What are the important aspects of performing exergy analysis on a thermodynamic system, such as an internal combustion engine, solar thermal systems, biogas systems, apart from studying its conventional characteristics?
An exergy of a part of the issue is equal to the maximum useful work that can be obtained taken from its given thermal equilibrium with the environment for, and without interface, rather than its own and the one of the environment . Such a final state of equilibrium is known as dead state. From another point of view, it can be considered as a measure of exergy of imbalance that existing between the considered the issue and the environment.
To be able to perform calculations of exergy, it is necessary to determine the ideal environment for reasonable model, which takes as reference, since the exergy always depend on the state of system and environment. It is also necessary to analyze the different possibilities of reached the dead state of equilibrium with the environment, in the wake of restrictions imposed on different analysis systems.
The exergy of every energy flow is (matter flow, heat, work,…) can be categorized in a similar way to classify their energy ,with some exceptions as shown in figure (1.5). Figure (1.5) shows thermo-mechanical exergy can be decomposed into a pressure on the basis of the part and the part of existing temperature. This decomposition has no real fundamental importance, and is only for convenience as will be described below. In fact, the decomposition is not even unique, which confirms to the lack of a deeper meaning. Consider a system in the state of thermodynamic state P, T which is subjected to a variety of operations reversed processes to end up at a state in thermodynamic equilibrium with the surroundings Po To, where equilibrium for pressure and temperature simply means equality [5].
See my theses:
Thesis An Energy and Exergy Analysis on the Performance of Wet Cool...
Exergy is the evaluation of a system based on the second law of thermodynamics.
Dear ,
Quantative evaluation of energy in a cycle or in a process can be done using the first law of thermodynamics. The direction of flow of heat or work is known from the second law of thermodynamics. However, it is equally important to assign the quality to the energy. Energy can be broadly classified into high grade and low grade energy. High grade form of energy are highly organised in nature and conversion of such energy to some other high grade form (W→W) is not dictated by the second law of thermodynamics. Conversion of high grade energy to low grade energy is not desirable. However, there may be some conversion to low grade energy as work is converted into other useful form. This is because of dissipation of heat due to friction (example: mechanical work → Electricity, some losses are there due to the friction in bearing of machineries). Thus both the first and the second law of thermodynamics are to be considered for analysis.Low grade energy such as heat due to combustion, fission, fusion reactions as well as internal energies are highly random in nature. Conversion of such form of energy into high grade energy (Q→W) is of interest. This is due to the high quality of organised form of energy obtained from low quality energy. Second law of thermodynamics dictates that conversion of 100% heat into work is never possible. That part of low grade energy which is available for conversion is termed as available energy, availability or exergy. The part, which according to the second law of thermodynamics, must be rejected is known as unavailable energy. Exergy analysis helps in finding the following:
- It can be used to determine the type, location and magnitude of energy losses in a system
- It can be used to find means to reduce losses to make the energy system more efficient
At this point, it is worth mentioning that the environment plays an important role in evaluating the exergy (composite property).
from:
http://nptel.ac.in/courses/112103016/8
Dear Thokchom,
Previously there was very useful and valuable information about idea behind of this concept. I would add some aspects that seems to be important when you are planning to do the exergy analysis:
1. Determine the purpose why you need to do this type of analysis - doing this analysis without clear purpose is waste of time. Or in terms of thermodynamics - entropy production. Knowing the purpose may help you understand where to focus and select one of specific methods like advanced exergy analysis, exergo-economic analysis, entropy generation minimization or exergy based life cycle analysis or one from many others.
2. Be sure that you have performed correct analysis according to first law of thermodynamics. While the correctness of second law analysis mainly depends on input data and used assumptions it is worth to spend time on the process parameters. You should focus on the process which produces correct answers.
3. Check for consistency of the expressions and ensure correctness of exergy an entropy balances. As there is different interpretations available in published materials. The results produced by different methods may differ from each other due to interpretations and mathematical limitations. There is conditions (variable or dynamic reference state) where there is ongoing discussion how to handle calculations and what assumptions to make.
Good luck,
Karolis
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