# Energy and Exergy Analyses of an Existing Solar-Assisted Combined Cycle Power Plant
## 4 Results and Discussion
### Investigating the Sources of Exergy Destruction
#### Irreversibility in the Combustion Chamber  
The combustion process is complex. Thus, the entropy generation during the combustion process is rather high due to the complexity of that process. It was reported that oxidation of fuel during the combustion process utilizes around 1/3 of the usable fuel energy [29]. This feature of the combustion process causes it to have the highest exergy destruction. The combustion process includes diffusion, chemical reaction, heat transfer, friction, and mixing. To implement all of these subprocesses, a considerable amount of the available energy is consumed. Most of this energy is unreachable (combustion activation energy, mixing, and diffusion).  
There are three major physicochemical subprocesses responsible for entropy production during the combustion process [29]:  
* Diffusion of reactants (mixing of fuel and air molecules) and chemical reaction (fuel oxidation) where energy is consumed to overcome the activation energy.
* Heat transfer between combustion products and other neighbors of particles; this is called \"internal thermal energy exchange\".
* Mixing of combustion products with other constituents.  
These processes cause exergy consumption (destruction) and thus result in a reduction in the system exergy. On one hand, all these processes destroy up to 40% of the useful exergy of the fuel. On the other hand, it was found that the dominant process of exergy destruction is the internal thermal energy exchange process. It was found that more than 2/3 of the exergy destruction in the combustion process occurs at the internal thermal exergy exchange process, while fuel oxidation is responsible for up to 30% of the exergy destruction, and the exergy destruction due to the mixing process is about 3% of the total exergy destruction of the combustion process [29].  
The thermodynamically irreversible combustion process is path-dependent. To get a quantitative solution for the total entropy production during the combustion process, correct information of the sequence of the combustion process and reactions must be offered.  
Many factors affect the exergy destruction in the combustion chamber. For example, the exergy destruction decreases with decreasing excess air and increasing preheating temperature. Mixing at a large temperature difference leads to high exergy destruction [30]. Also, the exergy destruction of the combustion chamber is affected by the molecular structure of the fuel, where the exergy destruction of the combustion chamber increases with the increase of the hydrocarbon chain length [31].  
An attempt was made to avoid this heat transfer by introducing the concept of reversible combustion, where it was proposed theoretically to preheat the reactants to the equilibrium temperature and partial pressures without a reaction, but it could not be achieved in practice [29].  
The major exergy destruction in the combustion chamber occurs during the phase of the internal thermal energy exchange between the system particles [29]. The unavoidability of the internal thermal energy exchange makes reducing the exergy destruction during the combustion process very difficult.