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Renewable Energy: PV peak lopping and system oversize

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  1.  
    • CommentAuthormike7
    • CommentTimeJul 31st 2011 edited
     
    Recent threads on pv have me wondering if it would be daft/sensible - if permissible - to fit significantly more panels than would deliver 4kW peak, and rely on the inverter being set to limit the output to the maximum 16A allowed. It would give you better output in less-than-full sun conditions, and also compensate for the decline in performance over time.

    As the price of panels fall they are a smaller proportion of the total cost, so it might be economic.

    Any thoughts?
    •  
      CommentAuthorDamonHD
    • CommentTimeJul 31st 2011
     
    My PV is over 5kWp nominal panel rating but ~4.5kWp inverter max output rating (nearer 4kW continuous).

    You can run the numbers and generally I think it may be wise to slightly 'under rate' the inverters providing you stay within their absolute max ratings.

    Rgds

    Damon
    • CommentAuthortony
    • CommentTimeJul 31st 2011
     
    Panels do net degrade significantly enough over time for that factor to be relevant.
    •  
      CommentAuthorDamonHD
    • CommentTimeJul 31st 2011 edited
     
    It's not primarily a question of the panels degrading IMHO, it's that panels spend relatively little of their time at maximum output, so it can be better to lose a little potential output at the top end and save on inverter and possibly DNO costs for example. When an inverter replacement is needed in 10 years or so it can be matched to the actual output of the panels around that point, but decline of non-thin-film panels seems to be <1%/y in general.

    Rgds

    Damon
    • CommentAuthormike7
    • CommentTimeJul 31st 2011 edited
     
    Posted By: tonyPanels do net degrade significantly enough over time for that factor to be relevant.


    My roof is only 17 degrees off horizontal, so might gather a bit of dirt if in my dotage I don't get up there to give the panels a wash every so often. I suppose that would reduce the output a bit more than the norm. But Damon is right in that the main advantage I was thinking of would be if the panels spend little time at their max rated output.
    •  
      CommentAuthorSteamyTea
    • CommentTimeAug 1st 2011 edited
     
    I have a graph somewhere that shows just how little time light levels go over 1kW/m^2, shall try to dig it out in the next few days, and I live in the 'best solar resource', though just got back from an Up Country trip and it was raining even before I go to the Tamar.

    Found it.
    Draw your own conclusions.
  7.  
    Is that probability based on daylight hours or a 24hr day?
    •  
      CommentAuthorSteamyTea
    • CommentTimeAug 2nd 2011 edited
     
    24 Hour day, and I should really have made it a Histogram, based it on Greater than lower bound and Less Than or Equal To upper bound (had been a long day). So just shift x-axis right by 50 Wm^-2.
    Does show just how little our light levels are high, but when they are they are there for longer because of out latitude.
    • CommentAuthorjamesingram
    • CommentTimeAug 2nd 2011 edited
     
    This is the kWh/m2/month for 1 kW of panels , from PVGIS for my area , london West of M25

    Nominal power of the PV system: 1.0 kW (crystalline silicon)
    Estimated losses due to temperature: 12.0% (using local ambient temperature)
    Estimated loss due to angular reflectance effects: 3.0%
    Other losses (cables, inverter etc.): 14.0%
    Combined PV system losses: 26.6%

    from http://re.jrc.ec.europa.eu/pvgis/apps4/pvest.php#
    •  
      CommentAuthorSteamyTea
    • CommentTimeAug 2nd 2011
     
    What levels do most systems start generating usable power?
  11.  
    Here, once the panels generate enough power to switch on the inverter, the power delivered to grid steadily increases from 0W.
    •  
      CommentAuthorSteamyTea
    • CommentTimeAug 3rd 2011
     
    Lloyd
    Any idea what the incoming W/m^2 is though, I have heard that it is 100, but never seen any data on it.
    •  
      CommentAuthorDamonHD
    • CommentTimeAug 3rd 2011
     
    ST, I seem to get some output at what I take to be about 10W/m^2 or 1% of peak, ie in a heavy rain shower.

    Rgds

    Damon
    • CommentAuthorbillt
    • CommentTimeAug 3rd 2011
     
    If my system is typical they will generate all the time in daylight. Yesterday mine started at 6.20 A.M. with 24W output - 12W psm insolation and stopped at 8.50 P.M. with 12W output and 5W psm insolation. Peak was 3770W at about 1.20.
  15.  
    Posted By: SteamyTeaLloyd
    Any idea what the incoming W/m^2 is though, I have heard that it is 100, but never seen any data on it.

    Are you thinking 100W/m^2 on a horizontal plane, or on the face of the panels?

    FWIW, our 3.9kWp of WSW facing (40 degree slope) PV raised its first LED pulse (one pulse per Wh) on the generation meter at ~0605 this morning. At that time our weather station's pyranometer reckoned that the W/m^2 on a horizontal plane was 14W/m^2.
    •  
      CommentAuthorSteamyTea
    • CommentTimeAug 3rd 2011
     
    Seems to be about 10 Wm^-2 on the horizontal plane then (about 6:15 AM down here this morning).

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