A Review on Solar Drying Devices: Heat Transfer, Air Movement and Type of Chambers

: Food waste is one of the biggest challenges we are facing nowadays. According to the Food and Agriculture Organization (FAO) of the United Nations, approximately one-third of all food produced in the world is lost at some stage between production and consumption, totaling 930 million tons of food per year. Meanwhile, 10.5% of humanity suffers from malnutrition, 26% are overweight and greenhouse gases derived from the food industry account for between 25 and 30% of total emissions (8 to 10% referring to food waste), exacerbating the current climate crisis. To address these concerns, there has been a growing inclination to seek alternatives to fossil fuels, including the adoption of solar energy across diverse sectors, including the food industry. Actions are needed in order to change these patterns. This review article aims to provide an overview of recent developments in the field of solar food dehydration and the types of dehydrators that have emerged. Extensive research and bibliographic analysis, including other review articles, have revealed a growing focus on investment in this area to develop solar dehydrators that are increasingly effective but as sustainable as possible.


Introduction
As is generally known, fossil fuels take millions of years to form.The world's heavy reliance on non-renewable energy sources in its energy matrix leads to a depletion of reserves as consumption surpasses production [1].The most alarming aspect of this reality is the environmental impact it carries, manifesting in numerous problems such as global warming (caused by excessive CO 2 emissions), acid rain (resulting from pollutants reacting with water vapor), air pollution and water contamination [2].Consequently, the availability of fossil fuels is under threat, putting global energy production at risk [3].Various agreements have already been established, such as the Kyoto Protocol and the Paris Agreement.The quest for alternative energy sources to replace fossil fuels is essential for environmental preservation and combating climate change [4].
The energy transition cannot be solved with the simple and sudden abandonment of fossil fuel sources.This process should provide for a gradual elimination to ensure stability, resilience and efficiency.The targets are well-defined: by 2030, global emissions related to the energy sector must reduce by 30% below 2019 levels and by 75% by 2040 to achieve the goal of zero net emissions by 2050 (United Nations Sustainable Development Goals).Although renewable energies are presented as alternatives, they currently do not produce enough energy to fully replace traditional sources.
Among renewable energy options, solar energy stands out as the most abundant.The sunlight that reaches Earth every day dwarfs all other energy sources on the planet, with a rate approximately 10,000 times greater than humanity's current energy consumption [5].This vast potential of solar energy could theoretically meet all of mankind's energy needs if it can be harnessed and stored in a cost-effective manner.
When solar radiation passes through the atmosphere, some of it is absorbed or scattered due to clouds, air molecules, aerosols and water vapor.As a result, the direct normal irradiance (DNI) represents the solar radiation that directly reaches the Earth's surface (Figure 1).olar 2024, 4, FOR PEER REVIEW 2 a rate approximately 10,000 times greater than humanity's current energy consumption [5].This vast potential of solar energy could theoretically meet all of mankind's energy needs if it can be harnessed and stored in a cost-effective manner.When solar radiation passes through the atmosphere, some of it is absorbed or scattered due to clouds, air molecules, aerosols and water vapor.As a result, the direc normal irradiance (DNI) represents the solar radiation that directly reaches the Earth's surface (Figure 1).The solar constant, approximately 1367 W/m 2 at the mean Earth-Sun distance at the top of the atmosphere, represents the value of solar radiation [6].Around 165 petawatts (PW) of solar energy are received on the Earth's surface.Out of this, about 30% is reflected back into space, while 47% is converted into low-temperature heat through various processes such as water evaporation (23%), wind (23%) and kinetic energy in waves (0.5% [7].On a clear day, at noon, the direct beam radiation on the Earth's surface can reach approximately 1000 W/m 2 .The harvesting of solar energy is influenced by factors such as location, season, time of day and weather conditions [8].Solar technologies offer a versatile range of applications, delivering heat, cooling, natural lighting, electricity and fuels, making them a vibrant research topic that attracts scientists to explore diverse approaches [9,10].Solar energy is becoming increasingly popular due to its abundance availability, cost-effectiveness and environmentally friendly nature.It is essentially free o charge, harnessing the sun's energy as a renewable and sustainable resource. Regardless, the utilization of solar energy to dry fresh food products is one of the oldest preservation techniques used by humans.The earliest recorded instance of drying is for vegetables, dating back to the 18th century, by Van Arsdel and Copley (1963).Drying involves two fundamental and simultaneous processes: the transfer of heat to evaporate the liquid and the transfer of mass as a liquid or vapor within the solid and as a vapor from the surface.During the drying process, moisture transfer occurs in two main stages external mass transfer, which involves the evaporation of moisture from the product's surface into the surrounding air and internal mass transfer, which refers to the movemen of moisture from inside the product towards its surface [11].
The conventional drying system, known as open sun drying, involves directly exposing food to the wind and sun, spreading it in a thin layer over the ground or using trays.This method is commonly used for agricultural goods and other products, serving The solar constant, approximately 1367 W/m 2 at the mean Earth-Sun distance at the top of the atmosphere, represents the value of solar radiation [6].Around 165 petawatts (PW) of solar energy are received on the Earth's surface.Out of this, about 30% is reflected back into space, while 47% is converted into low-temperature heat through various processes such as water evaporation (23%), wind (23%) and kinetic energy in waves (0.5%) [7].On a clear day, at noon, the direct beam radiation on the Earth's surface can reach approximately 1000 W/m 2 .The harvesting of solar energy is influenced by factors such as location, season, time of day and weather conditions [8].Solar technologies offer a versatile range of applications, delivering heat, cooling, natural lighting, electricity and fuels, making them a vibrant research topic that attracts scientists to explore diverse approaches [9,10].Solar energy is becoming increasingly popular due to its abundance, availability, cost-effectiveness and environmentally friendly nature.It is essentially free of charge, harnessing the sun's energy as a renewable and sustainable resource.
Regardless, the utilization of solar energy to dry fresh food products is one of the oldest preservation techniques used by humans.The earliest recorded instance of drying is for vegetables, dating back to the 18th century, by Van Arsdel and Copley (1963).Drying involves two fundamental and simultaneous processes: the transfer of heat to evaporate the liquid and the transfer of mass as a liquid or vapor within the solid and as a vapor from the surface.During the drying process, moisture transfer occurs in two main stages: external mass transfer, which involves the evaporation of moisture from the product's surface into the surrounding air and internal mass transfer, which refers to the movement of moisture from inside the product towards its surface [11].The conventional drying system, known as open sun drying, involves directly exposing food to the wind and sun, spreading it in a thin layer over the ground or using trays.This method is commonly used for agricultural goods and other products, serving the purpose of preserving them for later use, especially in the case of food, or as an integral part of the production process, as seen in wood and tobacco drying.However, it comes with several limitations and challenges.One of the major drawbacks of open sun drying is the susceptibility of the crops to various external factors.This includes damage caused by birds, rodents, dust, rain, direct exposure to radiation, insect infestations and microorganisms [12][13][14].Such issues can lead to significant post-harvest losses and negatively impact the overall quality of the dried products.Moreover, open sun drying requires a large area for the process to be efficient and it lacks the ability to control external drying parameters such as moisture content and temperature [15].This lack of control can further contribute to inconsistent drying results and may not be suitable for certain products that require specific drying conditions.Given these disadvantages, there is a need for more advanced and controlled drying methods to minimize post-harvest losses and ensure better preservation and quality of dried food and other products.
The advancement in sun drying techniques has led to the development of solar drying systems composed of closed devices that trap and utilize the sun's radiation to increase the internal temperature [16].The key difference between solar and solar drying lies in the utilization of equipment to collect the sun's radiation and trap it.Solar drying has found widespread application not only in agriculture but also in various industrial sectors, including the seafood, pharmaceutical, paper, ceramic and biomass processing industries [11].Numerous studies have been conducted over time to investigate the dehydration of different types of crops using both open sun drying and solar drying devices, either as additional means or for comparison purposes [17,18].
The primary advantages of solar dryers over traditional sun drying are focused on drying times, higher efficiency, improved hygiene, healthier end products and costeffectiveness [12].By utilizing solar energy, these systems offer a more controlled environment for the drying process, leading to better quality and reduced post-harvest losses.

Technology of the Dryer
Solar dryers work based on the principle of transmitting heat from a source to the product being dried and facilitating the transfer of moisture from the product's surface to the surrounding atmosphere [19].Successful food drying requires the removal of moisture from the product, with dry air absorbing it and air movement helping to carry it away [7].Due to the aim of utilizing free and renewable solar energy, various types of solar dryers have been presented in the literature [20][21][22][23][24][25][26][27].
Researchers have explored different approaches to enhance the efficiency of these devices, such as improving insulation, heat recovery, recirculation and optimizing operating systems.Furthermore, achieving similar results can be possible by substituting the system's energy supply with combined heat and power methods [28].Conventionally, solar dryers are classified in different ways according to heat transfer, air movement and type of chamber (Figure 2).

Mode of Heat Transfer
According to the incidence of the solar radiance as a working principle, the solar dryers can be classified into open sun, direct (with cabinet), indirect or hybrid [29]: 3.1.1.Open Sun Drying (OSD) Also called natural drying, in this method, solar radiation directly impacts the surface of the crop, which is generally spread on the ground (Figure 3).Short-wavelength solar energy was received for the majority of the day, along with natural air circulation facilitated by the wind.The absorbed radiation converted into thermal energy increases crop temperature, which is improved by the color of the product, facilitating dehydration.There are some energy losses from reflective, evaporative and convective modes, which decrease the efficiency of the process (Table 1).

Mode of Heat Transfer
According to the incidence of the solar radiance as a working principle, the solar dryers can be classified into open sun, direct (with cabinet), indirect or hybrid [29]: 3.1.1.Open Sun Drying (OSD) Also called natural drying, in this method, solar radiation directly impacts the surface of the crop, which is generally spread on the ground (Figure 3).Short-wavelength solar energy was received for the majority of the day, along with natural air circulation facilitated by the wind.The absorbed radiation converted into thermal energy increases crop temperature, which is improved by the color of the product, facilitating dehydration.There are some energy losses from reflective, evaporative and convective modes, which decrease the efficiency of the process (Table 1).

Lab model vacuum-assisted solar dryer
The final moisture content of 11.5 ± 0.5% was 360, 480 and 600 min in a vacuum-assisted solar dryer and 450, 600 and 750 min in an OSD.The temperature inside the vacuum chamber was 48 In direct solar drying, the sun is the only source of energy in all processes.The product can be exposed or protected, and solar radiation is incident on a transparent cover, typically made of plastic or glass (Figure 4).

Comparison between Solar
Tunnel, Solar-Cum Gas Dryer and OSD The model was the best drying model with the highest correlation coefficient.Figs Turkey 2018 [39] Strengths: independent of any source of energy; cheapest method; environmentally friend; Weaknesses: crops exposed to animals and weather changes; microorganism's contamination; discoloration by UV radiation; non-controlled drying.

Direct Solar Drying (DSD)
In direct solar drying, the sun is the only source of energy in all processes.The product can be exposed or protected, and solar radiation is incident on a transparent cover, typically made of plastic or glass (Figure 4).The glass reflects a portion of the solar radiation back into the atmosphere, while the remaining part goes into the drying chamber.Inside the chamber, some of the transmitted radiance is reflected back from the surface of the crop, while the rest is absorbed.This absorption leads to an increase in temperature inside the chamber and above the crop.The use of glass reduces convective losses to the environment.However, convective and evaporative losses still occur inside the chamber from the heated crop.The air entering the chamber through air holes and escaping through an aperture at the top of the cabinet takes the moisture away from the crop.Direct solar drying systems can be classified into various types, including cabinet-type, tunnel-type and greenhouse-type dryers, based on their specific configurations and designs.Each type offers distinct advantages and is suitable for different applications depending on factors such as the type of product being dried, local climate conditions and required drying efficiency (Table 2).The glass reflects a portion of the solar radiation back into the atmosphere, while the remaining part goes into the drying chamber.Inside the chamber, some of the transmitted radiance is reflected back from the surface of the crop, while the rest is absorbed.This absorption leads to an increase in temperature inside the chamber and above the crop.The use of glass reduces convective losses to the environment.However, convective and evaporative losses still occur inside the chamber from the heated crop.The air entering the chamber through air holes and escaping through an aperture at the top of the cabinet takes the moisture away from the crop.Direct solar drying systems can be classified into various types, including cabinet-type, tunnel-type and greenhouse-type dryers, based on their specific configurations and designs.Each type offers distinct advantages and is suitable for different applications depending on factors such as the type of product being dried, local climate conditions and required drying efficiency (Table 2).Strengths: protected crops; independent of any source of energy; cheap; environmentally friend; Weaknesses: limited to small scales; discoloration by UV radiation; moisture condensation inside transparent cover reduces transmittivity.

Indirect Solar Drying (ISD)
The principal differences between ISD and DSD are in their heat transfer and vapor removal methods.In indirect solar dryers, the crops are placed in trays or shelves inside an opaque drying cabinet in an independent unit from the solar collector (Figure 5).The solar collector is responsible for heating the atmospheric air, which is then conducted to the drying chamber.The air can be heated actively using a fan or passively through natural convection.The heated air is then transferred to the wet crop, where it evaporates moisture.This occurs because of the difference in moisture concentration between the drying air and the surface of the material.The drying process in ISD happens as water is exchanged between the product and the flowing hot air (Table 3).The solar collector is responsible for heating the atmospheric air, which is then conducted to the drying chamber.The air can be heated actively using a fan or passively through natural convection.The heated air is then transferred to the wet crop, where it evaporates moisture.This occurs because of the difference in moisture concentration between the drying air and the surface of the material.The drying process in ISD happens as water is exchanged between the product and the flowing hot air (Table 3).Strengths: better control over drying process; avoids direct exposition to sun, preserving quality; allows a lot of designs depending on the goal; Weaknesses: more expensive; requires higher temperatures; low drying rate, especially passive mode.

Hybrid Solar Drying (HSD)
Hybrid dryers are devices that combine two or more drying techniques, utilizing both direct solar radiation and electrical energy or stored heat, along with ventilators to ensure proper air circulation.These dryers can operate in forced convection or passive modes, depending on the design and application.The primary purpose of developing hybrid dryers is to overcome the limitations of other types of solar dryers and improve overall drying efficiency (Figure 6).This kind of dryer can use various heating processes, such as fossil fuel, gas, biomass, or electric heating, in conjunction with solar heating.They often incorporate photovoltaic (PV) panels to generate electricity, which can be integrated into the drying system.For example, PV modules can capture solar radiation and convert it into electricity, which can power fans for forced air circulation integrated with greenhouse dryers.It is suitable for single and combined techniques, as well as direct and indirect types.
both direct solar radiation and electrical energy or stored heat, along with venti ensure proper air circulation.These dryers can operate in forced convection or modes, depending on the design and application.The primary purpose of dev hybrid dryers is to overcome the limitations of other types of solar dryers and i overall drying efficiency (Figure 6).This kind of dryer can use various heating processes, such as fossil fuel, gas, b or electric heating, in conjunction with solar heating.They often incorporate phot (PV) panels to generate electricity, which can be integrated into the drying syst example, PV modules can capture solar radiation and convert it into electricity, w power fans for forced air circulation integrated with greenhouse dryers.It is suit single and combined techniques, as well as direct and indirect types.
The principal components of hybrid dryers include a drying chamber m materials such as aluminum or wood, a solar collector (e.g., a flat plate or other co to capture and convert solar radiation into thermal energy and an additional generator or accumulator for heat exchange.In cases where solar energy is utili PV module captures solar radiation and converts it into electricity, while the co absorb solar energy to increase the air temperature.The heated air is then direc the drying chamber to reduce the moisture content of the crop.Fans ensure fo circulation and are powered by the electricity generated from the PV module.The principal components of hybrid dryers include a drying chamber made of materials such as aluminum or wood, a solar collector (e.g., a flat plate or other collectors) to capture and convert solar radiation into thermal energy and an additional energy generator or accumulator for heat exchange.In cases where solar energy is utilized, the PV module captures solar radiation and converts it into electricity, while the collectors absorb solar energy to increase the air temperature.The heated air is then directed into the drying chamber to reduce the moisture content of the crop.Fans ensure forced air circulation and are powered by the electricity generated from the PV module.
By combining different energy sources and techniques, hybrid dryers offer more control over the drying process, allowing for better optimization of drying conditions and improved product quality (Table 4).

HSD
The total energy required is 89.9 kWh and the solar energy contribution is 66%.
Salted silver jewfish Malaysia 2016 [76] Solar-biomass HD Pretreatments like microwave blanching followed by brine solution dipping of carrots prior to drying affect the quality of dried carrots positively.
Carrot slices Bhopal (India) 2018 [77] Indirect HSD The indirect solar dryer performance was investigated with and without PCM, during the day and at night.n/a Tunisia 2017 [78] Strengths: continuous drying even without sun and during the night; reduces drying time because it does not depend on the weather; Weaknesses: bigger environmental footprint; running costs.

Mode of Air Movement
Another way to classify the types of solar dryers is by taking into account the air movement.They are classified into passive, when they use natural convection; active, where the utilization of an electric fan creates airflow [79]; and mixed-mode, when both types are used.

Passive Solar Dryers Systems
Often referred to as natural ventilation or convection solar dryers, they depend on the normal movement of the air that is heated by solar energy and spreads on the crop's surface.In passive-mode dryers, there are several types available.

Direct Passive Solar Dryers
In a direct passive solar dryer, the crop is protected with a transparent cover, allowing solar radiation to pass through.It is then converted into thermal energy within the drying chamber through the greenhouse effect.The primary objective of this type of solar dryer is to reduce the moisture content of the products and this is achieved through the process of evaporation by diffusion [80,81].

Cabinet Passive Solar Dryers
The passive solar cabinet dryers [82] are generally inexpensive and straightforward to construct.They consist of a small box, most of the time made of wood, painted black to better absorb the solar radiation transmitted through a plastic or glass cover.Normally, the products to dry are placed in aluminum or plastic trays with wire mesh or perforated at the bottom.The trays are spaced apart to ensure adequate airflow through the products.The base of the cabinet is designed with holes to allow ambient air to enter, pass through the product placed on the wire mesh trays and then escape through holes at the top of the cabinet, carrying away moisture vapors [83].
Compared to open sun drying and direct passive solar dryers, this type of dryer has demonstrated better efficiency and improved product quality.The enclosed cabinet design and controlled airflow help create a more favorable drying environment, minimizing the negative effects of external factors such as dust, insects and weather changes.The blackpainted surface of the cabinet facilitates better solar energy absorption, promoting more efficient drying.Some authors classify cabinet solar dryers into normal and reverse absorber types [84].The normal absorber represents the basic structure of a common passive solar cabinet dryer.The biggest disadvantages are the discoloration of the crop and the convective heat loss.In the reverse absorber cabinet, a reflector is placed under the drying chamber.The transparent cover is tilted at a 45 • angle to maximize solar radiation exposure.The absorber plate captures the solar radiation and directs it to the reflector, which then redirects the solar heat to the drying chamber.As a result, the hot air enters the cabinet and circulates through the crop, effectively removing its moisture content with the heated air.The hot air becomes humid due to moisture evaporation from the crop and is eventually released through a vent or exit hole in the cabinet.

Greenhouse Passive Dryers
The utilization of a greenhouse dryer is a way to optimize direct solar drying.It is based on a structure with extensive glazing walls and roofs (glass, fiber-reinforced polymers and polyethylene film) and is divided into dome types, which are better for maximum utilization of global solar radiation and roof types, which are better for suitable mixing of air inside.They can also be called tent dryers; they are designed with vents of appropriate size and position to have controlled air flow.The air flow into the dryer can be controlled by rolling or unrolling the cladding at the bottom edge of the front side.The drying chamber is heated by the incident solar radiation and the heated air becomes less dense than the ambient air, which leads to the dehydration of products.The crop is laid on the floor above plastic sheets or in trays.The remotion of moisture occurs by natural convection [22,27,31,[85][86][87].

Indirect Passive Solar Dryers
This is an indirect solar dryer that operates on the principle of natural convection.In the drying chamber, the crop is dried with the help of hot air provided by the solar air heater, and it passes out via an overhead vent.Crops are spread on trays without overlapping inside the drying chamber.The air flow rate is very low, as is the heat transfer [52,56,88-90].

Mixed-Mode Passive Solar Dryers
These dryers have the advantage of using both direct and indirect airflows.The passive mode of airflow depends on weather conditions combined with the greenhouse effect.Solar energy is captured directly in the drying chamber and indirectly through the solar air heater or collector.Such dryers can be used for a variety of crops that are suitable for low-temperature thermal drying (Figure 7) [21,[91][92][93][94][95].

Active Solar Dryers Systems
In the sequence of open sun drying and passive solar dryers, the active o to be built.They work on the principle of forced convection to transfer heat, u ventilation.They can sometimes incorporate external heaters to preheat in These dryers are suitable for crops with high water content and do not requir drying temperatures.There is a large variety of designs that can be divide indirect and mixed-mode dryers [100,101].

Direct Active Solar Dryers
The structure is almost the same as passive.The introduction of fans creates a forced draft in the dryer.They can be cabinet or greenhouse-type.

Indirect Active Solar Dryers
These kinds of devices have a separate collector and drying chamber.separate air heating unit, higher temperatures can easily be obtained with a co flow rate.Products dried in this dryer are found to have good nutrient qualit Studies show that the influence of drying air temperature on the variation versus drying time in the food process is more significant compared to the air-drying velocity [65,102,103].

Mixed-Mode Active Solar Dryers
Mixed-mode active solar dryers have almost the same design as passive the incorporation of fans or blowers.Both the solar collector and drying cham solar radiation, which makes the mixed-mode dryer more efficient for the dry due to its higher thermal rate (Figure 8; Table 5) [95,104].

Active Solar Dryers Systems
In the sequence of open sun drying and passive solar dryers, the active ones started to be built.They work on the principle of forced convection to transfer heat, using fans or ventilation.They can sometimes incorporate external heaters to preheat incoming air.These dryers are suitable for crops with high water content and do not require very high drying temperatures.There is a large variety of designs that can be divided in direct, indirect and mixed-mode dryers [100,101].

Direct Active Solar Dryers
The structure is almost the same as passive.The introduction of fans or blowers creates a forced draft in the dryer.They can be cabinet or greenhouse-type.

Indirect Active Solar Dryers
These kinds of devices have a separate collector and drying chamber.Due to the separate air heating unit, higher temperatures can easily be obtained with a controlled air flow rate.Products dried in this dryer are found to have good nutrient quality and color.Studies show that the influence of drying air temperature on the variation of moisture versus drying time in the food process is more significant compared to the influence of air-drying velocity [65,102,103].

Mixed-Mode Active Solar Dryers
Mixed-mode active solar dryers have almost the same design as passive ones, with the incorporation of fans or blowers.Both the solar collector and drying chamber receive solar radiation, which makes the mixed-mode dryer more efficient for the drying process due to its higher thermal rate (Figure 8     The more efficient method was conventional solar drying along with air recycling with a higher drying rate.
Pistachio nuts Iran 2020 [110] ISD-OSD Changes in weather during the day affect the water activity of dried products.

Type of chamber
Another way to divide the classification of solar dryers is based on the type of chamber, which can be a greenhouse or cabinet, as previously described.There are a variety of ways to dispose of the products to dry.They can be placed on trays or racks inside, or the chamber can be similar to a conventional oven, with various configurations.In the case of mixed-mode dryers, the chamber integrates both direct and indirect heating methods effectively.For hybrid dryers, the chamber is designed to accommodate both solar heating components and the additional energy source.The choice of chamber type is crucial to optimizing all the processes and achieving good drying results.

Hybrid Solar Dryers
This kind of dryer is, by definition, designed and constructed using direct solar energy and a heat exchanger.The products are dried under direct solar radiation and/or backup energy or stored heat when sunlight is not available.These types of dryers are used in single and mixed modes of drying.Several studies have been developed to test different techniques to improve solar dryers, considering the possible use of thermal storage materials, the deep bed drying method, improved solar collector designs and energy hybridization.They can be divided several ways, depending on their construction [107,122,123].

With Thermal Energy Storage (TES)
Due to the limitation of solar dryers operating only during sunlight hours, thermal storage emerges as a great solution.It allows the stored heat to be used during the night, ensuring continuous drying and preventing rehydration of the products.During offsunshine hours, microbial activity may lead to the growth of microorganisms and the extended drying periods can degrade the quality of agricultural products, resulting in poor product quality and spoilage [66,[124][125][126][127][128][129][130][131][132].
The integration of a TES unit is needed, and numerous studies have tested it.Some authors divided it into: Sensible heat storage (SHS): materials are heated to store excess solar energy, depending on their specific heat capacity, mass and temperature.The best properties of these materials are density, thermal conductivity and stability.For example, materials such as brick, aluminum, gravel, river rocks, concrete, granite and limestone can be used.The rock bed is the most common material for sensible storage used in solar dryer systems [133,134]; Latent heat storage (LHS): in this kind of material, solar energy is stored during the phase change process.The phase change materials (PCM) can be organic (such as paraffin, like wax n-alkanes and methyl groups) or non-paraffin types (like fatty acids, glycols, alcohols and esters), inorganic (salt hydrates and metallic) or eutectic composition [128,135]; Thermo-chemical energy storage (TCES): it is based on the principle that all chemical reactions either absorb or release heat.This process stores energy by using high-energy chemical processes.In this case, the heat stored depends on the amount of storage material, the endothermic heat of the reaction and the extent of conversion [136][137][138].

With an Auxiliary Unit
The dryer can operate on solar energy, but for additional heating, auxiliary units are used.The most common are fuelled with fossil fuel or biomass to reach and maintain the required temperature.Despite their effectiveness, their availability is limited, and they are associated with environmental pollution issues.Amer and Gottschalk [139] used electric resistances as auxiliary units in fresh chamomile drying; Matouk et al. [140] used them for onion slices; and Hossain et al. [141] used them for tomato slices [142].Ferreira et al. [143] applied 20 incandescent lamps, 100 W each, for drying banana slices.Suherman et al. [144] used SUS (stainless steel) plates as heat collectors for solar radiation and an LPG (liquefied petroleum gas) burner in seaweed drying.Many studies have been conducted on this type of dryer in various contexts [145].

With Photovoltaic (PV)
Solar dryers with PV assistance are probably the most widely used.Thermal energy can be obtained from solar radiation by using solar collectors and it is converted via PV panels into direct current electricity [146].These kinds of systems have a huge variety of possible configurations and can range from the simplest forms, such as powering fans to provide air circulation, to making a significant contribution to the decarbonization of electricity production.The PV-ventilated system is very common in greenhouse dryers [147][148][149][150][151][152].The integrated arrangement for applying thermal energy as well as electrical energy with a PV module is referred to as a hybrid PV/T system [18,73,[153][154][155][156][157][158][159][160][161].The integration of PV panels with solar dryers ensures a continuous and reliable power supply, reducing dependency on the grid and further promoting sustainability in the drying process.

With Heat Pump
Some authors defend that combining a solar thermal energy source, such as solar thermal collectors with a heat pump dryer, will assist in reducing the operation cost of drying and producing products of high quality [162].The aim of installing a heat pump is to solve the problem of the intermittent availability of solar radiation.Depending on weather conditions, four working modes can be chosen [163][164][165][166][167][168][169][170][171][172][173][174]: Solar energy heating mode, when solar radiation is sufficient during the daytime; Heat pump heating mode when solar radiation is unavailable; Solar-assisted heat pump heating mode, when solar radiation is insufficient during the daytime; Heat pump dehumidification mode when ambient humidity is high.Beyond all the drying designs associated with heat pump systems, some authors also consider solar systems with chemical heat pumps (CHP) and solar systems with dehumidification systems [13].
The chemical reactions in a CHP system are generally reversible, enabling the alteration of the temperature level of the thermal energy stored by chemical substances [175,176].These reactions are crucial for absorbing and releasing heat.Typically, the main components include an evacuation system, a storage tank, a chemical heat pump and a drying chamber.CHP can be categorized into solid-gas [177] and liquid-gas.
A solid-gas chemical heat pump unit consists of a reactor or adsorber, an evaporator and a condenser.Liquid-gas systems have at least two reactors: endothermic and exothermic.The high storage capacity, low heat loss and long-term storage of reactants and products are the principal advantages of CHP [178].
Regarding dehumidification systems, in general, fresh products have high moisture contents.Using a desiccant material, such as silica, alumina, pillared clay, or zeolite [179][180][181], may consume low energy and produce dry air to improve drying performance.The pressure difference of generated water vapor, even at low temperatures, can improve driving force that is proportional to the evaporation rate.As a result, energy efficiency can be potentially improved while maintaining product quality [182].
Heat pump dryers come in different types and their performance varies depending on the type.The ability to control the temperature of the drying air and humidity while recovering energy from exhaust is one of the primary advantages of heat pump dryers; however, the environmental impacts is still not well known [183].

With Geothermal or Waste Waters
This kind of dryer uses solar radiation in combination with a low-potential energy source, such as geothermal or wastewater.The installation allows for the combination of conventional or nonconventional energy sources [184].According to Ivanova and Andonov [185], it is possible to achieve continuous drying, even during the night, enabled by additional heating of the air during movement in the collector using this source of energy, in a clean and cost-effective mode, as renewable energies are used.The system includes a stainless-steel body, heat exchanger, piping, dehumidifier, blower and trays [186].
Based on the design, construction material used, energy backup systems and auxiliary heating units, several variants of solar dryers for drying foods have been described.These diverse configurations allow for customized solutions to suit specific drying requirements, optimizing energy efficiency and ensuring consistent drying performance across different applications.The integration of low-potential energy sources with solar radiation enhances the versatility and reliability of solar dryers, making them more sustainable and resilient in various operating conditions.
Several studies are being conducted to test different techniques for improving solar dryers, including the use of thermal storage materials, deep bed drying methods, enhanced solar collector designs and energy hybridization.As we can observe, there are a wide variety of solar dehydrators with different shapes and operating modes.The possibilities are so many that, in recent years, several authors have felt the need to write review articles on the subject (Table 6).Table 6.Overview of published review articles using solar energy dryers, since 2010.

Title
Year Ref.
Review of solar dryers for agricultural and marine products 2010 [187] A Review of Photovoltaic Thermal (PVT) Technology for Residential Applications 2010 [157] Solar dryer with thermal energy storage systems for drying agricultural food products: A review 2010 [131] Solar drying 2011 [188] The development of fruit-based functional foods targeting the health and wellness market_ a review 2011 [189] New Technologies of Solar Drying Systems for Agricultural and Marine Products 2012 [190] Solar drying of agricultural products: A review 2012 [191] Performance study of different solar dryers: A review 2014 [9] A Review of Solar Dryer Technologies 2014 [20] Solar greenhouse drying: A review 2014 [85] Osmotic dehydration of fruits and vegetables: a review 2014 [192] Applications of software in solar drying systems: A review 2015 [19] A review on indirect solar dryers 2015 [193] Direct Type Natural Convection Solar Dryer: A review 2015 [81] Performance enhancement of solar collectors-A review 2015 [194] A Review on Solar Drying of Agricultural Produce 2016 [13] Comparative energy-exergy and economic-environmental analyses of recently advanced solar photovoltaic and photovoltaic thermal hybrid dryers: a review 2022 [219] Performance improvement and advancement studies of mixed-mode solar thermal dryers: a review 2022 [220] A review study on recent advances in solar drying: Mechanisms, challenges and perspectives 2022 [221] Solar drying of fruits-A comprehensive review 2022 [222] A review of industrial food processing using solar dryers with heat storage systems 2023 [130] Thermal energy storage systems applied to solar dryers: Classification, performance and numerical modelling: An updated review 2023 [127] Designs, Performance and Economic Feasibility of Domestic Solar Dryers 2023 [92] Assessing the suitability of solar dryers applied to wastewater plants: A review 2023 [223] Performance enhancement techniques for indirect mode solar dryer: A review 2023 [224] A review on the latest developments in solar dryer technologies for food drying process 2023 [225] Progressive review of solar drying studies of agricultural products with exergoeconomics and econo-market participation aspect 2023 [226] A review of solar drying technology for agricultural produce 2023 [227] A review of the inflated solar dryer for improving the quality of agricultural product 2023 [228] Photovoltaic-thermal systems applications as dryer for agriculture sector: A review 2023 [229]

Advantages and Limiting Issues
The utilization of solar energy to dehydrate food remains attractive in terms of energy efficiency and the wide range of products that are suitable for the technique (Figure 9).

Advantages and Limiting Issues
The utilization of solar energy to dehydrate food remains attractive in terms of energy efficiency and the wide range of products that are suitable for the technique (Figure 9).Commonly, it is correct to refer to the principal advantages and disadvantages as follows in Table 7.
Table 7. Principal advantages/disadvantages of solar energy utilization in fresh products dehydration.

Advantages Disadvantages
Solar 2024, 4, FOR PEER REVIEW Commonly, it is correct to refer to the principal advantages and disad follows in Table 7.

Advantages Disadvantages
• Benefiting from solar energy as a renewable source for food consumption; • Dependence on climatic variables; • Possibility of utilization of recycled and low-cost materials; • Could be less efficient compared with electr dryers • Does not require specialized labor and promotes the reduction in crop losses; • Lack of investment in this type of processin • Extending the preservation of food.Dried foods are also easier to store and transport, promoting convenience in consumption, which is valued in nutrition due to the absence of added sugar and other additives.
• For products requiring continuous drying, a heating system is necessary • Contributes positively to sustainability and environmental preservation; • Improved food preservation, especially in underdeveloped countries where electricity could be unavailable; • Can generate income for small-scale producers and reduce losses;

Conclusions and Final Remarks
Nowadays, especially with awareness of the 2030 Agenda for Development global goals, food drying is a vital process for food preservation dehydration is a good solution, but concerns related to energy consumptio fuels persist.The use of solar energy, once widely employed in the past, importance.This trend is evident in the number of studies considered in article.Performance, design parameters, location, atmospheric conditions a crop are principal factors taken into consideration when choosing the bes drying purposes.The articles aim to synthesize the findings from various stud trends in the literature and provide insights and recommendations for furth and development in the field of solar drying.
Our paper reviewed the designs and mechanisms of various types of s with a main focus on heat transfer, air movement and types of chambers.A layouts have advantages and disadvantages, but commonly, mixed-mode types are more efficient in terms of time compared with direct and indi However, they are not entirely sustainable.Active circulation takes less time better final products than open-air and natural circulation, although it limitations in terms of product quantity.For large-scale crop drying, the green is the best method, but it requires more space.
The cited review articles are a valuable resource for researchers and p interested in understanding the current state of knowledge and advanceme drying technologies and applications.Commonly, it is correct to refer to the principal advantages and disadvantages as follows in Table 7.

Advantages Disadvantages
• Benefiting from solar energy as a renewable source for food consumption; • Dependence on climatic variables; • Possibility of utilization of recycled and low-cost materials; • Could be less efficient compared with electrical dryers • Does not require specialized labor and promotes the reduction in crop losses; • Lack of investment in this type of processing • Extending the preservation of food.Dried foods are also easier to store and transport, promoting convenience in consumption, which is valued in nutrition due to the absence of added sugar and other additives.
• For products requiring continuous drying, a backup heating system is necessary • Contributes positively to sustainability and environmental preservation; • Improved food preservation, especially in underdeveloped countries where electricity could be unavailable; • Can generate income for small-scale producers and reduce losses;

Conclusions and Final Remarks
Nowadays, especially with awareness of the 2030 Agenda for Sustainable Development global goals, food drying is a vital process for food preservation.Generally, dehydration is a good solution, but concerns related to energy consumption and fossil fuels persist.The use of solar energy, once widely employed in the past, is regaining importance.This trend is evident in the number of studies considered in our review article.Performance, design parameters, location, atmospheric conditions and type of crop are principal factors taken into consideration when choosing the best mode for drying purposes.The articles aim to synthesize the findings from various studies, identify trends in the literature and provide insights and recommendations for further research and development in the field of solar drying.
Our paper reviewed the designs and mechanisms of various types of solar dryers, with a main focus on heat transfer, air movement and types of chambers.All the dryer layouts have advantages and disadvantages, but commonly, mixed-mode and hybrid types are more efficient in terms of time compared with direct and indirect modes.However, they are not entirely sustainable.Active circulation takes less time and yields better final products than open-air and natural circulation, although it represents limitations in terms of product quantity.For large-scale crop drying, the greenhouse type is the best method, but it requires more space.
The cited review articles are a valuable resource for researchers and practitioners interested in understanding the current state of knowledge and advancements in solar drying technologies and applications.

•
Benefiting from solar energy as a renewable source for food consumption; • Dependence on climatic variables; • Possibility of utilization of recycled and low-cost materials; • Could be less efficient compared with electrical dryers • Does not require specialized labor and promotes the reduction in crop losses; • Lack of investment in this type of processing

•
Extending the preservation of food.Dried foods are also easier to store and transport, promoting convenience in consumption, which is valued in nutrition due to the absence of added sugar and other additives.

•
For products requiring continuous drying, a backup heating system is necessary • Contributes positively to sustainability and environmental preservation; • Improved food preservation, especially in underdeveloped countries where electricity could be unavailable; • Can generate income for small-scale producers and reduce losses;

Conclusions and Final Remarks
Nowadays, especially with awareness of the 2030 Agenda for Sustainable Development global goals, food drying is a vital process for food preservation.Generally, dehydration is a good solution, but concerns related to energy consumption and fossil fuels persist.The use of solar energy, once widely employed in the past, is regaining importance.This trend is evident in the number of studies considered in our review article.Performance, design parameters, location, atmospheric conditions and type of crop are principal factors taken into consideration when choosing the best mode for drying purposes.The articles aim to synthesize the findings from various studies, identify trends in the literature and provide insights and recommendations for further research and development in the field of solar drying.
Our paper reviewed the designs and mechanisms of various types of solar dryers, with a main focus on heat transfer, air movement and types of chambers.All the dryer layouts have advantages and disadvantages, but commonly, mixed-mode and hybrid types are more efficient in terms of time compared with direct and indirect modes.However, they are not entirely sustainable.Active circulation takes less time and yields better final products than open-air and natural circulation, although it represents limitations in terms of product quantity.For large-scale crop drying, the greenhouse type is the best method, but it requires more space.
The cited review articles are a valuable resource for researchers and practitioners interested in understanding the current state of knowledge and advancements in solar drying technologies and applications.

Figure 2 .
Figure 2. Schematic summary of the classification of solar dryers.

Figure 2 .
Figure 2. Schematic summary of the classification of solar dryers.

Solar 2024, 4 ,Figure 3 .
Figure 3. Schematic of working principle of open sun drying method.

Figure 3 .
Figure 3. Schematic of working principle of open sun drying method.

Figure 4 .
Figure 4. Schematic of working principle of direct drying method.

Figure 4 .
Figure 4. Schematic of working principle of direct drying method.

Figure 5 .
Figure 5. Schematic of working principle of indirect drying method.

Figure 6 .
Figure 6.Schematic of working principle of hybrid (solar-thermal) drying method.

Figure 6 .
Figure 6.Schematic of working principle of hybrid (solar-thermal) drying method.

Figure 9 .
Figure 9. Overview of the distribution of the publications and type of dried products, all over the world, consulted in this review article.

Figure 9 .
Figure 9. Overview of the distribution of the publications and type of dried products, all over the world, consulted in this review article.

Table 1 .
Published studies related to OSD experiments.

Principle of Study Principal Achievements Crop/Product Location Year Ref. The
final moisture content of 11.5 ± 0.5% was 360,

Table 1 .
Published studies related to OSD experiments.

Table 2 .
Published studies related to DSD experiments.

Table 3 .
Published studies related to ISD experiments.
Construction and study of a friendly solar ISD A low-cost and environmentally friendly way to make home-made snacks with recycled materials Various Portugal 2022 [7]A solar dryer with a flat plate absorber and thermal storage andThe economic performance of the dryer was analyzed based on the optimum cost of raw materials and the product sale price.

Table 3 .
Published studies related to ISD experiments.

Table 4 .
Published studies related to HSD experiments.

Table 5 .
Published studies related to different devices according to the mode of air mo

Table 5 .
Published studies related to different devices according to the mode of air movement.
UV sheet cabinet-type solar dryerForced convection drying is the most efficient way of drying when compared to natural and open sun drying.
• C, the dryer is suitable.
Solar 2024, 4, FOR PEER REVIEW 19 Design and analysis of different types of solar collector for solar air dryer: A review 2022 [215] Systematic Literature Review on Machine Learning Predictive Models For Indoor Climate In Smart Solar Dryer Dome2022 [216] The indirect solar dryers with innovative solar air heaters designs: A review article 2022 [217] A Comprehensive State-of-the-Art Review on the Recent Developments in Greenhouse Drying 2022 [218] Comparative energy-exergy and economic-environmental analyses of recently advanced solar photovoltaic and

Table 7 .
Principal advantages/disadvantages of solar energy utilization in fresh produ dehydration.

Table 7 .
Principal advantages/disadvantages of solar energy utilization in fresh products dehydration.