On Sat, 29 Sep 2018 23:07:27 -0700, 2G wrote:

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

Interesting stuff but, unless I totally misunderstood the presentation:

- the theoretical energy density of ZAB is around 170% of what LiFePO4
can do (0.3 kWh/kg vs 0.18 kWh/kg). no comparison with Li-ion made.
(page 4).

- page 18 quotes a measured 1 kWh/kg.

- page 32 quotes 0.0357 kWh/kg (0.41 kWh/litre) for an experimental ZAB
rig.

- page 36 quotes 1.9 kWh/kg (0.0036 kWh/litre) for a flexible cable
format battery.

.... all quite a way short of the 10 kWh/litre (12.5 kWh/kg) of
hydrocarbon energy storage.

The sort of high power performance needed for a car or aircraft motor
wasn't really discussed or apparent in the accompanying numbers, charts
and graphs.

Still, looks to be worth watching and the possibility of almost instant
recharge by replacing the zinc becomes interesting if a cell with high
power output can be designed for easy 'slip out/slip in' zinc replacement
without needing special equipment, e.g to deal with the (highly alkaline)
potassium hydroxide electrolyte.

The possibility of recycling the 'used' zinc externally sounds good too.


--
Martin | martin at
Gregorie | gregorie dot org