On Feb 12, 10:38 pm, Paul M. Eldridge
wrote:
Hi Dave,

OK, thanks for walking me through this, one step at a time; appreciate
your help. If I've got this right, with such a large number of ovens
in use, we can reasonably assume no more than one-third of these
elements will be energized at any one time, regardless of the length
of each "on" cycle, be it 1/120 of a second or one to two minutes. I
had envisioned this load would be more variable at these longer cycles
and that we could, in effect, "chop peak" and "fill valley" by slicing
it into increasingly finer increments. Perhaps with just 100 ovens
that might be possible, but with 100,000 it would make no discernable
difference. That seems to makes sense.

In terms of ensuring high power quality (and reduced appliance cost),
one alternative might be dual-wattage elements. One high power
element for quick start-up and a second, low-density companion that
would maintain the oven at its set temperature (similar to how some
hot water tanks operate). If, for whatever reason, this secondary
element couldn't keep up (e.g., repeated door openings), it would
temporarily throw things back to the primary element, then once again
resume command; it would still cycle on and off as required, but it
would be sized to more or less to run continuously and minimize any
further need for its bigger brother.

While such an arrangement might not reduce peak demand, it could still
offer some benefits in terms of reducing the strain on the local
distribution system. Would that sound reasonable?

Cheers,
Paul


No, it doesn't sound reasonable, because, as Dave and I have
repeatedly tried to explain to you:

1 - In your example, with 100,000 stoves cycling on and off randomly,
the load is already randomly distributed, at least after the initial
heat up period of 15 mins or so. You can use one element, two
elements, or 300 elements and it doesn't do anything to affect the
peak load or power distribution as long as the heating elements are
the same size and the duty cycle is the same.

2 - Assuming a lot of ranges/ovens come on around 5-7pm, if you wanted
to reduced the load at this time, you can do it by either:
a - Using smaller heating elements
b - Keeping the duty cycle from being 100% during the heat up period.

Either of those will reduce the heating capacity of the stove.
Option a permanently and option b during startup, meaning the oven
will take longer to get hot.

And I think the problem you're trying to solve here doesn't exist to
begin with. There are generally two problems that utilities are
concerned with regarding peak demand. One is they need a generator
and system big enough to handle the peak, requiring more capital
investment. And/or they need to buy power from somewhere else during
peak time and that power may cost more. AFAIK, none of these issues
typically occurs at 5-7PM due to home ranges. Around here, they
typically occur during very hot summer afternoon periods, when
commercial use is high and everyone has their home AC units running,
etc. Most people don't have their ovens going then, because it's hot
and they aren't planning on making a roast turkey to make the house
even hotter.






On Tue, 13 Feb 2007 01:53:38 +0000 (UTC), (Dave



Martindale) wrote:
No, the load on the utility (averaged over 100000 houses) is still 100
MW, no matter whether the ovens elements are cycling on and off every
1/120 sec or every 2 minutes. Because of the randomness of the
mechanical thermostat open/close, you'll never get more than about 1/3
of those ovens on at any one instant.

In fact, I will bet that the utility would be mightily *unhappy* to
have 100 MW of load all switch on for the last 1/3 of every half-cycle
of the line. That will distort the waveform on the grid.

Thus, if we can effectively reduce peak demand from by just ***ONE***
MW, the capital savings to the utility is a minimum of $367,000.00 US
($436,730.00 CDN); at 67 MW, the savings amount to $CDN 29.3 million.

But you haven't reduced peak demand at all. In fact, you've increased
it slightly due to losses in the triacs of the electronic control, and
distorted the current waveform.

Dave- Hide quoted text -

- Show quoted text -