Like solar, hydroelectric, wind, and geothermal power stations. Mining farms locate near those sources of clean/cheap electricity to make the operation more profitable. About 50% of the electricity generated worldwide is renewable and the rate keeps increasing.

Also, let's not forget that dirty energy gets wasted because it can't be stored so bitcoin mining is there to avoid that waste so this is another bonus.

Author of ´Bitcoin For Mere Mortals´ available @Amazon. Subscribe to my blog: albertoguerrero.net/subscribe and you'll get a copy of my book.