"Variable heat" electric range available anywhere?

Does anyone manufacture a "variable heat" electric range, where when you select the heat setting, it would have a constant heat at a certain temperature? (Like you can do with a gas range...)

This would be sort of like a dimmer switch for a light where you can adjust how much light is output from the bulb.

The way electric ranges work now is they go on and off, on and off.

Less heat means the "burner" goes on for a little while, then off for quite awhile. Then with more heat, the "burner" is on for a long time, then off for a little amount of time.

With a gas range, you can adjust the heat so it is constant - no off and on. Seems they could do this with an electric range as well....

Reply to
Bill
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Hi Bill,

There are electronic thermostats for electric heating applications that modulate output (I have some from Aube that control with my in-floor radiant heat), but they're fairly expensive. I suspect their high cost and perhaps concerns related to long-term reliability would limit their use elsewhere. Variable wattage control would be a nice feature from the utility's point of view (i.e., by smoothing out demand), but I suspect most consumers wouldn't care one way or the other.

Cheers, Paul

Reply to
Paul M. Eldridge

This makes no difference to the utility, because they have thousands of stoves and similar loads, which are all randomly cycling on and off, effectively averaging it all together, so smoothing out demand from one stove doesn't do anything. They would never see it.

The reason its done the way it is on electric ranges is it's a cheap mechanical switch, that cycles it on and off for varying periods. To make the heating more even would require turning it on and off rapidly, which is what is done in wall switch dimmers, which do it on each AC cycle. That requires electronics. And to do that for a dimmer, which is 600W max, takes a smaller, less expensive semiconductor than it would require for a range. It's possible someone offers it, but I haven't seen one.

Reply to
trader4

Looking at the product brochures at

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switching thermostats, too, just w/ models as fast as 15-20 second cycle times and solid-state switching instead of mechanical relays. They don't actually "modulate" output except in the sense of averaging, same as the range controls.

To do otherwise would require a mechanism to waste the "extra" power as a in a voltage-divider-type rheostat which would be quite inefficient and require quite large power resistors or other sinks. The mass of the burner element is made relatively large in electric stoves to make the average temperature reasonably constant. Better stoves control on higher frequency cycles and have better-designed burners to minimize the thermal cycling -- my Mom used to claim she could tell the difference between her stove and others in that regard. Whether real or simply perceived I have no idea... :)

Reply to
dpb

I'm not so sure about that. My utility, Nova Scotia Power, like virtually ever utility here in Canada, is winter peaking and these peaks typically occur around 17h30 when street lights start coming on and electric ranges are being used to prepare evening meals.

**Anything** that helps minimize concurrent demand, no matter how small, would be helpful from the utility's perspective, especially in light of the high percentage of homes that are electrically heated in this country.

Let's say the average electric oven draws 3.0 kW (my convection oven happens to be 5.5 kW). We might expect the typical household oven to operate at full power for the first ten minutes or so, then cycle on perhaps one third of the time thereafter (i.e., an average of one minute on for every two minutes off). Let's also assume there are

100,000 electric ovens in use province-wide during the suppertime peak (and here in Nova Scotia, virtually all ovens are electric since only 500 or so homes are currently served by natural gas).

If all 100,000 ovens were energized at the same time, we would expect this load to be 300 MW. We're assuming, of course, that as each of these ovens come up to temperature, the actual load at a 33% duty cycle, would be closer to 100 MW, and since these ovens are not all turned on at the same time, a coincidental peak of 100 MW is probably within spitting distance of the mark. If, however, each of these ovens were equipped with variable wattage controls and, again, assuming a 33% duty cycle, our coincidental peak should drop to just

33 MW.

On a typical winter's day, Nova Scotia Power's peak falls between

1,500 to 2,000 MW, so a 67 MW reduction in provincial demand would represent a peak savings of perhaps as much as 3 to 5 per cent. In theory, it would exceed the province's total installed wind capacity of some 60 MW (which, assuming a 40 per cent annual capacity factor, I take it might be closer to just 25 MW). Even if we were to cut the number of ovens in operation by half, the impact on a utility such as Nova Scotia Power is not exactly insignificant.

Cheers, Paul

Reply to
Paul M. Eldridge

This also means that you have eliminated the normal full on heating mode of the oven and reduced it to 1/3 of that. Which means now everyone has to wait 3X as long for the oven or burner to warm up, which few people are going to put up with. After that, the oven or burner will be cycling randomly anyway and the sum of them all cycling randomly is the same And presumably, this cooking load comes late in the day, like 6PM+, which is after industrial/commerical use is decreasing. With all the other loads I fail to see how this is going to make any difference in the generating capacity needed to meet peak demand or save the utiltiy even 5cents. It will mean a lot of pissed off users though, who can't get their oven hot in a reasonable time.

So, you have just as many ovens running longer. Unless you have proof that ovens are causing a peak demand that results in either higher capital cost for generators to meet peak capacity or are causing the need to kick in some higher cost energy source during dinner time, this is just a pipe dream.

Reply to
trader4

Hmm, good point. Because it uses a triac, I had assumed (incorrectly) that it works pretty much like a standard household dimmer.

I have the in-floor heat in my den set at 30C and when I started typing this, my Aube thermostat was showing three wavy bars indicating the floor was operating at 60 per cent capacity (and what I had thought to be 540 watts, versus 900 watts). Oddly, the thermostat will still cycle on and off because I can hear a loud "snap" when it does this; in fact, it just clicked off seconds ago and I can see there are now no bars shown on the display. In a few minutes, I expect to hear it click back on.

I took a look at one of the manuals and it does clearly state the bars indicate "the percentage of heating time required to maintain the desired temperature", so that seems to suggest you are correct.

Source:

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Ah, sure enough, "snap" and we're back to three bars again.

Cheers, Paul

Reply to
Paul M. Eldridge

Suppose you *did* have an oven with electronic variable power control, where temperature controlled the "on" time of a triac. This oven would still operate at full current until it came up to operating temperature, and then sit at 33% duty cycle after that. The only difference between this and what you have now is that the on/off cycle repeats every 1/120 second, rather than every couple of minutes. But that makes no real difference to the utility, which is looking at the load averaged over

100000 ovens.

In fact, the electronic control wastes a bit of power in the switching element, and consumes slightly *more* power than the non-electronic oven. The triac control also distorts the utility waveform into something that is less of a sine wave, which the utility also will not like (the power factor gets worse, so they need higher current capacity for the same billable watts).

Right - whether or not they have electronic controls.

Again, true with or without electronic controls.

If, however, each of these

This makes no sense. The 33% duty cycle has already been factored into the drop from 300 MW to 100 MW. You can't divide by 3 *again*. You need that 100 MW to keep all of the ovens at operating temperature.

This is all based on the assumption that you can somehow run all these ovens with electronic controls on 1/3 the average power you would need with conventional switching controls. That's nonsense - they need just as much energy, on average, to heat the same contents to the same temperature for the same time.

The only time it makes sense to use dimmer-like electronic power control is when the temperature swings with conventional controls are too large.

Dave

Reply to
Dave Martindale

No, please go back and re-read what I said; to whit:

"We might expect the typical household oven to operate at full power for the first ten minutes or so, then cycle on perhaps one third of the time thereafter..."

Followed by:

"If all 100,000 ovens were energized at the same time, we would expect this load to be 300 MW. We're assuming, of course, that AS EACH OF THESE OVENS COME UP TO TEMPERATURE, the actual load at a 33% duty cycle, would be closer to 100 MW, and since these ovens are NOT all turned on at the same time, a coincidental peak of 100 MW is probably within spitting distance of the mark...."

So there are two key points here:

a) the load on our utility during the suppertime peak is minimized due to the cycling of these elements at what I had estimated to be 33% and, secondly,

b) due to the fact these ovens are not all turned on at precisely the same time, the impact of that first ten-minute start-up is thereby diminished.

Nowhere did I say these ovens would operate at reduced power upon start-up. Each could continue to operate at full power for as long it takes to come up to temperature, then drop to the lowest wattage required to maintain a constant set temperaturer; if the oven element is rated at 3,000 watts and it normally cycles on one-third of the time, then it's fair to say a constant 1,000 watts is all that's needed to maintain a steady temperature from this point forward. There would be absolutely no inconvenience to the consumer whatsoever and the utility would still benefit from reduced aggregate load.

You claim these ovens would run longer but as I indicated above, they won't. In any event, according to U.S DOE EIA, the generating technology with the lowest capital cost would be a 230 MW advanced combustion turbine at a cost of $US367.00 per kW (O&M and T&D extra).

Source

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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. To this you would add the additional operational and maintenance costs (and this *is* the single most expensive way to generate electricity by conventional means), plus the added transmission and distribution expenses. Also, bear in mind, the utility continues to sell the same amount of energy as before, so there's no resulting loss in revenue, BUT it does get to pocket all these other savings.

Cheers, Paul

Reply to
Paul M. Eldridge

Hi Dave,

You're right. Clearly I was a couple neurons short in my thinking. Let me see if I can move closer to the mark this time or, failing that, embarrass myself further trying, as the case may be.

Our basic assumption is that these elements will operate 33 per cent of the time, once the oven reaches its set temperature. But this cycling will be random in nature, so our 100,000 ovens won't be cycling "perfectly" in the sense that only one-third will be energized at any one time. As the total number of ovens increase, I take it we'll move ever closer to this ideal scenario, but it's probably fair to say their combined load will fluctuate due to the unevenness in this cycling. If we were to take a series of snap shots, we might find that perhaps 50 per cent of these elements are energized, in which case our load at that particular moment in time is closer to 150 MW and not the 100 MW I had stated.

The point of this exercise was to determine if it might be possible to "smooth out" or flatten this load, so its net contribution to peak can be lowered. If we have 100,000 ovens running at a constant 1 KW each once they reach their set temperature, their combined load should remain fairly close to 100 MW (slightly more to account for the higher demand during start-up). Again, my thinking is that energy consumption should remain constant (or perhaps slightly more due to control related losses, as you suggest), but peak demand should be reduced.

Your concerns related to power quality are well taken. There may be ways to address that but I'm afraid I'm not very knowledgeable in this area.

Please let me know if I'm a little more successful this time out, or if I should be hiding my face. :-0

Cheers, Paul

Reply to
Paul M. Eldridge

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.

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

Reply to
Dave Martindale

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

Reply to
Paul M. Eldridge

Many (most?) electric ovens already do this. The pre-heat stage energizes both the bottom and broiler elements. The idea, though, is to pre-heat the oven faster, not to help the power company balance its load.

Reply to
Mike Hartigan

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.

Reply to
trader4

Hi Mike,

My apologies for asking this, but is this something relatively new or has it always been this way? I don't have a lot of experience with electric ovens because I've always used gas, but this time around I had to settle for a combination unit because an all gas version wasn't available.

I did experiment with mine just now and here's what I found. There are basically three heating options I can choose: "bake", "broil" and "convection" and as far as I can tell all three work independently of each other. When I turn on "broil" I can safely leave my hand on the lower bake element and it remains cool to the touch; likewise, the broiler never comes on when I select "bake" and neither of these two elements are used when I pick "convection" (that appears to be a third element hidden somewhere inside the oven's back wall).

In any event, this second element arrangement I had imagined would be used in bake (or convection) mode and would be very low wattage -- just the minimum required to maintain a steady operating temperature and effectively "lock out" the primary element that is used during the initial warm-up. It may be that we only need 500 or 1,000-watts during this extended cooking phase to keep things moving along.

There doesn't appear to be any real benefit in terms of utility-wide peak shaving (sorry to say it took a couple blows to the head to drive that point home), but there may be some benefit in terms of reducing the strain on the local distribution system. If, for example, my neighbour and I share the same pole transformer and our ovens both cycle on at roughly the same time (and we can safely assume there will be at least some overlap during their operation), the combined load of these two appliances might be 6 or 8 kW. However, if we don't turn our ovens on within ten minutes of each other (i.e., during that initial warm-up phase), with this dual wattage arrangement, our combined load may never exceed 4 or 5 kW and our steady-state operation may drop to just 1 or 2 kW.

It would seem that as we move closer to the point of use (i.e., from sub-station/feeder to local line, to individual pole transformer) the potential benefits to the utility become increasingly more attractive. And in the case of a large condo or apartment complex, I imagine the potential load reduction could result in some capital savings (i.e., smaller service requirements) and perhaps reduced monthly demand charges. Ideally, if dual wattage elements added an extra $50.00 to the cost of each appliance, this extra cost would be fully offset by these other capital savings and any additional savings in terms of reduced monthly demand charges would simply add extra gravy in the pot.

Cheers, Paul

Reply to
Paul M. Eldridge

And how are you going to coordinate not turning on ovens with your neighbor at the same time? Use a flag hanging out the window? This is simple physics. If you expect to limit the power, then the oven isn't going to heat up as fast. And that could be solved just as simply by just putting in an oven element that was say 25% smaller to begin with. You really only need the max when you're trying to take the oven from cold to operating temp. After, that, it cycles anyway and could easily get by with probably 1/2 the existing element capacity. Of course the downside is that instead of waiting 15 mins for the oven to heat up, now you're gonna wait a half hour and few people will put up with that to solve a problem that doesn't exist to begin with.

with this dual wattage arrangement, our

You still don't get it. The amount of energy that it takes to operate an oven is independent of whether you have 1 element or 40. You could have a 4000 watt element on for 15 mins or a 2000 watt element for 30 and it uses exaclty the same amount of energy. You can't just decrease the steady state amount of power and have the oven be just as hot.

You're focusing on one small nit here and ignoring everything else. Condos typically have all kinds of loads, AC, heat pumps, furnace blowers, electric water heaters, etc. Reducing some oven power isn;t going to be a big factor that now means smaller gauge wire or a smaller transformers can be used. And to reduce the oven loads, what you fail to realize is that you are either asking people to:

a - Wait longer for their ovens to warm up, because the power into them is being reduced to limit peak during start up.

b - Hang flags out the window so unit A can't start cooking dinnner at the same time as unit B

Who is going to put up with that?

Ideally, if dual wattage elements added an extra $50.00 to

What reduced monthly charges? You still need the same amount of energy going into the oven to make it hot. In fact, your idea could take MORE energy. By reducing the max power, its' going to take longer for the oven to get to 400 deg. While it's talking the extra

10 or 15 mins, heat is being lost out of the oven throught the walls, or even worse, people opening the door to see what's going on.
Reply to
trader4

There's nothing to prevent all of then from switching on at the same moment. The chance of this happening at times will be nearly 100%.

Peak demand is still 300MW, but most peaks can be expected to be very short. Well within the utility's capability.

Reply to
Mark Lloyd

There is no *technical device* to prevent them all from coming on at once. But the simple statistics of the situation mean that almost all the time, the number of ovens drawing power is 1/3 of the total plus or minus a few percent. The odds of having all of them on at once is incredibly tiny, so tiny that nobody would estimate or plan a peak load based on that event.

Just look at the probability distribution of a 100,000 random events each with a probability of being on of 1/3. The odds of having exactly

33,333 of them on is quite small, but the odds of somewhere between 30 and 35000 being on is nearly 1.

Dave

Reply to
Dave Martindale

Hi Mark,

Thanks for volunteering to be my "human shield" on this one; maybe I can use this time to lick a few of these self-inflicted wounds. ;-)

I suspect Dave's right. As you move into these very large numbers, these loads, on a system-wide basis, tend to "self-balance" (no doubt someone could quickly develop a computer model to confirm this). I'm now thinking their greatest impact may be in terms of the local distribution system, especially in predominately residential neighbourhoods, as their relative size and random behaviour would hold proportionately greater weight.

BTW, I picked 100,000 as our working number because NSP serves about

420,000 residential customers in this province and when you add in the contributions of the smaller municipal utilities, that final tally might reach upwards of 450,000 households; thus, we're expecting one out of every four and a half households to be operating their ovens during the suppertime peak and that estimate is likely to be a bit on the high side, even though we Nova Scotians are your stereotypical "supper-waiting-on-the-table-when-we-get-home-from-work" type.

And, hey, don't forget. I owe you one!

Cheers, Paul

Reply to
Paul M. Eldridge

...

...

...

The triac is essentially just a bi-directional gated switching circuit able to be controlled for either voltage polarity, so unless there is more internally than that, it is essentially just an enhanced switch. Less expensive dimmers are essentially the same, more expensive may include other circuitry to modify the waveform and phase to provide nearer a sinusoidal voltage, but I'd guess these thermostats don't have that sophistication.

Reply to
dpb

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