Performance Evaluation of PV / T Air Collector Having a Single-Pass Double-Flow Air Channel and Non-Uniform Cross-Section Transverse Rib

: In the present work, the electrical and thermal performances of a newly designed PV / T (photovoltaic / thermal) air collector, which was proposed and fabricated by the author, have been investigated experimentally in the natural weather conditions. The PV / T air collector has a single-pass double-ﬂow air channel. Also, a non-uniform cross-section transverse rib was attached at the back surface of the PV (photovoltaic) module to improve the heat transfer performance between the PV module and ﬂowing air. The experiment was carried out in an outdoor ﬁeld on a clear day with various air mass ﬂow rates ranges from 0.0198 kg / s to 0.07698 kg / s. In the results, it was found that the average thermal e ﬃ ciency of the PV / T collector increased from 35.2% to 56.72% as the air mass ﬂow rate increased. The average electrical e ﬃ ciency also increased from 14.23% to 14.81% with an increase in an air mass ﬂow rate, but the e ﬀ ect of air mass ﬂow rate on the increase in electrical e ﬃ ciency was inconsiderable. The average overall e ﬃ ciency, which represents the sum of electrical and thermal e ﬃ ciencies, was in the range of 49.44% to 71.54% and it increased as the air mass ﬂow rate increased. The maximum value of average overall e ﬃ ciency during the test period was found to be 71.54% at an air mass ﬂow rate of 0.07698 kg / s. From the results, it was conﬁrmed that the newly designed PV / T air collector provides a signiﬁcant enhancement in solar energy utilization.


Introduction
The PV (photovoltaic) module is one of the most widely used renewable systems and it has gained fast growth globally. It can convert 12-18% of the incident solar energy into useful electrical energy depending on its type and operating conditions. However, most of the incident solar energy either reflected or turn into thermal energy. This thermal energy dissipated into the atmosphere or leads to an increase in the temperature of the PV module, which reduces its efficiency.
To retrieve thermal energy and prevent a decrease in efficiency of PV module, Wolf suggested PV/T (photovoltaic/thermal) solar collector first [1]. The PV/T solar collector consists of a PV module and a solar thermal collector. The cooling mediums such as air and water in the solar thermal collector extract heat from the PV module, which reduces its temperature. In addition to this, heated fluid can be used to provide thermal energy for hot water supply, space heating, and so on.
The PV/T collectors can be categorized into two types: liquid-based PV/T solar collector and air-based PV/T solar collector. The liquid-based PV/T collector uses a liquid such as water and nanofluid as a cooling medium. Nualboonrueng et al. examined the performance of PV/T water collector under actual climate conditions in Bangkok to confirm the electrical and thermal energy supplied in tropical areas by the collector [2]. Jahromi et al. conducted an exergy and economic evaluation of a commercial

Description of PV/T Air Collector
The PV/T air collector used in this study is a newly designed collector, which has a single-pass double-flow air channel and non-uniform cross-section transverse rib at the back sheet of the PV module. Figure 1 shows the actual and exploded view of the proposed PV/T air collector. The PV/T air collector consists of glass, PV module, rib, insulator, and case. The length, width, and thickness of the glass were 1670 mm, 1040 mm, and 4mm, respectively. The PV module used in this study was a commercially available product. The length and width of the PV module were 1670 mm and 1000 mm, respectively. More detailed specifications of the PV module (Model: Q. PEAK BFR-G4.4) on the standard test conditions (Solar intensity 1000 W/m 2 , module temperature 25 • C, AM 1.5), which are obtained from the product catalog, are listed in Table 1. In the specifications, negative value of the temperature coefficient means the decrease in electrical efficiency according to the increase in cell temperature.

Description of PV/T Air Collector
The PV/T air collector used in this study is a newly designed collector, which has a single-pass double-flow air channel and non-uniform cross-section transverse rib at the back sheet of the PV module. Figure 1 shows the actual and exploded view of the proposed PV/T air collector. The PV/T air collector consists of glass, PV module, rib, insulator, and case. The length, width, and thickness of the glass were 1670 mm, 1040 mm, and 4mm, respectively. The PV module used in this study was a commercially available product. The length and width of the PV module were 1670 mm and 1000 mm, respectively. More detailed specifications of the PV module (Model: Q. PEAK BFR-G4.4) on the standard test conditions (Solar intensity 1000 W/m 2 , module temperature 25 °C, AM 1.5), which are obtained from the product catalog, are listed in Table 1. In the specifications, negative value of the temperature coefficient means the decrease in electrical efficiency according to the increase in cell temperature.   The upper and lower air channel have the same length of 1670 mm, width of 1000 mm, and different heights of 46 mm and 65 mm. The non-uniform cross-section ribs were attached at the back surface of the PV module in the form of a triangle in order to increase the heat transfer performance from the PV module to flowing air. The height, width, and length of each triangle of the rib have the same values of 8 mm. A total of 125 triangles were installed for one transverse rib. To prevent heat loss, the insulators were installed at the sidewall and bottom of the air channel with a thickness of 20 mm. Figures 2 and 3 show a schematic of the proposed PV/T air collector and a transverse triangular rib with more detailed dimensions.  The upper and lower air channel have the same length of 1670 mm, width of 1000 mm, and different heights of 46 mm and 65 mm. The non-uniform cross-section ribs were attached at the back surface of the PV module in the form of a triangle in order to increase the heat transfer performance from the PV module to flowing air. The height, width, and length of each triangle of the rib have the same values of 8 mm. A total of 125 triangles were installed for one transverse rib. To prevent heat loss, the insulators were installed at the sidewall and bottom of the air channel with a thickness of 20 mm. Figures 2 and 3 show a schematic of the proposed PV/T air collector and a transverse triangular rib with more detailed dimensions.    Figure 4 shows the experimental setup for PV/T air collector having a single-pass double-flow air channel and non-uniform cross-section transverse rib. The PV/T air collector is located at Engineering Building 2, Yongdang Campus, Pukyong National University, Busan, Korea (latitude: 35°6.98′, longitude: 129°5.39′). The experiments of the PV/T air collector were performed from 10:00 to 16:00 for a fixed value of air mass flow rate in an outdoor field on a clear day. Performance evaluations of the PV/T air collector were carried out with four different days with four different air mass flow rates ranging from 0.01977 kg/s to 0.07698 kg/s to get compatible results.     Figure 4 shows the experimental setup for PV/T air collector having a single-pass double-flow air channel and non-uniform cross-section transverse rib. The PV/T air collector is located at Engineering Building 2, Yongdang Campus, Pukyong National University, Busan, Korea (latitude: 35°6.98′, longitude: 129°5.39′). The experiments of the PV/T air collector were performed from 10:00 to 16:00 for a fixed value of air mass flow rate in an outdoor field on a clear day. Performance evaluations of the PV/T air collector were carried out with four different days with four different air mass flow rates ranging from 0.01977 kg/s to 0.07698 kg/s to get compatible results.  Figure 4 shows the experimental setup for PV/T air collector having a single-pass double-flow air channel and non-uniform cross-section transverse rib. The PV/T air collector is located at Engineering Building 2, Yongdang Campus, Pukyong National University, Busan, Korea (latitude: 35 • 6.98 , longitude: 129 • 5.39 ). The experiments of the PV/T air collector were performed from 10:00 to 16:00 for a fixed value of air mass flow rate in an outdoor field on a clear day. Performance evaluations of the PV/T air collector were carried out with four different days with four different air mass flow rates ranging from 0.01977 kg/s to 0.07698 kg/s to get compatible results.    Tables 2 and 3, respectively. Multi-function data logger Agilent 34972A --

Performance Evaluation
The main parameters used to analyze the performance of PV/T air collector were heat gain of air, thermal efficiency, electric generation, and electrical efficiency. The heat gain of air per unit area of PV/T air collector was calculated as follows: Ambient temperature was measured by T-type thermocouples located under the collector with the accuracy of ±1 • C during the experiment period. The outlet air temperature was also measured with the T-type thermocouples in an outlet air duct. The voltage and ampere generated by the PV module were measured with a power meter (PW3336-02) with an accuracy of ±0.1%. The air velocity was measured by hot-wire anemometer (KNOMAX 6531-2G) with an accuracy of ±0.015 m/s at the outlet air duct. Air mass flow rate was obtained by multiplying the cross-section area of air duct and air density by measured air velocity. The incident solar intensity on the surface of the collector was measured by pyranometer (MS-802) with an accuracy of ±2%. The experimental conditions and the properties of measuring devices are summarized in Tables 2 and 3, respectively.

Performance Evaluation
The main parameters used to analyze the performance of PV/T air collector were heat gain of air, thermal efficiency, electric generation, and electrical efficiency. The heat gain of air per unit area of PV/T air collector was calculated as follows: Energies 2020, 13, 2203 6 of 13 The thermal efficiency, which represents the ratio of the heat gain of air to the incident solar radiation, was calculated as follows: The electric generation per unit area of PV/T air collector was calculated as follows: where the ε PV is the PV cell's coverage factor. It is the ratio of the PV cell area to the overall PV/T solar collector area. The electrical efficiency, which represents the ratio of electric generation to the incident solar radiation, was calculated as follows: The PV/T air collector produces both thermal energy and electrical energy simultaneously. Thus, the overall efficiency needs to be confirmed and it was calculated as follows: Figure 5 shows the variation of ambient temperature and solar intensity for each experiment day with operating time.

Weather Conditions
The thermal efficiency, which represents the ratio of the heat gain of air to the incident solar radiation, was calculated as follows: The electric generation per unit area of PV/T air collector was calculated as follows: where the is the PV cell's coverage factor. It is the ratio of the PV cell area to the overall PV/T solar collector area.
The electrical efficiency, which represents the ratio of electric generation to the incident solar radiation, was calculated as follows: The PV/T air collector produces both thermal energy and electrical energy simultaneously. Thus, the overall efficiency needs to be confirmed and it was calculated as follows: Figure 5 shows the variation of ambient temperature and solar intensity for each experiment day with operating time.

Thermal Performance
In this section, the thermal performance of the PV/T air collector was investigated in the form of air temperature rise, heat gain of air, and thermal efficiency. Figure 6a shows air temperature rise, which means the temperature difference between inlet and outlet air in the PV/T air collector. The air temperature rise varied in the range of 15 10.92 • C for each air mass flow rate of 0.01977 kg/s, 0.02704 kg/s, 0.05402 kg/s, 0.07698 kg/s, respectively. From the results, it was observed that the air temperature rise increased with a decrease in air mass flow rate. This is due to the decrease in heat transfer time caused by the decrease in air velocity in an air channel. From the figure, it was also seen that the air temperature rise presented a changing trend similar to the solar intensity during the test period. This is because the increase in air temperature rise depends on the intensity of incident solar energy as shown in Figure 6b.

Thermal Performance
In this section, the thermal performance of the PV/T air collector was investigated in the form of air temperature rise, heat gain of air, and thermal efficiency. Figure 6a shows air temperature rise, which means the temperature difference between inlet and outlet air in the PV/T air collector. The air temperature rise varied in the range of 15 10.92 °C for each air mass flow rate of 0.01977 kg/s, 0.02704 kg/s, 0.05402 kg/s, 0.07698 kg/s, respectively. From the results, it was observed that the air temperature rise increased with a decrease in air mass flow rate. This is due to the decrease in heat transfer time caused by the decrease in air velocity in an air channel. From the figure, it was also seen that the air temperature rise presented a changing trend similar to the solar intensity during the test period. This is because the increase in air temperature rise depends on the intensity of incident solar energy as shown in Figure 6b.  The heat gain of air increased with an increase in the air mass flow rate. This is due to the increase in air velocity in an air channel of the PV/T air collector, which results in a higher heat transfer coefficient between the PV module and flowing air. The thermal efficiency was in the range of 30.97% to 37.44%, 37.4% to 44.64%, 45.34% to 56.67%, and 52.37% to 59.91% for each air mass flow rate. From the results, it was seen that the thermal efficiency significantly increased with an increase in the air mass flow rate. This is because the heat gain of air increased as the air mass flow rate increases, while the solar intensity remains  The heat gain of air increased with an increase in the air mass flow rate. This is due to the increase in air velocity in an air channel of the PV/T air collector, which results in a higher heat transfer coefficient between the PV module and flowing air. The thermal efficiency was in the range of 30.97% to 37.44%, 37.4% to 44.64%, 45.34% to 56.67%, and 52.37% to 59.91% for each air mass flow rate. From the results, it was seen that the thermal efficiency significantly increased with an increase in the air mass flow rate. This is because the heat gain of air increased as the air mass flow rate increases, while the solar intensity remains similar. The detailed values of thermal efficiency for PV/T air collector at different air mass flow rate are listed in Table 4 with operating time.  Table 4 with operating time.     Figure 8 shows the thermal efficiency with respect to air mass flow rate and the average thermal efficiency during the experiment period. From the figure, it was observed that the air mass flow rate has a strong effect on the increase in thermal efficiency, as stated previously. The average thermal efficiencies were 35.2%, 40.77%, 51.87%, and 56.72% for each air mass flow rate. Figure 8 shows the thermal efficiency with respect to air mass flow rate and the average thermal efficiency during the experiment period. From the figure, it was observed that the air mass flow rate has a strong effect on the increase in thermal efficiency, as stated previously. The average thermal efficiencies were 35.2%, 40.77%, 51.87%, and 56.72% for each air mass flow rate.

Electrical Performance
In this section, the electrical performance of the PV/T air collector for various air mass flow rates is presented in the form of electric generation and electrical efficiency. Figure 9 shows electric generation and electrical efficiency during test period at difference air mass flow rate. The electric generation varied in the range of 57.45 W/m 2 to 160.08 W/m 2 , 60.08 W/m 2 to 162.11 W/m 2 , 57.58 W/m 2 to 160.40 W/m 2 , and 49.56 W/m 2 to 164.03 W/m 2 with average values of 135.75 W/m 2 , 134.87 W/m 2 , 137.60 W/m 2 , and 133.35 W/m 2 for air mass flow rate of 0.01977 kg/s, 0.02704 kg/s, 0.05402 kg/s, and 0.07698 kg/s, respectively. As shown in the figure, the electric generation increased with increase in solar intensity and then decreased as the solar intensity decreased. The electrical efficiencies varied from 11.05% to 15.37%, 12.10% to 16.11%, 11.32% to 15.60%, and 11.32% to 16.27% for each air mass flow rate. Figure 10 shows the electrical efficiency with respect to the air mass flow rate and the average electrical efficiency during the experiment period. The average electrical efficiencies were 14.23%, 14.33%, 14.6%, and 14.81% for each air mass flow rate. From the results, it was seen that the electrical efficiency slightly increased with an increase in air mass flow rate. This is due to the fact that the increasing of the thermal performance with an increase in air mass flow rate leads to the lower temperature of the PV module, which results in higher electrical efficiency. However, the effect of air mass flow rate on the increase in electrical efficiency was inconsiderable. The detailed values of electrical efficiency for PV/T air collector at different air mass flow rate are listed in Table 5 with operating time.

Electrical Performance
In this section, the electrical performance of the PV/T air collector for various air mass flow rates is presented in the form of electric generation and electrical efficiency. Figure 9 shows electric generation and electrical efficiency during test period at difference air mass flow rate. The electric generation varied in the range of 57.45 W/m 2 to 160.08 W/m 2 , 60.08 W/m 2 to 162.11 W/m 2 , 57.58 W/m 2 to 160.40 W/m 2 , and 49.56 W/m 2 to 164.03 W/m 2 with average values of 135.75 W/m 2 , 134.87 W/m 2 , 137.60 W/m 2 , and 133.35 W/m 2 for air mass flow rate of 0.01977 kg/s, 0.02704 kg/s, 0.05402 kg/s, and 0.07698 kg/s, respectively. As shown in the figure, the electric generation increased with increase in solar intensity and then decreased as the solar intensity decreased. The electrical efficiencies varied from 11.05% to 15.37%, 12.10% to 16.11%, 11.32% to 15.60%, and 11.32% to 16.27% for each air mass flow rate.   Figure 10 shows the electrical efficiency with respect to the air mass flow rate and the average electrical efficiency during the experiment period. The average electrical efficiencies were 14.23%, 14.33%, 14.6%, and 14.81% for each air mass flow rate. From the results, it was seen that the electrical efficiency slightly increased with an increase in air mass flow rate. This is due to the fact that the increasing of the thermal performance with an increase in air mass flow rate leads to the lower temperature of the PV module, which results in higher electrical efficiency. However, the effect of air mass flow rate on the increase in electrical efficiency was inconsiderable. The detailed values of electrical efficiency for PV/T air collector at different air mass flow rate are listed in Table 5 with operating time.

Overall Efficiency
The PV/T air collector produces not only thermal energy but also electrical energy. Thus the overall efficiency, which means the sum of thermal and electrical efficiencies, was investigated.
Energies 2020, 13, 2203 11 of 13 Figure 11 shows the variation of overall efficiency during the test period. The overall efficiency was in the range of 46.24% to 52.08%, 52% to 58.97%, 60.94% to 70.80%, and 66.32% to 75.3% for air mass flow rate of 0.01977 kg/s, 0.02704 kg/s, 0.05402 kg/s, and 0.07698 kg/s, respectively.

Overall Efficiency
The PV/T air collector produces not only thermal energy but also electrical energy. Thus the overall efficiency, which means the sum of thermal and electrical efficiencies, was investigated. Figure 11 shows the variation of overall efficiency during the test period. The overall efficiency was in the range of 46.24% to 52.08%, 52% to 58.97%, 60.94% to 70.80%, and 66.32% to 75.3% for air mass flow rate of 0.01977 kg/s, 0.02704 kg/s, 0.05402 kg/s, and 0.07698 kg/s, respectively. Figure 12 shows the overall efficiency with respect to air mass flow rate and the average overall efficiency during the experiment period. The average overall efficiencies were 49.44%, 55.10%, 66.47%, and 71.54% for each air mass flow rate. From the results, it was found that the overall efficiency increased with an increase in air mass flow rate. The reason was that the increase in air mass flow rate leads to an increase in both thermal and electrical efficiencies for PV/T air collector.   Figure 12 shows the overall efficiency with respect to air mass flow rate and the average overall efficiency during the experiment period. The average overall efficiencies were 49.44%, 55.10%, 66.47%, and 71.54% for each air mass flow rate. From the results, it was found that the overall efficiency increased with an increase in air mass flow rate. The reason was that the increase in air mass flow rate leads to an increase in both thermal and electrical efficiencies for PV/T air collector.

Conclusions
In the present work, a newly designed PV/T air collector, which has a single-pass double-flow air channel and non-uniform cross-section rib, was fabricated and experimented. The thermal and electrical performance have been investigated experimentally on the natural weather conditions with various air mass flow rate. The major conclusions of this study can be summarized as follows: (1) The air temperature rise changed from 5.21 °C to 33.96 °C depending on the solar intensity and air mass flow rate. It increased with a decrease in air mass flow rate and the maximum air temperature rise was found at a low air mass flow rate of 0.01977 kg/s. (2) The thermal efficiency varied from 30.97% to 59.91% according to weather conditions and air mass flow rate. The maximum value of average

Conclusions
In the present work, a newly designed PV/T air collector, which has a single-pass double-flow air channel and non-uniform cross-section rib, was fabricated and experimented. The thermal and electrical performance have been investigated experimentally on the natural weather conditions with various air mass flow rate. The major conclusions of this study can be summarized as follows: (1) The air temperature rise changed from 5.21 • C to 33.96 • C depending on the solar intensity and air mass flow rate. It increased with a decrease in air mass flow rate and the maximum air temperature rise was found at a low air mass flow rate of 0.01977 kg/s. (2) The thermal efficiency varied from 30.97% to 59.91% according to weather conditions and air mass flow rate. The maximum value of average thermal efficiency during the test period was found to be 56.72% at an air mass flow rate of 0.07698 kg/s. From the results, it was found that the thermal efficiency increased significantly with an increase in air mass flow rate different from the air temperature rise. Also, a strong effect of air mass flow rate on the increase in thermal efficiency was confirmed. (3) The electrical efficiency varied from 11.05% to 16.27% according to weather conditions and air mass flow rate. The maximum value of average electrical efficiency during the test period was found to be 14.81% at an air mass flow rate of 0.07698 kg/s. The electrical efficiency increases slightly with an increase in the air mass flow rate and the effect of the air mass flow rate on the increase in electrical efficiency was found inconsiderable. (4) The overall efficiency varied from 46.24% to 75.3% according to the weather conditions and air mass flow rate. The maximum value of average overall thermal efficiency during the test period was found to be 71.54% at an air mass flow rate of 0.07698 kg/s. The overall efficiency increased with an increase in air mass flow rate because of the increase in both thermal and electrical efficiencies. (5) From the results, it was found that the PV/T air collector used this study provides a significant enhancement in solar energy utilization.
Author Contributions: All authors made equal contributions and efforts in writing the manuscript. All authors have read and agreed to the published version of the manuscript. Coverage factor (-)