*4.3. Parametric Analysis*

In this section, a parametric analysis has been carried out, taking into consideration the main important design parameters: the collector slope, water flow rate through the collector, number of collectors, and the solar tank size. In all analyses carried out below, the value of a single parameter is modified keeping the rest in the value corresponding to the base design.

#### 4.3.1. E ffect of Collector Slope

The inclination angle of the collector has a significant impact on the overall SACS performance. Figure 9 shows the variation in solar coverage with the collector field inclination in Baghdad. The evaluation based on a change in the tilt angle from 5◦ to 50◦ by a step of 5◦ was carried out in order to compute the optimum angle of the solar field that provides the highest solar fraction. The change in this variable shows that the tilt angles (15◦, 20◦, 25◦, 30◦) give a higher solar fraction, contrarily to the last three angles where solar fraction decreases. The reason for this di fference is solar radiation perpendicularity that provides optimal results during summer with low tilt angle values, which help capture more solar radiation, as reported by Shariah and Elminir [38,39]. The optimum tilt values giving higher solar fraction are between 15◦ and 25◦; therefore, operating at optimum value for tilt angles can readily expand the amount of solar energy incident and, thus, enhance both the thermal and economic e fficiency of the SACS.

**Figure 9.** Solar coverage variation with the solar collector tilt.

#### 4.3.2. Effect of Water Flow Rate

In the literature, the hot water flow rate values through solar collectors range from 20 to 80 L/h per m<sup>2</sup> of collector area are recommended for panels connected in parallel, as in our case. The variation of the solar fraction, with the hot water flow rate through the solar collector array, is indicated in Figure 10. The water flow rate varied from 20 to 55 (L/h)/m<sup>2</sup> of collector area. A change of water flow from 20 to 40 (L/h)/m<sup>2</sup> causes only a 0.9% increase in solar fraction; increasing the flow rate over an optimum value (40 (L/h)/m2) will lead to drops in a solar fraction of about 0.2%. It is evident that the results obtained depict small changes in solar fraction and allow us to affirm that this parameter does not present a significant impact on solar coverage; it is in alignment with the results obtained by Beckman [40,41].

**Figure 10.** Variation of the solar fraction with the flow of water circulating through the collector.

#### 4.3.3. Effect of Solar Field Area

The area and the number of solar collectors play an important role in determining the optimal configuration of the capture solar system. The collector surface has a decisive effect on the efficiency and feasibility of SACS. The simulation was carried out to establish the influence of this parameter on the overall performance of SACS under study, based on the collector's tilt angle 20◦. The area of each collector was 2.5 m2, the water flow rate was 40 L/h per m2, the solar tank volume was 30 <sup>L</sup>/m2; the lower and upper solar tank temperatures were Tlower = 75 ◦C, Tupper = 90 ◦C. Figure 11 depicts the variation of the solar fraction with the number of collectors installed. The evaluation involves changing the number of a collectors from 4 to 24 (10 m<sup>2</sup> to 60 m2) by a step of 2 (2 m2). It is clear that an increase in the collector surface area tends to enhance solar coverage due to the proportion between the captured energy from the ETC field and solar fraction, according to simulation results displayed in Figure 11. It is predicted that the solar coverage stays constant, especially at the higher solar surface field (>55 m2). As an example, an evacuated tube collector operating in Baghdad, inclination angle 30 degrees, presents a solar coverage of about 88.1% for 22 collectors (55 m2) and 88.3% for 24 collectors (60 m2). The stability in solar fraction SF (Solar Fraction), which was also achieved in published works Bahria and Assilzadeh [42,43], indicates that the system achieves its optimum level, and any additional increase in the surface field leads to overproduction of thermal energy, which can cause technological problems and significantly increase the initial investment. Therefore, with equal investment costs, the best design will be the one that offers the greatest coverage.

**Figure 11.** Solar coverage as a function of the number of collectors.

#### 4.3.4. Effect of Solar Tank Capacity

This section examines the influence of the solar tank capacity on the solar fraction. The literature recommends values of storage solar tank capacity from 20 to 100 <sup>L</sup>/m<sup>2</sup> of collector area for installations where the time delay between collection and consumption does not exceed 24 h. The solar fraction is not significantly affected by the change in storage tank capacity, as shown in Figure 12. It is clear that increasing the solar tank capacity has a slight effect on the solar fraction. A change in solar accumulator capacity from 10 to 55 <sup>L</sup>/m<sup>2</sup> of the collector area obtains an increase in solar coverage of only 60.6% to 61.1%, respectively, with this difference (0.5%) observed—that the effect of the solar tank size on solar coverage is not significantly high. The optimum capacity of the solar tank, 30 <sup>L</sup>/m2, gives solar coverage of 61.6%. Figure 12 depicts that the oversized solar tank will cause a decrease in solar fraction due to increases in thermal losses. The result in Figure 12 is in alignment with that of Beckman [40,41]. Therefore, it is not surprising that the optimal accumulator capacity is at the lower values of the recommended range.

**Figure 12.** Solar coverage as a function of storage tank volume.

#### 4.3.5. Effect of Solar Tank Temperature

Tables 7 and 8 present the results obtained for the variation of the solar fraction with the lower and upper temperatures of the solar tank that defines the operating of the absorption chiller with solar heat. The absorption chiller will operate with water from the solar accumulator tank when the upper temperature of the storage tank is between these limits and it is possible to completely cover the cold demand. Concerning the upper temperature, the value of 90 ◦C used in the basic design seems reasonable; a higher value of top solar tank temperature would improve the solar coverage somewhat, but it should be taken into account that the limit of 95 ◦C, imposed by the absorption chiller, cannot be exceeded. In fact, tank temperature affects, as well, the inlet temperature of the generator, since the hot water directly supplies the chiller generator. As for the lower temperature, the advantage of using values as small as possible is clear. This is because more solar heat can be used and the efficiency of the collector can be improved.


**Table 7.** Variation of the solar coverage with the lower temperature of the solar tank.

**Table 8.** Variation of solar coverage with an upper temperature of the solar tank.

