[olau]

Besides existing hydro, there's biomass (like coal plants, just without the coal), new pumped hydro, thermal electric (where you heat up rock and use the heat to drive a turbine, still no full-blown plant), and chemical batteries like lithium-ion or flow batteries. Chemical batteries are currently too expensive for longer term storage, but lithium-ion are already competitive for peak shaving.

So no new invention is needed, but some of this tech needs maturing/getting cheaper.

Extending the grid is also often helpful, albeit still somewhat expensive. I think we need someone to work on making that cheaper.

If you just need energy for heating, you can store the heat in a big insulated pond with an insulated floating lid on. That's cheap, and good enough for seasonal storage. There are several of those ponds in production already, village-sized ones, but they're going to build a town-sized one not far from where I live.