Performance of a photovoltaic ventilated solar tunnel dryer with an unglazed transpired cloth absorber

Transcription

1 African Journal of Agricultural Science and Technology (AJAST) Vol. 4, Issue 2, pp February, ISSN Oceanic Academic Journals Full length research Paper Performance of a photovoltaic ventilated solar tunnel dryer with an unglazed transpired cloth absorber Isaac Nyambe Simate* and Geoffrey Siwanzi Department of Agricultural Engineering, University of Zambia, P.O. Box 32379, Lusaka, Zambia. *Corresponding author. isaac.simate@gmail.com. Tel: Accepted 9 th February, 2016 A forced convection solar tunnel dryer utilizing both direct radiation and hot air convection on to the food was designed, constructed and tested under the Zambian weather conditions. The solar collector was unglazed and had a black cloth absorber through which air infiltrated. The solar collector and the drying tunnel were connected in series but separated by a wooden partition on to which fans were fixed. The fans were connected directly to a PV module and pumped air from the collector to the drying tunnel. Cabbage weighing 9.3 kg was dried and its moisture content reduced from 94% (w.b.) to 10% (w.b.) in 8 h. The results showed that the temperature of air at collector outlet approached the cloth temperature implying very high heat exchange effectiveness for the absorber. The temperature of air leaving the drying tunnel was only a few degrees less than that entering the drying tunnel resulting in uniform temperature distribution. The use of an unglazed cloth absorber in a solar tunnel dryer aims at reducing the capital cost of the dryer as it does away with the glazing and metal absorber. Key words: Solar collector, tunnel dryer, transpired, infiltrating air, cloth absorber. INTRODUCTION Imre (1995) has written extensively about solar dryer types including those with heat transfer by convection and direct radiation (mixed-mode), the type under which solar tunnel dryers fall. Mixed-mode solar dryers have higher drying rates compared to other types of solar dryers due the heating of the food by a combination of hot air from the collector and direct radiation from the sun (Garba et al., 1990). Solar tunnel dryers of the Hohenheim-type (INNOTECH- Tunneltrockner) have been widely used commercially in more than 55 countries to dry a variety of foods (Axtell, 2002). Their construction comprises a collector section connected directly to a drying tunnel in series. Both the collector and drying tunnel are covered with a clear UV stabilised polythene sheet and the collector absorber is painted black to absorb radiation (Schirmer et al., 1996). Air is sucked from the ambient and blown through the collector and over the absorber by 12 volts DC fans powered by a solar PV panel. Hossain and Bala (2007) used a solar tunnel dryer to dry fresh chilli and found that the drying time was much less than that for conventional sun drying and that the quality of the dried chilli was better than that dried using conventional sun drying. In this work, a Hohenheim-type of solar tunnel dryer was constructed and its collector was modified to use an unglazed transpired solar collector (UTC) with the absorber made of a black polyester cloth. The unglazed transpired solar collector has a dark perforated surface through which ambient air infiltrates as it is sucked by a fan. The perforations in the absorber provide a large surface area for heat transfer between the air and the absorber (Woods et al., 1994). For air heating, unglazed transpired collectors (UTC) are reported to have the highest efficiency (60 75%) and have the lowest cost (Christensen, 1998). The infiltrating effect of the air on the boundary layer on the absorber significantly reduces heat losses from the collector due to wind and therefore, the design of the

2 Afr. J. Agric. Sci. Technol. 581 UTC does away with the glazing. Leon and Kumar (2007) reported that unglazed transpired solar collectors were applied mostly in ventilation air heating in Northern America and Europe, but that their application in drying which requires higher delivery air temperatures had not been studied in detail. They developed a mathematical model for predicting the thermal performance of UTC with a mild steel absorber, for application in food drying. In this work, an experimental study was carried out to determine the performance of UTC with a cloth absorber in a solar tunnel food dryer. The use of a UTC made of a cloth absorber aims at improving the efficiency and reducing the cost of the dryer as the cloth material is cheaper than the metal absorber. Also, since the absorber is not covered with glazing, the cost of the dryer is further reduced. was fixed on one side of the tunnel with a flat metal bar and some bolts, whereas on the opposite side it was fixed to a metal tube which allowed for rolling the sheet up and down facilitating loading and unloading of the dryer. The completed dryer is shown in Figure 1 and a schematic arrangement of the main parts of the dryer is shown in Figure 2. MATERIALS AND METHODS Description A Hohenheim-type solar tunnel dryer was designed, constructed and installed at the Department of Agricultural Engineering of the University of Zambia in Lusaka, Zambia. The dryer consisted of a tunnel 4 m long and 1.8 m wide divided into two sections, the collector tunnel and the drying tunnel were 1.75 m and 2.25 m long, respectively. The collector and drying tunnels were made of flat galvanised sheets for the sides and corrugated sheets for the bottom. Two layers of corrugated sheets were used in the bottom of the tunnels, separated by an air space of 25 mm thickness as insulation for bottom heat losses. A frame made of 25 mm square tubes supported the sheets. The collector tunnel was covered with a black polyester cloth supported by a wire mesh. A 3 mm-hardboard was used to partition the collector and the drying tunnel. The other end of the collector was sealed with another piece of hardboard, thus making the only entry for air into the collector to be through the cloth. Three 12V DC computer cooling fans rated at 2.6 W each and powered by a 14-W solar PV module were fixed onto holes in the partitioning hardboard and pumped air from the collector to the drying tunnel. For the drying tray, a galvanised wire mesh was placed on top of the corrugated sheets in the floor of the drying tunnel. A plastic mesh for spreading the drying product was then placed on top of the wire mesh. This arrangement allowed air to flow both above and below the product eliminating the need to turn the product during drying. The end of the drying tunnel was covered with a fly-screen to stop flies from entering the dryer. The drying tunnel was covered with a 200 μm thick UV stabilised transparent plastic sheet. The plastic sheet Figure 1. Solar tunnel dryer with a cloth absorber. Figure 2. Arrangement of the main parts of the solar tunnel dryer. Operation The dryer was designed to operate with solar radiation as the only source of energy for both heating and running the fans. Solar radiation is absorbed by the black cloth, which in turn gets heated and transfers heat to the air that is infiltrating through it. The air is then blown through the drying tunnel and over and under the product arranged in a thin layer. The air does two functions; it supplies heat for some of the moisture evaporation and also carries away the evaporated moisture. In addition to heat transfer by convection of hot air from the collector, the product also receives

3 Simate and Siwanzi 582 direct radiation through the transparent plastic cover and is therefore operating as a mixed-mode solar dryer (Simate, 2003). Experimental procedure Solar drying of cabbage was done in March The cabbage was cut into strips of 5 mm width then blanched in boiling water for about 3 min. After draining excess water, the cabbage was weighed and then spread in a thin layer on the plastic mesh in the drying tunnel and the drying was then commenced. The weight of the cabbage was 9.3 kg. Samples of cabbage placed on small mesh trays were positioned at six locations in the drying tunnel as shown in Figure 3 to monitor the weight loss. The samples were weighed at 1 h intervals on an electronic balance, model PE 3000 (Mettler Instruments B.V., accuracy 0.01g). For the initial moisture content of cabbage, samples of raw cabbage were put in an oven at 70 C for 24 h as described by Barbosa-Canovas et al. (1997). Figure 3. Sample positions in the drying tunnel. The temperature and humidity of air were measured by thermocouples in the ambient, on absorber surface (cloth), at locations where air exited the collector (collector exit) and exited the drying tunnel (tray exit). The solar radiation was measured by a pyranometer CM11 (Kipp and Zonen) and the velocity of air inside the drying tunnel was measured by a temperature/airflow meter μp 2308 (Konders Instruments B.V.). The above measurements were recorded on a data logger Campbell Scientific CR1000. In order to measure of the performance of the collector, the heat exchange effectiveness, as defined by the temperature rise in the collector compared to the maximum possible temperature rise was determined by the following equation (Kutscher, 1994): ε HX = (T col exit - T ambient )/( T collector - T ambient ) 1 Where T col exit is the collector exit temperature, T ambient is the temperature of the surroundings, and T collector is the absorber cloth temperature. Another measure of collector performance is the instantaneous efficiency as defined by the heat delivered to the drying tunnel compared to the available solar radiation incident on the absorber. The efficiency was taken from Duffie and Beckman (2013) and is given by η = G C p (T col exit T ambient )/I 2 Where G is the mass airflow per unit collector area, C p is the specific heat capacity of air, T col exit is the collector exit temperature, T ambient is the temperature of the surroundings and I is the incident solar radiation. RESULTS AND DISCUSSION Collector and dryer performance Typical variations of solar radiation, temperatures, efficiency, relative humidity and moisture content are shown in Figures 4 to 7. Figure 4 shows the variation of solar radiation, air temperature in the ambient, air exiting the collector, air exiting the drying tunnel and the cloth temperature during the solar drying of cabbage. Ambient air temperature varied from to C while that of the cloth absorber varied from to 71.7 C. The temperature of air exiting the collector varied from to 63.6 C and was therefore only a few degrees less than the cloth temperature. The difference between ambient air temperature and temperature of air exiting the collector in Figure 4 ranged from to C indicating very high heat gain. The heat exchange effectiveness calculated using Equation 1 ranged from 0.66 to 0.98 and this indicated a very high rate of heat transfer since the cloth and collector exit temperatures differed only by a few degrees, a range of 2.39 to 8.1 C. During this experiment, solar radiation was intermittently disrupted by cloudy cover and therefore fluctuated quite a lot. The temperatures of the cloth and air exiting the collector and the drying tunnel did not appear to have similar margins of fluctuations as solar radiation. The collector in this case behaved like a thermal storage, resulting in relatively stable air temperature that changed little despite fluctuations in solar radiation. Another reason for the stable collector exit air temperature could be due to the automatic regulation of airflow effected by changes in solar intensity vis-à-vis PV output and fan speed and output. This observation has also been reported by Mastekbayeva et al. (1988). The temperature variations in Figure 4 show that the difference between the temperature at collector exit and tray exit (drying tunnel

4 Afr. J. Agric. Sci. Technol. 583 Figure 4. Changes in solar radiation, temperatures of cloth, ambient air, air at collector exit and air at tray exit with time for a typical day (15 March 2007) during solar drying of cabbage. Figure 5. Efficiency against mass flow rate (experiment ran on 19 March 2007). Figure 6. Relative humidity in the dryer for a typical day (15 March 2007) during solar drying of cabbage.

5 Simate and Siwanzi 584 Figure 7. Moisture content changes with drying time (experiment run on March 2007). exit) ranged from C at the start of drying at 10:50 h to 0.68 C at the end of the first day of drying at 16:20 h. At the start of drying, the cooling effect due to high evaporation from the fresh material lowered the temperature of air in the drying tunnel. At the end of the day most of the water had been removed from the drying material and therefore very little evaporation and consequently cooling was taking place and the temperature of air in the drying tunnel approached that of the collector exit. This result is consistent with what Imre (1995) reported, that the temperature of air in the drying tunnel is approximately constant and equal to the temperature of air at collector exit if the demand for evaporation of water from the drying food is satisfied by direct radiation on the food. Figure 5 shows the graph of efficiency against the airflow, with minimum and maximum values of efficiency being 22.6 and 74.8% respectively, with most of the values being higher than 50%. The low efficiency figures could be attributed to the low solar intensity at the beginning of the day as this could have resulted in the low cloth temperature and airflow. The general trend was that higher mass airflows resulted in higher efficiencies. This trend is consistent with results reported by Badache et al. (2012) who explained that it is due to the heat transfer capacity depending directly on the mass flux, therefore higher air velocities through the perforations transfer more heat from the absorber plate to the air. The trend is also consistent with the results of Zomorrodian and Barat (2010) who concluded that the convective heat transfer coefficient between absorber plate and cooling air is lower at lower air flows which results in a higher absorber surface temperature, and ultimately a higher absorber surface temperature increases radiative and convective heat losses from the top surface of the absorber. The relative humidity of air in the ambient, at collector exit and at tray exit is shown in Figure 6. During the first hour of drying, the relative humidity of air at tray exit was higher than that in the ambient due to high evaporation from the fresh cabbage. However, due to reduced evaporation from the cabbage as drying progressed, the relative humidity decreased to below the ambient value after 1.5 h and approached that of air at collector exit. With the relative humidity of air at tray exit being around 20%, the air coming out of the dryer still had a big capacity to carry more moisture. With the condition of air at tray exit of 20.75% relative humidity and C air temperature, the absolute humidity of air was kg water/ kg dry air, giving a water holding capacity of kg water/kg dry air before reaching the saturation point of kg water/kg dry air. This could indicate that the drying tunnel length could be made longer to take more material and utilise the full water holding capacity of the air. Alternatively, the collector could be made shorter to supply just enough heat to the drying tunnel. These adjustments to the dryer could be made by optimization. Simate (2003) carried out an optimisation of a mixedmode solar dryer which gave a shorter collector length for the mixed-mode compared to the indirect-mode dryer of the same capacity. To optimise this tunnel dryer however, would require correlations of airflow which is dependent on the incident solar radiation via the PV output and fan speed. Moisture content change with time Figure 7 shows the variation of moisture content of cabbage at various locations in the dryer with drying time. Sample 3 had the highest drying rate followed by samples 2 and 4, and then samples 1, 5 and 6.

6 Afr. J. Agric. Sci. Technol. 585 Samples 2, 3 and 4 were the closest to the fans and therefore received higher temperatures and airflow that translated into faster drying than the rest of the samples. On average though, by the end of eight hours of drying, all the samples had reached a moisture content of less than 10% (w.b.) which was suitable for storage. Conclusion A solar tunnel dryer of the Hohenheim-type, utilising a cloth absorber was constructed and tested. A mass of 9.3 kg fresh cabbage with a moisture content of 94% (w.b.) was dried to 10% (w.b.) in 8 h. The cloth absorber was been found to have very high heat transfer rate, with the temperature of air leaving the collector was only a few degrees below the cloth temperature and to C higher than ambient. The collector efficiency was in the range of 22.6 to 74.8% with most of the values being above 50% and therefore consistent with findings from other researchers. The dryer could be optimised if the correlations of airflow with solar radiation were developed. Kutscher CF (1994). Heat exchanger effectiveness and pressure drop for air flow through perforated plates, with and without crosswind. ASME Journal of Heat Transfer. 116, Leon MA and Kumar S (2007). Mathematical modeling and thermal performance analysis of unglazed transpired solar collectors, Solar Energy 81, Mastekbayeva GA, Leon MA and Kumar S (1988). Performance evaluation of a solar tunnel dryer for chilli drying. Paper presented at the ASEAN Seminar and Workshop on Drying Technology, 3-5 June, Phitsanulok, Thailand. Available from: < SOLAR%20DRYER-pitsanulok> [4 March 2006]. Schirmer P, Janjai S, Esper A, Smitabhindu R, Mühlbauer W (1996). Experimental investigation of the performance of the solar tunnel dryer for drying bananas. Renewable Energy. 7(2): Simate IN (2003). Optimisation of mixed-mode and indirect-mode natural convection solar dryers. Renewable Energy. 28(3): Woods JL, Mutuli DA, Kamanga MH (1994). The performance of an infiltrating solar collector. Renewable Energy. 1: Zomorrodian A and Barat M (2010). Efficient Solar Air Heater with Perforated Absorber for Crop Drying. J. Agri. Sci. Tech. 12: REFERENCES Axtell B (2002). Drying food for profit: A guide for small businesses. ITDG Publishing, London. Pp Badache M, Rousse D, Hélla S, Quesada G, Dutil Y (2012). Experimental and two-dimensional numerical simulation of an unglazed transpired solar air collector. Energy Procedia. 30: Barbosa-Canovas GV, Ma L, Barletta B (1997). Food Engineering Laboratory Manual, Technomic Publishing Company, Inc USA. Christensen C (1998). Transpired collectors (Solar preheaters for outdoor ventilation air). United States Department of Energy. Available from: < [15 March, 2008]. Duffie JA, Beckman WA (2013). Solar Engineering of Thermal Processes, John Wiley & Sons, Inc., Hoboken, New Jersey, Fourth Edition. Pp Garba MM, Atiku AT, Sambo AS (1990). Comparative studies of some passive solar dryers. In: Sayigh A.A.M. (Ed.), Proceedings of First World Renewable Energy Congress. 2: Hossain MA, Bala BK (2007). Drying of hot chilli using solar tunnel dryer. Solar Energy. 81: Imre L (1995). Solar drying. In: Mujumdar AS (Ed.), Handbook of Industrial Drying, Marcel Dekker Inc, New York. 1: INNOTECH Tunneltrockner. Solar Tunnel Dryer "Hohenheim" Available from: < [7 March 2006].