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  • Maximising Internal Use of Power

    I'm new here - 1st post.

    I did a bit of a search but didn't find much information about what other people have done to optimise internal power use of generated power.
    Where I live this is important as exported power is paid out at 1/3 the rate of imported power so it's much more effective to use it than sell it back.

    I already have a system so I though I'd share the configuration, reasoning behind the design and what I've done to maximise internal use of power.

    I'm fortunate to be able to design and build most of the equipment I need for monitoring and while capable of designing and building my own inverters for grid-tie and AC/DC + DC/AC conversion for battery storage elected to go with off the self units since they were reasonably priced and meet all the standards requirements to get signed off.

    I had 3 years of power use data prior to adding solar. I built my own power analyser to monitor the incoming mains and electric hot water heater. It's accurate to within +/- 0.1% and records the kJ used for every minute of the day. This gives me exact daily power use as well as when it was used. The minute data is stored into a PostgreSQL DB.
    I got 4 years daily kWh generation data from a friend just down the road.
    This allowed me to create a spreadsheet to dynamically model different sized systems and calculate cost vs payback.

    The best payback for our power use and usage habits was:
    24 x 260W Panel (6.24kW)
    1 x SB5000TL Inverter

    This system produces 45kWh on a good day.
    I was getting about 15% more power than expected. I discussed with the guy down the road I modelled my system off and he said - oh yeah, I forgot to tell you, I get some partial shading at certain times of the day.. I guess this helps to account for some of the difference since I'm due North facing with no shading (Southern Hemisphere).

    I then decoded to look at battery storage and it very quickly became apparent it was not cost effective. However, I went ahead anyway because what I had in mind meant an interesting engineering challenge.

    8 x Trojan T-105
    1 x Meanwell 2.4kW 48VDC Adjustable PSU
    1 x Meanwell 0.6kW 12DC PSU (in series)
    1 x Meanwell TS1500 Inverter

    I ended up designing and building a control system based on the AtMega2560 microcontroller. Because this is a failry basic 8-bit uP with relatively low resolution ADC's and peripherals I added 16 bit ADC's (4) and 16 bit DAC's + a 16 Bit CPLD based phase control firing generator (EPM3064) to get reasonably high resolution data and control.

    I use 2 of the ADC's to monitor the mains voltage and current and use the integral of the instanteaneous voltage x current over 2 seconds to get an accuate power reading even with relative high harmonic content. This value is used to create setpoints the the phase control and battery current.

    I'm using phase control with a 0.1' accuracy (about 2W) to control the power to the hot water element. There's a filter between the SCR and mains to reduce distortion.

    I can control the battery charge current to 10mA steps between +50A to -37.5A.

    The PSU's charge the batteries.
    The DC/AC inverter is always on with a server, firewall, network switches, fridge and a few other small load on a dedicated circuit in the house. It mean I could use a much cheaper non-grid-tie inverter.
    A DCCT on the battery feed monitors the battery current and the control loop adjusts the PSU voltage to keep the battery charge discharge at or less than the setpoint current.

    The control system gives priority to battery charging with any excess going into the hot water.
    During the day our import/export power sits at zero (within +/- 0.010kW of zero) as long as:
    1. There is enough solar energy to cover internal demand (otherwise we will be importing)
    2. The batteries are charging (otherwise we could be exporting)
    3. The hot water is less than max temp (otherwise we will be exporting)

    When solar energy is available the batteries are charged using the 3-stage method; Constant current up the voltage corrected boost level (59.28V) then constant voltage until the current drops to 3% AH rating then float at 54.00V.
    The setpoint current is reduced if solar energy is defficient in order to keep the import/export power on the mains as close to zero as possible.

    I'm using simple amp-second state of charge prediction. Every 500ms I take the battery current, correct the discharge/charge current for Peukert's law and add/subtract from a running total. If 50% DOD is reached the discharge is stopped. If 95% state of charge is not acheived then discharge is inhibitted until it is reached. I get about 98% round trip accuracy with the error being under predicting the remaining amp-seconds. Once float is reached for 1 hour the amp-seconds are reset to maximum. When reset they are always within 2% through the modelling anyway.

    The power is first maximised into battery charging and remaining power then into the hot water.
    During the evening the battery setpoint current is -10 amps. This guarantees 12.5 hours minimum time on batteries before 50% DOD. If more than -10A is demanded the control loop injects current from the charging inverter to hold the battery current at -10A.

    Using this method keeps our import/export power within +/- 0.010kW of zero for most of the day.

    All power use, solar generation, battery charge state, charge current, battery temperatures and temperatures from 12 sensors on our hot water cylinder are stored every minute into a database. A Delphi based Windows app allows trending, reporting and real-time views of the history and status.

    If anyone is interested in what I've done I'd be happy to send or publish the circuits, PCB's, C++ code and Delphi code (for Windows monitoring).

    Is there anyone else interested in trying to optimise thier internal power use?
    Last edited by wrsnz; 01-09-2018, 11:26 PM.

  • #2
    Sure you can.

    I bet you have tons of money to pay the 10's of thousands of dollars to have your equipment tested and listed. This is a DIY site and most might believe you, but I am a pro and know you are full of it.
    MSEE, PE

    Comment


    • #3
      Nice start - have seen many of your posts an know just to ignore you. Sometimes on "rare" ocassions people just aren't full of it. I believe you are an electrical engineer - so am I. I have over 25 years R&D and in field experience designing and building grid-tied systems and AC Drives at ABB...

      I am actually here to try to help and share some of my power electronics knowledge. If you can't deal with it and can only ever be negative, please don't bother posting to my threads or comments.

      Comment


      • #4
        Originally posted by wrsnz View Post
        I'm new here - 1st post.

        I did a bit of a search but didn't find much information about what other people have done to optimise internal power use of generated power.
        Where I live this is important as exported power is paid out at 1/3 the rate of imported power so it's much more effective to use it than sell it back.
        Welcome! The idea comes up, but usually the nuts and bolts of making it work dissuade serious effort to maximizing self-consumption. Most resort to a water heater or some other simple kind of opportunity load, and call it good enough. Please share more about what you are thinking.

        One glitch to be aware of... the forum spam protection doesn't like anything outside of simple ascii (no unicode characters). It will truncate anything you post if it hits one of those characters, without even warning you in preview. This tends to be more of a problem on Apple devices, but try to stick with simple text, even if punctuation suffers. Sorry!
        CS6P-260P/SE3000 - http://tiny.cc/ed5ozx

        Comment


        • #5
          Ah, I see - 85% of my post is missing and I didn't notice. Thanks for the heads-up. I did use unicode for +/- as a single character and it looks like this has erased the remained of my post. It took ages to type too.

          Have now restored the first post to what it should have been - should make sense now...
          Last edited by wrsnz; 01-09-2018, 11:20 PM.

          Comment


          • #6
            Originally posted by sensij View Post

            Welcome! The idea comes up, but usually the nuts and bolts of making it work dissuade serious effort to maximizing self-consumption. Most resort to a water heater or some other simple kind of opportunity load, and call it good enough. Please share more about what you are thinking.
            If you take the time I spent designing and building the control for doing this into account it's definately not worth it. Batttery cost alone kills it.
            For me this was a fun engineering challenge which was the only reason it was done.

            Comment


            • #7
              I see you were contacted by the Welcome Wagon representative.

              Your search for intelligent life is futile. It is the 12:00 problem. The VCR went out of production before people ever learned to set the time. Just go on youtube and look what the morons post for solar systems. And people gush and tell them how smart they are. Years ago I started looking for a site where advanced ideas were discussed. I thought the DIY people would come up with the ideas that commercial industry would later adopt as mainstream. I haven't found any such public forum. Energy management is the future as the utilities start changing the cost every 15 minutes. These systems still require some input from the user and people can't deal with the technology they have now. I have an off grid camp with 3KW of PV and a car battery and that provides for everything including hot water. All custom electronics. Wife's happy, she has a dishwasher with heated dry. I can't see myself ever having a bigger battery. All you will get here is, "Kid, you'll shoot your eye out."

              Comment


              • #8
                I kind of look at the situation as one of minimizing the need to use internal power rather than maximizing generation. I've found that doing so minimizes both what I need to generate and what I need to buy. I also try to only use electricity for tasks that cannot be done any other practical way. KISS also plays a big role in my life.
                Last edited by J.P.M.; 01-10-2018, 12:52 PM.

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                • #9
                  Your instrumentation accuracy is impressive, perhaps better than the problem requires. Balancing your input/output
                  is interesting, but most places is a financial looser when done with batteries as you found. A more favorable plan is
                  used here, the PoCo never pays me. For every KWH I send out, I get credit to take it back for free later. Annually
                  they zero out the account, I lose any credit. The PoCo is my free, infinite capacity, 100% efficient, zero maintenance
                  battery.

                  For years before generating any power, I made the effort to stop wasting power. Every circuit was monitored.
                  Equipment was redesigned or replaced, till consumption dropped below 5000KWH a year, under 10KWH a day
                  seasons the furnace didn't run. No life style changes were made, no discomforts tolerated, just stopped
                  throwing away energy.

                  Since then, solar generation many times that amount has been added. I heat the house with it. Resistance heat
                  currently is about twice the cost of propane, but heat pumps swing the advantage the other way. My generation
                  covers it all.

                  Currently am looking at a decentralized plant using the most efficient mini split heat pumps. With a capability well
                  below zero F, these should nearly eliminate resistance heating. Bruce Roe

                  Comment


                  • #10
                    As this thread continues, critical evaluation and skepticism of difficult to verify claims is welcome, but unrelated and unsubstantiated attacks on the credibility of forum members is not. Thanks to all who participate.
                    CS6P-260P/SE3000 - http://tiny.cc/ed5ozx

                    Comment


                    • #11
                      Originally posted by bcroe View Post
                      Your instrumentation accuracy is impressive, perhaps better than the problem requires.
                      Smart man Bruce and if you are in the industry you know is just about impossible. Not even utility test equipment for power is that accurate. 1% is about as good as it gets and that cost big bucks. Anyone in the biz especially Sensij should know better as 0.1% is fantasy.

                      Dumping power into batteries as you know is futile. No one wins except for the battery manufacture and the place he bought the batteries from. Every watt hour he stores and uses from batteries is costing him 4 to 10 times more than he can buy it from the POCO. Secondly if he lives in a State where excess iis only paid at wholesale prices, and he claims 1/3, going solar is just not justified as it means there is no chance for a payback. States with no Net Metering Laws mandated, electricity is DIRT CHEAP like Texas.

                      So what does he get for his trouble? From his own words a cold shower., and no money back from the POCO. He is loosing money. So yes he can help someone by demonstrating what not to do.

                      I call it like I see it. BS and everyone should take notice. Every engineer has a good idea what it takes to get 0.1% accuracy. To get power requires two measurements of voltage and current. Those sensors would have to be a minimum 0.05% or better. Then you have the A/D converter with its own accuracy issue, then you have the resolution issue to deal with with the A/D converters and processors. Time sampling then also comes into play as you do not know what happens between samples.

                      So yeah I call BS. If this were a professional forum, he would have been eaten alive because pros know it is 99.9% chance it is BS. I believe he built something, but his claims... I have used a lot of PQ analyzers/meters in my 40 plus years. Very few go below 1% and those cost 10's of thousands of dollars using 64 bit processors and sampling at 50 Mhz.
                      Last edited by Sunking; 01-10-2018, 03:35 PM.
                      MSEE, PE

                      Comment


                      • #12
                        Beleive what you want - I don't care, you aren't worth arguing with. You have no idea what test equipment I have access to, what my skill level is or how the system was calibrated. Proffessional engineering forum, bring it on!
                        Last edited by wrsnz; 01-10-2018, 07:26 PM.

                        Comment


                        • #13
                          Other than Sunking:

                          Thanks for your comments. It would be good if the PoCo's in my country were more on board with distributed generation like most other countries. NZ is a very small country with a high % of power coming from renewable resources. It has fairly poor infrastructure milked by high overseas ownership of almost everything. Our power price is also relatively high. We have no nuclear power.

                          I like the idea of credits for export power - that really makes sense.

                          In my situation there is no point exporting power since you get so little for it ($0.08/kWh export vs $0.299kWh import).

                          It would seem there is little need to optimise self-consumption outside NZ since the PoCo's are more reasonable.
                          For me this was more about a fun project and a challenge to see how far I could go with the power control...
                          Last edited by wrsnz; 01-10-2018, 07:27 PM.

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                          • #14
                            Originally posted by wrsnz View Post
                            The control system gives priority to battery charging with any excess going into the hot water.
                            During the day our import/export power sits at zero (within +/- 0.010kW of zero) as long as:
                            1. There is enough solar energy to cover internal demand (otherwise we will be importing)
                            2. The batteries are charging (otherwise we could be exporting)
                            3. The hot water is less than max temp (otherwise we will be exporting)
                            Originally posted by wrsnz View Post
                            It would seem there is little need to optimise self-consumption outside NZ since the PoCo's are more reasonable.
                            For me this was more about a fun project and a challenge to see how far I could go with the power control...
                            Many of the better (more expensive) commercially available charge controllers / inverter chargers are able to provide some relay control that offers similar logic to what it sounds like you've implemented. I think the Radian's "Grid Zero" mode achieves something like what you've done. That you've built it from scratch is impressive, and from that base you could probably fine tune the logic much more than what the existing options allow (by time of day, perhaps).

                            In other words, I think the need is real, but most don't try to extend it beyond that grid zero approach, since that can be easily bought and set up. Others have come through the forum with more elaborate ideas on managing daytime loads with only a minimal battery, and as I think PNPmacnab is saying, to be successful in that kind of system requires more technical understanding and engagement than most can handle.
                            CS6P-260P/SE3000 - http://tiny.cc/ed5ozx

                            Comment


                            • #15
                              In my case I dump excess power into heating ceramic bricks. Yes, using PV to create resistance heat is inefficient on a whole house scale, but it is very efficient when heating smaller spaces.

                              Its low tech but using timers and properly adjusted thermostats, I can convert many kWhs into comfortable heat in the space I'm in and do it efficiently.

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