(Plenary) Integrate to Extend the Reach of Solid Oxide Fuel Cells
Authors: Tew David , Cox-Galhotra Rosemary , Lecoustre Vivien , Radhakrishnan Rakesh , Soloveichik Grigorii L. 
Published: July 2021

Solid Oxide Fuel Cells (SOFCs) offer the potential for compelling thermo-economic value propositions in wide range of stationary and transportation applications through their potential for high conversion efficiency and their inherent fuel flexibility. Furthermore, many of the core SOFC materials, component, and system technologies have tremendous potential in electrolysis applications where fuels or chemicals could be produced using renewable electricity. However, to date, the wide-spread commercialization of SOFC/SOEC systems has been hampered by their high cost and durability challenges.

Fortunately, high temperature solid oxide-based energy conversion systems are well suited for integration with thermal-mechanical energy conversion systems in configurations that offer the potential for significant cost and thermodynamic performance synergies. Specifically, their integration with gas turbines or internal combustion engines offers the potential for ultra-high (>70%) efficiency at an attractive manufacturing cost (<$1/W). In these hybrid systems, engines or turbines are used to convert stack waste exergy to additional useful work resulting in higher overall efficiency and lower power-specific cost (e.g., $/kW) compared to SOFC-only systems.

Furthermore, in the case of SOFC and gas turbine hybrid systems, the SOFC stacks are typically located downstream of the gas turbine compressor and are at elevated pressure—enabling increased Nernst potential and decreased overpotentials and thereby higher power density at constant voltage. This benefit translates into a significant power-specific stack cost reduction. Additionally, the gas turbine compressor and recuperator may serve as the SOFC stack blower and recuperator—yielding further cost synergies via the elimination of duplicative balance of plant components.

In 2017, ARPA-E launched the INTEGRATE program to develop hybrid system concepts and enabling component technologies that if realized would result in 100kW-scale distributed generation systems with >70% fuel to electricity conversion efficiencies and system manufacturing costs of <$1/W. In the initial two-year Phase I portion of the program, nine teams developed a suite of hybrid system concepts. Given these concepts, the teams then proceeded to develop 1) durable SOFC stacks that are capable of operating at high (~4 bar) pressure, 2) high temperature (>600 °C) and low-cost heat exchangers, and 3) innovative control system approaches. In 2019, three of the original nine teams were selected to design, build and demonstrate 100kW-scale systems by 2023.

In the interest of focusing limited financial resources on foundational development risks, while avoiding the weight, volume, and fuel flexibility challenges associated with transportation applications and mitigating the financial risks associated with adopting new technologies at utility (e.g., >100MW) scale, natural gas fueled distributed generation (DG) systems were selected as the initial INTEGRATE application target. These DG systems would offer a fuel to electric power conversion efficiency that is roughly double that of the fossil fueled portion of the US electric grid. Furthermore, as renewable fuels such as hydrogen, ammonia, and bio- or electro- derived hydrocarbons (e.g., renewable natural gas or synthetic kerosene) become available, these systems would be capable of operating on them with relatively minor fuel processing system adjustments.

Moving beyond stationary DG applications, SOFC hybrid systems can leverage their fuel flexibility in the pursuit of a decarbonized of long-distance transportation sector—if the systems can be made both light and small enough. To this end, ARPA-E recently launched three programs that have taken on the challenge of developing ultra-efficient, yet compact and light weight carbon-neutral electrified propulsion systems for commercial aviation.

One of these programs is focused on carbon-neutral fuel to electric power conversion sub-systems—Range Extenders for Electric Aviation with Low Carbon and High Efficiency (REEACH). In the two-year Phase 1 of this program, six of nine selected teams will develop SOFC/gas turbine/battery hybrid systems that are fueled with renewable natural gas or synthetic kerosene. Phase 1 focuses on system conceptual design and fuel conversion component risk reduction, while full system design and prototype demonstration of a sub-scale fuel-to-electric power conversion device using a carbon-neutral liquid fuel is planned for Phase 2.

If the REEACH-envisioned systems are successfully developed, they would enable economically attractive all-electric commercial aircraft up to a narrow body airframe with dramatically reduced (ideally zero) net carbon emissions. Furthermore, INTEGRATE/REEACH hybrid system technologies would also have attractive value propositions in maritime, rail, or heavy-duty vehicle applications.