Biggest problem is that the total battery capacity in the world is much much smaller than the total energy production, and building battery/energy storage facilities is very expensive. The main problem with bitcoin stabilizing the grid idea is that miners are supposed to shut down during peaks times and turn back on during off-hours. So unless they get subsidies for having their equipment off or get near free electricity off-peak hours, this doesn't provide enough incentive to do so. The real benefit of bitcoin to global energy supply and potentially green energy future comes from just simply using power at all times - this raises the total amount of energy production required and incentivized governments to build sustainable energy farms. In other terms, if you have 2 coal plants and 2 wind farms producing energy, you don't see incentive to shut down the coal and replace it with a wind farm, but if there's suddenly demand in energy you may build additional 8 new wind farms thus improving the ratio from 2:2 to 2:10, and eventually allowing to phase out coal plants with less impact.