ANNALSVol. 13 No. 4

OF ARID ZONEDecember, 1974

SOLAR DRYING OF AGRICULTURAL PRODUC~SI. DRYING OF CHILLIES IN A SOLAR··

CABINET DRYER

H. P. GARG AND A. KRISHNAN

Central Arid Zone Research Institute, ]odhpiIt

( Received on 29-5-1974)

ABSTRACT

This paper reports tbe desiBn detail~ of a simplesolar .cabinet dryer developed at tbe Central Arid ZoneResearch Institute, Jodhpur. The dryer is based on theprinciple of hot box and is suitable for drying small agricultural

. produces like «billies, copra, peas, tobacco, etc. The basal areaof the dryer is 1.37 sq.m. and its volume is 0.324 Cu.m.

The solar cabinet dryer was used for drying CbiUiesin winter season having initial moisture content 91, 79 t~ 8Sper cent on wet weight basis. In the dryer the average airtemperature exceeded 22-8°C above the day time averagetemperature when chillies were kept inside. Field' (testsindicate that by the use of solar dryer the drying time forchillies can be reduce!l to ne~rly half of the open drying method.The quality of the chilli es dried by the solar dryer was. alsosuperior. Tbis result is useful since our country has areasunder chillies to the tune of 5.8 lakh hectares and tbis cashcrop is also being exported-

INTRODUCTION

';'0.--: •. " ••

. Spreading of agrkultural products in open sun on gropnd or on platform

for reducing their moisture contents either for storage or for consumption purposesis a traditional practice followed in various countries of the world since time

I~

286 I H.•:P. c,;~RQ AND A. KRISHNAN

immemorial. However this type of drying in tbe open s~n is a very slow processand is susceptible to various contaminations such as dirt, insects etc. The materialsare also exposed to adverse weather conditions. In view of these, there isdegradation of the quality of the materials dried.

Most of the above mentioned disadvantages can be remedied by usingartificial techniques for drying of the agricultural products. Mostly eitber oilfired or gas fired or electrically operated drver (Loerseche and Harry, 1955) are nsedin this connection. In view of tbe present energy crisis and shortage of fuel, it ishighly desirable that solar energy should be used in these dryers so that thesefuels used at present in the dryer viz. gas, oil and electricity etc. can be used~lternatively in some other development purposes.

Solar energy can be used for drying purposes either by collecting theenergy directly into a cabinet dryer where the material for drying isplaced or by convective type of dryer where hot air heated by solar energy isblown into a separate drying unit (Lof. 1961). The former cabinet type of solardryer is suitable for drying agficultural products wbere quantities involved aresmall, (Nahlawi, 1966) whereas the latter is suitable for drying large produces likepaddy. grounclnut. et~. and for use in grain storage systems (Buelow, 1958).The Division-of Wind Power and Solar Energy Utilisation of Central Arid ZoneResearch Institute, Jodhpur, plans to evolve a few suitable solar dryers of both thetypes mention~d .above and conduct extensive field tcsting and trials to find outtheir techno-economic feasibilities in drying different agricultural produces ..

As a preliminary study, a solar cabinet dryer of volume 0.32 cu. m.was used tor drying chillies during the winter season of 1974. In this connection,it may be mentioned that chilly is one of tbe most Important crop of Jodhpur'regio..n. This crop is also widely grown in other parts of India and occupiesan area of about 5.8 lakhs hectares in the country with an annual output of 4lakh tonnes of dry chillies (Singh and Ojha, 1972). This cash crop is extremelyuseful since apart from its wide use in home consumption the same is beingexported, outside India tbereby earning foreign exchange to our country.

In this paper the main design features of the solar cabinet tdryer developedat Central Arid Zone Research Institute. Jodhpur are presented and the results ofof the actual field tests conducted for drying chillies are discussed.

DESIGN AND INSTALLATION OF THE SOLAR. CABINET DRYER

Tbe principle of hot box has been used in aesigning the solarcabinet dryer which is made of wooden planks (25 mm. tbicb) having a

---------- •

A SOLAR CABINET DRYER 287 A

Fig. 2. Photograph of the chyer

A SOLARCo\BtNETJDRYER I 287

base area of 1.37 sq.m. and a volume of 0.324 cu. m. The dr.yer isprovided with a glass roof made of clear window glass (5 mm thick) at a fixedinclination of 23° from the horizontal (Garg and Gupta, 1967) so as toreceive maximum solar radiation year round at Jodhpur. The bottom of the dryeris insulated with 5 em. thick saw dust insulatioQ. A number of holes aredrilled in the base as well as on the sides of the dryer so th3t the humidwarm air can escape through the upper side holes thereby creating a partialvacum and inducing fresh air from the holes in the base. The inside walls ~s well asbase of t.he dryer is painted with Matt black paint for absorbing solar heat. Thedrying material can be placed on the perforated removable screen made of wiremesh which can be kept in the dryer through an openable door provided· on therear side Of the dryer. The schematic diagram of t he dryer _showing variousdimensions is presented in Fig. 1. The dryer was installed in the solar energyyard of the Central Arid Zone Research Institute on a cement platform. Thedryer was kept facing due south at a clear site. The photograph of the dryer isshown in Fig. 2.

OLASS COVER

WIRE MESH TRAY

VENTILATION HOLE

UPPER WOODEN COVE~

IN9Ul.ATlON

LOWER WOODEN COVER

LEG

9.50 mm----- ...•.•....c

OPENABLE DOORHINGE

I.t1OO0m",,..fig I SCHEMATIC DIAGRAM OF SOLAR CABINET DRYER

/MATERIALS AND METHOD

Two field trials of drying chilies in the s<;>larcabinet dryer one' in the'month of January, 1974-and another in the month of February, 1974 wereconducted. Equal quantities of chillies were kept inside as well as outside the

288 ' I H. P. GARG AND A. KRISHNAN

dryer keeping the area of exposure constant. The albedo of the freshly pickedup red chillies was measured by means of albedometer. Total bulk weights of thechilies were measured daily in the morning at 8 AM and in the evening at 6PM. Moisture contents of chillies during each day were computed from thedifference in these weights. Obviously, moisture contents obtained from bulkdifferences of weights would be more accurate than by estimating the same bysampling technique. The air temperatures inside the cabinet dryer weremeasured hourly by means of a calibrated bead type of thermistor.

The ambient air tempeatures were recorded in a thermohygrographkept in Stevenson screen of the agro-met obserntory near the solar energyyard. Total solar radiation on horiz·)ntal surface was automatically recordedwith a Kipp and Zonen pyranometer alongwith a Honeywell potentiometricrecorper.

RESULTS AND DISCUSSION

Figures 3 and 4 show the variation in percentage moisture content ofchillies taken on wet weight basis on successive days of exposure from thecommencement day in respect of the Test No. I and Test No. 2 respectively.

FIG.30 MOISTVfU; CONTENT IN CHIL.L.IES AFTER DI'FERENT NUN8EROF DAYS OF ExPOSURE IN SOL.AR CABINET DRYER & IN OPEN

I ,

8 9 10Oft DAYS

6 7NUMBER

432o

10

20

~'::L TEST ."

~ 90 o~_. O-o __ ~o

!~ 70. ~w~ ~CC60 ~;:)GI•.....!!~ ~O~:a OPENORYING~z 0

",040

"'-0- ~

30 '"

~ 0,I I , I , y---O

II 12 I~ 14 I~ 18

OF f:XPOSURE

~ SOLAR CABINET DRDR I 289

•••C)C!...Z•••IJII:IIICl.

30

20

10

o2 3 4

TEST No 2

"""'--9(I 7 8 9 10

NUMBER OF DAYS

FIe; 4 MOISTURE CONTENT IN CHILLIES AFTER DIFFERENT NuMBEROF DAYS OF f;XPOSURE IN SOLAR CABINET ORYER a IN OPEN

It is seen that the drying of chillies can be completed within 7 days in solarcabinet dryer .whereas the same will take 15 to 16 days in the open dryingmethod. Thus solar cabinet dryer reduces the drying time to less than half.The pattern of drying is similar in both the tests. In the solar dryer, thedrying Curve becomes 'steeper after 3 to 4 days from commencement when

. moisture content falls to 60 per cent or so. Similar feature occurs in respectof curve for open drying also roughly at the same moisture percentage.

Detailed climatic observations such as, total solar radiation on horizontal_ surface lKcalfm2• day), average day time temperatures ,oC) as well as maximum

and minimum temperatures lOC) of the day were reeorded during the wholeperi"d o( .Test No.2. Hourly values of the air ten~peratures insidethe dryer and the moisture contents of the chillies during morning andevening were also measured during each - day. These data are presented inTable 1.

292 HI P. GARG AND A. KRISHNAN

REFERENCES

Buelow, F. H. 1958, Drying grain with solar healed air, Qr. Bull. MechiganAgr. Exp. station, 41 (2) : 421-429.

Garg. H. P. and Gupta, C. L. 1967. Optimizing the tilt of flate platecollectors for India, J. Inst. Engrs. (India), 48 (1) : 21-28.

Lor, G. O. G. 1961, Use of solar energy for solar drying, Proc. of U. N. Conf.on New sources of Energy, Rome, 5: 248-255.

Loerecke, V. and Harry, W, 1955. Drying and dehydration of foods, Reinhold. Publishing Corporation, New York.

Nahlawi, N. 1966, The drying of yams with· solar energy, Thch. Rep. No~T.27 of Brace Res. Inst., Canada.

Singh, R.S. and Ojha, T.P. 1972, Drying characteristics of ground nut andchillies, J. Agr. Engg., 9 (2) : 50-61