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Heating and cooling: GSHP and the 3 year effect

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    • CommentAuthorjemhayward
    • CommentTimeApr 28th 2010
     
    Has anyone got any real information on this:
    It would seem that some GSHP istallations work well for about three years then the performance starts to drop off often quite alarmingly.
    The theory seems to be that the ground you are extracting heat from cannot replace that heat quickly enough so the net temperature of the land goes down, and the COP suffers accordingly.

    I've had various stories on this:
    1. its a complete myth
    2. its a theory not borne out in practice
    3. its only boreholes beneath buildings
    4. its only boreholes, not slinkys
    5. it all installations as the solar gain doesn't penetrate very far

    I probably can't afford a borehole anyway, but if I was going GSHP it would be my only option, though I suppose I could warm up my borehole in the summer if I pumped excess DHW from my solar panels down there?

    anyone with opinions, or even better, facts?
    • CommentAuthortony
    • CommentTimeApr 28th 2010
     
    For wrongly designed systems only:smile:
    • CommentAuthorevan
    • CommentTimeApr 28th 2010
     
    Inlaws have a borehole GSHP in Ireland, installed for nearly 3 years now. Last I heard it was being a bit expensive to run.. Will find out more and report back :)
    • CommentAuthorjemhayward
    • CommentTimeApr 28th 2010
     
    what constitues a "wrongly designed system". Being an amateur I have to trust the "experts" with huge amounts of my money.
    • CommentAuthortony
    • CommentTimeApr 28th 2010
     
    It is tempting to do too smaller area of buried pipes, or too shallow or not deep enough borehole.

    And even if designed right the above can and likely do all happen on occasion when installation time comes.
    • CommentAuthorjemhayward
    • CommentTimeApr 28th 2010
     
    Although the poor COP of ASHP means that GSHP can be twice as efficient, worries such as this, plus the cost of a borehole are making ASHP more appealing by the minute.
    Working on the assumption my electrons are carbon free, I can't see that the performance advantage of GSHP can pay for the extra installation cost over the expected system life.
    • CommentAuthorjon
    • CommentTimeApr 28th 2010
     
    I can't see how GSHP can work effectively in the long term unless it is significantly over-designed (so that gain equals output).
    • CommentAuthorGavin_A
    • CommentTimeApr 28th 2010
     
    Posted By: jonI can't see how GSHP can work effectively in the long term unless it is significantly over-designed (so that gain equals output).

    that's not over-designed, that's properly designed;)
  9.  
    Posted By: jonI can't see how GSHP can work effectively in the long term unless it is significantly over-designed (so that gain equals output).


    I did a small study on my system: we pull out 10x more heat per year than we put in during the airconditioning season. But if I work out the annual insolation on my small piece of land, then I find that I receive 7x more heat from the sun than the system extracts per year.

    Main problem with GSHP systems that aren't designed properly is that the rates of heat transfer are not properly considered so that performance during the peak demand can sag. It's not so much that the COP declines (it does, but not much) - it's that the capacity of the system diminishes since this is more temperature sensitive (this is to do with the characteristics of the refrigerant and the mass flow that's available at a particular temperature).

    I would think that in a UK kind of climate an ASHP would give good performance - though the damp weather in winter will have an effect on the number of defrost cycles. I know several people in Montreal who use ASHP systems in winter down to temperatures of -12C (this is the switch over point for the "dual rate" electricity tarrif).

    As for actual performance, I've had 5 winters out of my vertical borehole GSHP (water->air) and the performance is essentially the same as it was the 1st year.

    Paul in Montreal.
    • CommentAuthortony
    • CommentTimeApr 28th 2010
     
    In other words a well designed system will work for the foreseeable...
  11.  
    Posted By: tonyIn other words a well designed system will work for the foreseeable...
    Yes, it should. I've heard of problems in hot regions where GSHPs are mainly used for cooling where the ground warmed up - but this was because the conductivity of the rock was lower than expected and/or the loops were too short (i.e. cheap).

    Also, in a heating context, prolonged cold weather can exceed the rate of heat flow the ground loop was designed for and this will lead to the ground getting colder (and the capacity diminishing). Like everything in engineering, there's always a compromise between designing for worst case versus typical case. But on an annualized basis, a properly designed system should not exhibit any long term change in ground temperature (and hence capacity). I know the gs2000 program can do multi-year analysis to confirm this.

    Paul in Montreal.
    •  
      CommentAuthorfostertom
    • CommentTimeApr 28th 2010
     
    Posted By: Paul in MontrealI receive 7x more heat from the sun than the system extracts per year
    I'm surprised -it sounds like an alarmingly small multiple.

    Without GSHP, the ground spends half the year colder than the surface/air; half the year warmer - nett zero, or in fact slightly more loss than gain, because earth core heat has to be lost to space.
    Once you start to extract heat, the summer gain from surface/air to ground has to be increased above 'natural', to exceed instead of merely match/balance the winter 'natural' loss.

    The only way that can happen is by nett-depressing the subsoil temp, below 'natural'. 'Natural' is a fairly steady average between summer and winter surface/air temps. With GSHP, subsoil temp becomes a seasonally fluctuating number - well below 'natural' by the end of the winter, and hopefully getting back close to 'natural' by the end of the summer.

    A 'well designed' system won't get around that - the 'unnatural' nett gain from surface/air to subsoil can only happen because of nett-depressed subsoil temp.

    My suspicion is that systems that don't suffer in the way described, has more to do with fortuitous ground water movement. If you're lucky, your heat will be replenished not from solar input to your bit of surface, but by the heat contained in passing groundwater - which ultimately get their heat from solar input elsewhere/over a v wide area.
  13.  
    <blockquote><cite>Posted By: fostertom</cite>
    My suspicion is that systems that don't suffer in the way described, has more to do with fortuitous ground water movement. If you're lucky, your heat will be replenished not from solar input to your bit of surface, but by the heat contained in passing groundwater - which ultimately get their heat from solar input elsewhere/over a v wide area.</blockquote>

    I think you could be right where deep boreholes are used. For for installations using shallow (less than 2 metres deep) but large surface area, its a matter of correctly sizing, heat transfer and load calculation and ground water etc.

    The bottom line with gshp is that you dont want to extract more than is put in by solarr etc. which means a properly designed system as Tony said.
  14.  
    Posted By: fostertomI'm surprised -it sounds like an alarmingly small multiple.
    I used a very pessimistic calculation - I only considered the area of the land that I own - as someone asked what happened if all the neighbours also had a system.

    Posted By: fostertomA 'well designed' system won't get around that - the 'unnatural' nett gain from surface/air to subsoil can only happen because of nett-depressed subsoil temp.


    Sort of agree - but the overall energy balance is net zero as all the heat extracted from the ground will enter the house and leak out through the insulation and back into the air. If the heatpump is driven by fossil-fuel sourced electricity, then there's a net gain in heat into the atmosphere (from the stored solar energy that's in the coal/oil/gas etc.). A heatpump is called a heatpump because it just moves heat around.

    Posted By: fostertomMy suspicion is that systems that don't suffer in the way described, has more to do with fortuitous ground water movement.

    This definitely helps - especially in the case of rock that is not very conductive. As I said earlier, the rate of heat extraction is a variable in the design equation and this has to be factored in knowing the ground conditions. If it's done correctly, then the temperature does remain stable long term, but with annual fluctuations up and down as you described. There are some forums I follow where people post their longterm ground temperatures that clearly demonstrate this.

    Paul in Montreal.
  15.  
    I may have posted this before, it has some interesting reading about improved COP's when the ground temperature is raised by 5 degrees http://www.hsh-fiwihex.com/index.php?option=com_content&view=article&id=55&Itemid=71&lang=en
    • CommentAuthorCWatters
    • CommentTimeApr 29th 2010
     
    <blockquote><cite>Posted By: tony</cite>It is tempting to do too smaller area of buried pipes, or too shallow ...</blockquote>

    If most of the heat comes from the sun why does too shallow matter?
    •  
      CommentAuthorSteamyTea
    • CommentTimeApr 29th 2010
     
    The ground acts as a thermal store, so the closer to the surface the more uninsulated (by the ground) surface area there is and there is the thermal properties of ground( or stone), air and water to take into account.
    • CommentAuthorjemhayward
    • CommentTimeApr 29th 2010
     
    When I was researching earth sheltered houses I found some graphs of ground temperature at various depths vs surface temperature. They seemed to show that by the time you got to 1500mm below the surface the temperature was pretty constant all year round. Good news for earth sheltered house, but suggests the solar energy doesn't penetrate very far? (Of course, I cannot find a web ref now I need it!)
    • CommentAuthorCWatters
    • CommentTimeApr 29th 2010
     
    Posted By: SteamyTeaThe ground acts as a thermal store, so the closer to the surface the more uninsulated (by the ground) surface area there is and there is the thermal properties of ground( or stone), air and water to take into account.


    So it's basically it's acting much more like a long term interseasonal store than a short term day/night store? That makes sense. So the volume of the collector will be important. To me that suggests tricks such as coiling up the pipe to reduce the volume of the hole needed is a bad idea. Best spread the pipe out to utilise as much volume a possible.
  20.  
    Posted By: CWattersTo me that suggests tricks such as coiling up the pipe to reduce the volume of the hole needed is a bad idea. Best spread the pipe out to utilise as much volume a possible.
    There's two processes at work: amount of heat stored (i.e. ground volume) and rate of transfer of heat (i.e. pipe surface area). "Slinkies" optimize the latter - but you still sufficient of the former.

    Paul in Montreal.
  21.  
    Why metal fins not attached to t he pipe?
  22.  
    Posted By: bot de pailleWhy metal fins not attached to t he pipe?
    It wouldn't make any difference - the conductivity of the pipe is determined by the material and thickness of the wall. If you have a "slim jim" pond/lake heat exchanger, that will be metal - but normal ground-buried collectors are some kind of HDPE.

    All that said, there are "direct exchange" GSHP systems that use copper pipes and circulate the refrigerant directly. The advertised benefit of this is a smaller diameter and shorter borehole due to the increased conductivity - but you're still limited by the bulk conductivity of the rock the exchanger is buried in.

    Paul in Montreal.
    •  
      CommentAuthorSteamyTea
    • CommentTimeApr 29th 2010 edited
     
    Jeremy

    Was the graph a bit like this
      GSHP1.jpg
  24.  
    Posted By: SteamyTeaWas the graph a bit like this
    Even the borehole units are solar collectors as the temperature at the bottom of the borehole is equal to the average annual air temperature at that location. You have to go much deeper than 100m (in most cases) to get true geothermal heat. That's why the correct term for a GSHP is a ground exchange heat pump. The only difference between horizontal and vertical systems is the time constant of the solar storage: horizontal systems are on the order of days; vertical systems on the order of months.

    In the graph above it's showing a temperature of around 9C for "northern Europe". I'm in southern Canada (which is south of all of Northern Europe) - but the deep ground temperature here in Montreal is about 6.1C - in Vancouver (which is north of Montreal) it's closer to 14C. Both these figures happen to be are annual average temperature (and this is the temperature we measure at a depth of 150m).

    Paul in Montreal.
  25.  
    From companies I have contacted the majority have said that the actual drilling if it goes as planned is not that expensive. I had a quote for £25 per mtr last week so if I install a GSHP I will drill more than I get told to. The last thing you need after you have finished your ground etc is having your COP drop off because you saved £500 on drilling. It is not wise in my opinion to save a little money on the boreholes! They are the fuel tank after all!!!
    Beware of companies trying to undercut competitors by saying you only need 1 x 60 mtr hole when others say 2 x 100. Do your research and cover yourself. It is your money after all.:cry:
    •  
      CommentAuthorSteamyTea
    • CommentTimeApr 29th 2010
     
    Paul

    I am in West Cornwall, the rocks are hot much less than a 100m down, why the first uranium mine was here (may be some work re-opening it). Montreal and Vancouver have very different climates. I only have to travel 40 miles and the mean temperature can drop by 3 degrees. But I tend to agree that the seasonal integrated temperature probably goes a lot deeper that 15m. Boreholes tend to be more reliable (assuming system is designed right than shallow channels (there is a thread about it on here).
    Does anyone know where the thread is that was about monitoring GSHP performance, can't seem to find it. It had a link to some data.
    • CommentAuthormoggeridge
    • CommentTimeAug 26th 2010
     
    I would just like to add, although it is a million years late;

    1) Temps in the ground are directly related to surface temps and this can be extreme. Ie in Siberia, due to constant low tempertures and low thermal gradient, permafrost penetrates to 1 MILE!

    2) Systems will work when well designed. No system works well when not properly designed - even gas! Using 50w/m for calculating borehole lengths does not constitute god design.

    Mike
    • CommentAuthorjules
    • CommentTimeAug 27th 2010
     
    Rather off-topic I know, but near-permafrost dating from the last ice age has been discovered in an area of Poland 450m below the surface: ie in this particular region, the ground temperature drops steadily to about zero as you go down to 450m, and then rises again as you go lower (so don't try a borehole GSHP in Poland).

    Just goes to show how far down surface conditions can extend their influence over a long enough time, and also how well insulated the ground can be: this ground has been subjected to warming influences from below (geothermal) and above (post-ice age surface temps) for 10000 years, but is still close to freezing.
    • CommentAuthorchuckey
    • CommentTimeAug 27th 2010
     
    This discussion has triggered a thought :- If you get a contractor to dig your trench, put in your slinkies then re-fill the trench, how well is the soil compacted around your pipe?, what percentage of the pipes surrounding are actually air voids, with just the ground surface rolled/wacked down. This is where the ground water effect would be the greatest acting as a thermally conductive layer next to the pipe as well as being a source of more ground heat.
    Frank
    •  
      CommentAuthorSteamyTea
    • CommentTimeAug 28th 2010
     
    Don't they sit them in sand for this reason?

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