A comparative analysis on the uniformity enhancement methods of solar thermal drying

Accepted Manuscript A Comparative analysis on the Uniformity Enhancement Methods of Solar Thermal Drying

Shaymaa Husham Abdulmalek, Morteza Khalaji Assadi, Hussain H. Al-Kayiem, Ali Ahmed Gitan PII:

S0360-5442(18)30071-9

DOI:

10.1016/j.energy.2018.01.060

Reference:

EGY 12172

To appear in:

Energy

Received Date:

02 November 2017

Revised Date:

13 December 2017

Accepted Date:

11 January 2018

Please cite this article as: Shaymaa Husham Abdulmalek, Morteza Khalaji Assadi, Hussain H. AlKayiem, Ali Ahmed Gitan, A Comparative analysis on the Uniformity Enhancement Methods of Solar Thermal Drying, Energy (2018), doi: 10.1016/j.energy.2018.01.060

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Author group and affiliation Shaymaa Husham Abdulmalek1,a, Morteza Khalaji Assadi1,b, Hussain H. Al-Kayiem1.c, Ali Ahmed Gitan2,d,e, 1 Universiti

Teknologi PETRONAS, Faculty of Engineering, Mechanical Engineering Department, Solar Thermal Advanced Research Center, Bandar Seri Iskandar, Perak, Malaysia. 2

Tikrit University, Faculty of Engineering, Mechanical Engineering Department, Tikrit, Iraq.

[email protected], [email protected], [email protected], [email protected] [email protected], b

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A Comparative analysis on the Uniformity Enhancement Methods of Solar Thermal Drying Abstract: The uniformity of solar drying process and the quality of the product are inter-related parameters. Drying uniformity is influenced by the significant process air properties which are temperature, humidity and velocity. Accordingly, solar drying uniformity may be improved by integration with dehumidification system and/or optimizing the dryer design. These concepts were reviewed extensively in this paper by brush up the solar thermal hybrid dryers, the performance of solar assisted desiccant systems for dehumidification of drying air, the effect of geometrical parameters on drying performance, and the drying performance of different products. In the context of desiccant systems, the performance of drying is influenced by desiccant material, dehumidifier design and regeneration technique used. While, the issue of solar dryer design is related to drying chamber geometrical parameters, considering multiple drying chambers, and modeling and optimization of dryer design. Coming out with this comprehensive review may motivate to enhance the quality of product and drying performance in terms of cost and time. 1. Introduction 2. Solar Thermal Hybrid Drying Systems 3. Performance of Solar Assisted Desiccant Dryers 3.1. Effect of desiccant material 3.1.1. Solid desiccant material 3.1.2. Liquid desiccant material 3.2. Modeling effect of desiccant system on performance 3.3. Effect of dehumidifier design 4. Regeneration techniques of desiccant system 4.1. Integrated desiccant–solar collector regeneration system 4.2. Waste heat energy desiccant regeneration system 4.3. Solar air heater desiccant regeneration system 4.4. Solar water heater desiccant regeneration system 4.5. Ultrasonic desiccant regeneration system 4.6. Liquid desiccant regeneration systems 4.7. Phase change materials regeneration system 5. The Impact of Geometrical Parameters on Uniform Drying 5.1. Solar dryer design 5.1.1. Conventional chimney type drying chamber 5.1.2. Unconventional drying chamber design 5.2. The consideration of multiple dryers 5.3. Modeling and optimization of geometrical parameters 1

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6. Drying Product 7. Conclusion Acknowledgements References 1.

INTRODUCTION

The ability of save the food or any agricultural/fishery products, especially in countries with various seasons, is essential for long time storage and preservation. In developing countries, lack of suitable technology, such lack of marketing channels, improper transportation, high post-harvest losses, etc., causing a food loss from 10 to 40% [1]. To save the food for long time without deterioration, it should be dried and the moisture content should be reduced to a certain preselected level. Such drying process requires large energy consumption in terms of electrical energy or fusil fuel burning to supply the necessary thermal energy. Drying process is an energetic intensive activity and plays a significant role in many industrial applications such as food, textile and paper and in many other processing industries. One of the main heat sources that employed to reduce the conventional energy consumption for drying process is the solar energy. Basically, this method has been utilized traditionally for food drying purpose since long time. In many countries, open sun drying (OSD) is used in a small farms to produce 80% of dried food [2]. There are several disadvantage of open sun drying like dirt, wind, animals, insects and the occasional rain [3]. Solar dryer systems (SDS) have been used instead of open sun dryer this system is supply renewable energy and environmental friendly. Various studies reported that national energy consumption for industrial drying operations ranging from 10-15% for USA, Canada, France, and UK and around 20-25% for Denmark and Germany [4]. Significant energy saving can be achieved as efficient drying systems are designed by employing new techniques such as low energy dehumidification systems. Many previous works have come out with various approaches in order to reduce the energy consumption and enhance the performance of drying process. However, over 85 % of industrial dryers are of the convective type with hot air or direct combustion gases as the drying medium [4]. Within the framework of green technology performance development, solar drying uniformity is an important aspect that requires extensive investigation. The objective of the current paper is to develop one reference platform for the researchers who are intending to develop the uniformity and product quality of solar drying based on the temperature, the humidity and the velocity of the process air. Also, this work aims to presents the relevant techniques to these air properties which have a significant effect on the uniformity and performance of solar drying. These techniques include solar thermal hybrid drying, solar assisted desiccant dryers (SADD) and drying chamber design. Also, the paper presents comparative that allow the researchers to have insight information on the various enhancing techniques attempted previously.

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2.

SOLAR THERMAL HYBRID DRYING SYSTEMS

The hybrid solar thermal dryer consists of solar dryer connected with thermal back-up unit (TBU). The thermal back-up unit consists of burner and heat exchanger. The solar part is working with availability of solar energy within the day. The biomass heater is working when the absence of solar energy to maintain the continuity of drying process with uniform level of process air temperature. Maintaining uniform process air temperature is one objective of enhancing the solar drying uniformity and performance. A hybrid drying system was investigated experimentally with two new modifications of exhaust heat re-injection technique and thermal recovery sub-dryer [5], the experimental results demonstrated that the dryer in hybrid mode drying shortened drying time by 11.2 % compared with solar mode drying. The dryer in solar drying mode shortened the drying time by 76 % compared with open sun drying. In thermal mode, the exhaust heat reinjection unit enhanced the overall drying efficiency of red chilli by 6 %. In the hybrid and thermal modes, the recovery sub-dryer enhanced the overall drying efficiency of drying ginger by 23.7 % and 30.7 %, respectively. Furthermore, the recovery sub-dryer enhanced the overall drying efficiency of the system with respect to drying red chilli by 25.84 % and 29.7 % through the hybrid and thermal modes, respectively. Design, development and evaluation were performed for an indirect kind of natural convection solar dryer of integrated collector-storage solar and biomass backup heaters [6], The testing was carried out in three modes of operation (biomass, solar and solar–biomass), The values on average of the final-day moisture pickup efficiency were 15%, 13% and 11% in the solar, solar– biomass, and biomass modes of operation, respectively. A natural convection solar greenhouse dryer combined with a biomass thermal back-up heater was investigated experimentally [7], The material used for the flue gas ducts is 2 mm thick galvanized iron sheet metal. These ducts allow for the passage of the hot flue gas from the heater into the air being circulated for drying the product in the dryer. The gas then passes by way of the chimney to the outside atmosphere. It was found that, for drying coconuts it required 88 hours in open sun drying, 44 hours in solar mode, 30 hours in thermal mode, and 26 hours in hybrid solar thermal mode. A hybrid forced convection dryer system, which combines an unglazed transpired solar collector, rock bed, and a biomass gasifier stove with heat exchanger to provide the heat to the drying chamber was designed and study experimentally. The system was evaluated by drying chilli using air at 60 °C and flow rate of 90 m3/h. The temperature of hot air supplied was stable at 60 ±3 °C for about 21 hours during the entire drying duration [8]. The common practice was concluded in the hybrid solar dryers which are backed up with thermal source is to exhaust the flue gas to the ambient [9], This flue gases are still hot and carry considerable amount of thermal energy as waste. Also, the thermal energy of flue gas from a biomass thermal backup unit was utilized in terms of heat recovery criteria. A prototype of hybrid solar-thermal drying system was coupled with recovery dryer to yield a combination of the main dryer and the recovery dryer. The combination was investigated experimentally to evaluate the 3

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enhanced performance compared to the system without recovery. The investigations were conducted under two operational modes, hybrid mode (day and night) and thermal mode alone (night). Red chilli was utilized as drying material. The results of the thermal mode showed that the overall drying efficiency of the dryer was increased from 9.9 % without recovery dryer to 12.9 % with the recovery dryer. The overall drying efficiency of the hybrid drying without recovery dryer was 10.3 %, while it was increased to overall drying efficiency of 13% in the case of using hybrid dryer and recovery. The enhancement of the overall drying efficiency due to the recovery dryer was 25.84 % in the hybrid day and night drying, and was 29.7 % in the night thermal drying mode. This validated enhancement encourages the use of sub dryer as thermal recovery to optimize the utilization of fuel, and to increase the system capacity. In summary, different works which adopted hybrid drying systems are characterized in Table 1.

3.

PERFORMANCE OF SOLAR ASSISTED DESICCANT DRYERS

Numerous researchers have been done to achieve the efficient drying with less energy and less time with high product quality. Accordingly, significant energy saving can be achieved by employing new techniques such as low energy dehumidification systems. The desiccant material used in the dehumidification systems adsorbs moisture from process air and gets saturated with time and become less efficient, therefore it needs to be regenerated by a heat source. Solar radiation is used to regeneration the desiccant material in drying application. Solar assisted desiccant system (SADD) is used to dry the product with low temperature drying and continues drying even through in the absence of solar radiation. In other words, desiccant systems can provide low and uniform levels of humidity in the process air so and enhance the drying uniformity and performance. The dehumidification process of desiccant wheel dehumidifier is shown in Figure 1.

3.1. Effect of desiccant material The desiccant material has the ability to adsorb high levels of humidity from process air which is the demand of the uniform and efficient solar drying. Also, desiccants desorb humidity easily at low temperature through regeneration process. The desiccant materials may be classified into two main groups which are solid and liquid desiccant materials. Each kind of these materials has its own dehumidification technique. Basically, liquid desiccant systems are more complicated than the solid desiccant dehumidifiers [10]. However, it is convenient here to review how the desiccant materials affect the performance of drying.

3.1.1. Solid desiccant material The solid desiccant dehumidification has been reviewed for air conditioning systems [11]. Several solid desiccant materials have been considered such as silica gel, activated alumina, synthetic 4

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zeolite, composite materials, biodesiccants and activated carbons. The micro-porous structure of solid desiccant is an important property in considering the ability of adsorbing moisture from humid air. However, solid desiccant materials considered for solar drying application may have slightly different consideration regarding healthy dried products such as food drying. Also, the hot dried air is required for drying process unlike in air conditioning application where the dried air is preferred to be cold. The effect of composite desiccant material based on silica gel on solar drying performance was investigated [12, 13]. Silica gel composite has shown better drying performance than pure silica gel desiccant. Ten most commonly of the desiccant materials have been tested for optimum dehumidification process [14]. Of the different desiccants, in Molecular Sieve the silica gels B, 3A and RD always achieve higher levels of performance than other desiccants for air dehumidification under most operating conditions. The performance of continuous solar drying was investigated by considering two desiccant materials as a heat storage system in addition to their dehumidification function [15]. Molecular sieve 13 × (Na86 [(AlO2)86. (SiO2)106] 264H2O) as an adsorbent type and CaCl2 as an absorbent type. Comparing with the open sun drying, the adsorbent desiccant type has reduced the drying time by 25% while the absorbent desiccant type performed around 45% in drying time reduction. 3.1.2. Liquid desiccant material Liquid desiccant systems have attracted many researchers due to its low regeneration temperature in addition to its ability of absorbing organic and inorganic contaminants from the air [16]. The liquid desiccant dehumidifiers are used in many air conditioning systems [17] and in industrial applications such as drying of compressed air [18] and gelcast green bodies [19]. However, the effect of using liquid desiccant materials on product quality were considered for crop solar drying [20]. The results showed that these kinds of dehumidification systems can reduce the cracked seeds by 7% under low temperature drying of 45oC and 55oC. various liquid desiccant materials have been analyzed thermodynamically [21]. The three most commonly used liquid desiccant solutions, namely LiCl, LiBr and CaCl2 were evaluated against each other. The analysis has revealed that LiCl liquid desiccant system performs better and more stable than those using other candidates. Noteworthy outcomes of related works are listed in Table 2.

3.2. Modeling effect of desiccant system on performance The performance prediction of desiccant system may be affected by the mathematical model applied. In this context, the effect of model modification on performance prediction of novel silica gel haloid compound desiccant wheel has been investigated [22]. The developed model includes the effect of gas side resistance and the solid side resistance. It is found that this model agree better to the experiments than the former model which does not consider the solid side resistance. Based on artificial neural network technique, the modeling of desiccant wheel performance has been established [23]. Using a neural network toolbox of MATLAB with feed forward back propagation 5

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method, a neural network model has been developed based on the experimental observations on desiccant wheel. Several feed forward back propagation algorithms have been tested for the modelling of desiccant wheel. A training algorithm with least mean square error (MSE) represents the optimum model. The results of analyzing all training algorithms revealed that the training algorithm trainlm (Levenberg–Marquardt back propagation) is found most suitable.

3.3. Effect of dehumidifier design Different design configurations have been investigated in the previous works in order to enhance the performance of drying. Due to the simplicity in design and reduction in fabrication cost, the solid desiccant systems are used most commonly in solar food drying. Basically there are several solid desiccant systems used in various applications such as fixed bed, belt, axial flow wheel, radial flow wheel, cross flow bed and multi-pass wheel [10]. In the context of product quality and drying performance, rotary desiccant when integrated in hot air drying system has been studied [24, 25]. The rotary desiccant wheel is divided into adsorption and regeneration sections that work simultaneously by means of continuous rotation between the process humid air and a heated regeneration air stream. Silica gel is placed equally in four quarter of the wheel as shown in Figure 2 [24]. In comparisons with the pure hot-air system, the combined hot-air desiccant system has achieved shorter drying time considerably by about 25%. In comparison with the solar dryer without using a desiccant wheel, using of rotary desiccant wheel in the solar drying units increased the temperature of drying air from 65 oC to 82 oC while the humidity ratio decreased from 15 to 8.8 gwater/kgdry air [25]. A dehumidification system that is configured as desiccant beds has been used in solar drying in order to enhance the dryer performance for continuous drying process [26]. The desiccant system of silica gel beds (SGB) integrated into solar drying system is illustrated in Figure 3. Two modes of experiments have been carried out, namely with-dehumidification and without-dehumidification modes. The results exhibited reduction in drying time by around 20% when the dehumidification system is integrated.

4. REGENERATION TECHNIQUES OF DESICCANT SYSTEM The principles of dehumidification by using of desiccant materials are based on the ability of these materials to adsorb the moisture from process air. However, it is required to remove the adsorbed moisture from the desiccant material by regeneration process in order to maintain high dehumidification performance. Without regeneration process, the desiccant performance reduces gradually with process time until reaching equilibrium condition. Consequently, the absence of regeneration process causes non-uniform humidity levels in process air and hence affects the uniformity and performance of drying. Table 3 summarizes different techniques employed to regenerate the desiccant wheel used in different applications. For solar drying, the drying 6

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uniformity, performance and product quality depend on process air temperature which is related to the desiccant dehumidification rate and subsequently on regeneration characteristics such as regeneration temperature and flow rate. Various techniques have been utilized for regeneration of different desiccant systems [27]. 4.1. Integrated desiccant–solar collector regeneration system The desiccant material has been regenerated by locating it inside a solar air collector within tilted bed [28, 29] as shown in Figure 4. Metal mesh can be used to lay the desiccant bed on it in order to allow the generation air pass through the desiccant bed. In the regeneration process, the moisture removing rate augments with irradiation and decreases with air flow rate.

4.2.

Waste heat energy desiccant regeneration system

Saving energy issue motivates the researchers to exploit the waste heat energy from different sources in regeneration of desiccant systems. The regeneration of desiccant dehumidifier was carried out by utilizing microturbine-generator (MTG) exhaust gas [30]. In order to enhance drying efficiency, the condenser waste heat from refrigerator cycle has been employed to drive moisture off of the desiccant wheel dehumidification system [31, 32]. Using this hybrid heat pump-desiccant drying system, 30%-60% of energy can be saved. Based on a natural gas-fired reciprocating internal combustion engine, the waste heat from a microcogenerator has been utilized for regeneration of desiccant wheel dehumidifier [33, 34].

4.3. Solar air heater desiccant regeneration system The hot air required for desiccant regeneration can be produced by the solar air heater. Solar air heater with a compound parabolic concentrator collector (CPC) has been investigated for regeneration of desiccant dehumidification system [35]. The solar air heater has produced a regeneration temperature of 40 oC and 50 oC at flow rate of 0.03 kg/s and 0.003 kg/s respectively. The measurements revealed that using CPC solar air heater can produce hot air at 10 oC above the ambient air temperature at cloudy weather and 50 oC above the ambient temperature on sunny day.

4.4. Solar water heater desiccant regeneration system Solar energy can be employed to heat up the air by using solar water collectors. Both flat water solar collector [36] and parabolic trough solar collector (PTC) [37] can be used for regeneration of desiccant dehumidification system. A heat exchanger based on nanofluids can improve the heat transfer from the hot water and desiccant material for regeneration purpose. Solar energy was used 7

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to heat water with a solar collector and heat was transferred to the air through two heat exchangers to dry crushed oil palm fronds. The hot air is used to regenerate the desiccant and to increase temperature of drying air after dehumidification [38]. Low solar radiation was used to regenerate the dehumidifier to dry the kenaf core fiber [39].

4.5. Ultrasonic desiccant regeneration system An ultrasonic technology as a non-heating method can be employed for removing of moisture and improve drying process. The power ultrasonic induces an oscillating and acoustic microstreaming that will decrease the boundary layer thickness of moisture adjacent to the solid–gas interface and reduce the resistance of external mass transfer [40]. The ultrasonic method can be combined with another heating method for better performance of the desiccant system.

4.6. Liquid desiccant regeneration systems Different design concepts have been addressed for liquid desiccant regeneration systems in previous works. A tilted solar regeneration system has been developed to regenerate calcium chloride desiccant solution [41].The weak desiccant solution flows as a thin film over a blackened corrugated surface where its temperature increases while it moves down to the bottom. Thus, the concentration of desiccant solution increases due to evaporation of water. The results showed that the optimum liquid to air flow rate ratio is about 2.54. Another liquid desiccation regeneration system employs solar water collector combined with air heater [42]. The heated water from the storage tank of the solar heating system is circulated in a finned tube. Hot air from the air heater is blown through a packing of a honeycomb type for the purpose of regeneration of calcium chloride (CaCl2) solution. The test results exhibited that steady state operation condition can be achieved when a storage tank is used with the solar collector.

4.7. Phase change materials regeneration system Some dehumidification systems have been regenerated using phase change materials (PCM) as a heat storage system. Encapsulated phase change materials (EPCM) with silica gel desiccant particles have been placed in one dehumidification bed to enhance the performance of dehumidification system [43]. Mathematical modelling based on heat and mass transfer and numerical solution method have been proposed to express the dehumidification process through desiccant/EPCM bed. Careful choices of EPCM volume fraction and thermo physical characteristics have been found to increase the overall effectiveness of the desiccant dehumidifier with negligible loss in the dehumidification efficiency. Utilizing PCM as a heat storage was extended to liquid desiccant rather than solid desiccant. This technique has been employed in Liquid desiccant air conditioning (LDAC) systems to heat the weak solution using the stored energy before 8

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entering the thermal regenerator during off-sun hours [44]. The concentration of the spent solution at the thermal regenerator outlet varied between 26.91% to 27.06% (wt/wt). A fabric-PCMDesiccant model is developed to predict the temperature and moisture content of the microclimate air layer in the presence of a PCM-Desiccant packet [45]. The developed model is validated through experiments conducted on a wet clothed heated cylinder for the two cases of using (i) a PCM only packet and (ii) a PCM-Desiccant packet. Microclimate air temperatures and humidity content as well as PCM and desiccant temperatures were measured experimentally and were compared with predicted values by the fabric-PCM-Desiccant model. Good agreement was attained with a maximum relative error of 7% in measured temperatures. A decrease is observed in the humidity content of the microclimate air in the presence of the solid desiccant from 21.23 g/kg dry air to 19.74 g/kg dry air and an increase in the melted fraction of the PCM at the end of the experiment from 0.24 to 0.5.

5.

THE EFFECT OF GEOMETRICAL PARAMETERS ON UNIFORM DRYING

One of the challenges in solar dryer design problems is the drying uniformity which indicates to the efficient system and leads to high quality product. The design parameters of the solar drying system affect the drying uniformity significantly. For example air velocity distribution should be considered in drying chamber design in order to ensure uniform final moisture content of the dried products on the trays [39]. However, other parameters such as the arrangement of drying system components may influence the performance of drying. In this context, multi drying beds or even multi drying chambers require specific consideration in order to secure uniform drying.

5.1. Drying chamber design Drying chamber is the place where the product to be dried is located. Non open sun solar dryers have been classified according to the working principles. Three groups of solar dryers have been presented, namely direct dryer, indirect dryer and the integrated dryer [46]. In direct solar dryers, the product is exposed to the solar radiation directly in the drying chamber. On the other hand, the drying is carried out by air heated through solar heater in advance before entering the drying chamber. In contrast, integrated dryer utilizes solar heated air and direct solar radiation to dry the product. Indirect solar drying may maintain higher product quality after drying [47]. However, the design of drying chamber still under research in order to achieve uniform drying and increase drying performance.

5.1.1. Conventional chimney type drying chamber Chimney type drying chamber is commonly used for passive drying (natural convection mode) as shown in Figure 5 [48]. The updraft hot air is employed to dry the product located on one or more 9

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trays that are arranged vertically at the bottom of the chimney [49-51]. Chimney type drying chamber fed from multi pass solar air heater has been designed for deep bed drying application [52]. The challenge was to perform high quality drying with deep drying bed. It has been found that the air humidity and the drying rate increase with the increase in the product bed depth. The efficiency of a solar dryer can be improved further when the drying chamber is designed with an angled roof (the so called tent-dryer effect) [53]. Solar conduction dryer based on solar chimney concept is presented to produce dried whole and sliced turmeric rhizomes, The device utilizes solar power in a conductive manner as well as convective way for drying [54]. The product is placed on black painted treys distributed on two sides with a chimney at the middle as depicted in Figure 6. The drying is occurred due to the direct solar radiation falling on the product through transparent sheet cover in addition to the absorbed energy by the treys. However, some active drying systems (forced convection mode) have same chimney design [47, 55]. More focusing on structural design is required to ensure uniform air flow rate through the crop bed.

5.1.2. Unconventional drying chamber design The unconventional drying chamber design denotes to the drying chamber that does not follow chimney configuration in its design. A recirculation type integrated collector drying chamber (ICDC) solar dryer has been proposed for grain drying application [56]. The drying chamber is structured as a transparent cylinder contained a hopper inside as depicted in Figure 7. The hot air supplied from a stove is blown in order to carry the grain which circulate inside the drying chamber until get dried. This method is not suitable for all products due to the action of circulation that need low weight small particles and may destroy the original configuration of the product. A semi cylindrical shape solar dryer has been proposed for general orientations [57]. As shown in Figure 8, the drying chamber is a part of the semi cylindrical tunnel. The greenhouse type drying chamber has been proposed for efficient drying in both passive mode [58, 59] and active mode [38, 60, 61] as depicted in Figure 9 and Figure 10. In the context of drying uniformity, experiments have been conducted to prove the effect of unconventional drying chamber design on solar drying uniformity. The trays have been arranged vertically in three columns as shown in Figure 11 [38]. The numbers represents the position of the trays. The process air passes through each column laterally. The data collected from three experiments represent the moisture content of the product in three trays per column as presented in Table 4. The experimental results shows prove of drying uniformity per each column where the moisture content was around 28, 53 and 57 % in the first, second and third column respectively. In the context of maintaining homogenous drying conditions, a rotary column cylindrical drying chamber has been proposed for hygienic agricultural products [62] as shown in Figure 12. The rotating column contains holes to allow air to escape from the chamber. More hygienic product and uniform drying characteristics are obtained due to the rotation of the drying chamber. For 10

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continuous drying mode, an integrated desiccant solar drying chamber has been introduced as an indirect active solar dryer on sun hours and as closed desiccant drying chamber on sun off hours [63] as illustrated in Figure 13. In the context of maintaining uniform temperature distribution inside the drying chamber, the walls of drying chamber need to be insulated insulation material such as glass fibers [64]. Drying chamber for drying osmotically dehydrated cherry tomatoes with horizontal air flow over vertical arranged sieve-type trays has integrated with solar source heat exchanger [65]. This design has the advantage of allowing uniform distribution of air temperature in the drying cabinet as exhibited in Figure 14. The drying chamber is protected efficiently from insects, dust and rain, and good quality product can be achieved. A compound vertical and horizontal (mix mode) air flow has been modified for indirect solar dryer [66] as shown in Figure 15. Multi tray rack is used to arrange the trays vertically with different air flow direction either in vertical or in horizontal direction. However, an extensive comparison study between the effects of both flow directions on drying performance still uncovered in the literatures.

5.2. The consideration of multi chamber dryers The target of drying uniformity can be achieved only if the process air could flow over whole product bed area with same characteristics as long as the solar drying is occurred by the action of air flow (indirect solar drying mode). Logically, the process air properties may be influenced by the spent air of other part of product load. This interaction between process air and spent air could be controlled by developing proper design of drying chamber. Using multi chamber dryer may be considered to solve this problem or to enhance the uniformity of drying as a final result. Multi drying chamber of staircase type has been presented as a simple design of direct solar dryer [67] as shown in Figure 16. In this passive dryer type, the drying air flows through product trays in series. In other words, the spent air from the first tray is the process air for the next tray and so on. Consequently, the process air characteristics change downstream which subsequently lead to non-uniform drying. Also, a multi shelf drying chamber has been proposed to allow drying process to be performed locally at farm or even at home [68, 69]. The shelves are arranged one above the other on a tilted rack as illustrated in Figure 17. The multi shelf dryer have same working principles of the former staircase dryer. Therefore, the uniformity of drying still not perfectly improved. Multi trey drying chamber with vertical arrangement of treys is investigated [70, 71]. The hot air provided by solar air heater passes through four perforated treys in series from the bottom to the top of drying chamber as exhibited in Figure 18. As explained previously, this kind of multi drying chamber lacks a modification in its design to solve the problem of drying uniformity with large load and high quality product.

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5.3. Modeling and optimization of geometrical parameters The target parameters in solar drying optimization problem can be summarized by minimizing energy, cost, drying time and improving the quality of the products [72]. The optimization by numerical simulation has been carried out to optimize the operational and physical parameters of adsorption desiccant unit integrated in solar dryer for apricot drying [73]. The adsorption model has been defined by five non-linear partial differential equations that were employed to cover the adsorption kinetics and the simultaneous energy and mass transfer balances in order to investigate five basic parameters. These five variables are drying air temperature, desiccant temperature, interstitial velocity of gas flux through the adsorbent bed, the air absolute humidity, and molar ratio of sorbate/sorbent. An optimization of dryer geometry has been carried out for mixed-mode and indirect-mode natural convection solar dryers for maize [74]. The optimization procedures have been performed by running the simulation at different sets of physical parameters namely, collector length L, collector and drying chamber width W and grain depth B in order to give dryer cost. The optimization was based on minimizing dryer cost and getting the geometrical parameters that achieve this target. The drying cost (𝐶𝑑) is given as the ratio of annual cost (𝐶𝑎) to quantity of grain dried per annum (𝑄𝑔) as: 𝐶𝑑 =

𝐶𝑎

(1)

𝑄𝑔

The annual cost is defined as

(

𝑁

)𝜔(𝜔(𝜔 ‒ ‒1)1)

𝑖

𝐶𝑎 = 𝐶𝑇 + ∑𝑖 = 1𝑚𝑖𝜔

(2)

𝑁

The quantity of grain dried per annum is calculated from the throughput of the dryer per day and the number of days in a year the dryer is used and is given by (3)

𝑄𝑔 = 𝑄𝑑𝐷𝑎

In the context of improving drying quality and reducing drying time, a parametric study has been accomplished in order to optimize the design of solar drying of timber [75].

6.

DRYING PRODUCTS

The most important parameter from the drying process is the product quality. In this context study the performance of drying to investigate the uniformity of drying process system is the aim of this review article. Various types of products are gained from different solar drying systems which integrated with another heat source as listed in .

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First of all, the biomass has been used as a backup heater in the drying system to configure a solar-biomass dryer that has been suggested by many researchers to enhance the overall drying efficiency. Some researchers have used direct type natural convection solar dryer with simple biomass burner to dry fruit and vegetables in non-electrified areas of developing countries by fabricate experimental work. A compression done by using three mode, drying by using open sun, drying using the solar drying system and finally drying using hybrid system solar-biomass[6, 76, 77]. In these papers further improve the performance of both the solar and biomass components of the dryer are suggested. It found that the biomass supply useful heat to the drying by 27%. The results show that the hybrid system has low drying time [77]. The rectangular duct and flue gas chimney have been added to the burner to remove the gas and be as thermal storage with a conventional solar chimney[6]. To reduce consumption of biomass by ingesting solar energy application has been present, by Computational fluid dynamics technique is used to simulate the temperature and air flow distributions in the drying chamber [78]. The presented articles dry different type of product and different parameter present in these papers such as, drying efficacy and the drying time all the details summarize in Table 1. In the previous articles the evaluation of product quality has been done to test the ability of these systems to achieve the uniformity of the drying during the day and the night and in the same tray [6, 76-78]. By using PCM as heat storage and adding swirl element to the drying chamber to optimum distribution of air inside the drying room to achieve the uniform and continues drying even during the absence of solar radiation experimental have been carried out by [79]. The drying experiments have been carried out by both under natural conditions and by the dryer with swirl flow and without swirl flow at three different air velocities. The obtained moisture ratio values have been applied to six different moisture ratio models in the literature. The model is evaluated according to the value of (R & x2) which related to the thin layer model. Comparing with other model having the highest correlation coefficient (R) and the lowest Chi-square (×2) value has been determined as the most relevant one for each seeded grape drying status. A blower was used to force the heated air to the drying chamber for an indirect-mode forced convection solar dryer was designed and fabricated [80, 81]. The products dried in this work with the properties are present in . It is found that, Midilli and Kucuk model is convenient to describe the thin layer solar drying of mint [80]. However, the Page and modified Page models were found to be the best among others for describing the drying curves of thymus [80, 82]. Red seaweed dried and the system presented analytically by energy and exergy analysis [82], Seaweed is can be converted into energy such as biofuel oil, the product properties illustrate in . The first and second law of thermodynamics used for energy and exergy analysis of the solar drying process to study the performance for red chili and the (SEC) specific energy consumption equal to 5.26 kwh/kg [83], The drying process show in Figure 19 . 13

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The wasted heat from air conditioning system used to dry timber, the values 5.7 kg humidity was taken from the product at the end of 50h and12.5kg from the poplar timber at the end of 70h [84]. In this paper the results shows that the Specific humidity is a very significant factor in the psychometric analysis. A semi-continuous mode was performed to reduce the drying time by applying new technology in solar drying system by using desiccant material. In this study test the uniformity of the product have been done in different trays and different time. But the result show that it less quantity of product in the drying chamber. The drying rate at full capacity was 8.37 kg/h, which is twice that of open sun drying (4.23 kg/h).The hot air from the heat exchanger is used to regenerate the desiccant material [38]. 7.

Conclusion

This paper is focusing on the enhancement of drying uniformity and performance of solar dryers by considering the effect of thermal backup, dehumidification and design parameters of drying chamber. The related works of hybrid solar thermal dryers, each aspect of dehumidification using desiccant systems and drying chamber design have been reviewed extensively. The conclusion drawn from the review is:    



Biomass thermal backup can be integrated with solar dryer to achieve uniform drying in terms of process air temperature. Desiccant system needs to be considered extensively in terms of providing uniform humidity levels in solar dryers. The most important parameters involved in uniformity of dehumidification process are desiccant material type, dehumidifier design and regeneration method. The physical design of the solar dryer plays important rule in the drying uniformity and hence the product quality. The design parameters of drying chamber, the multi dryer issue, and optimization of modeled design parameters are strongly related to the enhancement of drying performance. The quality of the drying product is a main objective in designing an efficient solar drying system. Some lacking still need deep consideration such as the direction of process air inside the drying chamber and its effect on drying uniformity. Also, multiple drying chamber solar assisted desiccant dryer may have another scenario regarding the enhancement of drying performance and product quality.

Acknowledgment The authors would like to acknowledge the financial and logistic support to conduct the present work. The Solar Thermal Advanced Research center (STARc) in UTP is highly acknowledged for providing the needed facilities and space to carry out the work. 14

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References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.

Ong, K.S., Renewable Energy Energy Efficiency, Policy and the EnvironmentSolar dryers in the Asia-Pacific region. Renewable Energy, 1999. 16(1): p. 779-784. Murthy, M.V.R., A review of new technologies, models and experimental investigations of solar driers. Renewable and Sustainable Energy Reviews, 2009. 13(4): p. 835-844. Thoruwa, T.F.N., et al., Developments in Solar Drying Using Forced Ventilation and Solar Regenerated Desiccant Materials. WREC, 1996: p. 686-689. Mujumdar, A.S., Handbook of Industrial Drying. 2015, CRC Press: Boca Raton, Florida, USA. p. 1348. YASSEN, T.A., DEVELOPMENT AND EVALUATION OF SOLAR THERMAL HYBRID DRYING SYSTEM. 2016, Universiti Teknologi PETRONAS: Perak, Malaysia. Madhlopa, A. and G. Ngwalo, Solar dryer with thermal storage and biomass-backup heater. Solar Energy, 2007. 81(4): p. 449-462. Lokeswaran, S. and M. Eswaramoorthy, Experimental Studies on a Solar Drier System with a Biomass Back-up Heater. Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, 2013. 35(5): p. 467-475. Leon, M.A. and S. Kumar, Design and Performance Evaluation of a Solar-Assisted Biomass Drying System with Thermal Storage. Drying Technology, 2008. 26(7): p. 936-947. Yassen, T.A. and H.H. Al-Kayiem, Experimental investigation and evaluation of hybrid solar/thermal dryer combined with supplementary recovery dryer. Solar Energy, 2016. 134: p. 284293. Misha, S., et al., Review of solid/liquid desiccant in the drying applications and its regeneration methods. Renewable and Sustainable Energy Reviews, 2012. 16(7): p. 4686-4707. Sultan, M., et al., An overview of solid desiccant dehumidification and air conditioning systems. Renewable and Sustainable Energy Reviews, 2015. 46: p. 16-29. Jia, C.X., et al., Experimental comparison of two honeycombed desiccant wheels fabricated with silica gel and composite desiccant material. Energy Conversion and Management, 2006. 47(15–16): p. 2523-2534. Chua, K.J., Heat and mass transfer of composite desiccants for energy efficient air dehumidification: Modelling and experiment. Applied Thermal Engineering, 2015. 89: p. 703-716. Zhang, L.-Z., et al., Performance comparisons of honeycomb-type adsorbent beds (wheels) for air dehumidification with various desiccant wall materials. Energy, 2014. 65: p. 430-440. Dina, S.F., et al., Study on effectiveness of continuous solar dryer integrated with desiccant thermal storage for drying cocoa beans. Case Studies in Thermal Engineering, 2015. 5: p. 32-40. Gandhidasan, P., A simplified model for air dehumidification with liquid desiccant. Solar Energy, 2004. 76(4): p. 409-416. Rafique, M.M., P. Gandhidasan, and H.M.S. Bahaidarah, Liquid desiccant materials and dehumidifiers – A review. Renewable and Sustainable Energy Reviews, 2016. 56: p. 179-195. Yin, Y., et al., A proposed compressed air drying method using pressurized liquid desiccant and experimental verification. Applied Energy, 2015. 141: p. 80-89. Wang, X.-f., et al., Liquid drying of BeO gelcast green bodies using ethanol as liquid desiccant. Transactions of Nonferrous Metals Society of China, 2015. 25(7): p. 2466-2472. Alizadeh, S., et al., A Study of Solar Crop Drying System Using Liquid Desiccant. Canadian Journal on Environmental, Construction and Civil Engineering, 2012. 3(2): p. 63-70. Koronaki, I.P., et al., Thermodynamic analysis of a counter flow adiabatic dehumidifier with different liquid desiccant materials. Applied Thermal Engineering, 2013. 50(1): p. 361-373. 15

ACCEPTED MANUSCRIPT

22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43.

Ge, T.S., F. Ziegler, and R.Z. Wang, A mathematical model for predicting the performance of a compound desiccant wheel (A model of compound desiccant wheel). Applied Thermal Engineering, 2010. 30(8–9): p. 1005-1015. Parmar, H. and D.A. Hindoliya, Artificial neural network based modelling of desiccant wheel. Energy and Buildings, 2011. 43(12): p. 3505-3513. Madhiyanon, T., et al., Integration of a rotary desiccant wheel into a hot-air drying system: Drying performance and product quality studies. Chemical Engineering and Processing: Process Intensification, 2007. 46(4): p. 282-290. Kabeel, A.E. and M. Abdelgaied, Performance of novel solar dryer. Process Safety and Environmental Protection, 2016. 102: p. 183-189. Chramsa-ard, W., et al., Performance Evaluation of the Desiccant Bed Solar Dryer. Energy Procedia, 2013. 34(0): p. 189-197. Cheng, Q. and X. Zhang, Review of solar regeneration methods for liquid desiccant air-conditioning system. Energy and Buildings, 2013. 67: p. 426-433. Lu, S.M., et al., Development and experimental validation of two novel solar desiccantdehumidification-regeneration systems. Energy, 1995. 20(8): p. 751-757. Techajunta, S., S. Chirarattananon, and R.H.B. Exell, Experiments in a solar simulator on solid desiccant regeneration and air dehumidification for air conditioning in a tropical humid climate. Renewable Energy, 1999. 17(4): p. 549-568. Zaltash, A., et al., Laboratory R&D on integrated energy systems (IES). Applied Thermal Engineering, 2006. 26(1): p. 28-35. Wang, W.C., R.K. Calay, and Y.K. Chen, Experimental study of an energy efficient hybrid system for surface drying. Applied Thermal Engineering, 2011. 31(4): p. 425-431. Finocchiaro, P., M. Beccali, and B. Nocke, Advanced solar assisted desiccant and evaporative cooling system equipped with wet heat exchangers. Solar Energy, 2012. 86(1): p. 608-618. Angrisani, G., et al., Desiccant wheel regenerated by thermal energy from a microcogenerator: Experimental assessment of the performances. Applied Energy, 2011. 88(4): p. 1354-1365. Angrisani, G., et al., Experimental analysis on the dehumidification and thermal performance of a desiccant wheel. Applied Energy, 2012. 92: p. 563-572. Pramuang, S. and R.H.B. Exell, The regeneration of silica gel desiccant by air from a solar heater with a compound parabolic concentrator. Renewable Energy, 2007. 32(1): p. 173-182. Beccali, M., P. Finocchiaro, and B. Nocke, Energy performance evaluation of a demo solar desiccant cooling system with heat recovery for the regeneration of the adsorption material. Renewable Energy, 2012. 44: p. 40-52. Abu-Hamdeh, N.H. and K.H. Almitani, Solar liquid desiccant regeneration and nanofluids in evaporative cooling for greenhouse food production in Saudi Arabia. Solar Energy, 2016. 134: p. 202-210. Misha, S., et al., Performance of a solar-assisted solid desiccant dryer for oil palm fronds drying. Solar Energy, 2016. 132: p. 415-429. Misha, S., et al., Performance of a solar assisted solid desiccant dryer for kenaf core fiber drying under low solar radiation. Solar Energy, 2015. 112: p. 194-204. Yao, Y., W. Zhang, and B. He, Investigation on the kinetic models for the regeneration of silica gel by hot air combined with power ultrasonic. Energy Conversion and Management, 2011. 52(11): p. 3319-3326. Elsarrag, E., Evaporation rate of a novel tilted solar liquid desiccant regeneration system. Solar Energy, 2008. 82(7): p. 663-668. Alosaimy, A.S. and A.M. Hamed, Theoretical and experimental investigation on the application of solar water heater coupled with air humidifier for regeneration of liquid desiccant. Energy, 2011. 36(7): p. 3992-4001. Rady, M.A., et al., Study of heat and mass transfer in a dehumidifying desiccant bed with macroencapsulated phase change materials. Renewable Energy, 2009. 34(3): p. 718-726. 16

ACCEPTED MANUSCRIPT

44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56.

57. 58. 59. 60. 61. 62. 63. 64. 65.

Guo, Y., et al., Development and performance evaluation of a liquid desiccant air conditioning system with hybrid electrodialysis and thermal regeneration. 2016. Itani, M., N. Ghaddar, and K. Ghali, Innovative PCM-desiccant packet to provide dry microclimate and improve performance of cooling vest in hot environment. Energy Conversion and Management, 2017. 140: p. 218-227. Sharma, A., C.R. Chen, and N. Vu Lan, Solar-energy drying systems: A review. Renewable and Sustainable Energy Reviews, 2009. 13(6–7): p. 1185-1210. Vijayan, S., T.V. Arjunan, and A. Kumar, Mathematical modeling and performance analysis of thin layer drying of bitter gourd in sensible storage based indirect solar dryer. Innovative Food Science & Emerging Technologies, 2016. 36: p. 59-67. Bhardwaj, A.K., et al., Experimental investigation of an indirect solar dryer integrated with phase change material for drying valeriana jatamansi (medicinal herb). Case Studies in Thermal Engineering, 2017. Ayensu, A., Selected Proceeding of ISES 1995: Solar World Congress. Part IVDehydration of food crops using a solar dryer with convective heat flow. Solar Energy, 1997. 59(4): p. 121-126. Afriyie, J.K., et al., Mathematical modelling and validation of the drying process in a ChimneyDependent Solar Crop Dryer. Energy Conversion and Management, 2013. 67(0): p. 103-116. Ziaforoughi, A. and J.A. Esfahani, A salient reduction of energy consumption and drying time in a novel PV-solar collector-assisted intermittent infrared dryer. Solar Energy, 2016. 136: p. 428-436. Jain, D. and R.K. Jain, Performance evaluation of an inclined multi-pass solar air heater with inbuilt thermal storage on deep-bed drying application. Journal of Food Engineering, 2004. 65(4): p. 497-509. Afriyie, J.K., et al., Experimental investigations of a chimney-dependent solar crop dryer. Renewable Energy, 2009. 34(1): p. 217-222. Borah, A., K. Hazarika, and S.M. Khayer, Drying kinetics of whole and sliced turmeric rhizomes (Curcuma longa L.) in a solar conduction dryer. Information Processing in Agriculture, 2015. 2(2): p. 85-92. Mohanraj, M. and P. Chandrasekar, Drying of copra in a forced convection solar drier. Biosystems Engineering, 2008. 99(4): p. 604-607. Abu Bakar, R.b., et al., 2nd International Conference on Sustainable Energy Engineering and Application (ICSEEA) 2014 Sustainable Energy for Green MobilityPerformance of a Recirculation Type Integrated Collector Drying Chamber (ICDC) Solar Dryer. Energy Procedia, 2015. 68: p. 5359. Garg, H.P. and R. Kumar, Studies on semi-cylindrical solar tunnel dryers: estimation of solar irradiance. Renewable Energy, 1998. 13(3): p. 393-400. Perea-Moreno, A.-J., A. Juaidi, and F. Manzano-Agugliaro, Solar greenhouse dryer system for wood chips improvement as biofuel. Journal of Cleaner Production, 2016. 135: p. 1233-1241. Koyuncu, T., An Investigation on the performance Improvement of greenhouse-type agricultural dryers. Renewable Energy, 2006. 31(7): p. 1055-1071. Condorı́, M., R. Echazú, and L. Saravia, Solar drying of sweet pepper and garlic using the tunnel greenhouse drier. Renewable Energy, 2001. 22(4): p. 447-460. Condorı́, M. and L. Saravia, Analytical model for the performance of the tunnel-type greenhouse drier. Renewable Energy, 2003. 28(3): p. 467-485. Sarsilmaz, C., C. Yildiz, and D. Pehlivan, Drying of apricots in a rotary column cylindrical dryer (RCCD) supported with solar energy. Renewable Energy, 2000. 21(2): p. 117-127. Shanmugam, V. and E. Natarajan, Experimental investigation of forced convection and desiccant integrated solar dryer. Renewable Energy, 2006. 31(8): p. 1239-1251. Yahya, M., et al., Comparison of solar dryer and solar-assisted heat pump dryer for cassava. Solar Energy, 2016. 136: p. 606-613. Nabnean, S., et al., Experimental performance of a new design of solar dryer for drying osmotically dehydrated cherry tomatoes. Renewable Energy, 2016. 94: p. 147-156. 17

ACCEPTED MANUSCRIPT

66. 67. 68. 69. 70. 71. 72. 73. 74. 75. 76. 77. 78. 79. 80. 81. 82. 83. 84. 85.

Blanco-Cano, L., et al., Modeling the thin-layer drying process of Granny Smith apples: Application in an indirect solar dryer. Applied Thermal Engineering, 2016. 108: p. 1086-1094. Hallak, H., et al., The staircase solar dryer: Design and characteristics. Renewable Energy, 1996. 7(2): p. 177-183. Singh, S., P.P. Singh, and S.S. Dhaliwal, Multi-shelf portable solar dryer. Renewable Energy, 2004. 29(5): p. 753-765. Singh, P.P., S. Singh, and S.S. Dhaliwal, Multi-shelf domestic solar dryer. Energy Conversion and Management, 2006. 47(13–14): p. 1799-1815. Jain, D., Modeling the system performance of multi-tray crop drying using an inclined multi-pass solar air heater with in-built thermal storage. Journal of Food Engineering, 2005. 71(1): p. 44-54. Sekyere, C.K.K., F.K. Forson, and F.W. Adam, Experimental investigation of the drying characteristics of a mixed mode natural convection solar crop dryer with back up heater. Renewable Energy, 2016. 92: p. 532-542. McDoom, I.A., et al., Optimization of solar crop drying. Renewable Energy, 1999. 16(1–4): p. 749752. Hodali, R. and J. Bougard, Integration of desiccant unit in crops solar drying installation: optimization by numerical simulation. Energy Conversion and Management, 2001. 42: p. 15431558. Simate, I.N., Optimization of mixed-mode and indirect-mode natural convection solar dryers. Renewable Energy, 2003. 28(3): p. 435-453. Khater, H.A., et al., Optimization of Solar Kiln for Drying Wood. Drying Technology, 2004. 22(4): p. 677-701. Prasad, J. and V.K. Vijay, Experimental studies on drying of Zingiber officinale, Curcuma longa l. and Tinospora cordifolia in solar-biomass hybrid drier. Renewable Energy, 2005. 30(14): p. 20972109. Bena, B. and R.J. Fuller, Natural convection solar dryer with biomass back-up heater. Solar Energy, 2002. 72(1): p. 75-83. Sonthikun, S., et al., Computational fluid dynamic analysis of innovative design of solar-biomass hybrid dryer: An experimental validation. Renewable Energy, 2016. 92: p. 185-191. Çakmak, G. and C. Yıldız, The drying kinetics of seeded grape in solar dryer with PCM-based solar integrated collector. Food and Bioproducts Processing, 2011. 89(2): p. 103-108. El-Sebaii, A.A. and S.M. Shalaby, Experimental investigation of an indirect-mode forced convection solar dryer for drying thymus and mint. Energy Conversion and Management, 2013. 74(0): p. 109116. Fudholi, A., et al., Performances and improvement potential of solar drying system for palm oil fronds. Renewable Energy, 2015. 78: p. 561-565. Fudholi, A., et al., Energy and exergy analyses of solar drying system of red seaweed. Energy and Buildings, 2014. 68, Part A(0): p. 121-129. Fudholi, A., et al., Performance analysis of solar drying system for red chili. Solar Energy, 2014. 99(0): p. 47-54. Ceylan, I. and A. Ergun, Psychrometric analysis of a timber dryer. Case Studies in Thermal Engineering, 2014. 2(0): p. 29-35. Ali Mandegari, M. and H. Pahlavanzadeh, Introduction of a new definition for effectiveness of desiccant wheels. Energy, 2009. 34(6): p. 797-803.

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Table 1. Hybrid system in drying application Shorten the drying time System type

Exhaust heat re-injection technique Sub-drying through heat recovery Indirect natural convection Natural convection Hybrid forced convection Hybrid solar dryers with heat recovery

Solar (comparing with open sun)

Thermal

Hybrid 11.2% comaring with solar mode

76 %

27.5 %

Efficiency upon mode Type of Product

Thermal

hybrid

Red chili, ginger, waste palm oil and fish

30.7%

23%

YASSEN,T.A., 2016 [5]

Red chili

25.84%

29.7%

YASSEN,T.A., 2016 [5]

13%

11%

Pineapple

62 %

30 hours

66 %

References

26 hours

Coconuts

21 hours

Red chili at 60˚C Red chili

Solar

15%

Madhlopa, A. and G. Ngwalo, 2007 [6] Lokeswaran, S. and M. Eswaramoorthy, 2013 [7] Leon, M.A. and S. Kumar, 2008 [8]

9.9%

10.3%

Yassen, T.A. and H.H. Al-Kayiem, 2016 [9]

Table 2. Liquid desiccant material in different application Material lithium chloride (LiCl) LiCl solution Ethanol Liquid desiccant solution Liquid desiccant LiCl, LiBr and CaCl2 were evaluated against each other.

Users/recommender In air dehumidification ,the simple expression derived can be used to estimate the water condensation rate significant energy saving potential compared with the conventional compressed air cooling drying system In drying application, the green body changes from compressive stress to tensile stress In drying application, less energy consumption and better quality at(45 °C - 55 °C) Dehumidifier, liquid desiccant systems using LiCl seem to have better performance than those using LiBr and CaCl2,at Moisture ratio(15.9-2.8)%

19

References P. Gandhidasan, 2004 [16] Yonggao Yin, 2015 [18] Xiao-feng WANG, 2015 [19] Alizadeh, 2012 [20] I.P. Koronaki, 2013 [21]

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Table 3. Regeneration Techniques of Desiccant System Process parameters Regeneration Techniques

Application

A/C, drying

Desiccant material type

Insolation (W\m2)

Silica gel

-

3,4,7

Silica gel

300, 500, 700

Techajunta, S., S. Chirarattananon, 1999 [29]

Reg. temperature (°C)

Reg. time (hr)

Process air flow (kg\s)

Reg. flow rate

-

-

-

-

References

Lu,S.M., et al, 1995 [28]

Solar energy

Air-conditioning

100

6

Natural gasfired mode

Dehumidification

124

-

-

-

Single wheel

-

Zaltash, A., et al, 2006 [30]

Solar energy

Air-conditioning

72

2

0.007

0.15

Silica gel bed

300

Pramuang, S. and R.H.B. Exell, 2007 [35]

Solar energy

Air conditioning

100

-

-

-

Calcium chloride Solution

-

Elsarrag, E, 2003[41]

Solar energy

Air conditioning

59–144

-

580.8 (m3/h)

237.9 (m3/h)

Single wheel

-

Ali Mandegari, 2009 [85]

Air humidifier

-

-

-

-

-

Calcium chloride solution

-

Alosaimy, A.S, 2011[42]

Micro cogenerator

HVAC systems

65

-

-

2 - 60 (l/min)

Silica gel wheel

-

Angrisani, G., et al, 2011 [33]

Condenser coil

Drying

-

-

-

-

Desiccant wheel

-

Ultrasonic technology

Drying

-

-

-

-

Silica gel wheel

-

Solar energy

HVAC systems

-

-

-

-

Wheel

-

Beccali, M., 2012 [36]

Solar energy

Cooling

-

-

-

-

Wheel

-

Finocchiaro, P, 2012 [32]

Solar energy

Food production

-

-

-

-

Calcium chloride solution

-

Abu-Hamdeh , 2016 [37]

20

Wang, W.C., R.K. Calay, 2011 [31] Yao, Y., W. Zhang, and B. He, 2011 [40]

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Table 4. Final moisture content of the fronds sample for each experiment [38].

Table 5. The performance of solar drying of different products

Load (kg)

System Type Product

Initial moisture content (%)

Final Initial temp. moisture (°C) content (%)

Drying chamber temp. (°C)

140

Natural convection

Pineapple 66.9(db)%

-

11(db)%

41-56

-

Natural convection

Coconut

-

9.2

-

-

8.4

Thymus 95% and 85% and mint w.d

29

Red seaweed

90% w.d

Red chili

100

IP Drying up to Drying Efficiency 10% Time (%) (hr) w.b (W)

Ref.

Madhlopa, A. and G. Ngwalo, 2007 [6] Lokeswaran, S. and M. Eswaramoorthy, 2013 [7] Leon, M.A. and S. Kumar, 2008 [8]

13

-

26

-

-

60± 3

32.5

11.06

-

11 ± 0.5

39-54

34 and 5

-

-

El-Sebaii, A.A., 2013 [80]

30

10% w.d

48.6

15

-

247

[82],2014, Fudholi, A

80%

30

10%

44

33

13%

47.29

Fudholi, A., 2014 [83]

Indirect forced Palm oil convection fronds

60%

33

10%

49

22

19%

-

Dehumidifying Crushed bed of a oil palm desiccant fronds cooling system.

69%

-

29%

-

30 h and 40 mint

19%

-

Misha, S., 2016 [38]

-

In direct force Seeded convection grapes

-

-

-

-

-

-

-

Çakmak, G., 2011[79]

-

-

40

Hybrid forced convection An indirectmode forced convection Forced convection indirect type Forced convection indirect type

Red chili

53.4 76.7

21

172 Fudholi, 2015 [81]

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20-22

18

-

-

100

Hybrid system Natural convection Mixed mode hybrid natural convection Mixed mode hybrid natural convection Mixed mode hybrid natural convection Hybrid in direct force convection

Pineapple

-

-

53%

-

39

9%

-

Bena, B., 2002 [77]

Ginger

319.7(db)%

-

11.8(db)%

-

33

15.59%

-

Prasad, 2005 [76]

Turmeric 358.96(db)%

-

8.8(db)%

-

36

14.74%

-

Prasad, 2005 [76]

Guduehi 257.95(db)%

-

9.69(db)%

-

48

7.5%

-

Prasad, 2005 [76]

Rapper

-

0.34%

-

48

-

-

Sonthikun, 2016 [78]

34.26%

Figure 1. Schematic Diagram of the Counter Flow Desiccant Wheel

Figure 2. Rotating desiccant wheel [24]

22

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Figure 3. Desiccant bed solar dryer [26]

Figure 4. Integrated desiccant-solar collector regeneration system [29]

23

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Figure 5. Solar dryer of chimney type drying chamber [48]

Figure 6. Conduction drying system [54].

24

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Figure 7. Circulating type integrated collector drying chamber [56].

Figure 8. Semi cylindrical solar dryer [57].

25

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Figure 9. Passive greenhouse drying chamber [58].

Figure 10. Active greenhouse type drying chamber [38].

26

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Figure 11. Sensors position and flow diagram of drying air [38].

Figure 12. Rotary column drying chamber [62]

27

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Figure 13. Continuous duty drying chamber. 1, Blower; 2, Flat plate collector; 3, Drying chamber; 4, Insulation; 5, Absorber plate; 6, Bottom plate; 7, Transparent cover; 8, Desiccant bed; 9, Plywood;10, Air inlet; 11, Duct for air exit; 12, Drying trays; 13, Two-way fan; 14, Valve; 15, Plywood. [63]

Figure 14. Drying chamber for osmotically dehydrated cherry tomatoes [65].

28

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Figure 15. Indirect solar dryer with mix mode air flow [66].

Figure 16. Staircase drying chamber [67].

29

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Figure 17. Multi shelf drying chamber [68].

Figure 18. Multi trey drying chamber [70].

30

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Figure 19. Drying process

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How flow uniformity influencing the product drying quality has been clearly brought out. Previously reported works on the flow uniformity influencing in drying systems have been summarized and compared. New design approach for better flow uniformity in hybrid drying system is proposed.