sun and solar drying

7/28/2019 Sun and Solar Drying

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SUN AND SOLAR DRYINGH83IND INDUSTRIAL DEHYDRATION

Dr Hii Ching Lik

Department of Chemical &

Environmental Engineering

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Outline

Sun drying

Advantages & disadvantages of sun drying

Solar drying Solar dryers classification

Prospect of solar drying

Solar collectors Design calculation

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Sun Drying

This is the most fundamental method of drying forpreservation purpose

It is also know as natural drying

Only uses direct sunlight and wind Mostly use by smallholders/farmers, managed by

single family mostly

Some estates/plantations still use this method,

uses large land areas

Suitable for areas that receive high rate of solarinsolation and with longer daytime

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Examples of Natural dryer

Elevated platform

By hanging

Spreading on floor

surface

Inside a crib

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Examples of Sun Dried products

Some of these products are preferred by consumer to be

sun dried, due to better flavour/taste (proven) and even

appearance. 5


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Advantages & Disadvantages

Advantages Simple and cheap

Easy to set up

Easy to maintain

Uses renewable energy

Very low initial cost Suitable for small quantity

Little expertise is required

Can be applied anywhere

Disadvantages Labour intensive

Frequent mixing is needed

Large land area is required

Not efficient

Pests and insects attack Rains/dews

Contaminations i.e. smoke,dust, chemicals

Unpredictable weather

Extended drying duration Under dried product

Spoilage i.e. mouldHowever, this method is still widely

used by farmers worldwide due to

cost/price issue and level of

technology transfer. 6


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Solar Drying

In view of the various disadvantages of sun

drying, solar drying is used

Also, due to the current trend of high

fuel/energy cost, solar drying is favoured

Definition of solar drying

Drying in which the temperature of the drier air

has in some way been increased by the deliberatecapture of solar radiation (Fuller, 1993, Solar

drying of horticultural practices)

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Solar Insolation

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AverageInsolatio

n(kWhm-2d

ay-1)

Total Horizontal Solar Insolation and Sunshine

Hours for Some Developing Countries (by courtesy of Dr S. Jangam) 8


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Intensity and temperature profiles (date:20/07/2007)

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1000

8.30

9.00

9.30

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10.30

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11.30

12.00

12.30

1.00

1.30

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2.30

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5.00

5.30

Time of day

Intens

ity(W/m

2)

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60

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Tempe

rature(oC)

Intensity

Collector outlet temperatureDryer outlet temperatureAmbient air temperatureCollector inlet temperature

A Typical solar intensity plot with the temperature

variation in the dryer (by courtesy of Dr S. Jangam) 9


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Classification

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Direct type

Drying occurs primarily through the direct

absorption of solar radiation by the product

Indirect type

Drying is achieved primarily by forced or natural

convection, drying chamber is opaque Solar energy is absorbed by a separate collector

Mixed mode (hybrid)

Are those which combine the characteristics of

the above (direct/indirect)

It can also be used with external heat source(hybrid) i.e. with biomass heater, heatpump

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Passive Solar Dryers Direct type

Thin layer of product is placed inside the chamber

Inclined transparent cover

Air flow from bottom (not heated) and exit through

the top (back wall)

Wooden walls and insulated well

Direct solar cabinet

(Brace Research Institute)

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Indirect type

Drying chamber is opaque, sometimes is painted with

black paint to absorb heat

Shelves are usually used to hold the product

In some design chimney is excluded

A solar collector is used to generate warm air

Shelf type dryer

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Mixed mode

Transparent drying chamber with solar collector

The floor of the solar collector can be filled withrocks painted in black as thermal storage

Heat is released from the rocks to prolong drying

at night time or during bad weather

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Active Solar Dryers Direct type

Consists of 2 sections, namely the solar collector

and drying chamber (transparent)

Forced circulation with fan

Solar tunnel dryer

Solar collector

Drying

chamber

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Indirect type

Separate solar collector

Connected to a blower to distribute the warm air

Hot air is supplied to the drying chamber (not

transparent)

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Mixed mode

Greenhouse effect solar dryer, using transparent

UV stabilized plastic film

Auxiliary heater is used to heat up water as

thermal storage

Blower is used to circulate air flow

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Advantages (vs sun drying)

Faster drying rates due to elevated temperature

Product protection from rain/dews

Prevent spoilage due to pests and insects attacks

Prevent spoilage due to extended drying

Better product quality

Higher production rate

Smaller land area for installation Better temperature control

Can be used at all weather (mixed/hybrid type)

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Disadvantages

Initial cost is high to set up

Requires frequent maintenance

High maintenance cost

Availability of spare parts

Losses in nutritional quality due to overheating i.e. vitamins

Higher level of expertise is required Complicated design in some cases i.e.

incorporated with heat pump system

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Prospect of Solar Drying

Some factors that work against this technology: Lack of incentive (from government/buyer) to

improve quality

Taste preference

Price differential due to quality grades is small Poor weather conditions

Large areas of solar collectors are needed

Cost is high to install auxiliary heater/burner

Despite these difficulties, some successfulexamples can still be cited

Education is important, to teach farmers thebenefits of solar drying

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Guidelines in Design

Metrological: solar insolation, weather pattern, rainfall etc

Design: active/passive, tunnel/cabinet/tent, mixed/hybrid etc

Products: moisture content, equilibrium moisture content, maxallowable temperature

Dryer capacity: thick bed, thin layer, shelves Solar collector: air/liquid medium, single/double pass, insulation,

airflow, ducting etc

Drying chamber: single bed/shelves, loading/unloading,direct/indirect, ventilation etc

Construction: materials, insulation, spare parts

Airflow: natural/forced, fan/ventilator/chimney etc

Auxiliary component: heater, thermal storage, control etc

Cost: set up cost, maintenance/repair, operating, labour21


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Ventilator

Shelves

Tent dryer

Thermal

storage

Solar heat pump

system


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Solar Collectors

Many configurations can be used, and the

technology is still evolving

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Collector Efficiency ()

This is defined as the ratio of the useful energy (Qu) to theavailable solar energy incident upon the collector (Qo)

The quantity Qu is determined from the change intemperature between the inlet and outlet

= Qu/Qo = M*Cp*(Tout Tin)/Qo

M = mass flow rate of fluid (kg/s)

Cp = specific heat of fluid (J/kg.C)

Tout/in = temperature at outlet/inlet of collector

Collector efficiencies can range from 10-60%, dependingon many design factors

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Design Calculation

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Assumptions:

At all points inside the solar dryer the air temperature and the

air density is uniform

There is no leakage of air and warm air only exits at the top

Drying of grain is purely by natural convection

The significant resistant to airflow is only due to the grain bed

Data:

V = 0.008(P/h)0.87

P = gH

= 1.11363 0.00308T(C)h = 0.2 m

h1 = 1 m

h2 = 0.6 m

V = airflow (m/s)

P = pressure drop (Pa)

= density of air (kg/m3

)h = grain bed thickness

H = total height (m)

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sun and solar drying

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