Convective Solar Dryer

8/9/2019 Convective Solar Dryer

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CONVECTIVE SOLAR DRYER WITH A WOOD WASTE BACKUP HEATER FOR

DEHYDRATION OF FOOD

Madhlopa, A. and Ngwalo G.

University of Malawi – The Polytechnic, Private Bag 303,Chichiri, Blantyre 3

ABSTRACT

Appropriate technology for the conversion of solar radiation to thermal energy is vital for food

dehydration. Commercial producers of dried foods need reliable drying technologies for

continuity technologies for continuity of production. However, solar radiation is not available

whenever it is needed to drying. This limitation discourages many of these producers from

investing in solar technologies, including dryers. Consequently, it is necessary to improve the

system design to promote the application of solar dryers.

In the present study, a natural convection solar dryer with an integrated thermal mass and

sawdust backup heater has been designed and constructed, the dryer was tested in three modes of

operation by drying twelve batches of fresh pineapples (ananas comosus): solar, biomas and

solar-biomass, meteorological conditions were monitored during the dehydration process and

both fresh and dried pineapple slices analyzed for vitamin C and ash content.

Results show that the solar mode of operation was slowest (five days) in drying the samples,

with the solar biomass mode being fastest (3 days) under the prevailing meteorological

conditions (which were generally unfavorable from January through July). Samples were

successfully dried even under rainy conditions, with moisture content dropping from 669% to

11% (on dry basis db) when the dryer was operated in the solar-biomass mode. This level of

moisture content is suitable for safe storage and distribution of the dried fruit. It was also found

that 26% to 44% of the vitamin c was retained in the dried product, ash content varied between

0.5% and 0.6% (on wet bassis, wb), with no significant difference between ash content (db) in

fresh and dried samples, this shows that there was very little or no food contamination arising


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from dust. The final-day efficiencies of the system were 15+1, 11+ 1 and 13+2% for the solar,

biomass and solar-biomass modes of operation respective.

It is evident that the inclusion of a biomass backup heater enables drying to proceed under all

weather conditions. It appears therefore that this solar dryer overcomes the limitation of

intermittent dehydration of a food product. However, there is need for further work to optimize

the performance of the system.

INTRODUCTION

Dehydration is a common technique for the preservation of agricultural and other products,including fruits and vegetables .Techniques for drying including open–air, fuel–fired, electric

and solar dryers. The use of open-air drying is cheap but it often results in food contamination

and nutritional deterioration (Ratti and Mujumdar, 1997). Fuel-fired dryers (such as coal-or

wood-operated dryers) contribute to environmental degradation through deforestation and or

emission of gaseous and particular matter (UNEP, 1988: IPCC, 1995). The use of electric-dryers

is limited to areas where there is an electric power supply. In contrast, solar energy is not only

friendly to the environment but also available even in remote areas. Consequently, solar dryers

can be used in both urban and rural areas.

Numerous designs of solar dryers with or without thermal storage systems are reported in

(Chirarattananon et al. 1998; Goyal and Tiwari, 1997; Ayensu, 1997: Aboul-Enein et al., 2000;

Itodo et al., 2002, Madhlopa et al., 2002 and others). Thermal storage systems enable drying to

continue after sunset provided there is enough sunshine during the day. However, the intensity of

solar radiation is sometimes so low that the temperature of the thermal mass rises by a very small

(or no) margin above the ambient level. So, this still limits the continuity of the drying process in

a solar dryer with a thermal mass. It should be mentioned that discontinuity of the drying process

is worse off in a solar dryer without a thermal storage system. This reduces the rate of production

and can results in low quality of the solar-dried products.

Few studies have been conducted to overcome the limitation of the exclusive use of solar energy

in natural convention solar dryers. Bassey et al. (1987) used a sawdust burner to provide heat


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during bad weather and at night. In their study, the burner was not integrated to the dryer but

used steam as a heat transfer medium. Bena and Fuller (2000) designed a direct solar with an

integrated biomass backup heater. Their biomass burner performed well when operated on large

pieces of hard wood.

Fuelwood is a dominant source of energy, and commonly burned using inefficient technologies

in most developing countries (Kristoferson and Bokalders, 1991; Bena and Fuller, 2002).

Unfortunately, the heavy and inefficient consumption of fuelwood is contributing to

deforestration and other environmental problems (Kristoferson and Bokalders, 1991; Hyde and

Seve, 1993). It should also be noted that the use of large pieces of wood requires felling of treesfor the task, which aggravates the problem of deforestation. In contrast, the use of waste from

timber processing as a source of thermal energy does not require felling of trees specifically for

fuelwood. This approach can therefore contribute to a sustainable exploitation of wood resources

in many developing countries.

The solar dryer reported by Bena and Fuller (2002) does not have a thermal storage system for

capture solar radiation. In view of this, their system requires backup thermal energy from the

burner for over-night dehydration of the fresh food even when solar radiation is abundant during

the day. The air temperature in the drying chamber would fall to ambient level immediately after

sunset in the absence of backup heating. In this case, the drying food has to be removed from the

dryer everyday at sunset and re-loaded into the system in the morning, to avert moisture re-

absorption by the drying food product at night. This is inconvenient for a commercial producer

of dried foods.

In Malawi, open-air drying is the traditional method for preserving fruits and other fresh food

products. However, industrial type solar dryers have been applied to the drying of tobacco,

coffee, tea and fish but with limited success largely due to financial constraints and insufficient

solar data for designing appropriate crop dryers (Kafumba, 1994). Mumba (1995) designed a

photovoltaic forced-convection solar dryer for drying grain in rural areas where grid electricity

and fossil fuels are generally not available. He found that it was possible to construct a cost-

effective solar grain dyer using locally available materials in Malawi. More recently Madhlopa et


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al. (2002) developed a solar dryer with composite-absorber systems for dehydration of food. The

solar collection of this dryer had a removable metallic absorber systems for dehydration of food.

The Madhlopa et al. (2002) developed a solar dryer with composite-absorber systems for

dehydration of food. The solar collector of this dryer had a removable metallic absorber plate and

a fixed wooden bottom. This design provided novel flexibility in the adjustment of the thermal

characteristics of the dryer. It was found that the dryer was suitable for preservation of mangoes

( Magifera indicus) and other fresh foods. However, both of the dryers reported by Mumba

(1995) and Madhlopa et al. (2002) did not have thermal storage and backup heating systems.

This would limit their application in poor weather.

The main objective of the present study was to develop a solar dryer with a thermal mass and

backup sawdust burner, with the following specific objectives:

a. to design and construct a solar dryer for dehydration of fruits ,

b. to determine the efficiency of the dryer, and

c. to evaluate the quality(moisture content, ascorbic acid and ash content ) of the dried food..

An indirect solar dryer has been designed and constructed with a biomass backup burner (fig. 1)

the biomass burner was made of a drum fitted with a mild steel grill and horizontal baffle. The

grill was fitted 0.20m above the bottom part of the drum while the baffle was 0.06m above the

grill. The drum was integrated to a rectangular duct with 3 vertical baffles to increase the path

length of the flue gas and the amount of heat transferred to the drying chamber.

A truncated circular removable lid was lightly-fitted on the open end of the drum through which

biomass was loaded into and ash removed from the drum, with a rectangular steel door (with

vermiculite-filled cavity, 0.025m thick) fitted on the outer side of the lid, the door was perforated

(total area of perforation = 3.55 x 10-3 m²) on the bottom part for inlet of air into the combustion

chamber (drum). The area of the perforation was estimated based on the volume of air required

to burn 8kg of biomass in about 12 hrs (Ashrae, 2001). It was necessary for the combustion to

proceed for about 12 hrs to ensure that plenum temperature was higher that the ambient

temperature, and thereby sustain the drying process from sunset to sunrise the next day around


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steel bars (0.016m diameter) were placed across the drum and duct at 0.3 m apart, and an

expanded metal placed on top of the bars to enhance absorption and storage of thermal energy,

granite rock pebbles (total mass of 360 kg, about 0.025 m pebble diameter) were packed on top

of the expanded metal, with the top part of the rock layer covered with concrete (0.025 m thick)

to make hot air from the burner chamber infiltrate through the horizontal rock matrix before

rising into the drying chamber via a rectangular hole (1.00 x 0.09 m) at the front of the concrete

absorber. The top part of the absorber was painted matt black to increase radiation absorption.

Granite was used for thermal storage because it has a high thermal diffusivity of 1.27 x 10-6

m² s-

1 (Ayensu, 1997).

This flow pattern of hot air enables a reasonable amount of heat to be transferred to the thermal

mass (calculated thermal mass = 1.146 mj k -1

). It was found necessary to have a relatively small

thickness (0.10 m) of the horizontal rock-and-concrete structure due to consideration of support

strength the flue gas exited the burner through a chimney (constructed from a galvanized iron

sheet, 6 x 10-4

m thick) which was fitted to the duct, a hole was drilled at the bottom of the knee

bend into the chimney to drain out flue-gas condensate from the system (Ashrae, 2001).


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Fig. 4: A) Plan View Of The Biomass Burner, Showing Flow Of Flue Gas The Drum Through

The Rectangular Duct Into The Flue Gas Chimney, B) Longitudinal Section Of The Drum.


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The dryer has a drying cabinet mounted on a brick wall that encloses a biomass burner. The brick

wall around the burner has cavity (0.05 m thick) which is filled with vermiculite to reduce heat

loss through the wall. The exterior dimensions of the wall are 2.52mby 1.55m drying air enters

the system through a rectangular inlet (1.00 m x 0.09m) at the bottom part of the brick wall, gets

into contact with the bare part of the drum and rises into the drying chamber through natural

convention (forming an s-shaped flow pattern). It passes through the drying bed (of thickness x =

0.3m) and exits through a solar chimney (painted matt black, height = 1.2 m, diameter = 0.18 m)

or air outlet vents. The solar chimney is fitted on top of the drying chamber to augment

thermosiphoning during solar collection, with a pressure difference (_p) given by (Ayensu, 1997)

P= g (_-_)(h1 + h2)

Where g is the acceleration due to gravity, _ and _ are air densities at the inlet (ambient) and

inside the dryer, and h1 and h2 are heights as shows in fig. 2. At the design meteorological

conditions (table 1), the pressure difference arising from natural convention is _p = 1.04 n m-2

.

The drying chamber has three trays, each with a plastic mesh base, which slide horizontally

along wooden rails fixed to the vertical sides of the cabinet. These trays can be removed for

loading and cleaning the effective tray area 4.1m2 and accommodates about 20 kg of fresh

pineapple the top surface of the cabinet is opaque, and inclined at 16º to the horizontal to reduce

friction as the air rises, under natural convention, from the plenum through the drying bed, the

collector frame and drying cabinet were constructed were constructed from block board (0.02 m

thick) and covered with a painted galvanized iron sheet (6 x 10-4

m thick) to protect the wooden

components from weathering. Two wooden doors (1.20 m x 1.51 m) were fitted at the back of

the drying cabinet for accessing the drying chamber. Each door has an air vent (rectangle of 0.51

m by 0.10 m with semicircles on both short vertical sides) located at its top. A wire mesh was

fitted on the vents and collector inlet. An overhang was fitted over the outlet-air vents to prevent

rain drops from entering the drying chamber through the vents. A glass cover was fitted on the

top part of the collector and inclined at 16º to the horizontal to optimize solar collection at the

malawi polytechnic (15º 48’s, 35º 02’ e) where the system was tested. The dryer faced north,

with the flue gas chimney on the western side of the system, the drying cabinet was fixed to the


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brick wall with metal straps mortared to the top layer of the external layer of the brick wall (figs.

1-3).

Table 1: Design parameters for a solar dryer with biomass back-up heater.

dryer drying load and meteorological conditions

solar collector drying products

aperture, a = 2.2m2 initial mass of load, mi = 20.0 Kg

absorber: concrete slab initial moisture, mi = 85% (wet basis)glass cover final moisture, mf = 10% (wet basis)

thickness = 3mm

tilt angle = 16º meteorological conditions

concrete absorber=0.025m thick solar radiation, h = 21.0 mj m-2

day-1

thick granite rock = 360 kg inlet air temperature = 30 ºc:

plenum temperature = 40 ºc

drying chamber relative humidity, rh = 80%

effective tray area = 4.1 m2

solar chimney = 1.2 height, 0.18 m ǿ

0.18 m

biomass burner

drum

length = 0.89 m

diameter = 0.0.58 m

door = 0.55 m x 0.54 m

flue gas chimney

height = 2.12 m

diameter = 0.12 m


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EXPERIMENTATION

Sample preparation

Twelve batches of fresh pineapples (annas comosus) were procured from Limbe produce market.

These fruits were washed, peeled, sliced (disc-shaped slices, thickness of about 1 cm) and cored.

For each batch, the slices were weighed on digital top-loading balance (adam equipment co.,

model acw-6) and loaded into the dryer for dehydration.

Operational modes

The dryer was operated in three different modes: solar, biomass and a combination of the twoenergy sources. For the solar mode of operation, a batch of fresh pineapple slices was weighed

(on a balance), loaded into the dryer in the morning and re-weighed after 24 hrs and on the final

day of drying. For the biomass mode of operation, drying of a batch of pineapple slices started in

the evening. The aperture of the dryer was covered with a ceiling board (a black polythene paper

was pasted on the top part of the board ) which was parallel to and extended by 0.6m beyond

the perimeter on all sides of the grass cover, with an air gap of 0.6m between the board and

transparent cover (ashrae, 1998). To block radiation when the altitude of the sun was low, a

vertical removable ceiling board (wrapped in black polythene paper) was fitted on the eastern

side in the morning and western side of the collector in the afternoon. The location of this shield

was changed around solar noon, the burner was charged with 8 kg of sawdust or wood shavings

and ignited around sunset. Again, the batch was re-weighed after 24 hrs and on the final day of

drying. Due to lack of automated facilities for recording temperatures at night, the burner was

charged with biomass fuel at sunrise, with the solar aperture covered. For the mixed mode, the

burner was again charged once with the biomass fuel around sunset and re-weighed after 24 hrs

and on the final day of drying.

Metrological parameters

Twelve mercury-in glass thermometers were inserted horizontally into the dryer (six on the

eastern side and six on the western side of the drying chamber). On each side, three of the

thermometers measured the plenum air temperature (below the drying bed) while the other three

measured the temperature of the air above the drying bed (fig 2). Ambient air temperature was


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monitored by using a mercury-in glass thermometer placed in a room with louvered glass. The

louvers were kept open to allow free circulation of air. The flue gas temperature was also

monitored using a mercury in glass thermometer. A wet and dry bulb thermometer (kagaka

kyoeisha, model sllb) was used to measure the level of relative humidity while velocity was

measured by a casella low-speed air meter (n 1462). It was also found necessary to determine the

flow rate of drying air through the dryer in this case, the air meter was placed in the middle of

the air inlet to the drying chamber. This location was suitable for comparability of the

performance of the solar chimney and the vents on the dryer. One venting system was blocked

while measurements were taken using the other. The intensity of global solar radiation was

measured by a kipp & zonen pyranometer (cm 6b) mounted in the plane of the transparent cover,and connected to a kipp & conen solar integrator (cc 1

4).

Physico-chemical analysis

The moisture content of fresh and dried samples was determined by drying an accurately-

weighed (5 g) sample in an electric oven (griffin, serial number 0518171070, accurate to 0.001

kg) at 70 ºc for hours (aoac, 1990). The loss in moisture was calculated as a percentage of the

mass of the dry sample. The uniformity of drying at different levels of the trays was determined

by weighing all the pineapple slices from each tray. In addition, five samples were taken from

different positions of each tray to establish the uniformity of drying across each tray (fig, 5)


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Fig 5: position of five pineapple used for determination of drying uniformity across the bottom

tray.

Early studies indicate that vitamin c is particularly vulnerable to destruction under different

conditions (air, light and heat) and during processing (Drew and Ree. 1980: Fellows and

Hampton, 1992). So, the retention of vitamin c is used as a good indicator of the preservation of

all other nutrients (maeda and salunke, 1981).

Date Day or Weather Mean ambient Mean relative Daily solar

night temperature (ºc) humidity (%) radiation(mj m-2)

1/6/04 Day Partly cloudy with drizzles 17 78 16.6

1/6/04 Night N.a 15 N.a Nil

2/6/04 Day Cloudy with Showers 16 76 8.4


3/6/04 Day Partly cloudy 16 77 21.6


4/6/04 Day Cloudy with showers 16 82 13.5


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Air and flue gas temperature

The plenum air passes through the drying bed where it picks up moisture from drying food by

natural convention. Fig. 6 shows the variation of mean plenum and ambient air temperatures on

the typical days of drying. For the solar mode of operation, it is seen that the plenum and ambient

temperatures are almost equal in the morning but different during the day. In particular, the

plenum temperature is higher than the ambient temperature during the most part of the day. The

plenum-ambient temperature difference (excess temperature) was 17 ºc from 13:00 to 15:00 hrs

and 12 ºc at 17:00 hrs. This indicates that the dryer stored some of the captured solar energy for

drying to continue during periods of low insolation or at night. At a mean excess temperature 12

ºc at sunset, the thermal mass stored about 13.8 mj for food dehydration to continue at night.However, the excess temperature dropped down to 0 ºc by the next morning. Leon et al. (2002)

report that a temperature difference (between ambient and drying air) of at least 10 ºc is required

for drying to continue. This shows that there was generally significant drying around solar noon.

a

Present investigation, we could not conduct a full nutritional evaluation of the food product due

to the limitation of the scope of this study, consequently, the retention of vitamin c was used as

an indicator of nutritional quality. The concentration of vitamin c in fresh (c f ) and dried (cd)

pineapple samples was determined according to AQAC (1990), and expressed on dry basis ash


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content was determined by incinerating an accurately-weighted sample (5 g) in a muffle furnace

(carbolite glm 1) at 600 ºc. The content of ash was expressed as a percentage of the mass of the

original sample.

Data processing

Stored thermal energy

Stored thermal energy (q) at sunset was estimated by using excess temperature (_t) in the

plenum. It should be noted that when the thermal mass is hot, the plenum air temperature is

lower than that of the heated thermal mass. This shows that the excess plenum temperature is

also lower than that of the heated thermal mass. Consequently, the stored energy computed inthis study is a minimum value on a given day.

q = m _t (2)

Where m is thermal mass, and _t = ttm – ta

retention of vitamin c

The percentage retention (r) of vitamin c was computed by:

r = 100 cd /df (3)

Where cd is the concentration of vitamin c in dried samples on dry basis (mg)/100 g), and c f is

the concentration of vitamin c in fresh samples on dry basis (mg/100 g)

dryer efficiency

Drying efficiency varies with the nature and moisture content of the fresh product.

Unfortunately, there is lack of standard methods for testing solar dryers which adversely affects

their evaluation (Sodha et al., 1987), So, Leon et al. (2002) recommend the use of the first-day

and final day efficiencies for evaluation of drying efficiency. The efficiency (η) of the solar dryer

was computed

By using the equation reported by Bena and Fuller (2002) and Leon et al. (2002):


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η = wl/(ah + cm) (4)

Where w is the mass of water evaporated (kg), 1 is the latent heat of vaporization (mj kg

¹), a is

the aperture of the dryer (m²), h is the insolation on the dryer (mj m ²), c is the calorific value of

biomass (mj kg-1

) and m is the mass of biomass. In this study, the value of c were determined

using gallenkamp automatic adiabatic bomb calorimeter (cat. No. 15 cb 110) and corrected for

moisture content. The latent heat of vaporization was calculated using a temperature dependent

function reported by Jannot and Coulibaly (1998):

1=2.5018-0.002378 twp (5)

Where twp is the wet bulb temperature (ºc) of the drying in the plenum. This temperature was

estimated using the procedure recommended by ASHRAE (1994).

RESULTS AND DISCUSSION

Weather conditions

Table 2 shows average weather conditions during the dehydration of a typical batch of fresh

pineapples slices. The weather was very unfavorable on most of the days, dominated by cloudy

skies with occasional drizzles and showers. The levels of relative humidity were relatively high,

ranging from 76% to 82%, with mean ambient air temperatures varying between 15 ºc and 17 ºc.

It is seen that the daily global solar radiation on an inclined plane (16 ºc to the horizontal) was

variable, with values ranging from 8.4 mj m ² to 21.6 mj m ². So, the daily intensity solar

radiation was lower than the design value (21.08.4 mj m ²) on most days. These weather

conditions indicate that drying would not proceed quickly, resulting in spoilage of the pineapple

slices, if a biomass backup heater was not used. However, the daily intensity of solar radiation

was quite high on some days (typically 30.7 mj m ² on 22nd November 2004), which shows the

potential for using the dryer without backup heating under such weather conditions.


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Table 2: weather conditions during one of the tests of the loaded solar dryer.

b

c


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Fig 6: variation of mean plenum (t pm), flue and ambient air (ta) temperature with local time on

typical days for different modes of dryer operation: a) solar energy on 28th

July, 2004, b)

biomass on 30th November, 2004 and c) solar-biomass on 24th February, 2004.

For the biomass mode of operation, the plenum temperature was initially monitored without fire

in the burner. It was found that there was insignificant differences between the plenum and

ambient temperatures during the day, which indicated that the insulation cover over the solar

collector was effective in blocking off irradiation during the day. The burner was charged with

sawdust or wood shavings. It was observed that the combustion of wood shaving was more

satisfactory than that of sawdust. Consequently, wood shavings were used in the rest of the burner tests. With the burner in the morning, it is observed that the plenum temperature is higher

than ambient temperature immediately after 06: 00 h to 17:00 h with a maximum value of 41 ºc

at 12:00 h on a typical day, which is within the acceptable limits of the drying air temperature

(Bena and Fuller, 2002; leon et al., 2002). There is an excess temperature of atleast was 10 ºc at

17:00 h. This is attributed to the fact that the wood shavings burnt for about 10 hours with the

stored thermal energy being released into the plenum even after the completion of combustion.

In addition, it is observed that the flue gas temperature has a distinct maximum (53 ºc) at 09:00

h; which is relatively low. This indicates that most of the thermal energy was transferred to the

system before the gas exited the rectangular duct. Bena and Fuller (2002) report a flue gas an

excess temperature of 177 ºc, which was high. When the burner was charged with wood shaving

at sunset, there was an excess temperature of 6 ºc to 10 ºc the next morning but it dropped to zero

by sunset. This indicates that drying proceeded at night.

For the solar-biomas mode of operation, it is observed that the plenum temperature was higher

than ambient temperature from 06:00 h to 17:00 h, with the excess temperature increasing to

maximum value of 18 ºc around 09:00 h and 10:00 h, then dropping to 7 ºc at 17.00 h..

Moreover, the burner was re-charged with wood shavings at sunset. So, food dehydration

proceeded during both day and night.


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Drying time and uniformity

For the solar mode of operation, it was found that the dryer reduced the moisture content of a

typical batch of fresh pineapple slices from 669 % to 16% (on dry basis, db) in 5 days, with the

daily intensity of global solar radiation varying from 18.4 to 26.8 mj m ² on an inclined plane

(16º to the horizontal). It should also be mentioned that the drying product was not removed

every day from the dryer after sunset because the stored thermal energy enable drying to proceed

at night. So, the dryer can be exclusively powered by solar energy to dehydrate a fresh food to

the required level of moisture under favourable weather conditions.

For the biomass mode of operation (with a non-irradiated collector), it was observed that thedryer reduced the moisture content of a batch from 614% (db) to 13% (db) in 3 days. Prolonging

the dehydration process to 3.5 days resulted in an over-dried product. This clearly shows that the

rate of drying in this mode of dryer operation is higher than that of the solar mode, which is

attributed to the higher amount of energy supplied by the wood shavings (17.6 mj kg ¹r)

radiation.

The solar-biomass mode of operation was used to study the drying time and uniformity. Fig. 7

shows the variation of the mass and moisture content of a typical batch. It is observed that the

rate of drying was highest during the first day (24 hours), with mass of the batch dropping from

20.02 kg to 12.28 kg. The mass of the sample was 2.90 kg by the end of the third day, with

negligible drying after this. It is seen that the moisture content dropped from of 669% to 11%

(db) in 3.0 days, with the intensity of the daily global solar radiation varying from 15.2 mj m ² to

21.5 mj m ² on an inclined plane. Lower final levels of moisture content can be achieved in

shorter drying periods with higher intensities of global radiation. The solar dryer developed by

Bena and Fuller (2002) reduced moisture content of pineapples from 559% (db) to 11 % (db) in

3.5 days, under the prevailing meteorological conditions. Leon et al. (2002) recommended a final

moisture content of 10% (wet basis, db), which is equivalent to 11% (db) for evaluation of solar

dryers. So, the final moisture level obtained after drying a batch of pineapple slices for 3 days

was within acceptable limits when the solar dryer was backed by a biomass burner.


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Fig 7: variation of moisture content (%), db) and mass of a typical batch of pineapples (dried

from 21

st

to 24

th

June 2004) with drying time (day)

Figure 8 shows the rate of drying at different trays in the drying chamber. It is seen that samples

in the bottom tray dried fastest, with the top tray exhibiting the least drying rate. This drying

pattern is attributed to the flow of hot air which flows from the collector (in the bottom part) of

the dryer into the drying chamber in the middle part. Consequently, it is necessary to swap the

trays in order to ensure uniform drying.


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Fig 8: variation of moisture content at different trays (28th

June -1st July 2004)

The uniform of drying across the bottom tray is shown in fig 9. It is seen that samples from

positions a and b (in front of the tray) dried fastest with those at positions c and d (back of a tray)

exhibiting the least drying rate. The middle sample (position o) exhibited an intermediate rate of

drying. A similar pattern of drying uniformity was observed across the middle and top trays, with

the drying rate being most non-uniform across the top tray. These observations are attributed to

the flow pattern of the heated air which moves upwards and horizontally across the trays A

similar pattern of air flow is expected for the solar biomass operational modes. So, the front part

of the bottom tray receives the hottest air.


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Fig 9 uniformity of drying rate across the bottom tray (28th

June – 1st July 2004)

Concentration of vitamin c and ash

The concentration of vitamin c varied from 22 to 34 mg/100g (wb), with a mean value of 26+4

mg/100 g (wb). These results are in close conformity with the findings (25 mg/100 g, wb) of

Osborne and Voogt (1978). The retention of vitamin c varied from 26 to 44 %, probably due to

the high sensitivity of this type of vitamins to different processes (drying, sample preparation)

and conditions (Drew and Ree, 1980; Fellows and Hampton, 1992). Ash content varied between

0.5% and 0.6% (wb) which compares well with the value of 0.4% (wb) reported by Osborne and

Voogt (1978) for raw pineapples. The minor vitamins are probably due to the season and the

maturity stage of the fruit. There was no significant difference in the concentration of ash (db) in

fresh and dried samples, which indicates that there was very little (or no) contamination arising

from dust.

Air flow rate

For diurnal operation with the loaded dryer, the estimated air flow rate varied from 0.017 m3 s

-1

to 0.036 m3 s-

1 (mean value of 0.024 + 0.002 m

3 s-

1) when the dryer was operated with solar


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chimney, and from 0.030 m3 s

-1 to 0.048 m

3 s

-1 (mean value of 0.037 + 0.003 m

3 s-

1) when dryer

was operated with the air outlet vents. The two mean values of estimated flow rate were

significantly different (p-value = 0.00). It is seen that flow rate is slightly lower when the dryer

was operated with a solar chimney than air outlet vents. This is attributed to the pressure drop

between the dryer air inlet and the chimney air which is higher than that between the dryer air

inlet and the air vents. Moreover, samples provide resistance to air flow. It was also observed

that the air meter advanced one direction when the system was operated with a solar chimney,

which indicated that there was little or no reverse flow in this mode of operation. In contrast, the

air meter could occasionally stop and then advance in the opposite direction when the system

was operated with air outlet vents open. Bena and Fuller (2002) report an average air flow rate of0.038 m

3 s

-1 for a solar dryer operating in the biomass mode. These authors found considerable

variation in the flow rates for their dryer which had air outlet vents.

For nocturnal operation with an open solar chimney, it was observed that there was condensate

on a small portion of the absorber area directly below the air inlet to the chimney. This was

attributed to reverse flow at night when the chimney became cold and acted as a condenser for

the warm (and humid) air rising up from the drying bed. In view of this , the solar chimney was

blocked with card board papers (with vents opened) at sunset and opened (with vents closed) at

sunrise to avert reverse thermosiphoning. This approach proved to be effective.

It should also be mentioned that the flow rate of air through a solar chimney is affected by

several factors which include intensity of solar radiation, chimney length and cross-sectional area

(Ong and Chow, 2003). These authors found that air velocity increased with solar radiation and

air gap. It has been seen that air velocities produced by the size of the solar chimney in this study

are lower than those of the outlet air vents. Moreover, a solar chimney increases the production

cost of the dryer and is effective only during the day. Consequently, the dryer can still operate

satisfactorily with only air outlet vents.

Dryer efficiency

Table 3 shows the first-day (η1) and final-day (ηf ) efficiencies of the dryer. For the first-day

efficiency, it is seen that the solar mode of operation exhibits the highest mean value, with the


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biomass mode showing the lowest value. This is probably due to the fact that the burner loses a

relatively high proportion of the heat though the flue gas, which results in a relatively lower

efficiency than that for the solar mode of operation. For the final day efficiency, it is again

observed that the solar mode of operation is highest with the biomass mode being the lowest.

Table 3: variation of the first (η) and final (ηf) –day efficiencies.

Operational mode Efficiency

η (%) η (%)

Solar 20 + 2 15 + 1

Biomass 15 + 2 11 + 1

Solar-biomass 17 + 1 13 + 2

The calculated final efficiency of the solar mode of operation is comparable with typical

efficiency values of 10 – 15% for natural convention solar dryer (brenndorfer et al)., 1985). Bena

and Fuller (2002) report an efficiency value of 22% when their dyer was operated in the solar

mode. These authors state that this value is likely to be inflated because of the value of the area

of the solar aperture. Moreover, these authors to not report the first-day efficiency of their

system. Leon et al. (2000) states that the first-day efficiency may be taken as a consistent basis

for comparing the thermal performance of solar dryers. For the solar-biomass mode of operation,

it seen that the present calculated efficiency is higher than (8.6%) found by Bena and Fuller

(2002). This is attributed to the inclusion of a rectangular flue gas duct and thermal mass (with

granite rock pebbles) which increases the rate of heat transfer from the burner to the drying

chamber in the present dryer design. For the biomass mode of operation, the calculated

efficiency is again higher than that (6%) reported by Bena and Fuller (2002), for the same

reasons suggested for the soar-biomass mode of operation.


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CONCLUSION AND RECOMMENDATIONS

A convective solar dryer has been designed and constructed with a biomass backup heater. The

system was tested in three modes of operation (solar, biomass and solar-biomass), using fresh

pineapple slices, under different weather conditions. Results show that under favorable weather

conditions, it was possible to dry pineapple slices using solar energy as the only source of

thermal energy. The solar mode of operation yielded the highest thermal efficiency, with the

biomass mode of operation exhibiting the lowest efficiency. However the dryer performed most

satisfactorily with a backup heater (operated on wood shavings), yielding an overall efficiency

13+2%, and retaining 26% to 44% of vitamin c during the dehydration process. It appears thatthis solar dryer overcomes the limitation of intermittent dehydration of a food product. However,

there is need for further improvements to the design to optimize the performance of the system

and its assessment at pilot phase.

Nomenclature

A solar aperture of the dryer (m²)

C calorific value of biomass (mj kg

¹)

Cd concentration (db) of vitamin c in solar-dried sample (mg/100 g)

Cf concentration (db) of vitamin c in fresh sample (mg/100 g)

H daily intensity of global solar radiation (mj m ²)

L latent heat of vaporization of water (mj kg ¹)

M thermal mass (mj ºc ¹)

M mass of biomass (kg)

Q stored thermal energy (mj)

R retention of vitamin c (%)

Ta ambient temperature (ºc)

Twp mean plenum temperature (ºc)


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Twp wet bulb temperature of the drying air in the plenum ( ºc)

W mass of water evaporated (kg)

Greek letters

- efficiency of dryer

- air density (kg m ³ )

Subscripts

A ambient

Db dry basis

Wb wet basis

ACKNOWLEDGMENTS

The authors are very grateful to National Research council of Malawi for the financial support.

The Malawi Polytechnic is also acknowledged for the logistical and technical support. This work

has enabled the researchers gain some practical experience in the design, construction and testing

for solar dryers with biomass backup heaters.

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Documents

Convective Solar Dryer