Energy Conversion and Management 51 (2010) 813–820

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Energy Conversion and Management

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Design and performance evaluation of a new hybrid solar dryer for banana

B.M.A. Amer a,*, M.A. Hossain b, K. Gottschalk c

a Agricultural Engineering Department, Faculty of Agriculture, Cairo University, Giza 12613, Egyptb FMP Engineering Division, Bangladesh Agricultural Research Institute, Gazipur 1701, Bangladeshc Leibniz-Institut für Agrartechnik Potsdam-Bornim, 100 Max-Eyth-Allee, 14467 Potsdam, Germany

a r t i c l e i n f o

Article history:Received 9 July 2008Received in revised form 18 April 2009Accepted 21 November 2009

Keywords:Banana dryingHeat exchangerReflectorSolar dryerStorage energy

0196-8904/$ - see front matter � 2009 Elsevier Ltd. Adoi:10.1016/j.enconman.2009.11.016

* Corresponding author. Tel.: +20 2 23793704; fax:E-mail address: baherbmaa@yahoo.com (B.M.A. Am

a b s t r a c t

A hybrid solar dryer was designed and constructed using direct solar energy and a heat exchanger. Thedryer consists of solar collector, reflector, heat exchanger cum heat storage unit and drying chamber.The drying chamber was located under the collector. The dryer was operated during normal sunny daysas a solar dryer, and during cloudy day as a hybrid solar dryer. Drying was also carried out at night withstored heat energy in water which was collected during the time of sun-shine and with electric heaterslocated at water tank. The efficiency of the solar dryer was raised by recycling about 65% of the drying airin the solar dryer and exhausting a small amount of it outside the dryer. Under Mid-European summerconditions it can raise up the air temperature from 30 to 40 �C above the ambient temperature. The solardryer was tested for drying of ripe banana slices. The capacity of the dryer was to dry about 30 kg ofbanana slices in 8 h in sunny day from an initial moisture content of 82% to the final moisture contentof 18% (wb). In the same time it reduced to only 62% (wb) moisture content in open sun drying method.The colour, aroma and texture of the solar dried products were better than the sun drying products.

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1. Introduction

Sun drying of agricultural products is the traditional methodemployed in most of the developing countries. Sun drying is usedto denote the exposure of a commodity to direct solar radiationand the convective power of the natural wind. Sun drying offersa cheap method of drying but often results to inferior quality ofproducts due to its dependence of weather conditions and vulner-ably to the attack of dust, dirts, rains, insects, pests, and microor-ganisms [1]. Solar drying is an alternative which offers severaladvantages over the traditional method and it has been developedfor various agricultural products. Solar energy for crop drying isenvironmentally friendly and economically viable in the develop-ing countries [2,3].

In natural convection solar dryers, the air flow is due to buoy-ancy-induced air pressure, and the drying process needs some daysto complete, as a cabinet dryer needs 3–4 days to dry grapes [4].While in forced convection solar dryers the air flow is providedby using a fan either operated by electricity/solar module or fossilfuel [5]. Some researchers are opting for forced convection solartunnel drying for drying of various crops [6]. They reported that so-lar drying for grapes during the night period, it is necessary to de-velop a system having a back-up of thermal storage. An auxiliaryheat and forced convection are recommended for assuring reliabil-

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+20 2 35717355.er).

ity and better control, respectively. However, there exist someproblems associated with solar drying i.e. reliability of solar radia-tion during rainy period or cloudy days and its unavailability atnighttime. In a hybrid solar dryer, drying is continued during off-sunshine hours by back-up heat energy or storage heat energy.Therefore, drying is continued and the product is saved from pos-sible deterioration by microbial infestation [7,8]. Variability andtime-dependent characteristic of solar radiation make storage nec-essary for continuous operations of food drying [9]. The operationof a solar assisted dryer extended through the night hours andfound that thermal storage during the day can be used as a heatsource during the night for continuing drying of agricultural prod-ucts and also preventing their re-hydration from the surroundingair [10–13]. Continuous drying also prevents microbial growthduring drying [14]. Also, it was found that storage and auxiliaryheat supply can used to assess compatibility of solar energy tomeet the drying process temperature [15]. Misra et al. [16] re-ported that the advantage of storing solar heat several weeks foruse in grain drying was to enable drying to proceed independentlyof the fall weather conditions. This allowed management flexibilityin harvesting and drying the crops. The major disadvantage wasthat it required more hardware, in the form of a large heat storagestructure and heat recovery equipment, which could lead to exces-sive cost.

Some hybrid dryers were developed to control the drying airconditions throughout the drying time independent of sun-shineespecially at night when it is not possible to use the solar energy

Nomenclature

_m mass flow rate (kg/s)A surface area (m2)Cp specific heat (kJ/kg K)hL latent heat of vaporization (kJ/kg)I solar radiation (W/m2)Q amount of heat energy (kJ)t time (h)T temperature (K)g efficiency (decimal)

Subscriptsa airc collectord day time

f fang globali inleto outletp pumpn night timet totalw water

Table 1Component and specifications of the hybrid solar dryer.

Component Specifications

1. Solar collectora. Type Flat plateb. Area 5.04 m2c. Transparentsurface

Glass, 4 mm thick

d. Absorber Plate Corrugate sheet-metal (280 � 180 cm), 2 mm thickf. Collector tilt 0 (horizontal)g. Reflectors Brilliant aluminium (180 � 80 cm), 2 mm

thicknessh. Insulation Polystyrene 50 mm thick

2. Water tanka. Size 500 lb. Insulation Made of fibre-glass, 50 mm thick

3. Water pumpCapacity 20 l/h

4. Counter heat exchangera. Type of tube Copper (70 tubes)b. Dimensions 180 cm length and 15 mm thick

5. Drying chambera. Area 5.04 m2b. Height 20 cmc. Insulation Polystyrene 50 mm thickd. Tray 16 tray made of aluminium meshed, (90 � 70 cm)

6. Blowera. Type Axialb. Capacity 0.75 kW

7. Auxiliary HeaterCapacity 6 kW

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using alternative sawdust burner, [17] or by using a biomass stove[18]. It is reported that significant improvement was registeredafter the heater is added to the solar dryer during periods of lowsun-shine [19,20]. Tasmparlis [21] was found that using the hybridsolar dryer connected by heating unit (20 kW) was reduced thedrying time of the grapes to 30–40 h and the air velocity of0.8 m/s produced by fan was homogeneous but small, which re-sults in slow drying rates, hence large drying periods, also the qual-ity for the dried fruits was very high. A solar assisted hybrid drierwas developed in Asian Institute of Technology, Thailand for dryingof fruits and vegetables. The drier is a tunnel type and back-up en-ergy was provided with biomass burning during off sun-shine per-iod [22].

Banana (Musa sapientum L.) is one of the important tropical fruitin the world. The ripe fruit contains many of the necessary ele-ments that are essential for a balanced diet. Banana contains fat,natural sugars, protein, potassium and vitamins A, B complex andC. A ripe banana easily digests and it imparts quick energy. Bananacan also be used as medicinal fruit. It can help recover anaemia,blood pressure, brain power, constipation, depression, hangovers,ulcer, etc. [23]. Banana is a climacteric fruit with soft texture andit becomes more vulnerable to be spoiled during transportation,preservation and marketing. Due to high moisture content in bana-na, it is wounded and contaminated during handling and transpor-tation and quality is deteriorated at high temperature and relativehumidity. Both qualitative and quantitative losses occur duringstorage through loss of moisture, carbohydrates, vitamins, pestand disease and physiological disorders [24]. Dried banana is apopular food in many countries like Thailand. The postharvestlosses can be minimized by drying the ripe banana. Therefore,there is a scope of drying of banana in the tropical and subtropicalcountries.

An analytical model was developed for drying of sliced apple,peaches, cherries and mango in a solar cabinet dryer and in opensun drying method [25]. They were used the heat and mass bal-ance to help in designing a solar dryer, but without using it tocontrol the drying air conditions inside the dryer. Therefore, animproved and simplified model should be developed in this partof work.

Several studies have been reported on simulation of forced con-vection solar drying of agricultural products for different configu-rations of forced convection solar dryers [26,27].

Schirmer et al. [28] developed and tested a multi-purpose solartunnel dryer for drying of banana under hot and humid weatherconditions in Thailand. The capacity of this dryer was to dry about300 kg of whole (not slices) ripe banana in 3–5 days. The dryingtemperature was 40–65 �C. They reported that solar dried banana

was high quality products in terms of flavour, colour and texture.Phoungchandang and Woods [29] developed mathematical modelfor solar drying of whole banana in Thailand. The model agreedwell with the field data. Also, the drying time by these solar dryersfor banana takes some days and it needs to reduce. In addition, thequality of the dried bananas by the recent solar dryers is not highdepends on the non-stability for the drying air temperature duringthe drying process which causes by the variation of the solar inten-sity during the period of sun-shine. Finally, the efficiency of the re-cent solar dryers not high and it needs to increase by usingmovable reflectors to increase the solar radiation receivingcapacity.

So, the of objective of this study was to design and test contin-uously the hybrid solar dryer (day and night) by storing the solarenergy in water tank during the sun-shine time to reduce the dry-ing cost, improve the quality of the dried products, and to preventthe microbial growth during the drying.

Fig. 1. Schematic diagram of a solar hybrid dryer.

B.M.A. Amer et al. / Energy Conversion and Management 51 (2010) 813–820 815

2. Description of the solar dryer

A hybrid solar dryer was designed, constructed and tested atInstitut fur Agrartechnik Potsdam-Bornim, Germany during theperiod of 2005–2006. The solar dryer consisted with several parts:solar collector, drying chamber, heat exchanger and heat storageunit. A schematic view of the solar dryer is shown in Fig. 1. Thecomponents and specifications of the hybrid solar dryer are givenin Table 1.

3. The solar collector

The dimensions of the solar collector are 2.80 m � 1.80 m. Itconsisted of transparent cover, absorber plate, heat exchangerand insulation. The transparent cover is 4 mm thick clear glass sup-ported by 60 mm wooden frame. Absorber plate is fixed below200 mm of glass cover. The absorber plate is 2 mm thick corru-gated GI sheet painted in black. Solar radiation receiving capacitywas increased by providing the collector with three solar reflectormade of brilliant aluminium of size 1.80 m � 1.80 m and 2 mmthick. The collector had variable angles that could be changedaccording to the change of the sun’s angle during the day to collecthigher amount of sun rays that fall down on the solar collector. Inaddition, the collector was placed on six movable legs with

Fig. 2. A schematic diagram of th

150 mm wheel to turn the solar collector horizontally and changeits direction according to the change of the sun’s angle. The metal-lic reflectors were closed in night and in adverse weather to pre-vent heat loss through the glass during the drying process. Thesolar drying unit was insulated by 50 mm thick polystyrene.

There were three air controllers at the inlet, outlet, and just be-fore the suction opening of the air blower to control the air flow atthe inlet, outlet and mixed air. The three air controllers could becontrolled manually using three switches according to the quantityand the velocity of air needed using a small motor fixed on eachcontroller. The motors can be opened or closed using a special elec-tric key exists on an electric board fixed at the end of the solar dry-ing unit. A schematic diagram of the air damper configuration isshown in Fig. 2.

4. The drying chamber

The length and width of the solar dryer were same as the collec-tor (2.80 m � 1.80 m). It was located directly under the solar col-lector and 200 mm under the absorber plate. It was divided intoeight parts with equal dimensions. In each of the part there weretwo trays for drying. This allows the usage of 16 drying trays inthe drying unit. The drying air is passed across the fruits spreadin thin layers on 16 horizontally stacked trays and arranged in

e air damper configuration.

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two vertical columns. Each tray was made of wooden frame andplastic net with dimensions of 900 mm � 70 mm. The drying airwas heated up in the solar collector and passed to the dryingchamber through a curved metal part at the end of the solar dryingunit. This curved metallic part had a shape of a half horizontal cyl-inder with the same width and height of the solar collector and thedryer. Through this curved part, the direction of the air could bechanged inside the solar drying unit. The drying air came fromthe solar collector through curved part to the opposite directionand turning towards the drying unit and flew over and under allthe drying trays before exhausting from the outlet. To increasethe efficiency of the solar drying unit, some parts of the hot airwas mixed with the fresh air at the end of the solar dryer and flewthrough the collector again to the solar drying unit instead of exit-ing through the outlet opening.

5. Cross-flow heat exchanger and heat storage unit

The heat exchanger consisted of a 15 mm diameter coppertubes placed inside the solar collector, 100 mm below the glassand 100 mm above the absorber plate. The heat exchanger con-sisted of 70 tubes covered the whole area of the drying collector.These tubes were fixed and put over a metal holder of width20 mm at each side of the solar collector. Two ends of the coppertubes were connected to the water storage tank with 15 mm plas-tic tubes. The capacity of the water tank was 500 l. Water flew fromwater tank and circulated through the plastic and copper tubes bya small water pump of capacity 20 l/h. The heat exchanger gave apart of the heat collected during the hours of the sun-shine, whichwas carried by air inside the solar collector, to the water inside thecopper tubes. The water passed very slowly inside the pipes to beable to take the largest part of heat carried in the air contact withthe external surface of the tubes. This water was stored inside aplastic tank of 500 l volume and insulated by 50 mm fibre-glass.The heat stored during the day in the water tank could be usedagain at night. The temperature of this water could be raised byusing 6 kW water heaters located inside the tank to reach a desiredtemperature for drying during the night as well as in adverseweather for maintaining the temperature and the humiditythroughout the drying process.

6. Experimental procedure

Several experimental runs for different drying conditions for so-lar and sun drying of banana were carried out at Leibniz Institut fürAgrartechnik Potsdam-Bornim (ATB) campus, Germany during theperiod of June–October 2005 and 2006 (Mid-European summerconditions). Fresh and uniform size of ripe banana was purchasedfrom Potsdam supermarket. Before starting an experimental run,the whole apparatus was operated for at least 1 h to stabilize theair temperature and air velocity in the dryer.

Peeled bananas was cut into 4–5 mm slices with fruit slicer andthen placed them in single layer on the drying trays in the dryer.Slice diameter was 2.9–3.1 cm and weight varied from 8.5 to11.5 g. The dryer was full loaded with 30–32 kg of banana slices.To compare the performance of the dryer with that of sun drying,control samples banana slices were placed on trays in a single layerbeside the dryer in the open sun. Drying was started after comple-tion of the loading, usually at 09:00 h and discontinued up to reachthe final moisture content of banana slices. Weight loss of both thesamples in the solar dryer and the control samples in the open sunwere measured during the drying period in day time (09:00–17:00 h) at 5 min interval with an electronic balance (BP 310S, Sar-torius AG Göttingen, Germany) equipped with data logger. Duringthe day time, the positions of the collector and its reflector were

adjusted with the solar angle so that maximum solar radiationcan be captured by the solar collector as well as by the reflector.A data logger (Almeno 5590, Ahlborn Mess-und RegelungstechnikGmbH, Germany) was used to record the ambient air, collectorair, drying air (on different trays), inlet air and outlet air tempera-ture and relative humidities at 10 min interval. Temperatures atdifferent positions of glass cover, copper tube, absorber plate ofthe collector were also recorded at 10 min interval during the dry-ing period, the position of instruments for measuring air properties(temperature & relative humidity) in the hybrid solar dryer areshown in Fig. 3. A solar meter (Solarwatt, GmbH, Germany) wasused to measure the global solar radiation and total radiation (glo-bal+reflected from reflector) during the day time drying period.Velocity of drying air was measured with an anemometer (TA-5,Airflow Development Limited, England) at and when required.The moisture content of the banana slices were measured by dry-ing the samples in a vacuum oven at 70 �C until the weight of thedried sample became stable, a according to AOAC [30]. After com-pletion of drying, the dried banana samples were collected, cooledin a shade to the ambient temperature and then sealed it in theplastic bags.

7. Efficiency calculation

The thermal efficiency of the solar collector and system dryingefficiency of the solar dryer were calculated using followingformula:

(a) Collector efficiency during day time (when solar radiationwas available):

gcdg ¼_maCpaðTi � ToÞ

AcIgð1Þ

(b) Collector efficiency during night time:

gcn ¼_maCpaðTi � ToÞ_mwCwðTw � TiÞ

ð2Þ

(c) System drying efficiency of the solar dryer:(i) Solar dryer efficiency in day time

gdd ¼mwhL

AcItt þ Qf þ Q pð3Þ

(ii) Solar dryer efficiency in night time

Drying with hot water flow without using water heater

gnd ¼mwhL

Q f þ Q pð4Þ

Drying with hot water flow without using water heater

gnd ¼mwhL

Q h þ Q f þ Q pð5Þ

8. Collector performance

8.1. Day time heat collection without water flow

The experiments were carried from June to October 2005 underMid-European summer conditions with the ambient air temperature

Ig

g θ Reflector

Imaginary cover θ θ Ir β

Δx

Air out Drying chamber

SunI

Total energy (E)Reflection to ambient

Convection to ambient Radiation to skyCover

Radiation to cover Reflection to cover Air outConvection to air

Air inConvection to air

Receiver

Drying tray

Dryer floor

T1 – air properties at collector inlet T2 - air properties at the cover of collector in one side T3 - air properties at the cover of collector in the other side T4 - air properties inside the collector in one side T5 - air properties inside the collector in the other side T6 - air properties at collector outlet/dryer inlet T7 - air properties inside the dryer chamber (upper tray) in one side T8 - air properties inside the dryer chamber (lower tray) in one side T9 - air properties inside the dryer chamber (upper tray) in the other side T10 - air properties inside the dryer chamber (lower tray) in the other side T11 - air properties at dryer outlet

2 3

54

7

8

9

10

16

11

Fig. 3. Position of instruments for measuring air properties (temperature and relative humidity) in the hybrid solar dryer.

B.M.A. Amer et al. / Energy Conversion and Management 51 (2010) 813–820 817

ranged from 15 to 30 �C. Variations of ambient temperature, absor-ber temperature, collector air temperature with solar radiationwithout heat collection by water flow in the collector during a typ-ical sunny day is shown in Fig. 4. At the beginning of drying, collectorair temperature as well as absorber temperature increased with theincrease of solar radiation and it reached to a peak in the noon andthen decreased at slower rate in the afternoon with the decreaseof solar radiation. During this drying period average collector airtemperature was 54.31 �C which was 27 �C above the ambient tem-perature. During this typical sunny day variation of collector thermalefficiency with solar radiation is given in Fig. 5. Collector thermalefficiency rose up to 75% at the peak solar radiation of about700 W/m2. During the day time (09:00–18:00) average collectorthermal efficiency was 58.23%. This higher efficiency was founddue to the use of solar reflector.

Fig. 4. Variations of ambient temperature, absorber temperature, collector airtemperature with solar radiation without heat collection by water flow in thecollector during a typical sunny day.

8.2. Day time heat collection with water flow

When water flew in the collector from the water tank, thenwater was heated along with the air in the collector and storedin the water tank. By this way water temperature rose up and thishot water was circulated again at night for heating the air in thecollector. Variation of air and water temperatures with solarradiation for water flow in a typical sunny day is shown in Fig. 6.

Fig. 5. Variation of collector thermal efficiency with solar radiation.

Fig. 6. Variation of air and water temperatures with solar radiation for water flowin a typical sunny day.

Fig. 7. Variations of air temperatures on different trays during solar drying ofbanana.

Fig. 8. Variations of air and water temperatures with solar radiation in day andnight.

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At the end of the day (afternoon) water temperature in the watertank increased above 40 �C from initial morning temperature ofabout 15 �C. In this drying system, collector and drying chambertemperatures were little lower than those of collector heatingwithout water flow but not below the desired temperature (40–50 �C).

Variations of air temperatures on different trays are shown inFig. 7. At the beginning of drying temperature was low and in-creased slowly and then increased sharply with the increase of so-lar radiation. The reason might be that at the initial stage of drying,banana moisture content was high (about 82%, wb) and moremoisture was evaporated from the banana surface. Due to evapo-rating cooling, the increase of drying air temperature was slow.After 3 h of drying when free water was dried up from the bananasurfaces then, temperature rose sharply. There was no significantdifferent of temperatures among the trays but outlet air tempera-ture was low due to the effect of outside cool environment. Therewas a small temperature difference (max. 4 �C) for temperaturesmeasured at different locations inside the solar drying unit ontop, middle and bottom. This indicated uniform temperature distri-bution inside the present solar drying unit. Also, the temperaturedifference was (max. 2 �C) between air outlet of the solar collectorand the air inside the drying chamber.

The variations of the ambient air temperature, water tempera-ture, collector air temperature and the air temperature inside the

solar dryer with the solar radiation for a typical day during the so-lar drying of banana slices are shown in Fig. 8. During the day time(sun-shine hours) solar radiation was used to heat up the air in thecollector as well as heated the circulated water in the collector. Inafternoon (after 16:00 h) the intensity of solar radiation began todecrease and collector air as well as the drying chamber tempera-ture reduced. At 17:00 h, solar radiation reduced sharply and dry-ing air temperature reduced to 35 �C. Then water heater was madeon and water temperature in the storage tank was set at 70 �C. As aresult, water temperature rapidly heated up to 70 �C. This hotwater was circulated in collector and collector air and drying airtemperatures rose again about 50 �C. Water heater was made onup to next morning (09:00 h). At 09:00 h, intensity of solar radia-tion was increased and the water heater was made off. This proce-dure was continued up to the end of the drying.

9. Dryer performance

Fully loaded experiments were performed to check the capacityof the dryer. A single layer of 4–5 mm thickness of banana sliceswith initial moisture content 75–82% (wet basis) was used in thisstudy. It was found that the capacity of dryer was 30–32 kg of this

Fig. 10. Comparison of solar and sun drying of banana slices in bad weather (withauxiliary heating).

Fig. 9. Comparison of solar and sun drying of banana slices in sunny weather(without water heating).

Table 2Comparison of drying system efficiency.

Drying method Drying systemefficiency (%)

Only solar dryer without water heating 37.4Heating before using solar dryer (morning) 31.7Auxiliary heating without sun-shine (bad weather) 25.3Heaters using a low intensity of solar radiation

(afternoon)6.4

Fig. 11. Comparison of drying of banana in present study with other investigators.

B.M.A. Amer et al. / Energy Conversion and Management 51 (2010) 813–820 819

product. The system is worked as a solar dryer only, without heat-ing, during the sun-shine, but with using the water as a storagemedia for the heat during the time off the sun-shine. It was ob-served that drying air temperature rose from 25 to 35 �C abovethe ambient air and the drying time was about 10 h. The systemwas worked as a solar dryer with a heating unit to heat up thewater inside the tank, during the time of night and it can rise thedrying air temperature from 25 to 35 �C above the ambient airand the drying time was about 8 h. When heaters were used duringnight time with a low intensity of solar radiation in the morning ofnext day after sun-shine, drying time was extended to 26 h. Toachieve the high efficiency for this solar dryer comparing to thesun drying, a comparison of solar and sun drying of banana slicesin sunny weather (without water heating) is given in Fig. 9. For adrying time about 7 h, the moisture content of the banana slicesusing the solar dryer was 0.2 kg/kg (db) and for the dried slicesby the sun drying was 0.6 kg/kg (db). In addition, a comparisonof solar and sun drying of banana slices in bad weather (with aux-iliary heating) is given in Fig. 10. For a drying time about 27 h, themoisture content of the banana slices using the solar dryer waslower than the dried slices by the sun drying.

The drying system efficiency using different drying methods isgiven in Table 2. Using only solar drying (without water heating)in a sunny day, the drying system efficiency was found highest fol-lowed by using water heater with a low intensity of solar radiation.The efficiency of the dryer when using only as a solar dryer wasfound 37.4%, whereas, it found 31.7% when heating water beforethe sun-shine and beginning the drying process by the solar dryeronly. Also, it was found that 25.3% efficiency for using an auxiliaryheating source (6 kW) for 8 h with the solar dryer when the weath-er condition was bad. The efficiency was 36%, when it was usingheaters during night with a low intensity of solar radiation beforedark coming and in the next day after sun-shine, drying time was16 h.

A comparison of drying of banana of present study with otherinvestigators is shown in Fig. 11. It is observed from the figure thatdrying rate of banana of present study is higher than the studymade by Soponnarit et al. [31] and Bhattacharya et al. [32].Soponnarit et al. [31] dried banana in a solar cabinet dryer at thetemperature of 60 �C and Bhattacharya et al. [32] dried banana insolar-biomass hybrid drier at the temperature of 50–55 �C. Dryingtime of present study, study by Soponnarit et al. [31] and Bhattach-arya et al. [32] were 8, 10 and 12 h respectively. These variations ofdrying rate may be due to varietal differences and maturity of ba-nana. Therefore, the drying performance of the dryer in this studywas found similar to other dryers.

10. Conclusions

The efficiency of the solar dryer could be raised by recyclingabout 65% the drying air again in the solar dryer. It was found thatthe best condition for collecting the solar energy during the day bythe solar dryer is using the solar reflectors with holders to move itaccording to the sun angles during the day, and by turning thedryer also according to the sun angles. Under Mid-European sum-mer conditions it can raise up the air temperature between 30 and40 �C above the ambient temperature. The solar dryer was testedfor drying of ripe banana slices. The capacity of the dryer was todry about 30 kg of banana slices in 8 h in sunny day from an initialmoisture content of 82% to the final moisture content of 18% (wb).

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The dryer can also be used with an auxiliary heat source with thedryer when adverse weather conditions exist. Using the water tankwith the solar dryer, about 15 �C can be stored in water during thetime of sun-shine. During the night, the system transfers the storedheat from the water to the air inside the solar dryer and controlsthe air temperature through the drying process at night. The col-our, aroma and texture of the solar dried products were better thanthe sun drying products.

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