DESIGN AND FABRICATION OF A DOUBLE-CHAMBER SOLAR DRYER

temperature, ABSTRACT Object of research: This paper discusses the design and construction of a modified, cost effective solar dryer for use by the average Nigerian farmer or agri-business entrepreneur. Investigated problem: Local farmers often have a lot of difficulties in properly drying har-vested agricultural produce for storage and processing purposes, safely and efficiently due to capital constraints. There was therefore a need for an efficient, low cost solar dryer design to aid in this pursuit. Methodology : The designed dryer is a passive dryer that makes use of heat energy tapped through glass collectors placed over the drying and air inlet chambers respectively. The dryer was designed and constructed with carefully selected, inexpensive materials with dimensions of 40×40×60 cm for the drying chamber and dimensions of 10×80×40 cm for the heating chamber. The glass collectors (4 mm thick) were inclined at an angle of incidence of 17.26°. Okra slices were used to test the performance of the dryer. Results/Area of practical use: The study yielded a low cost (€145.60) modified solar dryer capable of drying agricultural produce in a safe and clean way. During eight (8) hours of drying under a temperature range of 39–45 °C, the initial weight of the okra slices reduced from 150 g to 9 g, 9 g, 10 g and 9 g on each of the four trays of the drying chamber respec -tively. Local farmers and agripreneurs will be encouraged to make use of this clean alter-native of drying food produce without the drawbacks from regular sun drying. Conclusion : There is need for more work to be done in terms of installation of solar panels to enhance dryer performance. More work should also be carried out on tests during the dry season for increased dryer

relative humidity, solar radiation and wind speed, amount of initial moisture content, type of dryer etc. [5].

The object of research
The object of this research is to design and construct a modified, cost effective solar dryer for use by the average Nigerian farmer or agri-business entrepreneur.

2. Problem description
For centuries, various nations have preserved fruits, meat, fish and other crops by drying. Drying is also beneficial for hay, copra, tea and other income producing non-food crops. With solar drying, the availability of all these farm produce can be greatly increased. Solar drying is the oldest method of food preservation wherein dried food is preserved and concentrated. Dried foods do not require any special storage equipment and are easy to transport. Studies show that food items dried in a solar dryer were superior to those which are sun-dried when evaluated in terms of taste, color and mold count. Solar dried foods can be stored at less cost while still providing excellent nutritive value [2].
All drying systems can be classified according to their temperature range i.e. high temperature range dryer and low temperature range dryer [3]. Nowadays, optimization of solar systems is used to reduce system total cost, increase life cycle savings and improve thermal efficiency. It is very demanding for optimal utilization of solar resources to meet the energy demands [6]. Several solar dryers, with different variations have been designed. In 2007, [7] designed a mixed-mode natural convection solar crop dryer (MNCSCD) for drying cassava and other crops in an enclosed structure. [1] (2020) designed an active indirect-mode solar dryers for drying cooking banana s in Nigeria. The dryers were constructed with special focus on the air inlet, to examine if the air inlet area had effect on the performance of the dryers. Solar drying generates high air temperatures and produces low air relative humidity. Alonge and Jackson [1] developed an indirect forced convection solar dryer for cassava in their bid to address concerns related to time of drying of cassava chip by other methods of drying.

3. Suggested solution to the problem
Passive solar dryers are also called natural circulation or natural convection systems. These dryers can be either direct (e.g. tent and box dryer) or indirect (e.g. cabinet dryer). Natural-circulation solar dryers depend for their operation entirely on solar-energy (Behera et al., 2017).
Active solar dryers also called forced convection or hybrid solar dryers utilize forced convection throughout the drying process to control temperature and moisture in wide ranges independent of the weather conditions. The use of forced convection can reduce drying time by three times and decrease the required collector area by 50 % [3].
Aim of research was to design and construct a passive modified two-chamber solar dryer for use by rural farmers.

Materials and Methods
The following factors were considered in the design of the solar dryer. 1. Materials' Selection: The materials used were sourced locally and at the cheapest price. They were selected on the basis of availability, cost, and durability amongst others.
2. Dimensions: The external dimensions of the solar dryer are 40×40×60 cm, with internal dimensions of 35×35×55 cm. The difference is due to the thickness of the metal used and the lagging material. The drying chamber is 57 cm in 36×36×56 cm with air passage out of the chamber through an outlet of 5 cm in diameter. Fig. 1 shows the external dimensions of the dryer.
3. Air Flow: The designed air vent to the solar collector aids in air suction to the drying chamber. The hot air is directed by the fan and it rises into the drying chamber. The heated air passes through the trays and around the farm produce reducing the moisture content and exits through the outlet near the top of the chamber. Thus the system is a passive solar system. The machine consists of the following parts: a. The Drying Chamber: This is designed to accommodate four layers of drying trays made of net cloth on which the produces are placed for drying.
b. Dryer Trays: Net cloth is used as the dryer screen to aid air circulation within the drying chamber. 4 trays of dimensions 30×30 cm are used in the design. The trays are spaced 10 cm apart.
c. The Air Inlet and vents: This is the opening through which air enters the system. The dimension of the air inlet is 22×35cm. For air to flow out of the solar dryer, two air vents were made at the back of the drying chamber. The vents have a diameter of 5 cm each. d. Fan: A fan with a blade height of 7 cm, standing 8 cm from the floor and powered by a battery is placed 20 cm in front of the air inlet to help in driving heated air into the drying chamber. The fan's speed is 1300-1550 rpm.
e. Glass plate Collector: Transparent glass is used to collect heat from the sun radiation. A glass covering of 4 mm thick is used to make the roofing of the chamber.
f. Battery: A battery of 12 volts is used to power the fan. The battery is detachable and can be charged when the dryer is not in use.
g. The Frame of the Dryer: The frame was constructed from square pipe. The pipe was cut and welded.
h. The Door and Vents of the dryer: The door of the dryer was constructed from a metal plate of 799 mm long and 599 mm wide for both the external and internal surfaces. The door was filled with lagging material to reduce heat loss, and joined to the main dryer with the use of hinges.  The dryer, being a tray loading type has the volume:
Considering the specie of okra (Hibiscus esculentus) used for this design, the bulk density was determined to be 670 kg/m 3 .
Using the formula: Density=Mass/Volume. where Ht=Total Heat required for vaporization; Hr=Heat required to increase air temperature to vaporization temperature; Hl=Latent Heat of Water at 60 °C=2359 kJ/kg [8] and where Tv=vaporization temperature, Tm=temperature of material (okra), L=Specific heat capacity of the material in kJ/kg°C=3.54 kJ/kg°C [9]. Therefore Optimum drying time for the okra was set at 4-12 hours (based on volume) at a range of 40-70 °C [10].
The angle of tilt of the solar collector was calculated using where lat Φ=latitude of collector location (Ibadan) β=10°+7.26°=17.26°. Fig. 2 shows the solar dryer after fabrication. The dryer has two chambers (one for heating air, and the other for proper produce drying). The fan can be powered by electricity or a small battery. Table 1 shows that there is a steady and uniform reduction in the weight of the okra slices during the drying process. The control, which is T 0 showed decrease from the initial weight of 150 g to 127 g, 93 g, 55 g and 15 g. Tray 1 reduced from the initial weight of 150 g to 9 g, while Tray 2 reduced from 150 g to 10 g. Tray 3 reduced from 10g while Tray 4 showed weight reduction from 150 g to 9 g. The drying temperature fluctuated between 390 °C to 450 °C. There was negligible color change in the produce while drying. Moisture content after 8 hours Adr=0.140/(8×60×60)=4.86×10 -6 kg/hr.

Solar dryer cost analysis
The dryer cost forty thousand naira (€ 80) to construct. Factoring in labor (30 % of material cost), overhead (10 % of material cost) and a modest thirty percent profit margin gave the final selling price of the dryer at seven two thousand, eight hundred naira only (N 72.800) or one hundred and forty five euros (€ 145.60). Table 2 shows the bill of materials used in the production of the solar dryer. Prices are shown both in Naira and in Euro (Exchange rate used: €1=N500). The result showed that there is reduction in the weight of the okra slices throughout the eight hours of the drying process. The control which is T 0 showed decrease from the initial weight of 150 g to 15 g over the drying period. Tray 1 reduced from the initial weight of 150 g to 9 g, Tray 2 reduced from 150 g to 10 g, Tray 3 reduced from 150 g to 10 g, and Tray 4 from 150 g to 9 g all in an 8 hour drying period. Negligible color changes were noticed throughout the drying period in the oven.
Solar dryer efficiency was calculated to be 16.2 %. This was due to the limiting fact that the project work was carried out during the rainy season. It is advised that further research work should focus on drying during the dry season.

Conclusion
Solar drying is a more effective drying medium for agricultural produce by small and medium scale farmers in Nigeria. The study yielded a low cost (€ 145.60) modified solar dryer capable of drying agricultural produce such as okra in a safe and clean way, to an acceptable moisture content, at medium heat ranges of 39-45 °C. Low solar dryer efficiency of 16.2 % was determined to be due to the rainy weather, hence the dryer should be used during the dry season for better and more effective performance. Solar panels can be incorporated into the dryer to increase drying efficiency and drying time. A regulator can also be mounted to regulate the speed of the suction fan.