Alternative Solar Dryer

Transcription

1 Alternative Solar Dryer Luiz Guilherme M. de Souza 1, Tiago S. da Silva 2, Luiz Guilherme V. M. de Souza 3, Amanda G. Cavalcante 4, Edinésio J. da Silva 5, Jailton W. Gomes Federal University of Rio Grande do Norte, Brazil 6 Federal Institute of Bahia, Brazil ABSTRACT It is presented a solar dryer by direct exposure for the production of dried fruit manufactured from scrap 200 liter polyethylene drums used. It will be described in some aspects of construction and assembly of the proposed solar dryer which has a main feature low cost. Its use is intended for processing widely available in our region fruits (mango, banana, guava, cashew, pineapple, tomato and others) in dried fruits and flour, increasing the shelf life of such foods. Such procedure will contribute significantly to alleviate the problems of hunger and misery of the poor population, since dried fruits and flour can be sold, generating employment and income. The results of tests with various kinds of tropical fruits that diagnose the viability of using such solar dryer are presented. The measured times for drying fruit with the proposed dryer are compared with those obtained for other solar dryers. KEYWORDS solar energy; solar dryer; tropical fruits; low cost. I. INTRODUCTION For thousands of years people have sun-dried fruit and vegetables to preserve for leaner times. New technologies have brought changed techniques but, at present, the increasing demand for healthy and low-cost natural foods and the need for sustainable income are bringing solar drying to the fore as a useful alternative for surplus products. The great challenge of the globalized world is currently producing food for a population that continues to grow and reaches six and a half billion inhabitants. Since farmlands are shrinking, the supply of irrigation water is scarce and production technologies are no longer able to give more leaps in productivity, it is believed that in the next decades the food supply in the world will grow unless the population. Brazil is an exception in this panorama and stands as one of the forces producing the third millennium. Currently, Brazil is the third largest fruit producer in the world, behind China (157 million tons) and India (54 million). Of the 39 million tons of fruit produced, approximately 45% are orange. Secondly, there is the banana, with 6.5 million tons [1]. The Brazilian fruit market moves now over 10 billion dollars a year (only with fresh fruit) and generates products of medium and high added-value, considering the fruit exploitation for exportation and noble fruit. Brazil's climate permits the production of all kinds of tropical fruit and some provide more than a crop per year. Because of the undeniable need to increase production and exportation of fruit, it becomes necessary to reduce the losses that occur throughout the production chain. In developing countries the losses are estimated in 50% for some products. This fact highlights the urgent need for simple and inexpensive procedures that can offer ways to maintain these highly perishable foods [2]. In recent years the solar drying has been growing through optimization process and cost reduction, causing a dynamic increase in the market of dried fruit. The emergence of other dried fruit such as banana, apple, papaya, pineapple and mango, even in small amounts, reinforces the assertion that the market is growing [3]. To combat this serious problem of wasting food, drying processes could be used to minimize the serious social problems associated with hunger and poverty. The use of solar energy is indispensable for being clean and widely available throughout Brazil (mainly in the Northeast region with the highest levels of social inequality), for having great potential, and for being easily manufactured. This paper presents a direct exposure, dryer working in a natural convection regime. It will be focused on manufacturing processes and assembly of the proposed dryer, manufactured from a scrap waste drum. The results of drying fruits, tests are presented, demonstrating the feasibility of using such dryer. The use of this type of dryer can contribute to a progressive increase in solar drying as a way of generating employment and income. Page 158

2 II. MATERIALS AND METHODS The foods chosen for testing solar dryers were pineapple, banana, papaya, mango and pepper. The pineapple, scientific name Ananascomosus, belongs to the Bromeliad family. Pineapple mainly contains potassium, and magnesium and calcium, as well as A, B1 and C vitamins. The Pacovan Banana is the largest known banana, reaching a 500g weight each fruit and having a length of 30cm. The banana is an herbaceous plant of the Musáceas family, originally from Africa, rich in A and C vitamins, fiber and potassium, and potential reduction of stress. The papaya is the fruit of the papaya plant (Caricáceas family). It is considered one of the most nutritious fruits. Rich in A, C and B complex vitamins, source of minerals like calcium, potassium and magnesium, it also has papain, a special enzyme that digests proteins, aiding in the absorption of nutrients and the proper functioning of the digestive tract. The mango tree, or hose as it is known, belong to the Anacardiaceae family and is grown in about five hundred varieties. Native of India, brought from Goa by the Portuguese, the mango is highly nutritious, rich in fiber, protein, minerals and vitamins (A, B, C). The term pepper refers to a group of cultivars of the Capsicum annum species. This kind of fruit can be found in different colors, the best known being the green, yellow and red pepper. The pepper is a much appreciated food, being rich in vitamins and minerals. In Figure 1 shows the chosen foods for drying tests. Fig. 1 Chosen food for drying tests. The direct exposure, dryer of the proposed drying system was constructed from a 200 liter polyethylene drum scrap, used for water storage or garbage disposal. The solar dryer has an inclination of 15.5 S and its internal area has 0.8 m². Figure 2 shows the various manufacturing and assembly operations for the manufactured direct exposure solar dryer. Fig. 2 Manufacturing and assembly operations for the built solar dryer. Page 159

3 The drying system was tested with three samples of fruits under natural convection. The collected data were solar radiation, relative humidity and initial and final temperatures of three spots located near the entrance, in the middle and near the exit the dryer. It was also measured the initial and final masses of each output load and the drying times for each type of dried product. To determine the thermal efficiency of the proposed solar dryer, the velocity at the exit of the dryer was measured, and by this parameter, the flow rate of circulating air was calculated. It was made a tunnel for a more accurate measurement of velocity and to this was used as an anemometer with read range of 0-20 m/s resolution of 0.1 m/s and an accuracy of ± 4.0%. The thermal efficiency can be determined using Equations 1 and 2. Also, the temperatures of the external surfaces of the dryer were measured to diagnose the level of heat loss, since the dryer was not built using insulation, focusing on the simplicity of manufacturing and technology transfer for poor communities. This initiative facilitates the use of the dryer in order to increase the shelf life of perishable foods and for generating employment and income. The necessary data for the study of thermal efficiency of the dryer were collected from a meteorological center installed in LMHES/ UFRN. (1) (2) Where: t = Thermal efficiency (%); Pu = power delivered by the system (W); A = dryer area (m²); I = global solar radiation (W/m²); m = mass flow (kg/s); cp = specific heat of the fluid (KJ/kg. C); ΔT = temperature difference obtained from the fluid system ( C). The efficiency of the drying process can be determined by the mass of product to be dried. The mass of the liquid is a function of the percentage of moisture of each product in natural. Equation (3) shows the parameters needed for such determinations. p ( m m inicial m água final ) (3) p Where: = process efficiency (%); m = mass (kg). III. RESULTS ANALYSIS Tests were conducted with the drying system in direct exposure and natural convection drying pineapple, banana, papaya, mango and pepper. The fruits were washed, peeled, if necessary, and cut. For each test three samples with equal masses were selected to be used as process control. Sample 1 is placed near the entrance of the dryer, sample 2 in the middle, and sample 3 near the exit of the dryer. Table 1 shows the results of the drying fruits, tests. Pineapples and peppers were cut transversely into slices of 1,0 cm and 0.5 cm respectively. The papayas and mangoes were sliced lengthwise with a width of 1.5 cm. The bananas were cut longitudinally into two parts. Figure 3 shows the foods in the drying process and dried with the proposed dryer solar. Page 160

4 TABLE I RESULTS OF DRYING PROCESS USING THE SOLAR DRYE. Product Minitial Mfinal Mdryf Drying time (kg) (kg) (kg) (h) Pineapple Banana Papaya Mango Pepper The mass losses of the total loads of the tested fruit, pineapple, banana, papaya, mango and pepper were respectively 79.4%, 64.3%, 82.1%, 76.1% and 89.6%, with residual wet mass of 7.6%, 15.0%, 5.6%, 3.6% and 10%, demonstrating the viability of the dryer to these fruits, since the residual wet mass was well below what indicates the literature for these products, between 15 and 20%. Drying times are obtained in three days of drying were competitive in relation to other dryers have been tested in LMHES / UFRN and other literature cited in the solar food dehydration and even lower for some fruits. In the samples, it was noted that the hourly mass suffered variation, and therefore not reached the saturation of air in the solar dryer. Table 2 shows the variations of mass in the samples. Product Pineapple Banana Papaya Mango Pepper TABLE III MASS VARIATION IN THE FRUIT SAMPLES. Sample Mlost Mwet i Mwet f (g) (g) (g) Mwf/Mwi I 40,1 44,72 4,62 0,10 II 42,4 44,72 2,32 0,05 III 42,0 44,72 2,72 0,06 I 16,0 20,02 4,02 0,20 II 15,6 20,02 4,42 0,22 III 14,8 20,02 5,22 0,26 I 30,0 31,32 1,32 0,04 II 30,0 31,32 1,32 0,04 III 29,4 31,32 1,92 0,06 I 23,2 27,20 4,00 0,15 II 22,2 25,20 3,00 0,12 III 21,2 22,90 1,75 0,08 I 4,7 4,7 0 0 II 4,7 4,7 0 0 III 4,7 4,7 0 0 Of all the fruits tested the pepper was the only one to be dried in just two days and the only one which has not showed residual moisture after the drying process. The average residual wet mass in the samples has been usually below the total load. The average meteorological parameters measured during the tests are shown in Table 3. TABLE IIIII AVERAGE PARAMETERS DURING THE TESTS. Parameter Pineapple Banana Papaya Mango Pepper Tamb ( C) 30,57 29,54 30,54 29,10 30,87 U (%) 62,76 66,93 65,86 69,67 68,14 I (W/m2) 815,76 809,93 845,75 762,43 760,67 For all the tested products, the average environmental temperature was around 31,0 C, average relative humidity of 65.3% and solar radiation of W/m², favorable conditions for the solar drying process. Such conditions significantly contributed to the efficiency of the drying process obtained with the proposed dryer. Figure 3 shows the evolution of the drying fruits drying process. Page 161

5 Fig. 3 Evolution of the tested fruits drying process. The efficiency of the drying process for each fruit tested in relation to the mass of the sample 2, located in the middle of the dryer, can be seen in the graph of Figure 4. This parameter was measured in every five hours of testing, in order to verify the performance of the proposed system during the drying process. Also, the masses of samples 2 are compared between the fruits in order to check for which fruit, taking into account their characteristics, the prototype has a better performance. Fig. 4 Average efficiency of the process at every five hours. For five hours of drying, the fruit with the larger mass loss was the pepper, much higher than the obtained for the other fruits. After ten hours of drying, the most efficient drying process was also for the pepper that was almost completely dry. After fifteen hours of drying pepper and banana were already dry and the pineapple, papaya and mango at the end of the drying process. The graph in Figure 5 shows the behavior pattern of the thermal performance of the tested direct exposure natural convection solar dryer. Fig. 5 Average thermal efficiency of the process. Page 162

6 Despite good performance of the drying process to the dryer proposed obtained, the average thermal efficiency of the dryer was very low, which is expected for a system for drying natural convection because the current flow is extremely low. Nevertheless, low thermal efficiency, all the fruits was dried within a few days, sometimes even lower than those reported in literature for the dehydrated solar food. Considering the masses of the samples obtained after drying was calculated the efficiency total the drying process shown in the graph of Figure 6. Fig. 6 The total efficiency of the drying process of fruits tested. The test results demonstrated the feasibility of using the built dryer. Other tests should be conducted with other fruits and larger loads. Drying times, the end wet mass, and mass loss for the tested products were consistent and competitive with the solar literature for dehydration of these foods. Another positive factor in the presented drying system is its low cost and may contribute to the socialization of solar drying as a means of generating employment and income. IV. CONCLUSIONS According to the objectives set, the conclusions and suggestions for the next experiments to be developed using the proposed solar dryer are presented. The proposed dryer was feasible to dry all the tested fruits; The manufacturing cost of the proposed prototype is low, providing a competitive cost/benefit in relation to other available conventional dryers; The processes of manufacture and assembly of the proposed dryer are simple and can be easily transferred; The drying times obtained for the tested products were competitive with those present in the literature for solar food dehydration; The products obtained in the drying had their shelf life considerably increased in relation to the fresh products while maintaining its good appearance, taste and integrity; The process of grinding using a residential blender for processing the dried fruits in dry flour, as was done with the pepper is simple, economical and provides the attainment of a good quality product; Despite the low thermal efficiency of the dryer, under 10%, the drying process proved itself to be feasible; For five hours of drying, the product with the best drying process efficiency was the pepper and the one with the worst efficiency was the banana. Evaluate the drying capacity of other kinds of food, like fish, shrimp, and other meats, and their drying efficiency with the proposed solar dryer; Adding to the system a layer of insulation to minimize heat exchange with the environment and carry out a new assessment of the prototype; Do a nutritional analysis of the dried fruits and of the produced flours. Page 163

7 REFERENCES [1] MAPA, Ministério da Agricultura, Pecuária e Abastecimento, Brasília, [2] J.B.S. Costa, Preparation and characterization of cashew flour by using a solar drying low cost, Master's Dissertation of the Graduate Program in Mechanical Engineering, UFRN, Natal, RN, [3] A.R.S. Costa, Solar drying system for drying fruits and modeling of banana in a drier static column, Doctoral Thesis Course Graduate in Chemical Engineering, UFRN, Natal, RN, [4] J.A. Duffie, and W.A. Beckman, Solar Engineering of Thermal Processes, Second Edition, New York, John&Sons, & Beckman, [5] H. J. L. Neto, Getting dried tomatoes through the use of an alternative low cost solar system, Master's Dissertation Program Graduate in Mechanical Engineering, UFRN, Natal, [6] A. Bejan, Convection Heat Transfer, John Wiley & Sons, New York, [7] A. S. Mujumdar, Handbook of industrial drying, Boca Raton, FL, USA: Taylor & Francis Inc., 2006 [8] C. Ratti, Advances in food dehydration, 1st ed., UK: CRC/Taylor & Francis; [9] V.P. Sethi, and S. Arora, Improvement in greenhouse solar drying using inclined north wall reflection, Solar Energy 83, Page 164