A review of solar dryers developed for grape drying

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Solar Energy 83 (2009) 1698–1712 www.elsevier.com/locate/solener

A review of solar dryers developed for grape drying K.S. Jairaj a,*, S.P. Singh b, K. Srikant a a

Chamelidevi Institute of Technology and Management, Khandwa Road, Indore 452020, Madhya Pradesh, India b School of Energy and Environmental Studies, Devi Ahilya University, Khandwa Road, Indore 452001, India Received 1 December 2008; received in revised form 13 May 2009; accepted 10 June 2009 Available online 17 July 2009 Communicated by: Associate Editor I. Farkas

Abstract This paper attempts to review various solar dryers developed exclusively for grape drying on a normal scale. Many popular varieties of solar dryers, certain typical models as well as traditional methods practiced for drying grapes are presented in this paper. Technical and economical results have proved that solar drying of grapes is quite feasible. Commercialization of solar drying of grapes has not gained momentum as expected, may be due to high initial investment and low capacity of the dryers. Even, the farmer’s acceptance of solar dryers developed is not encouraging. Exhaustive research and development work has to be carried out in order to make solar drying of grapes economical and user friendly. There has been a remarkable achievement in solar drying of grapes due to sustained research and development associated with the adoption of advanced technologies. A review of various solar drying models for grapes is thus necessitated. Ó 2009 Elsevier Ltd. All rights reserved. Keywords: Solar dryers; Grape drying; Raisins

1. Introduction Food is a basic need for all human beings along with air and water. Food problem arises in most developing countries mainly due to the inability to preserve food surpluses rather than due to low production. Agricultural yields are usually more than the immediate consumption needs, resulting in wastage of food surpluses during the short harvest periods and scarcity during post-harvest period (Ekechukwu and Norton, 1997). Hence, a reduction in the post-harvest losses of food products should have considerable effect on the economy of these countries (Sodha et al., 1986). More than 80% of food is being produced by small farmers in developing countries (Murthy, 2009). These farmers dry food products by natural sun drying, an * Corresponding author. Address: H.No. 45, Sanjana Park, Near Agarwal Public School, Bicholi Mardana Road, Indore 452016, Madhya Pradesh, India. Tel.: +91 9893744483; fax: +91 731 4243620. E-mail address: [email protected] (K.S. Jairaj).

0038-092X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2009.06.008

advantage being that solar energy is available free of cost, but there are several disadvantages which are responsible for degradation and poor quality of the end product. Certain variety of food products are not supposed to be dried by natural sun drying because they lose certain basic desirable characteristics. Experiments carried out in various countries have clearly shown that solar dryers can be effectively used for drying agricultural produce. It is a question of adopting it and designing the right type of solar dryer (Pangavhane and Sawhney, 2002). Fruits and vegetables constitute a major part of the food crops in developing countries. From the limited data available on post-harvest losses in fruits and vegetables, it is understood that the actual losses are much higher. The minimum reported loss is 21%, while some references indicate estimates of above 40–50% (Sodha et al., 1986). The most notable feature is that many varieties of fruits are seasonal and many of them are consumed in their dried form to a large extent which has been made possible by the process of drying. Grape is one of the world’s largest fruit

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crops. The world production of grapes is presently 65,486 million tonnes out of which India accounts for 1.2 million tonnes (Sharma and Adulse, 2007). Drying the grape, either by open sun drying, shade drying or mechanical drying, produces raisins (Fadhel et al., 2005). Raisin is a source of carbohydrates and it contains large amounts of iron, vitamins and minerals (Doymaz, 2006). Raisins are usually included in breakfast, cereals, dairy and bakery as well as confectionery products and more recently in nutritional bars (Ramos et al., 2004). Drying is quite a simple, ancient skill. It is one of the easily accessible and the most widespread processing technology. Drying is a dual process of:  Heat transfer to the product from the heating source.  Mass transfer of moisture from the interior of the product to its surface and from the surface to the surrounding air (Ekechukwu and Norton, 1999). India receives an enormous amount of solar energy: on average, of the order of 5 kW h/m2 day for over 300 days/ year (Sodha and Chandra, 1994). This energy can be used for thermal or electrical applications. Thermal drying, which is most commonly used for drying agricultural products, involves vapourisation of moisture within the product by heat and its subsequent evaporation from the product. Thus, thermal drying involves simultaneous heat and mass transfer (Ekechukwu, 1999). Drying is the most common form of food preservation and extends the food shelf life (Sarasavadia et al., 1999). Water, a major constituent of fruits, is important in controlling rates of deteriorative reactions, including those resulting in nutrient losses (Saguy and Karel, 1980). Drying is one of the methods used to preserve fruits by lowering moisture levels below which microorganisms cannot grow and reaction rates slow down (Mahmutoglu et al., 1996). Natural sun drying of fruits is still practiced largely unchanged from ancient times in many tropical and subtropical countries. This method is the cheapest and is successfully employed in grapes producing countries (Pangavhane and Sawhney, 2002). It is traditionally practiced because there is negligible cost in processing and work of spreading and turning of the grapes (Doymaz, 2006). Different methods adopted for solar drying of grapes are discussed in this paper. 2. Traditional drying methods Historically the production of raisin from grapes by open sun drying can be traced back to 1490 BC in Greece (Sharma and Adulse, 2007). Hence, grape drying using solar energy is an age old traditional method. Some of the traditional methods followed are:

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thus generating heat in the interior of the product as well as at its surface, and thereby enhancing heat transfer. The solar radiation absorptance of the product is an important factor in direct solar drying (Ekechukwu and Norton, 1999). In this method grapes are spread in a thin layer exposed to the sun as shown in Fig. 1. They are turned over at intervals to have uniformly dried raisins. This is the cheapest and most adopted method in India, without any capital investment but involves enormous manual labour. The drying time required for natural grapes is 20 days (Fuller and Charters, 1997) and for pretreated grapes is 8–10 days (Pangavhane and Sawhney, 2002). In this method there is scope for contamination of the dried grapes. The direct exposure to intense sun radiation will also result in colour deterioration (Pangavhane and Sawhney, 2002). The unexpected weather conditions further worsen the situation. 2.2. Open sun drying with cover Open sun drying of grapes covered with a plastic sheet is as shown in Fig. 2. In this method grapes spread on a platform are covered with a transparent sheet, so that they are protected from contamination to a certain extent and weather risk is reduced. They are turned over manually at intervals for uniform drying. The drying time, in this case, also gets reduced by a day (Pangavhane and Sawhney, 2002). The quality of raisins is much better in comparison to that when dried without a transparent cover. 2.3. Natural rack dryer The drying process using a natural rack dryer is also known as shade drying. The dryer consists of 8–10 racks, each 45 m long and 1.2 m wide. The spacing between these racks is about 25 cm. The dryer is usually oriented in the north-south direction lengthwise. The racks are covered on the top by an iron sheet which will be wider than the racks, to provide a better covering from rain and excessive solar radiation as shown in Fig. 3. In certain locations side curtains are provided to protect the grapes from rain and dust. The rack which is fabricated with the help of galvanized wire appears as a wire mesh. Grape bunches are laid on the mesh in thin layers. The ambient air acts as a source of heat for drying grapes. The spacing between two rack dryers is maintained such that grapes are exposed to direct

SUN GRAPES SPREAD FOR DRYING PLATFORM

2.1. Open sun drying without cover In open sun drying, part of the solar radiation may penetrate the material and be absorbed within the product itself,

Fig. 1. Open sun drying without cover.

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SUN GRAPES SPREAD FOR DRYING PLATFORM COVER

Fig. 2. Open sun drying with cover.

which are within the belt of good solar radiation, like India (Sodha and Chandra, 1994). Methods of obtaining thermal energy from solar radiation are changing day by day. Importance is being given to control the temperature and increase the efficiency of the drying system. Detailed studies have proved the superiority of solar dried grapes over naturally dried grapes (Gallali et al., 2000; Karathanos and Belessiotis, 1997; Tiris et al., 1996). Different systems that have been developed, fabricated and tested for effective grape drying are discussed. 3. Solar dryers In solar drying, solar energy is used as either the sole source of the required heat or as a supplement source (Ekechukwu and Norton, 1999). Solar dryers used for grape drying are broadly classified as shown in the Fig. 4.

SUN

RACK

IRON SHEET ROOF

Fig. 3. Natural rack dryer.

solar radiation during the morning and late afternoon hours. During the middle of the day, the grapes are shaded by the roof or by the racks above them. Solar radiation during early morning and late afternoon hours may contribute significantly to the heat required for such drying (Lof George, 1962). Location of the racks is preferred on an open high land, without any obstacles for the free flowing air. The racks are loaded with about 15–20 kg/m2 of grapes. The grapes get dried to a moisture content of about 13% within a period of 9–15 days. Amba and Anand have discussed about solar drying of grapes using natural rack dryers in detail and their observations are found to be similar (Amba and Anand, 1972). The traditional methods used for drying of grapes produce raisins of low quality which are unable to meet the market requirements. The use of industrial dryers for grape drying helped in improving the quality of raisins. The large initial and running costs of alternative fossil fuel powered dryers presented such financial barriers that they were rarely adopted by small farmers. The disadvantages of open sun drying, shade drying as well as mechanical drying, forced farmers in many countries to look for alternate drying methods which could be a cost-effective and hygienic way of preserving fruits. The solar dryer being costeffective with no running cost creates an absolutely hygienic situation for fruit preservation. The introduction of solar dryers can reduce crop losses and improve the quality of dried product significantly when compared to the traditional methods of drying (Yaldiz et al., 2001). Solar dryers, in which air is heated by solar energy, are the most viable option for most developing countries,

 In the direct type of solar dryer, grapes are exposed directly to solar radiation or a combination of direct solar radiation as well as reflected radiation.  In the indirect type of solar dryer, grapes are not exposed directly to solar radiation but air heated by solar radiation is made to flow through them.  In the mixed type of solar dryer, grapes are exposed directly to solar radiation and also hot air is allowed to flow through them.  In the natural circulation mode, air is heated and circulated through the grapes naturally by buoyant force or as a result of wind pressure or a combination of both.  In the forced circulation mode, heated air is circulated through the grapes using motorized fans or pumps.

3.1. Direct type solar dryers In the direct type of solar dryer, solar radiation passes through a transparent cover, usually glass, to be incident on the grapes placed for drying. The glass cover reduces direct convective losses to the surroundings and increases temperature inside the dryer. 3.1.1. Solar cabinet dryer A solar cabinet dryer loaded with grapes to be dried is shown in Fig. 5. It is a small hot box, usually made up

SOLAR DRYERS

DIRECT TYPE

INDIRECT TYPE

NATURAL CIRCULATION

MIXED TYPE

FORCED CIRCULATION

Fig. 4. Classification of solar dryers.

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SUN

SUN

CHIMNEY DOOR

TRANSPARENT COVER AIR OUT

WIRE MESH TRAY WITH GRAPES

REAR PANEL TRAY WITH GRAPES

INSULATION DRYING COMPARTMENT STEEL FRAME

HOLES FOR AIR FRONT PANEL

SIDE PANEL

Fig. 5. Solar cabinet dryer.

Fig. 6. Stair case solar dryer.

of wood and having a length of about three times its width. The sides and bottom of the cabinet are painted black internally for absorbing solar radiation transmitted through the glass cover. Ventilation holes are provided at the bottom and holes are also provided on the upper sides of the dryer. Grapes are spread on aluminum trays, having wire mesh at the bottom and exposed to solar radiation, the temperature of grapes rises resulting in evaporation of moisture. This warm moist air passes through upper ventilation holes by natural convection, creating a partial vacuum and drawing fresh air up through holes provided at the base of the dryer. Ambient air enters the cabinet through holes at the bottom, passes through the grapes spread on the wire mesh which are at a higher temperature due to solar radiation through the glass cover and escapes with moisture vapours through the upper ventilation holes. An advantage of this dryer is that the quality of dried grapes is improved by reducing contamination due to dust, insect infestation and animal or human interference. This dryer when tested for drying 10 kg of grapes required about 3–4 days (Sharma et al., 1986).

radiation passing through the glazed sheet converts it into heat and raises the temperature inside the dryer to vapourise water molecules contained in the grapes. Air enters by natural convection through holes at the bottom, warms up due to solar radiation and flows through the wire mesh of the first tray, thus warming up the grapes spread on the tray. The warm air proceeds through these ports and grapes spread on the second shelf and so on. Finally, air escapes out of the chimney, carrying with it any humidity introduced by the vapourisation process. A temperature gradient is thus established inside the dryer, thereby sustaining air mobility. The dryer efficiency is found to vary from 26% to 65%, depending on the weather conditions and ambient temperature. This dryer is able to dry grapes to the required moisture content in 3 days (Halak et al., 1996).

3.1.2. Staircase solar dryer A stair case solar dryer is shown in Fig. 6. Its length is 2 m, width 1 m, thickness 0.4 m with a total glazed surface area of 1.3 m2. It has the shape of a metal staircase with its base and sides covered by double walled galvanized metal sheets and the sandwiched cavity filled with an insulating material. The upper surface is covered with a polycarbon sheet to allow solar radiation to pass through and be trapped inside. The dryer has three shelves with the upper polycarbon glazed surface also divided into three equal parts which can swing open, to provide access to the three compartments inside the dryer. A chimney of 0.1 m diameter and 0.4 m length is located at the upper end of the dryer to ease air flow. The whole set-up is placed in a north-south alignment at 30° to the horizontal. The base of the dryer has four air entry points each of 8 cm diameter. The partition walls between compartments also have four port-holes for easy air flow. The solar dryer traps solar

3.1.3. Glass roof solar dryer A glass roof solar dryer is shown in Fig. 7. The dryer is covered by two long inclined glass roofs aligned lengthwise along the north-south direction. On the roof, a cap is provided with a longitudinal slot, so that moist warm air can escape out. Due to this, partial vacuum is created inside the dryer and fresh ambient air is sucked through holes provided on the side walls facing east and west below the drying platforms. The tray and inner walls of the dryer

SUN CAP WARM AIR

SHUTTER OPEN

GLASS ROOF GRAPES PLACED ON PLATFORM

Fig. 7. Glass roof solar dryer.

COLD AIR

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SUN

INCLINED SIDE FRAME WITH POLYTHENE SHEET

HOOD

FOLDABLE SIDE FRAME TRAY

MAIN BODY

WHEELS

Fig. 8. Foldable solar grape dryer.

are painted black. Shutters are provided in the lower portion of the wall below the glass roof and above the drying platforms, so that fresh air can enter into the dryer to control the inside temperature. Grapes which are placed on the trays get heated using solar radiation entering through the glass roof and enhances the inside temperature. A foldable solar grape dryer developed and fabricated by Nair and Bongirwar (1994) that works on the same principle as that of the glass roof dryer is shown in Fig. 8. Side walls of this dryer are fabricated using aluminum sheet and painted black from outside. This dryer is capable of drying about 100 kg of grapes at a time. Inside temperature of the dryer is found to be twice that of the ambient temperature at noon. The main drawback of this dryer is that an increase in outside wind velocity may reduce the inside temperature because heating takes place on the outer surface of the dryer. 3.2. Indirect solar drying of grapes Direct solar radiation in certain cases causes surface cracking of grapes being dried. Some crops such as sweet potatoes and grapes need to be protected from direct solar radiation to avoid undesirable discolouration in the resulting product (Diamante and Munro, 1993). These crops should therefore be dried in indirect solar dryers (Muhlbauer, 1986). There are many drying systems which use the indirect means of solar drying. These systems dry grapes in such a way that they do not come in contact with direct solar radiation. It has been shown that low temperatures are desirable for quality dehydration, which can easily be achieved by indirect solar drying (Gee, 1980). Two types of indirect solar drying systems are discussed. They are: Natural circulation type and Forced circulation type.

3.3.1. Indirect type conventional solar dryer This type of solar dryer has a solar collector for heating air and a drying chamber to accommodate trays over which grapes are spread as shown in Fig. 9. The solar collector uses a transparent foil cover and a black absorber sheet. The drying chamber is covered by a transparent foil which protects the grapes from rain and dust. The solar collector collects the solar energy and heats the air entering through an inlet. The maximum temperature recorded in the drying chamber is 50 °C when the ambient temperature is 30 °C. Heated air enters the drying chamber from beneath the tray and flows upwards through the grapes carrying moisture with it. This moist air goes out of the opening provided at the top. Ventilation is provided by natural convection inside the collector and drying chamber. This effect is further enhanced by a sucking effect at the air outlet caused by wind. The loading capacity of grapes is 25 kg/m2 of collector area. The drying time required is 7–8 days. This dryer is found to be effective, efficient and economical (Eissen et al., 1985).

3.3.2. Indirect natural convection solar dryer with chimney An indirect natural circulation solar dryer, which is often known as an indirect passive solar dryer, is shown in Fig. 10. It has a solar collector, drying chamber and a chimney. The solar collector with an area of 1.0 m2 uses a matte black painted copper plate 0.002 m thick as an absorber. A glass cover of 0.005 m thick is fixed over the copper plate with an air gap of 0.08 m for the air to enter. An insulation layer of thickness 0.08 m is provided at the bottom of the collector to minimize heat losses from the back of the collector. The drying chamber of size 1.0 m  1.0 m  1.5 m made of wood is fixed with a matte black painted galvanized iron cylindrical chimney of height 0.5 m and diameter of 0.1 m. The chimney increases the buoyant force imposed on the air stream, to provide a greater air flow velocity and thus, a more rapid rate of moisture removal. This moist air within the chamber goes out through the chimney. The drying trays are loaded

SUN

AIR OUT WIND

TRAYS WITH GRAPES

3.3. Natural circulation type

TRANSPARENT FOIL

In these solar dryers, air movement is due to natural circulation. Grapes get heated due to direct absorption of heat or due to high temperature in the enclosure and then moisture evaporated from the grapes escapes out of the chamber by natural circulation of air.

AIR INLET

SOLAR COLLECTOR

DRYING CHAMBER

Fig. 9. Indirect type conventional solar dryer.

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AIR OUTLET

SUN

CHIMNEY

DRYING CHAMBER GLASS COVER

TRAY

ABSORBER PLATE AIR INLET

INSULATING MATERIAL

Fig. 10. Indirect natural convection solar dryer with chimney.

and unloaded through the door, which forms one side of the drying chamber. The time taken to dry 1 kg of untreated grapes to 18% moisture content is only 72 h (El-Sebaii et al., 2002).

3.3.3. Multipurpose natural convection solar dryer This solar dryer consists of a solar flat plate air heater, flexible connector, reducer with plenum chamber, drying chamber and chimney as shown in Fig. 11. The solar air heater consists of an absorber with fins, glass cover, insulation and frame. The air duct beneath the absorber is made using an aluminum sheet 0.5 mm thick of size 1.95 m  0.73 m  0.03 m through which air is passed. The U-shaped corrugations are placed in the absorber plate parallel to the direction of air flow. The entire unit is placed in a rectangular galvanized iron box. The gap between the bottom of the air duct and the box is filled with glass wool

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insulation. A toughened glass plate 4 mm thick, 1.82 m  0.76 m in size is fixed on the frame of a rectangular box at a distance of 0.04 m above the absorber surface. The interior of the drying chamber of size 0.35 m  0.35 m  0.70 m is made using an aluminum sheet 0.5 mm thick and the outer cover using a GI sheet, with glass wool in between them. The drying chamber is fitted with five aluminum trays 0.33 m  0.33 m having a wire mesh bottom. A door is provided at the rear end of the drying chamber for loading and unloading the trays. A chimney provided at the top of the drying chamber creates the required draft through it. The drying air flow rate increases with an increase in ambient temperature by the thermal buoyancy in the collector. The collector efficiency of this solar dryer varies from 26% to 65%. The drying period of chemically treated grapes is found to be within 4 days (Pangavhane et al., 2002). 3.3.4. Indirect natural convection solar dryer with chimney and storage material An indirect natural convection solar dryer with chimney and storage material used for drying grapes is shown in Fig. 12. The solar dryer consists mainly of a flat plate solar air heater coupled to a drying chamber. The solar air heater with an area of 1.0 m2 uses a matte black painted copper plate 0.002 m thick as an absorber. A glass cover of 0.005 m thick is fixed over the copper plate with an air gap of 0.08 m. An insulation layer of thickness 0.08 m, composed of a wooden box filled with saw dust is used to minimize heat losses from the sides and back of the air heater. A gap of 0.1 m between the absorber and the insulation layer is filled with sand as a sensible heat storage material in order to obtain heated air even after sun set. The complete air heater unit is fixed into a metal frame. The drying chamber of size 1.0 m  1.0 m  1.5 m made

SUN

SUN

AIR OUTLET

TOP HOOD CHIMNEY CHIMNEY DOOR

G.I. BOX

ALUMINUM TRAY

GLASS WOOL REDUCER CUM PLENUM CHAMBER

DRYING CHAMBER

WIRE MESH TRAYS

FLEXIBLE CONNECTOR GLASS PLATE

GLASS COVER STEP

CORRUGATED ALUMINUM AIR DUCT ALUMINUM FOIL MATRIX AIR IN

GLASS WOOL

FRAME

G.I. BOX

Fig. 11. Multipurpose natural convection solar dryer.

ABSORBER PLATE AIR INLET STORAGE MATERIAL

INSULATING MATERIAL

Fig. 12. Indirect natural convection solar dryer with chimney and storage material.

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of wood is fixed with a matte black painted galvanized iron cylindrical chimney of height 0.5 m and diameter of 0.1 m at the top to enhance the buoyant force. Five drying trays of size 0.855 m  0.8 m are made using a wire mesh fixed on an aluminum frame to hold the grapes being dried. A spacing of 0.15 m is provided between the trays. The drying trays are loaded and unloaded through the door, which forms one side of the drying chamber. The time required to dry 1 kg chemically treated grapes to 18% moisture content is only 8 h. For drying 10 kg chemically treated grapes placed in three different trays to the required moisture content is 26, 28 and 30 h. A sensible heat storage material used in the solar air heater is sand with drying air temperatures at the inlet of the drying chamber in the range of 45.5–55.5 °C (El-Sebaii et al., 2002). 3.4. Forced circulation type In these types of solar dryers air is forced into or out of the drying chamber using a blower or fan which is electrically or mechanically operated. 3.4.1. Indirect type solar fruit and vegetable dryer This type of solar dryer has two different types of solar air collectors working at low and medium temperatures fixed on either sides of the drying chamber as shown in Fig. 13. Air is sucked through the collectors by an electrically driven ventilator, which is connected to a three phase induction motor of capacity 0.5–1 kW. Air heaters are covered by polyurethane sheet on the rear side to minimize conduction and convection losses. The drying chamber is divided into three parts, in which two parts are used as drying boxes and another for service. These two drying boxes are connected individually to each of the solar heaters operating at low and medium temperatures respectively. The material holding capacity of each drying box ranges from 500 to 3500 kg. Heated air from collectors is passed through the flat bed of material placed on the tray, which is kept inside an opaque drying chamber as shown in Fig. 14. Temperature measurements are made at regular intervals using sensors. During the drying of grapes, a constant air flow rate of 400 m3/h is maintained. It is found that the air heater operating at low temperatures is able to heat air from 55 to 60 °C, while the medium temperature

DATA ACQUISITION ROOM

SUN

FAN INSULATED PIPE FOR AIR

DRYING CHAMBER

MEDIUM TEMPERATURE COLLECTOR

LOW TEMPERATURE COLLECTOR

Fig. 14. Schematic layout of indirect multi-shelf solar fruit and vegetable dryer.

air heater is able to heat air from 75 to 80 °C at noon. The time for drying 90 kg of grapes varies from 5 to 7 days depending on weather conditions (Sharma et al., 1993). 3.4.2. Solar dryer with greenhouse as collector A solar dryer with greenhouse as a collector is shown in Fig. 15. It consists of a greenhouse of length 50 m, as a collector linked to a wooden stack chamber. The dryer has trays stacked inside a wooden shed. Trays of size 2 m  2 m are fixed in the wooden chamber to spread grapes. The fan and plastic film together forms an efficient solar collector system, which can increase the air temperature inside the greenhouse by about 20 °C. Heated air inside the greenhouse passes through the trays stacked in the wooden chamber. In order to obtain a regular air flow through the trays, a fan is placed on the rear side of the stack chamber. For practical purpose of loading, the size of the dryer shed is fixed at 2 m3. In order to avoid saturation of the outlet air and to keep the water gradient of the grapes small, the length of the shed is maintained at 2 m. It is observed that recycling of air is not financially viable in this type of dryer (Fohr and Arnaud, 1992). 3.4.3. Geodesic dome fruit dryer A geodesic dome designed as a mid size indirect fruit dryer in the forced air circulation mode is shown in Fig. 16. The dryer is built using 4 ft long wooden members.

SUN TRANSPARENT FILM

SUN DRYING CHAMBER LOW TEMPERATURE COLLECTOR

DRYING BOX

DATA ACQUISITION ROOM MEDIUM TEMPERATURE COLLECTOR

INSULATED PIPE FOR AIR

Fig. 13. Layout of indirect multi-shelf solar fruit and vegetable dryer.

DRYING SHED TRAYS BLACK FILM

GREEN HOUSE

Fig. 15. Solar dryer with greenhouse as collector.

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SUN TRANSPARENT SHEET

EXHAUST

TRAYS AIR IN

FAN

BLACK ABSORBER SHEET

Fig. 16. Geodesic dome fruit dryer.

It is covered by a transparent plastic sheet and fruit trays are placed inside the dome covered with a black absorber sheet. Thirteen trays of size 3 ft  6 ft are used at the same time for drying. The ground is covered with gravel which serves as thermal energy storage. Air between the two sheets gets heated up and is circulated around the black sheet using a blower. The blower location is such that air movement is from the top to the bottom in a spiral pattern. Hot air is then forced to pass through the product at the centre of the dome to exit at the top of the dome. The air flow pattern is as shown in Fig. 16. The blower is switched on during sunshine hours and switched off during the remaining period. About 70% of moisture from the grapes is removed in 230 h (Goswami et al., 1990). 3.4.4. Solar tunnel dryer with integral collector This type of solar dryer is used for large scale drying operations. The solar dryer consists of a solar collector and a tunnel dryer arranged as shown in Fig. 17. Grapes to be dried are spread in a thin layer inside the tunnel dryer. Heat is generated by absorption of solar energy by an absorber in the collector as well as by the grapes. The collector and the dryer are arranged in parallel when, capacity of the tunnel dryers has to be large. The solar dryer is 20 m in length and 2 m in width. The collector is also of the same length and 1 m in width. The holding capacity of the tunnel dryer is up to 1000 kg of grapes. Both the collector and dryer are covered with a transparent foil. Black plastic material is laid out between the walls of collector and

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dryer. To reduce conductive heat losses on the back side of the collector, a heat insulator is provided underneath the absorber foil. A radial flow fan with a vertical axle integrated directly into the frame of the collector is able to force drying air through the collector. The highly efficient fan with backwardly curved blades is driven by a 100 W AC motor. The drying air gets heated due to solar radiation in the collector and passes along its entire length. At the end of the collector, the drying air turns through 180° and enters the tunnel dryer. To achieve uniform air distribution over the cross section of the dryer, baffle plates are installed at the reversing section as shown in Fig. 18. A wire mesh covering provided at the inlet as well as at the outlet, prevents grapes from insect infestation. During rainy season the air outlets can be closed. The pay back period for this system ranges from 1 to 3 years and it can be used as a multipurpose dryer. The drying period is found to vary between 4 and 7 days depending on weather conditions (Lutz et al., 1987). 3.4.5. Solar air flat plate collector with obstacles Fig. 19 shows a simple solar air flat plate collector with obstacles; this system is an indirect blow-dryer that operates in the forced convection mode. The system has a solar air flat plate collector acting as a hot air generator, a drying unit and a fan. The solar collector has a polycarbonate, honeycombed, transparent cover of thickness 1 cm at the top, an absorber plate of aluminum sheet painted black with a BAFFLE PLATES DRYER FRAME

AIR OUTLET

COLLECTOR AIR INLET

FAN

Fig. 18. Top view of solar tunnel dryer.

FAN

SUN DRYING CHAMBER SUN GRAPES SPREAD ON PLATFORMS COLLECTOR

TRANSPARENT COVER

COVER

TRAYS

ABSORBER PLATE

DRYER

ABSORBER FOIL INSULATION AIR INLET INSULATION

ABSORBER FOIL

OBSTACLES

SEALING FOIL

Fig. 17. Side view of solar tunnel dryer.

Fig. 19. Solar air flat plate collector with obstacles.

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thickness of 0.4 mm and obstacles of 2 cm height fixed to a thin plate are placed on an insulator of polystyrene about 5 cm thick. The collector is placed at an inclined angle of 45o. The presence of obstacles in the air stream helps to extract maximum amount of heat from the absorber fixed between the cover and insulator. Hence, the temperature of air entering the drying chamber through the collector increases. This enhances the system efficiency with minimum load losses. Drying time of grapes to the required moisture content with an air flow rate of 31.3 m3/hm2, without obstacles in the flat plate collector is 13 h 20 min and with transverse-longitudinal obstacles in the collector is 5 h 50 min. Introduction of obstacles in the air channel is an important factor for improvement of collector performance which in turn reduces the drying time of grapes (Abene et al., 2004). 3.4.6. Solar multiple layer batch dryer This type of solar dryer consists of a flat plate solar air collector, a fan and a multiple layer batch dryer as shown in Fig. 20. The air collector has a transparent foil cover and a black corrugated metal absorber. Air is sucked underneath the absorber, to prevent dust contamination on the absorber surface. A centrifugal fan placed between the collector and the dryer is used to suck air. A fan with power rating of 0.8 kW is used, this fan can be driven either electrically or mechanically. Trays made of wire mesh are installed in a container, which serves as a multiple layer batch dryer. The loading capacity of the dryer is 500 kg of fresh grapes per m2 of dryer surface or 38 kg of grapes per m2 of collector surface. Initially the air flow rate is 2000 m3/h with an air velocity of 0.35 m/s in the dryer and later it is reduced to 1400 m3/h with an air velocity of 0.25 m/s. The drying of grapes is completed in 5–6 days (Eissen et al., 1985). A similar type of dryer used for drying 10 kg of grapes completed the drying process in two and a half days (Al-Juamily et al., 2007). 4. Hybrid solar dryer This type of solar dryer consists of a solar heating unit and a drying chamber as shown in Fig. 21. The heating unit

SUN

SOLAR COLLECTOR

TRANSPARENT FOIL AIR IN

SOLAR DRYER

FAN

WOODEN FRAME

Fig. 20. Solar multiple layer batch dryer.

is organized into two units with 10 evacuated tube collectors and two flat plate collectors in each unit. The flat plate collectors heat fresh air primarily and the high efficiency vacuum tube collectors act as secondary heating elements of the pre-heated fresh air for highest possible output temperature. This solar heating unit is detachable from the drying chamber and can be used for other heating applications when the dryer is not in use. The drying chamber is a major component of the solar hybrid dryer and its role is to modulate homogeneous drying conditions in the active drying space. The drying chamber can be divided into two parts; the upper part consists of a fan, electric heaters and a system for modulating the drying air velocity. The lower part contains trolleys with trays on which grapes are spread. There are four trolleys and each trolley has 30 drying trays. The loading capacity of grapes is 16– 18 kg/m2 on the trays. The drying cabinet is metallic whose sides are insulated using polyurethane foam. The drying operation is controlled using dampers. A 17 kW electric heater is placed in the upper part of the drying chamber, with another 3 kW heater placed at the entrance of the solar pre-heated air. Three dampers are provided, one at the entrance of fresh air in the solar unit, one at the entrance of pre-heated air in the drying chamber and another at the outlet to control the drying process and utilization of solar pre-heated air. The dryer is equipped with two fans, one of which is located in the drying chamber for maintaining air circulation within the drying cabinet and another is used to drive pre-heated air from the solar unit into the drying chamber. The quality of dried grapes obtained from this dryer is quite good. The drying period of grapes is greatly reduced to around 30–40 h (Tsamparlis, 1990). 5. Hybrid photovoltaic-thermal greenhouse dryer The hybrid photovoltaic-thermal integrated greenhouse dryer shown in Fig. 22 has been developed and installed at IIT, Delhi. The floor area is 2.50 m  2.60 m. The height at the centre is 1.80 m and height of the side walls are 1.05 m from ground with 30° roof slope. The dryer has been integrated with two PV modules on the south side roof. The PV module produces DC electrical power to operate a fan for forced mode operation and also provides thermal heating to the greenhouse environment. To provide air movement in the greenhouse dryer, an opening of 0.15 m height is provided at the bottom and a further 0.10 m is provided with wire mesh. Air moves from bottom to top through a three-tier system of perforated wire mesh trays after it becomes hot. The UV stabilized polyethylene sheet fixed over the structural frame of the dryer helps in trapping infrared radiation. It also prevents unnecessary circulation of ambient air and thus maintains a desired temperature inside the greenhouse. The value of convective heat transfer coefficient of grapes dried in a greenhouse, with the grapes fully mature and ripe is found to be 0.45–1.21 W/m2 K and for the grapes which are not fully

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SUN AIR OUT

DRYING UNIT FAN

EVACUATED TUBE

DOORS

HEATING UNIT

BLINDS

FAN

DRYER

FLAT PLATE AIR IN

TROLLEY

Fig. 21. Hybrid solar dryer.

of the solar dryer. The introduction of heat storage material in the air heater enhances the rate of drying and reduces drying time by nearly 12 h. The duration of time required for drying untreated grapes using different solar dryers as observed in Table 2 varies between 2.5 days to 12 days.

SUN PV MODULE GREEN HOUSE FAN

DOOR

Fig. 22. Hybrid PV-thermal greenhouse dryer.

mature and not completely ripe is 0.26–0.31 W/m2 K. Hence, fully mature and ripe grapes get dried earlier than the grapes which are not fully mature and not completely ripe (Barnwal and Tiwari, 2008).

6. Comparison of the methods adopted for drying of grapes A brief comparison of all the methods discussed earlier is tabulated. The observations of a few investigators who have worked on open sun drying of untreated grapes are presented in Table 1. Time required for drying untreated grapes by open sun drying method as observed from Table 1 is 20–31 days. A few investigators have used solar dryers for drying untreated grapes and their observations are summarized in Table 2. It is observed that grapes dried in solar dryers take lesser time to reach the safe level of moisture content for storage when compared to open sun drying and the quality of raisins produced are far more superior. Hence, it is feasible to use solar dryers for making raisins. The initial cost of a solar dryer can be got back in a short duration of time. The pay back period will be less when compared to the life

7. Chemical pretreatment of grapes The basic problem in grape dehydration has been the slow rate of moisture removal during the drying process. This is because the rate of moisture diffusion through the berries is controlled by the waxy cuticle of the grapes. Various chemical treatments carried out prior to the drying process have shown an increase in drying rate with reduction in drying time for the grapes to reach a safe moisture content required for storage. The dipping pretreatment not only reduces drying time but in certain cases also improves the quality of raisins produced (Pangavhane et al., 1999). Many investigators have used different chemical solutions for treating grapes along with some variety of oil combinations prior to drying. Some of their observations are summarized in Table 3. From Table 3 it is observed that dipping grapes for 2 minutes in an emulsion of 5% K2CO3 and 0.5% Olive oil enhances the drying rate. Also, time taken for drying chemically treated grapes to the required level of moisture content by open sun drying method is 5–12 days. A comparison of different types of solar dryers used for drying chemically treated grapes is presented in Table 4. It is a well established fact that chemical treatment of grapes prior to drying enhances the drying rate. The chemical solution which is suitable for reducing drying time as well as enhancing the quality of raisins is of utmost importance. The treatment with hot dipping solution causes cracking and perforation in the waxy cuticle thereby increasing the drying rate. However, the appearance, colour and texture of raisins are found to be poor. Among the various chemical treatments used at ambient temperature, the solution of K2CO3 with Olive oil is found to be

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Table 1 Comparison of open sun drying methods adopted for drying untreated grapes. S. No.

Drying characteristics

Quality of grapes

Duration for drying

References

1

Sultana grapes dried on plastic sheets spread on the ground Initial moisture content: 78% w.b. Temperature range: 23–35 °C Air humidity: 72%

15% moisture acceptable

740 h (31 days)

Karathanos and Belessiotis (1997)

2

Grapes dried on plastic sheet spread on concrete Initial moisture: 76% w.b. Dry bulb temperature: 22.6–24.2 °C Horizontal solar radiation: 22.2–23.7 MJ/m2

13% moisture acceptable

20 days (480 h)

Mahmutoglu et al. (1996)

quite popular. Hence, a right proportion of K2CO3 and Olive oil can be worked out to find the best possible combination for treating grapes. Among a variety of solar dryers used, the direct type of solar dryer is the cheapest with minimum drying time. However, the quantity of grapes that can be dried at a time is less and the quality of raisins produced is poor. The tunnel dryer is suitable for large scale production of raisins. The initial investment on a tunnel dryer is low and the calculated pay back period is less than 3 years. By introducing certain additional technical features to monitor drying air temperature and humidity inside the tunnel dryer, it is possible to enhance the quality of raisins produced to meet the international market standards. Among the indirect type solar dryers functioning in natural convection mode, the dryer designed by Pangavhane et al. is technically superior because all the important parameters have been taken into consideration while designing the collector and drying chamber (Pangavhane et al., 2002). The initial cost of this dryer is quite high when compared to its capacity. The dryer developed by El-Sebaii et al. is technically sound and economical. Provision is

made for sensible heat storage which facilitates the dryer to be used beyond sunshine hours (El-Sebaii et al., 2002). The initial investment is moderate and affordable by farmers. In the forced convection mode solar dryers, there is provision for controlling the temperature and humidity inside the drying chamber. This makes it possible to produce raisins of superior quality with much reduced drying time. The forced convection mode solar dryer developed by Yaldiz et al. is technically sound and economical (Yaldiz et al., 2001). The use of obstacles in the path of air passing through the air heater before entering the drying chamber enhances the drying rate of grapes thereby reducing the drying time to a large extent. The above mentioned method shows a notable reduction in the drying time of grapes but the quality of raisins produced is poor. The time duration for drying chemically treated grapes using solar dryers is found to vary from 5 h to 5 days depending on the type of solar dryer used and the process involved as seen in Table 4. The suitable air flow rates can be 0.5–1.5 m/s depending on the inlet drying air temperature (Yaldiz et al., 2001). It

Table 2 Comparison of different solar dryers used for drying untreated grapes. S. No.

Name of solar dryer

Drying characteristics

Quality of grapes

Air flow rate

Duration for drying

References

1

Tunnel greenhouse dryer

Grapes loaded in trays: 40 kg/m2 Initial moisture content: 76% w.b. Dry bulb temperature in dryer: 10–60 °C

13% moisture acceptable

Intermittent fan operation

12 days

Fuller and Charters (1997)

2

Indirect natural convection solar dryer

Grapes quantity: 1 kg Ambient temperature: 27–31 °C Inlet drying air temperature: 45.5–55.5 °C Max. solar radiation: 988 W/m2

18% moisture acceptable

Not mentioned

72 h

El-Sebaii et al. (2002)

3

Indirect natural convection solar dryer with storage

Grapes quantity: 1 kg Storage material: sand Ambient temperature:27–31 °C Inlet drying air temperature: 45.5–55.5 °C Max. solar radiation: 988 W/m2

18% moisture acceptable

Not mentioned

60 h

El-Sebaii et al. (2002)

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Table 3 Comparison of open sun drying methods adopted for drying chemically treated grapes. S. No.

Drying characteristics

Treatment

Quality of grapes

Duration for drying

References

1

Sultana grapes spread on plastic sheet Initial moisture: 78% w.b. Temperature range: 23–35 °C Air humidity: 72%

Immersed for 2 min. in solution of 2% KHCO3 and 0.2% Olive oil

15% moisture acceptable

179 h (7.5 days)

Karathanos and Belessiotis (1997)

2

Grapes spread over wire mesh on roof top Average diameter of grape: 0.024 m Initial moisture: 4.05 d.b. Ambient temperature: 31–43 °C Solar radiation: 1.10–2.93 MJ/m2h

Immersed for 2 min in emulsion of 5% K2CO3 and 0.5% Olive oil

15–17% moisture acceptable

7000 min (117 h)

Togrul and Pehlivan (2004)

3

Sultana grapes spread on plastic net Initial moisture: 74–78% w.b. Day temperature: 25–35 °C Relative humidity: 15–80%

Immersed for 3 min. in 2.5% sultafino oil, 2% K2CO3 and emulgator

18% moisture acceptable

Around 7 days

Lutz et al. (1987)

4

Sultana grapes spread over a grid support Initial moisture: 5–6.2 w.b. Temperature range: 20–45 °C

Immersed 2–3 times for 2–3 s in 1% NaOH solution at 90 °C

16% moisture acceptable

250 h (10 days)

Fadhel et al. (2005)

5

Sultana grapes spread on paved grounds Ambient temperature: 5–32 °C Wind speed: less than 5.4 m/s Total solar radiation: >630 W/m2

Dipped in a solution of 5% K2CO3 and 0.5% Olive oil

Acceptable

12 days

Tiris et al. (1996, 1994, 1995)

6

Thompson seedless spread on plastic net Initial moisture: 349.59% d.b. Temperature range: 25.9–40 °C Solar radiation: 605–673 W/m2

Dipped for 3 min into a solution of 2.5% K2CO3 and 2% dipping oil

17% d.b. browning colour

7 days

Pangavhane and Sawhney (2002)

has been shown by Abene et al. that working with low air flow rates is economical (Abene et al., 2004). Abene in his work has shown that the air flow rate has to be low when working at low temperatures and air flow rates can be high when working at higher temperatures. All solar dryers discussed so far are working at low temperatures and hence have lower air flow rates.

pricing. One of the most important criteria preferred in raisins is its colour. Perfectly green or grey-green coloured raisins are usually imported from Afghanistan. Addition of Olive oil to the chemical solution used for dipping grapes before drying helps in giving attractive colour, texture and appearance. 9. Conclusion

8. Quality parameters for raisins The quality of raisins produced is dependent on many factors. Chemical treatments carried out before harvesting also play an important role in deciding certain parameters. During harvesting the average maturity of berries in the vineyard has to be more than 22o Brix for good colour and better quality. Raisins of good quality should have the following characteristic features:  Good and uniform appearance in terms of its colour, size and smooth texture.  A higher pulp content and a pleasing taste without any sugar coat outside.  Intact skin and its outer layers, free from injuries, dust and foreign matter (Sharma and Adulse, 2007). Raisins meeting the above mentioned qualities are acceptable in the international market and fetch better

It has been established that solar drying of grapes is technically feasible and economically viable. The chemical treatment of grapes prior to drying decreases drying time required to reach the safe moisture content for storage. Addition of certain variety of oils to the chemical solution used for treating grapes enhances the quality of raisins. An indirect type of solar dryer with forced air circulation can be used to produce superior quality raisins acceptable in the international market. Drying time can be further reduced using the same system with heat storage material. Economically sound farmers capable of moderate investments can choose solar dryers according to their individual requirements. In order to encourage small and marginal farmers to use solar dryers, it is necessary to develop a simple, effective and economical natural convection solar dryer. A multipurpose solar dryer capable of drying a variety of agricultural products on a large scale would be a boon to small and marginal farmers.

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Table 4 Comparison of different solar dryers used for drying chemically treated grapes. S. Name of solar No. dryer

Drying characteristics

Treatment

Quality of grapes

1

Tunnel dryer

Sultana grapes Initial moisture: 74–78% w.b. Temperature: 25–35 °C Relative humidity: 15–80% Average radiation: 6 kW h/m2 day

Immersed for 3 min in 2.5% sultafino oil, 2% K2CO3 and emulgator

18% moisture Good quality

2

Tunnel dryer

Sultana grapes Initial moisture: 5–6.2 w.b. Max. product temperature: 60 °C

Immersed 2–3 times for 2–3 s in solution 1% NaOH heated to 90 °C.

16% moisture Best quality

3

Direct type natural convection solar dryer

Grapes Ambient temperature: 22–31 °C Relative humidity: 25–48% Solar radiation: 180–920 W/m2

Immersed for 1 h in a solution made from 7 g of Na2CO3/ l of water + 1 tsp. of Olive oil

14% moisture 1.5 m3/min. Acceptable quality

4

Indirect natural convection solar dryer

Sultana grapes Initial moisture: 5–6.2 w.b. Temperature range: 20–45 °C

Immersed 2–3 times for 2–3 s in alkali solution 1% NaOH heated to 90 °C

16% moisture Better quality

Not mentioned 77 h

Fadhel et al. (2005)

5

Indirect natural convection solar dryer

Thompson seedless Initial moisture: 349.59% d.b. Daily mean inlet temperature in dryer: 51.9–64.6 °C Solar radiation: 605–673 W/m2

Dipped for 3 min into a solution of 2.5% K2CO3 and 2% commercial dipping oil

17% d.b. Better quality

Not mentioned 4 days

Pangavhane et al. (2002)

6

Indirect natural convection solar dryer with storage

Grape quantity: 1 kg Storage material: sand Ambient temperature: 27–31 °C Drying air temperature:45.5–55.5 °C Max. solar radiation: 988 W/m2

Dipped for 60 s in boiling water with 0.3% NaOH and 0.4% Olive oil

18% moisture Not mentioned 8 h Acceptable quality

7

Forced convection solar dryer

Sultana grapes Ambient temperature: 10–32 °C Air inlet temperature:30–60 °C Relative humidity: 40–87% Solar radiation >630 W/m2

Dipped in a solution Acceptable quality of 5% K2CO3 and 0.5% Olive oil

Not mentioned 5 days

Tiris et al. (1996, 1994, 1995)

8

Forced convection solar dryer without obstacles

Grapes Solar radiation:520–960 W/m2 Ambient temperature: 7–27 °C Relative humidity: 40–90%

Not mentioned

31.3 m3/hm2

Abene et al. (2004)

Duration for drying References

1200 m3/h 600 m3/h

Around 5 days

Not mentioned 119 h

3 days

13 h 20 m

Lutz et al. (1987)

Fadhel et al. (2005)

Halak et al. (1996)

El-Sebaii et al. (2002)

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Acceptable quality

Air flow rate

82 h Yaldiz et al. 66 h (2001) 58 h 0.16 kg water/kg dry matter Acceptable quality Forced convection solar dryer 11

Dipped in solution containing 6% K2CO3 and Sultana grapes 0.5% Olive oil Packing density: 16 kg/m2 Initial moisture: 2.6–3.3 kg water/ kg dry matter Drying air temperature: 32.4– 40.3 °C Drying air humidity: 57.73– 75.11% Solar radiation: 790.3–802 W/m2

1.5 m/ s 1.0 m/ s 0.5 m/ s

Al-Juamily et al. (2007) 82 h Forced convection solar dryer 10

18% moisture Acceptable quality

Forced convection solar dryer with obstacle type – TL

Grapes Not mentioned Initial moisture: 80% Chamber temperature: 65 °C Relative humidity in the chamber: 30%

0.4 m/s

1711

References

9

Grapes Solar radiation: 520–960 W/m2 Ambient temperature: 7–27 °C Relative humidity: 40–90%

Not mentioned

Acceptable quality

31.3 m3/ hm2

5h 50 m

Abene et al. (2004)

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