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Renewable Energy: Flow rate calcs for solar collectors

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  1.  
    • CommentAuthorTim2
    • CommentTimeApr 2nd 2010
     
    My recently installed solar panels are making a racket today which may be a signs that the fluid is at or near boiling point even though the reported collector temperature is 60 to 70C. I would like to confirm that the flow rate is sufficient please.

    Panels are: Two of Ritter CPC18 connected in series
    Resol controller
    Gledhill Torrent 350L store
    24metres of 15mm pipe
    Wilo GOLD RS50 pump set on medium but altering the setting makes little difference to the flow rate
    Flow rate is reported at about 5 (litres per second I assume)
    Pressure is 2.5 to 3 bar

    Is there a rule of thumb calculation or even a table which specifies required flow rates please?

    Thanks

    Tim
    • CommentAuthorTim2
    • CommentTimeApr 2nd 2010
     
    Uh, no, that needs a correction. Just a very little thought suggests that 5 litres per second would be quite a lot through a 15mm pipe. So:

    Flow rate is reported at about 5 (litres per minute I assume)

    Tim
    • CommentAuthorCWatters
    • CommentTimeApr 2nd 2010
     
    I guess you could try calculating the maximium amount of energy that could be collected, assume it all goes into the water. That energy must match that extracted by the water...

    Energy extracted = 4186 x flow rate (in L/S) x Temperature rise (in C).

    The temperature rise is measured from input to output.

    Rearrange to give you the flow rate needed. That would be an over estimate for the maximium flow rate required to extract the energy. Solar panels aren't 100% efficient so they won't collect it all. If you operate at that flow rate the temperature rise would be lower.

    It's more complicated than that if the pump is temperature controlled. Basically the pump will switch on and off so that the average flow rate will match that calculated.
    •  
      CommentAuthorSteamyTea
    • CommentTimeApr 3rd 2010 edited
     
    How hot is the water in the store getting?

    Edit
    Just looked up your collector and it states:

    "Ritter CPC 18 OEM evacuated tub collector

    The evacuated tube collector CPC w OEM is a specialist system for efficient domestic water heating. It was developed especially for hot water production in one-family-houses. It can be used as a stand-alone collector which cannot be linked with additional modules. At 3.2 and 4.0 m2 aperture area, the modules are a manageable size and very efficient, and can be easily and quickly mounted."

    It is the "cannot be linked with additional modules" that I find interesting.
    • CommentAuthorCWatters
    • CommentTimeApr 3rd 2010
     
    I read it differently. Perhaps they mean it provides enough power on it's own so it's ideal for use where multiple modules can't be used.
    •  
      CommentAuthorSteamyTea
    • CommentTimeApr 3rd 2010
     
    Colin

    You have a point there and I cannot see a reason why they can't be in series
    • CommentAuthorwookey
    • CommentTimeApr 5th 2010
     
    This thread explains how to do solar flow rate calcs for a given pump: http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=5361
  8.  
    This is my first post, and not purely about flow rate, so re-direction welcomed ... has anyone tried, or heard about, using a hidden solar collector of 10mm copper run behind slates as a low-tech way of storing heat in the ground?
    If so I'd appreciate any information on the volume/depth/insulation required for the store to make this work.
    (The roof slope in question will be 15 m2, (100 metres of copper), SW facing & 36 degree slope from horizontal.)

    The other issue with this, (which seems to have been discussed at length elsewhere but I'm still not quite clear yet) is whether a ventilation gap is necessary if the roof is constructed with Thermofleece packed without voids between "I Beam" rafters. I think there has to be an internal VCL ...?
    If no gap is needed it will be easier to create a more "boxed in" pipe run for improved heat pick up. I won't put details here now but if anyone wants to know more about the options I'm weighing up, in order to give better advice then please say!
    Many thanks, Mike.
  9.  
    Mike, see this thread
    http://www.greenbuildingforum.co.uk/forum114/comments.php?DiscussionID=921

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