Nothing is impossible, just improbable.
That was Albert Einsteins take on the possibility of time travel.
As for free energy, I think we will eventually get to a point of developing practical Cold Fusion Devices, that is clean and sustainable energy but of course their is the initial cost to factor.
In my opinion the most obvious source of free energy in multiple forms, is our Sun; once we get to a decent technological level, we could partially surround the Sun in a some form of grid like network which would not even be noticeable from Earth but with this we could take advantage of several different types of energy to harness; a few examples being, Light (Photons), Radiation (Gamma) which would generate heat and cause chemical reaction with certain substances with would product more heat energy.
We then have Neutrinos of which pass through us everyday, billions of them which don't do use any harm but i travelling Neutrinos found a substance that it could not pass through, then that theoretically could generate heat; moving the movie 2012 to the side, Scientists are actually in India, in a deep underground chamber, seeing what Neutrinos could be doing to our Earths core.
After all that, Cosmologists and Theoretical Physicists, have already discussed the possibility, once we are at an established intergalactic level, of use traveling to distant Stars, encasing them completely and literally, using probably some form of Gravity Drive System to move the Star to where it would most be needed.
Passed that point, of us basically turning a whole Star into a super-massive Solar battery, if think that life as we know it would be so far advanced in almost everyway, that at our current level of understanding, we would lose all forms of point of reference for pretty much everything to do with living at such an advanced level of understanding and spiritual awareness.