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Fundamentals: Warm and fuzzy on Geothermal

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    <blockquote><cite>Posted By: Seret</cite>Certainly the performance of the borehole is dominated by geological properties,</blockquote>

    Definitely - that's why the professionals use loop-design software to calculate the appropriate depth/length of borehole for the heat extraction requirements. A "rule of thumb" used over here is "150 feet per ton of capacity". That translates to about 76W per meter.

    See http://198.103.48.154/eng/software_tools/gs2000.html for loop calculation software.

    Paul in Montreal.
    • CommentAuthorSeret
    • CommentTimeJan 23rd 2012 edited
     
    Well, I wasn't terribly happy with the specificity of anything I found, but I thought I'd post up some of the more interesting bits regarding the geothermal/solar thing, and then I'll shut up:


    GSHP systems rely on the fact that, under normal geothermal gradients of about 0.5oF/100 ft (30 oC/km) (Grant et al., 1982), the earth temperature is roughly constant in a zone extending from about 20 ft (6.1 m) deep to about 150 ft (45.7 m) deep (Hart and Couvillion, 1986). This constant temperature interval within the earth is the result of a complex interaction of heat fluxes from above (the sun and the atmosphere) and from
    below (the earth interior). As a result, the temperature of this interval within the earth is
    approximately equal to the average annual air temperature (Hart and Couvillion, 1986).
    Above this zone (less than about 20 feet (6.1 m) deep), the earth temperature is a damped
    version of the air temperature at the earth’s surface. Below this zone (greater than about
    150 ft (45.7 m) deep), the earth temperature begins to rise according to the natural
    geothermal gradient.

    - Chiasson "Advances in Modelling of Ground-Source Heat Pump Systems" 1999

    Below the so-called ‘neutral zone’ (some −20 m depth in Canada), the temperature field is governed by non-diurnal and non-seasonal changes. Temperatures below this depth are linked to surface climatic forcing and deep terrestrial heat flow. The energy supply for a heat exchanger can be tapped from the vertical geothermal heat flux, the horizontal conduction of heat, advective transport with groundwater (if present), and heat exchange between the ground surface and the atmosphere.

    - Majorowicz et al "Estimation of Shallow Geothermal Energy Resource in Canada" 2008

    So it seems that there is a mixture of sources for the heat, but that certainly below about 45m, radiogenic geothermal heat is significant.

    Posted By: tony99.95% solar generally with about 0.05% or less radioactive decay heat from the core


    I've not seen any numbers, but this (talking about depths of 10-100m) is interesting:

    Another factor related to thermal conductivity is the presence of overburden such as sand, clay and moraine. Such materials typically have thermal conductivities that are far lower than most rock types (Midttømme 1997), thereby acting as low-conductivity thermal blankets that will increase the subsurface temperature.

    - Slagstad et al "Factors influencing shallow (< 1000 m depth) temperatures and their significance for extraction of ground-source heat" 2008

    To my mind, if a layer of thermally insulating overburden will increase subsurface temperature, then at least 50% of the heat must be coming from somewhere other than overhead.
    • CommentAuthorEd Davies
    • CommentTimeJan 23rd 2012
     
    Posted By: SeretTo my mind, if a layer of thermally insulating overburden willincreasesubsurface temperature, then at least 50% of the heat must be coming from somewhere other than overhead.


    It would indicate that some heat is coming from below but I don't see any reason why it has to be 50%. Even a quite small percentage of heat from below will raise the temperature somewhat.
    • CommentAuthorSeret
    • CommentTimeJan 24th 2012
     
    Posted By: Ed Davies

    It would indicate thatsomeheat is coming from below but I don't see any reason why it has to be 50%. Even a quite small percentage of heat from below will raise the temperature somewhat.


    Because an insulating layer will inhibit heat flow in both directions. If more than 50% of the energy was coming from above the layer it would cause the temperature below it to fall, not rise. I presume not all the heat below the layer is necessarily coming from below though, there would be lateral heat transfer as well, and probably much more complex effects that a geologist could bore us all rigid with.
    •  
      CommentAuthorSteamyTea
    • CommentTimeJan 24th 2012
     
    When the temperature caused by the geothermal equals the temperature caused by the solar then there is no change in energy. This can be worked out from the density and heat capacity (note not specific heat capacity because of pressure). Below whatever that depth is, at that location, is geothermal. The physics is simple, what throws it all is the 'messiness' of geology. If there is an underground river you have to start taking into account the catchment area of the water, different rocks also have different heat capacities (especially true in the UK). And the most important point to my way of thinking is that if geothermal was so viable, it would be happening on a very large scale. I did read a paper about using the heat stored in natural gas to heat buildings. We currently have to compress, remove condensates and cool natural gas before we pipe it places. Not an argument to carry in burning, but better than not doing it for this transition fuel.
    • CommentAuthorEd Davies
    • CommentTimeJan 24th 2012
     
    Posted By: SeretBecause an insulating layer will inhibit heat flow in both directions. If more than 50% of the energy was coming from above the layer it would cause the temperature below it to fall, not rise.


    OK, if there's extraction of heat happening. It's not clear what the context of the Slagstad et al quote is: with heat extraction happening or with the only heat flows involved being the small geothermal flow upwards and the seasonal flows up and down. If it's only the natural flows then any insulating layer on top will tend to warm the ground below even if the geothermal flow is very small.

    Agreed, though, that if the presence of an insulating layer causes the ground to warm when there's significant heat extraction happening then a large proportion of the heat must be coming from below and around.
  7.  
    The heat under my lawn that my bores collect comes from the MOLES that are busy little B######S. It has nothing to do with solar or geothermal in my little mind and all to do with my little friends........... now to find the fork! :cry::devil::cry::devil::cry::devil::angry::angry::angry:

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