Wednesday, December 22, 2010

Further thoughts on ground loop

22 Dec '10: Some way back, there was a discussion of Ground loop, Horizontal, defrosting the ground.

If I had a garden or estate big enough for a long horizontal ground loop, I now know how I would do it.

Solar earth charging really is working for me, it is producing wonderfully consistent performance from the GSHP. But we all recognise that a horizontal loop would lose too much heat up to the atmosphere during the winter to hold any summer charge.
    If I had my chance to lay a horizontal ground loop, the first rule is to lay it as deep and as long as possible - that is agreed by all. But now with charging in mind, I would also lay, buried at a depth of about a meter below the final top surface and a meter above the slinkies, a slab of 100mm polystyrene along the length, using 1.2 x 2.4m rectangular slabs. This is so that sunbox injected solar heat would build up below and take a longer time to escape. It doesn't need to be thicker as the delta-T between charged and uncharged ground is not large. The lower surface would have to be very well levelled, or the foam would break up when the upper soil was rolled back and compacted.
    The greater solar heat that is the basis for all ground source heat pumps would easily rise from below the foam once the winter is set in and the region immediately under the foam is chilling. For a shallow horizontal ground loop, the chilling from the winter atmosphere above is much faster than that from the house - so Insulate!
   A similar requirement applies to Energy Foundations. If one is heating the ground below the slab, I would advocate putting a 'trenchfill' of foam around the perimeter - heat can build up below the slab and heat loss sideways would be reduced. There is no chance of it getting direct solar heat except that which is systematically injected. But when the injected heat is exhausted, the volume below the slab draws heat from the wider volume beyond. If the sizing of solar panels is well calculated, there is less risk of it being exhausted.

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