Engineering topics

Phillip J. Smith

Publications and source records attributed to Phillip J. Smith.

10 records

NASA NTRS2022

Lunar Equator Regenerative Fuel Cell System Efficiency Analysis

It is expected that future NASA mission energy storage requirements can be adequately met by regenerative fuel cells (RFCs). Aerospace RFCs are likely to package proton exchange membrane electrochemical conversion devices based on hydrogen, oxygen, and water. To optimize system designs and direct further development, the most useful system metric is round-trip efficiency (RTE). Improving RTE generally increases specific energy and reduces system mass. The following analysis models the impact of several factors on RTE for RFCs scaling from 0.1 to 50 kW. The analysis demonstrates that higher electrolyzer (EZ) operational pressure most negatively impacts both RFC RTE and specific energy. Reactant storage heating and power management losses are significant for RFCs of any scale, but solenoid valve power becomes a noticeable parasitic load for smaller RFCs. Spherical gas storage vessels benefit RTE by only 2 to 4 percentage points but increase specific energy by ~50 percent. Cryogenic reactant storage is shown to depress RTE to below 15 percent in all cases. For a given RFC scale, there is an RTE benefit to specifying larger EZs and recharging at higher rates—if the RFC can be coupled with a corresponding power source. Although fuel cell (FC) operation is less impactful on RTE than EZ factors, efficiency is maximized by sizing the FCs relatively large and keeping current density low. RFC design is a balance of many interrelated factors, but this analysis provides a starting point for difficult decisions that must be made when designing a lunar equator RFC.

regenerative fuel cell↗

NASA NTRS2022

Aerospace Regenerative Fuel Cell Fluidic Component Design Challenges

A discrete regenerative fuel cell (RFC) system is currently under development at the NASA Glenn Research Center and Johnson Space Center. The objective of this activity is to produce an energy storage system with a higher specific energy (W·hr/kg) than the state-of-the-art battery technologies then test that system within an environment simulating the temperatures and pressures expected over a full lunar day cycle on the lunar surface. The system is designed to be as flight-like as possible given resource and commercially available technology limitations. The RFC system is comprised of a fuel cell power system, a high pressure electrolysis system, fluidic and electronic balance of plant systems, avionics, power management and distribution system, and software. This technical memorandum discusses components and subsystems that present challenges in developing flight ready RFCs. To remedy identified issues, functional application test plans are presented for components and subsystems primarily constructed with commercially available options due to the budget and schedule constraints of this ground technology demonstration project.

fuel cell↗

NASA NTRS2021

Proton Exchange Membrane Fuel Cell Transient Load Response

Spaceflight mission profiles may not be limited to constant power loads. It is necessary for the associated power system to support changes in load in addition to being energy dense. H2-O2 proton exchange membrane (PEM) fuel cells can accommodate such requirements without significant current smoothing or additional devices to provide transient power. This testing involved current load step changes to characterize non-flow-through PEM fuel cell response to increasing and decreasing load step changes over a range of 0 to 0.5 A/sq. cm. It was determined that fuel cell output voltage stabilizes within 4 ms with no noticeable steady-state performance degradation, following exposure of the fuel cell to transient loads of up to ~1.5 A/sq. cm. Faster stabilization generally occurred following larger step changes.

fuel cell↗

NASA NTRS2021

Effect of Reactant Pressure on Proton Exchange Membrane Fuel Cell Performance

It is an objective of the National Aeronautics and Space Administration (NASA) to return to the moon and to create sustained lunar operations. Since many applicable lander and upper stage vehicles utilize cryogenic hydrogen and oxygen propellants, it is appealing to incorporate fuel cells into potential mission concepts. A fuel cell could generate electricity, heat, and water from the residual hydrogen and oxygen propellants. Such a concept depends on the capability of the fuel cell to utilize dry, propellant-grade reactants at reduced pressures compared to typical fuel cell operating conditions. This report describes a NASA evaluation of two passive water removal, non-flow-through proton exchange membrane (PEM) fuel cells: a twelve-cell 50 sq. cm active area stack and a seven-cell 150 sq. cm active area stack. Both stacks operated on research grade hydrogen and oxygen reactants supplied at pressures ranging from 20 psia up to the 45-48 psia design operating point, reliably supporting current densities of at least 450 mA/sq. cm at reduced pressures. Polarization curves, fitted with a semi-empirical equation for overpotential, show an increase in effective ohmic resistance and decrease in reversible cell potential, hydrogen crossover current, and Tafel slope when operating at lower pressures, resulting in a reduction in cell voltage at all current densities. Fitted parameters are compared to published values and rationalized with first principles explanations.

fuel cells↗

NASA NTRS2021

Effect of Reactant Purity on Proton Exchange Membrane Fuel Cell Performance

NASA has set a goal to return to the Moon and to establish a sustained lunar presence. Many applicable lander and upperstage vehicle concepts utilize cryogenic H2 and O2 propellants. This propellant selection enables appealing potential mission concepts wherein electricity is generated by operating a fuel cell on residual H2 and O2 propellants. This concept depends on the capability of the fuel cell to utilize dry, propellant-grade reactants with concentrations of up to 30 percent He present. This study consists of the evaluation of a 12-cell non-flow-through proton exchange membrane fuel cell stack of 50 sq. cm active area with passive water removal. This stack was supplied with three levels of reactant purity: >99.999, 99.1, and 70 mol%, with the remainder made up of He. The reactant humidity and flow-through rate were also assessed as performance factors, and the fuel stack repeatedly supported a load profile with current densities up to 500 mA/sq. cm (25 A). The fuel cell performed consistently over the course of testing, with cell voltages decreasing approximately 50 mV at the maximum current density when supplied with reactants with 30 percent He present.

electrochemistry↗

NASA NTRS2021

Electrolytic Co-deposition Neutron Production Evaluation

Co-deposition electrochemical cells are a simple means to examine novel nuclear reactions. In this study, palladium and deuterium atoms were co-deposited on a cathode at stoichiometric densities, forming dendritic morphologies. Bubble detector neutron dosimeters were used to measure equivalent dose levels during electrolytic deposition. Standard configuration cells expected to produce excess neutrons were denoted as experimental cells and contained an electrolyte consisting of palladium(II) chloride, lithium chloride, and heavy water. The control cells used copper(II) chloride, lithium chloride, and heavy water electrolyte. Experimental and control cells were supplied current, increasing from 0.1 to 100.0 mA over a period of 20 days. For test days 9 through 20, the standard configuration experimental cells exhibited significantly higher average neutron radiation than the controls at a 99% confidence level. Alternative configurations involving more increasing current over a shortened time scale, substitutions of water in place of heavy water or potassium chloride for lithium chloride, as well as inclusion of depleted uranium near the cell cathodes all resulted in inconclusive evidence for promoting neutron production.

neutron↗

NASA NTRS2021

Software Approach for the Regenerative Fuel Cell Project

The Regenerative Fuel Cell (RFC) project is developing a system that combines a fuel cell subsystem for energy production with an electrolysis subsystem for energy storage. The system is primarily being developed at the Glenn Research Center with support from Johnson Space Center under the Space Technology Mission Directorate / Game Changing Development Program. Currently, the intent is to operate the RFC system in an equivalent environment to a lunar equator location for a period of several lunar day/night cycles with minimal operator input. Demonstrating an automated RFC system in this environment is a step towards showing the extensibility to a flight system. This paper describes the RFC software team’s approach for development of the monitoring and control software that allows for the automated operation of the RFC system. We describe the rationale for the choice of Core Flight System architecture, as well as the plan for software/hardware integration.

Mathew J Mccaskey↗

NASA NTRS2020

Analysis of a 100-W Regenerative Fuel Cell Demonstration

A 100-W regenerative fuel cell (RFC) demonstration was successfully completed at the NASA Glenn Research Center on December 11, 2014. This effort was funded by Advanced Space Power Systems milestone and represented the first demonstration of passive laboratory-scale stacks arranged as an RFC, consisting of a non-flow-through fuel cell and a static-feed electrolyzer. Once developed, these stacks present the potential to minimize system mass, volume, and parasitic power in comparison to existing terrestrial electrochemical systems and nonnuclear power generation options. Testing involved the fuel cell powering a simulated load then recycling fuel cell product water by subsequently electrolyzing it to regenerate hydrogen and oxygen gases for fuel cell operations. A total of five cycles were completed over a 2-week period, yielding cell voltage data for stack performance and quality measures for regenerated reactants. Overall, round-trip efficiency was calculated to be 47.4 percent. Gas crossover was observed in electrolyzer gas products, as residual gas analysis showed 1.1 percent hydrogen in oxygen and 0.4 percent oxygen in hydrogen. Fuel cell product water was determined to be more acidic and conductive and have a higher fluoride content when compared to deionized water.

fuel cell↗

NASA NTRSDate not supplied

Lunar South Pole Regenerative Fuel Cell System Efficiency Analysis

It is expected that future NASA mission energy storage needs can be adequately met by regenerative fuel cells (RFCs). Aerospace RFCs package proton exchange membrane (PEM) electrochemical conversion devices based on hydrogen, oxygen, and water. To optimize system designs and direct further development, the most useful system metric is round-trip efficiency (RTE). Improving RTE generally increases specific energy and reduces system mass. The following analysis models the impact of several factors on RTE for RFCs at the lunar south pole scaling from 0.1 kW to 50 kW. The analysis demonstrates that higher electrolyzer operational pressure most negatively impacts both RFC RTE and specific energy. Optimal electrolyzer membrane thickness is evaluated as a function of operational current density and pressure. Reactant storage heating and power management losses are significant for RFCs of any scale but are reduced in impact at the south pole compared to colder equator applications. Solenoid valve power remains a noticeable parasitic load for smaller RFCs. Larger electrolyzers enable higher peak theoretical efficiencies when recharging at higher rates, if the RFC can be coupled with a corresponding higher power source. RFC design is a compromise of many interrelated factors, but this analysis provides a starting point for difficult decisions that must be made when designing a lunar south pole RFC.

regenerative fuel cell↗

NASA NTRSDate not supplied

Thermal Management Challenges and Technology Options for Lunar Regenerative Fuel Cells

Regenerative Fuel Cells (RFCs) enable lunar surface hardware, from pressurized rovers to surface habitats, to survive the lunar night by providing the required energy storage with a lower mass than state of the art battery systems. RFCs consist of fuel cell and electrolyzer electrochemical stack(s), a fluidic balance of plant, and electrical balance of plant. Fuel cells operate by converting the stored chemical energy of reactants into DC electrical power, heat, and water. When supplied DC power from an available external source, electrolyzers regenerate the reactants through electrolysis. The balance of plant is a critical part of the integrated system consisting of many individual components and provides fluid pressure management and conditioning, fluid storage, and thermal management. Component technology development is required to create an RFC system that will be capable of surviving the harsh lunar environment and maximize specific energy (W-hr/kg). One critical component requiring development includes a kW-scale high flux thermal switch. The primary requirement for the thermal switch is the ability to maintain low thermal resistance with the RFC heat rejection interface during the lunar day to maximize heat rejection while switching to a high thermal resistance during the lunar night such that the RFC can conserve as much thermal energy as possible. Different technology including freeze tolerant pumped loop, passive louvers, and variable conductance heat pipes were assessed as potential solutions. NASA is exploring development options through internal and external partnerships to develop this technology critical for a spaceflight RFC system.

Energy Storage↗