3 ms·

I don’t think the hydration of the clay is the important element here. Rather I suspect it’s simple just the sheer mass of clay, wet or otherwise, that’s involv

I don’t think the hydration of the clay is the important element here. Rather I suspect it’s simple just the sheer mass of clay, wet or otherwise, that’s involved. There’s a reason why ground source heat pumps work so well. It’s because the ground is such a fantastically huge heat sink/source that in most scenarios we consider it capable of sinking or sourcing a practically unlimited amount of heat. Unfortunately one of the scenarios where this breaks down, is when you stick a bunch of tunnels in the ground, then pump a crap ton of energy into those tunnels years round, and expect the ground to sink all the heat away. Turns out, if you do that, the ground itself starts heating up, and given that clay is a reasonable good insulator, it’s like wrapping all those tunnels in wool jumpers. I would point out as well that all these tunnels are “deep level” tunnels running at an average depth of 24 meters and getting as low as 67 meters. The heat of the sun on the ground surface will have approximately zero impact on the tunnel temperatures. 24 meters of clay is a lot of insulation to work with.

jjcob 1y ago
I think you are right. The graph at the bottom of the article shows that temperatures dropped around 2020 (presumably less traffic due to the pandemic). So the ground can take up more heat, but it can't do it quickly enough.
Y_Y 1y ago
My original comment was meant to be tounge-in-cheek, but I would like to add the advantage of wet clay is really in its thermal conductivity[0] which is roughly two to four times higher than dry clay. It will also have higher thermal capacity, so that high frequency fluctuations (like day/night) will be smoothed out. [0] https://www.mdpi.com/2571-8789/8/2/47 https://www.mdpi.com/2571-8789/8/2/47 (I don't think my comment implied that the sunshine makes it hot in the tunnel, only that when it's hot above ground it's nice to be cool below, like in the poster. And I did include a figure of 25m for average depth, I've spent enough time on their endless stairs to know that the TfL delved to greedily and too deep. This spiral staircase has 320 steps, I shudder.)

jjcob 1y ago

I think you are right. The graph at the bottom of the article shows that temperatures dropped around 2020 (presumably less traffic due to the pandemic). So the ground can take up more heat, but it can't do it quickly enough.

Y_Y 1y ago

My original comment was meant to be tounge-in-cheek, but I would like to add the advantage of wet clay is really in its thermal conductivity[0] which is roughly two to four times higher than dry clay. It will also have higher thermal capacity, so that high frequency fluctuations (like day/night) will be smoothed out. [0] https://www.mdpi.com/2571-8789/8/2/47 https://www.mdpi.com/2571-8789/8/2/47 (I don't think my comment implied that the sunshine makes it hot in the tunnel, only that when it's hot above ground it's nice to be cool below, like in the poster. And I did include a figure of 25m for average depth, I've spent enough time on their endless stairs to know that the TfL delved to greedily and too deep. This spiral staircase has 320 steps, I shudder.)