Hacker News Reader

It's not possible to lower the energy consumption of PoW like that, it has to constantly be mined to stay secure.

Peercoin is hybrid PoS+PoW, but for other reasons. Their university pages are well written for most people to read.

Sort of, if btc checkpoints eth. Hash the entire eth PoS state tree every hour or so and include that hash with block height in a bitcoin transaction.

That's polygon basically (which uses Ethereum for its checkpoints). It's one of the most used chains.