This study aimed at investigating the effect of thickness of glazing material on the performance of flat plate solar collectors. Performance of solar collector is affected by glaze transmittance, absorptance, and reflectance which results into major heat losses in the system. Four solar collector models with different glass thicknesses were designed, constructed, and experimentally tested for their performances. Collectors were both oriented to northsouth direction and tilted to an angle of 10° with the ground toward north direction. The area of each collector model was 0.72 m2 with a depth of 0.15 m. Low iron (extra clear) glass of thicknesses 3 mm, 4 mm, 5 mm, and 6 mm was used as glazing materials. As a control, all collector performances were analysed and compared using a glass of 5 mm thickness and then with glass of different thickness. The results showed that change in glass thickness results into variation in collector efficiency. Collector with 4 mm glass thick gave the best efficiency of 35.4% compared to 27.8% for 6 mm glass thick. However, the use of glass of 4 mm thick needs precautions in handling and during placement to the collector to avoid extra costs due to breakage.
In many countries, the use of solar drying systems for agricultural products to conserve vegetables, fruits, and other crops has been shown to be practical, inexpensive, and environmentally sound approach [
Flat plate solar collectors are special kind of heat exchangers that transfer heat energy from incident solar radiation to the working fluid [
Glazing is the top cover of a solar collector. It performs three major functions in particular: to minimize convective and radiant heat loss from absorber, to transmit the incident solar radiation to the absorber plate with minimum loss, and to protect the absorber plate from outside environment [
Glass is the principal material used to glaze solar collectors [
The major heat losses in the collector are from the front cover (glass cover), since the sides and the back of the collector are often adequately insulated [
Kalidasa et al., 2008, [
Energy absorbed by glass cover depends on temperature difference between glass and fluid, glass and plate, and glass and ambient:
The radiative heat transfer coefficients from the absorber to the glazing and from the glazing to the ambient are, respectively, given by
The convective heat transfer coefficients for air flowing over the outside surface of the glass cover were proposed by Kumar and Mullick [
Upward heat losses are greatly influenced by convective heat transfer from the upper outermost surface of a solar collector. This wind induced convective heat transfer has greater influence on upward heat losses in case of single glazed collectors
The thermal efficiency of a collector is the ratio of the useful thermal energy to the total incident solar radiation averaged over the same time interval. Mathematically, the efficiency (
Useful energy for a solar thermal collector is the rate of thermal energy leaving the collector, usually described in terms of the rate of energy being added to a heat transfer fluid passing through the receiver or absorber [
The area of the collector on which the solar irradiance falls is called the aperture area of the collector. Therefore, total energy received by collector (optical energy captured) can be described by
Accordingly, absorptance and transmittance are multiple effects of optical energy capture and, therefore, these factors indicate the percentage of the solar rays penetrating the transparent cover of the collector and the percentage being absorbed [
The rate of useful energy of the collector can be expressed by using overall heat loss coefficient and the collector temperature as (Yogi and Jan, 2000)
Since, it is difficult to define the collector average temperature in (
Finally, equation for efficiency of flat plate solar collector can be given by “
If it is assumed that
Four similar flat plate solar collectors were used in this study. Glazing materials used for experiments were low iron (extra clear) glass of thicknesses 3, 4, 5, and 6 mm. Collectors were constructed by using
Solar collector models with 3, 4, 5, and 6 mm glass thick, respectively.
Efficiency of the collectors was established by testing each collector with the same glass thickness (5 mm). The duration for this experiment was 5 days each for collector with similar glazing and with different glazing. Time of experiments was from 7:30 to 6:00 p.m. with an interval of data sampling of 10 minutes. Experiments were conducted at the University of Dar es Salaam at the College of engineering and Technology. Both collector models were placed on top of block Q building situated at the Department of Chemical and Mining Engineering.
The main objective of this experiment was to find out if there is significance performances difference between designed collector models with similar characteristics. Each collector model was tested for its performance by using 5 mm glass thickness.
Figure
Temperature profile of collector with similar glazing thickness.
Energy profile of collector with similar glazing thickness.
From Figure
ANOVA for collector with similar glass thicknesses.
Sum of squares | df | Mean square |
|
Sig. | |
---|---|---|---|---|---|
Between groups | 1.427 | 3 | 0.476 | 1.289 | 0.323 |
Within groups | 4.428 | 12 | 0.369 | ||
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Total | 5.854 | 15 |
From Table
Figure
Temperature profiles of collector with different glass thicknesses on 17/10/2011.
Solar intensity on 17/10/2011 and 18/10/2011.
Maximum temperature in both collectors occurred around noon when solar radiations were perpendicular to the collector surfaces. With single orientation of flat plate solar collector, the best performance occurs when the solar radiations are perpendicular to the collector surfaces, for this case at around noon.
Figure
Energy profile of collectors with different glass thickness on October 17, 2011.
The collector efficiency was evaluated by finding the area under the energy profiles curve and statistically tested for their difference. The results of the statistical analysis of variance (ANOVA) for collectors with different glass thickness were carried out to study the significance differences between their individual means and are reported in Table
Statistical analysis of performance of solar collectors with different glass thicknesses by using SPSS program.
Glass |
|
Mean | Standard deviation | Standard error | 95% confidence interval for mean | Minimum | Maximum | |
---|---|---|---|---|---|---|---|---|
Lower bound | Upper bound | |||||||
Glass, 3 mm | 5 | 32.7400 | 3.63015 | 1.62345 | 28.2326 | 37.2474 | 26.50 | 35.80 |
Glass, 4 mm | 5 | 35.4000 | 4.07001 | 1.82016 | 30.3464 | 40.4536 | 28.50 | 38.60 |
Glass, 5 mm | 5 | 30.4400 | 2.40583 | 1.07592 | 27.4528 | 33.4272 | 27.30 | 33.70 |
Glass, 6 mm | 5 | 27.8000 | 2.43002 | 1.08674 | 24.7827 | 30.8173 | 24.40 | 31.20 |
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Total | 20 | 31.5950 | 4.12546 | 0.92248 | 29.6642 | 33.5258 | 24.40 | 38.60 |
Model | ||||||||
Fixed effects | 3.21854 | 0.71969 | 30.0693 | 33.1207 | ||||
Random effects | 1.62083 | 26.4368 | 36.7532 |
The highest thermal efficiency as analysed by SPSS program was 35.4% in collector with 4 mm glass thickness, while the minimum performance was 27.8% in collector with 6 mm glass thickness. Collectors with 3 mm and 5 mm glass thicknesses were 32.7% and 30.4%, respectively. In the same way, a one-way between-subjects ANOVA was used to compare the effect of varying the thickness of glass materials on efficiency of collectors with 3, 4, 5, and 6 mm glass thicknesses. This was done to ascertain if the difference between collector’s means were significant.
From Table
ANOVA for collectors with different glass thicknesses.
Sum of squares | df | Mean square |
|
Significance | |
---|---|---|---|---|---|
Between groups | 157.625 | 3 | 52.542 | 5.072 | 0.012 |
Within groups | 165.744 | 16 | 10.359 | ||
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|||||
Total | 323.369 | 19 |
Results for Levene test for homogeneity of variance.
Levene statistic | df1 | df2 | Significance |
---|---|---|---|
0.462 | 3 | 16 | 0.713 |
Since, from Table
Table
Multiple comparisons test for collector mean efficiencies.
(I) Glass | (J) Glass | Mean difference (I − J) | Standard error | Significance | 95% confidence interval | |
---|---|---|---|---|---|---|
Lower bound | Upper bound | |||||
Glass, 3 mm | Glass, 4 mm | −2.66000 | 2.03558 | 0.572 | −8.4838 | 3.1638 |
Glass, 5 mm | 2.30000 | 2.03558 | 0.677 | −3.5238 | 8.1238 | |
Glass, 6 mm | 4.94000 | 2.03558 | 0.112 | −0.8838 | 10.7638 | |
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Glass, 4 mm | Glass, 3 mm | 2.66000 | 2.03558 | 0.572 | −3.1638 | 8.4838 |
Glass, 5 mm | 4.96000 | 2.03558 | 0.110 | −0.8638 | 10.7838 | |
Glass,6 mm | 7.60000* | 2.03558 | 0.009 | 1.7762 | 13.4238 | |
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Glass, 5 mm | Glass,3 mm | −2.30000 | 2.03558 | 0.677 | −8.1238 | 3.5238 |
Glass,4 mm | −4.96000 | 2.03558 | 0.110 | −10.7838 | 0.8638 | |
Glass, 6 mm | 2.64000 | 2.03558 | 0.578 | −3.1838 | 8.4638 | |
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Glass, 6 mm | Glass, 3 mm | −4.94000 | 2.03558 | 0.112 | −10.7638 | 0.8838 |
Glass, 4 mm | −7.60000* | 2.03558 | 0.009 | −13.4238 | −1.7762 | |
Glass, 5 mm | −2.64000 | 2.03558 | 0.578 | −8.4638 | 3.1838 |
Means plot of performances of solar collectors with different glass thicknesses.
Thermal properties of glass cover such as transmittance, reflectance, and absorptance are functions of collector performance. Therefore, the choice of collector glazing material should focus on increasing transmittance and reducing reflectance and absorbance. Generally, when increasing glass thickness, transmittance and convective losses decrease, while reflectance increases and vice versa. From this study, it can be concluded that 3 mm glass thickness gives high transmittance (low reflectance) and high convective losses and hence is worse performance compared to 4 mm. The 5 mm and 6 mm glass thicknesses gave low transmittance (high reflectance) and low convective losses and therefore gave poor performances compared to 4 mm glass thickness. Therefore, 4 mm glass thickness gave optimal transmittance and convective losses, and hence is the best glazing thickness for flat plate solar collector.
The solar collector models with different glazing thicknesses had been successfully designed, constructed, and tested in this study. The experimental data were compared graphically by using excel program and their performances were analysed statistically by using SPSS programme. From the results obtained, it could be concluded that the use of 4 mm glass thick improves the performance of air solar collector by 7.6% compared to 3, 5, and 6 mm glass thicknesses. However, the risk for glass breakage during construction is high when using thinner glass, 4 mm compared to 5 mm and 6 mm, especially when constructing larger collector with longer/wider span. Therefore, optimization of efficiency and runability owe to be made on whether to use 4 mm glass thickness with precaution to avoid extra cost due to glass breakage.
Collector area (m2)
Collector heat removal factor
Specific heat capacity of air (J/Kg·K)
Mass flow rate (kg/s)
Global solar intensity reaching collector surface (W/m2)
Useful energy gained by air (J/Kg·K)
Available solar energy on collector surface (J/Kg·K)
Temperature out of collector (°C)
Air inlet temperature (°C)
Heat loss coefficient (W/m2K)
Absorptivity
Transmissivity
Collector efficiency
Radiative heat transfer coefficient between glass and absorber plate (W/m2K)
Radiative heat transfer coefficient between glass and ambient (W/m2K)
Emittance of glass
Emittance of plate
Glass temperature (K)
Absorber plate temperature (K)
Wind speed (m/s).
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors wish to acknowledge the financial support from the Sida-UDSM-Food Security Programme Research Funds which is part of the Sida-UDSM Cooperation Programme for the period of 2009 to 2013.