Presented By:

Saurav Kumar Sahoo

M.Tech (FPEM) 1st Year

Regn. No. : 214014

NIFTEM, Sonepat, India

Guided By:

Dr. Prabhat K Nema

Assoc. Professor

Dept. of Food Engineering,

NIFTEM

Introduction Drying of agricultural produce found to be very

important unit operation to ensure proper

preservation or storage of food product after

harvesting.

Further, Solar Drying Systems is one of the well

known method of drying of food products, by

trapping the heat from sun followed by forced

blowing of heated air through the bed of

agricultural produce.

Solar Drying being a promising system because of

renewable source of heat, it also has certain

limitations, that acts as barriers for it to be used for

Commercial drying of food products, such as,

a) The system can supply very lower amount of

heat for drying purpose.

b) It can only be used in sunshine day hours.

c) The time required for desirable drying of

products is also very high.

A Solar Dryer of better efficiency ?

“Needs to overcome all these Problems”

Incorporation of a

PHASE CHANGE MATERIAL

in the Solar dryer.

(Very recent research under development)

What is a PCM ??? A phase-change material (PCM) is a substance

with the properties like:

a high heat of fusion (Latent Heat).

melting and solidifying at a certain temperature

is capable of storing and releasing large amounts

of heat energy during phase change.

Otherwise known as “Latent Heat Storage (LHS)

units”.

How it Works ??? Broadly, Heat energy are of two types:

1.Sensible Heat (changes Temperature)

2. Latent Heat (No change in Temp.)

PCMs changes its phase at a constant

temperature, by storing a large amount of latent

heat AND again changes back its phase by

releasing the stored heat, which is used for

heating or drying purpose. (Riffat et al., 2013).

Normally Solid-Liquid PCMs are used.

The thermal energy transfer occurs when a

material changes from solid to liquid, or liquid to

solid.

Initially, these solid–liquid PCMs perform like

conventional storage materials; their temperature

rises as they absorb heat. (Bal et al., 2010)

PCMs absorb and release heat at a nearly

constant temperature.

They store 5–14 times more heat per unit volume

than sensible storage materials such as water.

(Hale et al., 1971)

COMPARISON

OF ENERGY

STORAGE

BETWEEN PCM

(LHS UNIT) &

SHS UNIT

(Kaygusuz et al.,

1995)

Generally, PCMs are kept in containment to adopt

the volume changes during phase change.

A separate heat transfer medium must be

employed with heat exchanger in between to

transfer energy from the source to the PCM and

from PCM to the load.

The heat exchanger to be used has to be

designed specially, in view of the low thermal

diffusivity of PCMs in general.

Classification of PCM: (Riffat et

al.,2013)

Organic PCMs are generally straight chain

hydrocarbons (Alkane/Paraffin) or branched fatty

acids(Non-Paraffin) (CH3 (CH2)2nCOOH).

Generally, fatty acids have higher melting point

because of double bonds. It also have a higher

cost of about 2 to 2.5 times that of Paraffin.

Salt Hydrates (inorganic) have high volumetric

latent heat storage capacity and higher thermal

conductivity because of presence of water

molecules. But higher change in volume stands a

major problem in it. (Liquid-Vapour)

“Eutectics” are mixture of the both in required

proportion. Its use is relatively new and limited.

Selection Criteria of a PCM (Bal et al., 2010)

Melting temperature in the desired operating

temperature range.

High latent heat of fusion per unit volume

Small volume changes on phase

transformation and small vapour pressure at

operating temperatures to reduce the

containment problem.

1. Thermodynamic Properties:

Continued

…

Congruent melting for a constant storage

capacity of the material with each

freezing/melting cycle.

High thermal conductivity of both solid and

liquid phases to assist the charging and

discharging of energy of the storage systems.

Long-term chemical stability.

Complete reversible freeze/melt cycle.

Non-toxic, non-flammable and non-explosive materials for safety.

No degradation after a large number of freeze/melt cycle.

Compatibility (non-corrosiveness) with materials of construction.

2. Chemical Properties:

Large-scale availability of the

Phase Change Material.

Cost effective Material or Low

Cost PCMs.

3. Economic Criteria:

The dryer is most suitable for agricultural

products that are sensitive to direct exposure to

solar radiation such as chillies, fenugreek

leaves, onion, grapes, sweet potatoes and mint.

(Sharma et al., 1994)

The use of PCM in Drying purpose or in relation

to Agriculture is a concern of very recent

research and is still under progress.

Till date, some scientists like Butler, Garg and

Bal have already accomplished Solar Drying of

food using PCMs like Rock Bed, Wax etc., facing

major problems like inefficient Thermal

Conductivity.

Solar Dryer with PCM (Thermal Storage)

Vs

Solar Dryer without PCM (Thermal Storage)

The advantages of using PCM in Solar Dryer over

not using PCM are nothing but the Solutions to

the problems faced by Solar Drying System ,

discussed in the beginning.

Without the use of any Thermal Storage Unit (or PCM):

1. Lower amount of heat is available for drying purpose.

2. Drying can be done only in day hours, not in the absence

of Sunlight.

3. Drying Time is comparatively very high.

Continued

… As a solution to the said problems:

Since the Phase change material used in the

cabinet of the solar dryer stored a large amount

heat (equivalent to the latent heat of fusion)

before drying air is blown, large amount of heat

energy would be available in the air to pass

through the food products.

Hence, PCM can be used to solve our first

problem of using Conventional Solar Dryer for

Agricultural food products

1.

Continued

…

The Phase change materials starts to condense

from liquid to solid phase after the sunset, by

releasing the stored heat energy to the cabinet,

which can be used further for drying purpose

after the Sunset, effectively.

Since larger amount of heat is available for drying

of food products, ultimately drying time would

be lower.

2.

3.

Use of PCM in Solar Cooker to

facilitate Post-Sunset Cooking (Domanski et al., 1995)

Target topics for Next Seminar:

Practical orientation and assembly of

PCM used in a Solar dryer for drying of

Agricultural products.

Discussion about some particular

research undertaken on the topic.

REFERENCES

Bal L.M., Satya S. and Naik S.N., 2010, Solar dryer with

thermal energy storage systems for drying agricultural food

products: A review, Renewable and Sustainable Energy

Reviews 14,2298–2314.

Sharma VK, Colangelo A, and Spagna G, 1994; Experimental

investigation of different solar dryers suitable for fruit and

vegetable drying., Drying 94:879–86.

Sharma A, Tyagi VV, Chen CR, and Buddhi D. 2009; Review

on thermal energy storage with phase change materials and

applications. Renew Sustain Energy Rev 13(2):318–45.

Hale DV, Hoover MJ, and O’Neill MJ.,1971; Phase change

materials hand book, Alabaa Marshal Space Flight Center

Kaygusuz K, 1995; Experimental and theoretical investigation

of latent heat storage for water based solar heating systems.

Energy Conversion Management 36(5):315–23.

Butler JL and Troeger JM., 1980; Drying peanuts using solar

energy stored in a rockbed; Agricultural energy vol. I, solar

energy, ASAE Publication, St Joseph, Michigan.

Riffat S., Mempouoa B and Fangb W.,2013; Phase change

material developments: A Review, International Journal of

Ambient Energy.

Garg HP, Sharma VK, Mahajan RB and Bhargave AK.,1985;

Experimental study of an inexpensive solar collector cum

storage system for agricultural uses. Solar Energy;35(4):321–

31.