A convenient alternative to pumped hydro exists and is being implemented by at least one company in the US, it's essentially using trains and gravity to store energy. The basic strategy is to build a rail yard on a hill with a fairly steep incline, and load up many rail cars with rocks and dirt. To store energy, an electric motor winds up a cable and pulls the cars up the hill. To release the energy, simply release the brakes on the rail cars and let them roll back down. It scales linearly (more cars = more energy stored), can ramp up almost instantly, doesn't lose any energy over time, and uses cheap, existing technology and infrastructure. All you need is a big hill that nobody's using!