Urban wind turbines do they have a future? Or will they be white elephants?

Transcription

1 Urban wind turbines do they have a future? Or will they be white elephants? Presented by Brian Kirke As part of the What On Earth series, UniSA, 1 November 2012

2 WWEA* is optimistic about small wind (defined as either < 100 kw rated output or < 200 sq m swept area) WWEA releases Small Wind World Report World market for small wind turbines sees dynamic growth and total capacity reaches 440 MW In 2020, WWEA expects that the total installed small wind capacity will reach 3,800 MW, representing an almost tenfold increase compared with Ref: ew&id=348&itemid=40 * World Wind Energy Association

3 So people are putting up small wind turbines in urban environments

4 Taiwan Turby, Netherlands

5 Pixel building, Melbourne

6 4 MUCE 12 kw VAWTs on MBB Building, Hobart Notice that most Urban turbines are VAWTs (vertical axis). This is because (i) they don t need to yaw to follow changes in wind direction, so they are claimed to be better in turbulent wind conditions around buildings, and (ii) they are quieter than HAWTs because blades move more slowly (noise increases with 5 th power of blade velocity).

7 VAWTs can also get up to 30% more energy from vertical components of wind for 2 reasons: 1. Blades intercept some undisturbed wind on downwind pass, thus increasing effective swept area 2. Radial arms act like Wells turbines And generate some forward torque The Quietrevolutionwind turbine generator can utilisewinds from all directions unlike traditional HAWTs which need to track the oncoming wind. This is an important advantage in locations where winds are turbulent, gusty and constantly changing directions. Helical turbine being tested in a wind tunnel with axial flow

8 But SA Premier Mike Rann knew better. five SWIFT mini wind turbines are have been installed... on three Government buildings in the City, on the Central Market tower and one is pole mounted at the Environment Centre at Mawson Lakes At this point output of each of the turbines will be monitored against climatic conditions to better understand the value of this type of technology in a built up environment. (CAPITAL CITY COMMITTEE ADELAIDE, Annual Report ) Ignoring the fact that Modern HAWTs* have been designed to perform optimally in wind regimes found in open terrain; environments that typically exhibit low turbulence levels and predominantly laminar flow with relatively small vertical velocity components. (Victorian Urban Wind Resource Assessment, ATA, April 2009) *Horizontal axis wind turbines

9 And sure enough, an article in the Adelaide Advertiser, 11 Sept 2010, reported that Other urban turbines aren t doing too well, either. So what s the problem?

10 There are lots of problems! 1. Ignorance about wind in urban areas: (i) Less available energy than in flat, open terrain (ii) The wind around buildings is turbulent and unpredictable 2. Small wind turbines don t work as well as big ones due to Reynolds number effects 3. A lot of small wind turbines are not well designed: apparently designers haven t done their homework 4. Power curve data from manufacturers is often either lacking or based on theory, not actual measurement.

11 1. Ignorance about wind in urban areas Case studies of urban wind turbines overseas demonstrate poor economic performanceand long payback periods, a problem frequently attributed to a lack of accurate wind measurement preceding installation. Urban areas have, by the nature of their built-up topography, slower wind regimes than open rural areas. The number of obstacles in the path of the wind also makes it difficult to model the wind resource. Models and wind maps that do exist have a resolution far lower than the size of an average building, and turbulence can result in two adjacent locations having vastly different wind regimes. Ref: The Viability of Domestic Wind Turbines for Urban Melbourne, ATA, June 2007

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13 The Victorian Urban Wind Resource Assessment, prepared by the Alternative Technology Association (ATA), April 2009, found only one good site in Melbourne out of 15 surveyed.

14 The wind resource at MBB is W/m 2, and much less than 100 W/m 2 at the ACB. The corresponding mean wind speeds at each building were estimated at 5-6 m/s and 3-4 m/s, respectively. The MBB is right on the water front with hub height 51m. ACB MBB From CyclopicEnergy report Hobart Marine Board Building & ANZ Centre Building Wind study for a roof-mounted wind turbine installation Interim report -7 November 2010.

15 The wind around buildings is turbulent and unpredictable From CyclopicEnergy report Hobart Marine Board Building & ANZ Centre Building Wind study for a roof-mounted wind turbine installation Interim report - 7 November 2010

16 2. Small wind turbines don t work as well as big ones due to low Reynolds numbers lower stall angle, lower L/D ratio, lower max lift coefficient, higher drag coeff

17 Small wind turbines have trouble self-starting: Blade chord Re = Vc/νwhere V = relative velocity between blade and air, c = chord length (i.e. blade width), and ν( nu ) = kinematic viscosity 1.5E-5. e.g. 0.2m chord in 4 m/s wind, at λ= 2, 8 = 4 m/s, mean Re = 8 x 0.2/ 1.5E-5 100,000. VAWT won t self start! coefficient of performance ( efficiency) Tipspeed ratio = blade speed/wind speed

18 Small HAWTs have trouble self-starting, too According to David Wood, a small HAWT aerodynamics specialist, now Professor of Renewable Energy at the University of Calgary, who headed the Newcastle (NSW) wind turbine group for many years, Small HAWTs are typically designed to operate at tipspeed ratios of 7 to 10. This means fixed pitch blades are stalled when starting, so they are slow to get going. He considers variable pitch is too expensive U of Newcastle built a 1.94m diameter, 500W HAWT which operated for 10 years. It could start generating (cut in) at 3.4 m/s wind speed, but needed 4.7 m/s to start turning, although it would keep turning until wind dropped to 2.1 m/s. Relvelat High λ Lift Forward torque D, no forward torque at λ= 0 Relvelat startup

19 How to improve VAWT self-starting? Baker, J.R. (1983). Features to aid or enable self-starting of fixed pitch low solidity vertical axis wind turbines. J. Wind Eng & IndustAerodynamicsVol.15, , proposed the use of cambered aerofoils. Most VAWT designers favoursymmetrical aerofoilsbecause the angle of attack reverses as the turbine rotates. The NACA 0012 is a symmetrical aerofoil, and the Gö (Göttingen) 420 is a slightly cambered aerofoil. Baker argued that cambered blades work much better than symmetrical blades at positive incidence, especially at low Reynolds numbers (see next slide), and most of the energy is extracted on the upstream pass, so cambered blades with positive incidence on the upstream pass would give positive energy per cycle (revolution) for all tipspeed ratios, and would therefore self-start, unlike the NACA 0012.

20 Cambered aerofoilswork much better at positive incidence αthan symmetrical aerofoils, especially at low Reynolds numbers, but worse at negative incidence Highly cambered Aerofoil section NACA0015 symmetrical section at same Re

21 Kirke, B.K. and Lazauskas, L. (1991). Enhancing the Performance of a Vertical Axis Wind Turbine Using a Simple Variable Pitch System. Wind Engineering Vol.15, pp : Modelling showed that cambered blades would improve self-starting but reduce peak efficiency Symmetrical aerofoils: won t self-start. Cambered aerofoils: can self-start

22 We also showed that increasing solidity would improve self-starting (solidity σ= nc/r where n = number of blades, c = chord length and r = turbine radius. Thus for example a 2 m diameter turbine with 3 blades of 0.2m chord has σ= 3x0.2/1 = 0.6 Low solidity High solidity

23 Increasing blade thickness also improves starting torque. Increased mechanical strength is a further benefit, and there has been a trend in VAWT design from NACA 0012 sections in the early days to 0015 and 0018 sections.

24 Poorly designed turbines In an attempt to improve self-starting, many VAWT designers have gone to cambered blades with high solidity: large chord and/or many blades. But this reduces tipspeed ratio and hence efficiency, because blades are stalled most of the time performance coef fficient Cp prediction, solidity = 0.3 prediction, solidity = 0.4 prediction, solidity = 0.5 prediction, solidity = 0.6 field test, solidity = tipspeed ratio

25 Most of them have the camber the wrong way: convex on the outside looks right, but means negative incidence and poor performance on the upwind side where most of the power is extracted oops! Wrong! Right

26 no regard for parasitic drag Elliptical section arm should be aerofoil Blade with camber wrong way High drag bracket joining Blade to arm (unnecessarily rigid) Incredible high Drag arm!! Even with carefully designed aerofoil arms and minimum drag arm-blade connections, Parasitic drag loss is serious

27 Cp parasitic loss Cp predicted gross Cp measured gross Parasitic drag loss is important! It increases steeply with increased RPM. Predicted (modeled) efficiency did not allow for parasitic loss. When measured parasitic loss was deducted, predicted efficiency dropped from 50% to 27% λ Cp predicted net Cp measured net parasitic loss Cp λ

28 Misleading performance data The power curve apparently assumes 35% overall conversion Efficiency at all wind speeds. In fact the turbine efficiency is probably no more than 25% if perfectly matched to generator. Generator & inverter are not 100% efficient, so actual electrical output is probably half what the manufacturer claims. And data from the installer, IWE, is not the same as data from the manufacturer, MUCE.

29 So, we have poorly designed small turbines in the wrong places with misleading data no wonder they perform badly. What can be done? 1. Make them bigger to avoid low Reynolds numbers: it s hard to compete with small PV, but larger wind turbines are more cost-effective (about $4000/kW for small ones, $2000/kW for big ones) 2. Put them in the right places: open, exposed coastal, rural, hilltops and/or high towers to get good wind 3. Design VAWTs better: - Helical to even out torque and radial force fluctuations - Savoniusrotor for starting torque - Variable pitch for improved starting torque, higher efficiency, easier governing and less shaking

30 Make them bigger?? Some people think existing tall (and otherwise useless?) structures could be used to support large wind turbines in urban areas. But these structures would have to be designed for the extra loads.

31 Put them in the right places: in this map, brown is windy, blue is not. Southern, western and coastal areas are best

32 In this map, yellow is most windy, then green, blue and red is least windy. Some high country has good wind

33 Wind is very site-specific: high resolution data reveals big local variations

34 If high resolution data are not available, look at the local landscape, pick likely spots and monitor actual wind at hub height with an anemometer

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36 Better design: helical blades even out shaking, but reduce efficiency slightly and don t improve starting much The graph below shows actual power produced by a 7 kw fixed pitch helical VAWT. says Cut in at sustained 5m/s. At least they have real data but it s not impressive. Colston Hall, Bristol

37 Hybrid: Savoniusrotor for starting torque and Darrieus rotor for high tipspeed ratio performance Right site Wrong site! DS-1500 with solar panel for residence in Australia 25 C Q - λcurve for a Darrieus turbine showing the effect of a Savoniusauxiliary rotor with swept area 1/6 that of the main rotor. Torque, Nm Darrieus torque Savonius torque total torque λ

38 Variable pitch 1985: my first variable pitch VAWT: high starting torque but low efficiency. Feb 2012: hydrokinetic turbine with new pitch control system, high starting torque, 32% efficiency, reduced shaking August 2012: prototype VAWT with new pitch control system. The next step is to test its performance

39 0.45 Variable pitch hydrokinetic turbine test results to date: still room for improvement effic ciency Cp Feb Apr Nov-10 modelled tipspeed ratio

40 Variable pitch The water turbine has demonstrated a simple, effective variable pitch mechanism that starts easily and is more efficient than the competition. A demo wind turbine has now been built with simple aerodynamic overspeed control as well. Modellingby Dr Leo Lazauskas Torque coefficient Maximum torque Fixed pitch Maximum torque Variable pitch Variable pitch: high starting torque: Fixed pitch: low starting torque Ratio of blade speed to wind speed

41 conclusions Small wind turbines are generally not viable in urban areas except at exposed coastal or hilltop sites Wind turbines of a few kw capacity can be viable, especially in parallel with PV, at windy rural sites

42 Thank you for your attention Any questions?