The technical objective for regenerative fuel cells is to increase the efficiency and reliability of the core technologies needed to create a regenerative fuel cell with minimal mass and volume and the required safety and operability. These core technologies consist of a primary fuel cell, electrolyzer, and balance-of-plant components. The primary fuel cell creates electricity, water, and waste heat by the electrochemical conversion of hydrogen and oxygen. The electricity can be used to drive a load as needed, depleting the stored hydrogen and oxygen. These reactants are regenerated by the electrolyzer, which electrolyzes the product water using electricity generated from photovoltaic arrays. Balance-of-plant components such as transducers and valves manage all flows required for operation. This hydrogen/oxygen regenerative fuel cell can be used to provide continuous power during long shadowed periods for much lower mass than would be achievable from a combination of photovoltaics and batteries alone, with the power generation period limited by the size of the reactant tanks rather than the mass of the power plant.
The objective for primary fuel cells is to: 1) increase system lifetimes and reduce system mass, volume and parasitic power loads and 2) enable the use of scavenged propellants as a fuel cell reactant. Our approach is to develop advanced proton-exchange-membrane (PEM) fuel cells because they have the best combination of safety, technical maturity, compatibility with scavenged propellant, and performance needed for foreseeable human missions.
The objective for electrolyzers is to improve system reliability and efficiency using balanced, high pressure reactants to meet the expected needs of destination applications and life support systems. Our approach is to competitively select a system concept and then develop the stack, balance-of-plant, and MEA and integrate them into an electrolyzer system that works efficiently as a stand-alone unit and with the primary non-flow-through fuel cell.
To reduce system mass, volume and parasitic power, and increase system lifetimes for both primary and regenerative fuel cells, we are addressing the largest source of system failures and parasitic power losses: the balance-of-plant. Because we will be operating in the space environment, we are using pure oxygen rather than air as a reactant. This frees us from having to remove the non-oxygen elements of air from the system, and permits the use of capillary action to remove product water. Our design approach requires no recirculating reactants, and hence no requirement for providing either recirculation or external product water separation from two-phase reactant streams. Therefore, there is no need for the major components that provide these functions, and no resulting weight, volume, parasitic power, reliability, life, or cost penalties. These no-longer-needed components include water management pumps, water/gas separators, and injectors/ejectors for recirculation. As these are the system components most likely to fail, we are more likely to reach our goal of long-life maintenance-free autonomous operation without them. These components can also comprise 25-35% of total system mass, and contribute parasitic losses that reduce system efficiency and therefore require more reactants for a given mission. More reactants require bigger tanks which further increase the systems mass so their elimination offers a major advantage for launched systems. Since reactant recirculation is not required, this system is a “non-flow-through” design, reminiscent of the fuel cell technology used during the successful Project Gemini missions in the 1960’s.
Our technology development includes the following elements: 1) primary fuel cell stack development including advanced product water removal technology and advanced manufacturing processes, 2) balance-of-plant development including reactant management, controls, and thermal management, 3) electrolyzer stack development including advanced reactant feed mechanism, 4) membrane electrode assembly (MEA) development for primary fuel cell and electrolyzer stacks, and 5) system integration and testing. Stack technology development is conducted through competitively selected contracts to leverage industry’s manufacturing capabilities. Balance-of-plant development is conducted in-house to ensure that we can test and evaluate stacks regardless of the vendor and to facilitate integration with a variety of demonstration projects. MEA development is conducted in-house to address the unique needs of the non-flow-through and balanced high pressure stacks.
Fuel cell and electrolyzer stack development is accomplished by designing and building stacks made from progressively larger numbers of individual cells, using both small-scale (50 cm2) and large-scale (150 cm2) cross-sectional (active) areas. Detailed stack design concepts will be tested and evaluated on appropriately sized stacks; these design concepts include means to ensure compatibility with propellant-grade reactants, improve operational performance, and improve system efficiency and lifetime. Larger scale hardware is built to determine feasibility as needed, and also to provide required power levels as needed by demonstration projects. Balance-of-plant development includes the development of a stationary, “universal” system used to test vendor supplied stacks of various power levels and configurations. Mobile, autonomous systems are also built to support individual field or flight demonstrations. Emphasis is placed on using flight-ready hardware where possible to reduce future modifications to flight systems. MEA development is focused on improving catalyst formulations and chemical composition to maximize efficiency. MEA efficiency directly impacts stack efficiency, which determines how small reactant tanks can be (e.g. every 1% improvement in electrical performance translates directly into a 1% reduction in the mass and volume of reactants and a reduction in the mass and volume of the storage tanks).