Everything about Exergy Efficiency totally explained
Exergy efficiency (also known as the
second-law efficiency or
rational efficiency) computes the efficiency of a process taking the second law of thermodynamics into account.
Motivation
From the
second law of thermodynamics it can be shown 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.
Definition
The
exergy B balance of a process gives:
» so the exergy efficiency must always be greater than the energy efficiency.
Application
The destruction of exergy is closely related to the creation of entropy and as such any system containing highly irreversible processes will have a low exergy efficiency. As an example the combustion process inside a power stations gas turbine is highly irreversible and approximately 25% of the exergy input will be destroyed here.
For fossil fuels the free enthalpy of reaction is usually only slightly less than the enthalpy of reaction so from equations (3) and (4) we can see that the exergy efficiency will be correspondingly larger than the energy law efficiency. For example, a typical combined cycle power plant burning methane may have an energy efficiency of 55%, while its exergy efficiency will be 57%. A 100% exergy efficient methane fired power station would correspond to an energy efficiency of 98%.
This means that for many of the fuels we use, the maximum efficiency that can be achieved is >90%, however we're restricted to the Carnot efficiency in many situations as a heat engine is being used.
Carnot
A common misconception is that the exergy efficiency compares a given cycle to a
Carnot heat engine. This is incorrect because a Carnot engine is the most efficient heat engine possible, but not the most efficient device for creating work.
Fuel cells, for instance, can theoretically reach much higher efficiencies than a Carnot engine.
Second law efficiency under maximum power
Neither first or second laws of thermodynamics include a measure of the rate of energy transformation. When a measure of the maximal rate of energy transformation is included in the measure of second law efficiency it's known as second law efficiency under maximum power, and directly related to the
maximum power principle (Gilliland 1978, p. 101).
Further Information
Get more info on 'Exergy Efficiency'.
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