Any must match electricity production to consumption, both of which vary significantly over time. Energy derived from and varies with the weather on time scales ranging from less than a second to weeks or longer. is less flexible than , meaning it cannot easily match the variations in demand. Thus, without storage presents special challenges to .
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Electricity can be stored directly for a short time in capacitors, somewhat longer electrochemically in , and much longer chemically (e.g. hydrogen), mechanically (e.g. pumped hydropower) or as heat. The first pumped hydroelectricity was constructed at the end of the 19th century around in Italy, Austria, and Switzerland. The technique rapidly expanded during the 196.
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