geothermal energy storage system, repurposing a once-polluting resource extraction site into what they say amounts to a green energy battery. new study published in the journal Renewable Energy, the researchers from the University of Illinois Urbana-Champaign were able to make use of the deep subsurface structure, despite the fact that it doesn’t actually produce geothermal energy. That’s because they found it was the perfect place to build an artificial geothermal reservoir, which stores energy in the form of heat in the surrounding rocks