3.1. Variation of Solar Radiation, Ambient Temperature, SAHs Air Outlet, and Inlet Temperature Differences for Roughened and Smooth Solar Air Heaters, Respectively
Figure 3 illustrates the solar radiation (W/m
2) and ambient temperature (°C) through experiments since each experiment started from 9:00 am to 15:36 pm and the data were recorded every three minutes. For solar radiation, which was considered as the most important parameter in the experiment, for all days, the solar radiation curves followed a parabolic shape where the peak occurred at noontime. Despite the changeable and cloudy climate of Wuhan city, almost all clear days were selected to represent the experiment. The maximum value that was reached was 922 W/m
2 on 29 October, but the averages of the solar radiation for all days were 767.8, 754.8, 729.2, and 762.1 W/m
2 on 28 October, 29 October, 31 October, and 9 November, respectively. The performance of the SAHs was highly influenced by the environment, which involves many parameters like wind velocity and ambient temperature, etc. As plotted in
Figure 3, the ambient temperatures on the 28, 29, and 31 October were congruent, except that the ambient temperature on 9 November was lower than that of the other days. The ambient temperatures ranged from 26.7 to 32.9 with an average of 28.8 (°C) on 28 October, from 25.7 to 36.7 with an average of 30.1 (°C) on 29 October, from 26.8 to 32.9 with an average of 30.3 (°C) on 31 October and from 12.9 to 26.9 with average of 20.8 (°C) on 9 November.
Similar to the solar radiation curve pattern, the air outlet and the inlet temperature difference
(°C) follow a parabolic shape since
Figure 4 obviously shows (outlet–inlet) temperatures (°C) of roughened and smooth SAHs with CCu–BP versus daytime. The difference between the SAH outlet and inlet temperatures for either the roughened or smooth SAH reached a peak in the period from 12:00 to 13:00 because the solar radiation was also at its peak during this period. It is clear that the relationship between the temperature differences and airflow rates is inversely related where the highest differences occurred in case
A and ranged from 23.9 to 56.3 with an average value of 47 °C and from 22.3 to 57.3 with an average of 47.6 °C for smooth and roughened SAHs, respectively. The lowest differences were obtained in case
D and ranged from 6.2 to 22.8 with an average value of 18 °C and from 4.2 to 24.3 with an average of 18.6 °C for the smooth and roughened SAHs, respectively. On the other hand, the roughened SAH exhibited the best
when compared to the smooth SAH in case
C where the values ranged from 8.4 to 30.3 with an average of 21.7 °C and from 7 to 33.6 with an average of 26 °C for the smooth and roughened SAHs, respectively.
It can be inferred that under stable solar radiation, the new coating with a broken arc roughness could raise the difference between the outlet and inlet temperatures of the air across the SAH compared to the smooth duct due to the increase in the heat transfer coefficient due to the turbulence effect of the employed roughness. Furthermore, the RHAS’s was higher than that of the SSAH by 1.3, 8, 19.8, and 3.3% for the A, B, C, and D airflow conditions, respectively.
3.2. Useful Energy (W), Thermal Efficiency (%,) and Nusselt Number Variation Under Different Working Conditions
The thermal efficiency of the RSAH and SSAH with CCu–BP was investigated under four airflow rates for several days, then one sunny day was selected to represent each airflow rate due to the unstable weather of Wuhan city.
Figure 5 and
Figure 6 demonstrate the useful energy (W), which is calculated by Equation (5) and thermal efficiency (%), which is computed by Equation (11), respectively, under the same working conditions for both the RSAH and SSAH coated with CCu–BP. As seen in
Figure 5, RSAH showed a remarkable rise in useful energy compared to the SSAH for all airflow conditions due to the high-temperature differences
across the RSAH duct with respect to those across the SSAH duct. The useful energy averages were 405.18, 641.72, 711.25, and 761.20 (W) and 402.10, 599.46, 598.44, and 745.38 for the RSAH and SSAH, respectively, under working conditions
A,
B,
C, and
D.
Consequently, the instantaneous thermal efficiency of RSAH and SSAH had the same tendency of useful energy gained by the used fluid. RSAH clearly showed a higher instantaneous thermal efficiency (%) compared to the SSAHs as seen in
Figure 6. The instantaneous thermal efficiency for each airflow condition ranged as follows:
Condition A ranged from 20.5 to 30.3 with an average of 26.3 (%), and from 21.9 to 28.5 with an average of 26.1 (%) for RSAH and SSAH, respectively.
Condition B ranged from 20.4 to 59.5 with an average of 42.1 (%), and from 19.8 to 49.3 with an average of 39.2 (%) for RSAH and SSAH, respectively.
Condition C ranged from 21.1 to 67.3 with an average of 48.2 (%), and from 25.3 to 51.2 with an average of 40.7 (%) for RSAH and SSAH, respectively.
Condition D ranged from 18.1 to 76.9 with an average of 49.3 (%), and from 26.6 to 57.2 with an average of 48.4 (%) for RSAH and SSAH, respectively.
It was noticeable that either the useful energy or thermal efficiency was lower for the RSAH than for the SSAH at the beginning of the experiments, and this decline was increased by the decrease in the ambient temperature as in condition
D, which is because the cooled material of the Al ribs has a high heat capacity (921 J/Kg.k), so needs more energy to heat up compared to absorber without Al ribs then some of the absorbed solar radiation is lost for that purpose, which decreases the useful energy and consequently the thermal efficiency. The inverse effect occurred at the end of the experiment since the stored energy in the material of the ribs recovered to the working air and led to a higher useful energy and then a higher thermal efficiency. For more illustration,
Figure 7 shows the variation in the temperature parameters
∆T/GT with thermal efficiencies of SSAH and RSAH at the mass flow rates of A, B, C, and D. Furthermore, the empirical relations and regression coefficients between them are tubulated in
Table 3.
It can be concluded that the new coating with broken arc roughness enhanced the heat transfer coefficient between the absorber plate and working fluid (air), leading to a noteworthy improvement in useful energy (W). In addition, for both useful energy and thermal efficiency, there was a positive correlation with the increase in airflow rates. Besides, the thermal efficiency of RSAH under working conditions A, B, C, and D increased by 0.8, 7.4, 18.4, and 1.9%, respectively.
The comparison of the averaged thermal efficiencies of RSAH under airflow condition C for several days is tabulated in
Table 4. As illustrated in Equations (5)–(11), thermal efficiency is a function of many parameters such as solar radiation, working fluid temperature differences, heat removal factor, and overall losses coefficient. Consequently, thermal efficiency is massively affected by any variation of those variables. As seen in
Table 4, despite operating the RSAH under the same airflow, the averaged thermal efficiency varied because of the different amount of incident averaged solar radiation for each day and this clearly appeared on the 24 November, which showed the lowest averaged solar radiation and the lowest thermal efficiency. To calculate the lost energy, estimating the overall heat transfer coefficient(W/m
2.°C) is necessary and many parameters could affect the overall heat transfer coefficient (W/m
2.°C) since there is a direct relationship between U
los. and (T
p –T
a). The more (T
p – T
a) occurred, the more lost energy there was. The same manner was used for wind velocity, consequently the wind–heat transfer coefficient. (W/m
2.°C) as mentioned in [
35,
36]. Despite 11 October being the day with the highest incident solar radiation, the thermal efficiency was lower than on 31 October since on 11 October, the (T
p – T
a) was the highest; also, the wind heat transfer coefficient that dramatically increased the amount of lost energy by radiation and convection consequently decreased the thermal efficiency.
The influence of operating and the new coating with roughness parameters on the Nusselt number and heat transfer coefficient for broken arc rib roughened duct and smooth duct are discussed in the following part.
The variation of the heat transfer coefficient (W/m
2.°C) and Nusselt number with the four working conditions are plotted in
Figure 8 for the roughened and smooth SAHs. As predicted with the increase in the air flow rate (Reynolds number), heat transfer coefficient (W/m
2.°C), and Nusselt number increases for the RSAH since an improvement in turbulent strength takes place. This might be due to the role of artificial roughness, in addition to the more volumetric airflow rates, and the greater rise in the turbulent dissipation rate aside from the turbulent kinetic energy in the flow that brings an important development in wall heat transfer rates.
To conclude, the RSAH with CCu–BP showed a higher heat transfer coefficient (W/m2 °C), consequently, the Nusselt number was compared to the SSAH with CCu–BP for each working volumetric airflow rate.
3.3. Exergy Efficiency (%) and Improvement Potentials (W) for RSAH and SSAH with the New Coating
For both the RSAH and SSAH coated with CCu–BP, the second law efficiency (exergy efficiency) under different volumetric airflow rates was computed. The results are plotted in
Figure 9, and the averages are tabulated in
Table 5. Furthermore, the averages of exergy input (W), exergy destruction (W), and improvement potentials (W) in Equations (14)–(20) were used. It is obvious, as plotted in
Figure 9, that the exergy efficiency followed the change of instantaneous output or absorbed exergy (W) within the first hour of the experiment for all working conditions; the exergy efficiency and absorbed exergy showed approximate values, and reached its peak during the period of 12:30–13:30 with a big variation between RSAH and SSAH for the benefit of RSAH because at this time, the solar radiation was in peak and RSAH could maintain a higher difference between the air outlet and inlet than the SSAH. The RSAH with CCu–BP increased the exergy efficiency by 5.9, 20, 51.6, and 29.8% for conditions A, B, C, and D, respectively, compared to SSAH with the same coating.
The irreversibility of both RSAH and SSAH and their “improvement potentials” were estimated and the results revealed in
Table 5, where there is a reciprocal relationship between exergy destruction and exergy efficiency as well as a temperature difference since the exergy loss is minimal when the thermal efficiency is optimum. The RSAH showed lower energy destruction and improvement potentials compared to SSAH. Furthermore, for the RSAH, the airflow rate in case
D was the highest in energy destruction and improvement potentials compared to cases
A,
B, and
C because of the high-pressure drop across the duct. In contrast, the RSAH in case
C showed the lowest values of the exergy destruction and improvement potentials.
In brief, the RSAH with CCu–BP exhibited the highest increment of exergy efficiency (51.6%) under an airflow rate of 0.0244 m
3/s (condition
C) compared to the SSAH with the same coating. Since, in the case of condition C (Re = 2850), which lay in the transition region, from the laminar to turbulent flow showed a decline in exergy performance in this region (this behavior was noted in [
11,
30]). In the current study, the decline was much more for the SSAH, but for RSAH, the declination was lower because of the increase in turbulent intensity with the steady effect of the lost energy due to the increased flow rate of air. Therefore, in airflow condition C, the ratio between the RSAH and SSAH was higher and had the lowest values of exergy destruction and improvement potentials.