On very windy and sunny days the energy production of renewables skyrockets, but what happens to all that electricity that doesn’t get consumed? Is it all dispersed in the ground/heat or am I missing something?
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Most of the power plants connected to the grid does so with a generator. This includes coal, gas, hydro, nuclear and geothermal power plants but not wind or solar. A three phase generator is the exact same component as a three phase electric motor, it just depends on how it is installed. If the power production in a grid is higher then the power consumption then the generators will start working as motors and spin faster. Due to the high rotating mass of the generators and connected turbines it takes a lot of energy to change the speed so it takes some time for the generators to speed up.
Meanwhile the grid controllers will detect that the generators speed up because the frequency of the grid goes up. So they will start shutting down power production. Because solar and wind energy can not be stored they will not shut down this but they will reduce production from the other power plants starting with the most expensive ones. After some time the electricity production will be lower then the consumption and the generators will start slowing down and the grid frequency will return to normal.
So the power generated by wind and solar can be stored in the rotating mass of the generators of the other power plants, at least for shorter amounts of time. There is another concept which involves storing electricity in batteries which are connected to the grid and simulating a rotating generator. A few of these have been built, mostly small test plants. But there is a big production scale plant installed in South Australia.
In some cases though there is a need to stop production from wind and solar. When the grid controllers have extinguished all their options and the wind and solar plants is able to power the entire grid without any other power plants active and all the lines out are at capacity they will order these power plants to shut down as well. Wind turbines can change the pitch of their blades in order to optimize their power production based on the amount of wind. But it is also possible to change the pitch of the blades so the wind turbines produce less power. Even stopping the wind turbine entirely. In this case the wind energy is able to pass through the wind farm without getting collected. Solar cells can also safely be disconnected from the grid. The excess energy from the sun will then not be collected by the solar cells and just end up heating up the cells.
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