Elsevier

Renewable Energy

Volume 34, Issue 4, April 2009, Pages 1000-1008
Renewable Energy

Modelling and experimental validation of thin layer indirect solar drying of mango slices

https://doi.org/10.1016/j.renene.2008.08.006Get rights and content

Abstract

The thin layer solar drying of mango slices of 8 mm thick was simulated and experimented using a solar dryer designed and constructed in laboratory. Under meteorological conditions of harvest period of mangoes, the results showed that 3 “typical days” of drying were necessary to reach the range of preservation water contents. During these 3 days of solar drying, 50%, 40% and 5% of unbound water were eliminated, respectively, at the first, second and the third day. The final water content obtained was about 16 ± 1.33% d.b. (13.79% w.b.). This final water content and the corresponding water activity (0.6 ± 0.02) were in accordance with previous work. The drying rates with correction for shrinkage and the critical water content were experimentally determined. The critical water content was close to 70% of the initial water content and the drying rates were reduced almost at 6% of their maximum value at night. The thin layer drying model made it possible to simulate suitably the solar drying kinetics of mango slices with a correlation coefficient of r2 = 0.990. This study thus contributed to the setting of solar drying time of mango and to the establishment of solar drying rates' curves of this fruit.

Introduction

The dried mango represents a great proportion of dried fruit export in West Africa. In the case of Burkina Faso, the seasonal production of mango is about 47,600–54,000 tons and its exports have gone up from 602.1 to 4921.2 tons, respectively, from 1992 to 2005 [1]. In spite of the increase in these exports, great quantities of fruit (about 50% of the national production) are perishing under trees each season because of the lack of suitable transport means from the orchards to the big cities where drying sites are located. Moreover, the dryers used in town (electric dryer and gas dryer) cannot be used on the production sites because of lack of electricity and high cost of gas. However, the mango harvesting goes from April to July, which is the high solar radiation period. These favourable conditions of temperature and relative humidity could justify the use of solar energy as alternative to electric and gas energies or as extra source of energy for drying. It was in this context that this work was carried out. In the literature, few studies were undertaken on mango solar drying: While testing a solar dryer with natural convective heat flow, Gbaha et al. [2] studied the direct solar drying kinetics of plantain banana, sweet banana, cassava and mango. Madlopa, Jones and Kalenga [3] studied the indirect solar drying of mango slices using a natural convection solar dryer with a composite-absorber system. Touré and Kibangu-Nkembo [4] studied the free convection sun-drying of cassava, banana and mango. In Kenya, Gomez [5] carried out experiments on mango drying by sun-drying with mangoes cut into slices 0.76 cm wide and 1.5 cm thick. Most of these works did not deal with the thin layer solar drying modelling of mango slices whereas this mode of drying is generally the most used. Therefore, the objective of the current work is to model the indirect solar drying kinetics of mango slices in thin layer. The solar dryer used for the experimental validation of the drying model was designed and built at the laboratory; and the drying measures took place at the period of large production of mango (May–June). This study should contribute to the setting of thin layer solar dryer of mango and to the preservation process of this fruit.

Section snippets

Mangoes samples

The mangoes used in this study were purchased from a local fruit market of Ouagadougou (Burkina Faso). These mangoes were of “Amelie” variety, which is the most popularized in Burkina Faso as well as in the West Africa region. Good quality fruits were selected and washed into water to which a small quantity of natrium hypochlorite was added as disinfectant, rinsed with drinking water and peeled. The pulp was separated from the stone, and sliced into pieces of 8 mm thick and 5 cm wide. The slices

Hypothesis

To model the thin layer drying of mango slices, it was assumed that:

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    the transfers are unidirectional;

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    the dryer is cut off along its length (in the air flow direction) in sections of Δx thick;

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    the temperature and moisture content in each section are invariable;

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    physical parameters vary “step by step” in each tray and from one tray to another;

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    exchanges into each tray are determined by the exit conditions of drying air from the preceding tray;

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    the drying rates are known by experiment;

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    the humid air

Solar radiation and solar collector performances

The solar radiation data were simulated by adopting the concept of “typical day” of month. Also, typical days of year months are presented in Appendix 3. The assumption of “typical day” considers all the days in the month to be identical and the impact of the climatic risk are neglected. The concept of typical day is appropriate for the sites where the variation of the daily climatic conditions is not significant and which are characterized by high index of clearness [7]. This condition could

Conclusion

In this work, the solar drying of 8 mm thick mango slices was simulated by a thin layer drying model and experimentally validated with a indirect solar dryer prototype designed and constructed in laboratory. Results showed that 3 “typical days” of drying in harvest period of mangoes (May–June) were necessary to reach the range of water contents of conservation. Under metrological condition of May; 50, 40 and 5% of the product water was, respectively, evacuated at the first, the second day and

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