## 3 Energy Consumption Tests
### Energy Indicators  
Energy consumption is the relation between the operating time and the average power consumption of the elements. Figure 8 details the percentage of operating time during the 24-h tests of compressor and expansion valve for each layout for both refrigerants. The most significant effect than has been measured for both refrigerants is that the compressor operation fraction increases when using an indirect system. For this evaluation, the measured increment in compressor's operation time has been between 17.8% and 20.7% for R-134a and between 6.2% and 11.8% for R-507A. The reason for this increment is that the amount of refrigerant contained in the distribution lines (liquid and vapour) and in the cabinet's evaporator when using a direct expansion system is higher than when using an indirect system. In the direct expansion configuration, the amount of refrigerant contained in the system is higher, so the evaporation pressure rises slowly until the compressor is activated again. On the contrary, when using the indirect system, the amount of refrigerant contained in the secondary fluid heat exchanger is low, and therefore the low pressure increases rapidly and the compressor is activated more frequently. This effect can be observed by contrasting the evaporating temperatures in Figure 3 (direct expansion) and in Figure 4 (indirect expansion). It can be observed that the compressor enters into operation more frequently. Another important effect observed in Figure 8 is that the operation time of expansion valve and compressor are decoupled when using a direct expansion system, again due to the amount of refrigerant contained in the distribution lines and in the cabinet's evaporator, but they are coupled when using an indirect system. Figure 9 details the average power consumption of compressor (when it is in operation), of cabinet (including the three defrosting periods) and of secondary fluid pump during the 24-h tests. As can be observed, both the cabinet's and the compressor's power consumption remain practically constant for both refrigerants in direct expansion or indirect system. The maximum variation for R-134a is of 2% for the operation at 43.6 \({}^{\circ}\)C and of 5% for R-507A at 43.6 \({}^{\circ}\)C. Regarding the power consumption of the secondary fluid pump, it remains at a closer value, being negligible in relation to the compressor and cabinet. Figure 7: Average temperature differences between the evaporating temperature and air inside the cabinet and product temperatures. Finally, to compare the direct expansion and indirect configurations from an energy point of view, the energy consumption (kWh) of each element has been calculated from the power consumption measurements and operating time according to Equation (5) using a trapezoid integration method. In Equation (5), _'i'_ represents each energy consumer, '\(P_{C}\)' its power consumption and _'j'_ each sampled data. The expression is evaluated during the 24-h test. \[E_{i}=\frac{1}{36\times 10^{5}}\cdot\int_{0}^{24\text{h}}P_{C,i}(t)\cdot dt= \frac{1}{36\times 10^{5}}\cdot\sum_{j=1}^{24\text{h}}\biggl{\{}\biggl{[}\frac{P_{C,i}( j)+P_{C,j}(j-1)}{2}\biggr{]}\cdot[t(j)-t(j-1)]\biggr{\}} \tag{5}\]  
Figure 8: Operating time percentage of compressor and expansion valve during 24-h test. Figure 9: Average compressor’s, cabinet’s and SF pump power consumptions during 24-h test. The results are detailed in Figure 10. Concerning cabinet's energy consumption, no appreciable differences have been measured between the layout configurations and between both refrigerants. That indicates that the defrosting times are equivalent for both arrangements. In fact, the maximum deviation of defrosting times has been below 1 min for both refrigerants and both system layouts. Regarding the compressor's power consumption, an important increment in its energy consumption has been measured when going from a direct to an indirect system, the main reason being the longer time that the compressor is in operation (Figure 8).