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# A review on the use of sorption materials in solar dryers

Rock Aymar Dake, Kokouvi Edem N'tsoukpoe, Frédéric Kuznik, Babacar Lèye, Igor W.K. Ouédraogo

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Rock Aymar Dake, Kokouvi Edem N'tsoukpoe, Frédéric Kuznik, Babacar Lèye, Igor W.K. Ouédraogo.
A review on the use of sorption materials in solar dryers. Renewable Energy, 2021, 175, pp.965-979.
10.1016/j.renene.2021.05.071. hal-04918193

HAL Id: hal-04918193

https://hal.science/hal-04918193v1

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1 A review on the use of sorption materials in solar dryers

2 Rock Aymar Dakeª, Kokouvi Edem N'Tsoukpoeª (*), Frédéric Kuznikᵇ, Babacar Lèyeᶜ, Igor

3 W. K. Ouédraogoª

4 ªLaboratoire Énergies Renouvelables et Efficacité Énergétique (LabEREE), Département Génie Électrique,

5 Énergétique et Industriel, Institut International d'Ingénierie de l'Eau et de l'Environnement (2iE), 01 BP 594

6 Ouagadougou 01, Burkina Faso

7 ᵇUniversité de Lyon, INSA de Lyon, CETHIL UMR5008, F-69622, Villeurbanne, France

8 ᶜLaboratoire Eaux Hydro-Systèmes et Agriculture (LEHSA), Département Sciences et Techniques de

9 l'Ingénieur, Institut International d'Ingénierie de l'Eau et de l'Environnement (2iE), 01 BP 594 Ouagadougou

10 01, Burkina Faso

11 Abstract

12 Solar drying has received great interest as a viable alternative solution to fossil fuel dryers.

13 However, due to the intermittency of the solar resource, solar dryers suffer from discontinued

14 drying and thus, long drying time. In this paper, the use of sorption materials to enhance the

15 performance of solar dryers is reviewed. The main aspects regarding their use as thermal energy

16 storage or dehumidification materials are highlighted. The paper shows that solid adsorbents,

17 especially silica gel, are the most used materials. The use of composite materials, for instance

18 the mixture of bentonite, CaCl₂, vermiculite, and cement proved to be promising. The

19 incorporation of a sorption dehumidifier in a solar dryer, usually at the solar collector inlet,

20 generally leads to a reduction of the drying time of 15 to 30%, although values up to 50% and

21 even 64% have been reported. On the other hand, introducing sorption materials as thermal

22 storage in a solar dryer, mostly integrated at the top in the drying chamber, usually leads to a

23 reduction of the drying time in a range of 30 to 45%. There are still various aspects to be

24 investigated in depth before a large-scale utilization of these techniques in solar drying

25 technology.

26 **Keywords:** solar energy; solar dryer; sorption thermal storage; dehumidifier; desiccants;

27 adsorbents.

28 *Corresponding author:

29 E-mail address: edem.ntsoukpoe@2ie-edu.org ; alternative e-mail: n_christedem@yahoo.fr

30 (K.E. N'Tsoukpoe)
# 1. Introduction

In a portion of food with high moisture content, the presence of water can be observed in both bound and unbound forms. In contrast to bound water, which is an integral part of the molecular structure of the food [1], unbound water is involved in biological reactions and natural processes that lead to a decrease in food quality [2]. The ratio of the partial pressure of water vapor just above the product to the partial pressure of pure water at the same temperature gives the water activity, usually denoted $a_w$ [3,4]. This coefficient evaluates the amount of unbound water available in a product that can participate in chemical reactions and thus accelerate its deterioration. Therefore, high-moisture food materials are very sensitive to spoilage due to their high water activity. Figure 1 shows how microorganisms such as bacteria, yeasts, etc. develop according to the water activity in common food. For food with a water activity below 0.6, there is no proliferation of microorganisms. Thus, those products are less sensitive to spoilage.

<table><caption>Figure 1: Microorganisms growth as a function of water activity in common foods. Data from [5]</caption><thead><tr><th>a<sub>w</sub></th><th>0.6</th><th>0.65</th><th>0.75</th><th>0.80</th><th>0.87</th><th>0.91</th><th>0.95</th><th>1</th></tr></thead><tbody><tr><th>No microbial proliferation</th><td>Osmophilic yeasts (S. rouxii), few moulds</td><td>Xerophilic moulds (A. chevalieri), S. bisporus</td><td>Most halophilic bacteria</td><td>Most moulds, most saccharomyces, staphylococcosus</td><td>Most yeasts (condida, torulo psis), micrococcus</td><td>Salmonella, vibrio parahaemolyticus, lactobacillus moulds, yeasts</td><td>Pseudomonas, bacillus escherichia, some yeasts</td><td></td></tr><tr><th>Pasta, food with about 10 % moisture</th><td>Dried fruits containing 25-20 % moisture, honey, caramel</td><td>Jelly, nuts, molasses. Raw cane, sugar, some dried fruits</td><td>Jam, marmalade, fruits</td><td>Most fruit juice concentrates, maple, chocolate and fruit syrups, flour, rice, fruit cake</td><td>Fermed sausage (salami), dry cheeses, margarine</td><td>Some cheeses, cured meat, some fruit juice concentrates</td><td>Fresh and canned fruits, vegetables, meat, fish and other perishable foods</td><td></td></tr></tbody></table>

Among the various ways to control water activity in foods, reduction of water content is widely used. In this context, thermal drying remains the most common drying method, especially for agricultural products [5]. Many industrial and agricultural processes make use of thermal drying. For example, fruits, crops, and vegetables such as mint, mangoes, apples, rice, oilseeds, and carrots are dried with hot air [6]. Unfortunately, because they are highly energy-intensive (for example 65.45 MJ/kg of water evaporated for hybrid convective electric-gas dryer while drying some onion slices at 50 °C [7]), conventional industrial thermal drying methods increase final product cost. Consequently, solar drying has appeared to be a good alternative to sun
drying and all those expensive industrial drying methods [8]. It provides solutions to most of
their drawbacks, namely [9]:

55 - For sun drying: (i) damage to the products by rodents, birds and animals. (ii) 56 degradation through exposure to rain, storm and dew as well as wind. (iii) 57 contamination by dirt, dust, wind-blown debris and environmental pollution. (iv) 58 splitting of the grain, bleaching and loss of germination capability due to overdrying. 59 (v) insect infestation. (vi) growth of micro-organisms, and (vii) additional losses during 60 storage due to insufficient or non-uniform drying for sun drying.

61 - For industrial dryers (i) high initial investment and high energy costs. (ii) lack of 62 available skilled personnel for operation and maintenance, and (iii) lack of availability.

63 Convective thermal drying, which is commonly encountered in solar dryers, is governed by a 64 simultaneous heat and mass transfer in the product, accompanied by various physico- 65 biochemical transformations (shrinkage, changes in color, texture, odor, etc.) [10]. As heat is 66 supplied to the product at its surface, it diffuses into the solid essentially by conduction. This 67 heat supply causes the liquid or moisture in the product to move outward toward the product 68 surface and to vaporize before it is removed away by the carrier or surrounding gas [10]. Drying 69 is a very complex process and the dynamic of the heat and mass transfer may change a lot 70 during the process.

71 Since the solar resource is intermittent, conventional solar dryers suffer from discontinued 72 drying due to sunshine and non-sunshine hour cycles. Some studies have therefore introduced 73 hybrid solar drying. In hybrid solar dryers, in addition to solar energy, auxiliary energy sources 74 are also used to heat the drying air. For example, the auxiliary source may be an electric heater 75 [11] or a biomass back-up heater [12–14]. Besides auxiliary sources, thermal storage in solar 76 dryers has also been widely investigated.

77 The researches on thermal storage for solar dryers have been mostly oriented toward the use of 78 latent and sensible heat storage. Many authors [15–21] dealt with the use of rock as sensible 79 thermal storage in solar dryers. Also, because of its good sensible heat storage characteristics 80 (affordable, good thermal stability, and high specific heat), the use of sand has sometimes been 81 considered in some solar dryers [20,22]. Water has also been used as sensible thermal storage 82 in solar dryers [20]. Potdukhe and Thombre [23] used thermal oil as thermal storage material 83 in an agricultural solar dryer for drying products such as chili peppers and fenugreek leaves. 84 Phase change materials are also discussed in the literature as latent heat storage in solar dryers
[24–46]. However, although sensible heat storage is widely used, it faces major challenges such as low energy density and heat losses. Fortunately, phase-change materials greatly improve energy storage density but do not solve the problem of heat losses. Increasing the thickness of insulation would be helpful, but it also increases the size and cost of the system [47]. On the other hand, sorption materials used as thermal storage could be an interesting alternative to sensible heat storage and phase change materials. Moreover, they theoretically exhibit three times higher volume energy storage density than sensible heat storage materials and twice as high as latent heat storage materials [47]. They also have the advantage of reducing heat loss and allow seasonal storage [47]. Apart from their use as a thermal storage medium, sorption materials are also used in solar dryers as dehumidifiers. In this configuration, the sorption material lowers the relative humidity of the drying air before it enters the drying chamber. This enhances the overall drying performance of the dryer.

This review paper focuses on the use of sorption materials as thermal storage or dehumidifier in solar dryers. It covers forty years of the field and aims to point out what advantages have already been taken into account in the use of sorption materials in such dryers. Authors also identify gaps and challenges in the current research towards improving the performances of solar dryers and ensuring their continuous operation. First, a brief insight on some terminologies related to the concept of sorption and that of solar dryers is given, and then studies related to the use of sorption materials as thermal storage or dehumidifier, or even both in solar dryers are presented. Then, the findings are synthesized for critical analysis and to clearly show the gaps and rooms for improvement of the use of sorption materials as thermal storage and dehumidifier in solar dryers

## 2. Principles and terminology

### 2.1. Typology of solar dryers

Worldwide, there are different types of solar dryers. They fall into two main categories [5,48]: (i) passive solar dryers where the drying air circulates by natural convection and (ii) active solar dryers, often called forced convection solar dryers, where the circulation of the drying air is ensured by a blower. For Ekechukwu et al. [49], because of the system components that differ from one solar dryer to another as well as the way solar heat is used, three sub-categories of active and passive solar dryers can be listed: integral-type, distributed-type, and mixed-mode type (Figure 2). In integral or direct solar dryers, the product is directly exposed to solar radiation [50]. Direct solar dryers are simple and cheap to be constructed. They prevent the
product from contamination due to dust, rain, debris, etc. [51]. On the other hand, due to direct exposure to solar radiation, the product may become discolored, overheated, and hence, lose some qualities [52]. The drying capacity in such dryers may also be limited [51]. In indirect solar dryers, solar energy is first collected by a solar collector and is then passed onto the drying chamber, where the drying occurs. This offers better temperatures and air flow rate control, leading to better moisture removal and product drying [53,54].

<figure><img src="image_2.png" /><figcaption>Figure 2: Sub-categories of solar drying systems as classified by Ekechukwu et al. [49]. Reprinted from Energy Conversion & Management 40 (1999), O.V. Ekechukwu, B. Norton, Review of solar-energy drying systems II: an overview of solar drying technology, Pages 615–655, Copyright (1998), with permission from Elsevier.</figcaption></figure>

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## 2.2. Definitions related to sorption storage

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Concepts related to heat storage such as "sorption storage", "chemical storage" and "thermochemical storage" are generally misused in literature. In a study [55], the authors discussed the ambiguity associated with the use of these three expressions. It comes out from their study that when reversible chemical reactions are used to store heat in the limit of chemical bonds, the proper expression to use is that of "chemical storage". In that case, "thermochemical
reactions”, “photosynthesis” and “photochemical reactions” can provide the required energy for the storage (Figure 3).

<figure><img src="image_3.png" /><figcaption>Figure 3: Different types of sorption storage and chemical storage as described by N'Tsoukpoe et al. [55]. Reprinted from Renewable and Sustainable Energy Reviews 13 (2009), K. Edem N'Tsoukpoe, Hui Liu, Nolwenn Le Pierres, Lingai Luo, A review on long-term sorption solar energy storage, Pages 2385–2396, Copyright (2009), with permission from Elsevier.</figcaption></figure>

On the other side, when during the heat storage, a gas or a vapor is captured by a solid or a liquid the authors talk about “sorption storage”. The gas being captured is called “sorbate” and the host is referred to as “sorbent”. It is important to notice that when the sorbate integrates the molecular structure of the sorbent, the type of “sorption storage” is “absorption”. But if the gas remains at the surface of the host, then it is an “adsorption”. There are two types of adsorption. The first type is known as physisorption [56], and the second type, as chemisorption. Physisorption is a physical adsorption that mostly involves weak van der Waals forces and is reversible, while solid chemisorption is a chemical adsorption where valence forces are involved. Similarly, there are two types of absorption processes: physical absorption and chemical absorption, depending on whether there is any chemical reaction between the absorbate and the absorbent (chemical) or not (physical) [57].

In this paper, sorption storage covers both absorption and adsorption and encompasses chemical and physical phenomenon. Sorption materials signify both liquid and solid materials but also
composites materials, which are usually a combination of two of the previous type to overcome some limitations and improve some technical performances.

## 2.3. Basic principles of heat storage with sorption materials

The operating principle of sorption or thermochemical heat storage is based on reversible physico-chemical phenomena, which are used to store energy [58].

During the charging period, heat is supplied (1) to the material AB (solid or liquid), which absorbs the heat and decomposes into two products A (solid or liquid) and B (gas): this is an endothermic reaction. For the heat recovery in discharging period, the compounds A and B are put in contact; the reverse phenomenon occurs and AB is formed with a heat release (exothermic reaction).

$$AB + heat \rightleftharpoons A + B \quad (1)$$

Figure 4 shows the principle of thermal storage with solid sorption material (adsorbent).

<figure><img src="image_4.png" /><figcaption>Figure 4: Principle of energy storage by adsorption/desorption. Adapted from [59]</figcaption></figure>

## 2.4. Principles of the use of sorption materials as dehumidifier and thermal storage in solar dryers
167 Sorption materials can be used for two main purposes in solar dryers [60]: dehumidification and heat storage. The purpose of using sorption materials as dehumidifier is to lower the relative humidity of the drying air before it enters the drying chamber. In practice, a bed of a sorption material is provided in the path of the drying air, usually, at the entrance of the solar collector.
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171 As the drying air flows through the dehumidification bed towards the solar collector, the sorption materials ab/adsorb (depending on the type of material used) water molecules in the air, in order to reduce its relative humidity. This ad/absorption process which is exothermic, will also increase the drying air temperature and then enhance its overall drying potential. As for heat storage in solar dryers, the desorption process usually takes place during the day, by solar heat supply. This regenerates the sorption potential of the drying material. At night, the dried sorbent is introduced into the solar dryer, together with the wet product to be dried. The admission of air in the dryer is stopped so that the confined air can be recirculated between the bed of sorbent material and the product to be dried by the mean of a fan that can be powered by either a battery or another source of electricity. At that moment, adsorption process starts and the water molecules contained in the air are adsorbed by the sorbent. Since the process is exothermic, heat is released. This heat is transferred to the product to be dried, which releases water back into the air. These water molecules are again adsorbed by the sorption material and heat is again released. Thus, through this cycle, the drying of the product continues.
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185 **3. Review of the use of sorption materials in solar dryers**

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199 The use of sorption materials in solar dryers is subject to several conditions. First of all, since solar dryers are mostly used in agro-food applications [61], a crucial aspect to consider is toxicity. The ideal sorption material for an application in a solar dryer should therefore meet this first criterion which is that of non-toxicity, especially regarding human health hazards. It is very important because, in most applications, the drying air is in contact with both the sorption material and the product to be dried. Therefore, the transfer of toxic particles from the material to the product should be avoided. In this same line of maintaining product quality, one property to be considered in the selection is the handling of the material. Indeed, the material must allow good handling without risk of direct contact with the product to be dried, so that it does not alter its taste. Another property to be considered, which is not the least important, is the regeneration temperature of the material. It is important to ensure that the regeneration temperature of the material to be selected is in line with the temperatures achievable by the solar collector(s) integrated into the solar dryer. All these properties of the ideal sorption material for application in a solar dryer must be combined with the criterion of cost-effectiveness. More broadly, the
main parameters to be considered for an optimal choice of the sorption material for a solar dryer
application are as follows [62–64]:

202 high water uptake [kg(water)/kg(sorbent)];

203 high energy storage density under the operating conditions ;

204 regeneration at a relatively low temperature that is reachable by the solar thermal
collectors of the dryer;
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206 short regeneration time;

207 appropriate heat and mass transfer properties;

208 easy to handle i.e. nonpoisonous;

209 low investment cost;

210 thermal stability;

211 low environmental or lifecycle impact including disposal at the end of the lifetime.

212 These criteria are often in conflict with each other, which means that a good compromise for
213 an optimum must be made for the final choice.

214 **3.1. Review of applications of sorption materials as dehumidifier**

215 Silica gel is the most known [65,66] and used sorption material for physical adsorption of water
216 vapor especially due to its hydrophilic properties [47]. By far, it is also the material that has
217 been considered for dehumidification in solar dryers, both in direct and indirect forced
218 convection solar dryers [67–72]. Indeed, standard white silica gel is non-toxic and inert. It is
219 only when it is impregnated with a moisture indicator, such as cobalt chloride, which is
220 carcinogenic, that silica gel may become hazardous [73]. It is non-corrosive, odorless, thermally
221 stable, and requires a relatively low regeneration temperature. A combination of characteristics
222 that makes silica gel the preferred choice by most authors (Table 1). Calcium chloride aqueous
223 solution has also been considered for dehumidification in solar drying as it is also an abundant,
224 cheap, and non-toxic material. However, as a liquid material, it offers less handling possibility
225 compared to solids, namely silica gel. The regeneration of these materials used as dehumidifiers
226 in solar dryers is usually done with solar resource (Table 1).

227 Considering the works reported in the literature, most of the performances recorded in a solar
228 dryer integrated with a dehumidifier unit based on sorption material, ranged from 15 to 30%
229 reduction in drying time compared to operation without the sorption material integrated into the
230 dryer. Exceptional reduction in drying time of 50% and 64% were also reported by Punlek et
231 al. [68] and Misha et al. [71], respectively. In all these applications, forced convection solar
232 dryers have been of great interest for authors because the presence of the sorption bed creates
233 resistance (pressure drop) that is not favorable to natural convection [74,75].
<table><caption>Table 1: Key data and main results of some studies on the use of sorption materials for dehumidification in solar dryers. (wb=wet basis, db=dry basis.)</caption><thead><tr><th>Reference</th><th>Hodali and Bougard [67]</th><th>Punlek et al. [68]</th><th>Chramsa-ard et al. [69]</th><th>Misha et al. [70]</th><th>Misha et al. [71]</th><th>Dorouzi et al. [76]</th></tr></thead><tbody><tr><th>Country</th><td>Morocco</td><td>Thailand</td><td>Thailand</td><td>Malaysia</td><td>Malaysia</td><td>Iran</td></tr><tr><th>Type of work</th><td>Simulation</td><td>Experiment</td><td>Experiment</td><td>Experiment</td><td>Experiment</td><td>Experiment</td></tr><tr><th>Product</th><td>Apricots</td><td>Ginger slices</td><td>Chili</td><td>Kenaf core fiber</td><td>Crushed oil palm fronds</td><td>Tomato slices</td></tr><tr><th>Mass of product</th><td>300 kg</td><td>—</td><td>—</td><td>155 kg</td><td>231 kg</td><td>500 g</td></tr><tr><th>Initial moisture content of the product</th><td>5.2 g/kg (db)</td><td>—</td><td>82 % (wb)</td><td>55 % (wb)</td><td>69 %</td><td>93.6% (wb)</td></tr><tr><th>Final moisture content</th><td>0.25 g/(g(db))</td><td>—</td><td>13 % (wb)</td><td>&lt; 18% (wb)</td><td>29% (wb)</td><td>9.2 %</td></tr><tr><th>Drying time</th><td>44 h</td><td>4 h</td><td>19 h</td><td>15.75 h</td><td>10 h 55 min</td><td>5.5 h</td></tr><tr><th>Time saved</th><td>8 h / 15.4% (compared to without sorption material)</td><td>4 h / 50% (compared to without sorption material)</td><td>5 h / 20.8% (compared to without sorption material)</td><td>24 % compared to direct sun drying</td><td>64 % compared to direct sun drying</td><td>27 % (compared to without sorption material)</td></tr><tr><th>Electricity consumption saving</th><td>—</td><td>—</td><td>21.5 kWh</td><td>—</td><td>—</td><td>—</td></tr><tr><th>Type of dryer</th><td>Indirect</td><td>Indirect</td><td>Indirect</td><td>Indirect</td><td>Indirect</td><td>Indirect</td></tr><tr><th>Convection</th><td>Forced</td><td>Forced</td><td>Forced</td><td>Forced</td><td>Forced</td><td>Forced</td></tr><tr><th>Collector</th><td>Flat plate<br>20 m²</td><td>Flat plate<br>0.8125 m²</td><td>Flat plate<br>2.5 m²</td><td>Evacuated tubes<br>31.5 m²</td><td>Evacuated tubes<br>31.5 m²</td><td>Flat plate<br>1.1 m²</td></tr><tr><th>Drying chamber</th><td>2 m³</td><td>0.112 m³</td><td>—</td><td>10.2 m³</td><td>10.2 m³</td><td>—</td></tr></tbody></table>
<table><tr><td>Sorption material</td><td>Silica gel</td><td>Silica gel</td><td>Silica gel</td><td>Silica gel</td><td>Silica gel</td><td>Solution of CaCl<sub>2</sub></td></tr><tr><td>Regeneration</td><td>Solar</td><td>Solar</td><td>Solar</td><td>Solar</td><td>Solar</td><td>Solar</td></tr></table>
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**3.2. Review of applications of sorption materials as thermal storage**

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If authors working on dehumidifiers integrated to solar dryers has shown a very high interest for silica gel, this does not seem to be the case while considering thermal sorption storage integrated to solar dryers since achieving a high energy storage density and a sufficient temperature level during the discharging is a target (Table 2). To meet these requirements, some authors [77,78] proposed the development of composite materials. These researches are focusing on materials that have high porosity, a low cost, and a low regeneration temperature suitable to be integrated into solar dryers. Existing composite materials have already been identified to be capable of exhibiting excellent humidity adsorbing ability and attain deep dehumidification of moist air [79]. They are obtained by impregnating hygroscopic salt into the pores of porous desiccant host materials. Calcium chloride appears to be one of the most favored hygroscopic salts in this regard (Table 2), probably because of several advantages that it exhibits as sorption and thermochemical heat storage material [80]. It is easily available, cheap (maybe the cheapest salt hydrate), thermally and chemically more stable than several other salt hydrates, non-toxic, and offers a high hygroscopic desiccating and sorption capacity while it can be easily regenerated at relatively low temperature [81–83].

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Thoruwa et al. [77,78,84,85] used, for example, a solid composite material consisting of calcium chloride impregnated on bentonite in an indirect forced-convection solar dryer in Nairobi (subtropical highland climate, Kenya). The authors reported that this desiccant is capable of regeneration at 45 °C and has a high sorption capacity of 45% of its mass. They add that this mixture was suitable for the work, first, because it significantly extends the drying process at night and reduces aflatoxin contamination of the grain, and second, because it is a low-cost solid desiccant. During the experiments in Kenya, the authors used the material as thermal storage for drying 90 kg of maize. The dryer performed well, under crop harvesting conditions in Nairobi, drying maize from 38% (db) to 15% (db) within 24 h of harvest. One very important finding of their work that can be useful when one is about to make a thermal storage application of this composite material is that throughout the drying operation, 70% of the moisture content of the product was removed during the day while the remaining 30% was removed by thermal storage at night. Other authors, especially Patil et al. [86], who observed a load distribution of 68% for daytime drying and 32% for thermal storage, have confirmed these values.
<table><caption>Table 2: Key data and main results of some studies on the application of various sorption materials as thermal storage in solar dryers.</caption><thead><tr><th>Reference</th><th>Shanmugam and Natarajan [87]</th><th>Shanmugam and Natarajan [88]</th><th>Thoruwa et al. [78]</th><th>Dina et al. [89]</th><th>Dina et al. [89]</th><th>Phadke et al. [90]</th></tr></thead><tbody><tr><th>Country</th><td>India</td><td>India</td><td>Kenya</td><td>Indonesia</td><td>Indonesia</td><td>India</td></tr><tr><th>Type of work</th><td>Experiment</td><td>Experiment</td><td>Experiment</td><td>Experiment</td><td>Experiment</td><td>Experiment</td></tr><tr><th>Product</th><td>Green peas</td><td>Green peas</td><td>Maize</td><td>Cocoa bean</td><td>Cocoa bean</td><td>Grapes</td></tr><tr><th>Mass of product</th><td>20 kg</td><td>20 kg</td><td>90 kg</td><td>1 kg</td><td>1 kg</td><td>1.2 kg</td></tr><tr><th>Initial moisture content of the product</th><td>80 % (wb)</td><td>80 % (wb)</td><td>38 % (wb)</td><td>59.1%-60.4%</td><td>59.1%-60.4%</td><td></td></tr><tr><th>Final moisture content of the product</th><td>5 % (wb)</td><td>5 % (wb)</td><td>15 % (wb)</td><td>-</td><td>-</td><td>20 % (wb)</td></tr><tr><th>Drying time</th><td>21 h</td><td>19 h with reflective mirrors on sorption bed</td><td>Within 24 h</td><td>41 h</td><td>30 h/</td><td>57 h</td></tr><tr><th>Time saved</th><td>10 h (32.2%)</td><td>12 h (38.7%) - compared to without sorption material</td><td>-</td><td>25.45% compared to direct sun drying</td><td>45.45% compared to direct sun drying</td><td>47 h / 45% compared to direct sun drying</td></tr></tbody></table>
<table><tr><td></td><td colspan="6">– compared to without sorption material.</td></tr><tr><td>Type of solar dryer</td><td>Indirect</td><td>Indirect</td><td>Indirect</td><td>Indirect</td><td>Indirect</td><td>Indirect</td></tr><tr><td>Convection</td><td>Forced</td><td>Forced</td><td>Forced</td><td>Natural</td><td>Natural</td><td>Forced</td></tr><tr><td>Collector</td><td>Flat plate<br>2.88 m²</td><td>Flat plate<br>2.88 m²</td><td>Flat plate<br>–</td><td>Flat plate<br>0.1 m²</td><td>Flat plate<br>0.1 m²</td><td>Flat plate<br>1.4 m²</td></tr><tr><td>Drying chamber</td><td>1.44 m³</td><td>1.44 m³</td><td>–</td><td>0.125 m³</td><td>0.125 m³</td><td>0.512 m³</td></tr><tr><td>Sorption material</td><td>Bentonite + CaCl₂, vermiculite, and cement in a mass ratio of 6:1:2:1</td><td>Bentonite + CaCl₂, vermiculite, and cement in a mass ratio of 6:1:2:1</td><td>Bentonite + CaCl₂, vermiculite, and cement in a mass ratio of 6:1:2:1</td><td>Zeolite X</td><td>Solid CaCl₂</td><td>Silica gel</td></tr><tr><td>Form of the sorption material</td><td>Solid</td><td>Solid</td><td>Solid</td><td>Solid</td><td>Solid</td><td>Solid</td></tr><tr><td>Regeneration</td><td>Solar</td><td>Solar</td><td>Solar</td><td>Solar</td><td>Solar</td><td>Solar</td></tr></table>
Because of this good performance of the mixture of bentonite, CaCl₂, vermiculite, and cement in a mass ratio of 6:1:2:1, Shanmugam et al. [87,88] also used it for thermal storage in an indirect forced convection solar dryer in the hot and humid climatic conditions of Chennai in India. Experiments have shown that for the drying of 20 kg of green peas, the moisture content declined from 80% to 5% in 21 h. With the inclusion of a set of two reflective mirrors on the sorption bed, the drying time was reduced to 19 h. Similar results were recorded for the drying of 20 kg of pineapple. The safe moisture content was reached in 32 h with the thermal storage without the set of mirrors. With the two mirrors on the sorption bed, the drying time was reduced by 4 h leading to a drying time of 28 h.

Dina et al. [89] studied the performance of zeolite X compared to that of solid calcium chloride when each of them is separately used as thermal storage in the same natural convection indirect solar dryer. In general, zeolites are more hydrophilic than silica gels with a rate of water uptake of 0.3 kg per kg of materials [47]. However, for solar-driven systems, zeolites have a major drawback because of their high regeneration temperature which typically could reach more than 200 °C [47]. A fact that limits their use in solar dryers. Drying experiments were performed between April and June 2013 in Medan city (tropical rainforest climate, Indonesia) and have been classified into three groups. The first group is continuous solar drying integrated with zeolite X, the second group is continuous solar drying integrated with solid CaCl₂, and the third group is intermittent direct sun drying. Experiments showed that while the traditional direct sun drying required 55 h of intermittent drying, the use of zeolite X and that of solid CaCl₂, reduce the drying time to 41 h and 30 h, respectively. Besides, the authors pointed out the specific energy consumptions for direct sun drying, solar dryer integrated with solid CaCl₂, and solar dryer integrated with zeolite X. Their values were 60.4 MJ/kg of water, 18.94 MJ/kg of water, and 13.29 MJ/kg of water, respectively. Therefore, the authors concluded that a solar dryer integrated with sorption thermal storage makes drying using solar energy more effective not only in terms of drying time but also in terms of specific energy consumption. Zeolite X, with a 25.45% reduction in drying time, was, therefore, less efficient than solid calcium chloride (45.45%).

A 45% reduction in drying time was also reported by Phadke et al. [90] who used indicating silica gel as thermal storage in an indirect solar dryer. Indicating silica gels are silica gels that are capable of changing color depending on their state of charge (full of adsorbate or dry). This makes the monitoring of the state of a desiccant bed easier. Phadke et al. [90] used an indicating
silica gel which becomes pink when it absorbs moisture and blue when it is dry (Figure 5). In the study and for a constant air flow rate of 0.026 kg/s, the desiccant bed was regenerated in about 5 h during a normal day with solar energy. According to the authors, the quality of the grapes obtained from the solar dryer was excellent with good colors and taste in comparison to those dried directly under the sun.

<figure><img src="image_5.png" /><figcaption>Figure 5: Indicating silica gel bed used as thermal storage in a forced convection solar dryer in India [90].</figcaption></figure>

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Overall, and compared to dehumidification, thermal storage seems to bring a slight advantage in reducing the drying time of solar dryers. A range of 30 to 45% reduction in drying time has been reported by the different studies.

Apart from the nature of the material used for either dehumidification or thermal storage, the arrangement of the material within the dryer and sometimes even its conditioning or design innovations have often been influencing factors in the performance mentioned in this section. A review of these different configurations is made in the following section.

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# 4. Design aspects of the use of sorption materials in solar dryers

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## 4.1. Use of sorption materials as dehumidifier
In solar dryers where sorption materials are used as a dehumidifier, sorption beds are mostly located outside the drying chamber, in the path of the drying air. Figure 6 summarizes some configurations of this type encountered in experimental studies that have been reported in the literature.
<figure><img src="image_6.png" /></figure>

1. Solar collector

2. Air entrance to the collector

3. Drying chamber

4. Liquid desiccant bed

5. Connecting tube

6. PV Panel

7. Regeneration system's pump

8. Distribution pipe

<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /><figcaption>Figure 6: Some configurations of sorption beds located outside the drying chamber for air dehumidification (a) [76] (b) [91] (c) [92]</figcaption></figure>
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Let's take for example the dryer experimented by Misha et al. [70,71] (Figure 7). The material, silica gel, is packed in a rotating wheel with a diameter of 550 cm and a thickness of 20 cm. A special feature of this dryer is that it uses both water and air as heat transfer fluids. The desiccant wheel is regenerated with hot air produced through a heat exchanger between hot water coming from the evacuated tubes solar collector and the ambient air. No-loads experiment showed that the desiccant wheel performed poorly.

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<figure><img src="image_9.png" /></figure>

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<caption>Figure 7: Set up of an experimental indirect solar dryer equipped with silica gel wheel as dehumidifier and using both water and air as heat transfer fluids [70,71].</caption>

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Its thermal, regeneration and dehumidification effectiveness was 48%, 29%, and 6%, respectively. The authors noticed that the poor performance of the desiccant wheel might be improved by increasing the regeneration temperature above 65 °C, since the regeneration temperature appeared to range from 49 °C to 54 °C throughout the experiment, whereas the minimum regeneration temperature for better performance is 65 °C according to the manufacturer. Furthermore, the collector had an efficiency of 70%, and the heat transfer through the copper coil between the hot water from the solar collector and the water in the storage tank
was poor. In fact, despite the average temperature of the hot water from the tube solar collector was around 84 °C, the temperature of water in the tank reached approximately 65 °C. When used to dry 155 kg of kenaf fibers [70] the dryer exhibits a 24% gain in drying time compared to the performance of sun drying. However, this same dryer when used to dry 231 kg of crushed oil palm leaves [71] shows a 64% reduction in drying time. It highlights the influence of the product on the performance of a given dryer incorporating sorption materials.

In Figure 6a, Dorouzi et al. [76] used a bed of calcium chloride solution (concentration of 30% by mass) as a liquid desiccant. The desiccant bed was a 0.2 × 0.3 × 0.5 m galvanized iron chamber. The exhaust air from the drying chamber passes over the desiccant bed where part of its moisture is absorbed by the desiccant material. The desiccant regeneration process, when needed, was carried out by a PV-T solar collector which was of the free-flow type. It allowed the liquid to flow over the surface of the PV panel. A distribution pipe feeds the liquid desiccant uniformly over the surface of the solar panel so that it flows freely downwards and is regenerated by absorbing the accumulated heat from the panel. The concentrated solution is then fed to the desiccant chamber through the return pipe. During operation, when the relative humidity of the drying air raised above a given value, a relative humidity control unit activated the regeneration pump to re-concentrate the diluted solution. This increased the moisture absorption potential of the drying bed and caused the relative humidity of the drying air to drop back below the set point. The tests were carried out at three levels of drying air temperature (60, 65, and 70 °C) and at three levels of regeneration system pump relative humidity activation (18, 23, and 28%), which were selected based on the literature and the tests, respectively. For the drying of 500 g of tomato slices, the authors reported that the drying time was 27% shorter when applying the activation relative humidity of 18% instead of 28%. Besides, the increase of the drying air temperature from 60 to 70 °C leads to a reduction (about 10%) of the drying time.

Elsewhere, Kabeel and Abdelgaied [91] used a rotating silica gel wheel in a solar dryer as a dehumidifier. In their work, the cross-section of the wheel is divided into two parts (Figure 6 b). The first part has a central angle of 240° and is for air-drying. The second one with a central angle of 120° is dedicated to air regeneration. Therefore, the solar drying unit consists of two air paths, the air-drying path (1–3) and the air regeneration path (3–5). The drying air follows the path giving as follows: from state 1, it passes through the desiccant wheel, where its moisture is removed and the temperature is increased since the process is exothermic. Then through the solar collector, this hot and dry air is significantly heated from state point 2 to state point 3. A part of the drying air leaving the solar air collector is heated in an auxiliary air heater
from state 3 to state 4 for regeneration purposes. Finally, this warm air flows through the desiccant wheel to desorb the water vapor and regenerate the desiccant wheel and leaves at state 5. It appears from this work that the use of part of the hot air from the solar collector for the regeneration of the desiccant wheel results in a reduction of the operating cost of the dryer.

Punlek et al. [68] played on the geometric shape of the sorption bed. More precisely, the shape of the horizontal silica gel used in their study was optimized by simulation, and a V-shape bed was found more suitable from a technical point of view for the dryer (Figure 8).

<figure><img src="image_10.png" /><figcaption>Figure 8: V-Shaped silica gel bed integrated as a dehumidifier in a hybrid solar dryer designed to dry ginger slices in Thailand [68]. Adapted from food and bioproducts processing 87 (2009), Chantana Punlek, Rattanachai Pairintra, Sirinuch Chindaraksa, Somchai Maneewan, Simulation design and evaluation of hybrid PV/T assisted desiccant integrated HA-IR drying system (HPIRD), Pages 77–86, Copyright (2009), with permission from Elsevier.</figcaption></figure>

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With these optimization works, they were able to dry ginger slices to their safe moisture content after 4 h in an indirect solar dryer, which resulted in a 50% reduction in drying time (4 h).
But although the general trend in the literature regarding the use as dehumidifier of sorption materials is to locate the sorption bed before the drying chamber, Padmanaban et al. [93] in their work, reported a configuration in which the dehumidifier bed is located at the outlet of the solar collector (Figure 9). In their study, and unlike the other works where the dehumidification beds are used, and where the air intake into the dryer after sunset is stopped, the air intake is maintained at sunset. Here, a thermal storage based on phase-change materials located below the flat plate solar collector heats the air during the night and then the dehumidifier bed reduces its relative humidity. From an initial moisture content of about 52% (wb), the authors were capable of drying the samples of copra to a final moisture content (wb) of about 8% in 62 h. This represents a gain of 42 h compared to conventional solar dryers, which have a drying time of about 104 h.

<figure><img src="image_11.png" /><figcaption>Figure 9: Sorption bed placed at the inlet of the drying chamber – Desiccant assisted packed bed solar dryer for copra processing: use as a dehumidifier in the study of Padmanaban et al. [93].</figcaption></figure>

## 4.2. Use of sorption materials as thermal storage

Most studies using sorption materials for thermal storage have the sorption bed above the drying chamber. The typical configuration encountered in these case studies is shown in Figure 10. In this configuration, the sorption material is often packaged in small spherical bags and arranged on a tray perforated at several locations at the bottom. The perforated tray is covered with glass so that the solar radiations come in direct contact with the material. It is placed above the drying
chamber at a tilting angle close to that of the flat plate collector used. During daytime operation, while the hot air from the flat plate collector is used to dry the products stored in the drying chamber, the sorption bed is regenerated at the same time, with the incident solar radiation. During this phase of operation, the drying chamber is isolated from the tray containing the sorption material with e.g. plywood as done by Shanmugam et al. [87,88]. The moisture removed from the material is usually extracted and discharged to the surrounding through a valve (Figure 10b). At nightfall, the admission of fresh air in the dryer is stopped and the insulation inserted between the sorption bed and the drying chamber is removed. At this stage, a fan powered by a battery previously charged with a solar panel during the day is used to recirculate the air confined in the drying chamber on the sorption bed. Exchanges between the air and the material occur through the perforated holes at the bottom of the tray containing the material, and thus, the drying process continues. A limitation of this configuration is that it usually involves additional electricity consumption at night for the recirculation of the confined air. Batteries for the storage of electricity are therefore required for a proper operation of the dryer.
<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /></figure>

<figure><img src="image_14.png" /></figure>

<figure><img src="image_15.png" /><figcaption>Figure 10: Examples of sorption beds positioned on the top of the drying chamber of a solar dryer. (a) [90], (b) [87], (c) [78], (d) [77].</figcaption></figure>
Another challenge to consider is the position of the sun during the day. It can be seen that in the systems that have been listed, the bed is fixed in a given orientation and tilting. Depending on the position of the sun during the day, the bed receives less or more radiation. This influences the dynamics of regeneration in the bed. To mitigate this problem, Shanmugam et al. [87] integrated a system of reflecting mirrors in their prototype (Figure 11). Two reflective mirrors have been placed on the two sides of the sorption bed that are in the same orientation as the solar collector. These two mirrors reflect and concentrate solar radiation onto the bed to accelerate the regeneration process.

<figure><img src="image_16.png" /><figcaption>Figure 11: System of two reflective mirrors integrated on a sorption bed to improve the regeneration of the material [87].
Adapted from Applied Thermal Engineering 27 (2007), V. Shanmugam, E. Natarajan, Experimental study of regenerative desiccant integrated solar dryer with and without reflective mirror, Pages 1543–1551, Copyright (2006), with permission from Elsevier.</figcaption></figure>

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The sorption bed average temperature with the reflective mirror reached 86 °C at noon. This temperature is about 10 °C higher than the temperature without a reflecting mirror. Consequently, a higher drying potential of the sorption material is achieved and the temperature of the sorption bed is kept above the ambient temperature even the next morning. The mirrors
of Shanmugam et al. [87] are fixed in 90° positions, a tilt angle that might not be suitable with all the sun positions during the day.

Dina et al. [89] tested another configuration for thermal storage. In their design, the thermal storage was placed in an open steel container whose dimension is 30 cm×30 cm×5 cm (Figure 12) and placed just below the drying tray containing the product to be dried.

<figure><img src="image_17.png" /><figcaption>Figure 12: Sorption bed located inside the drying chamber - Application of adsorbent (zeolite X) as thermal storage in a natural convection indirect dryer for cocoa beans drying [89]. Adapted from Case Studies in Thermal Engineering 5 (2015), Sari Farah Dina, Himsar Ambarita, Farel H. Napitupulu, Hideki Kawai, Study on effectiveness of continuous solar dryer integrated with desiccant thermal storage for drying cocoa beans, Pages 32–40, Copyright (2014), with permission from Elsevier.</figcaption></figure>

The sorption bed is removable, which offers several advantages, especially concerning the regeneration of the material. Indeed, during the day, the bed can be removed from the drying chamber and exposed anywhere to maximize the solar radiation in case the regeneration takes place in the course of the sun. If this is not the case, regardless of the means of regeneration, the bed removability is still an advantage for controlling the bed's regeneration conditions outside the drying chamber, particularly the temperature, the humidity, and the regeneration air flow rate. In this case, Dina et al. [89] removed the sorption bed from the drying chamber during the day and allowed it to regenerate just by exposing it to sunlight. For small installations, this configuration may be feasible; even more so when there is a need to experiment several materials on the same dryer as Dina et al. [89]. However, an issue here is the heat losses resulting from the opening and closing of the drying chamber during the operations related to the replacement of the sorption bed. This could influence the efficiency of the dryer. A solution would be to opt for a non-removable bed, which would not require the opening and closing of the drying chamber as proposed by Phadke et al. [90]. On a prototype of an indirect solar dryer
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with forced convection, the authors integrated into the drying chamber, beds of indicating silica gel to serve as thermal storage (Figure 13).

<figure><img src="image_18.png" /><figcaption>Figure 13: Sorption bed attached to the walls of the drying chamber –: Forced convection solar dryer integrated with thermal energy storage units based on gravel and rectangular indicating silica gel beds in India [90]</figcaption></figure>

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The rectangular silica gel beds are attached on three sides inside the drying chamber with an industrial adhesive, so that during sunny hours, while the temperature inside the dryer rises, they also get rid of their moisture together with the products in the drying chamber. In the non-sunny times, as the ambient air is still being admitted in the dryer, when the temperature of the drying air decreases, the heat from the gravels is recovered and the air re-heated. After the blower is stopped at night, and there is no more admission of ambient air in the drying chamber, moisture from the products is now removed by the silica gel bed, which then acts as thermal storage. From the experiment, it came out that for a constant air flow rate of 0.026 kg/s, the desiccant beds were regenerated in about 5 h during a normal day.

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**5. Discussion and recommendation for future work**

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In this paper, many investigations on the use of sorption materials in solar dryers for performance improvement have been reviewed. These solar dryers have been proved technically more beneficial than classical solar dryers, specifically from the point of view of drying time and energy efficiency. With reference to these investigations, the following points can be highlighted (Figure 14):
<figure><img src="image_19.png" /><figcaption>Figure 14: Worldwide distribution of studies reported in the literature regarding the use of sorption materials in solar dryers as thermal storage (TS) or dehumidifier (DH)</figcaption></figure>

1. As for the use of sorption materials for dehumidification in solar dryers:

- Silica gel is by far the most used sorption material even though some applications of calcium chloride aqueous solution [76] have been found;

- Sorption beds are all located outside the drying chamber with a predominance for the location before the solar collector, except in a case where thermal storage is integrated into the solar collector and the air intake was not stopped at night.

- The incorporation of a dehumidification unit based on sorption material in a solar dryer usually leads to a reduction of the drying time of 15 to 30% compared to operation without the sorption material in the dryer, though values up to 50% [68] and even 64% [71] have been reported;

- The reduction of the drying time is highly affected by the product to bed dried;

- For the regeneration of the regeneration material, solar energy is the preferred resource. For a continuous regeneration, some authors proposed a rotating desiccant wheel [91], with the use of a part of the hot air from the solar collector for the regeneration;
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* Forced convection solar dryers have been of great interest for authors since the presence of the sorption bed creates resistance (pressure drop) that is not favorable to natural convection [74,75];

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## 2. As for the use of sorption materials for thermal storage in solar dryers:

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* Composite materials have been of great interest. More precisely, the mixture of bentonite, CaCl₂, vermiculite, and cement in a mass ratio of 6:1:2:1 was used several times. Authors [78], reported that the material was capable of regeneration at 45°C and could adsorb 45% of its mass. Besides composite materials, some applications of indicating silica gel [90], zeolite X [89], and solid CaCl₂ [89] were also found;

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* Sorption beds are most of the time located at the top of the drying chamber. This makes it possible to naturally recirculate the air confined in the drying chamber across the sorption bed and thus perpetuate the drying process after sunset and also absorbed maximum solar energy for regeneration. However, some authors [90] preferred to attach the sorption beds to the sidewalls of the drying chamber to take advantage of the hot air from the solar collector during the day and use it for regeneration purposes. Others [89] have used removable sorption beds, which offers several advantages, especially with regard to the regeneration of the material. Indeed, during the day, the bed can be removed from the drying chamber and exposed anywhere to maximize the solar radiation;

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* The integration of thermal storage based on sorption materials in a solar dryer usually leads to a reduction of the drying time in a range of 30 to 45% compared to the operation without thermal storage;

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* Some innovations regarding the regeneration procedure by solar energy were found in the literature, namely the inclusion of a set of two mirrors on the sorption bed placed at the top of the drying chamber. The inclusion of reflective mirrors increases the drying potential of the desiccant material by 20% and therefore reduces the drying time of the dryer [88].

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In both cases:

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* Little place for liquid desiccant has been found in the works reported in the literature. Instead, solid materials have been of great interest because they offer better handling, a property which is very important for use in solar dryers [62–64];
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* Only one numerical work was found, namely that of Hodali et al. [67] who numerically studied the optimized coupling between the collector, the sorption bed and the drying chamber for a dehumidification use;

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* Since silica gel is expensive, composite materials could be of great interest. In this regard, recent advances in materials for thermochemical and sorption storage which mostly focused on long-term heat storage but with several limitations could find here a field of application. The mixture of bentonite, CaCl₂, vermiculite, and cement in a mass ratio of 6:1:2:1 is promising. However, there is a need for further studies in order to better understand the material and to evaluate economic aspects, which were barely discussed in the literature.

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Finally, the most important challenges on the use of sorption materials in solar dryers systems include:

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* The elaboration of a comprehensive methodology for sizing the quantity of sorption material to be used according to the use (thermal storage or dehumidification), the climatic conditions of dryer operation, the targeted drying air conditions, and the product to be dried;

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* The identification or the development of cost-effective pure or composite sorption materials, for instance naturally and locally available materials.

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## 6. Conclusion

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The objectives of this review were to identify the current status of the research on the use of sorption materials in solar dryers and to provide future perspectives for better applications of such materials in solar dryers. From the literature review, the most commonly used materials for dehumidification and thermal storage were identified (as silica gel and CaCl₂-based material respectively), and the main conclusions and results regarding the various applications of sorption materials in solar dryers were highlighted. Although, further studies are needed to better understand influence of sorption materials on solar dryers' performances, typical drying time reductions compared to operations without sorption storage and dehumidification unit are 30-45% and 15-30%, respectively. This review has revealed the need to develop a comprehensive methodology to size the amount of sorption material to be used depending on the application (thermal storage or dehumidification), the operating climatic conditions of the dryer, the targeted drying air properties, and the product to be dried. Considering that composite materials have exhibited good results in various experiments in the literature, it appears
568 important to work on identifying or developing cost-effective pure or composite sorption
569 materials naturally and locally available.

570 Acknowledgement

571 This work was supported by the German Academic Exchange Service (DAAD) [grant numbers
572 57506852, 2019].

573
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831
Worldwide distribution of studies reported in the literature regarding the use of sorption materials in solar dryers as thermal storage (TS) or dehumidifier (DH)

<figure><img src="image_20.png" /></figure>

TYPICAL TIME-SAVINGS

<figure><img src="image_21.png" /></figure>

<figure><img src="image_22.png" /></figure>

Thermal storage: 30-45%

<figure><img src="image_23.png" /></figure>

Dehumidification: 15-30%

MATERIALS USED (in order of interest)

<figure><img src="image_24.png" /></figure>

Thermal storage:

(1)- Bentonite-clay mixture,

(2)- Silica gel,

(3)- Solid CaCl₂

Dehumidification:

(1)- Silica gel,

(2)- CaCl₂ solution

<figure><img src="image_25.png" /><figcaption>Solid materials</figcaption></figure>