CFD AND DOE STUDY THE EFFECT OF SOLAR CHIMNEY TOWER RADIUS, HEIGHT, CHIMNEY RADIUS AND HEIGHT

International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
ISSN 0976 – 6359(Online), Volume 5, Issue 5, May (2014), pp. 200-213 © IAEME
200
CFD AND DOE STUDY THE EFFECT OF SOLAR CHIMNEY TOWER
RADIUS, HEIGHT, CHIMNEY RADIUS AND HEIGHT
Manisha Jayprakash Dhanokar1
, Kiran Prakashrao2
, Parmar Ashwin1
1, 2
(Pillai HOC College of Engineering and Technology Rasayani)
3
(Assistant Professor, Pillai HOC College of Engineering and Technology Rasayani)
ABSTRACT
Solar chimney technology is a very promising solar thermal electricity generating technology.
In this project CFD technology is used to investigate the changes in flow kinetic energy caused by
the variation of tower flow area with height. It was found that the tower area change affects the
efficiency and mass flow rate through the plant. The divergent tower top leads to augmentations in
kinetic energy at the tower base significantly. Numerical calculations have been performed using
CFX. For this purpose, CFX solves the conservation equations for mass, momentums, and energy
using a finite volume method. Adaptive unstructured tetrahedral meshes were used in the present
study. The plants studied were modeled as an axis-symmetric model where the centerline of the
tower is the axis of symmetry. To simulate axis-symmetry, a 1 degree section of the plant is cut out
from the entire periphery. The results shows that the chimney height and Tower outlet radius and
base area are very important parameters for improving the gained power, and it is also important to
choose the region with suitable mean ambient temperature. And economically there are limitations to
collector and chimney sizes to get suitable profit output power and any increment in system size
becomes a small percentage increment in profit output power. The results compared with some
experimental data from other results researchers and there is a good agreement between simulated
and calculated results
Keywords: CFD, Optimization, Solar Chimney.
I. INTRODUCTION
In recent years, rapid developments of global economy and increase in population and living
standards have been posing great pressure on natural resources and the environment. Fossil fuels are
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being exhausted at a fast rate, and utilization of fossil fuels together with net deforestation has
induced considerable climate change in warming the atmosphere by releasing greenhouse gases
(GHG) which may produce many negative effects including receding of glaciers, rise in sea level,
loss of biodiversity, extinction of animals, and loss of productive forests, acidification of oceans etc.
Our over dependency on fossil fuels for our energy needs has lead us close to the energy crisis and
has caused various adverse effects on our environment as mentioned above. Therefore now the
biggest challenge before the energy pioneers and researchers is finding new ways of harnessing
energy from renewable sources that are sustainable and free of greenhouse gases (GHG) as well. The
drawback of most of the renewable power technologies has been their unreliability as they can’t
operate continuously for 24 hours or continuous operation is achieved only through hybrid systems
using fossil fuels along with renewable energy sources or through expensive and sophisticated
energy storage facilities. These drawbacks have kept the researchers busy in finding more and more
alternatives ways to power our future. Solar Chimney power technology is one of them and it
promises a better solution to our current energy problems.
Fig.1: SCPP working principle
The solar tower’s principle is shown in Fig 1. Air is heated by solar radiation under a low
circular transparent or translucent roof open at the periphery; the roof and the natural ground below it
form a solar air collector. In the middle of the roof is a vertical tower with large air inlets at its base.
The joint between the roof and the tower base is airtight. As hot air is lighter than cold air it rises up
the tower. Suction from the tower then draws in more hot air from the collector, and cold air comes
in from the outer perimeter.
Jörg Schlaich, Rudolf Bergermann, Wolfgang Schiel, Gerhardm Weinrebe 2000 [1] described
the functional principle of solar updraft towers and gave some results from design, construction and
operation of the first ever prototype built in Spain. S. Beerbaum and G. Weinrebe et al. 2000 [2]
conducted a techno-economic analysis on solar thermal power generation in India. In this study they
analyzed the potential and the cost-effectiveness of centralized and decentralized STE-generation in

202
India. Y.J. Dai, H.B. Huang and R.Z. Wang et al. 2002 [3] analyzed a solar chimney power plant
that was expected to provide electric power for remote villages in northwestern China.
M.A. dos S. Bernardes, A. Voß and G. Weinrebe et al. 2003 [4] carried out an analysis for the
solar chimneys aimed particularly at a comprehensive analytical and numerical model, which
describes the performance of solar chimneys. J.P. Pretorius and D.G. Kröger et al. 2005 [5] evaluated
the influence of a recently developed convective heat transfer equation, more accurate turbine inlet
loss coefficients, quality collector roof glass and various types of soil on the performance of a large
scale solar chimney power plant. Xinping Zhou, Jiakuan Yang, Bo Xiao & Guoxiang Hou et al.
2006 [6] built a pilot experimental solar chimney power generating equipment in China. They carried
out a simulation study to investigate the performance of the power generating system based on a
developed mathematical model.
Atit Koonsrisuk & Tawit Chitsomboon et al. 2007 [7] proposed dimensionless variables to
guide the experimental study of flow in a small-scale solar chimney: a solar power plant for
generating electricity.. T.P. Fluri, J.P. Pretorius, C. Van Dyk, T.W. Von backström, D.G. Kröger,
G.P.A.G. Van Ziji et al. 2008 [8] compared several cost models that were available in the literature.
Xinping Zhou, Jiakuan Yang, Bo Xiao, Guoxiang Hou & Fang Xing et al. 2008 [9] the maximum
chimney height for convection avoiding negative buoyancy at the latter chimney and the optimal
chimney height for maximum power output were presented and analyzed.
Cristiana B. Maia, André G. Ferreira, Ramón M. Valle & Márcio F.B. Cortez et al. 2008 [10]
carried out an analytical and numerical study of the unsteady airflow inside a solar chimney. The
conservation and transport equations that describe the flow were modeled and solved numerically
using the finite volumes technique in generalized coordinates. The numerical results were physically
validated through comparison with the experimental data. Tingzhen Ming, Wei Liu, Yuan Pan &
Guoliang Xu et al. 2008 [11] carried out numerical simulations to analyze the characteristics of heat
transfer and air flow in the solar chimney power plant system with an energy storage layer.
Marco Aurélio dos Santos Bernardes, T.W. Von Backström & D.G. Kro¨ger et al. 2008 [12]
compared the two comprehensive studies namely those of (Bernardes, M.A.d. S., Voß, A., Weinrebe,
G., 2003. Thermal and technical analyses of solar chimneys. Solar Energy 75, 511–524; Pretorius,
J.P., Kro¨ ger, D.G., 2006b. S. Nizetic, N. Ninic & B. Klarin et al. 2008 [13] analyzed the feasibility
of solar chimney power plants as an environmentally acceptable energy source for small settlements
and islands of countries in the Mediterranean region. For the purpose of these analyses, two
characteristic geographic locations (Split and Dubrovnik) in Croatia were chosen and simplified
model for calculation of produced electric power output is also developed. Atit Koonsrisuk & Tawit
Chitsomboon et al. 2009 [14] in their previous study found that the achievement of complete
dynamic similarity between a prototype and its models imposed the use of different heat fluxes
between them. Atit Koonsrisuk & Tawit Chitsomboon et al. 2009 [15] used dimensional analysis
together with engineering intuition to combine eight primitive variables into only one dimensionless
variable that establishes a dynamic similarity between a prototype and its scaled models.
II. MATHEMATICAL MODELS OF FLUENT
All the fluids investigated in this research are Newtonian. This means that there exists a linear
relationship between the shear stress, σij , and the rate of shear (the velocity gradient). In CFX, this
is expressed as follows:
ߪ௜௝ ൌ െ‫ߜ݌‬௜௝ ൅ ߤ ቆ
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൅
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߲‫ݔ‬௜
ቇ … … … … … … . .1

203
In FLUENT, these laws are expressed in the following form:
Law of Conservation of Mass: Fluid mass is always conserved.
߲ߩ
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൅
߲
߲‫ݔ‬௝
൫ߩ‫ݑ‬௝൯ ൌ 0 … … … … … . . .2
Newton’s 2nd
Law: The sum of the forces on a fluid particle is equal to the rate of change of
momentum.
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ሺߩ‫ݑ‬௜ሻ ൅
߲
߲‫ݔ‬௝
൫ߩ‫ݑ‬௜‫ݑ‬௝൯ ൌ
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ቈെ‫ߜ݌‬௜௝ ൅ ቆ
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൅
߲‫ݑ‬௝
߲‫ݔ‬௜
ቇ቉ ൅ ‫ܤ‬௜ … … … … … . . .3
First Law of Thermodynamics: The rate of head added to a system plus the rate of work done on a
fluid particle equals the total rate of change in energy.
߲
߲‫ݐ‬
ሺߩ‫݁ܪ‬ሻ ൅
߲
߲‫ݔ‬௝
൫ߩ‫ݑ‬௝‫݁ܪ‬൯ െ
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ቆߣ
߲ܶ
߲‫ݔ‬௝
ቇ ൌ
߲ߩ
߲‫ݐ‬
… … … … … . . .4
The fluid behaviour can be characterised in terms of the fluid properties velocity vector u (with
components u, v, and w in the x, y, and z directions), pressure p, density ρ, viscosity µ, thermal
conductivity λ, and temperature T. The changes in these fluid properties can occur over space and
time. H is the total enthalpy, given in terms of the static (thermodynamic) enthalpy, h:
III. GEOMETRIC MODEL
Geometric Model is created in ANSYS Design modeller which is ashon in Fig.2. and 3.
Fig.2: CFD Model of Solar chimney

204
Fig.3: Solar Chimney nomenclature
Fig.4: CFD Model of Solar chimney sector 1o
-Isometric View
IV. CFD MESHING
A QUAD mesh is generated using ANSYS Meshing preprocessor. Many different
cell/element and grid types are available. Choice depends on the problem and the solver capabilities.
First, the surface of the solar chimney is meshed with QUAD element. Then the QUAD element is
revolved for 1degree. No of elements is used for all the models 10 thousands.
Fig.5: Close View of solar chimney mesh
Tower Height
Tower inlet radius Roof Inlet radius
Roof outlet radius
Tower outlet radius

205
Following are the assumptions incurred on the present analysis:
• Flow is Turbulent
• Flow is steady and incompressible
• Coupled solver
Proper boundary conditions are needed for a successful computational work. At the roof inlet,
the total pressure and temperature are specified; whereas at the tower exit the ‘outlet’ condition with
zero static pressure is prescribed. The symmetry boundary conditions are applied at the two sides of
the sector while the adiabatic free-slip conditions are prescribed to the remaining boundaries,
consistent with the frictionless flow assumption. All test cases were computed until residuals of all
equations reached their respective minima. Moreover, global conservation of mass were rechecked to
further ascertain convergence of the test cases
Fig.6: Boundary zone names
V. RESULTS AND DISCUSSIONS
In Fig. 7, the gauge pressure distributions are seen to be nominally constant under the roof
before falling gradually in the tower portion to meet the hydrostatic pressure value at the tower top.
In Fig.8 shows the contours of temperature. rise of the temperature at the tower base is the response
to the abrupt velocity change, in accordance with the conservation of energy principle. Fig.9 the
velocity increases as it approaches the tower base.
Fig.7: Contours of static Pressure at Mid plane

206
Fig.8: Contours of Temperature at Mid plane
Fig.9: Contours of Velocity Magnitude at Mid plane

207
Fig.10: Contours of Velocity Magnitude Zoomed view
VI. OPTIMIZATION
A. Optimization Outlet radius
In Fig. 12, the gauge pressure distributions are seen to be nominally constant under the roof
before falling gradually in the tower portion to meet the hydrostatic pressure value at the tower top.
For the plants with AR greater than one, there are swift dips at the tower base; the severities of the
dips are proportional to AR. The dips are direct responses to the temperature dips because the density
is relatively unchanged in a low Mach number flow. Note that the ordinate is the gauge pressure
which was scaled such that pressure at the tower top is always zero. Figs. 13 show the distributions
of computed flow properties for different tower area ratio (AR). The abscissa of all plots is the scaled
flow path, equaling zero at roof inlet, one at tower top and 0.5 at tower base. As can be seen in Fig.
13 at any AR, the velocity increases as it approaches the tower base. In the tower portion, the
velocity distribution depends on AR. For models with AR smaller than one, the velocity keeps on
increasing and attains the maximum value at tower outlet. On the other hand, for models with AR
larger than one, the flow achieves its maximum velocity right after entering the tower, and then
decreases continuously afterward.

208
Fig.11: Optimization 1- Tower outlet radius variation
Fig.12: Comparisons of Pressure at different Area Ratio
Fig.13: Comparisons of velocity at different Area Ratio
Tower Height
Tower inlet radius-4m Roof Inlet Height-4m
Roof outlet Height-4m
Tower outlet radius
Varied from 2.83 m-16 m
Roof Radius 100 m
-14000
-12000
-10000
-8000
-6000
-4000
-2000
0
2000
-0.20 0.00 0.20 0.40 0.60 0.80 1.00
GaugePressurePa
Non-Dimensional Length
AR-1
AR-0.5
AR-0.75
AR-2
AR-4
AR-9
AR-16
-20
0
20
40
60
80
100
120
140
160
180
0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00
Velocitym/s
AR-1
AR-0.5
AR-0.75
AR-2
AR-4
AR-9
AR-16

209
Fig.14: Comparisons of Power at different Area Ratio
Fig.15: Effect of tower area ratio on Power for insulation = 800 W/m2
Fig.16: Effect of tower area ratio on the mass flow rate for insulation = 800 W/m2
-5
0
5
10
15
20
25
30
35
40
45
0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00
PowerW
AR-1
AR-0.5
AR-0.75
AR-2
AR-4
AR-9
AR-16
0
5
10
15
20
25
30
35
40
45
0 2 4 6 8 10 12 14 16 18
Power(W)
Outlet Diameter (m)
0
5
10
15
20
25
30
0 2 4 6 8 10 12 14 16 18
MassFlowRate(kg/s)
Outlet Diameter (m)

210
Fig. 15 shows the variation in dimensionless power, defined as the kinetic power divided by
the kinetic energy of the prototype at tower base. It is evident that high AR leads to augmentation in
power at the tower base. This suggests the potential of harnessing more turbine power from the high
AR system.
The effect of the tower area ratio on the mass flow rate is presented in Fig.16. The results
show that the mass flow rate rises and falls with AR. In relation to the constant-area tower,
convergent-top tower reduces mass flow rate and divergent-top tower increases mass flow rate.
B. Optimization Roof inlet Radius 4m and 2 m
The effect of the tower area ratio on the mass flow rate is presented in Fig. 17 for reducing
roof inlet 4m to 2m. The results show that the mass flow rate rises and falls with AR. In relation to
the constant-area tower, convergent-top tower reduces mass flow rate and divergent-top tower
increases mass flow rate. It shows the higher mass rate for 2m roof inlet.
Fig.17: Effect of tower area ratio on the mass flow rate Roof inlet radius 4 and 2m for
insulation = 800 W/m2
Fig. 18 shows the variation in dimensionless power, defined as the kinetic power divided by
the kinetic energy of the prototype at tower base. It is evident that high AR leads to augmentation in
power at the tower base. This suggests the potential of increasing turbine power from the lowering
the inlet height to 2m.
Fig.18: Effect of tower area ratio on Power on Roof inlet radius 4 and 2m for insulation = 800 W/m2
0
5
10
15
20
25
30
0 2 4 6 8 10 12 14 16 18
MassFlowRate(kg/s)
Outlet Diameter (m)
Roof Inlet radius 4m Roof Inlet radius 2m
0
10
20
30
40
50
60
70
80
0 2 4 6 8 10 12 14 16 18
Power(W)
Outlet Diameter (m)
Roof inlet radius 4m Roof inlet radius 2m

211
C. Optimization -Roof height
The effect of the roof height on the mass flow rate is presented in Fig19 for increasing 100m
to 200m for different roof height. The results show that the mass flow rate rises and increase roof
height up-to 150m then maintains constant. Fig. 20 shows the variation in dimensionless power
Effect of roof height on Power for insulation = 800 W/m2- 8 and 12m, defined as the kinetic power
divided by the kinetic energy of the prototype at tower base. It is evident that high Increase roof
height to augmentation in power at the tower base. This suggests the potential of increasing turbine
power from the increasing the tower height 150m.
Fig.19: Effect of Tower height on Mass flow rate for insulation = 800 W/m2
Fig.20: Effect of Tower height on Power for insulation = 800 W/m2
20
22
24
26
28
30
32
2 52 102 152 202 252
MassFlowRate(kg/s)
Tower Height (m)
Outlet Radius-12m
20.00
40.00
60.00
80.00
100.00
120.00
140.00
2 52 102 152 202 252
Power(W)
Tower Height (m)
Outlet Radius-12m

212
VII. CONCLUSION
A solar tower system with varying tower flow area has been studied and its performance has
been evaluated. The results show that divergent tower helps increase mass flow rate and kinetic
energy over that of the constant area tower. The tower area ratio of 12 to 16 can produce kinetic
energy as much as 100 times that of the constant area tower. For the convergent tower, the velocity at
the tower top increases but the mass flow rate decreases in a manner such that the kinetic power at
the top remains the same as the constant area case. For the divergent case, maximum kinetic energy
occurs at the tower base and this suggests the potential to extract more turbine power than the
constant area tower.
• The simulation convenient to predict the performance of the solar chimney and that can save
the cost of the experimental procedures.
• It is concluded that the mathematical model can predict the performance of the chimney
equipment well, and this approach is also applicable to different-scale solar chimney thermal
power generating systems.
• For a required electric power output, it can obtain many combinations of chimney and
collector dimensions by the simulation.
• The chimney couldn’t produce suitable power for low chimney height and small base area,
for low chimney height it is preferred to the desirable enhancement for increasing the
chimney power generation is reducing its exit area.
• The optimum collector area and chimney height could be chosen for required output power,
we only need to take some measurement at sites.
• It is convenient to choose the chimney site within acceptable annual average ambient
temperature.
• The chimney power increases rapidly as the sizes of collector and the chimney are increases,
but the percentage development in chimney power is reduced with an increase in their sizes
so that it will be useless economically.
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ISSN Print: 0976 – 6340, ISSN Online: 0976 – 6359.

CFD AND DOE STUDY THE EFFECT OF SOLAR CHIMNEY TOWER RADIUS, HEIGHT, CHIMNEY RADIUS AND HEIGHT

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CFD AND DOE STUDY THE EFFECT OF SOLAR CHIMNEY TOWER RADIUS, HEIGHT, CHIMNEY RADIUS AND HEIGHT