I suspect theoretical energy generation efficiency is in disproportion to the difference in potentials (IIRC the hotter something is, the more efficiently you c

I suspect theoretical energy generation efficiency is in disproportion to the difference in potentials (IIRC the hotter something is, the more efficiently you can extract energy from it - not more energy, but more in proportion - but IANAexpert). So while dams may catch rain efficiently, if they were 10x higher perhaps the extra potential energy usable is more than 10x