Our grid is alternating current, with a frequency of 50Hz. This is done as it then becomes very easy to step up and down the voltages to transmit it accross the country at high efficiencies.
The easiest way of keeping the grid at 50Hz is to turn over a turbine 50 times a second. Ofcourse the turbine can be powered in many different ways - by wind, water, gas or steam.
The drag on the turbine from the generator must equal the power being applied to the turbine by the wind, water, gas, steam - this ensures that the grid is kept at 50Hz. Tollerances are on the grid of +/- 0.5Hz - to ensure the safety of electrical equipment.
This sounds very easy - but it must be remembered that the drag on the turbine is constantly changing as people switch things on and off. The grid ramps up and down once a day as everyone wakes up.
The power applied to the turbines must also ramp up and down once a day at the same time as the electrical demand.
Of course occasionally someone in eastenders gets shot, or we loose a football match - and a few thousand kettles are switched on. The power required when these grid surges happen can be huge:
These surges also always come very rapidly.
Now, when these surges (or other events happen) the national grid calls for more energy - yes, partly from stored water resevoirs, but in the first instance from fossil.
Coal power stations (along with gas) have to be able to increase their demand by 10% within 10s and hold it for half an hour:
...and that's the rub - as more renewables come onto the grid this instability is going to increase. Renewables and nuclear can't load balance in this way - they just run - it's not possible to get the wind to blow a bit faster just because a football match is on.
This is one of the reasons why we will keep fossil (such as coal) on the grid.
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