Presentation of the exercise

The objective of the exercise is to model a 300 MW steam power plant in Thermoptim in order to assess its main characteristics.

This exercise is comprised of three parts:

in the first part (this one), we suppose that the plant uses a single circuit
in the second part (session S27En), we take into account an extraction and a reheat which occur in industrial machines. The model becomes much more realistic
in the third part (session S28), we quantitatively analyse the irreversibilities of both cycles through their exergy balances

Presentation of the exercise (pdf, 10 Ko)

(Session realized on 06/16/11 by Renaud Gicquel)

Steam power plants

Basic cycle (Rankine):
- compressed water P ≈ 80 - 300 bar
- three step heating (economiser, evaporator, superheater)
- steam expanded in the turbine (P ≈ 0.04 bar)
- then condensed

Steam power plants

Basic cycle (Rankine):
- compression 1-2 up to 80- 300 bar
- Échauffement jusqu’à envheating up to about 560°C
  - economiser 2-3a
  - evaporator 3a-3b
  - superheater 3b-3
- expansion 3-4’
- condensation 4’-1 at about 25°C

Steam power plant exercise

300 MW steam power plant

pump inlet: 29 °C, 0.04 bar
water mass flow-rate: 245 kg/s
high pressure: 169 bar
isentropic pump efficiency: 1
turbine inlet temperature: 542 °C
isentropic turbine efficiency: 0.85

Thermoptim

two screens at opening

diagram editor
simulator

Thermoptim

power plant components

pump
economiser, evaporator, superheater
turbine
condenser

Thermoptim

Setting the model parameters

When the simulator elements are created, Thermoptim initializes all points at a pressure of 1 bar and a temperature of 300 K, which must subsequently be modified depending on the data of the problem being studied.

The two next activities will show you how to set the parameters of the points and the processes, and the third one will remind you of the values which should be entered in Thermoptim.

Setting points

double-click on a link (arrow) or on a point table line
state of a small volume of matter

Setting processes

double-click on a component or on a process table line
fluid mass flow-rate

Setting the parameters of points and processes

Set now the parameters of the various points one by one, checking that you enter the values of the temperatures known (T1, T3), as well as those of the pressures, as Thermoptim does not set them automatically. In order to do that, follow these steps:

open point point 1 (liquid water), enter a pressure of 0.04 bar and choose “set the saturation temperature”, which sets the temperature to about 29 °C. The quality being by default equal to 0, click on “Calculate”
then open point 2, enter a pressure of 169 bar, and click on “Calculate”
open point 3a and enter a pressure of 169 bar, at the saturation temperature (351.2 °C), with a quality equal to 0, then click on “Calculate”
open point 3b in the same conditions, with a quality equal to 1, and calculate it
open point 3 and enter a pressure of 169 bar and a temperature of 542 °C, and calculate it
open point 4 and enter a pressure of 0.04 bar, then click on “Calculate”

Set then the process’s parameters, for which default options are valid : the compression and the expansion are adiabatic, calculated with the isentropic reference, and their isentropic efficiencies are known. In order to do that, follow these steps:

open process “pump”, whose isentropic efficiency is by default equal to 1, then click on “Calculate”, which sets the temperature of point 2 (29.37 °C)

open processes “economiser”, “evaporator” and “superheater”, and click on “Calculate”. The temperatures of their inlet and outlet points being known, their enthalpies can be calculated

open process “turbine”, and enter an isentropic efficiency of 0.85, then click on “Calculate”, which shows that point 4 is at the saturation temperature (29 °C), is quality being equal to 0.836

open process “condenser”, which can now be calculated, points 4 and 1 being known

The calculation of the overall balance requires to understand the notion of energy type, explained in the next activity.

Energy type choice

double-click on the energy type field

Balance and synoptic

Cycle on (T,s) chart

Setting an entropy chart (T,s)

Cycle on (T,s) chart

Refined cycle on (T,s) chart

Cycle on (h,P) chart

Cycle on Mollier (h,s) chart

Summary

This exercise has allowed you to study a simple steam cycle and to learn how to model it in Thermoptim. This is but a simplification (cycle without reheat nor extraction) of a 300 MW power plant.

What is interesting in using such a software package is that the substance properties are calculated with a high degree of accuracy and without any difficulty. Once the model built, you can the change the parameters very easily, which enables you to quickly make sensitivity studies on the influence of the various parameters.

This working manner involves however a risk: that you cannot 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 layout of the cycle on a chart appears also a very good means of checking the relevance of a model, since the general shape of the cycles is generally well known. An error of entry on the pressure or the temperature often appears obviously.

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