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General Doubt : Off-Grid system without Battery

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  • General Doubt : Off-Grid system without Battery

    I am designing a system for small industry where load is of around 25-30 KW.
    Since the industry works only in day time, I want to provide a complete solar solution to it.

    I have two question, it will be best if someone can guide me technicality of it.

    1)If we have to design a off-grid system(Solar Panels,Inverter) do we always need to provide battery for it? Even if load will be catered only through solar generation during sunny hours.If yes, then why?

    2)Can I design a system where battery will be very small and act like a supplementary source when there is insufficient solar irradiance, e.g if load is of 25 KW and solar panels producing 10 KW remaining load can be catered through battery, so battery required will be of 15 KW here. I am well aware of battery calculations, DOD factor,losses all, I am just giving simple example here as Diesel generator will be there for backup when solar is not present. But I want to cater most load through battery and solar only, that too only during sunny hours.

    Kindly guide me in these two conditions.

  • #2
    You have to provide a battery period no way around it.

    First thing you have to determine is HOW MANY WATT HOURS your plant uses in a day. That is required to calculate panel wattage required. Example lets say the plant uses 300 Kwh in a day, and you have 5 minimum Sun Hours. Since this is a battery system you have an efficiency factor of 66%. That means the panels must generate 300 Kwh / .66 = 455 Kwh in a day. With a 5 Sun Hour day means the panel wattage needs to be no less than 455,000 wh / 5 hours = 91,000 watts or 91 Kw.

    Let's just say a 90 Kw system. The demand is 30 Kw from the plant leaving you 60 Kw to hit the batteries with. It takes a lot of battery to be able to absorb 60 KW. Chose the right battery say an AGM that can take as much as a C/2 charge current to make the battery as small as possible. Charge current depends on Voltage of the battery. With a 30 Kw load is way more than 48 volts, more like say 500 volts and at 60 Kw is 1200 amps. So you go shopping for 2400 AH AGM batteries @ 500 volts.

    Good luck with that. You are talking about a 72,000 pound battery costing you $500,000 every few years replacing it. Another challenge is there is no solar battery equipment made to do this. All custom designed and manufactured. Everything out there is 48 to 6 volts @ 80 amp max.

    Good Luck. It will never happen.
    MSEE, PE

    Comment


    • #3
      We need a good sticky to just answer this repeated question: Why do I need a battery if I only want to run my load when sun is available?

      Comment


      • #4
        Originally posted by shrikantsol View Post
        I am designing a system for small industry where load is of around 25-30 KW.
        Since the industry works only in day time, I want to provide a complete solar solution to it.

        I have two question, it will be best if someone can guide me technicality of it.

        1)If we have to design a off-grid system(Solar Panels,Inverter) do we always need to provide battery for it? Even if load will be catered only through solar generation during sunny hours.If yes, then why?

        2)Can I design a system where battery will be very small and act like a supplementary source when there is insufficient solar irradiance, e.g if load is of 25 KW and solar panels producing 10 KW remaining load can be catered through battery, so battery required will be of 15 KW here. I am well aware of battery calculations, DOD factor,losses all, I am just giving simple example here as Diesel generator will be there for backup when solar is not present. But I want to cater most load through battery and solar only, that too only during sunny hours.

        Kindly guide me in these two conditions.
        Regardless of the size of your load what you are being misled about is the amount of sunlight you get daily to power something from solar pv. Just because the sun is up does not mean there is enough sunlight to make the solar panels work. Depending on where the system is in the world the best you can get is maybe 7 hours a day in the summer but usually less. The worst could be as little as 2 hours a day in the Winter.

        So unless you want to shut down your business after only a few hours each day or not have anything work on the days it is too cloudy you will need some type of energy storage system to run the loads. The most available storage system is a battery. The problem is that batteries do not last more then 5 years on average and are very expensive. The bigger your load the more you have to pay for batteries and charge controllers.

        What you are asking about generating 30 kw all day long to run a business will cost you many hundreds of thousands of US dollars. Less if you connect the solar pv system to the grid.

        And without the grid a solar battery system will never pay for itself compared to other forms of electrical generation at that scale.

        Comment


        • #5
          Originally posted by cebury View Post
          We need a good sticky to just answer this repeated question: Why do I need a battery if I only want to run my load when sun is available?
          Good idea. Unfortunately even with a really good sticky most people may still not understand because they have been misled into believing that as long as the sun is shining your solar panels will generate all the free electricity you need all day long. Which as you have learned is a false belief.

          Comment


          • shrikantsol
            shrikantsol commented
            Editing a comment
            Kindly guide me to literature regarding same. Sticky is a good option.
            Even if it will be little technical and complicated for many, it's best if we have such option so that those can understand, will be benefited.

            So looking forward to have understanding of this.

            Thanks.

        • #6
          OK your first wrong assumption you made and 90% before you was: You have assumed a 100 watt panel generates 100 watts with the sun shining on the panel surface, it will produce 100 watts. From that you conclude a huge error into assuming if you have 16 hours of Sunshine is 1600 watt hours. That would be a huge error. It would not work, and destroy the batteries shortly from severe undercharging.

          In practice you would discover what you thought was 16 Sun Hours is no more like 4 to 6 Sun Hours You thought you had 1600 watt hours but only get in real world is 4 to 6 Sun Hours in Summer when conditions are best. In winter months than can fall down to less than 1 Sun Hour and 3.2 is nation average So if you thought 1600 and now discovered it is only 500 you learn.

          Look at the graph below. On the left vertical axis is % of power of a solar panel. Lets pretend the panel is 100 watts. Bottom or horizontal is Time in hours of the day from morning to night So Sun Rise at 6am power = less than 1 watt,
          Come 8:00am power = 3 watts
          9:am power equal = 15 watts
          10 am power = 50 watts
          11am, 12am = 100 watts
          1pm power = 75 watts
          2 pm = 40 watts
          3 pm = 10 watts
          4pm = 3 watts.

          Conclusion. Panels only produce specified power for 2 hours around Solar Noon. Much less any other time of the day.

          So in the bell curve lies Watt Hours and when you add them up = 530 watt hours. We know the panel wattage is 100 watts. So for a 100 watt generator it would have to run for 5.3 hours to generate 530 WH. Sun Hours = 5.3.



          So why is a professional asking such simple fundamental questions. Strange!.
          Last edited by Sunking; 07-20-2016, 11:42 PM.
          MSEE, PE

          Comment


          • shrikantsol
            shrikantsol commented
            Editing a comment
            Thanks for prompt reply. I just joined this forum yesterday and I wasn't expecting such prompt replies, so really thanks for it.

            Secondly, thanks for pointing it out, but I am very well aware of solar irradiance and Solar Insolation. In my region worst sun hours in a year averages to 3.6 hours and during peak summer it's around 5-6 hours. So I tried to design system accordingly, taking into account losses as well. I am novice to off-grid solar system so I have few genuine doubts regarding it.

            I will write complete situation in your battery post, kindly guide me there.

        • #7
          Originally posted by Sunking View Post
          You have to provide a battery period no way around it.

          First thing you have to determine is HOW MANY WATT HOURS your plant uses in a day. That is required to calculate panel wattage required. Example lets say the plant uses 300 Kwh in a day, and you have 5 minimum Sun Hours. Since this is a battery system you have an efficiency factor of 66%. That means the panels must generate 300 Kwh / .66 = 455 Kwh in a day. With a 5 Sun Hour day means the panel wattage needs to be no less than 455,000 wh / 5 hours = 91,000 watts or 91 Kw.

          Let's just say a 90 Kw system. The demand is 30 Kw from the plant leaving you 60 Kw to hit the batteries with. It takes a lot of battery to be able to absorb 60 KW. Chose the right battery say an AGM that can take as much as a C/2 charge current to make the battery as small as possible. Charge current depends on Voltage of the battery. With a 30 Kw load is way more than 48 volts, more like say 500 volts and at 60 Kw is 1200 amps. So you go shopping for 2400 AH AGM batteries @ 500 volts.

          Good luck with that. You are talking about a 72,000 pound battery costing you $500,000 every few years replacing it. Another challenge is there is no solar battery equipment made to do this. All custom designed and manufactured. Everything out there is 48 to 6 volts @ 80 amp max.

          Good Luck. It will never happen.

          Thank you very much for your clear insight.

          I have little peculiar problem though, and I am trying to customize little different solution to it.

          Since I know how to provide a solar solution to on-grid system,I am able to calculate solar solution to off-grid little bit, but problem is again battery. Currently Industry runs on Diesel generator. So I don't have to worry about if there is no sun, since they already have generator it can be put into line. Now coming back to our discussion, I have this query what if I provide only minimal battery? By minimal battery I meant when during morning or evening when solar panels wont be able able to cater complete load I should be able to take remaining power from battery, I tried to give an example as well,Sorry, I wasn't clear to convey my message. So basically if the load is of 25KW ,and my panels producing say 10 KW, remaining is 15 KW, I want battery to provide.

          Now let's see the scenario
          Load is of around 25 KW
          Industry runs for 8 hours.
          typically only during 2 hours in morning and 2 hours in evening there could be problem of insufficient energy from solar panels.
          So I want to add battery so that only during these 4 hours of insufficient energy,battery will add remaining insufficient energy.
          Since Industry has Generator of their own, in cases when there is no sun for a day or something they may start it,but they want to use it as minimal as possible.

          Reason for all this is very simple, I don't want to unnecessarily burden more battery, as you said it's not even feasible. But if solution like the one I suggested can be worked out,it will be great help.

          And as commented on your previous post, I have taken into consideration losses,solar irradiance and insolation into calculating solar panels and system. But need to clear off-grid concepts little bit.

          Your help is highly appreciated.

          Thanks.



          Comment


          • #8
            A basic question for you: If you only have enough power to run your plant 4 hours during the day, where would the batteries be getting the power to run the plant during the other 4 hours?

            You have to charge the batteries with something in order for you to use the power later. Does that make sense? You are correct in that you should look into off-grid concepts more. I wish you luck in your attempt, but please don't start on it until you know it is feasible.

            Comment


            • shrikantsol
              shrikantsol commented
              Editing a comment
              Thanks for showing concern. Actually I am purposefully putting more panels to accommodate battery charging. Solar system capacity is almost little more than double of nominal load, so whatever excess will be generated will be put into batteries,which later will be used during energy shortage from panels. I am hope I am making some sense. Definitely I am trying all things to make this feasible, and once I am sure it will be only then I'll start the project.

          • #9
            Originally posted by shrikantsol View Post
            Now coming back to our discussion, I have this query what if I provide only minimal battery? By minimal battery I meant when during morning or evening when solar panels wont be able able to cater complete load I should be able to take remaining power from battery, I tried to give an example as well,Sorry, I wasn't clear to convey my message. So basically if the load is of 25KW ,and my panels producing say 10 KW, remaining is 15 KW, I want battery to provide.

            Now let's see the scenario
            Load is of around 25 KW
            Industry runs for 8 hours.
            typically only during 2 hours in morning and 2 hours in evening there could be problem of insufficient energy from solar panels.
            So I want to add battery so that only during these 4 hours of insufficient energy,battery will add remaining insufficient energy.
            Since Industry has Generator of their own, in cases when there is no sun for a day or something they may start it,but they want to use it as minimal as possible.
            That sort of design is possible. However:

            1) You need to take into account both power needed to run your load and power needed to charge the battery. (Really energy, which is power X time.) That means a larger array.
            2) You need very specific batteries that can handle very high charge and discharge rates, since your capacity will be lower. For example if you want a 2 hour battery (i.e. run for 2 hours before dying completely) then you will need to discharge at C/2, and likely charge at C/2. That means lead acids are out. You might be able to find a LiFePO4 battery that could fulfill that requirement.
            3) Lifetime will be lower when you charge and discharge at those rates.

            Note that an additional utility of this system will be better generator utilization; overall your generator run time will decrease if you can add storage, and will be better loaded (closer to rated power) while it is on.

            Comment


            • #10
              Originally posted by jflorey2 View Post

              That sort of design is possible. However:

              1) You need to take into account both power needed to run your load and power needed to charge the battery. (Really energy, which is power X time.) That means a larger array.
              2) You need very specific batteries that can handle very high charge and discharge rates, since your capacity will be lower. For example if you want a 2 hour battery (i.e. run for 2 hours before dying completely) then you will need to discharge at C/2, and likely charge at C/2. That means lead acids are out. You might be able to find a LiFePO4 battery that could fulfill that requirement.
              3) Lifetime will be lower when you charge and discharge at those rates.
              Thanks for support

              1)Yes, I have taken into account excess power needed for charging batteries.
              2)This is my major concern as well, I want batteries to be discharged quickly as well i.e c/2 batteries as suggested by you.

              Originally posted by jflorey2 View Post

              Note that an additional utility of this system will be better generator utilization; overall your generator run time will decrease if you can add storage, and will be better loaded (closer to rated power) while it is on.
              There is one more problem here. I want to keep generator off as long as possible, since what I understand is generator takes lots of fuel even during idling period.

              One more solution I was thinking is, giving system without batteries and keeping small generator just for reference voltage and frequency. I am not sure about will this work or not, since I am still technically unclear about need of secondary source of energy in off-grid system.

              It would be best if you can advice me on same.

              Comment


              • #11
                Originally posted by shrikantsol View Post
                One more solution I was thinking is, giving system without batteries and keeping small generator just for reference voltage and frequency. I am not sure about will this work or not, since I am still technically unclear about need of secondary source of energy in off-grid system.
                sure it will work for a very short bit. Till the load drops. So in your example above you would have a PV system producing 30kw, a load using ~30kw, and a small say 10kw generator. The load drops for a bit to say 10kw, PV is still producing 30kw, generator idles down but where does the extra 20kw go???
                there are some grid tie that can limit production to not back feed, though I don't know of any rated for off grid use like this.
                OutBack FP1 w/ CS6P-250P http://bit.ly/1Sg5VNH

                Comment


                • #12
                  Running a big load directly from pv is taking a gamble due to the dynamics of both the system load and the sunlight to generate power.

                  There needs to be a buffer in between to handle these dynamics like a battery system or grid connection.

                  A generator will not be able to change it's out put that quickly unless you have a bank of them where some are on all the time and others cycle depending on the need.

                  Comment


                  • #13
                    Originally posted by shrikantsol View Post
                    There is one more problem here. I want to keep generator off as long as possible, since what I understand is generator takes lots of fuel even during idling period.
                    Right. And the amount of time you can leave it off will be determined by the size of your battery bank. Small battery bank = frequent restarts. There's no free lunch here.
                    One more solution I was thinking is, giving system without batteries and keeping small generator just for reference voltage and frequency. I am not sure about will this work or not, since I am still technically unclear about need of secondary source of energy in off-grid system.
                    Won't work without a LOT of additional work. If you try that as you describe, you will overspeed the generator and damage it. You would need an additional dump load to ensure voltage/frequency do not rise above unsafe levels, and those are not available off-the-shelf to do what you want.

                    And in any case, at best all that will do is reduce the load on your generator; as you mentioned, it takes lots of fuel even during idling (or low load) operation.


                    Comment


                    • #14
                      Hello and Greetings.

                      I have 44 Dupont 117Watt Apollo Panels system and 2 string wiring configuration. Is it possible to use the Fronius Symo inverter 5.0-3-M and the smart meter to use this without batteries or grid? Any suggestions I would appreciate it. In my location they do not allow putting power back into the grid. I do have an available grid connect. Thx

                      http://www3.fronius.com/cps/rde/xbcr...8_snapshot.pdf

                      Comment


                      • #15
                        Originally posted by Results123 View Post
                        Hello and Greetings.

                        I have 44 Dupont 117Watt Apollo Panels system and 2 string wiring configuration. Is it possible to use the Fronius Symo inverter 5.0-3-M and the smart meter to use this without batteries or grid? Any suggestions I would appreciate it. In my location they do not allow putting power back into the grid. I do have an available grid connect. Thx

                        http://www3.fronius.com/cps/rde/xbcr...8_snapshot.pdf
                        For that Fronius inverter to work you still need a Grid to connect to. Once that is done then you can program it to not output anything.

                        If you have no grid then you will need to connect your solar panels to a charger and then that to batteries.

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