Mathematical modelling of a thin layer drying of apricots in a solar energized rotary dryer

In this paper we present a mathematical modelling of a thin layer forced solar drying of apricots. An indirect forced convection solar dryer consisting of a solar heater and a rotary column cylindrical drying (RCCD) cabinet was used in the experiments. Air heated by the solar air heater was forced through the apricots by an electrical fan. Moreover, the natural sun drying experiments were conducted for the comparison at the same time. Fourteen different thin layer mathematical drying models were compared according to their coefficients of determination (r,χ², RMSE) to estimate solar drying curves. The effects of the drying air temperature, velocity and the rotation speed of column on the drying model constants and coefficients were predicted by multiple regressions using a linear type model. Copyright © 2004 John Wiley & Sons, Ltd.     

Energy and exergy analyses of thin layer drying of mulberry in a forced solar dryer

This paper is concerned with the energy and exergy analyses of the thin layer drying process of mulberry via forced solar dryer. Using the first law of thermodynamics, energy analysis was carried out to estimate the ratios of energy utilization and the amounts of energy gain from the solar air collector. However, exergy analysis was accomplished to determine exergy losses during the drying process by applying the second law of thermodynamics. The drying experiments were conducted at different five drying mass flow rate varied between 0.014 kg/s and 0.036 kg/s. The effects of inlet air velocity and drying time on both energy and exergy were studied. The main values of energy utilization ratio were found to be as 55.2%, 32.19%, 29.2%, 21.5% and 20.5% for the five different drying mass flow rate ranged between 0.014 kg/s and 0.036 kg/s. The main values of exergy loss were found to be as 10.82 W, 6.41 W, 4.92 W, 4.06 W and 2.65 W with the drying mass flow rate varied between 0.014 kg/s and 0.036 kg/s. It was concluded that both energy utilization ratio and exergy loss decreased with increasing drying mass flow rate while the exergetic efficiency increased.     

Mathematical modelling of the thin layer solar drying of banana, mango and cassava

The main objectives of this paper are firstly to investigate the behaviour of the thin layer drying of plantain banana, mango and cassava experimentally in a direct solar dryer and secondly to perform mathematical modelling by using thin layer drying models encountered in literature. The variation of the moisture content of the products studied and principal drying parameters are analysed. Seven statistical models, which are empirical or semi-empirical, are tested to validate the experimental data. A non-linear regression analysis using a statistical computer program is used to evaluate the constants of the models. The Henderson and Pabis drying model is found to be the most suitable for describing the solar drying curves of plantain banana, mango and cassava. The drying data of these products have been analysed to obtain the values of the effective diffusivity during the falling drying rate phase.     

Experimental comparison of open sun drying and solar drying based on evacuated tube collector

In this article, an experimental comparison between open sun and solar drying is done. The thermal performance of evacuated tube based solar dryer is investigated with drying characteristics of Phyllanthus Emblica (Anvla), Aloe Vera, Aegle Marmelos (Bel) and leaves of Azadirachta Indica (Neem), Aegle Marmelos (Bel) and Psidium Guajava (Guava). In this setup, an evacuated tube collector, shell and tube heat exchanger and drying chamber are used. It was found that the maximum temperature difference between hot air and ambient air is 35.4°C and maximum efficiency of the setup is calculated as 55%. The average drying rate of Phyllanthus Emblica (Anvla), Aloe Vera and Aegle Marmelos (Bel) is measured as 0.46?g/min, 0.44?g/min, and 0.39?g/min respectively which are higher than that of Open Sun Drying. The leaves of Azadirachta Indica (Neem), Aegle Marmelos (Bel) and Psidium Guajava (Guava) also get dried with faster rates of 0.18?g/min, 0.17?g/min, and 0.14?g/min respectively.     

Studies on thin layer drying of bagasse

Thin layer drying experiments of bagasse were carried out to determine the drying kinetics and also to identify a suitable drying model. The drying experiments were conducted in a laboratory scale dryer for a wide range of air temperatures (80–120°C), velocities (0.5–2 m/s), humidity level of air (9–24 g water(kg of d.a)⁻¹) and product thickness (20–60 mm). Air temperature, velocity, humidity and mass of product were recorded continuously during experimentation. The data were fitted to the different semi-theoretical or empirical models and compared based on their correlation coefficient (r), chi-square (χ²), root mean square error and mean bias error values. Among the models considered, the Page model gave accurate predictions with a correlation coefficient of 0.99627. It is found that the drying of bagasse takes place only in the falling rate period, even though the product has high initial moisture content. Copyright © 2006 John Wiley & Sons, Ltd.     

Improvement in greenhouse solar drying using inclined north wall reflection

A conventional greenhouse solar dryer of 6 m² × 4 m² floor area (east-west orientation) was improved for faster drying using inclined north wall reflection (INWR) under natural as well as forced convection mode. To increase the solar radiation availability onto the product (to be dried) during extreme summer months, a temporary inclined wall covered with aluminized reflector sheet (of 50 μm thickness and reflectance 0.93) was raised inside the greenhouse just in front of the vertical transparent north wall. By doing so, product fully received the reflected beam radiation (which otherwise leaves through the north wall) in addition to the direct total solar radiation available on the horizontal surface during different hours of drying. The increment in total solar radiation input enhanced the drying rate of the product by increasing the inside air and crop temperature of the dryer. Inclination angle of the reflective north wall with vertical (β) was optimized for various selective widths of the tray W (1.5, 2, 2.5 and 3 m) and for different realistic heights of existing vertical north wall (h) at 25°N, 30°N and 35°N latitudes (hot climatic zones). Experimental performance of the improved dryer was tested during the month of May 2008 at Ludhiana (30.56°N) climatic conditions, India by drying bitter gourd (Momordica charantia Linn) slices. Results showed that by using INWR under natural convection mode of drying, greenhouse air and crop temperature increased by 1-6.7 °C and 1-4 °C, respectively, during different drying hours as compared to, when INWR was not used and saved 13.13% of the total drying time. By using INWR under forced convection mode of drying, greenhouse air and crop temperature increased by 1-4.5 °C and 1-3 °C, respectively, during different drying hours as compared to, when INWR was not used and saved 16.67% of the total drying time.     

Crop drying by indirect active hybrid solar – Electrical dryer in the eastern Algerian Septentrional Sahara

In the present work, a new specific prototype of an indirect active hybrid solar–electrical dryer for agricultural products was constructed and investigated at LENREZA Laboratory, University of Ouargla (Algerian Sahara). In the new configuration of air drying passage; the study was done in a somewhat high range of mass flow rate between 0.04 and 0.08 kg/m² s a range not properly investigated by most researchers. Experimental tests with and without load were performed in winter season in order to study the thermal behavior of the dryer and the effect of high air masse flow on the collector and system drying efficiency. The fraction of electrical and solar energy contribution versus air mass flow rate was investigated. Slice tomato was studied with different temperatures and velocities of drying air in order to study the influence of these parameters on the removal moisture content from the product and on the kinetics drying and also to determine their suitable values. Many different thin layer mathematical drying models were compared according to their coefficient of determination (R²) and reduced chi square (χ²) to estimate experimental drying curves. The Middli model in this condition proved to be the best for predicting drying behavior of tomato slice with (R² = 0.9995, χ² = 0.0001). Finally an economic evaluation was calculated using the criterion of payback period which is found very small 1.27 years compared to the life of the dryer 15 years.     

Experimental and simulated performance of a PV-ventilated solar greenhouse dryer for drying of peeled longan and banana

This paper presents experimental and simulated performance of a PV-ventilated solar greenhouse dryer for drying of peeled longan and banana. The dryer consists of a parabolic roof structure covered with polycarbonate plates on a concrete floor. Three fans powered by a 50-W PV module ventilate the dryer. To investigate the experimental performances of the solar greenhouse dryer for drying of peeled longan and banana, 10 full scale experimental runs were conducted. Of which five experimental runs were conducted for drying of peeled longan and another five experimental runs were conducted for drying of banana. The drying air temperature varied from 31 °C to 58 °C during drying of peeled longan while it varied from 30 °C to 60 °C during drying of banana. The drying time of peeled longan in the solar greenhouse dryer was 3 days, whereas 5-6 days are required for natural sun drying under similar conditions. The drying time of banana in the solar greenhouse dryer was 4 days, while it took 5-6 days for natural sun drying under similar conditions. The quality of solar dried products in terms of colour and taste was high-quality dried products. A system of partial differential equations describing heat and moisture transfer during drying of peeled longan and banana in the solar greenhouse dryer was developed and this system of non-linear partial differential equations was solved numerically using the finite difference method. The numerical solution was programmed in Compaq Visual FORTRAN version 6.5. The simulated results reasonably agreed with the experimental data for solar drying of peeled longan and banana. This model can be used to provide the design data and is also essential for optimal design of the dryer.     

Experiment and modelling of parameters influencing natural wind drying of willow chunks

The objective of this study was to investigate the parameters that govern the drying process of willow chunks. Indicative chunk drying trials were conducted to assess the potential of natural wind drying. Supportive model simulations were conducted to gain insight into the influence of different process parameters (particle size, pile depth) on drying. Natural wind drying experiments showed that willow chunks could be dried from 50% (wet basis) to around 10% within 5 months. Internal heating in the pile did not occur and dry matter losses were reasonably low (3.5–5%). The drying time of willow chunks depended on drying air conditions, particle size and pile dimensions (depth). The particle size of chunks should be large enough to create a low airflow resistance in the pile, but small enough to avoid that internal diffusion of moisture limits the drying process.     

Experimental characterisation and modelling of thin layer direct solar drying of Amelie and Brooks mangoes

Direct solar drying characteristics of Amelie and Brooks mangoes were experimentally determined using a solar dryer made up of four trays and used under weather conditions of fruit harvest period. Direct solar drying curves were established, fitted using 10 mathematical models and simulated with a direct solar drying model. Effective diffusivity, drying rates and drying efficiency were estimated for each drying day and each variety. Results showed that at least four days were necessary to reach the range of preservation water contents. Drying curves depended on variety and were suitably fitted by “two-term” and “Approximation of diffusion” models (with R² ≥ 0.9888, RMSE ≤ 0.0283, E ≤ 9.1283% and χ² ≤ 1.3314 × 10⁻⁴). Drying rates and drying efficiency significantly decreased with the number of drying days (respectively between 0 and 0.15 g kg⁻¹ s⁻¹ and between 0 and 34%) and were very close for the two varieties. Diffusivity weakly varied with variety and strongly decreased with the number of drying days between 2.7906 × 10⁻¹¹ and 1.8489 × 10⁻¹⁰ m²/s. Drying kinetics were suitably simulated by the direct solar drying model (with: Amelie: R² = 0.989 and E = 7.623%, Brooks: R² = 0.9924 and E = 4.961%). The final water content was about 24.83% for Amelie and 66.32% for Brooks and Amelie was the most suitable variety for direct solar drying.     

Simulation and optimisation of the ventilation in a chimney-dependent solar crop dryer

As published earlier on the performance of a chimney-dependent solar crop dryer (CDSCD) designed by the authors, the solar chimney can be combined with an appropriately inclined roof of drying chamber for ventilation improvement in the dryer. Mathematical models and a computer code are now developed to simulate the ventilation in relation to the design of the CDSCD. This is done for situations without any crop (no-load) in the dryer, to relate the ventilation to the external dimensions. The pressure-loss and bulk-fluid-temperature coefficients are deduced empirically from trials on the physical model. The simulation code predicts the ventilation to within 5% and the temperatures to within 1.5% of observed data, confirming the validity of the code as an effective design tool for the CDSCD. Results of parametric studies performed with the code indicate that, maximum airflow can be achieved when the inlet-exit area ratio is around 4:1, above which the system then approaches saturation without any real variation. The drying-chamber roof inclination and the chimney height are critical for the design in the geographical regions far from the equator, whereas the decisive parameters in the regions close to the equator are the drying chamber height and the area ratio of the dryer floor to chimney cross section. A high drying chamber with a short solar chimney is generally favoured at locations close to the equator, whereas a short drying chamber with a high solar chimney is suitable for regions far away from the equator.     

Performance of evaporator-collector and air collector in solar assisted heat pump dryer

A solar assisted heat pump dryer has been designed, fabricated and tested. This paper presents the performance of the evaporator-collector and the air collector when operated under the same meteorological conditions. ASHRAE standard procedure for collector testing has been followed. The evaporator-collector of the heat pump is acting directly as the solar collector, and the temperature of the refrigerant at the inlet to the evaporator-collector always remained below the ambient temperature. Because of the rejection of sensible and latent heats of air at the dehumidifier, the temperature at the inlet to the air collector is lower than that of the ambient air. Hence, the thermal efficiency of the air collector also increases due to a reduction of losses from the collector. The efficiencies of the evaporator-collector and the air collector were found to vary between 0.8–0.86 and 0.7–0.75, respectively, when operated under the meteorological conditions of Singapore.     

Optical confinement of the intermediate layer between Si and alumina substrate in thin film Si solar cells

Enhancement of the optical confinement effect by an intermediate layer (IML) between Si and alumina substrate in thin film Si solar cells was studied. The dependence of the optical confinement effect on refractive index of the IML and on thickness of Si was separately investigated by hemispherical reflectance measurement of the following two series of samples. In the first case, SiO_xN_y, SiN_x or TiO₂ was deposited as the IML in the multilayer, Si/IML/alumina. In the second case, Si layers with different thicknesses were formed. The study showed that in certain conditions the IML could enhance the optical absorption of Si layer in thin film Si solar cells.