Utilization of Hydrogen as an Energy Storage Vector by a Combined Reversible High Temperature Electrolysis and Compressor System Integrated with Upstream Biogas and Downstream E-Fuel Production

Current predictions towards a hydrogen society and the extensive use of hydrogen as a large-scale energy storage vector are emerging from the intermittent nature of renewable energy sources, especially wind and solar. The key issues to make these new solutions based on clean energy techno-economically viable are versatility, quick response time to changes in the price of electricity and high system efficiency compared to established technologies. Also, options such as the utilization (and capture) of CO2 and biogas would be desirable features to be connected to the future energy systems. When the price of electricity is high, biogas is, along with hydrogen, a very suitable fuel for high-temperature solid oxide electrolyzer. On the other hand, there is a huge global demand for sustainable and carbon-neutral fuels (e-fuels) in sectors that are difficult to electrify, such as aviation and maritime transport. One of the most feasible ways to produce e-fuels is to apply Fischer-Tropsch synthesis, where synthesis gas is converted into liquid hydrocarbons. Synthesis gas itself can be produced by feeding CO2 into a solid oxide electrolyser operating in co-electrolysis mode.

To enhance the utilization of hydrogen as an energy storage vector, VTT has developed a combined 10 kW reversible high temperature solid oxide electrolysis (rSOC) and compressor system for both hydrogen storage and e-fuel production. Depending on the price of electricity, the developed rSOC system can be operated in three different modes: 1) in fuel cell (SOFC) mode, where locally stored hydrogen or biogas is used to produce electricity for the local electricity grid 2) in electrolyzer (SOEC) mode, where ultra-pure hydrogen (suitable for vehicle applications according to ISO 14687 standard) is produced and compressed for storage at 350 bar 3) in co-electrolysis (co-SOEC) mode, where synthesis gas is produced and compressed for use in the production of e-fuels (or e-methanol) in downstream processes. Since the same electrolyzer-compressor system combination is used alternately for both syngas and high purity hydrogen production, high-precision system monitoring methods and an efficient hydrogen quality control measurement system have been developed to meet the strict hydrogen purity requirements of ISO-14687 standard. The developed rSOC system includes high-level instrumentation, which enables e.g. calculating enthalpy flows and efficiencies for all Balance of Plant (BoP) components, and monitoring the efficiency of novel heating methods to minimize the electrical energy required for system heating during the hydrogen/syngas production. Hydrogen safety and optimal system-level operation are essential parts of this demonstrated multifunctional energy concept, which can be further used in the design of larger-scale integrated modular multi-fuel and multi-purpose energy storage concepts and local emission-free electricity production.