Applying solar collectors is a popular tool for harnessing solar energy. In this work, a flat plate solar air collector was investigated under direct solar radiation in an endeavor to enhance the thermal efficiency of solar air collector with a slatted glass cover, perforated absorber aluminum sheets (porosity 0.0177, 0.0314 and absorber thickness of 1.25, 2.5 mm) which is the most suitable for a solar dryer. The effects of porosity and thickness on absorber performance of collector were evaluated. Six levels of air mass flow rates (0.0056 to 0.0385 kg m−2 s−1) were adopted. The tests were conducted in three replications on very clear sky days in September and October. The experimental results showed that thermal efficiency of collector was increased by an increase in the porosity of the absorber. The absorber with lower porosity showed a better thermal efficiency at lower air mass flux. In the minimum air flow rate, absorber efficiency with porosity 0.0177 and 0.0314 was 0.31 and 0.29, respectively, whereas at the maximum flow, efficiency showed an enormous change of 0.83 and 0.88, respectively. This solar air heater can be used for drying agricultural products, heating the space of greenhouse, and so forth.
Solar air heaters are inherently low in thermal efficiency due to low heat capacity and low thermal conductivity of the air in comparison to the liquid-type solar collectors [
In a study conducted by Whilier, 1964 [
In a research on the effect of wind direction on thermal performance, it was found that the thermal efficiency raised when the wind was perpendicular to the direction of grooves and the lowest one occurred when the wind was blowing along the grooves. The effect of this wind direction change was about 10 to 20 percent on thermal efficiency [
Using the FLUENT software to evaluate the numerical grid plates with heat transfer in parallel flow to the suction, it was revealed that thermal performance was dependent on the six dimensionless parameters. One of these dimension groups was
To enhance the heat transfer coefficient between the absorber and the air, three perforated aluminum sheets were evaluated with different porosities and thickness of 1.25 mm. To reduce heat losses from the upper surface, one layer of plain glass sheet cover was used. The results of this study introduced the two better porosities for absorber sheets for better thermal efficiencies under different operating conditions [
In order to decrease the radiation and convection losses and reduce the high effect of air stream blowing on the top of the collector and blowing direction, the present research was conducted on a slatted glass sheet cover (transpired cover) with metallic transpired absorber solar air heater for outdoor conditions.
This transpired solar air collector consisted of an optically transparent layer of several narrow glass sheets shaped in slatted form as a double glazing collector cover, a porous aluminum sheet absorber, a pressed wood framework covered by the bottom layer of insulation made of glass wool with a thickness of 50 mm. The pilot size experimental collector was illustrated in Figure
An overview sketch of the new solar collector.
The inlet air was introduced through the slots formed by the slatted collector cover. This air sucking recovers part of the short wavelength radiation absorbed by the glass sheets and causes a better cooling action for the pieces of glass sheet cover. Moreover, since the air passes downward throughout the absorber with more uniformly distributed pattern, the absorber plate would be more uniformly cooled by cooling fluid. Ten glass sheets (20
Side view of slatted glass cover.
Slatted glass cover and the connected sensors.
In this study two porous aluminum absorbers with effective surface area 106
Absorber plates and their porosities.
In order to maintain a uniform air flow rate through the absorber plate, along and across the air flow direction, cross section area between the absorber and glass cover in the direction of flow was kept constant,
Absorber was assembled into the collector body as a slanted plate. To install the glass cover, a rectangular-shaped wooden frame was made (internal dimensions 70
In this study, 12 smart temperature sensors (SMT-
A constant speed centrifugal fan (Parma, 1400 rpm, 50 Hz, Italy) was used as an air flow source connected to an inverter (N50-015SF, 1.5 kw, Korea) for altering the air flow rate. The air flow velocity was measured using an anemometer (Lutron, Taiwan) in a PVC pipe (10 cm ID) connected to the output duct of collector. The air flow velocity was converted to air flow rate by multiplying the mean air flow velocity by inside air duct cross section area. Furthermore, a silicon type pyranometer (Caselia, w, 0–2000 ± 1 w, UK) was used to measure solar radiation intensity.
The test rig was located on Faculty of Agriculture at Shiraz University. The inclination of collector and pyranometer by considering (
In each set of experiments (each absorber) thermal efficiency of collector was measured for six levels of air flow rate (0.0056, 0.0118, 0.018, 0.0235, 0.029, and 0.0385 kg m−2 s−1). To set each flow rate, an inverter was used to alter the engine Rpm, then the fan speed was changed and different flow obtained.
The tests were conducted (September-October 2010) in three replications on very clear sky days during 11 to 13 o’clock assuming the sun radiation and heat condition of environment show no significant changes during this time, [
For calculating the thermal efficiency of collector, (
In order to evaluate the significant effect of pertinent parameters on thermal efficiency, data was analyzed using SPSS software (version 16). The results were illustrated in Table
Test factors variance analysis of collector efficiency.
Variable | df | Sum of squares |
|
---|---|---|---|
|
5 | 2.781 | 15322.199** |
|
1 | 0.009 | 242.393** |
Th | 1 | 0.006 | 177.936** |
|
5 | 0.015 | 84.428** |
|
5 | 0.001 | 7.517** |
|
1 | 0.000 | 4.972 |
|
5 | 0.001 | 3.168 |
Error | 48 | 0.002 |
**Significance level of 1%.
Effect of cooling air flux on collector efficiency for different perforated absorbers.
It can be concluded that at lower air flows, the convective heat transfer coefficient between absorber plate and cooling air has a lower order of magnitude which results in a higher absorber surface temperature. Higher absorber surface temperature boosts the convective and radiative top heat losses. These results demonstrate a very good agreement with many other researchers’ investigations [
Due to low heat capacity of air, a low temperature difference between cooling air and absorber which is caused at a higher air mass flux, collector efficiency increases very slowly at high air flow rates. In other words, thermal efficiency increases at higher air flow rates due to greater contact volume of air flow rate which results in high rate of heat transfer coefficient and this reduces heat losses by radiation and convection that results in increase in efficiency. Therefore, an accurate justification needs to be made between blower power and collector efficiency rise at higher air flow rates.
Referring to Figure
Variations of normalized air temperatures, [(
The most porous absorber plate showed a better thermal efficiency compared with the others. This fact is also revealed in Figure
The maximum absorber thermal efficiencies of the collectors with absorber porosities of 0.0177 and 0.0314 were measured to be 0.83 and 0.88, respectively, at the highest air flow rates for the thicker absorber.
In the minimum air flow rate, absorber efficiency with porosity 0.0177 and 0.0314 was 0.31 and 0.29 respectively. This effect may be due to the lower heat transfer coefficient and higher absorber plate temperature which resulted in more convection and radiation heat losses.
A slatted glass cover and air solar collector with two different perforated absorber plates, two thicknesses and under air mass flow rates of 0.0056 to 0.0385 kg m−2 s−1 were tested. A maximum thermal efficiency of 0.88 was achieved for the most porous and thicker absorber plate at the highest air mass flow rate, but at very low air flow rates the absorber porosity showed a reversed effect on the efficiency. The absorber with less porosity illustrated a higher efficiency.
Porosity of absorber
Hole diameter
Cross section area between the absorber and glass cover
Internal diameter
The tilt angle of collector and pyranometer
Thermal efficiency of collector
Air mass flux per unit area of collector (
Air-specific heat capacity (
The radiation flux on the collector (
Outlet air temperature (
Inlet air temperature (