Since, and unless electricity has properties I’m not aware of, it’s not possible for electric power plants to produce only and EXACTLY the amount of electricity being drawn at an given time, and not having enough electricity for everyone is a VERY bad thing, I’m assuming the power plants produce enough electricity to meet a predicted average need plus a little extra margin. So, if this understanding is correct, where does that little extra margin go? And what kind of margin are we talking about?
In: Engineering
Excess electricity will speed up the turbines (let them speed up) in the power plants, which means the frequency of the voltage in the grid rises.
As this will be a problem if it increases (or decreases in case of lacking electricity) too much it is tightly controlled by reducing the amount of steam (or water) that reaches the turbines.
You can watch it happening live:
Edit for hopefully working link for everyone:
https://www.netzfrequenzmessung.de
This is for Germany (which is identical to all of mainland EU) so the target is 50.00 Hz.
It must be used. The exact amount generated is used. When demand fluctuates, it changes the load on the generators, which in practice changes the resistance to the spin of the turbines. Those are big and have a lot of inertia, so a slight increase or decrease in resistance can be absorbed by the system.
What happens is that the turbine speed changes, which changes the phase slightly. It averages out, but clocks that use phase to keep time drift a little as demand fluctuates.
A big problem with renewables is that they don’t have this feature. If you can’t increase usage by charging batteries etc, then you’ll oversuply the network, which will firstly make lightbulbs shine slightly brighter, and then start tripping fuses.
Traditionally power grids were comprised of baseload generators (coal, nuclear, etc), which could handle power fluctuations on the scale of hours to days via ramping up and down, and power fluctuations on the scale of seconds via the inertia of their turbines. Then things like hydro and gas turbines handled fluctuations on the scales of minutes.
[https://grid.iamkate.com/](https://grid.iamkate.com/) imma put this here and say nothing else other than power grid management is much much more complicated than anyone gives it credit for.
Most power plants work by rotating a big turbine which spins a shaft that spins the magnets in a generator. In the generator the rotating magnets creates a rotating magnetic field. And the windings in the generator which is hooked up to the three phase AC of the grid also produce a similar magnetic field. When these spin the same speed no current is produced by the generator. But if the generator gets ahead of the AC phase it produce power which also makes the AC speed up. Similarly if the generator starts slowing down the AC generated magnetic field will pull it back up to speed which slow down the AC.
This all means that all the turbines in all the power plants in the grid is all connected together and spins at exactly the same speed. And they have quite a lot of energy stored as rotating mass. If a single power station generates too much power the generators will spin faster. This takes up any excess power that is generated. But when the grid controllers slow down that power station and make it produce less power then is needed all the generators will release this energy as they slow down.
So there is a tiny bit of energy storage in the electricity grid, in the form of these big generators and turbines. It does not last for many seconds though so grid operators need to constantly increase or reduce power to meet the demand as accurately as it can.
This can be compared to driving a car. In order to maintain a fixed speed the engine needs to produce exactly as much energy as the car lose in drag and resistance. So these is a throttle position which works for the speed you want to go. But if you push the throttle a bit too hard or a bit too soft then the car is not going to instantly go super fast or instantly stop. You have some time to notice that the speed is not right and correct your throttle. And when there are changes in the driving conditions, going up or down a hill or going around a curve, just like there are different loads being applied to the electricity grid, you have time to adjust the throttle to meet this changing demand.
The excess energy is accelerating the turbines in the power plants, so the energy is stored in the inertia of the generators. The same happens when there is not enough production for the current demand, the energy comes from the inertia of the generators which causes them to speed down.
This acceleration can be measured in the grid frequency. If the frequency goes up, the operators know that they have to reduce power of power plants, and vice versa if the frequency down. Even huge demand spikes can be balanced within seconds, so the frequency doesn’t even change that much (normally less than 0.1 Hz). For this purpose, plants with very fast reaction speed (like hydropower) are used, also battery storage is a very good solution, because they could react within less than a millisecond (that’s not really necessary though).
In the UK grid, there are facilities for using surplus power – specifically the facilities in North Wales, where excess power is used to pump water up to a reservoir at the top of a mountain. This reservoir is a quick method of dealing with sudden demands on the grid as it can be “turned on” in a matter of seconds simply by opening (huge) valves.
[https://youtu.be/McByJeX2evM?si=7h9W2Wv7mfuCylIc](https://youtu.be/McByJeX2evM?si=7h9W2Wv7mfuCylIc)
In some cases, the electricity is stored for later use. While large-scale betteries are talked about as a new thing, there are older methods that work by changing the spare electricity into other forms of energy.
Here in Michigan, our two biggest power companies share ownership of the [Ludington Pumped Storage Facility](https://en.m.wikipedia.org/wiki/Ludington_Pumped_Storage_Power_Plant), which is a big reservoir uphill from Lake Michigan. It has been in use for 50 years to store energy from when the utilities’ nuclear plants are making “too much”, and then put that energy back onto the grid when it’s needed. Now that Michigan is investing heavily in wind and solar, it works well for those too.
When there’s “extra” electricity, the pumped storage facility uses that extra to pump water uphill, from the lake to the reservoir, converting electrical energy into potential energy. When the power is needed back on the grid, the storage facility lets the water run back downhill from the reservoir to the lake – spinning turbines on the way to make electricity just like a regular hydropower dam.
In Michigan’s Upper Peninsula, there’s also work being done on gravity storage using the old copper mines: using a winch to pull a heavy load up to the top of the mine shaft when there’s too much electricity on the grid, then letting the load lower back down to spin a turbine when the energy is needed again. (I understand there are places in Europe that do something similar with heavy trains on mountain slopes?)
Worth noting that some power companies have the infrastructure to store excess power off the grid and access it later during periods of higher demand. [Here](https://en.m.wikipedia.org/wiki/Raccoon_Mountain_Pumped-Storage_Plant) is an example of such a facility.
Most of the excess that you are thinking of doesn’t actually exist, it’s just extra capacity based on the forecasted demand for a given period of time.
Heres a comedic sketch regarding the issue. These guys are working with “using up” the excess power using regular appliances.
[Uti Vår Hage – Norsk Sluttstrøm (youtube.com)](https://www.youtube.com/watch?v=sAV2Ft2w928)
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