Field Experiments of the Silpakorn Type Solar Tunnel Dryers in Northern Thailand

Field Experiments of the Silpakorn Type Solar Tunnel Dryers in Northern Thailand1S. Janjai, 1P. Intawee, 1C. Chaichoet, 2P. Sittiamnuay

1Solar Energy Research Laboratory, Department of Physics, Silpakorn University,

Nakhon Pathom 73000, Thailand.2Department of Alternative Energy Development and Efficiency, Ministry of Energy, Bangkok 10330, Thailand.

AbstractFourteen Silpakorn type solar tunnel dryers were installed at the Royal Project sites in northern Thailand. The dryers were used to dry various agricultural products in the Project. The performance of one of these dryers was investigated under the field condition. The dryer was used to dry 70 kg of chillies. Various sensors were installed to monitor parameters affecting the performance of the dryer. The chillies could be dried in 3 days with this dryer, compared to 5 days needed for the natural sun drying. In addition, these chillies being dried in the dryer were completely protected from rain and insects and dried chillies were of high quality.Keywords: solar energy, solar drying, crop dryer

1. IntroductionDrying is an important post-harvesting process for tropical fruits and spices. It is not only aimed at the preservation of food but also for the modification of tastes and textures of dried product which increases their market values. Fruit and spices drying in the tropics is commonly achieved by spreading these products on beaten earth or mats directly exposed to solar radiation. Although, this method is cheap, it is susceptible to problems caused by rainfalls, animals and dusts. Most farmers in the tropics usually operate small-scale farms and cannot afford mechanical drying systems. An alternative solution to this problem is to take advantage of the abundance of solar energy by using solar drying technologies.A number of studies have been reported on the development of solar dryers [1-6]. However, most of these dryers are only for demonstration purposes. A few of them were practically used in the fields for a production of dried products. This is due to some the technical and economic problems of the dryers. In this work, we have developed a solar dryer named as the Silpakorn type solar tunnel dryer. This dryer has designed for drying fruits and spices with a loading capacity of 100 kg. The object of this paper is to present its performance of this dryer under a field condition.

2. Description of the dryerThis dryer is consisted of two parts, namely the solar collector and the drying tunnel. The collector and the drying tunnel have the area of 1.22 x 1.95 m2 and 1.22 x 8 m2

respectively. Both parts have a similar structure and they are connected directly without any air ducts as shown in Fig 1. The collector and the drying tunnel are covered by glass plates with a tilted angle of 5 degrees for drainage of water in case of rain. Loading and unloading of a product to be dried is done through the windows at a side wall of the drying tunnel.A dc-fan powered by a 15-watt solar cell module is used to suck ambient air into the collector tunnel and it is blown to the drying tunnel. All parts of the dryer were designed using a modular concept to facilitate the assembling and transportation of the dryer.

Glass platesWindow forloading products

Air outlet

Air inletSolar collector (1.95 m.)Drying tunnel (8 m.)

Back insulatorFan

Side wallSolar cell

1.22 m

Fig. 1 Schematic diagram of the Silpakorn type solar tunnel dryer.After designing and testing of all parts of the dryer, one prototype of the dryer was constructed at Silpakorn University (Fig. 2). This prototype was improved and tested to dry bananas and chillies. It performance was monitored and satisfactory results were obtained.

Fig. 2 Prototype of the tunnel dryerFig. 3 The tunnel dryer installed at Paksam site of the Royal Project in Chiang Mai

To disseminate the dryer, fourteen units of this type of dryer were constructed at Silpakorn University then transported and installed at 14 sites of the Royal Projects in northern Thailand. All sites are located in mountainous areas. The dryers have been using to dry various agricultural products such as bananas, chillies, coffee, etc.

3. Field experimentThe dryer installed at Paksam site in Chiang Mai was selected for the investigation of its field performance (Fig. 3). This dryer is normally used to dry chillies. To examine its performance, various measuring devices were installed at the dryer. A pyranometer of Kipp&Zonen (model CM 11) was used to measure global solar radiation. Thermocouples (k-type) were employed to monitor drying air and ambient air temperatures. Hygrometers of Electronik (model EE23) were used to measured relative humidity of air in the dryer and ambient air. An electronic balance of Sartorius (model E2000D) was used to weigh samples of the product being dried in the dryer for the determination of their moisture contents with the oven method. Voltage signals from the pyranometer, hygrometers and thermocouples were recorded with a data logger of Yokogawa (model DC 100) with the sampling rate of 10 minutes. For the output of the other devices, they were manually read out and recorded. The positions of the measurement are shown in Fig. 4