5.1. Model Validation
The dual-fluid PV/T model has been validated individually for the air and the nanofluid heat transfer fluids. This means that the air flow rate is set to zero when the nanofluid is to be operated, and vice versa when the air is operated independently. The model validation was performed by comparing the predicted PV and outlet air temperatures against the experimental data for the unglazed PV/T air heating system presented by Joshi et al. [
31]. For the purpose of the comparison, similar geometric and operational parameters were considered, as indicated in the reference [
31]. Other information that is not given in this research was taken from the study by Abu Bakar et al. [
14]. As can be seen in
Figure 3, the maximum deviation between the predicted and measured values did not exceed 2.25 °C and 1.98 °C for the PV and outlet air temperatures, respectively. These deviations can be attributed to the assumptions made during the model development. The published data have a satisfactory agreement with the simulation results delivered by the suggested model.
Meanwhile, the numerical model of a nanofluid heat exchanger was validated by comparing the predicted results with the experimental data taken from the uncovered nanofluid PV/T system reported by Rejeb et al. [
8]. To check the reliability of the model, a statistical parameter, the root mean square percentage deviation (RMSD), is used [
15]. The RMSD, which is the most frequently used parameter for error analysis, measures the deviations between the results predicted by a model and the measured results. The RMSD for the average PV surface and nanofluid outlet temperatures between the predicted and measured data was found to be 1.3% and 1.9%, respectively. The results derived from the model are consistent with the experimental data. It is concluded that the obtained results demonstrate the reliability of the model that is used for the performance prediction of the PV/T system.
where
n is the number of data points.
and
are the predicted and measured values, respectively.
5.2. Results Derived from Mathematical Model
The selection of an optimal fluid type and the optimal concentration of nanoparticles are important for a higher energy production from the PV/T collector. Based on the availability, cost, and inertness to the PV/T material, three metal oxide nanoparticles were selected: aluminum oxide (Al
2O
3), copper oxide (CuO), and silicon dioxide (SiO
2).
Table 4 shows the thermo-physical properties of the metal oxide nanoparticles used in this study [
32,
33,
34]. The influence of the nanoparticle concentrations on the collector’s performance is investigated by considering important thermo-physical properties such the viscosity and thermal conductivity. As depicted in
Figure 4, the viscosity ratio and thermal conductivity ratio increase with the increasing nanoparticle concentration. However, the highest percentage increase in the thermal conductivity was found with the CuO nanofluid, followed by the Al
2O
3, and SiO
2 nanofluids. The optimal concentration for the available nanoparticles in the base fluid (water) is around 0.75%; beyond this point, the aggregation of the nanoparticles and thermal diffusivity increased significantly. One of the reasons that the CuO nanofluid affords the highest heat transfer performance is that it has a lower specific heat and a slightly higher thermal conductivity compared to the aforementioned nanofluids. Based on the preceding outcomes, the CuO nanoparticles with a concentration of 0.75% in water are selected and employed as an optimal nanofluid throughout the rest of this study.
In order to locate the optimal flow rate of each fluid, both the nanofluid and air are operated independently when their counterparts are kept stagnant. Considering the proposed system configurations, the laminar, transition, and turbulent flow regions for the nanofluid are 0.006 kg/s, 0.015 kg/s, and 0.025 kg/s, respectively; and for air, they are 0.009 kg/s, 0.024 kg/s and 0.055 kg/s, respectively. Therefore, the mass flow rate of the nanofluid varied from 0 to 0.03 kg/s, and the air flow rate from 0 to 0.1 kg/s. The thermal and electrical efficiencies of the PV/T collector are predicted by operating the CuO nanofluid and air independently, as shown in
Figure 5. The efficiency values increased with an increasing flow rate of both fluids. However, the impact of the increase of the air flow rate on the PV/T efficiency patterns is small compared to the nanofluid flow rate. Furthermore, the percentage increase of the thermal and electrical efficiencies with the air flow rate is very small. On the contrary, the increase in the collector efficiency is notable even at a low mass flow rate of the nanofluid. Therefore, due to better thermal properties and a high heat removal capability, the nanofluid flow rate varied, as opposed to the air flow rate. This means that when both heat transfer fluids were operated simultaneously, the air flow was kept constant while the variable flow rate of the CuO nanofluid was considered.
Considering different heat transfer fluids, the daily PV module temperature is predicted under similar operating conditions. For a comparative analysis, four fluid modes were used: water, CuO nanofluid, water plus air, and CuO nanofluid plus air (
Figure 6). During the simultaneous mode of fluid operation, the air and CuO nanofluid flow rates were fixed at 0.055 kg/s and 0.025 kg/s, respectively. During the independent mode, the flow rate of either one of the two heat transfer fluids was set to zero, as described by Abu Bakar et al. [
14]. The predicted results show that the maximum PV module temperature with water, nanofluid, air plus water, and air plus nanofluid was 57.5 °C, 55.1 °C, 51.9 °C, and 48.6 °C, respectively. It is noted that the nanofluid (in either the simultaneous or independent mode) has an enormous potential as a heat transfer fluid compared to water. This indicates that a fluid with a high thermal conductivity can extract extra accumulated solar heat from PV cells and thus provide better and more targeted cooling. In addition, the simultaneous application of two fluids (air and nanofluid in particular) results in a significant reduction in the PV cell temperature. The results showed that the application of two fluids remarkably enhanced the total surface area of the heat exchanger. It should be noted that when a dual-fluid heat exchanger is used for the independent mode of fluid operation, it might affect the secondary fluid outlet temperature due to the primary fluid which may have been trapped in the pipe bends.
The influence of the variable flow rate of the nanofluid or water at a fixed airflow on the fluid temperature rise is presented in
Figure 7. When the fluids are to be circulated simultaneously, the temperature rise of both fluids decreased as the flow rate of the nanofluid increased. During the simultaneous operation of fluids, the temperature rise of both the CuO nanofluid and air was higher than the water and air. In both systems, the temperature rise of the liquid fluids (nanofluid and water) was smaller than that of the air. The discrepancy may be a result of the lower specific heat capacity of air. In addition, the findings demonstrate that the CuO nanofluid in combination with air can extract more accumulated solar heat from the PV/T system than water and air as a dual-fluid. This is anticipated to be due to the higher thermal conductivity and the lower specific heat of the nanofluid by dispersing CuO to the water, which removes solar heat faster than water.
Table 5 shows the variations of the total equivalent efficiency of a dual-fluid PV/T system against the variable CuO nanofluid flow rate at a fixed airflow of 0.055 kg/s. Meanwhile, the total equivalent efficiency is determined at a fixed quantity of the daily solar radiation (23.25 MJ/m
2 day) and ambient temperature (21.47 °C). When the nanofluid flow rate is set to vary between 0.005 kg/s and 0.030 kg/s at a fixed air flow rate of 0.055 kg/s, the total equivalent efficiency of the PVT collector was increased to 79.8% and 90.3% with water plus air, and with nanofluid plus air, respectively. It is noted that at the lowest nanofluid flow rate of 0.005 kg/s, the total equivalent efficiency was found to be as low as 82.6%, while under similar operating conditions using water plus air, the minimum value was 73.7%. The results show that when the fluids are operated simultaneously, a reasonably good total equivalent efficiency is achievable even at a low mass flow rate. In comparison with water plus air, the total equivalent efficiency of the PV/T system using nanofluid plus air as the dual-fluid was found to be approximately 10% higher. This can be attributed to the thermophysical properties of the nanofluid being sufficiently great to enhance the heat transfer behavior and thus increase the rate of heat removal from the PV module.
5.3. Results Derived from CFD Model
Figure 8 shows the variations of the convection heat transfer coefficient for various fluid flow rates and absorber temperatures. The convection heat transfer coefficient is calculated using the fluid average temperature and wall temperature, which are extracted from the ANSYS FLUENT software. To understand the influence of the nanofluid, the convection heat transfer coefficients of the CuO nanofluid (0.45%, 0.60%, and 0.75%) are discussed here, in comparison with those of water. The results indicate that the convection heat transfer coefficient (h_ft) between the circulating fluid and absorber wall increases for all heat transfer fluids with an increasing mass flow rate, as expected. However, initially, at an absorber temperature of 55 °C, the water shows a higher h_ft than that of the CuO nanofluid. The high specific heat of water may at least partly account for these results. Moreover, the low density and high thermal conductivity of the CuO nanofluid at a higher temperature enhances the random motion of the nanoparticles, and this ultimately results in an increase of the nanoparticle contact with the absorber surface and the heat transfer rate, respectively. It is observed that at a higher absorber temperature of 95 °C, the 0.75% CuO nanofluid has the highest heat transfer rate, followed by 0.60% CuO, 0.45% CuO, and water.
Since the absorber tubes are arranged in parallel, the temperature distribution and fluid flow through all the tubes can be taken as being the same. Therefore, the vicinity of a single pipe can be used to analyze the thermal behavior of the entire PV panel [
18]. The temperature distribution across the PV surface is predicted under the simultaneous and independent modes of fluid operation, namely: water, CuO nanofluid, water plus air, and CuO nanofluid plus air. In the simulation, the flow rates of the liquid fluid and air are considered to be fixed at 0.025 kg/s and 0.055 kg/s, respectively. The interface temperature between the PV module and both heat exchangers is presented in
Figure 9 and
Figure 10. Due to the fluid-to-solid and solid-to-solid coupling, the shadow effects can be clearly seen at the interfaces or common faces. The PV surface temperature has been reported taking four modes of fluid operation into account: with solely a water heat exchanger, the PV surface reached a temperature of 59 °C; with the use of the nanofluid, the estimated PV surface temperature is 56 °C; with water plus air as a dual exchanger, the PV temperature fell to 52 °C; and with nanofluid plus air this value further declined to 47 °C. This is attributed to the dual-fluid exchanger possibly covering most of the surface area of the PV module and ultimately contributing to a more efficient heat transfer. Furthermore, in the case of the simultaneous application of nanofluid plus air, in particular, an increase in the surface area of the heat exchanger is one among other possible explanations.
It is worthwhile to investigate the thermal performance of each fluid in a dual-fluid PV/T system when both fluids are operated at the same time, as shown in
Figure 11 and
Figure 12. In particular, a combination of the CuO nanofluid and air as a dual fluid is attractive because of the superior thermo-physical properties of the nanofluid relative to those of water. Because of the simultaneous operation, the thermal performance of each fluid is directly associated with its counterpart. Therefore, it is worth noting the contribution of each fluid to the overall performance of a dual-fluid PV/T system. In a situation where the mass flow rate of water or nanofluid increases while considering a fixed air flow rate, the extra solar heat is extracted by the fluid with an increasing flow rate [
15]. Therefore, a relatively small amount of solar heat remains to be removed by air as a second fluid. The observed increase in the mass flow rate of the nanofluid or water at a constant air flow rate had a significant impact on the amount of heat extracted by air. Hence, the total amount of accumulated solar heat extracted by a nanofluid plus air is higher than in the case of water plus air when used as a dual fluid.
The nano-engineered dual-fluid PV/T system is assessed in terms of effectiveness and reliability by comparing its performance with the previously reported collectors using conventional heat transfer fluids such as air, water, nanofluid, and water plus air (
Figure 13). As reported by Abu Bakar et al. [
14], the maximum thermal and electrical efficiencies of the PV/T collector with water plus air were 65.1% and 11.3%, respectively. In contrast, using the proposed PV/T collector, the predicted thermal and electrical efficiencies were found to be 8.4%, and 2.3%, respectively, higher than those of the aforementioned case. Compared to water-based and nanofluid-based PV/T systems [
11], the proposed system had a 26%, and 17.3% higher thermal efficiency, respectively. Furthermore, in the case of the reference PV module (without cooling) [
35], the electrical efficiency was found to be 6.61%. This may be attributed to the use of two fluids for the PV cells cooling, which consequently increased the overall surface area for the heat transfer, and hence ultimately improved the heat extraction from the PV cells. Specifically, introducing the CuO nanofluid along with air as a dual fluid increases the total efficiency per unit area because of their superior thermo-physical properties. Using the CuO nanofluid in combination with air for a PV/T system is promising considering the higher overall performance that can be achieved compared to a collector employing conventional fluids. In addition, a nano-engineered dual-fluid PV/T system offers a wide range of thermal applications depending upon the energy needs.