A Systems Perspective in the Conversion of CO2/H2o Feedstocks Using Intermediate-Temperature Reversible Solid Oxide Cell Technology for Energy Storage Applications
Authors: Braun Robert J. , Reznicek Evan 
Published: February 2020

Intermediate temperature, reversible solid oxide cells (ReSOCs) have the unique characteristic of being able to electrochemically convert steam/CO2 feedstocks into syngas or methane-rich fuel streams with high efficiency. Such electrolysis products can be further processed for manufacturing of synthetic fuels, such as synthetic natural gas or liquid fuels (via Fischer-Tropsch processes, for example). Alternatively, the electrochemical reduction products, including oxygen, can be stored for later use offering unique operational flexibility that could provide a critical solution to the challenges of intermittent renewable electrical energy supplies. Indeed, electrical energy storage is expected to become a critical component to the future energy infrastructure. Future electric grids will likely have a greater proportion of both intermittent renewable and distributed generation. Electrical energy storage could enable better management of energy generation and distribution, performing time-shifting and load-leveling services. Reversible solid oxide cells (ReSOCs) have the potential to provide efficient, scalable electrical energy storage. These cells can operate sequentially between electrolysis and fuel cell modes, producing power when it is needed and storing fuel when electricity is available. Leveraging C-O-H chemistry and operating at intermediate temperatures allows ReSOCs to be mildly exothermic in both operating modes, which simplifies balance-of-plant integration and thermal management. Prior laboratory tests performed with doped lanthanum-gallate (LSGM) electrolyte solid oxide button cells have shown very promising cell performance. However, realizing the promise of high efficiency ReSOC-based electrochemical conversion technology requires many additional considerations beyond cell performance. Reactant gas processing and thermal management in the stack periphery via heat exchangers and balance-of-plant equipment is critical to enabling the successful design and implementation of such systems. Furthermore, study of stack integration with the BOP offers several useful insights that can be leveraged to feedback into cell development activities and technology requirements.

In the present work, converting H2O/CO2 feedstocks into an energy dense fuel for storage and later use is explored via modeling and simulation of intermediate temperature (600°C) ReSOC systems using experimental test data from LSGM cells. Results from these tests have been used to calibrate a physically-based electrochemical cell-stack model which is subsequently used in system simulations. Common to all system configurations is tanked storage of both fuel (H2, CH4, and CO) and exhaust (H2O, CO2), which is key for enabling ReSOCs to function as stand-alone energy storage devices. An important parameter that influences both cell and system roundtrip efficiency performance and energy density is the pressure ratio between storage and stack operating pressures. Storage tank capital cost, which can be a significant proportion of system cost at the distributed scale (<1 MW), can be reduced by storing fuel and exhaust at high pressures (>50 bar). However, a large ratio between tank and stack pressure complicates the BOP design, leading to higher BOP costs. One solution to this problem is to increase stack operating pressure. While this may increase complexity and cost associated with the stack, it also offers benefits to the performance of ReSOCs as energy storage devices. Pressurization results in higher methane production in electrolysis mode, increasing the energy density of the stored fuel. It also allows the thermoneutral voltage, or the voltage above which the stack is thermally self-sustaining in electrolysis mode, to be closer to the Nernst potential. This means the stack can be operated at a lower over-potential, increasing efficiency. In this paper, we discuss benefits and demerits of operating ReSOCs at elevated pressure in the context of thermodynamic and economic performance. This analysis considers a 100 kW ReSOC system operating on a 16-hour round-trip cycle, suitable for distributed energy applications. A novel floating piston storage tank that could allow near-constant storage pressure and reduced overall tank volume is proposed and modeled. Off-design performance of balance-of-plant components is evaluated through the use of performance maps and flow correlations. We explore different tank and stack pressures through parametric studies, and evaluate different cases on the basis of system capital cost, round trip efficiency, and cost of stored energy. Thoughts and observations from this work that inform the setting of requirements for cell performance and operating characteristics are given.