can cryptocurrencies be energy efficient? Is there an intrinsic reason why cryptocurrencies currently take a lot of energy to keep working? Is there a way to make a cryptocurrency still widely used but make it energy efficient?
As the answer for this question, we have the Proof of Stake approach, firstly implemented by Peercoin (PPC). I think other coins as Ethereum have as well a proof of stake mining/minting approach

links: https://peercoin.net/minting and https://github.com/peercoin/peercoin/wiki/FAQ