Renewable energy resources are experiencing substantial growth. However, due to their intermittent and unpredictable nature, they cannot supply continuously; storage of electricity is perceived as the best way to accommodate these inherent drawbacks. The Blue Battery allows the storage of renewable generated electricity in a concentration gradient between fresh and salty water (H2O + NaCl), by creating an artificial gradient in times of oversupply of electricity with help of ion-exchange membranes in a process named Electrodialysis (ED). This concentration gradient can be used later to retain the energy by pumping the two streams of water back into the stack with membranes, thereby generating an electric potential in a process called Reverse Electrodialysis. Storage of electricity in such a way proceeds without any chemical reaction, allowing storage to become safe, cheap and especially highly scalable, as only water and salt are needed to increase storage capacity.