Monday, October 28, 2013

Enquiry about putting thermal energy UNDER the house

28 Oct 2013: David R writes: "I read with interest your blog posts as we are currently attempting to build a code 5 / 6 property in Northumberland, albeit somewhat remotely as we currently live in sunnier climes!
    I have been intrigued for a long time about the use of soil for inter-seasonal heat storage and followed the Viking House iterations in Ireland with interest. Your own endeavours with solar recharging of the boreholes seems to be a similar concept.
    As we are soon to break ground we really have to decide on the most appropriate (and affordable) technology and I am still looking along the lines of a below house thermal store. I saw you posted a link to the DHSS site and I have had some communication with them about their product, but they seem to be suggesting a huge amount of solar thermal tubes to dump heat below the building structure. We are already committed to a Viking House style passive slab for the foundation with 300mm of EPS insulation, so we could easily look at installing a thermal soil store beneath the foundations but my own grasp of the physics involved isn't good enough for me to work out if this would actually work in reality and whether the house footprint would be sufficient for a GSHP in this recharging scenario.
     I wondered if you had any thoughts on this DHSS type of set-up?"

David N-C replied same day: Thanks for reminding me of the Viking House. Very impressive. My project started with an existing house, so could not bury energy literally under the house and the idea started with thinking of a Future house under which we would do what you are thinking of :- putting solar energy under the house with a very well insulated floor slab to contain it. A few problems with that:
  • If you make the store insulated, its as expensive as building yourself an earth filled swimming pool (without the pleasure) and couldn't be big enough to meet the needs of a whole house. it would be well freezing by the end of Feb!
  • The house could need a volume of solid soil larger than the house itself if you have it enclosed with insulation.If it is smaller, you need to build up to a higher temperature, and cool down to a lower.
  • House could be designed to span structurally across the slab area so that shrinkage or swelling are not a problem, but this is expensive.
  • You must never allow the store to swell with frost heave. When the store reaches below 4degs, it might be moving to a point where it expands after it gets cooler - lifting the house! GSHPs can go below 4degs. You should avoid putting so much heat into it so that it becomes like a hard rock with large shrinkage cracks which tear your pipes apart and require abandonment when leaks occur - or which cause house to settle.
  • If it is uninsulated (like a borehole), this is safer, as the store has 'infinite' size and pulls energy in from outside in the cold spring months to stop it freezing. It is safer against overheating as the outside mass will stabilise it by allowing surplus heat to leak out.
  • For those reasons, we reason that vertical boreholes... comparatively easy and quick to drill... NEXT to the house.... are the best option, provided the soil below is right. Infinite size, no risk to the foundations, minimal seasonal heat loss, minimum cost, minimum land. Four shallow ones are better than one deep one if you include solar charging.


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