= Solution
Storage has multiple performance measures: <energy> and power <density>, efficiency, cycle/calendar life, duration, cost, materials, safety and siting. Lithium-ion batteries offer high <round-trip efficiency>, fast response and compactness, making them useful for vehicles and short-duration grid services. That does not make them optimal for every duty. Pumped hydro can suit large stationary <energy> stores where geography permits; thermal stores can serve <heat> directly; <Chemical fuel>[chemical fuels] or other long-duration systems can trade efficiency for cheap <energy> capacity. A technology that is excellent per kilogram may be poor for seasonal storage. “Best” requires a specified service and life-cycle comparison.
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