I wrote an article for California Sunday Magazine about BitTorrent founder Bram Cohen's new cryptocurrency, called chia. It uses the Bitcoin codebase, but Bram is replacing the energy intensive proof-of-work validation algorithm with a combination of proof-of-unused-hard-drive-space and proof-of-wall-clock-time, which does not require much electricity at all.
No one owns the Bitcoin network — the code is open source — so Cohen is free to build off of it to develop his spinoff, Chia Network. With Chia, Cohen aims to replace the energy-greedy part of Bitcoin's code, which rewards miners for generating trillions of numbers a second, with an energy-efficient system, which rewards miners (or "farmers" in Chia's parlance) based on the amount of unused hard-drive space they have on their computers and how long they've had it. If you opt in, the Chia Network will essentially populate that unused space with bingo cards. If Chia calls the numbers on your card, you'll be awarded newly minted chia.