The Homemade Heat Pump Manifesto

[pick1e]

Originally Posted by AC_Hacker

Also, you have to consider the geometry of heat flow. In the case of a bore hole, you have a pipe in the center. When you first begin your extraction, you get more heat from the earth immediately surrounding the pipe, then it is heat-depleted and needs to get heat from earth further away, etc. And as time passes, there is progressively less and less heat extracted, and the curve gets flatter.

AC Hacker,

Actually, as I look at this graph again, it actually confirms what I mean. In this test you were adding heat with a coffee cup heater, right? The graph shows the temperature leveling off... Think about it, if the borehole wasn't able to keep up with sinking the heat, the T would be shooting straight up and your water would be boiling! Instead, as it reaches a higher delta T, it is leveling off, so the borehole is actually keeping up and reaching equilibrium.

But as you pointed out, whether that would remain true for weeks or months is certainly a different story. My point is simply that the rate of heat transfer through the soil seems like it should depend on the delta T, not just be static.