Thermal Evaluation of a Double-Pass Unglazed Solar Air Heater with Perforated Plate and Wire Mesh Layers

: In this study, an experimental outdoor investigation of the thermal e ﬃ ciency and outlet air temperature was conducted on an unglazed, double-pass, solar air heater with a perforated absorber plate and packing wire mesh layers as a supplemental absorbent area. This was done to observe their e ﬀ ects on the thermal performance of the solar air heater. The double-pass collector was constructed with a bed height of 0.05 m, and a collection area of 1.5 m 2 . The height of the upper channel was ﬁxed at 0.015 m to improve the thermal e ﬃ ciency, and the outlet temperature at air ﬂow rates between 0.003 and 0.018 kg / s. The collector was mounted with a slope of 42 ◦ facing south, to maximize the intensity of solar irradiance during winter. The e ﬀ ects of the air ﬂow rate, ambient temperature, inlet temperature, outlet temperature, and solar intensity were experimentally investigated. The results showed that thermal e ﬃ ciency could be improved by increasing the air ﬂow rate, where the highest thermal e ﬃ ciency achieved was 86% at 0.018 kg / s. However, the temperature di ﬀ erence was increased to a maximum value of 38.6 ◦ C, when the air ﬂow rate was decreased to 0.003 kg / s. Furthermore, the results demonstrated a signiﬁcant improvement in the thermal e ﬃ ciency and outlet temperature; and when compared with previous research, the experimental results and the predictions for the outlet temperature using the theoretical model agreed.


Introduction
Energy is vital to human life. The rapid increase in the global economy and technological development, as well as population growth, has increased the demand for energy. Most energy is produced from nonrenewable sources, such as fossil fuel, because of the low cost of production and large supply. The fossil fuel production generates various hazards to the environment, and to human health-such as acid rain, global warming, and photochemical smog [1].
Utilizing a solar energy system for heating space, such as using solar air collectors, is a clean technique to decrease CO 2 emissions and resource consumption [2]. This type of heating system is low-cost and easy to maintain, but has the disadvantage of low thermal efficiency [3]. Solar air collectors have several advantages compared to application of other, liquid-type solar energy systems, such as no freezing or corrosion problems, less maintenance required, and ability to use a perforated plate system (which provides fresh air that improves the indoor air quality).
Solar air heaters can be classified into two basic modes: glazed collectors and unglazed transpired collectors. The unglazed transpired collector is relatively new solar technology that was developed in the 1980s for ventilation air heating. The absorber part of the unglazed transpired collector is perforated so that ambient air is continuously withdrawn through the perforations [4]. Much technological

Methodology
In this study, outdoor experiments setup was conducted and developed in Baghdad, Iraq. Solar air collector was thermal performance and tested for five cloudless days in April 2019, the slope of 42° facing south, thus, maximizing the solar intensity for winter days. The measurement equipment was a data-logger (EXTECH), an accuracy of ± (0.4% + 1 °C), model SDL200, using for obtained inlet, plenum and outlet temperatures and Pyranometer placed lower to the collector to sense the global solar irradiance (Pyranometer RK200-03, 0-2000w/m 2 Range, SN: R18021068) used for measuring solar intensity. The air velocity was measured and taken manually by HT-9829, 60-90 mA, hot wire anemometer, and solar intensity and temperatures were registered automatically every 30 minutes a day. A vacuum blower power of 0.75 kW, model 135C16A was used to discharge five air flows during five-day work, from 0.003 to 0.018 kg/s. A blower speed controlled by using the inverter drive speed control model: SV008iC5-1, 5A, 0.01-400Hz.

Instantaneous Efficiency of the Solar Collector
To evaluate the performance of the solar heater system, the first law of thermodynamic was applied, and the thermal efficiency was defined as the ratio of the useful energy to receive energy, and the useful energy was defined as the thermal energy, and receives energy as heat flux or solar radiation energy.
where useful energy gain and received energy in (W), ṁ is the air flow rate (kg/sec), Cp is the specific heat of the air (kJ/kg. K), Ac is the collector area in (m 2 ), (To) is the outlet air temperature (°C), and Ti is the inlet air temperature (°C) [19,20]. Equation (2) can be simplified to:

Construction and Design
The structure of the new transpired flat-plate collector was constructed and performed in Baghdad, Iraq 33.3152 • N, 44.3661 • E, double-pass-counter flow collector is shown in Figure 1. The double pass collector made of wood, with an area of 1.5 m 2 (1.5 m long and 1 m width). All edges and bottom of the collector were thermally insulated, except the upper side of the collector. Two ordinary panes of the plate (steel galvanized), the thickness of 4 mm were fixed parallel and upper the collectors to allow the inlet air to pass through these two plates in a space of 1.5 cm (upper channel), the flow then reversed direction to inter the bottom of the collector, lower channel (lower channel: The distance from Galvanized steel cover II to the back bed), high of 7 cm. The main components of the collectors were the six aluminum wire mesh layers (square hole area 2.2 × 2.2 mm 2 ) and galvanized steel perforated absorber plate, hole diameter 2 mm and pitch 4 mm. The collectors' bed, perforated plate and wire mesh layers were painted black before installation to increase the absorbing heat from the solar rays and improving the efficiency. A blower of 0.75 kW was connected with collectors by flexible (8 cm diameter) pipe, vary air mass flow rate from 0.003 to 0.018 kg/s controlled by controller speed inverter model: SV008iC5-1, 0.01−400 Hz, 5 A was fixed with air blower electricity, for more detail see Table 1.

Methodology
In this study, outdoor experiments setup was conducted and developed in Baghdad, Iraq. Solar air collector was thermal performance and tested for five cloudless days in April 2019, the slope of 42 • facing south, thus, maximizing the solar intensity for winter days. The measurement equipment was a data-logger (EXTECH), an accuracy of ± (0.4% + 1 • C), model SDL200, using for obtained inlet, plenum and outlet temperatures and Pyranometer placed lower to the collector to sense the global Sustainability 2020, 12, 3619 5 of 15 solar irradiance (Pyranometer RK200-03, 0-2000 w/m 2 Range, SN: R18021068) used for measuring solar intensity. The air velocity was measured and taken manually by HT-9829, 60-90 mA, hot wire anemometer, and solar intensity and temperatures were registered automatically every 30 min a day. A vacuum blower power of 0.75 kW, model 135C16A was used to discharge five air flows during five-day work, from 0.003 to 0.018 kg/s. A blower speed controlled by using the inverter drive speed control model: SV008iC5-1, 5 A, 0.01-400 Hz.

Instantaneous Efficiency of the Solar Collector
To evaluate the performance of the solar heater system, the first law of thermodynamic was applied, and the thermal efficiency was defined as the ratio of the useful energy to receive energy, and the useful energy was defined as the thermal energy, and receives energy as heat flux or solar radiation energy.
Thermal E f f iciency = Use f ul Energy Gain Received Energy (1) where useful energy gain and received energy in (W),ṁ is the air flow rate (kg/sec), C p is the specific heat of the air (kJ/kg K), A c is the collector area in (m 2 ), (T o ) is the outlet air temperature ( • C), and T i is the inlet air temperature ( • C) [19,20]. Equation (2) can be simplified to: It is clear from Equation (3), that there is a linear relationship between η th and (T o −T i )/I, the positive slope given with . mc p A c . The performance of flat plate solar collector operating under steady conditions can be successfully described Equation (4). Thermal efficiency is solved heat removal coefficient (F R ) as shown [19]: where (τα) is the effective transmittance absorbance product, (U L ) is the overall heat loss coefficient in W/ (m 2 K), and (F R ) is the collector heat removal factor which relays the actual useful energy gain to the useful gain that the collector surface was at the air inlet temperature. For this state Equation (4) gives a linear relationship between and (T i −T a )/I, but a negative slope of [−(F R U L )/I], and the origin order at F R (τα).

Ambient Temperature, Relative Humidity and Solar Intensity
Daily values of ambient air temperature, relative humidity and solar radiation incident were registered from 08:00 to 16:00 within a period between 23-27 April 2019 at Baghdad, Iraq. Figure 3, shows the total average for those parameters within the five days of the experimental work. The average ambient air temperature was increased slightly from 11 • C, with a solar intensity value of 340 W/m 2 , in the morning to 18 • C, with a solar intensity value of 110 W/m 2 , at the end of the day. The average maximum solar intensity was 820 W/m 2 at noon. The average relative humidity dropped, as opposed to the average ambient air temperature, during the day from 08:00 a.m. to 16:00 p.m., where the maximum value, 82%, was seen in the morning then falls to 52% in the evening.
shows the total average for those parameters within the five days of the experimental work. The average ambient air temperature was increased slightly from 11 °C, with a solar intensity value of 340 W/m 2 , in the morning to 18 °C, with a solar intensity value of 110 W/m 2 , at the end of the day. The average maximum solar intensity was 820 W/m 2 at noon. The average relative humidity dropped, as opposed to the average ambient air temperature, during the day from 08:00 am to 16:00 pm, where the maximum value, 82%, was seen in the morning then falls to 52% in the evening.   Figure 4 illustrates the ambient temperature (T a ), inlet temperature (T i ), bed temperature (T b ) and outlet temperature (T o ), with the time of the day from morning 08:00 to evening 16:00, for five different air flow rate from 0.003 kg/s to 0.018 kg/s. Temperature curves (T a ), (T i ), (T b ) and (T o ) had a similar manner with the solar intensity, increased to the maximum value at noon then decreased at the end of the day. The inlet air temperature (T i ) was higher than the ambient temperature (T a ). The ambient temperature was measured at a shadow place nethermost the bed, while (T i ) measured at entrance of the bed. The outlet temperatures (T o ) were greater than bed temperatures (T b ), (bed temperature means the temperature of the passing air inside the bed at the middle), for all days of experiments. This behavior was due to the double-pass of unglazed collectors or the counter-flow, which means that the passing air was preheated in the upper channel before the air flow reversed direction to enter the lower channel. Furthermore, (T b ) and (T o ) increase as the air flow rate was decreased from 0.003 kg/s to 0.018 kg/s, mainly due to the increase in the time of the moving air through the collector (from inlet section to the end of the collector). In other words, the air was able to carry more heat from the perforated plate and the wire mesh. Table 2 present the average value of inlet temperature, bed temperature, outlet temperature, and solar intensity with different air flow rate. which means that the passing air was preheated in the upper channel before the air flow reversed direction to enter the lower channel. Furthermore, (Tb) and (To) increase as the air flow rate was decreased from 0.003 kg/s to 0.018 kg/s, mainly due to the increase in the time of the moving air through the collector (from inlet section to the end of the collector). In other words, the air was able to carry more heat from the perforated plate and the wire mesh. Table 2 present the average value of inlet temperature, bed temperature, outlet temperature, and solar intensity with different air flow rate.    Figure 5 shows the temperature differences (∆T = T o -T a ) with time, for different air flow rate (from 0.003 kg/s to 0.018 kg/s) increased from the morning at 08:00 to reach a maximum temperature at noon, then afternoon the (∆T) slowly decreased to the end of the day at 16:00. The maximum temperature differences (∆T) is obtained 52.5 • C with a solar intensity of 925 W/m 2 , at an air flow rate of 0.003 kg/s, where the (T o ) of the flowing air through the collector increased with decreasing airflow rate  Figure 5 shows the temperature differences (ΔT = To -Ta) with time, for different air flow rate (from 0.003 kg/s to 0.018 kg/s) increased from the morning at 08:00 to reach a maximum temperature at noon, then afternoon the (ΔT) slowly decreased to the end of the day at 16:00. The maximum temperature differences (ΔT) is obtained 52.5°C with a solar intensity of 925 W/m 2 , at an air flow rate of 0.003 kg/s, where the (To) of the flowing air through the collector increased with decreasing airflow rate The results of temperature differences (as shown in Table 1) was compared with the previously unglazed transpired collectors published data, there was an enhancement in the heat transfer performance. The study of Kutscher [11] was obtained a maximum (∆T) of 32 • C for at I = 700 W/m 2 for an air flow rate of 0.02 kg/s and perforated absorber area of 3 m × 3 m. The research [28], for numerical values, obtained, by neglecting the natural convection loss have obtained a maximum (∆T) of 35 • C for at I = 800 W/m 2 , air flow rate of 0.015 kg/s and perforated absorber area of 2 m × 3 m. Rad and Ameri [29] obtained the highest (∆T) of 15 • C with a bed area of 2.79 m × 1.5 m at an air flow rate of 0.02 kg/s and I = 800 W/m 2 . Theoretical and experimental investigated [30], for unglazed perforated solar air collector, which is coupled to a capillary heating system, the (∆T) of 14.7 • C for ambient temperature (T a ) of 10 • C at . m = 0.005 kg/s, and bed area of 1.2 m × 2.2 m.

Thermal Efficiency (η)
From Equation (4), if the efficiency is plotted against the temperature parameter (T i −T a )/I, a straight line was made where the slope is indicative of (U L ), and the y-intercept is indicative of (τα). In fact, (U L ) is not a constant, but a function of the ambient temperature and the outlet temperature of the collector. Figure 6 presents the instantaneous thermal efficiency (η i ) of the collector versus (T i -T a )/I, for air flow rate of 0.018 kg/s. The maximum efficiency gained 86%, for air flow rate 0.018 kg/m 2 (Day 5), with the negative linear fit behavior slope B = −1.1782 and a correlation coefficient R 2 = 0.5566. The temperature parameter (T i −T a )/I is increased as the thermal efficiency decreased, increasing of (T i −T a )/I caused an increase in the heat loss from the upper cover to the surrounding, thus, thermal efficiency is reduced. There was a fluctuation of thermal efficiency value during Day 4, due to the part cloudy day, that effect on the solar intensity and thermal efficiency of the collector. Thus, the slope of the curve Day 4 seems straight line or low slope (not like other days), the constant A and B, are higher than constant A and B for Days 1-3, and 5.
(Day 5), with the negative linear fit behavior slope B = −1.1782 and a correlation coefficient R 2 = 0.5566. The temperature parameter (Ti−Ta)/I is increased as the thermal efficiency decreased, increasing of (Ti−Ta)/I caused an increase in the heat loss from the upper cover to the surrounding, thus, thermal efficiency is reduced. There was a fluctuation of thermal efficiency value during Day 4, due to the part cloudy day, that effect on the solar intensity and thermal efficiency of the collector. Thus, the slope of the curve Day 4 seems straight line or low slope (not like other days), the constant A and B, are higher than constant A and B for Days 1-3, and 5.
Replacing the slope by B and the origin by A, we obtain Adding Equations (6) and (3), we find the expression for (To) as a function of inlet air temperature (Ti) and the ambient temperature (Ta): where the variable λ is defined as: Replacing the slope by B and the origin by A, we obtain Adding Equations (6) and (3), we find the expression for (T o ) as a function of inlet air temperature (T i ) and the ambient temperature (T a ): where the variable λ is defined as: Replacing in Equation (7) Figure 7, there was a validation of the work conditions, for the double-pass, unglazed solar energy system. The different value between experimented and predicted data were between (0 to 5) • C, for all days of work. The experimental work and simulated work agreed. There was a mismatch between experimented and predicted curves, due to the outdoor work condition, that affected on the outlet temperature. They fluctuated for inlet temperature, solar intensity, wind speed and humidity are caused some unstable outlet temperature with time of the day.  Figure 7 shows experimental outlet temperatures (To, Measured) and simulated temperatures (To, Simulated) versus hours of the day at different airflow rate, that given by Equation (10). Clear from Figure 7, there was a validation of the work conditions, for the double-pass, unglazed solar energy system. The different value between experimented and predicted data were between (0 to 5) °C, for all days of work. The experimental work and simulated work agreed. There was a mismatch between experimented and predicted curves, due to the outdoor work condition, that affected on the outlet temperature. They fluctuated for inlet temperature, solar intensity, wind speed and humidity are caused some unstable outlet temperature with time of the day.

Effects of Airflow Rate
As estimated from Equation (3), the thermal efficiency of the collector can be improved by raising the air flow rate. The experimental results in Figure 8 show the thermal efficiencies for each day with different air flow rates raised from 0.003 kg/s to 0.018 kg/s. The weather condition of air inlet temperature (Ti) 21°C and the solar intensity 456 W/m 2 . Evidently, there was a fluctuation of thermal efficiency value during Day 4, that was due to the partly cloudy, that effect on the solar intensity and efficiency for Day 4.
The high efficiency is caused by: First, the passing air through the upper channel is preheated the inlet air temperature before entering the bed. Second, the larger heat transferred between the passing air and absorber area (perforated plate and wire mesh layers). Third, the perforated plate acts as air jet impingement, the passing air through the hole enhancement the convective heat transfer.

Effects of Airflow Rate
As estimated from Equation (3), the thermal efficiency of the collector can be improved by raising the air flow rate. The experimental results in Figure 8 show the thermal efficiencies for each day with different air flow rates raised from 0.003 kg/s to 0.018 kg/s. The weather condition of air inlet temperature (T i ) 21 • C and the solar intensity 456 W/m 2 . Evidently, there was a fluctuation of thermal efficiency value during Day 4, that was due to the partly cloudy, that effect on the solar intensity and efficiency for Day 4.
The high efficiency is caused by: First, the passing air through the upper channel is preheated the inlet air temperature before entering the bed. Second, the larger heat transferred between the passing air and absorber area (perforated plate and wire mesh layers). Third, the perforated plate acts as air jet impingement, the passing air through the hole enhancement the convective heat transfer.
The thermal efficiency in the present work was compared with the previously reported data. The proposed present new prototype of the unglazed solar collector showed significant improvement in the thermal efficiency, for air flow rate of 0.018 kg/s, as shown in Table 3.
intensity and efficiency for Day 4.
The high efficiency is caused by: First, the passing air through the upper channel is preheated the inlet air temperature before entering the bed. Second, the larger heat transferred between the passing air and absorber area (perforated plate and wire mesh layers). Third, the perforated plate acts as air jet impingement, the passing air through the hole enhancement the convective heat transfer.   Table 3. Shows significant improvement in the thermal efficiency with for air flow rate (0.018 kg/s).

Researcher Scheme Type of Solar Collector (Description) Area Airflow Rate (kg/s) η%
Kutscher et al. [10] Sustainability 2020, 12, x FOR PEER REVIEW 13 of 19 The thermal efficiency in the present work was compared with the previously reported data. The proposed present new prototype of the unglazed solar collector showed significant improvement in the thermal efficiency, for air flow rate of 0.018 kg/s, as shown in Table 3. As demonstrated in Figure 9, the efficiency increased continuously with the air flow rate, i.e., the heat transfer rate is directly proportional to the air flow rate. The increase in the air flow rate increased the convection heat transfer coefficient, thus, enhancing the heat transfer from the collector to the working fluid. At the maximum air flow rate, 0.018 kg/s, the outlet temperature (To) was at the minimum value. By lowering the air flow rate the outlet temperature (To) increased, due to the increase in the time of moving air, the air took more time to move from the inlet section to the end of the collector. Thus, the air carried more heat from the perforated plate and the wire mesh layers. Therefore, the application of the solar energy system in cold weathers should be at a low air flow rate to satisfy the thermal comfort of the buildings. At the minimum air flow rate, 0.003 kg/s, the maximum average temperature difference (∆Tave.), 38.6 °C , was achieved with average inlet As demonstrated in Figure 9, the efficiency increased continuously with the air flow rate, i.e., the heat transfer rate is directly proportional to the air flow rate. The increase in the air flow rate increased the convection heat transfer coefficient, thus, enhancing the heat transfer from the collector to the working fluid. At the maximum air flow rate, 0.018 kg/s, the outlet temperature (T o ) was at the minimum value. By lowering the air flow rate the outlet temperature (T o ) increased, due to the increase in the time of moving air, the air took more time to move from the inlet section to the end of the collector. Thus, the air carried more heat from the perforated plate and the wire mesh layers. Therefore, the application of the solar energy system in cold weathers should be at a low air flow rate to satisfy the thermal comfort of the buildings. At the minimum air flow rate, 0.003 kg/s, the maximum average temperature difference (∆T ave .), 38.6 • C, was achieved with average inlet temperature (T a ave. ) of 17.7 • C and average solar intensity (I ave. ) of 661 W/m 2 (Day 1 of the work). . A solar energy system that is designed for high efficiency would have a high air flow rate and low outlet temperature. During low outside temperature (climate degree), the building manager would feel that the air is "cold" and think the collector is not working well. Thus, to ensure reaching the required level of heating, a conventional heater would need to be attached to the air flowing to the building. However, by setting the solar energy system for high-temperature, lower air flow rates should be used. This can satisfy the heating requirement of the building, but additional ventilation might be needed. An optimized level of air flow rate, that satisfies the requirements of the building for both heating and ventilation, should be the aim of the manager.

Conclusions
The experimental outdoor study of the outlet temperature and thermal efficiency of a doublepass unglazed solar air heater, with a perforated plate and wire mesh layers as an absorber plate, were complete. Afterwards, the prototype was designed and built in Baghdad, Iraq, to improve the conventional unglazed transpired collector for the new design of double plates cover of the unglazed solar energy system, during five cloudless winter days, with a maximum solar intensity on the collector cover plate of 925 W/m 2 and outdoor air temperatures ranging between 10-20 °C .
Ambient air temperatures were associated with humidity ratio and solar intensity-the ambient air temperature increased slightly from the morning, when the solar intensity increased. At the end of the day the solar intensity decreased to minimum value, while the ambient temperature at maximum value of (20 °C ).
Temperature differences between the outlet and inlet temperatures with solar radiation curves increased when the air flow rate decreased. Outlet temperatures reached 72 °C at noon, with an average temperature rise of 52.5 °C, for an air flow rate of 0.003 kg/s. Thermal efficiency was increased as the air flow rate increased from 0.003 to 0.018 kg/s. However, average daily efficiency of 64.7% was measured with an air flow rate of 0.018 kg/s. Experimental results and the predictions of the theoretical model were complete, and agreed with the monitored data. The maximum different value between experimented and predicted data between 5 °C, for all days of work. For winter days, outlet temperature should be at least 45 °C, to ensure the thermal comfort of buildings in cold regions, so the optimal air flow rate used for this Figure 9. (a) Table of the average solar intensity and inlet air temperature, thermal efficiencies and temperature differences for five-day work; (b) variation of the average temperature differences and the average thermal efficiencies with the air flow rate for five-day work. A solar energy system that is designed for high efficiency would have a high air flow rate and low outlet temperature. During low outside temperature (climate degree), the building manager would feel that the air is "cold" and think the collector is not working well. Thus, to ensure reaching the required level of heating, a conventional heater would need to be attached to the air flowing to the building. However, by setting the solar energy system for high-temperature, lower air flow rates should be used. This can satisfy the heating requirement of the building, but additional ventilation might be needed. An optimized level of air flow rate, that satisfies the requirements of the building for both heating and ventilation, should be the aim of the manager.

Conclusions
The experimental outdoor study of the outlet temperature and thermal efficiency of a double-pass unglazed solar air heater, with a perforated plate and wire mesh layers as an absorber plate, were complete. Afterwards, the prototype was designed and built in Baghdad, Iraq, to improve the conventional unglazed transpired collector for the new design of double plates cover of the unglazed solar energy system, during five cloudless winter days, with a maximum solar intensity on the collector cover plate of 925 W/m 2 and outdoor air temperatures ranging between 10-20 • C.
Ambient air temperatures were associated with humidity ratio and solar intensity-the ambient air temperature increased slightly from the morning, when the solar intensity increased. At the end of the day the solar intensity decreased to minimum value, while the ambient temperature at maximum value of (20 • C).
Temperature differences between the outlet and inlet temperatures with solar radiation curves increased when the air flow rate decreased. Outlet temperatures reached 72 • C at noon, with an average temperature rise of 52.5 • C, for an air flow rate of 0.003 kg/s. Thermal efficiency was increased as the air flow rate increased from 0.003 to 0.018 kg/s. However, average daily efficiency of 64.7% was measured with an air flow rate of 0.018 kg/s. Experimental results and the predictions of the theoretical model were complete, and agreed with the monitored data. The maximum different value between experimented and predicted data between 5 • C, for all days of work. For winter days, outlet temperature should be at least 45 • C, to ensure the thermal comfort of buildings in cold regions, so the optimal air flow rate used for this prototype research between 0.003 to 0.013 kg/s, providing an air outlet temperature of around 47-53 • C.
A double-pass, counter-flow collector (to increase the air path, and therefore, gain more heat in addition to minimizing heat loss to the surroundings) was used to enhance the thermal performance. Non-uniform flow was obtained by passing air through wire mesh layers at the bottom part of the collector for increasing the absorbing heat. Suitable space heating and perfect preheating ventilation air required, if the ventilation air heated before entering the building.