## 5 Results and Discussion
Figure 9: The trend of storage sizes for evacuated tube collector (ETC) areas of 50% PES for all flow schemes. In Figure 12, it can be seen that auxiliary energy consumption of S-1 starts rising earlier in the day than S-2 and S-3 while auxiliary energy consumption of S-3 mostly fluctuates from zero to its maximum point many times a day. As is obvious from Equation (8), primary energy saving primarily  
Figure 11: PES vs. FPC area. Figure 12: Auxiliary energy consumption for the day of mid of June. Figure 13: Chiller COP and inlet/outlet temperature profile for auxiliary heater for the mid-day of June. depends on auxiliary energy consumption by the boiler in the system, so during operating hours of the building, the lowest (zero) value of S-3 auxiliary energy consumption (legend S3 aux cons) shown in Figure 12 gives a clear indication of the OFF condition of the boiler many times during the day and reflects higher PES than S-1 and S-2. However, in order to have a clearer and logical understanding that how boiler consumes energy to heat up the hot water up to the required temperature for chiller, Figure 13 explains the behavior of boiler and chiller component for all three schemes on operating hours of 15 June (i.e., 9 a.m. to 5 p.m). At the start of cooling operation, inlet temperature to the boiler was already higher than the minimum driving temperature of the chiller due to stored thermal energy in all three schemes. As time passes by, the hot water coming from the chiller combines and transfers heat with the water in the tank whose effect is to decrease the temperature of the fluid flowing towards the chiller and declining behavior of temperatures at the inlet of the auxiliary boiler was observed. Mechanism of modified flow schemes (S-2 and S-3) serves the purpose here for saving auxiliary energy by allowing the hot water to feed the boiler with higher temperatures than conventional scheme S-1. Figure 13 illustrates the transient behavior of inlet temperature to the boiler for all schemes throughout the operating hours of the day. It is also understood that the boiler's outlet temperatures will be equal to the minimum required temperature of the chiller at the times when the boiler's inlet is less than 109 \({}^{\circ}\)C for S-1 and S-2. COP for an optimized scheme is shown as the secondary axis of Figure 13. The dynamic profile of chiller's generator outlet temperatures is displayed in Figure 14 with minor differences in their average values stayed under 103 \({}^{\circ}\)C. Dotted lines are the average temperatures for boiler inlet for the day in the month of June shown in Figure 13 is the clear and strong clue that why and how boiler consumes energy for heating up the hot water up to the minimum driving temperature (109\({}^{\circ}\)) for the chiller in descending order of S-1, S-2 and S-3. In Figure 10, the probable reason for narrowing a gap between flow schemes curves (at higher collector areas) is that the boiler's inlet HTF attains 109 \({}^{\circ}\)C so early and operates the chiller with higher generator inlet temperatures more than the minimum required temperature. Returning fluid from chiller in case of S-2 and S-3 would remain above 109 \({}^{\circ}\)C for higher collector areas, so it does not matter if it is diverted or not because the inlet to the chiller will remain more than 109 \({}^{\circ}\)C that results in auxiliary unit remain in the OFF condition. The trend of primary energy saving with higher areas of FPC for all schemes is shown in Figure 11, where optimized flow scheme S-3 always performed better than the other two schemes. Monthly efficiencies in Figure 15 are plotted at optimal thermal storage and collector slope and for the areas of ETC (93 m\({}^{2}\)) and FPC (510 m\({}^{2}\)) by which 50% primary energy saving would achieve by using flow scheme-3 of the current study i.e., monthly efficiencies of ETC are plotted for 93 m\({}^{2}\) area, the optimal thermal storage size of 15 L/m\({}^{2}\) and 9\({}^{\circ}\) collector tilt.