AFAIK, the ground temperature will be the average air temperature over the year. It's certainly not 58F everywhere, because in Siberia there's permafrost, and in Denmark might likely be less than 58F, too.
Based on quick search, the average air temperature in Denmark is 8-9°C(46-48F)[1]. I suspect that you don't find that cold ground temperatures though. I found some local statistics about ground temperature[2], which shows that even around here the invention would barely work in the warmest months. And I'm living nearly thousand miles northwards from Denmark, from the same source the average air temperature here is -2°C.
I'm just giving this as more nordic perspective, seeing that most HNers probably are from US/warmer climate.
