Abstract
Power systems for human spacecraft have historically included fuel cells due to the superior energy density they offer over battery systems depending on mission length and power consumption. As space exploration focuses on the evolution of reusable spacecraft and also considers planetary exploration power system requirements, fuel cells continue to be a factor in the potential system solutions.
Substantial efforts are currently underway in the commercial markets to produce a proton exchange membrane (PEM) fuel cell capable of meeting terrestrial power demands in residential, commercial, and automotive applications. However, there are unique characteristics of spaceflight that can only be dealt with through specific engineering solutions.
From a systems perspective, removing product water from the cell stack and separating the water from the oxygen gas stream in a PEM fuel cell are two critical functions. One method to remove product water from the cell stack and subsequently separate the product water from the oxygen involves using components with no moving parts — a gas ejector and membrane gas-water separator. Tests are currently underway at the Johnson Space Center to evaluate and refine gas ejectors to satisfy the fuel cell requirement to circulate cathode reactant gas (oxygen) at 1 to 3 times the stoichiometric consumption flow rates in order to adequately remove water from the cathode. A gas-water separator utilizing hydrophobic and hydrophilic materials is also being evaluated to perform the function of separating the water from the oxygen gas stream. Analytical and experimental evaluations are continuing on the fuel cell components, including cell stacks, with the goal of developing a comprehensive design basis for a fuel cell powerplant capable of delivering 20 kW at approximately 28 VDC. Through the select critical component refinement in work at the Johnson Space Center, engineers are improving the readiness and reducing the technical and cost risks of a PEM fuel cell capable of operating in a space environment.
