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Heating and cooling: Borehole -- how much heat can he extract from it?

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  1.  
    • CommentAuthortony
    • CommentTimeMar 10th 2010
     
    My mate has a redundant borehole 105m deep that he wants to utilise in conjunction with a ground source pump

    Approximately how much heat will his heat pump be capable of producing?
    • CommentAuthorPikey
    • CommentTimeMar 10th 2010
     
    I have an uncle John who has a borehole connected to a GSHP, he told me the water underground was at 12 degrees - which should make for a very efficient system I would have thought! :smile:
  3.  
    Posted By: tonyApproximately how much heat will his heat pump be capable of producing?


    About 8kW.

    Paul in Montreal.
    • CommentAuthortony
    • CommentTimeMar 10th 2010
     
    Thanks
  5.  
    Calculation based on the "rule of thumb" used over here that says you need 150 feet per ton of capacity. 1 ton is 3413W (12500 BTU/h) and is a measure of cooling and heating capacity used by the air conditioning industry. It depends on the rock type, whether there's water and the ground temperature (to a lesser extent - that more determines the COP as it's the conductivity of the rock that is the limiting factor in actual capacity). Our vertical borehole is sized more aggressively than the 150 feet per ton rule (due to the rock type) but it will at least give you a good ballpark figure.

    Paul in Montreal.
    • CommentAuthormike7
    • CommentTimeMar 10th 2010
     
    Paul - does this rule of thumb depend to any extent on using summer cooling to restore temperature round the borehole?
  7.  
    Posted By: mike7Paul - does this rule of thumb depend to any extent on using summer cooling to restore temperature round the borehole?
    Not as far as I know. Our summer cooling load is less than 1/10 of the winter heating and our borehole temperatures seem to be stable on a year by year basis. I've been monitoring the system for 5 winters now and there is no decline in COP. Of course, the ground temperature changes from the beginning of the heating season to the end, but it has always returned to the same starting point each year - and last year we barely used the AC at all and last year's winter was the coldest in several years. This winter's heating performance was just fine.

    I would imagine one could get away with "short looping" to some extent if the cooling and heating loads were more balanced, but it seems the depth of the borehole is more related to the instantaneous capacity rather than the per-season load. For a given conductivity of the rock, there's only so much heat you can pull out without depressing the temperature too much (which reduces both COP and capacity). A knowledge of the type of ground the hole is drilled in is always a good idea for a proper system design.

    Paul in Montreal.
    • CommentAuthorDantenz
    • CommentTimeMar 10th 2010 edited
     
    And the rule of thumb here is 15 metre of drill depth for every 1kW of heat pump output, so enough to supply a 7kW heat pump which should be able to satisfy a peak heating load of around 8kW which is a property built to current regs. with a total heated area of 160m2.
    • CommentAuthorfinny
    • CommentTimeMar 10th 2010
     
    This thread is a perfect advert for this site!
    by my calcs..loads of moneys worth of specialist consultation in a few clicks!
    :bigsmile:
    • CommentAuthortopher
    • CommentTimeJul 15th 2010
     
    I have two 100 meter deep boreholes. They have about 10 meters of clay then the rest is gray shale and one quartz layer. Both were drilled through water sources at a depth of about 45 meters. Both produce 3 to 4 cubic meters per hour of water, but it does not emerge from the bore hole. During drilling, the compressed air produced this flow. I think it is fair to assume they are wet.

    Each bore hole has two black pipe loops (4 pipes per hole) in high density black polythene pipe. The outer diameter of the pipe is 32 mm the inner 26.2 mm. The pipes are loosely placed on the hole, there are no spacers. There will also be about 10 meters of trench from each bore hole at about 1 meter deep where the eight pipes will be joined to a manifold. From the manifold two pipes are connected to the heat pump.

    The bore holes are in south west France in the Vendée, department number 85.

    I have tried to find some look up table or simple computer programme to find the total heat available from this ground source. I would appreciate some help on this point please.

    Also on a matter of understanding, is the following correct? If a ground source heat pump with a COP of 5 and an out put of 10 Kw is running at is rated capacity of 10 kw, it is absorbing 2 kw (10/5) from the electrical supply, and 8 kw (10-2) from the ground. If not what are the correct numbers?
    •  
      CommentAuthorted
    • CommentTimeJul 15th 2010
     
    Posted By: topher
    Also on a matter of understanding, is the following correct? If a ground source heat pump with a COP of 5 and an out put of 10 Kw is running at is rated capacity of 10 kw, it is absorbing 2 kw (10/5) from the electrical supply, and 8 kw (10-2) from the ground. If not what are the correct numbers?


    Roughly that's right, although some purists might argue over the terminology. But the COP also depends on the temperatures involved as the manufacturer's quote of COP 5 will be for a specific setting.
    • CommentAuthormike7
    • CommentTimeJul 15th 2010
     
    topher - see if this helps:- http://canmetenergy.nrcan.gc.ca/eng/software_tools/gs2000.html
    • CommentAuthorDantenz
    • CommentTimeJul 15th 2010 edited
     
    Posted By: topher
    Each bore hole has two black pipe loops (4 pipes per hole) in high density black polythene pipe. The outer diameter of the pipe is 32 mm the inner 26.2 mm. The pipes are loosely placed on the hole, there are no spacers
    I have tried to find some look up table or simple computer programme to find the total heat available from this ground source. I would appreciate some help on this point please.

    A four probe collector (that is four pipes down the borehole) doesn't actually yield that much more energy than a 2 probe (2 pipes per hole), about 10% increase. The limiting factor is not the heat exchange rate of the pipe but more to do with the limited energy supply from the borehole. In the UK there is a requirement for the bore hole collector to be grouted in so the pipes should not be, as you put it, loosly placed in the hole. And you can look to your hearts content for a table or otherwise to predict your bore hole energy yield but it only ever be "rule of thumb guesswork"
    • CommentAuthortopher
    • CommentTimeJul 17th 2010
     
    Ted and Dantenz, Thanks for the quick responses, very useful.

    Mike7, I have had a look at the GS2000 tool and it looks excellent. I am a bit frightened about changing the language settings on my PC in case I mess everything up. Have you used it? Is it safe?
    • CommentAuthormike7
    • CommentTimeJul 18th 2010
     
    I have played with it a bit but couldn't quite get what I wanted from it, but it seems to be from a well respected source, so I'm sure it's OK. Paul in Montreal is the man who knows about these things for sure.
    • CommentAuthortony
    • CommentTimeJul 18th 2010
     
    Yes you have plenty of capacity and some to spare which is nice.

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