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Heating and cooling: GSHP is this slinky array ok??

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      CommentAuthoragu
    • CommentTimeJun 1st 2009 edited
     
    Firstly I know that GSHP aren't renewable as such but was unsure where else to put this disscussion so hope this is alright.

    I am specing to put a GSHP slinky under an astroturf pitch that is to be built, this will obviously have a subbase. What I wanted to know was if any problems would occur with this set up. I can't see any major reason why it shouldn't work but a couple of things I did think of are the following.

    1, If too much heat is extracted and the ground freezes could it cause the subbase to crack?
    2, being under the astro and subbase will the CoP drop off due to less solar gains on the ground?


    Can anybody advise if this will work fine ( I think it probably will ) or if there is anything major that I am missing or any reason if just wont work???

    THanks if advance

    Agu
    • CommentAuthortony
    • CommentTimeJun 1st 2009
     
    Problems will be that it will freeze more often and for longer than anywhere else near by and snow wont melt so well or as fast.
    • CommentAuthorralphd
    • CommentTimeJun 3rd 2009
     
    Freezing is not really bad; freezing 1g of water at 0C requires removal of 80 calories of heat. Cooling 1G of water from 1C to 0C requires removal of only 1 calorie of heat.

    -Ralph
    • CommentAuthorCWatters
    • CommentTimeJun 3rd 2009
     
    I think tony meant they wouldn't be able to play footie on it if it's frozen.
    • CommentAuthortrule
    • CommentTimeJun 3rd 2009
     
    The biggest problem you might have, aside from freezing which I doubt is a problem, is if you need to get access to the ground loop for any reason in the next 20-30 years.
    •  
      CommentAuthoragu
    • CommentTimeJun 4th 2009
     
    Thanks for the comments guys, some of the initial contractors that we have approached about the pitch are making noises that they wont g'tee it if we want to put the slinky underneath, but I could see any major problems with it. I think it's just covering their own backs and fear of something slighty different!
    • CommentAuthorralphd
    • CommentTimeJun 4th 2009
     
    Posted By: truleThe biggest problem you might have, aside from freezing which I doubt is a problem, is if you need to get access to the ground loop for any reason in the next 20-30 years.

    Assuming the heat pump is only running Nov-Apr, the ground will thaw every summer.

    -Ralph
    • CommentAuthortony
    • CommentTimeJun 4th 2009
     
    The action of digging arround under a playing surface may well lead to some settlement over time. For this reason I would avoid it -- how about a bore hole?
    •  
      CommentAuthoragu
    • CommentTimeJun 5th 2009
     
    It's cost Tony, no other need for a piling so the cost might be too much
    • CommentAuthorDantenz
    • CommentTimeJun 5th 2009
     
    I would avoid using the so called "slinky" method as this is a far more concentrated energy take from the ground over a smaller area, go with horizontal loop and over spec the amount of collector pipe and you should be Ok. Believe me, freezing is possible if all the conditions are right or rather if the whole system isn't correctly spec'd. Could also consider going for a heat pump output that will cover 100% of Peak Heating Load rather than the generalisation of 70-80% of PHL. This will mean that the heat pump has shorter running times which will therefore, give the ground more rest time and chance to re-load.
  11.  
    Posted By: DantenzCould also consider going for a heat pump output that will cover 100% of Peak Heating Load rather than the generalisation of 70-80% of PHL. This will mean that the heat pump has shorter running times which will therefore, give the ground more rest time and chance to re-load.
    Shouldn't make a difference if the collector is sized to the heat output. WIth a 80% output system, the ground collector would be sized as such. The key is the specific transfer capacity per metre of ground loop - doesn't matter if it's straight or slinky. A slinky is just a straight loop in a wider trench so it's really horses for courses. If you're freezing the ground too soon, it's because the ground loop is too small. Like everything, there's an engineering tradeoff between the cost of a right-sized loop versus seasonal COP (including any backup energy required). Sometimes it's cheaper to undersize and just pay for the lower COP / backup heat as the cost of the loop may never be recovered if going for a 100% solution. This is especially the case with a loop that's sized for 100% of the typical worst case winter conditions. BTW, there should always be a backup in case the heatpump is out of service for any reason.

    Paul in Montreal
    • CommentAuthorDantenz
    • CommentTimeJun 5th 2009
     
    Paul ... I just knew you would post on this, good on yer!

    The potential problem here is that the ground will be somewhat insulated from direct solar irradiation and may also receive less energy due to the fact that surface water cannot permeate down through the ground which also has a significant re-charge effect.

    </blockquote>Shouldn't make a difference if the collector is sized to the heat output. WIth a 80% output system, the ground collector would be sized as such. The key is the specific transfer capacity per metre of ground loop - doesn't matter if it's straight or slinky. A slinky is just a straight loop in a wider trench so it's really horses for courses. If you're freezing the ground too soon, it's because the ground loop is too small.Paul in Montreal</blockquote>

    Of course, the collector should always be sized according to the output of the heatpump and relative to the thermal conductivity of the ground. Problem is, determining the Lambda value of the soil; this is not an exact science at the best of times let alone trying to best guess what it is under astro turf. An 8kW heat pump (which has been sized to 80% of a 10kW PHL) will run constantly when the load is 8kW. This is the danger area for freezing the ground when you don't really know what the ground can yield. IMHO determining accurately what the ground can yield when there is a prolonged outake is nigh on impossible. This is the very reason why most sizing tools over spec the ground loop - there are so many variables.
  13.  
    Posted By: DantenzThe potential problem here is that the ground will be somewhat insulated from direct solar irradiation and may also receive less energy due to the fact that surface water cannot permeate down through the ground which also has a significant re-charge effect.


    Astroturf is probably no better an insulator than grass, in fact, probably worse since there is no transpirational cooling. Recently I did a calculation to rebuff a statement that a local official had made that GSHPs ultimate depress the ground temperature permanently. Where I live, a standard-sized lot is 7.5x33m so I calculated how much insolation is received over a full year versus the heat load model of my house and discovered I receive about 6X as much solar energy as I lose in heat over the year. Unless you're in an arid area, ground water will always get in under the astroturf (well, I'd be surprised if it didn't).

    Most of the sizing programs should allow for the ground conditions to be entered. For commercial installations, often a test loop is installed and the capacity actually measured.

    As for worst case conditions, the average temperature in Montreal for January was -14.6C - that's about the balance point of our GSHP so it effectively ran constantly for a month. The good side was that our hot water bill for the month was about Can$1.50 (one dollar fifty) as the GSHP preheats the hotwater tank :)

    Posted By: Dantenzhis is the very reason why most sizing tools over spec the ground loop - there are so many variables.
    Nothing worse than a disappointed customer.

    Paul in Montreal.
    •  
      CommentAuthoragu
    • CommentTimeJun 8th 2009
     
    Thanks for all the feedback everybody, when a decision is made I'll let you know what was decided

    Agu
    • CommentAuthorgyrogear
    • CommentTimeMar 16th 2016
     
    Just resurrecting this thread, as I shall soon have to relay my front lawn, and am wondering about the possibility of laying UFH etc. pipe under it, to collect some solar gain. The warm water would be used for preheating my DHW cylinder feed.

    The lawn is a rectangle, with the longer axis (seven meters) oriented North-South and the shorter axis (6 meters) oriented East-West. No shading. The surface slopes towards the south, by 60 cms. (I think that equates to a gradient of 7%).

    We get no snow in this region, and winter temps are generally mild (minimum temps, NOV-MARCH, °C = 6; 4; 4; 3; 4)
    Number of frost days, NOV-MARCH = 1; 4; 5; 5; 2).
    Townwater temp in winter = 11°C approx.

    The townwater arrives inside my crawlspace via a 30mm (?) black poly pipe.
    Since the CS is insulated, it stays around 15.8°C in winter.
    I have already thought of installing a coil or two of black polythene (say 50 meters) in CS, to catch a few degrees using a bypass on the DHW cold feed.

    So this lawn idea is an upstream add-on to that idea.

    If necessary, I would consider replacing the lawn grass with nylon grass - it has a black rubberized (perforated) backing that ought to absorb some sun, I'd have thought...

    All ideas gratefully received !

    gg
    • CommentAuthortony
    • CommentTimeMar 16th 2016
     
    Won't work,
    • CommentAuthorEd Davies
    • CommentTimeMar 16th 2016
     
    It's not obvious why the soil immediately under your lawn would be usefully warmer than the somewhat deeper soil the supply comes through. In summer it would be warmer but the heat wouldn't be terribly useful at that time of year. In winter it'd likely be colder most of the time (ie, all the time except the odd sunny day).

    Any chance of vertical south-facing solar thermal?
    • CommentAuthorgyrogear
    • CommentTimeMar 17th 2016
     
    Thanks for the feedback !
    It made me think a bit more...

    (Hope U don't mind, I'm off to try burying a temperature recorder under the lawn, and see what it comes up with...).

    Posted By: Ed DaviesAny chance of vertical south-facing solar thermal?


    Yes, I have got a back garden, so I *could* always sacrifice some of that, I suppose...
    In fact, I've been mooching around out there a few times this week...

    (It was the realization of having to dig a trench to/from the collector that got me thinking about buried pipes under the lawn in the first place...).

    gg
    • CommentAuthortony
    • CommentTimeMar 17th 2016
     
    Buried pipes need to be 2m deep and long enough not to cool the ground too much
    • CommentAuthorjfb
    • CommentTimeMar 17th 2016
     
    2m? How many heat pump installers in this country are going to that depth?
    Isn't 1.2m the max depth before trench needs shoring up during installation?

    How deep do they go in other countries?
    • CommentAuthorEd Davies
    • CommentTimeMar 17th 2016
     
    Am I right in assuming that the front of your house faces south but that you feel you can't put solar thermal on the front for whatever reason so any would have to be in the back garden (north) and hence well away from the house to avoid shading?

    What about the east or west sides? Pretty much useless in deepest winter but possibly useful at other times of year. Evacuated tubes rather than flat plate would, I think, be better for cases where the sun comes from an shallow angle.
    • CommentAuthorDarylP
    • CommentTimeMar 17th 2016
     
    jfb, that is part of the problem with GSHP performance not living up to claimed values....
    :shamed:
    ... along with other reasons I am sure...
    • CommentAuthorgyrogear
    • CommentTimeMar 17th 2016
     
    Posted By: Ed Daviesyou can't put solar thermal on the front for whatever reason so any would have to be in the back garden (north) and hence well away from the house to avoid shading?


    Yes, you got it in a nutshell ! Not just shading, but "potential neighbour effect" :sad:

    I could run the feed/return pipes in an insulated duct, buried in the garden, as far as the house wall.
    Initially, I thought I'd try the lawn scenario first, but it is looking like a non-starter !

    gg

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