Aerofólio S809 - 1 - s2 0 - s0960148109003048 - turbinaeolica

Aerofólio S809 - 1 - s2 0 - s0960148109003048 - turbinaeolica

(Parte 1 de 3)

Wind tunnel and numerical study of a small vertical axis wind turbine

Robert Howell*, Ning Qin, Jonathan Edwards, Naveed Durrani Department of Mechanical Engineering, University of Sheffield, Sir Frederick Mappin Building, Mappin Street, Sheffield S1 3JD, UK article i nf o

Keywords: Wind turbine VAWT HAWT Wind tunnel CFD abstract

This paper presents a combined experimental and computational study into the aerodynamics and performance of a small scale vertical axis wind turbine (VAWT). Wind tunnel tests were carried out to ascertain overall performance of the turbine and two- and three-dimensional unsteady computational fluid dynamics (CFD) models were generated to help understand the aerodynamics of this performance.

Wind tunnel performance results are presented for cases of different wind velocity, tip-speed ratio and solidity as well as rotor blade surface finish. It is shown experimentally that the surface roughness on the turbine rotor blades has a significant effect on performance. Below a critical wind speed (Reynolds number of 30,0) the performance of the turbine is degraded by a smooth rotor surface finish but above it, the turbine performance is enhanced by a smooth surface finish. Both two bladed and three bladed rotors were tested and a significant increase in performance coefficient is observed for the higher solidity rotors (three bladed rotors) over most of the operating range. Dynamic stalling behaviour and the resulting large and rapid changes in force coefficients and the rotor torque are shown to be the likely cause of changes to rotor pitch angle that occurred during early testing. This small change in pitch angle caused significant decreases in performance.

The performance coefficient predicted by the two dimensional computational model is significantly higher than that of the experimental and the three-dimensional CFD model. The predictions show that the presence of the over tip vortices in the 3D simulations is responsible for producing the large difference in efficiency compared to the 2D predictions. The dynamic behaviour of the over tip vortex as a rotor blade rotates through each revolution is also explored in the paper. 2009 Elsevier Ltd. All rights reserved.

1. Introduction

As a sustainable energy resource, wind energy is increasingly important in national and international energy policy in response to climate change. Many large scale commercial wind farms have been built in the UK and electricity generation from wind now exceeds 3.6 GW and is increasing rapidly. To meet its obligations under the Kyoto Protocol the UK Government has adopted a target for renewable energy generation of 10% of UK consumption by 2010, 15% by 2015 and an aspiration of 20% by 2020 [1]. Much of this, by necessity, must be met by wind energy. In addition, the EU Commission has set a European target for 2010 of 12% of electricity generation from renewable sources.

The two primary types of wind turbine are the horizontal axis

(HAWT) and vertical axis (VAWT) machines. The horizontal axis machines are highly developed and used in all current large scale wind farms. On the other hand, the majority of research on VAWT designwas carried out as long ago as the late 1970sand early 1980s, notably at the USA Department of Energy Sandia National Laboratories [2–5] and in the UK by Reading University, and Sir Robert McAlpine and Sons Ltd (through their subsidy VAWT Ltd) who erected several prototypes including a 500 kW version at Carmarthen Bay [6]. When it became accepted that HAWTs were more efficient at these large scales, interest was lost in VAWT designs and HAWTs have since dominated wind turbine designs. It is therefore not surprising that very little research can be found in the last couple of decades on the VAWT, its aerodynamics and the problem of the interaction of the blade structure with the unsteady aerodynamic loads. Their technical development lags significantly behind that of HAWTs. However, it has never been shown that HAWTs are fundamentally more aerodynamically efficient than VAWTs. Indeed it has been suggested that VAWTs may be more appropriate than HAWTs at very large scale (10 MWþ) due to the alternating gravitational loading on a HAWT blade becoming excessive. There are a number of substantial advantages over

E-mail address: (R. Howell).

Contents lists available at ScienceDirect

Renewable Energy journa l homepage: w.else m/lo cate/renen e

0960-1481/$ – see front matter 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2009.07.025

The VAWT has no need to constantly yaw into the local wind direction. Due to the relatively lower rotational speed, VAWTs are typically quieter than HAWTs. The manufacturing cost for a very large VAWT could be lower than that for an equivalent HAWT due to the simpler straight constant section blades compared to the complex threedimensional blade shape in HAWTs. The VAWT is also mechanically better able to withstand higher winds through changing stalling behaviour, offering a potential operational safety advantage during gust conditions.

Considerable improvements in the understanding of VAWT can be achieved through the use of CFD and experimental measurements. The aim of this paper is to illustrate some of the improved understandings of the aerodynamics of vertical axis wind turbine performance through wind tunnel testing and computational simulation of the flow field around the turbine.

One of the constraints used with the turbine developed here was that it was to be operated at realistic wind speeds. A median wind speed of 3.5 m/s at 10 m above ground was measured at the Sheffield University weather station between 1988 and 2007. Performance measurements were taken around this wind speed as well as some measurements at higher wind speeds, but it should be born in mind that these higher speeds are rather rare, particularly in the urbanenvironment. Forexamplejust 0.3% of thewindspeeds recordedat Sheffield University’s weatherstationwere above 12 m/ s. The investigators were interested in real world performance, not artificial 12 m/s ‘rated’ wind speeds often quoted by manufacturers.

2. Experimental set-up

The wind tunnel tests were conducted at the Aerodynamics

Laboratory at Sheffield University. The University low speed wind tunnel used for this study has a square test section of dimensions 1.2 m 1.2 m. The tunnel consists of a large bell mouth screened inlet with a contraction ratio of (2.5:1) before the flow encounters another screen and an array of honeycomb flow straighteners. The tunnel has a working section of length 3.0 m.

Any obstruction placed within a wind tunnel will alter the characteristics of the flow to some degree. If this obstruction is too largethen the area available tothe flow is significantly reduced, and so the speed of the flow around the model will increase. If the blockage ratio is high enough then the effects of the tunnel walls may begin to interfere with the flow over the model. The literature suggests that blockage ratios below about 6–7.5% have a negligible effect on the flow. However, the difficultly in this case comes in defining the frontal area of the turbine as it spins. The area of the wind turbine when it is stationary is small, but the area it sweeps out as it spins is significant. For the most conservative value of blockage ratio, the frontal swept area of the turbine could be used and using the wind tunnel dimensions, this would result a2 0 cm 43 cm turbine (i.e. area 0.086 m2). Clearly this would be a tiny model, and the difficulties in testing it would be numerous including the problem that the Reynolds number would be too much smaller than that of a realistic VAWT. It was therefore more practical and reasonable to assume the apparent frontal area of the turbine would lie somewhere between the small area of the components and the large area of the swept frontal area. It was decided that blockage ratios based on frontal swept area could be tolerated beyond 7.5%, so a notable amount of the frontal swept area would not actually be occupied by the turbine. Preliminary experiments with the turbine in the wind tunnel showed that there was no correlation between the rotational speed of the wind turbine and the approach velocity within the tunnel (measured five rotor blade chords upstream). Furthermore, during the testing of the Turby VAWT [7], a turbine with blockage ratio of 14% based on frontal swept area was used with a view to it having a ‘‘negligible blockage effect’’.

2.1. Aerofoil selection

Traditionally NACA 4-digit series have been employed for Darrieus-type VAWTs. For example, a NACA0015 profile has been used in the Sandia investigations. In the late 1970s, Healy investigated the effect of thickness on VAWT performance [8]. More recently, a systematic numerical study of various aerofoils including NACA 4- digit series based on the unsteady RANS solutions of the VAWT flows by the authors indicated that thicker sections performed much better under the flow conditions of interest. As a result of our simulations, the NACA0022 profile was chosen as the primary profile candidate for this research programme. In order to create a geometrically accurate profile the rotor blades were manufactured using CNC milling machine from high-density foam. In order to give the blades sufficient strength to withstand the centrifugal bending forces cause by the high rotational speeds of the turbine, the foam had to be of a minimum thickness. The NACA0022 profile was therefore created with a thickness of 2 m and a resulting chord of 100 m. This, combined with a height of 400 m (limited by the CNC machine), gave the blades an aspect ratio of 4 and gave the turbine a solidity of 1.0 for the three bladed turbine and a value of 0.67 for the two bladed turbine. These values are rather high given other research clearly shows a lower solidity will result in a higher performance coefficient. However, the aim of this research turbine was to provide validation data for computational methods and an understanding of the aerodynamic inefficiency (such as those generated by the tip flows and resulting vorticies) so maximumperformancein itselfwasunimportant.Aswillbeshown later, the low aspect ratio did indeed give rise to large tip vorticies. Testing was carried out using 300 m rotor arms resulting in a blockage ratio (based on frontal swept area) 16.7%.

During initial testing at some conditions, the turbine would suddenly reduce its rotation speed and eventually stop despite no changes to the applied torque. It transpired that the turbine blades were slowly rotating about an axis centred through the bolts that fix them to the support struts. This was caused by the centre of rotor lift not being aligned properly with the fixing bolts. It is clear that the centre of lift for VAWT rotor blades is constantly changing throughout every rotation so it was not a simple process to determine the best location of the fixing holes on the rotors. The maximum change in the rotor blade fixing angle was measured to be less than 5 and this small angle change caused a complete loss of lift and hence the eventual complete loss of power from the turbine.

A CAD model was used to carry out the design before manufacture, see Fig. 1. Two deep-grove ball race bearings were used to support the turbine and allow free rotation of the rotor shaft, where the lower bearing supporting the weight of the turbine by acting as thrust bearings. Two support arm/strut brackets were designed to fit on the main drive shaft so that they could be mounted anywhere along the heightof the shaft accommodating 2, 3 or 4 blades to accommodate future blades of different lengths (aspect ratios). Rotor blade radial arms/spokes were machined from aluminium bars and aerodynamically profiled with an elliptical leading edge (major to minor axis ratio of 2:1) and sharp trailing edge. All components were attached such that they could be assembled or adjusted with ease. The finished assembly is pictured in Fig. 1.

To measure the power output from the turbine a simple torque brake was employed. The torque applied to the turbine rotor drive

shaft was increased and decreased by changing the separation distance between two spring balances. The torque applied was calculated using the difference in the forces applied by the spring balances and the drive shaft radius. To calculate the power output, this torque was combined with the rotational speed of the turbine, itself picked up from a once per revolution optical tachometer. A feedback control system was not used for the control of the turbine so it was not possible to measure the characteristics of the turbine past its maximum torque (and thereforeminimum stable rotational speed). This is because beyond this limit any small increase in applied load (torque) causes a drop in the rotation speed of the turbine and a drop in the lift (and driving torque) it generates. If there is nocontrol systemtoreduce the applied torque to match the new aerodynamics condition, the turbine rotor will continue to drop in speed so the applied torque becomes more unmatched to the conditions of the slower rotating turbine.

3. Experimental results

This sectionpresents the results obtained from the wind turbine measurement campaign; however the effect of bearing and windage losses must first be explained. To determinethe amountof power lost in the system a series of spin down tests were conducted. To increase the moment of inertia of the system once the foam rotor blades had been removed, small weights were attached to the ends of the support spokes. The rotor system was manually spun up to slightly beyond the maximum speed used in the turbine performance testing and then allowed to freely decrease in velocity due to the effects of friction in the bearings and windage on the support arms and the added weights. As the speed reduced, the time and instantaneous rotor rpm were recorded. From this information, it was possible to calculate the bearing and windage losses of the system (with its known moment of inertia) without the rotor blades attached. In this paper, the power used to overcome these losses is actually included in the stated power output from the turbine. Clearly this is not useful power output, but it is still the power developed by the turbine in overcoming losses as well as generating useful work in the torque brake and is the correct value to be compared to the CFD results. In a turbine of this size the bearing losses are significant and so must therefore be carefully analysed.

Fig. 2 shows how the measured torque due to bearing friction and windage changed with tip-speed ratio as well as the torque applied by the brake for the same conditions. It is clear that at high

Fig. 1. CAD model of the turbine model assembly and the model turbine in the wind tunnel.

Fig. 2. Measured bearing and windage torque versus power torque developed by the turbine for a wind speed of 5.07 m/s.

tip-speed ratios, the torque caused by the losses generated in the bearings, support arms and windage in the system are double that of the applied braking torque. This means any error in the measurement of the windage torque will have a very significant effect on the performance coefficient. At the other end of the performance envelope, at a tip-speed ratio of around 2, the applied braking torque is more than double the windage torque, but none the less this torque ‘loss’ is still very significant.

(Parte 1 de 3)