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Heating and cooling: Solar assisted ground source heat pump

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    • CommentAuthorPeter Clark
    • CommentTimeJun 9th 2008 edited
     
    Does anyone know anything about ‘solar assisted’ heat pumps as described on this page?

    http://www.treehouseclapham.org.uk/ecobuilder39.html

    A COP of 6 is mentioned!

    This seems to be related to the AGS idea, use the sun to heat the ground, but then use the heated ground to supply a heat pump. Maybe easier than getting true ‘interseasonal’ storage as in AGS?
    •  
      CommentAuthorfostertom
    • CommentTimeJun 10th 2008 edited
     
    Posted By: Peter Clarkseems to be related to the AGS idea
    Yes, but if it's a sort of cut-down AGS, i.e. without the 'A' (Annualised) bit, i.e. relying on winter solar to deposit heat into the ground only shortly before retrieval, then the point is, how much solar heat is there at that time, when it's needed? If the system does store enough summer solar to bridge that winter shortfall, then it does qualify as
    Posted By: Peter Clarktrue ‘interseasonal’ storage as in AGS
    . By relying on a heat pump for retrieval, much less solar input is needed, whether current or inter-seasonal, because a heat pump is tolerant of low source temp. However, by settling for smaller solar input, you're locking yourself into buying electricity forever, to run the heat pump. True AGS stores plenty enough summer heat for the full winter's requirement, and stores it hot enough for simple conductive/convective retrieval, without need for powered heat pumping.
    • CommentAuthorPeter Clark
    • CommentTimeJun 10th 2008 edited
     
    Hi Tom,

    Yes, its not an AGS idea, I think, its a heat pump idea.

    IF you are going to use a GSHP, then this might be a good thing to do. A COP of 6 seems very good, I was just wondering if anyone had any experience of the reality and costs etc.

    Peter
    •  
      CommentAuthorfostertom
    • CommentTimeJun 10th 2008
     
    AGS isn't cut and dried - a bit of GSHP might play a part, perhaps as insurance against a really bad summer not providing enough input (lots of surplus input capacity is nicer), as a heat pump can forcibly extract heat even when source temp drops below desired supply temp. It could also help to exagerate the coolness of the cool part of the temp wave as it passes slowly through the subsoil, because AGS is just as good as an inter-seasonal store of coolth, as of heat.

    The capital and running costs of a heat pump could be traded against reduced capital cost of a smaller ground-storage mass and less apparatus to get heat into and out of the ground. At one extreme you could pay the capital and running costs of a traditional non-replenishing GSHP installation. At the other extreme you could pay the capital costs of a full-sized AGS system - with negligible running cost. Or you could pay for a half-sized GSHP as part of a smaller AGS system, but I suspect that way you'd end up paying almost twice-over because a half-sized GSHP and a smaller AGS system would cost much more than half the £s of a full-size one of each.

    Nevertheless, I think that GSHP techniques will play a supporting role within future AGS.
    • CommentAuthormbartosik
    • CommentTimeJun 10th 2008
     
    I have PV and GSHP and I have been looking at doing solar assist to GSHP for some time, I've been considering a variety of mechanisms. Being a retrofit, my options to do AGS are limited as it is not very practical to dig under an already built house. I am also looking into extracting heat from behind my solar panels and tiles. I just wish that my heat pump had been sized smaller, because after my insulation up grades it is already short cycling.

    I had thought a variety of ways:
    One way was to use a thermal store, when the thermal store contains enough energy, have heat pump circulate source water from there, either using the compressor or if the thermal store is hot enough circulate directly through water to a heat exchanger in place of the primary heat pump heat exchanger. Then when the heat store is low on energy, switch to the well. This allows the heat store to be replenished, and reduces the dilemma of heat store sizing (too big and temp is too low). When running from the thermal store the COP of the heat pump is higher. If the heat store gets a lot hotter the COP is a lot higher, but for a shorter time, when the heat store is larger the COP is less high but for longer before switching to the traditional GSHP source.

    For the moment I am concentrating on ways to suck heat from under my PV tiles and panels, because I have limited roof space left for solar thermal panels, and cooling the PV will make it more efficient.

    With a ground loop (rather than open loop) using solar thermal panels to dump heat into the ground may work, even better if the ground is protected from rain water draining the heat away (as in AGS).
  6.  
    Peter Clark asks "Does anyone know anything about ‘solar assisted’ heat pumps?".

    The answer can be found at http://www.icax.co.uk/interseasonal_heat_transfer.html

    This is a form of Renewable Heat which captures solar energy in summer, stores it in ThermalBank over the autumn, for use as space heating in winter.

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