How stable is ground temperature?

Question

Hi there,

Ive been wanting to construct a solar thermally powered sterling engine for some time now, and have recently switched designs from a parabolic dish mirror to a parabolic cylinder mirror. My design will be of a copper rod or other well thermally conductive rod placed at the focal point of the mirror, laid along its entire length. This will provide heat for the hot expansion cylinder quite adequately i think, but I was at a loss to think about what to do about a cold reservoir.

Then I thought, why not use the so-called thermal inertia of the ground? If I dig the cold cylinders underground, this should provide a constant heatsink. Additional cooling for the ground could be provided by driving metal heat sinks into the ground, cooled by summer breezes. The entire cool end of the system would be covered to prevent daytime heating directly from the sun.

My question is: Would the ground surrounding the cylinders start to heat up in any discernable way, or would it remain relatively constant for the entire day?

Thanks.

Answer 1

Very interesting idea.

I will start by saying that I am just giving you input and that I could only guess at the answer as well you.

That being said, I would say that the ground would not be heated up.  If you have a significant amount of ground surface area that your cold cylinders are in contact with then I would doubt you would heat the ground.  But I do like your idea of the heat sinks driven into the ground.

Is there any reason you don't want to put the cold reservoir above ground and use the air to cool it?  I anticipate your answer to be that you want the largest delta Temp possible and that underground will be cooler.  In rebutal to that, If you are putting heatsinks in the ground to use the air temperature to cool it than you have already accepted that the air will become cooler than underground, and if your heatsinks are working properly, on a hot day you will manage to heat up the underground rather quickly to the air temp.

Good luck and hopefully someone else has something more knowledgeable to say about heat dissipation in the ground.

Answer 2

Depends on depth and soil type.If you go down to your water table(in most places 50 ft it is very stable(usually around 60 degrees)