What is curtailment?

Curtailment is when a generator, most often wind or solar, is told to produce less than it could, so that energy is wasted rather than delivered. It happens when supply exceeds what the grid can take or carry at a given moment. It matters because curtailed energy is revenue lost and capacity paid for but not used, which weakens the case for building more.

How it works

There are two main causes. One is oversupply: at midday on a sunny day, solar output plus steady plants may exceed demand, and the operator has to reduce something to keep supply and demand matched. The other is congestion: the lines may not have room to carry the power from where it is made to where it is needed.

The effect feeds back on itself. As more renewable capacity is added in the same place without more storage or transmission, a growing share of each new plant's output is curtailed, so each additional plant adds less than the one before. Storage, flexible demand and new lines are the main ways to reduce it.

Example

Say a region has 1,000 MW of solar that could produce 800 MW at noon, and demand at noon is 600 MW, with 100 MW supplied by plants that cannot ramp down. Solar can deliver at most 500 MW, so 300 MW is curtailed, which is 300 / 800 = 37.5% of the available solar energy at that hour. If the region adds a 200 MW battery that absorbs the surplus, curtailment falls to 100 MW. This is an illustration.

How to model it in Machinations

Create a Source for available solar output and a Drain for demand. A Gate or Register limits delivery to the room available, and a second Drain collects the curtailed amount. Add a Pool for a battery as a Lever. Then Play over a day and chart curtailed energy as capacity grows.