Presentation of the exercise

This exercise shows how to model in Thermoptim a single pressure combined cycle whose sketch is shown below.

The low-capacity (11 MW) gas turbine is aeroderivative.

The purpose of the exercise is to model such a plant with Thermoptim and adjust the steam cycle flow rate to get the best performance. In a second step, we will prepare its exergy balance.

(Session realized on 06/21/11 by Renaud Gicquel)

Combined cycle exercise

gas turbine

intake: 15 °C, 1 bar, 45 kg/s
compression ratio: 16, η_p = 0.85
turbine inlet: 1120 °C , η_p = 0.85
turbine outlet : 1.3 bar

steam cycle

pump inlet : 24.1 °C, 0.03 bar, η_s = 1
HP: 120-118-115-110 bar
turbine inlet: 520 °C , η_s = 0.85
flow-rate unknown

Initial cycle

Setting the parameters of points and processes

The settings for the gas turbine are:

Points “air intake” (15 ° C, 1 bar) and “compressed air” (16 bar), process “air compressor” (polytropic reference, polytropic efficiency 0.85), which leads to an outlet temperature of 442.6 °C
Point “fuel” (15 ° C, 20 bar, substance “Montoir natural gas”
Process “combustion chamber” (no dissociation, Calculate lambda, with T = 1120 ° C)
Point “expanded gas” (1.3 bar), process “gas turbine” (polytropic reference, polytropic efficiency 0.85), which leads to an outlet temperature of 559.4 ° C

The settings for the steam power plant are:

Point “1” (0.03 bar, set saturation temperature, quality equal to 0), point “2” (120 bar), process “feedwater pump” (isentropic efficiency equal to 1)
Point “3a” (118 bar, set saturation temperature, quality equal to 0), point “3b” (115 bar, set saturation temperature, quality equal to 1), point “3” (110 bar, 520 °C), point “4” (0.03 bar)
Process “turbine” (isentropic reference, isentropic efficiency 0.85)

There is no need to change the types of energy, as the default values apply well

Heat exchanger preparation

Creation of the superheater

Superheater screen

First synoptic view

Second synoptic view

Third synoptic view

Solution synoptic view

Exergy balance

Summary

This exercise allowed you to study a single pressure combined cycle and learn how to model it in Thermoptim.

The interest to model cycles with a software package is that the properties of substances are calculated with great precision and without any difficulty. Once the model is developed, you can quite easily change its settings, allowing you to rapidly make sensitivity studies on the influence of various parameters.

This working manner involves however a risk: that you do not criticize the values provided by the software package, which can lead you to grant a disproportionate confidence to the results it provides.

It can happen that the model is not well built, and that its results are not right, and it is necessary to know to diagnose this type of errors. To do this, you should be able to check by yourself its results, at least in an approximate way.

It is in particular recommended to check the energy balances well. In order to do that, train to recompute them roughly, by summing on a bit of paper all the useful energies like all purchased energies, and by dividing them to obtain the cycle efficiency.

The plot of the cycle on a chart also proves a good way to control the relevance of a model, once we know the general shape of cycles, which can be learned quickly. An input error on the pressure or temperature is often quite obvious.

You will find below a link towards a note entitled “Model construction and verification methodology” intended to provide to the users of Thermoptim a certain number of recommendations based on past experience.