The use of renewable energy sources like solar and wind energy for the generation of electricity are expected to increase in the coming decades. However, these sources are intermittent. Therefore Electrical Energy Storage (EES) systems are needed in the near future to assure a reliable electricity supply. These systems should also be able to deliver power in longer periods of time with low generation from wind and solar energy sources. An example of such an EES system is the GreenBattery, developed by AquaBattery. This is a flow battery that stores the electricity by splitting salt water in an acid and a base. Because the main component of the GreenBattery is salt water, this battery is safe in use, as it cannot catch fire or explode. Furthermore, this makes the battery environmentally benign and cost competitive with other EES systems. The main focus of this thesis is to make a concept design of a GreenBattery that can be placed on a water like a lake. This concept is evaluated in a case study regarding the energy storage lake of the Delta21 project. This battery could provide a reliable backup electricity source for e.g. the Port of Rotterdam or stabilise the electricity output of renewable sources. Therefore, the main research question of this thesis is: ‘How can AquaBattery’s GreenBattery be realised on the energy storage lake of the Delta21 project, providing long term storage?’ To answer this question, first the design basis is explored. Based on this, concepts are generated using a morphological chart method and the most feasible concept is chosen using a multi-criteria analysis. This chosen concept is worked out in more detail, after which its technical and economic feasibility is investigated. The most feasible concept turned out to be a floating concept. This concept uses floating rigid tanks to store the different liquids present in the GreenBattery. The power unit, where the electricity conversion takes place, is placed on top of these tanks. Furthermore, the feasibility of integrating a solar photovoltaic system on the floating tanks is investigated, because of the large area available for multiuse on such an island. The resulting dimensions of the floating tanks is 98 by 98 by 3 m (length, width, draft). Four of these tanks can be coupled together to form one floating island. Such an island can store about 560 - 800 MWh of energy, depending on the required storage duration and about 3.2 MWp of solar panels can be placed on top of the tanks. In total, about 300 of such islands can be placed on the energy storage lake of the Delta21 project. Such an island is technically feasible, because the natural periods are larger than the expected wave periods. Therefore, the motions and forces of the island will be minimal. Furthermore, in terms of costs, the floating GreenBattery is cost competitive with other EES systems and therefore will be economically feasible as well.