The dependency of renewable energy systems on environmental influences such as sun or wind availability is one of the greatest challenges in the energy transition. For this purpose, it is important to develop electro chemical storage systems for long-term storing of renewable electricity in the form of hydrogen or methane. In the presented work, an electrolysis system with solid-oxide electrolyser stacks is designed. A solar thermal receiver is used to produce the steam supplied to the electrolyser stacks. On the other hand, a thermal energy storage, using a phase change material, is used for the extension of the operational hours during the night time. The system is optimized to minimize the levelized costs of hydrogen and compared to a system without a thermal energy storage. Finally, the cost sensitivity for the four main components of the cost structure is evaluated and discussed. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.