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Solar PV direct Pumping system. What size solar panel do I need?

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  • Solar PV direct Pumping system. What size solar panel do I need?

    Hello! I am working on a project where I need to pump water from one tank to another one. I am located in Rockford, IL and need to pump this system for a few hours. The pump that I am looking at using is a Shurflo 12 VDC pump (Model # 2088-514-145). I need 10 PSI and 2.90 GPM and the current needed for the pump is 5.6. I am not going to use any batteries as they are expensive, but I am wanting to use a Shurflo pump controller, 902-200 9300 series 12/24 V, to get a better performance from the pump. The one thing I am not sure of is what size solar panel do I need to run this pump? I calculated that I would need a 100 Watt pump, but I am not sure if this is correct.

  • #2
    On a bright sunny day one panel might be able to run a 100W pump for a few hours. Looking
    out the window by Rockford today, it might take half a dozen or more, does it need to run
    every day? You have no line power? Bruce Roe

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    • #3
      100 watts is the correct size you made it

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      • #4
        I'd go with a $20 buck converter and buy a bigger or extra panel. A 300ish watt grid tie panel is only about $200. A buck converter will convert the panels higher voltage to the pumps lower voltage at higher current. This combination will give you longer pump running and protect the motor. Not as good at low light levels as buck converters go into a death spiral when demand exceeds available panel power for a while. Just divide in half any rating for these modules. Youtube probably has some videos on how to convert these modules into a LCB if you are handy.

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        • #5
          Originally posted by PNPmacnab View Post
          I'd go with a $20 buck converter and buy a bigger or extra panel. A 300ish watt grid tie panel is only about $200. A buck converter will convert the panels higher voltage to the pumps lower voltage at higher current. This combination will give you longer pump running and protect the motor. Not as good at low light levels as buck converters go into a death spiral when demand exceeds available panel power for a while. Just divide in half any rating for these modules. Youtube probably has some videos on how to convert these modules into a LCB if you are handy.
          For a dc-dc step down buck converter, I know it will take in the power coming from the solar panels and you can adjust the voltage to what you need. What about the current? For the pump that I am looking at getting the maximum current it can take is 6.5 A, but I need 3.3 A if I run the pump at 2.2 GPM and 10 PSI. So will the buck converter also regulate the current that is coming from the panels?
          Last edited by ERIC5249; 03-25-2018, 01:53 PM.

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          • #6
            Originally posted by bcroe View Post
            On a bright sunny day one panel might be able to run a 100W pump for a few hours. Looking
            out the window by Rockford today, it might take half a dozen or more, does it need to run
            every day? You have no line power? Bruce Roe
            I do have a line of power I can hook this up to, but I want to make it fully solar.

            Comment


            • #7
              Watts in X 85% = potential watts out. Lower the voltage and it will be capable of more current. It is the voltage and load on the motor which determines the current. Throught most of the day expect less than 50% of panels output. I would lay the panels almost flat if I had sufficient panel power. E and W also works. Crappy output but more consistent throughout the day. Check open circuit voltage of panels. Converter has to be rated for more than that voltage.

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              • #8
                Originally posted by ERIC5249 View Post

                I do have a line of power I can hook this up to, but I want to make it fully solar.
                The sun may be shining today here in Rockford, but there are plenty of days it is not. Are
                you going to use line power then? Bruce Roe

                Comment


                • #9
                  Originally posted by PNPmacnab View Post
                  Watts in X 85% = potential watts out. Lower the voltage and it will be capable of more current. It is the voltage and load on the motor which determines the current. Throught most of the day expect less than 50% of panels output. I would lay the panels almost flat if I had sufficient panel power. E and W also works. Crappy output but more consistent throughout the day. Check open circuit voltage of panels. Converter has to be rated for more than that voltage.
                  Alright. I have a 150 W solar panel that has an optimum operating voltage of 22.9 V and an short circuit voltage of 9.05 A. I am thinking I would need a bigger solar panel to run this pump based off what you said. Half of the optimum operating voltage would give me 9 V. The solar panel that Shurflo recommended for the motor was a 75 W solar panel, but I think that would be insufficient to run this pump. I can see why they would recommend this as the pump can only handle 80 W. Unless I am wrong, I think this 150 W solar panel should be good enough for this pump.

                  Thank you for your input on buck converters, it is helpful.
                  Last edited by ERIC5249; 03-25-2018, 03:39 PM.

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                  • #10
                    Originally posted by bcroe View Post

                    The sun may be shining today here in Rockford, but there are plenty of days it is not. Are
                    you going to use line power then? Bruce Roe
                    No, I do not plan on using a line of power.

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                    • #11
                      Originally posted by ERIC5249 View Post

                      No, I do not plan on using a line of power.
                      So what will you do those cloudy days?

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                      • #12
                        Your math is a little off and your panel specs don't add up. Lets say for example power point is 9A @18V. 18V/12V X 9A = 13.5A due to current boost, so half power would be 6.75A X 85% = 5.7A. They specify a smaller panel when direct connecting because solar is a current source. With a larger panel the voltage on the motor could exceed 20V and a lot more current. In motors generally all you care about is current and you will never exceed the short circuit current of a panel. in direct connect A LCB will have either a current or voltage limit built in so that is controlled.

                        I have 560W of panels in my test set up and some days I am lucky to get 40W.

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                        • #13
                          [QUOTE=ERIC5249;n373587]

                          Alright. I have a 150 W solar panel that has an optimum operating voltage of 22.9 V and an short circuit voltage of 9.05 A.

                          The numbers you are giving are incorrect, as is your terminology. They would make your panel over 207 watts. Your first number would be Voc. or open circuit voltage.The number you want is Vmp. or voltage @ maximum power and should be in the 18.5 volt range The second number is short circuit voltage but the number you want is Imp. and should be around 8.1 amps. These numbers wont likely be seen in real world situations unless you are in cold, clear noontime, sunny situations.
                          2.2kw Suntech mono, Classic 200, NEW Trace SW4024

                          Comment


                          • #14
                            Originally posted by littleharbor View Post


                            The numbers you are giving are incorrect, as is your terminology. They would make your panel over 207 watts. Your first number would be Voc. or open circuit voltage.The number you want is Vmp. or voltage @ maximum power and should be in the 18.5 volt range The second number is short circuit voltage but the number you want is Imp. and should be around 8.1 amps. These numbers wont likely be seen in real world situations unless you are in cold, clear noontime, sunny situations.
                            Alright, I see what you are saying and I see how I am off. If these numbers are unlikely unless I am in good conditions like you said, then how would I go about getting these numbers? Would I need to combine solar panels in series or parallel to get what I need?

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                            • #15
                              It appears the pump controller needs higher voltage than a single 12 volt panel puts out. If you add another 100 watt 12 volt panel you need to series wire them.
                              2.2kw Suntech mono, Classic 200, NEW Trace SW4024

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