The project RESelyser develops high pressure, highly efficient, low cost alkaline water electrolysers that can be integrated with renewable energy power sources (RES) using an advanced membrane concept, highly efficient electrodes and a new cell concept. A new concept with a three electrolyte loop system will be developed demonstrating even higher performance than conventional two electrolyte loop systems. This three electrolyte loop system will use a new separator membrane with internal electrolyte circulation and an adapted cell to improve mass transfer, especially gas evacuation. Intermittent and varying load operation connected to an RES will be addressed by improved electrode stability and a cell concept for increasing the gas purity of hydrogen and oxygen especially at low power as well as by a system concept. Electrolysers up to 30 kW with 6 Nm^3/h hydrogen production will be realized in the project. The primary pressure of the electrolyser will be above 25 bar (without the use of a compressor) to reduce the power loss for hydrogen compression to a minimum. All components of the system will be analyzed for their costs and developed to reduce the system price such that hydrogen can be produced at 3000 /(Nm^3/h). An extrapolation to a primary electrolyser pressure of 100-150 bar is considered.