unreliable. At the same time, problems of balancing the daily peak-demand is still performed with expensive fossil fueled units. Sitting perfectly in between supply and demand is a solution with the potential of solving both problems: energy storage. Analysis The available large-scale energy storage technologies are analyzed and specifically their suitability for the Netherlands. The choice has fallen on Pumped Hydropower Storage (PHS); a proven technology. The exclusivity of hydropower for elevated countries is challenged with alternatives for PHS in the Netherlands. These alternatives are grouped according to their favor for either surface size with Storage Islands, or towards the depth with Underground PHS’s. Due to the uncertainty and low suitability for Underground PHS, the Storage Islands are preferred. Among the alternatives is a silt depot in the Maasvlakte which is losing its purpose, named the Slufter, which outperforms others in a cost-benefit analysis due to the low primary investment and highly suitability for this application. Transforming the Slufter from a silt depot to an energy storage system is worked out technically. The analysis of the Slufter consists of modeling major failure mechanisms using two finite-element method packages, solving the main technical and constructional issues, checking feasibility with a construction method and planning, mapping the risks and providing a strategy for integration within the power grid. Results Energy storage is realized by making a high reservoir of the Slufter at a level of +39m NAP that is enclosed by a 17 m upgrade to the current dams at +23m NAP in which the contaminated silt of 78 million m3 can be maintained. It was found that fast lowering the reservoir level (10 meters in 6 hours) is normative for dam stability and that a core helps in providing this stability. The profitable system generates power during the day and stores it in the night with the interaction between the high reservoir and the lower North Sea. The performance is similar to a medium plant with a peak power of 470 MW and storage capacity of 2.16 GWh; generation lasts 6 hours at maximum capacity. The system satisfies the peak-balancing needs of Zuid-Holland, Zeeland and largely Noord-Holland, while providing enough wind power balancing for Zuid-Holland till 2025. Using the imbalance between daily and nightly power prices, the system annually benefits €20 million from power trade and saves €8 million fuel costs. It confirms its potential with a competitive unit price of 54 €/MWh and a pay-back period of 30 years. The Slufter as a storage system fits the Port of Rotterdam’s ambition, offers a clean alternative to the current gas-fueled peak-generation-units and potential for the future power market in which variable renewable energy sources play a large role.