On Saturday, September 29, 2018 at 6:30:20 PM UTC-7, Martin Gregorie wrote:
On Sat, 29 Sep 2018 16:38:53 -0700, Bill.Daniels wrote:

But, an electric motor is more than three times as efficient as an
internal combustion engine - 98% vs. 33% - so they don't need as much
'fuel'.

Well, getting on that way - but certainly no better if you include the
generation and charging efficiencies.

Actually, all I was looking to do was to put numbers to the overall
efficiency of electric storage in terms of weight or volume compared with
hydrocarbon fuels. Batteries are much worse here than petrol.

And there's another demerit too: run the tank dry on a turbo or jet and
the empty tank is nice and light for the rest of the flight home, while a
battery that has been run flat is no lighter than when it was fully
charged.

A quick scan for zinc-air battery characteristics suggests they are only
suitable for low power applications, tend to have a high self-discharge
rate and don't like high temperatures or humidity. Nobody, that I found
anyway, is quoting energy density (kWh/kg) or volumetric efficiency (kWh/
litre).

Bottom line: I like the idea of electric powered vehicles. But in this
application the volume and weight of energy storage systems is all-
important, but electrochemistry is against any large improvement, at
least for rechargeable batteries using currently proven chemistry. So far
nobody has gotten better results from fuel-cells either, or we'd be
seeing them in cars.


--
Martin | martin at
Gregorie | gregorie dot org

Here is an interesting 2015 presentation on zinc-air batteries that indicates the possibility for very high energy densities (1200+ WHr/kg):
https://www.csm.ornl.gov/BLI8/presen...UNIST-BYI8.pdf

Tom