The myth of intermittency

It is widely perceived that because the wind resource is intermittent, the wind technology is not ‘reliable’ enough to be a major power source.

Watching a single wind turbine stop and start, it might seem logical to conclude that, as more of these machines are built, the result can only be an unreliable supply.

The entire electricity system is variable, like wind energy. Both supply and demand of electricity are infl uenced by a large number of planned and unplanned factors. The changing weather makes millions of people switch on and off their supply. Millions of others expect instant power for lights, TVs, computers.

Conventional power sources are intermittent.

On the supply side, no power station of whatever type is completely reliable. Large power stations that go off-line, whether by accident or for maintenance, do so instantaneously, causing immediate loss of power. When a fossil fuel or nuclear power plant trips unexpectedly, it takes a capacity of up to 1,000 MW off the network instantly. That is true intermittency.

Power systems have always had to deal with these sudden output variations, as well as variable consumption, and the procedures put in place by network operators can be applied to deal with variations in wind power production as well.

Variability and intermittency are different concepts.

Variations in wind energy are smoothed by the fact that there are hundreds or thousands of units in operation, making it easier for the system operator to predict and manage changes as they occur. The system will not notice the shut down of a 2 MW wind turbine, but it will have to respond to the removal of a 500 MW coal fi red plant or a 1,000MW nuclear plant. Wind energy does not suddenly trip off the system.

So the issue is not one of variability in itself, but how to predict, manage and ameliorate electricity variability and what tools can be utilised to improve effi ciency. Wind power is variable in output, but this can be predicted to an increasingly accurate extent.

The electricity system, not the turbine is what matters.

It is the net output of all wind turbines on the system or large groups of wind farms that matters for electricity needs. Wind power has to be considered relative to the overall variability of demand and the intermittency of other power generators.

  • The wind does not blow continuously in one place, yet there is little overall impact if the wind stops blowing somewhere – it is always blowing somewhere else.
  • Therefore wind can be harnessed to provide reliable electricity even though the wind is not available 100% of the time at one particular site. In terms of overall power supply it is largely unimportant what happens when the wind stops blowing at a single wind turbine or wind farm site.
  • The more wind farms that are built over a wider geographical location, the more reliable wind energy is.

The EWEA report “Large scale integration of wind energy in the European power supply” analyses these issues in depth. The report’s main conclusions are that the capacity of Europe’s power systems to absorb signifi cant amounts of wind power is determined largely by economics and regulatory rules rather than technical or practical constraints. Already today, it is generally considered that wind energy can meet in the region of 20% of electricity demand on a large electricity network without posing any serious technical or practical problems – as proven by the example of Denmark.

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