Simulation of solar drying of chilli in solar tunnel drier

Transcription

1 International Journal of Sustainable Energy Vol. 00, No. 00, Month 2005, 1 11 Simulation of solar drying of chilli in solar tunnel drier M. A. HOSSAIN*, J. L. WOODS and B. K. BALA FMP Engineering Division, Bangladesh Agricultural Research Institute, Gazipur 1701, Bangladesh School of Agriculture, Food and Rural Development, University of Newcastle upon Tyne, Newcastle upon Tyne, NE1 7RU, UK Department of Farm Power and Machinery, Bangladesh Agricultural University, Mymensingh 2202, Bangladesh (Received) This article presents modelling of solar tunnel drier by taking into account heat transfer in the collector and coupled heat and mass transfer within the drying unit. One set of equations was developed to predict cover temperature, absorber temperature and air temperature in the collector and another set of partial differential equations was developed to predict the air and chilli temperatures and moisture content for drying of green chilli in the solar tunnel drier. First set of equations was solved iteratively and the second set of equations was solved numerically on the basis of an exponential solution over the finite difference grid element using the outlet air conditions of the collector as inlet conditions of the drying unit. The simulated air temperatures in the collector and the drier agreed well with the observed air temperatures. Good agreement was also found between experimental and simulated moisture contents for drying of green chilli. Keywords: Collector; Green chilli; Simulation model; Solar drier 1. Introduction Chilli is an important spice and a potential cash crop in the world. It is dried for making chilli powder and for both short- and long-term storages. In Bangladesh, a large quantity of chilli is lost during the production season when the supply is abundant. Farmers do not get a proper return for their harvest during the peak period of harvest because of the low market price. There is an increasing interest in quality dried chilli for both the local market and the foreign market. In Bangladesh, chillies are traditionally sun-dried. In this method, drying rate is slow and cannot be controlled, and a low-quality dried product is obtained. Solar crop drying is environmentally friendly and economically viable in developing countries and a quality dried product can be produced using a solar drier (Mühlbauer 1986, Zaman and Bala 1989, Bala 1998). Solar drying systems must be properly designed in order to meet particular drying requirements of specific products and to give satisfactory performance. Designers should investigate *Corresponding author. mayubhossain@yahoo.com International Journal of Sustainable Energy ISSN print/issn X online 2005 Taylor & Francis DOI: / Techset Composition Ltd, Salisbury gsol tex Page#: 11 Printed: 24/8/2005

2 2 M. A. Hossain et al. the basic parameters such as dimensions, temperature, relative humidity, airflow rate and the characteristics of products to be dried. However, full-scale experiments for different products, drying seasons and system configurations are sometimes costly and not possible. The development of a simulation model is a valuable tool for predicting the performance of solar drying systems. Again, simulation of solar drying is essential to optimize the dimensions of solar drying systems and optimization technique can be used for optimal design of solar drying systems. Natural convection solar drier is low cost, can be locally constructed and does not require any power and energy from electrical grid or fossil fuels. However, the natural convection solar driers suffer from limitations due to extremely low buoyancy induced airflow inside the driers (Bala and Woods 1994, 1995). The high weather dependent risk and drying limitations due to extremely low buoyancy induced airflow of natural convection solar driers stimulated Mühlbauer and his associates at the Institute of Agricultural Engineering in the Tropics and Subtropics, University of Hohenheim to develop a solar tunnel drier in which a fan is providing the airflow required to remove the evaporated moisture. A photovoltaic module is required to operate the fan independent of electric grid. Numerous tests in regions of different climatic conditions have shown that fruits, vegetables, cereals, grain, legumes, oil seeds, spices and even fish and meat can be dried properly in the solar tunnel drier (El-Shiatry et al. 1991, Esper and Mühlbauer 1996, Bala 2000, Bala and Mondol 2001). Several studies have been reported on simulation of forced convection solar drying of agricultural products for different configurations of forced convection solar driers (Steinfeld and Segal 1986, Janjai et al. 1994, Bala et al. 1997, Ivanova and Andonor 2001, Hodali and Bougard 2001). No study on simulation of solar tunnel drier has been reported. The purpose of this study is to develop a simulation model for solar tunnel drier for drying of chilli to simulate temperature and moisture changes during drying to build up confidence in the model, and provide design data and also a simulation model for optimization. 2. Methodology 2.1 Solar tunnel drier A solar tunnel drier consists of a plastic-covered flat plate solar collector and a drying tunnel as shown in figure 1. The drier is arranged to supply hot air to the drying tunnel using two small fans powered by a photovoltaic module. The heated air passes over and under the products spread in a single layer in the drying chamber and thus moisture is evaporated and carried away from the products. 2.2 Analysis of collector performance Considering an element dx of the collector at a distance x from the inlet (figure 2), the energy balances on the collector components give the following equations Energy balances on the plastic cover. h cam (T c T am ) + h ca (T c T a ) + h rcs (T c T s ) h rpc (T p T c ) = α cs (1 + τ cs ρ ps )E (1)

3 Simulation of solar drying of chilli 3 Figure 1. Solar tunnel drier: 1, air inlet; 2, fan; 3, solar module; 4, solar collector; 5, side metal frame; 6, outlet of the collector; 7, wooden support; 8, plastic net; 9, roof structure for supporting the polyethylene cover; 10, base structure for supporting the drier; 11, rolling bar; 12, outlet of the drying tunnel Energy balances on the absorber plate. The bottom of the collector is provided with insulation, and heat loss through the bottom is neglected. The surface area of sides is small and heat loss through sides is negligible. The following equation gives the energy balances on the absorber plate. h pa (T p T a ) + h rpc (T p T c ) + h rps (T p T s ) = τ cs α ps E 1 (1 α ps )ρ ps (2) Energy balances on the air stream. Air flows inside the collector (between the cover and the absorber plate) along the length. Therefore, the following equation gives the Figure 2. Heat balances in the flat plate solar collector of depth, b.

4 4 M. A. Hossain et al. energy balances in the air inside the collector. bg a C pa dt a dx = h pa(t p T a ) + h ca (T c T a ) (3) Assume A = 2 h ca and B = T p + T c bg a C pa 2 Equation (3) can be solved to compute the rise in air temperature, T a, over the finite distance, x, as follows: T = T ai T ai 1 = (B T ai 1 )(1 e A x ) (4) The values of A and B are readily obtained by the iterative solution of equations (1) (4). The radiative heat transfer coefficients are given by Bala (1998) in the linearized form as: h rps = σ(t2 p + T s 2)(T p + T s ) (5) (1/ε p ) + (1/τ cl ) 1 h rpc = σ(t 2 p + T 2 c )(T p + T c ) (1/ε pl ) + (1/τ cl ) + (1/1 τ cl ) 2 h rcs = σ(t2 c + T s 2)(T c + T s ) (7) (1/ε cl ) + (1/τ cl ) 1 The convective heat transfer coefficient between the cover and the atmosphere is calculated from the correlation given by Sparrow et al. (1979). (6) Nu = 0.86Re 0.5 Pr 1/3 (8) For determining the convective heat transfer coefficient between the internal air stream and the cover or absorber, the following correlations are used (Knudsen and Katz 1958): for Re > 10,000, Nu = 0.023Re 0.8 Pr 0.4 for Re < 10,000 and Re Pr 2s/L > 70, Nu = 7.6 for Re < 10,000 and Re Pr 2s/L < 70, Nu = 1.85(Re P r 2s/L) 1/3 where the characteristic dimension is the hydraulic diameter of the drier. The sky temperature is taken as follows [ T s = T a (T ] dp 273) 1/4 (9) Analysis of solar tunnel drier performance The following equations are developed to describe the drying of a product in the solar tunnel drier. Consider an element dx of drying tunnel at a distance x from the inlet of the drying unit. The energy balances in the drier components are shown in figure Energy balances on the cover. Heat balance on plastic cover of drying tunnel is same as that on the cover of collector, and the temperature of plastic cover is T c = h camt am + h ca T a + h rcs T s + h rgc T g + α cs (1 + τ cs ρ gs )E h cam + h ca + h rcs + h rgc (10)

5 Simulation of solar drying of chilli 5 Figure 3. Energy balances in the solar tunnel drier of depth, b Energy balances on the product. The chilli in the solar tunnel drier is dried in a single layer thickness and heated air mainly flows above the chilli pods. The depth of the drier below the chilli bed is much smaller when compared with the depth above the chilli bed. Owing to low airflow, the temperature below the chilli bed will become higher first than the temperature above the chilli bed. Therefore, the airflow below the chilli bed will come to equilibrium with the chillies quite rapidly. It is therefore assumed that all airflow is above the chilli bed and the chillies to be standing on an impermeable adiabatic surface. It is also assumed that the tray is completely covered with chilli pods and there is no space between the adjacent pods. The following energy balance equation is developed for the drying of chilli in the solar tunnel drier as: T g t = [ ρ gz g (C v C l )( M/ t) + h ga + h rgc + h rgs ]T g ρ g z g (C pg + C pw M) + {α gsτ cs /1 (1 α gs )ρ gs }E + ρ g z g L g ( M/ t) + h ga T a + h rgc T c + h rgs T s ρ g z g (C pg + C pw M) (11) Assume, and m = [ ρ gz g (C v C l )( M/ t) + h ga + h rgc + h rgs ] ρ g z g (C pg + C pw M) Q1 n = (α gsτ cs /1 (1 α gs )ρ gs )E + ρ g z g L g ( M/ t) + h ga T a + h rgc T c + h rgs T s ρ g z g (C pg + C pw M) Therefore, T g = mt g + n (12) t Equation (12) can be solved to give the chilli temperature, T g2, over the time interval, t, as follows: T g2 = n ( m + T g1 n ) e m t (13) m

6 6 M. A. Hossain et al Energy balances of the air stream. Change in enthalpy of air is equal to the heat transferred convectively to the product and the heat supplied to air in the evaporated moisture. T a x = (h ca + h ga )T a ρ a z a V a (C pa + C pv H) + h ca T c + h ga T g ρ a z a V a (C pa + C pv H) (14) Assume, P = h ca + h ga ρ a z a V a (C pa + C pv H) and Q = h ca T c + h ga T g ρ a z a V a (C pa + C pv H) Therefore, T a x = PT g + Q (15) Equation (15) can be solved to give the air temperature, T a2, over the finite distance, x, as follows: T a2 = Q P + (T a1 Q P )e P x (16) 2.4 Drying rate equation The rate of change of moisture content of a thin layer product inside the drier can be expressed by an appropriate thin layer drying equation. The Page equation in the finite difference form can be expressed as (Bala 1983): where M = Ku t(m M e )(X + t) u 1 (17) X = [ ln (M M ] e/m o M e ) 1/u K Drying constant (K) and dynamic equilibrium moisture content (M e ) are obtained by Hossain and Bala (2002). 2.5 Mass balance equation Moisture lost by product is equal to moisture gained by air. The exchange of moisture between the product and the air inside the drier is given by ( ρ g dx M ) ( ) H dt = bg a dx dt (18) t x Change of humidity in the finite difference form can be written as ( )( ) ρg M H = x (19) bg a t 2.6 Solution procedure For a given incident solar radiation, ambient air temperature, relative humidity and air velocity, the mean cover temperature of the collector is computed using equation (1), assuming absorber plate temperature and air temperature inside the collector. Using this recent value of cover

7 Simulation of solar drying of chilli 7 temperature, the absorber plate temperature of the collector is calculated using equation (2). Using the recent value of cover temperature and absorber plate temperature, the air temperature inside the collector is determined using equation (4). The iteration process is continued until the solution is within the required accuracy. The chilli bed is divided into a number of sections (x = j x) along the length of the drier. The drying time is also divided into a number of intervals (t = i t). On the basis of the air temperature, relative humidity and airflow at the outlet of the collector or at the entry of the drier, the drying constant (K) and dynamic equilibrium moisture content (M e ) of the chilli are computed. Using these K and M e values, the changes of moisture content of chilli, M, for a time interval, t, are calculated using equation (17). Using the cover temperature, the absorber plate temperature and air temperature at the exit of the collector, the cover temperature of first section of the drier is calculated using equation (10). Using the recent value of cover temperature, air temperature and drying rate, the product temperature of first section of the drier is computed using equation (13). On the basis of the recent value of cover temperature and product temperature, the air temperature inside the first section of drier is estimated using equation (16). The change in air humidity is computed using equation (19). This process is repeated section-by-section until the end of the section is reached. This process is then repeated for each time increment. When air relative humidity exceeds 98%, the condensation routine deposits back the moisture from the over saturated air. Air and chilli temperatures are adjusted for this condensation (Bala 1983). The numerical solution was programmed in BASIC language. The radiation properties of plastic cover, absorber and chilli are taken from Bala and Woods (1994) and Anwar and Tiwari (2001). 3. Results and discussion The model was validated against the experimental data of green chilli. The details of the experiments are given in Hossain (2003). The observed and simulated air temperatures at the outlet of the collector at different times of a day are given in figure 4. Good agreement was found between observed and simulated air temperatures at the outlet of the collector. The observed and simulated air temperatures along the length of the collector are shown in figure 5. Figure 4. Observed and simulated temperatures at the outlet of collector at different times of a day.

8 8 M. A. Hossain et al. Figure 5. Simulated and observed temperatures along the length of the collector. Good agreement is found between observed and simulated air temperatures along the length of the collector. Figure 6 shows the comparison between observed and predicted air temperatures at the outlet end of the drier at different times of a typical day. Good agreement exits between observed and predicted air temperatures. The observed and simulated temperatures along the length of the drier are given in figure 7. It is observed from the figure that the predicted temperature at the outlet end of the drier is found to be little higher than the observed temperature. Figure 8 shows the experimental and simulated moisture contents during solar drying of green chilli in the solar tunnel drier. Good agreement was found between the experimental and simulated moisture contents. Figure 6. Observed and simulated temperatures at the exit from the drier at different times of a day.

9 Simulation of solar drying of chilli 9 Figure 7. Observed and simulated air temperatures the length of the drier. Figure 8. Experimental and simulated moisture contents in the solar tunnel drier during drying of green chilli (broken line indicates the end of a day). 3.1 Sensitivity analysis This model is found to be sensitive to solar radiation and air velocity, but not very sensitive to ambient temperature and relative humidity. No significant effect has been found for heat transfer coefficients in the prediction of air temperature and chilli moisture content. Bulk density has significant effect on air temperature and chilli moisture content. The time step used in this model is t = 60 s, as numerical solution is found to be insensitive for t 600 s. The position step used in this model is x = 1 cm, as numerical solution is found to be insensitive for 0.01 x 10 cm. 4. Conclusions The simulated air temperature at outlet of the collector and air temperature along the length of the collector agreed well with the observed data. The simulated air temperature in the drier

10 10 M. A. Hossain et al. was found to be almost constant along the length of the drier. The predicted air temperatures in the collector and drier agreed well with the observed air temperatures. Good agreement was found between experimental and simulated moisture contents of green chilli during drying. This model can be used for optimization and for providing design data of a solar tunnel drier. Nomenclature A, B constants A c area of collector (m 2 ) b depth of collector/drier (m) C p specific heat (kj/kg K) db dry basis E solar radiation (W/m 2 ) G a mass flowrate of air (kg/m 2 s) h c convective heat transfer coefficient (W/m 2 K) h r radiative heat transfer coefficient (W/m 2 K) H humidity ratio (kg/kg) i integer j integer k thermal conductivity (kj/m 2 s) K drying constant (min 1 ) L length of collector or drier (m) L g latent heat of chilli (kj/kg) L w latent heat of water (kj/kg) m, n integer M moisture content (db or wb, %) M e equilibrium moisture content (db, %) Nu Nusselt number P, Q integer Pr Prandtl number Re Reynolds number t time (min) T temperature ( C) T db dry bulb temperature ( C) T dp dew point temperature ( C) T wb wet bulb temperature ( C) u exponent of Page equation V air velocity (m/s) x space coordinate z thickness (m) Subscripts a air am ambient c cover g chilli l liquid

11 Simulation of solar drying of chilli 11 L p s S v w long wave absorber plate sky short wave water vapour water Greek letters α absorbance ε emittance ρ density (kg/m 3 ) ρ reflectance σ Stefan Boltzman constant (W/m 2 K 4 ) τ transmittance References Anwar, S.I. and Tiwari, G.N., Energy Convers. Manage., 2001, 42(5), 627. Bala, B.K., PhD thesis, University of Newcastle upon Tyne, UK, Bala, B.K. and Woods, J.L., Solar Energy, 1994, 53(3), 259. Bala, B.K. and Woods, J.L., Energy, 1995, 20(4), 285. Bala, B.K., Esper, A. and Mühlbauer, W., J. Energy Heat Mass Trans., 1997, 19, 145. Bala, B.K., Solar Drying Systems: Simulation and Optimization, p. 320, 1998 (Agrotech Publishing Academy: Udaipur, India). Bala, B.K., Final Research Report, Department of Farm Power and Machinery, Bangladesh Agricultural University, Mymensingh, Bala, B.K. and Mondol, M.R.A., Drying Technol., 2001, 19(2), 427. El-Shiatry, M.A., Müller, J. and Mühlbauer, W., AMA, 1991, 22(2), 61. Esper, A. and Mühlbauer, W., Plant Res. Dev., 1996, 44, 61. Hodali, R. and Bougard, J., Energy Convers. Manage., 2001, 42(13), Hossain, M.A. and Bala, B.K., Drying Technol., 2002, 20(2), 489. Hossain, M.A., PhD thesis, Department of Farm Power and Machinery, Bangladesh Agricultural University, Mymensingh, Bangladesh, Ivanova, D. and Andonov, K., Energy Convers. Manage., 2001, 42(7), 975. Janjai, S., Esper, A. and Mühlbauer, W., Renewable Energy, 1994, 4(4), 409. Knudsen, J.G. and Katz, D.L., Fluid Dynamics and Heat Transfer, 1958 (McGraw Hill Book Company: New York). Mühlbauer, W., Energy Agr., 1986, 5, 121. Sparrow, E.M., Ramsey, J.W. and Mass, E.A., Trans. ASME J. Heat Trans., 1979, 101, 199. Steinfeld, A. and Segal, I., Drying Technol., 1986, 4(4), 535. Zaman, M.A. and Bala, B.K., Solar Energy, 1989, 42(2), 167. Q2

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