K.A . Adesina
Food Science and Technology Department, Rufus Giwa Polytechnic, Owo
L. Alade
Food and Biochemical Engineering Department, Bells University, Ota, Ogun State, Nigeria
R.O. Ayodele
Mechanical Technology Department, Rufus Giwa Polytechnic, Owo


The efficiency of parabolic concentrator dryer over improved sun drying was investigated. Parabolic concentrator dryer was designed in form of a butterfly. It was fabricated by 1.5mm iron flat sheets, ¼ mild steel rods, 20mm and 40mm square iron steel pipes and 4 rollers trolley. Silver coated mirror was used as reflector. The focus was determine to be 36 inches from the centre of the parabola Performance evaluation was carried out during the raining season, July, between 12 noon and 5pm using fresh tatase pepper (Capsicum annum). Pepper sample was prepared by cutting and water blanched for 15mins at 250oC to destroy the vegetative flora of fresh tatase. 300g of sample was dried using parabolic concentrator and convectional sun drying. The daily and overall drying efficiency of the dryers was based on the percentage moisture evaporated obtained by sample weight difference. Drying curve obtained by parabolic concentrator agreed with the drying curve obtained by Arocho, 2004 was while a slightly deviated curve was found with sun drying method.  Parabolic concentrator method gave the drying efficiency of 66.66%, 33.33%, 50%, 33.33% while sun drying method was 23.33%, 59.09%, 33.33%, 20% per drying days. Overall efficiency of 93.33% was obtained by parabolic concentrator method while 86.66%. The trend revealed that parabolic concentrator dryer is efficient throughout the drying days based on the pepper sample used.

Keywords: Parabolic concentrator, sun drying, tatase (Capsicum annum), drying efficiency, butterfly design, moisture evaporated



In many cases, the drying of materials is the final operation in a manufacturing process carried out immediately prior to packaging or dispatch. Drying refers to the final removal of water and the operation often following evaporation, filtration or crystallization. In some case, drying is an essential part of the manufacturing process, as for instance in paper making or in the seasoning of timber. Though in the majority of processing Industries, drying is carried out for one or more of the following reasons as to reduce the cost of transport, to make a material more suitable for handling for example soap, powder, dye stuffs, fertilizers, to provide definite properties such as maintaining the free flowing nature of salt, to remove moisture content which may otherwise lead to corrosion for example, the drying of coal gas or of benzene prior to chlorination (Coulson and Richardson, 1991).

Drying is an excellent way to preserver food and solar food dryers are an appropriate food preservation technology for a sustainable world. Every year, millions of dollars worth of gross national product are lost through spoilage. Drying of crop can change this trend and is useful in most areas of the harvesting season. If drying of product were widely implemented, significant saving to farmers would be achieved and these savings could help strengthen the economic situation of numerous developing government as well as change the nutritional condition in these same countries (David and Whitfield, 2000).
Solar concentrator may be classified into two groups depending on their shape (Box concentrator, spherical concentrator, parabolic concentrator). These set of concentrator had been used amongst campers in the developed world and in India. Some West African countries such as Nigeria are becoming popular in the re-use of solar boxes (Bruce, 1974). A great deal of development work has been done on box concentrator as a cooker due to their simplicity to the cooking habits of most communities of the world (Adegoke and Fasheun, 1998). Another disadvantage of solar box concentrator is that its performance might be very poor during the tropical rainy season.           

Parabolic concentrators on the other hand are thermally efficient and high temperature attaining device (Bonazzi, 1990). Their development, production and use have been hampered by technicalities of constructing a parabolic reflector and the prohibitive cost of reflective material as well as the difficulties associated with the tracking of the sun (Bonazzi, 1990; El-kassaby 1991). In designing and fabrication of parabolic concentrator, high precision is however needed otherwise the reflecting sunlight would not be concentrated to the desired small area. A poor performance will result in a dryer that will not be able to heat to the needed high temperature. The recommended approached, with least of error, is to use a wooden template from the parabolic equation. Parabolic shape is most preferred simply because its exposed surface for sunlight collection is much larger than a spherical shape of comparable area or volume.

Therefore, the aims and objective of this work are to produce improved food solar dying equipment by designing and fabricating a parabolic concentrator dryer and to evaluate the performance of the parabolic reflector dryer.

Design considerations

  1. Parabolic Surface Wooden Template

An 8ft by 4ft plywood was cut into two halves such that each half was 8ft by 2ft. The 8ft edges were kept smooth and straight. Parabolic curve table was constructed using parabolic equation. X and Y axes were drawn on
each of the plywood. Y axes were drawn at the centre of each of the plywood such that it is perpendicular to X axes. 2 inches interval was marked on both y and x axis with x axis marked at both directions (positive and negative co-ordinate). All the points were joined together to obtain the parabolic surface and the curve was cut out by a jigsaw. The two plate wood pieces were mounted on each other by cutting out 8 inches long strip of width equal thickness of the wooden plate. The strip was cut along the Y-axis perpendicular to the X-axis of a plate while the other wood was in reverse. 

  1. Parabolic surface model

The wooden template was used to obtain a perfect parabolic curve for the metal model. The metal model was obtained by ¼ inch mild steel rod. Five rings made from the rod were placed round the wooden template at a spacing of 9 inches such that the diameters of the circle of each ring and their corresponding positions from the top of the dome of the wooden model is given in Table 1.



Table 1: Parabolic curve parameters

Circle No

Diameters (inches)

Position from the dome (inches)
















These rings were joined together by web of 18 pieces of 43 inches long connecting rods made of mild steel rods. This is to ensure that the metal model maintain perfect parabolic surface after the removal of the wooden template. The rings and the web of connecting rods were welded together at the back of the curve. The wooden template was removed after welding; hence the perfect parabolic surface.

  1. Design Type – Butterfly

In order to minimize the cost of fabrication, half of the metal parabolic curve was used. The half was further divided into two halves of ¼ circle and the two ¼ circles was used in the completion of parabolic dryer such that each ¼ take each wing of a Butterfly.

  1. Sheet surfacing

18 spaces were obtained from the web of connecting rods. These spaces were covered by 18 pieces of flat mild steel sheets; 9 sheets per wing. The dimension of the surface sheet was 43 inches x 7 inches x 4 inches (L x W x H).

  1. Reflector/Reflecting surface

Silver coated mirror was used as reflector for this design. 18 pieces of the same dimension 43 inches x 7 inches x 4 inches (L x W x H) was cut out to make the reflecting surface. These mirror plates were further pieces into squares of 4inches x 4inches and 3inches x 3 inches. This was done to ensure that the reflector when fixed on the surface will maintain the perfect parabolic curve. The reflectors were fixed to the surface by a water resistance silicone adhesive.

  1. Drying Chamber

Drying chamber was constructed with flat mild steel sheet using the dimension in 15 inches x 12 inches x10 inches (L x W x H). The chamber consists of two shelves/tray of and it was perforated by the sides and underneath. The perforation is to allow the passage of air flow in and out of the drying chamber for effective drying of food samples placed inside the chamber. The chamber is painted black for effective heat absorption for the drying process. The drying chamber was placed at the focal point of the parabola which was obtained to be 36 inches from the centre of the parabola.


Performance evaluation
Fresh tatase (Capsicum annum) was purchased from Oja-Oba market in Owo. It was washed in a large bowl. The pepper was cooked for 15mins at 250oC to destroy the vegetative flora of fresh tatase and drained. It was shredded into tiny shape so that an efficient drying procedure could reduce the moisture content to an inhibitory growth of spoilage micro- organism.300g of the cooked sample was collected for the performance experiment. The experiment was carried out in July (raining season) between the hours of 12 noon and 5pm.

In sun drying method, the sample was spread on a flat surface and exposed directly to the sun and was occasionally turned to ensure homogenous at every half hour. Similarly, in parabolic concentrator method, the sample was spread inside the perforated rays in the drying chamber. The parabolic was adjusted frequently for tracking of sunrays for effective drying. The weight moisture evaporated (dried) at every 1hr interval were determined by weight difference. Drying continues until constant weight of the dry matter was achieved. At the end of each drying day, the samples were cover with cellophane bag to prevent re-absorption of moisture.   

Results and discussion


Figure 1 : Drying curve for Parabolic Reflector drying
Figure 2:  Drying curve for Improved Sundrying

Figure 3: Drying efficiency in terms of % daily and overall moisture evaporated

The drying curve results for parabolic reflector dryer and improved sun drying method are shown in Figure 1 and 2 respectively. The drying curve obtained from parabolic concentrator dryer gave a perfect drying curve which compare favourably with the drying curve of similar works of Bala et al., 2003 for pineapple and Prachayawarakorn et al., 2003. The curve from sun drying method show slight constriction and deviated. The parabolic results indicate that parabolic concentrator dryer is efficient in drying pepper though constant weight was obtained in four (4) days for both methods.

Figure 1 shows the daily and overall moisture evaporated for parabolic concentrator drying and sun drying method. The trends revealed parabolic concentrator dryer to be more efficient throughout the drying days. The overall drying efficiency of parabolic reflector drying method was 93.33% while sun drying method was 86.66%. The performance evaluation was conducted during the raining seasons when the relative humidity of air is usually high and the temperature is low. Parabolic reflector drier drying time is expected to be more efficient in dry season when the relative humidity of air will be low and the temperature high.

Parabolic reflector dryer have some advantages over sun drying method. These include protection of the sample from rain and extraneous substances which can contaminate the food while sun drying method can easily get contaminated by any extraneous materials such as stone, leaves, sand, animal waste during drying process and also direct sun drying of some the food substances can affect the quality of the product.
Parabolic concentrator was found to be adequate for drying process as it gave a significant performance based on the sample verified over the convectional sun drying method. The initial cost of installing the parabolic reflector would be higher than the cost of sun drying method but the former is more advantageous than the latter. The service cost is however minimized as it does not require huge maintenance cost. Meanwhile the efficiency of the parabolic dryer need to be verified with other food samples to ascertain the extent of its applicability. With the present efficient, the drying chamber could be redesigned for enhance capacity. Performance evaluation needs to be carried out during the dry season to compare its efficiency.


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