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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

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    • CommentAuthorjamesingram
    • CommentTimeOct 29th 2017 edited
    Thats interesting steamy, it say up 30% rather than 30% under sizing being optimal though, I might update my sma inverter sizing software and see what it says ( not done it in a while)
    I know on my 4kW SMA inverter (4.32kW array) I've got has a setting, that i set, that is suppose to deal with this issue of low input outside best efficiency zone via some algorithm and increase early/late exposure generation. But this has to be set up via installer software so I'd imagine it's not often done
    • CommentTimeOct 29th 2017 edited
    You still have your SMA installer code don't you, so you can play about. Sometimes I wish I was still designing these systems but other times not.

    I do seem to remember reading somewhere that the system size is now determined by the module size, not the inverter size. Not sure if that was for FiTs only though or for the DNO.
    • CommentAuthorjamesingram
    • CommentTimeOct 29th 2017 edited
    If I can remember it!
    Re system considered size Its for FITS as people were doing what I did with regard to oversizing array but matching the inverter to the 4kW FITs limit for the banding. They clarified the definitions to stop it.
    Which is a shame because it result in lots of poorly design systems being fitted in terms of maximising inverter efficiency.
    I've always undersized the inverter 10-20% but was surprised by the suggestion of 5kW on a 3.6kW inverter (I do think easy of installation with regard to paperwork for the installer may be a factor here)
    the 16A DNO limit has always been a separate issue.

    Been doing some rooting around and found this
    Another recent article suggested a range of 1.15-1.3
    And also this from a UK university study from 2008 that suggests 10-40%
    "Conclusion The effects of time resolutions and inverter dynamic characteristics on optimum inverter sizing have been investigated in this paper and validated by simulation. Up to 3% inverter efficiency change as a funtion of voltage which needs to be considered in inverter sizing. The environmental data with time resolution higher than 10 minutes is recommended since low frequency data cannot guarantee accurate optimal sizing of inverter. The recommended guidelines for optimum inverter sizing is over-sizing the PV array 10% to 40% with respect to the inverter nominal power. This is different from the 130% rating given by the current guidelines specific to this inverter type. However, suboptimal orientation will require increased size of PDC,STC/Pinv,rated, which in extreme cases can be as much as 170% (north facing system). Fortunately, the losses due to suboptimal sizing are not too significant in the case investigated, as it is shown the efficiency for the given inverter are between 88.5% and 92.5% on an annual basis for all orientation"
    As to fiddling with my own inverter it a sma 4kW rather than one limited to 3.68 I can adjust the max output so could set the cut off below 4kW to whatever I want via the installer software, be interesting to set it to say 3kW or less over the winter and see what difference it made , trouble is there are so many variables it would be hard to isolate a clear improvement if the was one.
    Also as it would be a software rather than hardware output reduction in wonder if it would have the same effect on the efficiency curve?
    • CommentTimeOct 29th 2017
    Is it an old 4000TL, leave it alone, they are reliable on standard UK settings.
    • CommentAuthorjamesingram
    • CommentTimeOct 29th 2017 edited
    yes its a 4000TL set at 4kW with 'optitrac global peak' set ,still not sure what it does but reading up on it at the time it seemed the best setting to have it on.
    theres at lot on these inverters that isn't well advertised by the manufacturers such as the TLs having a inbuilt multifunction relay you can set to specific outputs.
    looking at the efficiency curve on the spec its pretty much flat until <0.2 rated power which seems pretty good.
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