Turndown ratio

The ratio between maximum and minimum stable operating load.

For a plant following a variable renewable resource, turndown determines how much of the generation profile can actually be used. A wider range captures more of a solar morning and evening and reduces the number of stops and starts.

The limit at the bottom of the range is normally gas quality rather than electrical capability. As load falls, hydrogen permeation into the oxygen stream is diluted into a smaller flow, so its measured fraction rises towards the safety limit.

Quoted turndown figures should be read carefully. They may refer to the stack alone rather than to the complete system with its balance of plant, and they usually describe a new plant rather than one whose separator has aged, which reduces the available margin over time.