Dear Khaoula

Normally it utilizes energy analysis to explore performance of the thermal system. Be that as it may, the energy analysis alone is deficient to portray some imperative perspectives to consider in energy use. To optimize the performance of the thermal system associated thermal system for optimal operating conditions, it is important to analyze the energy and exergy of the thermal system simultaneously.

Dear Khaoula.

I'm battening tell me give the difference between exergitic and energetic analysis. From the exergetic analysis we can defined the exergetic losses that allowed to defined where is the location of losses in result we can optimized the process. However energy efficiency it's not able to defined exactly the location of losses energy.

Dear khaoula

In most cases the energy analysis is enough to understand the system performance. But some times it is needed to find about the irreversibility sources (like exergy destruction) of the system, which can be obtained from exergy analysis.

Dear Dr. Hidouri,

Exergy Efficiency = [Energy Efficiency]/[maximum Energy Efficiency]

Energy analysis gives you a general overview that how a system is working. You know that energy efficiency is 20% and it is low, but you cant find which component has deficiency. In this regard, you should investigate the system by exergy analysis. By this method, you can determine which component has the highest exergy destruction and should be further studied.

In energy analysis the quality of energy would not be considered. For example, 1 kJ of heat energy is not as valuable as 1 kJ of electricity energy, but in energy analysis both of these kinds of energy are considered as the same.

Exergy of a kind of energy (based on its temperature, pressure and concentration than a reference condition) is the maximum ability of that kind of energy to do work.

Suppose you have different currencies. To make a fair comparison between these currencies, you need to convert all of them into a single currency and then make those comparisons.

Exergy is the base currency for which the comparison is made!

So, exergy analysis let us compare every kind of energies with each other, based on quality and quantity.

Regards

Mohammad Olfati

Fellows in the energy & thermodynamics area initially are interested in the concept of exergy analysis, they in general than find is useless to perform for physical systems. This indeed is due to lack of understanding of the exergy concept. Energy analysis itself even fails to find the efficiency of heat pump systems, therefore academics had to name the performance of heat pumps as COP but could not call it efficiency since it should be between 0 and 1. COP could be as high as 1400% if it was called efficiency, thus COP concept was made up. Second law assessment solves this problem by utilizing Gibbs relations and finally provides a real efficiency value between 0 and 1. Also exergy analysis is inevitable for thermochemistry and electrochemistry related processes. Such a strong tool definitely make any thermal system design more accurate and understandable.

Energy analysis is the traditional method of assessing the way energy is used in an operation involving the physical or chemical processing of materials and the

transfer and/or conversion of energy. This usually entails  performingenergybalances,whicharebasedonthe first law of thermodynamics  and evaluating energy efficiencies. This balance is employed to determine and reduce waste exergy emissions like heat losses and sometimes to enhance waste

and heat recovery.

The correct answer may be dependent on how you define the efficiency and for what kind of function you are measuring it. Just to give an idea how different the statement could be look for following examples:

1. You are looking for energy (quantity) production or transformation - system becomes more efficient there is better insulation or even close to adiabatic and more energy (fuel) is transformed in to product.

2. Looking how efficiently you transform the quality of energy(exergy) in order to produce or create some functionality. As example - reducing the inlet temperatures at heating system enables heat pump to operate with lower electricity (highest quality exergy) consumption

3. How effectively you are producing some physical product in terms of exergy monetary costs (see exergy- economics for more details)

4. What kind of environmental impact you are making in terms of producing some service or product which requires energy (See - exergy-environomics for more details)

5. What is impact of the system or component impact across the life cycle? (See exergy life cycle analysis ExLCA)

It is important to mention that performance indicators first of all should have the purpose (the management science is realy serious about Key performance indicators - KPI) . In order to be meaningful it must have purpose which drives the to "better". How the "better" is defined is the crucial part.

The efficiency according to first law of thermodynamics (FLT) could be skipped, bet without having correct solution in terms of FLT you are not able to obtain correct result in terms of second law (exergy).

Best regards.

Karolis

from the second law of thermodynamics it can be demonstrated that no system can ever be 100% efficient. When calculating the energy efficiency of a system, the figure found gives no indication of how the system compares to a thermodynamically perfect one operating under the same conditions. In comparison, the rational efficiency of a system can reach 100% because the work output is compared to the potential of the input to do work. The energy efficiencies of a heat engine are always smaller than its exergy efficiency

Is it correct to write the term "First law of thermodynamics" with a capital letter?

Thank you

Energy efficiency is important but it did not meaningful with out find out the exergy efficiency. For combined energy and exergy efficiencies give a meaningful evaluation of a system

In my opinion, exergy is no more than scaling of entropy generation. To make life simple there is no need for more terminologies.

Ineresting

Dear sir

Both of them are calculated the performance of system. But exergy efficiency are more precise.

In most cases energy efficiency is an enough tool to show performance of a system but it some cases not only energy consumption of two systems differs but also temperature of the heat source or kind of the required energy differs. In such cases exergy efficiency is a suitable tool. For example consider you are comparing three chillers, one of them uses electricity and the two others are absorption chillers which one of them uses high temperature steam and the next one uses hot water. It is clear that the energy efficiency is not a good tool to compare these three chillers but exergy efficiency is a good tool. Another example when you compare a kalina cycle which uses low level waste with a Ranklin cycle that uses high temperature and pressure steam, it is clear that energy efficiency is not meaningful.

Totally agree with Dr.Ali view.

The complete answers are given. I just want to mention this point, you need more to exergy efficiency in low temperatures heat recovery areas, where quality of energy is becoming more important than amount energy.