Solar Drying Process: A Review

Ashish Kumar1, Radhesh Garg and Pramit KumarMishra

Department of Mechanical Engineering, Rajshree Institute ofManagement & Technology, Bareilly

1ashish.05408@gmail.com

Abstract It is important to preserve cultivated crop for future

usage. Various natural forces hamper the product quality, if

dried in hostile open sun environment. Two fundamental problems

associated with drying process are: moisture content of the crop

which multiplies the chances of bacterial and fungal attacks and

the other one is drying rate which depends on the humidity of

ambience. In order to understand and control these parameters

authors have critically examined the secondary data. This

communication is an attempt to put solar assisted drying process

foremost in the developing countries.

Keywords: Drying rate, Moisture, Solar Crop Drying

Introduction Preservation of fruits, vegetables, and food are

essential for keeping them for a long time without further

deterioration in the quality of the product. Several process

technologies have been employed on an industrial scale to

preserve food products; the major ones are canning, freezing, and

dehydration. Among these, drying is especially suited for

developing countries with poorly established low-temperature and

thermal processing facilities [1]. It offers a highly effective

and practical means of preservation to reduce postharvest losses

and offset the shortages in supply. Drying is a simple process of

moisture removal from a product in order to reach the desired

moisture content and is an energy intensive operation [1]. Drying

involves the application of heat to vaporize moisture and some

means of removing water vapor after its separation from the food

products. It is thus a combined and simultaneous heat and mass

transfer operation for which energy must be supplied. [1] The

removal of moisture prevents the growth and reproduction of

microorganisms like bacteria, yeasts and molds causing decay and

minimizes many of the moisture-mediated deteriorative reactions.

It brings about substantial reduction in weight and volume,

minimizing packing, storage, and transportation costs and enables

storability of the product under ambient temperatures.

Solar radiation in the form of solar thermal energy is an

alternative source of energy for drying especially to dry fruits,

vegetables, agricultural grains and other kinds of material, such

as wood. This procedure is especially applicable in the so-called

“sunny belt” world-wide, i.e. in the regions where the intensity

of solar radiation is high and sunshine duration is long.

Classification of solar dryer

Drying equipment may be classified in several ways. The two most

useful classifications are based on:

(1)The method of transferring heat to the wet solids

(2) The handling characteristics and physical properties of the

wet material

A classification chart of drying equipment on the basis of heat

transfer is shown in Figure [2-6]

Three distinct solar dryer are may be classified as

1. Indirect type solar dryer

2. Direct type solar dryer

3. Hybrid solar dryer

Generally solar dryer are classified on the basis of quantity,

quality, types of material as well as source of energy . [7]

1) Open sun drying-

The crops are generally spread on the ground, mat, cement

floor where they receive short wave length of solar energy

and also natural air circulation. Energy receive by the

crops but radiation power depends only colors of crops so

huge amount of solar radiation wastage in this method. [2-6]

2) Direct solar drying-

The working principle of direct solar dryer is shown in

figure and also known as solar cabinet dryer .here the

moisture is taken away by the air entering in the cabinet

flow below and escaping through at the top as shown in

figure . In this process some energy absorbed and some

energy reflected into the atmosphere but wavelength of exit

energy is very low and less wastage of energy as compared to

open sun drying method. [2-6].

The following advantage of direct solar dryer over open sun

dryer method:

a) Simpler and cheaper to constructed than the indirect type for

the same loading capacity

b) Offer protection from rains, dews etc.

3) Indirect solar drying

These differ from the direct dryer with respect to the heat

transfer and vapor removal .figure describes the working

principle of indirect solar drying [2-6]. The crops in these

indirect soar dryer are located in trays or shelves inside

an opaque drying cabinet and separated rate unit termed as

solar collector is used for heating of entering air into

cabinet. The heated air is allowed to flow through over the

wet crop that provides the heat for moisture evaporation by

convective heat transfer between hot air and wet crop that

provides the heat for moisture evaporation by convective

heat transfer between the hot air and wet crop .the

advantage of indirect solar dryer are:

a) Offer a better control over drying and the product obtained

is of better quality than sun drying.

b) Can be operated at higher temperature, recommended for deep

layer drawing.

c) Highly recommended for photo –sensitive crops.

d) Have inherent tendency towards greater efficiency than

direct solar drying.

4) Hybrid solar drying

The hybrid solar dryer combine the features of direct and

indirect types solar energy dryers. Here the combined action of

incident direct solar radiation on the products to be dried and

air pre heated in a solar collector heater produces the necessary

heat required for drying process.

On the basis of mode of drying e.g. direct or indirect, solar

dryer may be classified as:

a) Passive dryer

b) Active dryer

a) Passive solar dryisng system-

In a passive solar dryer, air is heated and circulated naturally

by buoyancy force or as a result of wind pressure or combination

both. Passive drying of crops is still in common practice in many

tropical, sub-tropical regions especially in Africa and Asia or

in small agriculture communities. The passive solar dryer are

best suited for the small batches fruits and vegetable such as

banana, pineapple, mango etc.

Indirect –type passive solar energy dryers-

These are indirect –type dryers with natural convection air for

drying .in order to increase the capacity of a dryer i.e.

operates with one more than one layer of trays with crops with in

the available areas, the trays are generally placed in vertically

racks with some space in between consecutive trays. The

additional resistance generated for the air movement due to this

arrangement of the trays is achieved by the “chimney effect”.

Direct –type passive solar energy dryers –

The features of a typical direct –type passive solar dryer

are illustrated in figure. In these type of solar dryers the

exposure of the crop to the sun light. Two basic types of dryers

in this category can be identified cabinet and greenhouse dryers.

a) Solar cabinet dryers-The passive solar cabinet dryer are

generally simple and inexpensive unit having large

application for domestic purpose. These dryer are

constructed normally area 1-2 m2 and capacities of 10-20 kg.

Figure shows a typical solar cabinet dryer. The heat

transmitted through the glass cover and absorbed on the

blackened inner and crops as well. The required air

circulation is maintained by the worm moist air leaving via

upper area under the action of buoyancy force and generating

suction of fresh air base inlet. Pioneering works on solar

cabinet dryer were reported by the Brace research institute

[8].

Figure: Features of a typical

Figure :. A typical natural circulation

Integral –type natural -circulation

solar-energy cabinet dryer

Solar-energy dryer

Fig-a modified natural convection fig-

natural circulation solar energy cabinet with

Solar cabinet energy drier

chimney

A number of other design of passive solar cabinet dryer in

configuration to that developed by Brace Research Institute have

been built and tested for a variety of crops and location (fzekwe

1981)reported a modification of the typical design shown in above

figure, equipped with a wooden plenum guiding the air inlet and a

long plywood chimney to enhance natural circulation ,

acceleration the drying rate by about 5 times over open sun

drying shows a design by (heniksson and gustaffson 1986)with mesh

work and a chimney with black PVC foil facing the southwards

(sunlight).

a) Green house dryer-

These are also called as tent dryer. They are

designed with vent of appropriated size and position to have

a controlled air flow .they are cover poly ethylene sheet on

glazing surface. Earliest form of passive greenhouse solar

dryer as shown below (Brace research institute ). The length

–wise north- south alignment of the dryer had black coated

internal for improved absorption of solar radiation with the

ridge –cap over the roof for exit vent.

Fig- natural circulation glass – roof

fig-natural circulation polythene tent

Solar energy dryer

Dryer

Fig- natural circulation solar dome dryer

fig- a greenhouse type solar energy Dryer

[9-13] reported a horticulture greenhouse of clear plastic sheet

cladding over a semi- cylindrical metal frame as shown above.

A typical greenhouse type solar dryer with a transparent semi –

cylindrical chamber with a cylindrical solar chimney posted

vertically at one [13] . various experiment studies and modified

design of hemi-cylindrical solar tunnel dryer of grapes also few

researcher [14] have used polycarbonate cover for its

construction.

Hybrid type passive solar energy dryers –

A hybrid type passive solar dryer would have the

same typical feature as indirect and direct type and in addition

has glazed walls inside the drying chamber so that the solar

radiation impinges directly on the product as in the direct type

dryer as shown below. [9-13] Asian institute of technology

developed the widely reported solar rice dryer. Hybrid dryer

consisting air heater with a pile of granite functioning as an

absorber cum heat storage was developed [15]

Active solar drying systems-

Active solar dryer system is designed incorporation external

means like fans or pump, for moving the solar energy in the form

of air from the collector area to drying beds. These dryer have

major application in commercial purpose and also used fossil fuel

for better controlling. Active solar dryer largely use in highly

moisture content products likely kiwi fruits, papaya cauliflower

slices. These are classified as discuss below-

a) Indirect type active solar drying system-

These active dryer are discussed for indirect dryer section

have a separate collector and drying unit. they are four

basic component like a solar air heater , drying chamber a

fan for air circulation and ducting .due to the separated

air heating unit higher temperature can easily obtained with

a control on air flow rate . However as the efficiency of

collector decrease at higher temperature operation, an

optimum temperature and air flow rate has to be determining

to have a cost effective design. While most solar collector

are made up of metal or wood absorber with appropriated

coating , material like black polythene are also used as

they form an economic substitute . Shows a typical indirect

type active solar dryer.[16]

Fig-features of typical distributed type

fig-distributed active solar

Active solar energy dryer

dehydration with partial Air circulation

Dried agriculture product report-

Ser

ial

no.

Product name Moisture content Time Technology

1. Apples[17] Reduce moisture 82% 28h Solar tunnel

dryer2. Bananas[18] Dried 5-7 days Multipurpose

solar tunnel

dryer3. Cassava [19]

(ambient condition-

49.1kg,65.9kg,

162kg in 30-

Mixed mode

natural

77.8%relative

humidity and

temperature 25oc)

[19]

60h convection

solar dryer

4. Cauliflower[20] Forced

convection

colar dryer

using v-

grooves

solar

collector5. Copra[21] Reduced moisture

approx. 52%

82h Forced

convection

solar

drier ,tray

arrangement

bottom and

top

respectively6. Grape[22] Reduced moisture 43% Multipurpose

natural

convection

solar dryer7. Green peas(quantity-

1kg , 45.5-50.5oc)

[23]

Left moisture

content 5%

8-10h Indirect

type natural

convection

solar dryer8. Mangoes[24] Initial moisture 85%

and final moisture

content 13%

20h Indirect

type natural

convection

solar dryer9. Mangoes[25] Initial moisture 15h Natural

84%and left 27.6% convection

solar dryer

10. Onion slices[26] 7%moisture left Hybrid solar

drier11. Pineapple[27] Average value-15%,

11%,13%

(solar,biomass,both)

Hybrid solar

drier

Conclusion

Authors have critically examined the secondary data and come to a

point that it is important to preserve cultivated crop for future

usage. Various natural forces hamper the product quality, if

dried in hostile open sun environment. Authors also agreed on the

point that the use of mixed mode especially solar tunnel dryers

are efficient than other techniques. Authors would like to

suggest that some latent or sensible heat storage methods can

also be modeled and worked out.

References-

1. Mujumdar, A.S., 2007, Handbook of Industrial drying, Taylor

and Francis group, U.K.

Nandwani, S. S., 2007, Design construction and study of a

hybrid solar food processor in the climate of Costa Rica,

Renewable Energy, 32, pp. 427–441.

2. Sharma, V. K., Sharma, S., Ray, R. A., Garg, H. P., 1986,

Design and performance studies of a solar dryer suitable for

rural application, Energy Conversion and Management, 26, pp.

111-119.

3. Sharma, S., Ray, R. A., Sharma, K., 1987, Comparative study

of solar dryers for crop drying. Invention Intelligence, 22, pp.

105-113.

4. Sharma, S., Ray, R. A., Sharma, V. K., 1987, Comparative

study of solar dryers for crop drying, Invent Intell, pp.

105-113.

5. Sharma, G. P.; & Prasad, S. Effective moisture diffusivity

of garlic cloves undergoing microwave convective drying.

Journal of Food Engineering 2004, 65, 609–617.

6. Sharma, A., Chen, C.R., Lan, N. V., 2009, Solar-energy

drying systems: A review, Renewable and Sustainable Energy Reviews,

13, pp. 1185-1210.

7. Leon, M. A., Kumar, S., Bhattacharya, S.C., 2002, A

comprehensive procedure for performance evaluation of solar

food dryers, Renewable and Sustainable Energy Reviews, 6, pp. 367-

393.

8. Bolin, H.R., Salunkhe, D.K., 1982, Food dehydration by solar

energy, Crit. Rev. Food Sci. Nutri, 16, pp. 327-354. Brace

Research Institute, 1980, Types of solar agricultural

dryers, Sunworld, 4(6), pp. 181-999.

9. Leon, M. A., Kumar, S., Bhattacharya, S.C., 2002, A

comprehensive procedure for performance evaluation of solar

food dryers, Renewable and Sustainable Energy Reviews, 6, pp. 367-

393.

10. Lutz, K., Mulhbaner, W., 1986, Solar tunnel dryer with

integrated collector, DryingTechnology, 4, 583-603.

11. Mathew, S., Visavale, G., Thorat, B.N., 2010, Making

order in the Cabinet- Integrating

CFD in the Green energy design process for food industry

helps identify and fix causes

for uneven drying in a Solar Cabinet Dryer, 2010-Ansys India

Users Conference Bangalore.

12. Mekhilefa, S., Saidurb, R., Safari, A., 2011, A review

on solar energy use in industries

Renewable and Sustainable Energy Reviews, 15, pp. 1777–1790.

13. Miller, W.M., 1985, Solar drying coupled with solid

desiccant energy storage, Trans.ASAE, 1985, 28, pp. 649-656.

14. 649Reza, M. S., Bapary, M. A., Islam, M. N., Kamal, M.,

2009, Optimization of marine fishdrying using solar tunnel

dryer, Journal of Food Processing and Preservation, 33, pp.

47 - 59.

15. Ayensu, A., Asiedu-Bondzie, V., 1986, Solar drying with

convective self-flow and energy storage, Solar and Wind

Technology, 3(4), pp. 273-279.

16. Sreekumar, A. 2010, Techno-economic analysis of a roof-

integrated solar air heating system for drying fruit and

vegetables, Energy Conversion and Management, 51, pp. 2230-2238.

17. elicin, A.K., sacilik, K., 2005, an experiment study

for solar tunnel drying of apple tarimbilmleri, 11(2),

pp.207-211.

18. Sachimer , P, Janjai ,S. , Esper, A,Smitabhindu,R,

Muhlbaver ,1996,, experiment investigation of performance of

the solar tunnel dryer for drying biomass ,renewable energy

and sustainable energy review ,13,pp.1185-1210.

19. Forson ,F.K.Nazha, M.A., Akuffo ,F.O.,

Rajakaruna ,H.,2007, design of mixed made natural convection

solar crop dryer ; application of principle and rules of

thumb renewable energy ,32,pp. 2306-2319.

20. Kadam ,D. ,Samuel ,D,2006,convective flat plate solar

heat collector for cauliflower drying .biosystems

engineering ,93(2), pp.189-198

21. Mohanraj, M.,Chandrasekhar,P.,2008,drying of copra in

force convection solar drier , biosystemenginerring ,99,pp:

604-607

22. Pangavhane ,D.R. Sawhney , R.L. Sarvadia .P.N. ,2002,

design ,development and performance, testing , of a new

natural convection solar dryer energy .27,pp. 579-590.

23. El-Sebaii, AA.,Aboul-Enein,S.,Ramadan ,M.,El-

goharaj,H.G., 2002, experimental linvestigtion of an

indirect type natural convection solar dryer energy

conversion and management ,43,pp. 2251-2266.

24. Madhlopa ,A.,Jones ,S.A.,Saka ,J.,2002,a solar air

heater with composite –absorber system for food dehyderation

. renewable energy ,27,pp 27-37

25. Tour , S. , Kibangu ,N.Kembos.,2004,comparative study

of natural solar drying of cassava ,banana,and

mango ,renewable energy,29,pp. 975-990.

26. Sarsavadia ,P.N.,2007, development assisted dryer and

evaluation of energy requirement for drying of onion

renewable energy ,32 ,pp 2529-2547.

27. Madhlopa ,A. Ngwala , G.,2007 , solar dryer with

thermal storage and biomass back up heater solar energy ,81,

pp: 449-462.