J. Cent. South Univ. (2012) 19: 727−732 DOI: 10.1007/s11771­012­1064­8

Vertical axis wind turbine with omni­directional­guide­vane for urban high­rise buildings

W. T. Chong 1 , S. C. Poh 1 , A. Fazlizan 1, 2 , K. C. Pan 1

1. Department of Mechanical Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia; 2. UMPEDAC, Level 4, Engineering Tower, Faculty of Engineering, University of Malaya,

50603 Kuala Lumpur, Malaysia

© Central South University Press and Springer­Verlag Berlin Heidelberg 2012

Abstract: A novel shrouded wind­solar hybrid renewable energy and rain water harvester with an omni­directional­guide­vane (ODGV) for urban high­rise application is introduced. The ODGV surrounds the vertical axis wind turbine (VAWT) and enhances the VAWT performance by increasing the on­coming 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 increases by about 2 times. Simulations show that the installation of the ODGV increases the torque output of a single­bladed VAWT by 206% for tip speed ratio of 0.4. The result also reveals that higher positive torque can be achieved when the blade tangential force at all radial positions is optimized. In conclusion, the ODGV improves the power output of a VAWT and this integrated design promotes the installation of wind energy systems in urban areas.

Key words: vertical axis wind turbine; green architecture; omni­directional­guide­vane; wind­solar­rain water harvester; urban wind energy generation

1 Introduction

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 energy without direct emissions of greenhouse gases, and is 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 2008. 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 to be low (v∞<4 m/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 on­coming wind speed hence increasing the efficiency and performance of turbines. GOVARDHAN and DHANASEKARAN [4] have shown that the efficiency and starting characteristics of the Wells turbine (horizontal axis wind turbine) have been improved when compared with the respective turbines without guide vanes. Study conducted by HU and CHENG [5] also revealed that bucket­shape 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 %.

From the study conducted by CHONG [6] in Malaysia, wind energy generation (for electricity) could only be economically viable for isolated areas far away from the national grid system. There is great potential to site wind energy generators in urban areas [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

Foundation item: Project (RG039­09AET) supported by University of Malaya, Malaysia Received date: 2011−07−26; Accepted date: 2011−11−14 Corresponding author: W. T. Chong; Tel: +6012−7235038; E­mail: chong_wentong@um.edu.my

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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 [10] also reported that the cost of the hybrid system was found to be less than the individual photovoltaic and wind systems [10]. MÜLLER et al [11] proposed and architecturally demonstrated a wind energy converter which has a cylindrical form that facilitates current building design. GRANT et al [12] also reported and concluded that ducted wind turbines attached to the roof of a building have significant potential for retro­fitting onto the existing building with the minimum concern of visual impact.

2 Literature review

2.1 System design descriptions In this work, a novel shrouded wind turbine system

that integrates several green and renewable energy harvesting technologies (wind­solar 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. It has the capability to accelerate the on­coming 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 multi­sector 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 high­rise buildings to give an aesthetic green architecture design and also provide on­site 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 high­rise building with more emphasis on visual impact, safety, noise

Fig. 1 Illustrative view of high­rise building with hybrid renewable energy system integrated on it (Background is modified view of Tokyo City by Google [15])

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. 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. bird­strike. 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].

2.2 Computational fluid dynamics (CFD) solver The flow­solver is based on the two­dimensional

Navier­Stokes 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 the discretization of the Navier­Stokes equations using the finite volume method is carried out on each cell in the domain.

The shear stress transport (SST) k­ω turbulence model was used in the simulation because this model could produce more accurate and reliable results [16]. The SST k­ω 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 general­purpose two

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equation models, as demonstrated by MENTER et al [17]. The semi­implicit method for pressure linked equations (SIMPLE) algorithm adopted by PATANKAR and SPALDING [18] in their experiment was used to solve the conservation equations resulting from the discretization. Essentially, the SIMPLE algorithm links the mass conservation equation to the momentum equations via the pressure correction. This algorithm was chosen for this work because of its computational efficiency, robustness in iterating the coupled parameters and higher­order 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 FX­63­137 airfoil of the VAWT blade; hence, the force and the torque produced can be calculated. The average coefficient of moment, Cm,average and average power coefficient Cp,average

are obtained using the following equations [19]:

( ) ∑ =

=

= N C

N C

θ

θ θ

1 m average m,

1 (1)

average m, T average p, C R C ⋅ = (2)

where Cm is the moment coefficient, θ is the degree of rotation, N is the total number of degree of rotations and RT is the tip speed ratio.

The torque value is captured in non­dimensional form, i.e. the coefficient of moment at every degree of azimuth angle. After completing one cycle of revolution, all 360 sets of data are averaged. The average torque produced, T, is

R Av C T 2 average m, 2 / 1 ρ = (3)

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 omni­direction­guide­vane The shrouded feature of the ODGV is to enhance

the performance by increasing the on­coming wind speed before it interacts with the rotor blades. The ODGV is designed with four 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.

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 three different directions, i.e. 0°, 30° and 60°, as shown in Fig. 3. A Wortmann FX­63­137 five­bladed VAWT as shown in Fig. 4 is chosen for the experiment and the experimental set up is shown in Fig. 5.

Fig. 2 Design and dimensions of scaled ODGV

Fig. 3 On­coming wind direction (relative to ODGV)

Fig. 4 Laboratory experiment set­up for initial experiment

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

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Fig. 5Wortmann FX­63­137 airfoil five­bladed VAWT

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 are continuously recorded until the data reach the stabilized stage.

The rotor was in free­running condition where only inertia and bearing friction were applied. Rotational speed of the five­bladed Wortmann FX­63­137 airfoil VAWT was measured by a hand­held laser tachometer and readings were taken at 5 s intervals over 150 s.

3.3 Numerical simulation The study and investigation of the ODGV on the

single­bladed Wortmann FX­63­137 VAWT were conducted using a commercial computational fluid dynamics (CFD) package, Fluent 6.3. The ODGV integrated VAWT simulation [20] was adopted which utilized a single­bladed VAWT to simulate the performance of VAWT. 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 in Table 2.

Table 1 Computational parameters

Parameter Value

Airfoil chord length, c/m 0.045

VAWT rotor radius, R/m 0.24

Rotational speed/(r∙min −1 ) 2.896 3

Rotor rotation speed, θ & /(rad∙s −1 ) 0.303 3

Inlet velocity, v/(m∙s −1 ) 0.182

In the simulation, both of the cases were simulated using the same boundary conditions and Wortmann FX­63­137 single­bladed VAWT at RT=0.4. The 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 condition Value or description

Density/(kg∙m −3 ) 998.2 Viscosity/(kg∙m −1 ∙s) 1.003

Pressure/Pa 101 325 Space/Time 2D/Unsteady, 2nd order implicit

Viscous model k and ω (SST) CFD algorithm SIMPLE

Interpolating scheme (momentum)

2nd order upwind

Interpolating scheme (turbulence)

2nd order upwind

Residual error 1×10 −4

Inlet boundary type Velocity Inlet Reference frame Absolute Blade motion type Moving mesh (rotational) Outlet boundary tape Pressure outlet

Fluid type Water

Fig. 6Mesh for VAWT only

Fig. 7Mesh for VAWT with presence of ODGV

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 given in Table 3. In the computational grid generation, there are 86 498 cells and

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86 102 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° (the starting position of the VAWT blade) and the direction of blade rotation, Ω.

Table 3 Sets of generated mesh for simulation

Cell Quantity Region

With ODGV Without ODGV

Water tunnel 44 002 43 444

Rotary maingrid 12 991 13 081

Rotary subgrid 29 505 29 577

Total 86 498 86 102

4 Results and discussion

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 to 115, 94 and 109 r/min for the three configurations, i.e. 0°, 30° and 60°, 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 on­coming wind. By comparing these results, the VAWT gives the highest increment in rotational speed when the wind is blown from the direction of 0° 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 increment for each

Fig. 8 Experimental results for bare VAWT and integration of VAWT with ODGV in three configurations

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/(r∙min −1 ) Increment/%

No ODGV 51 —

ODGV 0° 115 125

ODGV 30° 94 84

ODGV 60° 109 114

Average — 108

4.2 Numerical simulation results The result obtained from the CFD simulation is the

coefficient of moment, Cm, of the blades on the centre of rotation. The simulation is 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. Figure 9 shows the coefficient of moment of the bare VAWT and VAWT with ODGV for azimuth angle from 0° to 359°. After obtaining the average moment coefficient, the average power coefficient can be calculated (Table 5).

Fig. 9 Plot of moment coefficient, Cm versus azimuth angle, θ, with and without ODGV

The simulation result shows that the average power coefficient is increased from 0.007 804 (without the presence of the ODGV) to 0.023 909 (with the presence of the ODGV), indicating 206% increment. By comparing the two curves in Fig. 9, the negative portion is minimized when the ODGV is introduced. This is because the vanes of ODGV guide the wind to attack the wind turbine blade at a better flow angle. Hence, the power generated is augmented by utilizing the ODGV.

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Table 5 Results for VAWT with and without ODGV

Parameter VAWT without ODGV

VAWT with ODGV

Average coefficient of moment, Cm,average

0.019 5 (5 blades)

0.059 8 (5 blades)

Rotor rotational speed, θ & /(rad∙s −1 )

0.303 3 0.303 3

Tip speed ratio, RT 0.4 0.4

Average power coefficient, Cp,average

0.007 804 0.023 909

5 Conclusions

1) An innovative device called omni­directional­ guide­vane (ODGV) that surrounds a VAWT is designed to improve the wind rotor performance. The shrouded design of the ODGV can minimize public concerns of installing a high­speed rotating wind turbine for on­site power generation and it is aesthetically friendly to an existing building. It also integrates several green and renewable energy harvesting technologies.

2) 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 single­bladed VAWT. With the presence of the ODGV, the power produced by the single­bladed VAWT increases by 206% at tip speed ratio of 0.4. The negative torque zone has been minimized, thus increasing the rotor torque.

3) 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 on­site and grid­connected power generation. The geometry of the ODGV can be further improved to match with the other types of VAWT. The long­term goal is the proliferation of wind and solar energy applications in populated urban areas or sub­urban regions, capable of supplying supplementary power to urban buildings.

Acknowledgments The authors would like to thank University of

Malaya for the assistance provided in the patent application of this design (WO2010098656­A2; WO2010098656­A3), and the research grant allocated to further develop this design under the project RG039­09AET and D000022­16001. 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, FP071­2010A).

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(Edited by YANG Bing)