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    I am trying to work out the best way to get my 10% renewables target for a new build project that we are doing. At the moment I have decided that I would like to try and use solar panels to achieve it but am coming up against very differing information. Basically I need to give our local council the amount of kWh per year per M2 output and I can't seem to find this figure.

    Our local council refers to an Energy Savings Trust document which "allows" in its % renewables calculations an output of 300 kWh per m2 pa. The Solar Trade Assocation gives some information on its website that our area of the country has around 1200 kWh available per year and solar panels are about 40-50% efficient and thus the output would be up to 600 kWh. The Renewable Energy Assocation says "A plane inclined at about 30 degrees, facing due south ranges from around 900 KWh/m2 per year in the North of Scotland to around 1,250 kWh/m2 in the South West of England".

    Turning to the manufacturers brochures - one in particular that I am considering claims that their panels are 95% efficient - If I could just multiply this by the available solar irradation then I will be home and dry with my calculations! But can someone please help me make sense of all these differing claims!

    • CommentAuthorDantenz
    • CommentTimeJul 28th 2008
    you can't be that exact when specifying the theoretical annual energy contribution that solar makes as you cannot guarantee how much solar gain there will be. Keep it simple and work on the basis that solar hot water heating will provide 50% of your annual hot water energy requirement which, for an average family of 4, is around 3,500 k/Wh. Solar therefore provides 1,750 k/Wh annually. Take 2 flat plate collectors of total absorber surface of 4m2 gives an output of approx. 440 k/Wh per m2. Ignore the quoted maximum efficiency ratings of solar panels as these figures only relate to "zero loss efficiency" ie when the panel fluid temperature is the same as ambient. As panels start to warm up the efficiency drops dramatically the hotter they get, an average efficiency of around 40-50% is more the norm.
    Dantenz - I can see what you mean - especially with the summer we've had so far. But I need to give a definitive figure to satisfy the local planning dept before they'll clear off my condition on my planning persmission for the new-build. The trouble is they use the EST document and as the solar panels may only just give me my 10% target, I am trying to clarify the situation and was hoping that the more favourable figures from the Solar trade assos and the renewable energy assocation would be correct.
    • CommentAuthoronm234
    • CommentTimeJul 30th 2008
    Try out this site is has a listing of some panels. If your panel is on it look in the 'typical solar yields' area
    • CommentAuthordrachir
    • CommentTimeAug 10th 2008
    The best site I have found is the Swiss SPF Institute at www.spf.ch Look at the Collector Tests and you will find a more realistic value for efficiency. They do state the maximum output to be expected from different collectors for IIRC an 'irradiance value' of 1000W/m2. Efficiency is nowhere near manufacturers' values! But -- big but -- it does have to be interpreted for UK use and you will need a value for annual 'insolation' for where you live. That's the Renewable Energy Ass. figures. I don't personally think anyone can predict the real-life output from a collector; the best you can do is try to work out a theoretical output based on test results adjusted for where you live.
    I'm struggling with this as well, since the tests referred to do not replicate the type of system I am installing. The best I can suggest is to take the DHW figures as a basis.
    • CommentAuthortony
    • CommentTimeAug 10th 2008
    The most important thing to think about is that all output is free

    therefore the panel is infinitely efficient -- a genuine case of something for nothing.
    • CommentAuthordrachir
    • CommentTimeAug 10th 2008
    A follow-up comment: after looking around the EST website (not at every page) I can't find anything specific about calculating a renewable energy contribution - it's all exhorting us to reduce consumption and make some contribution. It would be interesting to know just what EST document is being referred to.
    • CommentTimeAug 11th 2008
    Posted By: tonytherefore the panel is infinitely efficient -- a genuine case of something for nothing.

    Tony, that's abject nonsense. The cost of the panel is not zero so the output is not free. Plus you're confusing efficiency with energy return on investment.

    Paul in Montreal.
    • CommentAuthorGBP-Keith
    • CommentTimeAug 11th 2008 edited
    Work on about 980kWh per annum from 1 square metre of PVs for the UK and if a company is claiming 95% efficiency of their PVs then don't buy from them.
    • CommentAuthorEd Davies
    • CommentTimeAug 11th 2008
    Posted By: (GBP) KeithWork on about 980kWh per annum from 1 square metre of PVs for the UK...

    Are you sure? I think that's more plausibly the amount that 1 m² would receive in a year (so multiply by the efficiency of the panels to get the actual output) or, equivalently, a reasonable output for 1 kWp of panels (which would have an area of about 6 m²).
    agu - that's the correct document that our local authority uses.

    Page 17 table 2 gives a figure of 1200 kWh being the annual "contribution" of 4 m2 of panels - ie the 300 kWh per m2 that I was referring to and what our council will let us use unless I find some information to prove otherwise. The annoying thing is that with heat pumps the manufacturers "brag" that their machine had a COP of 4:1 and that isn't necessarily what it will give 24/7, 365 days a year but the council will be happy to use that figure but because such a figure isn't readily available for solar DHW I'm left having to use the lowest figure I can find!

    it seems to me at the moment that the 10% renewable target is designed for heat-pumps or perhaps bio-mass because of the way the calcs are done. I'm not convinced that this is the way to go if you end up having to ignore solar which at least is one of the renewables which requires very little fossil fuel input to run on a daily basis.
    • CommentAuthortony
    • CommentTimeAug 12th 2008
    Solar thermal and solar pv are the way forward, burning stuff is not.
    • CommentAuthordrachir
    • CommentTimeAug 14th 2008
    Thanks agu for the link to the EST document. The Planning Officer I'm working with would have a fit if I wanted to install one of the Thermal collectors shown on the front cover! They stick out too far. I think they are probably Riomay Ecotubes from what I can see.
    I suppose the attraction of GSHP to councils is that a fairly specific output can be calculated for 24/7/365 operation. Wind and Solar are far less predictable, but IMO far better solutions. I shall read the EST doc. with great interest.
    • CommentAuthorBargeman
    • CommentTimeAug 16th 2008

    You will need to have a SAP assessment for the new build at some stage. SAP includes a procedure (Appendix H) for assessing contribution from a solar thermal collector based on its aperture area, zero loss collector efficiency and linear heat loss coefficient, each of which is available from the EN 12975 performance report for a solar collector. By applying the angles of orientation and tilt the expected solar contribution can be estimated. This is the procedure used for Part L1A Building Regulation compliance so it should be acceptable to the planning officer.

    In practice the procedure is rather simplistic, so its main virtue is that it will satisfy building control.

    Self-build Mum,

    I have done alot of research myself into solar:

    Solar HW is a better investment than solar PV.
    Keith refers to insolation for the UK, then you multiply by the efficiency, it seems that 20% is the likely best result for PV and prob about 50% for Hot water. But be careful the solar industry seems to be full of b.s.

    Personally I would consider the money invested in renewables as you are planning would be better invested in better insulation/ more efficient equipment/lighting at present which will deliver more environmental benefit per pound, renewables will come in the next 5 years.

    It would be interesting to see anyone post actual data they have achieved from PV and solar HW.
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