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Fundamentals: Ground source heat pumps

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1 to 17 of 17

  1.  
    • CommentAuthortony
    • CommentTimeDec 2nd 2021
     
    Some friends of mine are describing ground source heat pumps as being powered by geothermal energy.

    For me they are labouring under a misapprehension! The energy that they use is solar energy.

    As heat is extracted from the ground, the ground cools and then solar energy gradually tries to bring the temperature back to where it was before. Extraction can slightly alter the equilibrium temperature in extreme cases.

    Geothermal energy is heat from hot spring, lava, volcanic activity, mud pools, geysers etc.
    • CommentAuthorSimonD
    • CommentTimeDec 2nd 2021
     
    It's funny as I was once told off for calling ground source heat pumps geothermal, yet the British Geological Survey says any heat derived from the ground, whether solar or otherwise is geothermal:

    https://www.bgs.ac.uk/geology-projects/geothermal-energy/
  3.  
    Posted By: SimonDthe British Geological Survey says any heat derived from the ground, whether solar or otherwise is geotherma

    Come on SimonD that is a very shallow reading of the BGS web page - Second para: "Low-grade heat stored in the shallow subsurface (<200 m) is largely derived from solar radiation that is absorbed by the ground".
    • CommentAuthortony
    • CommentTimeDec 3rd 2021
     
    ðŸ‘Â. Largely is an ocd reference to the fact that a tiny tiny amount of heat from radioactive decay and heat losses from rotational friction in the magma escapes all the time but this is of the order of parts per million.
    • CommentAuthorJonti
    • CommentTimeDec 3rd 2021
     
    but is it not a circular argument? When my wood burner is working does the heat come from the burner, the wood, the soil and solar energy the tree used to grow. At what point do you decide is the true source and is it really only a label anyway :confused:
    • CommentAuthorSimonD
    • CommentTimeDec 3rd 2021 edited
     
    Posted By: John Walsh
    Posted By: SimonDthe British Geological Survey says any heat derived from the ground, whether solar or otherwise is geotherma

    Come on SimonD that is a very shallow reading of the BGS web page - Second para: "Low-grade heat stored in the shallow subsurface (<200 m) is largely derived from solar radiation that is absorbed by the ground".


    Seriously John? :wink: They still define it as geothermal..

    Another elearning source on geothermal energy says:

    "‘Geothermal energy’ is often used nowadays, however, to indicate that part of the Earth's heat that
    can, or could, be recovered and exploited by man, and it is in this sense that we will use
    the term from now on."

    So while it might once have more technically meant the heat contained within the earth there seems to have been scope creep since then. I don't know, blame it on populism. You know where I stand :bigsmile:

    Posted By: Jontibut is it not a circular argument? When my wood burner is working does the heat come from the burner, the wood, the soil and solar energy the tree used to grow. At what point do you decide is the true source and is it really only a label anywayhttp:///newforum/extensions/Vanillacons/smilies/standard/confused.gif" alt=":confused:" title=":confused:" >


    Indeed :smile:
  7.  
    Mmm, if you took that line far enough then you'd be thinking that coal is also solar energy...

    Guess the point isn't where the heat originally came from - it's whether/how it can be renewed as fast as we use it.

    The ground 200m deep was originally warmed by the geothermal gradient. It is much nearer to the sunshine end of that gradient than the magma end. But if we extract that heat at a useful rate, the gradient is not steep enough to replace it in most places, from either the sun or the Earth's core. So that resource is not renewable, even if it is plentiful. Whereas the ground 1m deep can be re-warmed by the sun at a sensible rate.

    PS don't forget that the moon and sun's gravity flex the earth's surface a teeny tiny bit as it spins, which ends up as a little bit of heat. So I vote that a GSHP is a tidal energy collector...
    • CommentAuthorEd Davies
    • CommentTimeDec 3rd 2021
     
    Posted By: tonyLargely is an ocd reference to the fact that a tiny tiny amount of heat from radioactive decay and heat losses from rotational friction in the magma escapes all the time but this is of the order of parts per million.
    Closer to parts per thousand: averaged worldwide the heat from the core of the Earth is around 0.1 W/m² whereas solar input is of the order of 100 W/m².
    • CommentAuthortony
    • CommentTimeDec 3rd 2021
     
    Thx,
    •  
      CommentAuthordjh
    • CommentTimeDec 3rd 2021
     
    Posted By: WillInAberdeenThe ground 200m deep was originally warmed by the geothermal gradient. It is much nearer to the sunshine end of that gradient than the magma end. But if we extract that heat at a useful rate, the gradient is not steep enough to replace it in most places, from either the sun or the Earth's core. So that resource is not renewable, even if it is plentiful.

    There are a couple of ways to make it renewable though. Where I used to work there was a GSHP with boreholes. They used to cool the building in the summer and dump the heat in the boreholes, and then use it to heat the building in winter. Another approach is to use heat transferred by underground aquifers, which the explanations at the BGS site suggest is more common than I thought.

    PS don't forget that the moon and sun's gravity flex the earth's surface a teeny tiny bit as it spins, which ends up as a little bit of heat. So I vote that a GSHP is a tidal energy collector...
    :bigsmile: :clap: :clap:
    • CommentAuthorowlman
    • CommentTimeDec 3rd 2021 edited
     
    Posted By: djh

    PS don't forget that the moon and sun's gravity flex the earth's surface a teeny tiny bit as it spins, which ends up as a little bit of heat. So I vote that a GSHP is a tidal energy collector...
    http:///newforum/extensions/Vanillacons/smilies/standard/bigsmile.gif" alt=":bigsmile:" title=":bigsmile:" >http:///newforum/extensions/Vanillacons/smilies/happy/clap.gif" alt=":clap:" title=":clap:" >http:///newforum/extensions/Vanillacons/smilies/happy/clap.gif" alt=":clap:" title=":clap:" >




    Or;- Because of seafloor spreading, and Continental drift you could add "Friction heating " into the mix.

    :bigsmile::wink:
    • CommentAuthorEd Davies
    • CommentTimeDec 3rd 2021
     
    Posted By: owlmanOr;- Because of seafloor spreading, and Continental drift you could add "Friction heating " into the mix.
    Yeah, but that's driven by geothermal heat (in the narrower sense) anyway. :tongue:
  13.  
    There's also friction from the Earth's crust flexing as it rebounds from shedding the weight of the last ice age. So a GSHP is also a kind of hydropower collector.

    And DJH was storing up heat from office workers - that's bioenergy.

    Any more heat sources for a GSHP?


    From the BGS link, the typical geothermal gradient under the UK is ~27degC per km. With typical thermal conductivity of different rocks ~1W/mK, that gives a typical geothermal heat flow of ~0.03 W/m² under the UK. I don't doubt Ed’s figure of 0.1 W/m² for the Earth overall, but that shows that most of the geothermal heat flows are concentrated in volcanoes geysers etc like Tony said.*

    For comparison GSHP ground loops are sized for about 10 W per each m² of lawn I think. (That might be peak power rather than annual-average though, anyone know?)

    *Edit: came across a map showing heat flows of 0.03-0.06 W/m² across most of the UK but with a hot spot of 0.12 W/m² at the very tip of Cornwall.
  14.  
    Just 50km to the west of us there is a thermal lake which is about 4.4ha surface area and the spring feeding this lake comes in at 41 deg C at a flow rate of just over 400 lts / sec. (enough to change the water in the lake every 3 days) This IMO is geothermal heat.

    The heat gained by putting a slinky under the lawn is at best an inter-seasonal thermal store - just IMO
    • CommentAuthorwookey
    • CommentTimeJan 20th 2023
     
    As a caver, that 27C/km depth is quite hard to believe. That implies that a 200m deep cave (several of those in Yorkshire) would be 5C hotter at the bottom than the top. They aren't. The temp is just the same (or very close, within 1C). I guess this is because there is often a streamway involved and that swamps any other temp effects. Or maybe just the open air passage is enough to hide the 'warming rocks' signal.

    I don't know if the Austria average geothermal gradient is similar but the caves are much deeper and drier. Go down 600m or 800m there and there is still no temp difference to speak of. And there isn't much water moving heat either. Maybe 1C, but no more. Certainly not 5C or the 15-20C this number suggests. Camping at -650m the air temp is 3C. The air temp near the surface (out of the immediate entrance zone, influenced by current air temps) is 2C.

    The effect is observable in the world's deepest caves (Georgia, Caucasus) at 2km deep, where it is maybe 2-3C hotter at the bottom than the top (but not 50C).
    •  
      CommentAuthordjh
    • CommentTimeJan 20th 2023 edited
     
    I just looked at the BGS site and see that their claim appears to be wrong. I typed Busby 2014 into google and the first hit is https://nora.nerc.ac.uk/id/eprint/504718/ and that is specifically discussing "Deep onshore Mesozoic basins have favourable geothermal aquifers at depth comprising basal Permo-Triassic sandstones." The 27 °C/km value is given for the Cheshire basin and it says "These high temperatures only occur over a few square kilometers, but temperatures in excess of 50 °C are found over large areas creating a large geothermal resource."

    So I'm with Wookey :bigsmile:
  17.  
    The Eden Project geothermal well is 4.9km deep and produces 180â°C, so that's 35â°C/km.

    So indeed there are better sites, and worse sites.... guess there won't be so many geothermal projects in Yorkshire limestone!

    I missed it at the time, but apparently while testing the fracturing of the rock in the Eden Project geothermal well, they caused a very minor earthquake, leading to unfavorable comparisons with fracking gas wells. They seem to be treading carefully to avoid the technology getting a bad reputation.
    https://www.cornwalllive.com/news/cornwall-news/geothermal-earthquake-shakes-homes-near-6780130

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