Well, the extra-energy can be stored and sold later. I think that it isn't profitable mining bitcoin with this extra-energy because of the high price of the ASICs (and the maintenance). Not only free electricity is necesary to mine bitcoin, it's necesary a investment in machines and space.
Bye
Bye
You really can not store AC power in any meaningful way.... and AC to DC, then back to AC for transmission.... that isn't efficient either.
The power system, likely looses about 25 to 40 percent of the energy from generation to your door in things like line loss, along with losses at transmission.
While I am not saying the OP's idea is good or bad, since power systems are run by governments, and they manage to mess it all up at least 3/4 of the time..... storing electrical energy isn't as easy as you would think.
Quote from: http://cleantechnica.com/2013/10/05/a-potential-solution-for-long-distance-power-transmission-leakage/
Along the route, through the hundreds or even thousands of miles of high tension lines and towers (called pylons) significant but naturally-occurring electrical current losses of up to one-third of the original output are experienced which must be covered by 1) sending more current than is needed to ensure the required amount reaches the city after the losses are factored in 2) the installation of costly substations, with their hugely expensive transformer systems to recover and re-modulate the current.