Quick fuel cell question

[DaveDauria]

Ok I've got a quick fuel cell question. Basically what sorts of space vehicles/probes actually use them? The reason I ask is I was watching "The Colbert Report" and they had this guy(who came across as a complete buffoon) state that NASA uses them in satellites to explore planets.(My impression was that NASA uses solar panels and RTG's.) Anybody know?

[novaderrik]

i think the usefull life of a fuel cell is relatively short, and it needs to be "re-fueld" every once in a while.
i think the shuttle electrical systems are powered by fuel cells, but the shuttle is only designed to be up there for a couple of weeks at most.

[Bob B.]

The only space vehicles I know of that used fuel cells were Gemini (starting with Gemini 5), Apollo, and the Space Shuttle. Fuel cells are good for short duration flights but are bad for long duration because too much reactant must be carried. All deep space probes, to my knowledge, use either solar cells or RTGs. Solar cells are good out to about the orbit of Mars, beyond which the sunlight is too weak for them to work efficiently.

[Omicron Persei 8]

Solar cells are good out to about the orbit of Mars, beyond which the sunlight is too weak for them to work efficiently.

That's not entirely true. The new Jupiter mission JUNO will use three large solar panels for its endeavor.

The currently enroute comet landing ESA mission Rosetta will have an orbit that will take it beyond Jupiter's orbit, though it will be hibernating during this time.

[Bob B.]

That's not entirely true. The new Jupiter mission JUNO will use three large solar panels for its endeavor.

I was referring mainly to past missions, though I did not clearly specify so. Today's photovoltaics are more efficient than those used in the past. For instance, I believe Gallium Arsenide (GaAs) cells are nearly twice as efficient as Silicon (Si) cells.

[Larry Jacks]

Just to reenforce what others have already said, fuel cells have been used on US manned missions for 40 years, from Gemini through Apollo to the Shuttle. They weren't used on Skylab or the ISS to the best of my knowledge because solar panels and batteries were available to do the job.

Off hand, I don't know of any satellites that have used fuel cells. Using fuel cells for primary power would require carrying large quantities of hydrogen and oxygen, something that's difficult to store for long periods and has a high mass. For missions to the inner planets (out to Mars), solar panels make a much better solution than fuel cells. For missions much beyond Mars, RTGs have been the preferred solution to date. With newer, more efficient fuel cells, missions out to Jupiter could use solar panels if the electrical loads are kept low, although if you're looking to orbit Jupiter or one of its moons, the very high radiation levels might work agains solar panels.

It might be possible to use a form of regenerative fuel cell for geosynchronous satellites but I don't know of any that do. This type of fuel cell would use electrolysis to split water into hydrogen and oxygen when solar power is pelentiful (a geosynch satellite is in sunlight over 99% of the time) then use the H2/O2 to power the fuel cell during an eclipse. However, such a system might end up weighing more, taking more volume, and being more problematic than simply using batteries.

[Omicron Persei 8]

I=For instance, I believe Gallium Arsenide (GaAs) cells are nearly twice as efficient as Silicon (Si) cells.

Unfortunately they're so much more expensive.

[Larry Jacks]

Originally posted by Omicron
Unfortunately they're so much more expensive.

While the purchase cost of GaAs solar cells is higher, you need fewer of them to generate a given amount of electrical power. This represents a significant mass savings for the electrical subsystem, giving extra margin for propellant, instruments, etc. It's possible that you'd recoup that higher solar panel cost with savings from the lower launch mass (multiple thousands of dollars per KG). The cost of the solar cells is typically a very small percentage of the total mission cost.

[mugaliens]

For either long missions or short flights, the critical factor is the power requirement of system. P=V*I (Power (in Watts) = Volts x Current (amperes))

Converting from one voltage to another, and back and forth between DC and AC these days is trivial with modern switching power converters - highly efficient, too.

The real issue is the peak draw. Either way, solar panels or fuel cells, it needs to be big enough to provide the peak draw.

Whether fuel cells or solar panels are choice is dependant on the duration of the mission.

The size and weight of a photovoltaic panel is proportional to the peak draw.

The size and weight of a fuel cell, however, is proportional to the peak draw over time, or Energy (watt-hours), as you're exhausting expendibles to produce power.

Thus, for short duration missions, fuel cells provide a lower weight penalty, and readily operate in the penumbra.

For longer duration missions, such as most satellites and the ISS, photovoltaics provide a lower weight penalty, and use batteries to provide power while traversing the penumbra.

It's fairly mundane, from a financial and engineering point of view, to calculate the various fixed and variable costs associated with the variety of different power sources available.

Very long-duration missions within typical "sunlight range," such as 1 AU, should have the following:

1. Primary source: Photovoltaic panels capable of providing peak draw after twice mission duration and normal degredation (they're not nearly as efficient after being bombarded by micrometeorites).

2. Secondary source: Rechargeable batteries capable of providing normal operating requirements at twice the expected mission duration.

3. Tertiary source: Nuclear-fueled source (nuclear "battery"), like Cassini, Voyager, capable of providing basic life-support power requirements at twice the expected mission duration.