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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
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Transient Analysis of a Solar Dryer 2012
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
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Transient Analysis of a Solar Dryer 2012
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
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Transient Analysis of a Solar Dryer 2012
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.
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Transient Analysis of a Solar Dryer 2012
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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Transient Analysis of a Solar Dryer 2012
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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Transient Analysis of a Solar Dryer 2012
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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Transient Analysis of a Solar Dryer 2012
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.
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Transient Analysis of a Solar Dryer 2012
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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Transient Analysis of a Solar Dryer 2012
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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Transient Analysis of a Solar Dryer 2012
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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Transient Analysis of a Solar Dryer 2012
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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Transient Analysis of a Solar Dryer 2012
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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Transient Analysis of a Solar Dryer 2012
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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Transient Analysis of a Solar Dryer 2012
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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Transient Analysis of a Solar Dryer 2012
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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Transient Analysis of a Solar Dryer 2012
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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Transient Analysis of a Solar Dryer 2012
Reference1. Lecture notes of Renewable Energy Resources Course
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