To introduce the method, let's start with a simple and illustrative example: how a gas turbine works.

It is an internal combustion engine whose general principle is simpler than that of many other machines, making it a good basis for presenting the basic concepts of energy modeling.

Physical phenomena in a gas turbine

A gas turbine, or combustion turbine, is widely used in electricity production, aircraft propulsion, and cogeneration.

Its success stems from its relative simplicity and high efficiency: over 35% in simple cycle, up to 60% in combined cycle.

Figure 1.1: Diagram of a Gas Turbine

It is composed of three main parts (Figure 1.1):

1. The compressor raises the pressure and temperature of the intake air.

2. The combustion chamber, where fuel is injected and burned with this air, greatly increases the temperature of the combustion gases.

3. The turbine expands the hot gases while producing mechanical work.

Despite its apparent simplicity, this example highlights several important points:

• The components are strongly coupled: the compressor and turbine share a mechanical shaft, and fluids circulate from one element to another.

• If we set aside the fuel, which can be liquid, the working fluids are gaseous mixtures (air and combustion gases) whose thermodynamic properties (pressure, temperature, enthalpy) vary depending on the transformations they undergo. Unlike steam cycles, where the fluid changes state (liquid-vapor), the fluids in a gas turbine remain in the gaseous state.

• The compression and expansion phases are essential as they ensure conversion between mechanical energy and the energy within the fluid.

• Combustion is a complex physico-chemical phenomenon; modeling it allows for evaluating energy balances and estimating pollutant emissions, a key issue in reducing environmental impact.

• Internal flows in the components require advanced knowledge of fluid mechanics, a field we will not delve into here.

Energy Technologies as Assemblies of Components

The gas turbine illustrates an essential idea: any energy technology can be seen as an assembly of components through which one or more thermodynamic fluids pass and undergo various transformations.

Some components are dedicated to a specific function, while others may fulfill several roles depending on the operational phase. For example, in a piston engine, the same set of cylinder, piston, and head provides compression, combustion, and expansion in succession.

Two major challenges arise when analyzing or modeling such a system:

1. Fluids sometimes follow complex laws, such as the chemical reactions during combustion, which are difficult to model analytically.

2. The components are interconnected such that they cannot be calculated independently.

To overcome these difficulties, our methodology recommends separating the study into two parts:

1. Represent the overall architecture (systemic approach): describe the components, their connections, and fluid paths, independently of calculation details.

2. Model each component in detail (analytical approach): formulate the governing equations, taking into account boundary conditions provided by the system.

Let's now see how these remarks generalize.