A Comparison of Hot Dry and Transitional Season Solar Drying with A Modified V-Corrugated Solar Air Heater

2.1. Absorber Plate Modifications

A widely adopted approach for enhancing the thermal performance of solar air heaters is the use of corrugated or ribbed absorber plates, which increase the heat transfer surface area and promote airflow turbulence.

In a numerical study, Abedalh and Mohammed [5] reported that V-grooved corrugated absorber plates achieved 30–50% higher thermal efficiency than flat plates. Kumar et al. [7] experimentally investigated triangular corrugated absorbers under jet impingement and reported a thermal efficiency of 47.3%. In a related study, Assaye et al. [6] examined semi-circular transverse rib roughness on aluminium absorber plates and observed a thermal efficiency improvement exceeding 35%. Abdullah et al. [8] reported a 48% increase in thermal efficiency in a double-pass solar air heater through the use of delta wing turbulators.

Mahmood [15] reported significant improvements in outlet temperature and heat transfer coefficient in a double-pass unglazed solar air heater equipped with perforated plates and wire-mesh layers. Similarly, Karir et al. [16], in their review of arc-shaped rib roughness, concluded that turbulence-inducing modifications to absorber surfaces substantially enhance heat transfer performance.

2.2. Solar Dryers for Agricultural Products

Indirect solar dryers have been developed and experimentally evaluated for a variety of agricultural products. When compared to conventional open-sun drying techniques, Hossain and Bala [10] found that employing a solar tunnel drier to dry hot chilli improved drying performance and product quality. After reviewing several solar drying systems, El-Sebaii and Shalaby [9] found that adding thermal energy storage can increase drying continuity, lengthen drying times, and improve system performance. In Kano, Yahaya [17] developed and evaluated a multi-tray natural-convection dryer for yam drying and reported uniform product quality across trays, along with a significant reduction in drying time compared with open-sun drying.

In their assessment of solar dryers for marine and agricultural products, Fudholi et al. [4] concluded that indirect solar dryers consistently outperform open sun drying in terms of product quality, drying rate, and hygiene. For tomatoes and peppers, Forson et al. [11] developed a natural-convection indirect solar dryer that achieved more than 80% moisture removal within 48 hours. In a comprehensive review of solar energy drying systems, Ekechukwu and Norton [3] emphasized that appropriate collector design and effective airflow management are critical for achieving high drying efficiency.

2.3. Seasonal Performance Studies

Only a small number of research have looked into seasonal differences in solar dryer effectiveness. In their assessment of sun drying technologies, Janjai and Bala [18] underlined the necessity of taking environmental variability into account when designing and operating dryers, as well as the substantial impact of climatic circumstances on solar dryer performance. Bala and Woods [12] discovered that airflow velocity, solar radiation, and ambient temperature have a major impact on drier efficiency in their simulation of indirect solar drying. In a similar vein, greenhouse-style collectors can increase drying stability and heat retention, according to Jain and Tiwari [13].

2.4. Research Gap

The literature indicates that effective, hygienic, and rapid drying systems typically incorporate modified absorber plates, indirect drying chambers, well-managed natural or forced airflow, and transparent glazing materials.

Nevertheless, systematic research on tomato drying in northern Nigeria using a passive indirect solar dryer equipped with a V-corrugated absorber plate under varying seasonal conditions remains limited. By experimentally evaluating drying performance, moisture removal kinetics, and product quality during both hot-dry and transitional seasons, this study addresses the identified research gap. Table 1 summarizes selected solar dryer studies featuring modified absorber plates, highlighting the novelty of the present work.

3.1. System Description and Design

The experimental system was installed at Bayero University in Kano, Nigeria (11.9747° N, 8.4250° E). The device was a passive indirect solar dryer operating under natural convection, comprising a solar air collector, an insulated drying chamber, and an exhaust chimney.

3.1.2. Modified V-Corrugated Absorber Plate

The primary component was a modified V-corrugated absorber plate with a solar absorptance of 0.95, fabricated from a 1.0-mm-thick aluminum sheet and coated with matte black paint (Figure 2). The corrugated geometry was designed to enhance convective heat transfer by increasing the effective heat transfer area and promoting airflow turbulence [5,6].

The geometrical parameters of the modified V-corrugated absorber plate were as follows:

• Corrugation pitch, P = 25 mm

• Corrugation depth, D = 20 mm

• Corrugation angle, β = 60°

• Effective absorber area (aperture area), Ac = 1.5 × 0.9 = 1.35 m²

• Developed (actual) surface area ≈ 1.561.35 × sec(30°) ≈ 1.56 m² (∼16% increase over flat plate)

3.1.3. Drying Chamber and Chimney

The drying chamber, measuring 1.0 m in height, 0.6 m in width, and 0.5 m in depth, was constructed from 12-mm- thick plywood and insulated with 25-mm-thick polyurethane foam. The interior surfaces were lined with aluminum foil to enhance reflectivity and minimize heat loss. To ensure uniform airflow distribution, three perforated aluminum trays measuring 0.55 m × 0.45 m were installed with a vertical spacing of 15 cm between successive trays. To improve buoyancy-driven ventilation, a 1.2-m-tall PVC chimney with a diameter of 15 cm installed at the top of the drying chamber [11]. Figure 3 shows the solar dryer’s schematic configuration, including the collector, drying chamber, trays, and chimney.

3.2. Instrumentation and Calibration

Prior to the commencement of the study, all instruments were calibrated in accordance with ASHRAE Standard 93- 2003 [19]. Table 2 lists the instruments used for data collection, along with their specifications, accuracy, and calibration methods.

Throughout each drying experiment, data were manually recorded at hourly intervals between 09:00 and 18:00. The thermocouples were positioned at representative locations, including the collector inlet and outlet, the center of each tray, and the chimney exit. The anemometer was installed at the collector inlet and chamber outlet, while the pyranometer was mounted coplanar with the collector surface.

3.3. Experimental Procedure

3.3.1. Sample Preparation

Fresh Roma tomatoes (Lycopersicon esculentum) were procured from a local market in Kano. After selection based on uniformity in size, color, and ripeness, the tomatoes were washed with clean water and manually sliced to a uniform thickness of 5 mm using a stainless-steel knife. As illustrated in Figure 4, the initial moisture content was determined by oven-drying three 50 g samples at 105 ± 2 °C for 24 h [20]. The average initial moisture content was found to be 95.65% on a wet basis.

3.3.3. Load Tests (Drying Experiments)

For each drying run, 1.5 kg of tomato slices was evenly distributed across the three drying trays. Drying experiments were performed in triplicate during two distinct seasonal periods:

• Transitional season: October–November 2019

• Hot dry season: April–May 2021

To assess the dryer performance under two different seasonal settings, the experimental campaigns were carried out in various years. Due to operational and logistical limitations related to research efforts during the COVID-19 pandemic, the hot dry season tests were not completed on time. To guarantee consistency and comparability of the results, however, the same dryer configuration, apparatus, calibration processes, and testing technique were kept throughout both experimental cycles.

Drying was continued until the sample mass reached a stable value, corresponding to a final moisture content of approximately 10% (wet basis), which is generally considered safe for storage [11]. Hourly measurements included:

• Solar irradiance, I (W/m²)

• Ambient temperature, T_amb (°C) and relative humidity, RH (%)

• Collector inlet and outlet air temperatures, T_in, T_out (°C)

• Tray temperatures, tray1, tray2, Ttray3 (°C)

• Air velocity at collector inlet and chamber exit, v (m/s)

• Mass of each tray, m (g)

Figure 5 displays the experimental configuration used for the solar drying trials.

3.4. Data Reduction and Performance Evaluation

3.5. Statistical Analysis

All drying experiments were performed in triplicate. Results are reported as mean ± standard deviation (SD). SD was calculated as:

3.6. Uncertainty Analysis

Measurement uncertainties were propagated using the root-sum-square method [22]. The combined relative uncertainty in drying efficiency was estimated at ±5.2%, primarily arising from uncertainties in solar radiation measurements (±2.1%), mass measurements (±0.8%), and latent heat values (±1.5%).

4.1. Environmental Conditions

Solar radiation was significantly higher (p < 0.01) during the hot dry season (average 1250–1350 W/m²) compared to the transitional season (845–892 W/m²). Ambient temperatures were 5–6 °C higher and relative humidity 20–25% lower during the hot dry season. Table 3 summarizes the average environmental conditions recorded during the drying experiments.

4.2. No-Load Thermal Performance

Maximum absorber temperatures recorded during no-load testing were 82.7 °C in April and 83.7 °C in May, while lower values of 72.1 °C and 66.7 °C were observed in October and November, respectively. During peak solar radiation periods, the collector outlet air temperature exceeded ambient conditions by 25–30 °C, indicating effective heat transfer performance.

4.3. Drying Characteristics

4.3.1. Moisture Removal and Drying Time

Figure 6 illustrates the loss in weight of tomato slices following drying. The moisture content declined from an initial value of 95.65% (wet basis) to about 10% (wet basis) in all experimental trials, as assessed by weight loss. The corresponding drying times were:

1. October 2019: 22 h

2. November 2019: 16 h

3. April 2021: 12 h

4. May 2021: 10 h

The hot dry season reduced drying time by 45–55% compared to the transitional season.

4.4. Drying Efficiency

4.4.1. Daily Efficiency Variation

The daily drying efficiency for each experimental run is presented in Figure 7, Figure 8, Figure 9 and Figure 10. Drying efficiency was highest during the initial stage of each trial when free surface moisture evaporated rapidly. Thereafter, efficiency decreased as the removal of bound moisture required greater energy input per unit mass of water removed.

4.5. Product Quality Observations

4.5.1. Visual Assessment

1. Colour: Open sun-dried samples, exposed simultaneously on an adjacent tray, exhibited noticeable browning and darkening, whereas solar dried tomato slices retained their vivid red orange color (Figure 11).

2. Hygiene: Solar-dried samples exhibited no evidence of dust, insect debris, or contamination. In contrast, samples exposed to open sun drying showed visible dust accumulation.

3. Texture: Open sun-dried samples exhibited non-uniform drying, with some sections remaining moist, whereas solar dried slices showed uniform dryness and a leathery, flexible texture.

Although quantitative assessments of color and nutritional quality were outside the scope of this study, they are recommended for future research.

5.1. Effect of Seasonal Conditions on Drying Performance

The experimental findings demonstrate that the drying kinetics and efficiency of the passive V-corrugated solar dryer were significantly influenced by seasonal environmental factors, including solar radiation, ambient temperature, and relative humidity.

The average solar radiation during the hot dry season (April–May) exceeded 1250 W/m², with ambient temperatures approaching 40 °C and relative humidity below 35%. Moisture evaporation was accelerated under these conditions, leading to a strong vapor pressure gradient between the drying air and the tomato surface [12]. Consequently, drying time was reduced to 10–12 h, representing a 45–55% reduction compared with the transitional season.

The lower day-1 drying efficiency observed during the hot-dry season (29–31% compared with 45–55% in the transitional months) does not necessarily indicate inferior performance. Although a greater quantity of water was removed within a given time period, the corresponding instantaneous solar radiation input was also significantly higher. The hot dry season was more efficient in terms of energy consumption per kilogram of water removed (about 12–14 MJ/kg vs. 18–22 MJ/kg in the transitional season).

These results are in line with the observations made in the review by Janjai and Bala [18] and the modelling results of Bala and Woods [12], both of which emphasised the significant impact of sun radiation and ambient humidity on solar drying performance and drying kinetics

5.2. Role of the V-Corrugated Absorber Plate

The improved V-corrugated absorber plate enhanced the system’s thermal performance through three mechanisms:

1. The corrugated profile provided approximately 16% greater absorber surface area than a flat plate with the same aperture area, increasing the effective heat-transfer area.

2. The V-groove geometry enhanced convective heat transfer by disrupting the laminar sublayer and generating localized turbulence between the absorber surface and the airflow [5,6].

3. The corrugated duct improved thermal mixing, minimized temperature stratification, and increased heat- transfer efficiency.

Comparable performance improvements have been reported for rib-roughened absorbers [6], triangular corrugated configurations [7], and V-grooved absorber designs [5]. The maximum drying effectiveness of 54.66% attained in this investigation is comparable to the 40.2% reported by Abdullah et al. [8] for delta wing turbulators and the 47.3% reported by Kumar et al. [7] for a forced convection triangular corrugated absorber. The improved performance of the present system may be attributed to the combined effect of natural convection, which avoids fan-related energy consumption, and the corrugation geometry, which enhances buoyancy-driven airflow and heat transfer.

5.3. Comparison with Open-Sun Drying

The suggested solar dryer may cut drying times by roughly 70–85% compared to traditional open-sun drying, according to literature comparisons; however, since systematic open-sun drying experiments were not carried out as part of the current study, this estimate should be interpreted cautiously. In the same vein, Hossain and Bala [10] reported that the drying performance and product quality were enhanced when a solar tunnel dryer was employed in comparison to conventional open-sun drying methods. Forson et al. [11] observed a 60–75% reduction in drying time for tomatoes using a natural-convection in-direct solar dryer. Additionally, contamination from dust, insects, and animals, which is a major quality limitation of conventional open-sun drying, was effectively prevented by the enclosed system design [3,4].

5.4. Practical Implications for Smallholder Farmers

For rural, off-grid communities in semi-arid areas, the system examined in this study offers the following benefits:

1. No electricity is needed: By eliminating the need for fans or blowers, passive operation minimizes both capital and operating costs.

2. Local fabrication: The dryer can be constructed by local craftspeople using readily available materials such as wood, aluminum sheet, glass, and PVC tubing.

3. Year-round usability: Farmers can preserve food even during periods of moderate solar radiation, as the system remains operational throughout the transitional season, although drying occurs more rapidly during the hot dry season.

4. Scalability: To enhance drying capacity for larger crop loads, several collectors could be connected in parallel.

5.5. Limitations of This Study

Several limitations should be acknowledged:

1. No systematic measurements of moisture loss or drying time were conducted for open-sun-dried samples, although side-by-side visual comparisons were performed. As a result, the reported 70–85% reduction should be presented as a literature-based estimate rather than a direct experimental finding.

2. Lycopene, vitamin C, and other heat-sensitive nutrients were not measured. Theoretically, their retention is expected to be improved due to shorter drying times and moderate temperature conditions; however, this has not yet been experimentally verified.

3. The experiments were conducted exclusively in Kano, Nigeria; therefore, variations in microclimatic conditions or latitude may lead to different performance outcomes in other locations.

4. The collector was operated for approximately 60 h in total; therefore, the long-term deterioration of the glazing and absorber coating was not assessed.

5. Drying efficiency was calculated based on total incident solar energy. However, some studies report moisture extraction rate or energy efficiency instead; therefore, comparisons with such studies should be made with caution.