## 3 Results and Discussion  
### Experimental Results  
Figure 8 shows the inlet temperature \(T_{i}\), outlet temperature \(T_{o}\) and absorber temperature \(T_{a}\), as well as ambient temperature \(T_{amb}\), and global solar radiation measured experimentally from 6 a.m. to 6 p.m. on 1 March 2020 for both a CPC absorber without fins (Figure 8a) and a CPC absorber with segmented fins (Figure 8b). It can be seen that the increase in the outlet temperature is due to the increase in the global radiation. It can also be seen that the CPC collector with a finned absorber produces a larger temperature difference, \(\Delta T\) = \(T_{o}-T_{i}\), as a result of the increase in the available energy (power) on the absorber surface. Figure 9 shows \(\Delta T\) and the thermal efficiency \(\eta_{t}\) for the absorbers without and with segmented fins. The efficiency \(\eta_{t}\) was obtained using the following equation:  
\[\eta_{t}=\frac{\dot{m}C_{p}\Delta T}{A_{a}I} \tag{1}\]  
where \(\dot{m}\) is the mass flow rate, \(C_{p}\) is the specific heat capacity, \(A_{a}\) is the aperture area of the CPC collector and \(I\) is the global solar radiation. As previously mentioned, it can be seen in Figure 9 that \(\Delta T\) is larger for the CPC with the absorber with segmented fins, which results in an increase in the efficiency of the CPC collector. The average \(\eta_{t}\) of the CPC absorbers without fins and with segmented fins on 1 March 2020 was 42% and 57%, respectively, under the given experimental conditions. The average \(\eta_{t}\) of the CPC absorber  
Figure 8: Experimental measurements of the inlet, outlet and absorber temperatures in the CPC collectors, ambient temperature and global solar radiation on 1 March 2020: (**a**) absorber without fins; (**b**) absorber with segmented fins. Figure 7: Mesh of CPC absorber models: (**a**) CPC absorber without fins; (**b**) CPC absorber with segmented fins; (**c**) CPC absorber with continuous fins. without fins is similar to the \(\eta_{t}\) obtained using the same CPC system with a fixed tilt angle of 21\({}^{\circ}\) on an early winter day (\(\sim\)43%) [16]. The thermal efficiency \(\eta_{t}\) can also be defined as:  
\[\eta_{t}=\frac{Q_{u}}{A_{a}I} \tag{2}\]  
where \(Q_{u}\) is the useful heat of the collector, which is defined as  
\[Q_{u}=F_{R}A_{a}[I(\tau\alpha)-U_{L}(T_{i}-T_{amb})] \tag{3}\]  
where \(F_{R}\) is the heat removal factor, \((\tau\alpha)\) is the transmittance-absorptance product and \(U_{L}\) is the heat loss coefficient. Equation (3) is known as the Hottel-Whillier-Bliss equation. By substituting Equation (3) into Equation (2), \(\eta_{t}\) can be expressed as  
\[\eta_{t}=F_{R}(\tau\alpha)-F_{R}U_{L}\bigg{(}\frac{T_{i}-T_{amb}}{I}\bigg{)} \tag{4}\]  
Figure 10 shows the experimental \(\eta_{t}\) versus \((T_{i}-T_{amb})/L\) for the CPC collectors on 1 March 2020. Equation (4) was fitted to the experimental data as shown in Figure 10. The intercept \(F_{R}(\tau\alpha)\) of the fitted curve with the ordinate axis for the CPC with segmented fins (0.584) is larger than that of the curve for the CPC without fins (0.405); the ratio of the two values is 1.45. This indicates that \(F_{R}\) for the CPC with segmented fins is 45% larger than that of the CPC without fins (considering that \((\tau\alpha)\) is constant), which in turn indicates that the heat transfer from the absorber tube to the fluid is greater for the CPC with segmented fins when compared to the CPC without fins. On the other hand, the slope \(F_{R}U_{L}\) of the curve for the CPC with segmented fins is 11.065 W/\({}^{\circ}\)C m\({}^{2}\), while for the CPC without fins it is 6.881 W/\({}^{\circ}\)C m\({}^{2}\). The ratio of these two values is 1.61, which indicates that the \(U_{L}\) is larger for the CPC with segmented fins when compared to the CPC without fins.