1. Introduction
Commercial refrigeration units play a crucial role in modern society, being widely employed for satisfying various human needs. However, supermarket refrigerating applications predominately rely on hydrofluorocarbons (HFCs), such as HFC-404A and HFC-507A, as refrigerants. These working fluids feature a Global Warming Potential (GWP) being thousands of times more environmentally damaging than carbon dioxide, leading the commercial refrigeration sector to be a major direct driver of global warming. To reduce the HFC consumption and, thus, significantly mitigate the carbon footprint of food retail stores in Europe, the EU F-Gas Regulation 517/2014 [
1] was issued. This legislative act aims at progressively decreasing the HFC supply by 79% by 2030 in relation to the average levels in 2009–2012. Additionally, the EU F-Gas Regulation 517/2014 imposes a limit in terms of GWP
100 years for the refrigerants used in multipack centralized refrigeration systems with a rated capacity above 40 kW equal to 150
since January 2022. The fight against HFCs is intensifying on global perspectives as well, as 197 countries recently agreed to bring the production and consumption of HFCs down by more than 80% over a 30-year period [
2]. Therefore, the selection of a long-term refrigerant is becoming arduous for engineers and end-users in as strategic a sector as that of supermarket applications. This challenge is further complicated in warm climates where, for climate reasons, refrigeration reclaim has a considerable impact on economic, energy, and environmental perspectives.
Commercial refrigeration plants using carbon dioxide as the sole refrigerant (R744) are perceived to be one of the most promising candidates with which to replace the currently employed units [
3]. Being that R744 is non-flammable, non-toxic, and environmentally friendly (i.e., negligible GWP), in fact, this working fluid is bound not to be subject to any future restrictions. This refrigerant is also readily available and inexpensive, as well as features more favorable thermo-physical characteristics compared to HFCs [
4]. However, as CO
2 presents a low critical temperature (about 30.98 °C), the heat rejection process through the high pressure heat exchanger (i.e., gas cooler) can commonly take place in transcritical conditions. These running modes feature large differences between rejection and absorption pressure, leading the conventional transcritical R744 supermarket refrigeration systems to have very poor energy efficiencies with a rise in outdoor temperature. As shown in [
5], in fact, the aforementioned systems can energetically compete with refrigerating units employing man-made working fluids at external temperatures up to about 25 °C. Therefore, commercial “CO
2 only” refrigeration plants need a more sophisticated system architecture so as to perform equivalently to, or better than, HFC-based solutions in warm locations [
3]. As a result of the entry into force of the EU F-Gas Regulation 517/2014, many measures with the purpose of enhancing the performance of such HFC-free units at severe running modes have been developed [
3], such as:
parallel compression [
6,
7,
8], which represents the first step towards of the adoption of “CO
2 only” supermarket refrigeration systems in high ambient temperature countries;
cold thermal energy storages [
9,
10], which allows reducing the energy consumption by shifting a part of the refrigeration load from more adverse (i.e., daytime) to more advantageous (i.e., night-time) operating conditions;
the implementation of the recovery of part of the available expansion work via two-phase ejectors [
11,
12,
13], giving rise to a significant enhancement in overall thermodynamic performance. The conventional expansion valve, in fact, is responsible for the largest irreversibilities in basic transcritical CO
2 refrigerating cycles [
14] and, thus, for the significant penalization in their efficiencies as the cooling medium temperature goes up.
In order to reduce the aforementioned inefficiencies, Fazelpour and Morosuk [
14] recommended the adoption of an expedient aimed at reducing the temperature of R744 exiting the gas cooler. As showed in [
15], this target can be achieved with the aid of a dedicated mechanical subcooling, which permits the refrigerant to going into the evaporator with a lower quality and, thus, leads to an increment in refrigerating effect. Also, as described by many researchers [
16,
17,
18], an optimal high pressure, which maximizes the coefficient of performance (COP), has to be evaluated as a function of the gas cooler exit temperature as transcritical running modes occur. As revealed in [
15], the integration of the dedicated mechanical subcooling also allows decreasing the optimal heat rejection pressure, giving rise to an additional improvement in performance. The benefits from the adoption of a dedicated mechanical subcooling are summarized in
Table 1.
Additionally, an in-depth overview on the “CO
2 only” solutions using the dedicated mechanical subcooling was recently presented by Llopis et al. [
28]. On the one hand, the findings listed in
Table 1 reveal that such “CO
2 only” supermarket refrigeration plants are expected to offer promising performance in warm locations. On the other hand, it is also possible to notice that conventional energy-based methods are predominately employed for evaluating the performance of the aforementioned systems. This is due to the fact that such assessments offer simplicity with respect to their adoption, as well as intuitive interpretation of the results obtained, favoring their wide adoption. However, the thermodynamic performance of any energy system can be more appropriately evaluated by applying a conventional exergy analysis. Such an evaluation, in fact, allows bringing to light the location, the magnitude, and the sources of the inefficiencies caused by the irreversibilities taking place in the investigated system. More appropriate conclusions aimed at properly evaluating the thermodynamic performance of any energy system can be drawn with the aid of the advanced exergy analysis [
29,
30,
31]. Unlike the conventional exergy evaluation, in fact, the implementation of this thermodynamic tool enables revealing: (1) the real improvement potential associated with the selected system via the assessment of the avoidable exergy destruction of its components; (2) the mutual interdependencies among the system components via the evaluation of their mexogenous exergy destruction. As a consequence, at present the advanced exergy analysis is widely recognized as the most suitable thermodynamic method to adequately evaluate the performance of any energy system. Furthermore, as mentioned above, state-of-the-art transcritical R744 refrigeration systems have taken center stage in as a crucial sector as that of supermarket applications. To the best of the author’s knowledge, a few investigations combining these key research topics are still available and none of these involves “CO
2 only” supermarket refrigeration plants outfitted with dedicated mechanical subcooling, as summarized in
Table 2. Therefore, this study is intended to take steps towards this scientific gap by appropriately assessing the thermodynamic performance of a promising HFC-free solution, such as the commercial transcritical R744 booster refrigeration system employing a R290 dedicated mechanical subcooling, with the aid of one of the most powerful thermodynamic tools, i.e., the advanced exergy assessment.
First of all, the advanced exergy analysis has been applied by selecting the external temperature of 40 °C as well as the typical operating conditions of the investigated solution, as the ones suggested in the open literature [
8]. At a later time, a study involving the effect of the most influential parameters on the performance of the whole system, i.e., the high stage compressor efficiency, the gas cooler/condenser approach temperature, the R744 subcooler exit temperature and the outdoor temperature, has also been implemented. It is worth remarking that the approach temperature of a heat exchanger is defined as the difference between the outgoing hot fluid temperature and the ingoing cold fluid temperature. In addition to the Introduction, the present work presents five additional sections. In
Section 2 the investigated solution and the assumptions in common in all the implemented evaluations are described, while the main concepts related to both the conventional and the advanced exergy assessment are presented in
Section 3. The results obtained are shown and discussed in
Section 4 and
Section 5, respectively. Finally, the conclusions are given in
Section 6.
5. Discussion
At the outdoor temperature of 40 °C the results of the conventional exergy analysis show that high stage compressors, the MT evaporators, and the gas cooler/condenser present the highest exergy destruction rates, contributing each for about 20% to the total exergy destruction rate (). Significant irreversibilities can also be ascribable to the LT evaporators ( = 8.2% of ) and the subcooler ( = 6.6% of ). In addition, despite the presence of the mechanical subcooling loop, the high pressure expansion valve features a contribution of 6.4% to at the selected external temperature, similarly to the R290 compressor. The conduction of the advanced exergy analysis has led to a better understanding of the real potential improvements achievable by the evaluated system. First of all, it has been found that only 59% of the irreversibilities occurring in the investigated solution can be actually avoided. This can be attained by mainly enhancing its components. In addition, the designer should focus even more on the high stage compressors, as these are responsible for 31.8% of the total avoidable irreversibilities taking place in the selected system. On the contrary, the contribution on the part of the high pressure expansion valve to the total avoidable exergy destruction rate () is negligible. Furthermore, the basic analysis has indicated the gas cooler/condenser as a component, which needs to be substantially improved. However, only about half of its inefficiencies are actually avoidable and mainly by reducing the irreversibilities owing to the simultaneous interaction among the components, as well as improving the high stage compressors. Additionally, the R290 compressor and the subcooler feature 9.6% and 4.4% of , being roughly half of their avoidable inefficiencies exogenous. The thermodynamic performance of the former can be incremented by reducing the inefficiencies brought about by the concurrent interaction among the components. The enhancement of the gas cooler/condenser would lead to reductions in irreversibilities associated with both the R290 compressor and the subcooler. Furthermore, approximately one third of the avoidable exergy destruction related to the high stage compressors is due to the other components and mainly associated with the increase in medium temperature. Although the conventional exergy analysis has suggested that large improvements can be accomplished by enhancing the MT evaporators, only half of their irreversibilities can be actually avoided. In particular, the MT evaporators and the LT evaporators, respectively, cause 17.1% and 7.8% of , being improvable uniquely by enhancing the heat exchangers themselves.
Finally, the sensitivity analyses have revealed that:
the increment in high stage compressor efficiency by 10% at the external temperature of 40 °C would imply a decrease by 6.6% in and by 11.1% in , respectively. In particular, the avoidable irreversibilities related to the high stage compressors and the gas cooler/condenser would reduce by 28% and 13.5%, respectively;
in comparison with the scenario relying on ΔTappr,GC = 2 K, and would respectively increase by 5.3% and 8.2% as a gas cooler/condenser approach temperature of 5 K is adopted at the outdoor temperature of 40 °C. Also, such an increment would cause growths in and by up to 4.1% and 3.1% as well as increases in and by up to 2% and 6.9%;
as the external temperature is taken as 45 °C, and grow by 8.3% and 9.6%, respectively. In particular, , and have been found to be 9.1%, 7.4%, and 5.6% higher, respectively. Finally, the aforementioned parameter does not affect significantly;
it has been showed that the optimal temperature of R744 exiting the subcooler from the energy perspective is similar to the value minimizing the total avoidable irreversibilities at the outdoor temperature of 40 °C.
Gullo et al. [
32] applied the advanced exergy analysis to a CO
2 booster refrigeration system with parallel compression at similar boundary conditions as the ones used in this study. Consistently with the outcomes available in the open literature, all the evaporators can be enhanced uniquely through the reduction in inefficiencies occurring in the components themselves in both investigations. Although the improvement in high stage compressors would allow reducing the majority of its avoidable irreversibilities, further enhancements can be obtained by enhancing the MT evaporators in both studies and increasing the irreversibilities in the remaining components in the configuration with parallel compression. As for the gas cooler/condenser, although in both investigations this component can be improved by reducing the inefficiencies occurring in the other components, discordant outcomes have been found. In fact, a significant enhancement in performance of the MT evaporators and a substantial worsening in irreversibilities occurring in the other components are required to improve the gas cooler/condenser operating in the solution with parallel compression. On the other hand, this component is mainly affect by the simultaneous interaction of all the components and high stage compressors in the configuration with dedicated mechanical subcooling.
6. Conclusions
Thanks to its favorable environmental and safety properties, carbon dioxide as the sole refrigerant for supermarket refrigerating systems has taken center stage worldwide. However, commercial “CO2 only” refrigeration plants need to implement some expedients in order to be able to outperform HFC-based systems in warm/hot climates. This target can be properly accomplished by adopting a dedicated mechanical subcooling, leading this technology to be in the spotlight.
In this paper, the thermodynamic performance of a transcritical CO2 booster supermarket refrigeration unit equipped with a mechanical subcooling loop relying on R290 has been exhaustively investigated with the aid of the advanced exergy analysis. This method is currently considered the most effective thermodynamic tool to implement such evaluations. Therefore, in the present work two of today’s most relevant key research topics have been combined for the first time ever to the best of the author’s knowledge. The subcooler outlet temperature has been firstly set to 15 °C and the cooling capacities have been selected equal to 97 kW at the evaporating temperatures of −10 °C and to 18 kW at −35 °C.
The application of the advanced exergy analysis has provided additional and useful information, which could foster the spread of the investigated system. It can be concluded that:
only 59% of its inefficiencies can be actually reduced and mainly by enhancing its components;
it is crucial that the manufacturers promote the diffusion of more efficient high stage compressors;
close attention needs to be devoted to the gas cooler/condenser. Its performance is improvable mainly by decreasing the irreversibilities due to the simultaneous interaction among the components;
focus on the performance of the R290 compressor, MT and LT evaporators is also necessary. In particular, about half of the avoidable inefficiencies occurring in the R290 compressor are mainly due to the concurrent interaction among the components and to the gas cooler/condenser. On the contrary, all the evaporators need for a reduction in their temperature difference;
the approach temperature of the gas cooler/condenser and the outdoor temperature have also been found to affect the thermodynamic performance of the selected solution.
As future work an advanced exergoeconomic analysis will be applied to the investigated system to suitably investigate the connection between the costs related to the equipment and its thermodynamic inefficiencies. However, it is worth remarking that, although realistic operating conditions have been adopted, the proposed work would significantly benefit from the validation of the results obtained against field measurements.