I don't think it's possible to be energy efficient in a proof-of-work blockchain? Whatever energy saving you come up with will be negated by having to match competing miners' increasing throughput. If you are not running on 100% you are losing out?
From the perspective of the hardware manufacturers that's an advantage. If you're making the hardware and you come up with a 10% energy savings, everyone must now replace their old hardware or be left in the dust. Hardware manufacturers will make a killing. The prisoner's dilemma you are thinking of only applies to the miners, it's a benefit for the parties selling them the hardware.
Something something shovels in a gold rush...
If all hardware was equivalent and an unlimited amount were available efficiency wouldn't matter, but neither of those things are true.
Bitcoin network bandwidth remains the same, and energy usage has continued to increase in a race to the bottom in terms of work and waste for bitcoin mining - this is by design of how Satoshi's difficulty algorithm reduces efficiency with increase in hash rates.
Stuff like this is a much more interesting use of cryptocurrency and blockchain technology in my opinion, instead of burning electricity for PoW you can do meaningful work while collecting a reward.