In the context of cycling duty, two main operating schedules are prevalent: load following, in which the unit remains in service but changes load to match the system requirements, and two-shift operation, in which some units are shutdown during the night and the weekend/holiday.

The present paper analyses the economic benefits obtainable from the two-shift operation of fossil fuel power units with respect to load following. In particular, two different types of thermoelectric power units are considered:

• conventional steam power units;

• advanced fossil fuel power units (i.e. gas-steam combined cycle units).

Firstly, a thermodynamic analysis is carried out evaluating the off-design heat rate of the power units during the part load operation.

Subsequently, the economic hourly and annual savings, obtained by shutting down one unit, are performed.

Appreciable benefits are found for a large range of the system load requirements, thus indicating the two-shift operation as a strategy that can be pursued.

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