Electrolysis is a well-known process to split water (H₂O) into hydrogen (H₂) and oxygen (O₂) by using electrical energy. If the water can be converted into steam by waste heat from other processes or steam is directly available at an industrial facility, it is more efficient to perform a high temperature electrolysis (HTE) and convert the steam directly. Steam is commonly supplied at elevated pressure, and the industrial demand for hydrogen product gas is at high pressure too. Therefore, the overall process efficiency can be increased by operating the HTE at elevated pressure level because steps of expansion and compression of the involved gases are saved. As technique for HTE, solid oxide cells (SOC) are used. So far they are mainly known and used as fuel cells (SOFC). SOC are usually made of different ceramics. They consist of a membrane for gas separation, a hydrogen electrode and an oxygen electrode. The SOC is easy scalable. So called Stacks are a series connection of multiple cells, and these Stacks can as well be connected in series or in parallel to fulfill the requirements of the specific application.

Sunfire presents a system approach for a pressurized SOC system. It is shown how Stacks and Hot-BoP components can be operated at elevated pressures. First results of a prototype with a nominal electrolysis DC input power of 10 kW and an operation range of up to 15 bar are shown. Main objectives are the control of the different pressure sections, stack performance and thermal behaviour at varying pressure levels.

Acknowledgement
The research leading to these results has received funding from the EU’s Seventh Framework Programme for the Fuel Cells and Hydrogen Joint Technology Initiative under grant agreement n°621210 for the HELMETH project (www.helmeth.eu).