What is intermittency?

Intermittency describes sources whose output rises and falls with conditions outside human control, such as wind speed and sunshine. It matters because a grid must match supply and demand at every moment, so an unsteady source needs help from other supply, storage or flexible demand. It is the main reason that a plant's size alone does not show how useful it is.

How it works

Output from such a source changes over several time scales: seconds as clouds pass, hours as the sun sets, days as weather systems move and seasons as the year turns. Spreading plants over many places and mixing wind with solar smooths the total, because their lulls rarely coincide.

The costs of intermittency appear elsewhere in the system. Backup plants must be kept ready and are used less, storage must be built, and the grid must be able to ramp other supply quickly. The more of the supply that is variable, the more these balancing costs matter.

Example

Say a region relies on 1,000 MW of wind. On a windy day it produces 800 MW, but on a calm day only 100 MW, while demand stays at 600 MW. On the calm day there is a shortfall of 500 MW to cover with other plants or storage. If a second wind farm of 1,000 MW in a different region has calm days that coincide only half the time, the combined output on a day when one is calm is typically higher, so the expected shortfall falls. This is an illustration.

How to model it in Machinations

Create a Source whose rate comes from a Register using a random value, so that the output varies from step to step. Add a Drain for demand and a Pool for storage or a backup Source that fills the gap. Make wind capacity and storage size Levers, then Play many times and chart how often a shortfall occurs.