1. Introduction

Cooling is part of twenty-first century life. Air conditioning, food conservation, industries such as steel, chemicals, and plastics depend on cooling. By mid-century people will use more energy for cooling than heating [1]. Almost all cold is produced by vapor-compression refrigeration and requires large amounts of electricity for its production. And since electricity is still overwhelmingly produced by burning fossil fuels, the rise in cold production will inevitably

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

increase both fuels consumption and power plant emissions. A climate-change irony is that cooling makes the planet hotter. Besides the development of new cooling devices using renewable energy, an important way to reduce refrigeration power consumption is through the energy efficiency improvement of vapor-compression cycles and their associated elementary processes. The processes of compression and expansion play a central role in air-conditioning, refrigeration and cryogenics. An important question still remains: How to define the efficiency of these processes by taking into account the constraints of the first and second laws of thermodynamics? The answer will be discussed in this paper.

temperatures. However, according to Eq. (1), E\_ <sup>Q</sup> is positive due to the fact that heat is removed from a cooled object, and thus Q has a negative sign in Eq. (1). The energy and exergy balances \_ of a reversible refrigerator (RR) are presented in Figure 1. One can notice that the directions of energy and exergy flows are opposite below T0. This means that the exergy of a heat flow at T<T0 is looked upon as a product of the refrigeration system rather than as feed. The exergy transfer of a RR characterizes the rate of transformation of power W to exergy of heat flow \_ Q\_ (exergy of produced cold). Given that the system presented in Figure 1 is reversible, the minimum power W\_ min necessary to maintain a cooling rate Q equals \_ E\_ Q. Obviously it is not the case for a real (non-reversible) refrigerator, where E\_ <sup>Q</sup> is lower than W by the value of \_

Exergy Flows Inside Expansion and Compression Devices Operating below and across Ambient Temperature

http://dx.doi.org/10.5772/intechopen.74041

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The thermo-mechanical exergy equals the maximum amount of work obtainable when the stream of substance is brought from its initial state to the environmental state, defined by pressure P0 and temperature T0, by physical processes involving only thermal interaction with the environment [3, 4]. The specific thermo-mechanical exergy eP,T is calculated according to:

The value of eP,T may be divided by two components: thermal exergy eT due to the temperature difference between T and T0, and mechanical exergy eP due to the pressure difference between P and P0. It is important to emphasize that this division is not unique, because eT depends on pressure conditions and eP in its turn depends on temperature conditions. As a result, the division has no fundamental meaning and leads, as will be illustrated further, to ambiguities for the exergy efficiency definition. By conventional agreement [4], eT and eP are

Figure 1. Energy (a) and exergy (b) balances of a reversible refrigerator (RR).

eP,<sup>T</sup> ¼ ½h Pð Þ� ; T h Pð Þ <sup>0</sup>; T0 � � T0 � ½ � s Pð Þ� ; T s Pð Þ <sup>0</sup>; T0 (2)

eT ¼ ½h Pð Þ� ; T h Pð Þ ; T0 � � T0 � ½ � s Pð Þ� ; T s Pð Þ ; T0 (3)

eP ¼ ½h Pð Þ� ; T0 h Pð Þ <sup>0</sup>; T0 � � T0 � ½ � s Pð Þ� ; T0 s Pð Þ <sup>0</sup>; T0 (4)

exergy losses D. \_

defined as:

2.2. Thermo-mechanical exergy

The introduction of exergy, the thermodynamic function that takes into account the quality as well as the quantity of energy, has paved the way for a unified approach to the concept of efficiency, a subject pioneered by Grassmann [2]. Serious difficulties concerning the practical application of this concept to sub-ambient systems, however, retarded the acceptance of exergy analysis by the air-conditioning and refrigeration engineering profession. One can mention, in particular, the difficulty of formulating a coefficient of exergy efficiency (CEE) for elementary processes such as compression and expansion. The coefficient should evaluate the exergy losses, quantify the extent to which the technical purpose of an elementary process is achieved, as well as quantify the exergy consumption within the process. Finally, a uniquely determined value (not several) should be assigned to the coefficient. This paper examines some important points pertinent to these issues and presents a definition of the CEE for the thermodynamic evaluation of expansion and compression devices operating below and across ambient conditions. The definition is based on the concept of transiting exergy, introduced by Brodyansky et al. [3], that allows non-ambiguous computation of two metrics: exergy produced and exergy consumed.
