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STC vs. PTC Wattage Ratings Tests

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  • STC vs. PTC Wattage Ratings Tests

    As far as I can tell, the biggest difference between Standard Test Conditions (STC) and PVUSA Test Conditions (PTC) is that PTC tests are performed in air which is 5 degrees cooler, and moving at 1 m/s. This cooler air and forced convection should allow the module to provide higher output, but it seems the opposite is true. I found a typical module with Pmax 220W STC is only 194W by the PTC test method.

    Can anyone explain what I'm missing?

    Thanks,
    Art

  • #2
    The solar radiation really heats up the panel, think of PTC as much more indicative of real life.
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    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
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    • #3
      Mike,
      Yes, high heat will lower the output of the cells. But having a 1 m/s 20C breeze blowing over them should keep them cooler than the 25C air in the STC test, right? It seems as though the STC test provides higher output than the cooler PTC test. What am I missing?
      Art

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      • #4
        Originally posted by Vandergraaff View Post
        Mike,
        Yes, high heat will lower the output of the cells. But having a 1 m/s 20C breeze blowing over them should keep them cooler than the 25C air in the STC test, right? It seems as though the STC test provides higher output than the cooler PTC test. What am I missing?
        The STC test uses a flashlamp to test the panels output. panels never get a chance to get warm. PTC actually matches real life harvest.
        Powerfab top of pole PV mount (2) | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
        || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
        || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

        solar: http://tinyurl.com/LMR-Solar
        gen: http://tinyurl.com/LMR-Lister

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        • #5
          Ah, now that makes sense! A flash test is what we use to test satellite solar arrays as well.

          Thanks Mike, for clearing this up.
          Art

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          • #6
            Originally posted by Vandergraaff View Post
            Mike,
            Yes, high heat will lower the output of the cells. But having a 1 m/s 20C breeze blowing over them should keep them cooler than the 25C air in the STC test, right? It seems as though the STC test provides higher output than the cooler PTC test. What am I missing?
            Sounds like you now understand. However even with 20 C and breeze, the panels wil be much higher than ambient temp. Just like your car on a winter day in bright sunshine. Even though it is cold outside, inside the car is fairly warm just like a solar cell. They are black, on a roof reflecting heat, and sealed up. Even in winter the cell temp can exceed 25 C
            MSEE, PE

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            • #7
              isn't 1000W/m2 unrealistic as well. do you get that much in the sahara desert or middle east?

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              • #8
                STC stands for Standard Test Conditions which are 1,000 watts per square meter solar irradiance, 1.5 Air Mass and 25 degrees C. cell temperature.

                PTC stands for PV USA Test Conditions which were developed at the PV USA test site at Davis, California. PTC are 1,000 watts per square meter solar irradiance, 1.5 Air Mass, and 20 degrees C. ambient temperature at 10 meters above ground level and wind speed of 1 meter per second.

                I recorded an hourly average reading of 1001 watts/m2 one hour only last summer - and it is sunny here.

                Some places in the SW USA probably see a few more.

                Russ
                [SIGPIC][/SIGPIC]

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                • #9
                  Originally posted by russ View Post
                  STC stands for Standard Test Conditions which are 1,000 watts per square meter solar irradiance, 1.5 Air Mass and 25 degrees C. cell temperature.

                  PTC stands for PV USA Test Conditions which were developed at the PV USA test site at Davis, California. PTC are 1,000 watts per square meter solar irradiance, 1.5 Air Mass, and 20 degrees C. ambient temperature at 10 meters above ground level and wind speed of 1 meter per second.

                  I recorded an hourly average reading of 1001 watts/m2 one hour only last summer - and it is sunny here.

                  Some places in the SW USA probably see a few more.

                  Russ
                  That day was most likely a bright sunny day with a fair amount of high cumulus clouds adding some reflection to the direct radiation.
                  NABCEP certified Technical Sales Professional

                  [URL="http://www.solarpaneltalk.com/showthread.php?5334-Solar-Off-Grid-Battery-Design"]http://www.solarpaneltalk.com/showth...Battery-Design[/URL]

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                  • #10
                    Unless I'm missing something, which is entirely possible, the 1000W/m2 is a convenient number, a standard used for comparison.

                    We are really interested in power over time more than instant power. Most of the solar influx charts I see (such as those from http://rredc.nrel.gov), describe power in terms of kWh/m2/day. Since we are typically looking for "full solar day equivalents", those kWh/m2/day number's units all cancel out and they directly provide us "full solar day equivalents".

                    If we used some standard that was more realistic in terms of instantaneous power, like 600 or 800W/m2, we would have to make adjustments from the tables.

                    Of course, the sun does not shine as a binary switch, if the table shows, say 4 kWh/m2/day your real life experience might be something like on an average day:

                    8am: 0.05kWh
                    9am: 0.15kWh
                    ...
                    11-1: 0.8 kWh
                    ...
                    3pm: 0.15kWh
                    4pm: 0.5kWh

                    All of which would add up to a total of 4 kWh. Number are, of course, for illustrative purposes only.

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                    • #11
                      Originally posted by russ View Post
                      STC stands for Standard Test Conditions which are 1,000 watts per square meter solar irradiance, 1.5 Air Mass and 25 degrees C. cell temperature.

                      PTC stands for PV USA Test Conditions which were developed at the PV USA test site at Davis, California. PTC are 1,000 watts per square meter solar irradiance, 1.5 Air Mass, and 20 degrees C. ambient temperature at 10 meters above ground level and wind speed of 1 meter per second.

                      I recorded an hourly average reading of 1001 watts/m2 one hour only last summer - and it is sunny here.

                      Some places in the SW USA probably see a few more.

                      Russ
                      Just for those who did not get the point of the highlighting, there is a lot bigger difference between 25C cell temperature and 20C ambient temperature than just the temperatures would indicate. Show me a panel that only goes 5C above ambient in full sun and I will show you one that is refrigerated or water cooled!

                      The STC measurement can be done with a flash lamp, while the PTC numbers require either continuously illuminating the panel or computing (from experiment) an offset temperature at which to conduct the flash tests.

                      Q: How close to the Equator are you for your 1000 w/m2 measurement? I assume that there is some reduction in insolation corresponding to the low sun angle at solar noon, even without clouds, dust, etc.
                      SunnyBoy 3000 US, 18 BP Solar 175B panels.

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                      • #12
                        Originally posted by kevinc_63366 View Post
                        Small correction -
                        8am: 0.05kW/m2
                        9am: 0.15kW/m2
                        ...
                        11-1: 0.8 kW/m2
                        ...
                        3pm: 0.15kW/m2
                        4pm: 0.5kW/m2

                        Those are instantaneous readings - to become kWh you need to add the time factor.
                        Comments in the text.
                        [SIGPIC][/SIGPIC]

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                        • #13
                          Please presume the time period between data points is 1 hour...

                          Is my basic premise correct? That since the national data is presented in kW/m2/hr for a given month, and that panel STC numbers are based on 1kW/m2, that the units cancel out and one can simply read values, like 3.2, as 3.2 hours of equivalent full power irradiation per day?

                          The charts intention was to simply showed that those 3.2 equivalent full power hours ramped over many more hours. Some of those hours might exceed that hourly power rating, most won't, but you make it up over the day. Kind of like my corals in my aquarium: I can light them with a bright new bulb for 8 hours a day, or a dimmer older bulb for longer. As long as the total photon count remains about the same over a day, they are happy.

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                          • #14
                            Originally posted by kevinc_63366 View Post
                            Please presume the time period between data points is 1 hour...

                            Is my basic premise correct? That since the national data is presented in kW/m2/hr for a given month, and that panel STC numbers are based on 1kW/m2, that the units cancel out and one can simply read values, like 3.2, as 3.2 hours of equivalent full power irradiation per day? When one has hours in it and the other does not you can not cancel.

                            The charts intention was to simply showed that those 3.2 equivalent full power hours ramped over many more hours. Some of those hours might exceed that hourly power rating, most won't, but you make it up over the day. More or less correct though it is a difficult way to get there.
                            Sun hours are a gimmick to make it easy for the customer I suppose.

                            Kw/m2/day is an instantaneous reading - if you have 1000 watts reading continuously for one hour then you have the kWh.

                            You are going the correct direction.
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