There are a few (experimental) power station that get their power from the temperature gradient of the earth crust

There are more than "experimental" - In Nuclear Free New Zealand we've been seriously using the Wairakei Geothermal power station since 1958, and we are currently increasing the amount of geothermal electricity we generate.

It's a "sustainable" resource, but it's sadly not a renewable one.

Remembers... Hmmm Wairakei and Perl are linked for me... I first learned Perl in a tutorial at a Uniforum conference in the hotel at Wairakei (just around the corner from the powerstation) :o)