Design, fabrication and performance evaluation of solar dryer for banana
© Hegde et al. 2015
Received: 27 February 2015
Accepted: 6 July 2015
Published: 28 July 2015
An indirect, active-type, environmentally friendly, low-cost solar dryer was designed to dry various agricultural products. The dryer was built by locally available, biologically degradable, low-cost materials. The dryer consists of solar flat plate air heater with three layers of insulation, drying chamber and a fan with a regulator to induce required air flow in the system. Banana is the chosen crop for the experimentation since it is high in production and also has substantial loss in India. Also, dried bananas are having good nutritive value which makes it as essential diet.
The experiments were conducted to dry banana slices and to study its drying characteristics like rate of drying and quality of dried banana in terms of taste, colour and shape. The dryer has the following features: two different air flow configurations (air flow between glass cover and absorber plate called as the top flow and air flow between absorber plate and the bottom insulation of solar collector called as the bottom flow), forced flow with variable flow rates from 0–3 m/s and two different mounting schemes (conventional trays and wooden skewers).
In the top and bottom flow experiments, the bottom flow provided about 2.5 °C higher chamber temperatures than the top flow for the same solar energy input. The efficiency of top flow configuration was found to be 27.5 %, whereas the efficiency of bottom flow configuration was found to be higher at 38.21 %. The results also agree well with the theoretical calculations performed as 60 W of energy can be saved for the same energy input.
The drying rate was found to increase when wooden skewers were used instead of conventional trays. At the end of the day, the total difference in moisture content is found to be 3.1 % which is considerable knowing that the rate of drying drastically decreases with time. Banana dried at 1 m/s air flow rate was of the best quality in terms of colour, taste and shape when compared to drying at 0.5 and 2 m/s air flow rate while the weather condition and ambient conditions were almost the same for all the cases with negligible difference.
KeywordsSolar drying Design of solar dryer Thermal analysis Experimentation Top flow and bottom flow Conventional trays and wooden skewers Varying air flow rate
In many parts of the world, awareness is growing about renewable energy which has an important role to play in extending technology to the farmer in developing countries like India to increase their productivity. Poor infrastructure for storage, processing and marketing in many countries of the Asia-Pacific region results to a high proportion of waste, which average between 10 and 40 % . Although India is a major producer of horticultural crops, many Indians are unable to obtain their daily requirement of fruits and vegetables and the Human Development Index (HDI) is very low. Considerable quantities of fruits and vegetables produced in India go to waste owing to improper postharvest operations and the lack of processing . This results in a considerable gap between gross food production and net availability . Reduction of postharvest losses is essential in increasing food availability from existing production . Traditional techniques used in food preservation are drying, refrigeration, freezing, salting (curing), sugaring, smoking, pickling, canning and bottling. Among these, drying is especially suited for developing countries with poorly established low-temperature and thermal processing facilities. It offers a highly effective and practical means of preservation to reduce postharvest losses and offset the shortages in supply.
Drying is a method of dehydration of food products which means reducing the moisture content from the food to improve its shelf life by preventing bacterial growth . It is still used in domestic up to small commercial size drying of crops, agricultural products and foodstuff such as fruits, vegetables, aromatic herbs, wood etc. contributing thus significantly to the economy of small agricultural communities and farms [4–6].
Hii et al.  have shown that sun drying (laying the crops under direct sunlight) is economical, but the product obtained by it is of lower quality due to contamination by dust, insects, birds, pets and rain. Also, loss of vitamins, nutrients and unacceptable colour changes due to direct exposure to ultraviolet rays, and it takes long time to dry. Solar dryers are specialized devices that control the drying process and protect agricultural products from damage by insect pests, dust and rain. Umogbai et al.  made a comparison between sun drying and solar drying and obtained that solar dryers generate higher temperatures, lower relative humidity, lower product moisture content and reduced spoilage during the drying process than sun drying. Rajeshwari and Ramalingam  have demonstrated that the drying time in case of solar dryers compared to open air drying reduced by about 20 % and produces better quality dried products. Solar dryers are available in a range of size and design such as tunnel dryers, hybrid dryers, horizontal- and vertical-type dryers, multi-pass dryers and active and passive dryers [10–17]. Hii et al.  classified solar dryer according to their heating modes and the manner in which the solar heat is utilized, namely forced air circulation or active solar dryers and natural air circulation or passive solar dryers. Three distinct sub-classes of either the active or passive solar drying system can be identified depending upon the design or working principle of the dryer, mode of drying and type of product to be dried, namely integral or direct mode, distributed or indirect mode and mixed mode solar dryers. It should be noted that sunlight may affect certain essential components in the product, e.g. chlorophyll is quickly decomposed. If available places are scarce, indirect mode types of dryers are preferred for drying larger quantities. In such case of indirect mode, nutritive value of the food product and colour is retained.
Mohanraj and Chandrasekar  and Banout and Ehl  concluded that forced convection solar dryer is more efficient than natural convection dryers. Also, products can be dried faster in the case of forced convection solar dryer than in the case of natural convection solar dryer, and end products obtained from forced convection drying have a superior quality.
From the literature survey, it is evident that though there are many dryer designs which involve the flow of air between glass cover and absorber plate in the collector  and also in some other designs, the flow is maintained between absorber plate and bottom insulation . However, there is no comparison of the performance done between these two cases in a single setup. Hence, there is a requirement for comparative study to address the relative performance of the above-mentioned cases and hence to arrive at a better and efficient flow configuration.
Indian Horticulture Database 2013  shows that banana is the most important fruit crop in India, accounting for 32.6 % of the total fruit production. Almost the entire production is used fresh, and hence, the entire production is subjected to the postharvest losses of 17.87 %. Banana is the chosen crop for the experimentation since it is high in production and also has substantial loss in India. Also, dried bananas are having good nutritive value which makes it as essential diet [23–25].
Most of the thin-layer drying of fruits is carried on using stainless steel meshed trays . In practice, trays have many disadvantages which include sticking of the dried products to the trays, difficulty in loading and unloading, hygiene among others. Hence, an innovative way of placing the bananas in the trays is devised in which wooden skewers are used to hold the fruits. Though the various literatures on drying banana included the studies on the optimum slice thickness, solar-assisted dryer for bananas and effect of various pre-treatments and temperatures on banana among others [26–29], the effect of the air velocities on the moisture removal rate and quality of the dried bananas obtained has not been studied. As air velocity also plays an important role in drying of food products [30–33], there is a need to address the effect of air velocity on drying of banana.
In the present study, low-cost indirect-type solar dryer was designed and constructed using locally available environmentally friendly materials to compare the performance of the flat plate collector for the top flow and the bottom flow of air both theoretically and experimentally, to compare the performance in the form of moisture removal rate and dryer efficiency for different banana mounting methods such as wooden skewers and on conventional trays and to dry banana using different air velocities 0.5, 1 and 2 m/s and to compare the quality of the end product in terms of taste, texture, colour and final moisture content.
The design process of the dryer first involved the collection of the climatic data of the study location, i.e. Bengaluru. Further, the other important data such as insolation was studied and calculated as per the collector configuration. For the initial phase of dryer design, many existing designs were studied and some of the design parameters were determined. The performance of the dryer was then analysed [34–37]. Once the dimensions of the dryer were fixed, an appropriate axial fan was selected to obtain the required flow rates.
Climatic data collection
Bengaluru is located in Karnataka, India, at a latitude of 12° 58′ North and longitude of 77° 34′ East. Solar radiation over the year on horizontal surface in Bengaluru is found to be 666.635 W/m2 . Total solar radiation on a 13° tilted surface is calculated as 676.367 W/m2.
The dryer was constructed using plywood, stainless steel mesh, wooden skewers, clear glass, galvanized iron sheet and axial fan for operation of the dryer which are locally available with low cost.
In the design, a flat plate collector with an area of 1.6 × 0.6 m2 is considered. The performance of the collector is described by an energy balance that indicates the conversion of solar radiation into useful energy gain and losses. The thermal analysis was done to calculate the heat gain and losses for flow of air between glass cover and absorber plate which is the top flow and flow of air between absorber plate and bottom insulation which is the bottom flow [3, 36, 41]. Figures 1 and 2 show the typical configuration of top flow and bottom flow, respectively.
Top loss (W)
Side loss (W)
Bottom loss (W)
Specification of the dryer
Specification of the dryer
Absorber plate dimension
1.6 × 0.6 m
Glass cover thickness
Insulation total thickness (bottom)
Gap between absorber plate and glass cover
Gap between absorber plate and insulation
Number of trays
0.3 × 0.6 m
Distance between trays
Tilt angle of the collector
13° due south
Flow over the absorber plate (top flow) and flow beneath the absorber plate (bottom flow)
Case A: Air is allowed to pass between the absorber plate and glass cover (Fig. 4). The air passage between absorber plate and bottom insulation is blocked using a cardboard with adhesive tape and glue.
The temperature readings were taken, and the losses and gain are calculated and compared.
Conventional trays and wooden skewers
where 77.2 % is the initial moisture content of banana variety selected.
Varying air flow rate
The air flow though the dryer was varied using a speed regulator for the axial flow fan. The experiments are conducted, and repeatability tests are also made. As the experiments have been done on consecutive days, there is a very little change in atmospheric temperature and solar radiation.
Based on the test results of drying banana slices kept in trays and wooden skewers, varying velocity tests were conducted only on banana slices attached to skewers. The velocity of air flow is maintained as 0.5, 1 and 2 m/s in the collector region (0.0169, 0.0338 and 0.0676 m3/s volume flow rate) for a duration of 16 h of drying time for consecutive days with each day 8 h from 9 am to 5 pm. Every hour the weight of the banana slices was measured, and the moisture content and the efficiency of the dryer were calculated. At the end of the day, banana slices were stored in air-tight bags. After drying for 16 h, the dried banana samples obtained from using 0.5, 1 and 2 m/s were compared in terms of taste, texture, colour and final moisture content.
Results and discussions
Comparison of the top flow and bottom flow
Thus, it can be concluded from this experiment that the bottom flow configuration is more efficient than the top flow configuration.
Comparison of conventional trays and wooden skewers
It is clear from Fig. 11 that the rate of moisture removal is better with the skewers at every stage of time. At the end of the day, the total difference in moisture content is found to be 3.1 % which is quite considerable knowing that the rate of drying drastically decreases with time. In terms of the weight of moisture removed, it is 825 g for trays while it is 864 g for skewers, the difference being 39 g.
The cumulative efficiency continuously drops as the rate of moisture removal drops even though the input energy is the same because of this falling rate period. Starting off with better efficiency, the skewer configuration maintains the higher efficiency throughout the day over the tray-type configuration. At the end of the day, the efficiencies are 8.45 and 8.06 % for skewer and tray type, respectively.
Flow rate for drying of banana
The maximum temperatures achieved with 0.5, 1 and 2 m/s are 49.5, 45 and 41 °C, respectively, with almost similar ambient air temperature for all the velocities. At the end of 2 days of drying, i.e. 16 h, the moisture content in the bananas is 34.98, 29.63 and 36.04 % for 0.5, 1 and 2 m/s, respectively. If the absolute moisture removal rate is considered then the moisture removal rate is fastest with the velocity of 1 m/s, followed by 0.5 and 2 m/s as seen in Fig. 13.
In the top and bottom flow experiments, the bottom flow provided about 2.5 °C higher chamber temperatures than the top flow for the same solar energy input. The efficiency of top flow configuration is found to be 27.5 % and the total heat loss or the case is found to be 201.9 W, whereas the efficiency of bottom flow is found to be higher at 38.21 % and the total heat loss is found to be 139 W. The experimental results are in excellent agreement with the theoretical values with the savings of 62 W energy. Hence, the bottom flow configuration is more efficient. The drying rate is found to be increased when skewers are used instead of conventional trays with ease of loading and unloading of banana in the case of skewers. At the end of 16 h of drying, about 3.1 % difference in moisture content is obtained between the two configurations which is significant. The result also shows that the banana dried at 0.0338 m3/s volume flow rate (velocity of 1 m/s over the collector) is of the best quality in terms of colour, taste and shape when compared to drying at 0.5 and 2 m/s flow rate for the same solar energy input and atmospheric conditions.
We are thankful to the Principal, RV College of Engineering, Head of the Department, Mechanical Engineering for their support throughout the work. We are thankful to Professor CS Prasad for sharing with us the technical knowledge and helping us to achieve the results. We are also thankful to Professors M S Krupashankara and Dr. J R Nataraja for their invaluable support and encouragement.
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