EWT for vertical loop seasonal variation

Discussion in 'Geothermal Loops' started by richard kellogg, Aug 25, 2016.

  1. richard kellogg

    richard kellogg New Member

    I am new to geothermal heating/cooling and have been trying to educate myself. So I have a question for the experts about the variation in entering water temperature from a vertical loop over a season.

    For a vertical loop, how much of the temperature variation is due to the heat being added/removed to/from the loop field, and how much variation would be there without any loop heat being added/removed by the load?

    For example, look at the EWT_source graph from this pair of 400 ft wells - http://welserver.com/WEL0383/ (3rd graph from bottom of page). This shows a sinusoidal like yearly variation from around 50F in summer to around 30 F in winter. So my question is, if the heat pump were not running (but the circulator was - so that the EWT = LWT = average temp along the well), what would this graph look like (e.g. still a sine like wave, but with a mean of maybe 60 F, and peak maybe in august of around 61F- 62F)?

    Thanks for responses.
     
  2. urthbuoy

    urthbuoy Well-Known Member Industry Professional Forum Leader

    Only the top 20' or so of soil has any seasonal temperature fluctuation.
     
  3. richard kellogg

    richard kellogg New Member

    So are you are saying 100% of the temp variation is due to the heat being added/removed?
     
  4. urthbuoy

    urthbuoy Well-Known Member Industry Professional Forum Leader

    Not really. I'm just stating what I know and one can extrapolate from there.

    But yes, the fluctuation is vertical boreholes is going to be mostly from heat of extraction/rejection by the heat pumps.
     
  5. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    WEL 383 used to be heat only, however since this year they have a small A/C fan coil. We measure small seasonal fluctuation down to 30ft in our ground, no fluctuation below. The short term swing in the loop comes from heat pump cycling, although the EWT is only recorded when the heat pump is running. But still the temp sensor is mounted on the outside of a copper line, and might have a bit of an inertia. The seasonal swing comes from loop recovery during the summer, and this year also from heat rejection into the loop during A/C.

    The 5 spikes are power interruption for the monitoring system.
     
  6. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    http://welserver.com/WEL0662/

    Here you can see when they go from heating into cooling mode. Small load, 400ft borehole.

    http://welserver.com/WEL0712/
    Similar house, but horizontal slinky loop field. It does not swing as rapidly, since it is connected to more ground, thus the loop has a higher thermal inertia.
     
  7. richard kellogg

    richard kellogg New Member

    OK, so let me say repeat what I think you are implying - to make sure I'm following you correctly. Ignoring the new A/C fan coil, last year during the heating season, the well temperature went down to near 30F, because the heat pump was removing heat, and not due to cold weather above ground. Then after heating season was over, the well temperature rebounded, because heat was no longer being removed.

    Thanks for the reply.
     
  8. richard kellogg

    richard kellogg New Member

    When they switch from heating to cooling (around 3 months ago), there is a rapid change in EWT of about 15 deg F. Is that real? If so, what is the explanation?
     
  9. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    Yep.
    Loops recover through solar gain (vertical gain from above), horizontal movement of heat through the loop field, and some vertical heat raising from below.
     
  10. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    If you check the system was not running much before and after the change over. You also need a delta T to extract or reject heat. So the loop temp has to be above (a/c) or below (heating mode) the actual ground temp. I also depends on the borehole thermal resistance how high the delta T during switch over is.
     
  11. richard kellogg

    richard kellogg New Member

    I was thinking the same thing.

    I was also wondering what that deltaT should be. So can you comment on what a typical deltaT is for a vertical hole that has been heating, that then switches to cooling? (I know you said it depends on thermal resistance, but I'm looking for ballpark answer, e.g less than 5 deg, or typically 10 deg.)
     
    Last edited: Aug 27, 2016
  12. urthbuoy

    urthbuoy Well-Known Member Industry Professional Forum Leader

    This is a "rabbit hole" of questioning that is a bit wide of the mark.

    The delta T is applicable to the EWT and LWT at the heat pump. It is not that applicable to an arbitrary point in the borehole vs soil temps unless you want to start talking about finite difference approximations and such.

    At the heat pump? Delta of 4C.
     
  13. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    Chris is right. It becomes academic.
    The delta t is dictated by the amount of heat taking our or rejected into the loop by the heat pump, and varies depending on the amount of water you decide to push through. The the end of the day, the amount of heat extracted from the loop by the heat pump should be similar than the amount of heat extracted by the loop from the ground (in heating mode). The amount of heat is a function of the water flow and the delta T. The lesser your water flow, the higher the delta T, and vice versa.
    Between 5 and 10 degrees F is a good number, more on the 5F around the 30F loop temp for heating, and more around 8F(+/-2F) when the loop is warmer. Delta T is also higher in A/C ode since no the compressor heat must also be rejected, when in heating mode compressor heat gets rejected into the conditioned space.
     
  14. richard kellogg

    richard kellogg New Member

    I understand the EWT/LWT deltaT times the flow rate is the heat removed or added to the loop field. But I thought you were implying in the previous post,


    that there needs to be a deltaT between the water temp (presumably the average of EWT and LWT), and the surrounding ground. It was that delta T I was asking about.
     
  15. richard kellogg

    richard kellogg New Member

    I don't know the meaning of this sentence.

    See my response to docjenser. I was not referring to the temp difference between EWT and LWT, rather between the ground next to the bore hole, and the average temp of the water in the pipe.
     
  16. urthbuoy

    urthbuoy Well-Known Member Industry Professional Forum Leader

    It means you need to know math. A lot of math (calculus). Or let a computer program figure it out for you.

    As an example: let's start with which point in the ground you are interested in (depth, distance from borehole, etc.)? And what type of material (thermoconductivity/thermodiffusivity)? And how close are your boreholes together? Specific heat of the loop fluid? Etc. It goes on.

    So, for the purposes of this forum (and the business world in which we work) - we don't really care. Our work is built on what the academics and in-situ studies have shown are good models. Our design programs/guides/etc are built on these models. We then use our own regional experience to fine tune these models. Ideally based on site monitoring.

    The results we get basically give us a heat flux in one way or another based on numbers such as what you are asking. To figure out those original numbers (real world vs theoretical) you need to work backwards through that heat flux (we can measure the totals of this) to get that delta T you're asking.

    So in summary, this isn't really relevant. But if you want to go down that "rabbit hole" - here is a start http://docs.lib.purdue.edu/cgi/viewcontent.cgi?article=1044&context=ihpbc
     
  17. richard kellogg

    richard kellogg New Member

    OK, I get it. Thanks for the link.

    It's relevant to my understanding (which of course, is not relevant to anyone but me). The reason for the original question, is to understand why there is such a swing in temp from winter to summer. And what the ground temp is relative to what's being measured (i.e EWT, LWT).

    It is my (perhaps incorrect) belief, that EWT is pretty close to the ground temp, because the water will have had a lot of time (typically several minutes) to transfer heat with the ground.
     
  18. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    I it typically is. But you also take heat out of it, so this is a moving target. In order to move heat, you need a differential between 2 media. Now if you make your loop real large, that differential can be much less. Or if the conductivity is very good, like a borehole in granite. Or your convection is very good like typically in a pond loop.
     
    richard kellogg likes this.

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