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General: PVGIS puzzle

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      CommentAuthorfostertom
    • CommentTimeOct 18th 2012 edited
     
    Bit puzzled about http://re.jrc.ec.europa.eu/pvgis/apps4/pvest.php# >click PV Estimation

    Almost all the excellent PVGIS output data comes as 'per m2 of collector'
    e.g kWh of incident irradiation per m2 per day
    or W of incident radiation per m2.

    But PV output is given as kWh per day for the proposed system, having previously specified the kWp size of the system. Not related to m2 of collector.
    I can look up polycrystalline collectors and see that they're about 6.25m2 per kWp, so if I specify the size of the proposed system as 1/6.25 = 0.16kWp, that should give PV output as kWh per m2 of collector?

    i suspect a falacy. Is the kWp of a system directly related to m2 of a particular collector make/model? Surely a system's kWp depends on orientation, inclination, location etc?
  2.  
    Theres a useful help section on the site that might answer your question
    http://re.jrc.ec.europa.eu/pvgis/apps4/PVcalchelp_en.html
    • CommentAuthorSeret
    • CommentTimeOct 18th 2012
     
    No the kWp figure is the boiler plate value. Actual output will depend on the details of the installation, but kWp is just the total rating of all panels in the array.
    •  
      CommentAuthorfostertom
    • CommentTimeOct 18th 2012
     
    OK so kWp is only a measure of the collector's performance, nothing to do with installation conditions.

    So is my method good?
    Posted By: fostertomlook up polycrystalline collectors and see that they're about 6.25m2 per kWp, so if I specify the size of the proposed system as 1/6.25 = 0.16kWp, that should give PV output as kWh per m2 of collector
    • CommentAuthorSeret
    • CommentTimeOct 18th 2012 edited
     
    What is it you're trying to work out?
    •  
      CommentAuthorfostertom
    • CommentTimeOct 18th 2012
     
    For an actual site, have uploaded horizon files etc, specified collector azimuth and inclination, so am getting back great figures for solar irradiation incident per m2, and total kWh/m2 incident per day - for the months of Dec and Jan is what I'm interested in (Hence the horizon is critical to low-angle sun path).

    From that we can go into ST and see how much of that we can usefully collect, for space heating a near-PH house, and DHW, minimising wipe-out from simultaneous radiant losses to cold scenery and sky, by lowering the thermofluid temp in the panels - we can afford to take that very low, for space heating a near-PH house.

    At the same time, want to compare that with how much PV electricity we cd generate per m2, to make warm water for space heating and/or DHW, because although PV has lower nom collection efficienct than ST, it's immune to wipe-out from simultaneous radiant losses to cold scenery and sky, in fact does better when cold.

    That's why I want PV output in kWh/m2/day.
    •  
      CommentAuthorSteamyTea
    • CommentTimeOct 18th 2012
     
    Have a look at the bit that Ed did about heating water from PV. It is relevant to what you want.
    What you are up against is that PV in effect can dump all the power to the grid while ST can't. So you have different operating conditions to start with (regardless of other efficiencies and temperatures).
    • CommentAuthorjms452
    • CommentTimeOct 19th 2012
     
    Have you got a link to Ed's thread?

    Does make me think that a big PV array in conjunction with a small heat pump could work well in a well insulated building.

    i.e. 15-20% x4 is better than solar thermal without the summer overheat problem (albeit much more pricy)
    •  
      CommentAuthorfostertom
    • CommentTimeOct 19th 2012
     
    Ed's stuff is great - can't find it on GBF, but on his blog
    http://edavies.me.uk/2012/01/solar-per-area/
    http://edavies.me.uk/2012/01/pv-et-flat/
    http://edavies.me.uk/2011/12/pv-heating/

    Looks like, for the house we're considering, with reasonably optimum collector site, for Dec/Jan
    45kWh/day incident solar on 44m2 abs max
    15kWh/day space heating requirement, with MHRV and after occupancy and solar (thro windows) gains, plus (modest in this case) DHW requirement
    5.7kWh/day PV production from 6.25kWp installation (39m2).

    So PV clearly isn't enough for Dec/Jan space heating in a near-PH house - that's settled.
    Moving on to ST (no not you) - see what we can collect (after simultaneous re-radiation loss) and store for long enough, at the useable min store temps necessary - 30% end-to-end efficiency should do it.
    Useable min store temp is set by use of extg rad system (after house insulation etc vastly oversized). If (as on another project) the heat-input 'radiators' were the entire external wall surface, then useable store min temp cd go really low, like 25C, in which case 30% efficiency shd be easy.
    Of course some part of the storage has to be higher at DHW temp.
    • CommentAuthorEd Davies
    • CommentTimeOct 19th 2012
     
    Posted By: fostertom: “Ed's stuff is great - can't find it on GBF, but on his blog”

    Thanks. I hope the blog posts stand alone but this thread should give some useful context:

    http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=8396
    •  
      CommentAuthorSteamyTea
    • CommentTimeOct 19th 2012
     
    Posted By: jms452work well in a well insulated building

    Anything will :wink:
    •  
      CommentAuthorfostertom
    • CommentTimeOct 19th 2012
     
    What he means is it won't work at all in a non-insulated building - it's a possibility that only kicks in when you're super-insulated.

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