Advanced Exergetic Analysis is a Modern Tool for Evaluation and Optimization of Refrigeration Systems

Advanced Exergetic Analysis is a Modern Tool for Evaluation and Optimization of Refrigeration Systems

Tatiana Morosuk (Technische Universität Berlin, Germany) and George Tsatsaronis (Technische Universität Berlin, Germany)
DOI: 10.4018/978-1-4666-8398-3.ch003
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Abstract

In the last decades an exergetic analysis became increasingly popular because this analysis identifies the location, magnitude and sources of thermodynamic inefficiencies. A conventional exergetic analysis, however, does not consider (a) the real potential for improving a system nor (b) the interactions among the components of the system. The interactions among different components of the same system can be estimated and the quality of the conclusions obtained from an exergetic evaluation can be improved, when the exergy destruction (irreversibilities) within each system component are split into endogenous/exogenous and avoidable/ unavoidable parts. We call this advanced exergetic analysis. The purpose of this chapter is to demonstrate that the advanced exergetic analysis is a practical method that allows engineers to extract useful information and conclusions and to develop ideas and solutions that cannot be suggested by other methods. In this chapter the conventional and advanced exergetic analysis are applied to an air refrigeration machine.
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Introduction

Exergy-based methods is a general term that includes the conventional and advanced exergetic, exergoeconomic, and exergoenvironmental analyses and evaluations (Tsatsaronis & Morosuk, 2009). It is well known that conclusions obtained through the evaluation of an energy-conversion system using exergy-based methods are unique and cannot be obtained by using other methods. However, conventional exergy-based analyses do not consider (Tsatsaronis, 1999)

  • The real potential for improving a system nor

  • The interactions among the components of the system.

The interactions among different components of the same system can be estimated and the quality of the conclusions obtained from an exergetic evaluation can be improved, when the exergy destruction within each (important) system component is split into endogenous/exogenous and avoidable/ unavoidable parts. The most important publications up to date in the field of the advanced exergy-based methods are summarized and generalized in (Tsatsaronis & Morosuk, 2008, 2009).

Conventional and advanced exergy-based analyses have already been applied to different energy-conversion systems (power, cogeneration, and refrigeration), for example by (Tsatsaronis & Morosuk, 2008, 2009, 2010, 2011; Petrakopoulou, Tsatsaronis, & Morosuk, 2011; Morosuk & Tsatsaronis, 2008, 2009a, 2009b, 2011). The interconnections among components of these systems are usually not very strong and the advantages of the advanced exergy-based methods could not have been fully demonstrated. In this paper conventional and advanced exergetic analyses are applied to an air refrigeration machine. We selected this kind of a refrigeration machine as an academic example of an energy-conversion system with very strong interconnections among components.

The purpose of the chapter is to emphasize that

  • Exergy-based methods are practical methods that allow engineers to extract useful conclusions, and

  • The informations provided by advanced exergy-based analyses help engineers to develop ideas and solutions that cannot be suggested by other methods.

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Case Study

Figure 1 shows the air refrigeration machine that is analyzed here. The machine consists of the compressor (CM) driven by expander (EX) and electrical motor (EM, =0.9), the heat exchanger (HE) where the working fluid is cooled by water, the expander, and the refrigerator (R) where the working fluid is heated by air. The refrigeration capacity of the machine is assumed to be =100kW. The compressor and the expander are turbomachines with a theoretical pressure ratio =5. The assumed operation conditions of the air refrigeration machine are given in the column “Real process” of Table 1.

Figure 1.

Schematic of a simple air refrigeration machine

Table 1.
Values of parameters assumed for the different operation conditions of the air refrigeration machine considered in this paper (Figure 1)
ComponentParameter, UnitTheoretical ProcessProcess with Unavoidable Thermodynamic InefficienciesReal Process
CM [-]10.950.8
HE [K]0110
[%]015
EX [-]10.950.8
R [K]0320
[%]015

Key Terms in this Chapter

Exergy of Fuel: Represents the exergetic resources expended to provide the exergy of product.

Exergy of Product: Desired result achieved by a component (system) expressed in exergy terms.

Exergy Loss: Transfer of exergy from the overall system to its surroundings. This exergy transfer is not further used in this or another system.

Exergy: Maximum theoretical useful work obtainable from a system as this is brought into complete thermodynamic equilibrium with the exergy reference environment while interacting only with this environment.

Exogenous Exergy Destruction: Difference between the total exergy destruction and the endogenous exergy destruction within one component.

Unavoidable Exergy Destruction: Part of the total exergy destruction within a component that cannot be reduced due to technological limitations associated with the availability and costs of materials and manufacturing methods regardless of the amount of investment.

Avoidable Exergy Destruction: Difference between the total exergy destruction and the unavoidable exergy destruction within one component.

Exergy Destruction: Refers to the exergy destroyed due to irreversibilities within a component (system).

Endogenous Exergy Destruction: Exergy destruction within one component when all other system components are assumed to operate with exergetic efficiencies of 100%, and the total system produces the same product.

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