potential and development of solar drying …...solar drying is promising and most preferred method...

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Potential and development of solar drying technology: A Review

R.T. Ramteke1, C.N. Gangde

2 and S.R. Kalbande

2

1PhD Scholar Department. of Unconventional Sources of Energy and Electrical Engineering, CAET,

Dr.P.D.K.V., Akola, Maharashtra, India. 2Department. of Unconventional Sources of Energy and Electrical Engineering, CAET, Dr.P.D.K.V.,

Akola, Maharashtra, India.

Abstract- Solar energy is one of the promising heat sources for meeting energy demand without

putting adverse impact of environment. Hence it plays key role for sustainable development. This

paper presents developments and potentials of solar drying technology for drying of agriculture

produce. Detail description and fundamentals of different types of solar air heating systems like

natural convection direct dryer, forced circulation type dryer, green house type air heaters, solar

dryers with and without heat storage medium have been carried out and presented in present review

paper. It is found that natural circulation type solar tunnel dryer, wind ventilated type solar dryers

with heat storage units and green house dryers are more suitable for rural applications. The use of

solar collector’s performance enhancement technique accelerates drying rate and promotes overall

efficiency of drying system. Solar collectors works by absorbing the direct solar radiation and

converting it in to thermal energy which can be stored in the form of sensible heat or latent heat or a

combination of sensible and latent heat. During the investigation rock bed storages have been found

to be low type thermal heat storage, while phase change materials have been found to be higher heat

thermal storage. For better thermal performance of solar air heater with high latent heat and with

large surface areas for heat transfer required. The researcher’s designs going to the integration

between solar energy collection and thermal storage to reduce the loss, volume and system cost.

Key Words - Solar drying, phase change medium, latent heat storage, sensible heat storage

I. INTRODUCTION

Solar drying is promising and most preferred method for drying agricultural products. Drying

crops by solar energy is of great economic importance the world over. Solar energy being simple to

use, free, renewable, clean, non polluting and inexhaustible has received wide spread attention in

recent times. It provides well abundant energy source if utilized efficiently. But this energy is time

dependent energy source with an intermittent character. Most of the crops and grain harvests are lost

to fungal and microbial attacks these wastages could be easily prevented by proper drying, which

enhances storage of crops and grains over long periods of time. However, solar drying systems must

be properly designed to match particular drying requirements of specific crops, which can increase

the efficiency of a system.

The drying of agricultural produce under the sun is a common practice most especially in

developing countries like India where more than 3300 to 3700 hours of bright sunshine per year

available in North-West and West coastal regions of the country and 2900 hour over central part of

India except Kerala, Kashmir and Assam. In brief, there are nearly 250-300 days per year available

to get useful sunshine [1]. This method is characterized by a number of constraints like; lack of

process control, non-uniformity in the drying rate, soaking by rain, theft and vandalism,

contamination by dust, rodents, and other domestic animals . Also prolong open sun drying often

caused deterioration of vital ingredients like vitamins, minerals and sensory characters of dried

product and thus less market value Solar drying is alternative for drying varieties of agricultural

produce at superior drying rate compared to open sun drying method with improved quality of dried

product at a relative low cost.

Various types of solar air-heaters are being used for different applications; among them flat-

International Journal of Modern Trends in Engineering and Research (IJMTER) Volume 03, Issue 06, [June– 2016] ISSN (Online):2349–9745; ISSN (Print):2393-8161


plate collectors are extensively used in low-temperature solar energy, because they are relatively

simple, easy to operate and have low capital costs. Solar air heaters have many attractive advantages

over liquid heaters regarding the problems of corrosion, boiling, freezing and leaks. Solar air heater

without thermal storage is extensively used for drying agriculture products. Basically most of the

agriculture products are getting dried at low temperature (50–60 0C) and this can be easily achieved

in flat-plate type solar air heater.

There are many types and designs of solar dryers available for drying of agricultural

commodity. They mainly include natural convection cabinet dryer and forced circulation solar dryer

based on array of flat plate air heaters. The main drawbacks of forced circulation solar dryer are

requirement of more ground space under air heating array, high power requirement to circulate the air

and the high initial cost. The mechanical (fuel) operated dryer are more efficient than open sun

drying, provide uniform high quality dried produce, with many more advantages. The limitation of

this drier is that it cannot be used in the areas where electricity is not available. The major drawbacks

of natural convection dryer are higher drying time, uneven drying and comparatively low capacity.

Indirect dryers are suitable for colour sensitive produce, as the produce is not exposed directly to the

sun. These types of dryer are suitable for drying various industrial products, fruits, vegetables and

spices like turmeric, ginger etc.

This paper presents the potentials of direct solar dryers, indirect solar dryers, solar dryers

with heat storage media, solar dryers without heat storage media, solar tunnel dryers for solar drying

of fruits, vegetables, spices, cereals, grain, legumes, medicinal plants, fish etc, with a view to

providing some of the benefits to farmers.

1.1 Classification of Solar Dryers

The dryer can be classified into two major groups, namely:

a) Passive solar-energy drying systems (conventionally termed natural-circulation solar drying

systems) and,

b) Active solar-energy drying systems (most types of which are often termed hybrid solar dryers).

Three distinct sub-classes of either the active or passive solar drying systems can be

identified namely:

• Direct-type solar dryers;

• Indirect-type solar dryers; and

• Hybrid solar dryers.

Solar energy dryers can broadly be classified into direct, indirect and hybrid solar dryers.

The working principle of these dryers mainly depends upon the method of solar- energy collection

and its conversion to useful thermal energy for drying.

1.1.1 Direct Solar Drying (DSD)

Here the moisture is taken away by the air entering into the cabinet from below and escaping

through at the top exit as shown in Figure 1.1 Sharma [2]

“Figure 1. Working principle of direct solar drying”



1.1.2 Indirect Solar Drying (ISD)

The crops in these indirect solar dryers are located in trays or shelves inside an opaque

drying cabinet and a separate unit termed as solar collector is used for heating of the entering air into

the cabinet. Shown in Fig. 1.2 Sharma [2]

“Figure 2. Working principle of indirect solar drying system”

1.1.3 Hybrid Solar Drying (HBD)

The hybrid solar dryers combine the features of the direct and indirect type solar- energy

dryers. Here the combined action of incident direct solar radiation on the product to be dried and air

pre-heated in a solar collector heater produces the necessary heat required for the drying process.

II. NATURAL CONVECTION SOLAR DRYER

In the natural convection solar dryers the airflow is established by buoyancy induced airflow

while in forced convection solar dryers the airflow is provided by using a fan either operated by

electricity/solar module or fossil fuel. Natural convection solar drying has advantages over forced

convection solar drying, one of which is that it requires a lower investment. Due to low cost and

simple operation and maintenance, natural convection solar dryer appears to be the obvious option

and popular choice for drying of agricultural products. In such a dryer, the crop is contained within a

cabinet in a relatively thin bed, air flows as a result of the buoyancy forces resulting from the

temperature differences up through the crop bed. The drying rates achieved to date with these dryers

have not, generally, been very satisfactory .

The simplest of solar cabinet dryer was reported by Fudholi et al. [3], it was very simple,

and consists essentially of a small wooden hot box. Dimensions of this dryer was 2 m × 1 m where

the sides and bottom were constructed from wood and metal sheets. A transparent polyethylene sheet

was used as cover at the upper surface. Air holes were located on the sides of the dryer for

circulation.

Singh et al. [4] investigated a small size domestic natural convection solar dryer. It is mainly

consists of a hot box, base frame, trays and shading plates. A transparent window glass (4 mm thick)

was fixed as glazing. It was fixed to the hot box with an aluminum angle. In order to provide air

circulation in the dryer, 40 holes with total area of 0.002 m2 were drilled in the top and sides of the

dryer. A flat sheet of 5 cm thick of thermocole was used as insulator. Mursalim et al. [5] evaluated a

modified cabinet dryer with natural convection system. The dryer had a single transparent plastic

cover and the sawdust was used as an insulating material. The drying chambers walls were build of

plywood painted black with dimensions 120 cm × 80 cm × 40 cm (long, width, and height). For air

flow, 12 holes were provided at the bottom.

Ezekoye and Enebe [6] evaluated a domestic direct mode solar dryer with

constructed components mounted on a skeleton and screwed such that the various components are

detectable and dismantled for easy movement. Optimum temperature of 67.0°C is achieved with

the dryer at no-load test and could dry pepper and groundnuts for a maximum of five and eight days

respectively.



2.1.1 Natural circulation greenhouse dryers

These are also called as tent dryers and are basically modified greenhouses. They are

designed with vents of appropriate size and position to have a controlled air flow. They are

characterized by extensive glazing by the transparent cover of polyethylene sheet. Fig.3 shows the

earliest form of passive solar greenhouse dryer by the Brace Research Institute, with slanted glass

roof, allowing direct solar radiation over the product. The length-wise north-south alignment of the

dryer had black coated internals for improved absorption of solar radiation with the ridge-cap over

the roof for exit vent. Doe et al. [7] later designed the widely reported poly-ethylene tent dryer,

illustrated in Fig.4 consisting of a ridged bamboo framework clad with a clear polythene sheet over

it. A black poly-ethylene sheet was also spread on the floor inside the tent to enhance the absorption

of solar radiation. The air flow into the tent was controlled by rolling/ unrolling of the cladding at the

bottom edge of front side and the vents at the top served as the exit for the moist exhaust air.

Sachithananthan et al. [8] reported a horticultural greenhouse of clear plastic sheet cladding over a

semi-cylindrical metal frame (Fig.5). The modification were with a black galvanized iron sheet

absorber at floor, inlet vents along the full length of both sides of base and exit with plastic nets at

the top to protect from insects and dust.

Fleming et al. [9] reported a typical greenhouse type solar dryer with a transparent semicylindrical

chamber with a cylindrical solar chimney posted vertically at one end and a door for air inlet and

access to the chamber at other end as shown in Fig.6.

Rathore et al. [10] has conducted various experimental studies on a modified design of

hemicylindrical solar tunnel dryer for drying of grapes also few researchers have used a

polycarbonate cover for its construction. Afriyie et al. [11] has reported the study of simulation

and optimization of a chimney ventilated solar crop dryer.

Kalbande et al. [12] designed and fabricated a solar-biomass hybrid dryer. A biomass

combustor retrofitted to natural convection solar tunnel dryer used solar energy for drying during day

time and Prosopis juliflora as fuel in biomass combustor cum hot air generator during off sunshine

hours and thus extends the working time of the dryer. Experiments have been conducted to test the

performance of the dryer, turmeric had been dried. The results indicate that the drying is faster. The

quantitative analysis showed that the traditional drying i.e. open sun drying took 4 to 15 days to dry

the rhizomes while solar biomass drier took only 1.5 to 4 days for the samples having different

treatments and produced better quality produce. The maximum efficiency of biomass combustor and

solar-biomass hybrid dryer was found to be 79.79 and 14.00 respectively.

“Figure 3. Natural-circulation glass-roof “Figure 4. Natural-circulation polythene tent

dryer solar-energy dryer”



“Figure 5. Natural-circulation solar “Figure 6. A greenhouse type natural circulation

dome dryer” solar–energy dryer”

2.2 Forced convection solar dryers

Adding a small fan powered by a photovoltaic system to provide the required air flow will

give drying rates much higher than those achieved by an indirect natural convection solar

dryer. Various design of indirect solar dryer had been developed and tested. These designs had been

recommended for commercial purposes. The indirect type solar dryers include the chamber-type

dryers (tray and rack type dryer, bin type, and tunnel type), chimney-type dryers and wind-ventilated

dryers. Solar dryer which have demonstrated their potentialities for small-scale agro-industrial

applications are discussed below.

Bolaji [13] investigated an indirect solar dryer using a box type absorber collector. The

dryer consists of an air heater, an opaque crop bin, and a chimney. The box-type absorber collector,

made of a glass cover and black absorber plate, was inclined at angle of 200 to the horizontal to allow

the heated air to rise up the unit with little resistance. He reported that the maximum efficiency

obtained in the box-type absorber system was 60.5%. He found also that the maximum average

temperatures inside the collector and drying chamber were 64 and 57 ◦C, respectively; while, the

maximum observed ambient temperature was 33.50C.

Al-Juamily et al. [14] constructed and tested an indirect-mode forced convection dryer for

drying fruits and vegetables in Iraq. The solar dryer consists of a solar collector, a blower, and a

solar drying cabinet. Two identical air solar collectors having V-groove absorption plates of two air

passes and a single glass cover were used. The total area of the collectors was 2.4 m2. Two types of

fruits and one type of vegetables were dried using this dryer. These were grapes, apricots, and beans.

The moisture content of apricot had been reduced from 80% to 13% within one day and a half of

drying. Moreover, the moisture content of grapes had been reduced from 80% to 18% in two and a

half days of drying. Finally, the beans moisture content had been reduced from 65% to 18% in 1 day

only. They concluded that the air temperature is the most effective factor on drying rate. The effect of

variation of the speed of air inside the drying cabinet was small and may be neglected.

Karim and Hawlader [15] studied the V-groove, fins and flat-plate collectors for crop drying

applications. The results showed that the V-groove collector had 7–12% higher efficiency than flat-

plate collector. The double pass operation of the collector improved the efficiency of all three

collectors. The efficiency of all the air collectors is a strong function of airflow rate. The flow rate

0.035 (kg/m2

s) was considered optimal for solar drying of agricultural product.



“Figure 7. Schematic description of the solar air heater (a) cross-section view perpendicular to the flow direction; (b)

cross-section view along the flow direction; and (c) schematic description of the cross-corrugated absorbing plate and

bottom plate [53]”.

Sarsilmaz et al. [16] conducted experiments on drying of apricots in a newly developed rotary

column cylindrical dryer (RCCD) equipped with a specially designed air solar collector to find the

optimum drying air rate and rotation speed of dryer, to maintain uniform and hygienic drying

conditions and to reduce drying times. The systems were constituted of three parts: air blow region

(fan), air heater region (solar collector) and drying region (rotary chamber).

Ehiem et al. [17] designed and developed an industrial fruit and vegetable dryer consisting

of a drying chamber, blower and heat exchanger unit for reducing vegetable wastage and improving

their storage conditions. Using tomatoes of small, medium and large sizes; mean drying capacity of

258.64 kg for tomatoes per batch at an average drying chamber temperature of 50°C and varying air

flow rates (18.3 m/s, 18.8 m/s and 19.5 m/s); drying time and drying rate of 0-14 hr and 40 g/hr were

achieved respectively with thermal efficiency of 84 %.

Bolaji et al.[18] utilized the benefits of using a rotary wind ventilator for forced convection

solar dryer in remote area where electricity and other power sources are non- existent. The dryer is

made of solar air-heater of black absorber plate insulated with foam material to minimize heat losses.

An absorber mesh screen is place midway between the glass cover and the black absorber plate for

effective air heating as solar radiation that passes through the transparent cover is absorbed by both

the mesh and black-plate. The drying chamber holds drying trays in layers, a rotary wind ventilator

located at the top circulates the heated air in the drying chamber. The performance of the dryer with

and without the wind ventilator led to considerable reduction in drying time and increased system

efficiency in comparison to open sun drying method.

Bala et al [19] dried pineapple using solar tunnel drier at Bangladesh Agricultural University,

Mymensingh, Bangladesh. The drier consists of a transparent plastic covered flat plate collector and

a drying tunnel connected in a series to supply hot air directly into the drying tunnel using two d.c

fans operated by a solar module. With a loading capacity of 120-150kg and 8 experimental tests, the

use of the solar tunnel drier leads to considerable reduction in drying time in comparison to open sun

method and dried products were completely protected from contamination. Also, the proximate

analysis indicates that dried products were of good quality compared to open sun dried products.

2.3 Solar dryers without heat storage system

A widespread literature reviewed various types of solar air heater without thermal storage and

presented in this section.



Chandrakumar and Jiwanlal [20] developed and evaluated the performance of a forced

convection mixed-mode solar dryer using smooth and rough plate solar collectors. The study was

conducted to compare the performance of the smooth and the roughed plate collectors in conducting

drying. The thermal performance of the smooth solar collector was found to be poorer because

of low convective heat transfer from the absorber plate to air. Artificial rib roughness on the

underside of the absorber plate has been found to considerably enhance the heat transfer coefficient.

Under no-load conditions the roughed absorber plate of the dryer attained a temperature of

69.2°C when it was studied. The maximum air temperature in the dryer under this condition was

64.1°C. The dryer could dry 3 kg of grapes having an initial moisture content of 81.4% to final

desired moisture content of 18.6% within 4 days while it was 8 days for open sun drying. Sahu and Bhagoria [21] reported effect of broken transverse ribs on absorber plate of a solar

air heater. Integral rib roughened absorber plates were prepared by fixing wires of 1.5 mm diameter

over one side of absorber plate. Roughness geometry was having pitch (P) ranging from 10 to 30

mm, height of rib (e) was 1.5 mm and duct aspect ratio was 8. Investigated range of Reynolds

number was 3000–12,000. Heat transfer coefficient enhancement over smooth duct was reported to

be 1.25–1.4 times and maximum thermal efficiency of the order of 83.5% was obtained.

Saravanakumar and Mayilsamy [22] have presented the thermal performance of flat plate solar

air heater (FP-SAH) with and without thermal storages. A forced convection solar collector

integrated with the different SHS material was developed and tested for its performance. The system

consists of a FP-SAH with heat storage unit and a centrifugal blower to increase collector and 𝜂

(10–20%). Gravel with iron scraps gives better efficiency than other storage materials. Forced

convection solar collector was more suitable for drying high quality dried product even in a cloudy

climate.

Chabane et al. [23] investigated thermal performance of a single pass solar air heater with fins

attached. Longitudinal fins were used inferior to the absorber plate to increase the heat exchange and

render the flow fluid in the channel uniform. The effects of mass flow rate of air on the outlet

temperature, the heat transfer in the thickness of the solar collector and thermal efficiency were

studied. Experiments were performed for an air mass flow rate of 0.012 kg/s. Maximum efficiency

was obtained by using five longitudinal fins and without using fins. The maximum efficiency levels

obtained for the 0.012 kg/s with and without fins were 40.02% and 34.92% respectively. A

comparison of the results of the solar collector with and without fins shows a substantial

enhancement in thermal efficiency.

Several research works have been explored for dryers having thermal energy storage

materials either under the collector absorber plate or integrated to the drying chamber for accelerated

drying process. The various forms in which thermal energy can be stored are sensible heat, latent

heat, thermo chemical or combinations of these. The following sections discuss each of these forms

in detail.

2.4 Solar dryers with heat storage system

Gutti et al [24] designed and constructed a natural/forced convection solar vegetable dryer

using locally available materials and incorporated with a heat storage unit for ensuring drying

during night hours. The solar collector absorber plate was made of corrugated aluminium,

painted black to improve its absorption and insulated with thick plywood to prevent heat loss. The

drying cabinet structural frame was built from a well- seasoned gmelina wood and plywood sheet

and outlet vent (chimney) provided towards the upper end at the back of the cabinet to facilitate and

control the convection flow of air through the dryer. Forced convection drying system gave higher

drying capabilities compared to the natural convection system and the stored useful heat used

for about six hours in drying during the night.

Ayyappan and Mayilsamy [25] integrated a natural convection solar tunnel dryer with

sensible heat storage material (sand) for copra drying. Experiments were performed with and

without the heat storage material. The system reduced the moisture content of copra from

52% (w.b.) to 7.2% (w.b.) in 52 hrs and 78 hrs respectively with and without the heat storage



material. It also produces copra of high quality and market value for the farmers than the sun dried.

In open sun method, moisture content was reduced from 52.3% (w.b.) to about 7.8% in 172 hrs. The

average solar tunnel drier thermal efficiency was estimated to be about 18% in both drying modes.

Jain [26] presented a transient analytical model to study solar crop dryer having reversed

absorber plate type collector and thermal storage with natural flow. The developed model was solved

to compute the air temperature and various functional components of the drying systems for the

climatic condition of Delhi, India. The parametric study involved the effect of width of airflow

channel and height of packed bed on the crop temperature. A reversed absorber plate of 1m length

with 0.15 m packed bed could dry 95 kg of onions from a moisture content of 6.14 to 0.27 kg

water/kg of dry matter in a 24 hrs drying period.

2.4.1 Sensible Heat Storage

Sensible heat is stored by raising the temperature of a solid or liquid media. Some of the solid

mediums used are metals, rock, concrete and liquid medium are oil-based liquids, water, and molten

salts.

Singh and Bansal [27] fabricated solar air heater from broken glass pieces and evaluated with

four typical cases (i) top surface blackened, (ii) all glass pieces blackened and (iii) bottom surface

blackened. From the study it was concluded that the efficiency of all three systems goes on

increasing with increasing mass flow-rate. Porous types of air heaters always give better performance

over the nonporous types, since the bottom absorbing type of air heater is better than the top

absorbing type of air heater. For the case of non-porous types of air heating collectors, the thin and

thick absorbing plates are better for bsorption at the top and bottom, respectively, while a porous type

air heater has an optimum plate thickness (i.e. 0.05 m).

Tiwari et al. [28] have experimentally evaluated a crop dryer cum water heater and crop dryer

rock bed storage. They reported energy balance equations for each component of the system have

been used to predict the analytical results. On the basis of the analytical results, it is observed that the

drying time is significantly reduced due to the increase in thermal energy on the collector by the

reflector. The system can be used to provide hot water in case the drying system is not in operation.

The water heater below the air heater systems will act as a storage material for drying the crop during

off-sunshine hour.

Chauhan et al. [29] studied the drying characteristics of coriander in a stationary 0.5

tone/batch capacity deep-bed dryer coupled to a solar air heater and a rock bed storage unit that is to

receive hot air during sunshine and off-sunshine hours, respectively. The theoretical investigation

was made by writing the energy and mass balance equations for different components of the dryer-

cum-air-heater-cum-storage and by adopting a finite difference approach for simulation. The results

revealed that for reducing the moisture content 28.2% (db) to 11.4% (db) the solar air heater takes 27

cumulative sunshine hours, i.e. about 3 sunshine days, whereas the solar air heater and the rock bed

storage combined take 31 cumulative hours, i.e. about 2 days and 2 nights at an air flow velocity of

250 kg/h m2, and recommended that the heat stored in the rock bed can be used effectively for

heating the inlet (ambient) air for off-sunshine drying of agricultural products.

Shanmugam and Natarajan [30] studied regenerative desiccant integrated solar dryer with and

without reflective mirror. An indirect forced convection with desiccant integrated solar dryer had

been built and tested. The main parts were: a flat plate solar air collector, a drying chamber, desiccant

bed and a centrifugal blower. The system was operated in two modes, sunshine hours and off

sunshine hours. During sun shine hours the hot air from the flat plate collector is forced to the drying

chamber for drying the product and simultaneously the desiccant bed receives solar radiation directly

and through the reflected mirror. In the off sunshine hours, the dryer was operated by circulating the

air inside the drying chamber through the desiccant bed by a reversible fan. The dryer was used to

dry 20 kg of green peas and pineapple slices.

Mohanraj and Chandrasekar [31] performed an indirect forced convection solar drier

integrated with heat storage material; it was designed, fabricated and investigated for copra drying.

One side of the collector was connected to the blower with the help of reducer and the other side was

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attached with drier cabin as shown in Fig.8. The 100-mm gap between the absorber and insulation

was filled with sand mixed with aluminum scraps to store the heat during sunshine hours and to

obtain hot air during off sunshine hours. The experiments with heat storage material were conducted

for 8 h during potential sunshine hours and 4 h during lean or off sunshine hours the chili was dried

from initial moisture content 72.8% to the final moisture content about 9.2% and 9.7% (wet basis) in

the bottom and top trays, respectively. Thermal efficiency of the solar drier was estimated to be about

21% with specific moisture extraction rate of about 0.87 kg/kWh.

“Figure 8. Schematic of solar air heater integrated with storage unit used for copra drying [41]”.

Kamble et al. [32] Studied drying of chilli using solar cabinet dryer coupled with gravel bed

heat storage system. The solar cabinet dryer coupled with gravel bed heat storage system was

evaluated for drying of green chilli. The loading capacity of the dryer was about 15 kg of fresh

produce per batch. An exhaust fan was provided in the drying chamber. The average temperature was

observed in the range of 25 to 55°C. Drying time for drying green chilli from initial moisture content

of 88.5% (w.b) to 7.3% (w.b) was estimated to be 56 h in solar dryer whereas 104 h was observed in

the open sun drying. Drying time due to introduction of heat storage system was extended by 4 h

after sunset. Drying efficiency of the solar cabinet dryer was found to be 34 %.

Saxena et al. [33] designed and carried out performance of a solar air heater with long term

heat storage system. ‘‘Granular carbon’’, has introduced as a long term heat absorbing media inside

solar heater. The thermal performance evaluation of solar heater has been carried out on four

different configurations by operating it on natural and forced convection. The thermal behavior of the

system has also been evaluated by operating it on auxiliary power by placing a halogen tube (300 W)

inside the inlet and outlet ducts. Because of using halogen lights the system is feasible to perform in

night or bad climatic conditions. The thermal performance of all new configurations was found better

in comparison of conventional solar air heater on both natural and forced convection.

2.4.2 Latent Heat Storage

Solar air heater with PCM (Phase change media) based thermal storage system is the solar

energy collection for it’s off sunshine hours use. A phase change material is a solid and melted

which stores energy. The melting temperature may be fixed or may vary over a small range. The

stored energy is recovered upon solidification of the liquid. In this system, solar energy stored in

thermal energy storage system (TESS), during sunny days and recovered later at night or during

cloudy days.

Farid et al. [34] constructed a latent heat storage module consisting of 45 cylindrical capsules

fixed vertically in 15 rows. The capsules made of copper tubes 0.335-m long and external diameters



of 31.8 mm were fixed in an insulated rectangular duct. Three commercial waxes having melting

temperatures of 44 0C, 53

0C, and 64

0C were selected. Each of the three sets of 15 tubes was filled

with different wax. For comparison purposes, experiments were also done with a single commercial

wax, having a melting temperature of 53 0C, in all the tubes. During heat charge, hot air flowed

across the capsules such that the melting temperature of the waxes decreased in the flow direction.

Air flow direction was reversed during heat discharge. Experimental measurements showed some

improvement in the heat transfer rates during both heat charge and discharge when three types of

PCMs were used. There was no improvement in the heat transfer rate during the sensible heat storage

period, while a maximum increase of 15% was observed during the latent heat period.

Fatah [35] developed a simple solar air heater integrated with thermal energy storage system.

A set of copper tubes were filled with thermal energy storage material and placed as an absorber.

Different sensible heat and latent heat storage materials were studied. The results indicated that the

heater filled with PCMs with 51 and 43 0C melting temperatures gives the best performance;

otherwise the system daily average efficiency varies between 27% and 63%.

Fath [36] designed and analyzed thermal performance of a simple design solar air heater with

built-in thermal energy storage system. The heater absorber consists of a corrugated set of tubes filled

with a phase change material (paraffin wax) as a thermal energy storage material. In this study, the

absorber projected area was used 1.0 m2, the length being 150 cm and the width 67 cm. The depth of

the heater is taken to be equal to 7.5 cm. The system shows a 63.35% daily average efficiency an air

flow rate of 0.02 kg/s, and the hot air outlet temperature (50C above ambient temperature) extended

for about 16 h, as compared to 38.7% and 9 h, respectively, for the conventional flat plate air heater.

For an air flow rate of 0.01 kg/s, the hot air outlet temperature continues for 21 h of the 24 h/day.

Enibe [37] have been designed and evaluated the performance (on natural convection) of

a passive solar powered air heating system which had potential applications in crop drying and

poultry egg incubation that consist of single-glazed flat plate solar collector (FPSC) integrated with

paraffin type PCM heat storage system. The phase change material (PCM) was prepared in modules

equispaced across the absorber plate. The system was tested under daytime no-load conditions with a

ambient temperature range of 19 0C– 41

0C and a daily global irradiation range of 4.9–19.9 MJ m

-2.

The peak temperature rise of the heated air was about 15 0K, while the maximum airflow rate and

peak cumulative useful efficiency were about 0.058 kg s−1

and 22%, respectively.

Takakura and Nishina [38] tested polyethylene glycol and CaCl2 ·6H2 O as PCMs in

greenhouse heating for 7.2 m2 ground area. They compared conventional greenhouses with PCM

storage type greenhouses. The efficiency of the greenhouse with PCM storage integrated with

solar collector was 59% and able to maintain 8 0C inside the greenhouse at night, when the outside

temperature dropped to −0.6 0C. A microcomputer control system has been developed in order to

establish more accurate and more sophisticated control for solar greenhouse systems.

Benli and Durmus [39] studied the thermal performance of solar air collectors heating system

with phase change material for space heating of a greenhouse. CaCl2·6H2O was used as PCM in

thermal energy storage with a melting temperature of 29 0C. In this system, the hot air delivered by

10 pieced solar air collectors was passed through the PCM to charge the storage unit. The stored heat

was utilized to heat ambient air before being admitted to a greenhouse. Through this study, they

concluded that the solar air collectors and PCM system created 6–9 0C temperature difference

between the inside and outside the greenhouse. The system worked more efficiently in day with high

solar radiation air temperatures. The proposed size of collectors integrated PCM provided about 18–

23% of total daily thermal energy requirements of the greenhouse for 3–4 h, in comparison with the

conventional heating device.

III. CONCLUSIONS

A review of potential and development of solar air heater for natural convection direct solar

dryer, forced circulation indirect type solar dryers, tunnel type solar dryers, solar dryers without heat

storage material, air heating system with sensible and latent heat storage material have been carried



out and concluded that Cabinet type dryer may be useful for domestic applications for drying fruits

and vegetables. For preservation of nutrients of dried products, indirect type forced convection solar

dryers are preferred and therefore recommended for drying mostly perishables. These indirect type

forced convection solar dryers have superior drying rate but their power requirement renders them

inappropriate for rural and small scale operations.

Since solar air heater is the most important component of the indirect solar drying system,

improvement of the solar air heater would led to better performance of the drying system. Therefore;

more studies to investigate and improve the thermal performance of double pass flat, V- corrugated

and finned plate solar air heater is still of considerable interest. The integration of heat storage units

in drying systems accelerates drying during low intensity solar radiation periods and night hours.

The use of sensible heat storage materials is most preferred due to their availability, simplicity in

construction and low cost.

The natural-circulation type solar tunnel dryers, wind ventilated type solar dryers with heat

storage units and greenhouse dryers have the advantage of low cost and simplicity in both on-

the-site construction and operation and therefore recommended for rural applications. For a better

thermal performance of solar air heater a phase change material with high latent heat and with large

surface area for heat transfer is required. The researcher’s designs going to the integration between

solar energy collection and thermal storage to reduce the heat loss, volume and system cost.

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