Analysis of Pressurized, Tubular, Proton-conducting, Ceramic Reversible Fuel Cells for Efficient, Low-Cost Energy Storage
Authors: Colella Whitney 
Published: July 2021

This work analyzes pressurized, tubular, proton-conducting, ceramic reversible fuel cells (PreTProCRFC) for efficient, low-cost energy storage. PreTProCRFC have the potential to facilitate low carbon energy economies and power-to-gas applications. In the power-to-gas concept, excess electricity from wind turbines and solar photovoltaic arrays is sent to high temperature electrolyzers, like the PreTProCRFC, that store energy as hydrogen (H2) and oxygen (O2) for short or long durations (> 4 hours). When electricity is needed, the PreTProCRFC runs as a fuel cell and consumes the stored H2 and O2 to generate electricity. In this way, PreTProCRFCs have the potential to significantly augment the growth of renewables globally. This research analyzes the potential for PreTProCRFCs as viable short or long-term energy storage devices. This work develops thermodynamic and techno-economic analyses (TEA) for these PreTProCRFC systems and identifies cost drivers important to their R&D and commercialization path. This work also explores the value propositions presented by pressurized, tubular designs, for different segments of the energy storage market.

Approach:

This work is conducted in close collaboration with project partners Special Power Sources Inc. (SPS), which currently manufactures tubular ceramic fuel cells for research and niche energy applications, and Kansas State University (KSU), which is developing novel, proton-conducting ceramic materials. To execute this research, Gaia works with SPS and KSU to identify and analyze cell, stack, and system engineering performance data. Gaia then develops and deploys custom computer models and data sets that include, but are not limited to, chemical engineering process plant designs of PreTProCRFC systems and detailed TEA models. Gaia also builds on existing U.S. DOE modelling tools, such as the H2A H2 production analysis modelling tools and existing DOE electrolysis case studies.

Results:

Modelling results indicate that the primary cost drivers for the lifecycle energy storage costs of PreTProCRFC systems include, but are not limited to,

(1) the system’s ramp rate in fuel cell mode (i.e. the electric power output per unit time);

(2) the system’s ramp rate in electrolysis mode (i.e. the electric power input per unit time);

(3) the quantity of effective heat reuse between exothermic fuel cell modes and endothermic electrolysis modes;

(4) the efficiency of thermal storage between fuel cell and electrolysis modes;

(5) the electricity consumed per unit of hydrogen produced in electrolysis mode;

(6) the electricity consumed per unit of oxygen produced in electrolysis mode;

(7) the capital costs of the stack;

(8) the capital costs of the surrounding balance of plant (BOP) subsystems;

(9) the electrolysis outlet pressures for hydrogen and oxygen; and

(10) the marginal increase in system cost with higher electrochemical outlet pressures.

Regarding (1) and (2), when connected to the electricity grid, and receiving intermittent renewable electricity to store, PreTProCRFC systems need the capability to respond rapidly. Electricity storage devices that can either receive and/or supply electricity more quickly can demand a higher premium ($/kWh price). In particular, the electricity balancing market is a submarket within the overall electricity market responsible for reconciling instantaneous differences in electricity supply and demand, and, consequently, sees the highest prices. The proposed SPS PreTProCRFC stack design is tubular, with less total sealant area, and with a close match of stack material thermal expansion coefficient, so it is expected that this design will more readily electrically ramp compared with other designs. (A main failure mechanism of high temperature electrochemical systems under fast-ramping conditions is mechanical cracking of the stack’s materials, due thermal expansion coefficient mismatches.)

Regarding (3), (4), (5), and (6), a unique feature of high temperature electrolysis (compared with low temperature electrolysis) is that electricity consumption by the electrolyzer can be displaced by heat consumption, in a one-to-one ratio. In a PreTProCRFC system, heat released during the exothermic fuel cell reaction can be stored and reused to provide heat for electrolysis. Efficient design of this heat reuse subsystem is crucial to the PreTProCRFC system achieving high round-trip efficiencies, a necessity for the energy storage market.

Regarding (7) and (8), primary cost drivers for PreTProCRFC system also include stack capital costs, stack power density, and system BOP costs. Over time, stack capital costs are estimated to decline more rapidly with R&D and more high volume manufacture, compared with BOP capital costs, because the BOP already incorporates many mass-produced components.

Regarding (9) and (10), the SPS PreTProCRFC stack is also unique in that it has the capacity to supply hydrogen and oxygen at pressure. Model results indicate that electrochemical compression has the most compelling cost advantage over mechanical compression in the lower pressure range included in the proposed stack design.