A primary candidate for free energy in icy moons is thermal venting at the bottom of the liquid oceans; they do have rocky cores, after all. If Jupiter's tidal forces can cause the volcanism on Io, then it's reasonable to assume that they can also cause the rocky interior of Europa to produce volcanoes that vent heat and interesting ions in to the liquid water.
There's also a surprising amount of electrolysis going on in the ice of Europa, because Jupiter has such a terrifying electrical field. I doubt that's enough to sustain an ecosystem, but it's enough for me to fantasize about giant upside-down forests of filter-feeders digging their roots upwards to get at the free oxygen.
The preliminary results we're seeing on Pluto should also adjust your expectations in favor of ice-moon habitability; there, we see active tectonics on a Kuiper Belt Object even without the tidal forcing of a nearby planet. It seems that a giant pile of silicates and water ice provide a great deal of dynamism all on their own.
How does the free energy on an ice moon compare to the amount the sun shoots down at earth?
This is the first in a series of posts I am putting together on a personal blog I just started two days ago as a collection of my musings on astrobiology ("The Great A'Tuin" - sorry, I couldn't help it), and will be reposting here. Much has been written here about the Fermi paradox and the 'great filter'. It seems to me that going back to a somewhat more basic level of astronomy and astrobiology is extremely informative to these questions, and so this is what I will be doing. The bloggery is intended for a slightly more general audience than this site (hence much of the content of the introduction) but I think it will be of interest. Many of the points I will be making are ones I have touched on in previous comments here, but hope to explore in more detail.
This post references my first two posts - an introduction, and a discussion of our apparent position in space and time in the universe. The blog posts may be found at:
http://thegreatatuin.blogspot.com/2015/07/whats-all-this-about.html
http://thegreatatuin.blogspot.com/2015/07/space-and-time.htm