Storage by Duration

A 4-hour battery solves the daily solar curve. A 12-hour pumped hydro facility bridges overnight demand. But what handles a week-long winter weather event with no wind and no sun? That multi-day gap is the hardest unsolved problem in grid decarbonization.

Storage technologies map naturally to timescales. Short-duration (0-4 hours): lithium-ion batteries dominate. Fast response, declining costs, broad deployment. Handles daily solar shifting and frequency regulation. Medium-duration (4-24 hours): pumped hydro is the incumbent. Some emerging technologies (iron-air batteries, compressed air energy storage, gravity-based systems) target this range. Handles overnight demand and short weather events. Long-duration (24+ hours to seasonal): no proven, scalable, affordable technology exists today. Candidates include hydrogen (electrolysis plus storage plus fuel cells or turbines), compressed air in underground caverns, and thermal storage. The challenge is that these assets cycle infrequently, so capital costs must be extremely low per kWh.

Quantify the gap. At 80% renewable penetration, the grid can manage most days with 4-hour storage. At 90-100%, multi-day low-wind, low-sun events (Dunkelflaute in German) require tens of GWh to hundreds of GWh of stored energy per region.

Apply cost logic. A lithium-ion system scaled to 100 hours of duration would cost $20,000-$30,000/kW at installed, turnkey prices (roughly $200-300/kWh), about double the raw cell price used elsewhere in this curriculum, since installed price also covers power electronics, enclosures, and labor. Hydrogen storage in salt caverns: roughly $1-5/kWh of capacity.

Why is the long-duration gap considered the defining challenge for deep decarbonization?

Last 10% is the hardest. Going from 50% to 80% renewables is mostly a matter of building more solar, wind, and 4-hour batteries. Going from 80% to 100% requires handling multi-day weather events without fossil backup. No current technology does this economically.

Storage duration is the dimension that separates achievable near-term targets from the full decarbonization challenge.

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