Controlling Peak Demand

[cnil1]

I'm in Arizona and considering solar.

Our POCO is proposing new rate plans that charge a high penalty (about $16/kW) for "peak demand". This is defined as the highest kW usage over one hour during peak time over the course of the month. So, even if you conserve electricity 99% of the time, if you have one bad hour when you use a lot of electricity, then your bill skyrockets.

In my house, for better or worse, I have 3 A/C units. My peak demand is high (up to 18kW - $288 per new rates) and almost entirely a function of air conditioning. Therefore, in order to fully evaluate solar, I need to learn what products / options are available to regulate the "peak demand" of my A/C units, so I can see what my "without solar" bill could be with better peak demand control.

One option could be programmable thermostats, but it would likely be a tough job to program each of 3 thermostats with the type of hourly or even minute by minute granularity needed to limit peak demand. I am not sure such a highly programmable thermostat even exists.

Another option could be some sort of peak demand controller that hooks directly to my A/C power distribution setup and simply shuts off A/C power when the combined 3 A/C unit use reaches a given number of kW during the hour. Ideally, such a device would have some programmability, too. I've tried a Google search for such a device, but the units I have found seem more geared to large office buildings and the cost is not presented on the website.

Your thoughts?

[bcroe]

Starting and stopping big motors isn't as simple as breaking the main AC power feed. There are surges getting going, and gas
pressures that must dissipate before restart after shut down. Air conditioning systems are designed to deal with them, but you
will interfere with the process by cutting off the power. I suggest instead working your control by breaking the COLD wire from
the thermostat. Bruce Roe

[peakbagger]

Darn I was hoping someone would bring up putting in an ice plant. What you need is Energy Management System used on commercial systems, unfortunately I am not aware of small one but expect they are out there. You may end up having to put in small PLC and programming it yourself The big issue is figuring out what hour is peak hour. Usually the utility figures out the peak hour after the fact so you would need the EMS to manage for anytime the peak might occur. I expect you could program in outdoor temp and set a ramp rate on temp rise over the course of the day. If the ramp exceed a certain set point it time to bump the temps up in the house by breaking the "cold" wire with a PLC output. The definition of peak is also important, how do they define the peak and what is the sampling frequency?. I would guess they just look at the total power purchased over a one hour period and call that peak. If the sample rate is shorter, then the goal is to cycle the AC units so that all three units are never operating at the same time. If they use a short interval switching the AC units to variable speed drives may also avoid a peak.

Thinking about it if the ramp rate in heat gain is predicting a peak on a particular day the best option may be to over cool the space a few degrees prior to the predicted peak hour and then let the units coast during the predicted peak hour. I know of commercial customers who turn on emergency generators during predicted peak events so they don't set a one year capacity peak.

In any case the devils in the details, the EMS software has to be developed hand in hand with the rate sheet.

[HX_Guy]

I'm in Arizona and considering solar.

Our POCO is proposing new rate plans that charge a high penalty (about $16/kW) for "peak demand". This is defined as the highest kW usage over one hour during peak time over the course of the month. So, even if you conserve electricity 99% of the time, if you have one bad hour when you use a lot of electricity, then your bill skyrockets.

In my house, for better or worse, I have 3 A/C units. My peak demand is high (up to 18kW - $288 per new rates) and almost entirely a function of air conditioning. Therefore, in order to fully evaluate solar, I need to learn what products / options are available to regulate the "peak demand" of my A/C units, so I can see what my "without solar" bill could be with better peak demand control.

One option could be programmable thermostats, but it would likely be a tough job to program each of 3 thermostats with the type of hourly or even minute by minute granularity needed to limit peak demand. I am not sure such a highly programmable thermostat even exists.

Another option could be some sort of peak demand controller that hooks directly to my A/C power distribution setup and simply shuts off A/C power when the combined 3 A/C unit use reaches a given number of kW during the hour. Ideally, such a device would have some programmability, too. I've tried a Google search for such a device, but the units I have found seem more geared to large office buildings and the cost is not presented on the website.

Your thoughts?

The problem is not all 3 AC's running at the same time, the problem is all of them running within the same hour.

APS' demand charge is based on a 60 minute period between 3pm - 8pm (the on-peak period is being changed from the current 12pm-7pm). Notice I said a 60 minute period, and not "an hour" because most people tend to think each top of the hour, but from what I was told by APS, it's a rolling 60 minutes. The demand is averaged out over the 60 minutes, it's not a one time peak spike.

So in your situation, say your 3 AC's pull 5kW while operating. If all three are on for 30 minutes at the same time...your hourly demand would be 7.5kW (5kW + 5kW + 5kW = 15kW divided by 60 minutes multiplied by 30 minutes ). As you can easily see, you can run all three at the same time...or you can run one for 20 min, then the next for 20 min, then the next for 20 min and the net effect is still 7.5kW of demand in that hour.

What you would really have to do is wait for a new 60 minutes to start to really control the demand, and that's why there is so much backlash about this Demand charge...it's almost impossible to control. I doubt most of the population even really understand how it works. APS shouts out "Shift, Stagger & Save"...making people think to stagger their appliances, as in wait till one shuts off until the next one turns on...but again that really needs to happen the next hour, not within the same hour.

The only real option is a load controller, which basically will not let items turn on if the demand has been hit within that 60 minutes. That sure sounds like a fun way to live, no?

[adoublee]

You indicate highest usage over an hour. Use this to your advantage - if you have 30 minutes over extra high usage, 30 minutes of extra low usage is going to bring the average over the hour at "average" power/demand. This should equate to lower hanging fruit than worrying about motor surges which will represent seconds of that hour. Do you know if your one hour is rolling or is set periods?

Keep in mind your dryer and your stove might represent peak setting moments if they run when the AC is running.

The very first thing I would be doing would be to buy and eGauge (up to twelve 120V or six 240V metering inputs) which will help inform you what is driving peaks, and you might be able to use it's alerts to initiate some shedding functions in the future.

[moontower]

Shameless plug: Check out our mobile app, SolarTrackr, which integrates with eGauge. http://www.moontowerinnovations.com

[adoublee]

The only real option is a load controller, which basically will not let items turn on if the demand has been hit within that 60 minutes. That sure sounds like a fun way to live, no?

And better home sealing, insulation, inverter driven HVAC that can run at part load, cooking in an outdoor kitchen, efficient appliances that use less energy on-peak...

But I do get your point. People who care about their bill will understand cost of service, off-grid living considerations, and a level of energy-awareness which I don't think is all bad. Their are pros and cons to a long demand period - at least one bad second won't spike a bill. I also think the rolling hour allows for more ability to respond and impact a bill than having set time blocks that might not end when you want to plan your highest power times to be. I also like it better than another utility response of very large fixed charges which decouple impact to the grid from the bill paid (more like a tax designed to cover the cost of whatever they decide they want to build).

[organic farmer]

I would assume that your peak need for A/C would be mid-day, when your solar system is producing the most power.

We have loads on timers so they can only come on during day-light, to avoid them ever running on battery.

I would think you could do the same with your A/C units.

[HX_Guy]

I would assume that your peak need for A/C would be mid-day, when your solar system is producing the most power.

We have loads on timers so they can only come on during day-light, to avoid them ever running on battery.

I would think you could do the same with your A/C units.

That may be ok in other parts of the country, but not in the Phoenix area. It's still 105 degrees or more at 7:00 PM when the solar isn't doing much for you to offset that demand.

[J.P.M.]

I would assume that your peak need for A/C would be mid-day, when your solar system is producing the most power.

We have loads on timers so they can only come on during day-light, to avoid them ever running on battery.

I would think you could do the same with your A/C units.

Assuming A/C means air conditioning and that use is more f(desired interior temp.) than strictly what's going on at any instant outside the dwelling, depending on location, climate and dwelling thermal characteristics, demand for air conditioning often and usually loosely lags the exterior temp. profile by a few hrs. or so, more so for well insulated and/or thermally massive dwellings.

One very uncommon but very effective way to shift an A/C load is to increase the effective thermal mass of a dwelling. That and the more common sealing cracks etc. to reduce infiltration and insulation.

Thermostats usually do a fair job of interior temp. regulation. Putting A/C loads on timers when the building is unoccupied unoccupied has some usefulness.

[organic farmer]

Arizona does sound like a very inhospitable place to live.

[fengshui]

This is where stationary storage is a big win. Program your Tesla Powerwall to supply supplementary power when demand exceeds 95% of previous maximum.

[J.P.M.]

Arizona does sound like a very inhospitable place to live.

Pay your money, take your choice. Some like it hot. I think most of New England is beautiful and I lived in upstate NY for a long time, with some of the best friends and neighbors one could possibly hope for, but I wouldn't live in such a climate again, and I definitely will not go near such places in the winter - cold climate winters have pretty always defined inhospitable to me. On the other hand, I travel to the desert on a regular basis, particularly in the summer, to places like Death Valley, Yuma, Phoenix and parts of NM where I also lived for several years. Still trying to warm up and dry out from when I lived in the northeast I guess. I can always manage to cool off now, but I couldn't get warm in Buffalo in the winter, sometimes it seems for months on end with typical and common 10-14 day stretches of no beam solar radiation in the winter. Different strokes as they say.

[wblanford]

I am on SRP in Phoenix and have been considering solar. SRP has a demand rate E27P that is optional for non-solar customers, but required for new solar customers, which I have been on for the last 4 months. SRP's demand rate uses the highest usage for the month averaged over a half hour period starting on the hour and half hour during the peak hours. My house is two stories with a new 2 stage 2 ton heat pump for the upstairs and for the down stairs. I have my hot water heater and pool pump on timers so they do not operate during the peak hours. For the summer, I cool the upstairs until the start of the peak hours. Then the set point is raised so the upstairs unit does not operate until the end of the peak hours. The down stairs thermostat is kept at a fixed temperature. The main high usage appliances are the electric clothes dryer and the electric oven/cooking elements. We wash and dry the clothes during off peak hours. Baking is more of a challenge. We try to start the baking ten minutes before the hour or half hour to spread the usage over two half hour periods. I also try to turn off as much other usage during baking.

My peak usage was 3.5 kW in May, 4 kW in June, 4.5 kW in July and 4 kW in August.

Living with demand is a pain and definitely a reduction in the quality of life. It also increases the conflict in our house.

For more info see: https://www.solarpaneltalk.com/forum...o-from-phoenix

[adoublee]

I am on SRP in Phoenix and have been considering solar. SRP has a demand rate E27P that is optional for non-solar customers, but required for new solar customers, which I have been on for the last 4 months. SRP's demand rate uses the highest usage for the month averaged over a half hour period starting on the hour and half hour during the peak hours. My house is two stories with a new 2 stage 2 ton heat pump for the upstairs and for the down stairs. I have my hot water heater and pool pump on timers so they do not operate during the peak hours. For the summer, I cool the upstairs until the start of the peak hours. Then the set point is raised so the upstairs unit does not operate until the end of the peak hours. The down stairs thermostat is kept at a fixed temperature. The main high usage appliances are the electric clothes dryer and the electric oven/cooking elements. We wash and dry the clothes during off peak hours. Baking is more of a challenge. We try to start the baking ten minutes before the hour or half hour to spread the usage over two half hour periods. I also try to turn off as much other usage during baking.

My peak usage was 3.5 kW in May, 4 kW in June, 4.5 kW in July and 4 kW in August.

Living with demand is a pain and definitely a reduction in the quality of life. It also increases the conflict in our house.

For more info see: https://www.solarpaneltalk.com/forum...o-from-phoenix

Interesting.

Have you looked at the Sunbandit system for PV hot water system, and do you think it would benefit you>

Assuming you have a front loading washing machine that should reduce time that electric dryer would need to operate if it spins water out well. Have you looked to see if there is a way to run your dryer at lower wattage for longer periods of time - maybe if your dryer runs until a sensor says it's dry you could install a lower wattage element in it?

[wblanford]

Running the dryer in the summer before 1 pm is not a problem. The off peak rate is only about 4 cents / kWh. I personally think hanging the clothes on a line would be fine, but my wife likes the dryer.

[wblanford]

I have a 50 gallon hot water heater on a timer which restricts the operation to off peak hours. We never run out of hot water. The off peak rate is only about 4 cents / kWh. It is difficult for solar to compete with this. There are only two of us at home.