There are efforts RIGHT NOW to solve this problem. So just like how BOA wastes electricity right now, and is trying to reduce its electricity use, so too are cryptocurrencies.

For example, one way to do it is to power the blockchain via "wasted" electricity from other sources, that was going to do nothing anyway. For example, a heater literally sends electricity through wires, and "wastes" that energy to create heat. One could imagine a heater that uses the electricity/heat from mining for "free".

Or even at a more basic level, imagine a electric dam, that produces so much power during certain times in the day, that it has to sell it for negative prices on the market. Thats Free energy right there!

Another example is Proof of Space algorithms. The way it works is instead of using GPU heavy hashing algorithms, you use "Space" intensive hashing algorithms.

The reason why you do this is because there is a TON of extra hard disk space that is just lying around doing nothing. We could take that extra space for "free" and use it to secure a blockchain (while allowing the user to reclaim the space at any time with no effort), and it would cost very little electricity.