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

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 [1–9].

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 [1–3, 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 𝐿-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.

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 𝑇_𝑑, collector 𝑇_𝑐, collector case (or greenhouse chamber) 𝑇_ca, and environment 𝑇_𝑎 are 41.56°C, 47.38°C, 48.81°C, and 31.69°C, respectively.

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]): 𝜌=1.1324 kg/m³, 𝑐_𝑝=1.0066 kJ/kg·°C, 𝜇=1.9036×10⁻⁵ kg/m·s, 𝜈=16.9616×10⁻⁶ m²/s, 𝑘=0.02719W/m·°C, Pr=0.70524. These properties are used to calculate the thermal characteristics and performance of the system.

Grashof Number, Gr Gr=𝑔𝛽𝐿³Δ𝑇𝑣², (1) where 𝜈=𝜇/𝜌, 𝛽=1/𝑇, Δ𝑇=𝑇_𝑐−𝑇_𝑎, and 𝑇=(𝑇_𝑐+𝑇_𝑎)/2. 𝐿 depends on the section of the cabinet under analysis. The heat transfer coefficient is represented as ℎ=𝑘Nu𝐿, (2) where the following Nusselt numbers Nu were used depending on the section or area of the cabinet under consideration.

For horizontal plate:

 Above heated plate: Nu=0.54Ra_𝐿^1/4for10⁴≤Ra_𝐿≤10⁹,Nu=0.15Ra_𝐿^1/3for10⁷≤Ra_𝐿≤10¹¹. (3a)(3b)

 Below heated plate:Nu=0.274Ra_𝐿^1/4for10⁵≤Ra_𝐿≤10¹⁰. (3c)

 For vertical heated plate: Nu=0.68+0.67Ra_𝐿^1/41+(0.492/Pr)^9/164/9Ra_𝐿≤10⁹, (3d)

where Ra_𝐿=Gr∗Pr. 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 𝑄_rad,in: 𝑄_rad,in=𝛼𝐴_𝑐𝐼, (4) where, 𝛼 is the absorptivity of the metal; 𝐴_𝑐 is the surface area of the collector exposed to sunshine, and it is equivalent to 0.7 m². 𝐼 is the total incident solar input, and it is expressed as 𝐼=hourlyradiationdailyradiation×Solarirradiation, (5) 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 𝑄_cb,loss: 𝑄_cb,loss=Δ𝑇_cb∑𝑅_cb, (6) where Δ𝑇_cb is the temperature difference between the collector and the environment. ∑𝑅_cb=1/ℎ_cb𝐴_cb+𝑥/𝑘𝐴_cb 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). 𝐴_cb is the base area of the collector and its value is 0.5 m². ℎ_cb is calculated from (2) and (3b).

Useful Heat in the Collector 𝑄_𝑢: 𝑄_𝑢=𝑚̇𝑐_𝑝𝑇_𝑐−𝑇_𝑎=𝑄_rad,in−𝑄_cb,loss. (7)

Collector Efficiency 𝜂_𝑐: 𝜂_𝑐=𝑄_𝑢𝑄_rad,in×100%. (8)

3.1.2. Cabinet Analysis

For the analysis of the drying cabinet, the properties of the air are evaluated at the bulk temperature 𝑇=(47.38+41.56)/2+273^∘C, 𝛽=1/𝑇=3.15×10⁻³K⁻¹, Δ𝑇=47.38^∘C−41.56^∘C, Pr=0.7042, 𝑘=0.02756W/m⋅^∘C, 𝜌=1.1147kg/m³, 𝑐_𝑝=1.0066kJ/kg⋅^∘C, 𝜇=1.9261×10⁻⁵kg/m⋅s, 𝜈=17.463×10⁻⁶m²/s.

Heat Absorbed by Radiation in the Cabinet 𝑄_rad,in: 𝑄_rad,in=𝛼𝐴_𝑑𝐼, (9) where, 𝐴_𝑑=1.5416m² 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 𝑄_gc,in: 𝑄_gc,in=Δ𝑇_gc∑𝑅_gc, (10) where, Δ𝑇_gc=𝑇_ca−𝑇_𝑑 and ∑𝑅_gc=1/ℎ_gc𝐴_gc+𝑥/𝑘𝐴_gc. ℎ_gc is the heat transfer coefficient at the inside vertical wall of the cabinet and it is calculated using (2) and (3d). 𝐴_gc=0.05 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 𝑄_rad,out: 𝑄_rad,out=𝜀_𝑚𝜎𝐴_𝑑𝑇⁴_𝑑−𝑇⁴_𝑎, (11) where, 𝜀_𝑚 is the emissivity of the metal, 𝜎=5.669×10⁻⁸W/m².𝐾⁴ is the Stefan-Boltzmann constant and 𝐴_𝑑 = 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 𝑄_vc: 𝑄_vc=Δ𝑇_𝑑𝑎∑𝑅_vc, (12) where Δ𝑇_𝑑𝑎=𝑇_𝑑−𝑇_𝑎 and ∑𝑅_vc=1/ℎ_vc𝐴_vc+𝑥/𝑘𝐴_vc. 𝐴_vc=1.2916m² is the area through which heat is transferred by convection and conduction from the side walls of the cabinet to the environment. ℎ_vc 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 𝑄_tc: 𝑄_tc=Δ𝑇_𝑑𝑎∑𝑅_tc, (13) where, Δ𝑇_𝑑𝑎=𝑇_𝑑−𝑇_𝑎 and ∑𝑅_tc=1/ℎ_tc𝐴_tc+𝑥/𝑘𝐴_tc. 𝐴_tc=0.25m² is the area through which heat is transferred from the lower side of the top of the cabinet to the environment. ℎ_tc 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 𝑄_bc: 𝑄_bc=Δ𝑇_𝑑𝑎∑𝑅_bc, (14) where Δ𝑇_𝑑𝑎=𝑇_𝑑−𝑇_𝑎 and ∑𝑅_bc=1/ℎ_bc𝐴_bc+𝑥/𝑘𝐴_bc. 𝐴_bc=0.25m² is the area through which heat is transferred from the base wall of the cabinet to the environment. ℎ_bc 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 𝑄_loss: 𝑄_loss=𝑄_vc+𝑄_tc+𝑄_bc+𝑄_rad,out. (15)

Useful Heat in the Cabinet: 𝑄_𝑢=𝑚̇𝑐_𝑝𝑇_𝑐−𝑇_𝑑=𝑄_rad,in+𝑄_gc,in−𝑄_loss. (16)

System Efficiency 𝜂_𝑠: 𝜂_𝑠=𝑄_𝑢𝑄_rad,in+𝑄_gc,in×100%. (17)

3.2. Drying Analysis

Percentage Moisture Removed from Product, 𝛾%: 𝑚𝛾%=^𝑤−𝑚_𝑑𝑚_𝑤100%wetbasis, (18) where 𝑚_𝑤 and 𝑚_𝑑 are the masses of wet and dried products, respectively.

Final Percentage Moisture Content of Product, 𝑚_𝑓: 𝑚_𝑓=100−𝛾𝑚100^𝑖, (19) where 𝑚_𝑖 is the initial mass of the product.

Amount of Moisture Content Removed, 𝑚: 𝑚𝑚=^𝑤𝑚_𝑖−𝑚_𝑓100−𝑚_𝑓. (20)

Quantity of Heat Used in Evaporating Moisture, 𝑞_𝑒: 𝑞_𝑒=𝑚ℎ_fg, (21) where ℎ_fg is the latent heat of vaporisation and it is defined by Youcef-Ali et al. [19] as: ℎ_fg=4.186×10³597−0.56𝑇_𝑑. (22)

Average Drying Rate, 𝑚_ave: 𝑚_ave=𝑚𝑡, (23) where t is daily sunshine hours.

Drying Time, 𝛿:𝛿=𝑚ℎ_fg3600𝐼𝐴_𝑇𝑡𝜂_𝑐𝜂_𝑠. (24)

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].

Figure 2 

Ambient, collector, collector case, and food cabinet temperatures for Day 1.

Figure 3 

Ambient, collector, collector case and food cabinet temperatures for Day 2.

Figure 4 

Ambient, collector, collector case, and food cabinet temperatures for Day 3.

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%.

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, 𝑘 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).

Table 3 

Mathematical models applied to drying curves.

Table 4 

Cost of constructing a unit solar dryer.

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

𝐴:	Surface area, m2
𝑐𝑝:	Specific heat at constant pressure, kJ/kg·°C
𝑔:	Acceleration due to gravity, m/s2
Gr:	Grashof number
ℎ:	Heat transfer coefficient, W/m2·°C
ℎfg:	Latent heat of vaporisation, MJ/kg
𝐼:	Solar insolation of collector, W/m2
𝑘:	Thermal conductivity, W/m·°C
𝐿:	Length, m
𝑚̇:	Mass flow rate, kg/s
𝑚:	Mass, kg
Nu:	Nusselt number
Pr:	Prandtl number
𝑞𝑒:	Heat of vapourisation, MJ
𝑄:	Heat transfer rate, W
Ra𝐿:	Rayleigh number
𝑥:	Thickness of metal sheet, m
𝑋𝑚:	Percentage moisture content at any time t, %
𝑡:	Sunshine hours (Time), hrs
𝑇:	Temperature, °C
Δ𝑇:	Temperature difference, °C
Σ𝑅:	Total thermal resistance, °C/W.

Greek Letters

𝜈:	Kinematic viscosity, m2s
𝜇:	Dynamic viscosity, kg/m·s
𝛽:	Coefficient of thermal expansion K−1
𝛿:	Drying time, days.
𝜏:	Transmittance
𝜌:	Density, kg/m3
𝛼:	Absorptivity
𝜀:	Emissivity
𝜎:	Stefan-Boltzmann constant, W/m2K4
𝜂:	Efficiency, %
𝛾:	Moisture removal, %

Subscript

𝑎:	Ambient
ave:	Average
bc:	Base of cabinet
𝑐:	Collector
ca:	Collector case
𝑑:	Drying cabinet, dried product
𝑑𝑎:	Drying cabinet to environment
𝑓:	Final
gc:	Collector case (greenhouse) to cabinet
𝑖:	Inlet, initial
𝑚:	Metal
loss:	Heat loss to the environment
radout:	Radiation outward
radin:	Radiation into
tc:	Top of cabinet
𝑇:	Total
𝑢:	Useful
𝑣𝑐:	Vertical sides of cabinet
𝑤:	Wet product.