numerical simulation on the … int. j. mech. eng. & rob. res. 2014 adarsh s and ajith c menon,...

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Int. J. Mech. Eng. & Rob. Res. 2014 Adarsh S and Ajith C Menon, 2014

NUMERICAL SIMULATION ON THE PERFORMANCEEVALUATION OF A SOLAR UPDRAFT TOWER

Adarsh S1* and Ajith C Menon1

*Corresponding Author: Adarsh S,[email protected]

The solar updraft towers (solar chimney) are non-conventional power plant with hightechno-economic importance. In this work, the performance evaluation of a solar updraft toweris carried out based on the parameters such as roof angle, inlet height and for different irradiationvalues using ANSYS Fluent 14.0. The results are presented in the form of contour plots ofvelocity and temperature. The effects of angle of attack, inlet height and irradiation are discussedin this work.

Keywords: Solar updraft towers, Performance evaluation, Roof angle, ANSYS

INTRODUCTIONThe energy demand is increasing day by daydue to the increase in population andindustrialization. The depletion of conventionalenergy sources such as hydrocarbon fuels, depromotion of atomic energy sources andassociated pollution factors demands theresearch in the field of renewable/non-conventional sources of energy. To meet thecurrent energy crisis solar energy is playingan important role. There are many ways tocapture power from this non-conventionalsource of energy. The important method issolar power plants for producing electricity, forthose solar panels are using since decades.There is an alternative for producing power

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Int. J. Mech. Eng. & Rob. Res. 2014

1 Marian Engineeering College, Kazhakuttom, Thiruvananthapuram, India.

from solar energy and it is known as solarupdraft tower. It is a green energy producingsystem worked based on natural flow of air.

The solar updraft tower consists of threeessential elements-glass roof reflector-chimney/tower and wind turbine. Solarradiation heats the air beneath a very widegreenhouse-like roofed collector structuresurrounding the central base of a very tallchimney tower. The resulting convectioncauses a hot air updraft in the tower bythe chimney effect or stack effect. This airflowdrives wind turbines placed in the chimneyupdraft or around the chimney base toproduce electricity. The system is shown inFigure 1.

Research Paper

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The collector constructed mainly by usingtransparent PVC sheets or glass. It is mainlyplaced several meters above the ground. Itmay be sloped from inlet to base of the towerand the solar radiation reflecting inside thecollector provides internal reflection andcauses the temperature difference by meansof greenhouse effect. The tower act as athermal engine in solar updraft tower powerplant. The velocity of the updraft or buoyancydriven flow is mainly determined by the heightof the tower and the velocity of the air will beperpendicular to the height of the tower. Theturbine converts the mechanical output in theform of rotational energy into electricity withthe help of a generator coupled with the turbine.The turbine used is pressure staged turbineThe main principles used in this system areGreen house effect, Buoyancy and Naturalconvection.

From literature survey it is clear that the mainparameters which lead to the key efficiency ofthe Solar Updraft Power Plant System(SUPPS) are chimney height and collectorarea. Several scientists have studied on the

performance parameter of this SUPPS inwhich the main studies are in 2014 Padki andSherif (1992) studied mathematically thethermo fluid analysis and also studied thegeometrical and environmental behaviour andfound that the cylindrical chimney andcircumferential collector can achievemaximum efficiency. In 2013, Bernardes et al.(2003) provided fundamental studies for theSpanish prototype in which the energybalance, design criteria and cost analysis werediscussed and reported preliminary test resultsof the solar chimney power plant. Schlaichet al. (2005) developed a comprehensivethermal and technical analysis to estimate thepower output and examine the effect of variousambient conditions and structural dimensionson the power output. Schlaich (1995)presented theory, practical experience, andeconomy of solar chimney power plant to givea guide for the design of 200 MW commercialsolar chimney power plant systems then Minget al. (2010) presented a thermodynamicanalysis of the solar chimney power plant andadvanced energy utilization degree to analyzethe performance of the system, which canproduce electricity day and night. So from allthese surveys it is clear that the parameters ofthe system is having prominent role inperformance characteristics. Only few havestudied on the roof top angle, inlet height andalso different irradiation values on this system.In this project ,the numerical simulation of aSUPPS is done to study the influence of theabove said parameters.

PROBLEM DEFINITIONThe effect of roof top angle for 0, 5, 10, 15values, then for various inlet height values(12.7, 15,17, 19.5) cm and for various

Figure 1: Schematic Diagram of SolarUpdraft Tower

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irradiation values (700, 800, 900, 1000) W/m2

is studied. The model is a small prototype ofsolar updraft tower which has a dimensionshown in Figure 2.

The Figure 2 shows the dimensions andgeometry of the system and it has a towerheight of 194.6 cm, collector diameter of 150cm, inlet height of 19.5 cm and is changed tovarious values like 12.7, 15, 17 and 19.5 cmand the angle values changes from 0-15 (0,5, 10, 15) values and the effects are studied.The modeling has done with ICEM CFDsoftware and numerical simulations areconducted using a commercial CFD packageANSYS FLUENT 14.

NUMERICAL MODELLINGThe Finite Volume Method (FVM) is one of themost versatile discretization techniques usedin CFD. The main governing equations usedare continuity equation, momentum equationand energy equation.

1. Continuity equation

0

i

i

xu

t

2. Momentum equation

j

ij

ij

jii

xxpTTpg

xuu

tu

3. Energy equation

j

jpp

xTuc

tTc

j

jj

iij

jj xpu

tpT

xu

xT

x

4. Turbulent kinetic energy k

ij

kux

kt

bkj

effkj

GGxk

x

5. Equation for energy dissipation

ij

uxt

k

CGCGk

Cxx bk

jeff

j

2

231

For the present study, a two dimensionalsteady state analysis is considered. The RNGk-e turbulence model is used and the radiationmodel used is DO(Discrete Ordinates). TheDO model is used since the solar radiationreaches on the semi transparent collectorcover as an radiation beam from outside thecomputational domain that is simulated usingthe solar ray tracing algorithm. Thesimulations are conducted for almost 7000-8000 iterations and solved for a convergencevalue of 10^-6.

Figure 2: Dimensions of System

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RESULTS AND DISCUSSIONGrid Independent StudyGrid independent study is performed forgetting a favourable number of mesh for themodel selected. Grid independent study is didfor five grids, named Grid 1, Grid 2, Grid 3,Grid 4, Grid 5 and Grid 6 whose description isexplained in the table below.

The Figure 4 shows the maximumtemperature variation at different angles andfor different irradiation values.it is shown thatthe maximum temperature in the domainincreases with increase in angle andirradiation values. For maximum value ofirradiation the temperature will be maximumand also temperature increases with increasein angle, since the surface area is more forradiation to incident and thus the temperaturealso increases.

S. No. Grid Number of Cells Velocity (m/s)

1. Grid 1 6822 0.657

2. Grid 2 8029 0.66

3. Grid 3 9234 0.675

4. Grid 4 10443 0.67

5. Grid 5 11650 0.6701

6. Grid 6 14064 0.67

Table 1: Description of Grids Studied

Grid independence is obtained at Grid 4since velocity at Grid 4, Grid 5 and Grid 6 isexactly similar and we consider the lessnumber of mesh in order to reduce the solvingtime and also to get the results faster, so gridnumber and the results are analyzed for allthree parameters, mainly with the influence ofperformance parameters like velocitytemperature and mass flow rate of the systemwith respect to the change in angle values,change in inlet height and change in irradiationvalues. The results are plotted in curves as wellas in colour contours.

Effect of AngleFigure 3 shows the mass flow rate for variousirradiation incident on top at various roof topangles and it is shown that there is an optimumangle for maximum mass flow rate for differentirradiation values. Here 7.5 is the optimumangle for all irradiation values to obtainmaximum mass flow rate.

Figure 3: Mass Flow Variation with Angleand Irradiation

Figure 4: Temperature Variation withAngle and Irradiation

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Effect of IrradiationThe Figure 5 shows the temperaturedistribution on top wall is given, it is clear thatfrom the graph the temperature on the top wallis increasing with increase in irradiation values(700, 800, 900, 1000)W/m^2, the temperatureincreases since more energy is falling on thewall as the irradiation increases. In bottom wallalso the temperature distribution is increasingwith increase in irradiation values as shown inFigure 6. It is because the radiation causes

the temperature to increase temperature inbottom wall.

Effect of Inlet HeightThis chart shows that at low values of inletheight the flow is not initiating since not muchair is replacing as the inlet height is less andthe mass flow rate is increasing at 17 cm andis only getting till angle 10 and in angle 15 it isnot getting because of the recirculation formedinside the domain in angle 15 as the space

Figure 5: Temperature Distribution on TopWall with Various Irradiation Values

Figure 6: Temperature Distribution onBottom Wall with Various Irradiation

Values

Figure 7: Effect of Mass Flow Rate withChange in Inlet Height

Figure 8: The Recirculation Formed Insidethe Domain by Increasing Angle

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inside the domain is more but increasing inletheights facilitating in good flow because ofmore air coming inside the system. Therecirculation zone is shown in Figure 8.

The velocity contours for different anglevalues at a fixed irradiation is shown in Figure9. It shows that the velocity is increasing asthe angle increasing but in angle 15 case therecirculation zone formed inside the domainis large and it causes an obstruction to the flow.

The results shows that there is an optimumvalue of angle for maximum mass flow rate theefficiency is approximately same for allirradiation values. The inlet height has aprominent role in the output gain, mass flowrate is obtained only at higher inlet heights and

for optimum angle value, at higher anglesrecirculation inside the domain will obstruct theflow. The efficiency that we obtained in thiscase is of range 40-48%, the main losses arereverse flow in inlet and recirculation zoneinside domain.

CONCLUSIONThe efficiency of the system increases withincrease in the collector area. The mass flowrate is always proportional to the height of thetower. With the increase in roof angle, therecirculation zone near the tower inletdecreases, whereas the recirculation zone inthe collector domain decreases. It is found thatthe main loss occurring in the system is throughthe radiation loss at inlet. With large scale Solar

Figure 9: The Velocity Contours for Different Angle Values at a Fixed Irradiation Value

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updraft tower we can achieve good results. Theefficiency of the tower in this system is in therange of 40-48%.

REFERENCES1. Bernardes M A, dos S, Voss A and

Weinrebe G (2003), “Thermal andTechnical Analyzes of Solar Chimneys”,Sol. Energy, Vol. 75, pp. 511-524.

2. Ming T Z, Zheng Y, Liu W and Pan Y(2010), “Simple Analysis on the ThermalPerformance of Solar Chimney PowerGeneration Systems”, J. Energy Inst.,Vol. 83, No. 1.

3. Padki M M and Sherif S A (1992), “AMathematical Model for Solar Chimneys”,in Proceedingsof 1992 InternationalRenewable Energy Conference, Vol. 1,pp. 289-294, Amman, Jordan.

4. Schlaich J (1995), The Solar Chimney,Edition Axel Menges, Stuttgart, Germany.

5. Schlaich J, Bergermann R, Schiel W andWeinrebe G (2005), “Design ofCommercial Solar Updraft TowerSystems e Utilization of Solar InducedConvective Flows for Power Generation”,J. Sol. Energy Eng., Vol. 127, p. 117.

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