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    • CommentAuthorskyewright
    • CommentTimeSep 1st 2017 edited
    PVGIS is getting a face (& data) update...


    Edit: Changed format to give a clickable link
    • CommentTimeSep 1st 2017
    Seems to work well, and pretty accurately, for me.
    • CommentAuthorMackers
    • CommentTimeSep 1st 2017
    Thats great, ill have to try it and see how its been improved
    • CommentTimeSep 2nd 2017 edited
    A brilliant tool, made better it seems.

    For me, its value has been in quantifying the small but still significantly useable radiation during the deep-winter months. At those seasons, the sun is low in the sky and obstruction by surrounding buildings, trees and distant hilltops is critical.

    PVGIS allows the observed local horizon (as viewed from any given point of interest) to be uploaded and incorporated in the results that come back as download. You upload a string of numbers as a txt file: these numbers represent measured angle of elevation of the horizon at given compass bearing. The system assumes they are equally spaced around the full 360o, so If you upload 36 numbers it decrees that they are spaced at 10o intervals. With a jagged horizon you need them closer than that, so upload a string of 120 numbers, giving 3o intervals. But you don't need to measure elevation at 120 points - just the SE to SW sector in deep winter, so 40 points measured, the rest uploaded as zeros or whatever you like - the sun doesn't 'see' the rest in deep winter so doesn't matter.

    How to measure the horizon's angle of elevation at 3o intervals (the first number is taken as due East)? Obviously a theodolite will do it, and you have to relate it to true solar North, not map North or magnetic (compass) North.

    I've experimented with taking photos with the camera exactly level. The print can be overlaid with a transparent sheet gridded in degrees upward and sideward from the print's exact centre point, which you've created e.g. by photographing a brick wall head-on, measuring the brick module and distance from wall, converting that by trigonometry into degrees. Then it's a problem of hoisting the camera aloft to perhaps a roof position (point of interest) that doesn't exist yet. Drones obviously, but haven't tried that yet.

    Anyone got any bright ideas, remote hardware etc, to elegantly acheive this? My efforts so far v Heath Robinson!
    • CommentTimeSep 2nd 2017
    I think there are tools in Google Earth for measuring elevation.
    This is not the one I remember, but may do the job:
    • CommentTimeSep 3rd 2017
    Thanks ST. That one gives the longitudinal profile along a path - maybe there's a tool for the circular horizon profile from a fixed point.

    Unfortunately map data, let along Google Earth, is no good for this purpose. It really has to be measured direct from the actual point of interest. It's completely sensitive to tiny local variations, like a building or a tree, tho I suppose map data/Google Earth could be reliable for a more distant land-formed horizon - but even then, whether it's bare land or tall-wooded makes all the difference.

    Just moving the point of interest a couple of metres can make a massive distance to the patch of clear sky it 'sees', where that is (as usual) defined by nearby buildings/trees.
    Posted By: fostertom
    Anyone got any bright ideas, remote hardware etc, to elegantly acheive this? My efforts so far v Heath Robinson!

    I prepared our custom horizon using a simple home made clinometer, but maybe that counts as Heath Robinson. ;)
    • CommentTimeSep 3rd 2017
    I bet I'm more Heath Robinson than you!
    I really meant doing it photographically.
    • CommentTimeSep 4th 2017 edited
    Posted By: skyewrightI prepared our custom horizon using a simple home made clinometer
    Did it significantly improve PVGIS's estimate?
    Posted By: SteamyTea
    Posted By: skyewrightI prepared our custom horizon using a simple home made clinometer
    Did it significantly improve PVGIS's estimate?

    At a daily level it much better replicated when we 'loose' the sun in the early evening behind the ground that rises steeply to our NW, However, because of the time of day that happens it didn't make a big difference to the annual figures.
    Since our horizon is less than 5 degrees from E to W we probably aren't a tough test case...
    Posted By: fostertomI bet I'm more Heath Robinson than you!
    I really meant doing it photographically.

    I'm sure I've seen pictures of a device that looked a bit like one of those old '3-d viewers' but instead of having a slot for a stereo pair of images it had a properly curved inclination grid so you could look through it 'see' the angles in the view.
    I can't recall where I saw that. It might possibly have been in something on or linked from www.builditsolar.com
    • CommentTimeSep 4th 2017
    Posted By: SteamyTeaDid it significantly improve PVGIS's estimate?
    It wouldn't 'improve' PVGIS's figures, only reduce them for when the sun is blocked. As skyewright says, would make v little difference to annual figures, which are totally dominated by when the sun is high in the sky.

    But for deep winter, making use of what little there is (plenty, with a low southern horizon and suitable 10-day or so storage, for a not-quite-PH's space heating demand - or a 'done the best we can' refurb), all at low elevation, that shading is critical, maybe affecting the siting of collectors, if not ruling it out.

    Even then any shading below 5-7o elevation doesn't matter because there's so very little power in the sun at such low elevation (in any season). Above that, well worth it.
    • CommentAuthorJeremy S
    • CommentTime1 day ago edited
    I did an analysis of the effect of the hill to the SE of us on available winter insolation w.r.t. orientation of the house (we were needing convince planners that 'matching the granularity' of nearby buildings, rather than facing S, would affect our solar gain - which it did by about 15%). I used free GIS software (QGIS with GRASS). Part of the process required generation of a 'skyline' from topographic data - a digital terrain model (DTM). I refined the output using PVGISv4

    If anyone's interested I could dig out the files and any notes I made to create a 'how to', but.. I don't think this forum is the place to post it (or at least not solely), as it would be better in a wiki (as would 80% of the content here, IMO).
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