OVERFED EVAPORATORS AND PARALLEL COMPRESSION IN COMMERCIAL R744 BOOSTER REFRIGERATION SYSTEMS AN ASSESSMENT OF ENERGY BENEFITS

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1 PAPER ID 1039 DOI: /iir.gl OVERFED EVAPORATORS AND PARALLEL COMPRESSION IN COMMERCIAL R744 BOOSTER REFRIGERATION SYSTEMS AN ASSESSMENT OF ENERGY BENEFITS Paride Gullo(a*), Giovanni Cortella(a), Silvia Minetto(b) and Alessio Polzot(a) a b University of Udine, DPIA, Via delle Scienze 206, Udine, Italy, gullo.paride@spes.uniud.it National Research Council, ITC, Corso Stati Uniti 4, Padua, Italy, silvia.minetto@itc.cnr.it (*) Corresponding author ABSTRACT The entry into force of the EU F-Gas Regulation 2014 pushes European Countries to use natural refrigerants. Carbon dioxide (R744) is one of the most promising long-term alternatives to the currently employed refrigerants in commercial refrigeration. On the other hand, the use of CO2 supermarket refrigeration systems in warm climates requires specific technical solutions to make them competitive with conventional HFC-based plants. In the European market, different layouts have been currently proposed, which have been mainly derived from the basic booster configuration. In this paper, the performance of a commercial CO2 refrigeration solution employing medium temperature (MT) overfed evaporators, a CO2 refrigeration configuration using parallel compression and the system which combines both of the previously technologies were theoretically compared. Three different warm climate locations were selected, i.e. Rome (Italy), Valencia (Spain) and Seville (Spain). The results showed that the benefits deriving from a higher evaporating temperature in the MT display counters, allowed by overfed evaporators, outweighed those related to parallel compression. Furthermore, the investigated R744 refrigerating plants performed an annual energy saving ranging from 2.9% to 12.3% over a R404A multiplex refrigeration system. The use of a de-superheater on the part of the combined solution led to an additional drop in the annual electricity consumption by %. Keywords: De-superheater, Overfed Evaporator, Parallel Compression, Transcritical CO2 Supermarket Refrigeration Systems, Warm Climates. 1. INTRODUCTION The development of transcritical R744 refrigeration systems for warm climates has become the most important key research area in supermarket applications in the last few years. In fact, thanks to its negligible global warming potential (GWP) and the safety level associated with its use (i.e. non-flammability and non-toxicity), R744 is prone to neither be phased down nor be phased out. Additionally, carbon dioxide features cheapness and good thermo-physical properties. A further push towards the use of CO2 has been provided through the issuing of some regulations aimed at withdrawing the high-gwp refrigerants (European Commission, 2014), as well as the taxes imposed by some Countries on the purchase of the latter. On the other hand, the performance of the CO2 refrigeration solutions is more sensitive to the external temperatures than the one associated with configurations employing all other working fluids. High cooling medium temperatures lead to the occurrence of transcritical operations, which worsen the energy efficiency of carbon dioxide refrigerating machines substantially. On the contrary, the use of such natural working fluid allows achieving higher COPs than the ones obtained by the traditional refrigerants in subcritical running modes (Sawalha, 2008). This has largely prompted its adoption in cold climates. However, large enhancements are still supposed to be attained in terms of cost, energy consumption and heat recovery. Focusing on the improvement of CO2 booster refrigeration solutions for warm outdoor temperatures, many researchers have theoretically investigated some of the currently proposed configurations (Gullo et al., 2016a; Gullo et al., 2016b; Hafner and Hemmingsen, 2015; Hafner et al., 2014; Polzot et al., 2016; Polzot et al., 2015; Karampour and Sawalha, 2015). The target is to push the so-called CO2 equator, which refers to an efficiency limit beyond which CO2 refrigeration systems are not as advantageous as the conventional solutions, further south. Gullo and Cortella (2016) and 12th IIR Gustav Lorentzen Natural Working Fluids Conference, Edinburgh, UK, 2016

2 Gullo et al. (2015) implemented a thermoeconomic analysis in order to examine the cost-effectiveness of different transcritical R744 refrigeration solutions. The advanced exergy evaluation was applied to a CO 2 booster supermarket refrigerating plant with parallel compression by Gullo et al. (2016c). Minetto et al. (2014a) presented an innovative method based on the overfeeding of multiple evaporators by employing an ejector as a pump. At the outdoor temperature of 16 C and at typical MT operating conditions, the experimental data demonstrated that the power input can be dropped by 13% over a basic CO 2 refrigerating unit. The CO 2 single-stage cycle using an overfed evaporator operating in Bari (Italy) consumes slightly less than the one employing an auxiliary compressor. The combination of these two technologies drives to a large energy saving over a basic R744 refrigeration system, as theoretically proven by Minetto et al. (2014b). The energy assessment fulfilled by Girotto et al. (2004) suggests that a R744 refrigeration system consumes 10% more energy than a R404A direct expansion solution (DXS) in North of Italy. The main goal of this paper is to quantify the energy saving which can be separately achieved through the use of an auxiliary compressor and that associated with the adoption of MT overfed evaporators. Furthermore, the increase in the performance associated with the simultaneous usage of the previously mentioned technologies, with and without a de-superheater, have also been investigated. A comparison with a R404A multiplex system, which serves both the MT load and the LT one, has also carried out in different European warm climates. 2. METHODS 2.1. Investigated solutions Figure 1a depicts a R744 refrigeration system in which the liquid collected in the low pressure tank is pumped back to the liquid receiver to overfeed the MT evaporators. Thanks to the absence of the superheating, the entire heat transfer area is optimally used being completely at the evaporation temperature. The refrigerantside heat transfer does not drop as typically happens in the superheated area and consequently the evaporating temperature can be increased. Such configuration behaves in the same way as the one presented by Minetto et al. (2014a) since, in both cases, the main energetic benefit can be attributed to the medium pressure increment and the pump power input in the system represented in Figure 1a can be neglected. Figure 1 - (a) Schematic of a R744 refrigeration system with overfed evaporators (OV); (b) Schematic of a R744 refrigeration system with parallel compression (PC). The configuration with the parallel compression (PC) is sketched in Figure 1b. In this solution, the whole amount of vapour generated by the high-pressure (HP) expansion valve is compressed from the intermediate pressure to the high one.

3 Figure 2 schematizes a refrigerating plant using MT overfed evaporators and the parallel compression (OVPC). The effect of the adoption of a de-superheater at the LS compressors discharge on the overall performance of the most technologically advanced configuration has also been investigated. Figure 2 - Schematic of a R744 refrigeration system which combines overfed evaporators and parallel compression without (OVPC) and with de-superheater (OVPC-D) Simulation models The design cooling capacities were set to 120 kw and to 25 kw for the MT and the LT loads (Girotto et al., 2004), respectively. The deviation from the design operating conditions due to the changes in the outdoor temperatures was taken into account by adopting the equation proposed by Zhang (2006): Load factor = (1 (1 min) (30 t ext ) (30 5) ) (1) in which min refers to the minimum fraction of design load (equal to 0.66 for MT and to 0.8 for LT) and t ext is the outdoor temperature. The outcomes obtained were compared to both those of a R404A direct expansion system and the ones performed by a conventional booster (CB). The multiplex configuration presented a minimum condensing temperature of 25 C and an approach temperature of the condenser equal to 10 K. As regards the R744-based configurations, an approach temperature of 3 K was selected in the event that the condensing process was occurring, otherwise such value was set to 2 K in case of the occurrence of the gas cooling process. A transition operating region was defined similarly to that selected by Gullo et al. (2016a). These running modes allowed the system to move from the subcritical conditions to the transcritical ones gradually. Furthermore, they reduce the subcooling degree from 2 K to a null value, as well as the approach temperature of the high pressure heat exchanger. The main differences between CB and the configuration represented in Figure 1a is the evaporating temperature value, which was equal to -10 C (Girotto et al., 2004) in the first case (and for PC and DXS) and to -6 C (Girotto, 2012) in the second one (and for OVPC and OVPC-D). The low temperature of -35 C was selected (Girotto et al., 2004) for all the selected configurations, while the power input associated with the fans of all the high pressure heat exchangers was computed as equal to 3% of the heat rejected (Karampour and Sawalha, 2015). Furthermore, a degree of superheating of 5 K was assumed for both the useful superheating, which took place in the evaporators, and the external one, which occurred in the suction lines. It is worth

remarking that the solution in Figure 1b and that in Figure 2 were respectively run in the same way as a conventional booster system and as the configuration in Figure 1a in subcritical conditions.

4 remarking that the solution in Figure 1b and that in Figure 2 were respectively run in the same way as a conventional booster system and as the configuration in Figure 1a in subcritical conditions. Engineering Equation Solver (EES) was employed to implement the models of all the selected cycles (F-Chart Software, 2015). All the simulations were based on steady state conditions and on the basic thermodynamic relations. All the expansion valves were modelled as isenthalpic devices, while the heat losses into the surroundings and the pressure drop occurring in all the components were assumed negligible. All the CO 2 booster configurations were optimized as a function of the high pressure in transcritical conditions. Although the intermediate pressure for the systems using an auxiliary compressor is an additional optimization parameter (Bell, 2004; Minetto et al., 2005; Gullo et al., 2016a; Gullo et al., 2016b), the current tendency is to keep it fixed (or slightly variable) in order to avoid high pressure values inside the supermarket and make the expansion process more stable (Minetto et al., 2015). In this study, the intermediate pressure for PC, OVPC and OVPC-D was chosen equal to 35 bar. The global efficiencies of all the compressors were delivered from BITZER Software (BITZER, 2016). All the selected compressors were semi-hermetic reciprocating ones and all their suggested technological limits were respected. The conventional booster configuration and the one with the auxiliary compressor had a minimum condensing temperature of 9 C, whereas it added up to 10.5 C in case of adoption of overfed evaporators. In the latter case, the evaporator outlet quality and the pump efficiency were selected equal to 0.9 and to 0.5, respectively Outdoor temperatures The evaluation of the energy consumption of the system under investigation was based on the weather trend in three different European cities: Valencia (Spain), Seville (Spain) and Rome (Italy). According to Figure 3, the outdoor temperature was below 20 C for 70.2% of the time in Rome, 59.2% in Seville and 61.5% in Valencia. It varied from 20 C to 30 C for about 31.5% of the time in Seville, for 36.7% in Valencia and for 27.8% in Rome. The external temperature exceeded or reached the value of 30 C for more than 9% of the time in Seville. Figure 3 - Temperature bins for the selected locations (Remund et al., 2014). 3. RESULTS 3.1. Effect of the use of overfed evaporators on the power input of OV in comparison with CB The increment in the medium temperature of the display cabinets leads to a reduction in the power input associated with the HS compressors. Although a low electricity consumption can be attributed to the LS compressors, the adoption of such technology causes an increase in the power needed to run them. It is worth mentioning that, in comparison with a conventional evaporator, the heat transfer area of an overfed evaporator does not need to be enlarged. In fact, the substantial enhancement in its overall heat transfer coefficient counterbalances the growth in its heat transfer area due to the decrease in the heat exchanger temperature difference.

Figure 4 compares the percentage difference of the total power input and the power input required by the HS compressors of OV with the corresponding values belonging to CB.

5 Figure 4 compares the percentage difference of the total power input and the power input required by the HS compressors of OV with the corresponding values belonging to CB. The LS compressors of OV consumed 24.4% more electricity than those of CB, value which was independent of the outdoor temperature. The average percentage difference in HS compressors power input was equal to about 8.4%, while it was equal to 6.6% in the case of the total amount. The comparison was fulfilled over the outdoor temperatures range varying from 28 C to 40 C, which referred to transcritical operations for both the investigated solutions. Figure 4 - Reduction in energy consumption associated with OV in comparison with CB Annual energy consumption The results in terms of annual consumption of the all investigated solutions are shown in Figure 5. In comparison with CB, OV consumed at least 7.2% less energy in the selected locations. The use of the parallel compression allowed achieving an energy saving by 4.3% in Rome, by 5% in Valencia and by 5.8% in Seville. Encouraging outcomes could be associated with the simultaneous adoption of the overfed evaporators and that of an auxiliary compressor, especially in cities characterized by high outdoor temperatures for a long time over the year. OVPC, in fact, exhibited a reduction in the annual energy consumption by 11% in Seville, by 10.6% in Valencia and by 10.1% in Rome over CB. Taking into account DXS as the baseline, the use of CB was not sufficiently satisfactory in none of the chosen Spanish locations, whereas it had a slightly lower power consumption in Rome. The energetic benefits related to the increment in the medium temperature brought down the electricity consumption by 9.6% in Rome, by 6.4% in Valencia and by 4.4% in Seville. The energy saving associated with PC added up to 2.9% in Seville, by 3.9% in Valencia and by 6.5% in Rome over DXS. OVPC consumed 12.3% in Rome, 9.7% in Valencia and 8.2% in Seville less energy than DXS, respectively. The pump power input could be completely neglected. Figure 5 - Annual energy consumption [MWh] of the investigated refrigeration solutions.

3.3. Influence of the de-superheater on the annual energy consumption of OVPC Figure 6 shows the percentage decrement of the annual energy consumption associated with OVPC and OVPC- D over CB.

6 3.3. Influence of the de-superheater on the annual energy consumption of OVPC Figure 6 shows the percentage decrement of the annual energy consumption associated with OVPC and OVPC- D over CB. In comparison with the latter, OVPC-D exhibited 12.5% in Rome, 12.8% in Valencia and 15.1% in Seville larger energy saving. The use of a de-superheater led to an annual energy consumption drop by more than 9 MWh over OVPC and by more 50 MWh over DXS. Figure 6 - Effect of the adoption of a de-superheater on the part of OVPC in comparison with CB. 4. DISCUSSION AND CONCLUSIONS In this paper, the performance assessment of a R744 refrigeration system with MT overfed evaporators (OV), that of a booster cycle with an auxiliary compressor (PC), as well as the one performed by a R744 refrigeration plant which combines both the aforementioned technologies (OVPC) has been theoretically conducted. The typical running modes of a supermarket have been simulated through the selection of a design MT load of 120 kw at -10 C and a design LT load of 25 kw at -35 C. The cooling capacities have been changed according to the outdoor temperature and the performance of all the compressors has been extrapolated from some manufacturers data. The effect of the overfeeding of the MT evaporators on the compressors power input has firstly investigated in transcritical operating conditions. OV leads to an increment of the LS compressors power input by 24.4% over a conventional booster configuration (CB). On the other hand, the total electricity consumption and that associated with the HS compressors drop on average by 8.4% and by 6.6%, respectively. It s worth underling that the amount of the refrigerant flowing through the LS compressors is much lower than that drawn by the HS compressors. Furthermore, a comparative evaluation has been carried out by taking into account the outdoor temperatures trend in Rome (Italy), Valencia (Spain) and Seville (Spain). OV exhibits a decrement in the annual energy consumption ranging from 4.4% to 9.6% in comparison with a R404A direct expansion (DXS) configuration, while the one related to PC varies from 2.9% to 6.5%. The results clearly demonstrate that the contribution to the increase of the performance of a CO 2 booster refrigeration system induced by an increment in the medium pressure outweighs the one induced by the use of an auxiliary compressor. The simultaneous use of such technologies drives at least to an energy saving by 8.2% over DXS. Additionally, the scenario involving a de-superheater (OVPC-D) on the part of OVPC has also been studied. DXS consumes at least 10.4% more energy than OVPC-D, while the latter achieves an energy saving by at least 2.3% over OVPC. It can be concluded that: the adoption of overfed evaporators is one of the most promising technologies in order to foster the spread of R744 in locations characterized by high outdoor temperature for a long time over the year; the coupling of an auxiliary compressor with MT overfed evaporators can substantially improve the performance of a basic booster system, as well as it allows outperforming a R404A multiplex configuration even in warm climates;

7 although some energy benefit can be associated with the use of a de-superheater, its employment would imply an increase in the total capital investment and therefore a suitable trade-off between the economic and the energetic aspects should be considered. NOMENCLATURE CB Conventional R744 booster system MT Medium temperature DXS Direct expansion system OV R744 booster system with MT overfed evaporators EES Engineering Equation Solver OVPC R744 booster system with MT overfed evaporators and parallel compression GWP Global Warming Potential (kg CO 2 kg 1 refrigerant) OVPC-D R744 booster system with MT overfed evaporators, parallel compression and de-superheater HFC Hydro-fluorocarbon PC R744 booster system with parallel compression HP High pressure t Temperature ( C) HS High stage TOT Total LS Low stage Subscripts and superscripts LT Low temperature ext External REFERENCES Bell, I., Performance increase of carbon dioxide refrigeration cycle with the addition of parallel compression economization. In: Proceedings of the 6 th IIR Gustav Lorentzen Conference on Natural Working Fluids; Glasgow, United Kingdom. BITZER, BITZER Software Version Available at: < [accessed ]. European Commission, Regulation (EU) No 517/2014 of the European Parliament and of the Council of 16 th April 2014 on fluorinated greenhouse gases and repealing Regulation (EC) No 842/2006. F-Chart Software, Engineering Equation Solver (EES), Academic Professional version Available at: < [accessed ]. Girotto, S., CO 2 refrigeration in warm climates, efficiency improvement. In: Proceedings of the ATMOsphere Europe 2012; Brussels, Belgium. Girotto, S., Minetto, S., Nekså, P., Commercial refrigeration system using CO 2 as the refrigerant. International Journal of Refrigeration 27, Gullo, P., Cortella, G., Polzot, A., 2016a. Energy and environmental comparison of commercial R744 refrigeration systems operating in warm climates. In: Proceedings of the 4 th IIR Conference on Sustainability and the Cold Chain; Auckland, New Zealand. Gullo, P., Elmegaard, B., Cortella, G., 2016b. Energy and environmental performance assessment of R744 booster supermarket refrigeration systems operating in warm climates. International Journal of Refrigeration 64, Gullo, P., Elmegaard, B., Cortella, G., 2016c. Advanced exergy analysis of a R744 booster refrigeration system with parallel compression. Energy 107, Gullo, P., Cortella, G., Comparative Exergoeconomic Analysis of Various Transcritical R744 Commercial Refrigeration Systems. In: Proceedings of the 29 th International Conference on Efficiency, Cost, Optimization Simulation and Environmental Impact of Energy Systems; Portorož, Slovenia. Gullo, P., Elmegaard, B., Cortella, G., Energetic, Exergetic and Exergoeconomic Analysis of CO 2 Refrigeration Systems Operating in Hot Climates. In: Proceedings of the 28 th International

8 Conference on Efficiency, Cost, Optimization Simulation and Environmental Impact of Energy Systems; Pau, France. Hafner, A., Hemmingsen, A.K., R744 refrigeration technologies for supermarkets in warm climates. In: Proceedings of the 24 th IIR International Congress of Refrigeration; Yokohama, Japan. Hafner, A., Hemmingsen, A.K., Van de Ven, A., R744 Refrigeration system configurations for supermarkets in warm climates. In: Proceedings of the 3 rd IIR International Conference on Sustainability and the Cold Chain; London, United Kingdom. Karampour, M., Sawalha, S., Theoretical analysis of CO 2 trans-critical system with parallel compression for heat recovery and air conditioning in supermarkets. In: Proceedings of the 24 th IIR International Congress of Refrigeration; Yokohama, Japan. Minetto, S., Girotto, S., Rossetti, A., Marinetti, S., Experience with ejector work recovery and auxiliary compressors in CO 2 refrigeration systems. Technological aspects and application perspectives. In: Proceedings of the 6 th IIR Ammonia and CO 2 Refrigeration Technologies Conference; Ohrid, Macedonia. Minetto, S., Brignoli, R., Zilio, C., Marinetti, S., 2014a. Experimental analysis of a new method of overfeeding multiple evaporators in refrigeration systems. International Journal of Refrigeration 38, 1-9. Minetto, S., Girotto, S., Salvatore, M., Rossetti, A., Marinetti, S., 2014b. Recent installations of CO 2 supermarket refrigeration system for warm climates: data from the field. In: Proceedings of the 3 rd IIR International Conference on Sustainability and the Cold Chain; London, United Kingdom. Minetto, S., Cecchinato, L., Corradi, M., Fornasieri, E., Zilio, C., Theoretical and Experimental Analysis of a CO 2 Refrigerating Cycle with Two-Stage Throttling and Suction of the Flash Vapour by an Auxiliary Compressor. In: Proceedings of IIR International Conferences Thermophysical Properties and Transfer Processes of Refrigerants; Vicenza, Italy. Polzot, A., D Agaro, P., Cortella, G., Gullo, P., Supermarket refrigeration and air conditioning systems integration via a water storage. In: Proceedings of the 4 th IIR Conference on Sustainability and the Cold Chain; Auckland, New Zealand. Polzot, A., D Agaro, P., Gullo, P., Cortella, G., Water storage to improve the efficiency of CO 2 commercial refrigeration systems. In: Proceedings of the 24 th IIR International Congress of Refrigeration; Yokohama, Japan. Remund, J., Lang, R., Kunz, S., Meteonorm, Meteotest, Bern (Switzerland). Sawalha, S., Theoretical evaluation of trans-critical CO 2 systems in supermarket refrigeration. Part II: System modifications and comparisons of different solutions. International Journal of Refrigeration 31, Zhang, M., Energy Analysis of Various Supermarket Refrigeration Systems. In: Proceedings of the International Refrigeration and Air Conditioning Conference; Purdue, USA.

Energy and environmental performance assessment of R744 booster supermarket refrigeration systems operating in warm climates

Downloaded from orbit.dtu.dk on: Nov, 1 Energy and environmental performance assessment of R booster supermarket refrigeration systems operating in warm climates Gullo, Paride; Elmegaard, Brian; Cortella,