Model of Hydrogen Production by Combination of Water Electrolysis and Unstable Sources of Energy

Water electrolysis is accepted as direct method for hydrogen production from water. The emission pathway of hydrogen is related to the source of electric energy and process of water electrolysis just define energy efficiency or amount of hydrogen produced from energy supplied. For green hydrogen production almost exclusively, renewable energy sources are necessary. But unstable production is typical for most frequently mentioned wind and solar power station. Currently four types of water electrolyzers are available: Proton exchange membrane water electrolysis (PEMWE), Alkaline water electrolysis (AWE), Membrane alkaline water electrolysis (MAWE) and high temperature solid oxide water electrolysis (SOWE). Each of mentioned type exhibits different properties and requirements if operated in combination with renewable power sources. Proper selection of electrolyzer type and its size is crucial for economic production of hydrogen. PEMWE shows higher flexibility in operation changes. Its warming up to operating temperature is only few minutes or seconds due to the high current densities, thin ion conductive membrane and circulation only of demineralized water. Its production range is limited by hydrogen permeation through membrane at low production and electrolyzer stability at high production. The production range starts usually from 5% and finish at short term 120% of designed capacity. AWE represents oldest technology with high durability proved by large number of installations. But old electrolysers were designed for continual operation with limited possibility to moderate its production. Newly developed units are better adapted to intermittent operation. Parasitic currents in electrolyte distribution channels are main reason for efficiency decrease at low loading. Their minimum operation range varies from 1% up to 30% depending of manufacturer and size. Despite it, the request for circulation of concentrated hydroxide and low conductivity of separator make warm up period significantly slower with low efficiency. Therefore, frequent phase off and on affect the total amount of produced hydrogen. MAWE is newest electrolyzer type. It combines the concept of ion selective membrane usage as separator and alkaline environment to avoid platinum metals need. Thanks to the ion-selective membrane the startup dynamics is faster but durability of membrane limits its operation temperature. SOWE as process with operating temperature 800 °C is not suitable for irregular operation. But it high efficiency make this process interesting for combination of power sources to keep permanent run. Nuclear and renewable power sources are one of possible combination. Mathematical model capable to calculate actual and integral production of hydrogen on the base of known power sources production and elektrolyzer parameters was created. Input data of elektrolyzer behavior for the model come from an in-house measurement and from literature. Various data of for unstable power sources involve a photovoltaic power-plant and train recuperation energy from braking on the 15 min interval were taken. Created model indicates production capacity of given combination and also the running time and amount of utilized energy. With consideration of investment cost i tis able to estimate most suitable sizes. Acknowledgment: This project is financed by the Technology Agency of the Czech Republic under grant TO01000324, in the frame of the KAPPA programme, with funding from EEA Grants and Norway Grants.