S. Janjai

SOLAR DRYING OF PINEAPPLE USING SOLAR TUNNEL DRIER

Field level experiments on solar drying of pineapple using solar tunnel drier were conducted at Bangladesh Agricultural University, Mymensingh, Bangladesh. The drier consists of a transparent plastic covered flat plate collector and a drying tunnel connected in a series to supply hot air directly into the drying tunnel using two dc fans operated by a solar module. This drier has a loading capacity of 120–150 kg of pineapple and a total of eight drying runs were conducted. In all the cases the use of the solar tunnel drier leads to considerable reduction of drying time in comparison to sun drying. The pineapple being dried in the solar tunnel drier were completely protected from rain, insects and dust, and the quality of the pineapple dried in the tunnel drier was of quality dried products as compared to sun dried products. Proximate analysis also indicates that the pineapple dried in the solar tunnel drier is a good quality dried product for human consumption.

Experimental investigation of the performance of the solar tunnel dryer for drying bananas

The multi-purpose solar tunnel dryer was used to dry bananas under the hot and humid weather conditions of Thailand in order to investigate its performance. The dryer comprises a plastic sheet-covered flat plate collector and a drying tunnel. The dryer is arranged to supply hot air directly to the drying tunnel using three fans powered by a 53 W solar cell module. The products to be dried are spread in one layer on a plastic net in the drying tunnel to receive energy from both the hot air supplied by the collector and incident solar radiation. This dryer can be used to dry up to 300 kg of ripe bananas in each drying batch. In investigating the performance of the dryer, seven drying tests were conducted at the Royal Chitralada Projects in Bangkok during March–May 1995. Teh temperature of the drying air from the collector varied between 40 and 65°C during drying and the bananas could be dried within 3–5 days, compared to the 5–7 days needed for natural Sun drying. In addition, the bananas being dried in the solar tunnel dryer were completely protected from rain, insects and dust, and the dried bananas were of high quality in terms of flavour, colour and texture. As the fans are powered by the solar module, the dryer could be used in rural areas where there is no supply of electricity from grid. The pay-back period of the dryer is estimated to be about 3 years when the dryer is locally produced.

Equilibrium Moisture Content and Heat of Sorption of Longan (Dimocarpus longan Lour.)

The equilibrium moisture contents of longan were determined experimentally using the dynamic method at temperatures ranging from 30 to 50°C and water activity ranging from 11 to 97%. The sorption isotherm curves of longan were sigmoidal in shape and decreased with increased temperature at constant relative humidity. Five selected isotherm models were tested to fit the experimental isotherm data of longan. The GAB model fitted the best to the experimental data of longan and the modified Oswin model was the next to the GAB model. The agreement between the best-fitted models and experimental data was excellent. The isosteric heats of sorption, determined from equilibrium data using the Clausius-Clapeyron equation, were found to be a function of moisture content.

Solar Drying Technology

Drying is the oldest preservation technique of agricultural products, and sun drying is still widely used for preservation of agricultural products in the tropics and subtropics. Previous efforts on solar drying of cereal grains, fruits, vegetables, spices, medicinal plants, and fish are critically examined. Recent developments of solar dryers such as solar tunnel dryer, improved version of solar tunnel dryer, roof-integrated solar dryer, and greenhouse-type solar dryer for drying of fruits, vegetables, spices, medicinal plants, and fish are also critically examined in terms of drying performance and product quality, and economics in the rural areas of the tropics and subtropics. Experimental performances of different types of solar dryers, which have demonstrated their potentialities for drying of fruits, vegetables, spices, medicinal plants, and fish in the tropics and subtropics, are addressed, and also the simulated performances of the different types of solar dryers are discussed. The agreement between the simulated and experimental results was very good, and it is within the acceptable limit (10%). The simulation models developed can be used to provide design data and also for optimal design of the dryer components. A multilayer neural network approach was used to predict the performance of the solar tunnel dryer. Using solar drying data of jackfruit and jackfruit leather, the model was trained using back propagation algorithm. The prediction of the performance of the dryer was found to be excellent after it was adequately trained and can be used to predict the potential of the dryer for different locations and can also be used in a predictive optimal control algorithm. Finally, prospects of solar dryers for drying of fruits, vegetables, spices, medicinal plants, and fish in the tropics and subtropics are discussed.

Determination of Angstrom’s turbidity coefficient over Thailand

Values of the Angstrom’s turbidity coefficient, β, at 53 meteorological stations covering Thailand were determined by using three different methods. A selection of the methods was based on input data available at each station. It was started with the calculation of β at Nakhon Pathom (13.81 °N and 100.4 °E) using narrow-band spectral irradiance data obtained from a multi-filter rotating shadow band radiometer. Langley’s method was employed to calculate β from the spectral data. The values of β derived from this method were used as references to validate a method for computing β from broad-band direct irradiance proposed by Louche et al. (Solar Energy 38(2)89). It was found that this method was valid for a tropical climate. Then Louche et al.’s method was used to calculate β at meteorological stations situated at four main cities, namely Chiang Mai (18.78 °N, 98.98 °E) located in the north, Ubon Rachathani (15.25 °N, 104.87 °E) in the northeast, Songkhla (7.20 °N, 100.60 °E) in the south and Bangkok in the central region. Based on values of β of these cities, a new model relating β to visibility, suitable for the tropical climate was developed. This model was used to estimate β at the other 48 meteorological stations where the visibility was routinely observed. Finally, seasonal variations of β were investigated. It was found that for the stations in the north, the northeast and the central region, the values of β are relatively high in the dry season (November–April). They decrease in the wet season (May–October). For most stations in the south, β was relatively low and remained nearly constant all year round. It was also inferred that the northeast monsoon and the southwest monsoon had a strong influence on the seasonal variations of β.

A new model for computing monthly average daily diffuse radiation for Bangkok

A model for computing monthly average daily diffuse radiation for Bangkok Thailand was developed. The development of the model was based on the analysis of global and diffuse radiation data collected from four solar monitoring stations located at different parts of Bangkok, with the data collection period ranging from 4 to 8 years. The monthly average daily ambient temperature and relative humidity were also incorporated in the model. To evaluate its performance, the model was used to compute monthly average daily diffuse radiation of an independent data set. The model performed better than those found in the literatures.

Solar Drying of Peeled Longan Using a Side Loading Type Solar Tunnel Dryer: Experimental and Simulated Performance

This article presents experimental and simulated results of drying of peeled longan in a side-loading solar tunnel dryer. This new type of solar tunnel dryer consists of a flat-plate solar air heater and a drying unit with a provision for loading and unloading from windows at one side of the dryer. These are connected in series and covered with glass plates. A DC fan driven by a 15-W solar cell module supplies hot air in the drying system. To investigate the experimental performance, five full-scale experimental runs were conducted and 100 kg of peeled longan was dried in each experimental run. The drying air temperature varied from 32 to 76°C. The drying time in the solar tunnel dryer was 16 h to dry peeled longan from an initial moisture content of 84% (w.b.) to a final moisture content of 12% (w.b.), whereas it required 16 h of natural sun drying under similar conditions to reach a moisture content of 40% (w.b.). The quality of solar-dried product was also good in comparison to the high-quality product in markets in terms of color, taste, and flavor. A system of partial differential equations describing heat and moisture transfer during drying of peeled longan in this solar tunnel dryer was developed and this system of nonlinear partial differential equations was solved numerically by the finite difference method. The numerical solution was programmed in Compaq Visual FORTRAN version 6.5. The simulated results agreed well with the experimental data for solar drying. This model can be used to provide the design data and it is essential for optimal design of the dryer.

Finite Element Simulation of Drying of Longan Fruit

A two-dimensional finite element model has been developed to simulate moisture diffusion in longan fruit during drying and moisture diffusivities of the components of longan fruit determined experimentally are used in this simulation. Shrinkage of the flesh of longan during drying is also taken into account. The finite element model is programmed in Compaq Visual FORTRAN version 6.5. This finite element model satisfactorily predicts the moisture diffusion during drying. Moisture contents in the different components in the longan fruit during drying are simulated. Moisture content profiles of the longan fruit are also predicted. Knowledge gained from this study will be useful in the understanding of the transport processes in the different components of the longan fruit.

A model for calculating global illuminance from satellite data

A model for calculating global illuminance on horizontal surfaces from meteorological satellite data was developed. The data used for developing the model are global illuminance measured at four solar monitoring stations situated in different parts of Thailand and 8-bit digital data from visible channel of GMS-5 satellite covering the whole country for the period of 1–2 years. Values of normalized global illuminance defined as a ratio of global illuminance to clear sky global illuminance were calculated. These values were used to correlate with those of cloud index derived from the satellite data. From the correlation, a model relating the normalized global illuminance to cloud index was established. The performance of this model was investigated using an independent illuminance data set. It was found that the global illuminance calculated from the model agreed well with that obtained from the measurement, with a root mean square difference of 5.38 klux or 7.0% of the mean values.

Moisture Diffusivity Determination of Different Parts of Longan Fruit

The components of the longan fruit were modeled as sphere for seed, cylinder for seed stalk, and slab for seed coat, shell, and flesh. The moisture diffusivities were determined by minimizing the sum of square of derivations between the predicted and experimental values of moisture content of thin layer drying under controlled conditions of air temperature and relative humidity. Experimental data were obtained from thin layer drying of the components in the form of sphere for seed, cylinder for seed stalk and slab for seed coat, shell and flesh. Air temperatures of 50, 60, 70, and 80°C and relative humidity in the range of 1.5–13.33% were used. The mean diffusivity of flesh of longan fruit increased with temperature, but the diffusivities of shell, seed coat, seed, and seed stalk were independent of temperature. The moisture diffusivities of shell and seed coat were much lower than those of the other parts of the longan.