## 5 Conclusions  
This work analyses the energy impact of converting an R-134a or R-507A direct expansion refrigeration system to an indirect one using a medium temperature cabinet for fresh product. Systems were tested under laboratory conditions at a product temperature in the commercial cabinet of 2 \({}^{\circ}\)C at three water inlet dissipation temperatures (23.3, 32.8 and 43.6 \({}^{\circ}\)C), thus covering the common conditions of a supermarket. The experimental evaluation covered 24-h energy consumption tests.  
From the experimental evaluation, it has been concluded that conversion to an indirect system provokes a reduction in the evaporating level to compensate for the additional heat transfer in the secondary fluid heat exchanger. This temperature reduction varies from 0.0 to 2.3 K for R-134a and from 0.0 to 2.9 K for R-507A. It has also been verified that the expansion valves operate with a reduced average degree of superheat in the indirect layout.  
With respect to the working temperatures in the cabinet, both systems operated in a similar way, with the only exception being the average temperature of the heat exchanger of the cabinet. The use of the secondary fluid heat exchanger allowed for increasing the average temperature of this heat exchanger from 12.7 to 12.9 K with R-134a and from 16.1 to 21.3 K with R-507A. This temperature increment could have an influence on the defrosting periods; however, no appreciable differences were found in the experiment.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & & **Refrigerant Charge (kg)** \\ \hline
**Element** & **Volume (m\({}^{3}\))** & **R-134a** & **R-507A** \\ \hline Liquid line & 0.00067 & 0.761 & 0.636 \\ Vapour line & 0.00130 & 0.011 & 0.023 \\ DX evaporator & 0.00780 & 1.622 & 1.547 \\ SF evaporator & 0.00035 & 0.073 & 0.070 \\ Refrigerant charge DX & & 5.41 & 6.51 \\ Charge reduction & & 2.32 & 2.14 \\ Charge reduction (\%) & & 42.9 & 32.8 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Volumes, refrigerant charge and charge reduction in the experimental plant.  
Figure 12: Refrigerant charge reduction from the DX system to an indirect system.  
Regarding the energy evaluation during the 24-h tests, no appreciable differences were found in the average power consumption of cabinet and compressor between the two system layouts; however, large increments were measured in the operation time of the compressor. For R-134a it increased between 17.8% and 20.7% and for R-507A between 6.2% and 11.8%. That resulted in an overall increment in the energy consumption of the system when moving to an indirect configuration, varying between 22.0% and 22.8% for R-134a and between 27.7% and 38.7% for R-507A.  
Finally, the experimental refrigerant charge reduction of the conversion of this system to an indirect layout was 42.9% for R-134a and 32.8% for R-507A.  
Accordingly, it can be concluded that the conversion from a direct expansion system to an indirect configuration will reduce to a large extent the refrigerant charge of the system; however, the energy impact of using a secondary fluid indirect system is large, and therefore both aspects must be taken into consideration when proposing the conversion.  
The authors gratefully acknowledge the Ministerio de Economia y Competitividad of Spain (project ENE2014-53760-R.7, grant FPI BES-2015-073612), the Jaume I University of Spain (project P11B2015-66) and the Generalitat Valenciana of Spain (grant ACIF/2017/194), for financing this research work.  
[PERSON], [PERSON] and [PERSON] designed and built the experimental plant and performed the experimental analysis. [PERSON] wrote the paper, which was revised and corrected by [PERSON], [PERSON], [PERSON] and [PERSON].  
The authors declare no conflict of interest.