Computational Fluid Dynamics (CFD) Study for an Augmented Vertical Axis Wind Turbine (AVAWT)

Transcription

1 Computational Fluid Dynamics (CFD) Study for an Augmented Vertical Axis Wind Turbine (AVAWT) R. Nobile 1, Dr M. Vahdati, Dr J. Barlow, Dr A. Mewburn-Crook 3 1 Technologies for Sustainable Built Environment Centre University of Reading, JJ Thompson Building, Whiteknights, Reading, RG6 6AF r.nobile@reading.ac.uk 3 Matilda s Planet Manufacturing Ltd One Green Place, Unit 36, Kenfig Industrial Estate, Margam Port Talbot, Swansea, SA13 2PE mewburncrook@btinternet.com ABSTRACT In this paper a numerical study is conducted in analysing the flow inside and around a stator of an Augmented Vertical Axis Wind Turbine (AVAWT). Several simulations are presented (using the ANSYS CFX 12. software) for different sized stators, vane angles and conical surfaces on the top and bottom of the stator. A Shear Stress Transport (SST) turbulence model is used during the simulations. The fluid domain is discretised with an unstructured tetrahedral mesh and the quality of the mesh is achieved by using metric quality tools. The aim of the paper is to understand if the presence of a stator around the rotor of a VAWT can be beneficial to the energy output of the machine. The diffuser/concentrator theory is revisited, as it can play an important role in the development of small wind turbines in the built environment. Keywords: Vertical axis wind turbine, Diffuser augmented wind turbine, Computational fluid dynamics, Built environment, Airfoil, Small wind turbine List of symbols c z R U Ω λ airfoil/blade chord number of stator blades radius of stator undisturbed velocity rotational speed (rad/s) tip speed ratio λ = ΩR/ U D c σ θ V t s diameter of the stator stator vane spacing stator angle to the radius (degree) relative wind speed thickness of the blade span of the blade 1. INTRODUCTION In the last few decades, the production of electricity from wind turbines has seen a rapid growth in many countries around the world. The major drivers are the recent need to reduce CO 2 emissions into the atmosphere and meet the growing demand for electricity [1]. Although, the expansion and installation of wind turbines is mainly focused on rural and open areas, more recently, the attention has also moved to the built environment. The benefits are mainly generation of electricity on the site where it is needed, and the reduction in transmission and cable costs [2]. It also obviates the need for a tower and reduces visual intrusion. Building Integrated Wind Turbine 1

2 The progress of small wind turbines in the built environment has been mainly focused on Horizontal Axis Wind Turbines (HAWTs) than Vertical Axis Wind Turbines (VAWTs). But several studies have shown that VAWTs are more suitable for urban areas than HAWTs [2], [3], [4], [5]. The advantages are mainly: omni-directional without a yaw control, better aesthetics to integrate to buildings, more efficient in turbulent environments and lower sound emissions [6]. Another important concept that has been revisited in the field of wind energy is the diffuser/concentrator theory that, for small wind turbines, can bring several benefits in the future generation of electricity in urban areas. This paper presents an aerodynamics study of a stator of an Augmented Vertical Axis Wind Turbine (AVAWT). The aim of the research is to provide a general understanding on how the airflow behaves inside and around the stator of the AVAWT for different combinations. 2. DIFFUSER AUGMENTED WIND TURBINE (DAWT) The diffuser theory, in the wind energy field, has been around for a period of 5 years without having any industrial success, as the process is not well understood [7]. Its major disadvantage being that it was unidirectional. A first theoretical study was carried out, at the end of the sixties, by Lilley and Rainbird [8] in Cranfield, UK. They concluded that using Diffuser Augmented Wind Turbines (DAWTs) has some benefits over bare wind turbines. Wind tunnel experiments were conducted by Foreman et al in USA and Igra et al in Israel in the seventies [7]. Foreman et al [9] stated that a DAWT was able to produce four times the power of a bare wind turbine and Igra et al [1] also concluded a similar statement with tests using a shrouded wind turbine. Abe et al, 25[11] in analysing a small wind turbine with a flanged diffuser found the power coefficient Cp higher than the Betz limit (16/27). The use of a flange at the exit of a diffuser has the advantage of creating separation zones with low pressure. These low pressure regions contribute to the increase in the wind streams conduction or mass flow rate into the diffuser. All of the studies concerned with DAWTs are mainly conducted for HAWTs. Therefore, very little has been carried out in the field of VAWTs considering the advantages of using diffusers. The main reasons that have stopped the development of a DAWT are the high costs related to additional materials [7] and the progress of HAWTs in power and efficiency [12]. In published studies, Thomas et al [13] developed a VAWT with omni-directional diffusion in The authors claimed that the diffuser around the stator had an improved factor of two when compared to a bare VAWT. Furthermore, a vertical diffuser has the advantage of starting the rotor at low and moderate wind speeds. Another important published study in 199, on the benefits behind the use of an augmented stator for a VAWT has been conducted by Mewburn-Crook [3]. With multiple tests in the wind tunnel, and on a full-sized field prototype in , the author proved an increased mass flow around the rotor, reduction in tip losses and a large amount of power from a small rotor. 3. METHODOLOGY In order to overcome the few disadvantages of the aerodynamics of a straight three-bladed vertical Darrieus wind turbine, a stator with 8 directed guide vanes is proposed and analysed. Several numerical simulations are carried out using a Computational Fluids Dynamics (CFD) package. The effect of different guide vanes, diameter and conical surfaces is clarified in this Building Integrated Wind Turbine 2

3 numerical study. Specific points around the stator are selected and analysed in terms of velocity intensity and direction. The effects of different layouts of the stator on the dynamics of the flow are presented in tabulated and graphed forms. A steady flow with constant wind speed U of 1 m/s is considered for all numerical simulations. This assumption is not consistent with the real case where the wind turbine will operate, but it is necessary for a first approach to the problem. A Shear Stress Transport (SST) turbulence model is used during the simulations. The fluid domain is discretised with an unstructured tetrahedral mesh and the quality of the mesh is improved by using metric quality tools. For all the simulations the number of interactions in the solver control is set between 2 and 3 steps and the residual accuracy for the Root Mean Square (RMS) is 1. e -4. The objective of the research is to show the best configuration of the stator in terms of local wind speeds, as the power output of a wind turbine is highly dependent on the wind speed. 4. COMPUTATIONAL FLUID DYNAMICS (CFD) STUDY The evaluation of the performance of a wind turbine can be achieved with the use of Computational Fluid Dynamics (CFD) software. CFD has the potential to predict fluid flow and related phenomena by resolving specific mathematical equations. The results obtained can help on improving design and understanding of the local area to be analysed. The advantages are mainly reduction in costs compared to running experimental tests in the wind tunnels. CFD can be used for the development or improvement of new or existing wind turbines. Validation can take place through wind tunnel tests. 4.1 Airfoil Selection The airfoil selected during the fluid dynamics study is a NACA 18, as several authors have shown that the NACA 4 digit series with thicker sections has better aerodynamics performance for the Darrieus-type VAWTs [14]. The stator, as shown in Table 1 and Figure 1, is composed of 8 blades with thickness t of 18mm and a cord c of 1mm. The blade span s is 26mm, but since the geometry is symmetric only half is studied to reduce calculation time required by the solver. The 3-D model of the stator was configured with ProEngineer 4.. The diameter D c of the stator from the middle of the cord is 55mm. This large diameter was chosen as the AVAWT has been designed to be installed in tall buildings in the built environment and generate a power between 5 and 1 kw. Characteristics of NACA18 for the blades of the stator c t s s t D c c Table 1: Properties of the blade of the stator 4.2 Mesh Selection and Boundary Conditions Figure 1: Half stator at to the radius and conical surface A mesh in CFD has the function of dividing the geometry into many elements that are used to create control volumes to be solved by the solver. During the simulations the fluid domain is Building Integrated Wind Turbine 3

4 discretised with an unstructured tetrahedral mesh and the mesh quality is measured by using a metric quality tool called Skewness. Published studies suggest the need to keep this value below to.98 provides a reasonable mesh, as poor mesh quality can lead to inaccurate solutions and convergence problems [15]. For all simulations the mesh was generated with the CFX-Mesh option that is provided in ANSYS 12.. The fluid domain was divided into two sub-domains: the fluid domain and the stator domain. The fluid domain was generated into a cube of 2mx2mx6.3m with an automatic mesh, while the mesh of the stator was accurately refined with the facing space and inflation options available in ANSYS 12.. This accuracy was necessary to allow a better understanding of the physics involved during the simulations. The boundary conditions for the stator were mainly constant velocity at the inlet of the fluid domain, standard atmospheric pressure at the exit of the fluid domain, standard wall conditions for all the surfaces of the stator and symmetrical plane for studying half part of the model. 5. NUMERICAL SIMULATIONS In this section of the paper a number of numerical simulations were conducted with ANSYS 12.. Three different numerical simulations were conducted and analysed using ANSYS 12.: stator with different vane guides orientation, stator with different diameters and stator with different conical surfaces. 5.1 Stator with Variable Vane Guides The guide vane as shown in Figure 1 consists of 8 NACA18 vertical blades that are designed to be mounted around the periphery of a rotor. In this case the angle θ to the radius was incremented by 3 for an interval between -9 and 9. In Table 2 the results are listed for different vane guide orientations and 4 positions around the stator are analysed with a distance σ of 3 mm from the internal diameter. θ (deg.) V av. Pos.1 Pos.2 Pos.3 Pos Wind Speed Stator with different guide vane orientations Rotor Position (Degrees) Table 2: Wind speed for different vane guide orientations and positions around the stator Figure 2: Wind speed for different vane guide orientations As shown in Figure 2 the wind speed is highly dependent on the guide vanes orientation. The best performances in terms of orientations are given by 3 and. In both cases there is an increase in the local wind speeds which can be beneficial to the power generation of the wind turbine. However, in all the other combinations, the blades of the stator can have a negative effect on the wind speeds available on the rotor. The negative effects are mainly due to the development of eddies and low wind speeds that are not favourable to the rotor. Building Integrated Wind Turbine 4

5 5.2 Stator with Different Diameters From the previous study the 3 orientation of the blades of the stator was chosen and different diameters were analysed. The diameter was incremented by 4mm each time and 4 specific positions were selected, as in the case above. Table 3 shows the results obtained for different diameters. D c V av. Pos. 1 Pos.2 Pos.3 Pos Wind Speed Stator with different diameters Rotor Position (Degrees) Table 3: Wind speed for different diameters and positions around the stator Figure 3: Wind speed for different diameters Figure 3 shows that use of different diameters does not have a significant effect on the wind speed which approaches the rotor of the wind turbine. 5.3 Stator with Conical Surface Finally, in the last case a number of numerical simulations with conical surface were conducted with different angles φ to the horizontal. The range was set between 3 and 5 with an increment of 5 for each case. The introduction of converging surfaces on the top and bottom has the scope of increasing the performance of the turbine in terms of mass flow rate and wind speed. φ (deg.) V av. Pos. 1 Pos.2 Pos.3 Pos Table 4: Wind speed for different conical surface orientations and positions around the stator Wind Speed 15 1 Stator with different conical surface orientation Rotor Position (Degrees) Figure 4: Wind speed for different conical surface orientation Building Integrated Wind Turbine 5

6 As shown in Table 4 and Figure 4 the conical surface around the stator has a positive effect on the wind speed for specific orientations. It shows that the best performance is achieved for an orientation of the conical surface of 3 to the horizontal. 6. DISCUSSION In the several simulations demonstrated, it can be noted that the performance of a VAWT can be improved with the introduction of a stator. Figure 5 and 6, obtained with ANSYS 12., are few of the several simulations able to illustrate wind speed intensities, directions and presence of eddies or circulation zones that can increase the rate mass converging inside the stator. The best combination is achieved with the blade orientation of to the radius and conical surface that is inclined 3 to the horizontal plane. The 8 vertical blades combined with conical surfaces have the advantage of forming concentrator ducts upwind, and diffusers downwind. In addition, Figure 5 shows that the stator has created in Position 2 and 4 wind speeds perpendicular to the blades of the rotor that are not found in the case of a bare VAWT. Figure 5: Wind speed at to the radius and conical surface (Top view) Figure 6: Wind speed at to the radius and conical surface (Front view) Therefore, the use of a stator with guide vanes and conical surfaces around the rotor of a VAWT can have a positive effect on the power generation of the machine in urban areas where there are low wind speeds and turbulence problems. As the wind power generation is a cubic function, a slight increase in wind speed will generate a substantial increase in power. 7. CONCLUSIONS This paper has shown how the use of an augmenter or stator around the rotor of a VAWT has the potential of increasing mass flow and capture area of the rotor, which can lead to an increase in the power developed by the wind turbine. It has also shown that the use of the stator will improve the efficiency with which the rotor will extract energy of the flow. The several numerical simulations, conducted with ANSYS 12., show that the blade orientation and the introduction of conical surfaces have a substantial impact on the wind speed, direction and formation of circulation zones inside the stator. These factors can have a positive or negative effect on the performance of the wind turbine if they are not selected carefully. The best performance in terms of wind speed and direction is mainly achieved for orientations of and 3 respectively. In both cases the wind speed intensity is increased between 1% and 2% to the free stream. Building Integrated Wind Turbine 6

7 The advantages of having a stator around a rotor of a VAWT are: 1. Safety grid around the rotor in case of failure of the rotor 2. Converge more fluid around the rotor 3. Create eddies and pressure changes that can have positive effects on the rotor 4. Start the rotor at low and moderate wind speeds In the future development of wind turbines the use of VAWTs can be more appropriate than HAWTs, as VAWTs can overcome the problems related to the excessive gravitational loading on the root of the blades that limits the increase in size of HAWTs. Finally, VAWTs should be considered for the future development of floating offshore wind turbines, as they have a lower centre of gravity. 8. REFERENCES [1] K. Pope, V. Rodrigues, R. Doyle, A. Tsopelas, R. Gravelsins, G. Naterer, and E. Tsang, Effects of stator vanes on power coefficients of a zephyr vertical axis wind turbine, Renewable Energy, vol. In Press, Corrected Proof. [2] S. Mertens, G.A. van Kuik, G.J. Van Bussel, and D.U.O.T. TU Delft, Wind energy in the built environment: concentrator effects of buildings, 26. [3] A. Mewburn-Crook, The Design and development of an augmented vertical wind turbine, Apr [4] S. Stankovic, N. Campbell, and A. Harries, Urban Wind Energy, Earthscan Publications Ltd., 29. [5] C.J. Ferreira, G. van Bussel, and G. van Kuik, 2D CFD simulation of dynamic stall on a Vertical Axis Wind Turbine: verification and validation with PIV measurements, 45th AIAA Aerospace Sciences Meeting and Exhibit/ASME Wind Energy Symposium, 27. [6] C. Hofemann, C.J. Simao Ferreira, G.J. Van Bussel, G.A. Van Kuik, F. Scarano, and K.R. Dixon, 3D Stereo PIV study of tip vortex evolution on a vawt, European Wind Energy Association EWEA, 28. [7] D.G.J.W.V. Bussel, The science of making more torque from wind: Diffuser experiments and theory revisited., Journal of Physics: Conference Series, vol. 75, 27, p [8] D.G. Phillips, P.J. Richards, and R.G.J. Flay, Diffuser Development For A Diffuser Augmented Wind Turbine Using Computational Fluid Dynamics, Department of Mechanical, Engineering, the University of Auckland, New Zealand. [9] R.A. Oman, K.M. Foreman, and B.L. Gilbert, Investigation of diffuser-augmented wind turbines, Jan [1] O. Igra, The shrouded aerogenerator, Energy, vol. 2, Dec. 1977, pp [11] K. Abe, M. Nishida, A. Sakurai, Y. Ohya, H. Kihara, E. Wada, and K. Sato, Experimental and numerical investigations of flow fields behind a small wind turbine with a flanged diffuser, Journal of Wind Engineering and Industrial Aerodynamics, vol. 93, Dec. 25, pp [12] S. Eriksson, H. Bernhoff, and M. Leijon, Evaluation of different turbine concepts for wind power, Renewable and Sustainable Energy Reviews, vol. 12, Jun. 28, pp [13] R.N. Thomas and others, Vertical windmill with omnidirectional diffusion, Google Patents, [14] R. Howell, N. Qin, J. Edwards, and N. Durrani, Wind tunnel and numerical study of a small vertical axis wind turbine, Renewable Energy, vol. 35, Feb. 21, pp [15] ANSYS Meshing: Application Introduction, 29. Building Integrated Wind Turbine 7