what you are referring to is a liquid diet. keep in mind those greens powders can have a ton of calories per scoop if you get a poor brand. the one i use only has 25 cals per scoop.
you don't need food to workout, your body has plenty of storage to get you through a workout. I fasted for 19 days with no food and was able to workout daily
if you click my link below i published an Ebook on fasting strategies, i include the liquid fasting too