Or in a pinch: just run big-ass space heaters. Seriously. It's a stupid way to burn off excess power, but it's dirt simple and cheap. Just have a big array of resistive heaters out in an empty field somewhere with a high fence around it. Need to burn off an extra GW? Run it through massive heating elements and burn burn it off. It's a stupid waste of good energy, but as an emergency backup, it's not a bad option. It's trivially easy to dispose of huge amounts of excess electricity if you just run the mother-of-all space heaters. Run your stupid giant resistive heater at the bottom of a lake for even better effect.