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Selling home, solar inspection issue with VOC measurement.

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  • #31
    Originally posted by bcroe View Post

    I'd throw a saw tooth current drain (dummy load) on it at perhaps 5 HZ, low enough frequency
    to ignore system inductance but high enough to have minimal thermal change. Watch the
    waveform of the voltage relative to the current; it will probably be pretty flat. Better measure
    voltage at the panel terminals so resistance of the copper doesn't affect it. Bruce Roe
    I would too, but lacking the required equipment and expertize, I'll forego the attempt. I usually measure stuff and then see if I can use what book knowledge I have to make sense of the results to explain things, perhaps in a semi-empirical way and hopefully learn stuff along the way.

    For example, I've for 2 strings of 8 ea. 327's and a spec sheet for them that says the Voltage drop per panel per deg. C. of panel temp. change is -0.1766 V/deg. C, making a spec sheet Voltage change of (0.1766)*(8) = ~1.413 v/deg. C. per string. After a lot of quasi steady state measurements of near simultaneous string Voltage and measurements of all 16 individual panel temps. under clear skies at daily min. incidence angle both summer and winter, and recording as many variables as I've got the means to measure (roof ambient air temp. GHI, wind vector, dew point and other stuff) , I calc'd and use a voltage drop/deg. C constant of 1.478 V/deg. C. per string. That's got some uncertainty in it (mean = 1.478 V/deg. C, std. dev. 0.123 V/deg. C, N = 60), but it seems to produce consistent results when applied to other conditions that are useful in my attempts to measure array fouling, and other things such as possible measurements of panel deterioration as f(time), and other stuff. All that leads me to the confirming conclusion that, as a practical matter, array mpp Voltage is pretty much only f(array temp.) as most all the cogent and technically accepted literature seems to say.

    Science is about precision and more about absolutes. There's no variation in Avogadro's Number for example. Most of engineering is about approximations and looking for explanations about real world variations, and ultimately accepting that for all our attempts at accuracy, in the end, and in spite of our best efforts, everything we measure is no better than an approximation.

    Bottom line for engineering: Is the damn thing safe ? And does it accomplish the required task for a reasonable price ? The rest of the application decisions involve economics and politics, which, in the real world, will always govern.

    Comment


    • #32
      Originally posted by bcroe View Post

      Actually I think my idea FAILS, because its changing current instead of irradiance. Oh well. Bruce Roe
      I thought you were after plotting one of those IV curves for the currently present irradiance. In that case it would work- your variable current drain would 'scan' IV curve so reading voltage would give you second coordinate for IV points. As you noted it would need to be measured at the panel for the best results. To get set of curves you'd repeat the process for multiple irradiance levels but it needs to be done automatically/fast to avoid heating up the panel.

      Comment


      • #33
        Originally posted by max2k View Post
        As you noted it would need to be measured at the panel for the best results. To get set of curves you'd repeat the process for multiple irradiance levels but it needs to be done automatically/fast to avoid heating up the panel.
        The panel heats up when the sun hits it, whether it is open circuit, short circuit, or something in between. Producing electricity should even lower the cell temp relative to what the open circuit temperature would be in the same conditions.

        Digging deep enough into the weeds, the effect of irradiance on Voc can be seen, as the IV curves suggest. For what the OP is looking at, I would stay focused on the temperature effect since it is well known and easy to estimate, and explains most of the voltage drop from the STC values that are observed. (More later, if I can find time)
        CS6P-260P/SE3000 - http://tiny.cc/ed5ozx

        Comment


        • #34


          Picking this back up, I've taken another walk through the models that do a good job of describing PV behavior for conventional c-Si modules. For everything written below, reference Solar Engineering of Thermal Process (Duffie and Beckman), or several other sources in which the same concepts are discussed.

          One of those most accepted is the single diode model, in which the equivalent circuit of the PV cell is as shown: single diode.JPG



          The equation that describes this circuit at constant temperature and irradiance is: equation.JPG



          Voc is the condition with I = 0, which collapses the equation down to

          0 = IL - I0 [exp(Voc/a) - 1] - Voc/Rsh

          When temperature and irradiance vary, they affect each of those parameters differently.

          As implemented by NREL's System Advisor Model

          IL (photogenerated current) responds proportionally to irradiance, and also has a temperature coefficient.
          I0 (reverse saturation current) increases exponentially with temperature
          a increases proportionally with temperature
          Rsh increases proportionally with irradiance.

          The net result of all this is a Voc that responds strongly to temperature, but can also be affected by irradiance.

          Using the CEC parameters for Canadian Solar's CS6X-320P module, IV curves can be generated for conditions that might represent the real world. If we artificially de-couple temperature from irradiance and hold it constant as irradiance is varied, the following chart is produced for the sample conditions: iv.JPG




          This matches the general patterns of the IV curves shown on the module data sheets, and gives us an analytical path forward for estimating the change in Voc due to irradiance. There are publications of experimental results which show this same effect, with Voc responding nearly linearly with irradiance before falling off a cliff as bcroe described in his moonlight test.

          Assuming they are in the CEC database, the same kind of chart could be made for the OP's actual panels and the actual conditions at the time of the Voc measurement.
          Last edited by sensij; 07-25-2017, 04:39 AM.
          CS6P-260P/SE3000 - http://tiny.cc/ed5ozx

          Comment


          • #35
            Originally posted by sensij View Post

            Picking this back up, I've taken another walk through the models that do a good job of describing PV behavior for conventional c-Si modules. For everything written below, reference Solar Engineering of Thermal Process (Duffie and Beckman), or several other sources in which the same concepts are discussed.

            One of those most accepted is the single diode model, in which the equivalent circuit of the PV cell is as shown: single diode.JPG



            The equation that describes this circuit at constant temperature and irradiance is: equation.JPG



            Voc is the condition with I = 0, which collapses the equation down to

            0 = IL - I0 [exp(Voc/a) - 1] - Voc/Rsh

            When temperature and irradiance vary, they affect each of those parameters differently.

            As implemented by NREL's System Advisor Model

            IL (photogenerated current) responds proportionally to irradiance, and also has a temperature coefficient.
            I0 (reverse saturation current) increases exponentially with temperature
            a increases proportionally with temperature
            Rsh increases proportionally with irradiance.

            The net result of all this is a Voc that responds strongly to temperature, but can also be affected by irradiance.

            Using the CEC parameters for Canadian Solar's CS6X-320P module, IV curves can be generated for conditions that might represent the real world. If we artificially de-couple temperature from irradiance and hold it constant as irradiance is varied, the following chart is produced for the sample conditions: iv.JPG




            This matches the general patterns of the IV curves shown on the module data sheets, and gives us an analytical path forward for estimating the change in Voc due to irradiance. There are publications of experimental results which show this same effect, with Voc responding nearly linearly with irradiance before falling off a cliff as bcroe described in his moonlight test.

            Assuming they are in the CEC database, the same kind of chart could be made for the OP's actual panels and the actual conditions at the time of the Voc measurement.
            Now, folks, stick a fork in it.

            Sensij: Couldn't have done it better myself.

            Comment


            • #36
              Originally posted by sensij View Post
              This matches the general patterns of the IV curves shown on the module data sheets, and gives us an analytical path forward for estimating the change in Voc due to irradiance. There are publications of experimental results which show this same effect, with Voc responding nearly linearly with irradiance before falling off a cliff as bcroe described in his moonlight test.

              Assuming they are in the CEC database, the same kind of chart could be made for the OP's actual
              panels and the actual conditions at the time of the Voc measurement.
              My simplistic view is of a diode. A diode will of course start forward current conduction at lower voltage
              and increase conduction with a little more voltage. An irradiated panel generates a current, which can
              be observed by shorting the terminals. That is not very useful (zero output power), so allowing the
              output short to increase in resistance will result in current at some voltage. However continuing increase
              will result in some current being stolen by the diode. At open circuit we see Voc, the voltage drop across
              the diode with all the current flowing through it, again not very useful. To get useful power, we find a
              point in between to maximize I x V (MPPT).

              So I would expect the panel at MPPT to be cooler, because about 80% of the available current is
              taken away instead of being dissipated internally. With a shorted load, I'd it expect it even cooler.
              Bruce Roe

              Comment


              • #37
                So, does this revelation change, in any way, the tenor of the responses seen in posts #18, #20, #22, and #24? A rhetorical question.

                Comment


                • #38
                  Originally posted by bcroe View Post

                  ... With a shorted load, I'd it expect it even cooler.
                  Bruce Roe
                  that would contradict law of energy preservation- where would that converted energy disappear to? IMO the panel will be at exactly same temp as in 0 current case- the converted energy will simply heat up its internal resistance component.

                  Comment


                  • #39
                    Originally posted by max2k View Post

                    that would contradict law of energy preservation- where would that converted energy disappear to? IMO the panel will be at exactly same temp as in 0 current case- the converted energy will simply heat up its internal resistance component.
                    I don't agree. At anything like MPPT the energy that goes to the external load is not burned in the
                    panel, so the panel is cooler than open circuit. The energy went to the load. Bruce Roe

                    Comment


                    • #40
                      Originally posted by AzRoute66 View Post
                      So, does this revelation change, in any way, the tenor of the responses seen in posts #18, #20, #22, and #24? A rhetorical question.
                      IMO you were right and everybody was wrong if it makes you feel better . I didn't expect myself Voc to be that much dependent on irradiance until I checked the data sheet but I think everyone understood that as well. This actually caused quite useful discussion and I think at the end at least I learned few new things. Who was right does not mean much to me so from that point of view your contribution is a little different- you made all of us to revisit a well misunderstood concept .

                      Comment


                      • #41
                        Originally posted by bcroe View Post

                        I don't agree. At anything like MPPT the energy that goes to the external load is not burned in the
                        panel, so the panel is cooler than open circuit. The energy went to the load. Bruce Roe
                        when load is short circuited the energy it dissipates is 0 so it must go elsewhere. I was referring to this specific case only.

                        Comment


                        • #42
                          Originally posted by AzRoute66 View Post
                          So, does this revelation change, in any way, the tenor of the responses seen in posts #18, #20, #22, and #24? A rhetorical question.
                          Are you expecting a rhetorical answer? Bruce Roe

                          Comment


                          • #43
                            Originally posted by max2k View Post

                            IMO you were right and everybody was wrong if it makes you feel better . I didn't expect myself Voc to be that much dependent on irradiance until I checked the data sheet but I think everyone understood that as well. This actually caused quite useful discussion and I think at the end at least I learned few new things. Who was right does not mean much to me so from that point of view your contribution is a little different- you made all of us to revisit a well misunderstood concept .
                            I think it is important to stress that temperature and irradiance cannot be so easily decoupled in the real world as they are in the analysis I provided. Even within the CEC model, if I attempt to keep environmental conditions constant and change only effective irradiance by changing the panel tilt (for example), the impact of cell temperature on Voc overwhelms the impact of irradiance, and except at the extremes, the Voc is largely determined by the simple temperature coefficient. So, no, I don't think it changes any of the advice to the OP, but I do appreciate that the truly accurate response is more nuanced than early thread replies suggested.

                            (In other words, the information provided by suncalc.org doesn't do anything to help the OP reconcile the electrician's measurements with what reasonable expectations of the system Voc should be)
                            Last edited by sensij; 07-25-2017, 03:38 PM.
                            CS6P-260P/SE3000 - http://tiny.cc/ed5ozx

                            Comment


                            • #44
                              Originally posted by sensij View Post

                              I think it is important to stress that temperature and irradiance cannot be so easily decoupled in the real world as they are in the analysis I provided. Even within the CEC model, if I attempt to keep environmental conditions constant and change only effective irradiance by changing the panel tilt (for example), the impact of cell temperature on Voc overwhelms the impact of irradiance, and except at the extremes, the Voc is largely determined by the simple temperature coefficient. So, no, I don't think it changes any of the advice to the OP, but I do appreciate that the truly accurate response is more nuanced than early thread replies suggested.

                              (In other words, the information provided by suncalc.org doesn't do anything to help the OP reconcile the electrician's measurements with what reasonable expectations of the system Voc should be)
                              ok, the actual IV curves from any data sheet (not a model) readily show Voc dependence on irradiance in 5-7 V range with irradiance in 200 - 1000 W/m2. To get the same drop from temp the same panel needs to be heated by 5/30x0.004 = 41C OP conditions were low irradiance in the morning and I doubt his panels were at 66C at the time. If anything he should've benefited from colder morning temp in Voc sense. IMO the only feasible explanation of his low Voc readings is low irradiance in the morning. Am I missing something here?

                              Comment


                              • #45
                                Originally posted by max2k View Post

                                ok, the actual IV curves from any data sheet (not a model) readily show Voc dependence on irradiance in 5-7 V range with irradiance in 200 - 1000 W/m2. To get the same drop from temp the same panel needs to be heated by 5/30x0.004 = 41C OP conditions were low irradiance in the morning and I doubt his panels were at 66C at the time. If anything he should've benefited from colder morning temp in Voc sense. IMO the only feasible explanation of his low Voc readings is low irradiance in the morning. Am I missing something here?
                                Read the first post again. Ambient temp was 75-80 deg (25 deg C) at the time of the measurement, on the way to a day in which temp would be well over 100. The NOCT rating of the panel is made at 20 deg C ambient, yielding a cell temp of ~46 deg C, so if rooftop temp was equal to the 25 deg C ambient temp, you would already expect the cell temp to be over 50 deg C. Rooftop temp is typically warmer than ambient, so even if you don't get all the way to the 66 deg C you are estimating, *most* of the reduction in Voc observed can be explained by temperature.
                                CS6P-260P/SE3000 - http://tiny.cc/ed5ozx

                                Comment

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