Dear FEFLOW developers,

I did a literature search on the socalled Internal Borehole Resistance that must be specified along with the Borehole Thermal Resistance if one wants to specify the latter - but I came up empty-handed. On page 854 of the FEFLOW book, there seems to be some sort of definition. There are also papers outlining a multipole method for determing resistances in a borehole but it is not easy to get something of practical use from these papers.

So to put it shortly:

Is there a closed-form expression for calculating the Internal Borehole Resistance or at least a good approximation? I mean the sensible argument in the associated white paper is that the Borehole Thermal Resistance can be specified directly - which is great if one has done a thermal response test. But the thermal response test does not give you the Internal Borehole Resistance. It would be nice if the manual elaborated a bit further on this rather complicated topic.

Thank you in advance!