Fredbob711 said:
Correct me if I'm wrong here,[...]
So battery energy density doesn't quite need to reach that of current fuels, just needs to reach the point where a given volume (? might not be the right word) of input energy medium, be it gas or battery, gives you equivalent energy output from the engine.
You are correct - below I provide one way to estimate the real-world difference in energy per unit mass for the two systems.
Subsea said:
It would be interesting to see the comparison of the efficiency of gas vs electric /power to weight.
Sac Arrow said:
Except that petroleum fuels exceed a battery's energy density by orders of magnitude.
ettsn said:
OK, so I'm working on the assumption that we can get a rough estimate of effective energy density ratio by comparing the miles per pound a pure electric car can travel versus a pure internal combustion engine car of similar masses traveling at similar speeds. I did some net searching and came up with the following (there is probably a better match than the Cadillac, but I didn't want to spend too much time on something that is only a rough estimate anyway):
2015 Cadillac CTS-V
19 mpg highway (
EPA highway avg is ~50 mph)
6.1 lb/gal
3.1 miles/lb
Curb weight 4217 lbs
$69,900
Tesla Model S
Battery weight 1200 lbs
330 mile range at 50 mph (per chart)
0.275 miles/lb
Curb weight 4785 lbs
$69,900
So the ratio of energy density (in miles/lb) for these two ground vehicles yields about a factor of 11.3; basically
one order of magnitude energy density difference for estimation purposes. At least it isn't several orders.
By the way, a careful read of the news article linked in the OP indicates a factor of 2 improvement over other electric motors in power density. The factor 5 is over non-electric engines.
You can buy electric motors for electric ultralights from places like
Joby Motors. Their power density is maybe half that of the Siemens motor, but still good enough for experimenting on electric aircraft. Joby's site has a link to a Youtube video of an electric Lazair that uses their motor; it is worth watching for anyone interested in seeing what is possible today.
Battery energy density is going to be barrier for the indefinite future as a primary energy source. On the other hand, one might argue for electric hybrids for the following reasons:
- You can run electric motors for brief periods well over their continuous duty ratings, something not easily done with internal combustion engines, allowing STOL or even VTOL operations otherwise not possible. Getting over the proverbial 50 ft obstacle would require only 6 secs of overdrive duty at an avg 500 fpm ascent rate and 1 minute to get to 1000 ft AGL at an average 1000 fpm. Batteries would need to store only enough energy for such operations.
- You can easily run an electric motor in reverse with no added mechanical complexity, which would make for some interesting possibilities for STOL operations of small planes.
