Energy-saving bitcoin mining methods can only be obtained by innovating computer processors or chips regarding performance and power requirements, I think in the future there will emerge the latest generation of computers that are capable of sophisticated computing but with low electricity consumption.
Low electricity consumption is a very relative statement. Computers of today have extremely low consumption compared to first computers (which occupied multiple
rooms in univerisities) while they are extremely more powerful. Sure, you can go further and probably in 50 years people will not believe how we could be using what we are using today.
Anyway, mining can be eco-friendly if it is powered with a dedicated river or wind turbine power station.