Pumped Hydro: The strongest contender (4:06). By using existing mine shafts and voids as reservoirs, researchers hope to avoid the bureaucracy and environmental impact of surface-level dams. However, challenges like fractured rock, chemical reactions, and structural instability make this a complex site-specific engineering task rather than a simple scalable solution (5:12–6:30).
Gravity Storage: While concepts like those formerly developed by Gravitricity (using heavy weights in shafts) are physically sound, they face significant hurdles in commercial deployment (6:32). The video notes the liquidation of Gravitricity as an example of these ideas struggling in the "valley of death" between demonstration and commercial reality (7:05).
Compressed Air (CAES): An older technology being revisited with modern, more efficient designs (8:10). Using underground caverns for air storage is more cost-effective than surface tanks. Companies like Hydrostor are exploring "hydrostatic compensation" to manage pressure and structural integrity, though efficiency remains lower than lithium-ion batteries and heavily dependent on geology (9:10–10:12).
Conclusion:
The video concludes that while abandoned mines offer a vast, existing infrastructure opportunity for energy storage, they are not a "silver bullet" (10:53). The energy transition will require a "messy, complex mix of solutions" rather than a single breakthrough, and these mine-based ideas will likely evolve significantly or face challenges that prevent widespread adoption (11:16–11:40).