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ANALYSIS OF A GEODESIC DOME SOLAR FRUIT DRYERD. Y. Goswami a , A. Lavania b , S. Shahbazi b & M. Masood ba Department of Mechanical Engineering , University of Florida , Gainesville, FL, 32611b Department of Mechanical Engineering , North Carolina A &T State University ,Greensboro, NC, 27411Published online: 17 Apr 2007.

To cite this article: D. Y. Goswami , A. Lavania , S. Shahbazi & M. Masood (1991) ANALYSIS OF A GEODESIC DOME SOLAR FRUITDRYER, Drying Technology: An International Journal, 9:3, 677-691, DOI: 10.1080/07373939108916703

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D R Y I N G TECHNOLOGY, 9 ( 3 ) , 6 7 7 - 6 9 1 ( 1 9 9 1 )

ANALYSIS OF A GEODESIC DOME SOLAR FRUIT DRYER

D. Y. Goswami Department of Mechanical Engineering

University of Florida Gainesville, FL 32611

A. Lavania, S. Shahbazi and M. Masood Department of Mechanical Engineering North Carolina A & T State University

Greensboro, NC 27411

Key Words and Phrases: Dryer, Geodesic Dome, Solar Energy

ABSTRACT

This paper analyzes use of geodesic domes as low cost and practical solar energy drying systems, especially for developing countries. A geodesic dome solar fruit dryer was designed, construcied and tested for drying grapes. The structure was constructed using wooden members, covered with transparent tedlar plastic and a black absorber inner shell. Fresh air is heated in the space between the outer shell and the absorber sheet. The hot air then passes through the fruits on the trays and exits from the top. The dryer was tested for drying grapes under two different modes, namely natural convection and forced air flow. In addition, theoretical results were obtained by thermal-electrical simulation and considering the heat and mass transfer phenomena between the air and the grapes.

INTRODUCTION

Use of solar energy for grain drying has been studied very extensively, and as a result, there

are many commercial units available in the market today. In contrast, research on the use

CopyrightO 1991 by Marcel Dekktr, Inc.

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678 GOSWANI ET AL.

or solar energy for drying fruits has not been so extensive (Huang, 1986). One reason is that

most fruit dryers in the industrialized countries are run on fossil fuels. In the developing

countries fruits and vegetables are generally dried in the open sun and thus are exposed to

din and insects. Also i t takes longer to dry in the open sun. Consequently, fruits and

vegetables are not available off season in these countries. The situation in the developing

countries can be turned around by introducing low-cost, practical technology for preserving

these fruits and vegetables for off-season consumption. The use of solar energy to dry fruits

and vegetables is an appropriate technology that can help to achieve the above goal. This

teclinology can also help the industrialized countries in reducing the use of fossil fuels and

consequently slowing down the "greenhouse effect." Solar drying systems for fruits are

somewhat different than the grain drying systems. This is because fruits have tender and

turgid physical properties which do not allow them to be tumbled around by mechanical

equipment. The solar fruit dryers can be classified in the following major categories:

I. Cabinet type of dryers as shown in Figure 1 or some modification of it. such

dryers are batch type and usually small.

2. lndustrial type of dryers in which the system utilizes solar energy and fossil

fuel. These dryers usually are of continuous type and have large capacities.

Figure 2 shows an industrial type parallel flow fruit dryer.

3. Greenhouse type of dryers. These dryers are usually batch type, multipurpose

and usually of medium capacity. A greenhouse type of solar dryer can be set

as an indirect or a direct dryer depending on whether a layer of opaque

absorber is placed between the outer transparent glazing and the product, or

not. Such a dryer can be set up to work in the natural convection or forced

convection mode.

Lawond (1966) reported on a cabinet dryer which had 1.3 sq. meter drying bed. Guitierrez

1981) used a 10 x 12 feet glass house for drying fruits. He used a forced- air configuration

in which the greenhouse air enters under the thermal absorber, travels to the back of the

greenhouse and then rises to the top. This configuration provided a temperature rise of 20-

25 ' F above ambient. Another greenhouse-type of solar dryer was designed and studied by

tluang (1986). This unit was made up of a large multi-directional solar collector with a

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ANALYSIS O F G E O D E S I C DOME SOLAR FRUIT DRYER

O u t l e t P a r t Transparent Cover

Dry ing Trays

Transparent Cover n s u l a t o r

Air Heater

Transparent Cover

F i g . 1 . Cabine t t y p e S o l a r D r y e r w i t h s e p a r a t e a i r h e a t e r

' ~ r u i t ' c a r t s

F i g . 2 . I n d u s t r i a l t y p e p a r a l l e l f l o w t u n n e l d r y e r w i t h h e a t from s o l a r energy and f o s s i l f u e l

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GOSWAMI ET AL.

( a ) P e r s p e c t i v e view o f dome. ( b ) T o p view o f dome.

Fig. 3. Geodesic - Dome Solar Fruit Dryer.

transparent cover outside and interchangeable inner chambers with black absorbing surfaces

for drying and curing purposes. Two types of drying c ~ ~ g u r a t i o n were examined inside the

greenhouse - perforated and rotating drum, used for drying grains and peanuts and a bulk

rock for curing tobacco. Curing tobacco in this facility was reported to give good quality

tobacco with 47 to 54% fuel saving over the conventional bulk-barn curing. In indirect solar

dryer, the fruit is not exposed to direct sun radiation. Most designs in this category are

made up of a hot air solar collection system and a drying chamber. These dryers result in

higher temperature than cabinet type of dryers. The air is circulated through the system

either by natural convection or by means of a mechanical blower. In industrial type of

dryers, the product can be dried by solar energy as well as fossil fuels. These dryers have

large capacity, run continuously and provide the shortest drying time (Cruss and Christie,

1921; Christie and Nichols, 1927; Mark, 1938; Gentry, Miller and Claypool, 1965; Radda and

Gentry. 1969; Groh, 1978; Thompson, Chhinnan, Miller and Knuston. 1981).

This study analyzes the use of a greenhouse type of solar fruit dryer. The geometric shape

of the greenhouse determines the amount of energy collected at any location. It was

estimated by simulation that a geodesic dome would perform better than the conventional

gable and semi-cylindrical shapes. Figure 3 shows the structure of a geodesic dome. A

geodesic dome can be made from small, lightweight members, yet it is a very strong and

laterally stable structure. A complete design procedure for a geodesic dome solar fruit dryer

is given by Goswami et, al. (1989). A geodesic dome type of solar fruit dryer was designed

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ANALYSIS OF G E O D E S I C DOME SOLAR FRUIT DRYER

Transparent Sheet

Black Absorber Sheet

Fruit Trays

a. Forced Air Configuration

Air in

b. Natural Air Configuration

Fig. 6. Schematics of Forced and Natural

Air Configurations of the Dryer

procedure for the geodesic fruit dryer used in present investigation is given by Goswami et.

al. (1989).

EXPERIMENTAL METHODS

In the present study a number of experiments were performed with the two drying

arrangements shown in Figure 6. Grapes were selected for this experiment since they are

very popular in this country as well as many developing countries. A seedless grape variety

with a medium skin strength was used in this experiment. One fruit tray was filled with 20

pounds of grapes and the rest of the trays were covered with a plastic sheet. Grapes were

weighed every morning before sunrise and every evening after sunset. Twelve

thermocouples were used to record temperature distribution inside the dome. Temperature.

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COSUAMI ET AL.

Fig. 7. Percent Moisture Reduction vs. Time for Forced and Natural Air Flow.

100 -

relative humidity of the air entering and leaving the dryer, air flow rate and the solar

radiation were recorded every hour. During the forced air mode of operation the fan was

turned on during the day and was cut off during the night time to prevent blowing humid

night time air through the product. From the measurements of the relative humidity and

the entry and exit temperatures of the air, increase in the moisture content of the air was

computed and compared with the actual weight loss of the grapes. Figures 7 shows the

percentage of moisture lost, obtained from the weight lost measurements, for the forced air

and the natural air configurations respectively.

- -

-

Reduction in the grape moisture content was determined from two other methods also as

explained below:

Q- --0 Natural Air Flow Aprll 29 - May 9, 1989) 1-$ orced Air Flow

1) Reduction in the grape moisture content was calculated from the change in the

relative humidity, specific volume and air flow measurements as follows:

75 - (March 29 - April 13. 1989) J - C - .- - .a - < 50- CL -

- E - .a - VI .- o 25 - Z

- -

0 50 1 00 I50 200 250 Tirne(Hr)

m = Y(W,/v, - W i h )

where

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ANALYSIS OF GEODESIC DONE SOLAR FRUIT DRYER

m = increase in the moisture content of the air (kg H20/hr.)

V = volumetric flow rate of air (m3/hr.)

W = humidity ratio

v = specific volume (m3/kg)

subscript "ow is for the air leaving and "i" is for air entering the dryer.

2) Moisture removed was also calculated as the difference between the current moisture

content M(t) and the initial moisture content of the grapes M(o) (Henderson and

Perry, 1976). Moisture content at any time, M(t), was determined from the following

relation:

Equation 2 describes the drying mechanism in the falling-rate period. M(t) is the moisture

content, after a period of time t, M(o) is the moisture content at time zero, and M(e) is the

equilibrium moisture content, determined from the relative humidity of the drying air.

Constant F represents the shape factor for the product being dried. If the grape kernels

were placed on the tray apart from each other and dried as separate units, then F should

be equal to 6/n2. But, we filled the tray with grapes to a depth of 0.09 meters, thus creating

a slab of .09 meters thickness. Constant F is given as 8/$ for a slab (Hnderson and Perry,

1974).

The parameter k is given as D, (n2/d2) where d is the thickness of the slab in feet and D,,

is the mass diffusivity of the product. The mass diffusivity for grapes is given by Mohsenin

(1980) as 601.11 ftz/hr (55.84 m2/hr).

Equation 3 is an imperical relationship between equilibrium moisture content M(e),

equilibrium relative humidity (rh), and temperature (T). This equation has been obtained

by Henderson and Perry (1974) by curve fitting the experimental data for various

agricultural products. The values of equilibrium constants C and n for raisins are given as

7.13 x and 1.02, respectively (Hendenson and Perry, 1974).

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RESULTS AND DISCUSSION

GOSWAUI ET AL.

Figures 7 shows plots of the moisture removed from the grapes over the entire drying cycle

for the forced air and the natural air configuration, respectively. The two experiments

shown in these plots were conducted at different times, thus data points in the graphs can

not be compared. However, the general trend of the two curves can be compared. These

figures show that there is very little difference between the two air flow configurations. If

the two curves are superimposed, the natural flow arrangements shows slightly better results

for moisture reduction. Data for one day was recorded and the moisture loss was calculated

based on equations 8 and 9. Figure 8 compares the weight loss calculated from these

equations with the actual weight loss measured at the end of the day for forced air

configuration. Figure 9 shows the temperatures, relative humidities and the solar radiation

for this experiment. Similar plots for the natural air configuration are shown in Figures 10

and I I. Points labeled A represent weight loss based on actual weight measurements since

the weight was measured only twice a day, just before sunrise and just after sunset, a curve

between these points would be misleading. Therefore, the figure shows only points for

measured values A. curves B and C represent weight loss calculated from equations 8 and

9 respectively. It can be seen that there is a large difference between measured weight loss

A and curve B which was found to be due to the moisture picked up from the ground.

This was verified by recording the relative humidity and the temperature of the air just

above and below the tray. Curve A represents the measured value. Since the weight was

measured only at the start and the end of the experiment a line cannot be drawn for A since

it is not known how the moisture was removed with time during the experiment. At the end

of the experiment there is very little difference between A & C. The difference between

the curves A and C in Figure 11 can he explained as follows:

a) The drying equations were based on a thin-layer drying concept. The thin layer is

assumed to dry uniformly and with no gradient in properties through the layer. This

does not hold, however for a finite mass depth.

b) The temperature, the relative humidity and the specific volume of the air change as

it passes through a finite mass. The change in the state of the air affects progressive

change in the drying rate because the temperature change affects diffusivity, and the

relative humidity affects the equilibrium moisture content, M(e).

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ANALYSIS OF GEODESIC DOME SOLAR FRUIT DRYER

Meosured Values Analytical (Eq. 2) Indirect Measurements (Eq. 1 )

Clock l i m e

Fig. 8. Cumulative Amount of Moisture Removed for Forced Air Configuration.

,--. U - $ 35 " !?

2 rr o Outside f o-+ Inside

8 10 12 14 16 18 20 Clock Time

80 - J 70 , Outside

o--o Inside 3 60 .- u '5 50 : 40

t 34 .- t; - 20

10 8 10 12 14 16 18 20

Clock Time

Fig. 9. Temperature, Relative Humidity and Solar Radiation for Forced Air Configuration Experiment (Fig. 8).

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GOSWAMI ET A L .

D p 1.0 0

5 [L

2 0.5 0 A Measured Values 3 - - 0 Analytical (Eq. 2) '" .- 0 I

0.0 7 9 11 13 15 17 19

Clock l ime

Fig. 10. Cumulative Amount of Moisture Removed for Natural Air Configuration.

7 9 1 1 13 15 17 19 Clock l ime

50 - 2 40 .- 30

E 2 20 Q- o Outside

Inside

-90 3 3 70

, o Outside .- D .- E 2 50

Q .- t; 30 - U IT

7 9 11 13 15 17 19 Clack Time

Clock Time

Fig. 11 . Temperature, Relative Humidity and Solar Radiation for Natural Air Configuration Experiment (Fig. 10).

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ANALYSIS OF GEODESIC DOME SOLAR FRUIT DRYER 689

Tests in natural convection mode showed that about half of the moisture content from the

grapes was removed within six sunny days. More than seventy percent of the moisture

content was removed within ten continuous days of mixed sunny and cloudy conditions. This

compares well with the drying of a similar crop by tunnel dryer using a 100 watts fan (Sodha

et. al, 1987). The color, the flavor and the nutritional qualities of the raisins dried in [he

geodesic dome are better because the fruits are not exposed to direct solar radiation.

Furthermore, geodesic dome dryer does not use any electricity in the natural convection

mode.

Geodesic dome type structure can be easily built with wood or bamboo, in a short period

of time. Tests showed that a ventilator which was used for natural air circulation

arrangement was a better choice than a fan. Also, it is convenient for the developing

countries where electricity may not be available. The geodesic greenhouse structure tested

in this study gave an average of 22' C rise in temperature during the summer months and

about 10' C rise during the winter months.

Nomenclature

Symbols

area (m2) thermal capacitance (M/ ' c ) specific heat (H/kg'C) equilibrium constants thickness of slab = 9.14 cm mass difhsivity = 55.86 m2/hr. shape factor = 8/n2 for a slab drying constant = Dv (n2/d2) (h") moisture content at time t = 0 equilibrium moisture constant on dry basis (%) increase in the moisture content of the air (kg H,O/hr.) relative humidity air temperature ('R) time volume (m3) volumetric flow of air (m3/hr.) specific volume (m3//k) humidity ratio

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G O S V A U l ET AL.

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