I don't want to use double energy. I'm at 15. Can I cash them in?

Double energy? Perhaps you mean the star booster? They fill up your star (above your scene when you finish playing) twice as fast. Helpful when you want to speed through a new scene to get to the next story clue. But you don't want that, you can just activate it to give yourself a full bar of energy or top of what you currently have. That would be how you would "cash it in". Have to wait 30 min. before you cash the next one.