Just use it for applications that need heat directly instead of electricity. Heating your home, your hot water, preheating your oven. And tbh even at 30% and that is better than the 0% we have when there's nowhere for renewable energy to go and it has to just be shut off. If the sand battery can be made very quickly and easily and cheaply the inefficiency doesn't matter so much at the moment because we just need fucking somewhere to put it