Comparative study of transcritical vapor compression configurations using CO2 as refrigeration mode base on simulation

https://doi.org/10.1016/j.applthermaleng.2012.10.018Get rights and content

Abstract

Despite of the excellent efficiency of the CO2 transcritical cycle in heat pump mode, there is a loss of efficiency in refrigeration mode compared with the subcritical vapor compression cycle with HFC refrigerants. This reason has lead several researches to propose ways to improve the transcritical cycle performance using CO2 as refrigerant in refrigeration mode, mainly associated to cycle configurations changes. In this paper, a comparative study and energetic simulation of most common configurations for transcritical single stage cycle using CO2 as refrigerant has been carried out. In order to make the comparison, a cycle components modelization has been proposed and the simulation results are used to find the optimum configuration for a single stage vapor compression transcritical system.

Highlights

► A comparative study and energetic simulation of most common configurations for transcritical single stage cycle using CO2. ► The characterization of each of the configurations has been done under steady state conditions. ► In DEC configuration, the superheating helps the COP to increase around of 4.5% in the firsts 1.8 °C approximately. ► The configuration resulting to be the most efficient is the one which uses a turbine as an expansion device. ► Superheating degree in DEC and DEC + IHE configurations is beneficial in both cases.

Introduction

Over the last decades, the carbon dioxide has been proposed again in refrigeration as a low GWP alternative for HFC refrigerants. The excellent environmental properties are its main advantages (GWP = 1 and ODP = 0), as well as the efficient thermodynamic and transport properties, taking into account that R744 is neither flammable nor toxic [1], [2], [3]. However, one of the implied disadvantages of using this refrigerant in transcritical cycle is the low energy performance presented in the refrigeration facilities in comparison with the subcritical installations working with HFC refrigerants. In this context, it is well known that one way to improve R744 transcritical cycle performance is associated to take advantage of throttling process losses. Sarkar et al. [4] noted that in an exergetic analysis, the replacement of the expansion valve by a turbine is the only available option to improve the performance of the system to reduce the irreversibilities of the expansion process. High pressure in this cycle could be argued as another disadvantage which could constitute a special danger in the case of accidental rupture. Talking about transcritical and subcritical cycles comparison, it has to be noted that energy performance of transcritical cycles is commonly matched to subcritical cycles assuming that the same gas cooler outlet temperature versus condenser outlet temperature often disfavors transcritical cycles. A fair basis for comparison is to assume equal mean temperature differences in the heat exchangers, consequently for assessing the trends in the temperature approach [5].

The dependence of COP on variables such as: the evaporation temperature, gas cooler outlet temperature, discharge pressure [6], [7], and isentropic efficiency [8] have motivated the use of various configurations which improve the cycle's efficiency, either introducing an internal heat exchanger (IHE), expansion devices such as ejectors [9], turbines [10], multi-stages compression [11] or arrangements that promote a subcooling at the gas cooler exit incrementing the cycle's efficiency [12], [13]. In experimental works, various contributions can be found in relation to transcritical systems. For instance, in a transcritical single stage cycle, Cabello et al. [14] develop an energetic evaluation of a refrigeration plant using a configuration which employs an internal heat exchanger obtaining an increase in the COP. Fornazieri et al. [15] propose an optimization in the throttling system for a transcritical cycle. This work is carried out in a theoretical and experimental manner, developing a configuration which uses a double stage expansion system. Yongchan [16] studies energetic improvements through the manipulation of various parameters such as: expansion valve opening, length of the internal heat exchanger, compressor speed and the charge of refrigerant; proving that the presence of the internal heat exchanger in the system decreases the optimum discharge pressure and increases the refrigeration capacity.

Therefore, various manners of increasing the COP in a transcritical system by using basic thermodynamic concepts such as the increase of the refrigerating effect in the evaporator can be highlighted; the use of devices which allow the fewest losses in the expansion stage, or even the recovering of the work on the expansion to decrease the facility energy consumption.

Hence, this work is focused on modeling and evaluating the energy performance of six single stage transcritical cycle configurations which can be considered the most representative ones according to the literature review. Several of those configurations have been studied by Cavallini and Neksá [17], however, they have not introduced internal superheating in their configurations. In this paper, an analysis for choosing the best configuration introducing internal superheating has been made. It is of great importance because in real installations, this parameter is usually used in order to improve the Coefficient of Performance, furthermore for taking control over the cooling capacity.

This paper is organized as follows: In Section 2, the proposed configurations are presented; Section 3 is focused on presenting the modeling features; Section 4 shows the results of the modeling, and finally Section 5 summarizes the main conclusions of the work.

Section snippets

Configurations

The main objective of this paper is to evaluate the energy performance of different configurations for a single stage transcritical vapor compression cycle. In the literature review there is a great amount of several proposals [11] based on single and multiple compression stages. This paper focuses on modeling configurations of single stage compression cycle and the configurations analyzed are the following:

  • a)

    Basic cycle (BC)

  • b)

    Dual expansion cycle (DEC)

  • c)

    Cycle with internal heat exchanger (IHEC)

  • d)

    Cycle

Configuration model

The characterization of each of the configurations has been done under steady state conditions. In Table 2, the characteristic equations of the modeling are presented, denoting “i” for sub-index of the component inlet and “o” for the sub-index of the outlet. For components such as the turbine working as an expansion device, it has been considered an isentropic efficiency of 60% [10]. An internal heat exchanger effectiveness of 50% has been considered [25]. For all configurations, the isentropic

Results

According to the proposed operating conditions (see Table 1), and to the characterization presented in Table 2, the main results of the energetic simulation performed are shown in this section. The modelization developments and simulations have been made in the Engineering Equation Solver software (EES).

Fig. 7 shows the Coefficient of Performance evolution varying the discharge pressure where pressure limits are 7.3 and 15 MPa. It is observable that after 12 MPa the COP presents a slightly

Conclusions

In this paper a comparative study in terms of energy of different transcritical system configurations has been carried out. The main conclusions of this work are the following ones:

  • 1.

    The configuration resulting to be the most efficient is the one which uses a turbine as an expansion device. On the other hand, using an internal heat exchanger in a cycle which uses a turbine produces a detrimental effect on the COP of the configuration.

  • 2.

    The presence of internal heat exchanger in all configurations

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