Difference Between Energy and Exergy

The key difference between energy and exergy is that the energy is a quantitative measurement whereas the exergy is a qualitative measurement.

Energy is the ability of a substance to perform work. It is hence a quantitative parameter. But exergy gives the maximum work that a substance can perform. Therefore, it is a qualitative parameter. However, we measure both of these parameters in order to determine the total useful work that we can extract from a thermodynamic system or a substance.

CONTENTS

1. Overview and Key Difference
2. What is Energy
3. What is Exergy
4. Side by Side Comparison – Energy vs Exergy in Tabular Form
5. Summary

What is Energy?

The energy of a particular substance is the ability of that substance to do work. It is a quantitative property. We can transform energy into a substance in order to perform a work or to heat the substance. Moreover, it is a conserved quantity. According to the law of conservation of energy, we can only convert one form of energy into another but cannot create or destroy it.

Figure 01: We can convert solar energy into electricity.

The SI unit for the measurement of energy is joule (J). There are several forms of energy that we know; potential energy, kinetic energy, elastic energy, chemical energy, thermal energy, radiant energy, etc. More importantly, we can divide the energy stores in a particular substance into two categories as potential energy which is stored in the substance and kinetic energy which is determined by the movement of the substance. These two categories are sufficient to describe all other forms of energy.

What is Exergy?

Exergy is a thermodynamic term which describes the maximum useful work that a substance can perform during the process of bringing a system into equilibrium with a heat reservoir. In other words, it is the maximum useful work that we can extract from a system when it reversibly comes into equilibrium with its environment. Moreover, it is the capacity of that system to do physical work.

More importantly, exergy is a measure of energy quality. Here, quality means the ability to perform physical work. An example of such physical work is the ability to overcome resistance against the movement of an object. In addition to that, we define this quality with respect to the environment of the system or substance.

Apart from that, we can destroy the exergy of a substance or a system (unlike energy) during transformations. For example, energy can be converted into low-temperature waste heat. This reduces the quality of energy. The sum of output and the waste heat always equal to the total input. However, the unit of measurement for exergy is same as for energy.

What is the Difference Between Energy and Exergy?

Energy of a particular substance is the ability of that substance to do work while exergy is a thermodynamic term which describes the maximum useful work that a substance can perform during the process of bringing a system into equilibrium with a heat reservoir. Moreover, energy of a substance refers to the work it can perform whereas exergy refers to the maximum work that can perform. Unlike energy, exergy can get destroyed during a process. In addition to that, energy of a substance is a quantitative parameter whereas exergy is a qualitative parameter. This is the key difference between energy and exergy.

The below infographic presents the difference between energy and exergy in tabular form for a quick reference.

Summary – Energy vs Exergy

Energy and exergy are two related parameters of thermodynamic systems or substances. Although there are many differences between them, we can define the difference between energy and exergy as; energy is a quantitative measurement whereas exergy is a qualitative measurement.

Reference:

1. “Energy.” Wikipedia, Wikimedia Foundation, 26 Aug. 2018. Available here  
2. “What Is Exergy?” Exergy Economics, 25 Jan. 2016. Available here 

Image Courtesy:

1.”12648929455″ by TAFE SA TONSLEY   (CC BY 2.0) via Flickr

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