Experimental Analysis of Potato Slices Drying Characteristics using Solar Dryer

INTRODUCTION

It has been proved to prevent bacteria, yeast, mold and enzymes spoiling a food, its moisture content should be reduced to 10-20%. At the same time the nutritional values and the flavour are concentrated and maintained (Scanlin, 1997). The typical traditional method to reduce the moisture content is to use open sun drying. It involves spreading the agricultural products in a thin layer such as crops, fruits, vegetables and tobacco on the ground and turned regularly. Turning the products continuously until the food moisture content reduced to the required moisture level (dried) for storage purpose. This method is still widely used in the developing countries. However, there exist many problems associated with open sun drying. It has been seen that open sun drying has the following disadvantages. It requires long drying time and large surface area. Since the products are open to the air they could easily get damaged by the hostile weather conditions, degradation by overheating, infestation by animal and contamination by foreign materials. Crops are also susceptible to re-absorption of moisture if they are left on the ground during periods of no sun and the drying process cannot be controlled. These could lead to slow drying rate, contamination and poor quality of dried products and loss in production.

The aforementioned problems can be avoided if agricultural products are dried in a sheltered area or chamber using a stream of heated air by solar energy to reduce its relative humidity. Moreover, speed of operations from harvest to storage forced the implementation and use of heated air for drying (Hall, 1980; Ergunes et al., 2005). Consequently, various drying devices were implemented for drying crops such as woodfuel driers, oil-burned driers, electric dryers and solar dryer. However, the high cost of oil and electricity and their scarcity in the rural areas of most third world countries have made some of these driers very unattractive. Therefore, interest has been focused mainly on the development of solar driers (Veziroglu, 1989).

Based on the mode of air flow solar dryers are classified into two groups natural convection and forced convection dryers. Forced convection dryer requires external fan or blower to create the air current in the dryer which require power source. Whereas in natural convection with the proper design of the dryer the air current is generated due to density gradient of the air along the dryer, i.e., does not require power source. However, low air flow rate and the long drying time result in low drying capacity (Bala et al., 2005). As a result natural convection solar dryer is limited for drying small amount of agricultural products for family consumption. If large amount of products are required to be processed, then forced convection dryers should be used. Performance of forced convection maize and bean solar dryer was evaluated by Gatea (2010, 2011). Stiling et al. (2012) used concentrating solar panels to improve solar drying for Roma tomatoes and showed a considerable increase in drying rate on sunny days. This study focused on natural convection solar dryer so that it can be used anywhere else as it does not need power input except the solar radiation.

The objective of this study was to analyze experimentally the drying characteristics of potato slices using free convection solar drying system. It is mainly concentrated on the practical field test performance of a locally manufactured, solar dryer. The experiment was conducted in the Faculty of Technology, Addis Ababa University in Ethiopia.

EXPERIMENTAL SETUP

The solar collector is parallel piped shape with dimension of L = 2 mxW = 1 mx0.14 m having 80 mm channel depth, 40 mm gap between the absorber plate and glass cover, 20 mm thick fiber glass insulation was used at the bottom of the collector to reduce the back and edge losses. The solar collector which is inclined at an angle of 12° from the horizontal is oriented along the N-S direction. Blackened stainless steel flat sheet of thickness 1 mm is used as the absorber plate. The solar collector is covered with 4 mm thick commercial glass. The lower end face of the collector (1x0.08 m) is the air inlet whereas, its higher face end is connected to the rectangular duct of the chamber. For the study of the drying characteristics of potato slices in the solar dryer, measuring instruments and thermocouples are used and are all connected to data logger. Figure 1 shows the location and types of the sensors applied.

The drying chamber has 1.2x1.04x0.55 m outer dimensions. Out of the three shelves/trays only two trays TR1 and TR2 were placed inside the drying chamber. The depth of each tray is 19 mm with wire mesh at the bottom. The two drying trays TR1 and TR2 have areas of A₁ = 0.39 m² and A₂ = 0.42 m², respectively, with a total area of 0.81 m². The relative positions of the three trays, TR1, TR2 and TR3 are 0.18, 0.32 and 0.46 m, respectively, above the drying chamber base (hot air entry point). Holes were drilled on the drying chamber for inserting the rod-shaped measuring probes.

The potato slices were characterized by analyzing their moisture content and drying rate curves both on wet and dry basis. The initial moisture content on wet and dry basis of the potato slice was determined by AMB (AMB 310) moisture balance. The AMB balance test was set at Mode 1, with strobe time interval of 2 sec and drying temperature 160°C. Then, slices of potato samples were placed on the AMB moisture balance tray. Samples of the potato of weight ω_O were dried in the moisture balance at 160°C until the dried sample weight, ω_d became stable. The moisture content on wet basis of the potato used was 81.56% (42.3% moisture content on dry basis).

Fig. 1:	Schematic representation of the drying setup

The moisture content, dry basis, Md_o of the potato is expressed as:

(1)

For the determination of the moisture content, dry basis, Md_i of the potato at any time t_i during the drying process, the following equation is used:

(2)

or moisture content, wet basis, of the potato at any time t_i during the drying process where ω_i is the weight of the potato at time t_i is expressed as:

(3)

The moisture content on dry basis and wet basis is related by Eq. 4 and 2:

(4)

The determination of the potato weight was done by weighing the drying tray with its load of potato at any time in the drying process. For the determination of the instantaneous drying rate RDd_i, dry basis, Eq. 5 is applied:

(5)

where, t_i-1 and t_i are successive times corresponding to when two successive measurements of a drying material are made.