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Journal of Energy
Volume 2013 (2013), Article ID 479894, 8 pages
http://dx.doi.org/10.1155/2013/479894
Research Article

Analysis and Testing of a Natural Convection Solar Dryer for the Tropics

The Department of Mechanical Engineering, University of Lagos, Akoka, Yaba, Lagos 101017, Nigeria

Received 12 December 2012; Revised 20 February 2013; Accepted 20 February 2013

Academic Editor: Mattheos Santamouris

Copyright © 2013 A. O. Adelaja and B. I. Babatope. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Solar dryers are imperative for the tropical and sub-Saharan African countries, which are faced with the duo challenges of inadequate electrical energy supply, which has severely limited the application of conventional refrigeration as a means of preservation of agricultural produce, and the need to make produce competitive in the international market. In this study, a cost-effective natural convection solar dryer was developed; the thermal and drying analyses were done and tested to obtain some performance evaluation parameters for the system in order to examine its efficiency and effectiveness by drying some plantain fillets. The collector and system efficiencies are found to be 46.4% and 78.73%, respectively, while a percentage moisture removal of 77.5% was achieved at the 20th hour in order to give final moisture contents of 15.75% in the product, which still maintained its integrity. With a cost of about $195.00, it has been affordable for the small- and medium-scale enterprises as well as for private use in domestic applications.

1. Introduction

Sun drying is the most common, economical and environmentally friendly method of preservation of agricultural produces in the tropical and sub-Saharan African regions. Open sun drying, however, has some disadvantages, of which are, undue exposure of produce to weather elements such as rain, ultraviolet rays of the sun, and contamination by wind-borne dirt and dust. Others include infestation by insects, rodents, pests and other animals as well as degradation by mucus and bacteria. These, for a long time, had made agricultural produce consumed locally, and those meant for export to fall short of the required hygienic and international standards, and hence, they are very often uncompetitive. Obviously, lack of effective preservation technology is a major stumbling block for farmers in the tropics and sub-Saharan African regions, as electricity is either inadequate or unaffordable to the small-scale farmers, thus discouraging refrigeration of the farm produce. This has left most farmers, particularly those engaged in small- and medium-scale farming, with no other option than to depend on one of the most abundant and free primary energy resources—the sun. Faced with the challenges of inadequate supply of electricity, on one hand, and the need to make farm produce competitive in the international market, on the other hand, the closest solution option is the use of solar dryer. This drying technique is in no way a new concept. Over the past four decades, there has been a steady evolution of solar dryer technology from natural convection types (direct and passive mode) to forced convection types [19].

The interest in the passive mode of drying rather than the direct type is the advantage it offers over the latter as it protects the produce from the harmful effects of the ultraviolet radiation of the sun, which destroys some nutrients and vitamins in the produce. Numerous works on natural convection solar types have been done by various researchers [13, 5, 6, 8, 9].

In most of these studies, emphasis has been on the construction and testing of the system while, in some other cases, the drying analysis and/or simulation was also done. This study presents the thermal analysis as well as the system’s evaluation during drying of plantain fillets. The next section deals with the description of dryer and material selection followed by the design analysis in Section 3. Experimental procedure and testing are discussed in Section 4, while results and the discussion are in Section 5. Section 6 presents the cost analysis, and the conclusions drawn in Section 7.

2. Description of Dryer and Material Selection

The system is an -shaped solar dryer which operates on the principle of natural convection (Figure 1). The base, which comprises the collector channel and collector case compartment, is connected to the food cabinet. The collector and cabinet are made of mild steel painted black on the inside and outside surfaces. This enables maximum absorption and emission of radiant heat energy by the system. The paint also serves to reduce the tendency of the metal to corrode. To facilitate the build-up of heat in the system, the collector is covered with a box which has a transparent glass at the top surface—greenhouse effect. The inlet air comes in through the metal collector channel heated and naturally rises through the food cabinet and exits through the shutter of the chimney, which is adjusted between 42 cm2 and 84 cm2 space in order to control the flow and the temperature in the cabinet. The product, plantain, is cleaned and cut into slices with a thickness of about 3 mm and weighed and spread on the first rack of the cabinet over which the hot air from the collector flow and remove moisture from the produce in the process.

479894.fig.001
Figure 1: Diagram of solar dryer.

This solar dryer is clearly different from most of the natural convection types in that it receives useful heat energy through indirect mode as a result of heated air flowing through the solar collector by buoyancy effect. Radiant heat energy is additionally absorbed by this cabinet without the exposure of the produce (article intended for drying) to the ultraviolet rays from the sun. However, some available indirect-mode natural convection solar dryers are made of plywood, but the collector covers are made of transparent glass [8]. This receives useful heat into the cabinet through the collector. On the other hand, the mixed-mode solar dryers [8, 9] are made of glass; both the collector and the cover of the cabinet are made of glass. The disadvantage these may have, though they seem to be more efficient than the indirect mode, is that the ultraviolet rays of the sun reaching the produce may destroy some nutrients and vitamins.

The design parameters and consideration used for the construction of the equipment are listed in Table 1. The properties of air used are evaluated at 312.54 K [15], being the mean of the temperatures of air at the inlet and exit of the cabinet taken over a period of eight days during no load conditions. The average temperatures recorded in cabinet , collector , collector case (or greenhouse chamber) , and environment are 41.56°C, 47.38°C, 48.81°C, and 31.69°C, respectively.

tab1
Table 1: Design considerations and assumptions.

3. Design Analysis

In this work, focus will be on the thermal and drying analyses only.

3.1. Thermal Analysis

The performance of a solar system and collector is described by an energy balance that indicates the conversion of incident solar energy into useful energy gain and various losses. The thermal analysis here follows the procedures as in Duffie and Beckman [16].

Some Properties of Air. The following air properties taken from the average temperature in the cabinet are used for the analysis (extracted from Holman [15]):  kg/m3,  kJ/kg·°C,  kg/m·s,  m2/s, W/m·°C, . These properties are used to calculate the thermal characteristics and performance of the system.

Grashof Number, Gr where , , , and . depends on the section of the cabinet under analysis. The heat transfer coefficient is represented as where the following Nusselt numbers were used depending on the section or area of the cabinet under consideration.

For horizontal plate:

 Above heated plate:

 Below heated plate:

For vertical heated plate:

where . However, the analysis will not be shown in detail as the values of the heat transfer are more important and are calculated using the above Nusselt numbers. In all cases, the air in the environment is assumed to be still; therefore, the heat transfer coefficients, at the outside of the dryer, are taken as zero. Also, the analysis is based on the assumption that the shutter is fully opened; that is, the cross-sectional area is 0.0084 m2.

3.1.1. Collector Analysis

The above properties of air are used for the analysis of the collector.

Heat Absorbed by Radiation in the Collector : where, is the absorptivity of the metal; is the surface area of the collector exposed to sunshine, and it is equivalent to 0.7 m2. is the total incident solar input, and it is expressed as where the ratio of the hourly and daily radiation is approximately 0.17 (1/6) [10].

Heat Loss by Convection and Conduction through the Base of the Collector : where is the temperature difference between the collector and the environment. is the thermal resistance offered by the wall and film at the inside of the collector taking the heat transfer coefficient at the outside of the wall as zero (still air). is the base area of the collector and its value is 0.5 m2. is calculated from (2) and (3b).

Useful Heat in the Collector :

Collector Efficiency :

3.1.2. Cabinet Analysis

For the analysis of the drying cabinet, the properties of the air are evaluated at the bulk temperature , , , , , , , , .

Heat Absorbed by Radiation in the Cabinet : where, is the total area of the cabinet through which radiant heat is absorbed.

Heat Transfer through the Wall of the Collector Case (Greenhouse) into the Cabinet : where, and . is the heat transfer coefficient at the inside vertical wall of the cabinet and it is calculated using (2) and (3d).  m2 is the area of the portion of the vertical wall separating the cabinet and the collector case.

Heat Radiated to the Environment by Cabinet : where, is the emissivity of the metal, is the Stefan-Boltzmann constant and = 1.5416 m2 is the total surface area of the cabinet through which heat is exchanged by radiation with the environment.

Heat Transfer from the Cabinet through the Side Wall : where and . is the area through which heat is transferred by convection and conduction from the side walls of the cabinet to the environment. is the heat transfer coefficient on the inside of the cabinet around the specified area and it is obtained using (2) and (3d).

Heat Transfer from the Cabinet through the Top Wall : where, and . is the area through which heat is transferred from the lower side of the top of the cabinet to the environment. is the heat transfer coefficient on the inside of the cabinet around the specified area and is obtained from (2) and (3c).

Heat Transfer from the Cabinet through the Bottom Wall : where and . is the area through which heat is transferred from the base wall of the cabinet to the environment. is the heat transfer coefficient at the top surface of the base wall inside the cabinet around the specified area, and can be evaluated from (2) and (3a).

Total Heat Loss through the Walls of the Cabinet :

Useful Heat in the Cabinet:

System Efficiency :

3.2. Drying Analysis

Percentage Moisture Removed from Product, : where and are the masses of wet and dried products, respectively.

Final Percentage Moisture Content of Product, : where is the initial mass of the product.

Amount of Moisture Content Removed, :

Quantity of Heat Used in Evaporating Moisture, : where is the latent heat of vaporisation and it is defined by Youcef-Ali et al. [19] as:

Average Drying Rate, : where t is daily sunshine hours.

Drying Time,

4. Experimental Procedure and Testing

The assembly was carefully checked to minimise heat loss. It was positioned in such a way that the food cabinet does not cast a shadow on the collector. The plantain fingers were sliced to about 3 mm thickness and were weighed to determine the initial mass. Four thermometers were placed at strategic locations on the assembly to measure the temperatures of the ambient, collector, collector case, and cabinet. The chimney shutter was adjusted to between 42 cm2 and 84 cm2 space in order to control the mass flow rate, hence, the temperature within the cabinet. The experiment was carried out in October. The first reading was taken at about 10:00 each day, when the sun was just coming up and subsequently on an hourly basis till around 17:00.

5. Results and Discussion

The general trend in the profiles of the temperatures obtained shows increases from 10:00 to between 13:00 and 15:00 then decline (Figures 2, 3, 4 and 5). The collector channel and collector case temperatures have higher values than the cabinet and ambient. Though the collector case temperature is always higher, the collector had higher values in Figures 2 and 5 before 14:00. Maximum temperatures of 58°C, 58°C, 52°C, and 37°C were recorded for the collector, collector case, cabinet, and ambient, respectively. Of particular interest is the temperature obtained in the cabinet, 52°C, which is in the range proposed as the optimum temperature for drying fruits and vegetables [20, 21].

479894.fig.002
Figure 2: Ambient, collector, collector case, and food cabinet temperatures for Day 1.
479894.fig.003
Figure 3: Ambient, collector, collector case and food cabinet temperatures for Day 2.
479894.fig.004
Figure 4: Ambient, collector, collector case, and food cabinet temperatures for Day 3.
479894.fig.005
Figure 5: Ambient, collector, collector case, and food cabinet temperatures for Day 4.

From (1) to (24), the results used for the evaluation of the system are presented in Table 2. The collector efficiency obtained was 46.4% and system efficiency was 78.73%.

tab2
Table 2: Performance evaluation of the system.

The collector efficiency obtained here is greater than 40% by Tarigan and Tekasakul [22]. Figure 6 shows the profile of the percentage moisture content. In general, it follows an exponentially decaying pattern before remaining constant at the 20th hour, which indicates about 78% moisture content removal. This profile pattern is exponentially decaying, and when compared with Henderson and Pabis [17] and Logarithmic models [18] constants, was obtained as shown in Table 3. It can be seen that the Logarithmic model better predicts the moisture content profile when compared with the Henderson and Pabis model. A critical observation of the experimental data and the models shows some noticeable disparity between the second and ninth hour. A closer look further reveals that the maximum deviation takes place during the fifth and sixth hour. At the beginning of the experiment, it was easier for the produce to lose much of its water content than much later. Moreover, the rate of water moisture loss is proportional to the intensity of the heat energy, hence, the temperature of the cabinet which was always at its peak between thirteen and fifteen hours. Before and after this peak period, the rate of loss was low. It was during this initial period that the model failed to predict the experimental data. The cost of constructing a unit system from locally available materials is tabulated in (Table 4).

tab3
Table 3: Mathematical models applied to drying curves.
tab4
Table 4: Cost of constructing a unit solar dryer.
479894.fig.006
Figure 6: Percentage moisture content removal in the dried plantain fillet compared with two drying models.

6. Cost Analysis

The materials were locally available, and the cost for the construction of a unit is tabulated below in the local currency, Naira, and the equivalent in US dollars. The cost per unit is affordable for small and medium manufacturing enterprises (SMMEs) [23].

7. Conclusion

The analyses are comprehensive but exclude the structural aspect, mainly because the system is not really a heavy-load carrying structure. Assessing the design based on the performance evaluation during the testing, collector and system efficiencies of 46.4% and 78.73%, percentage moisture removal of 77.5%, final moisture content of plantain chips of 15.75%, and average drying rate of 0.184 kg/hr show that the system is both efficient and effective. However, mass production of the system will further reduce the unit cost. To encourage domestic applications in the rural communities, government could subsidise or give a loan with low interest rates.

Nomenclature

Surface area, m2
Specific heat at constant pressure, kJ/kg·°C
:Acceleration due to gravity, m/s2
:Grashof number
:Heat transfer coefficient, W/m2·°C
Latent heat of vaporisation, MJ/kg
Solar insolation of collector, W/m2
:Thermal conductivity, W/m·°C
:Length, m
Mass flow rate, kg/s
Mass, kg
:Nusselt number
:Prandtl number
Heat of vapourisation, MJ
Heat transfer rate, W
Rayleigh number
Thickness of metal sheet, m
Percentage moisture content at any time t, %
Sunshine hours (Time), hrs
Temperature, °C
Temperature difference, °C
Total thermal resistance, °C/W.
Greek Letters
Kinematic viscosity, m2s
Dynamic viscosity, kg/m·s
Coefficient of thermal expansion K−1
Drying time, days.
Transmittance
Density, kg/m3
Absorptivity
Emissivity
Stefan-Boltzmann constant, W/m2K4
Efficiency, %
Moisture removal, %
Subscript
Ambient
Average
Base of cabinet
Collector
Collector case
Drying cabinet, dried product
Drying cabinet to environment
Final
Collector case (greenhouse) to cabinet
Inlet, initial
Metal
:Heat loss to the environment
:Radiation outward
:Radiation into
:Top of cabinet
Total
Useful
Vertical sides of cabinet
Wet product.

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