Look up pumped storage for example - you convert power to "potential energy". Entirely doable by moving in-situ available mass.
The power system on a battery powered rover consists of a BIG battery and an electric motor; using an ICE, the power system consists of the electric motor, a small battery, a small generator/starter, a small ICE, a fuel tank, AND the fuel.
Quote from: Warren Platts on 01/05/2014 01:27 amThe power system on a battery powered rover consists of a BIG battery and an electric motor; using an ICE, the power system consists of the electric motor, a small battery, a small generator/starter, a small ICE, a fuel tank, AND the fuel. Presumably a gaseous hydrogen tank? Figure 5% mass fraction for that.
Yes, I was thinking of mainly mobile, mass-limited applications (rovers/miners). However, consider perhaps the early stages of a lunar base that must survive 2 weeks of darkness. Batteries would be the most efficient end-to-end system, at least as far as electricity is concerned, but you'd have to haul in a huge load of batteries, most likely from Earth. Any heating you did would have to come from electric heaters, and any shaft power you needed would require electric motors.
So you propose to pump water up to a reservoir high on a mountain during the day, and then letting it run down in the night?
* He had/has a web site documenting his efforts. If anyone is interested I'll try and find it.
Quote from: joek on 01/05/2014 02:34 am* He had/has a web site documenting his efforts. If anyone is interested I'll try and find it.Yes please.
Well, I was thinking hydrogen peroxide since that's what the MoonEx lander proposes to use. The tank would be relatively simple then. The only question is how many kg of H2O2 you would need to run a 400 W ICE for 24 hours....
Quote from: go4mars on 01/05/2014 02:43 amQuote from: joek on 01/05/2014 02:34 am* He had/has a web site documenting his efforts. If anyone is interested I'll try and find it.Yes please.Ditto!
Quote from: A_M_Swallow on 01/05/2014 01:07 amOn a long term off-Earth mission the steam produced by the ICE will need recycling. So the complexity and mass of the condenser, water tank and water need including.Why would a long-term mission need to recycling the water produced by the ICE? It's not like you can recycle the water back into hydrogen and oxygen, and robotic missions have no need for water. Now if you were talking about a manned mission recovering the waste water might make sense but that wasn't mentioned.
On a long term off-Earth mission the steam produced by the ICE will need recycling. So the complexity and mass of the condenser, water tank and water need including.
I think ICE is a better choice for PSRs but to be fair, a solar powered rover (or later, digger, driller, hauler, maintenance mech for the ISRU, etc. etc.) might well "dip in" to the PSR to do some work, then "climb back out" by traveling back to sunlight, and get recharged and then go back in, many many times... as long as nothing bad happened.
The higher energy density of the H2 ICE means it has a lot more dwell time before it has to refuel, IMHO anyway.
Quote from: Lar on 01/05/2014 12:31 amI think ICE is a better choice for PSRs but to be fair, a solar powered rover (or later, digger, driller, hauler, maintenance mech for the ISRU, etc. etc.) might well "dip in" to the PSR to do some work, then "climb back out" by traveling back to sunlight, and get recharged and then go back in, many many times... as long as nothing bad happened.That is the plan for RPM: to do one "dip in", climb out, recharge, and then go for broke as long as it lasts within the PSR.
Quote from: Warren Platts on 01/05/2014 06:33 pmQuote from: Lar on 01/05/2014 12:31 amI think ICE is a better choice for PSRs but to be fair, a solar powered rover (or later, digger, driller, hauler, maintenance mech for the ISRU, etc. etc.) might well "dip in" to the PSR to do some work, then "climb back out" by traveling back to sunlight, and get recharged and then go back in, many many times... as long as nothing bad happened.That is the plan for RPM: to do one "dip in", climb out, recharge, and then go for broke as long as it lasts within the PSR. Why is that the plan? Why not dip in multiple times?
Interesting info, Warren. Does energy density get any better if you are using liquid H2 and O2 in the tanks and combusting the boiloff? It may take more energy to make the fuel for refilll (since you have to chill it to liquefy it) but energy out in the sun might be worth spending profligately if it improves energy density.I have this visual of a digger (with smaller tanks) pulling a trailer 3x the size it is with big fuel tanks on it Of course all that complexity adds mass and failure points.
I figured for a 200 kW excavator to run for 10 hours, it would take about 1385 kg of H2+O2 to run:Energy density of H2/O2 = 13 MJ/kg
Quote from: Warren Platts on 01/05/2014 08:21 pmI figured for a 200 kW excavator to run for 10 hours, it would take about 1385 kg of H2+O2 to run:Energy density of H2/O2 = 13 MJ/kgThat appears to be using 8:1 O/F stoichiometric mixture, 1/9th of hydrogen's ~120MJ/kg LHV. As you wrote earlier IVF ICE runs 1:1 rich, only 1/16th of total propellant mass is burned. 120MJ/kg / 16 = 7.5MJ/kg.
The good news is 200kW terrestrial excavators are quite big 30+ ton machines and in 1/6th gee you could use that power to move even bigger machine. Traditionally a lot of energy is wasted into heat by not trying to recover it when you lower or turn the boom. Substantial amount of that can be recovered by regenerative breaking (see terrestrial example).
{snip}Good luck building heat pumps and radiators ( convection cooling is not terribly effective in vacuum ) All the theoretical energy density figures of fuel sources are pretty much meaningless in a real system design.
Sounds like we will have to pump the gasses to the surface. A hot mixture of steam, water, hydrogen, hydrogen monoxide, hydrogen peroxide and oxygen may need pipes made from stainless steel.
I dont know how to put this exactly but .. with all these figures of 40% efficiencies ( which are fiction anyway ) etc and 100 kilowatt outputs, people dont realize that you are effectively pumping 60kw of heat directly into a closed system somewhere, continously, and all the rest of 40kw ends up as heat too. Or as anyone that is briefly familiar with thermal design of spacecraft would say, "in this house, we obey the laws of thermodynamics!"