I haven't studied it, but the general compaint seems to be it takes a fair amount of power to run the computers to generate coins, validate transactions. (enough to power over 1,000,000 american homes, by some accounts[1]) China has some of there mines near hydropower (the cheapest power source).

[1] https://blockgeeks.com/bitcoins-energy-consumption/

https://motherboard.vice.com/en_us/article/qkvxk3/chinas-big...

https://qz.com/1026605/photos-chinas-bitcoin-mines-and-miner...