$1 billion BMS Ooops...

On Thu, 04 Mar 2021 11:55:17 -0800, Mark Mocho wrote:

Energy density is the "elephant in the room" that determines a lot about
electric vs. internal combustion. The Tesla S uses a 100 kWh battery
that weighs 1,375 lbs. 100 kWh is approximately the same energy
contained in 2.1 gallons of AvGas. So, you have a 2 gallon capacity
(about 13 lbs.) in a 1,375 lb. container. Makes perfect sense to me.
NOT!

And we aren't even bringing up the environmental impact of producing the
battery and then disposing of it when its life is used up.

Here are some free-wheeling thoughts inspired by what I've read about the
Aptera hybrid road vehicles: the prototype used an all-electric drive
chain connected to a battery with a 40 mile range when fully charged. It
also carried a small ICE generator pack - on ling trips the rig was said
to average 120 mpg.

So, how would a similar set-up work for us?

Say, use a pylon-mounted electric motor coupled to a battery capable of
take-off olus a 2000 ft climb (i.e. a somewhat higher than normal winch
launch) and carry a small ICE generator pack to be run during and after
launch to recharge the battery.

A modern 20cc 2-stroke can knock out 2.5 hp at 9000rpm (around 1.8 kWh,
so with a 40% efficient generator you can recharge the battery at a 0.7
kWh rate from a unit with a guestimated weight of 1.5 Kg (750 g motor
plus the same weight for the generator) plus fuel at around 9,7 kWh/litre
(thats 12 kWh/kg) so something like 1.2 kWh/liter of fuel can be put back
into the battery after launch (assuming motor efficiency 25% and
generator efficiency 40%). Now, scale the system up a bit and use a 200cc
generator set and you've got an equipment weight of 15 kg plus fuel and a
recharge rate of around 12 kWh. Time to recharge a 20 Kwh launch battery
is around 1.7 hours, so a fair time to be listening to the (muffled)
engine behind you, but a much lighter system than a pure electric system
(launch battery + 15kg generator set) would be.

What did I miss?


--
Martin | martin at
Gregorie | gregorie dot org