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Solar dryer

A solar dryer is a device that harnesses solar energy to remove moisture from substances, primarily agricultural products such as fruits, vegetables, grains, and fish, by heating air and facilitating evaporation in a controlled environment, thereby preserving food more efficiently and hygienically than traditional open-air sun drying.^[1] These systems operate on the principle of converting solar radiation into thermal energy, which warms the air in collectors or directly on the product, increasing its capacity to absorb moisture while reducing relative humidity to accelerate the drying process.^[2] The drying mechanism involves two main phases: a constant-rate period where surface moisture evaporates rapidly, followed by a falling-rate period where internal diffusion limits the process, typically requiring energy inputs around 2258 kJ per kg of water evaporated at standard conditions.^[3] Solar dryers are classified into direct types, where products are exposed to sunlight within the enclosure; indirect types, using separate solar collectors to heat air before it passes over the items; and hybrid systems combining solar with auxiliary energy sources for consistent performance.^[2] Passive variants rely on natural convection for airflow, achieving temperatures of 54–82°C suitable for small-scale operations like drying 35–40 kg of sliced fruits in a day, while active systems incorporate fans for forced convection, enabling larger capacities and uniformity.^[1] Innovations such as double-pass collectors, thermal storage with rocks or pebbles, and photovoltaic-ventilated designs further enhance efficiency, reducing drying times by up to 50–66% compared to open sun methods and minimizing post-harvest losses, which exceed 30% in developing regions. Recent advancements as of 2025 include integration of phase change materials (PCMs) for improved thermal storage, IoT-enabled smart monitoring, and AI optimization for enhanced performance and product quality.^[2]^[3]^[4]^[5] Applications span food preservation for subsistence farmers, including drying grapes to raisins in 17 hours or chilies in 100–150 kg batches, as well as non-food uses like timber seasoning to 12% moisture content over 10–40 days.^[2] Key advantages include environmental sustainability through renewable energy use, cost savings on fuel or electricity, protection from contaminants like dust, insects, and rain, and improved product quality with retained nutrients, color, and texture, though challenges such as variable solar availability and potential overheating must be managed.^[1]^[6] These systems are particularly vital in sunny equatorial belts with average insolation of 3.82 kWh/m²/day, supporting food security and economic viability for rural communities.^[3]

Fundamentals

Definition and Purpose

A solar dryer is a device that harnesses solar energy to accelerate the drying of materials, particularly perishable goods such as food, by evaporating moisture content without relying on fossil fuels or electrical power.^[1] These systems typically involve a controlled enclosure where solar radiation heats air or the materials directly, facilitating efficient moisture removal while minimizing exposure to environmental contaminants.^[7] The primary purpose of solar dryers is to preserve the quality and nutritional value of products by reducing their moisture levels, thereby inhibiting microbial growth, enzymatic reactions, and spoilage that lead to post-harvest losses.^[8] This method enables longer storage periods and easier transportation of goods, which is especially vital in regions with limited access to electricity or refrigeration, supporting food security for small-scale farmers and subsistence communities.^[1] By maintaining hygiene and product integrity, solar drying helps retain essential nutrients and market value compared to traditional preservation techniques.^[7] Unlike open sun drying, where products are spread directly under sunlight and remain vulnerable to dust, insects, rain, and animals, solar dryers provide an enclosed, controlled environment that protects the materials throughout the process, often significantly reducing spoilage losses, which can reach up to 70% in open methods, and shortening drying times.^[7] In developing countries, where over 80% of food is produced by smallholders and undernourishment affects a significant portion of the population, solar dryers play a crucial role in enhancing food security through the drying of fruits like apples and bananas, vegetables, grains, and even meat or herbs.^[8] Solar dryers exist in variations such as direct, indirect, and hybrid types, all centered on this core objective of solar-powered moisture evaporation.^[7]

History and Development

The practice of open-air sun drying has been employed for millennia in agricultural preservation, dating back to prehistoric times when communities relied on solar energy to reduce moisture in food products. Evidence from historical records indicates that as early as 12,000 B.C., Egyptian tribes along the lower Nile dried fish and poultry using the intense desert sun to extend shelf life and support food security.^[9] Similarly, in ancient Mesopotamia around 2000 B.C., sun drying was integral to processing grains and other staples amid the region's arid climate, enabling storage and trade in early civilizations. These rudimentary methods laid the foundation for solar drying, which remained the dominant technique for centuries due to its simplicity and reliance on abundant natural resources.^[10] By the 19th century, the limitations of open-air exposure—such as contamination and weather dependency—prompted innovations in enclosed solar drying systems, particularly in California where favorable climate conditions supported commercial fruit production. Large-scale sun drying operations for apricots, prunes, and raisins emerged in the 1870s and 1880s, with early enclosed evaporators like the Alden Fruit Dryer representing initial attempts to protect produce while harnessing solar heat.^[11] Following World War II, energy shortages accelerated advancements in passive solar collectors for drying applications, with the first modern solar dryer prototypes developed in the 1950s to address post-war fuel scarcity and improve efficiency over traditional methods.^[12] The 1970s oil crises heightened global interest in renewable energy alternatives, leading to a surge in solar drying research and adoption, especially in developing countries where access to conventional fuels was limited. The Food and Agriculture Organization (FAO) played a pivotal role in the late 20th century by promoting affordable, simple solar dryer designs through technical guidelines and projects, aiming to enhance food preservation and reduce post-harvest losses in rural areas. During the 1980s and 1990s, key milestones included the prototyping of indirect solar dryers in India, such as the low-cost natural convection model developed by Das and Kumar in 1989, which separated the product from direct sunlight to minimize quality degradation while optimizing heat utilization.^[10] Entering the 2000s, solar drying evolved through integration with greenhouse technologies, creating enclosed environments that combined solar heating with controlled ventilation for more consistent results in diverse climates.^[13] Since around 2010, developments have emphasized active solar systems incorporating fans for enhanced airflow and thermal energy storage materials like phase-change substances to extend drying periods beyond sunlight hours, motivated by international climate objectives such as the United Nations Sustainable Development Goals (SDGs) for zero hunger and sustainable agriculture.^[14] In the 2020s, innovations include advanced phase-change materials (PCM) integration, reducing drying times by up to 70% and improving thermal efficiency by 10–20%, alongside hybrid photovoltaic-ventilated designs and sustainable materials, with the global market reaching USD 3.16 billion as of 2024.^[15]^[16] These advancements align with broader efforts to mitigate food waste, a critical issue in global supply chains where solar dryers preserve perishable goods more effectively than open-air methods.^[17]

Principles of Operation

Solar Heating Mechanisms

Solar dryers harness solar radiation, which consists of direct beam radiation from the sun's direct path, diffuse radiation scattered by the atmosphere, and global irradiance as the total incident energy on a surface.^[18] These components are captured by the dryer's collector surfaces to initiate heating.^[19] Absorption occurs primarily through blackbody-like surfaces or selective coatings on the collector, where materials such as painted metal absorbers convert incoming solar energy into thermal energy with high efficiency. Selective coatings, often achieving solar absorptivity greater than 0.9, minimize reflection in the visible and near-infrared spectrum while reducing thermal re-emission.^[20] Common materials include black-painted steel or aluminum alloys designed to approximate ideal blackbody behavior.^[19] Heat transfer within the system involves three main modes: radiation from the sun directly to the absorber, conduction through solid materials like the collector plate, and convection via air flow over heated surfaces. Radiation provides the initial energy input, while conduction distributes heat internally, and convection circulates warmed air to maintain uniform temperatures.^[18] These modes collectively elevate the internal environment to support the drying process.^[19] Solar dryers employ passive or active heating strategies to optimize energy capture. In passive systems, natural convection drives air circulation, often aided by solar chimneys that create buoyancy from temperature gradients without external power. Active systems, conversely, use forced convection through fans or blowers to enhance airflow and heat distribution, improving uniformity in larger setups.^[18] This distinction allows adaptation to varying solar conditions and scales.^[19] Collectors in solar dryers typically achieve temperatures between 40°C and 70°C, influenced by ambient conditions and design elements like glazing. Transparent covers made of glass or polycarbonate create a greenhouse effect by allowing shortwave solar radiation to enter while trapping longwave thermal radiation, thereby reducing convective and radiative heat losses. Flat-plate collectors often operate in the 20–70°C range under standard solar irradiance.^[19]

Drying Process Physics

The drying process in solar dryers involves the removal of moisture from a material through evaporation and diffusion, governed by fundamental heat and mass transfer principles. This process typically progresses in two main stages: the constant rate period and the falling rate period. During the constant rate period, moisture evaporates from the surface of the material at a steady rate, as long as free water is available on the surface and the drying conditions remain constant; the rate is controlled by external factors such as air temperature, humidity, and velocity.^[21]^[22] In the subsequent falling rate period, the drying rate decreases as surface moisture depletes, and moisture removal shifts to internal diffusion from the material's interior to the surface, often limiting the overall process.^[23]^[24] Psychrometrics plays a critical role in characterizing the air's moisture-holding capacity during drying. Key parameters include the humidity ratio, which represents the mass of water vapor per unit mass of dry air and indicates the air's absolute moisture content, and the wet-bulb temperature, which is the lowest temperature achievable by evaporative cooling and reflects the thermodynamic limit of drying under adiabatic conditions.^[25]^[26] These properties, often visualized on a psychrometric chart, help predict evaporation rates and ensure the air remains unsaturated to absorb moisture effectively from the material. Solar heating serves as the primary energy source to elevate air temperature, enhancing its capacity to drive evaporation.^[27] Mass transfer during drying is primarily described by Fick's first law of diffusion, which states that the diffusive flux J of moisture is proportional to the concentration gradient:

J = -D \frac{dc}{dx}

where D is the diffusion coefficient, c is the moisture concentration, and x is the position within the material; this law applies especially in the falling rate period where internal diffusion dominates.^[24]^[28] Concurrently, heat transfer to the air follows the energy balance equation for sensible heating:

Q = m C_p \Delta T

where Q is the heat transferred, m is the mass of air, C_p is the specific heat capacity of air, and \Delta T is the temperature change; this quantifies the energy required to raise air temperature for evaporation.^[29] These coupled processes ensure that heat input facilitates both evaporation and vapor transport away from the material. Air flow, whether by natural convection (driven by buoyancy from temperature differences) or forced convection (aided by fans), is essential for removing water vapor from the material's surface and maintaining low relative humidity in the drying chamber, ideally below 60% to maximize the driving force for evaporation.^[30]^[31] Higher air velocities enhance convective mass transfer coefficients, accelerating drying rates, while controlled humidity prevents reabsorption of moisture.^[32] To preserve product quality, the drying process must avoid issues such as case hardening, where rapid surface drying creates a impermeable outer layer that traps internal moisture, leading to uneven drying and potential microbial growth; this is mitigated by maintaining moderate temperatures and humidity levels.^[33]^[34] Additionally, controlled temperatures below critical thresholds (typically 60-70°C for sensitive materials) minimize nutrient degradation, such as vitamin C loss, by reducing thermal breakdown and oxidation reactions during moisture removal.^[35]^[36]

Types of Solar Dryers

Direct Solar Dryers

Direct solar dryers feature a configuration where the material to be dried is exposed directly to solar radiation within an enclosed structure, typically a shallow insulated box or cabinet with a transparent cover such as glass or plastic sheeting that allows sunlight to penetrate while trapping heat through the greenhouse effect.^[37] The drying chamber includes perforated trays or racks for holding the product, along with air inlets at the bottom and vents at the top to facilitate airflow.^[2] This setup leverages basic principles of solar heating, where radiation is absorbed by the product and chamber surfaces, raising internal temperatures.^[37] In operation, sunlight passes through the transparent cover to heat the product directly, typically achieving temperatures of 50-60°C, which accelerates moisture evaporation without requiring external energy input.^[38] Natural convection drives ventilation, with cooler ambient air entering through bottom vents, absorbing humidity from the warming product, and exiting as moist air via top openings or chimneys, thereby removing vapor and preventing stagnation.^[2] Some designs incorporate simple enhancements like fans for improved airflow, but the core process relies on buoyancy-induced circulation.^[38] Examples of direct solar dryers include simple box-style cabinets used for drying herbs, fruits, vegetables, and grains, such as those employed in Southeast Asia for spices and traditional crops.^[37] Historical applications include greenhouse-style tent dryers used in Bhutan for drying items like noodles, with calls for portable designs to meet the needs of nomadic yak herders drying cheese and produce.^[39] These dryers offer high simplicity and low construction costs, often using locally available materials, making them accessible for small-scale users, while providing faster drying rates and protection from environmental contaminants compared to open-air methods.^[2] However, the direct exposure to sunlight poses risks of contamination if seals are inadequate, ultraviolet degradation of sensitive nutrients, and limited control over conditions during cloudy weather.^[37]

Indirect Solar Dryers

Indirect solar dryers utilize a separate solar collector to heat ambient air, which is then conveyed to an enclosed drying chamber containing the material to be dried, preventing direct exposure of the product to sunlight. The collector typically consists of a flat plate with a black absorber surface that captures solar radiation to warm the air passing over or through it, with the heated air directed to the chamber via insulated ducts or pipes.^[40]^[41] In operation, the preheated air enters the drying chamber at temperatures ranging from 50-70°C and relatively low humidity, facilitating moisture evaporation from the product through convection without the risks associated with direct solar exposure. Airflow is driven by natural convection, often aided by a chimney that creates a draft through thermal buoyancy, or by mechanical means such as a fan for forced circulation, ensuring consistent movement over the material. This mediated heating process maintains controlled conditions within the chamber, promoting efficient drying while keeping the product shielded.^[40]^[41] Notable examples include tunnel dryers adapted for grains, where long, tunnel-like chambers allow continuous or batch processing of crops like paddy or maize, with heated air flowing longitudinally over trays of product. Designs from 1980s studies, such as natural convection systems from the Asian Institute of Technology referenced in FAO documents, dried grains like paddy from high initial moisture levels to safe storage conditions in 1-2 days, with similar applications explored in resource-limited settings including Africa.^[42]^[41] These dryers offer specific advantages, including superior protection of the product from environmental contaminants like dust and rain, as well as more uniform drying due to the controlled airflow and temperature distribution. However, they present challenges such as increased design complexity from the need for separate collector and chamber components, along with potentially higher construction and maintenance costs compared to simpler systems.^[40]^[42]

Hybrid Solar Dryers

Hybrid solar dryers integrate solar energy collection with auxiliary energy sources or thermal storage systems to enhance reliability and extend operation beyond daylight hours. These systems typically feature a solar collector, such as flat-plate or evacuated tube absorbers, paired with backup heating mechanisms like electric heaters, biomass burners, or liquefied petroleum gas (LPG) combustors. Alternatively, they incorporate sensible heat storage materials, including rocks, gravel, or bricks, or latent heat storage via phase-change materials (PCMs) like paraffin wax or hydrated salts, which absorb and release thermal energy during phase transitions to maintain drying temperatures. This configuration allows for controlled airflow and heat distribution, often through insulated chambers and ducts, distinguishing hybrids from purely solar-dependent designs.^[43]^[44]^[45] In operation, solar energy serves as the primary heat source during peak sunlight, heating air or the drying chamber directly or indirectly, while auxiliary systems activate automatically during low insolation, nighttime, or adverse weather to sustain temperatures between 40–70°C. Forced convection is commonly employed via photovoltaic-powered fans or blowers, which circulate heated air uniformly across the product, reducing drying times by 30–50% compared to natural convection setups and improving product quality by minimizing exposure to contaminants. Sensors and simple controllers monitor humidity and temperature, triggering backups as needed, enabling continuous 24-hour drying cycles for moisture-sensitive crops.^[46]^[47]^[48] Recent advancements in the 2010s and 2020s have focused on hybrid designs tailored for agricultural applications in regions like India and China, where intermittent sunlight poses challenges. In India, a solar-biogas hybrid dryer developed in 2020 for onion slices utilized biogas as a nighttime backup, achieving uniform drying and nutrient retention superior to open-sun methods. Similarly, a 2021 hybrid active greenhouse solar dryer integrated evacuated tube collectors with electric auxiliaries, drying high-moisture produce like tomatoes continuously while leveraging the greenhouse structure for passive heat gain. Collector-greenhouse hybrid systems in various regions have incorporated PCM storage for extended drying of fruits and herbs. As of 2025, further innovations include AI-enhanced controls for hybrid systems and advanced PCM integrations, reducing drying times by up to 70% and improving nutrient retention. These examples highlight hybrids' role in reducing post-harvest losses in tropical climates.^[49]^[50]^[51]^[52]^[15] Hybrid solar dryers offer weather-independent performance and higher throughput capacities, often processing 50–200 kg of produce per batch, which supports commercial-scale operations and consistent product quality through regulated conditions. However, they incur higher initial costs compared to direct solar dryers, due to auxiliary components and controls, alongside increased energy consumption from backups, which can raise operational expenses in electricity-scarce areas. Despite these drawbacks, their ability to operate reliably enhances economic viability for perishable goods drying, with payback periods of 2–4 years in sunny regions.^[48]^[53]^[54]

Design Considerations

Key Components and Materials

Solar dryers consist of several essential components that facilitate the capture, transfer, and utilization of solar energy for drying processes. The solar collector, also known as the absorber, is a primary component responsible for capturing solar radiation and converting it into heat to warm the air or the product directly. It is typically constructed from materials like black-painted galvanized iron sheets or wood to maximize absorption of sunlight.^[55]^[56] The drying chamber serves as the enclosed space where the product is placed for dehydration, protecting it from direct sunlight, dust, and pests while allowing controlled airflow. This component is often made from plywood coated with reflective materials such as aluminum foil or white emulsion paint to minimize heat loss and prevent moisture absorption.^[56]^[57] Trays or racks within the chamber hold the material to be dried, usually in multiple layers for efficient space utilization; these are commonly fabricated from food-grade stainless steel mesh or wire mesh to ensure hygiene and adequate air circulation.^[56]^[57] Vents and ducts are critical for regulating airflow and moisture removal, with inlet and outlet openings facilitating natural or forced convection. These may include flexible insulated conduits connecting the collector to the chamber or holes positioned at the base and upper sides for buoyancy-driven exhaust.^[55]^[57] Optional components such as fans or chimneys enhance circulation; fans, often axial and solar-powered, provide forced convection in active systems, while chimneys promote natural draft for humid air expulsion.^[55]^[56] Recent designs incorporate phase change materials (PCM), such as paraffin wax, for thermal storage to maintain temperatures during low insolation periods.^[15] Suitable materials for solar dryers emphasize durability, thermal efficiency, and environmental suitability. Absorbers utilize black-painted metal sheets or wood for optimal heat retention.^[55]^[57] Glazings, which cover the collector to trap heat, are typically tempered glass, UV-stabilized polycarbonate, or polyethene sheets to allow solar transmission while resisting weathering.^[55]^[57] Frames are constructed from aluminum or galvanized iron (GI) sheets for structural integrity and resistance to corrosion in humid conditions.^[55]^[56] Insulation materials like foam, polythene, or air layers are incorporated in walls and bases to minimize thermal losses and maintain elevated temperatures within the system.^[55]^[57] Corrosion-resistant options, such as stainless steel plates, are preferred for components exposed to moisture to ensure longevity.^[55] For cost-effective constructions, particularly in rural settings, locally sourced materials like bamboo for frames or wooden elements are utilized to reduce expenses without compromising basic functionality.^[55]^[56] Advancements like nanofluid use in collectors can further boost thermal efficiency.^[58]

Efficiency and Performance Factors

The efficiency of solar dryers is primarily evaluated through key metrics such as drying rate and thermal efficiency, which quantify the system's ability to remove moisture using solar energy. The drying rate, typically expressed in kg of water evaporated per square meter per hour (kg water/m²/h), measures the speed of moisture removal from the product and varies based on design and conditions; representative values range from 0.08 kg water/m²/h in later drying stages to 0.5–0.8 kg water/m²/h under optimal solar insolation for agricultural products.^[59]^[60]^[61] Thermal efficiency (η) is calculated as the ratio of energy used for water evaporation to the total solar energy input, expressed as η = (energy for evaporation / solar input) × 100, with typical values ranging from 10% to 30% for most systems, though optimized designs can reach up to 60%.^[62]^[63]^[64] Several factors significantly influence these performance metrics. Optimal orientation, such as south-facing with a tilt angle approximately equal to the local latitude (±15°), maximizes solar irradiance capture and can enhance overall efficiency by ensuring consistent heating.^[62] Airflow velocity, ideally maintained between 0.5 and 2 m/s, promotes uniform heat distribution and moisture removal, with velocities outside this range potentially reducing drying rates due to insufficient convection or excessive energy loss.^[62] Loading density affects performance by balancing heat exposure; higher densities (e.g., 162 kg per unit) can increase efficiency to around 12.3% by minimizing unused space, but excessive loading may lead to uneven drying.^[62] Losses from leaks or poor insulation further degrade efficiency by allowing heat escape, often reducing thermal performance by 10-20% if not addressed through sealed designs.^[62] Performance testing of solar dryers relies on standardized assessments to ensure reliability. Uniformity of temperature across the drying chamber is evaluated to confirm even heating, typically targeting variations below 5°C to avoid hotspots that could damage products.^[65] Moisture content reduction curves, plotting time against remaining moisture (e.g., from initial 80% to final 10% wet basis), provide insights into drying kinetics and help validate efficiency claims under controlled conditions.^[62] Recent optimizations have improved performance, particularly through the integration of reflectors that boost effective irradiance by 20-50%, as demonstrated in designs achieving up to 49.79% irradiance increases during peak seasons.^[62] These enhancements, combined with refined airflow management, underscore the potential for solar dryers to approach higher efficiency benchmarks in practical applications.^[65]

Applications

Agricultural and Food Processing

Solar dryers play a vital role in agricultural and food processing by enabling the preservation of perishable produce in tropical and subtropical regions, where high humidity and rainfall often lead to spoilage. They are commonly applied to a variety of food types, including fruits such as mangoes and grapes (processed into raisins), vegetables like tomatoes and onions, grains including rice and maize, and proteins such as meats and fish. These applications support smallholder farmers by facilitating on-farm drying, which minimizes dependency on unreliable weather conditions and protects crops from contaminants like dust, insects, and animals.^[66]^[67] In agricultural contexts, solar dryers significantly mitigate post-harvest losses, which can reach 20-50% for fruits, vegetables, and other perishables in tropical areas due to inadequate drying infrastructure. For instance, in India, solar dryers have been widely adopted for copra (dried coconut kernel) production, reducing moisture content from over 50% to safe levels (around 7-8%) while preventing mold growth and quality degradation that affects marketability. Similarly, in Africa, particularly in regions like Uganda and the Democratic Republic of Congo, solar drying of cassava chips has lowered losses by improving drying uniformity and reducing exposure to environmental hazards, thereby enhancing farmer incomes through better-quality flour production. These benefits are especially pronounced for smallholder farms, where direct solar dryers are suited for home-scale operations due to their simplicity and low maintenance.^[43]^[43]^[68] The drying process using solar dryers typically involves batch operations, where produce is loaded into trays for controlled exposure to solar-heated air, achieving safe moisture levels (below 10-15%) faster than traditional methods. For example, fruits like mango slices or grapes can be dried in 2-3 days compared to 5-7 days under open sun, while grains such as rice reach 14% moisture in about 10 hours versus 19 hours in direct sunlight. This accelerated process not only shortens exposure time but also preserves nutritional quality, with solar drying retaining more nutrients than open-air methods, such as up to 90% of β-carotene in certain green leafy vegetables and around 74% of ascorbic acid in fruits like mango, outperforming open-air methods that suffer greater degradation from UV exposure and oxidation.^[67]^[69]^[70] A notable case study from Bangladesh illustrates the community-scale impact on fish processing, where solar tunnel dryers have been implemented to dry small fish species like Bombay duck and silver carp. These systems reduce drying time from 20 hours under open sun to 9 hours per batch, handling 120-150 kg and yielding higher-quality products free from contamination, which increases market value due to improved color, texture, and hygiene. In coastal communities, such dryers address annual fish losses of over 300,000 metric tons by enabling reliable preservation during monsoon seasons, supporting livelihoods for thousands of processors.^[71]^[67]

Industrial and Other Uses

Solar dryers find significant application in industrial sectors for processing materials such as herbs, spices, timber, textiles, and pharmaceuticals, where controlled moisture removal enhances product quality and reduces energy costs. In the spice industry, large-scale tunnel solar dryers are utilized for drying turmeric, offering an efficient alternative to traditional methods by retaining essential compounds like curcumin while minimizing contamination.^[72] Similarly, solar kilns are employed in the timber industry to dry lumber, achieving substantial moisture reduction through low-cost solar heating and ventilation systems that prevent defects like warping.^[73] For textiles, solarized cylindrical can dryers have been integrated into manufacturing processes to dehydrate fabrics, demonstrating reliable performance in industrial-scale operations.^[74] In pharmaceuticals, solar dryers process medicinal herbs by providing gentle, uniform drying conditions that preserve bioactive components for subsequent formulation.^[75] As of 2025, hybrid solar-biomass systems have gained traction for reliable timber and sludge drying, reducing energy use by 50-70% in pilot projects.^[76] Beyond core industrial uses, solar dryers address diverse needs in waste management and construction. They are applied to dry sewage sludge in wastewater treatment facilities, significantly reducing volume and aiding stabilization through direct or indirect solar exposure.^[77] In brick production, solar systems dry clay bricks efficiently, supporting sustainable manufacturing by harnessing ambient solar radiation to accelerate the process without fossil fuels.^[78] Emerging applications in waste management extend to other sludges, where solar drying promotes eco-friendly volume reduction and pathogen control. For off-grid areas, solar-powered laundry dryers enable reliable clothes drying, utilizing passive or active solar mechanisms to operate independently of electrical grids.^[79] Commercial implementations often feature capacities of 100-500 kg, particularly in Southeast Asia for tea processing, where solar dryers support bulk operations in regions with abundant sunlight. Active hybrid solar dryers, combining solar collection with auxiliary heating, are adapted for factory environments to maintain consistent output during variable weather, ensuring uninterrupted industrial drying.^[43]^[53] The underlying drying physics, involving heat transfer and moisture diffusion, enables these systems to adapt to a range of industrial materials.

Advantages and Challenges

Benefits

Solar dryers offer substantial environmental benefits by harnessing renewable solar energy, which results in zero direct emissions and a reduced carbon footprint compared to conventional fuel-based drying systems that contribute to greenhouse gas emissions.^[80]^[81] This technology also helps curb deforestation by minimizing the need for fuelwood in drying processes, preserving forests and biodiversity in regions dependent on wood for agricultural post-harvest activities.^[81] Economically, solar dryers feature low operating costs due to the free availability of solar energy, with payback periods typically ranging from 1 to 2 years depending on design and location.^[81] They enhance product value by extending shelf life and improving marketability, such as increasing the price of dried fruits like bananas by up to three times compared to open-air sun drying.^[81] In terms of quality, solar dryers provide uniform drying conditions that better preserve the color, flavor, and nutritional content of products, including higher retention of vitamins A and C in fruits and vegetables.^[81] Drying rates are generally 30-50% faster than traditional open-sun methods, reducing exposure to contaminants and spoilage risks.^[81] Socially, solar dryers empower rural communities, particularly women in developing regions, by enabling efficient food preservation that supports local livelihoods and reduces post-harvest losses, thereby contributing to food security.^[80]^[81] This aligns with United Nations Sustainable Development Goals related to zero hunger and affordable clean energy.^[80]

Limitations and Solutions

One primary limitation of solar dryers is their dependence on intermittent solar radiation, which restricts operation to daylight hours and can halt drying during cloudy weather or at night, potentially leading to moisture reabsorption and microbial growth in the product.^[19] This intermittency also results in inconsistent drying rates, extending overall processing time compared to continuous mechanical systems.^[18] Additionally, direct solar dryers expose products to ultraviolet radiation and environmental contaminants, causing discoloration, nutrient degradation, and quality loss, while indirect types may suffer from convective heat losses that reduce efficiency.^[19] Scalability poses another challenge, as many passive designs are suited only for small-scale applications, limiting industrial adoption due to insufficient capacity and the need for large surface areas.^[57] To address intermittency, hybrid solar dryers integrate auxiliary energy sources such as biomass burners, electric heaters, or photovoltaic panels, enabling continuous operation and reducing drying time—for instance, forced convection hybrids have shortened processing to as little as 7 hours for certain crops.^[19] Thermal energy storage systems, including sensible heat storage like rock beds or latent heat via phase change materials (PCMs), store excess daytime heat for nighttime use, improving overall efficiency and product uniformity.^[18] For quality preservation, indirect dryer configurations with insulated walls and reflective surfaces minimize direct exposure and heat losses, while advanced airflow designs, such as back-pass solar collectors, enhance heat transfer and prevent contamination.^[57] Economic barriers, including higher upfront costs for hybrids, are mitigated through low-cost materials like recycled components and government incentives, though expertise gaps require targeted training programs for widespread implementation.^[18]

References

[1]
Solar Dryer - Climate Technology Centre and Network (CTCN)
Solar dryers are devices that use solar energy to dry substances, especially food. Traditional methods of food drying is to place the foodstuffs in the sun in ...
[2]
[PDF] Solar Drying: Fundamentals,Applications and Innovations
Drying using solar radiation, i.e. drying under direct sunlight, is one of the oldest techniques used by mankind to preserve agriculture based food and ...
[3]
(PDF) Principles, Classification and Selection of Solar Dryers
Solar radiation in the form of solar thermal energy, is an alternative source of energy for drying especially to dry fruits, vegetables, agricultural grains ...
[4]
4.10. Improved sun-drying and solar drying: basic considerations ...
The principles underlying improved sun-drying and solar drying are explained in simple terms and attention drawn to the basic questions which the decision-maker ...Missing: definition | Show results with:definition
[5]
Solar Drying - energypedia
Jun 14, 2022 · Drying vegetables, fruits and meat with thermal energy enables longer storage times and easier transportation (by decreasing the moisture ...Introduction · Open-Drying · Solar Dryers · Project examples
[6]
Solar drying - an effective means of food preservation - ScienceDirect
Numerous investigations have shown that solar drying can be an effective means of food preservation since the product is completely protected during drying.Missing: definition | Show results with:definition
[7]
history notes--Mesopotamia through Shakespeare - The Food Timeline
"There is evidence that as early as 12,000 B.C., Egyptian tribespeople on the lower Nile dried fish and poultry using the hot desert sun.
[8]
Mesopotamian Creation Myths - The Metropolitan Museum of Art
Apr 1, 2009 · They ate grass for nourishment and drank water from ditches. Later, the gods created sheep and grain and gave them to humankind as sustenance.
[9]
Historical Review and Recent Trends in Solar Drying Systems
Sep 10, 2012 · products have been dried by the sun and wind in the open air for millions of years. Solar drying is one of the potential decentralized thermal ...
[10]
THE ALDEN FRUIT DRYER. — Tuolumne Independent 4 December ...
This, we believe, is the first fruit-dryer erected in any of the mountain counties, and we hope its success will be the means of leading to the ...
[11]
Shall we get to know some facts regarding types of Solar Dryers?
May 4, 2021 · Did you know that Solar drying technology has a history of 70 years? Solar drying technology was invented around 1950's as a solution to the ...
[12]
[PDF] A solar dome dryer for drying of fish - FAO Knowledge Repository
This paper presents the results, analysis, and discussion of four experimental trials conducted in Aden, comparing sun drying and solar dome drying of three ...Missing: history | Show results with:history
[13]
Open Sun and Greenhouse Drying of Agricultural and Food Products
In this paper, a comprehensive review of open sun drying and greenhouse drying of various products are presented.<|control11|><|separator|>
[14]
Recent trends on energy-efficient solar dryers for food and ...
Apr 7, 2024 · This work comprehensively reviews the state-of-the-art research carried out on solar dryers for energy efficiency enhancement using various alternative ...
[15]
Goal 7: Energy - United Nations Sustainable Development
Goal 7 is about ensuring access to clean and affordable energy, which is key to the development of agriculture, business, communications, education, healthcare ...Missing: post- dryers active heat<|control11|><|separator|>
[16]
A Review on Solar Drying Devices: Heat Transfer, Air Movement ...
Direct solar drying systems can be classified into various types, including cabinet-type, tunnel-type and greenhouse-type dryers, based on their specific ...
[17]
Solar dryers: A review of mechanism, methods and critical analysis ...
This review examines the mechanisms and methods applicable to solar drying, including indirect and direct solar drying, hybrid systems combining solar drying ...<|control11|><|separator|>
[18]
Theoretical Design of a Multilayer Based Spectrally Selective Solar ...
The absorption rate of the coating in the solar spectrum can reach above 0.9, but its emissivity in the infrared spectrum is also beyond 0.6. The overall ...
[19]
Mass Transfer - an overview | ScienceDirect Topics
For convenience, drying processes in solids are often divided into two periods – constant rate period and falling rate period. Constant rate drying period ...
[20]
Kinetic Models for Drying Techniques—Food Materials
The process is often divided into two periods of drying which are the constant drying rate period and the falling drying rate period [36] .
[21]
Drying Kinetics of a Single Biomass Particle Using Fick's Second ...
Mar 23, 2023 · The falling rate period was the main drying process, during which internal diffusion dominated the moisture transfer in the material [26]. ...
[22]
Moisture Transfer in Solid Food Materials: A Review of Mechanisms ...
Thus, moisture transfer in the falling rate period of drying can usually be described by a diffusion model based on Fick's second law.<|control11|><|separator|>
[23]
Psychrometry Applications in Grain Drying Operations
Nov 20, 2023 · Psychrometry plays a crucial role in grain drying operations by helping us understand and control the drying environment.
[24]
[PDF] THE PSYCHROMETRIC CHART: Theory and Application
Two and a half cubic meters of lumber is being dried at 60°C dry bulb temperature and 52°C wet bulb temperature. The drying rate of the lumber is 12.5.<|control11|><|separator|>
[25]
[PDF] Characterizing Dryer Performance Using Psychrometric Charts
Electronic psychrometric charts, solid state humidity sensors, and advances in control software are all available for you to do a better job of drying brick.
[26]
Modelling Heat and Mass Transfer Processes during Drying
... of drying processes normally uses the Fick's second law of diffusion and heat conduction models to describe the heat and mass transfer processes, respectively.
[27]
Spray dryer Heat and Mass Balance calculation - PowderProcess.net
Heat and Mass Balance is the tool at the disposal of Engineers to calculate the amount of heat entering the dryer, the amount of heat leaving the dryer, and ...
[28]
Effect of air velocity, temperature, and relative humidity on drying ...
In general, the drying rate and the overall moisture transfer coefficient increased with increasing air velocity, drying temperature, and decreasing RH.
[29]
The Effect of Air Relative Humidity on the Drying Process of Sanitary ...
Oct 30, 2023 · The results indicate that the higher the air's relative humidity, the slower the moisture removal. Herein, aspects of the product quality after ...
[30]
Effects of drying temperature, airflow, and cut segment on drying rate ...
This indicated that the airflow supplied to the drying chamber increased the value of and reduced the activation energy, and hence the specimen dried faster ...
[31]
Mechanistic understanding of case-hardening and texture ...
It has been found that case-hardening is particularly enhanced with higher drying temperatures, higher air velocities in the drying chamber and lower relative ...
[32]
Introduction to Food Dehydration | MU Extension
Jan 20, 2021 · This condition is known as case hardening and leads to a moldy food product. Temperatures that are too high at the end of the drying period may ...
[33]
[PDF] INFLUENCE OF PROCESS CONDITIONS AND DRYING PRE-TREAT
However, drying can cause some undesirable changes in fruits including browning and oxidation reactions, case hardening and degradation of nutritional compounds ...
[34]
Impact of Three Different Dehydration Methods on Nutritional Values ...
Oct 14, 2020 · The REV-dried samples had much higher retention of the nutritional values (vitamin C, β-carotene) compared to the conventional air-drying process.
[35]
[PDF] Solar-Energy Drying Systems - IntechOpen
May 11, 2012 · A direct solar dryer is one in which the material is directly exposed to the sun's rays. This dryer comprises of a drying chamber that is ...<|control11|><|separator|>
[36]
[PDF] Direct Solar Dryer - Lund University Publications
The study presents the design, construction, and performance evaluation of a direct solar dryer with a focus on enhancing the post-harvest preservation of ...Missing: configuration | Show results with:configuration
[37]
[PDF] Barriers and Drivers for Wider Adoption of Solar Dryers in Rural ...
A direct solar dryer of the greenhouse type in Jakar, Bumthang. ... Greenhouse dryers (also called tent dryers) are an example of commonly used direct solar ...
[38]
Solar Dryer - an overview | ScienceDirect Topics
Indirect solar dryers are categorised into two systems, namely, natural and forced-convection systems, depending on the approach applied to pass the air over ...Missing: configuration | Show results with:configuration
[39]
Drying methods - Natural and solar drying
An improved technology in utilizing solar energy for drying grain is the use of solar dryers where the air is heated in a solar collector and then passed ...Missing: indirect tunnel
[40]
(PDF) A review on indirect solar dryers - ResearchGate
However, drying these products directly under the open sun has many drawbacks such as debris, rain, blowing wind, insect infestation, human and animal ...
[41]
Review of solar dryers for agricultural products in Asia and Africa
Aug 15, 2020 · In a hybrid dryer, agricultural materials are dried under direct solar radiation and/or back-up energy or stored heat in the absence of sunlight ...
[42]
Thermal Evaluation of a Hybrid Dryer with Solar and Geothermal ...
Sep 30, 2019 · The design and construction of a hybrid dryer that worked with 80% of thermal solar and low enthalpy aerogeothermal energy and 20% with electric power was ...
[43]
A comprehensive review of solar dryers incorporated with phase ...
Nov 20, 2023 · Solar dryers incorporated with phase change materials (PCMs) are gaining importance as they are characterized by higher efficiencies and shorter ...
[44]
Assessment of a LPG hybrid solar dryer assisted with smart air ... - NIH
Oct 13, 2024 · This hybrid solar dryer (HSD) uses indirect forced convection and a controlled auxiliary heating system (LPG) to regulate both temperature and ...
[45]
Experimental and exergy evaluation of a PCM integrated active ...
Jul 1, 2025 · The system incorporates divergent ducts operated by photovoltaic power to enhance drying uniformity as well as thermal performance.
[46]
A Comprehensive Review of the Hybrid Solar Dryers - ResearchGate
Aug 6, 2025 · In this assessment paper, we reviewed unique forms of hybrid oriented solar dryers with respect to unique layout adjustments to be able to increase their ...
[47]
Design and Development of Solar-biogas Hybrid Dryer for Onion ...
Jun 5, 2020 · A solar-biogas hybrid dryer was designed, developed and tested for onion drying with a capacity of 8 kg/batch. Solar energy was utilized as ...
[48]
Sustainability assessment of hybrid active greenhouse solar dryer ...
Oct 2, 2021 · The hybrid active greenhouse dryer is developed especially for drying high moisture agro and non-agro-based produce. Evacuated tube solar ...Missing: examples 2010s 2020s
[49]
Study of the Solar Energy Drying Device and Its Application in ...
The collector-greenhouse type hybrid solar dryer is suitable for dry the materials of which have higher water content and need higher temperature to be dried, ...Missing: 2010s 2020s
[50]
A comprehensive review of hybrid solar dryers integrated with ...
Apr 15, 2024 · Although the drying method is simple to operate and low-cost, it still has many limitations, including low drying efficiency and degraded ...
[51]
A Review on Hybrid Solar-Biomass Dryer for food processing ...
Dec 7, 2022 · Hybrid dryers like solar- biomass gives more advantages to farmers. They are independent of climate changes, cost effective and more efficient.
[52]
Solar Drying for Domestic and Industrial Applications: A ...
Jan 12, 2025 · This article reviews the classification of solar dryers, including direct (DSD), indirect (ISD), and hybrid (HSD) systems, examining key components like solar ...
[53]
Design, fabrication and performance evaluation of solar dryer for ...
Jul 28, 2015 · The dryer consists of solar flat plate air heater with three layers of insulation, drying chamber and a fan with a regulator to induce required ...<|control11|><|separator|>
[54]
(PDF) Recent advances in solar drying system: A Review
Aug 9, 2025 · Active solar dryers are ideal for drying crops with high moisture content like papaya, kiwi fruits,. brinjal, cabbage and cauliﬂower slices [29] ...
[55]
Drying Performance of a Combined Solar Greenhouse Dryer ... - MDPI
... drying rate curve (Figure 8). The average daily drying rate is 0.24 kg water/m2 h for the second drying day, and it is 0.08 kg water/m2 h for the last drying ...
[56]
Semicontinuous solar drying of sludge from a waste water treatment ...
Aug 7, 2025 · The achieved drying rates were 2.02kg water/m2 h and 0.49kg water/m2 h in September and February, respectively. Under a given insolation rate ...
[57]
Review of solar crop drying efficiency and its interconnection with ...
Aug 20, 2025 · Forced convection was observed to enhance airflow and efficiency (up to 70.2%) when supported by proportional thermal input, whereas natural ...
[58]
[PDF] Thermal performance evaluation of an indirect solar dryer - IIETA
In terms of drying, the thermal efficiency is defined as the ratio between the latent heat of vaporization of the water of the product and the quantity of ...<|control11|><|separator|>
[59]
Solar drying technologies: A review of design, efficiency, and environmental impacts
### Summary of Solar Dryer Efficiency and Performance Factors
[60]
Solar dryers for food applications: Concepts, designs, and recent advances
Summary of each segment:
[61]
(PDF) Review on solar dryers for drying fish, fruits, and vegetables
Jun 3, 2022 · This paper mainly focused on the conventional methods to advanced solar drying technologies for perseverating fish, fruits, and vegetables<|control11|><|separator|>
[62]
Performance evaluation of cassava drying technologies: a case ...
Aug 9, 2025 · The objective of this study was to assess the performance of cassava sun drying on a raised platform and drying in a direct passive solar dryer.
[63]
Recent Progress and Performance of Solar Dryers used for on-Farm ...
Nov 20, 2021 · Bolaji [27] designed an indirect solar dryer with a box type absorber collector. The dryer was made up of a collector with glass cover and black ...
[64]
Effect of Traditional open sun-drying and solar cabinet drying on ...
Jun 25, 2025 · *β-Carotene retention was maximum when dried in solar drier with forced convention (80 %) and least when dried in shade (50 %). The interaction ...
[65]
Solar tunnel fish dryer for seasonal application in the perspective of ...
The solar tunnel dryer significantly reduces fish drying time compared to traditional sun drying methods. Bangladesh loses approximately 307,500 MT of fish ...
[66]
Solar tunnel drying of turmeric (Curcuma Longa Linn. Syn. C ...
Aug 10, 2025 · Solar tunnel drying method is an effective alternative to traditional open sun drying, where retention of curcumin, volatile oil and oleoresin ...
[67]
Basics of Solar Lumber Drying | Oklahoma State University
Solar drying of lumber is one of the more efficient and the least expensive methods to reduce moisture content of wood.Solar Drying · Greenhouse Kilns
[68]
Textile drying using solarized cylindrical can dryers to demonstrate ...
Textile drying using solarized cylindrical can dryers to demonstrate the application of solar energy to industrial drying or dehydration processes: Phase I.
[69]
Solar drying of medicinal herbs: A review - ScienceDirect.com
Jul 15, 2021 · Moisture from food can be removed to a required level using a solar dryer to reduce the destructive effect. The percentage of moisture content ...
[70]
Solar drying of wastewater sludge: A review - ScienceDirect.com
Drying constitutes an important process for wastewater sludge management, as it can reduce the mass and the volume of the product and consequently the cost of ...
[71]
Cleaner production of geopolymer bricks using Solar-LPG hybrid dryer
Feb 25, 2024 · This research aims to develop and build a hybrid solar-LPG dryer system for drying geopolymer bricks, a cutting-edge technology for making bricks.
[72]
"Solar Powered Electric Clothes Dryer" by Roger Mai and Alan Swe
This project aims to provide an alternate solution to the wall-powered electric clothes dryer by designing one which operates entirely off the grid. URL: https ...
[73]
[PDF] TECHNOLOGY FACTSHEET SOLAR DRYERS
Benefits to economic/social and environmental development. Solar drying is not simply a method for substituting fossil fuels by solar energy, but it is a.
[74]
Review of solar dryers for agricultural products in Asia and Africa
Aug 15, 2020 · Solar drying has positive impacts on quality as well as economic, environmental, and social aspects. •. Financial and technical efforts are ...