Sylvia Else wrote:
mm wrote:
On Thu, 29 Oct 2009 23:35:06 +1100, Sylvia Else
wrote:

OTOH, if they were to raise their thermostat's setpoint
that degree (or two?) all the time, they would probably not
notice the difference.
But, as I've commented elsewhere, as soon as the AC is powered up it
will stay on for longer to bring the temperature down again.

There is an energy saving,

Radio-controlled AC turn-offs are not meant to be energy saving.
They're meant to be peak-load lessening. It's meant to prevent
brown-outs because of too much load at one time.

And I'm arguing that they do not achieve that goal.

How effective they are at doing this currently is something that
you would have to investigate in each particular locality
where such tariffs exist. Note that the extent to which the
tariff is adopted (by customers) will directly impact how
effective this technology can be (i.e., if very few folks
"subscribe", then the utility doesn't have much of a tool
at its disposal).

Air conditioners are very reactive loads. As the power system
is stressed, you get voltage sag. For a resistive load, this
is fine -- the decrease in voltage creates a propportional decrease
in *load*.

But, AC compressor motors are essentially "constant power"
devices -- as the voltage decreases, the amount of current
that they require goes up. And, the relationship (phase)
of that current requirement is not coincident with the
supplied *voltage* (i.e. reactive).

Motors *starting* are even worse. (e.g., preventing
ACbrrs from *starting* is more effective than turning *off*
existing ACbrrs, all else being equal).

Note that, to avoid a "catastrophic event", it may only be
necessary to shift a load by a small time interval --just
long enough for alternate supplies to come on-line. E.g.,
imagine a high tension line tripping and effectively
resulting in a reduction of supply -- sag -- so existing
load has to be carried by other -- redundant -- lines.
Until additional lines can be brought into service, the
system is stressed. If you can shed loads at the consumer
you can avoid shedding *all* the consumers (i.e., a major
outage).

The system can *always* break. You are just trying to reduce
the number of scenarios/criteria under which it is brittle.

I think much of this is the basis for the move (in the US) to
upgrade the power distribution infrastructure (aka "Smart
Grid"). No idea how things are done elsewherein the world
but, here, the infrastructure is really pretty outdated.

[N.B. I don't know enough about the "Smart Grid" issue
to comment one way or the other -- I'm just hypothesizing
as to its intent]