Monday, January 23, 2012

Graph of Underground Temps

23 Jan 2012: While I am in Austria, I have the chance to think about the project... (while speeding downhill on a blizzard..). It's time I updated the graph of ground temperatures now that I have observed 3 winters and two summers with detailed data.
I have noticed that when the winter comes, there is a rapid fall off in temperature... a steep slope, even if the borehole has been charged. This is a temporary effect because the heating activity is cooling the area immediately around the pipes. There is a nadir at some point, usually in January, and then things improve and there is a longer slower slope for the temperature to climb back up. At the nadir, the delta-t between the borehole and the larger surroundings is big enough to draw energy more quickly from the wider surroundings.
   The exact size of this phenomenon depends on many factors, e.g. whether you have a colder/warmer than usual autumn or spring, the size of the house and how much heating you have to give it, the depth of the borehole, the availability of alternative heating (e.g. biomass in main living space), the size of solar panels you use for charging, and even the nature of the surroundings for receiving spring and summer sunshine - a tarmac road or open field nearby.
Temperatures from Sept 2009 to Jan 2012
The Surya system started in March 2010. The right hand side of the graph
demonstrates the revised shape of borehole temperatures. A rapid decline
in autumn, with a long slow climb-out from January to midsummer.
Then a flattish top until the autumn heating season restarts.
  With Solar charging, there is still the usual steep autumn fall, but it pulls out sooner, and with January, the ground temperature might be rising already because of improving sunshine towards the February end of the month. Once the ground has reached an optimum in summer, it did not just continue rising, presumably because heat is moving outwards into the mass surrounding (which is precisely what one wants it to do). If it remained too close to the pipe, the ground temp would rise, but it seems to stick at just under 14ºC. If I tested it at teatime, it might be 16 or 17ºC, but the methodology is to test at midnight, some hours after both the Sun and the GSHP stopped adding or subtracting. So one has a rather long flat top to the curve until the heating season restarts in October.
  So, looking at the top graph again, the uncharged borehole would gradually get lower in energy levels until it reached a low enough temperature that the delta-T was pulling enough energy in from surroundings to reach a new equilibrium. Being at a lower temperature it makes the GSHP less efficient, in fact, it might cause owners to consider that it was malfunctioning. The charged borehole will have higher nadirs that last a shorter time, with a quicker climb-out to summer temperature. The peaks will not get literally higher, but they get wider at the top, e.g. they seem to consolidate - increasing the energy level by staying at about 14ºC but to a wider radius.

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