As you can see, this system would preserve most of the current Proof Of Work characteristics of bitcoin mining but with a twist that allow the mining pools to put their system to rest for a significant period of time, thus saving energy and money. I've chosen the duration of the two phases in an arbitrary manner, so that, for every day of competition we'd 10 have days of minimal energy consumption.
While I can't see any major drawbacks to this system, I know how tricky game theory can be, so I'd love to hear your feedback on the idea.
Except that if you reduce all miner's energy costs by 90%, they will all use their vastly increased profits to buy many times more hashpower to maintain their share of the pie. You end up with 10x more hashpower sitting idle 90% of the time.
In the end, you'll always have: cost of total energy spent on mining = block reward + fees ...