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Experimental Investigations of Performance of Solar Tunnel Drier for Drying of Aonla

M. A. Haque
Assistant Professor
Department of Farm Power & Machinery, Bangladesh Agricultural University, Myrnensingh.

B. K. Bala
Professor
Department of Farm Power & Machinery, Bangladesh Agricultural University, Myrnensingh.

Abstract/ Executive Summary:

This study was conducted to investigate the performance of the forced convection solar tunnel drier for drying of aonla (Emblica officinalis; F. Euphorbiaceae) under Bangladesh conditions. The drier consists of a plastic foil covered flat plate collector and a drying tunnel. The drier is arranged to supply hot air directly into the drying tunnel using two d.c. fans powered by a 40 W solar cell module. The products to be dried are spread in one layer on a wire mesh in the drying tunnel to receive energy from both hot air supplied by the collector and incident solar radiation on products. The variation in the air flow rate due to variation in the solar insolation helped to regulate the drying temperature. This resulted in minimum variation in the drying air temperature. This drier can be used to dry upto 200 kg of aonla for each drying batch. The temperature of the drying air at the collector outlet varied from 32°C to 39°C during drying and the aonla could be dried within 3-4 days in the tunnel drier compared to 4-5 days needed for natural sun drying. In addition the aonlas being dried in the solar tunnel drier are completely protected from rain. insects and dust. and the dried aonlas were of high quality in terms of color and texture. As the fans are powered by a solar module, the drier could be used in rural areas where there is no supply of electricity from grid.

Keywords: Experimental Investigations, Performance, Solar Tunnel Drier, Aonla

Location: Hohenheim University

Start Date: 00-00-1996

End Date: 00-00-1996

Program Area: Postharvest and Agro-processing

Non-commodity: Drier

Objectives:

To investigate the performance of the solar tunnel dryer for drying aonla under Bangladesh conditions.

Methodology:

The solar tunnel dryer developed at the institute for Agricultural Engineering for the Tropics and Subtropics, Hohenheim University was used in this work. It consists of a plastic foil-covered flat plate solar collector, a drying tunnel and two small axial flow fans as shown. To simplify the construction and to reduce the costs, the solar collector is connected directly to the drying tunnel without any additional air ducts. The complete frame of the tunnel is placed on the brick lined floor. The floor of the collector is painted black to absorb solar radiation. Both the collector as well as the dryer are covered with a transparent PE plastic foil which is fixed with the frame using plastic rope and foam. For the drying tunnel. a wire mesh is placed 10 cm above the floor where product to be dried is spread. The PE plastic foil can be open from either side to load and unload the product to be dried. This fixing method is designed to facilitate the replacement of the foils. Two small fans powered by a 40 Watt solar cell module are installed at the back side of the collector to suck ambient air into the collector. The solar energy absorption area of the collector is 10 x 2 m². The drying area of the drying tunnel has approximately the same dimension. All parts of the dryer and the metal frame were designed using the modular concepts which facilitate the easy installation and transfer of the dryer. This solar tunnel dryer uses solar energy both in the thermal form for drying processes and electrical form for driving the fans by means of the solar collector and solar module respectively. Therefore the dryer could be used in rural areas where there are no supply of electricity. The solar tunnel dryer under this investigation was installed at the yard of the workshop of the department of Farm Power and Machinery, Bangladesh Agricultural University, Mymensingh. The dryer was not shaded by trees or buildings during 8:30 am to 2:30 p.m. The test was carried out during latter half of October. 1996. Parameters affecting the performance of the dryer were measured by equipment. The k-type thermocouple was used to measure drying air temperature along the flow direction of the dryer. A pyranometer was used to measure the global radiation at the inlet of the dryer. The relative humidity and temperature of the ambient air were measured with a RH meter and a digital thermometer respectively. The velocity of the drying air was measured with an anemometer at the outlet of the dryer. Weight loss of the product during drying was measured with an electronic balance. The sun dried control samples were weighed as well. All these data were monitored at one hour interval. The average size of aonla was approximately 2 cm in diameter. Aonla were cut into pieces and blanched followed by thawing and rinsing under a ceiling fan for a few minutes. This product was then spread on the wire mesh of the dryer in one layer. The drying was started after completing the loading. usually at 8:30 am and the drying was stopped at 2:30. Afterwards the samples were collected and kept in a closed room. These were again put into the dryer in the next morning. To compare the performance of the tunnel dryer with that of the natural sun drying. control samples of aonla were spread on trays near the dryer and dried simultaneously under the same weather condition.

Source of Information: Journal of the Institution of Engineers, Bangladesh; Agricultural Engineering Division, Vol. 24/AE, No.1, Dec. 1996

Article Link (Online Journal):

Funding Source:

Total Budget (Tk.) :

Achievement/Findings:

Potentiality of the solar tunnel drier for drying aonla in Bangladesh was demonstrated by the tests conducted for this purpose. As the dried aonla has a good market in this subcontinent as a medicinal ingredient and the product dried in this study has better quality than that of traditional drying, it is a worth adoption. The drier is simple in construction and it can be constructed using locally available materials by the local craftsman. Two small electric d.c. fans and a solar module need to be purchased for the tunnel drier. The solar tunnel drier is operated by this photovoltaic solar module independent of electrical grid. The photovoltaic system has the advantage that the temperature of the drying air is automatically controlled by the solar radiation. The variation in the air flow rate due to variation in the solar insolation helped to regulate the drying temperature. Further study should be carried out for optimizing the photovoltaic driven solar tunnel drier for its efficient operation.

Knowledge Management: Journal