Honestly, what is better? Batteries? Nukes? Flywheels? Ultracapacitors? Laser beams? Fuel cells?
Quote from: savuporo on 01/04/2014 08:12 pmQuote from: Warren Platts on 01/04/2014 08:03 pmApples and oranges. Oppy's battery has a specific energy of about 500 kJ/kg. GH2/GO2 has a specific energy of 13 MJ/kg; These numbers have very little relevance to the system efficiencySince we're talking about the Moon and the PSRs are where the action is on the Moon, the relevant number is how many hours a lithium battery powered rover can operate in a PSR compared to an ICE-electric hybrid of the same mass. Under this measure of efficiency, I believe the ICE-electric hybrid would last longer, but I am open to correction.
Quote from: Warren Platts on 01/04/2014 08:03 pmApples and oranges. Oppy's battery has a specific energy of about 500 kJ/kg. GH2/GO2 has a specific energy of 13 MJ/kg; These numbers have very little relevance to the system efficiency
Apples and oranges. Oppy's battery has a specific energy of about 500 kJ/kg. GH2/GO2 has a specific energy of 13 MJ/kg;
Quote from: Warren Platts on 01/04/2014 09:14 pmHonestly, what is better? Batteries? Nukes? Flywheels? Ultracapacitors? Laser beams? Fuel cells? If your primary source of energy is electricity generated by solar or nuclear, you're not going to beat battery-electric. Everything else has a round-trip efficiency under 50% (usually well under), while electric is going to be above 85%.
Quote from: Lee Jay on 01/05/2014 12:01 amQuote from: Warren Platts on 01/04/2014 09:14 pmHonestly, what is better? Batteries? Nukes? Flywheels? Ultracapacitors? Laser beams? Fuel cells? If your primary source of energy is electricity generated by solar or nuclear, you're not going to beat battery-electric. Everything else has a round-trip efficiency under 50% (usually well under), while electric is going to be above 85%.I don't think efficiency is the key metric, I think energy density and ease of storage / refuel are more important metrics. Optimise for cost, not efficiency.
If you're running on otherwise-to-be-vented H2 and O2, then you could look into high-specific-power devices like ICEs and turbines engines, especially when you have a severe mass penalty. However, if this were a surface situation and thus has a much less severe mass penalty than something like a turbopump has for a stage, then I'd be looking to long-term reliability and efficiency, and thus fuel cells. A 500kW fuel cell is not a large device at all. I rode in a prototype Hydrogen fuel cell car and the fuel cell was in the center console between the two front passengers, yet it produced over 100kW.As I said, if the source of power were not excess H2 and O2, such as solar, I'd be shocked if you could beat battery-electric.
{snip}As I said, if the source of power were not excess H2 and O2, such as solar, I'd be shocked if you could beat battery-electric.
Quote from: Lee Jay on 01/05/2014 12:54 am{snip}As I said, if the source of power were not excess H2 and O2, such as solar, I'd be shocked if you could beat battery-electric.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.
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.
A 500kW fuel cell is not a large device at all. I rode in a prototype Hydrogen fuel cell car and the fuel cell was in the center console between the two front passengers, yet it produced over 100kW.
"is a relatively large fuel cell for flight applications and because all power is produced as electricity (as compared to per 10% for the IC engine) it must be converted via motors to shaft power with their attendant switching systems and losses. This grows the fuel cell to address conversion efficiencies."
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.
I would think it would depend heavily on application. Fixed installations might trade for higher end-to-end efficiency (e.g., centrally generated power and storage).
Quote from: Warren Platts on 01/04/2014 09:14 pmHonestly, what is better? Batteries? Nukes? Flywheels? Ultracapacitors? Laser beams? Fuel cells?I would think it would depend heavily on application. Fixed installations might trade for higher end-to-end efficiency (e.g., centrally generated power and storage). Mobile and mass-limited applications might trade for lower end-to-end efficiency and higher power density (e.g., portable generator or automobile).ULA's ICE application appears to be in the latter category--a good portion of the end-to-end efficiency penalty has already been paid for by the presence of the stage's H2/O2.