This is not a new idea. I used such a system at the ABS several decades ago for the census data. Less frequently used data was automatically migrated to a slower, higher capacity device (I think it was a helical scan tape library). When an access request was made the data was staged back again to a high speed disk. The system automatically managed what was where.
The Australian National Computational Infrastructure has two automated tape libraries with a total of about 6500 tapes and a system to automatically manage the migration of files.
Using such systems to save energy, rather than money, is a clever twist on this old idea. The algorithms used to do the staging would only need to be tweaked slightly, if at all, to take into account energy use.