[pfdietz]

I've wondered if such systems could act as thermal batteries. That is, use industrial heat pumps to generate moderate temperature steam, inject that into fracked rocks, then recover later as geothermal steam. The storage time constant of the rocks scales as the square of the linear dimensions, and can easily be many years (that's why geothermal works.)

For extra fun, do this in rocks that contain subeconomic levels of some interesting mineral resource and use the hot fluids to transport that mineral out of the formation for recovery.