I mentioned on the climate section of the issues page that pumped hydro is already in use. Wikipedia has an article that explains how it works and some issues with it. A tl;dr version is that it’s in use generally in naturally hilly areas, and has been for over a century, so this is existing proven technology, not cutting edge stuff waiting on technology breakthroughs or startup funding.
Minnesota, of course, is not hilly, but that doesn’t mean we have no hills, just that we don’t have so many areas that are naturally suitable. We will have to, in all likelihood, pay more for such facilities since more of the facility needs to be man-made.
This is said in the way of being open that our costs are likely to be more than other areas needed to pay to build these facilities, but then again, the price of doing nothing is basically the whole planet. Moreover, something even science deniers would have to admit, Minnesota has no fossil fuels. None. Every lump of coal or drop of oil gets imported. That means every lump and drop represents money leaving the state. Any power we can generate from within the state represents money that is staying here. It represents home-made power that insulates us from any shocks in fossil fuel markets. Anyone who lived through the oil shocks of the 1970’s will get this intuitively.
So yes, pumped hydro might prove expensive, but it’s money that we will spend on ourselves, and will let us do our part to fight global warming. Our diminishing moose population will thank us, though our emerald ash borers may have a contrary opinion.