A review on solar tunnel greenhouse drying system

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Renewable and Sustainable Energy Reviews 56 (2016) 196–214

Contents lists available at ScienceDirect

Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser

A review on solar tunnel greenhouse drying system Rajendra Patil a,n, Rupesh Gawande b a b

Department of Mechanical Engineering, Bapurao Deshmukh College of Engineering, Wardha 442102, Nagpur, India Department of Mechanical Engineering, Rashtrasant Tukadoji Maharaj Nagpur University, 440001 Nagpur, India

art ic l e i nf o

a b s t r a c t

Article history: Received 7 August 2015 Received in revised form 9 November 2015 Accepted 22 November 2015

This paper mainly deals with review of research and development work on solar tunnel and greenhouse type dryers operating in natural and forced convection mode by different researchers. The comprehensive explanation, basics and earlier work performed on solar tunnel greenhouse type dryers has been presented briefly. A technical and economical assessment shows that solar tunnel and greenhouse dryers appear the most gorgeous option for use in rural areas. Trials on solar tunnel and greenhouse type dryer show not only massive fuel savings but also great worth addition due to improved quality of dried product in terms of color, aroma and taste. However application of such dryers has not picked up due to higher capital investment, long payback period and lack of confidence in the technology. Nevertheless incessant research and development work should be carried out to overcome these factors. It is hoped that this review work may be valuable and appropriate for further development work. & 2015 Elsevier Ltd. All rights reserved.

Keywords: Greenhouse dryer Tunnel dryer Solar energy

Contents 1. Introduction . . . . . . . . . 2. Previous work on solar 3. Conclusion . . . . . . . . . . References . . . . . . . . . . . . . .

.............................. tunnel and greenhouse type dryers . .............................. ..............................

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1. Introduction Drying is the process of removal of moisture from the product to a specified value due to application of heat energy. In short, drying is a combination of heat and mass transfer [1]. Many drying methods are available for drying of agricultural products. Sun drying is the earliest method of drying agro-commodities and having several disadvantages like spoilage of produce due to wind, dust, rain, insects, birds, storms etc. Also open sun drying requires large land and drying time [2]. Hence to protect agricultural products from damage and to control the drying process, efforts had been made to improve sun drying to solar drying. Solar drying is a technological process and works on principle of greenhouse effect. Basically solar drying can be classified into two types of low temperature and high temperature dryers [3]. Further it may be classified into direct, indirect, mixed mode and n

Corresponding author. Tel.:- +919604825862.

http://dx.doi.org/10.1016/j.rser.2015.11.057 1364-0321/& 2015 Elsevier Ltd. All rights reserved.

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hybrid type. In direct solar dryers, collection of solar energy and drying of product takes place in an enclosed insulated structure. The simplicity and low cost of dryer makes it attractive for end users. However, control of drying temperature and poor quality of dried product are main limitations. In indirect dryers, air is heated in air heater and then flows over a product bed. The better controlled of temperature in dryer results in efficient operation. High cost and complex construction are the drawbacks of such dryer. In mixed mode, products are exposed directly to solar radiations and hot air from solar collector. It is a combination of direct and indirect solar dryers while in hybrid solar dryers, heat energy from solar radiations and fossile fuel or biomass is used for drying the agricultural product [4]. Solar dryers can also be classified according to the movement of air as natural and forced convection dryers. In natural (passive) convection dryer movement of air takes place due to density difference, while in forced convection mode (active) fan or blower is required to move the air through air heater and over the product. Active solar dryers are more efficient

Patil, R. Gawande / Renewable and Sustainable Energy Reviews 56 (2016) 196–214

Roof Cap

197

Exhaust

Solar Radiation

Drying Platform Glass Roof

Shutter Open

Cold Air

Fig. 1. Passive glass – roof type greenhouse dryer.

than passive dryers in terms of speed and quality of drying. Though active dryers are better than passive dryers they are superb for drying of minute batches and not suitable in rural areas due to requirement of power for operating the fans. These limitations along with the fact of reducing drying time, encourages the researchers to build up new designs like tunnel and greenhouse type dryers. Tunnel is an enclosed duct except at entry and exit end and used for heating of air in numerous applications including drying. However greenhouse is a structure used for production of vegetables/flowers along with effective drying of large quantity of agro-commodities. Hence greenhouse structure can be used all over the year making it more reasonable and have low operating cost [5]. The dual function of greenhouse and dryer improves the return rate of investment [6]. Kumar et al. gives the detailed types of greenhouse and suggested the even span and Quonset shape of greenhouse for maximum utilization of solar energy [7]. The solar tunnel and greenhouse comes in the group of direct and mixed type dryers and can be effectively operated in both natural as well as forced convection mode [5]. Greenhouse dryers are also called as tent dryers and basic form of passive greenhouse dryer with inclined glass roof was presented by Brace Research Institute. The schematic of dryer is shown in Fig. 1 [8]. Several researchers have done an excellent review work on solar drying technology for drying of agricultural produce. A comprehensive review of various designs of direct, indirect and mixed mode solar cabinet dryers were presented by Sharma et al. [9] and they mentioned that processing of agricultural produce to a processed food can be a wise alternative to small land holder farmers. Farmers can make extra money for value addition to the product. Bal et al. [10] summarize a detailed review of research and development work on solar dryers with thermal energy storage system for drying of agricultural food products. The thermal, physical, chemical and kinetic property of heat storage materials were discussed for an appropriate selection of material for a particular application. The review study shows that thermal energy storage system increases the utility as well as reliability of solar dryer and latent heat storage system stores more heat per unit volume than sensible heat storage system. Heat storage with phase change material is a smart option. El-Sebaii et al. focused on a numerous types of solar air collectors and solar dryers with and without heat storage media. From the available literature, they conclude that indirect mode forced convection solar dryers are superior in quality and also recommends a more investigation on sensible/latent heat storage media for accelerating the drying rate

[1]. Further, the research work carried by different scientists in the area of solar dryers used for grape drying was reviewed by Pangavhane and Sawhney [11]. The various types of solar dryers were discussed technically as well as economically and showed that solar drying of grape is feasible. They suggest that solar dryer should have maximum utilization factor and recommended forced convection mode of drying for better control and reliability. Fudholi et al. [12] presented an inclusive review of different designs, working principles and details of construction of solar dryers for drying of marine and agricultural products. Study revealed that the technical directions in the upgrading of solar drying systems for marine and agricultural produce are compact collector design, integrated storage, high efficiency and long life of drying system. The author also focused on water based solar collectors whereby water to air heat exchanger can be used. A review article of dome shape and roof even type solar greenhouse dryer was recently presented by Prakash and Kumar [5]. The study reveals that greenhouse dryer under forced convection mode was better for high moisture products and greenhouse drying should be adopted to overcome the restrictions of traditional open sun drying. PV – integrated dryers can be used in remote areas. The main aim of this paper is to summarize the research and development work for solar tunnel and greenhouse type dryers. The comprehensive explanation, basics and earlier work performed on solar tunnel greenhouse type dryers were presented briefly. It is hoped that this review work may be valuable and appropriate for further development work.

2. Previous work on solar tunnel and greenhouse type dryers The solar tunnel drying system is simple in construction and consists of a collector as well as drying tunnel connected in series. Both collector and drying tunnel are covered with transparent glasses or plastic sheets. The collector element contains black color metallic absorber plate while drying chamber has number of trays for loading the product to be dried. On the other hand a greenhouse is basically an enclosed structure covered with stabilized UV resistant polythene sheets, acts itself as a solar collector and drying chamber. Such dryers are used for drying of multiple agricultural products at low cost. The solar tunnel dryer of capacity 100–300 kg was developed in the mid of 1980s at Hohenheim University, Germany and known as Hohenheim solar tunnel dryer. The Hohenheim tunnel dryer unit has been tested in 30 countries

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Patil, R. Gawande / Renewable and Sustainable Energy Reviews 56 (2016) 196–214

Heat absorber Clear fiberglass

Drying Racks Drying Racks Perforated

Plenum Air Ducts

Fig. 2. Green house solar dryer (a) load supporting wall design and (b) shell design (not to scale).

like India, Thailand, Turkey, Uganda and Australia etc. for drying of vegetables, fruits, herbs and indicates that this design is good enough for small land holders in terms of consistency, price and consumer – sociability in operation. Several investigators have explored on solar tunnel and greenhouse dryers for drying of various agricultural products. Huang and Bowes [13] developed two types of greenhouse namely the load supporting wall and shell type solar dryer as shown in Fig. 2. In the load supporting design, exterior as well as interior walls were used to hold tobacco bulk racks and fiber glass was used to transmit the solar radiations. The stagnant air space between the transparent outer surface and the heat absorber acts as an insulation layer. The concrete slab and block foundation served as an additional heat storage system. The corrugated fiberglass was used in shell design to form greenhouse with built-in gravel energy storage system for night time usage. The gravel system was used to store excess solar energy. In this design instead of walls, portable frames were used to support tobacco racks. Results showed that the load supporting wall design was better and able to save 15–25% fuel as compared to the shell type design. On the other hand the shell design showed better saving at 33–51% as compared to the commercial unit. The study showed that drying of tobacco in both designs was feasible and gives better performance due to use of energy storage system. The solar tunnel greenhouse dryer for grape drying has been investigated by many researchers namely Lutz et al., Eissen et al., El-Shiatry et al., Gentry et al. and Fohr and Arnand. Lutz et al. designed and constructed a multi-use solar tunnel dryer for drying of grapes, vegetables and medicinal plants at Greece. For minute holding capacities (50–300 kg) collector and tunnel can be linked in series (Fig. 3), while for large quantities (up to 1000 kg) collector and tunnel of dryer have to be set in parallel (Fig. 4). A radial fan operated by AC motor was used to move the air at 3 m/s over the product. The dryer was tested for drying of 1000 kg of seedless grapes and they found that tunnel dryer took only 4–7 days for removal of moisture up to safe limit of 9% (w.b). The quality of dried grapes was excellent in terms of color, flavor and taste. This dryer was effectively tested in Greece, Egypt, Yugoslavia and Saudi Arabia for drying of grapes, peppers, dates and onions. Finally economic evaluation of solar tunnel dryer gives a payback period in the range of 2–3 years [14]. Instead of polythene foil, author used a costly air bubble foil which increases the cost of drying and payback period. In parallel arrangement of dryer, cost of drying increase due to requirement of baffles to turn the air at require

Air Outlet

Dryer

Collector Frame Fan

Air Inlet Fig. 3. Solar tunnel dryer for small quantities.

Baffle Plate

Dryer Collector Frame Fan

Air outlet Air Inlet

Fig. 4. Solar tunnel dryer for large quantities.

velocity by 180°. Also more power is required to move the air over the product. Eissen et al. [15] considered a similar type of tunnel dryer for drying of 25 kg/m2 of fresh grapes and found that drying was completed in 5–6 days with better quality raisins. The design of solar dryer allows consistent drying and absolute protection of grapes from insects, dust and rain. A solar tunnel dryer (20 m length) for drying of 100–200 kg of food per day under forced convection mode was proposed by ElShiatry et al. [16]. The solar collector (1 m wide) and drying chamber (2 m wide) was arranged in parallel (Fig. 4) to determine drying period, electric power consumption, quality of produce in terms of color, flavor, reconstitution properties, microbial count

Patil, R. Gawande / Renewable and Sustainable Energy Reviews 56 (2016) 196–214

and life expectancy. The dryer was tested for drying of tomatoes, grapes, onions, potatoes, basil and wild marjoram under the metrological conditions of Egypt. A radial fan operated by 150 W AC motor was used to circulate the air over the product. A 70% and 40% of reduction in drying time for grapes and other crops within STD was observed as compared to natural sun drying. The highest electric consumption of 19.5 kW h for drying of 1000 kg of grapes was recorded; while only 4.4 kW h was consumed by radial fan for 200 kg of basil. The reconstitution test shows the excellent result for potatoes and onions while only 75% of initial moisture content was reached by tomatoes. The faster drying rate was achieved since STD receives heat energy from collector as well as from solar irradiations. A parallel arrangement of dryer and high electric consumption increases the cost and payback period of dryer; however a better dried product was obtained. The greenhouse solar dryer for drying of grapes was studied by Fohr and Arnaud [17]. It consists of (Fig. 5) a few trays, fan, plastic film and a collector of 50 m greenhouse allied in face of the grape stack hall. The fan was placed in the rear wall of stacked chamber to achieve consistent air flow over the trays. Result showed that recycling of air was not economically feasible. Amir et al. [18] studied a solar tunnel dryer under forced convection mode for drying of coffee beans. The solar collector and a drying chamber were connected side by side. The centrifugal blower was used to move the hot air into the drying tunnel through U-shaped duct. An air stream rate of 400–900 m3/h was maintained by a centrifugal blower to achieve air temperature in the range of 40–60 °C. For drying of 500–600 kg of coffee beans, dryer took 50 h while traditional open sun drying required 75 h for the same moisture removal. The drying technique for coffee under polythene solar tunnel dryer was also evaluated by Chapman et al. at Thailand. It consists (Fig. 6) of solar collector, a drying chamber and centrifugal blower. The dryer of 7 m  4 m  2 m in size was constructed and Greenhouse

Shed

Transparent film Fan

2m x 2m Stake of trays

Black film

Fig. 5. Solar dryer with greenhouse.

Clear Polyethylene Cover

Drying Table

Fig. 6. Polyethylene solar tunnel dryer.

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enclosed with a polythene sheet of 200 μm. Result showed that moisture removal rate for coffee drying was 1.5 times faster than open air drying and observed an increase in efficiency of 20–40%. An increase in temperature of air by 4–5 °C was also recorded within the dryer. The cost of construction of dryer was about USD 190 while cost to operate the electrical blower was less than 1 USD for 12 h daily. Also Ochratoxin (OTA) forming fungi was not observed in solar dried samples [19]. From above results, STD was a right option for drying of small and medium capacity agricultural products at faster drying rate. Schirmer et al. [20] developed a multi-purpose solar tunnel dryer for drying of banana under active mode at Thailand. The dryer consists of a collector and drying tunnel in which 300 kg of banana is spread in thin layer. Both collector and drying chamber are joined in series and covered with UV plastic sheet. Three fans powered by 53 W PV module were used to ventilate the tunnel dryer. The dryer was capable of producing the sufficient and continuous flow of hot air temperature between 40 and 65 °C. The quantitative examination showed that traditional sun drying had taken 5–7 days to dry the bananas while solar dryer took only 3–5 days and produced better quality of produce. The anticipated payback period of dryer is about 3 years. Result shows that the most effective factor on the moisture removal rate was the temperature of air inside the dryer. The introduction of STD seems to be means of earning extra money for worth addition to the product. Gauhar et al. [21] fabricated a reduced size of solar tunnel dryer based on the Hohenheim dryer design. The AIT (Asian Institute of Technology, Bangkok) solar tunnel dryer consists of a solar collector (4 m length), drying tunnel (4.30 m length) and five radial flow fans (14 W; 130 m3/h) to drive the moist air out of the dryer. For hygienic and ergonomic reasons, the drier stands on a 75 cm high brick plinth. Both the collector and tunnel are covered with a 0.2 mm thick, UV-stabilized polythene sheet. No-load tests were conducted with both AC and DC/Photovoltaic operation of the fans and reported that during AC operation of fans, the drying temperature fluctuates with changes in solar insolation, while solar PV operated fans, reduces this variation as air flow rate is directly related to the solar radiations. They also conducted experiments separately, with AC and DC driven fans, for drying the same quantity of chili (19.5 kg) at almost like weather conditions. The chili took 3 days to get dried up in dryer with AC fans while only 2 days are required for drying with PV operated fans. The efficiency of the dryer for DC/PV operation was estimated at 14.20% against 9.32% for AC operation. The full load capacity of AIT dryer is about 85 kg of raw chilly per batch and payback period of two years was estimated. The AIT dryer performs well with PV driven DC fans, because they considerably diminish the fluctuations in the drying air temperature with fluctuating solar irradiation. Author does not utilized rated capacity of dryer thus full drying potential of air inside the dryer was not used. The economics of AIT dryer gives a pay-back age of around two years and it can be reduced significantly if dryer is utilized for multiproducts. The AIT dyer is specially designed for small land holder farmers. Garg and Kumar [22] reported the thermal performance for collector of a semi-cylindrical STD (Fig. 7). The thermal performance was investigated under natural and forced convection mode under metrological conditions of Delhi. The dryer essentially consists of a solar collector and drying chamber in which the crop is spread in thin layer. In forced convection mode, air at 5 m/s was circulated at different flow rates and increase in air temperature has been found out with the help of computer programme FORTRAN77. Result showed that for different flow rate of air, temperature in dryer was varied from 25 to 32 °C. This shows that for improving the performance of dryer under forced convection mode, air flow rate should be fixed as per the requirement of

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Patil, R. Gawande / Renewable and Sustainable Energy Reviews 56 (2016) 196–214

Solar Radiation

Solar Radiation Outlet Air

Air from collector

Plastic Cover Fan

L

Humid Air

Drying Chamber Air Collector r

Collector

2r

Inlet Air Fig. 7. Semi-cylindrical solar tunnel drying system.

drying temperature in dryer. In other words this analysis will be helpful in proper design of STD. Under natural convection mode, the performance of STD was investigated at different tilts of collector (5–45°) and found that for a small collector tilt, high temperature and small flow rate was observed; however a low temperature and high flow rate of air was noticed for more collector tilt. For a tilt of 15°, flow rate of 1000 m3/h and temperature of 35 °C was observed. In other words, for more collector tilt, buoyancy effects are more in the direction of flow with increase in flow rates and drop in drying temperature. The study reveals that value of collector tilt is important not only in creating natural air currents within collector but also in effective operation of drying. Also air temperature and flow rate was optimized with respect to length of collector and radius of cover. However different researcher suggests the equation for collector tilt (Tilt ¼latitude þ15°) to improve the performance. A prototype of a greenhouse effect (GHE) solar dryer was developed by Abdullah et al. [23] for investigating performance of 1.1 t Robusta coffee at Indonesia. A UV stabilized polythene sheet (1.5 mm thick, 70% transmissivity) was used to form greenhouse enclosure and consists of components like absorber, two heat exchanger, blower and chimney. A blackened steel plate was used to increase the thermal performance of dryer. They reported a drying time of 58 h in the greenhouse unit as compared to 70 h in conventional drying method and showed a drying efficiency of 57.4% in greenhouse as compared to 21.1% of conventional drying. Also a specific energy of 5.5 MJ/kg and 11.6 MJ/kg was recorded in greenhouse and conventional drying method respectively. The analytical study for two greenhouse dryers under active mode for drying of pepper was presented by Condori and Saravia [6]. The first greenhouse has single drying chamber and other greenhouse has two drying chambers. In single chamber system, whole greenhouse is used as a drying chamber, whereas in double chamber system a greenhouse is divided into two drying chambers and fan is located between both of them in a transparent partition wall. For experimentation a drying area of 50 m2 was considered in both dryers. Result showed that the production of double chamber drying system compared to single chamber dryer was increased by 87%. The double chamber dryer is superior to single chamber due to improved energy consumption. In single chamber design, dryer uses only a minute fraction of incoming energy during last drying phase, while in double chamber design, the chamber containing the partially dried product also works as a

Fig. 8. Sectional view of tunnel greenhouse dryer.

preheating collector for the remaining chamber improving the evaporation speed. The essential changes required for improving the performance and to reduce the drying cost are very simple and economical. Condori et al. [24] constructed and investigated a low cost tunnel greenhouse dryer under forced convention mode for drying of pepper and garlic at Argentina. The dryer consists (Fig. 8) of a drying chamber with transparent plastic walls; a line of carts with a number of stacked trays containing the product and an electrical fan to move hot air from greenhouse into tunnel. They reported that a nonstop production was possible with least energy consumption and labor cost, since some carts with dried product come out of the tunnel every day, while the equal quantity of fresh product is fed by the other tunnel. Also the installation can be used as a greenhouse for small production when it is not used as a dryer. The average temperature increase in collector was about 25 °C above the ambient temperature, while the maximum difference between ambient and dryer RH was about 20%. The overall efficiency of dryer can be improved by increasing greenhouse optic parameters (transmmitivity and absorptivity) and heat removal rate. This can be done by reducing the dryer height or by using sufficient insulation. The average production efficiency was 1 kg of dried product for 5 kg of fresh product. The key features of this design are: (a) high drying efficiency due to counter flow circulation between air and product. (b) Partial mechanization of carts for product handling lowers the labor cost and (c) improved product quality and constant production is possible by use of a conventional heater. The analytical study of tunnel greenhouse dryer for describing its performance was presented by Condori et al. Considering the greenhouse dryer as a solar collector, an analytical linear relation among the drier outlet temperature and the solar irradiation was established. This is a common result that can be useful to any greenhouse dryer without a load. This relation was also confirmed with experimental records and shows good connection. The comparison of greenhouse tunnel dryer with single chamber and double chamber dryer has been done and result showed an improvement of about 160% and 40% with respect to the single chamber and double chamber dryer respectively [25]. The Hohenheim solar tunnel dryer at Mymensingh, Bangladesh was tested by Bala et al. [26] for drying of 150 kg of pineapple. It consists (Fig. 9) of a flat plate collector, drying chamber and two fans to circulate required air over the product. A plastic sheet like sloping roof was fixed over the drying tunnel chamber to prevent product from insects and rain. The moisture content of pineapple was reduced from 87.32% (w.b) to 21.52% (w.b) in 20 h of drying in tunnel dryer while it took 20 h to bring down the moisture content of equivalent sample to 21.52% (w.b) in traditional sun drying. A maximum temperature of 64 °C was observed for insolation of 580 W/m2. During drying, air flow rate was varied from 0.2 to 0.3 m/s. A good quality of dried pineapple with higher amount of protein and vitamin-C was obtained. This design permits a uniform drying due to least variation of drying air temperature during

Patil, R. Gawande / Renewable and Sustainable Energy Reviews 56 (2016) 196–214

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2m 12 11 1. Air Inlet 2. Fan 3. Solar Module 4. Solar Collector 5. Side Metal Frame 6. Outlet of Collector

7. Wooden Support 8. Plastic Net 9. Roof Structure 10. Base Structure 9 11. Rolling Bar 12. Outlet of Drying tunnel

10

8 7 5

6

20m

4

3

2

1

Fig. 9. Solar tunnel dryer.

the drying period. The drying temperature was kept almost constant by regulating the air flow rate inside the dryer. The increase in temperature due to high solar radiation was controlled by increasing the air flow rate over the product; while decrease in temperature due to low solar radiation was compensated by increase in temperature by lowering air flow rate. Also a multilayer neural network approach to forecast the performance of STD for drying of jackfruit and leather was introduced by Bala B. K. [27] in 2005. The dryer essentially consists of a flat plat solar collector, tunnel unit and two DC fans operated by PV module to give desired air flow rate. During trail they found that initial moisture content of jackfruit and leather was reduced to the desired level of 24% and 10% in 14 and 19 h respectively. Out of 16 experimental runs, the data collected from seven runs was used to train the ANN model. The ANN model with seven inputs, one output and two hidden layers was found to be able to forecast the performance of dryer after adequate training. Result showed a good accord between predicted and experimental moisture content of jackfruit and leather. Author presents a new option and tool for solving complex mathematical models and enables very speedy and effortless simulations. To predict the accurate performance, a care should be taken in selection of data for adequate training of the model. Bala et al. [28] proposed a study on exergy analysis of tunnel dryer for drying of jackfruit leather under active mode. The dryer consists of flat plate solar collector connecting in series to a drying tunnel with 15° of tilt of the roof slope. Experiments have revealed that 50 kg of jackfruit can be dried out to 11.88% (w.b) from initial moisture content of 76% (w.b) in 2 days. Due to variation of solar insolation between 100 and 600 W/m2, the energy efficiency of collector and dryer varies in the range of 27–43% and 32–65% respectively, while the overall efficiency of STD was in the range of 39–48%. The temperature in the collector also varies in the range of 43–58 °C. Exergy analysis of tunnel dryer gives an efficiency of 41.42% and showed that almost 50–58% of available energy was lost in collector and drying chamber of dryer. The exergy study revealed that exergy efficiency depends upon drying air temperature and time. Since exergy calculates the available energy at different locations in the system, it acts as a measure of quality and grade of energy. In future the concept of exergy should

be used in design of solar drying systems with optimal drying conditions. The comparative analysis for solar tunnel dryer is presented in Table 1. A roof type greenhouse dryer (Fig. 10) was developed by Tiwari and Jain [29] in 2001 for drying of cabbage and peas under the metrological conditions of New Delhi, India. A greenhouse was made from PVC pipes and covered with UV plastic sheet. An air vent of 0.043 m2 was used at roof of greenhouse for passive mode while a fan of 225 mm diameter was provided at the sidewall to move the air at 5 m/s during active mode of drying. During testing of greenhouse, experimental data was used to find convection heat transfer coefficient and then empirical relation was developed to show convection heat transfer coefficient as a function of drying time with aid of two term exponential curve model. Result showed that convection mass transfer coefficient was doubled in forced convection mode as compared to natural convection mode of drying. Also they reported that a moisture removal rate was higher in the commencement of drying and it becomes stable after 20 h of drying time. In GHD under forced convection mode the values of heat transfer coefficient are doubled (35–15 W/m2 °C) during constant rate of drying. The moisture removal rate in active mode is considerably high due to decrease in RH within the greenhouse. A slow rate of drying was observed in GHD under natural mode due to poor ventilation in dryer compared to OSD and GHD under forced convection mode. The mass transfer coefficient is a strong function of moisture removal rate. A similar type of greenhouse was used by Tiwari et al. [30] in 2004 for estimation of convection mass transfer coefficient during drying of Jaggery (800–2000 g) by using regression analysis and found that moisture removal rate was lower in active method of drying due to low drying temperature. However the convection heat and mass transfer coefficients in active method was higher than passive method of drying. Tiwari and Kumar [31] developed a thermal model to envisage moisture removal rate, greenhouse air temperature and product temperature in drying of Jaggery (product of sugarcane juice) under passive mode during February 2004. The mathematical models were developed with energy balance equations at Jaggery surfaces and solved by MATLAB software. The fair agreement between predicted and experimental values for greenhouse and

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Table 1 Comparative analysis of Hohenheim type solar tunnel dryer under passive and active mode. Author

Physical features of the dryer Type/size/shape/drying capacity

Thermal performance Efficiency/heat transfer coefficients/temp. and RH/ drying time

Economics Payback period/ reliability

Gauhar et al. [21]

The design of AIT tunnel dryer (8.30 m  1.80 m) and its concert for drying of 19.5 kg chili was presented. Experiments were conducted at no-load and load circumstances using AC and PV driven DC fans.

2 years.

Garg and Kumar [22]

Thermal performance for collector of semi-cylindrical STD for different flow rates (400–1600 m3/h) in active mode and for different length (10–40 m), radius (1–2 m) and tilt (0–45°) in passive mode was presented.

Bala et al. [26]

A PV operated STD of size 20 m  2 m has been studied for drying of 150 kg of pineapple under active mode.

Bala et al. [27]

A theoretical basis for drying process report by means of ANN was presented. A STD with 20 m  2 m was developed for drying of 120–150 kg of jackfruit and leather.

Bala et al. [28]

The energy and exergy analysis of STD (20 m  1.8 m  380 mm) for drying of 50 kg of jackfruit leather under active mode was presented.

Senadeera et al. [34]

A passive STD (12 ft  3 ft) was designed and tested for its concert for drying of chilly using two chimneys. One chimney is made from GI sheet while other is from wood having dimensions of 1 ft (diameter) and 4 ft (height).

Hossain et al. [35]

Optimization technique for designing dimensions of STD for drying of chili under active mode was presented. For optimization, basic mode dryer with both collector and tunnel of 18 m2 with 80 kg capacity was used.

Lertsatitthanakorna et al. [47]

A mixed mode kind of STD (6.2 m  1.8 m) was evaluated under active mode for drying 30 kg of pupae.

Srisittipokakun et al. [56]

A parabolic STD (12.20 m  1.22 m) for drying of 100 kg Andrographis paniculata under active mode was evaluated.

Amunugoda et al. [64]

For continual drying, author has come up with a modified design of STD (14 m  2 m), which uses biomass stove (1.2 m  2 m) for providing supplementary heat energy. A biomass stove was attached in between collector and drying tunnel.

Initial moisture content of 85% was reduced to 10% (w.b) in 2 days with DC fans while 3 days were required for AC fans. For air flow rate of 130–650 m3/ min, temperature inside the dryer varies in the range of 30–70 °C. The air velocity of 5 m/s was kept constant during different flow rates and temperature in range of 25– 32°C was observed. The study revealed that a proper angle of tilt, flow rate and air temperature plays a significant role in proper design of STD. The moisture content was reduced from 87.32% to 21.52% (w.b) in 20 h of drying in dryer. A maximum temperature of 64 °C was observed for insolation of 580 W/m2. Air flow rate was varied from 0.2 to 0.3 m/ s. Initial moisture content of jackfruit (78.12%) and leather (82.44%) was reduced to 24% and 10% (w.b) in 14 and19 h respectively. The guess of ANN model has been found outstanding. The average temperature of 54 °C was reached in collector. The overall energetic and exergetic efficiency of 42.5% and 41% was observed. Exergy analysis showed that almost 50–58% of energy was lost in dryer. The overall drying and pick up efficiency for wood chimney was obtained as 16% and 20% as compared to 11% and 13% for GI chimney respectively. The collector efficiency at no load condition for wood chimney (68%) was found to better than GI chimney (64%). This technique gives relatively bigger collector area (Design I- 118 kg, 26.60 m2, Design II- 115 kg, 26 m2) thus helps in cost saving by 15.89%. Design I gives a maximum temperature of 64.89 °C with high moisture removal rate of 2.68 kg/kg per day. The moisture content was reduced from 79% to 5% (w. b) in 570 min and overall efficiency of 19.68% was observed. The study reported a least drying time at flow rate of 0.30 kg/s. The drying time was reduced by 40% as compared to traditional drying. During experimentation, temperature variation of 35–75 °C and maximum solar insolation of 1050 W/ m2 was recorded. The moisture content of product was reduced from 75% to 7% wet basis in 12 h. The supplementary heat maintains a good temperature range of 40–55 °C through the dryer giving excellent performance. Results of modified STD are very encouraging and decreases the drying time by 1–1.5 days. Air flow rate in the range of 0.09–0.25 m3/ s was maintained during operation.

Fig. 10. Cabbage and peas drying (a) natural convection and (b) forced convection.

2–2.5 years.

2–3 years/reliable.

Highly economical.

2–3 years/reliable.

3 years.

3 years.

1.4 years. Economical.

1.5 years

Patil, R. Gawande / Renewable and Sustainable Energy Reviews 56 (2016) 196–214

Fig. 11. Greenhouse for Jaggery drying under natural and forced convection mode.

Jaggery temperature was presented interms of coefficient of correlation and root mean square. Result showed that for 2 kg of Jaggery, moisture evaporation rate was high on the first day; however on second and third day of drying, the moisture removal was found to be 12.8 g and 11.7 g respectively and occurs from the core of the Jaggery due to diffusion from inside to outside surface. On the fourth day, a negligible amount of moisture about 9 g was evaporated, resulting in complete drying of Jaggery. Similar performance of drying was noticed for 0.75 kg of Jaggery. The study revealed that the drying time required for 2 kg Jaggery was 4 days while only 2 days were required for drying of 0.75 kg of Jaggery. This shows that drying time depends mostly on total mass of the product to be dried. The similar greenhouse was also tested by Tiwari and Kumar [32] to find effect of size and shape on mass transfer coefficient for Jaggery drying of different shapes (0.03  0.03  0.01 m3, 0.03  0.03  0.02 m3 and 0.03  0.03  0.03 m3) in month of February–March 2004 under free and forced convection mode. An air vent of 0.0722 m2 was used at roof of greenhouse for passive mode while a fan of 120 mm diameter was provided at the sidewall to move the air at 5 m/s during active mode of drying. The experiments were carried out for 0.75 and 2 kg of Jaggery pieces and showed that mass transfer coefficient was more for Jaggery piece of dimension 0.03  0.03  0.02 m3 and 0.03  0.03  0.03 m3 in free and forced convection mode respectively. The even span type of greenhouse for Jaggery drying has been shown in Fig. 11. The study revealed that mass transfer coefficient decreases with increase in the mass of Jaggery and shape of Jaggery pieces also plays a major role. Tiwari et al. presented energy and exergy analysis of greenhouse for drying of fish (Prawn). They tested the greenhouse in both natural and forced convection mode to calculate drying air temperature, moisture removal rate and surface temperature of prawns. The predicted values show good accord with experimental values. They also developed an analytical expression for exergy efficiency and found to be lower than energy efficiencies under natural and forced convection mode. Result showed that drying of prawns was quicker in forced convection mode [33]. The comparative analysis for roof type even span greenhouse is presented in Table 2. Senadeera and Kalugalage [34] evaluated a solar tunnel dryer with two chimneys (Fig. 12) for drying of chillies (15 kg) under the metrological conditions of Moratuwa, Sri Lanka. Parallel pass form of solar collector was developed to allow air to flow through top and bottom of the absorber plate. Two chimneys with metal duct were used to achieve passive air flow. One chimney (diameter 1 ft,

203

height 4 ft) was made of GI sheet and painted black on inner and outer side; while other chimney made from wood and covered with polythene sheet to form a greenhouse effect. Author used a flow controlling unit stuck between the collector and tunnel. The air flow rate was controlled by two metal flaps which are operated by the levers. This unit may be useful in improving the performance of dryer when it was not tested for its full load capacity. The performance of both chimneys in terms of collector efficiency was compared during no load test while the main idea of load test was to compare the drying rate by managing the flow rate throughout the drying tunnel. Result showed that wooden frame chimney with polyethylene sheet was more efficient than GI sheet due to fast removal of moisture with higher drying and pick up efficiency. For collecting maximum solar radiations and easy loading/ unloading convenience, author designed the roof of collector and drying tunnel in curved shape. Optimization of STD using solar energy for drying of chilly without color loss was presented by Hossain et al. [35] in 2005. For optimization, basic mode of STD with 10 m length and 1.80 m width of 80 kg capacity was used. The optimized dimensions for collector were determined by exploratory and pattern search technique under controlled conditions. This technique gives two types of optimum designs. For design – I, both collector and drying tunnel are 14 m long, 1.9 m wide while design – II gives dimensions of 13 m long, 2 m wide for collector and drying chamber. The simulation and economical model gives the drying capacity of 118 kg and 115 kg for optimized designs – I and II respectively. For the above drying capacity the drying efficiency was found to be almost constant (27%) while a payback period of 4 years for basic mode and 3 years for optimized mode dryer was evaluated. The optimized Model-I was superior due to bigger collector area and high moisture removal rate since hot air passes over and under the product during drying. However drying rate and efficiency can be improve by increasing heat input using transparent glasses having higher transmmitivity. Author gives an excellent technique for design of solar tunnel dryer for drying of several agricultural products economically. The predictions from this technique were outstanding, but the mathematical models were complex because they require data on drying kinetics and information of physical and thermal properties of the product. Hossain and Bala [36] also developed a mixed mode type of tunnel dryer (12 m long, 1.80 m wide) for drying of chillies. The solar dryer consist of collector and drying chamber made of simple metal sheets and timber frames. Solar collector and drying tunnel was enclosed with transparent UV plastic cover. Two electric fans operated by 40 W photo-voltaic panels were used to distribute the air in solar dryer. During experimentation an average temperature rise of 21.62 °C at the exit of the collector was observed. Result showed that drying of 80 kg of chillies was completed 13 h prior than traditional sun drying. Elicin and Sacilik [37] developed a solar tunnel dryer to learn the kinetics of apple under climatic conditions of Turkey. The dryer was covered with UV plastic sheet and oriented in east–west direction as shown in Fig. 13. During experimentation a significant difference between values of temperature and RH of 13.1 °C and 9.2% with respect to ambient conditions were observed respectively. Result showed that moisture content of organic apple has been reduced from 82% (w.b.) to 11% (w.b.) in 1.5 days only; while open sun drying took 2 days for the same moisture removal. Experimental results were validated by mathematical models like Page, Logarithmic, Wang and Singh. A page model was found to be the top model for explaining the drying performance of apple. The brighter and excellent yellow color apple slices were obtained during drying. Sacilik et al. developed a mathematical model for the same greenhouse for drying of tomatoes. Result showed that tomatoes were dried within four days in STD while open sun

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Table 2 Comparative analysis of roof type even span greenhouse under passive and active mode. Author

Physical features of the dryer Type/size/shape/drying capacity

Tiwari et al. [29]

During constant rate of drying heat transfer coefficient 1–1.5 years. varies from 25 to 10 and 17 to 8 W/m2 °C for OSD and GHD respectively while this value ranged from 8 to 2 W/ m2 °C for above cases during falling rate of drying in passive mode. However during active mode, value of heat transfer coefficient changes from 38 to 15 W/m2 °C. A roof type even span greenhouse (1.2  0.8 m2) having capacity Experimental investigation showed that for three days of 1.5 years. Less Cost drying, rate of moisture removal in passive mode was of 2000 g was developed and studied for evaluation of mass transfer coefficients for drying of Jaggery in natural and forced slow in comparison with that of active mode. Result showed that in natural mode, mass transfer coefficient convection mode. changes from 1.29 to 1.41 W/m2 K while due to high temperature and moisture removal rate in forced convection drying, this value was significantly increased to 1.3 to 1.46 W/m2 K. A thermal model for Jaggery drying was presented to predict the Result showed that the drying time for 2 kg Jaggery was 1–1.5 years. performance of roof type even span greenhouse (1.2  0.8 m2) found to be 2 days more as compared to 0.75 kg of Jagunder passive mode. Jaggery piece of 0.03  0.03  0.01 m3 was gery. This shows that drying time depends mostly on total mass of the product to be dried. kept on tray of 0.4  0.24 m2 in east–west direction for experimentation. Two different designs (M I-1.62 m2 and M II-1 m2) of natural The thermal efficiency of Model I (21.46%) and II (18.29%) 2–5 times extra compecirculation STD for rainy and high RH places were proposed. The was found be higher under unloaded and with chimney tent than OSD. Low cost. effect of trays, chimney and tilt angle was investigated. Product condition. An increase in temperature of 5–9 °C was observed. The arrangement of chimney and tilt improves loading rate is 10 kg/m2. the performance of dryer by 2–5%. A roof type even span (2.5 m  2.60 m) PV/T integrated green- The convective heat transfer coefficients for GR-II (0.45– 1–1.5 years. house dryer for drying 100 kg of grapes under active mode was 1.21 W/m2 K) was higher than GR-I grapes (0.26–0.31 W/ presented. Five trays were used for loading of (8 kg) GR-I and m2 K). The desired moisture content was achieved in GR-II grapes. 96 h. 1.5 years. During passive mode, air temperature with INWR A modified even span greenhouse (6 m  4 m) with INWR for (61.4 °C) remained much higher as compared to when drying of bitter gourd (18 kg) under passive and active mode was presented. A north wall with aluminum reflector was used INWR was not used (55.6 °C). With use of INWR, 13.13% of drying time was saved. During active mode, air temto increase the availability of sun rays over the product. perature with INWR (56 °C) remained much higher as compared to when INWR was not used (51.7 °C) and 20% of drying time was saved. The experimental convective and evaporative heat A new approach for papad drying in roof type greenhouse 1–1.5 years. (1.2 m  0.8 m) under forced convection mode was proposed. A transfer coefficients were associated with dimensionless circular shape wire mesh tray was used for loading the product. equations in terms of Nu, Pr and Re Nos. These were observed to change from 0.739 to 0.786 W/m2 °C and 21.37 to 25.42 W/m2 °C respectively for 23.5 g sample.

Tiwari et al. [30]

Tiwari et al. [31]

Koyuncu [39]

Barnwal et al. [46]

Sethi et al. [49]

Kumar [62]

Thermal performance Economics Efficiency/heat transfer coefficients/temp. and RH/drying Payback period/reliability time

The study of a roof type even span greenhouse for drying of 300 g cabbage and peas under natural and forced convection mode was done. The greenhouse dryer with 1.2  0.8 m2 was placed in east-west direction and observations were noted for 33 h of continuous drying.

Fig. 13. Solar tunnel dryer for organic apple and tomatoes.

Fig. 12. Solar tunnel dryer with chimney.

drying took five days for the same moisture removal. Experimental results were validated by 10 different models and diffusion model is found to be the best model [38]. The study reveals that drying of small quantity of organic apples and tomatoes with STD was feasible. Further the uneven shape of dryer results in improper mixing of air and thus lowers the drying rate. The quality of dried produce can be further enhanced by adopting mixed mode type of dryer. In this study author does not contribute regarding various efficiencies and heat transfer rate. Koyuncu [39] developed and tested two different designs of greenhouse dryer with and without chimney for loaded (Pepper) and unloaded conditions under natural convection mode at

Patil, R. Gawande / Renewable and Sustainable Energy Reviews 56 (2016) 196–214

205

6 30

50

PV/T Collector

Chimney (GI Sheet)

Inlet of Air 3m 2m

Door Outlet of Air

Door

1.1m

100

Plastic Cover Sheet

4m

0.25m

Air Inlet Channel

65°

100

20

Exhaust Fan Fig. 16. Front view of greenhouse dryer.

100

Fig. 14. Schematic presentation of Model I.

25

58

20

3. Tray

4. Tray Frame

2. Tray 1. Tray Black Surface

175

Insulator Fig. 17. Greenhouse dryer. Fig. 15. Schematic presentation of Model II.

University of Samsun, Turkey. In addition, the effect of trays, chimney and tilt angle was also investigated. The Model-I has a drying area of 1.62 m2 and consist of three trays for loading the crop with air inlets at front and rear end. A slope of 65° was given to Model-I greenhouse as shown in Fig. 14; while Model-II was deliberately constructed as steps of a staircase. Model-II has a drying area of 1 m2 and consists of seven steps, air inlet at front and rear side and air outlet channel at top side as shown in Fig. 15. An increase in temperature of 5–9 °C was observed during testing of Models-I and II under the same environmental conditions. The investigation shows that thermal efficiency of both Models I and II was found be higher under unloaded and with chimney condition. When Pepper is loaded at rate of 10 kg/m2, efficiency of greenhouse decreases due to reduction in velocity and flow rate of air. The arrangement of chimney leads to increase the performance of dryer by 2–5%. In Model-I though temperature of all trays is same, a different drying rate was observed in view of the fact that bottom tray receives the maximum heat from the black absorber surface, while the top tray receives a less amount of heat as well as cold and moist air. On the other hand, for Model-II, the drying rate of top (fourth) step was higher as compared to steps available on the right and left side. This happened due to shape and position of dryer as top step receives more energy than other steps. Also an ideal tilt of 65° gives the less surface area at the top of greenhouse. This is desirable since air at the top of dryer has high humidity and low temperature thus improves the drying rate due to low quantity of produce. In short proposed designs are appropriate for drying small batches of crops only. The greenhouse dryers are 2–5 times extra competent than open sun drying and commercially viable due to low capital and maintenance cost. The study reveals

that both designs of natural convection tunnel greenhouse proposed by author gives a much better controlled conditions for drying and found to be excellent for areas that have a raining climate and high RH. A hybrid photovoltaic thermal (PV/T) air collector integrated with a greenhouse was developed by Nayak and Tiwari [40] under the metrological conditions of New Delhi, India. In hybrid PV/T greenhouse dryer (Fig. 16) 16 photovoltaic modules were used to operate the fans with the help of DC battery. To find outlet air temperature, a thermal modeling was also done for hybrid PV/T air collector. MATLAB software is used for calculation of useful thermal energy and to evaluate thermal performance of hybrid PV/T air collector. They compared the results of PV/T air collector with PV/T without air flow and found an increase in thermal efficiency of 25.5% when air was made to flow. They also reported that overall efficiency of hybrid PV/T with air flow is more (72%) than PV/T without airflow (41%) for 3 m length of PV module. Numerals of studies have been presented on greenhouse tunnel dyers by Janjai Serm in a period of 2003–2012. A PV-ventilated greenhouse dryer (Fig. 17) with concrete floor for drying of chilies was developed by Janjai et al. [41] in 2003. The parabolic shape of greenhouse was covered with polycarbonate sheet. A solar PV module of 53 W was used to operate three fans to aerate the dryer. To study its performance, the greenhouse was used to dry 4 batches of chilies in December 2003–March 2004. They reported a maximum air temperature of 65 °C in dryer and found that drying time in the dryer for drying of 100–150 kg of chilies was considerably less than traditional open sun drying method. Author used the concrete floor for heat storage which helped to decrease variation of air temperature due to the instability of solar radiation. The use of solar PV cell unit helps to adjust indirectly the drying air temperature. Further Drying efficiency of greenhouse

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12.0m

2.5m 9.0m Fig. 18. Roof integrated solar drying system.

Fig. 19. Pictorial view of the large-scale solar greenhouse dryer with LPG burner.

can be increased with loading capacity. The performance evaluation of roof – integrated solar dryer for drying of rosella flower and chilly under forced convection mode was presented by Janjai et al. [42] in 2008. The arrangement of greenhouse is shown in Fig. 18. The dryer consists of two arrays solar collector (one at south and other at north), drying bin and electric powered fan to force the air over the product bed. The dryer has three compartments: first for the drying bin, second for preparation of produce to be dried and third for storage of dried product. The polycarbonate sheet was used to cover the roof of dryer. Field-level experiments confirmed that drying in the roof-integrated solar dryer results in major reduction in drying time compared to the traditional drying method and a quality dry produce is obtained. The dryer has been found cost efficient with a payback period of 5 years. Janjai et al. [43] proposed a PV greenhouse dryer under active mode to dry longan as well as banana and at the same time the sample of above produce were also dried in open sun. The experimental data was validated by developing partial differential equations. During 10 full scale investigational runs, drying air temperature changes in the range of 31–58 °C in drying of longan while a temperature change from 30 to 60 °C during drying of banana was observed. Experiments have revealed that longan and banana can be effectively dehydrated in 3–4 days, while it took 5–6 days for traditional method under similar conditions. A high quality dried produce was obtained. Experimental performance of parabolic roof type greenhouse dryer for drying of chili, banana and coffee under active mode was presented by Janjai et al. [44] at Champasak in Lao PDR. The dryer was covered with polycarbonate sheet and used 50 W solar PV module to run nine DC fans to circulate the air. Seven experimental runs for banana, chili and coffee were carried out in period of September–December 2007. The moisture of 1000 kg of banana was decreased from 68% (w.b) to 20% (w.b) within 5 days, while the moisture of the sun-dried banana samples reduced to 29% (w.b) in the same time. For 300 kg of chili moisture was reduced from an initial value of 75% (w.b) to a final value of 15% (w.b) in 3 days while the moisture content of the open sundried samples was decreased to 42% (w.b) in the same period. The moisture content of 200 kg of coffee was decreased from an initial value of 52% (w.b) to a final value of 13% (w.b) within 2 days whereas the natural sun-dried samples required 4 days to reach the desired value of moisture content. Partial differential equations for heat and mass transfer during drying of above products have been developed and establish a sensible conformity with experimental results. This model can be helpful for providing design statistics for the greenhouse dryer at any other locations. The reimbursement period of 2.5 years was estimated on the basis of production scale, capital and operating cost of drying system. Later

on Janjai Serm [45] tested the same dryer (Fig. 19) at Nakhon Pathom in Thailand for drying of 1000 kg of tomatoes. For uninterrupted drying process, he used a 100 kW of LPG burner to supply hot air during night operation and in rainy or cloudy days. A system of partial differential equations for heat and mass transfer was developed for dehydrated tomatoes. The simulated outcomes are in acceptable range with the experimental data. Uninterrupted and faster rate of drying is the novel feature of parabolic roof type greenhouse. Also the rate of drying was uniform in all trays due to heating of air by supplementary heat source. To improve the performance of dryer, it is advisable to recirculate the exhaust air coming from the dryer. Author does not focused on energetic and exergetic analysis of drying system. The comparative analysis for semi-cylindrical greenhouse dryer is presented in Table 3. Barnwal et al. [46] developed and tested a hybrid PV thermal greenhouse dryer for drying of Thompson seedless grapes at IIT, New Delhi, India. They used a roof type even span of greenhouse covered with UV stabilized plastic sheet shown in Fig. 20. For experimentation, only 8 kg of seedless grapes were used and result showed that heat transfer coefficient for GR-II grapes are superior than GR-I grapes thus needed less drying time due to higher moisture removal rate. The grapes were physically sorted in two grades, GR-I (green and premature) and GR-II (yellow and mature). The convective heat transfer coefficient for GR-I and GR-II grapes was found to be in the range of 0.26–0.31 W/m2 K and 0.45–1.21 W/m2 K respectively. Author used PV module of 84 W capacity but 20 W module is adequate to drive the installed fan. The topography of place of work permits a roof slope of 30° for maximum collection of solar irradiation. The performance of greenhouse can be enhanced if it is tested for its full load capacity. Lertsatitthanakorna et al. [47] conducted an experimental analysis to investigate the performance of a mixed mode type forced convection solar tunnel dryer (Fig. 21) for drying of silkworm pupae under weather conditions of Mahasarakham, Thailand. The dryer consists of a flat plate collector, drying chamber and fan to supply the required air over the product. Both collector and drying tunnel was covered with glass of 3 mm thickness. The performance of dryer was tested for different air flow rates of 0.20, 0.30, 0.43 and 0.50 kg/s and they reported the least drying time at flow rate of 0.30 kg/s which reduced the drying time by 40% as compared to traditional drying. The drying time for all flow rates in STD gave minimum drying time as compared to traditional sun drying since pupae in the dryer received thermal energy both from collector and solar irradiation, whereas control samples received energy only from sun rays. During testing, the highest drying and overall efficiencies of 30.14% and 19.68% were observed respectively. Experiments showed that the drying of pupae was very

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207

Table 3 Comparative analysis of semi-cylindrical greenhouse dryer under passive and active mode. Author

Physical features of the dryer Type/size/shape/drying capacity

Thermal performance Efficiency/heat transfer coefficients/temp. and RH/drying time

Condori et al. [6]

Evaporation rate in two different types of active mode greenhouse dryers, the single and double chamber system was presented. For comparing the performance drying area of 50 m2 was considered. Pepper was dried in both dryers. A simple, low cost tunnel greenhouse dryer (13 m long, 7 m wide and 3.70 m high) for drying of red pepper and garlic under active mode was studied.

Result showed that double chamber system was 87% more efficient than single chamber system. The key feature of double chamber design is maximum utilization of incoming solar energy. Results of drying analysis with pepper and garlic Condori et al. [24] showed that the pepper has been dried from 6.5 to 0.2% (d.b) in 6.5 days and garlic from 1.8 to 0.2% (d.b) in 7.5 days. The maximum drying air temperature of 60 °C was recorded. The average drying efficiency of 25% was observed. Elicin et al. [37] A direct type natural convection STD (1.8 m  2.5 m  8 m) for The moisture content was reduced from 82% to 11% (w. drying of apple was presented. The dryer was covered with b) in 28 h for STD, whereas traditional method took polyethylene sheet and have two trays to accommodate 2.5 kg of 32 h. The drying air temperature in dryer was larger produce. than the atmospheric temperature, whereas the relative humidity in dryer was lesser than the ambient RH. Experimental work was validated by a page model. The desired moisture content of chili, banana and coffee Janjai et al. [44] A parabolic roof type greenhouse (7.5 m  20 m  3.5 m) for was achieved in 3, 5 and 2 days respectively which is far drying of chili, banana and coffee under active mode has been superior as compared to traditional drying method. studied. Rated capacity of dryer is 1000 kg. Janjai et al. [45] The experimental and simulated study for roof type greenhouse Temperature variation of 35–65 °C was observed. The faster and uniform drying was found to be outstanding (8 m  20 m  3.5 m) under forced convection mode was presented. A novel feature of 100 kW – LPG gas burner was used to in all perspective. Uniform drying. supply additional heat source during off sunshine hours. For solar insolation of 940 W/m2, the maximum temRathore and Panwar A novel design of a hemicylindrical greenhouse with heat pro[51] tecting north wall for drying of seedless grapes (320 kg) under perature of 65 °C was observed in the dryer. Moisture passive mode was presented. content was reduced from 85% to 16% (w.b) in 7 days. Ayyappan and Mayil- A Quonset shape solar tunnel dryer of size 4 m (w)  10 m (L)  Moisture content was reduced from 52% to 8% (w.b) in 57 h and gives 20% of drying efficiency. Also the RH samy [54] 3 m (H) for drying of 5000 coconuts per batch under natural inside the dryer was (30%) less than ambient air (60%). A convection mode was proposed. maximum temperature of 67 °C was recorded for air velocity of 0.3–1 m/s during drying. The maximum drying air temperature of 67 °C was Dulawat and Rathore A mixed mode, semi-cylindrical STD (3.75 m  16 m  2 m) for [58] drying of 500 kg tobacco under forced convection was developed. observed. Tobacco was dried from 138.11% (d.b) to 8.69% Three trays and 12 flat plate collectors were used for enhancing (d.b) in 8 h. the performance of dryer.

Fig. 20. Hybrid PV thermal greenhouse dryer. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)

effective, and drying period was reduced to about 570 min from 945 min of the open sun drying. Also a slight decrease in PUFA (polyunsaturated fatty acid) and no variation in lipid content of the dried silkworm pupae were observed. The study reveals that efficiency of collector is firmly dependent on flow rate due to fact that mass transfer between air and produce increases due to increase in air flow rate. In this study author focused on different air flow rates only; however drying time not only depends on air flow rate but also depends on the drying air temperature. Derbala et al. [48] conducted experiments for drying of squash in a solar tunnel dryer for investigating the effect of slice thickness on drying rate and relationship between air temperature, solar insolation and relative humidity. The experiments on 200 kg of

Economics Payback period/ reliability 2–3 years/reliable

2–3 years/reliable

1.5–2 years/reliable

2.5 years/reliable

0.65years/reliable

1.5–2 years/viable

0.5–1 year/reliable

2–5 year/high cost

squash were conducted in month of July–August 2008 under the environmental conditions of Romania and found that only 8 h were required for drying of 0.5 cm thick slice of squash. However for the same moisture removal 11 h was required for drying of 1 cm thick slice of squash. They also found that the maximum difference between air temperature and ambient was 30 °C at 14.20 h in both 0.5 cm and 1 cm thickness of dried squash. Result showed that as air temperature increased from 30 °C to 40 °C, relative humidity of air decreased from 30% to 15%. The solar insolation changes from minimum value of 300 W/m2 to 700 W/ m2 of maximum value. The study revealed that moisture removal rate also depends on slice thickness of produce to be dried. Sethi and Arora [49] designed and developed a modified even span greenhouse with INWR. The performance of improved greenhouse for drying of bitter gourd was tested under passive and active mode at Ludhiana, India. During Passive mode of drying, air temperature with INWR (61.4 °C) remained much higher as compared to when INWR was not used (55.6 °C) whereas in active mode of drying, air temperature increased by 1–4.5 °C. Result showed that the desired moisture content of 7% (d.b) was achieved in 12 h during active mode while it took 15 h for the same moisture removal under passive mode. The study revealed that performance of dryer with INWR under active mode was superior to passive and open sun drying due to strong effect of airstream velocity over the product surface. In this study author proved that all transmitted solar energy in the greenhouse was not utilized for crop drying, major portion of this energy is lost to surrounding

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Fig. 21. General view of solar tunnel dryer.

Fig. 22. Rotating and tunnel dryer.

through north side and this energy can be effectively utilized by INWR concept. Sultana et al. [50] evaluate the drying performance of solar tunnel and rotating dryer. The studies were conducted on three marine fishes: Silver Jew fish, Bombay duck and Ribbon fish under the metrological conditions of Bangladesh. A new design of rotary type dryer (Fig. 22) was developed and it consists of Thai aluminum structure bearing hooks, S.S pipes, bearings, and stainless steel rings. To hang the large size fish, hooks and steel structure was used while for small fishes, round shaped plastic net was used. A 0.75 HP motor with speed regulator was used to operate the dryer at 90 RPM. When dryer rotates, drying process was improved due to movement of ambient air over the fishes (40 kg). A solar tunnel dryer of drying area 20 m2 based on Hohenheim design was developed for drying the fish. It consists of a solar collector and drying tunnel covered with polythene sheet. To maintain the required air flow rate over the fishes, four DC fans operated by PV cells were used. During experimentation the air temperature and humidity inside the dryer varied in the range of 31.33–47.78 °C and 33.83–68% respectively. The initial moisture content of fish samples was in the range of 78.7% (w.b)–85.47% (w.b) and gets reduced to 16% (w.b) within 20 sunshine hours in

rotary dryer and while 31 sunshine hours in solar tunnel dryer. Both rotary and STD showed the opposite relationship between humidity and temperature of drying air. The RH inside the STD was minor compared to outside the tunnel which accelerates the drying rate by absorbing larger amount of moisture from the fish. Result showed that performance of rotating dryer was superior to that of solar tunnel dryer. Solar drying experiments for seedless grapes were conducted at Udaipur, India. For this purpose Rathore and Panwar [51] designed and developed a walk-in type hemicylindrical solar tunnel dryer as shown in Fig. 23. It consists of hemispherical metallic structure covered with polythene sheet. To generate natural current of air inside the dryer, four chimneys were provided on the top of dryer. During experimentation, the maximum heat loss was found in northern side of tunnel; hence a heat protective wall was placed on the north side. The seedless grapes of 320 kg were dried from initial moisture content of 85% (w.b) to final moisture content of 16% (w.b) in seven days, while open sun drying took 11 days for the same moisture level. Result showed that air temperature inside the dryer increases rapidly and attains the maximum value of 65 °C. A solar tunnel dryer was designed and constructed by Palled et al. [52] in 2010 for drying of 1000 kg grapes under the

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GI Pipe Class B

Lateral Support

Hoops Cross Connectors Chimney Ridge line S E

100 Poly grip assembly

No

rth

Wa

W

N

ll

200 D O O R

G.I. Pipe Class A

Foundation Pipes

75

Exhaust fan

Fig. 23. Schematic of natural convection solar tunnel dryer.

metrological conditions of Raichur, India. During experimentation they found a maximum temperature of 62 °C inside the dryer, while the maximum observed ambient temperature was 35.5 °C. Result showed that moisture content of grapes has been reduced from 82% (w.b) to 14% (w.b) within 60 sunshine hours; however for the same moisture removal 140 drying hours were required in traditional drying and shows a net saving of 57.14% in drying time. Palled Vijaykumar [53] developed and tested a semi-cylindrical solar tunnel dryer at Raichur district in northern Karnataka, India. The dryer was covered with polythene sheet of 200 μm and investigation was done for drying of 350 kg of chili per batch. Two exhaust fans were used to regulate the air flow through the dryer, while three chimneys were provided at the roof top for removal of warm moist air. A highest temperature of 58.5 °C was observed at 14:00 h within the dryer which was 41% higher than the maximum surrounding temperature at the same time. Result showed that moisture content of chili has been reduced from 76% (w.b) to 9% (w.b) within 50 sunshine hours, while traditional open sun drying took 105 sunshine hours for the same moisture removal. The dehydration ratio of the chili was found to be 4:1, signifying that four parts of the new product was reduced to single part of the finishing product after drying. Ayyappan and Mayilsamy [54] developed a greenhouse solar tunnel dryer for drying of copra in natural convection mode under the metrological conditions of Pollachi, India. The dryer was covered with 200 μm UV stabilized polythene sheet and metallic racks were used for keeping 5000 coconuts per batch in layers for drying. During experimentation they found that the moisture content of copra has been reduced from 52.2% (w.b) to 8% (w.b) in 57 h. Also the RH inside the dryer was less than ambient air. This indicates that air inside the dryer was capable to carry more moisture from the product. Thus the efficiency of dryer can be increased by recirculating humid air again or by adding more quantity of product on metallic trays. Later on they used sand as a sensible heat storage material and carried out the experimental investigations on the same STD for drying of same quantity of copra under natural convection mode. Result showed that use of heat storage material (sand) improves the heat absorption capacity within the dryer which leads to achieve higher temperature even in night. This helps in faster drying of copra and improves the

performance of dryer by reducing drying time and increasing thermal efficiency. Moisture content of copra has been reduced from 52.3% (w.b) to 7% (w.b) in 52 and 78 h with and without heat storage material respectively; while open sun drying took 172 h for the same moisture removal. A maximum temperature of 61 °C with sensible heat storage material and 52 °C without heat storage material was observed. High quality copra was produced as compared to OSD method [55]. Author used a Quonset greenhouse without any post for receiving more energy and ensuring better use of inside space. But design of a greenhouse mainly depends upon the latitude of the place and solar flux, so for latitude of Pollachi (10.39°N), a roof type even span greenhouse may be an intelligent option for enhancing the performance of existing dryer. The drying rate was reduced by 5 h due to use of sensible heat storage material; however this can be further reduced by recirculating the moist air again over the product bed. Srisittipokakun et al. [56] manufactured a parabolic shaped solar tunnel dryer for drying of Andrographis paniculata at Nakhon Pathom, Thailand. The dryer consists of flat plate collector, a drying tunnel and three fans driven by 15 W photovoltaic module. Two experimental runs were conducted for Andrographis paniculata in period of June 2011 and result showed that the dryer can dry 100 kg of Andrographis paniculata from an initial moisture content of 75% (w.b) to 7% (w.b) in just 12 h. High quality dried Andrographis paniculata was obtained. The photograph of experimental setup is shown in Fig. 24. The drying rate in the solar tunnel dryer was significantly higher than that of the traditional drying. A solar greenhouse dryer was designed, constructed and installed by Almuhanna [57] at King Faisal University, Saudi Arabia. The developed dryer (Fig. 25) was used for drying of dates. The dimension of greenhouse was 2 m (L)  1 m (w)  1.2 m (H) and covered with a flat fiber glass sheet of 800-μm thickness having transmmitivity of 77.48%. To make best use of solar radiations existing within the greenhouse, glass sheets were inclined at 30°. An electric fan of 0.31 m in diameter was used to maintain 5.5 m3/ min of air flow. The experiments were conducted in month of October 2010 to evaluate the thermal performance and stability of solar greenhouse in terms of useful heat gain available for drying process and thermal efficiency. The anticipated thermal balance for the solar dryer was well validated with experimental results.

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Fig. 26. Semi-cylindrical solar tunnel dryer. Fig. 24. Solar tunnel dryer.

Fig. 27. Schematic diagram of the large-scale polycarbonate cover greenhouse solar dryer.

Fig. 25. Solar greenhouse dryer.

The dryer was effectively operated for 5 days, but 55 h of bright sunshine were used in thermal performance analysis. During experimentation, the temperature rise of 14.1 °C and air RH drop of 9.6% compared to ambient air was observed. Maximum solar insolation recorded was 1000 W/m2. Result showed that 12.335 kW h of solar energy was available inside the greenhouse and only 7.414 kW h was transformed into useful heat gain. He also concluded that the overall thermal efficiency of green house dryer was 60.11% and almost 39.89% of the solar energy available inside the dryer was lost. Instead of plastic sheet, author used a transparent glass for more absorption of solar irradiations. The prevention of loss of thermal energy within the dryer and utilization of high transmmitivity (more than 0.80) glasses may be helpful in improving the thermal performance of dryer. Dulawat and Rathore [58] designed and tested a semicylindrical solar tunnel dryer under forced convection mode at Udaipur, India for drying processed tobacco. The photograph of solar tunnel drying system with flat plate collectors is shown in Fig. 26. The hot air from 12 flat plate collectors (size-2 m2 each) forced over the tobacco by using blowers. Cement concrete floor was painted black for maximum absorption of solar irradiation. Three trays of 57.24 m2, 39.75 m2 and 25.44 m2 sizes at bottom, center and top respectively were used for loading of tobacco. A useful temperature rise of 18–20 °C and 30 °C was achieved during no-load, without collector and no-load with collector conditions respectively, whereas a temperature rise of 26.56 °C was recorded during full-load condition. The moisture content of tobacco has been reduced from 138% (d.b) to 8.7% (d.b) within 8 h in STD and 12 h in traditional open sun drying. Author made the efforts for

nonstop and automatic loading of product inside the dryer so the problem of toxicant gases of tobacco with workers was minimized to larger extent. Use of 12 flat plate collectors improves the drying performance with proper mixing of air and by maintaining uniform drying rate. However the cost of dryer is more. Result showed that this type of system can be effectively implemented for industrial production. Experimental performance of a large scale greenhouse type solar dryer for drying of chili under the metrological conditions of Thailand was investigated by Kaewkiew et al. in 2012. Dryer consists of a hemispherical tunnel covered with stabilized polythene sheet shown in Fig. 27. Nine DC fans operated by three PV solar panels were used to remove the humid air. The capacity of dryer is 1000 kg but for experimentation only 500 kg of chili was dried in the greenhouse. Result showed that the moisture content of chili has been reduced from 74% (w.b) to 9% (w.b) within 3 days; however traditional open sun drying needed five days for the same moisture removal. The payback period of 2 years was predicted [59]. The dryer was not tested for its full load capacity and result showed that the exit air from the dryer has drying potential for recirculation to dehydrate the product. In order to improve the living standards of rural peoples in Cameroon, a double pass solar tunnel dryer (Fig. 28) with heat storage system was developed by Berinyuy et al. [60] in 2012 for drying of leafy vegetables and other agricultural products. The major components of dryer were solar collector, a drying chamber and basalt rocks as heat storage material. Both the collector and the tunnel are covered with two layers of plastic sheet and orientated towards south with an inclination of 6°. The dryer was investigated for drying of cabbage, pepper, amaranth and bitter leaf. The 17 kg of cabbage was dried from 95% (w.b) to 9% (w.b) in five days and gives efficiency of 17.68%. Result showed a significant reduction (30–50%) in drying time for high moisture products. They also concluded that heat storage material permits uninterrupted drying of produce during periods of rainfall and low

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Chimney Corrugated Sheet

Deflector

Tray

Black Stone (Heat Storage)

Collector

Dryer

Fig. 28. Double pass natural convection solar tunnel dryer with heat storage material.

sunshine. The tilt of 6° to dryer was given for maximum collection of solar energy. The double pass arrangement in collector absorber helps in attaining the uniform temperature inside the dryer. Even though the initial cost is moderately high, the running cost of dryer is low. The quality of final produce was good enough in taste and color. Kagande et al. [61] investigated a solar tunnel dryer for drying of tomatoes under forced convection mode at Zimbabwe's scientific research and development centre (SIRDC). The transparent polythene sheet of 200 mm was used to cover both collector and drying tunnel. From local market, 50 kg fresh tomatoes were purchased and sliced to 3–5 mm thickness and then placed on a tray in a single layer. No pretreatment was given to tomatoes. Moisture content of sliced tomatoes has been reduced from 93.70% (w.b) to 6.29% (w.b) using air temperature in the range of 32–56 °C in 15 h only. The mean ambient conditions for wind speed, relative humidity and solar radiation were 0.77 m/s, 29% and 558.30 W/m2 respectively. A more drying time was observed due to low solar intensity (558 W/m2) and inconsistence between two days of drying. Solar dried tomatoes are found to be superior in terms of color, flavor and taste than oven dried tomatoes. During last part of drying process for tomatoes, a discolourization and growth of offflavors may take place with elevated temperature. To avoid above problem, author used a thermostat and set it at a temperature of 60 °C. Author used a cylindrical shape for collection of maximum solar irradiations but a special arrangement was required for covering and removing the plastic sheet. Due to use of special arrangement for plastic sheet and thermostat, the cost of drying slightly increases. Result showed that drying technology is moneymaking for both small and large scale farmers in Zimbabwe. The effect of convective and evaporative heat transfer coefficient under forced convection mode for greenhouse drying of papad was investigated by Kumar Mahesh [62] at Hisar, India. The experimental setup for greenhouse drying of papad in forced convection mode is shown in Fig. 29. The greenhouse of was constructed by PVC pipe and a fan is used to provide air velocity of 5 m/s. The UV plastic sheet was used to cover the roof type even span greenhouse. For all papad samples, he observed the variation of 6.08% in values of convective heat transfer coefficients; while the variation of 23.97% was noticed in values of evaporative coefficient. A linear regression analysis were used to determine the constants (C and n) in the Nusselt number expression and found to be 0.996 and 0.194 respectively. The experimental error in terms of percentage uncertainty was also evaluated and found to be in the range of 23–45%. Later on, same greenhouse was used by Kumar [63] in 2014 for drying of khoa to study the effect of size on

Fig. 29. Greenhouse for papad drying under active mode.

Fig. 30. Greenhouse for khoa drying in passive mode.

heat and mass transfer coefficients under passive mode. Fig. 30 shows the experimental setup for drying of khoa samples. Study reveals that both heat and mass transfer coefficient increases with reduction in size of khoa samples. An increase of 59% and 52% was observed in average value of convective heat and mass transfer coefficient by reducing size of khoa sample size from 0.0000675 m3 to 0.0000075 m3 respectively. The experimental error in terms of percent uncertainty has also been evaluated and found to be 33.54%. Size of khoa plays a significant role in its drying. A modified solar tunnel dryer with biomass heating source for day and night operation was presented by Amunugoda et al. [64] at Colombo, Sri Lanka. The dryer consists of a solar collector, drying tunnel, two axial fans operated by PV solar panel and heating unit. The heating unit essentially contains a combustion chamber, heat exchanging bottom plate, removable roof and chimney. The heating unit was located between the collector and drying tunnel. A number of tests were carried out on drying of Pineapple slices, Papaya, Mango and Jackfruit at five different

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Exhaust Vents

End Frame

UV Stabilized Polyethylene film Door

10 4 Fig. 32. Solar tunnel greenhouse dryer.

Fig. 31. Rack type greenhouse-effect solar dryer.

locations in Sri Lanka. During trial at various locations, they found that due to addition of heating unit, a temperature difference of 22–23 °C can be maintain between ambient and drying air during night. Result showed that, modification in dryer appreciably reduced the drying period by 1–1.5 days compared to solar tunnel dryer. A biomass stove for providing supplementary heat energy in drying is an intelligent option for increasing the performance and to shorten the drying duration. The modified design of STD prevents the product from direct contact with smoke and thus preserves the product quality. During commercialization of drying technology author tried to identify the field problems that stands as obstacles for acceptance of solar dryers. Wulandania and Elsamila [65] developed a new rack type hybrid greenhouse solar dryer for drying of wild ginger under the metrological conditions of Indonesia. The dryer consist of a transparent building with 144 trays, blowers (80 W, 04 Nos.), biomass stove and heat exchanger. The photograph of rack type greenhouse dryer is shown in Fig. 31. The performance of greenhouse was obtained for three conditions i.e. without product, with 21 kg and 60 kg of wild ginger and they found that 60 kg of ginger was dried in 30 h at 47 °C. Result showed that drying of 60 kg of wild ginger gives the best performance showing drying efficiency of 8% and total energy consumption of 29 MJ/kg vapor. For continuous operation of greenhouse a biomass fuel (wood) was used in biomass stove. Result showed the fair quality of produce due to pale color, shrinkage and hardness. The new design proposed by author does not maintain the uniform drying temperature throughout the racks due to energy loss from transparent building to surrounding. The lower drying efficiency of 8% shows that the full load capacity of greenhouse is not utilized and distribution of energy may be improper. Arun et al. [66] designed and developed a natural convection solar tunnel dryer for investigating the drying characteristics of tomatoes. Experiments for drying 30 kg of tomatoes were carried out in month of June 2014 under the metrological conditions of Pollachi, India. The solar tunnel dryer of size 4 m (w)  10 m (L)  3 m (H) was constructed using locally available materials and shown in Fig. 32. The exhaust vents were used at roof top for removal of humid air. To

improve the performance of STD, cement flooring was coated with black paint. A metallic racks were used for loading of tomato slices. During trial they observed that the drying air temperature was more than surrounding temperature by 22–25 °C. Also the maximum solar intensity of 737 W/m2 and ambient temperature of 39 °C was recorded during experimentation. Result showed that the moisture content of tomato slices has been reduced from 90% (w.b) to 9% (w.b) in 29 sunshine hours, whereas traditional open drying took 74 h for the same level. Basunia et al. [67] designed and constructed (12 m  2 m) a semicircular solar tunnel dryer for drying of fish. The dryer consists of solar collector and a drying tunnel enclosed with 0.2 mm thick plastic sheet. The trial on 51.5 kg sardines with initial moisture content of 66.5% (w.b) was taken under the metrological conditions of Oman. The capability of dryer was 100 kg but due to unavailability of fish on trial day merely 51.5 kg of fish was spread on a wire mesh net. Moisture content was decreased from 66.5% (w.b) to 15.3% (w.b) within 3 days in STD, while 7 days were required in open sun drying. This shows a significant reduction of drying time in comparison with open sun drying. As drying area of STD was not entirely utilized, the average drying efficiency of 29.5% was observed. During testing, average air temperature and relative humidity in collector and drying tunnel were found to be 44.5 °C, 44.35% and 52.7 °C, 37.1% respectively. Result showed that solar tunnel dryer can be used for drying of fish and other agro-commodities. The performance of solar tunnel dryer for drying of 300 kg limes was also reported by Basunia et al. [68] in 2013 under the weather conditions of Oman. The dryer consists of a solar collector (6 m  2 m) and drying tunnel (10 m  2 m) covered with stabilized plastic cover. The drying air was forcefully supplied (0.1–0.3 m/s) in drying chamber with the help of solar operated fan. During experimentation an increase in drying air temperature of 5–30 °C above ambient was observed. The performance of dryer was compared with open sun drying and found that solar dryer took only 70 h for reducing moisture content of limes from 86% (w.b) to 10% (w. b) whereas the open sun drying took more than 30 days.

3. Conclusion The significant outcomes from this study are summarized below: 1. Numerous field tests in region of different climatic conditions have shown that solar tunnel and greenhouse dryers are ideally suitable to preserve vegetables, food crops, estate crops and marine products. 2. Greenhouse effect (GHE) technology appreciably improves the quality of produce and reduces the drying time as compared to the traditional open sun drying method. 3. From the referred literature, it has been observed that solar tunnel and greenhouse dryers were effectively operated under natural and forced convection mode but an auxiliary heat

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4.

5.

6.

7.

8.

energy and forced convection are strongly recommended for assuring consistency and better control for drying. The desired drying air temperature can be achieved by simply adjusting the collector length and by altering the air flow rate in solar tunnel and greenhouse dryers. The PV ventilated solar tunnel greenhouse dryers are smart and reliable alternative to the natural sun drying method but such dryers must be optimized for proficient operation. Experiments reveal that solar food processing should not be viewed as merely an energy saving method, but it has great value addition because of unmatched food quality it can deliver. The drying system should have a maximum utilization factor and must have a backup of thermal storage during night operation. Computer simulation models should be used in designing and optimization of solar drying systems.

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