Re: Enlightenment
- From: rkshullat@xxxxxxxxxxxxxxxxx
- Date: Tue, 24 Jul 2007 15:44:55 +0000 (UTC)
David Friedman <ddfr@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
In article <f7qti5$5nr$1@xxxxxxxxxxxxxxxxxxxxxxxxxx>,
rkshullat@xxxxxxxxxxxxxxxxx wrote:
Bill Snyder <bsnyder@xxxxxxxxxxx> wrote:
On 20 Jul 2007 03:00:57 GMT, "David Dyer-Bennet" <dd-b@xxxxxxxx>
wrote:
But do you really think the synthesis of liquid fuel is the most
likely solution? It's true that it provides more energy per pound
than current batteries--but battery technology is improving. And
using the energy to synthesize fuel then burning the fuel to get
back energy seems like an awfully kludgy, and probably
inefficient, approach.
Direct burning of fuel, compared to burning of fuel in a powerplant,
generating electricity, transmitting it long-distance, and using it to
charge a battery? I'd guess the first was *much* more efficient.
Not so, AFAIK. There are efficiencies of scale to consider. A large
modern power station might turn 40% - 45% of the heat content of its
fuel into electricity. (And it's possible to do better; the last I
heard the DOE was talking about 60% as a reasonable goal a generation
or so down the line.)
Transmission losses are fairly trivial: the big transformers in a
substation have efficiencies in the 99% area. A switching power
supply/charger would be in the 70% to 90% range. I'm on shakier
ground with the percentage of battery charge that you get back when
you discharge it, but I think that if you do it slowly enough you can
get above 90% there.
Altogether, I'd expect at least 25% efficiency for the
fuel/electricity/power-at-wheels process, which is roughly what
ordinary cars manage.
A paper at http://www.evworld.com/library/fcev_vs_hev.pdf claims about
90% efficiency for transmission to the home, 92% efficiency for the
power supply/charger, 80% efficiency for the charge-discharge cycle and
90% efficiency for the drive train, for 59.6% overall. Factoring in
52.5% efficiency for the latest combined cycle plants gives an efficiency
of 31.3%. Regenerative braking is claimed to "give back" about 10%, for a
final efficiency of 34.4%. But the real advantage would come from not using
fossil fuel to produce the electricity in the first place. The disadvantages
are that battery technology still needs improvement to match the cost and
reliability of liquid fuels and we'd need tremendous improvement in all
aspects of the electrical grid to support large numbers of electric
vehicles.
Note that the discussion started with the idea of using power from some
other source, such as nuclear, to synthesize the fuel--that was the idea
I was questioning. With that version you cut out the 52.5% for making
electricity from fossil fuel, since you are starting with electricity,
and put in a factor on the other side for the energy losses in
synthesizing your fuel.
That gets the efficiency of the battery alternative up to almost 70% on
your calculations. I find it hard to believe that the process
electricity to synthetic fuel to power in an automobile can be more
efficient than that, and expect it's much less.
I suspect that efficiency is going to matter less than convenience. A
synthetic liquid fuel that used the same delivery mechanisms and was
compatible with existing engines would have a tremendous convenience
advantage over electric vehicles. I suspect that for an all-electric
vehicle to get wide adoption it's going to need a couple of hours
duration at normal city driving speeds and be rechargeable overnight.
That's going to mean something in the neighborhood of 100 kWh of
electricity stored in an 8 hour period. Given the efficiency of the
process, that implies a 60 amp, 240 volt circuit. An upgrade is going to
be needed for most residences and, if sales take off, for most
neighborhoods. On the good side, "fuel" cost would be roughly comparable
to gasoline at current prices.
I rough-calculated that it would take about a 50% increase in overall
electric capacity to replace IC engines with battery-based electric,
even allowing for the improved efficiency of electric engines over IC.
A "battery swap" system at existing gas stations might be a better model,
with most home chargers being used mainly to "top off". That would require
less pervasive change to the electrical grid and commercial areas tend to
have higher capacity to start with. It also greatly increases the
convenience and allows longer trips.
Robert
--
Robert K. Shull Email: rkshull at rosettacon dot com
.
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