## 4 Results and Discussion  
After optimization of the SHW for maximum exergy efficiency, we found 3.72% exergy efficiency. The values of parameters at optimum condition are used to examine the effect of certain parameters on the system performance. Effect of various parameters on exergy efficiency and thermal efficiency is shown in the following figures (Figures 4-8). Figure 4 shows the effect of fluid inlet temperature on exergy and thermal efficiency. With the increasing fluid inlet temperature, the amount of heat transfer to the fluid (water) from the absorber plate reduces due to lesser temperature difference. Regarding the exergy efficiency, it increases with inlet temperature to the maximum value after which it reduces. Exergy efficiency depends on inlet temperature by two factors: destruction of exergy due to the temperature difference between the sun and the absorber and the exergy loss due to heat loss to the surrounding. With the increasing inlet temperature, the temperature difference between the sun and the absorber reduces, which causes lower exergy destruction. On the other hand, with the increasing inlet temperature, heat loss increases that cause higher exergy loss. In combination of exergy reduction due to the two mentioned factors, we obtain the gradual change in exergy with a maximum value at a certain point. Similar result for thermal and exergy efficiency was reported for solar collectors [9; 10; 14; 26]. Figure 5 depicts the effect of ambient temperature on the two efficiencies. When the ambient temperature increases, the temperature difference between the absorber and the ambient reduces, which results lower heat loss. Exergy efficiency decreases significantly with the ambient temperature. As the ambient temperature increases, the amount of flow exergy reduces, for which the exergy efficiency reduces. Increasing ambient temperature from 301 K to 311 K results in an exergy efficiency drop from approximately 4% to 3%. [PERSON] et al. found same pattern for FPC [14]. Figure 4: Variation of exergetic efficiency and thermal efficiency with fluid inlet temperature. Optical losses reduce as the optical efficiency increases, hence the thermal and exergy efficiency increases (Figure 6). Increasing optical efficiency from 0% to 100% increases the exergy efficiency from 0% to 4.9%.For FPC, it was reported that the exergy efficiency increases with optical efficiency in similar manner [14]. Figure 7 shows the variation of exergy and thermal efficiency with the irradiance. Exergy efficiency increases at almost steady rate between the irradiance levels 300 W/m\({}^{2}\) to 1000 W/m\({}^{2}\) which is similar to the result reported for FPC [14]. Thermal efficiency initially increases rapidly but later the rate of change of efficiency decreases, which reaches saturation level at around 800 W/m\({}^{2}\) irradiance. Figure 5: Variation of exergetic efficiency and thermal efficiency with ambient temperature. Figure 6: Variation of exergetic efficiency and thermal efficiency with optical efficiency. Controlling the mass flow rate in this type of passive heating system is not possible. The analysis shows that (Figure 8) the exergy efficiency decreases with the increased mass flow rate of water, while the thermal efficiency almost remains constant. A Similar pattern of exergy efficiency with respect to mass flow rate is reported by [PERSON] et al. [11]. Figures 9-13 show the dimensionless exergy destruction and loss components with respect to various parameters. Here, it can be seen that around 75% of the exergy destruction occurs due to the temperature difference between the sun and the absorber plate. Hence, increasing the absorber plate temperature can reduce this portion of exergy destruction. Basically, without using any solar concentrating device this portion of destruction cannot be minimized much. [PERSON] et al. [11] reported that about 73% of the exergy destruction is caused in similar case for flat plate collectors.