| email@example.com wrote:
|> On Wed, 25 Mar 2009 21:44:03 -0400 Michael A. Terrell
|> | If homes were still fed with DC, the generators would have to be a
|> | couple miles from your home. Also, it doesn't have to pulse to charge a
|> | battery.
|> If using Edison's 220/110VDC split system, even a couple miles would be way
|> too far. Today would could do transmission, distribution, service, and
|> utilization, and different DC voltages and keep it DC all the way. It would
|> still not be as cheap as AC, but it is possible to do.
| You could do these things but, in practice, it would be a rather stupid
| step backwards (and somehow involving changes from AC to DC, back to AC
| etc, ad nauseum. Inefficient and bloody expensive.
| For distribution, service, utilization, and different DC voltages, there
| is no comparison, AC wins hands down. With regard to switching, again AC
| wins. For transmission- it is a balance between line and terminal costs
| (point to point systems, not grids) or an asynchronous connection is
| needed- then HVDC has advantages. These factors have been known for
| roughly 80 years or so. As for generation. DC is severely limited- so
| what do we do- generate AC (simpler, cheaper and more efficient) step it
| up or down with simple transformers and then use it in loads that
| generally don't care whether it is AC or DC or can be handled by an
| induction motor in a more efficient and far less complex machine than a
| DC machine.
| So where to use DC? In those places where it provides a cost or
| technical advantage or a specific need for DC (long distance HV
| transmission, long cable systems, asynchronous links between grids,
| aluminum pot lines, electronic power supplies, etc.)
| However, for voltage level changing, switching, and most utilization,
| AC wins. That is a lesson that was learned over 100 years ago and
| modern electronics hasn't changed the situation. Wishful thinking to
| the contrary.
DC went from "you can't do that with DC" to "OK, now you can do that, but why
would you, since it costs so much" for a lot of things.
I just can't figure what motivated Edison with DC so much other than trying
to recoup what he had already invested, and not wanting to admit that he was
wrong. That, or he though electric lights would be the only use for any
form of electricity. Motors were around in plenty in those days, so he should
have seen some alternative uses. I guess he had a comfy box to think in.
Edison's only hope of more distant power generation (essential when trying to
exploit renewables like hydro) was some kind of transmission that could drive
motor-generator sets at local areas. Maybe MVDC (8800/4400V) to drive big
motors that turn his LVDC (220/110) generators? That would still be quite a
nightmare to manage (motor genset maintenance vs. transformer maintenance).
Even I tried to dseign some kind of DC transformer. The best I came up with
still involved rotating parts (think of a Faraday Homopolar generator with
an equivalent motor piggybacked). Still a maintenance nightmare compared to
what AC gets to use.
OTOH, extremely long electrical transmission is, IMHO, not a good idea, for
two reasons. It wastes energy (where applicable ... you are stuck if the
power mover source is a hydro dam, for example, or to get power from wind
farms where the wind blows the most) in warming the transmission lines. And
it is an exposure to terrorism. But where we must transmit power, HVDC can
come out ahead despite its costs when considering not only the losses and
risks in long distance transmission, but also managing reactive power flow.
I'm guessing some costs for HVDC will come down, and I hope we do see more
of HVDC transmission ... for where there is no choice but to do that. But
I also want to see more localized generation where that is possible. It is
still a balance. For example, transporting coal from PA/WV/KY to burn in
localized power plants in the east and northeast would not make sense if that
transporting is too costly or too exposed to terrorist attack (e.g. taking
out a major rail bridge may deplete a power plant's 30 day supply, whereas a
transmission line over the same river could be put back in just a few days).
BUT ... I still want to call that 240/120 VAC electricity coming into my home
as TWO phases ... and connect more and more loads to BOTH phases.
AND ... I want to push for a cabinet full of computer boards/blades powered
by a big single power supply (one per cabinet) that feeds 12 VDC to each of
the boards, which is fed by 480 VAC (2 or 3 phases).
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