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Experimental investigation and CFD simulation studies of a laboratory scale solar chimney for power generation
Sustainable Energy Technologies and Assessments 13 (2016) 13–22
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Sustainable Energy Technologies and Assessments journal homepage: www.elsevier.com/locate/seta
Original Research Article
Experimental investigation and CFD simulation studies of a laboratory scale solar chimney for power generation Shiv Lal a,⇑, S.C. Kaushik a, Ranjana Hans b a b
Centre for Energy Studies, Indian Institute of Technology Delhi, 110016, India Department of Electrical Engineering, Appeejay Satya University, Gurgaon 122102, India
a r t i c l e
i n f o
Article history: Received 4 July 2014 Revised 13 November 2015 Accepted 13 November 2015
Keywords: Solar chimney Collector Power generation technology Energy and exergy
a b s t r a c t In this communication, thermal performance of the laboratory type solar chimney for power generation is studied for a warm and semi-arid climate of Kota, India. Mathematical and Computational Fluid Dynamics (CFD) modeling are used to calculate the specific parameters, energetic and exergetic efficiencies. The predicted results are validated through experimental studies and statistical assessment shows that predicted temperatures are observed very close to measured data. The temperature variation along the collector height is also examined. The maximum air temperature and velocity in the collector area are found to be 42.4 °C and 12.2 m/s respectively at 1400 h of the typical day. The maximum solar radiation is measured to be 820 W/m2 at 1200 h. The maximum ambient temperature is found to be 42 °C at 1400 h of the typical day. The temperature of the collector surface is approximately 4–6 °C higher than the hot air temperature at the peak hours of the typical day. The high energy efficiency is estimated to be 3.5% at 1200 h of the day and reduced by morning and evening hours. The exergy efficiency is also low and found to be 8% at the same time. The turbine installation location is decided by the maximum velocity point, which is estimated with the help of CFD simulation as to be 0.25–1 m inside the chimney pipe. The effect of chimney height, inlet temperature and the solar radiation is also evaluated and model equation for performance measurements is developed. The diameter of the chimney is very low as 0.2032 m and velocity generated at the entry of the chimney (exit of collector) is appropriate to produce small power and it can be used as a small power plant. Published by Elsevier Ltd.
Introduction The limited reserves and higher cost of conventional energy sources are responsible for research and development of renewable energy technology. The sustainable development of the human society is directly linked with the renewable energy options. The solar chimney based power plant seems to be an alternative for electrical power generation, recently which is gaining wider interest in the research community around the world. It consists of three functional components, i.e. collector, chimney and turbo-generator. The solar insolation is first utilized in order to heat in the canopy type collector, within which air is heated and collected at the center and it flows through a large chimney due to stack effect. The hot air provides the requisite kinetic energy which is converted into electricity by an air turbo-generator. ⇑ Corresponding author. Tel.: +91 8447300192. E-mail address: [email protected] (S. Lal). http://dx.doi.org/10.1016/j.seta.2015.11.005 2213-1388/Published by Elsevier Ltd.
The first pilot solar chimney power plant was commissioned in 1982–83, by the Ministry of Research and Technology of the Federal Republic of Germany at Manzanares. It consists of 46,000 m2 collector area (244 m diameter), and 195 m chimney height and 10 m chimney diameter [1]. The air velocity was measured as 15 m/s at no load condition, and produced 50 kW energy in the peak hour (36 kW base load). Haaf [2] performed the test results of the plant and in continuation of that the theoretical modeling have been developed to evaluate the collector efficiency, and pressure losses due to friction in turbine section. Schlaich [3] has written a concept book of solar chimney power plant (The solar chimney: electricity from the sun) and reported solar chimney for hundreds of MW power generation. It gives the design data for Indian climatic conditions, but there is no progress in this field in India till now. India is having suitable climate and barren land for this project. The SCPP performance is substantially influenced by two parameters, i.e. geometry and solar insolation and these two have
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S. Lal et al. / Sustainable Energy Technologies and Assessments 13 (2016) 13–22
Nomenclature Ach AColl C Cd Cp FR G Hch hf Is K _ m Pa DP ch Qs Q_ u Ra Ta Tf To
chimney inlet area, m2 collector area or absorber surface area, m2 constant as recommended for natural draft as 0.0342 coefficient of discharge 0.65–0.70 for high rise chimney specific heat, J/kg K heat removal factor gravitational acceleration, m/s2 chimney height, m convective heat transfer coefficient, W/m2 K solar insolation on collector, W/m2 constant of integral approximation mass flow rate of air in collector, Kg/s atmospheric pressure, N/m2 total pressure drop, N/m2 total solar energy input in the collector, W useful heat (solar energy used to increase the energy of air, W Rayleigh number ambient air temperature, K average hot air temperature, K collector outlet temperature, K
been examined by various researchers. Mullet [4] developed analytical method to design and evaluate overall performance and efficiency of SCPP. Some governing equations are developed for chimney performance [5] and then extensive studies are carried out by [6,7]. Padki and Sherif [8] considered the effect of geometrical parameters on chimney performance. Dai et al. [9] performed a case study of SCPP in North-Western region of China and concluded that SCPP could produce 110–195 kW electricity with 200 m height, 10 m diameter, and 1, 96, 270 m2 collector area. Krisst [10], Kulunk [11], Pasumarthi and Sherif [12,13] and Zhou et al. [14] developed pilot/scale experimental models of SCPP. Nizetic et al. [15] analyzed the feasibility of solar chimney power plant as an environmentally acceptable option for island countries in the Mediterranean region. They found out that 550 m chimney height and 82 m diameter with 1250 m collector diameter would produce 2.8–6.2 MW power with a total investment cost of 60.0 Mo€. The solar chimney is not yet exploited commercially for electric power generation although electricity produced from SCPP is economically viable and cheaper than conventional power production [16]. Gupta and Kaushik [17] evaluated the exergetic performance of a solar air heater and presented its parametric study, whereas an exergy of pump is also considered. Thermodynamic study with exergy analysis was carried out by Petela [18] and his simplified model consisted of solar collector, chimney and turbine. Some pilot SCPP models were developed at various places in the world for e.g. Spain, China, and Botswana etc. Bejan et al. [19] used constructional theory for optimizing the geometry of solar chimney to accomplish the aim of increasing power production over the occupied area by the power plant and analyzed the height/radius ratio, maximum mass flow rate, maximum power under fixed area and volume constraints and losses. A numerical investigation through computational fluid dynamic is carried out by Xu et al. [20] and validated the CFD model by experimental results. The study on interior for different-climatic zones are suitable for plant cultivation under a large solar chimney; also being carried out. Asnadi and Ladievardi [21] performance analyzed the proposed SCPP in Iran and found that it can produce 10 to 28 MWh/month of electrical power. Ghalamchi et al. [22] reviewed the experimental studies on geometrical and climate effect on the performance of a small solar chimney.
DT UL V ch V_ v ol
W ch Wo Wt
a gcoll gtg go
hch hcoll
qa qch qcoll s e
temperature difference between average hot air temperature and ambient temperature, K overall heat transfer coefficient, W/m2 K maximum air velocity at chimney inlet, m/s volume flow rate, m3/s power contained in the flow at bottom of chimney, W total power produced by the plant, W maximum mechanical power produced by wind turbine, W absorptivity of the absorber surface collector efficiency generator efficiency that contains the transmission and generator efficiency overall efficiency of the SCPP chimney nozzle angle, Degree collector (canopy) slope angle, degree ambient air density, kg/m3 air density in the chimney, kg/m3 density of the air at the outlet of the solar collector, kg/m3 transmissivity of the cover constant of mean temperature approximation
A lot of experimental work is required in the various countries. There is an immense opportunity for R&D, particularly in the large size collector system, chimney system design and low wind power generators. This paper deals with the experimental analysis in the series of experimental evaluation of solar chimney for power generation in India. The energy and exergy analysis are also presented. Material and methods Modeling of solar chimney power plant Many factors affecting the solar chimney power plant performance like the type of collector, materials used in collectors, soil contents under the collector, collector diameter, chimney height/ radius, performance of turbo-generator and its controlling system. Here a simple method is reported which gives us an account of solar collection, useful work and electrical power output to evaluate the overall performance of SCPP. The overall efficiency of the SCPP is given by [3]:
go ¼ gcoll gch gtg
ð1Þ
Total power produced by the plant is given by:
W o ¼ Q s go
ð2Þ
Eqs. (1) and (2) shows that the efficiency and power output, where the overall efficiency is the multiplication of collector, chimney and turbo-generator efficiencies. To find out the individual efficiencies of individual system some equation’s are derived in the next sections. Solar collector The energy balance equation for solar collector is given as:
_ p ðT o T i Þ Q_ u ¼ F R AColl ðs a Is U L DTÞ ¼ mc
ð3Þ
The assumption for the above equation is that the maximum heat loss considering through upward direction approximately 40–60%. So the maximum scope is available in cover material and layers to reduce the heat loss. Solar insolation on the collector is given by,
Q s ¼ Is Acoll
ð4Þ
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S. Lal et al. / Sustainable Energy Technologies and Assessments 13 (2016) 13–22
The efficiency of the solar collector is given by Eqs. (3) and (4) as:
gcoll ¼ gcoll ¼
Q_ u Qs
ð5Þ
F R AColl ðs a Is U L DTÞ Is AColl
gcoll ¼ F R s a: where F R ¼ h
U L DT Is
ð6Þ ð7Þ
hf þU L hf
i, where F R is the heat removal factor
and it comes in the derivation of Q_ u for a simple solar collector. And, hf is the convective heat transfer coefficient between absorber surface and the air,
DT ¼ T f T a
ð8Þ
where T f is the average hot air temperature in the collector (K). The assumption for the above equation is that the absorber surface temperature is equal to the average air flow temperature. The measurement of average temperature of air and absorber surface is meticulous; So Zhou et al. [14] proposed a simple formula to calculate the average hot air temperature. Again, the T f can be given by [14],
T f ¼ eT a þ ð1 eÞT o
ð9Þ
where T o is the collector outlet temperature and e denotes the constant of mean temperature approximation and its value is recommended as 0.25 by [23]. Solar chimney The hot air exit from collector enters into the chimney and stack effect generates between the inlet and outlet of the chimney where height plays an important role. In other words chimney height develops induced flow. The orthographic view of chimney is shown in Fig. 1(b). The chimney is a pressure tube with low friction because of its optimal surface volume ratio. The maximum kinetic energy is found out at the bottom of chimney because it works as a throat of the venturi. The chimney efficiency is given by [3]
W gch ¼ _ ch Qu
W ch ¼ DPch V ch Ach
ð11Þ
where DPch is the pressure difference (N/m ) produced between chimney base (Section ‘‘Introduction”) and the ambient (Section ‘‘ Material and Methods”) [see Fig. 1(b)], and product of velocity and area is represented by volume flow rate. The assumption for Eq. (11) is that the total pressure energy is converted into kinetic energy. Pressure difference due to induced flow of air it is given by, 2
_ p 2mC _ p U L Acoll þ2mC
where W ch = power contained in the flow at bottom of chimney (W). And, W ch is given by Schlaich [3] as follows,
ð10Þ
Z
DPch ¼ g
Hch
0
ðqa qch ÞdH
ð12Þ
where g, qa , qch are gravitational acceleration (m/s2), ambient air density (kg/m3) and air density in the chimney respectively. Solution of Eq. (12) is given by,
DPch ¼ g qch K Hch ðT o T a Þ=T a
ð13Þ
Schaich [3] proposed the equation for maximum velocity (buoyancy flow with effect of density change by temperature) is given by,
V ch ¼
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2gHch ðT o T a Þ=T a
ð14Þ
From Eqs. (11) and (14), Wch is given by:
W ch ¼ g qch V ch Ach Hch ðT o T a Þ=T a
ð15Þ
Substituting the value of V cmax from Eq. (14) into Eq. (15) and we get, 3=2 W ch ¼ 1:414 qch Ach g 3=2 Hch
To Ta Ta
3=2 ð16Þ
The value of W ch from Eq. (15) and Q u from Eq. (3) putting in Eq. (10) and we get the value of chimney efficiency. The theoretical efficiency of chimney is given by,
1:414 qch Ach g 3=2 Hch 3=2
gch ¼
T o T a Ta
3=2 ð17aÞ
F R Acoll ðs a Is U L DTÞ
Or it can be written as,
gch ¼
ðT o T a Þ p 1:414 qcoll Ach g 3=2 H3=2 ch T o T a
F R Acoll ðs a Is U L DTÞ
3=2
ð17bÞ
Turbine Low velocity vertical air turbines have always been located at the base of solar chimney where air force converts maximum 2/3 times of air flow into mechanical power and finally produces electrical power [3]. The maximum mechanical power produced by the turbine is given by [3],
Wt ¼
2 W ch 3
ð18Þ
The above equation can further be expressed by, W ch ¼ gch :Q_ u ; ch [9] and Q_ u ¼ gcoll Acoll Is putting into Eq. from Eq. (10), gch ¼ CgH p T o (18) and get:
Wt ¼
2 g g Hch Acoll Is 3 coll C p T a
ð19Þ
Again the electrical power produced by generator is given by [9],
Fig. 1. Photographic view of solar chimney.
gg ¼
We Wt
ð20Þ
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We ¼
gtg ¼
2 g g g Hch Acoll Is 3 coll g C p T a
ð21aÞ
We W ch
ð21bÞ
where gtg ¼ gt gg = combined turbine and generator efficiency and W e is the electrical power output, assumed gg ¼ 90%. Exergy analysis The exergy of solar radiation enters into the solar collector and it converts into thermal (heat) exergy. The exergy of solar radiation input to the solar chimney is given by [18] as follows:
"
Ex;rad
4 # 4 To 1 To þ ¼ I s As 1 3 Ts 3 Ts
ð22Þ
ð23Þ
Exergy utilization efficiency to raise the inside air temperature is as follows:
Wsolar ¼
Ex;heat Ex;rad
ð24Þ
Inside the solar chimney, hot air has the exergy potential to generate the chimney effect. This process is considered as a steady flow process and exergy can be derived by the simplification of the Petela Equation. The exergy balance in a steady flow process is given as:
Ex;in þ Ex;heat ¼ Ex;out þ Ex;work þ I_
ð25Þ
There is no work out in this solar chimney then,
Ex;work ¼ 0
ð26Þ
Exergy of air entering to the chimney is given by:
_ in ½ðhin ho Þ T o ðsin so Þ Ex;in ¼ m
ð27Þ
Exergy out from the chimney outlet is given by:
_ out ½ðhout ho Þ T o ðsout so Þ Ex;out ¼ m
ð28Þ
_i¼m _ out ¼ m _ f. And mass balance is m Substituting the values of heat exergy, work exergy, input and exit exergy, we get:
I_ ¼ Ex;in þ Ex;heat Ex;out
ð29Þ
Above equation can be written as,
I_ ¼
1
To Tf
_ f ½ðhout hin Þ T o ðsout sin Þ Q_ u m
ð30Þ
ð31Þ ð32Þ
And again the total irreversibility of the system is given by
I_ ¼
T out _ T _ f cp ðT out T in Þ þ m _ f T o cp ln out Qu m Tf T in Pout _ f R ln m P in
ð34Þ
Then entropy generation in the system is given by,
T out _ T _ f cp ðT out T in Þ þ m _ f T o cp ln out Qu m 1 Tf T in Pout _ f R ln ð35Þ m Pin
1 S_ gen ¼ To
The exergy efficiency of the chimney is given by,
wchimney ¼
Ex;out Ex;in þ Ex;heat
ð36Þ
But in actual practice supplied exergy is only heat exergy, than
Ex;out Ex;heat
wchimney ¼ 1
ð37Þ I_
Ex;heat
; or
T o S_ gen wchimney ¼ 1 1 TTo Q_ u
ð38Þ
ð39Þ
f
The net exergy efficiency of solar chimney can be written as:
wnet ¼ Wsolar wchimney
ð40Þ
CFD model development A two dimensional model of experimental solar chimney has been developed in design modular associated with ANSYS workbench 14.0, where the half model is used for simulation because it is axis symmetry. The model has been meshed in meshing software and mesh also refined at the glass and soil surface for improving the results at edges. The total number of nodes and elements observed 55063 and 53742 respectively. The grid independence test have also been performed and found that, the medium mesh given smooth and better results than coarse mesh. On the other hand the fine mesh and medium mesh performed similar results and fine mesh takes higher converging time, so we have selected medium mesh for the study. The DELL workstation of 64 GB RAM with parallel processing and 1 TB hard disk has been used for the simulation in FLUENT. There are two models working in the simulation as one for the turbulent fluid flow and another for radiation heat transfer. The Realizable k-epsilon turbulent model with standard wall functions have been used to simulate the two dimensional model of the solar chimney and discrete ordinance (DO) model has been used for the radiant heat transfer in the system. Experimental set-up
And total rate of the energy received by the solar chimney absorber area is expressed by Eq. (4). In above equation of irreversibility, ðhout hin Þ and ðsout sin Þ can be find out by,
hout hin ¼ cp ðT out T in Þ T out Pout R ln sout sin ¼ cp ln T in Pin
I_ ¼ T o S_ gen
wchimney ¼
Exergy output is follows:
To Q_ u Ex;heat ¼ 1 Tf
From the definition of irreversibility
1
ð33Þ
The experimental solar chimney was constructed on the solar park of Rajasthan technical university Kota, India (25.18°N 75.83°E). Figs. 1 and 2, shows photographic and schematic view of the experimental solar chimney for power generation situated on experimental site. The Solar chimney power plant consists of three major components such as: Solar Collector, Chimney and turbine. The collector cover and chimney are made through transparent polyethylene and PVC respectively. The dimensions of the experimental solar chimney for power generation are presented in Table 1. The primary structure was built with steel strips, channels and angles and 12 legs are grouted in earth surface. The whole structure consists of two circular areas numbered as 1 and 2 and polyethylene was fixed over the frame. The inner frame and external frames are tilted at two different angles 10° and 5°.
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S. Lal et al. / Sustainable Energy Technologies and Assessments 13 (2016) 13–22
20
Chimney 800
Transparent cover
40
5 Air flow
Air flow
40
1200 All dimension in cm Fig. 2. Schematic diagram of solar chimney power plant.
Table 1 Specification of solar chimney set-up.
Table 2 Uncertainties of the experimental parameters.
S. No.
Description
Dimension
Parameters
Units
Uncertainty
1. 2. 3. 4. 5. 6.
Collector diameter Collector area Chimney height Chimney diameter Angle of inclination for 1, 2 Height of opening (inlet)
12 m 113.04 m2 8m 0.2032 m 10°, 5° 0.4 m
Uncertainty in the temperature measurement (K-type thermocouples used) Uncertainty in the solar energy measurement
°C
0:17
W/ m2 m/s
0:17
Instrumentation and measurement The experimental study is conducted during a typical day of May 2013. The global and diffuse solar radiation incident on a horizontal surface were measured by Solar Pyranometer (Model: SP Lite-2 type; Make: Kipp & Zonen, Delfi Poland) respectively. Calibrated K-type thermocouples are used to measure the air temperature at various positions in SCPP. Hot wire anemometer is used to measure the velocity at the inlet and outlet of the collector. Experimental uncertainty The error or uncertainties during the experiments may be occurring due to instrumentation, sensor, climatic condition,
Uncertainty in the air velocity measurement
0:17
calibration method, method observation and testing, and method of evaluation. The total experimental uncertainties of different parameters during the experiment on SCPP are presented in Table 2. Results and discussion Validation of predicted data through experimental data A Computational Fluid Dynamics (CFD) model has been developed to simulate solar chimney for power generation. The CFD results are validated with experimental results. From validated study, it is revealed that CFD results are found to be very close to experimental results with Mean Biased Error (MBE) (0.14–0.22)
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S. Lal et al. / Sustainable Energy Technologies and Assessments 13 (2016) 13–22
and Root Mean Square Error (RMSE) (0.15–0.26). It is found that predicted temperatures are very close to the experimental temperatures, as shown in Fig. 3.
Absorber earth surface
Cover
Collector outlet
48 46 44
Approximately 300–325 clear sky days are observed in Kota district of Rajasthan state. The environmental parameters such as global, diffuse solar radiation, and ambient air temperature are monitored for a typical day of May 2013. The Fig. 4 reveals the relation between global, diffuse solar radiation, and ambient air temperature of the typical day of May month. It is found that, the maximum global radiation is observed by 820 W/m2 at 12:00 h, whereas the maximum diffuse solar radiation is 270 W/m2. As far as the ambient air temperature is concerned, it found to be maximum of 42.2 °C at 14:00 h and minimum by 34.1 °C at 4:00 h.
Temperature, °C
Observation and measurement of environmental parameter
42 40 38 36 34 32 30 0:00
2:00
4:00
6:00
8:00
10:00
12:00
14:00
16:00
18:00
20:00
22:00
Time, hour Fig. 5. Average temperatures at various positions in solar chimney.
Observation and measurement temperatures at various positions height=0.2m
height=0.35m
height=0.00
48 46 44
Temperature, °C
Fig. 5 reveals the temperature variation for a typical sunny day. As far as the absorber surface temperature; it has to be observed maximum by 46.3 °C at 14:00 h and minimum (41.5 °C) at 8:00 h. The cover temperature also measured by 40.5 °C (maximum) at 14 h and 34.6 °C (minimum) at 5 h. In solar collector surface temperature at 12 noon of a typical day is also reported along with particular heights as 0.2 m and 0.35 m, and noted that the surface temperature is always higher than the other heights. When going to the upward side the air temperature will be reduced; but
42 40 38 36 34
To_exp
To_cfd
Tsoil_exp
Tsoil_cfd
Tc_exp
Tc_cfd
32 30 -6
-5
-4
-3
47 45
Temperature, °C
-2
-1
0
1
2
3
4
5
6
Horizontal distance from centre, m Fig. 6. Air temperature at 0.00, 0.2 and 0.35 m above the ground inside collector.
43 41 39 37 35 33
8:00
9:00
10:00
11:00
12:00
13:00
14:00
15:00
16:00
Time, hour Fig. 3. CFD model validation.
Ambient air
70
Global Solar radiation
at the closure of the cover plate, it is found to be higher than adjacent layer of free stream air which is shown in Fig. 6. The absorber surface temperature is higher means the air temperature inside the collector will be higher, so the improvement and modification is required in surface color and absorption capacity to improve the absorber surface temperature. The contour of temperature variation inside the solar chimney is presented in the Fig. 7. It shows the maximum temperature at the bottom (soil) and reduces it within the vertical upward direction. Collector outlet velocity
Difffused Radiation
800 65 700 600
55
500
50
400
45
300
40
200
35
100 0
30 0:00
2:00
4:00
6:00
8:00
10:00
12:00
14:00
16:00
18:00
20:00
22:00
Time, hour Fig. 4. Solar radiation and temperatures for a typical day of May 2013.
Radiation, W/m2
Temperature, °C
60
The chimney velocity is equivalent to the collector outlet velocity. The CFD and experimental collector outlet velocity for a typical day is presented in Fig. 8. The maximum and minimum theoretical velocity at the exit of the chimney is observed to be 12.2 m/s and 6 m/s respectively (between 8 AM and 4 PM) for a sunny day. This velocity can be used for small power generation and it is validated the statement ‘‘The air velocity in a solar chimney is required minimum 15 m/s for commercial generation” [2]. And again the velocity and volume flow rate can be increased by increasing the collector and chimney dimensions. The CFD results for outlet velocity is higher than the experimental value, it happens because we have not considered the surface roughness and other physical and thermal properties of the soil. In CFD a general dry soil (thermal conductivity = 0.5 W/m. K) is being used for the simulation. The velocity vector and contour is shown in Figs. 9 and 10 respec-
S. Lal et al. / Sustainable Energy Technologies and Assessments 13 (2016) 13–22
19
Fig. 7. Temperature contour of solar chimney for power generation in morning hour.
Vi
Vo_exp
The chimney efficiency can be directly calculated by the above equation in percentage.
Vo_cfd
16 14
Effect of chimney inlet temperature on mass flow rate of hot air
Velocity, m/s
12 10 8 6 4 2 0
8:00
9:00
10:00
11:00
12:00
13:00
14:00
15:00
16:00
Time, hour Fig. 8. Velocity variation for a typical day of May 2013.
The effect of chimney inlet temperature on mass flow rate inside the chimney is shown in Fig. 12. Therefore model equation for chimney velocity from [5] is used to calculate the mass flow rate. Actually the chimney inlet temperature means the collector outlet temperature. It is seen that, the mass flow rate in chimney increases with increase in chimney outlet temperature, it means chimney inlet temperature increases due to high solar radiation. Finally, it increases the velocity in the system and increases the mass flow in the chimney. The mass flow or volume flow with high kinetic energy produces high power, which can be used to generate the electrical power by the help of wind turbine. Effect of solar radiation on output power for existing set-up
tively. It is seen that the maximum velocity found at vena contracta which appears slightly above the chimney entering edge. It is estimated by 0.25–1 m inside the chimney. So, it is the best location for a turbine, where found highest kinetic energy of wind. Effect of chimney height on efficiency We used the model equation of [9] and found the effect of chimney height. From the regression method, it is found that the chimney efficiency is solely depends on chimney height, it is shown in Fig. 11. A polygenic equation is developed for minimum error for maximum value of R2 = 0.999. The modeled equations is presenting as follows.
gch ¼ 0:0027H4ch 0:0347H3ch þ 0:1616H2ch 0:2868Hch þ 0:1775
The chimney power, turbine power and electrical power production by the designed solar chimney directly depends upon the availability of solar radiation. Fig. 13 shows the effect of solar radiation on all three powers. From the regression method, it is found that the dependence of power on solar radiation is not linear, it is found polygenic. The equation is developed for minimum error for maximum value of R2 = 0.99. The modeled equations are presented as follows.
W ch ¼ 0:1415I3s þ 1:1016I2s 0:4107Is þ 1:7176 W t ¼ 0:0943I3s þ 0:7344I2s 0:2738Is þ 1:145 W e ¼ 0:0849I3s þ 0:661I2s 0:2464Is þ 1:0305
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Fig. 9. Velocity vector inside the solar chimney for power generation.
Fig. 10. Velocity contour inside the solar chimney for power generation.
Energetic and exergetic efficiency of SCPP: The overall energetic and exergetic efficiencies are reported in Fig. 14. The highest energy efficiency is evaluated by 3.53% at 1200 h of the day and reduced by morning and evening hours. The exergy efficiency is calculated by 8% at the same time of
1200 h. The energy efficiency is lower than exergy efficiency because the exergy losses in the turbo- generator are not included in the calculation; else the turbo-generator and solar chimney efficiency both are included in the energy efficiency evaluation. The exergy and energy efficiency of the solar chimney power plant are mainly depending on the performance of solar collector. In this
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3.50
Collector energy efficiency
3.00
25
Exergy efficiency
20
2.00
Efficiency, %
Efficiency, %
2.50
Overall energy efficiency
1.50 1.00
15
10
0.50 5
0.00
5
10
15
20 50 100 Height of Chimney, m
200
500
1000 0 8:00
9:00
Fig. 11. Effect of chimney height on chimney efficiency.
10:00
11:00
12:00
13:00
14:00
15:00
16:00
Time, hour Fig. 14. Energy and exergy efficiency of SCPP for a particular day.
Hch=10
Hch=50
Conclusion
Hch=100 m
1.40
Mass flow rate, kg/s
1.20 1.00 0.80 0.60 0.40 0.20 0.00
40
45
50
55
Chimney outlet temperature, °C Fig. 12. Effect of chimney outlet temperature on mass flow rate.
Wch
Wt
We
Poly. (Wch)
Poly. (Wt)
Poly. (We)
780
790
820
10
Power out put, W
9 8 7 6 5 4 3 2 1 0
562
744
Solar radiaon, W/m2 Fig. 13. Effect of solar radiation on power output.
study, the efficiency of solar collector is found to be in the range of 10–19.7%. The highest collector efficiency has been estimated as 19.7% at mid of the day and minimum at morning. The energy and exergy is observed low for low temperature application of solar energy system. The maximum losses occur through collector cover and it can be reduced by proper selection of collector cover material and number of glazing. The efficiency of the system can be slightly increased by improving the absorption capacity and application of thermal energy storage system.
The solar chimney power plant technology is not new and global researchers contributing so many research papers, but no commercial power plants exist till now. This paper contributes experimental evolution in this field, and in situ data is collected from laboratory type solar chimney for micro power generation installed at the Rajasthan Technical University, Kota India. It is concluded that the high rise chimney and a lot of collector area (in Kilometers) required for MW power generation and it is a feasible solution for sustainable development. The ambient temperature, solar radiation and wind speed is being measured as environmental parameters and maximum global solar radiation is found to be 820 W/m2 at 12 h of typical days. The higher solar radiation produces a sufficient greenhouse temperature to develop stack effect which improves the wind flow speed in upward direction. The maximum earth surface temperature is measured with approximately 46.3 °C at 2 PM, and it has to develop a hot air at the collector outlet of 42.36 °C temperature and 12.2 m/s velocities. The air temperature profile at different height reveals the heating zones under the collector cover. The sufficient air velocity is generated for small power generation. The energy efficiency is found very low ( 3:5%) and the exergy efficiency also estimated stumpy ( 8%) because of the sun temperature is very high and trapping temperature is very low. The CFD model is been developed for demonstration of temperature and velocity profile inside the solar chimney. The turbine installation place is decided by the maximum velocity point, which is estimated by 0.25–1 m inside the chimney pipe. The effect of chimney height, inlet temperature and the solar radiation are also evaluated and develop the model equation for performance measurements, where producing higher mass flow rate is due to solar radiation is the measure constraint of higher performance. Acknowledgements The author gratefully acknowledges University College of Engineering, Rajasthan Technical University, Kota, Rajasthan (India) and IIT Delhi (India), for sponsorship under a quality improvement program of government of India. References [1] Haff W, Friedrich K, Mayr G, Schlaich J. Solar chimney. Int J Sol Energy 1983;2 [Part 1: 3–20, Part 2: 141–161]. [2] Haaf W. Solar chimneys – Part II: preliminary test results from the manzanares pilot plant. Int J Sol Energy 1984;2(2):141–61.
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[3] Schlaich J. The solar chimney: electricity from the sun. Stuttgart: Axel Menges publisher; 1995. [4] Mullett LB. Solar chimney – overall efficiency, design and performance. Int J Ambient Energy 1987;8:35–40. [5] Padki MM, Sherif SA. Fluid dynamics of solar chimneys. Forum on industrial applications of fluid mechanics, vol. 70. New York: ASME; 1988. p. 43–6. [6] Padki MM, Sherif SA. Solar chimney for medium-to-large scale power generation. In: Proceedings of the Manila international symposium on the development and management of energy resources. Philippines: Manila; 1989. [7] Padki MM, Sherif SA. On a simple analytical model for solar chimneys. Int J Energy Res 1999;23:345–9. [8] Padki MM, Sherif SA. A mathematical model for solar chimneys. In: Proceedings of the international renewable energy conference in faculty of engineering and technology. University of Jordan; 1992. [9] Dai YJ, Huang HB, Wang RZ. Case study of solar chimney power plants in Northwestern regions of China. Renewable Energy 2003;28:1295–304. [10] Krisst RJK. Energy transfer system. Altern Sources Energy 1983;63:8–11. [11] Kulunk HA. Prototype solar convection chimney operated under humid conditions. In: Veiroglu TN, editor. Proceedings of the 7th Miami international conference on alternative energy sources, vol. 162. 1985. [12] Pasumarthi N, Sherif SA. Experimental and theoretical performance of a demonstration solar chimney model – Part 1: mathematical model development. Int J Energy Res 1998;22(3):277–88. [13] Pasumarthi N, Sherif SA. Experimental and theoretical performance of a demonstration solar chimney model – Part II: experimental and theoretical results and economic analysis. Int J Energy Res 1998;22(5):443–61.
[14] Zhou X, Yang J, Xiao B, Hou G. Simulation of a pilot solar chimney thermal power generating equipment. Sci Technol 2007;32:1637–44. [15] Nizetic S, Ninic N, Klarin B. Analysis and feasibility of implementing solar chimney power plants in the Mediterranean region. Society 2008;33: 1680–90. [16] Fluri TP, Pretorius JP, Dyk CV, Backstro TWV, Zijl GPAGV. Cost analysis of solar chimney power plants. Sol Energy 2009;83:246–56. [17] Gupta MK, Kaushik SC. Exergetic performance evaluation and parametric studies of solar air heater. Energy 2008;33:1691–702. [18] Petela R. Thermodynamic study of a simplified model of the solar chimney power plant. Sol Energy 2009;83:94–107. [19] Koonsrisuk A, Lorente S, Bejan A. International journal of heat and mass transfer constructal solar chimney configuration. Int J Heat Mass Transfer 2010;53(1–3):327–33. [20] Xu H, Karimi F, Yang M. Numerical investigation of thermal characteristics in a solar chimney projects. J Sol Energy Eng 2014;136. http://dx.doi.org/10.1115/ 1.4024742 [011008 (1–7)]. [21] Asnadi A, Ladievardi SM. Solar chimney power plant performance in Iran. Renewable Sustainable Energy Rev 2012;16(5):3383–90. [22] Ghalamchi M, Kasaeian A, Ghalamchi M. Experimental study of geometrical and climate effects on the performance of a small solar chimney. Renewable Sustainable Energy Rev 2015;43:425–31. [23] Hirunlabh J. Study of natural ventilation of houses by a metallic solar wall under tropical climate. Renewable Energy 1999;18(1):109–19.
Contents lists available at ScienceDirect
Sustainable Energy Technologies and Assessments journal homepage: www.elsevier.com/locate/seta
Original Research Article
Experimental investigation and CFD simulation studies of a laboratory scale solar chimney for power generation Shiv Lal a,⇑, S.C. Kaushik a, Ranjana Hans b a b
Centre for Energy Studies, Indian Institute of Technology Delhi, 110016, India Department of Electrical Engineering, Appeejay Satya University, Gurgaon 122102, India
a r t i c l e
i n f o
Article history: Received 4 July 2014 Revised 13 November 2015 Accepted 13 November 2015
Keywords: Solar chimney Collector Power generation technology Energy and exergy
a b s t r a c t In this communication, thermal performance of the laboratory type solar chimney for power generation is studied for a warm and semi-arid climate of Kota, India. Mathematical and Computational Fluid Dynamics (CFD) modeling are used to calculate the specific parameters, energetic and exergetic efficiencies. The predicted results are validated through experimental studies and statistical assessment shows that predicted temperatures are observed very close to measured data. The temperature variation along the collector height is also examined. The maximum air temperature and velocity in the collector area are found to be 42.4 °C and 12.2 m/s respectively at 1400 h of the typical day. The maximum solar radiation is measured to be 820 W/m2 at 1200 h. The maximum ambient temperature is found to be 42 °C at 1400 h of the typical day. The temperature of the collector surface is approximately 4–6 °C higher than the hot air temperature at the peak hours of the typical day. The high energy efficiency is estimated to be 3.5% at 1200 h of the day and reduced by morning and evening hours. The exergy efficiency is also low and found to be 8% at the same time. The turbine installation location is decided by the maximum velocity point, which is estimated with the help of CFD simulation as to be 0.25–1 m inside the chimney pipe. The effect of chimney height, inlet temperature and the solar radiation is also evaluated and model equation for performance measurements is developed. The diameter of the chimney is very low as 0.2032 m and velocity generated at the entry of the chimney (exit of collector) is appropriate to produce small power and it can be used as a small power plant. Published by Elsevier Ltd.
Introduction The limited reserves and higher cost of conventional energy sources are responsible for research and development of renewable energy technology. The sustainable development of the human society is directly linked with the renewable energy options. The solar chimney based power plant seems to be an alternative for electrical power generation, recently which is gaining wider interest in the research community around the world. It consists of three functional components, i.e. collector, chimney and turbo-generator. The solar insolation is first utilized in order to heat in the canopy type collector, within which air is heated and collected at the center and it flows through a large chimney due to stack effect. The hot air provides the requisite kinetic energy which is converted into electricity by an air turbo-generator. ⇑ Corresponding author. Tel.: +91 8447300192. E-mail address: [email protected] (S. Lal). http://dx.doi.org/10.1016/j.seta.2015.11.005 2213-1388/Published by Elsevier Ltd.
The first pilot solar chimney power plant was commissioned in 1982–83, by the Ministry of Research and Technology of the Federal Republic of Germany at Manzanares. It consists of 46,000 m2 collector area (244 m diameter), and 195 m chimney height and 10 m chimney diameter [1]. The air velocity was measured as 15 m/s at no load condition, and produced 50 kW energy in the peak hour (36 kW base load). Haaf [2] performed the test results of the plant and in continuation of that the theoretical modeling have been developed to evaluate the collector efficiency, and pressure losses due to friction in turbine section. Schlaich [3] has written a concept book of solar chimney power plant (The solar chimney: electricity from the sun) and reported solar chimney for hundreds of MW power generation. It gives the design data for Indian climatic conditions, but there is no progress in this field in India till now. India is having suitable climate and barren land for this project. The SCPP performance is substantially influenced by two parameters, i.e. geometry and solar insolation and these two have
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S. Lal et al. / Sustainable Energy Technologies and Assessments 13 (2016) 13–22
Nomenclature Ach AColl C Cd Cp FR G Hch hf Is K _ m Pa DP ch Qs Q_ u Ra Ta Tf To
chimney inlet area, m2 collector area or absorber surface area, m2 constant as recommended for natural draft as 0.0342 coefficient of discharge 0.65–0.70 for high rise chimney specific heat, J/kg K heat removal factor gravitational acceleration, m/s2 chimney height, m convective heat transfer coefficient, W/m2 K solar insolation on collector, W/m2 constant of integral approximation mass flow rate of air in collector, Kg/s atmospheric pressure, N/m2 total pressure drop, N/m2 total solar energy input in the collector, W useful heat (solar energy used to increase the energy of air, W Rayleigh number ambient air temperature, K average hot air temperature, K collector outlet temperature, K
been examined by various researchers. Mullet [4] developed analytical method to design and evaluate overall performance and efficiency of SCPP. Some governing equations are developed for chimney performance [5] and then extensive studies are carried out by [6,7]. Padki and Sherif [8] considered the effect of geometrical parameters on chimney performance. Dai et al. [9] performed a case study of SCPP in North-Western region of China and concluded that SCPP could produce 110–195 kW electricity with 200 m height, 10 m diameter, and 1, 96, 270 m2 collector area. Krisst [10], Kulunk [11], Pasumarthi and Sherif [12,13] and Zhou et al. [14] developed pilot/scale experimental models of SCPP. Nizetic et al. [15] analyzed the feasibility of solar chimney power plant as an environmentally acceptable option for island countries in the Mediterranean region. They found out that 550 m chimney height and 82 m diameter with 1250 m collector diameter would produce 2.8–6.2 MW power with a total investment cost of 60.0 Mo€. The solar chimney is not yet exploited commercially for electric power generation although electricity produced from SCPP is economically viable and cheaper than conventional power production [16]. Gupta and Kaushik [17] evaluated the exergetic performance of a solar air heater and presented its parametric study, whereas an exergy of pump is also considered. Thermodynamic study with exergy analysis was carried out by Petela [18] and his simplified model consisted of solar collector, chimney and turbine. Some pilot SCPP models were developed at various places in the world for e.g. Spain, China, and Botswana etc. Bejan et al. [19] used constructional theory for optimizing the geometry of solar chimney to accomplish the aim of increasing power production over the occupied area by the power plant and analyzed the height/radius ratio, maximum mass flow rate, maximum power under fixed area and volume constraints and losses. A numerical investigation through computational fluid dynamic is carried out by Xu et al. [20] and validated the CFD model by experimental results. The study on interior for different-climatic zones are suitable for plant cultivation under a large solar chimney; also being carried out. Asnadi and Ladievardi [21] performance analyzed the proposed SCPP in Iran and found that it can produce 10 to 28 MWh/month of electrical power. Ghalamchi et al. [22] reviewed the experimental studies on geometrical and climate effect on the performance of a small solar chimney.
DT UL V ch V_ v ol
W ch Wo Wt
a gcoll gtg go
hch hcoll
qa qch qcoll s e
temperature difference between average hot air temperature and ambient temperature, K overall heat transfer coefficient, W/m2 K maximum air velocity at chimney inlet, m/s volume flow rate, m3/s power contained in the flow at bottom of chimney, W total power produced by the plant, W maximum mechanical power produced by wind turbine, W absorptivity of the absorber surface collector efficiency generator efficiency that contains the transmission and generator efficiency overall efficiency of the SCPP chimney nozzle angle, Degree collector (canopy) slope angle, degree ambient air density, kg/m3 air density in the chimney, kg/m3 density of the air at the outlet of the solar collector, kg/m3 transmissivity of the cover constant of mean temperature approximation
A lot of experimental work is required in the various countries. There is an immense opportunity for R&D, particularly in the large size collector system, chimney system design and low wind power generators. This paper deals with the experimental analysis in the series of experimental evaluation of solar chimney for power generation in India. The energy and exergy analysis are also presented. Material and methods Modeling of solar chimney power plant Many factors affecting the solar chimney power plant performance like the type of collector, materials used in collectors, soil contents under the collector, collector diameter, chimney height/ radius, performance of turbo-generator and its controlling system. Here a simple method is reported which gives us an account of solar collection, useful work and electrical power output to evaluate the overall performance of SCPP. The overall efficiency of the SCPP is given by [3]:
go ¼ gcoll gch gtg
ð1Þ
Total power produced by the plant is given by:
W o ¼ Q s go
ð2Þ
Eqs. (1) and (2) shows that the efficiency and power output, where the overall efficiency is the multiplication of collector, chimney and turbo-generator efficiencies. To find out the individual efficiencies of individual system some equation’s are derived in the next sections. Solar collector The energy balance equation for solar collector is given as:
_ p ðT o T i Þ Q_ u ¼ F R AColl ðs a Is U L DTÞ ¼ mc
ð3Þ
The assumption for the above equation is that the maximum heat loss considering through upward direction approximately 40–60%. So the maximum scope is available in cover material and layers to reduce the heat loss. Solar insolation on the collector is given by,
Q s ¼ Is Acoll
ð4Þ
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S. Lal et al. / Sustainable Energy Technologies and Assessments 13 (2016) 13–22
The efficiency of the solar collector is given by Eqs. (3) and (4) as:
gcoll ¼ gcoll ¼
Q_ u Qs
ð5Þ
F R AColl ðs a Is U L DTÞ Is AColl
gcoll ¼ F R s a: where F R ¼ h
U L DT Is
ð6Þ ð7Þ
hf þU L hf
i, where F R is the heat removal factor
and it comes in the derivation of Q_ u for a simple solar collector. And, hf is the convective heat transfer coefficient between absorber surface and the air,
DT ¼ T f T a
ð8Þ
where T f is the average hot air temperature in the collector (K). The assumption for the above equation is that the absorber surface temperature is equal to the average air flow temperature. The measurement of average temperature of air and absorber surface is meticulous; So Zhou et al. [14] proposed a simple formula to calculate the average hot air temperature. Again, the T f can be given by [14],
T f ¼ eT a þ ð1 eÞT o
ð9Þ
where T o is the collector outlet temperature and e denotes the constant of mean temperature approximation and its value is recommended as 0.25 by [23]. Solar chimney The hot air exit from collector enters into the chimney and stack effect generates between the inlet and outlet of the chimney where height plays an important role. In other words chimney height develops induced flow. The orthographic view of chimney is shown in Fig. 1(b). The chimney is a pressure tube with low friction because of its optimal surface volume ratio. The maximum kinetic energy is found out at the bottom of chimney because it works as a throat of the venturi. The chimney efficiency is given by [3]
W gch ¼ _ ch Qu
W ch ¼ DPch V ch Ach
ð11Þ
where DPch is the pressure difference (N/m ) produced between chimney base (Section ‘‘Introduction”) and the ambient (Section ‘‘ Material and Methods”) [see Fig. 1(b)], and product of velocity and area is represented by volume flow rate. The assumption for Eq. (11) is that the total pressure energy is converted into kinetic energy. Pressure difference due to induced flow of air it is given by, 2
_ p 2mC _ p U L Acoll þ2mC
where W ch = power contained in the flow at bottom of chimney (W). And, W ch is given by Schlaich [3] as follows,
ð10Þ
Z
DPch ¼ g
Hch
0
ðqa qch ÞdH
ð12Þ
where g, qa , qch are gravitational acceleration (m/s2), ambient air density (kg/m3) and air density in the chimney respectively. Solution of Eq. (12) is given by,
DPch ¼ g qch K Hch ðT o T a Þ=T a
ð13Þ
Schaich [3] proposed the equation for maximum velocity (buoyancy flow with effect of density change by temperature) is given by,
V ch ¼
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2gHch ðT o T a Þ=T a
ð14Þ
From Eqs. (11) and (14), Wch is given by:
W ch ¼ g qch V ch Ach Hch ðT o T a Þ=T a
ð15Þ
Substituting the value of V cmax from Eq. (14) into Eq. (15) and we get, 3=2 W ch ¼ 1:414 qch Ach g 3=2 Hch
To Ta Ta
3=2 ð16Þ
The value of W ch from Eq. (15) and Q u from Eq. (3) putting in Eq. (10) and we get the value of chimney efficiency. The theoretical efficiency of chimney is given by,
1:414 qch Ach g 3=2 Hch 3=2
gch ¼
T o T a Ta
3=2 ð17aÞ
F R Acoll ðs a Is U L DTÞ
Or it can be written as,
gch ¼
ðT o T a Þ p 1:414 qcoll Ach g 3=2 H3=2 ch T o T a
F R Acoll ðs a Is U L DTÞ
3=2
ð17bÞ
Turbine Low velocity vertical air turbines have always been located at the base of solar chimney where air force converts maximum 2/3 times of air flow into mechanical power and finally produces electrical power [3]. The maximum mechanical power produced by the turbine is given by [3],
Wt ¼
2 W ch 3
ð18Þ
The above equation can further be expressed by, W ch ¼ gch :Q_ u ; ch [9] and Q_ u ¼ gcoll Acoll Is putting into Eq. from Eq. (10), gch ¼ CgH p T o (18) and get:
Wt ¼
2 g g Hch Acoll Is 3 coll C p T a
ð19Þ
Again the electrical power produced by generator is given by [9],
Fig. 1. Photographic view of solar chimney.
gg ¼
We Wt
ð20Þ
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S. Lal et al. / Sustainable Energy Technologies and Assessments 13 (2016) 13–22
We ¼
gtg ¼
2 g g g Hch Acoll Is 3 coll g C p T a
ð21aÞ
We W ch
ð21bÞ
where gtg ¼ gt gg = combined turbine and generator efficiency and W e is the electrical power output, assumed gg ¼ 90%. Exergy analysis The exergy of solar radiation enters into the solar collector and it converts into thermal (heat) exergy. The exergy of solar radiation input to the solar chimney is given by [18] as follows:
"
Ex;rad
4 # 4 To 1 To þ ¼ I s As 1 3 Ts 3 Ts
ð22Þ
ð23Þ
Exergy utilization efficiency to raise the inside air temperature is as follows:
Wsolar ¼
Ex;heat Ex;rad
ð24Þ
Inside the solar chimney, hot air has the exergy potential to generate the chimney effect. This process is considered as a steady flow process and exergy can be derived by the simplification of the Petela Equation. The exergy balance in a steady flow process is given as:
Ex;in þ Ex;heat ¼ Ex;out þ Ex;work þ I_
ð25Þ
There is no work out in this solar chimney then,
Ex;work ¼ 0
ð26Þ
Exergy of air entering to the chimney is given by:
_ in ½ðhin ho Þ T o ðsin so Þ Ex;in ¼ m
ð27Þ
Exergy out from the chimney outlet is given by:
_ out ½ðhout ho Þ T o ðsout so Þ Ex;out ¼ m
ð28Þ
_i¼m _ out ¼ m _ f. And mass balance is m Substituting the values of heat exergy, work exergy, input and exit exergy, we get:
I_ ¼ Ex;in þ Ex;heat Ex;out
ð29Þ
Above equation can be written as,
I_ ¼
1
To Tf
_ f ½ðhout hin Þ T o ðsout sin Þ Q_ u m
ð30Þ
ð31Þ ð32Þ
And again the total irreversibility of the system is given by
I_ ¼
T out _ T _ f cp ðT out T in Þ þ m _ f T o cp ln out Qu m Tf T in Pout _ f R ln m P in
ð34Þ
Then entropy generation in the system is given by,
T out _ T _ f cp ðT out T in Þ þ m _ f T o cp ln out Qu m 1 Tf T in Pout _ f R ln ð35Þ m Pin
1 S_ gen ¼ To
The exergy efficiency of the chimney is given by,
wchimney ¼
Ex;out Ex;in þ Ex;heat
ð36Þ
But in actual practice supplied exergy is only heat exergy, than
Ex;out Ex;heat
wchimney ¼ 1
ð37Þ I_
Ex;heat
; or
T o S_ gen wchimney ¼ 1 1 TTo Q_ u
ð38Þ
ð39Þ
f
The net exergy efficiency of solar chimney can be written as:
wnet ¼ Wsolar wchimney
ð40Þ
CFD model development A two dimensional model of experimental solar chimney has been developed in design modular associated with ANSYS workbench 14.0, where the half model is used for simulation because it is axis symmetry. The model has been meshed in meshing software and mesh also refined at the glass and soil surface for improving the results at edges. The total number of nodes and elements observed 55063 and 53742 respectively. The grid independence test have also been performed and found that, the medium mesh given smooth and better results than coarse mesh. On the other hand the fine mesh and medium mesh performed similar results and fine mesh takes higher converging time, so we have selected medium mesh for the study. The DELL workstation of 64 GB RAM with parallel processing and 1 TB hard disk has been used for the simulation in FLUENT. There are two models working in the simulation as one for the turbulent fluid flow and another for radiation heat transfer. The Realizable k-epsilon turbulent model with standard wall functions have been used to simulate the two dimensional model of the solar chimney and discrete ordinance (DO) model has been used for the radiant heat transfer in the system. Experimental set-up
And total rate of the energy received by the solar chimney absorber area is expressed by Eq. (4). In above equation of irreversibility, ðhout hin Þ and ðsout sin Þ can be find out by,
hout hin ¼ cp ðT out T in Þ T out Pout R ln sout sin ¼ cp ln T in Pin
I_ ¼ T o S_ gen
wchimney ¼
Exergy output is follows:
To Q_ u Ex;heat ¼ 1 Tf
From the definition of irreversibility
1
ð33Þ
The experimental solar chimney was constructed on the solar park of Rajasthan technical university Kota, India (25.18°N 75.83°E). Figs. 1 and 2, shows photographic and schematic view of the experimental solar chimney for power generation situated on experimental site. The Solar chimney power plant consists of three major components such as: Solar Collector, Chimney and turbine. The collector cover and chimney are made through transparent polyethylene and PVC respectively. The dimensions of the experimental solar chimney for power generation are presented in Table 1. The primary structure was built with steel strips, channels and angles and 12 legs are grouted in earth surface. The whole structure consists of two circular areas numbered as 1 and 2 and polyethylene was fixed over the frame. The inner frame and external frames are tilted at two different angles 10° and 5°.
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S. Lal et al. / Sustainable Energy Technologies and Assessments 13 (2016) 13–22
20
Chimney 800
Transparent cover
40
5 Air flow
Air flow
40
1200 All dimension in cm Fig. 2. Schematic diagram of solar chimney power plant.
Table 1 Specification of solar chimney set-up.
Table 2 Uncertainties of the experimental parameters.
S. No.
Description
Dimension
Parameters
Units
Uncertainty
1. 2. 3. 4. 5. 6.
Collector diameter Collector area Chimney height Chimney diameter Angle of inclination for 1, 2 Height of opening (inlet)
12 m 113.04 m2 8m 0.2032 m 10°, 5° 0.4 m
Uncertainty in the temperature measurement (K-type thermocouples used) Uncertainty in the solar energy measurement
°C
0:17
W/ m2 m/s
0:17
Instrumentation and measurement The experimental study is conducted during a typical day of May 2013. The global and diffuse solar radiation incident on a horizontal surface were measured by Solar Pyranometer (Model: SP Lite-2 type; Make: Kipp & Zonen, Delfi Poland) respectively. Calibrated K-type thermocouples are used to measure the air temperature at various positions in SCPP. Hot wire anemometer is used to measure the velocity at the inlet and outlet of the collector. Experimental uncertainty The error or uncertainties during the experiments may be occurring due to instrumentation, sensor, climatic condition,
Uncertainty in the air velocity measurement
0:17
calibration method, method observation and testing, and method of evaluation. The total experimental uncertainties of different parameters during the experiment on SCPP are presented in Table 2. Results and discussion Validation of predicted data through experimental data A Computational Fluid Dynamics (CFD) model has been developed to simulate solar chimney for power generation. The CFD results are validated with experimental results. From validated study, it is revealed that CFD results are found to be very close to experimental results with Mean Biased Error (MBE) (0.14–0.22)
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S. Lal et al. / Sustainable Energy Technologies and Assessments 13 (2016) 13–22
and Root Mean Square Error (RMSE) (0.15–0.26). It is found that predicted temperatures are very close to the experimental temperatures, as shown in Fig. 3.
Absorber earth surface
Cover
Collector outlet
48 46 44
Approximately 300–325 clear sky days are observed in Kota district of Rajasthan state. The environmental parameters such as global, diffuse solar radiation, and ambient air temperature are monitored for a typical day of May 2013. The Fig. 4 reveals the relation between global, diffuse solar radiation, and ambient air temperature of the typical day of May month. It is found that, the maximum global radiation is observed by 820 W/m2 at 12:00 h, whereas the maximum diffuse solar radiation is 270 W/m2. As far as the ambient air temperature is concerned, it found to be maximum of 42.2 °C at 14:00 h and minimum by 34.1 °C at 4:00 h.
Temperature, °C
Observation and measurement of environmental parameter
42 40 38 36 34 32 30 0:00
2:00
4:00
6:00
8:00
10:00
12:00
14:00
16:00
18:00
20:00
22:00
Time, hour Fig. 5. Average temperatures at various positions in solar chimney.
Observation and measurement temperatures at various positions height=0.2m
height=0.35m
height=0.00
48 46 44
Temperature, °C
Fig. 5 reveals the temperature variation for a typical sunny day. As far as the absorber surface temperature; it has to be observed maximum by 46.3 °C at 14:00 h and minimum (41.5 °C) at 8:00 h. The cover temperature also measured by 40.5 °C (maximum) at 14 h and 34.6 °C (minimum) at 5 h. In solar collector surface temperature at 12 noon of a typical day is also reported along with particular heights as 0.2 m and 0.35 m, and noted that the surface temperature is always higher than the other heights. When going to the upward side the air temperature will be reduced; but
42 40 38 36 34
To_exp
To_cfd
Tsoil_exp
Tsoil_cfd
Tc_exp
Tc_cfd
32 30 -6
-5
-4
-3
47 45
Temperature, °C
-2
-1
0
1
2
3
4
5
6
Horizontal distance from centre, m Fig. 6. Air temperature at 0.00, 0.2 and 0.35 m above the ground inside collector.
43 41 39 37 35 33
8:00
9:00
10:00
11:00
12:00
13:00
14:00
15:00
16:00
Time, hour Fig. 3. CFD model validation.
Ambient air
70
Global Solar radiation
at the closure of the cover plate, it is found to be higher than adjacent layer of free stream air which is shown in Fig. 6. The absorber surface temperature is higher means the air temperature inside the collector will be higher, so the improvement and modification is required in surface color and absorption capacity to improve the absorber surface temperature. The contour of temperature variation inside the solar chimney is presented in the Fig. 7. It shows the maximum temperature at the bottom (soil) and reduces it within the vertical upward direction. Collector outlet velocity
Difffused Radiation
800 65 700 600
55
500
50
400
45
300
40
200
35
100 0
30 0:00
2:00
4:00
6:00
8:00
10:00
12:00
14:00
16:00
18:00
20:00
22:00
Time, hour Fig. 4. Solar radiation and temperatures for a typical day of May 2013.
Radiation, W/m2
Temperature, °C
60
The chimney velocity is equivalent to the collector outlet velocity. The CFD and experimental collector outlet velocity for a typical day is presented in Fig. 8. The maximum and minimum theoretical velocity at the exit of the chimney is observed to be 12.2 m/s and 6 m/s respectively (between 8 AM and 4 PM) for a sunny day. This velocity can be used for small power generation and it is validated the statement ‘‘The air velocity in a solar chimney is required minimum 15 m/s for commercial generation” [2]. And again the velocity and volume flow rate can be increased by increasing the collector and chimney dimensions. The CFD results for outlet velocity is higher than the experimental value, it happens because we have not considered the surface roughness and other physical and thermal properties of the soil. In CFD a general dry soil (thermal conductivity = 0.5 W/m. K) is being used for the simulation. The velocity vector and contour is shown in Figs. 9 and 10 respec-
S. Lal et al. / Sustainable Energy Technologies and Assessments 13 (2016) 13–22
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Fig. 7. Temperature contour of solar chimney for power generation in morning hour.
Vi
Vo_exp
The chimney efficiency can be directly calculated by the above equation in percentage.
Vo_cfd
16 14
Effect of chimney inlet temperature on mass flow rate of hot air
Velocity, m/s
12 10 8 6 4 2 0
8:00
9:00
10:00
11:00
12:00
13:00
14:00
15:00
16:00
Time, hour Fig. 8. Velocity variation for a typical day of May 2013.
The effect of chimney inlet temperature on mass flow rate inside the chimney is shown in Fig. 12. Therefore model equation for chimney velocity from [5] is used to calculate the mass flow rate. Actually the chimney inlet temperature means the collector outlet temperature. It is seen that, the mass flow rate in chimney increases with increase in chimney outlet temperature, it means chimney inlet temperature increases due to high solar radiation. Finally, it increases the velocity in the system and increases the mass flow in the chimney. The mass flow or volume flow with high kinetic energy produces high power, which can be used to generate the electrical power by the help of wind turbine. Effect of solar radiation on output power for existing set-up
tively. It is seen that the maximum velocity found at vena contracta which appears slightly above the chimney entering edge. It is estimated by 0.25–1 m inside the chimney. So, it is the best location for a turbine, where found highest kinetic energy of wind. Effect of chimney height on efficiency We used the model equation of [9] and found the effect of chimney height. From the regression method, it is found that the chimney efficiency is solely depends on chimney height, it is shown in Fig. 11. A polygenic equation is developed for minimum error for maximum value of R2 = 0.999. The modeled equations is presenting as follows.
gch ¼ 0:0027H4ch 0:0347H3ch þ 0:1616H2ch 0:2868Hch þ 0:1775
The chimney power, turbine power and electrical power production by the designed solar chimney directly depends upon the availability of solar radiation. Fig. 13 shows the effect of solar radiation on all three powers. From the regression method, it is found that the dependence of power on solar radiation is not linear, it is found polygenic. The equation is developed for minimum error for maximum value of R2 = 0.99. The modeled equations are presented as follows.
W ch ¼ 0:1415I3s þ 1:1016I2s 0:4107Is þ 1:7176 W t ¼ 0:0943I3s þ 0:7344I2s 0:2738Is þ 1:145 W e ¼ 0:0849I3s þ 0:661I2s 0:2464Is þ 1:0305
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Fig. 9. Velocity vector inside the solar chimney for power generation.
Fig. 10. Velocity contour inside the solar chimney for power generation.
Energetic and exergetic efficiency of SCPP: The overall energetic and exergetic efficiencies are reported in Fig. 14. The highest energy efficiency is evaluated by 3.53% at 1200 h of the day and reduced by morning and evening hours. The exergy efficiency is calculated by 8% at the same time of
1200 h. The energy efficiency is lower than exergy efficiency because the exergy losses in the turbo- generator are not included in the calculation; else the turbo-generator and solar chimney efficiency both are included in the energy efficiency evaluation. The exergy and energy efficiency of the solar chimney power plant are mainly depending on the performance of solar collector. In this
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S. Lal et al. / Sustainable Energy Technologies and Assessments 13 (2016) 13–22
3.50
Collector energy efficiency
3.00
25
Exergy efficiency
20
2.00
Efficiency, %
Efficiency, %
2.50
Overall energy efficiency
1.50 1.00
15
10
0.50 5
0.00
5
10
15
20 50 100 Height of Chimney, m
200
500
1000 0 8:00
9:00
Fig. 11. Effect of chimney height on chimney efficiency.
10:00
11:00
12:00
13:00
14:00
15:00
16:00
Time, hour Fig. 14. Energy and exergy efficiency of SCPP for a particular day.
Hch=10
Hch=50
Conclusion
Hch=100 m
1.40
Mass flow rate, kg/s
1.20 1.00 0.80 0.60 0.40 0.20 0.00
40
45
50
55
Chimney outlet temperature, °C Fig. 12. Effect of chimney outlet temperature on mass flow rate.
Wch
Wt
We
Poly. (Wch)
Poly. (Wt)
Poly. (We)
780
790
820
10
Power out put, W
9 8 7 6 5 4 3 2 1 0
562
744
Solar radiaon, W/m2 Fig. 13. Effect of solar radiation on power output.
study, the efficiency of solar collector is found to be in the range of 10–19.7%. The highest collector efficiency has been estimated as 19.7% at mid of the day and minimum at morning. The energy and exergy is observed low for low temperature application of solar energy system. The maximum losses occur through collector cover and it can be reduced by proper selection of collector cover material and number of glazing. The efficiency of the system can be slightly increased by improving the absorption capacity and application of thermal energy storage system.
The solar chimney power plant technology is not new and global researchers contributing so many research papers, but no commercial power plants exist till now. This paper contributes experimental evolution in this field, and in situ data is collected from laboratory type solar chimney for micro power generation installed at the Rajasthan Technical University, Kota India. It is concluded that the high rise chimney and a lot of collector area (in Kilometers) required for MW power generation and it is a feasible solution for sustainable development. The ambient temperature, solar radiation and wind speed is being measured as environmental parameters and maximum global solar radiation is found to be 820 W/m2 at 12 h of typical days. The higher solar radiation produces a sufficient greenhouse temperature to develop stack effect which improves the wind flow speed in upward direction. The maximum earth surface temperature is measured with approximately 46.3 °C at 2 PM, and it has to develop a hot air at the collector outlet of 42.36 °C temperature and 12.2 m/s velocities. The air temperature profile at different height reveals the heating zones under the collector cover. The sufficient air velocity is generated for small power generation. The energy efficiency is found very low ( 3:5%) and the exergy efficiency also estimated stumpy ( 8%) because of the sun temperature is very high and trapping temperature is very low. The CFD model is been developed for demonstration of temperature and velocity profile inside the solar chimney. The turbine installation place is decided by the maximum velocity point, which is estimated by 0.25–1 m inside the chimney pipe. The effect of chimney height, inlet temperature and the solar radiation are also evaluated and develop the model equation for performance measurements, where producing higher mass flow rate is due to solar radiation is the measure constraint of higher performance. Acknowledgements The author gratefully acknowledges University College of Engineering, Rajasthan Technical University, Kota, Rajasthan (India) and IIT Delhi (India), for sponsorship under a quality improvement program of government of India. References [1] Haff W, Friedrich K, Mayr G, Schlaich J. Solar chimney. Int J Sol Energy 1983;2 [Part 1: 3–20, Part 2: 141–161]. [2] Haaf W. Solar chimneys – Part II: preliminary test results from the manzanares pilot plant. Int J Sol Energy 1984;2(2):141–61.
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