Investigation of heat transfer enhancement for a model of solar central tower project in Baghdad city.

1. Investigation of heat transfer
enhancement for a model of
external receiver solar power plant
By
Mahmoud Sh. Mahmoud
M.Sc. Mech. Eng. / Thermal power (2001)
Supervised by
Asst. Prof. Dr. Ahmed F. Khudheyer
Asst. Prof. Dr. Qusai Jihad Abdul Gafor

2. Motivations:
Seeking to obtain energy with minimal expenses and
pollution is still a challenge that is being worked on.
Energy production especially in developing countries
like Iraq often falls short of energy requirements
which results in frequent power failure. The grow in
energy consumption is continual, on the other hand
fossil fuel is limited, so it is essential to consider other
energy sources like renewable energy especially solar
power to keep up and try to meet the energy demands
in the future. The present work is conducted by
building up a (CSP) model with a novel receiver
design to enhance the thermal performance of it.

3. Fig 1: shows that Iraq position in area of over than 3000
hours of bright sunshine yearly [1].

4. Introduction:
The renewable energy sources are available and used in many applications
now a days. One of these applications is the power generation. The solar energy is the
most available renewable energy sources used in the world. There are two familiar
ways to convert the solar energy to electricity, direct and indirect. The direct way is
presented by using photovoltaic cells (PV). The indirect way is presented by using the
solar energy as a heating source for the working fluid that is used in power plant. One
of the most common ways to heat the working fluid by solar energy is to concentrate
solar energy. there are many concentration types, like Fresnel reflectors, trough type,
compound parabolic collectors (CPC), and heliostat concentrators.

5. Concentrated solar power (CSP) thermal systems using heliostats for power
generation shows better capability for improvement in performance as well as
reducing the cost compared to other (CSP) systems.

6. Aims and Novelty for the present work
The main objectives are to increase the (SPTS) performance:
 Increase reflected sun rays from the heliostats toward the solar receiver.
 Enhance solar receiver thermal efficiency.
The originality of the present work could be summarized as:
 Individual automated dual axis tracking system design to enhance solar plant
performance.
 A novel solar receiver design consisting of staggered configuration pipes is used to
investigate its effect on the receiver thermal efficiency.

7. Problem Description
Central tower solar power system is considered for the present study. The main losses
in the traditional solar receiver are due to the presence of gaps between receiver pipes
that lead to losing some of the incident sun rays. Seeking for improving and achieving
the thermal efficiency of the system, sources of losses must be defined and minimized.
Fig 2: (a) Traditional
solar receiver. Fig 2: ( (b) novel
designed solar
receiver.

8. Literature review
Solar power
plant receiver
design
A review of previous researchers and studies investigated the main effecting
parameters on the central tower solar power system performance are shown in the
following categories.
Optimization
and Heat
Transfer
Enhancement
Receiver
Coating
Tracking
system
Heliostat
field

9. Solar
power
plant
receiver
design
J. Yellowhair et al. (2015), Testing and optical modeling of novel concentrating solar receiver
geometries to increase light trapping and effective solar absorptance.
N. H. Abu-Hamdeh and K. A. Alnefaie (2016), Design considerations and construction of an
experimental prototype of concentrating solar power tower system in Saudi Arabia.
M. Hazmoune et. al (2016), 3D Simulation Study of a Receiver on a Solar Power Tower.
S. S. Alrwashdeh (2018), The effect of solar tower height on its energy output at Ma’an-
Jordan.
A. K. Khlief et. al (2018), Design a New Receiver for the Central Tower of Solar Energy.
Solar power
plant design

10. Optimization
and
Heat
Transfer
Enhancement
P. Xu et. al (2014), Numerical simulation and experimental study of the tube receiver’s
performance of solar thermal power tower.
D. Potter et. al (2015), Optimized Design of a 1 MWt Liquid Sodium Central Receiver System.
N. Karwa et. al (2016), Receiver shape optimization for maximizing medium temperature CPC
collector efficiency.
A. Piña-Ortiz (2018), Experimental analysis of a flat plate receiver for measurement of low
thermal power of a central tower solar system.
Optimization
and Heat
Transfer
Enhancement

11. Receiver
Coating
A. Hall et. al (2012), Solar selective coatings for concentrating.
A. H. Alami et. al (2018), Enhancement of spectral absorption of solar thermal collectors by bulk
graphene addition via high-pressure graphite blasting.
S. A. Sakhaei and M. S. Valipour (2019), Investigation on the effect of different coated absorber
plates on the thermal efficiency of the flat-plate solar
collector.
Receiver
Coating

12. Tracking
system
M. M. Arturo and G. P. Alejandro (2010), High-precision solar tracking system.
K. K. Chong and M. H. Tan (2011), Range of motion study for two different sun-tracking
methods in the application of heliostat field.
K. Malan (2014), A heliostat field control system.
S. patil Pratik Pawar et. al (2018), Solar Tracking System Using Arduino.
W. M. Hamanah et. al (2020), Heliostat dual-axis sun tracking system: A case study in KSA.
Tracking
system

13. Heliostat
field
C. J. Noone et. al (2012), A new computationally efficient model and biomimetic layout.
K. Lee and I. Lee et. al (2019), Optimization of a heliostat field site in central receiver systems
based on analysis of site slope effect.
Heliostat field

14. From the above mentioned studies, it is obvious that there are many studies
that deals with solar central tower.
The present work aims to:
• Fabricate new staggered configuration for the absorber heat exchanger and,
• Design a new dual axes tracking system.
Present Study scope

15. Mathematical Model
and
Numerical Analysis

16. The governing equations
The present study can be described according to the assumptions, steady incompressible, viscous fully developed
turbulent flow through a pipe, three-dimensional with no slip condition, single phase, conjugate heat transfer, and
variable thermal physical properties for the working fluid with the bulk temperature.
Continuity Equation:
𝜕 𝜌𝑢𝑧
𝜕𝑧
+
1
𝑟
𝜕 𝑟𝜌𝑢𝑟
𝜕𝑟
= 0
Momentum Equation:
r - component:
𝜕 𝜌𝑢𝑧𝑢𝑟
𝜕𝑧
+
1
𝑟
𝜕 𝑟𝜌𝑢𝑟𝑢𝑟
𝜕𝑟
= −
𝜕𝑃
𝜕𝑟
+
𝜕
𝜕𝑧
𝜇eff
𝜕𝑢𝑟
𝜕𝑧
+
1
𝑟
𝜕
𝜕𝑟
𝑟𝜇eff
𝜕𝑢𝑟
𝜕𝑟
− 𝜇eff
𝑢𝑟
𝑟2 + 𝑆𝑟
𝑆𝑟 =
𝜕
𝜕𝑧
𝜇𝑒𝑓𝑓
𝜕𝑢𝑧
𝜕𝑟
+
1
𝑟
𝜕
𝜕𝑟
𝑟𝜇𝑒𝑓𝑓
𝜕𝑢𝑟
𝜕𝑟
− 𝜇𝑒𝑓𝑓
𝑢𝑟
𝑟2 −
2
3
𝜕
𝜕𝑟
𝜌𝑘
Effective viscosity is: 𝜇eff = 𝜇𝑡 + 𝜇
Turbulent viscosity is: 𝜇𝑡 = 𝐶𝜇𝑓𝜇𝜌(𝑘/𝜀)

17. The governing equations
z - component:
𝜕 𝜌𝑢𝑧𝑢𝑧
𝜕𝑧
+
1
𝑟
𝜕 𝑟𝜌𝑢𝑟𝑢𝑧
𝜕𝑟
= −
𝜕𝑃
𝜕𝑧
± 𝑔 +
𝜕
𝜕𝑧
𝜇eff
𝜕𝑢𝑧
𝜕𝑧
+
1
𝑟
𝜕
𝜕𝑟
𝑟𝜇eff
𝜕𝑢𝑧
𝜕𝑟
+ 𝑆𝑧
𝑆𝑧 =
𝜕
𝜕𝑧
𝜇eff
𝜕𝑢𝑧
𝜕𝑧
+
1
𝑟
𝜕
𝜕𝑟
𝑟𝜇eff
𝜕𝑢𝑟
𝜕𝑧
−
2
3
𝜕
𝜕𝑧
(𝜌𝑘)
Energy Equation:
𝜕 𝜌𝑢𝑧𝑇
𝜕𝑧
+
1
𝑟
𝜕 𝑟𝜌𝑢𝑟𝑇
𝜕𝑟
=
𝜕
𝜕𝑧
𝜇eff
𝜎𝑛,𝑡
𝜕𝑇
𝜕𝑧
+
1
𝑟
𝜕
𝜕𝑟
𝑟
𝜇eff
𝜎𝑛,𝑡
𝜕𝑇
𝜕𝑟
Using finite volume method (FVM) to discretize the governing equations for the current study.

18. Thermal efficiency
q
˙
inc. = IN × AH × NH × ηcos ⋅ ηsha ⋅ ηblo ⋅ ηatt ⋅ ηspil ⋅ ρref
𝜂𝑡ℎ𝑒𝑟𝑚𝑎𝑙 =
𝑞𝑔𝑎𝑖𝑛
𝑞𝑖𝑛𝑐.
=
𝑚𝐶𝑝𝛥𝑇
𝑞𝑖𝑛𝑐.
𝑞
˙
gain = 𝑞
˙
inc. − 𝑞
˙
loss

19. Numerical analysis
The governing equations are solved numerically with computational
fluid dynamics (CFD) using ANSYS FLUENT 2019R1 commercial
package, according to the mentioned assumptions and boundary
conditions for the present study.
Higher-order differential governing equations that were
analyzed using the SIMPLE algorithm with (10-15) convergence.
Upwind scheme was used in this model. The initialization was
selected as a hybrid for twenty iterations to save the
initialization values used for running.

20. ANSYS code validation
For justifying the ANSYSFLUENT 2019R1 commercial package performance, a validation process was carried
out for two different cases
Fig 3: Code validation

21. Design Considerations
Design
Considerations
Solar Receiver
Profile
Series Parallel Staggered
Continuous
Cross
Pipe material
Pipe
diameter
Working
fluid
Inlet
position
Horizontal
Downward
Upward
Vertical
Downward
Upward
Entrance
length
Heliostat
Position
Offset
Tracking System
Solar Tower height

22. Design considerations conclusions
From analysis of the design consideration, the present study description had been recognized. The outcomes of the
design considerations are listed below:
1. Staggered pipes configuration for the receiver, having totally (54) pipe divided into two rows with dimensions
(50*50) cm.
2. Copper is selected to be the pipe material with (9.5 mm) inner diameter and (0.5 mm) thickness.
3. Heat transfer working fluid is selected to be (water) with variation in thermo-physical properties with bulk
temperature.
4. Central tower high is (4.5 m).
5. Heliostat reflecting material is glass (4mm) thickness, with dimensions (80*80) cm equipped with automated dual
axis tracking system.
6. Heliostat horizontal radius is (7 m) from tower base.
7. Offset angle between heliostats (opt.) is 30o, axisymmetric arrangement with the central tower normal line.
8. Duel-axes tracking system had been used.

23. Receiver’s final design
Fig 4: Receiver schematic

24. Mesh generation
Factor Value Recommended Status
Skewness 0.2  0.25 Excellent
Orthogonal element 0.88  0.9 Excellent
Element quality 0.84  0.88 Excellent
Aspect ratio 1.81  2 Excellent
Table 1 Finest mesh metrics factors.
Fig 5: Computational domain mesh

25. Mesh Independence
Case No. of elements T (oC)
1 79125 38.58
2 1213250 41.63
3 6174048 47.112
4 7698288 47.161
5 27074104 47.86
Table 2 Mesh independence results.

26. Boundary conditions
The boundary conditions for the present study are:
o Location: Al-Nahrain University / Baghdad Latitude (33.28° N) and Longitude (44.38° E).
o Solar radiation (W/m2) reflected per each heliostat is measured during the average recommended day for each
selected month.
o Inlet temperature (oC) is measured during the average recommended day for each selected month.
o Turbulent flow, mass flow rate = 0.04 kg/s.
o Velocity profile
o Working fluid "Water" with variable properties as a function of temperature.

27. CFD models validation
Fig. 6: Comparison among CFD models

28. Numerical results validation
Fig. 8: Numerical results validation with Blasius correlation
[48].
Fig. 7: Numerical results validation with Dittus-Boelter
correlation [48]

29. Experimental
Work

30. Design and manufacturing of the experimental rig that had
been used in this study was presented, in addition to
instrumentation, measurements, calibrations, test procedure,
and processing gathered data. The rig was assembled, and
tests were performed at Al-Nahrain University/Baghdad with
Latitude (33.3152°N) and Longitude (44.3661°E) during a
clear sky or partially clouded weather conditions according to
the selected tested dates. The presented work is initialized
from January 2018, utilizing central solar tower south
oriented with heliostat automated duel axis tracking system.

31. Rig Design
Rig Design
Solar Receiver
Assembly
Flat Sheet
Copper pipes
Stainless steel
threaded studs
Coating process
Heliostat
Reflector
Material
Tracking
System
Control board
Base
linear actuators
LDRs sensing
system
Movable
central tower
Instruments
Digital flow
meter
Solar intensity
data logger
48 channel
Temperature
data logger

32. Experimental rig setup
Tower alignment in vertical position.
1
Casting the tower base using cement and sand mortar.
2

33. Experimental rig setup
Locate the tower height and the position (offset angle
and radius) for each heliostat.
3
Casting the heliostats bases using cement and sand
mortar.
4

34. Experimental rig setup
Connecting the tracking system control board
with each heliostat and setting the LDRs sensors
with auxiliary mirror.
5
Manually setting the tracking system for both
heliostats to reflect the sun rays toward the
testing board and observe the reflected image
and giving feedback till the accurate positioning
is achieved.
6

35. Experimental rig setup
Instilling the receiver on the tower and
connecting the input and output water hoses.
7
Additional fixing for the receiver using stainless
steel strands to avoid wind effect on it.
8

36. Experimental rig setup
Connecting all instrumentations to the system
and prepare it for operating.
9

37. Experimental rig setup
System is operated according to the
experimental procedure.
10

38. Results

39. Results
Fig. 9: Comparison between dual and single axis systems
for 16 August.
Fig. 10: Hourly enhancement for dual and single axis tracking
systems for selected months.

40. Results
Fig. 11: heliostat number one location at 9:30 in the azimuth and tilt directions for July (summer season).

41. Results
Fig. 12: Friction factor comparison between Numerical
and Blassius equation.
Fig. 13: Pressure distribution for fluid flow
in staggered solar receiver.

42. Results
Fig. 14: Comparison between numerical and experimental hourly direct solar beam radiation (W/m2) at 16
August.

43. Results
Fig. 15: Experimental measurements for ambient, outlet, maximum receiver surface temperatures distribution
verses concentrated solar irradiance.

44. Results
Fig. 16: Comparison between experimental and numerical
working fluid outlet temperature.
Fig. 17: Numerical and experimental maximum surface
temperature.

45. Results
Fig. 18: Staggered receiver surface temperature
contour for 16 August
Fig. 19: Staggered receiver surface temperature
contour for 17 July.

46. Results
Fig. 20: Temperature distribution along ZX plane at 12:00 and Y=0.05 m.

47. Results
Fig. 21: Experimental and numerical surface temperature along the receiver.

48. Results
Fig. 22: Comparison of experimental and numerical average
Nusselt number versus Reynolds number
Fig. 23: Local Nusselt number validation.

49. Results
Fig. 24: Numerical and experimental thermal efficiency results

50. Results
Fig. 25: comparison between present study efficiency versus numerical, one row, and evacuated tube.

51. Conclusio
ns

52. Conclusions
1. The staggered configuration receiver gave 10.13%, 5.5%, and 1.5% thermal efficiency enhancement compared
with conventional parallel, series, and staggered continuous configurations for the receiver, respectively.
2. The numerical results showed that the Copper pipe used in building up the solar receiver had given the highest
outlet temperature with 361.7K compared with Aluminum, and Steel.
3. Automated dual axes tracking system was selected for the heliostats that provided the highest reflected solar rays
with an average enhancement percent was 49.19%, 26.65%, 71.91%, and 42.28% for May, June, July, and August,
respectively.
4. Optimization of tower height, heliostat radius, and offset angle were performed, and the results show that 4.5 m,
7m, and 30o respectively are the optimum values.
5. To ensure best thermal performance, a coating process was done to reduce the reflectivity and emissivity as well as
increase the absorptivity of the receiver by using Ebonol C to oxidize the receiver pipes and then coated with
black matt paint, this process had enhanced the thermal performance by 13.97%.

53. Conclusions
6. The highest thermal efficiency for July at 12:00 noon was 86.88% while minimum value was 75.1% for May.
Thermal efficiency enhancement for June, July, and August with that for May are 11.45%, 15.68%, and 13.32%
respectively. The average error percent between numerical and experimental results for May, June, July, and
August are 1.28%, 1.33%, 1%, and 1.13% respectively.
7. An empirical correlation for Nusselt number had been formulated
𝑁𝑢𝑦 = 1602 ∗ 𝑅𝑒0.024463 ∗ 𝑃𝑟 −2.32751 ∗ 𝑦0.160764
The correlation conditions are, fully developed, turbulent flow (Re > 5500), L/D  3150 and Pr  7.56.
8. The present design thermal efficiency compared with one row and evacuated tube mentioned in previous studies
was increased to 8% and 10.92% respectively.

55. Publications

56. Videos