Renewable Energy Vol. 2, No. 6, pp. 577 586, 1992 0960 1481'92 $5.00 +.l)0 Printed in Great Britain. Perganlon Press Ltd

I N V E S T I G A T I O N OF A N I N D I R E C T TYPE M U L T I - S H E L F SOLAR F R U I T A N D VEGETABLE D R Y E R

V. K. SHARMA,* A. COLANGELO and G. SPAGNA

ENEA C.R.E., Trisaia, Area Energetica-Dipa'rtimento SIRE, Divisione Ingegneria Sperimentale, Italy

(Received 17 January 1992 : accepted 27 February 1992)

Abstract Design details and the performance studies carried out with indirect type multi-shelf fruit and vegetable dryer, are reported. The system investigated comprised plastic covered flat plate collector, drying box, and thermally and acoustically insulated pipes, joining the two. Experimental results for drying a variety of fruits with and without chemical pretreatment and under different drying conditions have been analysed. In drying the fruits it has been observed experimentally that use of chemical pretreatment offers not only a significant increase in drying rate but also higher dryer efficiency with better quality dried end product.

INTRODUCTION

Since fruits are grown in hot sunny areas, and also since they are harvested and dehydrated during the time of the year that solar radiation is abundant, solar drying as a means of food preservation seems to be the most promising and modest approach for pres- ervation of various agricultural products. Solar de- hydration offers the advantages of faster drying than regular sun drying, greater retention of vitamins especially vitamin A and C, minimizing damage from inclement rains, etc. [1-8].

Keeping in mind the basic purpose of this research paper, attempts have been made to provide a design for fruit and vegetable drying, suitable for implemen- tation with limited financial resources. The paper pre- sents experimental investigations of solar assisted dryer developed at ENEA Research Centre, Policoro (Italy). The process of dehydration of fruits and vegetables has been analysed to see the factibility of replacing both artesanal and industrial drying.

INDIRECT TYPE MULTI-SHELF SOLAR FRUIT AND VEGETABLE DRYER

The idea underlying the design of an indirect type solar dryer (Fig. l) described here is to heat a body of air using a solar air heater and then let this air pass through a fiat bed of material placed on trays kept inside an opaque drying chamber, attached with a solar air collector. The system isbased on the principle of forced convection. The experimental set-up consists

* To whom all correspondence should be addressed.

of a fan, double glazed solar air heater operating at medium temperature, and a drying chamber with a column of trays to hold the material to be dried. Design and function of the indirect type solar drying system is described examplarily for a collector and dryer arranged in series.

SOLAR AIR COLLECTOR

Double glazed solar air collector used to supply hot air at medium temperature, shown in Fig. 2, is well insulated on the rear side by 6 cm thick layer of insu- lation. Solar collector with net absorbing surface area of 20 m 2 and installed horizontally is kept on a fixed iron stand inclined at an angle of 30 ° with the hori- zontal facing south all the time. The solar air heater consists of a double transparent cover provided by two separate layers of honeycomb shaped poly- carbonate sheets held apart at a distance of 10 ram. A stagnant air gap of 30 mm between the covers and the U-shaped corrugated sheet is achieved by providing a proper support after every 1 m length of both the components. The absorber plate is coated dull black to absorb the incoming solar radiation. Air is made to flow below the absorbing sheet. An approximately 60 mm thick polyurethane sheet sand- wiched between two asbestos sheets is provided at the rear side of the collector to minimize conduction and convection losses. Outlet air from the collector is made to pass through thermally and acoustically well insu- lated rectangular air ducts (30 x 30 cm).

Air is sucked through the air collector by an elec- trically driven fan. Model VR 164 (maximum air flow

577

578

South

V. K. SHARMA et al.

1 Data acquisition room 2 Indoor d~/ing box 3 Low temperature collector 4 Medium temperature collector 5 Insulated pipes for air

- - ~ I

2 5

Fig. I. Layout for drying system investigated.

capacity 1500 m3/h) was used for supplying the desired air flow rate. The fan is run directly by a 400 W, 380 V, three phase induction motor.

MULTI-SHELF DRYING BOX

A prototype drying chamber has been fabricated out of a galvanized iron box of dimensions 90 × 90 x 120 cm. The drying cabinet houses a single column of seven product shelves (each shelf with an area of 0.64 m2). Each shelf is kept on a frame fixed to the side walls of the drying box. The drying trays can be easily removed to load or to change the position of the trays. The shelves are placed 15 cm apart from each other so as to ensure a uniform air circulation under and aroond the product. The ambient air heated

by the solar air collector is ducted to the drying chamber at the bottom of the drying box. The air will transfer its sensible heat to the shelves, and to the product on the shelves on its passage through the cabinet, and will leave at the top of the cabinet, kept open. Air flow is forced by the blower. In order to minimize thermal losses due to conduction and con- vection, etc. the drying box is well insulated from all the four sides. The drying box is connected to a solar air heater operating at medium temperature. The material holding capacity of the box depends mainly on the bulk density of the crop to be dried and ranges from 75 to 100 kg. The schematic diagram for the drying chamber is presented in Fig. 3.

The design details for the solar air heater and drying chamber are presented in Tables 1 and 2.

Polyurethane sheet sandwiched between aluminium sheets, thickness 0.06 m /

/ ~ / / x T " ~ / ,,/ ',, / V"~ x

Galvanized iron sheet painted black

Polycarbonate sheets

J .Io.o~ x / x • ).o4 1.14

Xo~7 f

2m

Fig. 2. Section of solar air collector operating at medium temperature.

0.05 m

Indirect type multi-shelf dryer

Iron sandwiched panel with insulation

s Wooden frame with plastic net

1 .2m

0.9 m

Fig. 3. Drying box.

Hot air inlet from solar collector

579

Table 2. Design details of multi-shelf drying chamber

Type of drying box

Gross dimensions of cabinet No. of products shelves used Dimension of each drying

shelf Net surface area available

for drying Material used for

construction of drying cabinet

Material used for construction of drying shelves

Mode of air flow

Air space between plenum chamber and bottom of first drying tray

Space between two drying trays

Air duct

Opaque box with forced air circulation

0.9 x 0.9 x 1.2 m Seven 0.8 x 0.8 m

4.5 m"

Galvanized iron sheet

Plywood

Transversal air flow in the upward direction through the drying material

10 cm

15 cm

Thermally insulated circular air duct was used, connecting rectangular air duct from the collector and the drying box through centrifugal blower

Table 1. Double glazed single pass solar air heater

Type of collector

Gross dimensions Net area of absorbing

surface Absorber material

Glazing material No. of glazing Thickness of glazing Insulation material Thickness Collector tilt angle with

horizontal Heat transfer fluid Mode of air flow

Blower specification

Air [low rate Location of the test

Longitude Latitude Height

Non-porous conventional solar air heater

lOx2xO.14m 9.90 x 1.95 m 2

Black painted galvanized iron sheet

Polycarbonate Two 1.5 mm Polyurethane 0.06 m 30"

Air Forced air circulation

parallel to the absorber 0.4 kW three phase

induction motor (model VR 164)

500-1500 m3/h ENEA-C.R.E. Trisaia,

Policoro (MT) 16 38'E 40 09'N 30 m AMSL

INSTRUMENTATION

The various operating temperatures were recorded by means of PT 100 sensors at regular intervals of 10 min. The solar intensity was measured by means of a Pyranometer placed at the plane of the collector. Air flow rate was calculated from the air speed measured at the air inlet section, by using a photoelectric type air flow meter.

A centrifugal fan coupled with a 400 W, three phase induction motor of variable speed was used for sup- plying air flow to the assembly. All data is measured by using an automatic data acquisition system (HP 86 data processor connected with a HP 3497 data logger). Output from the inst rumentat ion used to measure and characterize the ou tdoor environment and climatic condit ions as well as the relevant plant 's performance data was recorded by the data processor unit placed inside the service room measuring 5 × 2 m 2 and located approximately 50 feet away from the solar air collector and the drying box.

E X P E R I M E N T S

A series of experiments were conducted during summer days in the Policoro climate during 1991 at ENEA-C.R.E . Trisaia. The main objective is to estab-

580 V. K. SHARMA et al.

lish the effect of many external parameters that effect the drying process such as temperature, humidity, velocity of air stream, state of subdivision of the solid, etc. The experimental observations were continued until the product acquired approximately constant weight, i.e. it attained its equilibrium moisture content. The products chosen for the study were grapes, plums, tomatoes and figs.

Below is given the brief procedural description for the preparation of the sample to be dried.

Grapes

The grapes harvested manually were checked care- fully to discard spoiled ones, in order to prevent infec- tion of intact grapes by bacteria or fungi. To increase the water permeability of the coat the grapes were dipped in boiling water containing 0.3% soda (NaOH) and 0.4% olive oil, for a period of 10, 20, 30 and 40 s. To see the effect of the above treatment as a function of dipping time, samples prepared with and without any pretreatment as described above were placed on drying trays. Throughout the drying period a constant air flow rate [1350 m3/h] was maintained. Quality of the drying product was judged in terms of taste, colour aroma and texture.

Plums

The drying tests using plums were conducted during the month of October 1991, using the solar air col- lector at medium temperature. The samples were dipped for 15, 30 and 45 s in boiled water containing 0.3% of NaOH. Separately, another sample was pre- pared to see the effect of using olive oil along with NaOH as well. The samples were placed uniformly on a tray. For ventilation, an air flow rate value of 1350 m3/h was maintained throughout the drying period. Under identical climatic and operational conditions, depending on the duration of chemical pretreatment, the drying process could be completed within 3-7 days. However, it has been observed that drying rate was very slow in cases where no chemical treatment was given.

Tomatoes The drying tests were carried out with the "Italpil"

variety. Two separate samples were prepared using fresh tomatoes purchased from the market. In one of the samples, tomatoes used were in their original shape and size while the second sample was prepared using tomatoes cut into pieces. The main objective of using the second sample was to see the effect of the membrane formed on the drying rate, i.e. on the pro- cess of dehydration. Both the samples were placed on a drying tray in a single layer. A constant air flow rate of 1350 m3/h was maintained during the drying period.

The test was conducted during the month of October 1991.

F/ffs Fig was another seasonal fruit dried. Samples were

prepared, as discussed above for tomatoes. The test was conducted during the month of October 1991.

RESULTS AND DISCUSSION

The dehydration of a few fruits and vegetables was carried out in the solar drying system designed and fabricated at ENEA-C.R.E., Trisaia. Experimental tests have shown that the quality of the product is influenced by many factors such as chemical pre- treatment, drying conditions, physiological charac- teristics, etc. For example, chemical pretreatment of fruits effects not only the quality of the dried end product but drying period as well. Drying temperature and air flow rate are other important parameters effecting the drying process. The tests were performed during September-October 1991.

The solar insolation, ambient temperature, relative humidity, etc. measured on the day of experi- mentation are shown in Fig. 4. The outlet air tem- perature for the solar air heater is presented in Fig. 5. As shown in the figure, depending on the weather conditons and air flow rate applied, the solar air heater can provide hot air at an temperature of 55-75 C at noon hours.

Hot air from the collector is used to dry a variety of fruits and vegetables. A series of experiments using

Oct. 12, 1991

Solar radiation (W/m 2 / 1 O)

- - - - Temperature (°C) 80

• ~ - - - Humidi ty (%)

70 •~• . . . . ssss

60 o - "

5o

40

30

20

10

0 8 110 112 114 116 118

T ime of the day

Fig. 4. Climatic conditions, 12 October 199l.

80

70

60

50

40

30

20

Oct. 10, 1991 Air f low rate = 1350 m 3 / h

~., - - " - - Tin (*C)

\ ~ Tout (°C)

\ l - - - H in(%)

Q~ - - - - Hou t (%) v, I t

Time of the day

l:ig. 5. Thermal performance of solar air collector.

different materials were conducted during the months of September-October 1991. The first set of experi- ments was conducted using grapes as the drying material. Grapes with initial moisture content of 80% were dried to achieve a final moisture content of 15%.

The total weight loss for different samples, lneasured instantaneously throughout the drying operation is presented in Fig. 6. It is clear from the curves that the moisture content of the sample decreases exponentially with drying time. To calculate

Indirect type multi-shelf dryer 58 I

the amount of water evaporated, all the samples were weighed at an regular interval of 3 h using a physical balance.

Similarly, the variation of water loss (%) as a func- tion of drying period was observed and the same has been presented (SDP/PI ; %) in Fig. 7. Observations concerning drying rate (DP/PP*h) as a function of drying time, recorded for each sample twice or three times a day, are presented in Fig. 8.

The dehydration results of another fruit tried, i.e. plums, are satisfactory. The experiment was con- ducted to dry different samples of the product with initial moisture content of 85%. From the experience gained, it is possible to say that to minimizc drying period as well as to obtain good quality dried product, chemical pretreatment is very' important. As shown in Figs 9 11, it is compulsory that during chernical pretreatment process, samples must be kept dipped in hot boiling mixture at least for a period of 35 40 s. Figures 12 14 represent results for tigs drying. No chemical pretreatment is required for drying of tiffs fruit.

Tomatoes, the only vegetable dehydrated, provided very good results without posing any problems. It was observed during the experiment that if arranged properly, the product could also be dried a little faster. Results obtained from the drying of tomatoes are presented in Figs 15 17.

After performing these tests on a laboratory scale, the dryer was also tested while loaded to its full capacity. The experiment was conducted using grapes treated chemically and spread uniformly on different drying trays. During drying, it was observed that the

Drying test w i th grapes Oct. 1-4, 1991

- - . - C1 (grams)

C2 "

220 . . . . . . C3 "

200 - - - C4

180 . : .~ - - - - C5 " 160 ~'. " ~ " ~ - • O ~ o ~ . ~

, , o c; 120 ~ """ " - -" ~ " "~ . . . . without treatment °°% 100 0 . 3 % NaOH + 0 . 4 % olive oil for 10 sec 80 \ ~ ' ~ °" . . . . . . C3 0 . 3 % NaOH + 0 . 4 % olive oil for 20 sec

",,? ~'~...~ "" . . . . . . C4 0 . 3 % NaOH + 0 . 4 % olive oil for 30 sec 60 ~ ' - ' ~ . . . . . . . . . C5 0 . 3 % NaOH + 0 . 4 % olive oil for 40 sec 40

Drying period (days)

Fig. 6. Evolution of weight loss of the product. Drying test with grapes, 1 4 October 1991.

582 V .K. SHARMA et al.

v

E_

Drying test with grapes

8 0

70

60

50

40

30

20

10

0

(CA + C5) / . . ~ ~ 0ct. 1-4,1991

1 / ~ , , , " " C1 without treatment / J , , . ' " C2 0.3% NaOH + 0.4% olive oil for 10 s e c

/ / .," C3 0.3% NaOH + 0.4% olive oil for 20 s e c

/ / ,.c CA 0.3% NaOH + 0.4% olive oil for 30 s e c

/ /,.'" C5 0.3% NaOH + 0.4% olive oil for 40 s e c

/ / - - - - C1

"' . - C2 "

F _ . . ~ o . ~ " ° - -

~ . ~ - . - - - C4

. " " " C5

Drying time (days)

Fig. 7. Evolution of total water loss. Drying test with grapes.

highest drying rate is possible at the first or second shelf from the bottom, and the lowest at the top shelf. This is because the bot tom shelves receive the greatest heat supply from the incoming hot air f rom the solar

air collector, while the top shelf receives a reduced amount of heat supply as well as cooler and more humid air. Consequently, the drying rate is the lowest at the top shelf. It is therefore necessary that to avoid

&

C3

2 0

18

16

14

12

10

8

6

4

2

0

Drying test with g r a p e s

Oct. 1-4, 1991

C1 without treatment C2 0.3% NaOH for 15 s e c

C3 0.3% NaOH for 30 s e c

C4 0.3% NaOH + 0.4% of olive oil for 30 sec C5 0.3% NaOH for 45 s e c

--] C1

m c2

m cs

1 1 1 2 2 2 3 3 3 4

Drying time

Fig. 8. Drying rate of the product. Drying test with grapes, 1-4 October 1991.

180

160

140

120

100

80

60

4O

2C

Indirect type multi-shelf dryer

Drying test with plums

Oct. 4 - 8 , 1991

- - - - C1 (grams)

C2

- - - - - - C 3 H

\ \ - - - CA "

:.~ ~. ~ . . . . . . C5 " -:, ~ • ~- ,,¢- . . . . . . .

_ ~ . :..~ • ,~ ~ ~ C1 w i thout t r e a t m e n t ~ . . ~ . . ~ ~ C2 0.3% NaOH for 15 see

~ - ' - - " c C3 0.3% NaO for 30 sec C4 0.3% NaOH + 0.4% olive oil for 30 sec

- - ' - ~ C5 0.3% NaOH for 45 sec

Drying per iod (days)

Fig. 9. Evolution of weight loss of the product. Drying test with plums, 4-8 October 1991.

583

over drying of the product at the lower shelf as well as to achieve uniform drying conditions inside the drying box, depending on the quantity and type of the product being dried, lower shelves must be exchanged by the upper ones, after a certain period of continuous drying.

As dehydration of fruits and vegetables involves a compromise between retaining nutritional quality and preventing spoilage, the finished product was evalu- ated not solely on economic grounds but with nutritional criteria as well. For this purpose, the dried

samples were tested for their culinary and organo- leptic characteristics. From the tests realized it was observed that the quality of the product using forced convection solar drying technology was much better as opposed to the open air drying.

CONCLUSIONS

The experimental results very clearly indicate the importance of chemical pretreatment in the drying mechanism. It is evident from the experimental results

13-

E3

80

70

60

50

40

3(]

20

10

0

Drying materials: plums

Date: Oct. 4-8 , 1991

S ' ~ .... . . . . .

Drying per iod (days)

C1 without treatment C2 0.3% NaOH for 15 sec C3 0.3% NaO for 30 sec C4 0.3% NaOH + 0 .4% ol ive oil for 30 sec C5 0.3% NaOH for 45 sec

- - - - C1

C2

- - - - - - C 3

- - - - C4

. . . . . . C5

Fig, 10. Water loss of the product. Drying material : plums, 4-8 October 1991.

584 V . K . SHARMA et al.

A

& 13_

Q _ E3

14

12

10

0 m

1

Drying material: plums Oct. 4--8, 1991

C1 without treatment C2 0.3% NaOH for 15 sec C3 0.3% NaOH for 30 sec C4 0.3% NaOH + 0.4% of olive oil for 30 sec

5 0.3% NaOH for 45 sec

[ ~ C1

[ "_,2. M c5

1 1 2 2 2 3 3 3 4 4 4 5

Drying period (days)

Fig. 1 l. Drying rate of the product. Drying material : plums, 4~8 October 1991.

Drying materials: figs Oct. 10-15, 1991

160 - - . - C1 (grams)

140 , C2 (grams)

~ ~ C1 Product in original shape 120 and size

• ~ . C2 Product cut into two pieces lOO

60

40

I I I I I I I I I I I 20 I i~22333~44556 Drying periods (days)

Fig. 12. Evolution of total weight loss of the product. Drying material : figs, 10-15 October 1991.

Drying materials: figs Oct. 10-15, 1991 9o

C1 As such / 80 - - ' " C2 Cut in 70 t O ~

I *

E I " 50 • ~ °

a ° 1

30 t " t

20

10

0 P' I I I I I [ L I I I J I I I 1 1 2 2 2 3 3 3 4 4 4 5 5 6

Drying periods (days)

Fig. 13. Water loss of the product. Drying material : figs, 10- 15 October 1991.

12

Indirect type multi-shelf dryer

Drying material: figs Oct. 10-15, 1991

585

a~

r -

&

n E3

10

2

C1 As such C2 Cut into two pieces

- - - ] C1

I I c2

1 1 1 2 2 2 3 3 3 4 4 4 5 5

Drying period (days)

Fig. 14. Drying rate of the product. Drying material: figs. 10 15 October 1901.

200

180

160

140

120

100

80

60

40

20

o;

. °~

\ \

\ k

Drying materials: tomatoes Oct. 10-15, 1991

C1 (grams) ~ - - C2 (grams)

C1 Product in original shape and size

C2 Product cut into two pieces

100

90

8O

70

E -~ 60 1 3 -

50

40

30 ° ~ o ,~ .o

20

"*" 10

I I I I I I I I I

1 1 2 2 2 3 3 3 4 .4 4. ; ; 6 0 Drying period (days)

15. Evolution of weight loss of the product. Drying Fig. 16. material: tomatoes, 10 15 October 1991.

Drying materials: tomatoes Oct. 10-15, 1991

/ /

/"

/"

/"

/"

i i

1 2 2

o,~

C1 - - . - C2 C1 As such C2 Cut into two pieces

J f

I I I q I I I I I I

2 3 3 3 4 4 4 4 5 6 Drying period (days)

Fig. Water loss of the product. Drying material: tomatoes. 10 !50c tobe l 1991.

586 V . K . SHARMA et al.

A

rt

O- E3

11 I- 10

9[-

8 -

7 i _

6 -

5 -

4 -

3 -

2 -

1 -

0 - - 1

Drying material: tomatoes Oct. 10-15, 1991

C1 As such C2 Cut into two pieces

[--]

m c2

1 1 2 2 2 2 3 3 4 4 4 5 5 6

Drying period (days)

Fig. 17. Drying rate of the product. Drying material: tomatoes, 10-15 October 1991.

that the samples C3, C4 and C5, treated with a chemi- cal process lasting for 30-40 s, were dried to a safe moisture level for storage just after 3-4 days of con- tinuo.us drying, whereas sample C1, without any chemical pretreatment, could not be dried even after a period of 25-30 days of continuous drying. Drying time can thus be reduced considerably by giving proper prechemical treatment for an appropriate time period to the samples before the start of the drying process.

REFERENCES

1. T. H. Jackson and B. B. Mohammed, Sun-drying of fruits and vegetables: food and agricultural industries series. Agricultural Service Bulletin No. 5. Food and Agricultural Organisation of the United Nations, Rome (1969).

2. W. Eissen and W. Muhlbauer, Development of low-cost solar grapes dryers. Proc. Int. Workshop on Solar Drying and Rural Development, Bordeaux, France, 263 pp. (1983).

3. V. K. Sharma et al. Design and performance studies of a solar dryers suitable for rural applications. Energy Convers .~gmt 26(1), 111 118 (1986).

4. V. K. Sharma, G. Rizzi and H. P. Garg, Design and development of an augmented integrated solar collector with rock storage system for heating applications. Energy Convers. Mgmt. 31,369 (1991).

5. G. Lai and A. Colangelo, L'Essiccazione dei Prodotti Ortofrutticoli." L 'Impianto S A D A A P a Collettori Solari ad Aria. Gennaio 1988, RTI/FARE(88)/3.

6. Solar Drying of Agricultural Products. CNRE Technical Meeting Stuttgart, Federal Republic of Germany, %11 September 1987. CNRE Bulletin No. 19 (1988).

7. G. Lai, A. Colangelo and G. Spagna, ]Campagna Speri- mentale Sull'Impianto de Essiccazione Solare S A D A A P del C.R.E. Trisaia. RTI/FARE (90).

8. M. A. S. Malik et al., Solar crop drying. A Study Under UNDP Global Project (GLO/80/003) ; Studies on Testing and Demonstration of Renewable Energy Technologies, submitted to the World Bank, Washington, U.S.A. (1983).