Performance analysis of solar updraft tower using Ansys fluent

The solar updraft tower (SUT) is a renewable energy power plant design concept for generating electricity by utilizing convective air flow due to low temperature heat absorption from incoming solar radiation. A parametric study on the performance power output of the SUT at various canopy angles and different insolation levels were carried out to investigate the relationship between them with respect to key parameters i.e. in terms of updraft tower velocity, temperature, pressure and height to choose turbine position for maximum air velocity absorption and energy conversion using Ansys-fluent software version 15 for analysis. The results obtained were analyzed comparatively to draw conclusions. The results identified using simulation software mentioned above were validated with earlier results and identified significant improvement in the power generated at various collector angles, when compared with the earlier works on SUT.


Introduction
Allocation Solar Updraft Tower (SUT) utilizes three basic governing principles i.e. greenhouse effect, convection and wind power generation to convert incoming solar radiation into electricity [1]. As direct and diffuse solar radiation strikes the surface of glass roof or canopy, reflection, refraction, transmission and absorption take place depending on the incidence angle and optical characteristics of the canopy which can be measured in terms of physical parameters like refractive index, thickness and extinction coefficient [2]. Apart of the transmitted radiation through canopy was absorbed while another part gets reflected back towards the canopy and undergoes further reflection [3][4][5][6]. This leads to multiple reflections in-between canopy and surface of the ground so that enhanced energy absorption takes place at the surface of the ground due to transmittance-absorptance product of the ground resulting in production of hot air beneath the canopy. The buoyant air rises up in plant chimney by natural convection and initiates forced convection by allowing more amount of air to flow with in the vicinity of canopy [7][8][9][10][11][12]. Through mixed convection, the lower part of canopy gets heated up due to presence of warm air while a part of energy is absorbed by the cooler part of the ground surface in addition to radiation exchange between warm ground surface and canopy [13][14][15][16][17][18][19][20]. Further, due to natural and forced convection, heat exchange occurs RESGEVT 2020 IOP Conf. Series: Materials Science and Engineering 937 (2020) 012022 IOP Publishing doi: 10.1088/1757-899X/937/1/012022 2 over canopy surface with adjacent ambient air. While the air parcels flow upward from canopy to chimney, its temperature increases and velocity remains constant due to increase in height of the canopy [21,22]. Finally, air parcels get cooled at the terminal walls of the chimney where heat energy is converted into kinetic energy [23][24][25][26][27][28][29][30]. The difference in pressure across the chimney base and its outlet can be known by measuring density difference of respective air parcels which in turn depend on associated inlet and outlet temperatures across the chimney. The requisite pressure difference required to drive the turbine can be regulated by minimizing chimney's entrance, exit and friction losses. As the air passes through the turbine, the kinetic energy associated with it drives the turbine blades which in turn drive the generator [30][31][32][33][34][35].

Methodology
The objective of this work is to develop a simplified design model of solar updraft tower power plant to investigate the effects of the canopy angle engulfment and turbine location on fluid flow. Decreasing the overall tower height would decrease the overall construction cost and improve the appearance of the chimney which leads to decreased power output and reduced efficiency of the system. This indicates that there is a scope for optimization. Thus, in search of optimizing the same, the goal of this work is to conduct parametric study and identify maximum power output by varying canopy orientation with respect to ground at various insolation levels using simplified two-dimensional simulation model based on Ansys fluent version 15 software [2].

Solar updraft tower design
The experimental prototype of SUT design can be carried out in four different phases such as geometry phase, meshing phase, solution setup phase and modelling phase which are presented here as follows.

Phase-I: Geometry Selection
Ansys fluent 15 software pack with full version was used for modelling SUT design with prototype dimensions as shown in table 1. The corresponding geometry can be modelled by opening fluent in Ansys 15 workbench wherein by selecting, geometry → 2D analysis model → XY plane → optional selection of angles and parameters can be further selected for modification. Using this procedure, SUT design based on the opted parametric dimensions displayed in table 1 can be simulated. After attaining the desired tower structure, one can save the same by procedural clicking of options in menu bar → concept → sketches → surface → generate → save the SUT design in a folder.

Phase-II: Meshing
After completing the geometry, select the mesh option in the Ansys workbench. Meshing is a process that involves dimension setup, refinement, face mapping of the desired structure (SUT) usually achieved by fragmenting the desired component into similar individual panel components to study the uniform distribution effect of applied load on the entire structural composition. Since each element has its own stiffness while loading, the stress developed on the entire structure can be calculated by summing up all individual stiffness's of the components to attain global stiffness matrix.
The mesh of each part in SUT is generated separately as its components can be split into three different zones i.e. canopy zone, transition section between canopy and tower zone. A secondary refinement of the mesh can be considered when panel size is too wide or sensitive to obtain better quality of the mesh. As an example, a strong pressure gradient will exist at the transition zone i.e. between canopy and tower zone where secondary refinement is recommended due to its sensitiveness in the computational domain. Finally, skewness ratio is examined to check desired mesh quality. Here the system geometry considered for examination consists of tower with height 2 meters (m) and 0.4 m diameter having a base length of 3.2 m and provision to vary not only the orientation of canopy angle between 0 • -10 • but also the height of the inlet between 0.127 m to 0.17 m.

Phase-III: Solution Setup
Once the geometry and meshing were done for desired SUT structure, general settings for the solution setup has to be applied by opting Ansys workbench 15 → click on Solution setup → launch fluent with double precision. The general setting applied for examination is showed in table 2.

Phase-IV: Modeling
After completing the above three phases successfully, the presumptions required for the SUT modeling has to be carried out by choosing requisite options and setting desired options such as Energy equation → ON, K-Epsilon model with standard mode with buoyancy effect [6], select air as material (boussinesq) [7], Operating conditions → 300K, insertion of boundary conditions like insolation (heat flux) values at different parts, setting solution initialization as standard initialization. Finally, clicking on 'Run calculations' by insertion of desired value for number of iterations to be carried out.

Results and Discussion
By following the above-mentioned procedure, simulation results can be obtained where contour maps and graphs pertaining to different parameters under consideration can be observed for desired SUT power plant structure. Here it is intended to analyze the variations in basic physical parameters such as temperature, pressure and velocity by varying the canopy angles and insolation levels to explore their interdependence for extracting maximum power output. The following observations were noticed [8] while carrying out parametric study on the original design of the simulation prototype.

Contour maps at 0º canopy angle and 600 Wm -2
Figure 1a, 1b and 1c represents contour maps of variation in pressure, temperature and air velocity at 0º canopy angle and 600 Wm -2 while table 3 displays variations in pressure, temperature and velocity with respect to variation in height at 0 • canopy angle at 600 Wm -2 insolation. From figure 1a, it can be observed that the pressure is least at center of the tower and increases with increase in the height of the tower. Pressure varies from inlet to the outlet of the tower which creates an updraft and leads to mass flow in the tower wherever pressure difference occurs. From figure 1b, it can be noticed that, as the heat flux emanate from the base, the temperature below the canopy is identified to be more than the outside temperature and the temperature at inlet of the tower is identified to be maximum whereas with the increase in height of the tower, there is a decrease in the value of temperature. From figure 1c, it is clearly evident that air velocity remains constant throughout the tower but there is a change in velocity at middle and bottom of the tower with maximum velocity at a height of 0.3 m, indicating suitability for turbine placement position to attain maximum turbine efficiency.       From figure 2a, it can be observed that the pressure at the inlet of the tower is low and gradually increase at the outlet of the tower. Thus, pressure difference is created in the tower. From the figure 2b it can be noticed that at 800 Wm -2 , pressure at the inlet of the tower is low and gradually increase at the outlet of the tower there by creating pressure difference in the tower. From figure  2c, it is clearly evident that the pressure at the inlet of the tower is low and gradually increases towards the outlet of the tower. Thus, pressure difference is created in the tower.

Temperature contours obtained for different insolation values
Figure 3a, 3b and 3c represents contour maps of variation in temperature at 5º canopy angle with respect to varying insolation at 600 Wm -2 , 800 Wm -2 and 1000 Wm -2 while table 5 displays variations in temperature with respect to variation in height of the tower at 5 • canopy angle at different insolation levels of 600 Wm -2 , 800 Wm -2 and 1000 Wm -2 . From figure 3a, it can be observed that at 600 Wm -2 insolation, change in temperature is low with maximum temperature at the base due to absorption and reflection of solar radiation beneath the canopy. From figure 3b i.e. at 800 Wm -2 insolation, no significant change with respect to figure 3a except that tower temperature slightly increases when compared to temperature beneath the canopy. From figure 3c i.e. at 1000 Wm -2 , it is clearly evident that temperature inside the tower is more than the canopy temperature.       , 4b and 4c represents contour maps of variation in velocity profile at 5º canopy angle with respect to varying insolation at 600Wm -2 , 800Wm -2 and 1000Wm -2 while table 6 displays variations in velocity with respect to variation in height of the tower at 5 • canopy angle at different insolation levels of 600 Wm -2 , 800 Wm -2 and 1000 Wm -2 . From figure 4a, a maximum velocity at the center of the tower can be observed that at 600 Wm -2 and 800 Wm -2 insolation levels, indicating suitability of turbine placement for attaining maximum efficiency whereas at 1000 Wm -2 , maximum velocity at the inlet periphery of the collector can be clearly noticed from figure 4c, indicating desirable placement of turbine to extract maximum efficiency with an option to place more than one turbine.  . Figure 5c. Pressure contour maps of SUT plant at 1000 Wm -2 at 10º canopy angle. , 5b and 5c represents contour maps of variation in pressure profile at 10º canopy angle with respect to variation in insolation at 600 Wm -2 , 800 Wm -2 and 1000 Wm -2 while table 7 displays variations in pressure with respect to variation in height of the tower at 10 • canopy angle at different insolation levels of 600 Wm -2 , 800 Wm -2 and 1000 Wm -2 . At 600 Wm -2 , it can be observed that the pressure at inlet of the tower is high and gradually decreases at the outlet of the tower from figure 5a thereby allowing air flow within the tower due to pressure difference. By examining the figure 5b and figure 5c i.e. at 800 Wm -2 and 1000 Wm -2 insolation levels, pressure at the inlet of the tower is observed to be low and increases gradually at the outlet of the tower leading to occurrence of pressure difference within the tower.      Figure 6a, 6b and 6c represents contour maps of variation in temperature profile of SUT at 10º canopy angle with respect to variation in insolation at 600 Wm -2 , 800 Wm -2 and 1000 Wm -2 while table 8 displays variations in temperature with respect to variation in height of the tower at 10 • canopy angle at different insolation levels of 600 Wm -2 , 800 Wm -2 and 1000 Wm -2 . At 600 Wm -2 insolation i.e. from figure 6a, it can be observed that the temperature at the base is high due absorption of incoming radiation and reflection from base beneath the canopy whereas at 800 Wm -2 and 1000 Wm -2 insolation, no significant change with respect to figure 6a, except that the inside temperature of tower is slightly more than temperature corresponding to area underneath canopy.    , 7b and 7c represents contour maps of variation in velocity profile (blue color in the legend) of SUT at 10º canopy angle with respect to variation in insolation at 600 Wm -2 , 800 Wm -2 and 1000 Wm -2 while table 9 displays variations in velocity with respect to variation in height of the tower at 10 • canopy angle at different insolation levels of 600 Wm -2 , 800 Wm -2 and 1000 Wm -2 .

Velocity contours obtained for different insolation values
At 600 Wm -2 insolation level i.e. from figure 7a maximum velocity at the center of the tower can be observed, indicating the suitability of turbine placement for maximum efficiency. From figure 7b, at 800 Wm -2 maximum velocity inside the tower can be examined in addition to distribution of velocity profile across the tower due to enhanced recirculation of air inside the tower. Although velocity remains constant, velocity maximum can be noticed at the inlet of the collector thereby indicating suitability of turbine placement beneath the canopy for attaining maximum efficiency of SUT while a constant velocity profile with a maximum velocity at the inlet of the collector. Here it should be noticed that power generated by the turbine may decrease due to recirculation of air.

Conclusion
Simulation was conducted using the Ansys fluent software, which solved the energy equation using the Boussinesq approximation. The standard k−epsilon turbulence model was used together incorporating the solar ray tracing model for a more accurate result.
The inlet and outlet ambient temperature was assumed at 300 K. All walls were considered as adiabatic except for the solar collector and the soil cover beneath the solar collector. The solar collector modeling parameters were defined as non-slip, convective heat transfer and radiation. And as for the soil layer, it was assumed as a thermal storage with a heat absorption coefficient of 0.8. The numerical model was also validated using previous numerical results obtained from IIT Dhanbad and Marian College of engineering Thiruvananthapuram.  15 The parametric study was conducted for the angle of inclination of the slope collector, with fixed height of the solar tower configuration with either a fixed entrance or a fixed exit and different solar radiation for the optimized geometry. The objective of varying these angles and insolation is to determine the most suitable position for installation of the power generation turbine. It was important to have a high velocity at the upstream of the turbine. The power generation turbine utilized the updraft velocity, which means the greater the velocity, the faster the turbine can turn to generate more power.       Figures 8a, 9a and 10a represents the parametric relationships of air velocity, temperature and pressure with respect to variation in height at 5 • canopy angle while figures 8b, 9b and 10b represents the parametric relationships of air velocity, temperature and pressure with respect to variation in height at 10 • canopy angle. From these figures, one can clearly notice that, it is possible to determine the most suitable position for installation of the power generation turbine by deploying site specific design consideration values in Ansys fluent software to extract maximum efficiency from SUT. This simulation was carried out to analyze the performance of the SUT using a two-dimensional steady state and energy equation. The air flow inside the updraft tower was assumed as to be planar where it was simulated with k-epsilon model  18 using Ansys Fluent software. The above results are validated and selection of canopy angle depending on the latitude and longitude of a specific location can yield desired results in successful implementation of SUT power plant.