PSA: batteries V.S. fuel cells for general purposes

[Jesrad]

Did you know ? In KSP, liquid fuel + oxydizer are a much denser form of energy storage than batteries, by a ratio of either 8 or 4 depending on whether you compare LFO to small batteries or to the 4k variant. But whether this ratio is enough to compensate the extra mass and complexity of lugging fuel cells and tankage around, versus the operational simplicity of batteries, may depend on the use context. Let's find out !

LFO converted to electricity by a fuel cell is 80,000 energy / ton

Batteries hold less: 10,000 (small batteries) or 20,000 (Z-4k) energy / ton.

However, batteries are not constrained by power requirement, whereas fuel cells only output 30 (fuel cell) or 75 (array) energy/second for each additional ton. This means that, in the energy-intensive uses found in KSP, which are ion propulsion and ore drilling + ore conversion, the extra output-constrained mass may obviate the better density edge of fuel cells, in some contexts.

For example, in ion propulsion you need 94414 energy to expend an entire PB-750 Xenon tank. That amounts to 4,8 tons of Z-4k batteries (96k charge), or 1.18 ton of LFO, which is 1.31 ton of actual tankage. But then you need at least 6 fuel cells or half a fuel cell array to power a single ion engine at full thrust from LFO, for a total of 1.5 or 1.6 ton, and that is still ahead of batteries. You may use lower throttle settings off a single fuel cell for better efficiencies, but that lowers a LOT the already paltry TWR of the whole contraption. Smaller Xenon tanks require the same power (and same mass of fuel cell) but less LFO/battery mass, so there is a balance point at which batteries become more efficient: this tipping point is at 197 Xenon, which is less than half of the smallest tank in the game. If you have no power source to recharge the batteries, in ion propulsion the fuel cells + LFO will beat batteries everytime. IF you do have some form of external power (solar panel / RTG) then you might recharge batteries between two maneuvers, in which case you'd have to calculate how high a deltaV this precise amount of 197 Xe would give your ship, and how it compares to the deltaV maneuvering you're planning to need at most, in order to decide between batteries and fuel cells. FYI that's equivalent to 96 seconds of ion thrust.

To recap: if all your maneuvering will always be done in less than 96 seconds of ion thrust, you should favor big batteries, or else use fuel cells and a LFO tank containing about 20 times less LFO units than you have of Xenon (1 Round8 for each PB150, 1 FLT200 + 1 Round8 for evey PB750, and so on). Simply said, if your TWR times 96 seconds is more than the maximum amount of deltaV you'll need in any single maneuver, go with batteries, otherwise go with cells.

Ore drilling and processing is more energy intensive: 15 energy/sec per drill and double that for conversion of a single unit worth of fuel (1 LFO is 0.005 ton, 1 monoprop is 0.004 ton). The thing is, with an ISRU the fuel cells basically become rechargeable. According to wiki data, mining 0.5 ore yields 1 LFO unit, at a total energy cost of anywhere between 12 and 6000 energy for mining, and 30 energy on top of that for conversion. The same unit of LFO gives off 400 energy once converted by a fuel cell. This means that you can only "refuel at a profit", energy-wise, using a level 3+ (advised 4 or 5) engineer off a moon (assuming ~5% ore concentration) operating a single drill, or one unmanned drill off an asteroid (75% or better ore concentration). In these conditions, battery size is meaningless and fuel cells must instead be compared with whatever panel and RTG setup you may consider in their place (and applying the squared distance from Kerbol to your panel output, using Kerbol-Kerbin distance as unit).

To recap: if your mining conditions clear the thresholds above (<400 energy per half unit of ore mined) go with fuel cells, otherwise you will need panels or RTGs.

If you see any mistake in the maths above please let me know

Edited June 22, 2015 by Jesrad

[Warzouz]

Same as real life, isn't it ?

[carlorizzante]

I didn't get a word. But... cool stuffs. I like it.

I'll use it when I'll get there. Thanks.

[Sharpy]

To recap: if your mining conditions clear the thresholds above (<400 energy per half unit of ore mined) go with fuel cells, otherwise you will need panels or RTGs.

If there's enough sunlight, just go with the panels.

[annallia]

If there's enough sunlight, just go with the panels.

Depends on where you are mining, they finally work correctly, the further away from the sun you are the less effective the panels get.

[Laie]

I admit that I don't understand what you're trying to get at. You seem to treat batteries as a power source, as if they could be drained only once. But of course, their key feature is that they can be recharged, over and over again.

In a nutshell: batteries are no supply, they are a buffer.

[Jesrad]

There's little point refilling big batteries for feeding a ion engine once you've expended all the xenon Sometimes you shouldn't bother recharging batteries.

[Superfluous J]

There's little point refilling big batteries for feeding a ion engine once you've expended all the xenon Sometimes you shouldn't bother recharging batteries.

You mean there's little point of bringing a big battery for feeding an ion engine, when a small battery + enough power generation to create infinite energy weighs less.

Someone (me?) should do an analysis of RTGs vs solar panels vs fuel cells vs batteries. If you need X total power doled out in Y seconds, what should you use to generate it? Broken down by mass and by cost.

[Wiseman]

This is a really interesting breakdown. Like Laie, I had always considered batteries to just be a buffer, and had never considered just bringing enough charge up from Kerbin and not worrying about constantly keeping my batteries full. Thanks for doing all the math on this!

[klgraham1013]

Pretty good read there. Another thing to consider when building my ion contraptions.

[Warzouz]

I think I read a topic on fuel loss of fuel cells. I don't remember well but it seems that someone identified the fuel is burnt in fuel cells as soon as the battery "tank" (real battery or consummer buffer) losses a fraction of it's capacity. Even if the energy generated by the fuel cell was bigger than the consummed energy on the battery. So part of the generated energy was lost (because battery "tank") don't have enough storage).

Efficiency of fuel cells might be lower than expected.

I'm very NOT sure of what I say, but I think it's worth checking, though.

[Foxster]

PSA: Please don't issues PSAs.

If you have something to say then say it. Announcing it is rather pretentious and just a tad annoying.

Having said that...interesting, thanks. I too had not considered batteries as a disposable resource, simply assuming the choices were either solar panels, atomic decay or fuel cells. You could stage them off like other resources.

Edited June 22, 2015 by Foxster

[Captain Vlad]

Plus it also has the tendency to sound 'official' when it, in fact, isn't.

[Laie]

There's little point refilling big batteries for feeding a ion engine once you've expended all the xenon Sometimes you shouldn't bother recharging batteries.

Solar panel or RTG are cheap, lightweight, and basically a one-time fee for infinite power. Yes, I will never need infinite power. But I find that it's considerably more lighweight than any finite solution.

Above you wrote "If you have no power source to recharge the batteries..." -- my nitpick is that I don't see why I should go without a power source in the first place. So the whole calculation seems to be baseless.

About mining, the 5% / 3-star rule is good to know. Coming to think of it, "good to know" would be a better label than PSA.

[katateochi]

Is "PSA" like the new "life hack" or something?

Aside from that, interesting post OP. I've been favouring fuel cells to power rovers recently. In my tests two fuel cells produces enough to power 4 ruggedized wheels a couple lights and 3 SAS modules and with two oscarB tanks that gives about 3 hours of continual driving time. But the main reason I prefer fuel cells in this case is they are less prone to breakage on a rover than solar panels (that and not being affected by distance to the sun). Obviously the rover can't operate indefinitely like this as it could with solar panels, but this particular rover is indented to return to base to refuel (it also has a couple solar panels as a fall back).

Re using RTGs, I have a problem (from a role playing point of view) with using RTGs on small crewed craft. Basically I don't allow them on small manned craft, on larger craft they have to be put well away from the crew sections (Kerbals are green enough as it is), so that's another advantage to fuel cells.

[Jesrad]

I just went with the other threads starting with "PSA" that were around

Someone (me?) should do an analysis of RTGs vs solar panels vs fuel cells vs batteries. If you need X total power doled out in Y seconds, what should you use to generate it? Broken down by mass and by cost.

Oh I did that one some time ago. Best parts, energy-wise, are:

• TR2L for wheels if you need speed VS energy, and RoveXL3 if you just need roving capacity VS energy
  
• Okto, Hecs and Mk2 drone for autopilots
  
• Comm88 for antenna
  
• Always go for the larger reaction wheel you need, than multiple smaller - mind that torques add up just like energy consumption
  

As for ion engine propulsion, it's from memory but I think there's a sweet point on power requirements: at Kerbin distance from Kerbol you should need either 6 OX4 solar panels and 2k battery, or 4 OX4 panel and 6k worth of battery, per engine, for best results (there's no situation in Kerbin space where you would need bigger batteries IIRC). I'll try to find back the exact numbers.

On general purposes PB-NUK become more interesting than solar panels when light intensity drops below 10% of Kerbin's, which should happen at 2.3 K.U. or 31,315,000 km from Kerbol (Dres and beyond). Substitute them with the "sweet spot" above for ion propulsion, or overall for mining/ISRU.

I'll see to adding fuel cells properly to the analysis.