jpeeJournal of Power and Energy Engineering2327-59012327-588XScientific Research Publishing10.4236/jpee.2026.144003jpee-150754ArticleEngineeringThermal Analysis and Performance Evaluation of a Natural Convection Solar Dryer for Agricultural Products in ChadAbdelkerimAbdelkerim Ahmat12MahamatAbdallah Ban-Nah2TahirAbakar Mahamat3GastonTamba Jean4 Laboratory for Research in Electronics, Electrotechnics and Energy (LR3E), National Higher Institute of Science and Technology of Abéché, INSTA, Abéché, Chad National Higher Institute of Science and Technology of Abéché, Abéché, Chad University of Ndjamena, Ndjamena, Chad University of Douala, Douala, Cameroon
The authors declare no conflicts of interest regarding the publication of this paper.
Post-harvest losses of agricultural products remain a significant challenge in semi-arid regions such as Chad, where high ambient temperatures and limited preservation technologies considerably reduce food availability and quality. Solar drying has emerged as a sustainable and energy-efficient method for preserving fruits and vegetables by reducing moisture content and inhibiting microbial growth. This study investigates the thermal performance and drying behavior of an indirect natural-convection solar dryer under the climatic conditions of N’Djamena. The experimental setup consists of a flat-plate solar air collector coupled with a drying chamber equipped with multiple trays. Key parameters—including solar radiation, ambient temperature, air velocity, relative humidity, and product mass—were continuously monitored throughout the drying process. The thermal efficiency of the solar dryer, the evolution of moisture content, and the drying kinetics of okra were evaluated. Thin-layer drying behavior was analyzed using the Page model to describe the variation of moisture ratio with drying time. Experimental results indicate that the solar dryer achieved temperatures ranging from 50˚C to 65˚C, significantly exceeding the ambient temperature (34˚C - 42˚C). Under these conditions, the moisture content of fresh okra decreased from 80% to approximately 6% within 8–10 hours, whereas traditional open-sun drying required 2 - 3 days to reach comparable moisture levels. The thermal efficiency of the system varied between 25% and 40%, depending on solar radiation intensity and airflow conditions. These results demonstrate that natural-convection solar dryers can substantially reduce drying time, enhance product quality, and minimize contamination risks. Consequently, this technology represents a practical and sustainable solution for reducing post-harvest losses and improving food preservation in semi-arid regions such as Chad.
Solar DryerNatural ConvectionThermal PerformanceMoisture RatioPage ModelChad1. Introduction
Agricultural production in sub-Saharan Africa experiences significant post-harvest losses due to inadequate preservation and storage techniques [1]. In Chad, losses of fruits and vegetables during storage and transportation are estimated to reach 30% - 40% of total production [2]. These losses are primarily caused by high ambient temperatures, insufficient preservation facilities, microbial contamination, and inadequate processing technologies [3].
Traditional open-sun drying remains the most widely used preservation method in rural areas [4]. However, this technique presents several limitations, including exposure to dust, insects, and environmental contaminants [5]. In addition, drying conditions are highly dependent on weather variability, which often results in uneven drying, prolonged drying times, and deterioration of product quality and nutritional value [6].
Solar drying represents a promising and sustainable alternative to traditional drying methods [7]. By utilizing abundant solar energy, solar dryers provide controlled drying conditions that enhance moisture removal while protecting products from environmental contamination [8]. Among the different solar drying technologies, indirect natural-convection solar dryers have received considerable attention due to their simplicity, low operational cost, and suitability for rural applications [9].
These systems operate based on basic heat transfer mechanisms and buoyancy-driven airflow [10]. Solar radiation is absorbed by the collector surface, heating the air inside the collector. The heated air then flows naturally through the drying chamber due to density differences between hot and ambient air [11]. This airflow removes moisture from the product and transports it outside the drying chamber through a chimney or exhaust opening.
Several experimental studies have demonstrated that solar dryers can achieve drying temperatures between 50˚C and 65˚C, which are appropriate for drying many agricultural products while preserving their nutritional and sensory qualities [12]. Furthermore, mathematical modeling techniques have been widely used to describe the drying behavior of agricultural materials [13]. Thin-layer drying models such as the Page model are commonly employed because of their ability to accurately represent moisture reduction during the drying process [14].
Despite the increasing interest in solar drying technologies, relatively limited research has been conducted on the thermal performance and drying kinetics of solar dryers under the specific climatic conditions of Chad [15]. The country benefits from high solar irradiance and semi-arid climatic conditions, which provide excellent potential for solar-based food preservation technologies [16].
Therefore, the main objective of this study is to analyze the thermal performance and drying characteristics of an indirect natural-convection solar dryer for agricultural products operating under the climatic conditions of N’Djamena.
The specific objectives of this research are:
1) To characterize the temperature distribution inside the solar collector and drying chamber under real solar conditions.
2) To analyze the reduction of moisture content and drying kinetics of okra during the drying process.
3) To evaluate the thermal efficiency of the solar dryer and compare its performance with traditional open-sun drying methods.
4) To assess the potential of solar drying technology for improving food preservation and reducing post-harvest losses in semi-arid regions.
By addressing these objectives, this study contributes to the development and optimization of solar drying systems adapted to the climatic conditions of Chad. The findings provide valuable insights for improving post-harvest preservation techniques and promoting sustainable agricultural technologies in developing countries.
2. Materials and Methods2.1. Study Area and Climatic Conditions
The experimental investigation was conducted in N’Djamena, the capital city of Chad, located in the Sahelian region of Central Africa (12.11˚N latitude and 15.05˚E longitude). The region is characterized by a semi-arid climate with high solar irradiance and elevated ambient temperatures throughout most of the year, making it particularly suitable for solar drying applications.
During the experimental campaign, meteorological conditions were carefully monitored. The average daily ambient temperature ranged from 34˚C to 42˚C, while the relative humidity varied between 20% and 45%. The global solar radiation intensity ranged between 600 W/m2 and 1000 W/m2, with peak radiation occurring around midday.
These climatic conditions are highly favorable for solar drying systems because high solar radiation improves heat generation within the collector, while low relative humidity enhances the moisture removal capacity of the drying air.
Meteorological parameters including solar radiation, ambient temperature, and relative humidity were recorded throughout the drying experiments to evaluate the influence of environmental conditions on the dryer performance.
2.2. Description of the Solar Dryer
The experimental setup consisted of an indirect natural-convection solar dryer designed to dry agricultural products using heated air generated by solar energy.
The solar dryer is composed of three main components:
a solar air collectora drying chamberan exhaust chimney
Solar Collector
The solar collector is a flat-plate air collector designed to capture solar energy and convert it into thermal energy for heating the drying air.
The collector includes:
a black-painted galvanized steel absorber plate to maximize solar radiation absorptiona transparent glass cover that allows solar radiation to enter while reducing convective heat lossesa thermal insulation layer placed beneath the absorber plate to minimize heat losses to the surroundings
Ambient air enters the collector through an inlet opening located at the lower part of the system. As the air flows along the heated absorber plate, it gains thermal energy and its temperature increases significantly.
Drying Chamber
The heated air from the collector flows into the drying chamber where the agricultural products are placed. The drying chamber is designed to ensure uniform airflow distribution across the product layers.
The chamber contains several perforated trays made of metallic mesh, allowing the hot air to circulate freely around the product. The products are spread in thin layers to promote efficient moisture removal.
Chimney
A vertical chimney is installed at the top of the drying chamber. The chimney plays a crucial role in enhancing airflow through the buoyancy-driven natural convection mechanism.
The airflow is generated due to the density difference between hot air inside the dryer and the cooler ambient air. As the heated air rises through the chimney, fresh air is continuously drawn into the collector, maintaining a continuous drying airflow.
All components of the solar dryer were constructed using locally available materials, including wood, galvanized steel sheets, and transparent glass, making the system affordable and easily reproducible for rural applications.
2.3. Experimental Material and Sample Preparation
Fresh okra (Abelmoschus esculentus) was selected as the agricultural product for the drying experiments because it is widely cultivated and consumed in Chad.
The okra samples were obtained from a local agricultural market in N’Djamena on the same day of the experiment to ensure freshness and minimize deterioration before drying.
Prior to the drying experiment, the following preparation steps were carried out:
1) The okra pods were washed thoroughly with clean water to remove dust and impurities.
2) Surface moisture was removed by draining the samples.
3) The pods were cut into uniform slices of approximately 5 - 7 mm thickness to ensure homogeneous drying conditions.
Uniform slicing is important because the drying rate depends strongly on the surface area exposed to the drying air.
The prepared okra slices were evenly distributed on the drying trays to avoid overlapping and ensure uniform airflow around the samples.
2.4. Instrumentation and Measurement Devices
Several measuring instruments were used to monitor the drying process and evaluate the thermal performance of the solar dryer. The instruments and their measurement accuracy are summarized in Table 1.
Table 1. Instrument and measurement devices.
Instrument
Parameter measured
Accuracy
Pyranometer
Solar radiation
±5 W/m
2
Thermocouples
Air temperature
±0.5˚C
Digital balance
Product mass
±0.01 g
Hygrometer
Relative humidity
±2%
Anemometer
Air velocity
±0.1 m/s
Temperature Measurement
Temperature measurements were performed using K-type thermocouples installed at different locations within the solar drying system:
ambient air temperaturecollector inlet temperaturecollector outlet temperaturedrying chamber air temperature
These measurements allowed the evaluation of the temperature distribution throughout the drying process.
Solar Radiation Measurement
Solar radiation intensity was measured using a pyrometer installed near the experimental setup. The sensor was positioned to measure the global horizontal solar radiation incident on the collector surface.
Relative Humidity Measurement
Relative humidity inside the drying chamber and in the ambient environment was measured using a digital hygrometer.
Air Velocity Measurement
Air velocity at the dryer outlet was measured using a digital anemometer positioned near the chimney outlet.
2.5. Experimental Procedure
Fresh okra samples were selected for the drying experiments. The samples were first washed and cut into uniform pieces to ensure homogeneous drying conditions.
The initial mass of the product was measured using a digital balance before placing the samples on the drying trays. The experiments were carried out during sunny days from 08:00 to 18:00.
During the drying process, the following parameters were recorded at 30-minute intervals:
solar radiation intensityambient temperatureair temperature at different points of the dryerrelative humidityproduct mass
The drying experiment continued until the moisture content of the product reached a stable value.
2.6. Thermal Analysis
The thermal performance of the solar dryer was evaluated using energy balance equations. The solar energy received by the collector is expressed as:
Qsolar=I×A
where I is the solar radiation intensity (W/m2) and A is the collector area (m2).
The useful heat transferred to the air is calculated using:
Qu=m·Cp(Tout−Tin)
where:
m · is the mass flow rate of air (kg/s)Cp is the specific heat capacity of air (1005 J/kg∙K)Tout and Tin are the outlet and inlet air temperatures respectively.
The thermal efficiency of the solar dryer is defined as:
η=m·Cp(Tout−Tin)IA2.7. Drying Kinetics
The drying behavior of the product was analyzed using the moisture ratio (MR), defined as:
MR=Mt−MeM0−Me
where:
M0 is the initial moisture contentMt is the moisture content at time tMe is the equilibrium moisture content.
The drying kinetics were modeled using the Page model:
MR=exp(−ktn)
where k and n are empirical drying constants
The goodness of fit between the experimental and predicted values was evaluated using statistical indicators such as:
coefficient of determination (R 2)root mean square error (RMSE)
3. Results and Discussion3.1. Solar Radiation Variation
Solar radiation plays a key role in determining the thermal performance of a solar drying system. Figure 1 shows the variation of solar radiation intensity during the experimental day.
Figure 1. The variation of solar radiation.
The results indicate that solar radiation increased gradually from morning until midday. The radiation level was approximately 300 W/m2 at 08:00, reaching a peak value of about 950 W/m2 around 13:00. After midday, the radiation decreased progressively due to changes in solar position and atmospheric conditions [12][17].
Such radiation levels are typical for semi-arid regions and provide favorable conditions for solar drying applications [16]. High solar radiation improves the heating of the solar collector, which increases the air temperature entering the drying chamber and consequently enhances the drying rate [8][18].
These results are consistent with those reported in previous solar drying studies conducted in tropical and arid climates, where peak solar radiation values between 800 and 1000 W/m2 were commonly observed during clear-sky conditions [19][20].
3.2. Temperature Distribution inside the Solar Dryer
The variation of temperature inside the solar dryer and the ambient air is presented in Figure 2.
Figure 2. The variation of temperature inside the solar dryer and the ambient air.
The ambient temperature during the experimental period ranged between 34˚C and 42˚C. However, the air temperature inside the solar dryer was significantly higher due to solar heating. The collector outlet temperature reached values between 50˚C and 65˚C during peak solar radiation hours [12].
The temperature difference between the ambient air and the drying chamber reached approximately 20˚C, which is sufficient to create natural convection airflow inside the system [10][18].
The increase in air temperature improves the moisture evaporation rate from the product surface. Higher temperatures enhance the diffusion of moisture from the internal structure of the product to its surface [13][21].
These observations confirm that the solar dryer provides a controlled drying environment compared to traditional open sun drying, leading to improved drying efficiency and better product protection [19].
3.3. Moisture Content Reduction
The evolution of moisture content of the product during the drying process is shown in Figure 3.
Figure 3. The evolution of moisture content of the product during.
The results show a rapid decrease in moisture content during the first hours of drying. The initial moisture content of the product was approximately 80% (wet basis). After 10 hours of drying, the moisture content decreased to about 6%, indicating a significant reduction in water content [22].
The drying process can generally be divided into two main stages [21]:
1) Constant-rate drying period
2) Falling-rate drying period
During the initial stage, moisture evaporates rapidly from the product surface due to high temperature and low relative humidity inside the drying chamber [17]. In the second stage, the drying rate decreases because moisture diffusion from the internal structure of the product becomes the limiting factor [19].
These results indicate that the solar dryer significantly accelerates the drying process compared with traditional open sun drying methods, which may require several days under similar climatic conditions [20].
3.4. Drying Kinetics
The drying kinetics were analyzed using the moisture ratio (MR), which represents the normalized moisture content during drying.
Figure 4 presents the variation of the moisture ratio with drying time. The results show an exponential decrease of MR with time, which is typical for thin-layer drying processes.
Figure 4. The variation of the moisture ratio with drying time.
The experimental data were fitted using the Page model, expressed as:
MR=exp(−ktn)
The model showed good agreement with the experimental results, indicating that it can effectively describe the drying behavior of the product in the solar dryer [21].
The constants k and n obtained from the fitting process reflect the influence of temperature and airflow conditions on the drying process. These parameters are commonly used to characterize the drying kinetics of agricultural products and are strongly affected by drying conditions such as air temperature, velocity, and relative humidity [17].
These findings are consistent with previous studies on solar drying of agricultural products, where the Page model has been widely used due to its accuracy and reliability in predicting drying behavior during thin-layer drying processes [19][20].
3.5. Drying Rate Analysis
The drying rate curve is presented in Figure 5. The drying rate is defined as the rate of moisture removal from the product per unit time.
Figure 5. The drying rate curve.
The results show that the drying rate was highest at the beginning of the drying process and gradually decreased with time. This behavior can be explained by the rapid evaporation of free water located on the surface of the product during the initial stage of drying [17].
As drying progresses, moisture migration from the interior of the product to the surface becomes slower, resulting in a decreasing drying rate [19]. This phenomenon is mainly controlled by internal moisture diffusion within the product structure.
The falling-rate period dominated the drying process, which is typical for most agricultural products subjected to thin-layer drying conditions [20][23].
3.6. Thermal Efficiency of the Solar Dryer
The thermal efficiency of the solar dryer was calculated using the energy balance equations described in the methodology section.
Figure 6 shows the variation of the thermal efficiency during the drying process. The results indicate that the efficiency ranged between 25% and 40%, depending on solar radiation intensity and temperature difference between inlet and outlet air. [24]-[28]
The efficiency reached its maximum value during the midday period when solar radiation was highest [17]. This increase in efficiency is directly related to the higher solar energy input absorbed by the collector, which increases the temperature of the drying air.
Figure 6.The variation of the thermal efficiency during.
These efficiency values are comparable to those reported in previous studies on natural convection solar dryers [19][20]. The relatively high efficiency observed in this study demonstrates the effectiveness of the dryer design for agricultural drying applications, particularly under the favorable solar conditions found in semi-arid regions [23].
3.7. Comparison with Traditional Sun Drying
Compared to traditional open sun drying, the solar dryer provides several advantages, including higher drying temperatures, faster moisture removal, better protection from contamination, and improved product quality [19].
The drying time was reduced from 2 - 3 days using open sun drying to approximately 8 - 10 hours using the solar dryer, demonstrating the effectiveness of the system in accelerating the drying process [20].
This significant reduction in drying time contributes to improving food preservation and reducing post-harvest losses in agricultural products [23].
3.8. Implications for Solar Drying Applications in Chad
The climatic conditions in Chad, characterized by high solar radiation and elevated ambient temperatures, make solar drying a highly suitable technology for agricultural product preservation [16][19].
The results of this study demonstrate that the use of solar dryers can significantly improve drying efficiency and product quality [17][20]. The adoption of such technologies can therefore play an important role in enhancing food security and supporting rural agricultural activities [23].
Future work should focus on optimizing dryer design, improving airflow distribution, and integrating thermal storage systems to enable drying during periods of low solar radiation, thereby further increasing the effectiveness and applicability of solar drying technology [29][30].
4. Conclusions and Future Work4.1. Conclusions
This study presents the thermal performance and drying efficiency of an indirect natural-convection solar dryer designed for agricultural products in N’Djamena, Chad. Experimental measurements were conducted to evaluate temperature distribution, moisture content reduction, drying kinetics, drying rate, and thermal efficiency of the system. The main findings are summarized as follows:
1) Temperature Performance:
The solar dryer achieved air temperatures between 50˚C and 65˚C, significantly higher than the ambient temperature (34˚C - 42˚C). This temperature difference ensured effective natural convection airflow inside the drying chamber, which is consistent with the operating principles reported for solar drying systems [18].
2) Drying Rate and Moisture Reduction:
The moisture content of the product decreased from 80% to 6% within 8 - 10 hours, demonstrating a significant reduction in drying time compared with traditional sun drying, which typically requires 2 - 3 days [22].
3) Drying Kinetics:
The experimental moisture ratio data were successfully modeled using the Page model, showing excellent agreement and confirming its suitability for thin-layer drying analysis of agricultural products [21].
4) Thermal Efficiency:
The thermal efficiency of the solar collector ranged between 25% and 40%, peaking during midday when solar radiation was highest. These values are comparable to efficiencies reported for similar indirect solar dryers in previous studies [17].
5) Advantages over Traditional Drying:
Compared to open sun drying, the solar dryer ensures faster drying, better product protection from contamination, and improved product quality [19]. It therefore represents a sustainable solution for reducing post-harvest losses and enhancing food security in Chad [20].
Overall, the study demonstrates that solar dryers are technically feasible, energy-efficient, and suitable for rural agricultural applications in semi-arid regions. These findings confirm the potential of solar drying technology as an effective approach for improving agricultural product preservation in developing countries [23].
4.2. Future Work
Several opportunities exist to further optimize the performance of solar drying systems:
1) Design Optimization: Improving the geometry of the collector and drying chamber to enhance air circulation and uniform temperature distribution. Computational Fluid Dynamics (CFD) simulations could be employed for design refinement. [24]-[28].
2) Hybrid Drying Systems: Integrating auxiliary heating sources, such as biomass or photovoltaic-thermal (PVT) systems, to enable drying during cloudy periods or low solar radiation.
3) Thermal Energy Storage: Incorporating latent or sensible heat storage materials in the dryer design to maintain high drying temperatures during non-sunlight hours.
4) Product-Specific Drying: Investigating the drying behavior of various agricultural products (e.g., okra, mango, tomatoes) under similar climatic conditions to develop standardized drying protocols.
5) Economic and Environmental Assessment: Conducting life cycle assessment and cost-benefit analysis to evaluate the economic feasibility and sustainability of large-scale solar dryer implementation in Chad and similar regions.
6) Automation and Monitoring: Implementing low-cost sensors and automated control systems to regulate airflow, temperature, and moisture for optimized drying processes.
By addressing these research directions, solar drying technologies can be further improved and widely adopted, contributing to food preservation, post-harvest loss reduction, and sustainable energy utilization in semi-arid countries.
Acknowledgements
The authors would like to express their sincere gratitude to the technical staff and colleagues who contributed to the experimental work and data collection for this study. Their assistance and valuable discussions greatly contributed to the successful completion of this research.
The authors also acknowledge the support provided by their respective institutions for facilitating the laboratory resources and experimental equipment required for the development and testing of the solar dryer system.
Special thanks are extended to the local agricultural community in N’Djamena, Chad, for their cooperation and for providing the agricultural products used in the drying experiments.
Finally, the authors appreciate the constructive comments and suggestions from anonymous reviewers, which helped improve the quality and clarity of this manuscript.
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