- Email: [email protected]
New design of solar chimney (case study)
Author’s Accepted Manuscript New design of Solar Chimney (Case study) Omer Khalil Ahmed, Abdullah Sabah Hussein
www.elsevier.com/locate/csite
PII: DOI: Reference:
S2214-157X(17)30274-5 https://doi.org/10.1016/j.csite.2017.12.008 CSITE247
To appear in: Case Studies in Thermal Engineering Received date: 24 October 2017 Revised date: 21 November 2017 Accepted date: 29 December 2017 Cite this article as: Omer Khalil Ahmed and Abdullah Sabah Hussein, New design of Solar Chimney (Case study), Case Studies in Thermal Engineering, https://doi.org/10.1016/j.csite.2017.12.008 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
New design of Solar Chimney (Case study) Dr Omer Khalil Ahmed
Abdullah Sabah Hussein
Technical Institute / Hawija
Technical College / Kirkuk
Northern Technical University
Northern Technical University
[email protected]
[email protected]
Abstract: The solar chimney power plant has a promising future in the world. Α new design of solar chimney is offered including both PV panels with solar chimney plant for electricity generation. Two experimental models of a hybrid solar chimney were built and designed (systems A&B). System (A) had a collector glass roof cover and a PV panel as an absorber with a chimney of 2 m height while system (B) is similar to system (A) but with PV panel as collector roof cover and plywood as an absorber in the base of the chimney. Two similar experimental models were built to achieve the performance of these new designs. Practical tests were conducted in Kirkuk (35° 28' latitude and 44° 24' longitude), northern Iraq. The results showed that system (A) had higher thermal gain than system B while the daily average of electrical power in system (B) was (75.6 W) higher than system (A) (79 W). This is because the high thermal gain raised the operating temperature of the PV panel which led to a decrease in its power output. The results also presented that system (A) converted thermal power to kinetic power with daily average (0.008 W) because of the great thermal gain which made air less dense in turn increased its velocity more than system (B) (0.006 W) which had lower kinetic power. The total useful power produced by the system (B) is greater than the useful power produced from the system (A).
Keywords: Solar chimney, New design, Solar cell.
1
NOMENCLATURES As
Cross-sectional area of the duct [m2]
cp
Specific heat of air [J/kg.K]
I
Current [A]
m air
Air mass flow rate [kg/sec]
Pele
Electric power produced by solar cell [W]
Pkinetic
Kinetic energy of air [W]
V
Voltage [V]
v
Air velocity [m/s]
vav
Average air velocity [m/s]
ρair
Air density [kg/m3]
1- Introduction: Energy is essential for economic and human development. Using renewable energies like solar for the production of electricity reduces pollution and saves our environment in our plant. Electricity energy can be produced from solar energy by two ways; solar cells and solar thermal power plants. Moreover, there is also another method, namely solar chimney [1]. The efficiency of solar cell is affected by the rise of the cell temperature, especially in hot climates. The rise in the temperature of the solar cells causes the reduction in their electrical efficiency. A solar chimney or solar tower is a type of passive solar design that can be used to produce electricity[2]. The diligent development of the concept has included the investigation of new methods to increase the parallel solar chimney efficiency and capacity to reduce the cost of this type of power plant. Martí-Herrero and Heras-Celemin [3] presented a theoretical study of the solar chimney for air ventilation of room, where the glass surface is replaced by the solar cell. Zou et al.[4] offered a numerical investigation of the solar enhanced passive air system. The numerical results of this system show that the novel design can keep solar cell temperature under 75 oC. Sh-eldin et al. [5] used the solar chimney technique to improve the solar cell system efficiency by using the particle swarm optimization. The numerical results show that the PV system efficiency has positively been affected by increasing the chimney height. Mekhail et al. [6] studied a small mathematical model of a solar chimney power plant to predict the output power of a bigger one. Al-Taaie et al. [7] studied a numerical model with different collecting angles (0°, 15°, and 30°) by using Fluent software. The results showed that the velocity increased and reached the maximum value at a 2
collector angle (30°). Yelpale et al. [8] carried out an experimental analysis of the solar chimney which integrated the solar panels to increase the efficiency of the solar panel by increasing the chimney height. Hussain [9] presented a new idea of hybrid geothermal/PV/Solar chimney which was suggested to be built in the south region of Libya. Geothermal hot water was pumped and circulated through pipes on the soil under the collector roof. PV was added to the system to replace the glass roof. Koonsrisuk and Chitsomboon [10] used iteration techniques to study the performance of solar chimney. The results showed that the optimum ratio between the turbine extraction pressure and available driving pressure for the solar chimney was 0.84. Tongbai and Chitsomboon [11] studied the effects of different parameters on the performance of solar chimney for building ventilation such as inclination angles, channel gaps, solar intensities, vertical chimney attachment heights and channel expanding angles. Hu et al. [12] analyzed the effects of the divergent chimney on the performance of solar chimney. Nakielska and Pawłowsk [13] offered experimental analysis on solar chimney located in Poland. Two models were used in this investigation conducted in the University Science and Technology. 2. Methodology: From the previous studies, it can be concluded that solar chimney has a low power conversion efficiency so that researchers worldwide can strive to enhance its efficiency [14]. From this point of view, the author proposes a novel approach to develop this technique. This approach includes combining following technologies: 1- Hybrid glazed PV / Solar Chimney (Fig. 1-a) 2- Hybrid PV / Solar Chimney (Fig. 1-b). In Figure (1-a), the base of the chimney is covered with solar cells and the solar radiation heats the air found in the space between the glass cover and solar cell on the ground. The resulting convection causes a hot air updraft in the tower by the chimney effect. The solar radiation is incidents of the solar cell installed on the floor of the chimney and produces the electric power. The moving air in the space between the glass cover and solar cell leads to cool the solar cell and improves its efficiency. In Figure (1-b), the glass cover in the classical solar chimneys is replaced by solar cells. These cells generate electricity and act as an absorber surface, where the air is heated by the energy radiated from the back surface for the solar cell and hot air that flows under the solar cell working to cool the solar cell. The kinetic energy of flowing air can be used to produce the electricity using the turbine. 3
(a) Hybrid glazed PV / Solar Chimney
(b) Hybrid PV / Solar Chimney Figure (1): New design of solar chimney In this study, a new way of enhancing the solar chimney by hybridizing the collector was carried out through integrated solar cell in two different experimental models. One was by using a solar PV panel as an absorber with a glass roof and the second was by using plywood as an absorber with PV panel as the covering roof. Comparing these two models to reach the best performance and maximum power output is the main object of this article. 3. Experimental setup: The experimental set-up was built and tested in Kirkuk, a city north of Iraq (Lat.35.46 oN and long 44.39 oE) at the height of 350 m from the sea level. It was orientated to the south and the tilt 4
angle of the solar cell in both systems was 35°. The collector was tested in outdoor conditions. Two experimental systems were built for this purpose. System A consisted of a solar collector with PV panel as an absorber with 148 cm long and 67 cm wide dimensions with emissivity of (ε = 0.95)[15]. The solar panel was fixed inside an aluminum box entirely insulated from three sides of the collector. The collector had a 4 mm thick transparent glass cover mounted at the distance 10 cm from the solar cell to gain the maximum amount of energy from the solar radiation as shown in Figure 2. The glass was fitted with a mounting sealant to block air leakage. A rectangular duct was attached vertically to the collector to work as a chimney 200 cm long and 67 cm wide dimensions. The duct (chimney ) was totally insulated with glass wool which had a thermal conductivity of (k = 0.46 W/m.K)[16]. The dimensions of area duct in both systems were 67 cm * 10 cm. System B had the same dimensions and experimental setup as system A with two major differences that the glass roof was replaced with PV panel while the base of the duct was replaced with plywood that had a thermal conductivity (k = 0.12 W/m.K)[17] as shown in Figure 3. Twenty digital temperature sensors were fixed on the two systems in selected places as shown in Figure 2a and 3a. Five sensors were placed along air stream for each system; two sensors were fixed on the glass cover for system A while the other two were fixed on the PV panel for system B. Two sensors were also fixed on the absorber (PV panel) for system A and the other two on the absorber (plywood) for system B. The last two sensors were fixed on the back of insulation for each system to find the thermal power. Four holes for each design were used to measure the air velocity along the air duct as shown Figures 2a and 3a. The specifications of the solar cell modules used are shown in Table 1. Table 1: Solar cell specification of the PV/T air collector
Subject
Unit
Maximum power (Pmax)
150W
Maximum voltage (Vmax)
17.9 V
Maximum current (Imax)
8.38 A
Open voltage (Voc)
21.9 V
Short circuit current (Isc)
9.01 A
Module size
1480*670*35 mm
Cell type
Poly-crystalline Silicon 5
(a)Photographic picture
(b) Schematic diagram
Figure (2): System (A)
(a) Photographic picture
(b) Schematic diagram Figure (3): System (B) 6
For the air velocity, a hot wire anemometer type (MY-300) was used to measure the velocity of the air stream through four holes in the duct of each system as shown in Figures 2b and 3b. The purpose was to find the kinetic power. For electric current and voltage measurements from the PV panel, two multi-meters type (SM-20) fixed on each system were used to measure the electric current and one multi-meter to measure the voltage while small light bulbs were used to reach the PV panel peak load. The electricity generated by the cells was stored in batteries. The voltage and current were measured by using DC voltmeter and digital ammeter, respectively as shown in Figure (4). Solar radiation was measured by the solar power meter [Solar meter SM206] which was put at the same height as the solar cell.
Figure (4): Solar panel with accessories 4. Mathematical calculations: In order to compare between the two systems (A and B), it is necessary to compute their output power which has two different forms that include electrical and kinetic energies: A– The electrical power: It is expressed by[15]:
Pele V .I
(1)
B- The kinetic Power: It is obtained from the flow speed ( vair ) and expressed by[18]: 1 2 air .v air m 2
(2)
m air air .vair .As
(3)
Pkinetic
Where:
7
C- The total useful power: The total useful power from the solar chimney can be expressed as a summation of the electrical power output Pele from the solar cell and kinetic energy Pkinetic of flowing air[19]: Pusefull Pele Pkinitic
(4)
5. Results and discussions: Both systems were set facing south direction with the inclined angle to gain the maximum solar radiation readings taken at clear days. At the beginning of each test, the glass cover and solar cell were thoroughly cleaned. The measurements of temperature and flow rate of air were recorded at each hour. The experiment was conducted for four months from March to July 2017. 5.1 Solar cell temperature: Figure (5) shows the variation of solar cell temperature for two designs (A and B), when they were tilted at an angle of 45 with the horizontal line. It was noticed in the two models that there was an increase in the temperature of the solar cell from the beginning of the test until it reached the highest temperature at midday coupled with increased solar radiation values. The temperature of the cell surface then began to decrease due to the decrease in the amount of solar energy and the increase in heat losses in the afternoon. The temperature of solar cell for system A with a glass cover recorded its highest value at 90 oC while solar cell of system B reached its ultimate value (67 oC) at 12 noon. The existence of the glass layer leads to the increasing temperature of the solar cell in system A. These results are similar to the results of references [15][20]. 95
Temperature (C)
90 85 80 75 70 65 60 55 50 9
10
11
12
13
14
15
16
Time (Hour) Panel Temp (System A) Panel Temp (System B) Figure (5): Comparison of Solar panel temperature for both designs at the angle of 45o 8
5.2 Air velocity: The air velocity is an important parameter for calculating the performance of solar chimney. Incident solar radiation is motivated heat transfer throughout the body of the fluid with a performing change in the air density. The buoyancy effect causes natural flow of the air relative to the system test solid walls. The fluid increased under the effect of buoyancy forces to reach the outlet of the solar chimney. Figure (6) shows a comparison between the average velocities of the two systems. The average velocity is defined as the summation of four readings at the same time divided by its number as [21]: 4
vav vi / 4
(5)
i 1
It was noticed that the average velocity and solar radiation increased with time. Towards the end of the day, the outlet air temperature reached its minimum value because of the reduction in the absorbed radiation and an increase in heat losses. As can be observed from Figure 6, the speed levels in design A were greater than those in design B. This was because the glass cover increased absorbed solar energy. These results correspond to the results of previous researchers such as[8][22]. 0.7
Velocity( m/s)
0.6 0.5 0.4 0.3 0.2 0.1 0 9
10
11
12
13
14
15
16
Time (Hour) Average velocity (system A) Average Velocity (system B) Fig.(6) Variation of average air velocity for two systems during the day. Figure (7) shows the variation of air velocity at the base of the solar chimney (Position of the wind turbine). It was observed in Figure (7) that the experimental value of this velocity reached its maximum at 2 p.m. and tapered off during the end of the operation period for both designs. This trend is typical of classical solar chimneys [23][24].The glass cover of the system A reduced the heat losses to the surrounding and inhibited radiation losses from the solar cell. This variation 9
in radiation energy creates the "greenhouse effect" in the chimney by increasing the energy absorbed inside the solar chimney. Figure 8 shows the changing in air velocity along the air duct from the entrance to the chimney exit. This figure also confirmed that the speed levels in design A were higher than the other design. It was also noticed that the velocity of air increased as it passed through the air duct as a result of its absorbing energy from the absorber surface (solar cell) in both systems [4]. This conclusion is useful in determining the optimal location of the wind turbine [25]. 0.7
Velocity (m/s)
0.6 0.5 0.4 0.3 0.2 0.1 9
10
11
12
13
14
15
16
Time (Hour) Vchin (System A) Vchin (System B) Figure (7): Variation of air velocity at the entrance of the chimney for two systems
Velocity (m/s)
0.6 0.5 0.4 0.3 0.2 0.1 0 0.5
1
1.6
3.6
Path (m) System A
System B
Figure (8): Changing the air velocity along the air duct 5.3 Electrical Power: Figure (9) indicates that the electrical power was produced from the PV panels on each of the two systems. It can be observed that the electrical power in system B was higher, which reached 108 W because the solar chimney provided a cooling effect to the PV panel which increased its electrical power output efficiency. In contrast, system A had a higher thermal gain due to the 10
greenhouse effect and presence of the glass cover which decreased its electrical power output efficiency so that it had lower power than system B which reached 79 W. It shows that the electrical power is a function of solar radiation and heat gain. Increasing panel temperature leads to a drop in the electrical energy when the temperature of the panel becomes higher than the design temperature (reference temperature equal to 25 oC). This agrees with experimental results of [26][27]. The presence of the glass cover also reduces the solar radiation incident on the surface of the solar cell.
Electrical Power (W)
120 100 80 60 40 20 0 9
10
11
12
13
14
15
16
Time (Hour) System (A) System (B) Figure (9): The variation of electrical power produced from the two systems. 5.4 Kinetic power: Figure (10) explains the converted power from thermal to kinetic. It was noticed that system A has a higher kinetic power than system B because the great thermal gain made the air less dense in turn increased the air velocity. The solar radiation was transmitted through the glass cover of system A and it heated the solar cell below. The heat was transferred from the solar cell in the form of convection and radiation to the air above it. The cold air entered the system and was heated by the hot ground (Solar cell). In both new designs, the solar energy can be used as an auxiliary source of electrical power. Therefore, the beneficial energy in both designs consisted of wind energy and energy produced from solar cells. Table 2 shows the amount of power produced during the day. It was observed that the daily power rate produced in system B was higher than the amount of energy produced in model A. It was noted that if a real model is built, the power from the wind is the major part of the power produced from the new design and play an important role in generating power. 11
Kinetic Power (W)
0.018 0.016 0.014 0.012 0.01 0.008 0.006 0.004 0.002 0 9
10
11
12
13
14
15
16
Time (Hour) K.E (System A)
K.E (System B)
Figure (10): Kinetic energy of the two systems during the day Table 2: Electrical, Kinetic, Useful power (Watt) for two systems Time
System A
System B
9
Kinetic power 0.006
Electrical power 18.2
Total power 18.206
Kinetic power 0.004
Electrical power 41.82
Total power 41.824
10
0.008
32.85
32.858
0.004
75.768
75.772
11
0.010
37.44
37.450
0.007
104.39
104.397
12
0.016
79.98
79.996
0.009
108.78
108.789
13
0.006
64.41
64.416
0.004
108.78
108.784
14
0.009
34.408
34.417
0.008
95.85
95.858
15
0.004
30.756
30.760
0.006
52.8
52.806
16
0.003
7.317
7.320
0.003
16.65
16.653
0.008
38.170
38.178
0.006
75.605
75.610
Daily average
6. Conclusions: In the present article, the performance of a new design of solar chimney was discussed. From the results of the previous sections, the following results were obtained: 1- The temperature of the solar cell for system A with a glass cover recorded its highest value at 90 oC, while the solar cell of system B reached its ultimate value (67 oC) at 12 noon. 12
2- The speed levels in design A were greater than those in design B. This is because the glass cover increased the absorbed solar energy. 3- The electrical power in system B was higher than system A which reached 108 W because the solar chimney provided a cooling effect to the PV panel which increased its electrical power output efficiency. 4- The daily useful power produced in system B was higher than the amount of energy produced in model A. Also, it was noted that the major part of the power was produced by solar cells. 7- References: [1]
O. K. Ahmed and A. Ahmed Hassen, Principle of Renewable energies, First edit. Baghdad: Foundation of technical education, 2011.
[2]
M. A. Al-Dabbas, “A performance analysis of solar chimney thermal power systems,” Therm. Sci., vol. 15, no. 3, pp. 619–642, 2011.
[3]
J. Martí-Herrero and M. R. Heras-Celemin, “Dynamic thermal simulation of a solar chimney with PV modules,” Int. Conf. “Passive Low Energy Cool. Built Environ., no. May, pp. 891–896, 2005.
[4]
Z. Zou, H. Gong, J. Wang, and S. Xie, “Numerical Investigation of Solar Enhanced Passive Air Cooling System for Concentration Photovoltaic Module Heat Dissipation,” J. Clean Energy Technol., vol. 5, no. 3, pp. 3–8, 2017.
[5]
M. Sh-eldin, K.Sopian, F. O. Alghoul, A. N. Abdlla, A. M, and A. Muftah, “The Optimum Solar Chimney Structure to Improve Photovoltaic System Efficiency,” Comput. Appl. Environ. Sci. Renew. Energy, pp. 261–266, 2010.
[6]
T. Mekhail, A. Rekaby, M. Fathy, M. Bassily, and R. Harte, “Experimental and Theoretical Performance of Mini Solar Chimney Power Plant,” J. Clean Energy Technol., vol. 5, no. 4, pp. 294–298, 2017.
[7]
A. Al-taaie and A. J. Jubear, “Numerical Simulation of the Collector Angle Effect on the Performance of the Solar Chimney Power Plant,” Al-Khwarizmi Eng. J., vol. 12, no. 2, pp. 79–89, 2016.
[8]
S. J. Yelpale, P. M. M. Wagh, and P. N. N. Shinde, “Integration of Solar chimney with PV Panel for Improving Performance of Polycrystalline Solar PV system,” Int. J. Emerg. Technol. Adv. Eng., vol. 4, no. 8, pp. 669–674, 2014.
[9]
A. O. Chikere, H. H. Al-Kayiem, and Z. A. A. Karim, “Review on the enhancement 13
techniques and introduction of an alternate enhancement technique of solar chimney power plant,” J. Appl. Sci., vol. 11, no. 11, pp. 1877–1884, 2011. [10] A. Koonsrisuk and T. Chitsomboon, “Mathematical modeling of solar chimney power plants,” Energy, vol. 51, pp. 314–322, 2013. [11] P. Tongbai and T. Chitsomboon, “Enhancements of Roof Solar Chimney Performance for Building Ventilation,” J. Power Energy Eng., vol. 2, no. 6, pp. 22–29, 2014. [12] S. Hu, D. Y. C. Leung, and M. Z. Q. Chen, “Effect of Divergent Chimneys on the Performance of a Solar Chimney Power Plant,” Energy Procedia, vol. 105, pp. 7–13, 2017. [13] M. Nakielska and K. Pawłowski, “Natural Ventilation using Windcatchers,” in Energy and Fuels, 2016, vol. 51. [14] A. B. Kasaeian, S. Molana, K. Rahmani, and D. Wen, “A review on solar chimney systems,” Renew. Sustain. Energy Rev., vol. 67, no. January, pp. 954–987, 2017. [15] O. K. Ahmed and Z. A. Mohammed, “Influence of porous media on the performance of hybrid PV/Thermal collector,” Renew. Energy, vol. 112, pp. 378–387, 2017. [16] O. Khalil Ahmed, “Experimental and numerical investigation of cylindrical storage collector (case study),” Case Stud. Therm. Eng., vol. 10, pp. 362–369, 2017. [17] O. K. Ahmed and Z. A. Mohammed, “Dust effect on the performance of the hybrid PV/Thermal collector,” Therm. Sci. Eng. Prog., vol. 3, pp. 114–122, Feb. 2017. [18] D. O. K. A. Al-Jibouri, “FEASIBILITY OF USING WIND ENERGY FOR IRRIGATION IN IRAQ,” Int. J. Mech. Eng. Technol., vol. 5, no. 5, pp. 62–72, 2014. [19] A. El-ghonemy, “Solar Chimney Power Plant with Collector,” J. Electron. Commun. Eng., vol. 11, no. 2, pp. 28–35, 2016. [20] A. Shahsavar and M. Ameri, “Experimental investigation and modeling of a direct-coupled PV/T air collector,” Sol. Energy, vol. 84, no. 11, pp. 1938–1958, 2010. [21] O. K. Ahmed, “Assessment of Wind Speed for Electricity Generation in Makhool Mountain in Iraq,” Int. J. Inven. Eng. Sci., vol. 2, no. 2, pp. 5–10, 2014. [22] P. Guo, Y. Wang, Q. Meng, and J. Li, “Experimental study on an indoor scale solar chimney setup in an artificial environment simulation laboratory,” Appl. Therm. Eng., vol. 107, no. July, pp. 818–826, 2016. [23] A. K. Husain, W. S. Mohammad, and A. Jassim Jubear, “Numerical Simulation of The 14
Influence of Geometric Parameter on The Flow Behavior in a Solar Chimney Power Plant system,” J. Eng., vol. 20, no. 8, pp. 88–108, 2014. [24] M. O. Hamdan, “Analysis of a solar chimney power plant in the Arabian Gulf region,” Renew. Energy, vol. 36, no. 10, pp. 2593–2598, 2011. [25] T. P. Fluri, “Turbine Layout for and Optimization of Solar Chimney Power Conversion Units by,” University of Stellenbosch, South Africa, 2008. [26] M. Y. Othman, S. A. Hamid, M. A. S. Tabook, K. Sopian, M. H. Roslan, and Z. Ibarahim, “Performance analysis of PV/T Combi with water and air heating system: An experimental study,” Renew. Energy, vol. 86, pp. 716–722, 2016. [27] C. G. Popovici, S. V. Hudişteanu, T. D. Mateescu, and N.-C. Cherecheş, “Efficiency Improvement of Photovoltaic Panels by Using Air Cooled Heat Sinks,” Energy Procedia, vol. 85, no. November 2015, pp. 425–432, 2016.
15
www.elsevier.com/locate/csite
PII: DOI: Reference:
S2214-157X(17)30274-5 https://doi.org/10.1016/j.csite.2017.12.008 CSITE247
To appear in: Case Studies in Thermal Engineering Received date: 24 October 2017 Revised date: 21 November 2017 Accepted date: 29 December 2017 Cite this article as: Omer Khalil Ahmed and Abdullah Sabah Hussein, New design of Solar Chimney (Case study), Case Studies in Thermal Engineering, https://doi.org/10.1016/j.csite.2017.12.008 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
New design of Solar Chimney (Case study) Dr Omer Khalil Ahmed
Abdullah Sabah Hussein
Technical Institute / Hawija
Technical College / Kirkuk
Northern Technical University
Northern Technical University
[email protected]
[email protected]
Abstract: The solar chimney power plant has a promising future in the world. Α new design of solar chimney is offered including both PV panels with solar chimney plant for electricity generation. Two experimental models of a hybrid solar chimney were built and designed (systems A&B). System (A) had a collector glass roof cover and a PV panel as an absorber with a chimney of 2 m height while system (B) is similar to system (A) but with PV panel as collector roof cover and plywood as an absorber in the base of the chimney. Two similar experimental models were built to achieve the performance of these new designs. Practical tests were conducted in Kirkuk (35° 28' latitude and 44° 24' longitude), northern Iraq. The results showed that system (A) had higher thermal gain than system B while the daily average of electrical power in system (B) was (75.6 W) higher than system (A) (79 W). This is because the high thermal gain raised the operating temperature of the PV panel which led to a decrease in its power output. The results also presented that system (A) converted thermal power to kinetic power with daily average (0.008 W) because of the great thermal gain which made air less dense in turn increased its velocity more than system (B) (0.006 W) which had lower kinetic power. The total useful power produced by the system (B) is greater than the useful power produced from the system (A).
Keywords: Solar chimney, New design, Solar cell.
1
NOMENCLATURES As
Cross-sectional area of the duct [m2]
cp
Specific heat of air [J/kg.K]
I
Current [A]
m air
Air mass flow rate [kg/sec]
Pele
Electric power produced by solar cell [W]
Pkinetic
Kinetic energy of air [W]
V
Voltage [V]
v
Air velocity [m/s]
vav
Average air velocity [m/s]
ρair
Air density [kg/m3]
1- Introduction: Energy is essential for economic and human development. Using renewable energies like solar for the production of electricity reduces pollution and saves our environment in our plant. Electricity energy can be produced from solar energy by two ways; solar cells and solar thermal power plants. Moreover, there is also another method, namely solar chimney [1]. The efficiency of solar cell is affected by the rise of the cell temperature, especially in hot climates. The rise in the temperature of the solar cells causes the reduction in their electrical efficiency. A solar chimney or solar tower is a type of passive solar design that can be used to produce electricity[2]. The diligent development of the concept has included the investigation of new methods to increase the parallel solar chimney efficiency and capacity to reduce the cost of this type of power plant. Martí-Herrero and Heras-Celemin [3] presented a theoretical study of the solar chimney for air ventilation of room, where the glass surface is replaced by the solar cell. Zou et al.[4] offered a numerical investigation of the solar enhanced passive air system. The numerical results of this system show that the novel design can keep solar cell temperature under 75 oC. Sh-eldin et al. [5] used the solar chimney technique to improve the solar cell system efficiency by using the particle swarm optimization. The numerical results show that the PV system efficiency has positively been affected by increasing the chimney height. Mekhail et al. [6] studied a small mathematical model of a solar chimney power plant to predict the output power of a bigger one. Al-Taaie et al. [7] studied a numerical model with different collecting angles (0°, 15°, and 30°) by using Fluent software. The results showed that the velocity increased and reached the maximum value at a 2
collector angle (30°). Yelpale et al. [8] carried out an experimental analysis of the solar chimney which integrated the solar panels to increase the efficiency of the solar panel by increasing the chimney height. Hussain [9] presented a new idea of hybrid geothermal/PV/Solar chimney which was suggested to be built in the south region of Libya. Geothermal hot water was pumped and circulated through pipes on the soil under the collector roof. PV was added to the system to replace the glass roof. Koonsrisuk and Chitsomboon [10] used iteration techniques to study the performance of solar chimney. The results showed that the optimum ratio between the turbine extraction pressure and available driving pressure for the solar chimney was 0.84. Tongbai and Chitsomboon [11] studied the effects of different parameters on the performance of solar chimney for building ventilation such as inclination angles, channel gaps, solar intensities, vertical chimney attachment heights and channel expanding angles. Hu et al. [12] analyzed the effects of the divergent chimney on the performance of solar chimney. Nakielska and Pawłowsk [13] offered experimental analysis on solar chimney located in Poland. Two models were used in this investigation conducted in the University Science and Technology. 2. Methodology: From the previous studies, it can be concluded that solar chimney has a low power conversion efficiency so that researchers worldwide can strive to enhance its efficiency [14]. From this point of view, the author proposes a novel approach to develop this technique. This approach includes combining following technologies: 1- Hybrid glazed PV / Solar Chimney (Fig. 1-a) 2- Hybrid PV / Solar Chimney (Fig. 1-b). In Figure (1-a), the base of the chimney is covered with solar cells and the solar radiation heats the air found in the space between the glass cover and solar cell on the ground. The resulting convection causes a hot air updraft in the tower by the chimney effect. The solar radiation is incidents of the solar cell installed on the floor of the chimney and produces the electric power. The moving air in the space between the glass cover and solar cell leads to cool the solar cell and improves its efficiency. In Figure (1-b), the glass cover in the classical solar chimneys is replaced by solar cells. These cells generate electricity and act as an absorber surface, where the air is heated by the energy radiated from the back surface for the solar cell and hot air that flows under the solar cell working to cool the solar cell. The kinetic energy of flowing air can be used to produce the electricity using the turbine. 3
(a) Hybrid glazed PV / Solar Chimney
(b) Hybrid PV / Solar Chimney Figure (1): New design of solar chimney In this study, a new way of enhancing the solar chimney by hybridizing the collector was carried out through integrated solar cell in two different experimental models. One was by using a solar PV panel as an absorber with a glass roof and the second was by using plywood as an absorber with PV panel as the covering roof. Comparing these two models to reach the best performance and maximum power output is the main object of this article. 3. Experimental setup: The experimental set-up was built and tested in Kirkuk, a city north of Iraq (Lat.35.46 oN and long 44.39 oE) at the height of 350 m from the sea level. It was orientated to the south and the tilt 4
angle of the solar cell in both systems was 35°. The collector was tested in outdoor conditions. Two experimental systems were built for this purpose. System A consisted of a solar collector with PV panel as an absorber with 148 cm long and 67 cm wide dimensions with emissivity of (ε = 0.95)[15]. The solar panel was fixed inside an aluminum box entirely insulated from three sides of the collector. The collector had a 4 mm thick transparent glass cover mounted at the distance 10 cm from the solar cell to gain the maximum amount of energy from the solar radiation as shown in Figure 2. The glass was fitted with a mounting sealant to block air leakage. A rectangular duct was attached vertically to the collector to work as a chimney 200 cm long and 67 cm wide dimensions. The duct (chimney ) was totally insulated with glass wool which had a thermal conductivity of (k = 0.46 W/m.K)[16]. The dimensions of area duct in both systems were 67 cm * 10 cm. System B had the same dimensions and experimental setup as system A with two major differences that the glass roof was replaced with PV panel while the base of the duct was replaced with plywood that had a thermal conductivity (k = 0.12 W/m.K)[17] as shown in Figure 3. Twenty digital temperature sensors were fixed on the two systems in selected places as shown in Figure 2a and 3a. Five sensors were placed along air stream for each system; two sensors were fixed on the glass cover for system A while the other two were fixed on the PV panel for system B. Two sensors were also fixed on the absorber (PV panel) for system A and the other two on the absorber (plywood) for system B. The last two sensors were fixed on the back of insulation for each system to find the thermal power. Four holes for each design were used to measure the air velocity along the air duct as shown Figures 2a and 3a. The specifications of the solar cell modules used are shown in Table 1. Table 1: Solar cell specification of the PV/T air collector
Subject
Unit
Maximum power (Pmax)
150W
Maximum voltage (Vmax)
17.9 V
Maximum current (Imax)
8.38 A
Open voltage (Voc)
21.9 V
Short circuit current (Isc)
9.01 A
Module size
1480*670*35 mm
Cell type
Poly-crystalline Silicon 5
(a)Photographic picture
(b) Schematic diagram
Figure (2): System (A)
(a) Photographic picture
(b) Schematic diagram Figure (3): System (B) 6
For the air velocity, a hot wire anemometer type (MY-300) was used to measure the velocity of the air stream through four holes in the duct of each system as shown in Figures 2b and 3b. The purpose was to find the kinetic power. For electric current and voltage measurements from the PV panel, two multi-meters type (SM-20) fixed on each system were used to measure the electric current and one multi-meter to measure the voltage while small light bulbs were used to reach the PV panel peak load. The electricity generated by the cells was stored in batteries. The voltage and current were measured by using DC voltmeter and digital ammeter, respectively as shown in Figure (4). Solar radiation was measured by the solar power meter [Solar meter SM206] which was put at the same height as the solar cell.
Figure (4): Solar panel with accessories 4. Mathematical calculations: In order to compare between the two systems (A and B), it is necessary to compute their output power which has two different forms that include electrical and kinetic energies: A– The electrical power: It is expressed by[15]:
Pele V .I
(1)
B- The kinetic Power: It is obtained from the flow speed ( vair ) and expressed by[18]: 1 2 air .v air m 2
(2)
m air air .vair .As
(3)
Pkinetic
Where:
7
C- The total useful power: The total useful power from the solar chimney can be expressed as a summation of the electrical power output Pele from the solar cell and kinetic energy Pkinetic of flowing air[19]: Pusefull Pele Pkinitic
(4)
5. Results and discussions: Both systems were set facing south direction with the inclined angle to gain the maximum solar radiation readings taken at clear days. At the beginning of each test, the glass cover and solar cell were thoroughly cleaned. The measurements of temperature and flow rate of air were recorded at each hour. The experiment was conducted for four months from March to July 2017. 5.1 Solar cell temperature: Figure (5) shows the variation of solar cell temperature for two designs (A and B), when they were tilted at an angle of 45 with the horizontal line. It was noticed in the two models that there was an increase in the temperature of the solar cell from the beginning of the test until it reached the highest temperature at midday coupled with increased solar radiation values. The temperature of the cell surface then began to decrease due to the decrease in the amount of solar energy and the increase in heat losses in the afternoon. The temperature of solar cell for system A with a glass cover recorded its highest value at 90 oC while solar cell of system B reached its ultimate value (67 oC) at 12 noon. The existence of the glass layer leads to the increasing temperature of the solar cell in system A. These results are similar to the results of references [15][20]. 95
Temperature (C)
90 85 80 75 70 65 60 55 50 9
10
11
12
13
14
15
16
Time (Hour) Panel Temp (System A) Panel Temp (System B) Figure (5): Comparison of Solar panel temperature for both designs at the angle of 45o 8
5.2 Air velocity: The air velocity is an important parameter for calculating the performance of solar chimney. Incident solar radiation is motivated heat transfer throughout the body of the fluid with a performing change in the air density. The buoyancy effect causes natural flow of the air relative to the system test solid walls. The fluid increased under the effect of buoyancy forces to reach the outlet of the solar chimney. Figure (6) shows a comparison between the average velocities of the two systems. The average velocity is defined as the summation of four readings at the same time divided by its number as [21]: 4
vav vi / 4
(5)
i 1
It was noticed that the average velocity and solar radiation increased with time. Towards the end of the day, the outlet air temperature reached its minimum value because of the reduction in the absorbed radiation and an increase in heat losses. As can be observed from Figure 6, the speed levels in design A were greater than those in design B. This was because the glass cover increased absorbed solar energy. These results correspond to the results of previous researchers such as[8][22]. 0.7
Velocity( m/s)
0.6 0.5 0.4 0.3 0.2 0.1 0 9
10
11
12
13
14
15
16
Time (Hour) Average velocity (system A) Average Velocity (system B) Fig.(6) Variation of average air velocity for two systems during the day. Figure (7) shows the variation of air velocity at the base of the solar chimney (Position of the wind turbine). It was observed in Figure (7) that the experimental value of this velocity reached its maximum at 2 p.m. and tapered off during the end of the operation period for both designs. This trend is typical of classical solar chimneys [23][24].The glass cover of the system A reduced the heat losses to the surrounding and inhibited radiation losses from the solar cell. This variation 9
in radiation energy creates the "greenhouse effect" in the chimney by increasing the energy absorbed inside the solar chimney. Figure 8 shows the changing in air velocity along the air duct from the entrance to the chimney exit. This figure also confirmed that the speed levels in design A were higher than the other design. It was also noticed that the velocity of air increased as it passed through the air duct as a result of its absorbing energy from the absorber surface (solar cell) in both systems [4]. This conclusion is useful in determining the optimal location of the wind turbine [25]. 0.7
Velocity (m/s)
0.6 0.5 0.4 0.3 0.2 0.1 9
10
11
12
13
14
15
16
Time (Hour) Vchin (System A) Vchin (System B) Figure (7): Variation of air velocity at the entrance of the chimney for two systems
Velocity (m/s)
0.6 0.5 0.4 0.3 0.2 0.1 0 0.5
1
1.6
3.6
Path (m) System A
System B
Figure (8): Changing the air velocity along the air duct 5.3 Electrical Power: Figure (9) indicates that the electrical power was produced from the PV panels on each of the two systems. It can be observed that the electrical power in system B was higher, which reached 108 W because the solar chimney provided a cooling effect to the PV panel which increased its electrical power output efficiency. In contrast, system A had a higher thermal gain due to the 10
greenhouse effect and presence of the glass cover which decreased its electrical power output efficiency so that it had lower power than system B which reached 79 W. It shows that the electrical power is a function of solar radiation and heat gain. Increasing panel temperature leads to a drop in the electrical energy when the temperature of the panel becomes higher than the design temperature (reference temperature equal to 25 oC). This agrees with experimental results of [26][27]. The presence of the glass cover also reduces the solar radiation incident on the surface of the solar cell.
Electrical Power (W)
120 100 80 60 40 20 0 9
10
11
12
13
14
15
16
Time (Hour) System (A) System (B) Figure (9): The variation of electrical power produced from the two systems. 5.4 Kinetic power: Figure (10) explains the converted power from thermal to kinetic. It was noticed that system A has a higher kinetic power than system B because the great thermal gain made the air less dense in turn increased the air velocity. The solar radiation was transmitted through the glass cover of system A and it heated the solar cell below. The heat was transferred from the solar cell in the form of convection and radiation to the air above it. The cold air entered the system and was heated by the hot ground (Solar cell). In both new designs, the solar energy can be used as an auxiliary source of electrical power. Therefore, the beneficial energy in both designs consisted of wind energy and energy produced from solar cells. Table 2 shows the amount of power produced during the day. It was observed that the daily power rate produced in system B was higher than the amount of energy produced in model A. It was noted that if a real model is built, the power from the wind is the major part of the power produced from the new design and play an important role in generating power. 11
Kinetic Power (W)
0.018 0.016 0.014 0.012 0.01 0.008 0.006 0.004 0.002 0 9
10
11
12
13
14
15
16
Time (Hour) K.E (System A)
K.E (System B)
Figure (10): Kinetic energy of the two systems during the day Table 2: Electrical, Kinetic, Useful power (Watt) for two systems Time
System A
System B
9
Kinetic power 0.006
Electrical power 18.2
Total power 18.206
Kinetic power 0.004
Electrical power 41.82
Total power 41.824
10
0.008
32.85
32.858
0.004
75.768
75.772
11
0.010
37.44
37.450
0.007
104.39
104.397
12
0.016
79.98
79.996
0.009
108.78
108.789
13
0.006
64.41
64.416
0.004
108.78
108.784
14
0.009
34.408
34.417
0.008
95.85
95.858
15
0.004
30.756
30.760
0.006
52.8
52.806
16
0.003
7.317
7.320
0.003
16.65
16.653
0.008
38.170
38.178
0.006
75.605
75.610
Daily average
6. Conclusions: In the present article, the performance of a new design of solar chimney was discussed. From the results of the previous sections, the following results were obtained: 1- The temperature of the solar cell for system A with a glass cover recorded its highest value at 90 oC, while the solar cell of system B reached its ultimate value (67 oC) at 12 noon. 12
2- The speed levels in design A were greater than those in design B. This is because the glass cover increased the absorbed solar energy. 3- The electrical power in system B was higher than system A which reached 108 W because the solar chimney provided a cooling effect to the PV panel which increased its electrical power output efficiency. 4- The daily useful power produced in system B was higher than the amount of energy produced in model A. Also, it was noted that the major part of the power was produced by solar cells. 7- References: [1]
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Influence of Geometric Parameter on The Flow Behavior in a Solar Chimney Power Plant system,” J. Eng., vol. 20, no. 8, pp. 88–108, 2014. [24] M. O. Hamdan, “Analysis of a solar chimney power plant in the Arabian Gulf region,” Renew. Energy, vol. 36, no. 10, pp. 2593–2598, 2011. [25] T. P. Fluri, “Turbine Layout for and Optimization of Solar Chimney Power Conversion Units by,” University of Stellenbosch, South Africa, 2008. [26] M. Y. Othman, S. A. Hamid, M. A. S. Tabook, K. Sopian, M. H. Roslan, and Z. Ibarahim, “Performance analysis of PV/T Combi with water and air heating system: An experimental study,” Renew. Energy, vol. 86, pp. 716–722, 2016. [27] C. G. Popovici, S. V. Hudişteanu, T. D. Mateescu, and N.-C. Cherecheş, “Efficiency Improvement of Photovoltaic Panels by Using Air Cooled Heat Sinks,” Energy Procedia, vol. 85, no. November 2015, pp. 425–432, 2016.
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