Review of solar crop drying efficiency and its interconnection with meteorological factors

Crop dryers (CDs) play a vital role in agriculture, automotive, marine, and mining industries, particularly agriculture, where they help meet global nutritional needs [1, 2]. They are essential for drying fruits, vegetables, and medicinal herbs to market standards, as crops often retain up to 90% moisture, requiring drying to 7–15% moisture content (MC) [3]. With food processing accounting for 30% of global energy consumption and carbon emissions, energy-efficient drying solutions are necessary [4]. Solar crop dryers (SCDs) are becoming popular due to low maintenance and quick payback periods, sometimes within 200 days [5]. However, they face challenges such as weather dependence and slow drying times. Conventional crop dryers (CCDs) are more reliable, reaching temperatures up to 600 °C, while unconventional dryers rely on lower temperatures and solar power [6, 7]. Open sun drying (OSD) is cost-effective but suffers from contamination and long drying times [8].

Drying occurs in two stages: the constant rate period, where moisture is removed steadily, and the falling rate period, where moisture extraction slows [9]. Performance parameters include drying rates, initial MC, evaporated moisture, drying efficiency, final MC and mass. Forced convection drying in CCDs, with higher air velocity and lower humidity, enhances heat and mass transfer and is more efficient than natural convection dryers [10]. However, a critical balance for SCDs is essential as they are low-temperature devices. Drying can occur on-site or off-site, using natural or forced convection methods, which include direct, indirect, mixed, and hybrid modes [11]. Enhanced OSD versions, like direct solar crop dryers (DSCDs) and indirect solar crop dryers (ISCDs), improve drying efficiency [12,13,14]. ISCDs, though more complex, offer better quality [15, 16]. Mixed-mode dryers combine direct and indirect modes for higher efficiency but at a higher cost [17]. Hybrid systems integrating solar energy with photovoltaic (PV) or heat exchangers improve efficiency for large-scale operations [18,19,20].

This work identifies key deficiencies in SCDs resulting in low drying efficiencies and longer drying times. Therefore, this work reviews how meteorological factors, ambient air temperature, relative humidity, drying rates and conditioned drying air velocity affect the drying efficiency of various SCDs. It comparatively analyses key parameters in SCDs to identify shortcomings of drying air velocity, temperature, and RH at the chamber inlet, interior, and outlet to ambient air and RH. The study also examines heat collection enhancement techniques, both individually and collectively, to improve drying efficiency. Data on drying efficiencies, chamber designs, and payback periods from various sources are analysed to identify trends influencing high drying efficiency, considering chamber designs and heat retention methods.

The literature surveyed provided all data material for comparative analysis of various solar crop dryers. Data reduction through grouping and discrimination was performed to isolate non-essential data for analysis. Meteorological conditions, including ambient air temperature and relative humidity (RH), were the main parameters influenced by solar intensities and were further comparatively analysed. In turn, these meteorological conditions were instrumental in determining the drying chamber inlet, inside the chamber, and outlet conditions (temperature and RH), with drying efficiency as a function of these meteorological conditions. The performance of SCDs with corresponding payback periods was comparatively analysed and discussed exhaustively, from which conclusions were drawn. Google Scholar and ResearchGate were the two search engines used to extract relevant published work per the keywords: Solar crop dryers, drying efficiency, crop drying rates, final moisture content, etc. The grouping and discrimination focused on articles with any design types and with the ultimate focus on the following aspects, drying air velocity, ambient to chamber outlet air temperature and RH, drying rates, drying efficiency and payback periods. The data set was reduced on the basis of major contribution and similar results were excluded to avoid duplication and repetition with the focus on design and operating parameters. The final 72 papers were selected of the total data set of 143 articles, focusing on diversity of reported results. The timeframe mainly considered was almost three decades, but two older articles (1986–1987) were included to draw attention to their innovative approaches and unique results at that time compared to the modern innovation [21].

This section explores key aspects that are considered central to the performance of crop dryers. The subsequent sections classify and examine various types of crop dryers, considering the following: Sect. 3.1—Design parameters of solar crop dryers; Sect. 3.2—Performance of solar air collectors as standalone systems; Sect. 3.3—Direct solar crop dryers; Sect. 3.4—Indirect solar crop dryers; and Sect. 3.5—Mixed-mode solar crop dryers.

3.1 Solar crop dryer design parameters

This section examines design aspects that are integral to enhancing the performance of SCDs, with a focus on input parameters required to achieve high crop quality, reduce drying time and costs, and improve reliability. That is all in an effort to bridge the gap between UCDs and controlled crop dryers CCDs.

Numerous studies have investigated SCDs with the aim of improving drying efficiency. Chanda et al. [22] concluded that mixed-mode drying, which combines direct and indirect methods is the most effective approach. Tiwari et al. [23] reported that a dryer equipped with two evacuated tube solar collectors (ETSCs) and a heat exchanger achieved temperatures of up to 73.6 °C, although its efficiency declined under loaded conditions. Lingayat et al. [11] also noted that mixed-mode systems significantly reduce drying time. Ekechukwu and Norton [24] classified cabinet and greenhouse dryers as the two main types of mixed-mode systems. Mühlbauer [7] emphasised that solar dryers must meet key performance criteria, including maintaining crop quality, affordability, and operational reliability, however, their effectiveness can be limited in regions with high solar intensity due to meteorological constraints. Tyagi et al. [25] identified essential design elements for SCDs, such as forced convection, perforated trays, and thermal energy storage (TES) systems to sustain drying efficiency. Furthermore, several techniques, either in conjunction with TES or used independently can be employed to modify drying chambers, solar collectors, or both, in order to enhance overall performance [5, 26,27,28,29].

3.2 Solar air collector

This section examines the performance of SACs as standalone systems, discussing various aspects such as the intensity and interrelationship of key influencing parameters. It also highlights potential innovations aimed at improving their performance.

Kamarulzaman et al. [17] categorised SACs into air-based and liquid-based systems, the latter requiring heat exchangers. The absorber surfaces have been modified in various configurations to enhance heat transfer [9, 30]. Additionally, SACs are classified as either concentrating or non-concentrating types, with standard drying chambers typically constructed from cost-effective materials. Majumder et al. [31] evaluated a SAC featuring a semi-hexagonal solar absorber plate integrated with a helical spring and honeycomb passage, designed to improve absorptivity, as illustrated in Fig. 1a and b. The findings indicated that larger tilt angles reduced performance, reinforcing the recommendation to align the tilt angle with the local latitude (± 15°). Over a three-day period, air temperature increases were averaged at 36.02  °C, 37 °C, and 39.2 °C, corresponding to heat gains of 283.1 W, 308.75 W, and 321.82 W, respectively. While earlier studies suggested a negative correlation between air velocity and heat gain, this study observed that higher air velocities (up to 1.9 m/s) led to increased heat gain. In particular, thermal efficiency improved by 15.8% as air velocity increased from 1.3 m/s to 1.9 m/s, with efficiencies ranging from 24.73% to 40.5%. However, the benefits of increased velocity diminished beyond this point.

a SAC pictorial view, b Schematic view [31]

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Akpinar [32] investigated convective heat transfer coefficients (CHTC) for various crops, finding that potatoes exhibited the highest CHTC, while pumpkins had the lowest. The study showed that air velocity positively influenced CHTC, whereas temperature had a minimal effect. Hans et al. [33] analysed forced convection SACs incorporating mild steel chips, demonstrating that efficiency improved with increasing air velocity, though only up to a certain threshold.

3.3 Direct solar crop dryers (DSCD)

This section provides a brief overview of direct solar crop dryers, focusing on the key parameters that influence drying efficiency. It also distinguishes between naturally aspirated and forced convection designs.

3.3.1 Natural convection

Direct solar crop dryers are an improved alternative to open sun drying (OSD), as they utilise enclosed structures to capture solar radiation and retain heat, thereby enhancing the drying process. Typically, square or rectangular in shape, these dryers are equipped with a transparent polyethylene cover that allows sunlight to enter the chamber. Airflow is driven naturally by buoyancy differences, which carry evaporated moisture out through an exhaust [34]. Afriyie et al. [15] investigated a direct chimney-dependent solar crop dryer (CDSCD) under both loaded and unloaded conditions. The system featured a single-pass drying chamber with glass covers exposed to direct solar radiation and was designed with optimised inlet and exhaust manifold dimensions to support efficient drying. The tests, which involved varying roof inclinations and solar chimney configurations, revealed that higher exhaust air temperatures achieved under no-load conditions enhanced drying performance by increasing air velocity.

The most effective configuration featured a slight roof inclination combined with a solar chimney. Under loaded conditions using cassava, air velocity was reduced due to resistance from the drying trays. However, the highest average air velocity (0.18 m/s) was recorded with a setup incorporating an absorber, a smaller roof inclination, and a solar chimney. Afriyie et al. [35] observed that the performance of chimney-dependent solar crop dryers (CDSCDs) was influenced by several factors, including geographic location, roof inclination, chimney height, and the chamber’s surface area. Drying rates were initially high but decreased as surface moisture was removed. Notably, drying continued even after heat lamps were turned off. RH was found to significantly affect the drying process, with lower RH levels enhancing moisture removal [36,37,38].

Chauhan et al. [39] tested a greenhouse dryer, shown in Fig. 2, and found that the forced mode using a 12 V DC exhaust fan, achieved higher efficiency but had a longer payback period (2.35 years) compared to the natural mode (1.68 years).

Greenhouse dryer under natural and forced modes [39]

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The final moisture content (MC) reached 0.12 in 12 h under forced mode, and 0.14 in 14 h under natural mode, while sun drying took 21 h to reach 0.25 MC, emphasising the superior efficiency of the forced mode. Ekechukwu and Norton [40] highlighted the limitations of naturally circulated, direct-type dryers, including crop damage and slow moisture removal rates. It was proposed that incorporating solar chimneys could enhance airflow and thus improve moisture extraction. As a result, chimney-assisted dryers demonstrated better overall performance.

Spall and Sethi [20] investigated a modified stacked tray dryer featuring an aluminium reflective north wall, which outperformed both natural and forced convection models lacking this feature. The single-pass airflow system incorporated a front-loading design. Using carrots as the test crop, It was reported that winter thermal efficiencies of 25.88% (natural convection) and 23.98% (forced convection) with the reflective wall, compared to 20.91% and 19.63%, respectively, without it. The dryer measured 1.25 m × 1.5 m × 1.09 m, with a tray area of 1.61 m². The reflective wall significantly enhanced drying efficiency by increasing solar radiation by 49.79% in summer and 37.58% in winter. During winter, air temperatures reached 55.8 °C (natural) and 52.4 °C (forced) with the reflective wall, compared to 54.6 °C and 47.6 °C without it. Moisture content reduction improved by 513.86% under natural convection and 381.43% without the wall, with equilibrium moisture content achieved three hours earlier.

Hidalgo et al. [2] investigated a direct trapezoidal-shaped SCD for drying green onions, integrated with a 10 W PV module and tested under both natural and forced convection. The dryer measured 0.3 m in length, 0.7 m in width, and 1.2 m in height, featuring inclined faces at 90° and 45°, respectively. The system utilised eight 24 V fans, each with a nominal diameter of 120 mm, to circulate air. Under forced convection, air temperatures ranged from 37.3 °C at the inlet to 37.5 °C at the outlet, with RH values of 59.4% and 58.2%, respectively. Although airflow slightly increased the temperature, RH remained largely unchanged. In contrast, natural convection exhibited a greater temperature and RH differential, resulting in more effective moisture removal. Green onions with an initial MC of 91.30 ± 0.63% and an initial weight of 1.64 ± 0.15 g dried to 0.38 ± 0.12 g under natural convection. Under forced convection, the initial MC was 90.84 ± 0.33%, with drying reducing the weight to 0.24 ± 0.02 g.

Ekechukwu and Norton [24] evaluated a solar chimney crop dryer over the course of a year under varying weather conditions. Faster drying was observed during the dry season, attributed to lower RH, higher temperatures, and stable solar radiation, which enabled crops to reach a safe MC within 17 h. Conversely, the wet season was characterised by higher drying air RH and lower temperatures, resulting in slower drying rates. During this period, drying took six days to reduce MC to 30% (dry basis), which is above the safe storage level of 20% (dry basis). Additionally, sporadic weather and moisture reabsorption during the night further delayed the drying process. Trust and Okot [41] compared DSCD with open-air drying for sweet potatoes and cassava. Over a four-day period with daily 10-h drying sessions, more than 60% moisture removal was achieved, reaching a safe MC suitable for storage. Drying rates were recorded as 0.21 g/h for DSCD and 0.17 g/h for open-air drying of sweet potatoes, and 0.29 g/h for DSCD and 0.22 g/h for open-air drying of cassava.

3.3.2 Forced convection

This drying method improves the direct passive dryer by using a fan or blower to enhance airflow, expelling moisture efficiently and reducing drying time compared to OSD and DSCD in natural convection [34]. Tiwari and Tiwari [42] studied a greenhouse dryer with three semi-transparent PV modules and two 12 V, 1A fans, achieving 68.5% thermal efficiency over four seasons and a 1.23-year payback period. The semi-transparent design improved crop colour by reducing direct sunlight, aligning closely with theoretical predictions. Pochont et al. [43] compared a hybrid greenhouse dryer in active and passive modes for drying red chilli alongside an OSD method. The active mode showed superior drying rates with an initial MC of 80% and a weight of 2 kg. Drying times were 24 h for OSD, 12 h for passive drying, 9 h for the active crossflow pattern, and 11 h for the mixed-flow pattern. Final MC values were 24.24%, 24.24%, 18.70%, and 16.67%, wet basis (wb) for OSD, passive mode, active mixed-flow, and active crossflow, respectively.

Mbacho et al. [44] presented a forced convection greenhouse tomato SCD using a Clay-CaCl2-TES system. Nighttime RH was 94.4% for the ambient and 59.9% for the drying chamber, daytime RH was 72.2% for ambient and 38.4% for the drying chamber with TES. The TES system improved the drying rate and reduced drying time. The final moisture content was achieved over 27 h in three phases: solar drying (94.3% to 25.5%), nocturnal drying (25.5% to 16.4%), and solar drying (16.4% to 8.3%).

3.4 Indirect solar crop dryers (ISCD)

This section discusses indirect solar crop dryers in brief detail prioritising aspects influential in drying chamber performance, while distinguishing between natural and forced convection dryers. The SAC integrated system protects crops from sunlight, improves drying rates over DSCD, and enhances heat transfer by switching from direct to indirect heat. It can be configured with single or double ducts and a modified absorbing surface, increasing contact area for better heat exchange and drying efficiency [28, 31, 33, 36, 45].

3.4.1 Natural convection

Jain and Tewari [46] studied an ISCD using Phase Change Material (PCM) as a TES medium under natural convection. The system had a 1.5 m² SAC with reflectors for improved air heating. Air passed through PCM-filled cylindrical tubes, storing heat during sunlight and releasing it post-sunset, extending drying by 5–6 h. MC decreased from 4.8 kg/kg (db) to 0.11 kg/kg (db) for mint leaves, with 28.2% efficiency and a 1.5-year payback. Atia et al. [47] highlighted that stable drying chamber temperatures with PCM improve crop quality.

Musembi et al. [48] designed an ISCD that dried apples efficiently, achieving a maximum air temperature of 47.5 °C and reducing 886.64 g of sliced apples to 135.1 g. The ambient RH was 51.3%, the inlet RH of the drying chamber was 40%, and the outlet RH was 54.3%, indicating how the drying process affects the air's MC. The outlet RH is higher as the air absorbs moisture from the crops. The system efficiently reduced the crops' initial MC from 86% to 8.12% in 9 h, achieving an overall SAC efficiency of 17.89%, demonstrating significant moisture removal quickly.

Lingayat et al. [36] studied a chimney-integrated ISCD for drying banana slices. The chamber had meshed-stacked trays powered by a V-corrugated SAC in natural convection mode. Under load, air temperature dropped from 55 °C (lower tray) to 44 °C (upper tray). The SAC outlet temperature was 61.2 °C with crops and 81 °C unloaded, while the drying chamber exit temperature reached 78 °C. These variations highlight how load and temperature affect performance, improving efficiency when the dryer is not loaded. Abubakar et al. [49] found that a non-sequential air dryer achieved 25.35% efficiency for yam, with uniform air injection and independent chimneys, ensuring consistent drying of sliced yam. The tray-by-tray air injection design avoided issues of sequential drying, which causes uneven results. Over 16 h across two days, the dryer removed 78% of the crop's moisture, with a drying rate of 2.55 × 10⁻⁵ kg/s.

3.4.2 Forced convection

Boughali et al. [50] developed a hybrid forced convection ISCD (solar-electrical) with a 12.8 kg capacity. It used a 3.75 kW electric heater, a 40 W exhaust fan (200 mm, 1400 rpm, 0.325 m³/s airflow, and a 2.45 m² solar air collector). Tomato crop trays (0.4 m²) were spaced 0.12 m apart. Observations over three winter days showed that as air velocity increased in the drying chamber, the air temperature decreased. The dryer combined a SAC and resistor heater to heat air, compensating for low solar heating in winter. It had six stacked trays, with air flowing in one direction and exiting at the bottom. On winter afternoons, without the electric heater, the temperature increased by 13 °C with an intake velocity of 1.5 m/s. Among three air velocities (1 m/s, 1.5 m/s, and 2 m/s), the highest efficiency (51.6%) occurred at 1 m/s with a mass flow rate of 0.0405 kg/m²/s. At higher temperatures (50 °C to 75 °C), drying time decreased from 14 to 6.5 h. Increased air velocity reduced the SAC's contribution, and the dryer had a payback period of 1.27 years.

Kontaxakis et al. [51] compared an ISCD and OSD for drying grapes, showing that the ISCD, with three fans, reduced drying time from 12 to 7 days and reached temperatures up to 53.5 °C, creating a more efficient environment for drying. The ISCD achieved lower RH during sunshine hours, dropping to 11%, compared to the OSD's 58.7% RH.

Goud et al. [9] studied a novel ISCD enhanced with three PV-powered inlet fans for drying green chilli and okra with fans installed at the inlet duct. With a 1.05 \(\times \) 0.4 \(\times \) 2 m³ chamber and a corrugated SAC, the dryer achieved efficient drying with an air velocity controlled by the inlet fans. Drying efficiency was influenced by MC, with higher efficiency in the initial stages when surface moisture content (SMC) was readily available. Efficiency decreased as core moisture content (CMC) required diffusion. Chilli had a drying efficiency of 36.28%, while okra had a higher efficiency of 52.75% due to more available SMC. The average RH in the drying chamber was 35.13% for chilli and 38.68% for okra, with inlet RH at 14.35% and 16.55%, respectively. Drying rates were 0.4414 kg/h for chilli and 0.99 kg/h for okra. Forced convection drying reduced chilli’s MC to 0.238 kg/kg (db), outperforming natural convection.

Sharma et al. [52] found that a corrugated SAC improved drying performance in an ISCD system, enhancing air circulation. The theoretical thermal efficiency of the dryer with the corrugated absorber was 71.3% in summer, while winter efficiency ranged from 68.03% to 82.85%. Hadibi et al. [53] experimentally analysed the performance of onion-forced convection, ISCD, under three modes: stationary solar electric drying (SED), intermittent solar electric drying (ISED), and ISED with TES material. A 10 kg batch of onion was examined using a compact system. All modes utilised both SAC and an electric heater, alternating every hour. The final moisture content (0.2 kg/kg db) was achieved in 12, 19, and 17 h for SED, ISED, and ISED-TES, respectively. The moisture extraction rate was lower in the intermittent solar electric drying (ISED) mode compared to stationary solar electric drying (SED), but increased slightly with TES integration. The drying efficiencies were 21.05% for SED, 35.69% for ISED, and 35.39% for ISED-TES.

Amirtharajan et al. [54] studied a PV-integrated SCD, comparing it to natural drying and OSD with peanuts as the test crop. In contrast to Eltawil et al. [55], system performance improved with higher air mass flow and solar radiation. The MC reduction to 71.5% (wb) was achieved in 37 h for the solar dryer. The study concluded that the thermal and overall efficiencies of the hybrid dryer were 48.23% and 60.86%, respectively. El Khadraoui et al. [56] studied the thermal behaviour of a forced convection ISCD with two SACs: one without and one with a PCM cavity. The PCM-equipped SAC stored latent heat during the day and released it at night, extending drying time and reducing overall drying duration. The drying chamber featured six stacked perforated trays for sequential drying. The presence of PCM caused a temperature difference of 5 °C to 19 °C overnight, enhancing performance. The PCM-equipped SAC delivered a uniform heat output of 100 W/m², supplying 16.87 kW of heat for continuous nighttime operation. This resulted in a drying efficiency of 33.9%. The drying chamber temperature was higher with PCM, and the RH during the day was 66.7% for the SAC with PCM, compared to 78.6% for the SAC without. At night, the SAC without PCM matched ambient RH, while the SAC with PCM had an RH 17%-34.5% lower.

In Ohagwu et al. [57], drying chamber efficiencies for simulated and experimental setups were 89 and 90.3%, respectively, with an average drying temperature of 41.2 °C and a drying rate of 0.36 kg/h. The experiments were conducted under no-load and loaded conditions to investigate the temperature requirements for drying paddy rice.

3.5 Mixed-mode solar crop dryers (MMSCDs)

These systems dry crops faster than DSCDs and ISCDs, with heat supplied from the SAC and directly from the sun through the transparent cover [23, 28].

3.5.1 Natural convection

Forson et al. [58] studied an MMSCD under loaded conditions using a lab simulation. The setup involved naturally aspirated air heated by a double-duct SAC, with a 3 mm thick glass cover to allow solar radiation. Cassava chips (initial MC: 201.7%) were tested at two loading densities: 2.8 kg/m² (room temperature) and 9.13 kg/m² (preheated air). Ten infrared lamps (100W each) simulated solar radiation (464 W/m²). Thermocouples and RH measurements tracked data. The drying process was improved with combined direct and indirect heating. In 24-h cycle tests, MC reductions ranged from 607.3 to 840 g, with nocturnal moisture removal from 7.1% to 29.9%.

Despite sporadic curves (with infrared lamps off), a faster drying rate was observed. Nocturnal moisture removal was due to residual heat, temperature, pressure, and RH, slowing as MC decreased over time. For capacity evaluation, 3 kg of the same crop was loaded into the dryer and tested for 21 h, reducing the final MC to 21.1%. The nocturnal drying contribution was 23.9% at an optimal 18 kg/m² loading density. During this test, the pick-up and dryer efficiencies peaked at 28.2% and 22.5%, respectively.

César et al. [59] studied a natural convection MMSCD/ISCD coupled with a SAC for drying tomatoes. The dryer operated in mixed mode with a movable drying chamber cover. Perforated crop trays were arranged to allow heated air to pass through, creating a temperature gradient between the trays, allowing heated air to pass through and evaporate the crop moisture. In a two-day test, the MMSCD reduced the initial tomato weight from 7090 ± 197 g to 792.5 ± 17.67 g, achieving a safe MC after 17 h. By the end of day one, the MC was reduced to 0.449 ± 0.035 g/g (db), and by day two, it was 0.054 ± 0.005 g/g (db).

When operated as an ISCD, tomatoes' initial and final weights were 7285 ± 157 g and 692.5 ± 15.07 g, respectively. Final moisture contents on days 1, 2, and 3 were 2.23 ± 0.04, 0.4 ± 0.05, and 0.07 ± 0.001 g/g (db). The ISCD process lasted 26 h longer than the mixed-mode dryer due to a 10 °C higher temperature and larger solar collection area. Mixed-mode drying had a higher average efficiency (5.47% vs. 4.48%), but indirect mode had a higher thermal efficiency (10.66% vs. 8.80%).

In a study by Abubakar et al. [60], integrating TES material with a chimney crop dryer and SAC preconditioning increased drying efficiency by 13%. The drying time was reduced by 2 h (from 18 h), and the average drying efficiencies were 29.15% with TES and 25.35% without TES.

Sekyere et al. [61] investigated MMSCD drying under natural convection. This was expanded in Sekyere et al. [62] with simulations on drying air inlet/outlet openings under solar hybrid and backup heating modes.

Using a backup electric heater during low solar insolation, four drying modes were tested with 2.262 kg of pineapple slices: (1) solar insolation by day and electric heater by night, (2) continuous electric heater, (3) hybrid mode, and (4) simulated solar energy only. Mode 4 had the lowest thermal energy build-up, while Modes 1–3 improved drying rates with the electric heater, especially at night. Moreover, it was found that the safe MC values were 144%, 106%, 184%, and 155% (db) at 23, 19, 10, and 7 h for modes 4, 1, 2, and 3, respectively. Mode 3 was the most effective, achieving the lowest MC in the shortest time, with the highest RH difference between inlet and outlet air, indicating efficient moisture evaporation from solar and electric heating. Forson et al. [45] studied MMSCD-natural convection with a double-duct SAC for cassava chips. Drying efficiencies were 3.5%, 5.5%, and 12.3% for crop loads of 49.1 kg, 65.9 kg, and 162 kg, respectively, indicating efficiency increased with crop load.

The drying efficiency improved with higher crop loads, although the drying time for the 162 kg load (35.5 h) was slightly longer than the 49.1 kg and 65.9 kg loads (both 34 h). The final MC of 17% was reached in 34 h for the 65.9 kg load, while the 49.1 kg and 162 kg loads reached 17.3%. Ambient RH affected the drying, with the lowest RH (66.2%) for the 65.9 kg load and higher RH (67.1% and 72.1%) for the 49.1 kg and 162 kg loads, requiring more thermal energy. Ellis et al. [26] showed that integrating sensible TES material (flake salt) into an MMSCD dryer, significantly improved sorghum drying. The TES dryer reduced MC from 30 to 8% (wb) in six hours, while the TES-free dryer took 24 h to reach 13% (wb). TES integration boosted drying efficiency by 21.1%, providing more consistent results.

3.5.2 Forced convection

Lutz et al. [6] described two types of forced convection SCDs. The smaller dryer, with a 300 kg crop capacity, had a drying chamber arranged in series with the solar collector. With a 1000 kg capacity, the larger dryer featured a parallel arrangement and measured 20 \(\times \) 2 m, powered by a 100-W fan. Experiments in Greece showed solar insolation between 3.2 to 7.5 kWh/m²/day (avg. 6 kWh/m²/day), with ambient temperatures ranging from 25 °C to 35 °C and RH from 15 to 80%. Continuous ventilation with high air velocity (3 m/s, 1200 m³/h) was essential for the first two days to prevent grape discolouration and enhance moisture removal. The air humidity was 55–60%, indicating higher moisture absorption. Afterwards, airflow was reduced to 600 m³/h, with ventilation limited to daylight hours. The drying chamber's temperature peaked at 50–60 °C, and exhaust air humidity dropped to 16%, signalling reduced moisture absorption and dryer efficiency.

Malik and Kumar [63] investigated the performance of an MMSCD vertical system for drying turmeric slices under varying loads. Using forced convection with a 0.12 m diameter, 12 V fan circulating air at 2.2 m/s, air passed through solar radiation-absorbing plates (1.08 m² and 1.13 m²). Turmeric slices (2, 3, and 4 mm thicknesses) had surface temperatures of 56.1 °C, 56.4 °C, and 55.3 °C, respectively, while chamber air temperatures were 56.4 °C, 56.8 °C, and 55.8 °C. Solar radiation heat was 490.88, 502.46, and 483.77 W/m², with 8.01, 9.37, and 8.67 g/h drying rates.

The study tested 3 mm thick turmeric slices under natural and forced convection drying, both in fully loaded conditions. Surface temperatures were 55.10 °C (natural) and 55.14 °C (forced), with ambient air temperatures of 55.90 °C and 55.80 °C, respectively. Solar radiation averaged 391.54 W/m² (natural) and 379.06 W/m² (forced). Drying rates were 41.12 g/h (natural) and 42.32 g/h (forced). Moisture content decreased to 26.7% (natural) and 13.6% (forced). Forced convection proved more efficient but costlier at 561.12 INR/kg vs. 475.10 INR/kg for natural convection. In the study by Deepak and Behura [64], a mixed-mode solar tunnel crop dryer integrated with TES was developed for cucumber drying. The dryer design featured two aluminium wire mesh trays and a 50 mm thick insulator at the chamber's bottom to minimise heat loss. The system included a pre-conditioning SAC powered by a centrifugal blower, initiating forced convection. Pebbles in the drying bed absorbed solar energy, helping maintain chamber temperature. The innovative system used Lauric acid as a PCM within the SAC, stored and transferred latent heat to the drying air, enhancing drying efficiency, particularly during periods of low solar radiation. The MMSCD tunnel-type dryer with TES was tested for loaded and unloaded crops under three conditions. Solar radiation averaged 262.9, 383.26, and 371 W/m² for Tests 1, 2, and 3, respectively. The goal was to assess dryer performance with no storage (Test 1), sensible heat storage (Test 2), and both sensible and latent heat storage (Test 3). Ambient wind speeds ranged from 2.7 to 4.1 m/s, and temperatures were 26–31 °C. Test 2 recorded the highest drying chamber temperature (63.4 °C), while Test 3 had a lower average temperature (50.89 °C). The time to reach the desired MC was 12 h for Test 1, 9 h for Test 2, and 6 h for Test 3. Test 1 reached a final MC of 6%, while Tests 2 and 3 achieved 5%. Test 3 had the highest drying rate (0.76 kg/h) and efficiency (14.2%). Energy storage, particularly both sensible and latent heat, enhanced drying performance, especially at low solar intensity, improving drying rate and efficiency.

Hamdi et al. [65] studied an MMSCD greenhouse-type dryer under forced convection. They found that the efficiency of a 2 m² SAC with a corrugated absorber was influenced by solar insolation, peaking at noon and decreasing in the evening. The initial moisture content of grapes reduced from 5.5 kg/kg to 0.22 kg/kg over 128 h, reinforcing the importance of solar intensity in optimising dryer performance. Eltawil et al. [55] investigated a forced convection, PV-powered MMSCD-tunnel dryer for drying potato slices. The system, powered by a 12 V, 80 W DC fan, induced forced convection, allowing preconditioned air from the SAC to enter the drying chamber. The study assessed performance with and without a thermal black curtain, which improved heat retention and drying efficiency. The PV-powered system offered a sustainable, energy-efficient drying solution through solar power and forced convection.

The study found an inverse relationship between drying air temperature and airflow rate, with higher airflow leading to lower temperatures. Drying efficiency was highest with a thermal curtain, achieving 34.29% efficiency compared to 28.49% without it at 0.0786 kg/s airflow. The thermal curtain also improved thermal efficiency (19.07%) over no curtain (16.71%), demonstrating its role in heat retention and reducing heat loss, enhancing the drying process.

El Khadraoui et al. [66] studied a novel MMSCD-greenhouse system powered by a single flat plate SAC, revealing key findings. The drying air temperature at the SAC outlet peaked around 12:30 PM before decreasing in the afternoon due to solar radiation. In contrast, the drying chamber temperature continued rising until 3:00 PM, owing to the greenhouse soil's thermal inertia. The RH inside the greenhouse remained lower than the ambient RH, improving drying efficiency. Compared to open sun drying, the MMSCD system with forced convection was more stable and efficient. For red pepper, the initial MC of 12.15 kg/kg (db) dropped to 0.17 kg/kg (db) in the greenhouse dryer, while it reached 0.19 kg/kg (DB) in the open sun drying, reducing drying time from 24 to 17 h. For grapes, the initial MC of 5.49 kg/kg (db) dropped to 0.22 kg/kg (db) in the greenhouse, compared to 0.20 kg/kg (db) in sun drying, reducing drying time from 67 to 50 h. The greenhouse dryer had a payback period of 1.6 years.

Chauhan et al. [67] analysed a PV-assisted modified MMSCD-greenhouse dryer under forced convection, as shown in Fig. 3. The dryer, with dimensions 1.5 \(\times \) 1.0 \(\times \) 0.5 m³, featured an insulated drying chamber and a 12 V DC fan powered by a 40 W PV module, enhancing the drying process.

a Pictorial view of a greenhouse dryer, b Schematic diagram of a greenhouse dryer [67]

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The dryer with the integrated SAC achieved a maximum air temperature of 64.8 °C, higher than the non-SAC setup. The SAC setup showed a 32.2% temperature difference between SAC and ambient air, compared to 22.79% without it. Elevated SAC temperatures were partly due to the solar-absorbing wall. The heat transfer coefficient (HTC) was higher with the SAC, indicating faster heat transfer and moisture removal. However, more heat loss occurred through the exhaust fan, while the heat utilisation factor and coefficient of performance (COP) were 10.1% and 7.9% higher, respectively, with the SAC. Some of the design parameters and their core effects, which are integral in the chamber performance are detailed in Table 1.

A review of studies spanning years identified key challenges in SCDs driven by technological progress: (1) maintaining high drying air temperature to reduce RH, (2) increasing air velocity for faster MC removal, (3) narrowing productivity gaps between CCDs and UCDs, and (4) managing intermittent thermal energy supply. The following discussion is based on the reviewed literature.

4.1 Effects of drying air velocity

Drying air velocity is categorised into natural and forced convection modes, with forced convection exhibiting higher velocities. Air velocity ranges from 0.18 m/s (natural) to 3 m/s (forced). From the literature sources reporting their findings over time, it is found that: 1) Literature consistently indicates that for lower SCD operating temperatures, slower air velocities are preferable. 2) The air velocity should vary with the constant and falling rates periods, such that high air velocities are desirable during the constant rate periods and low air velocities with a falling rate period. Hence, a balance between air velocity and temperature is crucial for low-temperature dryers like UCDs, where higher velocity enhances cooling due to increased convection [68]. 3) The literature sources also show that there is an efficiency gain with an increase in air velocity for SAC, but a reverse for the chamber [6, 15, 31, 50]. This suggests that since these systems operate under subsonic conditions, a divergent section between the SAC outlet and chamber inlet is desirable. Higher velocities in low-temperature systems are detrimental, reducing chamber temperature and increasing relative humidity, which negatively impacts crop quality and drying time. While higher air velocities enhance drying during the initial constant rate period, they must decrease during the falling rate period to preserve chamber temperatures for effective moisture extraction [52]. Therefore, relatively high air velocities are better suited for thermally enhanced system, PCM and reflectors amongst other are integrated in the chamber. However, there are no defined optimal air velocities for either period, constant or falling rate. Therefore, a study aimed at establishing a range of design air velocities or a parametric model for a given system based on the operating conditions is crucial.

Moreover, the inlet-to-outlet area ratio of the drying chamber also influences drying air velocity, as noted earlier, higher drying air velocities in the chamber are undesirable for low temperature SCDs. Larger inlet and smaller outlet areas promote higher velocities, while a reverse configuration reduces it. Sekyere et al. [62] found that an inlet-to-outlet area ratio of 1:1.5 was more effective than 1:1 or 1:3. Therefore, for SCDs as low temperature devices, a divergent rather than a convergent air passage with set limits is desirable to prevent chamber excessive cooling, which affects crop quality and drying time. Forson et al. [58] suggested that the top channel of double-ducted solar air collectors should have a larger depth-to-width ratio than the bottom channels. The top channels receive direct sun rays which translate to high heat transfer rates, and larger portions of heat absorbed by increase air volumetric flowrate compared to the bottom channels.

Different drying air mass flow rates are required for constant and falling periods due to varying surface moisture and inner moisture extraction rates. To address this, possible solutions include installing adjustable annular drying systems, using variable-speed fans under forced convection, or heating exhaust humid air to reduce density and improve airflow [15]. Chimney-integrated dryers have been shown to improve airflow, especially in natural convection systems, with heated chimneys enhancing buoyancy differences [40]. To maintain buoyancy and reduce airflow obstructions, dryer performance is typically higher under unloaded conditions [55, 64]. Therefore, larger and fewer perforated cells subject to crop types and loading density in crop trays facilitate airflow. Forced convection can enhance drying, but the air velocity-to-heat ratio limits it. Solutions like compounded solar collection (larger or multiple collection surfaces), hybrid systems, and exhaust recirculation may increase efficiency, although their feasibility remains uncertain [43, 46].

4.2 Ambient air temperature and relative humidity

Intermittent heat supply is a significant challenge in SCDs, impacting crop quality [47]. Forson et al. [58] demonstrated that residual heat in well-designed drying chambers can be used during low or intermittent solar radiation periods, particularly in the initial drying phase. Unloaded SCDs typically operate at higher temperatures due to the absence of moisture (no load), while the loaded SCDs increases RH, leading to unstable and intermittent heat supply rates [36]. Figure 4 shows the maximum and minimum ambient temperatures recorded at various test locations.

Ambient air temperature for various SCDs

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The ambient temperature relative to the SCDs generally shows the extent to which heat is required. Heat energy demands are intensive for SCD operating under low ambient air temperature compared to the one exposed to higher ambient air temperature. Higher ambient temperatures correspond to locations with greater solar intensity, varying by region [69].

The data were gathered across different seasons, with winter typically having much lower temperatures. The highest recorded ambient temperature was 45.8 °C, and the lowest was 15 °C [56, 67]. Per the findings from the literature, system operating under high ambient temperatures are advantageous over those operated under a low ambient temperature. Hence, far less modifications are required to improve performance where the ambient temperature is higher. Moreover, most SCDs were tested in regions where the maximum temperature exceeded 25 °C, providing favourable conditions with minimal need to heat the drying air. Research indicates that increasing the drying air temperature can shorten drying time, potentially replacing CCDs [25].

However, the biggest challenge is the intermittency of solar radiation and poor absorption characteristics of the collectors. Therefore, more effort is necessary in the field of solar absorption to enhance the collection efficiency. While SACs do not fully utilise heat, there are methods aimed at maintaining chamber temperatures above ambient levels. Ambient conditions, especially RH, play a significant role in SCD performance. Where RH is high, some initial heat supplied is utilised in reducing RH, while low RH and high ambient air temperature ensures rapid crop moisture extraction without delay.

For instance, reducing RH from 51.3% to 40% improves drying efficiency [48], suggesting an RH gradient between the crop moisture and the surrounding at low RH values. Therefore, RH values have direct impact on drying time, crop quality and heat rate supply to the chamber. Moreover, high heat can drop RH to 11–8.4% on sunny days but rise to 53% during off-sunshine hours, slowing moisture extraction [51, 59]. The heat rate-RH relationship suggests that due to the intermittent nature of solar radiation, high heat retention techniques in the chamber are necessary to alleviate the adverse effects of high RH to reduce drying time and improve crop quality. Figure 5 illustrate the ambient RH parameter responsible for high ambient in the chamber for various systems.

Range of maximum and minimum ambient RH at various SCD test locations

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Low chamber temperatures and forced convection can hinder moisture removal [2, 17]. Forced convection modes work better with increased heat injection to balance the air velocity-to-heat ratio. Figure 4 illustrates the maximum and minimum temperatures at SCD test sites, with the highest recorded at 45.8 °C [56, 67]. The higher value of RH increases the energy demand to dry crops and are undesirable in SCDs. Figures 4 and 5 also show an inverse correlation between high RH and low temperatures. This explains why ISCDs and MMSCDs, which preheat air using SAC, are typically more effective. Ellis et al. (2020) found RH between 54 and 72%, with temperatures ranging from 26.5 °C to 29.5 °C. Similar trends were observed by César et al. [59] and Kontaxakis et al. [51], showing that seasonal and geographical factors also affect drying rates.

4.3 Drying chamber inlet air temperature and relative humidity

Figure 6 shows that the maximum RH observed in MMSCDs [67] and ISCDs [48] were 57% and 51.3%, respectively. This suggests that (1) Direct heating in MMSCDs increases air inlet temperature in Fig. 6. (2) Unloaded chambers maintain a high temperature due to lack of heat absorption by a load and high RH due to MC evaporation, and (3) Forced Convection helps replace humidified air and maintaining low RH. The 57% and 51.3% RH values corresponded to inlet temperatures of 47.9 °C and 47.5 °C. These temperature-RH values suggest that MMSCDs are more energy efficient compared to ISCDs due to high useful heat utilisation but may also depend on other influencing factors during the operation.

Drying chamber air temperature for various SCDs

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Simulations revealed that coupling dryers with alternative heat sources like bio-waste raised the inlet temperature to 100 °C [57]. Using heat from renewable energy (RE) devices could improve the drying chamber’s conditions, as systems with reflectors and TES demonstrate. Though SCDs are low-temperature devices, adding auxiliary heating like electric resistance heaters or investigating waste heat sources to improve drying rates and system efficiency is already shown in the literature, and it is a promising prospect. SCDs are also mode-sensitive, with performance varying between natural and forced convection [2]. Hybrid setups with additional heat sources, such as resistance heaters, have achieved higher inlet temperatures, surpassing ISCDs and MMSCDs purely powered by SAC [50]. Therefore, there is a strong necessity to experiment with various external sources to support SACs, while considerate of environmental impact.

4.4 Drying chamber air temperature and relative humidity

Figure 7 shows the chamber drying air temperature for various SCDs, with MMSCDs and ISCDs generally operating at higher temperatures under natural convection. The highest drying temperatures were 78 °C for ISCDs and 73.5–75 °C for MMSCDs, with ambient RH of 25.33% and 48.02 to 65%, respectively [36],Lasisi et al., 2020; [59]. Modifications to these systems highlight the need for additional heating.

Key observations on drying chamber temperatures for various SCDs

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A general correlation between the highest ambient temperatures in Fig. 4 and the highest chamber temperatures in Fig. 7 can be observed. These correspond to geographical locations with known high solar intensities as they vary with location [69]. The minimum and maximum ambient temperatures of 15–17 °C and 28–29.5 °C led to the lowest chamber temperature of 40 °C [56, 70]. This finding shows a direct correlation between ambient and chamber temperatures, reiterating the adverse effects of low ambient air temperatures. Furthermore, techniques like TES have been shown to mitigate nocturnal temperature reductions during drying. However, a permanent solution is to provide additional reliable heating sources as support during active drying periods during the daytime [51, 57].

Figure 7 also shows that MMSCDs, operating under forced convection, and DSCDs, under natural convection, maintain higher temperatures. ISCDs, under forced convection, exhibit moderate temperatures. MMSCDs' high heat rates sustain temperatures even with high drying air velocity, while DSCDs, with low air velocity, retain temperatures longer. ISCDs' high air currents increase cooling, leading to reduced performance. Techniques for improving SCD conditions include (1) Heat Retention [64], (2) Reflective Materials [20], (3) Compact Design [67] and (4) Integrated solar Chimneys to attain air density and RH [15]. These additions and modifications improve heat generation and SCD performance.

4.5 Drying rates and final moisture content

Figure 8 presents the drying rates for various systems under natural and forced convection. The highest 1891 g/h drying rate was observed for okra crop drying in the ISCD under forced convection. However, Fig. 8 also show that for SCDs under forced convection can achieve low drying rates where balanced heating and cooling is not provided. In contrast, the lowest drying rate of 0.17 g/h was found in natural convection DSCDs [9, 41]. These findings suggest that systems with high heat rates, such as ISCD and MMSCD, can maintain high drying temperatures even with increased air velocity. This allows for the removal of MC through the combination of air current and heat. However, the air velocity increase is limited due to the mode sensitivity and intermittent solar radiation patterns, leading to low-temperature operations.

Drying rates for various SCDs under natural and forced convection modes

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Figure 9, which plots the chamber RH values offers additional insights and shows that RH vary significantly across systems, highlighting differing drying conditions. A comparative analysis of drying duration, based on Figs. 7 and 8 data, reveals that systems with low RH, such as those using ISCD or MMSCD configurations, typically achieved lower final MCs faster.

Range of chamber RH for SCDs tested under different locations

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Figure 10 presents final moisture content versus drying time under natural convection. These systems often used high heat rates, additional heat sources, TES, and modified SAC, aligning with trends in Fig. 10, where low RH systems had shorter drying times [36, 59]. Optimised heat rates and drying air velocity enable faster drying, reducing nocturnal drying requirements. SCDs operating at low ambient temperature (Fig. 4) and high RH (Fig. 5) resulted in lower chamber inlet (Fig. 6) and chamber temperatures (Fig. 7). Figure 10 shows OSDs had the longest drying times, followed by DSCDs and ISCDs, emphasising the impact of heat input on drying efficiency.

Final moisture content versus drying time for various systems for natural convection

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The data in Fig. 11 shows that the shortest drying time of 6 h was achieved by the MMSCD enhanced with Thermal Energy Storage (TES) under natural and forced convection. The shorter drying time is illustrative of the advantages of enhanced heat input rates. Meanwhile, systems in Fig. 10 show poor heat input and longer drying times. The longest drying time occurred with a 130 kg crop in a larger greenhouse dryer for systems in Fig. 11, which suggests that under forced convection, the loading density can be increase, indicating a strong possibility for mass production [65]. In Fig. 10, 50% of these systems achieved a final MC below 20%. In Fig. 11, 75% of systems reached the same MC. This demonstrates the effectiveness of forced convection with sufficient chamber heating. Additionally, OSD systems took significantly longer to reach the desired MC compared to DSCD, ISCD, and MMSCD systems, emphasising the role of heat input and system configuration in reducing drying times and improving efficiency.

Final moisture content versus drying time for various systems for forced convection

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4.6 Drying chamber outlet air temperature and relative humidity

Figure 7 indicates that most systems maintaining elevated chamber temperatures near 70 °C were MMSCDs, which benefit from direct and indirect heating. Figure 12, on the other hand, demonstrates the effects of direct heating, where the chamber exit temperature peaked at 58.2 °C [2]. The higher exit air temperature is beneficial in reducing suspended water particles and increasing buoyancy for air current through the chamber. This suggests that direct heating combined with natural convection provides an optimal chamber temperature.

Key observations on the range of drying chamber outlet temperature for various SCDS

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The figure also shows the effects of indirect heating using SAC under natural convection, achieving a maximum temperature of 53.5 °C [48]. Furthermore, this temperature profile indicates that a significant increase in heat supply is necessary for forced convection systems to maintain a similar temperature level as observed with direct and indirect heating in natural convection mode [44]. It has been found in the literature that the overall buoyancy effect can be enhanced by heating the exhaust air, which aids in maintaining a higher temperature within the drying chamber.

4.7 Drying efficiency and payback periods

Figure 13 shows drying efficiencies for various systems. The MMSCD under natural convection had the lowest efficiency (3.5%), improving to 12.3% as crop load increased from 49.1 to 162 kg. High ambient RH (67–72.1%) limited heat utilisation despite shorter drying times for partially loaded systems [45]. In contrast, the DSCD achieved 49.5% efficiency with similar RH values (66–72%) and higher initial moisture content (91.46% wb) than MMSCD (67% wb). Key factors include loading density, chamber design, chamber area and crop moisture content, which enhanced heat utilisation and efficiency [2, 35, 71].

Drying efficiency under natural mode

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These values indicate low practical heat energy utilisation in most systems, as drying efficiencies above 20% were primarily observed in ISCDs and MMSCDs, often enhanced with PCM, while unmodified systems had efficiencies below 20%. Notably, 90.9% of systems with efficiencies above 20% were ISCDs and MMSCDs. Figure 13 shows most systems had efficiencies below 20%, with few achieving 20–30%, and one nearing 50% (DSCD), indicating generally low heat energy utilisation, as normalised drying efficiency is generally between 20 and 30% [72]. In contrast, Fig. 14 shows higher efficiencies for forced mode systems, most exceeding 20%, suggesting better heat utilisation. The highest efficiency, 70.2%, was achieved by a DSCD in forced convection mode, followed by an ISCD at 62.5%, surpassing ISCDs and MMSCDs [2]. This DSCD performance support the assertion that high performance is dependent or various factors beyond the system type and heat-to-air ratio. The lowest efficiency, 6.55%, was recorded by an MMSCD [63]. Hybrid ISCDs and MMSCDs maintained efficiencies above 30%, with 73.4% and 47.4% of systems reaching efficiencies of 20% and 30%, respectively. While MMSCDs and ISCDs are typically more efficiency, the above discussions and values are testament that the operating conditions are integral as efficiency in not solely dependent on design. There must be a balanced heat-to-air velocity ratio for enhanced performance.

Drying efficiency under forced mode

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The payback period varies from system to system, with those operating under natural mode exhibiting shorter payback periods than those operated by forced convection systems. These may be linked with additional modifications and system components in the forced convection systems. However, the higher drying rates and efficiency justify these longer payback periods in forced convection systems. Generally, the payback period for natural convection mode systems ranges from 1.5 to 1.96 years and 0.46 to 2.35 years for forced convection systems. A hybrid (ISED) and TES integrated system in forced convection mode showed the lowest payback period. Therefore, a considerable overlap necessitates a review of the choice of design and modifications to enhance the SCD performance.

This study examined the impact of meteorological conditions on crop drying efficiency.

• For natural convection SCDs, the general trend was that ISCDs and MMSCDs collectively accounted for 90.9% of systems with drying efficiency, ranging from 20 to 30%. Systems above 30% efficiency were enhanced with TES or hybrid modes.

• In forced convection SCDs, the general trend was that ISCDs and MMSCDs accounted for 73.4% and 47.4% drying efficiency, respectively, with 20% to 30% and higher efficiencies. Systems above 30% efficiency were hybrid models.

• Higher drying efficiency was linked to regions with high solar intensity, low RH, and elevated temperatures, reducing heat requirements and improving efficiency. Hence, current designs are challenged with low heat injection, which explains why TES-integrated and hybrid systems are effective.

• The payback period for natural convection SCDs ranged from 1.5 to 1.96 years, with MMSCDs taking longer. Hybrid and TES systems under forced convection reduced drying time and increased efficiency, with minimum payback periods of about 0.7 years. Without these modifications, payback periods ranged from 1.6 to 7.72 years.

• There is an imbalance of drying air velocity-to-heat ratio in high-performance systems. Despite that, forced convection systems (TES-integrated and hybrid) showed better efficiency, drying rates, and shorter payback periods than natural convection systems.

• Drying air velocity minimum and maximum values are bound by the low heat rate injection with about 0.18 to 3 m/s, reported in SCDs. Thus, a crucial imbalance exists between the drying air velocity-to-heat ratio in the drying chamber.

• Uneven crop drying remains challenging, but non-sequential drying in chamber trays addresses issues in low-temperature SCDs.

• The inlet-to-outlet ratio in SACs, with smaller inlets and larger outlets (1:1.5), improved performance. Supersonic conditions slowed air currents and increased RH, while heated solar chimneys eliminated buoyancy issues by heating exhaust air.

• While forced convection systems had higher performance, natural convection SCDs maintained a favourable drying environment with reduced RH during the daytime, achieving 49.5% and 70.2% efficiency in the DSCDs.

• A compact chamber design, with minimal surface area relative to solar radiation collection, is needed for optimal drying efficiency by minimising RH and maintaining appropriate chamber temperatures.

• In regions with low solar intensity or during winter, improve the use of TES, reflectors, and SAC positioning in existing designs.

• Integrating TES into chamber walls helps absorb and release heat during low heat input periods.

• Exhausted drying air in single-pass systems, exiting at high temperatures, can be repurposed to regulate the chamber's internal environment—further research into reusing humid exhaust air is needed.

• Hybrid systems with TES perform well; consider using waste heat from other renewable devices, like water distillers, to support SCDs, especially to balance drying air velocity and heat input.

• The optimal drying air velocities for different crop drying periods are unclear, revealing a research gap as velocity needs change between constant and falling-rate periods.

• Smaller drying chambers with more solar exposure improve drying efficiency. More research on the optimal chamber-to-SAC ratio is needed, particularly for solar-powered systems to enhance heat input.