Performance Evaluation of a Mixed-Mode Tomato Solar Dryer and Modeling of the Dying Process

Transcription

1 Nigerian Journal of Solar Energy, Vol. 27, Solar Energy Society of Nigeria (SESN) All rights reserved. Performance Evaluation of a Mixed-Mode Tomato Solar Dryer and Modeling of the Dying Process 1 Abdulrahman, M.B., 2 Yahaya D.B. *3 Rikoto I.I. and 2 Adamu A.A. 1 Department of Mechanical Engineering Federal Polytechnic, Mubi Adamawa State 2 Department of Mechanical Engineering Bayero University, Kano 3 Sokoto Energy Research Centre, Usmanu Danfodiyo University, Sokoto Abstract - This work presents the result of performance evaluation and modeling of the drying process of a mixed-mode solar dryer. The mixed-mode solar dryer was tested at the Bayero University, Kano Nigeria (latitude 12.1 o N). The three tests carried out were, No loading test (stagnation test), loading test and the open sun test. The temperature rise inside the collector for No load test was 94 o C by 1300hrs at solar radiation of 1125W/m 2 on the 6th April, 2013, while temperatures of 84 o C and 49 o C were obtained inside the dryer chamber during the loading test at 1300hrs and 1700hrs respectively. The 95% moisture content of tomato was reduced to 10% in 16hrs of solar drying compared to open sun drying that took about 36hrs. The rapid rate of drying in the solar dryer revealed its ability to dry slice tomato rapidly to safe moisture level. Regression analysis of the moisture content of the tomato samples on wet basis were carried out using three models namely; Newtonian, quadratic and Authors approximation. The models were compared using their coefficient of determination (R 2 ). Regression models and statistical analysis of the results have shown that the author s approximation model has a better fit to experimental drying data of tomato with coefficient of determination R 2 of compared to other models. The results indicate that solar dryer had a significant higher temperature than open air drying. Drying of tomato in a mixed mode solar dryer leads to considerable reduction in the drying time as compare to open air drying. Keywords: Performance evaluation, mixed-mode solar dryer, modeling, open air drying, moisture content 1. INTRODUCTION One of the millennium development goals is the eradication of hunger and poverty. The application of solar dryers in developing countries can reduce post harvest losses and significantly contribute to availability of food in these countries. Estimations of these losses are generally cited to be of order of 40 percent, but they can under a very adverse condition be nearly 80 percent. A significant percentage of these losses are related to improper and/or untimely drying of food stuff such as cereals, grains, pulses, tubers, meat and fish (Gatea, 2010). In developing countries like Nigeria, agriculture is a major source of employment and income to rural farmers. It offers great opportunities to stimulate economic growth. Capitalizing on these opportunities requires modification of agricultural processing systems and application of sustainable energy technologies. Solar drying is an excellent way to preserve food. Drying crops by solar energy is of great economic importance especially where most of the crops and grain harvests are lost to fungal and microbial attacks (Gatea, 2011). These wastages could be easily prevented by proper drying, which enhances storage of crops and grains over long periods of time. If solar drying of produce were widely implemented, significant savings to farmers would be *Corresponding author Tel: mobash2007@gmail.com 8 achieved and these savings could help strengthen the economic situation as well as change the nutritional condition of numerous developing countries. Considerable research and development activities have taken place to identify reliable and economically feasible alternative clean energy sources. The choices for the alternate energy sources are: energy from sun, wave, wind and geothermal etc. Solar energy being the most promising could provide an alternative means of preserving perishable crops. Open air drying or natural sun drying has several disadvantages such as spoilage of product due to adverse climatic condition due to rain, wind, moist, dust, storm etc. Loss of material to birds and animals, deterioration of the material by decomposition, insects and fungus growth in open air drying is quite common. The open air drying process is labor intensive, time consuming and requires large area (Parikh and Agrawal, 2011). Nouhou et al, (2005) studied on the passive solar drying of tomato halves. The study was based on the continuous drying of tomato halves using the open air drying layout, rock bed collector solar dryer, steel plate absorber solar dryer and the chimney type absorber solar dryer. The drying experiments were conducted at ambient temperatures between C and C, relative humidity between 32% and 76%, wind speed between 1.5 and 4.5m/s and insolation ranging from 100 to 1080W/m 2 for six consecutive days at Sokoto Energy Research centre. Samples of 1kg each of sliced tomato of average

2 thickness 1.5cm were spread on the respective drying racks. An exponential model was fitted into the drying process with coefficient of determination ranging between 74.4% and 99.2%. The moisture content of the dry sample was found to vary between 6.5% and 97%. Eze and Ekechukwu (1999) studied how tomato preservation can be improved using solar dryer. The study was carried out at the National Center for Energy Research and Development, University of Nigeria, Nsukka, Nigeria. The results obtained from the experiments indicate that tomatoes can be successfully dried at temperatures attainable within the solar dryer with a mean wind velocity 2.5m/s. The moisture content, water activity and microbial load significantly reduced while vitamin C content increased during drying, concluded by recommending that rather than advising the Nigeria rural farmers to apply sodium metabisulphite as a preservation in tomatoes processing, they should be encouraged to use salt is very common. Mathematical modeling and computer simulation of the drying of agricultural products are now widely used in agricultural engineering research. A simulation model is a powerful tool for predicting performance and can help designers to optimize the dryer geometry at various operating conditions without having to test the performance of the dryer experimentally at each condition (Bala and Woods, 1994). When one visits land of the southern zone of Kano state i.e. Kura, Rano, Garin-Mallam and Bunkure local government areas, will observe large hectares of land been cultivated with tomatoes by peasant farmer in Kadawa and kwonar Gafan. The farmers around these villages are predominantly involved in tomato farming between December and April of every year. These farmers, during the post-harvest period, after first and second picking of the tomatoes, there will be tomatoes in excess which will result in excess losses due to lack of storage, preserving and processing facilities. The only way out is to slice and spread on the ground/soil which is unhygienic and laborious, coupled with low quality of the product. During interview with some farmers at Kadawa lamented lack of preservation techniques in the area is their major constrain. They lose a lot of their tomatoes due lack of storage and preservation facilities. A basket of tomatoes which sold N2500 at the first picking will be sold at N 250-N350 in the peak period of cultivation. These problems prompted the authors to think of a way out for the peasant farmers. It is in line with these above observations that there was need to carry out the design, construction and testing of the passive mixed-mode solar dryer with the strongly believed that its impact will be felt in the above communities. The objective of this research was to carry out performance analysis and establish the best fitting model for the drying process of tomato using the mixed mode solar dryer, designed and constructed at the Department of Mechanical Engineering Bayero 9 University Kano. The regression models used are: Newtonian, Quadratic and Authors approximation. The drying regime is on wet basis. The coefficients of determination of the various samples are to be determined using these models. The equations for the drying process on wet regime are given by; (1) Where, =Moisture content (%), Mass of the sample before drying (g), Mass of the sample after drying (g) 2. MATERIALS AND METHODS 2.1 Mixed-Mode Solar Dryer The mixed-mode solar dryer used in this experiment for drying of tomato slices was developed and tested at the Department of Mechanical Engineering, Bayero University, Kano (latitude 12.1 o N), in the month of April, 2013.The tests were done so as to determine the performance of the solar dryer by monitoring the moisture loss and the drying rate of the samples placed on the tray of the drying chamber. The dryer consists of three main sections coupled together. These are: solar collector, drying chamber and inlet and outlet vents. Figure 1 shows the section of the mixed-mode solar dryer. The thin drying models that are reported in the literature were used separately to fit the mixed-mode solar dryer and open air experimental data for tomato slices to determine the best curve fitting. The three models used were Newtonian, quadratic and author s approximation. The performance evaluation was carried out using three tests namely; No load testing (stagnation test), Load testing of the solar dryer using tomato slices and open air drying test. The first test with no load on the drying trays was conducted to determine the maximum stagnation temperature of the dryer. This is done in order to know if there was sufficient temperature difference between the solar collector and the ambient, which was required for effective drying through the dryer. Measurements at an interval of one hour were taken at various points between the hours of 0800hours and 1700hours.The second experiment was conducted with load. Equal weights of tomato samples were used for comparison of both solar dryer and open air drying. The slice tomato products were loaded at 0800hours and drying took about an hour to achieve steady state of conditions. Hourly readings were taken from 0900hours to 0500hours to determine the hourly moisture loss, tomato weight loss and hourly drying rate (H.D.R) for both solar and open air drying. Also signs for dryness were checked within the intervals, at the end of day the products were packed in a polythene bag and product weight was recorded. The product drying time for open air drying and in the mixed- mode solar drying were also studied simultaneously.

3 velocity (m/s), Air flow rate (m 3 /s) and Weight change of the tomato (Kg/h). Performance evaluation test for mixed-mode solar dryer in tomato drying were performed between 6 th and 11 th /04/2013. Plate 1 show the constructed solar dryer used in the performance evaluation. Fig. 1 sectional view of the mixed-mode solar dryer 2.2 Experimental Procedure: Fresh and uniform sized ripe tomatoes were purchased from student village market in the new campus of Bayero University, Kano. The tomatoes were cut into halves with a sharp knife and then placed in a single layer on the drying trays with the cut side up in the dryer. A set of three experimental tests were conducted during the period of April (6 th, 8 & 9 th, 10 &11 th ), 2013.The experiments commence by 0800Hours and continued until 1700hours. The drying was continued on subsequent days until the desired moisture content (about 10% Wb) was reached. Weight loss of both the samples in the solar dryer and the control sample in the open sun were measured during the drying period at one hour intervals with an electric/digital balance (Mettle PM6100). Mercury in-glass thermometers was used to record the ambient air, collector air, drying air on the tray (drying chamber), inlet air, and outlet air temperatures. Relative humidity was determined by using Digital humidity/temperature and clock meter (CTH 288) and a microprocessor digital meter HT-6290 Humidity/temperature meter at one hour interval during the drying period. A solar Meter SL 100 was used to measure the global solar radiation. The velocity of drying air was measured with an anemometer TA-5, Airflow. The moisture content of the tomato sample was measured by drying the sample in an air ventilated oven at 65 0 C in Agronomy Department Bayero University Kano and it was found to be 95.6 %( wet bases). The following parameters were recorded hourly during drying processes: Ambient air temperature, relative humidity (R.H), Dryer Inlet and outlet air temperatures and respective relatives humidity (RH), Solar collector temperature and its glass cover temperature, Drying chamber temperature, and its glass cover temperature, Global solar radiation (W/m 2 ), Air 10 Plate 1: Front view of the designed and constructed mixed mode solar dryer 2.3 Mathematical Modeling of Solar Drying Curves The most widely investigated theoretical model in the drying of different foods is given by the solution of Fick s second law (Sacilik et al., 2005). Fick s law is often used to describe a moisture diffusion process, (2) Where m is the local moisture content on a dry basis, t the time, x the spatial coordinate and D eff is the effective diffusivity in m 2 /s. To apply Fick s law, the food product is usually assumed to be unidirectional, to have uniform initial moisture content and to have internal moisture movement as its main resistance to moisture transfer. The solution of Fick s law for a slab is as follows (Gökhan et al, 2009); (3) where M t, M e, M o are the moisture content at time t, the equilibrium content and the initial moisture content, respectively, H is the half-thickness of the slab in tomato sample. For long drying times (setting n=1), Equation (9) can be further simplified to a straight line equation as; (4) Thus, the moisture ratio (MR) was simplified to M t /M o as written in the following form: (5) ---

4 Where; M o is sample mass at the beginning, M e is equilibrium moisture content and M t is sample mass in time t. The effective diffusion coefficients are typically determined by plotting experimental drying data in terms of ln(m t /M 0 ) versus time (Gökhan et al, 2009) The values of effective diffusivity in the range of m 2 /s can be found in literature. sufficient temperature difference between ambient temperatures and collector temperature. This indicates prospect for better performance than the open air sun drying. 2.4 Method of Linear Regression Analysis Three models were used in the curve fitting of the drying of 2kg mass tomato samples. All the three models have time as independent variable and mass as dependent variable. The regression models were shown in Table 1 Table 1: Thin-layer drying models Model Model name Model Reference No. equation 1 Newtonian (Saeed et al,2008) 2 Quadratic Fig. 2: V ariation of temperatures of inlet, outlet air of dryer, collector, ambient, and solar radiation with time during the stagnation test on April 6, Authors approximation) *where, ᾳ is the drying constant and a, b, c, d are equation constants and t is the drying time in hour and M o = mass of tomato sample at the beginning of drying. 3. RESULTS AND DISCUSSION 3.1 Solar Collector Performance From the results of the experiments, the diurnal variation of temperatures of the solar collector inlet air, outlet air, collector air, ambient air, and solar radiation were plotted. One typical day of April is shown in Figure 2. It is observed that the rise in air temperature due to the generated air flow rate in the collector is sufficient for the purpose of most agricultural products drying (Gatea, 2011). For the no load test, the maximum temperature recorded at the collector was 94 C, which corresponds to the maximum ambient temperature of 49.1 C and at a solar irradiance level of 1125 W/m 2, at 13:00hours. The daily solar radiation was relatively high with an average of W/m 2. For the average ambient air temperature of 41.6 C, average inlet air temperature of 47.2 C, the average air temperature at the collector at no load conditions was recorded as 75.4 C. It was observed that during the stagnation test there was no significant difference between collector temperature and drying chamber (tray). This was because the heated air from the collector did not encounter any moisture in the drying chamber. Temperature rise of air was o C during the stagnation test, which shows that there was Load testing using solar collector Two equal samples of about 2kg of fresh sliced tomato of 1.5cm thick were arranged on the drying bed in a single layer of the dryer as well as on the ground for sun drying, and the figure 3 and figure 4 were plotted using the readings obtained during the experiments. Average drying chamber temperature for load testing was found to be about 60 o C which is higher the value obtained by Lawrence et al., (2013). This value indicates more prospect of solar drying. Fig. 3 : Variation of air temperature in the dryer during drying of g fresh sliced tomato to g in the first day.

5 Fig.4: Variation in the dryer temperature during the 2 nd day of drying of g tomato To 97.77g (Final wieght after loss of moisture) 1100hours.The maximum wind speed was recorded at 1300hours with a value of 1.74m/s. The highest values of relative humidity were recorded in the morning at 0900hours with a decreasing trend until end of the experiment. This decrease in relative humidity was as result of increase in the temperature inside the dryer due increase in solar radiation (Bukola and Ayoola, 2008). The highest ambient temperature recorded was C at 1200hours. 2kg of tomato were spread for the open air drying as well as inside the solar dryer. At the end of the first day of drying as shown in the figures reveal that the mixed-mode solar dryer is the fastest as far as moisture removal is concerned. About g of water out of 2kg (69%) and 1132g of water (58%) was removed for solar drying and open air drying respectively. It took the mixed-mode solar dryer 16hours to dry tomato from 95% moisture content to 10% its safe moisture content level, The open air layout drying sample took three days before drying the produce completely to its safe moisture content. The shortest drying time obtained using mixed-mode solar dryer this was due to upward convection of air through the bed from the collector and the extra-energy received through the drying chamber. Food safety is the one of the most important properties governing the stability of the health of the consumer. Field evidence showed that the tomato that was dried using solar dryers does not present any sign of moldiness. This may be probably due to less exposure to microorganisms because of dryer chamber higher temperature as compared to ambient temperature. Similar experiments were repeated for other days that are 10 th and 11 th, April, Almost the same results were obtained with first experiment. Therefore the mixedmode dryer seems to provide most attractive solution. Fig.5: Hourly moisture loss for tomato from g g Fig. 3, 4 and 5 presented the test results for load test of drying tomato slices in the dryer as well as open air on 8 th and 9 th April, Figures 3 and 4 show the hourly variation of drying parameters when drying g of sliced tomato such as ambient temperature and its relative humidity, dryer inlet and outlet temperatures with their relative humidity, collector temperature and its glass temperature, drying chamber temperature and its glass temperature, air velocity, flow rate and solar radiation. While their corresponding hourly weight losses in solar dryer and open air shown in fig.5. It was observed in the first day (8/4/2013) was that highest solar radiation recorded between 1100 hours and 1300hours with average value of 1301w/m 2 recorded at Fig.6: Result of regression analyses for Newton, Quadratic and Cubic Models, for mixed-mode solar Drying on the 1 st day (8/4/2013) 12

6 Fig. 9: Result of regression analyses for Newton,Quadratic and Qubic Models, for open air drying on the 2 nd day (9/4/2013). Fig. 7: Result of regression analyses for Newton,Quadratic and Qubic Models, For mixedmode solar Drying on the 2 nd day (9/4/2013). Fig. 8: Result of regression analyses for Newton,Quadratic and Qubic Models, for open sun Drying on the 1 st day (8/4/2013). The drying data as the moisture content versus drying time were fitted to the drying models. Figure 5 to figure 8 present results of regression analyses carried out by using the EXCEL computer program and the variations of moisture content versus drying time for the three mathematical models. They also showed drying model constants and the coefficient of determination (R 2 ). The coefficient of determination (R 2 ) was one of the important criteria to select the best equation in the solar drying curves of the dried samples of tomato. According to the figures, the Authors approximation model showed good agreement with the experimental data and gave the best result for tomato samples as similarly observed by Nouhou et al., The highest value of R 2 can be used to describe the drying behavior of tomatoes in the range of the tested drying conditions. 4. CONCLUSION A mixed-mode solar dryer was designed, constructed and its performances for drying tomato have been investigated. Modeling of tomato drying process was carried out. The field experiments showed that the performance of the solar dryer was encouraging based on the temperature generated. The maximum drying temperature through the drying time at no load test was 94 o C. The moisture content of the fresh tomato dried in the dryer as found range from 94.9 to 96%. The drying time was 16 hours for the mixed-mode solar dryer, and about 36hours for the open sun drying. Regression Analyses of the moisture content of the tomato samples on wet basis were carried out with the three Models namely; Newtonian (Exponential), Quadratic, and Authors approximation (cubic). The models were compared using their coefficient of determination, of the regression models, according to statistical analysis results; the Author s approximation drying model has shown a better fit to experimental drying data of tomato with coefficient of determination R 2 of as compared to other models. REFERENCES Bala, B. K. and Woods, J.L. (1994). Simulation of the indirect natural convention solar drying of rough rice. Journal. of Solar Energy, 53:259. Bukola, O.B. and Ayoola, P.O. (2008). Performance evaluation of mixed mode solar dryer. AU.J.T, 11(4) :

7 Eze, J.I. and Ekechukwu, O.V. (1999). Improved studies of tomato preservation using a solar dryer. Nigerian Journal of Renewable Energy, 7(1&2): Gatea, A. A. (2010). Design, construction and performance evaluation of solar maize dryer. Journal of Agricultural Biotechnology and Sustainable Development, 2(3): Gatea, A. A. (2011). Performance evaluation of a mixedmode solar dryer for evaporating moisture in beans. Journal of Agricultural Biotechnology and Sustainable Development, 3(4): Gökhan, G., Necdet, Ö., Ali, G. (2009). Solar Tunnel Drying characteristics and Mathematical Modeling Tomato, Journal of Thermal Science and Technology 29 (1): Lawrence, D., Folayan C.O. and Pam, G.Y. (2013). Design, construction and performance evaluation of mixed mode solar dryer. International Journal of Engineering and Science (IJES) 2(8): Nouhou, I., Momoh, M. and Ganda Y.M, (2005). An experimental study of passive solar drying of an agricultural produce. Nigerian Journal of Renewable Energy, 15: Nouhou, I., Dauda, A., Nuhu, U. (2012). Drying behavior of Roselle (Hibiscus sabdariffa.l). Nigerian Journal of solar Energy, 23: Parikh, D. and Agrawal, G.D. (2011). Solar Drying In Hot and Dry Climate of Jaipur, India. International Journal of Renewable Energy Research, IJRER, 1(4): Sacilik, K., Keskin, R. & Elicin, A.K., (2006). Mathematical modeling of solar tunnel drying of thin layer organic tomato, Journal of Food Engineering., 73: Saeed, I. E., Sopian, K. and Zainol-Abidin, Z. (2008). Thin-Layer Drying of Roselle (I): Mathematical Modelling and Drying Experiments. Agricultural Engineering International: the CIGR E-journal. Manuscript FP Vol.X. 14