This paper presents an experimental analysis for comparisons of conventional flat plate solar collectors and collectors integrated with different numbers of baffles. Heat transfer between absorber plate and drying fluid (air) has been one of the major challenges in the design and operations of the indirect solar dryer systems. In this experiment, efficiency of air flat plate solar collector integrated with 2, 3, 4 and 8 baffles was studied and compared with the ordinary collector. The results showed that integrating solar collector with baffles significantly increased the efficiency of the system. It was noted that collector with 2, 3, 4 and 8 baffles had a mean efficiency of 29.2%, 31.3%, 33.1% and 33.7% respectively while with no baffles was 28.9%. The analysis showed that when there were less than four baffles in the collector, heat transfer was dominant over pressure drop and hence high efficiency. However, when the number of baffles exceeded four, the effect associated with an increase in pressure drop highly observed compared to heat transfer coefficient, thus resulted to insignificant increase in efficiency. Therefore, the optimum number of four baffles was commended for the designed model for optimum efficiency.
Open sun drying for the preservation of agricultural, food and many other pro- ducts has been popular in many societies from ancient time [
It is known that important quality verdict made by consumers on dried food in the market is its visual appearance and sensory quality, which includes: colour, flavour, taste, texture and aroma [
Indirect cabinet solar dryer with forced convection flow is one of the best solar drying technologies which can produce high-quality products and eliminate the risk of discolouration and distraction of vitamins [
The solar collector is a kind of duct heat exchanger that transfers heat energy from incident solar radiation to the fluid passing through it. It absorbs solar radiation through glazing, converting it to heat energy and transferring it to a working fluid [
Baffles provide an additional heat transfer surface area and promote air turbulence in the collector. The presence of baffles causes the air flow to separate, reattach and create a reverse flow which increasing the heat washing action. The main concept of integrating collector with baffles is to reduce dead zones and increase heat transfer area.
Important of baffles in solar dryers system was also reported by Pona [
The thermal efficiency of the flat plate solar collector is the ratio of the energy out of the collector to the total incident solar radiation averaged over the same time interval. Mathematically, the efficiency ( η ) of a collector is expressed by Equation (1) [
η = usefulenergy solarenergyavailable (1)
The outlet energy (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.
Q u = m ⋅ C p ⋅ ( T o − T i ) (2)
The area of the collector on which the solar irradiance falls is called the aperture area of the collector. Therefore, total energy received by the collector (optical energy captured) can be described by Equation (3).
Q i n = I ⋅ A (3)
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.
Q i n = α ⋅ τ ⋅ I ⋅ A (4)
The rate of useful energy of the collector can be expressed by using overall heat loss coefficient and the collector temperature as Equation (5).
Q ˙ u s e f u l = Q ˙ i n − Q ˙ l o s s = α ⋅ τ ⋅ I ⋅ A − U L ⋅ A C ⋅ ( T c − T a ) (5)
Since it is difficult to define the collector average temperature in Equation (5), it is convenient to define a quantity that relates the actual useful energy gain of a collector to the useful gain if the whole collector surface were at the fluid inlet temperature [
F R = m ˙ ⋅ C p ⋅ ( T o − T i ) A ⋅ [ α ⋅ τ ⋅ I − U L ⋅ ( T i − T a ) ] (6)
Finally, the equation for the efficiency of flat plate solar collector can be given by “Hottel-Whillier-Bliss equation” [
η = F R ⋅ α ⋅ τ − F R ⋅ U L ⋅ ( T i − T a I ) (7)
If it is assumed that τ and α are constants for a given collector and flow rate, then the collector efficiency is a linear function of the three parameters defining the operating condition: Solar irradiance (I), Fluid inlet temperature (Ti) and collector outlet temperature (To). Thus, the performance of a Flat-Plate Collector can be approximated by experimentally measuring these three parameters, and the efficiency can be calculated by using summarized Equation (8) [
η = m ˙ ⋅ C p A ∗ [ T o − T i I ] (8) (Yeh et al., 1998)
Four similar flat plate solar collector models were constructed by using Ptero- carpus timber which functions as the frame of collector and insulator. The thickness of side walls of the collectors were 2 inches with black painted marine plywood as absorber materials. Absorber plates were well protected with food- standard waterproof material and both collectors were covered by 4 mm glass thickness. Geometrical specifications of collectors were: collector length to width ratio 2 (length 1.2 m and width 0.6 m), depth 12 cm and length of baffles covered 85% of collector width. Both Collectors were oriented to the north-south direction and tilted to an angle of 6˚ (latitude angle of Dar es Salaam) with the ground toward the north direction to receive maximum solar radiation and to avoid rainy water accumulation inside the collector (
The temperatures in the collector were measured by using multichannel XR5-SE data logger connected with PT940 temperature sensors whilst ambient temperatures were measured by CEM DT-172 temperature and humidity data logger. On the other hand, solar intensity and air flow rate were respectively measured by using PCE-SPM solar radiation meter and Testo-425 Hot Wire Thermo-Anemometer.
Air flows in the collectors were controlled by extract fans which were attached to the outlet duct. Experiments were conducted daily from 7:00 am before sunrise to 6:00 pm after sunset. Data were filtered by removing the data with some interruptions such as rainy days and power cuts.
Collector models were integrated with 2, 3, 4, 8 baffles respectively, their efficiencies were measured and compared with the collector with no baffles (conventional collectors). Baffles were equally spaced over the collector geometry as shown in
Measuring devices | Accuracy |
---|---|
XRe-SE data logger | ±0.15˚C from 10˚C to 40˚C; ±0.3˚C from −25 to 85˚C |
PCE-SPM solar radiation meter | ±10 W/m2 or ±5% (more accuracy in highest value) |
CEM DT-172 temperature and humidity data logger | Temp ±1˚C and Humid ±2% |
Testo-425 hot wire thermal anemometer | ±(0.03 m/s) |
for ordinary collectors and with 2, 3 and 4 baffles. It can be seen that collector with 4 baffles gave high temperature along the day compared to the ordinary collector and with 2 and 3 baffles. The model with 4 baffles gave an average temperature of 42.7˚C while that with 2, 3 and without baffles gave 42.1˚C, 41.6˚C, and 40.1˚C respectively. On the other hand, the maximum temperature reached in collector with 4 baffles were 52.2˚C while that of the collector with 2, 3 and without baffles were respectively 50˚C, 51˚C, and 48˚C.
On the other hand,
During the morning, the sky is normally covered by clouds, which resulted into high fluctuation of solar intensity compared to the afternoon. In some cloudy days, the fluctuations were prolonged along the day as indicated in
The rate of change in temperature at the interval of 10 minutes is shown in
of solar intensity.
Generally, collected energy varies with the fluctuation of solar intensity and when solar intensity increases, the collector’s energy also increased. Low energy during the morning and sunset were caused by the low solar incidence angle to the collector surface and the fact that during the morning most of the collected heat was used in preheating the system. In addition, fluctuations of energy during the afternoon were due to poor thermal heat storage behaviour of the absorbing material.
The results of the statistical analysis of variance (ANOVA) for the collector with a different number of baffles (different ratio of baffle space to collector length) which was carried out to study the significance differences between their individual means are reported in
A one-way between subject ANOVA was used to compare the effect of varying
N | Mean efficiency | Std. deviation | Std. error | 95% Confidence interval for mean | Min. | Max. | |||
---|---|---|---|---|---|---|---|---|---|
Lower bound | Upper bound | ||||||||
No-baffles | 10 | 29.00 | 1.93129 | 0.61073 | 27.5084 | 30.2716 | 28.20 | 30.40 | |
2-baffles | 7 | 29.214 | 1.91349 | 0.72323 | 27.4446 | 30.9840 | 29.10 | 31.70 | |
3-baffles | 10 | 31.290 | 2.34400 | 0.74124 | 29.6132 | 32.9668 | 29.80 | 34.80 | |
4-baffles | 10 | 33.140 | 2.67798 | 0.84685 | 31.2243 | 35.0557 | 30.30 | 38.70 | |
8-baffles | 3 | 33.700 | 3.77227 | 2.17792 | 24.3292 | 43.0708 | 30.60 | 37.90 | |
Total | 40 | 30.970 | 2.92340 | 0.46223 | 30.0351 | 31.9049 | 26.10 | 38.70 | |
Model | Fixed effects | 2.37842 | 0.37606 | 30.2066 | 31.7334 | ||||
Random effects | 0.97645 | 28.2589 | 33.6811 |
baffles number on the efficiency of the collector. The comparisons were done to ascertain if there is a significant difference between collector’s efficiencies.
From
Since the significance value for Levene test for equity of variance is 0.729 which means (P > 0.5), therefore, equal variance assumed (Tukey HSD) were used for multiple comparisons of means of collector efficiency.
The purpose of introducing baffles in the collector duct was to create turbulence so as to increase heat transfer coefficient and hence the collector’s efficiency. In this study, baffles played a big role in collector heat transfer rate by forcing air to take longer meandering trajectory than the normal length of collector and forcing air to circulate through space left between baffles. On the other hand, the presence of baffles caused the flow to separate, re-attach and create a reverse flow which was increasing the heat transfer action from the absorber to air as. The flowing air was also forced by baffles to pass onto the warm wall of the absorber surface which resulted in good heat transfer and a considerable increase in output temperature. It was further observed that reducing the baffles spacing by increasing the number of baffles considerably increases
Sum of squares | df | Mean square | F | Sig. | |
---|---|---|---|---|---|
Between groups | 135.313 | 4 | 33.828 | 5.980 | 0.001 |
Within groups | 197.991 | 35 | 5.657 | ||
Total | 333.304 | 39 |
Levene statistic | df1 | df2 | Sig. |
---|---|---|---|
0.510 | 4 | 35 | 0.729 |
(I) Baffles | (J) Baffles | Mean difference (I-J) | Std. error | Sig. | 95% confidence interval | |
---|---|---|---|---|---|---|
Lower bound | Upper bound | |||||
0-baffles | 2-baffles | −0.32429 | 1.17210 | 0.999 | −3.6941 | 3.0456 |
3-baffles | −2.40000 | 1.06366 | 0.183 | −5.4581 | 0.6581 | |
4-baffles | −4.25000* | 1.06366 | 0.003 | −7.3081 | −1.1919 | |
8-baffles | −4.81000* | 1.56567 | 0.031 | −9.3114 | −0.3086 | |
2-baffles | 0-baffles | 0.32429 | 1.17210 | 0.999 | −3.0456 | 3.6941 |
3-baffles | −2.07571 | 1.17210 | 0.406 | −5.4456 | 1.2941 | |
4-baffles | −3.92571* | 1.17210 | 0.016 | −7.2956 | −0.5559 | |
8-baffles | −4.48571 | 1.64126 | 0.069 | −9.2044 | 0.2330 | |
3-baffles | 0-baffles | 2.40000 | 1.06366 | 0.183 | −0.6581 | 5.4581 |
2-baffles | 2.07571 | 1.17210 | 0.406 | −1.2941 | 5.4456 | |
4-baffles | −1.85000 | 1.06366 | 0.424 | −4.9081 | 1.2081 | |
8-baffles | −2.41000 | 1.56567 | 0.545 | −6.9114 | 2.0914 | |
4-baffles | 0-baffles | 4.25000* | 1.06366 | 0.003 | 1.1919 | 7.3081 |
2-baffles | 3.92571* | 1.17210 | 0.016 | 0.5559 | 7.2956 | |
3-baffles | 1.85000 | 1.06366 | 0.424 | −1.2081 | 4.9081 | |
8-baffles | −0.56000 | 1.56567 | 0.996 | −5.0614 | 3.9414 | |
8-baffles | 0-baffles | 4.81000* | 1.56567 | 0.031 | 0.3086 | 9.3114 |
2-baffles | 4.48571 | 1.64126 | 0.069 | −0.2330 | 9.2044 | |
3-baffles | 2.41000 | 1.56567 | 0.545 | −2.0914 | 6.9114 | |
4-baffles | 0.56000 | 1.56567 | 0.996 | −3.9414 | 5.0614 |
*The mean difference is significant at the 0.05 level.
the collector efficiency.
From this study, a collector with 8 baffles gave the highest efficiency of 33.7%, while that of 4, 3, and 2 and with no baffles were 33.1%, 31.3%, 29.2% and 28.9% respectively.
the efficiency of the solar collector. It is clear that as the ratios of baffle spacing to collector length was reduced the efficiency of collector were greatly increased, however, when the ratios become small (many baffles), the efficiency was reduced. This is due to the fact that, in collector without baffles (ratio of 1), there was the direct passage of air in the medium of the collector from the inlet toward outlet which was associated with many dead zones and therefore reducing the effectiveness of the collector.
On the other hand, when there is less number of baffles in the collector, in this case, less than 4, there was a high influence in heat transfer while pressure drop was insignificant. However, when baffles exceed this number, the associated increase in pressure drop was becoming higher than the heat transfer coefficient which results into insignificant increase in efficiency.
For the best performance, a number of baffles should be optimized to ensure the increase in collector efficiency is economical in order to avoid a high increase in air pumping power.
The study conducted [
・ Thermal efficiency of air flat plate solar collector with different numbers of buffles has been successfully studied and compared to conventional collectors.
・ Installation of buffles in the air passage gives a promising aproach in tackling the effect of dead zones in the collector and hence improves the overall efficiency of the system.
・ The optimum number of baffles has to be considered during design to avoid excessive effect in pressure drop which will increase the operation cost.
Bakari, R. (2018) Heat Transfer Optimization in Air Flat Plate Solar Collectors Integrated with Baffles. Journal of Power and Energy Engineering, 6, 70-84. https://doi.org/10.4236/jpee.2018.61006