Online course and simulator for engineering thermodynamics

Basic Thermoptim notions


This page presents the basic notions that should be known before using Thermoptim.

Thermoptim provides a modeling environment integrating in a deeply interconnected way a diagram editor / synoptic screen, interactive charts, simulation functions and an optimization method.

This tool was created to facilitate and secure the modeling of energy conversion technologies.

This pagepresents the main concepts that it is important to know to be able to work with Thermoptim. However, to take full advantage of the possibilities of the software package, we recommend that you also consult the reference manuals and the getting started examples available in the documentation.

Thermoptim is composed of:

  • a core comprising the main elements, which already make it possible to model many energy systems,

  • but which can be extended to represent additional elements not available, which makes this environment very largely customizable.

Three categories of extensions can be made:

  • substances, to add fluids not available in the core

  • components representing specific energy technologies, such as solar collectors or fuel cells

  • pilots or drivers, which are small programs that take control of Thermoptim and thus make it possible to control the calculations it performs.

To refer to these extensions, we talk about external classes, a class representing an element of Java code, and the external adjective indicating that they are external to the core.

To fully understand how Thermoptim works, three basic concepts must be present in mind:

  • substances, which make it possible to characterize the different fluids involved

  • points, which represent an elementary particle of matter

  • processes, which are used to determine the processes undergone by the fluids in the various components, such as compression, expansion or heating


To represent the different fluids that flow through the systems studied, Thermoptim defines four types of substances:.


Thermoptim includes around twenty pure gases.

We can define as many compound gases as desired, obtained by mixing the pure gases available.

Compound gases are divided into two categories:

  • protected gases whose composition cannot be modified by a user

  • the others, called unprotected.

The reason for this distinction is simply to prevent a modeling error from changing the composition of a known gas, such as that of air or natural gas.

The properties of the gases are based on the ideal gas model: the law Pv = rT and a development of the thermal capacity Cp of the gas as a function of the temperature.

The perfect gas corresponds to the particular case where Cp is a constant.

Of course, this type of model is only valid to represent the properties of the fluid far from its liquid-vapor equilibrium curve.

Condensable vapors

The third type of substance complements the two previous ones: twenty condensable vapors, which can not be mixed, Thermoptim not being able, in the general case, to calculate the properties of a mixture of vapors.

Some substances, such as water, appear both as pure gases and as condensable vapor: these are two different models, which are selected according to the problem to be solved. The names of gases always include their chemical formula, which is used in particular in combustion calculations, and those of vapours are usually the common names. For example, water is called H2O as gas, and water as steam.

External substances

External subsances are defined outside of the Thermoptim core, hence their name.

They can either be simple substances, entirely calculated in the external classes of Thermoptim, or else mixtures, whose calculation can be carried out in specific software, coupled with Thermoptim.

For a Thermoptim user, a substance is simply characterized by its name

Substance selection screen

This figure shows the substance selection screen, with lists of condensable vapours deployed.

This figure shows the screen for defining the composition of a compound gas, here the natural gas of the Montoir de Bretagne LNG terminal, with the names of the pure gases that compose it, and their molar and mass fractions.


Thermoptim also defines points, which represent a small amount of fluid, whose thermodynamic state can be calculated in an open or closed system, when pressure and temperature are known, for example.

This figure shows the screen of a point, which can be calculated in an open or closed system, or to determine the properties of moist gases, which are modeled as a mixture of water and dry gas considered ideal.

To set a point, you must give it a name, here 3a, enter the name of the substance associated with it or choose it from one of the lists presented above, which are displayed when you double-click in the name field.

It is then necessary to define a sufficient number of state variables, often pressure and temperature, checking that the selected calculation method (here, P and T known) is the correct one.

When the substance is a condensable vapor, it is possible to set either the saturation temperature, knowing the pressure, or the saturation pressure, knowing the temperature. It is then necessary to specify the quality, here equal to 0.

The point can then be calculated, by clicking on the Calculate button


Processes correspond to thermodynamic evolutions undergone by a substance between two states. A process associates therefore two points such as defined previously, an inlet and an outlet point. Moreover, it indicates the mass flow rate involved, and allows one to determine the variation of energy involved in the course of the process.

The most common processes have been modeled and are directly accessible in the core.

Knowing the state of the fluid at the process inlet, Thermoptim can then solve either the direct problem or the reverse problem. In the first case, knowing the characteristics of the process, it calculates the state at the end of the evolution and the energies involved, and updates the downstream point. In the second case, it identifies the values of the parameters of the process so that the chosen evolution leads to the state of the downstream point as defined.

This figure shows the screen of an expansion process, which represents a steam turbine. Other process screens are structured in a similar way.

In the upper left, appear the name of the process (here turbine), its type (here expansion), the type of energy that automates the establishment of energy balances (here useful), and an option to set or not the flow that crosses the process. If it is not set, that of the process located just upstream is automatically propagated.

In the left part, appear in summary the reminders of the thermodynamic state of the upstream and downstream points. Clicking a show button opens the screen of the corresponding point. In the upper right, the buttons for navigation, opening and closing and calculating the process appear.

It is in the lower right part that the configuration options specific to each process are located. Compressions and expansions can be calculated in open or closed systems, taking an adiabatic or non-adiabatic reference, and an isentropic or polytropic model, with a set or calculated compression ratio. We have chosen here an adiabatic expansion, of isentropic efficiency equal to 0.85. The calculation of the downstream point is carried out taking into consideration the known efficiency .

Since a point does not allow to specify the flow involved, it may be necessary to create particular processes, called process-points.

A process-point connects a point with itself, and specifies the mass flow rate to be taken into account. It therefore corresponds technologically to a small pipe, and allows in particular to represent fluid inputs or outputs.

Other features


The fluids involved cross the machines forming more or less complex networks that must be described. Processes correspond to a part of these circuits. They are complemented by three types of nodes, which make it possible to describe the elements of the network where the mixtures and divisions of fluids take place. In a node, several fluid branches are connected together to form a single vein.

Three types of nodes exist in the Thermoptim core: mixers, dividers and phase separators for two-phase fluids.

The simulator

This figure shows the screen of the Thermoptim simulator, which gives access to the list of points, at the top left, to that of the processes, just below. The list of nodes appears at the bottom left, but there are none in this example. Heat exchangers can also be defined, but we will not talk about them at this time. A double-click in one of the lines displays the selected point or process screen.

Diagram editor

This figure shows the screen of Thermoptim's diagram editor, with an enlargement of the palette icons.

With the exception of the arrow on the left used to bring the mouse back to its normal state, we can distinguish, from left to right, the icons that allow you to select the different components and place them on the working plane :

  • the A makes it possible to write a text

  • the processes of the core, namely the process-point, the heat exchange (heating or cooling of a fluid), the compression, the expansion with work, the combustion chamber and the isenthalpic throtling

  • the following block gives access to the three nodes: mixers, dividers and phase separators

  • the next icon is the component giving the synthetic balance

  • the Q serves to represent a heat source, intended, essentially for educational purposes, to show that the systems generally exchange heat with the outside

  • the following block gives access to the three external components

  • finally, the last two icons allow you to zoom in or out the diagram

The different components are connected by thin black lines oriented in the direction of circulation of the fluid. The state of the fluid is indicated there.

A double-click on one of the processes displays its screen. Double-clicking on one of the oriented links opens the corresponding point.

The blue links here represent connections to external sources, and the cycle balance appears in a small block.


Access to thermodynamic charts is via the Interactive Charts menu line in the Special menu on the simulator screen.

When you double-click on the line "double-click here to choose the chart", the list of available chart types is proposed. One can thus choose the desired one.

copyright R. Gicquel v2022.1

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