# Energy-Based Thermo-Hydraulic Performance of Roughened Absorber in Solar Air Heater Duct
## 4 Result and Discussions
### The Effect of Solar Insolation  
The exergy efficiency of novel roughness in the solar air heater is studied for different values of insolation to understand the influence of solar insolation on the SAH performance. Figure 11 plots of exgetic efficiency at solar insolation from 600 W/m\({}^{2}\) to 1000 W/m\({}^{2}\) for constant rib parameters (\(p/e=10\), \(e/D=0.0289\)). As described in the previous section, similar plots are obtained here. Over the whole range of temperature increase parameters, the maximum exergy efficiency occurs at 1000 W/m\({}^{2}\) of solar insolation. This is because exergy associated with insolation is larger at higher insolation, which translates into higher exergy of air. Nevertheless, according to [PERSON] et al. (2019), exergy loss from the absorber to the environment is independent of insolation. Exergy efficiency plots are observed to peak at different values of temperature increase parameters. The exergy efficiency grows with the temperature increase parameter and, after achieving a maximum value, decreases with further increase in the temperature increase parameter. The sudden drop in exergy efficiency implies that substantial exergy is wasted due to high pumping power requirements.  
The exergy efficiency of conical protrusion ribs is compared to semi-spherical dimple ribs ([PERSON] et al., 2017), V ribs ([PERSON] et al., 2017) and discrete V-down ribs ([PERSON] et al., 2017), as depicted in Figure 12. The exergy efficiency of the conical protrusion ribs, semi-spherical dimple ribs, V ribs and discrete V ribs is presented with respect to the Reynolds number. Conical protrusion ribs have similar exergy efficiency trends to those of the semi-spherical dimple ribs, V ribs and discrete V ribs. At low Reynolds numbers, the exergy efficiency of the conical protrusion ribs is higher than those of other rib types. The net exergy efficiency of conical protrusion ribs is then surpassed by the exergy efficiency of the discrete V ribs. This occurs because of the larger friction factors in the conical protrusion ribs, which lead to high pumping power requirements. When compared to typical V ribs, the sharp corners of conical ribs enhance flow turbulence. The exergy efficiency of the conical protrusions decreases under the exergy efficiency of semi-spherical dimple ribs and V ribs because of the high pumping power requirements, especially at high Reynolds numbers.  
Figure 11: Exergy efficiency as function of insolation.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \(e\)/\(D\) = 0.0289, \(I\) = 1000 W/m\({}^{2}\) & \(\Delta T\)/\(I\) = 0.010 & \(\Delta T\)/\(I\) = 0.015 & \(\Delta T\)/\(I\) = 0.020 & \(\Delta T\)/\(I\) = 0.025 & \(\Delta T\)/\(I\) = 0.030 \\ \hline Enhancement & \(p/e=6\) & 1.11 & 1.25 & 1.34 & 1.37 & 1.27 \\ factor & \(p/e=8\) & 1.15 & 1.32 & 1.47 & 1.62 & 1.72 \\ (\(\eta_{\rm{erg}}/\eta_{\rm{exgs}}\)) & \(p/e=10\) & 1.16 & 1.33 & 1.48 & 1.64 & 1.75 \\  & \(p/e=12\) & 1.15 & 1.3 & 1.43 & 1.55 & 1.59 \\ \hline \hline \end{tabular}
\end{table}
Table 5: The enhancement factor in exergy efficiency for varying rib pitches.