Spinning Reserve and Energy
Date: Winter 2012-2013
Electrical power is continuously generated at power stations and distributed. If the distributed power is not consumed and at the generation end, power is being continuously generated, what happens to the generated power?
Thank you for an excellent question.
First, consider that power consists of voltage and current. In simple terms, voltage x current = power. When power demand changes with time (such as when many air conditioners or other "loads" are switched on or off) the supplied voltage remains relatively constant while the current changes significantly.
Second, consider energy and power. Energy is required to do mechanical work and can be measured in joules. One joule = one newton meter. Power is the rate at which energy can be produced. Power can be measured in (joules per second) = watts. If you want to lift a 10 newton weight a distance of 100 meters, that will require 10 x 100 = 1000 newton meters of work. If you do this with a machine producing 1 watt of power, it will require 1000/1 = 1000 seconds. If you do the same amount of work with a more powerful machine producing 100 watts of power, it will require 1000/100 = 10 seconds. In these two cases the amount of work done is the same but the time required varies inversely with the power that is used.
Most traditional power plants (excluding solar) rely on rotating mechanical generators. The rotating speed mainly determines the voltage, while current demand from loads causes mechanical drag on the shaft of the machine. As the load current demand changes, this results in varying load on the mechanical generators. With no adjustment of the mechanical driving power, that would tend to cause the generator speed to increase or decrease. That would change the rotating speed, thus the voltage and the frequency of the alternating (AC) voltage. However the speed of the generators are closely controlled to prevent slow variations, and the "inertia" caused by the rotating mass of these rotating machines stores energy. On a very short time scale, the "dynamic" energy storage in the rotating mass of these machines can either absorb or deliver energy; this prevents rapid changes. Thus the voltage produced is nearly constant even as the power and load current is varied. This is the way in which small variations in power demand are accommodated.
Larger changes in power demand are usually a little slower. For this, power utilities (companies that generate the power) typically have different types of power plants.
On one extreme are nuclear plants. These generating plants generate a relatively constant amount of power (and current) which is difficult to change. Typically, nuclear power plants provide the minimum or "baseline" power level which is constantly required. This might be the level which would be used during the middle of a night when most people are sleeping and outdoor temperatures are moderate.
Another type of power plant is one which is powered by natural gas. These can be started up or adjusted or shut down quickly in order to accommodate peak demand variation. These peaks can happen during very hot or very cold weather, or when business is very active. Coal is another fuel which is commonly used. These plants can also be switched on or off but may be slower to change than the natural gas fired plants.
There are also wind and solar plants, which current and power output vary due to weather conditions. Because the available output from these is often unpredictable, they mostly cannot replace other power plants. The best that these plants can do is to reduce the usage of coal or gas fired plants so as to save fuel and exhaust emissions.
In summary, the power that is generated will generally not exceed the power which is demanded. Rather, the current will vary according to demand. This is accomplished with a combination of energy storage (mostly in mechanical inertia) and with power plants which can be started or stopped according to demand.
Please let me know if this answers your questions.
Robert M. Zwicker
I think you misunderstand how a generator works. A generator
(whether a large one in a power station, or a small gasoline powered
one) is a device that converts rotational mechanical power, into
electrical power. If more electrical power output is required because
the electrical load increases, then the mechanical power driving the
generator also must be increased. Conversely, if less electrical power
is needed from the generator, then less mechanical power is needed
to drive the generator.
Most generators (including all generators in power stations) have a
feedback control system that controls the amount of mechanical power
driving the generator in order to ensure the generator's output power
matches the amount of electrical power the load is consuming.
This feedback principle is similar to what happens when a car being
driven at a constant speed, suddenly starts going down a hill. Less
power is now needed to keep the same speed, so as the car starts to
speed up, the driver releases the gas pedal a little to reduce the
engine's power and keep the car's speed constant.
The thing to remember is that the amount of mechanical power
needed to drive a generator, increases as the generator's electrical
output increases, and vice versa. What impressed me as a young guy
(many!) years ago, was a demonstration where a small generator was
driven by a set of bicycle pedals. The little generator was connected to
a number of lamps. When none of the lamps were connected, it was
easy to pedal very fast, but as each lamp was connected to the
generator, it became harder and harder to pedal. Similarly, as the
lights were one by one switched off, it became easier and easier to
drive the generator with the pedals. In sort, the amount of electrical
power needed to light the lights, was reflected in the amount of leg
So what happens to the electrical power a generator produces, when
the electrical load is reduced? The answer is that the generator
becomes easier to turn, and therefore less mechanical power is
needed to drive the generator, and it now generates less power to
match the requirements of the new (lighter) load.
To start, we need to be clear on the difference between power and energy. It is useful to think in terms of an automobile in this case. We fill the gas tank, and the amount of gasoline in the tank represents how much energy the car can use. We know this intuitively because if we run the car too much, it uses up the energy in the gas tank and we have to refill it. The amount of energy we use over a specific time represents power. In other words, power=energy/time. For cars, we represent this power as horsepower (hp). We may have a car rated at 250hp, but the car does not use this amount of power except when the engine is loaded to its capacity such as when we slam the accelerator pedal to the floor or we have to pull a heavy load. When the car is idling or simply cruising on the highway, it is not using its advertised power of 250hp. In these cases, we know that the gas tank will not empty as quickly as if we run the car full out at 250hp, but we still list the car as, "having 250 horsepower."
Similarly, power plants generate power based on how this power is drawn. If the generator is spinning, and there is no load on it, no power flows from the generator. You will still talk of a power plant as a "10 megawatt power generation plant" in the same way you would talk of the horsepower available in your car. However, if there is no load on the generator, no energy is consumed (the gas tank level stays the same).
Kyle Bunch, PhD, PE
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Update: November 2011