Vertical axis wind turbine with omni-directional-guide-vane for urban high rise application

Transcription

1 Vertical axis wind turbine with omni-directional-guide-vane for urban high rise application W. T. CHONG 1,*, S. C. POH 1, A. FAZLIZAN 1, 2 and K. C. PAN 1 1. Department of Mechanical Engineering, University of Malaya, Kuala Lumpur, Malaysia; 2. UMPEDAC, Level 4, Engineering Tower, Faculty of Engineering, University of Malaya, Kuala Lumpur, Malaysia. Abstract: A novel shrouded wind-solar hybrid renewable energy and rain water harvester with an omni-directional-guide-vane (ODGV) for urban high-rise application is introduced. The ODGV surrounds the vertical axis wind turbine (VAWT) and enhances the VAWT performance by increasing the on-coming wind speed and guiding it to an optimum flow angle before it interacts with the rotor blades. An ODGV scaled model was built and tested in the laboratory. The experimental results show that the rotational speed of the VAWT increase by about 2 times. Simulation works show that the installation of the ODGV increases the torque output of a single-bladed VAWT by 206% for tip speed ratio, TSR = 0.4. The result also revealed that higher positive torque can be achieved when the blade tangential force at all radial positions is optimized. To conclude, the ODGV improves the power output of a VAWT and this integrated design promotes the installation of wind energy systems in urban areas. Keywords: vertical axis wind turbine, green architecture, omni-directional-guide-vane, wind-solar-rain water harvester, urban wind energy generation 1 Introduction 1 The interest in utilizing renewable energy sources in order to meet energy demand is growing rapidly. Renewable energy such as solar energy, wind energy, hydro energy and geothermal energy offers a clean, no direct emissions of greenhouse gases, and cost effective alternative to fossil fuel. Wind power is growing fastest among the energy sources. Wind energy has continued as the most dynamically growing energy source in the year The global wind installations reached 121,188 MW in year 2008, after 59,024 MW in year 2005, 74,151 MW in year 2006, and 93,927 MW in year 2007 as reported in World Wind Energy Report 2008 [1]. This is due to the concern about the limited fossil fuel resources and their impact on the environment. Malaysia experiences low wind speed throughout the year. In most of the areas, the wind speed is recorded as low (V < 4m/s for more than 90% of total hours) and unsteady [2]. Extracting wind energy by using conventional wind turbines in this condition would not be suitable. But over the decades, wind energy technology has developed rapidly in new dimensions [3]. Many researchers had studied and reported different designs of ducted or funneled wind turbines which increase the on-coming Foundation item: Project (RG039-09AET) supported by University of Malaya; Received date: ;Accepted date: Corresponding author: Chong Wen Tong (W.T. CHONG); Tel: ; chong_wentong@um.edu.my wind speed hence increasing the efficiency and performance of turbines. Govardhan and Dhanasekaran have shown that the efficiency and starting characteristics of the Wells turbine (horizontal axis wind turbine) have improved when compared with the respective turbines without guide vanes [4]. Study conducted by Hu and Cheng also revealed that bucket-shape ducted wind turbine tested in the field improved the flow around the generator. It was estimated that the power extraction efficiency increased by about 80 % [5]. From the study conducted by Chong (2006) in Malaysia, wind energy generation (for electricity) could only be economically viable for isolated areas far away from the national grid system [6]. There is great potential to site wind energy generators in urban areas according to research done by the European Union [7]. Generally, urban areas have weak wind conditions and turbulence due to the presence of high rise buildings. Thus, the wind energy generation systems for urban regions need to overcome these disadvantages. On top of that, in order to design a wind energy generation system that can be used in urban areas, other factors must be considered such as visual impact, acoustic pollution, structural issues, safety problems due to blade failures and electromagnetic interference [8]. A demonstrative study for the wind and solar power hybrid system was conducted at Ashikaga Institute of Technology in Japan. It was confirmed that there is a complementary relationship between wind and solar energy after one year of operation [9]. Eke et al (2005) also reported that the cost of the hybrid system was found to

2 be less than the individual photovoltaic and wind systems [10]. Müller et al (2009) has proposed and architecturally demonstrated a wind energy converter which has a cylindrical form that facilitates current building design [11]. Grant et al (2008) also reported and concluded that ducted wind turbines attached to the roof of a building have significant potential for retro-fitting onto the existing building with minimum concern of visual impact [12]. 2 Literature Review 2.1 System design descriptions In this paper, a novel shrouded wind turbine system that integrates several green and renewable energy harvesting technologies (wind-solar hybrid energy generation system and rain water collector) is proposed to be used on high rise buildings. The system is designed to overcome the weak wind and turbulent conditions in urban areas which are not suitable for conventional wind turbine operation. It has the capability to accelerate the on-coming wind to improve the energy output and starting characteristic of the wind turbine. Moreover, the ODGV also acts as a shield to prevent the wind turbine blades from flying off and causing public injury in case of blade failure. The system is also designed to provide optimum surface area for solar panel installation on top of the harvester (outer surface of the ODGV upper wall) to generate extra energy from sunlight. In addition, rain water can be collected through the flow paths formed by the multi-sector arrangement of the inclined solar panels. The rain water flows towards the center of the system and the collected rain water can be stored in a storage tank for various purposes. The system is designed to complement the building architecture with minimal visual impact so as to overcome the issue concerning public acceptance. Another advantage of the ODGV is that it can be retrofitted to an existing building and gives an aesthetic appearance. It can be designed to blend into the building architecture or built on top (or between upper levels) of high rise buildings to give an aesthetic green architecture design and also provide on-site green power to the buildings. An illustrative view of its possible integration is shown in Fig. 1. This design is also intended to integrate a hybrid renewable energy system on top of a high-rise building with more emphasis on visual impact, safety, noise pollution and improvement on starting behavior of the wind turbine [13]. This patented design overcomes the inferior aspect of low wind speed by guiding and increasing the speed of high altitude wind through the ODGV [14]. The system can be of cylindrical shape or any shape of design, depending on the building architectural profile, such as in the shape of an ellipse, etc. Generally, the system utilizes the advantages of the Malaysian climate, i.e. high solar radiation and high rainfall over a year for green energy generation and free water supply. The ODGV collects the wind stream radially from any direction. When the wind stream flows from a larger area to a smaller area, it creates a venturi effect to guide and increase the wind speed before entering the wind turbine. The mesh is mounted at the outer side of the ODGV to avoid foreign objects from striking the VAWT, e.g. bird-strike. The wind turbine has a common rotating axis with the ODGV and it is coupled with the generator through the power transmission shaft and mechanical drive system for generating electricity. The power generated from the wind turbine and solar panel is stored in a battery bank or fed into the electricity network line [15]. Fig. 1 Illustrative view of a high-rise building with the hybrid renewable energy system integrated on it. The background is a modified view of Tokyo City by Google [15] 2.2 Computational Fluid Dynamics (CFD) Solver The flow-solver is based on the two dimensional Navier-Stokes equations which formulate the principles of conservation of mass, momentum and energy in the form of partial differential equations. The computational domain is divided into cells and discretization of the Navier-Stokes equations using the finite volume method is carried out on each cell in the domain. The SST (Shear Stress Transport) k-ω turbulence model was used in the simulation because this model could produce more accurate and reliable results [16]. The SST k-ω model is also known to have reduced sensitivity to far field values of turbulence frequency, ω, and a more balanced performance for a wide range of flow types compared to other general-purpose two equation models, as demonstrated by Menter et al [17]. The SIMPLE (Semi-Implicit Method for Pressure Linked Equations) algorithm adopted by Patankar and Spalding in their experiment was used to solve the conservation equations resulting from the discretization [18]. Essentially, the

3 SIMPLE algorithm links the mass conservation equation to the momentum equations via the pressure correction. This algorithm was chosen for this study because of its computational efficiency, robustness in iterating the coupled parameters and higher-order differencing schemes are available for this algorithm. The convective terms are numerically differentiated with the linear upwind differencing scheme, striking a balance between accuracy and computing cost. The CFD package, Fluent is capable of capturing the surface pressure on the Wortmann FX airfoil of the VAWT blade; hence the force and the torque produced can be calculated. The average coefficient of moment, C m,average and average power coefficient C p,average are obtained using the following equations [19] : C m, average 1 = N θ = N θ= 1 C m ( θ) ( TSR)( C ) C p average m, average, (1), =, (2) where C m is the moment coefficient, is the degree of rotation, N is the total number of degree of rotations and TSR is the tip speed ratio. The torque value is captured in non-dimensional form, i.e. the coefficient of moment, C m at every degree of azimuth angle. After completing one cycle of revolution, all 360 sets of data were averaged. The average torque produced, T is Fig. 2 Design and dimensions of the scaled ODGV 3.2 Initial experiment An initial experiment has been conducted for a scaled down ODGV system which has the same dimensions as the CFD model. The initial experiment is to analyze and compare the performance of wind rotor with and without the aid of the ODGV. In the experiment, a custom built ODGV model is tested with the wind blowing in 3 different directions, i.e. 0 degree, 30 degrees and 60 degrees as shown in Fig. 3. A Wortmann FX bladed VAWT as shown in Fig. 4 was chosen for the experiment and the experimental set up is shown in Fig. 5. T 1 = 2 Cm, average ρ Av R, (3) 2 where ρ is the density of fluid, A is the projected area of the turbine, v is the velocity (free stream) of fluid and R is the radius of the turbine. 3 Methodology 3.1 Design of omni-direction-guide-vane The shrouded feature of the ODGV is to enhance the performance by increasing the on-coming wind speed before it interacts with the rotor blades. The ODGV is designed with 4 pairs of guide vanes placed uniformly around a cylinder with tapered feature at outer radial band. The vanes in each pair are tilted at angles of 20 and 55 as shown in Fig. 2. This design allows it to capture wind blowing from any direction without the need of a yawing mechanism. Fig. 3 On-coming wind direction (relative to the ODGV) The 3 m/s wind stream is generated from three industrial stand fans arranged in parallel. The other reason that the test is not conducted in a wind tunnel is due to the fact that air blown by industrial fans will perform in a similar manner to the field environment where the wind stream is swirling and turbulent. The performance of the wind rotor is measured by recording the rotational speed over time using a laser tachometer. The rotor rotational speeds were continuously recorded until the data reached the stabilized stage.

4 computational domain in Fig. 6 shows the boundary conditions only with the VAWT. On the other hand, the computational domain in Fig. 7 shows the boundary conditions of the VAWT with the presence of the ODGV. Table 2 Computational conditions Computational Conditions Density kg/m 3 Fig. 4 Laboratory experiment set-up for initial experiment Fig. 5 Wortmann FX airfoil 5-bladed VAWT The rotor was in free-running condition where only inertia and bearing friction were applied. Rotational speed of the 5-bladed Wortmann FX airfoil VAWT was measured by a hand-held laser tachometer and readings were taken at 5 second intervals over a period of 150 seconds. 3.3 Numerical simulation The study and investigation of the ODGV on the single-bladed Wortmann FX VAWT were conducted using a commercial computational fluid dynamics (CFD) package, Fluent 6.3. The ODGV integrated VAWT simulation was adopted from the investigation carried out by Oler et al which utilized a single-bladed VAWT to simulate the performance of VAWT published by the Sandia National Laboratories [20]. The simulations of the VAWT were conducted for the cases with and without the presence of the ODGV. The computational parameters are summarized in Table 1 and the simulation computational conditions are tabulated as Table 2. Table 1 Computational parameters Parameter Airfoil chord length, c VAWT rotor radius, R Rotational speed, RPM Rotor rotation speed, Inlet velocity, v Value m 0.24 m rpm rad/s m/s In the simulation, both of the cases were simulated using the same boundary conditions and Wortmann FX single-bladed VAWT at TSR = 0.4. The Viscosity Pressure Space/Time Viscous Model CFD algorithm Interpolating Scheme (momentum) Interpolating Scheme (turbulence) 1.003x 10-3 kg/m.s 101,325 Pa 2D/Unsteady, 2 nd order implicit k and ω (SST) SIMPLE Residual error 1 x 10-4 Inlet Boundary Type Reference Frame Blade Motion Type Outlet Boundary Tape Fluid Type 2 nd order upwind 2 nd order upwind Velocity Inlet Absolute Moving Mesh (Rotational) Pressure outlet Water 3.4 Computational grid The computational domain was meshed by using the Gambit software. The quantity of meshes that was generated for the simulation is shown in Table 3. In the computational grid generation, there are cells and cells of mesh created in the computational zone for the cases with and without ODGV respectively. The meshes generated in the rotary maingrid and rotary subgrid are the moving mesh zone to simulate the dynamic characteristics of the VAWT. The domains as shown in Fig. 6 and Fig. 7 indicate moving mesh zones and the azimuth angle of 0 degree (the starting position of the VAWT blade) and the direction of blade rotation, Ω. Table 3 Sets of generated mesh for simulation Region Cell Quantity With ODGV Without ODGV Water tunnel Rotary maingrid Rotary subgrid Total

5 4 Results and Discussions Fig. 6 Mesh for VAWT only 4.1 Initial experiment results The experiment was carried out to compare the performance of the VAWT, without and with the use of the ODGV, and the results are shown in Fig. 8. The experimental results show that the rotational speed of the VAWT increases from 51 rpm to 115 rpm, 94 rpm and 109 rpm for the three configurations, i.e. 0 degree, 30 degrees and 60 degrees respectively. From Fig. 8, the rotational speed of the VAWT increases more rapidly compared to the bare VAWT, this trend shows that the ODGV improves the response of the wind turbine to on-coming wind. Comparing these results, the VAWT gives the highest increment in rotational speed when the wind was blown from the direction of 0 degree of the ODGV. The most important finding is that the presence of the ODGV successfully increases the rotational kinetic energy of the wind rotor. The stabilized rotor rotational speed and percentage increment for each configuration are summarized in Table 4. Overall, the increment of rotational speed is about 108%. The feasibility of the ODGV to improve the performance of the wind rotor has been proved via this simple experiment. Table 4 Summarized experimental results Configuration Rotational speed % increment No ODGV 51 rpm - 0 degree ODGV orientation 30 degrees ODGV orientation 60 degrees ODGV orientation 115 rpm rpm rpm 114 Fig. 7 Mesh for VAWT with presence of the ODGV Average 108 Fig. 8 Experimental results for the bare VAWT, and the integration of VAWT with the ODGV in the three configurations, i.e. 0 degree, 30 degrees and 60 degrees orientation

6 4.2 Numerical simulation results The result obtained from the CFD simulation is the coefficient of moment, C m of the blades on the centre of rotation. The simulation was done for a few revolutions and iteration is done at every degree of azimuth angle. The solution is converged at every time step and for every revolution. Fig. 9 shows the coefficient of moment, C m comparison between the bare VAWT and the VAWT with the ODGV for azimuth angle from 0 to 359. After obtaining the average moment coefficient, the average power coefficient can be calculated. Table 5 shows the summarized results calculated from the simulation data obtained for the cases of the VAWT with and without the presence of the ODGV respectively. Fig. 9 Graph of moment coefficient, C m versus azimuth angle, θ with and without the ODGV The simulation result shows that the average power coefficient is increased from (without the presence of the ODGV) to (with the presence of the ODGV), indicating 206% increment. By comparing the two graphs in Fig. 9, the negative portion of the graph is minimized when the ODGV is introduced. This is because the ODGV s vanes guide the wind to attack the wind turbine blade at a better flow angle. Hence, the power generated is augmented by utilizing the ODGV. Table 5 Summarized results for the VAWT with and without ODGV Parameter Average coefficient of moment, C m,average Rotor rotational speed, VAWT without the ODGV (5 blades) Result rad/s Tip speed ratio, TSR 0.4 Average power coefficient, C p,average VAWT with the ODGV (5 blades) Conclusion An innovative device called an omni-directional guide vane (ODGV) that surrounds a VAWT such as Darrieus rotor or H-rotor was designed to improve the wind rotor performance. The shrouded design of the ODGV can minimize public concerns of installing a high-speed rotating wind turbine for on-site power generation and it is aesthetically friendly to an existing building. It also integrates several green and renewable energy harvesting technologies. The initial experiment shows that the ODGV is able to increase the rotational speed of the VAWT by up to 125%. From the CFD simulations, the ODGV is capable of augmenting the torque and power output of a single-bladed (Wortmann FX airfoil) VAWT. With the presence of the ODGV, the power produced by the single-bladed VAWT has been increased by 206% at TSR = 0.4. From the graph of the torque coefficient versus azimuth angle, the negative torque zone has been minimized, thus increasing the rotor torque. The ODGV integrated wind power generation system improves the power output of a VAWT and it has great potential to be sited in urban areas for on-site and grid-connected power generation. The geometry of the ODGV can be further improved to match with the other types of VAWT. The long-term goal is the proliferation of wind and solar energy applications in populated urban areas or sub-urban regions, capable of supplying supplementary power to urban buildings. 6 Acknowledgments The authors would like to thank University of Malaya for the assistance provided in the patent application of this design (WO A2; WO A3), and the research grant allocated to further develop this design under the project RG039-09AET and D Special appreciations are also credited to the Malaysian Meteorology Department for providing useful weather data and Malaysian Ministry of Higher Education (MOHE) for the research grant (Fundamental Research Grant Scheme, FP A). References [1] World Wind Energy Report 2008, In World Wind Energy Association: (2008). [2] TONG C W, ZAINON M Z, CHEW P S, KUI S C, KEONG W S, CHEN P K. Innovative power-augmentation-guide-vane design of windsolar hybrid renewable energy harvester for urban high rise application//the 10th Asian International Conference on Fluid Machinery.Kuala Lumpur: AIP

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