Evaluation of the performance of a cabinet type solar dryer

Energy Convers. Mgrat Vol. 30, No. 2, pp. 75-80, 1990 Printed in Great Britain. All rights reserved

0196-8904/90 $3.00 + 0.00 Copyright © 1990 Pergamon Press pie

EVALUATION OF THE P E R F O R M A N C E OF A CABINET TYPE SOLAR D R Y E R S A N JAY S H A R M A , V. K. S H A R M A , R A N J A N A J H A and R. A. RAY Centre of Energy Studies, Indian Institute of Technology,Hauz Khas, New Delhi-ll0 016, India (Received 10 August 1987; received for pubfication 8 November 1989)

Abstract--In the present work, an attempt has been made to analyse a cabinet type solar dryer. The transient equations for the different components of the system are written, and their solutions are attempted within the frameworkof periodicanalysis. This model is capableof predictingthe instantaneous temperature inside the dryer, the moisture content and drying rates. The results obtained from the theoretical and experimental observations are reported in this research paper. Solar energy

Crop drying

Cabinetd r y e r

Thermalperformance

INTRODUCTION

Solar energy devices have the most potential application for heating air for various applications, including grain drying, timber drying and space heating. The technical feasibility of solar drying has been well demonstrated by a number of investigations, mainly due to the low temperature requirement of the applications. According to Forster and Peart, a number of designs of solar collection systems are available, which can be used for solar drying applications. A detailed information on the experimentation and design of various solar crop dryers has also been reported by Sayigh. Dryers of the designs such as tubular, parabolic, concentric and collector cum storage system have been evaluated for grain, vegetables and fruit drying applications. These dryers have in fact significant potential in the near future because of their limited drying capacity. The economic feasibility of solar drying is strongly influenced by the operating parameters, design characteristics and absorbing surface. In spite of the tremendous utility of solar energy for drying applications, it has not been adopted as a common system till now. In the present research paper, we have developed a mathematical model for a solar cabinet dryer. The experiments were carried out to verify the mathematical model. The materials dried were compared with open dried material for their culinary and organoleptic characteristics, and the results obtained are included in this paper. MATHEMATICAL

MODELLING

OF S O L A R C A B I N E T

DRYER

Since the behaviour of the solar dryer system is inherently transient, we have considered the transient energy balance equations at the two nodes of the collectors. The energy balance at the glass cover is

dL

MgCg--~

= S ( f ) - h r ( T p - T g ) - U , ( T g - Ta).

(l)

The energy balance at the grain surface is

drp

M, Cp-~

= S ( t ) - h r ( T o - T , ) - he(To - Ta) - he(T, - T~).

(2)

In the second equation, we have assumed that the air which enters the dryer gets heated to the plate temperature, and the evaporative and conductive heat transfers take place from the grain to the ambient. This assumption implies that the hot moist air escapes from the dryer to the ambient, carrying the heat and moisture with it. The glass is heated only by radiative heat transfer. The heat losses from the bottom, being small, are ignored. The transient equations (1) and (2) were solved numerically on a digital computer using the finite difference technique. 75

76

SHARMA et al.:

CABINET TYPE SOLAR DRYER ANALYSIS

The overall efficiency is defined as

r/

Total heat evaporated Total solar insolation

-

Mev. L

fo

' H.dt

EXPERIMENT

The experimental studies have been done in the winter season. The test procedures are the same as those performed by earlier workers and within the limit of availability of the laboratory facilities. Inlet and outlet temperatures were measured by copper-constantan thermocouples at a regular interval of half an hour. The ambient temperature was recorded by means of an automatic potentiometric recorder along with a copper-constantan thermocouple wire. The solar intensity was measured by means of a pyranometer placed at an inclination of 28 ° facing due south. The cover glass was kept clean so that the dust factor remained unity. The materials dried were compared with open dried material for culinary and organoleptic characteristics.

0

0

a_ °

I]

---I---F--I--

J Pipes for b o t t o m ventiLotion holes

InsuLation

Sectional view

0

0

0

0

I[ III I

II Ill I

I. I I I

0

0

iil ili

iil

0

lJ

0

0

0

,I, ,l, lil lj, I

Back view

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

Top view Fig. 1. Sectional view of convectional solar cabinet dryer with direct heating mode.

SHARMA et al.: CABINETTYPE SOLAR DRYER ANALYSIS

77

Table 1 Location Latitude Longitude Height Total collector area Area of each glass cover Number of glass covers used Area of absorbing surface of each tray Number of trays used Thickness of inner insulation Diameter of air inlet and outlet pipe Area of window Number of windows Mode of air flow Duration of experiment Number of articles dried

Centre of Energy Studies, liT, Delhi, Hauz Khas, New Delhi-110 016, India 28.58°N 77.28°E 216m AMSL 195 × 66cm = 12,870cm2 = 1.28m2 96 x 62 cm = 5952cm2 = 0.59 m2 Two 57 x 57 cm = 3249cm2 = 0.32 m2 = Three 5 cm 1.4 cm 60 x 60 cm = 0.36 m2 Three Natural 2 months (a) Turnip (b) Green peas (c) Cauliflower

S T R U C T U R E OF S O L A R C A B I N E T D R Y E R T E S T E D The solar dryer used for carrying out this work was fabricated at the Indian Institute of Technology, New Delhi. The schematic picture of the solar dryer is shown in Fig. 1. This is essentially a wooden rectangular box divided lengthwise into parallel channels of equal width. The dryer is made of glass which provides a substantial screening effect against ultraviolet light, thus reducing photodegradation of the drying product. Solar heated air passes through the perforated wire mesh on which the material to be dried is spread. For proper and uniform ventilation, the ventilation holes were fitted with short lengths of plastic. The different construction parameters are shown in Table 1. D R Y I N G OF D I F F E R E N T A R T I C L E S The details about the preparation and drying of the articles are discussed below:

(a) Drying of turnip Six kilograms of fresh turnip were taken, and chips of equal size and thickness were prepared. Three kilograms of the sample were kept in the tray inside the dryer at the maximum temperature region, and 3 kg of the sample were kept outside the dryer in another tray. Weight of the sample and temperature inside and outside the dryers at different hours of the day were recorded at regular intervals of time. Readings of weight and temperature were noted until a constant value of weight of sample inside and outside the dryer was observed.

(b) Drying of green peas Six kilograms of fresh green peas were taken. Three kilograms of the sample was kept in the tray inside the dryer at the maximum temperature region, and 3 kg of the sample was kept outside the dryer in another tray. The readings of weights and air temperatures were taken inside and outside the dryer at different hours of the day at regular intervals of time. Readings of weight and temperature were noted until a constant value for weight of the sample inside and outside the dryer was observed.

(c) Drying of Cauliflower Six kilograms of fresh cauliflower were taken. Three kilograms of the sample was kept inside the dryer at the maximum temperature region, and another 3 kg of the sample was kept outside

78

SHARMA et al.:

CABINET TYPE SOLAR DRYER ANALYSIS 800 I ~

SoLar insoLation mperoture

80

600

7O

==

8

°m

=o 60 (2. E o i - 5O

400

o= c o (/)

40

200 ~

30

20

I 12

8

I 16

~

II

I 20

I 24

I 28

0 32

T i m e (h)

Fig. 2. Meteorological data for the day of experimentation.

.Q "o

30

g z6 ~. 22

30 ko

~ ~o

8

16

12

Time

20

i

24

(h)

Fig. 3. Variation of the moisture content with time of day for different loads, 1O0 i--

Green

!

peas

I -Open sun dryino I I - D r y i n g inside the dryer

80--

~ 60 ~

40

~

2o

0

I 2

I 4

I 8

I I I 12 16 20 Drying time (h)

I 24

I 28

Fig. 4. Thermal performance studies o f the solar cabinet dryer.

I 32

S H A R M A et al.:

CABINET TYPE SOLAR D R Y E R ANALYSIS

79

CauLifLower I 100

--~ Open sun drying

11" --~ Drying

inside the dryer

8O

60 c o

40 "6

=E

20

I 4

8

I

I

I

I

I

I

I

I

12

16

20

24

28

52

:56

40

Drying time

(h)

Fig. 5. Thermal performance studies of the solar cabinet dryer.

the dryer in another tray. The readings for weight of the sample and air temperature were taken inside and outside the dryer at regular intervals of time until a constant value of weight of sample inside and outside the dryer was observed. RESULTS AND DISCUSSION The solar cabinet type dryer geometry is shown in Fig. 1. The energy balance equations were solved for a typical day in late summer. The solar insolation and the ambient temperature, as measured at New Delhi, for a typical day at half hourly intervals (Fig. 2) are taken. The predicted plate temperature for no load reaches a maximum of 80-85°C during the noon hours, while with a load of 20 kg of wheat, the maximum temperature is about 45-50°C. This decrease in plate temperature is because of the fact that with a drying load, a major portion of the heat is utilized in the evaporation of water, thus limiting the temperature rise. It was also observed that the equilibrium moisture content is reached very rapidly for small loads, whereas it is much slower and takes a much longer time for higher load values (Fig. 3). The results obtained from the experimental observations using turnip, cauliflower and green peas, are reported in Figs 4-6. Two samples of equal weight for the above mentioned articles were taken and were chemically treated for the drying processes. From the comparative studies of the

Turnip

100

I

~t

--=.-Open sun

drying dryer

80

60 c 0 U

40

,-t

o 20

~E

I

I

I

I

I

I

I

I

I

4

8

12

16

20

24

28

32

36

Drying time

(h)

Fig. 6. Thermal performance studies of the solar cabinet dryer.

SHARMA et al.: CABINET TYPE SOLAR DRYER ANALYSIS

80

moisture content recorded while drying the products under open air conditions and in the newly fabricated solar cabinet dryer (Figs 4-6), it is evident that the drying of all the articles gets greatly enhanced inside the solar dryer. It was also observed that the products dried in the solar dryer were superior in quality as compared to the open air dried products (i.e. culinary and organoleptic characteristics of the solar cabinet dried products were much improved compared to the respective products dried in open air under natural conditions). REFERENCES 1. 2. 3. 4. 5. 6.

T. R. C. A. V. R.

A. Lawand, Sol. Energy 10, 158 (1966). H. B. Exell, Renewable Energy Rev. J. 1, (1980). W. Hall, Drying Farm Crops. Dyall Book Depot, Ludhina, India. L. Lydersen, Mass Transfer in Engineering Practice. Wiley-Interscience, New York. K. Sharma S. Sharma, R. A. Ray and H. P. Garg, Energy Convers. Mgmt 26, 111 (1986). H. B. Exell, Sun Wld 4, 186 (1980).