Investigating the Thin Layer Drying Characteristics of Vegetable Kales in a Natural Convection Solar Cabinet Dryer, under the Climatic Conditions of Maseno, Kenya

Investigating the Thin Layer Drying Characteristics of Vegetable Kales in a Natural Convection Solar Cabinet Dryer, under the Climatic Conditions of Maseno, Kenya

*Onyinge George O. Department of Physics and Materials Science,

Maseno University, Box 333, Maseno, Kenya,

Oduor Andrew O. Department of Physics and Materials Science,

Maseno University, Box 333, Maseno, Kenya,

Othieno Herrick E. Department of Physics and Materials Science,

Maseno University, Box 333, Maseno, Kenya,

Abstract The growth of fruits and vegetables is becoming an important component of the agricultural industry sector, as sale of dried fruits and vegetables assume commercial scale. However due to lack of processing considerable amount of losses (between 30% and 40%) of these seasonal products occur in many developing countries. Drying is one of the most common methods of preservation of agricultural products. It removes sufficient moisture from the food and prevents decay or spoilage. Majority farmers in developing countries use open-sun method for drying their harvest despite the existence of more efficient methods of drying. This method is time consuming and exposes the produce to destruction by birds, animals and contamination. The areas surrounding Maseno university is agriculturally rich and known for production of vegetables and fruits among other agricultural products. In the wet seasons there is significant waste due to lack of preservation facilities to prolong their shelf life and yet during dry seasons these products are short in supply. In the present work, a model of natural convection solar dryer with twin collectors was designed and tested with and without load under the climatic conditions of Maseno, Kenya for drying of Vegetable kales. Results of test under no load revealed that the humidity in the drying chamber was reduced by 31.4%, while the chamber temperature was raised by 11.5 o C at average air flow rate of 0.39 m/s. The moisture content in the kales was reduced from 84.1% to less than 10% in about 15 hours of drying for an average air flow of

0.39 m/s. The maximum thermal efficiencies obtained were 22.51

% and 25.52 % for collector 1 and 2 respectively. Quality tests performed on both solar dried and wet samples revealed more than 50 % retention of all the nutrients in the solar dried vegetable kales.

Keywords: indirect mode, natural flow, cabinet solar dryer, twin collectors

Nomenclature

T2: Out let temperature of collector 1 T3: Ambient temperature

T6: Drying chamber temperature T7: outlet temperature of collector 2 1 L: top left tray

1 R: top right tray

6 L: middle left tray 6 R: middle right tray 10 L: bottom left tray

10 R: bottom right tray

I R: incident solar radiation

R Hc: relative humidity of air in chamber R Ha: relative humidity of ambient air d1: day 1

d2: day 2

d3: day 3

d4: day 4

d5: day 5

d6: day 6

w. b.: wet basis

INTRODUCTION

Drying, one of the most common methods of preservation of agricultural products in developing countries substantially reduces the weight and volume of the product thus minimizing packaging, storage and transportation costs, [1]. Although most developing countries have a huge potential of solar energy for agricultural drying applications, rural small scale farmers use traditional methods to inefficiently dry agricultural produce. [2]. These methods have the advantage that their requirement of solar and wind energy is readily available in nature and therefore require marginal capital investment, a fact that makes them the most viable option of drying agricultural products at both small and commercial scale in developing countries, [3]. Some of the disadvantages of these methods include: contamination of the product by dust, destruction by insects and micro-organisms, pecking by birds, loss of some nutrients (vitamin A), [4]. They are also labor intensive and totally dependent on weather conditions,

[5] and therefore a cause of the huge post harvest losses of between 30 and 40 % currently encountered in these countries, [6]. These methods involve laying the product in the sun where the product absorbs part of the radiation falling

on its surface and the remaining is reflected back depending on its color. The absorbed radiation is converted into thermal energy raising the temperature of the product causing evaporation of moisture, [3].

Drying is energy intensive due to the large value of latent heat of water evaporation and the use of solar energy in industrial and agricultural drying applications can lead to a significant saving of conventional fuels and reduction in pollution, [7]. In particular the development of appropriate solar technologies for agricultural applications in rural areas of developing countries can minimize post harvest losses and increase food supplies, [8]. A better alternative to open sun drying method is the use of solar dryers which are more efficient and results in better quality dried products. A solar dryer is an enclosed unit in which the product is protected from damage by birds, insects, micro-organisms, pilferage and unexpected rainfall, [9].

Drying is a heat and moisture transfer process between the material and air, the heat is transferred to the surface of the product through conduction and convection from the adjacent air at a temperature above the material, [3]. Solar drying is a process in which moisture is removed from a product by heat in the presence of a controlled flow of air. It involves the application of heat to the product to increases the vapor pressure of the moisture in the product above that of the surrounding air, creating thermal and pressure gradients causing the moisture both liquid and vapor to move to the surface of the product before the water vapor is transferred to the surrounding air by evaporation, [10]. As air at relatively lower humidity than the moisture content in the material is passed through the material, the air absorbs moisture from the material and its absolute and relative humidity increases. The efficiency of a solar dryer is affected by relative humidity in the air, the moisture content of the product to be dried, its quantity and thickness, [11]. The solar radiation intensity incident on the material varies with seasons, time of the day and length of exposure, ambient air temperature and wind speed.

METHODOLOGY

Description of the solar dryer

The indirect mode solar dryer was designed to dry vegetable products under the climatic conditions of Maseno, Kenya. Its main components were drying cabinet and two solar collectors. The materials used in the construction include: well seasoned cedar wooden bars, G.I. sheets, transparent glass and polythene sheets, nails, black paint, wire mesh, Aluminum gauze, PVC waste pipes, foam sheets, glass glazing of 5 mm thickness and a 32 cm tall chimney of 15cm diameter. The solar collector system consists of two collectors each with glazing area of 2.5 m2 made of wooden frames covered with glass and connected to the drying chamber via air ducts made of plastic waste pipes. The roof and sides of the drying chamber were covered with G.I sheets internally insulated with foam material covered polystyrene. The drying chamber consists of 20 trays each measuring 1.0 x

1.0 x 0.2 m, spaced 20 cm apart. The trays were of Aluminum screens on wooden frames and a 40 cm tall chimney with an inner diameter of 15 cm.

Design Parameters Of The Solar Dryer

The design parameters were decided based on the amount of moisture to be removed and the required air flow rate. Accordingly the length and width of chamber, size of the flat plate collectors, height and diameter of the chimney were calculated. The drying chamber was provided with a door 1.6 x 0.75m to facilitate loading and loading of the dryer.

1. The collector tilt for maximum incident solar radiation normally taken as the latitude of the location, 0 o for Maseno, but in this case to allow rain off a value of

   10o was used.

2. The ratio of length to width of the air heater was taken as

   1.5 and the length of the drying chamber Ls is then given

   The growth of fruits and vegetables is becoming an important by

   L Adc s w

   (1)

   component of the agricultural industry sector, as sale of dried

   fruits and vegetables assume commercial scale. However due

   where

   Adc and w are the area and width of the collector

   to lack of processing, considerable amount of losses (between 30% and 40%) of seasonal fruits occur in many developing countries. [12]. Drying reduces the moisture content in these products to safe levels after harvest to prevent growth of moulds and bacterial action and allow storage over an extended period. The area around Maseno University is agriculturally rich and known for production of vegetables and fruits among other agricultural products. In the wet

   respectively.

3. The aggregate thin drying layer thickness hL 200mm was used

4. The expected mean temperature difference of heated air at the collector outlet and the ambient value is given by

seasons there is significant waste due to lack of preservation facilities to prolong their shelf life and yet during dry seasons these products are short in supply. In the present work a model of indirect natural convection cabinet solar dryer was designed and tested for thin layer drying of vegetable kales under the climatic conditions of Maseno, Kenya.

T 2 (T T ) It

I

b c

o

where is a dimensionless parameter that ranges between 0.14 and 0.25 I t the intensity of radiation incident on the plane of the collector, Io the maximum

(2)

intensity of the source of radiation/ solar constant value

(1367 W/m2) , Tb the boiling temperature of water, Tc the critical temperature of water. The values were assigned as follows:

t

o 2

0.20 , Tb Tc 100 C , I 500W / m giving a

10. Thermal efficiency of the solar collector was obtained according to the equation

c

mCpa (To Ti ) 100

Ac I

(7)

value T 14.6o C .

1. The quantity of moisture to be removed mw was obtained according to the relation

   where m is air mass flow rate, C specific heat capacity of air, Ti collector inlet air temperature, To outlet air temperature, I incident solar radiation and Ac the collector

   mw ww

   mi mf

   1 mf

   (3)

   area.

   11. The system efficiency for the solar dryer was calculated

   where mi initial moisture content, m f

   is final moisture

   according to the equation

   content, ww is initial product

   p

   WL

   IA P

   (8)

   c f

   mass, from literature the values were assigned as follows:

   mi 0.73 , mf 0.15 , ww 50kg , from which

   mw 34.12kg is obtained.

2. The latent heat of evaporation is estimated using the equation

where W is weight of water evaporated from the product, L latent heat of vaporization of water , Pf power used to drive the fan

12. The effective moisture diffusivity for agricultural products

P T T

0.38

is given by

c

b

L R T T In c c pt

(4)

2 D

t g c b

105 T

T 1.38

k ef

r 2

(9)

where Rg gas constant of water vapor, Pc critical pressure

where k is the drying rate constant, r is the

of water,

Tpt

temperature of product and given by

characteristic thickness.

Tpt

0.253To

T 312.6K , Rg 287.1 J/kg K

EXPERIMENTAL PROCEDURES

a

1. The total volume of air needed to remove the moisture was then obtained using the relation

   FULL LOAD TESTS UNDER NATURAL FLOW

   Tests were done on the indirect mode natural flow solar dryer

   V mw Lt RaTa

   pa a o f

   A C P (T T )

   where Ra is specific gas constant

   (5)

   Pa the partial pressure of

   shown in figure 1, under no load conditions between 9:00 hrs and 16:00 hrs in the month of April and May 2014, to determine the temperature profile, relative humidity and solar radiation at different locations and moisture ratio profile of

   dry air in the atmosphere Cpa the specific heat capacity of air

   kales in the solar dryer. During the experimental drying tests, temperatures readings were taken for the ambient air and of

   at constant pressure Tf Ta 0.25T

   the temperature of

   air at various locations in the drying system (collectors and

   inside the drying chamber) using type J and K thermocouples

   air leaving the drying chamber

   To the ambient temperature

   connected to a Fluke 2286 data logging system at regular

   Lt the latent heat of vaporization of water. The values were assigned as follows: Ra 287J / kgK , Ta 298K

   intervals of 10 minutes between 0900 hrs and 1600 hrs local time. The intensity of the incident solar radiation incident on the surface of the collectors was measured at one minute

   Cpa

   1005J / kgK , Tf

   302.65K , Pa

   101325N / m2

   intervals using a portable solarimeter placed horizontally on

   one of the surfaces of the collectors. The relative humidity of air outside and inside the drying chamber was measured

2. The expected volume air flow rate was then obtained using

   VA

   the relation

   .

   v (6)

   t

   where t is total time needed to dry a given sample of the product.

   using a Psychrometer at one-hour intervals. The air flow velocity and volume flow rates were measured at the collector outlets and chimney at one-hour intervals using an anemometer model 8360. The dryer was loaded with 10 kg Kales with initial moisture content 84.41 % w. b. laid in thin layers onto each tray. The initial mass of kales loaded on the six control samples were recorded thereafter these samples

3. The chimney length was taken to be length ( 4.8m ), a value of 0.32 m.

1 of the collector

15

were removed from the dryer and their weight measured at the at two-hourly intervals using an electronic balance. The mass of the kales during the drying hours was recorded up to the stage when no significant weight loss occurred after three

consecutive weighing, at which point drying was stopped. Weight measurements were also taken from open sun-dried

control samples up to the same moisture content.

Figure 1: Pictorial view of the indirect cabinet solar dryer designed at the Department of Physics and Materials Science, Maseno University, Kenya.

RESULTS AND DISCUSSION

The results of tests under no load for the natural convection solar dryer presented in figures 2 – 5, reveal that the humidity of air in the chamber has a decreasing trend with increasing solar radiation. The maximum and minimum relative humidity of air in the chamber were 77.1% at 9:00 hrs and 44.3% at 16:00 hrs respectively against maximum and minimum relative humidity values of the ambient air of 75.7% at 10:00 hrs and 46.9% at 15:00 hrs respectively, a

reduction of 31.4% in relative humidity in the drying chamber below the maximum ambient value, (figure 3). The maximum and minimum chamber temperatures were

38.8 o C at 1600 hrs and 33.7 o C at 9:00 hrs respectively against corresponding ambient maximum and minimum values of 31.6o C at 15:00 hrs and 27.3 o C at 9:00 hrs respectively, (figure 5). Thus it was observed that the

temperature of air in the drying chamber was increased by

11.5 o C above the ambient value at average air flow rate of

0.39 m/s. The observed maximum and minimum solar radiation during the selected days were 1212 W/m2 and 61 W/m2 respectively (figure 4), while the maximum and

.

minimum values of humidity in the ambient air during the month of April 2014 were 95.4% and 29.6% respectively,

(figure 3).

Variation of humidity of ambient and chambe air and the incident solar radiation

90

80

Relative humidity of air (%

70

60

50

40

30

20

10

0

9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00

Time of the day (hrs)

1400

Incident solar radiation (W/m

1200

1000

800

600

400

200

RHc (%)

RHa (%)

Solar radiation

0

Figure 2: shows variation of relative humidity of ambient air and drying chamber with time under no load.

Variation of relative humidity with time forselected days of the month of April 2014

100

90

80

RHa d1(%)

RHa d2(%)

RHa d3(%) RHa d4(%) RHa d5(%) RHa d6(%)

70

RH (%)

60

50

40

30

20

10

0

9:00 10:00 11:00 12:00 13:00 14:00 16:00

Time of the day (hrs)

Figure 3: Variation of relative humidity of ambient air for selected days of April 2014

Variation of Solar radiation with time for selected days in April 2014

1400

1200

Solar radiation intensity (W/m2

1000

IR d1(W/m2) IR d2(W/m2)

IR d3(W/m2)

IR d4(W/m2)

800

600

400

200

0

9:00 10:00 11:00 12:00 13:00 14:00 16:00

Time of the day (hrs)

Figure 4: shows variation of solar radiation for selected days of April 2014

Temperature profile for collectors and chamber with incident solar radiation under natural flow

80 1400

70 1200

Incident solar radation (W/m

60

1000

Temperature (oC)

50

800

40

600

30

400

20

T2

T3

T6

T7

S o la r ra d ia tio n

10 200

0 0

9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00

Time of the day (hrs)

Figure 5: shows temperature profiles of the different locations in the dryer system with time under natural flow with no load.

FULL LOAD TESTS

The results tests on the natural convection solar dryer under full load, revealed that the maximum and minimum chamber temperatures were 36.8 o C at 1:00 hrs and 28.3 o C at 9:00 hrs respectively against corresponding ambient

was also observed that the maximum solar radiation for the first and second drying days in were 1080 W/m2 and 1016 W/m2 at 1:03 hrs while the minimum solar radiation values

maximum and minimum values of 31.5 o C at 1:00 hrs and

22.4 o C at 9:00 hrs for the days of drying in April 2014. It

for the first and second days were 30 W/m2 at 16:00 hrs

and 290 W/m2 at 10:00 hrs respectively, (figure 4).

Variation of Moisture ratio for open sun drying and different

trays in the dryer during the two days of Kales drying

1R

6R

10R

1L

6L

10L OS

Time of the day (hrs)

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0

-0.1

Moisture Ratio

10:00

12:00

14:00

16:00

8:00

10:00

12:00

14:00

16:00

Figure 6: Variation of moisture in the Kales with time for the top, middle and bottom trays.

Variation of drying rates of top, middle and bottom trays with Moisture content

3

2.5

Drying rate (g/s

2

1L(g/s)

6L (g/s)

10 L (g/s)

1.5

1

0.5

0

0.00 0.20 0.40 0.60 0.80 1.00

-0.5

Moisture ratio

Figure 7: shows variation of drying rate with moisture content for top, middle and bottom trays

Moisture Content of the kales

The results (figure 6), indicate a reduction in moisture content of vegetable kales from an initial value of 84.14 % w. b. to 60 %, 54%, 48%, 28%, 24%, 22% and 14% for

middle right, middle left, top left, bottom left, bottom right, top right trays and open sun respectively at the end of the first 6 hrs of drying. It is observed that the moisture ratio was further reduced on the second day from 46% to 10% in the middle right tray, from 38% to 20%, from 13% to 0%

and from 13% to 2% in the middle right tray, bottom left, top right trays and the bottom right tray respectively, in the solar dryer and from 13% to 10% in open sun drying in a further 7 hours. It is observed that the final moisture content of kales attained is a function of the tray location; lower moisture contents were obtained in the top and bottom trays than in the middle during the first hours of drying. This can be explained by the fact that at the initial stage drying rate is controlled by the rate of evaporation of moisture from the surface of the kales, a process that

wholly depends on external factors such as humidity, temperature and air flow rate. At the bottom of the chamber the humidity of air is minimum while temperature is maximum. The values of air flow rate and temperature at top trays near the chimney are also comparatively higher than those of the middle trays. It is observed that while a final moisture content of zero for kales was obtained in the solar dryer, the open sun dried samples attained a final moisture content of 10% in for a similar duration of drying. From figure 7 it is observed that at the initial stage of drying there is rapid decrease in the drying rate kales with moisture content followed by an increase and then a constant rate drying period.

rate followed by the middle and top trays. It is also observed that open sun drying has the lower value of drying rate constant compared to solar dried kales.

Effective moisture diffusivity (Def)

The effective moisture diffusivity for Kales were computed according to equation 9, and found to be: 4.90 108 m2/s (1R) and 5.55108 m2/s (1L) for top trays, 5.84 108 m2/s (6R) and 6.28108 m2/s (6L) for the middle trays, 1.41107 m2/s (10R), 1.01107 m2/s

(10R) for the bottom trays and 2.27 108 m2/s. The top

Drying rate constant (k)

From the graphical plots of In M. R. versus drying time

trays had higher values compared to the middle and bottom trays. The mean effective moisture diffusivities of the kales

(figure 12), the drying rate constants were computed for

are

5.23108

m2/s,

6.06 108

m2/s and

drying of kales in top, middle, bottom trays and for open sun drying and obtained as follows: 0.121 hr-1(1R), 0.137hr-1 (1L), 0.155 hr-1 (6R) 0.144 hr-1 (6L), 0.347 hr-

1(10R), 0.250 hr-1(10L) and 0.0561 hr-1(OS). Thus the

mean drying constants for the top, middle and bottom trays were: 0.129 hr-1, 0.150 hr-1 and 0.299 hr-1 respectively. It is deducible that drying rate constant varies with tray location in the dryer with bottom trays having the highest drying

1.21107 m2/s for the top, middle and bottom trays respectively. It is observed that the effective moisture diffusivity of kales is highest for bottom trays and least in the top trays of the solar dryer. However the values of the effective moisture diffusivities of kales in the solar dryer were all greater than for open sun drying.

Graphical plots of -In M.R. versus time for Kales

4.5

4

3.5

3

– In M. R.

2.5

2

1.5

1

0.5

0

drying

1R

6R

10R

1L

6L

10L OS

-0.5 0.0 5.0 10.0 15.0 20.0

Time (hrs)

Quality tests

Figure 8: shows graphical plots of In M. R. versus time for drying of Kales

<>The results of quality tests conducted on solar dried kales and control (wet) samples revealed that the content of Vitamin C in the solar dried kales was 90.9 mg per 100g as compared to 120mg per 100g in the wet samples, 76% being retained in the solar dried sample. The solar dried sample contained 4.275% tannius compared to 7.7 % in the wet sample, with 55% being retained in the dried sample.

The carbohydrate content was found to be 7.5 g per 100g as compared to 10 g per 100g in the wet samples, with 75 % being retained. The protein content was also tested in the solar dried sample and found to be 3.0 g per 100g as compared to 3.3 showing retention of 91 %. The mineral content in the solar dried sample were Ca: 130.0 mg/ 100g, Mg: 33.8 mg/ 100g, Fe: 1.7 mg/ 100g and Zn: 0.47 mg/ 100g, an almost 100% retention of these minerals.

Table 1: Results of quality test conducted on solar dried and wet vegetable kales

Test	Wet sample	Solar dried sample	Comments
Vitamin C (Ascorbic acid ) for 11.87 m/s and 30 m/s sample Titers	120mg per 100g	90.9 mg per 100g	76 % retained
Tannius	7.8 %	4.275 %	55% retained
Carbohydrates	10 g/ 100g	7.5 g/100g	75 % retained
Protein	3.3 g/ 100g	3.0 g/ 100g	91 % retained
Minerals	Ca : 130.0 mg/ 100g Mg: 33.8 mg/ 100g Fe : 1.8 mg/ 100g Zn : 0.48 mg/ 100g	130.0 mg/ 100g 33.6 mg/ 100g 1.7 mg/ 100g 0.47 mg/ 100g	100 % retained

CONCLUSIONS

The temperature of the drying chamber in the indirect natural solar dryer was increased by an average of 11.5 o C above the ambient values while the relative humidity of air in the chamber was reduced by an average value of 31.4% at 0.39 m/s air flow rate for a mean incident solar radiation of 792 W/m2 under no load conditions. The thermal efficiencies of the collectors were found to vary between 5

REFERENCES

1. Mohamed M. A., Gamea G. R. and Keshek M. H., Drying Characteristics of Okra by different solar dryers J. Ag. Eng. 27(1): 294-312, (2010).

2. Azhrarul K. and Hawlader M. N. A., Mathematical modeling and experimental investigation of Tropical fruits drying Int. J. of Heat and Mass transfer 48 (23, 24): 4914- 4925, (2010).

3. Jairaj K. S., Singh S. P. and Srikant K. A., Review of solar dryers developed for grape drying, Solar Ener. 83 (9): 1698-1712, (2009).

4. Madhlopa A., Jones S. A. and Kalenga-Saka J. D., A solar air heater with composite absorber systems for food dehydration, Ren. Ener. 27: 27-37, (2002).

5. Venkata R. V.S., Iniyab S. and Goic R., A review of solar drying technologies, Ren. and Sus. Energy Reviews 16 (5): 2652-2670 (2012).

6. Sami S., Etesami N. and Rahimi A., Energy and Exergy analysis of an indirect solar cabinet dryer based on mathematical modelling results, J. Ener. 36 (5): 2847-2855 (2011).

   % and 25.5 %. Drying of Kales occurs in the falling and constant rate period with moisture content being reduced from 84.14% w. b. to less than 5.0 % w. b. and 10% in the solar dryer and open sun drying respectively in 13 hours. The overall quality and appearance of the solar dried kales was found to be good.

7. Chandrakumar B. P. and Jiwanlal B. L., Development and performance evaluation of mixed mode solar dryer with forced convection, Int. J. of Ener. and Envi. Eng., 4: 23 (2013).

8. Gatea A. A. Design and construction of a solar drying system, a cylindrical section and analysis of the performance of the thermal drying system African J. Agri. Res. 6 (2): 343-351, (2011).

9. Ogunkoya A. K., Ukoba K. O. and Olunlade B. A., Development of a low cost solar dryer Pacific J. of Sci. and Techn. 12(1): 98-101 (2011)

10. Sharma A., Chen C. R. and Vu Lan N., Solar energy drying systems: A Review, Ren. and Sust. Ener. Review, 13, 1185-1210, (2009).

11. Singh S. and Kumar S., Testing method for thermal performance based rating of various solar dryer designs, Elsevier 86 (1): 87-98 (2012).

12. Sadodin S., Kashani T. T., Numerical investigation of a solar green house tunnel dryer for drying of Copra, Dept. of Ener. Convers., Faculty of Mech. Eng., Semnam University, Semnan, Iran,

13. Yolanda B., Gran A. and Miranda A., SD sim: A Novel simulator for solar drying processes, Automatic Control Dept. Technical University of Catalonica UPC, Bacelona, Spain, (2011).