Orbiter fuel cell improvements

The orbiter's fuel cells provide electric power for the orbiter and water for the crew. Ninety-six fuel cells in three stacks convert hydrogen and oxygen into electrical power, water, and heat via an alkaline electrolyte. The fuel cells require approximately four overhauls (at about $3.5 million per overhaul) and four repairs (at approximately $100,000 per repair) each year. With continuing overhauls and repairs, the current inventory of fuel cells could support current shuttle flight rates beyond 2012. If the flight rate increases to 12 per year or more, additional fuel cells will be needed. Two distinct upgrades—longer-life alkaline fuel cells and proton exchange membrane (PEM) fuel cells—are being considered to replace the current cells.

Longer-Life Alkaline Fuel Cells
This upgrade, proposed by International Fuel Cells and Boeing, would entail replacing the current fuel cells with modified alkaline cells. The modified fuel cells would operate at reduced reactant temperatures and would be designed to inhibit corrosion and improve reliability. Their electronic controls would also be upgraded to enable new monitoring capabilities and to preclude obsolescence. The lifetime of the upgraded fuel cells is estimated at 5,000 hours. The present fuel cells are certified to 2,600 hours before overhaul. In reality NASA is experiencing an average overhaul time for the current fuel cells of 2,100 hours. It should also be noted that the current fuel cells are operating in the vehicle for an average of only 1,200 hours before they must be removed to repair system component failures.

The contractors estimate that certification of the units to fly on the shuttle would cost about $14 to $17 million, with a production cost of approximately $3 to $4 million for each of the four power plants, assuming that many of the current fuel cell components are reused. The development of advanced alkaline fuel cells could begin in 1999. The contractors estimate that the certification program would take three years, with the first production unit delivered a year later. Because the longer-life alkaline fuel cells appear to be straightforward engineering modifications of the existing orbiter fuel cell power plants and the changes are relatively minor, these estimates of cost and schedule should be reasonably accurate.

If this upgrade were implemented, the primary benefit would be to reduce operations and maintenance costs and time. NASA estimates yearly savings from reducing the number of overhauls and annual repairs would be $22 million. The current fuel cells have flown successfully with an excellent reliability record, so the new cells would have no major functional or safety advantages. The advanced alkaline cell could, however, support longer missions and an increased flight rate, and the associated electronics upgrade could enable improved health monitoring of the fuel cells.


Sent from my iPad