What happens to generated electricity if it is not used?

Question

Let's take for example hydroelectricity produced at a dam and not consumed by any household or industry... Will the electricity need to be stored? What will happen if it is not stored? Does it flow like water and get wasted back to the earth? What happens to it?

Answer 1

TL;DR: a small portion of the excess energy is stored in the form of rotational kinetic energy, however, generator controls make it so these amounts are not significant.

electricity generation

(Al least up to now) electricity production is mainly produced by rotating machinery ( I.e. a rotor rotates in the stator). (The one notable exception to electricity is solar energy that uses the photovoltaic effect, and its becoming more and more common.)

In the usual case of rotating machinery, the process usually utilizes Faraday's law of induction (i.e. the production of voltage in a time varying magnetic field). So, somehow (either falling water from a hydroelectric dam, or by creating steam in a thermal plant), a large shaft is rotated inside a magnetic field, and this produces voltage and current flow.

grid frequency

The AC grid frequency is another parameter which is very important. All AC grids have a specified frequency that they operate (for example in the US is 60 Hz, while in the Europe is 50 Hz). That frequency of the AC voltage is determined by the speed that the shafts rotate.

Rotating of shafts

Regarding the rotational speed $\omega$ and rotational acceleration $\alpha$ of the shaft, it can be described by the following equation

$$\Delta M = I \alpha = I \frac{d \omega}{dt}$$

where:

• I is the mass moment inertia of all rotating masses
• $\alpha$ is the angular acceleration $(\frac{d \omega}{dt})$
• $\Delta M$ is the total torque on the shaft. This can be thought to be be equal to $$\Delta M = M_{gen}-M_{load}$$

where:

• $M_{gen}$ is the generating torque (e.g. the torque on the shaft from the water on the rotating blades)
• $M_{load}$ is the torque which comes from the electric load and its resisting rotation.

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To me a useful analogy for the $M_{gen}$ and $M_{load}$ is a bicycle. In the bicycle the rider applies $M_{gen}$ on the pedals. The $M_{load}$ is determined by factors like the inclination.

So when a rider starts from rest on a level surface he/she will be able to accelerate faster the wheels, compared to a starting from rest on an uphill.

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Bottom line is that if:

• $M_{gen}>M_{load} \rightarrow M_{gen}-M_{load}=\Delta M>0$ then the system accelerates
• $M_{gen}=M_{load} \rightarrow M_{gen}-M_{load}=\Delta M=0$ then the system does not change its kinetic state.
• $M_{gen}<M_{load} \rightarrow M_{gen}-M_{load}=\Delta M<0$ then the system decelerates

Tying it all up.

When energy is produced in a hydroelectric dam, $M_{gen}$ will be applied on the torque, and there will be also $M_{load}$ from the electric load on the grid. In a steady state condition $\Delta M =0$. In that case the speed of shaft remains constant and does not change.

When appliances turn off, the what happens is that $M_{gen}>M_{load} \Rightarrow \Delta M>0$, therefore the shaft will accelerate. The consequence is that the frequency of the grid will change. Sub-second changes to generators on the grid are controlled in a distributed fashion by automatic generation control (AGC), which monitors for these changes in frequency and adjusts $M_{gen}$ accordingly.

The inertia of the spinning rotors on the grid can be said to "store" the excess energy, however in reality this inertia only creates a temporary buffer allowing for AGC to kick in and reduce $M_{gen}$.

So, although a small portion of the energy can be said to be stored in the form of rotational kinetic energy, there is not in fact any meaningful amount of "excess energy".

Answer 2

The main problem is that all the terminology associated with electricity was formalized before anyone knew what electricity was. Early on, there were three competing theories about what electricity was. Each contributed terminology that only made sense in terms of it's own theory, and all three of them were horribly wrong!

So we have an entire branch of engineering where the words don't mean what they mean in the other branches of engineering. Actually, we have two branches where this is true. We were building airplanes for 30 years before anyone figured out why they flew, and aero engineering has much the same problem.

When you generate electricity, you aren't creating or transforming anything at all. All you are doing is pushing on electrons. Whether they move or not depends on whether they have a path to travel along that doesn't push back as much.

So you have to understand that you don't actually generate electricity in a generator, you don't store electricity in a battery, and you don't consume electricity in a motor, that's just the unfortunate terminology that English adopted. It fails completely when you attempt to make analogies using these terms. You have to swap out the vocabulary when you make physical analogies to other physical systems.