Evaluation and demonstration of direct solar potato dryer

Abu Tefera, Wolelaw Endalew and Brihan Fikiru*

Bahir Dar Agricultural Mechanization and Food Science Research Centre
P.O. Box 133, Bahir Dar, Ethiopia
abuteffera@yahoo.com   ;   abuteffera@gmail.com
* Food, Medicine and Health Care Administration and Control Authority of Ethiopia
P.O. Box 5681, Addis Ababa, Ethiopia

Abstract

This study was conducted to evaluate the performance of two models of direct solar potato dryers and to demonstrate to farmers around potato producing areas, in Amhara Regional State. Wooden box dryer and Pyramid shape dryer models were evaluated. Dryers were compared with open sun drying methods. Temperature, relative humidity as well as the rate of moisture removal as expressed by loss-in-weight were recorded and analyzed.

Results showed that on the average there was a 10-20 ⁰C temperature difference between ambient condition and the drying chambers. Besides, the weight of sliced potato which was initially 0.90 kg was reduced to about 0.19 kg within two days. This showed an overall reduction in drying time by 2-3 hours compared to open sun drying. This result, however, was not perceived to be large enough under existing testing condition. But considering other benefits of the driers like protecting the drying material against contaminants, dust, and insects resulting in better quality product, this result is acceptable. On the other hand, comparing the performances of the two driers, Pyramid dryer was found better in creating more conducive drying environment with optimal temperature and lower relative humidity. Moreover, considering manufacturing costs, simplicity in design to manufacture in rural area from almost any kind of available building materials by locally available workmen, Pyramid dryer is better than Box dryer. Demonstration and practical training on the use of solar dryers and methods of food preparation out of the dried potato slices was provided for a group of farmers. Participant farmers actively participated in the potato menu preparation and informal sensory evaluation. They have showed high interest in diversified potato utilization as it enhances their feeding habits. This dryer was found suitable for drying small quantities (10-15 kg) of agricultural products which suits best for household level. Therefore, Pyramid (pyramid shape) dryer models should be recommended for further promotion.

Key words: dried potato, dryer model, farmers, potato processing

Introduction

Drying as a means of preserving agricultural products has been practiced since ancient times. It is one of the postharvest operations for biological materials as quality of these materials is influenced by drying. Crops such as fruits, vegetables and cereals can be well preserved after removing free water by means of drying. Moreover, the main purpose of drying agricultural product is to store for longer periods, minimize storage and packaging spaces and reduce weight to handle and transport. Open sun drying with the application of traditional knowledge is still widely practiced throughout the world and largely unchanged since ancient times. This method employs spreading the crop on the ground and turning regularly until the product is sufficiently dried. However, it has inherent limitations in that it requires large amount of space and extended drying time. The crop could also be damaged because of the hostile weather conditions, could be contaminated with foreign materials, and its quality could degrade by overheating. Besides, products will be subjected to series insect and fungal infestation and also might be susceptible to re-absorption of moisture. In such conditions, solar operated crop dryers appear to be viable alternative to the traditional open sun drying where quicker and controlled drying process can be attained and crops can be well protected during drying. 

Solar thermal technology is a technology that is rapidly gaining acceptance as an energy saving measure in agriculture application. It is preferred to other alternative sources of energy such as wind, because it is more abundant, inexhaustible, and non-polluting (Akinola and Fapetu  2006). In many parts of the world there is a growing awareness that renewable energy has an important role to play in extending technology to farmers in developing countries to increase their productivity (Waewsak et al 2006). But the performance of solar dehydration process can be affected by many variables such as amount of sun light, relative humidity, air movement and type of crop to be dried. In this regard, different types of dryers have been developed and used to dry agricultural products to improve shelf life (Esper and Muhlbauer 1996). Most of the dryers in use to date utilize electric power which is expensive and unavailable for smallholder farmers in Ethiopia. Simple natural convection dryers can suit better to dry fruits and vegetables in remote areas of the country.  

Potato is one of the most widely grown tuber crops in Amahara region. Recent survey results reveal that more than 71,325 ha of land in Amhara region were covered by potato in the year 2001/02. It accounts 90% of the total land coverage by root and tuber crops and about 68% of the volume of root crop production. Out of this produce, about 70% was used for household consumption (CSA  2003). In spite of its wide adaptation, it is perishable which often results in high postharvest losses. Drying potato using simple natural convection dryer may be a viable option to reduce postharvest loss and assist farmers to prepare a variety of potato based food products. The objective of this study was, therefore, to determine the performance of two models of direct solar dryers in drying potato and thereby demonstrate better performing dryer to potato producing farmers in Amhara region. 

Materials and Methods

Description and construction of the direct solar dryers

Numerous types of solar dryers were developed to reduce post harvest losses and to improve the product quality. Among different existing simple solar dryers, wooden box and pyramid dryers were selected for farther modification to suit for household level.  Solar dryers manufacturing drawings were prepared and the units manufactured  at Bahir Dar Agricultural Mechanization and Food Science Research Centre. 

Box solar dryer

Pioneering work on solar box dryers was reported by the Brace research institute, Canada (Ekechukwu  1999). Photo 1 illustrates the essential features of solar wooden box dryer.  The drying chamber is made of a simple wooden box with its outer dimension of 1.81 m x 0.83 m x 0.82 m. The upper part of the dryer is covered with a single layer of 4 mm thick glazing transparent glass sheet; with a surface area of 1.72 m by 0.86 m. But the effective area of the glazing collector is 1. 17m². The solar collector (mirror) is oriented facing south and tilted at 21.3^o to the horizontal so as to receive maximum solar radiation during the desired season of use. The best stationary orientation is due South in the Northern hemisphere and due North in Southern hemisphere. This is approximately 10^o more than the local geographical latitude (Bahir Dar is located 11.4^oN). 

The drying chamber has partition in the middle where each partition has three shelves and a total of six sliding trays are inserted for placing the dried products. Each tray is 10 mm deep with wire mesh in the bottom and its effective area was 0.44 m². The relative positions of these trays are: the bottom tray (T3), the middle tray (T2) and the top tray (T1) which is located at 32cm, 21cm and 10cm above the interior bottom floor of the dryer respectively.

Photo 1:  Box solar dryer

The drying trays inside the drying box are constructed from a double layer of fine chicken wire mesh with a fairly open structure to allow drying air to pass through the drying stuff. The solar dryer has a total of 80 air inlet and outlet vents at the bottom with a diameter of 12 mm.  Inlet vents located at the bottom and outlet vents provided towards the upper end at the back and at both sides of the cabinet to facilitate and control the convection flow of air through the dryer. Access door to the drying chamber is also provided at the back of the cabinet. The material used for the construction of the direct absorption dryer is wood, chipboard, 4mm mirror and sawdust between 5cm thick walls as insulating materials. These materials are easily available at the local market. Its weight was 75kg and production cost was about ETB 2075.

Pyramid  solar dryer

Many designs of solar dryers have been developed for the preservation of fish. One of such dryers is the one designed by Doe et al (1977) in Bangladesh. This design was reconstructed to be suitable for potato slice drying. Photographic view of modified one is shown in Photo 2. This dryer has 1.70 m xx 1.48 m x 1.10 m overall dimensions. The front and back sides of the dryer are covered with a transparent plastic sheet; the effective area of the collector glazing is 3.28 m². The dryer has four shelves with sliding trays where the products to be dried are placed. Each tray is 10 mm deep with wire mesh in the bottom and their effective area for tray 1,2,3, and 4 are 0.59 m², 0.74 m², 0.89 m², and1.05 m², respectively. The relative positions of these trays are: the bottom tray (T4) located at 48cm, the next tray (T3) located at 62cm, the middle tray (T2) located at 77cm and the top tray (T1) located 90cm above the internal bottom floor of the dryer. The drying trays are placed inside the drying box which is constructed from a double layer of fine chicken wire mesh with a fairly open structure to allow drying air to pass through the material to be dried.

Photo 2: Pyramid shape solar dryer

The dryer has one rectangular shape air inlet and two triangular shaped outlet windows. The inlets have a total area of 0.05m² while the outlets have an area of 0.06 m².  Inlet vents are located under the edge of the door, whereas the outlet vents are located on the upper end at both sides of the cabinet to facilitate and control the convection flow of air through the dryer. Access door to the drying chamber is also provided at the front of the cabinet (Photo 2). The materials used for the construction of this solar dryer were wood, plywood, and plastic sheet. These materials are easily available in the local market.  Its weight is 20 kg and production cost is about ETB 568.72. The design is simple, can be disassembled and assembled easily during transporting time.

Dryer Performance Evaluation

The performance of dryers was evaluated at Bahir Dar Agricultural Mechanization and Food Science Research Centre. Fresh potato was purchased from the local market to use for the experiment. Before drying, the potatoes were washed, peeled, cut into slices of 58 mm average diameter and 2.8 mm average thickness. The samples were blanched for about 10 minutes in 87ºC water, and then the surface water was removed using wire mesh filter. Finally, the samples were placed on the drying tray in a single layer in each dryer. Open sun drying was used as a check.

Air temperature and relative humidity of both ambient and dryer environment were recorded using temperature and relative humidity sensors which were placed at different locations inside the dryer. The sensors (Model 100 Watchdog data logger of Spec Ware ^TM) were configured to record data at 10 minute interval. The hourly weight difference of the sample was recorded for two consecutive days between 4:00 and 9:00 local time. Conventional air and sun radiation were used to dry potato slices. The test was conducted with dryer inlets and outlets fully opened. 

After evaluation in the center, the selected one was demonstrated for farmers. The dryer with necessary working hand tools and goods were provided to the group of farmers selected in Adet woreda at Inawera kebele. The group have 60 farmers which was over half of participant farmers were women’s. During demonstrated time, technique of operation and handling of dryer and also methods of potato processing taken as major focus area. Chips, Cribs, injera, local arika, bread, and starch extraction and making atemet from potato were some of the activities demonstrated to farmers. Finally farmers tested their dishes and give their comments.

Results and discussion

The two dryers were first compared with open sun drying between May 12 to 14/2010 without loading potato slices. The result of temperature and relative humidity are shown on fig. 3 & 4 respectively.  The three consecutive day’s time average temperature value at the peak time (20:03 hr) was found to be 29.4, 44.0 and 61.9^OC for ambient (open sun), Box and Pyramid dryers, respectively. The overall day time (12:00–24:00) average temperature was found to be 27.3, 38.0 and 48.1^OC for open sun, Box and Type 2 dryers (Figure 1). Box and Pyramid dryers had 10.7 to 20.8^OC higher temperature values than open sun drying, respectively. The solar dryers were well energized during day time to a temperature of 38 to 60^OC, which is suitable for drying fruits and vegetables. During night time the temperature inside the dryers lowered and was almost equal to the ambient condition.

Figure 1. Inside and outside temperature of solar dryers within trial periods

The day time (12:00–24:00) relative humidity for the open sun drying, Box and Pyramid dryers was 36.6%, 24% and 13.1%, (Figure 2). The night time relative humidity was higher than the day time values. A fairly lower value was obtained in Pyramid dryer both during day time and night time which shows that the potential of the drying air helps to remove from the potato slices.

Figure 2. Relative humidity inside and outside solar dryers within trial periods

In other hand, it was tried to see the difference in temperature among trays arranged vertically while fully loaded with potato slices on both dryers. The result shows that the temperature in the upper tray was higher than the trays in the middle and bottoms (Table 1).

The reason is the direct sun radiation fully falls first on the top tray then through time expands to the rest of the trays. This observable fact is common to this type of solar dryer, but to obtain uniform final moisture content, the trays has to be rotated frequently within determined intervals.  

The results on the moisture content of potato slices showed that in Box dryer the upper tray (Tray 1) exhibited the most rapid drying (Figure 3). In the first day, after 4 hours of drying the moisture content of the potato slice dropped from 78.9% to about 68.4%, 69.7%, 72.3%, and 75.7% (wet basis) for Tray 1,Tray 2, Tray 3 and open sun, respectively.

During the second day, the moisture content decreased gradually to 17.2%, 24.9%, 28.0% and 40.6% for respective tray locations. The final moisture content for the above respective trays was 8.2%, 17.0%, 18.1% and 20.8% on wet basis. This condition was also happen on type II dryer. These moisture contents indicated that the first tray drying time was faster than the other trays. The moisture content for potato in the open sun tray showed slow drying time requiring 2 to 3 hour getting a stable dried material.

Figure 3. Moisture content of potato slices on different trays of Box (left) and Pyramid (right) dryers

The results of the drying rate analysis indicated that Tray 1 expectedly had the highest drying rate during the first 3 hours (Figure 4). However, as it got dried its drying rate decreased from 1.07 up to 0.167%.  The drying rate of the potato on the other trays was less than Tray 1 after hours because the drying air absorbed less moisture from the trays.

Figure 4. Drying rate curves for potato on a dry weight basis.  A. Box (left)    B.  Pyramid (right) dryers

Figure 6 plays the variation of air temperature with vertical distance from the bottom of the drying chamber. The major drawback of the shelf-type dryer is uneven drying. As a result of the migration of the drying front, the materials at the upper trays dried first, while the middle trays delayed in drying resulting in uneven drying.

Demonstration

Due to its optimal performance, simplicity, low cost and weight Pyramid dryer was selected for demonstration. Demonstration was carried out for selected 60 farmers in Adet woreda at Inawera kebele. The dryer along with different methods of potato processing using dehydrated and not dried potato was demonstrated for various potato producing farmers. Both men and women households were involved during the demonstration.  They were actively participated in solar driers utilization; potato menu preparation and informal sensory evaluation (Photo 3).They have showed high interest diversified potato utilization as it enhances their feeding habits.  

Photo 3: farmer’s participation during demonstration time

It was then observed that most farmers due to low production cost, light weight, and manufacturing simplicity point of view showed interest in using the dryer together with the new methods of home processing of potato slices. Participant farmers actively participated in the potato menu preparation and informal sensory evaluation. They have showed high interest diversified potato utilization as it enhances their feeding habits.

Conclusions

• From the production cost, weight, transportation and manufacturing simplicity point of view Pyramid dryer is preferable than Box solar dryer. Farmers said that they can produce this dryer from local materials using their own skill.

  
  

• Drying time reduced up to 2-3 hour compared to open sun drying, but this difference was not large enough under existing weather condition; however, the dried material is protected against contamination by dust, and insects. As a result, the product quality will be high.

  
  

• Uneven draying was seen among different trays on both dryer types. But the problem of irregular drying can be alleviated by rotating the drying shelves.

  
  

• Farmers got knowledge on the advantage of processing potato. They were able to use potato slices in different mode of food preparation, so that they are upgrading their feeding habits and saved major crops to use at home.

  
  

• In general the design of Pyramid dryer is found suitable for drying small quantities (10-15 kg) of agricultural products which are best for household level. Therefore, this dryer should be recommended for further promotion.

References

Akinola A O  and Fapetu  O P  2006. Exergetic Analysis of a Mixed-Mode Solar Dryer.

Central agricultural census commission 2003 Ethiopian Agricultural sample enumeration. 2001/02 (1994E.C) Result for Amhara Region statistical report Adis Ababa

Esper A and Muhlbauer W 1996 Solar tunnel dryer. Plant Research and development 44(4):16-64.

Ekechukwu O V a N B 1999 Review of solar-energy drying system II: an overview of solar drying technology, Energy Conversion and Management 40 pp 615-655, Pergamon.

Doe P E Ahmed M, Muslemuddin Sachithanathan K 1977 A polyethylene tent dryer for improved Sun drying of fish, Food technology in Australia, 29 pp 437 – 441.

Waewsak  J, Chindaruksa S and Punlek C 2006 A mathematical modeling study of hot air drying for some agricultural products. Thammasat International Journal of Science and Technology  11(1): 14-20. 

Received 7 September 2013; Accepted 27 October 2013; Published 1 December 2013

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