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  • How to - Weather Station Pyranometer

    Hello,
    I have completer weather station from Davis and it also includes Solar Radiation Sensor (pyranometer).
    (Vantage Pro2 Plus with 24-Hr Fan Aspirated Radiation Shield)

    I am confused with installation of pyranometer .

    How do I install Pyranometer, shall I install straight horizontal (on pole along with rain collector) or should I install at the same angle as Solar panels (on solar structure)

    I guess, if I install horizontal flat, then it would measure GHI, in that case I need to convert GHI to PoA, which I do not know how to convert.

    For PR calculation, which one is better method?

    Horizontal or at same angle of solar panel?

    I saw many solar installations and they almost always install at Panel angle. Like photo shown below.

    Please suggest and help.

    Thanks
    Sanket


    pyran.jpg

    maxresdefault.jpg

  • #2
    If you want to measure what your panels see, install with panels.

    If you want a standard measure that others can use / compare, install level (recommended) Is that a bubble level on the pyranometer ?
    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

    Comment


    • #3
      For many reasons, the standard way of reporting solar radiation is in the horizontal plane. That's why the Davis pro II plus has the mounting brackets as they do.

      Even though it's a (one time) PITA, if you're planning on communicating with anyone that's knowledgeable about panel/array performance and how the solar resource changes as f(array location/orientation, solar position, time), you will almost always be dealing in GHI and doing a conversion to P.O.A. That's the lingua franca.

      If you only need P.O.A. a sensor mount in the plane of the array will work, but know that the reading may be affected by sensor location. One way to minimize such effects is to keep the sensor mounted to the Davis but add a gimbaled bracket. In any case, I'd keep the sensor on/at/near the weather station and not mount it on the array.

      As for an algorithm to convert GHI to POA, there are many of varying sophistication. Understand that the atmosphere is like water in the ocean, it's always in motion and never the same. All algorithms are approximate. I've found most to have limitations.

      If you have the interest, about the most thorough but still manageable treatment of the solar resource for practical engineering use is Duffie & Beckman: "Solar Engineering of Thermal Processes".

      Mounting the sensor in the plane of the array is fine and will probably work well enough but accuracy of the output will probably suffer for several reasons, two of which are likely increased albedo effects from the sensor "seeing" more of the surroundings, and also, unless you have a thermometer very near the sensor, no good way to adjust for the sensor temp. which, if attached to a panel will be above amb. temp. by something like 20-30 C or so. By the time the inaccuracies and perhaps more importantly the uncertainties introduced by off horizontal mounting are estimated, my semi informed opinion is that the variation introduced is at least as much as the uncertainty introduced in the algorithms that convert GHI to POA. The algorithms at least may be a bit more consistent in their variation.

      I've got a Davis Pro II plus exactly as you describe. It's a work horse and hasn't failed or dropped any data in almost 7 years of continuous operation measuring and logging data in 1 minute increments. It's mounted about 1.2m north of my array at about the same elevation as the north (top) of the array on the N-S C.L. I started this rodeo by getting the pyranometer recalibrated every year as they can drift some. I figured out after a couple of years it was just as good and no more expensive to simply buy a new sensor every 2 yrs. and calibrate the new against the old by mounting the new where the IR sensor normally is and swapping sensor leads for 20 clear days for about 15 minutes or so on either side of solar noon and at solar noon around the summer solstice, and swap which one is before/after solar noon to minimize some of the (hopefully small) variation caused by before/after variances which ought to be somewhat or mostly symmetrical around solar noon on uniiformly clear days. Then, correct for temp. from the Davis readings.

      Also, after looking at the sensor tabs in the line from the sensor to the Davis, I'd bet the instrument drift is mostly or at least partially due to the line connections that show some corrosion at the attachment points. Sending the sensor out for calibration was not a successful endeavor anyway. Took too much time and so required a 2d sensor anyway.

      Welcome to the neighborhood.

      Holler back.
      Last edited by J.P.M.; 07-30-2020, 01:01 PM.

      Comment


      • #4
        @Mike, yes that bubble level on pyranometer. But it is not useful when installing PoA.


        @J.P.M, did you install Davis Pyranometer PoA or GHI? if PoA, can you suggest good gimbal for tilting pyranometer?

        Regarding GHI to PoA, still I could not understand executable formula. I do have Long/lats, Azimuth (0 in my case), panel installation angle but then I do not know how to calculate it with time (as it affect the output) and if any other parameter input is required in GHI to PoA formula.

        My current Davis station logs irradiation data into SQL server and my solar power generation is also being logged into SQL.
        As of now I am calculating PR on daily/weekly/monthly basis with the formula:

        PR= sum of total production / avg PoA of that selected period in kWh * installed capacity


        This gives me PR in my software. But I was not sure about installation method of pyranometer for this daily/monthly PR calculation.

        I agree that every method has margin of error, but wanted to select less error model with given infrastructure and available parameters.


        Comment


        • #5
          bump

          Comment


          • #6
            Originally posted by teknas View Post
            @Mike, yes that bubble level on pyranometer. But it is not useful when installing PoA.


            @J.P.M, did you install Davis Pyranometer PoA or GHI? if PoA, can you suggest good gimbal for tilting pyranometer?

            Regarding GHI to PoA, still I could not understand executable formula. I do have Long/lats, Azimuth (0 in my case), panel installation angle but then I do not know how to calculate it with time (as it affect the output) and if any other parameter input is required in GHI to PoA formula.

            My current Davis station logs irradiation data into SQL server and my solar power generation is also being logged into SQL.
            As of now I am calculating PR on daily/weekly/monthly basis with the formula:

            PR= sum of total production / avg PoA of that selected period in kWh * installed capacity


            This gives me PR in my software. But I was not sure about installation method of pyranometer for this daily/monthly PR calculation.

            I agree that every method has margin of error, but wanted to select less error model with given infrastructure and available parameters.

            1.) I prefer to work with GHI and use an algorithm for POA that I developed, tweaked and polished from ~ 1977 onward.
            I do have a small fixture I made to enable the pyranometer to be mounted to the underside of the array facing downward. I did that some years back to measure the incident insolation on a roof mounted array with ~ 6" of clearance between edge of the sensor "top" surface and the roof deck. My array has ~ 12" or so clearance and the fixture allows the sensor surface to be ~ 6" below the bottom of panel. (BTW and FWIW, best irradiance I could measure under full sun at solar noon in the summer under my array was ~ 30 W/m^2 or ~ 3% of frontside irradiance. At the time one blowhard treehugging poster was spreading B.S. about how good bifacial panels would be for std. residential setups.

            2.) Get a hold of a copy of Duffie & Beckman and all the details and what's necessary for a decent GHI to POA conversion algorithm will be made clear.
            Not a knock, but given what seems to be your current knowledge level of the solar resource, if I was you, and if you're task oriented more than knowledge curious, I'd think I'd be better off mounting the sensor in a POA orientation and not screw around with a conversion algorithm, which is a PITA to get reasonably close to a good approx. of POA irradiance and not for the feint of heart. If I did put the Davis sensor in POA orientation, I'd be sure to put it about 0.5+ m or so away from the array with the plane of the sensor surface about 6" or a bit more above the plane of the array. That way the sensor irradiance will have a small(er) probability of getting enhanced irradiance from reflections off the array, but if close to the array, may just see some of the albedo the array sees. Also, not mounting the sensor to the array frames or supports will probably tend to keep the sensor temp. closer to ambient temp.
            As I wrote prior, my entire Davis is near the array w/the irradiance sensor in the standard (horizontal) orientation. The advantage of mounting the irradiance sensor in/on the Davis as it comes from the factory is that the thermometer is very near giving sensor temp. corrrections a better chance of being a bit more accurate.

            If I wanted a gimbal mount for POA sensor mounting, I'd simply make a fixture using an additional Davis sensor mounting bracket, some aluminum angle, a couple of bolts, nuts and lock washers, a 4 ft. level and mount it to the roof next to the array. Easy peasy with a bit of thought.

            3.) I don't know what SQL or PR refer to.

            4.) I agree with wanting less error. I've found I usually get less error in modeling and most everything else for that matter, by knowing what I'm looking for, not relying on others too much and keeping things as simple as possible.

            For your situation, skip the algorithms - bottom line for most of this stuff is that it ain't rocket science but it does need some knowledge of the fundamentals. At your current state of knowledge, you'll get more accuracy and less aggravation with a POA mounted sensor. Just mount it near, but not attached to the array with simple fixturing. Make it high enough to avoid albedo from the array and hopefully near the rest of the Davis instrumentation, which is probably best located near the array.

            Good luck.
            Last edited by J.P.M.; 07-31-2020, 12:18 PM.

            Comment


            • #7
              quick reply:
              PR = Performance Ratio
              SQL is just the database where I store the data of power generation and irradation.

              Comment


              • #8
                Originally posted by teknas View Post
                quick reply:
                PR = Performance Ratio
                SQL is just the database where I store the data of power generation and irradation.
                Thank you.

                FWIW, I believe the pyranometer shown in your top photo is a Kipp & Zonen.
                Last edited by J.P.M.; 07-31-2020, 12:52 PM.

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