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Pyranometer with modbus connection

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  • Pyranometer with modbus connection

    Hi,

    I was interested in collecting some data from a pyranometer, and wondering if anyone had any experience with any pyranometers that are solid. We like Modbus in general so that would be the preferred method but interested in any recommendations for quality instruments.

    Thanks, John

  • #2
    Depends on price. For quality, factory support and durability, Eppley is about as good as it gets. But you better have deep pockets.

    Lots of different types and mfgs. depending on application. I used and maintained an Eppley PSP and an Eppley "Black & White ands a

    Kipp & Zonnen are also quality instruments with a somewhat lower price.

    For a lot less $$ and almost as much accuracy and precision, but probably more frequent calibration, there are a lot of silicon photodiode instruments. About the most frequently found in the field are from Li-Cor. A decent reputation and a favorite due to it's relatively (compared to Eppley) low price and available support.

    Lots of folks, including a few around here use a Davis Pro II plus weather station. For a relatively low admission price ( ~~ $800 on up, depending on bells/whistles/software) you'll get a decent weather station that's easy to use and record/store data that's about as accurate as warranted, including a pyranometer that's reasonably accurate. Just plan on replacing the pyranometer every couple of years for ~ $125-150 or so and calibrate it against the old one.

    A word on pyranometer accuracy: Even the Eppley's will be not much better than +/- a % or 2, (And they'll tell you 5% officially, in writing) and even then that's only with frequent (1X/yr. or so) recalibration. That's not a knock, just reality of the task.

    The silicon devices are almost as good (maybe +/- 2% or better if you're lucky, but much less precision as f(cloud/atmospheric conditions), for a lot less $$, and have what can be considered the advantage of being mostly insensitive to off horizontal orientation and operation.

    FWIW, if you have a PV array, or even a single panel and a way to get a reasonably accurate estimate of that array's (panel's) cell temp., measuring the array's current can give a pretty accurate estimate of irradiance under a clear sky. I've got one of the Davis instruments next to my array. After corrections for array temp. and pyranometer temp. the array's current under clear skies is just about directly proportional to what the Davis pyronameter reads to a degree I found surprising.

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    • #3
      Wow J.P.M that's a lot of experience you have with these - thanks! I have heard the midrange value level is the Davis before. When you say the silicon devices, these are the ones that do not have a clear plastic dome, or is this the component inside the device that is silicon?

      Good to know about the DC current measurement too on the panel, interesting. What I have been working on is a neural network module for our monitoring platform that can zero in on performance based on weather and just watt-hours in a given time frame. But I have not added the pyronometer input for that learning, so am interested in testing that.

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      • #4
        Originally posted by jwrgorman View Post
        Wow J.P.M that's a lot of experience you have with these - thanks! I have heard the midrange value level is the Davis before. When you say the silicon devices, these are the ones that do not have a clear plastic dome, or is this the component inside the device that is silicon?

        Good to know about the DC current measurement too on the panel, interesting. What I have been working on is a neural network module for our monitoring platform that can zero in on performance based on weather and just watt-hours in a given time frame. But I have not added the pyronometer input for that learning, so am interested in testing that.
        FWIW, you're welcome. Short ans. to your questions, at least as the answers may relate to the solar energy application of pyranometers: Common silicon devices are smaller and usually not under a dome. Perhaps coincidentally, the smaller devices cost less. Davis is close to the bottom on price.

        On array (panel) current as a measure of irradiance, two things: Cell temp. is not ambient temp. and, any estimates/correlations using that method including temp. corrections, will be of est. P.O.A. (Plane Of Array) irradiance which, while it may be bottom line what you're looking for, will not be the same as most published irradiance data which is usually reported as G.H.I (Global Horizontal Irradiance) with the user then transposing/converting the GHI to POA for the orientation of the array or device using any one of available algorithms or models. See NREL for more info.

        I suppose from a technically correct standpoint, the silicon diode devices like the Davis and Li-Cor and simpler stuff are not considered true pyranometers because of the definitional requirement of true pyranometers requiring a response to the incoming solar radiation that is independent of the wavelength of the incoming solar radiation and also be independent of the angle of incidence of all radiation for beam, diffuse and any albedo sources. The silicon devices' response are sort of representative but not truly wavelength independent. See a response curve for the graphic. Most folks don't make that distinction, or maybe need to.

        Perhaps interestingly, the silicon devices may actually be closer to the response of a PV array to irradiance than a true pyranometer, but not give as true a picture of the value of GHI or POA irradiance as the thermopile devices. That probably doesn't matter a whole lot for day/day backyard stuff but good to keep in mind if things require more detail and accuracy.

        Best suggestion I can give for answers to your questions is to first see the first few sections of "Solar Engineering of Thermal Processes", by Duffie & Beckman, ISBN # 0-471-51056-4. The info there may be a bit outdated with respect to the more recent silicon devices, but still good background info. Then, armed with that background, see the product literature from outfits I mentioned which, while not bad, is from those with skin in the game and stuff to sell. Caveat Emptor.
        Last edited by J.P.M.; 01-09-2018, 04:00 PM.

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        • #5
          Hi J.P.M., that makes a lot of sense. I am all for a lower cost option, especially when the physical device is similar in function to the device creating the electricity you're benchmarking - but making sure we're dealing with apples is important. Just so I get this right - the P.O.A. irradiance - is that often taken from a unit mounted in the same plane as the array OR is it calculated from a horizontal mounted unit? I can see both datapoints being interesting. Also the thermopile devices that are more frequency independent, those are always horizontally mounted?

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          • #6
            Originally posted by jwrgorman View Post
            Hi J.P.M., that makes a lot of sense. I am all for a lower cost option, especially when the physical device is similar in function to the device creating the electricity you're benchmarking - but making sure we're dealing with apples is important. Just so I get this right - the P.O.A. irradiance - is that often taken from a unit mounted in the same plane as the array OR is it calculated from a horizontal mounted unit? I can see both datapoints being interesting. Also the thermopile devices that are more frequency independent, those are always horizontally mounted?
            Short answers: Purists use horizontal orientation for resource assessment needs like determining clearness indices and such like. They convert the GHI to POA for determining efficiency and other needs in solar energy generation work/estimating. Thermopile type devices are always horizontally oriented.
            .
            P.O.A., Plane Of Array irradiance, is a necessary quantity when dealing with arrays or other flat plate solar devices and their efficiency. GHI, Global Horizontal Irradiance is a necessary quantity for investigating things solar resource availability.

            Historically, because most solar resource tools - that is pyranometers - were once mostly of the thermopile type that, because of their design, and the idea that natural convection currents inside the instrument (under the dome) are different in different orientations and so affect the devices sensitivity and accuracy as well, and also due to the time honored convention of reporting GHI, the standard has developed of using all pyranometers a horizontal orientation. But, the rub with that was/is that because most solar collectors were not horizontal, a very large body of knowledge developed around and dealing with converting GHI to POA irradiance. About the best concise but reasonably complete treatment of the basics of all that is in Duffie & Beckman. See NREL for additional info.

            Now, to your questions:

            I suppose POA day long insolation as gathered by silicon devices mounted on an array will probably be sufficient, but will probably be different than day long total insolation that starts with daylong GHI from the same device in a horizontal orientation (including the Davis weather station pyranometers), and then converted that GHI to POA via readily available models and algorithms. The cosine response of the device in the plane of the array may well be slightly different and albedo certainly will be different, maybe more, maybe less, depending on array location and orientation. There are other things that will affect the readings horizontal to POA as well. Purist types I know, and most all of the literature use GHI as the basis from any and all types of pyranometer - silicon and thermopile type - and convert to POA as needed. The amount of software available to do the conversion modeling has small variations if the models are comprehensive and the weather data used is reasonably accurate, particularly with respect to precipitable water vapor and atmospheric turbidity.

            Starting with GHI and converting just keeps everyone on the same page, dealing with apples from the same orchard and working to mostly the same assumptions. After a while, you'll get to appreciate the advantages.

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            • #7
              Thanks J.P.M. - didn't think of convection currents inside the instrument, that's a lot fine detail. OK will stick with GHI for now, I am reviewing here as well : https://pvpmc.sandia.gov/modeling-st...oa-irradiance/

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              • #8
                Originally posted by jwrgorman View Post
                Thanks J.P.M. - didn't think of convection currents inside the instrument, that's a lot fine detail. OK will stick with GHI for now, I am reviewing here as well : https://pvpmc.sandia.gov/modeling-st...oa-irradiance/
                The Sandia reference you include is a good start, but there's a lot of detail not shown that will be needed to get the most out of it. I don't want to sound like a broken record, but Duffie & Beckman is still the best place to find most of what you'll need in one place.

                Using GHI and converting to POA will be more work up front, but the extra detail may be worth the effort if you need more than POA at one orientation - for comparison at different orientations for example. D & B have a good discussion of that.

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