How do fuel cells comapre to batteries on rovers?

[DDE]

So, basically, I'm considering a Mun Buggy (pun attempted); and I'm thinking what the better choice of power source is. RTGs are excluded at the start, and I have great doubts about solar; if I can have enough gas to outlast my own patience, then why bother with infinite power?

[PT]

It depends on your consumption but wild guess is few hours (if my math checks out) at most. Unless you want to drive fuel truck, that is. But do not forget capacitor.

Example:

Small Oscar B tank have 18 liquid fuel, Fuel Cell eats 0.0016875 per second, this gives 3 hours (10666 seconds) of 1.5 electricity per second. This gives you equivalent of ~4 small solar panels (at Kerbin orbit). 16k charge total in 300kg or so. Comparable battery bank would weight twice as much and be horrible in shape, size an/ord part count.

However have in mind your place of operation when comparing fuel cells to solar panels as their power output depends on distance from Kerbol. To get same power output as in Kerbin zone you will need twice as many panels on Duna and ten times as many on Dres.

Do not forget to check your rover power usage before sending it. Smallest wheels eat 1 electric charge per second, add some for probe core and reaction wheels and you get around 5ec/sec. With just one power cell you will need battery to drive around. Science transfer with antena will eat anywhere from few dozens to few hundreds.

(numbers from wiki, might be wrong)

[Kermanzooming]

i can see fuel cells as a measure to keep operating during the night but not as the only energy supply; once you run out of fuel, there is no practical/economical way to refuel so you will have a dead rover.

That won't happen with solar panels.

[Geschosskopf]

1 hour ago, DDE said:

So, basically, I'm considering a Mun Buggy (pun attempted); and I'm thinking what the better choice of power source is. RTGs are excluded at the start, and I have great doubts about solar; if I can have enough gas to outlast my own patience, then why bother with infinite power?

Well, it depends on the number of wheels on the rover and what you're trying to do with it.  Since 1.1, wheel power consumption takes inertia into account.  They only draw their max power when working hard, such as accelerating or climbing a hill.  When just cruising along at max speed, they draw considerably less juice, and of course if you're not going max speed and coasting a lot, they don't draw much at all.  Because of this, it's quite possible to design solar or even RTG rovers that have a total generating capacity about of about 1/2 or less than the total max draw of all the powered wheels, and have a fairly big (usually a few thousand EC) battery to provide excess juice for accelerating and hill-climbing.  Also, you can disable the motor on some of the wheels if you don't need the extra horsepower at the moment.  You can, in this way, make a rover that never runs out of juice, at least if the sun is shining.

The size of the battery determines how much hard work you (or the size of the hill) you can do before you have to pause for a recharge.  It usually takes some testing to figure out the idea size of battery and the amount of generation capacity for a given rover design.  If you have enough battery just barely to climb the biggest hill you're likely to meet (considering generation while doing so), then as you coast down the other side, the generators will charge it back up for the next hill.

Now, as to what you use for a generator, you have the options of RTGs, solar, and fuel cells.  RTGs don't make much juice so it's very expensive to make rovers with lots of them.  Still, it's often a good idea to have 1 as an emergency, always-there battery charger, plus it will keep a probe core alive if you park the rover for any length of time.  But really, you're main options are solar and fuel cells, or a combination of both.

Solar panels work just fine on Mun (slightly better than on Kerbin due to no atmosphere), at least when the sun is shining.  But Mun's nights last about 3 of Kerbin's days so you might have to park and warp ahead that long if you intend to go any great distance.  But OTOH, it's dangerous driving in the dark so you might want to wait out the night anyway, regardless of how you power the rover.  Remembering that you don't need generation for more than about 1/2 the total wheel draw, a few of the OX-State-XLs should do you pretty well, provided you have enough battery.

Fuel cells are kinda overkill for small rovers.  The big one makes way more juice than you need most of the time, they require fuel tanks, and the tanks eventually run dry.  Now, if you have a medium rover (8 or more powered wheels) that has to do a lot of hill-climbing, a big fuel cell can be quite handy.  It will keep you going without having to stop and recharge the battery.  Just turn the fuel cell on when you really need it and shut if off the rest of the time.  Now, if you've got a huge rover with multiple ginormous wheels on it, fuel cells are pretty much required.

[Aegolius13]

Unless you have absolutely no space, I'd stick a static solar panel or two on the roof, at least as a supplement.  They'll generate enough to make up for their mass pretty quickly.  

Plus driving at night is annoying, so the panels are likely to be useful when you're cruising around anyway.

[baldamundo]

Yeah, honestly I find it really hard to see when the stock fuel cells are worth taking. Maybe if you're at Eloo or somewhere, but solar panels normally seem a much better bet. The hydrogen fuel cells that a couple of mods (like Universal Storage) add are much more attractive though - especially since you can combine it with a water splitter and solar panels to make it renewable.

[foamyesque]

5 hours ago, Kermanzooming said:

i can see fuel cells as a measure to keep operating during the night but not as the only energy supply; once you run out of fuel, there is no practical/economical way to refuel so you will have a dead rover.

That won't happen with solar panels.

That depends on how large your rover is :v

[Kermanzooming]

14 hours ago, foamyesque said:

That depends on how large your rover is :v

Don't know how to interpret this... do you mean that if your rover is small enough you can refuel it on some practical and economical way?

 

[baldamundo]

30 minutes ago, Kermanzooming said:

Don't know how to interpret this... do you mean that if your rover is small enough you can refuel it on some practical and economical way?

 

I think he means having a massive rover with its own ISRU for refuelling

[Kermanzooming]

1 minute ago, baldamundo said:

I think he means having a massive rover with its own ISRU for refuelling

Of course, my bad; as always in KSP, when in doubt go bigger.

[Sharpy]

ISRU and a 4+ -star engineer. You break even on fuel consumption:production with negligible surplus on 3 stars. Autonomous rover without the engineer can't run ISRU on fuel cells, period.

But otherwise, at Kerbin distance from the Sun, fuel cells lose to solar panels bad. The fuel cells start making sense at Dres. Other than that, yeah, if you're going for non-reusable electric  power source, fuel cells give you the best energy density. I once built a craft with upper stages of some 30,000m/s of delta-V (the whole thing had something like 60,000m/s; rescue kerbal from retrograde solar orbit), by creating an extreme asparagus-staged set of fuel cell powered ion boosters. .

Just if you use more than, say, 4 fuel cells, make sure to use the 'fuel cell battery'. It looks like 6 fuel cells. It weighs as much as 6 fuel cells. It gives power of 12 fuel cells

[PrathamK]

Stick a Small panel or two and a docking port, have lander a port , a fuel cell or have a RTG on rover, either, dock with lander when ever you are short of power, when the night is going to start, and while transmitting, energy and heavy antennas on lander will help.

Edited April 24, 2017 by PrathamK