Start Date: August 2011
Scott Barnett, Materials Science and Engineering, Northwestern University; Robert Kee, Robert Braun, Engineering Division, Colorado School of Mines
The objective of this research program is to carry out fundamental studies for a solid oxide flow battery (SOFB) – a novel energy storage device that resembles a solid oxide fuel cell but operates more like a flow battery. A unique storage chemistry is utilized, enabled by device operation at relatively high pressures and/or low temperatures. If successful, the device will ultimately improve the round-trip efficiency, system lifetime, and cost effectiveness of large-scale energy storage.
Figure 1 shows a simplified schematic of the proposed SOFB energy storage system. These cells will require new, or at least optimized, oxygen-ion-conducting ceramics and membrane-electrode architectures. The membrane electrode assembly (MEA) is based on advanced ceramic components, which are designed to operate at 600-750°C. In the SOFC mode, electricity is produced (discharge) as the fuel flows from the SOFC Fuel tank through the MEA, with the exhaust collected in the solid-oxide electrolysis cell (SOEC) “fuel” tank. In the SOEC mode, the flow reverses direction, with electricity stored in chemical form (charging) in the SOFC Fuel tank. Oxygen flows through the lower channel. Oxygen ions are transferred through a dense ceramic membrane within the MEA.
The proposed SOFB device has differences and similarities with solid oxide fuel cells and flow batteries. Compared to flow batteries, SOFBs have a higher operating temperature, and the storage fluids are gaseous, not liquid. And unlike solid oxide fuel cells, SOFBs can operate reversibly at lower temperatures and higher pressures.
Several activities are planned in three broad, closely coordinated, tasks: (1) experimental testing at Northwestern University, (2) cell-to-stack level modeling at Colorado School of Mines (CSM) and (3) system modeling at CSM.
All three tasks are fundamental to the success of the R&D effort. Stack- and system-level modeling is needed to establish material and cell performance targets that are congruent with SOFB systems meeting grid energy storage requirements. Cell-level materials testing, development and modeling are needed to establish the feasibility of reaching the targets. Experimentally verified cell models will also be incorporated into system models, making them quite realistic. These system models will examine thermal characteristics and the integration of other balance-of-plant components, and will be utilized for overall optimization of system design, operating conditions and assessment of dynamic operating capabilities.
Task 1: Solid Oxide Cell Development and Testing (Northwestern University): This task involves materials and cell fabrication, as well as performance and efficiency cost- effectiveness evaluation. Minimizing cell area-specific resistance will be a critical factor for achieving high efficiency in a cost-effective device. The experimental effort concentrates on studying SOFB electrochemical processes under unique high-pressure (3-15 atm), low-temperature (600-750°C) operating conditions, developing new materials and microstructures designed for these conditions, along with accelerated studies of degradation mechanisms. Optimized material sets will be utilized in a small-stack demonstration test.
Task 2: Develop and Validate Fundamental Theory/Modeling Framework and Cell-Level Models (Colorado School of Mines): A fundamental theory and modeling framework for reversible solid oxide cells will be developed and validated. The framework will include thermodynamic theory and cell architecture. Cell-level modeling will assist the quantitative interpretation of experimental results and help guide experimental lines of investigation. These models will be measured and validated with laboratory experimentation. Results from the models can be used to predict important cell electrode characteristics, interpret experiments and guide design improvement. The cell models will also be incorporated into the stack models.
Task 3: System Modeling, Design, Analysis and Optimization (Colorado School of Mines): Scale-up studies will be performed to minimize the system footprint and the capital and operating costs. Realizing the novel high-efficiency SOFB concept requires an understanding of numerous system-level considerations, including (1) appropriate operating conditions, (2) storage state points, (3) thermal management, (4) pumping requirements, and (4) system dynamics and mode-switching operating strategies.
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