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Green Building Bible, Fourth Edition
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Renewable Energy: Solar decision and other technical questions

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    I wonder if the knowledgeable folk on here can help.

    I live in a listed building and so I can't put solar panels on the south facing roof of the house. I can however put panels onto an outbuilding at the rear of the house, but its roof is east west facing. The roof is also shaded by the house in the winter months when the sun is low, I guess from November to Feb. I have a heatbank connected to mains gas and to a multi-fuel stove. The heatbank has an unused solar coil installed.

    The questions are:

    1. Am I better to install solar water heating connected to the solar coil (with a buried solar duo pipe between the house and the outbuilding, or better to install solar PV with a buried cable between the house and outbuilding?

    My thoughts are that solar thermal is a much more efficient way to heat water, but PV gives me a wider range of uses and I can export unused electricity to the heatbank via an immersion heater (there is an unused immersion heater boss low down on the heatbank that I could use - separate from a higher immersion heater connected to mains).

    2. The west facing roof of the outbuilding receives sun for about an hour longer, so should I go for panels on both sides of the roof or just on the Western side?

    3. If I opt for solar water heating, does anyone know if I would have problems with heat loss by burying the solar duo pipes underground between the house and outbuilding? I measure the run of the pipe to be 20 meters underground.
    • CommentAuthorEd Davies
    • CommentTimeMar 7th 2013
     
    Posted By: Pile-o-Stonesolar PIR
    PV?

    Posted By: Pile-o-StoneMy thoughts are that solar thermal is a much more efficient way to heat water
    Traditionally, yes, that has been the thinking. However, with the current fairly low cost of PV panels it's a lot less clear-cut. For relatively low temperatures and/or bright sunshine solar thermal still wins but in the trickier circumstances of high temperatures and less than ideal sunshine PV can be better. Essentially, this is because the losses from PV don't increase with temperature. Your combination of east/west and long runs might also tip the balance in favour of PV.

    There are various boxes which can match the power fed into an immersion to excess of PV generation over other power uses in the house (Immersun is one of them). What I don't know is if/how they work with the PV and inverter in a separate building from the tank. Would be interested to know as some friends are considering PV on a separate E/W garage roof.
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      CommentAuthorfostertom
    • CommentTimeMar 7th 2013
     
    Very concisely put Ed - thanks.
  4.  
    Posted By: Pile-o-StoneMy thoughts are that solar thermal is a much more efficient way to heat water
    Traditionally, yes, that has been the thinking. However, with the current fairly low cost of PV panels it's a lot less clear-cut. For relatively low temperatures and/or bright sunshine solar thermal still wins but in the trickier circumstances of high temperatures and less than ideal sunshine PV can be better. Essentially, this is because the losses from PV don't increase with temperature. Your combination of east/west and long runs might also tip the balance in favour of PV.

    There are various boxes which can match the power fed into an immersion to excess of PV generation over other power uses in the house (Immersun is one of them). What I don't know is if/how they work with the PV and inverter in a separate building from the tank. Would be interested to know as some friends are considering PV on a separate E/W garage roof.


    Thanks for the above. I was all set for solar thermal until I saw some information on the immerson product and thought what a great idea it was. It's also more appealing to just lay a thick electrical cable between the outbuilding and the house and then have little or no maintenance. The solar heating option seems to have a lot more worries such as dumping unwanted heat, ant-freeze levels, laying expensive pre-insulated pipes, water boiling, leaks, using mains electricity for the pump, whether to go for pressurised or drainback, etc. etc.

    PV it is then. :)
  5.  
    Does anyone know if there is a regulation covering the size/rating of electrical cable that connects solar PV back to the meterbox?

    I'd be looking at armoured cable between the house and outbuilding and while I would more than likely get an installer in for the install, I think I'd rather dig the channel and install the cable myself.
  6.  
    Yes - all wiring must meet BS7671 Wiring Regulations.

    Best get in touch with a qualified electrician, get them to specify the size of the cable (which depends on length of run and maximum current). Strictly speaking, an electrician should not certify cabling that wasn't installed by them, so you need to come to some arrangement, probably for you to dig and lay but leave the trench open for inspection by the electrician before back-filling.
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      CommentAuthorSteamyTea
    • CommentTimeMar 7th 2013 edited
     
    Posted By: Pile-o-StoneDoes anyone know if there is a regulation covering the size/rating of electrical cable that connects solar PV back to the meterbox?
    If you are going to go down the FITs route, and no reason why you should not, then you need to make sure that the cable has less than a 1% volt drop. Probably 6mm^2 cable, but would have to check that for buried armoured cable, cost it at 10mm^2.
    • CommentAuthorSeret
    • CommentTimeMar 7th 2013 edited
     
    Posted By: Pile-o-Stone
    PV it is then. :)


    Whoa there. You'll probably want to run some numbers before you decide to go ahead. You mentioned you've got shading issues. I would suggest the first place to go would be PVGIS:

    http://re.jrc.ec.europa.eu/pvgis/apps4/pvest.php#

    Put in your location, the orientation of panels and if possible include a horizon file (click the help button on that page for more details). This will spit out an estimated amount of energy that the panels will produce. There's no sense forking over four-figure sums if you aren't going to get a useful amount of power out of it.

    As for running cable, it would be the AC cabling that you'd be running, not the DC. So the rules are simply those for running a normal 230V AC domestic supply to your outbuilding.
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      CommentAuthorted
    • CommentTimeMar 7th 2013
     
    When you say you cannot put panels on the main roof - have you asked and been refused?

    You would still need planning permission for PV on an out-building which is within the curtilage of a listed building.

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