Electronic – Is electricity wasted when energy generated by power plant is not in immediate demand

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After my second series physics course I have begin to wonder how power plants control energy production to match the usage of a city.

There are several possible answers in my mind listed below:

  1. Does it always over-produce the amount of energy that a city needs so that a certain amount of electricity is wasted?

  2. Does it over-produce the amount of energy but stores the excess energy in molten salt, or pumping water?

  3. Does it match exactly the current energy need?

The reason that this question has come to my mind is that if option 1 or 3 is true, then solar power and wind power is completely useless, as the generated energy must be used immediately, but I have to list as an option as I often don't see dams or molten salt reservoir around solar and wind power generators.

Best Answer

Number 3 with a bit of 2. Conservation of Energy forces the energy input to the grid to equal the energy out in losses / load usage or storage within the system at all times with no exceptions.

When there are small mismatches in the input/output energy, this mismatch is accommodated through the rotational inertia of the electric grid. (this inertia is present in all of the grid connected generators AND motors/loads that are spinning. In the case of North America or Europe, these grids have a very large effective inertia). If there is excess generation, the grid frequency increases. If there is excess load, the grid frequency drops.

Most generators use a control technique that controls the energy input to the prime mover based on the grid frequency. Low grid frequency --> increase energy input; high grid frequency --> lower energy input. This is called Frequency Droop control.

Different generator prime movers have different time responses. Thermal nuclear or coal plants may have a time response on the order of 1/2 - 1 day. Natural gas or water turbines can slew power much, much faster.

Slow responding plants provide "base load", while faster responding plants are "peakers" and follow the variations in the load while the system's inertia makes up the difference.

Small, isolated grids can be more tricky to integrate larger portions of wind or solar; however, it is quite possible. For example, there are many villages in Alaska which get their electricity from a combination of diesel generator sets and wind turbines. Some of these villages get 50+% of their electricity from the wind through a combination of careful system control and "dump loads". These dump loads use the excess electrical energy to heat the water for the village's district heating system.