Objectives Dryer

Transient Analysis of a Solar Dryer 2012

Addis Ababa University

Faculty of Technology

Energy Center

Solar Thermal Systems Engineering [ETEC – 6408]

Assignment I

Transient Analysis of a Solar Dryer

Submitted to: Dr. Ing. Abebayehu Assefa

Submitted by: Hulunayehu Dejene[GSR/0569/04]

Million Merid[GSR/0570/04]

Mohammed Shikur[GSR/0571/04]

Tsion Berhanu[GSR/0579/04]

June 3rd, 2012

O BJECTIVES OF THE PROJECT WORK 3

1


METHODOLOGY 3

1. INTRODUCTION 4

2. HOW SOLAR DRYER WORKS 4

3. THE PRINCIPAL TYPES OF SOLAR DRYERS 5

A. SOLAR CABINET DRYER 5B. SOLAR GREEN HOUSE DRYERS 6C. INDIRECT SOLAR DRYER 6

5. OPERATION AND MAINTENANCE 7

6. FUNDAMENTALS OF DRYING 8

7. USES OF SOLAR DRYER 9

7.1 ADVANTAGES 97.2 LIMITATIONS 9

8. THEORY ON TRANSIENT ANALYSIS OF THE SOLAR COLLECTOR APPLIED FOR THE SIMULATION 10

8.1. ENERGY BALANCE ON EACH COMPONENT OF THE SOLAR DRIER 12A. ABSORBER PLATE 13B. AIR STREAM 16C. GLASS COVER 16

9. SOLUTION METHOD 17

10. WORKSHOP AND ASSEMBLY DRAWING OF THE SOLAR DRYER 18

REFERENCE 18

2


Objectives of the Project Work

The main objective of this project is to investigate the thermal analysis of a solar drier system

by doing the following.

1. Using Transient Analysis of the solar collector for a dryer, evaluate the hourly

variations of temperatures Tao1 (temperature of the air at the outlet), Tg1 (the glazing

temperature), TP1 (the plate temperature) and m the mass flow rate of air for a year

and simulate the results with MATLAB. Finally represent the results graphically for

the days January 17, April 15, July 17 and October 15.

2. Design a solar dryer using AUTOCAD with a work shop and assembly drawing of

the solar dryer considering optimum conditions.

Methodology

Reading different literatures to understand the basic types of solar dryer, performance,

operation and working principles.

Limitation

3


1. Introduction

Drying has been used for centuries as a method for food preservation. Fluid extraction from a

material is known as drying; by this the water is removed from the solids to a certain level

with different techniques. Hence drying is moisture migration from solids in a specific period

of time. The drying system is the factors that influence the drying such as the moisture

content of the material that is intended to be dried.

Drying is a mass transfer process resulting in the removal of water or moisture from another

solvent, by evaporation from a solid, semi-solid or liquid (hereafter product) to end in a solid

state. To achieve this, there must be a source of heat, and a sink for the vapour produced.

Solar energy can be used as an important and environmental compatible source of renewable

energy. The use of solar energy for drying effectively reduces the problems arising from

generating energy by convention method. This is because the use of the conventional energy

source for drying purposes is costly and hazardous to environment.

2. How Solar Dryer works

Air is drawn through the dryer by natural convection. It is heated as it passes through the

collector and then partially cooled as it picks up moisture from the produce. The produce is

heated both by the air and directly by the sun.

This solar thermal energy dryer generates two-way thermal energy from the direct thermal

energy of solar radiation through its glass roof and thermal energy from its solar radiation

collector.

The solar energy will pass through the glass roof to the collector, generating hot air. The

ventilation system installed in the system will force the air flow passing through the drying

chamber. This drying system can be operated by solar energy and/or electric power to allow

drying during non uniform sunlight or high humidity.

In open air Solar drying the heat is supplied by direct absorption of solar radiation by

material being dried. The vapour produced is carried away by air moving past the

material, the air motion being due either to natural convection resulting from contact

with the heated material or to winds.

4


Solar dryer make use of solar radiation, ambient temperature, relative humidity. Heated air

is passed naturally or mechanically circulated to remove moisture from material placed inside

the enclosure.

The collector comprises the following components

A Transparent glass cove to prevent thermal energy to release to the environment

when the wind flows through the collector.

A space between the glass roof and the collector act as a film for prevent the loss of

thermal energy

An absorber made of a matte black painted corrugated metal sheet

An Insulation to prevent the loss of thermal energy at the bottom and on the sides of

the collector

Many variation of solar dryers offer the processor a wide range of opportunity to select

various type of solar dryers depending on degree of control over the drying process and

handling of the material.

3. The principal types of solar dryers

A. Solar Cabinet dryer

Solar Cabinet dryer mainly consists of a drying cabinet. One side of the cabinet is glazed to

admit solar radiation, which is converted in to low grade thermal heat thus raising the

temperature of the air, the drying chamber, and the produce. Usually the sun light shines

directly on the material being dried. The moisture evaporated by solar heat is removed by air

circulation. This is accomplished either by designing to encourage natural convective air flow

or by forcing circulation with fans or blowers. The material to be dried is placed in shallow

layers on trays inside the drying cabinet. Proper air vents are provided for displacement of

hot air.

Fig.1Solar cabinet dryer

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B. Solar green house dryers

Solar green house dryers are characterized by having extensive glazing on their south

facing side while the other sides are well insulated. Inside the enclosed area, some means are

provided to store the daytime excess heat. Vents are strategically sized and positioned to

control air flow. A well designed greenhouse dryer permits a greater degree of control over

the drying process than the solar cabinet dryers and should be used where relatively large

quantity of product is to be dried.

Fig.2 Solar green dryer

C. Indirect Solar Dryer

In this type of dryer, the produce is placed on trays inside an opaque drying chamber to which

is attached an air type solar collector. The sun does not shine directly on the material to

be dried, instead the air heated in solar collector is ducted to the drying chamber for

dehydration. Air circulation can be by natural convection; however it is often forced by

blowers. These dryers result in higher temperature than the cabinet dryers or sun

drying, and can produce higher quality product.

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Fig.3 Indirect solar dryer

In many large scale commercial drying operations, air type solar collectors are being

combined with fuel fired dehydrators in order to reduce the fuel consumption and yet

maintained fine control of the drying conditions. The solar heated air is used directly if it is

hot enough, otherwise, the fossil fuel system boost the air temperature to the required

level Thus the effect of fluctuations in energy output from the collector is less serious since

the fuel fired system is automatically controlled to provide specific optimum temperature.

4. Performance

Performance of a solar dryer varies for different types of dryer operated at different

temperatures. Basic requirement for installation of a solar dryer is the open space free from

any obstacle to the solar radiation. For indoor drying/dehydration processes by solar heated

air, air heating collectors are to be installed in open space on ground, on roof or on a

terrace and ducted to the drying chamber.

5. Operation and maintenance

Operation of solar dryers is very simple and it is considered to be maintenance free.

However for efficient performance cleaning of the glazing, regular coating of absorbing

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surface and instruments and controls is essential.

6. Fundamentals of Drying

The Drying Curve

For each and every product, there is a representative curve that describes the drying

characteristics for that product at specific temperature, velocity and pressure conditions. This

curve is referred to as the drying curve for a specific product. Fig…. shows a typical drying

curve. Variations in the curve will occur principally in rate relative to carrier velocity and

temperature.

Fig.4 Drying curve

Drying occurs in three different periods, or phases, which can be clearly defined.

The first phase, or initial period, is where sensible heat is transferred to the product and the

contained moisture. This is the heating up of the product from the inlet condition to the

process condition, which enables the subsequent processes to take place. The rate of

evaporation increases dramatically during this period with mostly free moisture being

removed. In some instances, pre-processing can reduce or eliminate this phase.

8


The second phase, or constant rate period, is when the free moisture persists on the

surfaces and the rate of evaporation alters very little as the moisture content reduces. During

this period, drying rates are high and higher inlet air temperatures than in subsequent drying

stages can be used without detrimental effect to the product. There is a gradual and relatively

small increase in the product temperature during this period. Interestingly, a common

occurrence is that the time scale of the constant rate period may determine and affect the rate

of drying in the next phase.

The third phase, or falling rate period, is the phase during which migration of moisture

from the inner interstices of each particle to the outer surface becomes the limiting factor that

reduces the drying rate.

7. Uses of solar dryer

Solar dryers can be utilized for various domestic purposes. They also find numerous

applications in industries such as textiles, wood, fruit and food processing, paper,

pharmaceutical, and agro-industries.

7.1 Advantages

Solar dryers are more economical compared to dryers that run on conventional

fuel/electricity.

The drying process is completed in the most hygienic and eco-friendly way.

Solar drying systems have low operation and maintenance costs.

Solar dryers last longer. A typical dryer can last 15-20 years with minimum

maintenance.

7.2 Limitations

Drying can be performed only during sunny days, unless the system is integrated with

a conventional energy-based system.

Due to limitations in solar energy collection, the solar drying process is slow in

comparison with dryers that use conventional fuels.

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Normally, solar dryers can be utilized only for drying at 40-50oC.

8. Theory on Transient Analysis of the Solar Collector applied for the

simulation

For the investigation of the transient thermal-analysis of a solar dryer using the available

hourly-averaged solar radiation data - global and diffuse radiation intensities and ambient

temperature - for a year (8760 hrs), the theoretical analysis given in the following sections

was used.

The solar radiation energy incident on the collector surface which is inclined at an angle β to

the horizontal, defined in terms of the global radiation Gr, the diffuse radiation Dr, the beam

radiation factor Rb and the ground reflectivity factor ρ (=0.2), is given by:

I N=Rb (Gr−Dr )+0.5 × (1+cosβ ) Dr+0.5 (1−cosβ ) Gr [1]

Where Gr=Global radiation

Dr=Diffuse radiation

β=Collector slope

The flux collected per unit time is given by:

I c=I N × ( τg α p ) [2]

Where τ g=¿Transmissivity of glass

α p= Absorptivity of panel

For calculating the above mentioned parameters, the following assumptions were made.

Location = Addis Ababa

Latitude (∅) =9.02°

Collector facing south (γ =0° ¿

The following dimensions and parameters were used for the system simulation. Results of the

thermal analysis are to be shown graphically for representative days of each month.

The following dimensions and parameters were used for the system simulation.

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Name Symbol Value

Collector area [m2]

[L=2, W=1m]Ac 2.0

Collector perimeter [ m ] P 6

Collector slope (south facing) β 15º

Back insulation thickness [ m ] tbe 0.03

Edge insulation thickness [ m ] te 0.03

Depth of edge [ m ] De 0.05

Emissivity of panel [ - ] εp 0 .10

Emissivity of glass [ - ] εg 0 .88

Conductivity of insulation [ W/mK ] Ki 0.045

Absorptivity of panel [ - ] αp 0.90

Transmisivity of glass [ - ] τg 0.96

Space between panel and glass [m] S12 0.040

Mass x Sp. heat of panel [ J/K ] (m cp)p 16500.0

Mass x Sp. heat of air [ J/K ] (m cp)a 10050.0

Mass x Sp. heat of glass [ J/K] (m cp)g 18200.0

Wind velocity [ m/s] Vw 2.5

The value of IN can be calculated after finding each of the unknown parameters in Equation

(1).

Beam radiation factor is given by:

Rb=cos θi

cos θz

[3]

Wherecosθ i is the angle of incidence and is given by:

cos∅ i =cos (∅−β ) cosωcosδ+sinδsin (∅−β )[4]

The hour angle ω is calculated as follows:

ω=( ST−12 )× 15° [5]

And the collector slope (β) is given to be 15°.

The value of the declination angle δ for each day of the year starting from day one of the year

which is January 1 where N=1 is given by:

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δ=23.45 sin ¿ [6]

Then, we can find the value of the angle of incidence by using the parameters calculated

above.

The zenith angle θ zcan be calculated by using the following formula.

cosθ z=cosωcosδ cos∅+sin∅ sinδ [7]

Finally, using the data for the total global radiation and diffused radiation for each day and

each hour of the year, the total incident radiation (IN) and the flux collected (Ic) can be

calculated for each hour of the year.

8.1. Energy balance on each component of the solar drier

A solar air heater is a flat plate collector with an absorber plate, a transparent cover at the top

and insulation at the bottom and the sides.

The transient thermal performance of the solar collector is evaluated by applying energy

balance on its components.

Fig.5. Solar drier schematic diagram

Energy balance on the absorber plate, the air stream and the glass cover are performed based

on the thermal circuit indicated below.

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Fig.6. Thermal circuit of solar dryer

a. Absorber Plate

Energy balance on the absorber plate is expressed as:

(m c p )p

d T p

dt=Ac I c−Ac U pg ( T p−T g )−Ac U a {T P−

T ao+T ai

2 }−Ac U pab (T p−T a )−A1U pae¿ -Ta) [8]

Where A1=collector perimieter ×depth of the edge=P× de

U pg=collector plate to glass heat transfer coefficient [kW/m2]

U pab=heat transfer coefficient from collector plate to the ambient through the bottom side [kW/m2]

U pae= heat transfer coefficient from collector plate to the ambient through the edge [kW/m2]

U a=¿heat transfer coefficient of the air [kW/m2]

The plate temperature T p at any time ( t+ Δτ ) can be determined from conditions at time t and

the energy absorbed by the plate and energy losses.

Then we have to find the unknown parameter in equation (8) using the expressions below.

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The overall heat transfer coefficient from the plate to the glass, Upg, by convection and

radiation is given by:

U pg=[0.06−0.017β

90 ] Ka

S12

GrL

13+ε pg σ (T p

2+T g2 ) (T p+T g ) [9]

GrL is the Grashof number GrL and is given by:

GrL1 /3=

g βv (T p−T g)L3

ν2 [10 ]

Where g=gravitational acceleration =9.81m/s2

ν= kinematic viscosity of the air

βv = the volume of expansion of the air and is calculated as follows:

β= 2T ao+T ai

[11 ]

The overall emittance factor εpg for the absorber plate and the glass is obtained from the

relation:

ε pg=1

1ε p

+1ε g

−1[12]

Where ε p=Emissivity of the plate

ε g=Emissivity of the glass

The heat transfer coefficient Ua of the air is defined as:

U a=0.664Ka

S12 [ P rG rLcos β

P r+0.9524 ]0.25

[13]

Where Pr=C p ν

Ka

[14 ]

Ka=Thermalconductivity of air

S12=¿Distance between cover and absorber

ν=¿ kinematic viscosity of the air

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The properties in the above equations are evaluated at the film temperature Tf given by:

T f =T ao+T ai

2[15]

The heat transfer coefficient from collector plate to the ambient through the bottom side, Upab,

is given by:

U p ab=K ¿

tb

[16]

Where K ¿=¿Thermal conductivity of the insulator

t b= Bottom insulation thickness

The heat transfer coefficient from the collector plate to the ambient through the edge, Upae, is

given by:

U pae=U pab( Ae

Ac)[17 ]

Where Ae=P ×t be

t be=Edge insulation thickness

Due to the transient nature of the radiant and convective heat transfer coefficients, Equation

(8) is a non-linear equation and its solution requires an integration scheme that linearises the

given differential equation within a given time step.

The plate-temperature at time (t + ∆τ ) is evaluated from available data at time t and incident

solar radiation and thermal losses during time interval ∆τ .The temperature of the plate at time

(t + ∆τ ) in terms of the absorbed useful incident radiation on the surface of the plate, the heat

losses through the glass cover and the collector edge, the quantity of heat absorbed by the air

stream and temperatures of the collector components at time t is obtained from equation (8)

as follows:

T p1=Ac I c

( mc p )p

Δτ⏟

A 11

+{1−( Ac U pg

(m c p )p

+Ac U a

(m cp ) p

+Ac U pab

( mc p )p

+A1U pae

2 (m c p )p)Δτ }⏟

A12

T po+( Ac U pg

(m c p )p

Δτ )⏟A13

T go+( Ac U a

2 ( mc p )p

Δτ )⏟A14

Tao 0+( Aa U a

2 (m c p )p

Δτ )⏟A15

T ai 0+{( Ac U pab

( mc p )p

+Ac U pae

2 (m c p )p)Δτ }⏟

A16

T ao [18]

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b. Air Stream

Considering heat transfer from the collector plate to the air-stream, heat transfer from the air-

stream to the glazing and heat transfer to the air entering the collector, energy balance on the

stream yields:

(mc p )adT ao

dt=Ac U a(T p−

T ai+T ao

2 )−Ac U a(Tai+T ao

2−T g)−(m cp )a (T ao−T ai ) [19 ]

where the mass flow rate of air at exit from the collector, m , is determined from the solar

flux collected per unit area, Ic, the temperature difference between the heated air at Tao and

the ambient (incoming) air temperature at Tai as:

m=Ac [ qab−UL (T p−T a) ]

c pa (T ao−T ai )[20]

The temperature of the air stream entering the solar collector at the ambient conditions at

time t gains energy from the incident radiant energy on the collector plate. In relation to the

inlet air-stream temperature, the heat losses from the plate to the glass cover and the collector

edge, the temperature of the air-stream at outlet from the collector at (t +∆τ ) is determined

from:-

T ao 1=1−∆ τ

(m c p)a[ Ac Ua+(m c p )a ] T ao0+

∆ τ

(m c p ) a [ ( mc p ) a−Ac U a ] Tai 0+[ AcUa

(m c p ) a∆ τ ]T p0+[ AcUa

( mc p ) a∆ τ ]T g0[21]

c. Glass Cover

Energy balance for the glass cover, assuming single glazed collector yields:

dT g

dt=Ac I N (1−τg )+ Ac U pg (T p−T g )−Ac U ga (T g−T a )[22]

The glazing temperature at time (t + τ ) in relation to the absorbed incident radiation on the

surface of the glass, the heat losses from the collector plate to the glazing and heat losses

from the glazing to the surrounding at time t is given by:

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T g 1=Ac I N (1−τg )

( mc p )g

Δτ⏟

A21

+{1−( Ac U pg+ Ac U ga) Δτ

(m cp )g}⏟

A22

T g 0+( Ac U pg

(m c p )gΔτ )⏟

A 23

T p0+( Ac U ga

(mc p )g

Δτ )⏟A24

T a 0[23]

9. Solution Method

The year-round transient analysis of the collector model is performed using the given solar

radiation data and the solar collector parameters. The temperature variations of the collector

panel, the air stream leaving the collector and the glass cover are estimated starting on

January 01 at 01:00 am. The following steps are applied in the determination of the

temperature variations over the year.

Step 1:

Evaluate temperatures Tp1 of the plate, Tao1 of the air stream at exit, Tg1 of the glass cover and

the mass flow rate maof air at exit at time (t +Δτ ) ,starting the system simulation by assuming

the following initial temperatures for the various components of the solar-collector model.

1. Ambient temperature Ta0 = value read from solar radiation data on January 01 at 01:00 am;

2. Glass temperature Tg0 = Ta0 + 0.25;

3. Plate temperature Tp0 = Ta0 + 1;

4. Temperature of air at collector inlet Tai0 = Ta0; and

5. Temperature of air at collector outlet Tao0 = Ta0+0.5;

Step 2:

Since the assumed temperatures for the various components in Step 1 are too crude, they have

to be refined by taking average values of the assumed temperatures and the evaluated

temperatures at time (t + Δτ ) and substituted for temperature values at time t. Steps 1 and 2

are performed only for January 01 at 01:00 am.

Step 3:

Temperature variations Tp1, Tao1, Tg1 and the mass flow rate maduring a period of 24 hours

over the year are then estimated by substituting the new values of Tp1, Tao1 and Tg1,

respectively, for Tp0, Tao0, Tg0 for the next time interval.

10.Workshop and assembly drawing of the solar dryer

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Reference1. Lecture notes of Renewable Energy Resources Course

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