Hi,
7km long deep closed 1dm diameter cylinder with peltier walls and water inside, also little turbogenerators for steam inside could give 1GWt as permanent energy source, go for it
details:
7km deep give about 160 celcium for about 500m.
could use peltier effect walls there
with water inside
boiling water never exceeds 100 celcium
gives about 50 celcium of permanent temperature difference
profit
