## 4 Results and Discussion  
It is obvious that the overall efficiency (thermal plus electrical) of the photovoltaic/thermal (PV/T) system is affected greatly by the fluid type and the flow rate; therefore, tracking of the optimal flow rate against each heat transfer fluid is important. Due to this reason, in the first phase of the experiments,the optimal flow rates for air and water were tracked by varying the fluid flow rate from laminar to turbulent ranges. With the help of a control panel, the fluid flow through the pump was controlled and monitored using a digital flow meter, and six flow rates were selected, two for each flow regime: laminar, transitional, and turbulent. In the case of water plus air as dual fluid, either fluid was varied while the second fluid was kept fixed in different flow regimes as shown in Figures 3 and 4. It is noticed that a higher fluid flow rate caused higher thermal efficiency, as expected; however, considering pump power consumption and the vibration effect in the photovoltaic (PV) module, the highest yield optimal flow rates for solely air or solely water were 0.055 kg/s and 0.032 kg/s, respectively. In addition, for the dual-fluid heat exchanger when both fluids are operating simultaneously, the optimal flow rates are 0.04 kg/s for air and 0.024 kg/s for water, considering both flow schemes such as varied air and fixed water, and fixed air and varied water. The selection of optimal flow rates for the dual-fluid is purely based on performance and electricity consumption. The electricity cost to pump water/air simultaneously at high flow rates of 0.032 (kg/s)/0.055 (kg/s) is significantly higher than that of 0.024 (kg/s)/0.04(kg/s), and the difference between the overall efficiencies using both flow schemes was marginal. The aforementioned optimal flow rates against different modes of fluid operations are used to evaluate the PV temperature at daily variable ambient temperature and irradiance. For a comparative performance analysis, a PV module without a cooling medium is also taken into consideration during experiments. Figure 5 shows that the PV temperature for all cases varies directly with incident solar radiation; however, the individual PV module showed the highest temperature. This is because the individual PV module was cooled by the surrounding air. The maximum PV temperature with the reference case, air, water, and water plus air heat exchangers was found to be 69.4, 52.3, 50.1, and 44.4 \({}^{\circ}\)C, respectively. It is concluded that the higher specific heat made the water type heat exchanger more effective than the air type heat exchanger as a result of minimizing the PV module temperature. Furthermore, the dual-fluid heat exchanger (water plus air) caused more solar heat to be extracted from the PV module because of its larger surface area for heat exchange than that of an exchanger using solely water or air. Figure 3: Variations of overall efficiency of the dual-fluid PV/T system at variable air flow rate and fixed water flow rate. The simultaneous fluid operation of water and air helped to remove the trapped, accumulated energy from the PV module, even during solar noon operation. As shown in Figure 6, introduction of air with water caused a significant improvement in the heat transfer rate. This additional heat drawn from the PV module has a benefit in the form of increased thermal gain. In other words, with a greater heat transfer rate, there will be greater opportunities to keep the PV module temperature close to standard conditions. In addition, the use of the dual-fluid heat exchanger mitigates the non-uniformity of the heat exchanger. Figure 4: Variations of overall efficiency of the dual-fluid PV/T system at variable water flow rate and fixed air flow rate. Figure 5: Daily photovoltaic (PV) module temperature plotted against different fluid operations. of the temperature distribution, which usually results from incident solar radiation intermittency.