From the very start of electricity, supply and demand had to be balanced.
Initially, it could be done by changing the output of the generators. The shifts in load were slow, mostly between day and night, and predictive.
This was the case through VWII, but after the war some fundamental changes, both load as well as of generation, made the balancing more demanding. On the load side the introduction of air conditioning significantly increased the amplitude between the bottom, referred to as the baseload, and the peak of demand. On the generation side nuclear power emerged. These plants had basically no flexibility and had to run all the time. Further, the increasingly large coal plants and the first generations of combined cycle gas turbines (CCGT) had limited flexibility. These generators, so called “baseload generators”, needed to be supplemented with more flexible generation to follow the load, from the baseload all the way to the peak power demand. Hydro power was the main flexible resource.
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