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  • Demand load controller

    I apologize if this is not the correct category for this thread but it seems kinda a grey area. So I am 2 months deep in my solar system (self-install) in AZ and i am tied to the utility company here. Due to the fact I am grid tied and the way my utility company (SRP) charges me, it behooves me to stay within the < 3kwh range during peak times (2p-8p). Currently I have no way of monitoring my (new) ac units. 1 is a variable speed and the other is a 2 stage unit so they can spin very slowly if need be. The problem is that I cannot tell them to do that on a schedule. I have smart thermostats but that just tells them to turn on and off during the times of the day based on temps not Kwh. I looked at the Sense controller and that is pretty slick but it can't shut things down and at that point it may be too late. I am looking for something that I can tie into at least one of my AC units at the meter and give it rules to throttle <3kwh during peak times. I did find something called Schneider Electric EER260LLCR Wiser Large Load Controller and Disconnect and i may pull the trigger on it but I was hoping there may be someone else that monitors their kwh and has some recommendations for me. thanks

  • #2
    What do you mean by 3KWH, or do you really mean 3KW? Bruce Roe

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    • #3
      I think we are saying the same thing, basically SRP charges me a "demand rate" (and to to make it more complicated it is tiered) so basically the lowest pricing structure is in the <3kw range. They take the highest kw used in a 30 min rage during the peak time (in the 2p - 8p range) and multiply that by demand rate ($7.89 for 0-3kw used). Hence why i want a demand controller to actually throttle my AC units. I really don't care about anything else. Hope that clarifies. thanks

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      • #4
        Explain your 2 stage more...1 or 2 compressor? Copeland Ultra? What does t'stat Y2 trigger?

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        • #5
          These are the units here. They come from a company called Day and Night. Fairly still new (within the last 3 years). I found my utility company offers 3 devices to purchase; Inergy Smart Panel 3000, Brayden Automation Energy Sentry, and Dencor 300C NEO. The first one is $2600, for that kinda money i will buy a battery. Looking for a less expensive DIY solution so I can run my AC below 86 during peak times.
          Attached Files
          Last edited by dstinson; 07-05-2019, 07:06 PM.

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          • #6
            The first is listed as 17 SEER, the second I think is about 19. This is not the worst, but the
            units I am running are 25 to 33, a very substantial amount more efficient than yours. You
            might check your options there. Bruce Roe

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            • #7
              The 2 stage model could have second stage toggled easy enough, but for variable speed, I don't know of what may be implemented there.

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              • #8
                Hi. I'm from Phoenix, AZ and got into same problem with SRP as topic starter after DIY solar installation of 12.96kW grid-tied system

                I'm on "Customer Generation" plan from SRP with peak time 2pm-8pm where demand charges apply at summer. My system covers almost 100% of daily electricity needs and in some months I have even negative balance of kilowatt-hours consumed. Between 2-6pm demand is almost fully covered with solar generation and thus not countable

                However from 6pm to 8pm sun is weak or there is no sun. Demand charges jump to $150 and more per month for occasional spikes happening even one time per month

                I looked at Demand Control systems Smart Panel 3000 and Brayden Automation Energy Sentry suggested above. They are very expensive. However basically they can help only with periodic turning off A/C when demand is high. I was thinking about similar DIY system consisting of following:

                (A) Current transformers on house main line to measure amperage in that line
                (B) Current transformers on solar line to measure solar production
                (C) Sensor for logical 1 or 0 on A/C compressor control line from thermostat
                (D) Control block with USB interface, input channels for A, B and C and relay to control A/C compressor connected between thermostat and A/C.
                (E) Cheap laptop, MiniPC or even Raspberry PI caccessible from remote computer. It will be installed in NEMA3 box together with Control block D next to electric panel with A, B and C inside
                (F) Script running on that computer and communicating with interface D

                Total cost of equipment is ~$100-200 considering "free" old computer pulled out of dusty pantry and wiped out of cobwebs

                Script will have following algorithm:
                1. If time is outside 2-8pm, do nothing. Maybe just monitoring and saving data to log file about momentary parameters. Relay in control interface is "Normal On" connecting A/C to thermostat and allowing the latter control A/C as normal
                2. If it's peak time above, then measure current A and B
                3. If B>=A, do nothing. Solar production 100% covers demand and the latter is not countable
                4. If B<A then A is a current demand amperage. Calculate "Momentary demand kW"=A*240V
                5. Calculate cumulative "Demand kWh" by measuring momentaty demand every 20 seconds and adding so sum consumed electricity for those 20 seconds
                6. SRP measures demand for billing purposes every 30 minutes. So it's from xx:00 to xx:30 and from xx:31 to xx+1:00 every hour. At each 00 minutes and 30 minutes of hour new period starts when demand calculation goes to zero and then starts growing. Based on that, control system compares cumulative demand for half hour from #5 above with preset allowed maximum. E.g. 3kWh per half hour
                7.If cumulative demand exceeds that maximum, then computer gives relay command to turn off compressor in A/C until the end of half hour period. Thermostat still believes that A/C is working, but in reality compressor in it turned off saving energy
                8. At next half hour period relay gives control of compressor back to thermostat, A/C turns on and entire cycle repeats
                9. At 8pm relay permanenttly swithches to "Normal On" position fully restoring thermostat control and program goes to sleep until 2pm next day

                Certainly that algorithm is not ideal. However it will allow A/C to run only if billed "half hour" demand doesn't exceed some preset maximum. E.g. 3kWh, or 5 kWh which is still affordable. It should prevent any spikes more than 7kWh in half hour as besides A/C there are no other appliances in the house to create such high demand

                Unfortunately this system won't eliminate demand charges. But it can significantly reduce them

                In parallel to that, I'm thinking about hybrid system with battery backup to help covering high demand. Unfortunately all systems I saw so far (e.g. Tesla PowerWall) technically switch entire house "off grid" for certain time to run fully on batteries. That requires powerful and expensive transfer switch on main AC line in the house and very large expensive battery supplying 15kWh or more to run A/C and etire house for a couple of hours during peak time

                More interesting solution might be using partially battery energy during peak time and partially energy from Utility (SRP) keeping demand within 3kWh limit per half hour. 3kWh demand tier is pretty affordable and might be the best solution. However I couldn't find any battery inverter with backfeed to grid similar to solar grid-tied inverters. Thought about using the latter with batteries instead of solar panels. It would need a battery bank with DC output at least 150V. Otherwise inverter won't start. It's pretty dangerous battery and not very reliable. Also MPPT in solar inverter will try to squeese as much juice of batteries as it can exceeding demand and feeding to grid. That will quickly exhaust battery. At each moment energy production from inverter should be equal to demand or slightly lower allowing some small demand charges. So it needs additional control system

                Any ideas about that?

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                • #9
                  so I can tell you what i did as I was the one who originally posted this. I went down the route of a demand controller and called a few guys and sure they wanted to sell me one for about 3k. And while it would be nice to have this, the sales guy told me he doesn't even have one on his house. I have gas for heat, oven, and water heater, so I really only needed to throttle my AC units. What I did was plug in my old SMA inverter that was in my garage so I now i have 2 SMA inverters and added more panels so during peak i can run both units and give the middle finger to SRP. And sure between 6-8pm I do throttle 1 ac unit to 80 degrees so I stay within the 3kw range. It's cheaper to buy panels than it was to invest in a demand controller. Batteries are too expensive now in my opinion and sure that would solve the problem altogether but I cant afford that.

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                  • #10
                    Thank you, Dstinson. I also considered that option you just described. There are several problems to that:

                    All south part of my roof and flat part of north side already covered with panels. There is no room for more there or even on other structures in the yard
                    10-12 panels can be installed only on north side of A-shaped roof. That side has pretty good solar irradiation after 5pm in Summer and can give additional 2-3kW of power before sunset when it's most needed. However between 7-8pm there is almost no sun even if panels look directly to it. So demand during that hour will be still high

                    Throttling A/C to 80 degrees is not an option for me as everybody in the house starts complaining about getting hot. The only solution should be technical keeping temperature within comfort level

                    Batteries are very expensive to run 5ton/7kW A/C entirely off grid even for one hour. Hybrid solution (3-5 kW from SRP and the rest from batteries) would be best. But I cannot find any grid-tied battery inverters capable for such operation. They are "all or nothing" requiring transfer switch and running either 100% from grid or 100% from batteries

                    So my idea is to piggyback existing solar inverter for that purpose. Connection might look like this:

                    Battery Charger with time controlled by computer - Battery 24V/7kWh - Random cheap 24VDC to 240VAC 5kW inverter from Harbor Freight-Rectifier with big capacitor to get 240V DC-PWM controller with computer interface to provide amperage of DC current depending of demand-diode to prevent DC backflow from solar panels-MPPT channel of solar inverter connected in parallel with solar string

                    Since this addition will start only when there is no sun, panels at that moment will be not in use. Solar inverter will technically feed from batteries believing it's still using solar power. PWM with computer and demand measuring system will give just enough DC current to it in order to cover entire or part of demand at that moment. Balance of demand between battery and utility can be flexibly adjusted programmatically

                    Second Inverter in the middle and rectifier might be replaced with 120-200V battery bank providing DC directly. However there is a problem how to charge such nonstandard battery and also safety issues having 200V on battery terminals

                    Any better ideas?



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                    • #11
                      most of the stuff you are suggesting are over my head, but I have been following this guy on youtube 'jehugarcia' and he uses multiple batteries to make a powerwall and he built one specifically with a timer for TOU options, it was brilliant but way too complex for me. He has lots of videos out there and shows you how to do most of the wiring. You might find it interesting.

                      I also would be every interested to see what you come up with. If you find a way around demand fees with an alternate solution i for one would love to hear about it.
                      Last edited by dstinson; 06-04-2020, 12:20 PM.

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                      • #12
                        Originally posted by dstinson View Post
                        ...............I have been following this guy on youtube 'jehugarcia' and he uses multiple batteries to make a powerwall and he built one specifically with a timer for TOU options, it was brilliant ............
                        Jehu has a way of making things look simple. He leaves out important details most of which are integrated into hybrid inverters which are UL approved for interacting with the grid. For example most hybrid inverters have TOU timers and grid zero mode where the battery is used over the grid to optimize self consumption of solar. To do that current sensors are used.

                        None of them can control loads. the closest thing that I have seen on the horizon in California is some of the DER programs that provide incentives for shutting down certain devices when the grid is stressed. For example I get a credit each month of $5 for allowing Sonoma Clean Power to turn off my EV charging station at those times. The irony is that the times that the grid is stressed are not times that I would usually charge my EVs.

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                        • #13
                          Amy ideas of using EV as backup energy source during peak time to mitigate demand? All Electrical Vehicles have powerful battery. But is it accessible for drawing energy out of it for "off label" purposes like feeding external inverter?

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                          • #14
                            There are some Chademo solutions out there but they are expensive. There were rumors that Tesla was going to enable access to their car packs but those proved not to be true.

                            In my opinion, the most likely things we will see in California will be DER programs to incentivize putting load on the grid during the belly of the duck curve. That could include EV charging and charging hybrid inverters and Powerwall like battery packs.

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                            • #15
                              Originally posted by phx View Post
                              Amy ideas of using EV as backup energy source during peak time to mitigate demand? All Electrical Vehicles have powerful battery. But is it accessible for drawing energy out of it for "off label" purposes like feeding external inverter?
                              OpenEVSE has PV diverter functionality.

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