[bits elided to trim this to manageable size]

Sylvia Else wrote:
D Yuniskis wrote:
Sylvia Else wrote:

There's not much point in having a comfort feature if it's likely
to get turned off at the time of greatest need.

It's not "turned off", per se. Rather, it is *deferred*.
I.e., maybe 10 minutes later your ACbrrr will kick in
instead of at the (slightly) earlier time when the
thermostat called for cooling.

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.

Yes, but not *much* longer.

The AC has to run long enough to pump out the heat that's flowed in
during the time it was turned off. Heat is ariving in the building at a
rate that's largely a linear function of the difference between inside
and outside temperature, and the energy required to pump it back out is
a slimilarly linear function. So to calculate the average energy

Yes. But, the ACbrr is typically sized such that it can remove
heat much faster than heat infiltrates the home. E.g., in the
hottest portions of the summer, our ACbrrr rarely hits a 30%(?)
duty cycle.

Coonsider that the amount of hysteresis on many (most?) thermostats
is only a few degrees, at most, so we are essentially calling on
the HVAC system to maintain a comfort region of 2 or 3 degrees.
(some thermostats allow this to be adjusted but I suspect
you will find most users don't know this nor do they even know
why they might *want* to adjust it!). Deferring the onset of
the ACbrrr just increases the hysteresis temporarily. The
overall duty cycle remains largely unaffected.

consumption - ie power - you just look at the rate at which heat is
flowing in. That rate is minimally altered by deferring the turn on of
the AC. As I've observed, the rate is slightly lower because the average
difference between the inside and outside temperatures is slight lower.
On a very hot day, which is when this load shedding mechanism is likely
to be most used, the saving will be modest, because the change in
average temperature will be a small fraction of the total temperature
difference.

Again, you're not trying to *save* anything. You *do* save by
increasing the hysteresis that is acceptable to you (assuming you
don't, at the same time, alter the setpoint lower). You save
*more* by increasing the setpoint. What the utilities want is
for you to shift *when* you use that electricity.

I.e., if you shift your usage by 10 minutes, the utility still gets
the same amount of money from you -- because you are still
purchasing the same amount of electricity (essentially). *But*,
they have saved money by not having to overload their distribution
or production system in that 10 minute window!

A "thought experiment" grin :

Imagine all ACbrrrs require a 30% duty cycle to maintain their
respective residences at the "right temperature" (whatever
*that* is -- it may be different from one residence to the
next). Now, imagine that all of these were synchronized such
that they ALL turned on their compressors at the exact
same instant EACH TIME. The ACbrrr is the largest single
load in most homes -- it can account for up to 30% (?) of the
peak power capabilities of most homes. Most homes typically
don't use anywhere near their *capacity* ON AVERAGE.

Yet, this pathological behavior would have the utility seeing
a *huge* demand... followed by negligible demand (the load that
the rest of your house represents)... followed by that same
huge demand, etc.

And, this would happen while businesses were (elsewhere)
placing *their* huge demands on the system.
As I said:

The goal isn't to save energy (though I think the laws of thermo
say you *do* save in this case). Rather, the goal is to
get you to *shift* your energy consumption (in time).

The infrastructure is sized for some percentage above nominal.
Of course, going too far *beyond* that causes things to *break*
(hence the blackouts that become newsworthy).

But, things like diesel/coal/gas fired plants that are there
*deliberately* to respond to these short term fluctuations in
demand could, theoretically, be eliminiated if the demand could
be "leveled".

But the demand cannot be levelled by short term adjustments to things
like airconditioner demand. The overall demand is higher on hot days.

Sure it can! Yes, the overall demand is higher on hot days.
But, letting each *huge* load (ACbrrr) operate independantly
of each other (as they do currently) means that there is a high
probability (certainty?) that several of them will engage
concurrently. If all of them coincide, then you get a big peak!
Even if that load is only present for a *minute*, the system
has to handle it -- or, "shed it" (by blowing fuses).

Deferring an airconditioner load by ten minutes won't alter that.
Getting people to defer their airconditioner loads until night time
would be better, but of course that's not going to happen, at least not

As I said, that is how some businesses operate, in a perverse
distortion brought about by economics.

until and unless airconditioners based on a large cold sink become the
norm. That might be a bit of an own goal anyway, environmentally,
because the cold sink would have to be kept cool against the possibility
that the day following would be hot. There would always be some leakage,
so the overall energy consumption of such systems would be higher, even
if they were running on cheaper power.

I suppose it could be argued that some transient peaks arise from a
disproportionate number of AC thermostats switching to on at the same
time, but given the number of ACs around, I'd be surprised if that were
really an issue - the probability of a significant deviation from the
short term average due to such an effect would have to be very low.

It isn't. Hence the motivation for these tariffs.
See if EPRI (Electric Power Research Institute) has any
publications on the subject (I know they have, I am just
not sure if they are available to the public -- I'm not
sufficiently motivated to go dig through my files... : )

Nuclear power plants, for example, like to put out a steady amount
of power (can we *please* not let this discussion digress into
the pros and cons of nuclear power? : ) which doesn't lend
itself to rapid response. If you had fill-in-the-blank power
source that was ecnomical to operate and had this characteristic,
then you would have a big incentive to coerce users into
adapting their usage patterns to match.

Also, it is important to define what crteria you actually want to
optimize. E.g. "efficiency" can be defined in a lot of different
ways -- many of which are inconsistent with each other :-/

While nuclear, and indeed coal, plants cannot respond rapidly to changes
in demand, that is not the reason they are not used to handle peak
loads. It's purely a question of economics. It is not cost-effective to
have such plants lying idle - if that's going to happen, you don't build
them, because they're too expensive to be used that way. You build less
capital intensive plant for that purpose - typically gas or oil powered
(though what we do when the gas/oil runs out isn't clear).

Exactly! You want to size your load to (ideally) fit your capacity.
(sort of like matching source impedance to load). You don't want
excess capacity (if it will never be needed) so you want to get
your capacity to exactly fit your load and then you want that load to
*behave* (i.e., if it is likely to increase, then you will need
excess capacity -- which you don't *want*).

Note that the utilities aren't doing this for *your* benefit
(at least not directly). If they were, they could elect to
automatically set your thermostat higher when you are away
at work, etc. They want to sell you power. They just want
to sell it when it costs them the least!