(Invited) A New Method for Combining P2X and Electricity-Balancing Using SOECs
Authors: Jensen Søren Højgaard , Kær Søren Knudsen , Simon Araya Samuel , Chattopadhyay Kabitri , Hauch Anne , Frandsen Henrik Lund , Skafte Theis Løye , Graves Christopher , Mogensen Mogens Bjerg 
Published: May 2020

Solid Oxide Electrolysis Cells (SOEC) can be used for efficient power-to-X (P2X) conversion. Here X symbolizes the myriad of end products that can be made using green power, H2O and CO2. The SOEC converts H2O and CO2 into gas (most often H2 and/or CO) which can be catalyzed into a broad range of fine- and speciality-chemicals, as well as bulk transportation fuels.
Dynamic operation of the SOECs can help balance the electricity consumption with the electricity production from intermittent energy sources, such as wind and solar. P2X can therefore assist the on-going transition to a non-fossil energy infrastructure by substituting fossil-based chemicals and fuels with sustainable alternatives, while simultaneously providing energy balancing services.
It is important to realize that the requirement for electricity balancing ranges in time from milliseconds to months. Batteries offer quick response times and are increasingly being used to provide fast ancillary services to the electrical grid. Unfortunately battery-based electricity storage costs hundreds of $ per kWh of capacity, which renders them impractical for large-scale storage exceeding time-scales of about one hour. This is where dynamic electrolysis operation with part load enters the scene.
During operation of SOECs at part load, i.e. below the thermo-neutral point of operation, the endothermic reaction heat will dominate the internal heat balance and generate thermal gradients in the SOEC stack. The gas exiting the stack will therefore be colder than the gas entering the stack, such that an external (electric) heater is required to pre-heat the gas before the gas can enter the stack. The thermal gradients cause temperature variations inside the stack that increases with increasing stack size. It is an advantage to avoid thermal gradients, as these cause thermo-mechanical stresses and variations in the cells’ resistance across the stack. Thermo-mechanical stresses may lead to cracks in the brittle ceramic cells and delamination at interfaces. Varying cell resistance causes variations in current density and electrode overpotential across the stack, which increases the risk of accelerated degradation. Thus, thermal gradients hinder an increase of the footprint (i.e. the size) of SOEC stacks. Consequently, upscaling to MW-systems faces considerable complexity related to wiring, piping and heat management of hundreds of smaller kW-sized stacks. This limits cost reductions related to economy-of-scale, in particular if the SOEC MW-systems are to be used for dynamic operation and electricity balancing.
In this work, we demonstrate theoretically and experimentally how pseudo thermo-neutral operation can be achieved at part load. The uniform temperature profile is established by overlaying the DC voltage in electrolysis mode with a rapid alternating voltage, such that the Joule heat balances the endothermic reaction heat. The alternating voltage causes the stack to be operated a few milliseconds in SOFC (or SOEC) mode before it returns to SOEC (or SOFC) mode. The frequency of the alternating voltage is around 20 – 100 Hz. This means the stack temperature and outlet gas composition is not affected by the alternating voltage. During SOFC mode, excessive heat is generated to balance the endothermic SOEC operation. As the SOFC operation produces more heat than the SOEC operation consumes per time interval, a net-production of fuel is realized.
By controlling the time and cell voltage in respectively SOEC and SOFC mode the overall load can be continuously changed from 0% (corresponding to OCV) to 100% (corresponding to DC operation at thermo-neutral voltage) while keeping the gas outlet temperature equal to the gas inlet temperature.
The operation method therefore enables very fast load variations without having to change the stack temperature or gas compositions. This enables dynamic operation of MW SOEC stacks to enter the market for fast electricity balancing, where batteries are currently applied.
Importantly, the operation method seems to decrease the stack degradation rate, which we speculate is caused by desorption of impurities that otherwise adsorb on the surface of the electrodes.
The potential for the novel operation method to combine chemicals and fuel production with electricity balancing at time scales spanning from milliseconds to months, can hopefully help commercialize and upscale the SOEC technology towards MW-systems.