Use of excess solar array power by Regenerative Fuel Cell energy storage systems in low Earth orbit

Regenerative Fuel Cells (RFC's) are a competing energy storage system technology for a number of low-earth-orbit applications. The system is comprised of an electrolyzer which utilizes solar array power to convert water into hydrogen and oxygen reactants, a fuel cell that recombines the reactants back into water and produces power during eclipse, and associated controls and reactant storage. Round-trip electrical efficiencies of RFC systems are typically lower than competing battery energy storage systems. This results in larger solar arrays for the same application, with inherent drag, mass, and cost penalties. However, the increase in solar array size can be limited, if not totally eliminated, because of the ability of RFC systems to use excess solar array power. With a RFC system, any excess solar array power can be converted directly into reactants. In some cases the quantity of reactants can be significant. This is in addition to the normal quantity of reactants produced during the insolation (sunlit) portion of a typical orbital cycle. Assuming sufficiently sized reactant storage tanks, the additional reactants can be used to reduce the required input power for all orbital cycles throughout the year, effectively reducing the required size of the solar array. With battery energy storage systems, excess solar array power is typically shunted and not utilized. For this paper, the International Space Station (ISS) application was chosen for evaluation and comparison of battery and REC energy storage systems. This selection was based on the authors' familiarity with the ISS design and the availability of a detailed in-house computer model specific to the ISS electrical power system (SPACE). Combined altitude and orientation effects, seasonal variations, and beginning-of-life solar array performance were examined for individual orbits at and above specified reference points. Charging characteristics of the battery system were also investigated. The evaluation allowed a comparison of the solar array size required with the existing battery energy storage system to the projected solar array size required with a possible RFC system. The results of the examination indicated that no increase in solar array size would be necessary for the ISS if outfitted with a RFC energy storage system, in spite of the lower round-trip electrical efficiency. For orbits with a minimum of excess power, the battery energy storage system used only 73% of the available solar array power as compared to 100% usage for a RFC system. The usage by the battery system was far less for the orbits with higher amounts of excess power. In terms of solar array size, a 59% efficient RFC system thus becomes equivalent to an 80% efficient battery system. Thus, for the ISS and possibly other low-earth-orbit spacecraft, there would be little if any solar array size advantage as a result of a difference in round-trip electrical efficiency between battery and RFC energy storage systems.