Windmeup.org

Understanding inflow angles

2010-06-16T15:40:00.003+10:00

So, we've been using WAsP for a while now, and we've come up with some great layouts optimised for energy production - however what is this pesty inflow angle that keeps coming up in the turbine manufacturer's siting reports? Is this something that really has to be considered?

The inflow angle is the angle (off the horizontal) at which the mean flow comes into the rotor, and we want to keep this as close to the horizontal as possible. It affects both the turbine loading, and the aerodynamic efficiency of the rotor (the power curve). As per IEC 61400-1 edition 3, the turbine manufacturers' design requirement for inflow angles are +/- 8 degrees (from the horizontal). These inflow angles are used as the design basis for the aeroelastic load calculations, so they really drive the fatigue loads. In practise, the more critical inflow angle is positive (coming up into the rotor), and this is usually the only one calculated by the turbine manufacturer as this is conservative. The reason for this is predominantly due to the rotor tilt being positive, and therefore a slight negative inflow angle (coming down into the rotor) can sometimes be a beneficial thing for fatigue loads.

Here are some outputs from WAsP Engineering 2.0 of inflow angles as it comes over a North-to-South ridge (wind from the west), it shows the positive inflow angle as it climbs over the ridge (green) and negative as it descends back into the valley (red). For this theoretical project, you can see the turbines right on the crest of the ridge where the flow is relatively neutral before it descends again - coincidentally this is where the high wind is and where we should be aiming for.

Sometimes it can't be avoided putting turbines in higher inflow angles, particularly for more complex sites where we are trying to optimise capacity, however in practise you should avoid exceeding inflow angles of +/- 4 degrees to minimise loads and maintain aerodynamic efficiency (power curve). If you exceed this you'll have to dive into more detail on the loading, and the accuracy of the power curve will be called into question (the manufacturer's power curves are also usually linked to a narrow range of inflow angles).

I'll devote a future post to the aerodynamics of inflow angles, and how this affects turbine loading and the power curve. For a taste, just think positive inflow into a rotor with rotor tilt - we're starting to venture into helicopter aerodyanmics!

Melbourne offshore wind: WAsP in the southern hemisphere

2010-06-08T13:20:00.004+10:00

Following on from my last review of WAsP 10, Risoe have now fixed up the southern hemisphere issue for their map editor, and more importantly for their Google Earth plug in. Now even guys in the southern hemisphere can visualise their latest layout in 3D rendered Google Earth brilliance!

Check out a preview below, where you can see the purely ficticious offshore project that I quickly knocked up in my hometown of Melbourne, Australia. Purely a figment of my imagination, however it would be rather nice...

WAsP 10 review

2010-03-30T16:33:00.003+11:00

The latest version 10 of Risoe's wind-analysis software WAsP has been released for a few months now, and after working with it for a while I thought I'd post a few thoughts and comments on how it's working.

WAsP is a micro-scale wind modeling software package used to estimate wind speeds over a region based on measured wind speeds at discrete points (typically met masts). WAsP also forms the backbone for other wind packages such as Windfarmer and Windpro. The orographic-flow model used by WAsP is the 'BZ-model' of Troen (1990), which in simple terms places a high-resolution 2D polar grid over the measured wind source to estimate the flow perturbations over the region - so this isn't a CFD package. Importantly, the resolution and accuracy of the grid reduces the farther away from the initiation point. The flow model used to estimate the perturbation also assumes that there is no flow separation, and therefore the validity breaks down with more complex terrain.

One of the biggest changes in this release in my opinion is the new Google Earth integration, and it works very well (in the northern hemisphere at least - see below). In a few simple clicks, WAsP will send your live turbine layout and met mast locations into Google earth so you can visualise the turbines and masts, as well as your topographic and roughness assumptions, and any resource grid you run. It's a well implemented feature and actually models your turbine tower height and rotor diameter based on your .wtg model. Eye candy aside, this can really help in layout development as it allows you to have your land boundaries and other GIS data in Google Earth (as .shp files for example) and develop your layout around it. It also allows you to bring the satellite imagery back into WAsP as an overlay image.

Official demo site in Portugal - wind resource grid and turbines visible.

There are also some great user friendly changes in WAsP, such as the turbine diameter circles around your positions (this was my favourite), and the added reference locations (which allows you to check how the wind modeling is handling certain features of interest, such as other masts).

workspace view with orography, turbines, and spacing visible

Unfortunately a minor bug in WAsP still hasn't been fixed, and that is that it assumes all map projections are in the northern hemisphere. What this means is that for projects in the southern hemisphere, the WAsP map editor can't convert between datums and projections, and the Google earth integration doesn't work! The actual wind modeling isn't affected however, so it's more of a cosmetic issue that can be overcome with another GIS package. Risoe have told me they are working on a fix for this however.

Overall however I think this is an excellent and constantly evolving software package, and is well
worth the one-off EUR 3300 license fee. For users of Windfarmer and Windpro, I thoroughly recommend looking into your base flow model again!

note: I have no affiliation with Risoe/WAsP and have fully paid for my licensed copy! :)

LIDAR-based wind turbine control system

2010-03-13T11:56:00.003+11:00

There's been a little bit of interest in laser based anemometry, or LIDAR, wind turbine control systems. It's been the precept of Risoe's Windscanner program below, and several Risoe-DTU papers at EWEC 2009. It even made this month's Economist technology quarterly. As rotors get bigger, I believe this concept will become more and more import - both for energy production optimisation, and for loads reduction on the blades and drive train.

So what is LIDAR? LIDAR is simply the use of a laser in the same way a RADAR is used, by measuring the time it takes for pulses to return after they have bounced off minute particulates carried in the air, a profile of the wind speed can be developed. If used in an array, a three-dimensional picture of the column of approaching wind can be more accurately estimated.

How does this help a wind turbine? At the moment, wind turbines estimate the wind speed based on a nacelle-based anemometer - typically ultrasonic, or a traditional cup anemometers. The wind speed measured here is used as the basis for the controller to estimate the wind speed over the entire rotor and set the appropriate pitch setting of the blades to optimise the angle of attack. Wind turbine blades operate most efficiently at only one angle of attack setting - too much, or too little reduces the aerodynamic efficiency and therefore the energy output.

The bigger rotors get, the less accurate it is to estimate the entire wind speed of the column of wind based on one single anemometer input. So, a LIDAR can help by providing a more accurate distribution of wind speeds of the incoming wind. Armed with this, the controller can then anticipate more accurately the pitch settings to use (possibly even individual blade pitch settings). In the case of an extreme gust, the blades could then feather and dramatically reduce the loads on the turbine, where as a traditional system will only know a gust after it's experienced it.

There are some definite practical engineering problems with integrating this system however. If you base it on the ground, how do you yaw it efficiently to align with the turbine? If you base it on the nacelle, you have to place it in the rotating hub and allow for the complications of rotating a delicate laser array continuously. Whether or not the increased cost and complication of this system will be balanced by the production increase and possible cost reduction in structure optimisation: only industry experience will tell.

350.org international day of climate action this 24 October 2009

2009-10-16T10:13:00.003+11:00

350.org are organising numerous actions across the globe this coming Saturday 24th of October 2009, you can see a full map below of where you can find them.

For those of you unaware, 350.org is trying to raise public awareness of the science of climate change, and prompting open public debate about our CO2 emissions - specifically returning from our rapidly increasing 390+ ppm of CO2 to 350ppm to avoid the serious impact of climate change. The key message here is that we have already passed the safe limit, and we need to get back - not argue about how much further we can go.

Key to returning CO2 levels to 350ppm is the future energy sources we use as a planet, a significant part of this mix will be wind energy - the most cost-effective and globally accessible source of renewable energy we have at the moment. It's a great message, and well worth getting out and showing your local support!

View Actions at 350.org

extreme cyclonic weather

2009-07-20T10:38:00.000+10:00

Based on some recent experiences, I thought I'd recap on extreme cyclonic weather and how it influences wind-turbine site analysis.

Cyclonic weather relates to weather systems that rotate clockwise around low-pressure systems in the Southern hemisphere, and anti-clockwise in the Northern hemisphere. Once they exceed a set level (and there are various classification systems) then they are called: typhoons in the Pacific, Hurricanes in the Atlantic, and tornadoes when they are land based. Tornadoes can be spawned from typhoons/cyclones/hurricanes when they hit land.

Research in the field is fairly limited, however they seem to be characterised quite well by the Holland model: a warm centre surrounded by colder air with no fronts. Extreme winds occur in the outer eye wall. As the cyclone rotates, centripetal acceleration is balanced by the pressure gradient (suction internally), therefore the lower the eye pressure the higher the wind speeds.

example cyclone passage over a weather station

So, how do we deal with them? In my opinion they affect wind turbines in three ways:

extreme wind speeds
extreme changes in direction as the eye passes
grid-outages, and the turbines ability to 'center' with no external power

On the extreme wind side, the Vref (10-minute average) and Ve50 (3s gust) should be investigated closely, difficult to get data however any cyclone tracking station data should have estimates of these speeds. These are typicaly at 10m height, so you have to extrapolate them to hub height, a wind shear of around 1.2 is typically representative. However how do you estiamte the one-in-fifty cyclone strength? To do this, I did a Gumble analysis on peak cyclone wind speeds in the area. Failing this, the local building codes may also give you an idea of the extreme wind speeds (however not recommended!).

Extreme wind direction changes can be estimated by looking at the cyclone's speed along the path, and estimating the angle change over time. The peak rate of change can be compared to the IEC standards.

Grid outages are typically coincidental with cyclone passages, as they knock down power lines in the area. Although a turbine might be rated to class I extreme winds (DLC 6.1) the load case with loss of power (DLC 6.2) doesn't use safety factors as it's an accidental load case - therefore the loads are significantly lower. For maximum survivability then, it's worth investigating a UPS (a diesel backup generator for example) to kick in when a cyclone is coming to keep the turbine centering into the wind. These are really 'soft systems', where the turbines are manually stopped switched over to the UPS, and not put back into run again until the cyclone has passed.

For further reading, I strongly recommend checking out Risoe report RIS-R-1544.

power-line mounted turbines?

2009-06-19T10:28:00.003+10:00

Check out this winner of the 2008 red dot design awards by Nils Uellendahl, it's a concept for a helical turbine that mounts directly onto over-head power lines. It's quite interesting in two ways:

it generates electricity directly into the line using induction
It's actually horizontally mounted, however is actually a traditional vertical-axis turbine

I'm sure there are some large issues with the electrical connection and grid stability (to say the least!), and with the additional loading of the power lines, however it's a great concept.

24 October 2009 - an international day of climate action

2009-06-19T09:54:00.004+10:00

In preparation for the December '09 Copenhagen round of international discussions on climage change, 350.org is busy preparing local actions around the world on the 24th of October to reiterate the goal of a return to 350ppm of CO2 - a worthy aim for any climate change negotations indeed!

The Copenhagen round will be keenly watched by the renewable-energy industry, as it could well provide a huge boost to meet any meaningful CO2-reduction target. It's fair to say that the wind industry will be set to receive the lion's share of any boost, as it's commercially the most ready technology we have available at the moment. A 2010 boost would be welcome news to an industry that is starting to feel the effects of the GFC through hard-to-find financing.

Big rotors king for 2010+

2009-05-25T13:51:00.007+10:00

A few years back, it was quite common to see various turbine manufacturers releasing their latest Class IA turbine, followed rather quickly with a class II and III rotor. This was typically just a longer blade with minimal nacelle changes. The Vestas 2WM, Siemens 2.3 MW, and GE 1.5 MW platforms are good examples of this.

This was understandable at the time, as there were numerous easily exploitable IA sites around, with demand in the market for suitable class IA turbines. However now in 2009, as we are rapidly shooting past 100 GW of installed capacity around the world, the number of high-wind sites that are also close to the grid and easily developable are becoming quite rare (although there is no shortage of high-wind sites in the world) - a quick chat with any developer will tell you that! The dominant sites for 2010+ are looking to be for class II and class III.

Preempting this, we are seeing the latest generation of turbines being released with this in mind. No longer are turbine manufacturers jumping straight in with their smaller-rotor IA machines. The first release looks to be the II/IIIA specific turbine, with the IA coming later as an after thought (if at all). Have a look at the new Repower 3.xM, Vestas V112-3MW, and GE 2.5XL. Adding to this the last iteration of existing nacelles with new class III rotors: Siemens 2.3-101, Vestas V100-1.8, and the Nordex N100-2.5 ; big rotors are set to be king for 2010+.

Picture of the SWT-2.3-92 I took in the US. Progression from Class IA (82m rotor) to class IIA (92m rotor) to class IIIA (101m rotor) in around 5 years.

density correction of stall-controlled power curves

2009-03-13T16:43:00.003+11:00

For energy estimates at non-standard IEC conditions (p=1.225 kg/m3), the power curve should always be density corrected. For stall-controlled turbines this can be quite tricky.

If you follow the IEC 61400-23 recommendations (which are for power-curve testing) then a simplistic ratio approach is recommended. However, this standard is intended for normalising datasets at 'near sea level' conditions - not for detailed energy estimates. In reality, a stall-controlled turbine will be tuned for site, with the pitch setting feathered for lower density so that rated power is always hit.

For example, if we take a 2000m site (p=0.99 kg/m3) and correct it using IEC measurements we see a large decrease in energy (around 25%) - whereas the actual power curve will be around 10-15% decreased after adjusting the pitch settings. In this case, the pitch settings were feathered by around 2 degrees to achieve this power curve.

Therefore, the site-specific power curve should always be used for energy estimates, and this should be sourced from the manufacturer. For sites at higher altitude (I would say above 1000m), then tested power curves should be insisted on as the aerodynamics of the turbine will change (particuarly for stall-controlled turbines) which invalidated the original testing. I'll go into more detail on stall-controlled turbine aerodynamics in a future post.

Clean-coal busters - this is reality strikes back!

2009-03-05T15:10:00.002+11:00

I love it, the organisation thisisreality.org have come back with a vengeance at all that clean-coal spin! Check out their website to see how they are trying to educate the public and dispel the mistruths of the glossy clean-coal spin doctors!

The credit-crisis for engineers

2009-02-23T11:33:00.002+11:00

For those of you developing wind farms right now, I'm sure you are well aware of the scarcity of credit at the moment, making it difficult to finance wind farm development. Even for no-brainer projects with excellent wind resource, great grid connections, and full planning permits - it's difficult to pry any cash from lenders' white-knuckled hands.

The very talented Jonathan Jarvis has done a great summary of the credit crisis as part of his thesis, allowing even the pragmatic engineer that doesn't like to get involved with the rubbery and subjective world of finance to understand!

The Crisis of Credit Visualized from Jonathan Jarvis on Vimeo.

The destructive spin of 'Clean coal'...

2009-02-16T10:24:00.004+11:00

If you're reading this blog, I'm preaching to the converted, however there is a great article in Scientific American here summarising the destructive spiral of events that the supposedly 'clean-coal' technology has had on the planet.

'Clean-coal': lot's of spin and hollow promises from the coal industry that not only is delaying renewable-energy development, but actively blocking it with hundreds of millions of dollars of spin and lobbying in the US alone (let's not even talk about all that wasted 'research' funding subsidies the coal industry is receiving) - the ever distant promise of clean coal, let's just do nothing until that arrives. Sounds very familiar to other fairy tales of the modern age like hydrogen-powered cars, and cold-fusion generation to name a few.

It doesn't take a genius to figure out that it won't be cheaper to run a coal-fired power station whilst:

scrubbing all of the CO2 out of the emissions (that's over 11-billion tonnes a year),
compressing the CO2 (yes, the same 11-billion tonnes every year)
transporting the CO2 to a suitable storage site (how do you transport 11-billion tonnes of CO2a year anyway?)
pumping and storing that same CO2 underground so that it won't leak... for all future generations of mankind (that's more stringent than nuclear waste storage, however there's 11-billion tonnes of it every year to deal with)

How stupid does the coal industry think the public are?? Obviously fairly stupid. For a great laugh, check the latest spin commercial from the US coal industry (are there people out there that actually buy this??). Oh man, I would love to meet one of the geniuses in the marketing team behind this one...

High-wind hysteresis losses explained

2009-02-13T11:02:00.004+11:00

For anyone who has been presented with energy-estimate reports, a little entry called 'high-wind hysteresis' in the losses table is often brushed over. I've seen values from 0-4% depending on the site, so what actually is it?

In simple terms, it's the turbine's control-system lag between shutting down in high wind speeds, and starting up again. Most modern pitch-controlled turbines will shut down at 10-minute average wind speeds above 20-25m/s, with 3s-gust speeds a little higher than this (typically around 5m/s higher); to prevent the turbine from starting up again during the high wind (just to shut down again), a re-start speed of around 5 m/s less than this value is specified. Physically, this is controlled through discrete parameters in the turbine controller that specify this number.

An example of this is shown below in some historic 10-minute average hub-height wind data:

If we assume a modern turbine (such as the V90) was running here, the turbine would shut down at 25m/s and remain shutdown until the extreme event had passed and decreased below 20m/s. However from an energy estimate perspective, if we are running this data through a power curve, the turbine is assumed to be producing energy whenever the wind speed is below 25m/s and therefore doesn't consider this data. It is this value that we are trying to estimate.

This analysis should be conducted on a representative time series of at least a year to be meaningful. To provide a more detailed analysis, the 3s-gust speeds should also be used, as the turbines will often have a higher 3s-gust shutdown or startup speed; so both scenarios can be considered together, and a more accurate hysteresis loss estimated.

Empirically, the higher the wind speeds and the 'gustier' the sites (particularly inland mountainous sites), the higher the hysteresis loss that can be expected.

Energy Revolution blueprint - wind energy practicalities

2009-01-05T13:38:00.007+11:00

Greenpeace has issued their Energy Revolution blueprint to provide a basis for reducing mankind's CO2 emissions.

There's some great stuff in their blueprint, too much to go into in any detail here, however a key part of their energy strategy involves wind energy. In fact, they recommend increasing fromcurrent levels of around 100GW of installed capacity, to 2733 GW in 2050! This will require a massive increase in: identification and development of sites, local government approval of projects, production of wind turbines, and development of grid infrastructure. A great challenge for our industry indeed!

I've done some rough calculations based on the supplied data, and some of my ideas are:

We will need to significantly ramp up the wind industry in the next few years to meet this target of 2733 GW - more than tripling our current rate of installation to around 60 GW per year. That would involve installing around 30,000 2MW-turbines every year, being around a thousand projects commissioned every year - that's alot of sites that need development.
Onshore wind will definitely be key, however offshore wind will need to grow significantly to around 20% of the total by 2050 - 547 GW of offshore wind energy, that's thousands of large individual offshore projects. Significant development in site acquisition studies, and energy estimation will be a key element of this.
This will transform the wind turbine-manufacturing industry from around US$30b/year today to over $100b/year! Plenty of room for new manufacturers, and novel design concepts. Reducing the capital cost will be the key driver, with smart developers looking at the realistic and accurate costs for generated energy over the projcet's life ($/MW.hr)
Operational and Maintenance (O&M) will become a serious industry, from current levels of around US$4b/year to US$100b/year in 2050. This will be a serious industry, surpassing the turbine-manufacturing industry, and resulting in some interesting market and industry movements. Maybe the wind industry will go the way the aircraft industry went, with more money in operations, and leasing being the predominant business model?

Ambitious targets, and exciting times for the wind industry. Our future energy needs will require a portfolio approach, with the renewable-energy industry being increasingly important with all its different technologies healthily competing to displace the old-world technologies.

The true cost of coal

2009-01-05T11:43:00.006+11:00

"Coal burning contributes more to climate change than any other
fossil fuel. Coal-fired power stations pump vast amounts of CO2
into the atmosphere each year, 11 billion tonnes to be precise.
This amounts to 72% of CO2 emissions from power generation
and 41% of total global emissions of CO2 from fossil fuels."

Greenpeace has released a great report: 'the true cost of coal' looking at the true cost of using coal for electricity generation. It takes a good look at coal use throughout the world, and some of its current and future affects to the environment. Some of the key points I took out of the report were:

Coal use is our single biggest emitter of CO2
Coal is used for 40% of the word's energy supply - and expected to rise
The true annual cost of coal use has been estimated at EUR360 billion in 2007 - with an estimated 150,000 deaths! That would pay for 3 times our total current level of wind energy - in a single year.
Australia has a dirty history with coal, being the world's largest exporter, and relying on coal-fired power stations for 80% of its energy supply.
Carbon Capture and Storage (CCS) is a false promise from the coal industry, with more of a focus on diverting public attention away from coal's current and future impact. CCS is not expected to be commercially available until at least 2030, and even this is technically unproven, will drastically increase the cost of coal-fired power generation, and it is still unsure how all this CO2 will actually be stored safely underground... for ever. The sooner we drop this red herring the better.

My next article will focus on both the: Greeenpeace Energy Revolution blueprint to radically reduce CO2 emissions; and the IPCC assumptions for energy supply for the various scenarios. And what this means for wind energy.

The best of 2008

2009-01-04T19:08:00.003+11:00

"I'm not one to attribute every man - activity of man to the changes of the climate. There is something to be said also for man's activities, but also for the cyclical temperature changes to our planet."

-Sarah Pallin, 2008

Hmm... thanks for that Sarah

Wind-powered cars and boats anyone?

2008-12-22T12:34:00.006+11:00

The application of wind energy for the generation of electricity is a relatively new phenomenon of the last 100 years or so. By far the most dominant human application of wind energy has been transport throughout our history - predominantly sail-powered boats over the last few millenia. Could transport applications of wind power be coming back?

UK-based Greenbird are working on a wind-powered car, that recently clocked up 60mph (27m/s) during a preliminary speed trial in the Australian desert. Although the vehicle is aimed at breaking speed records, the concept is an interesting one, using a carbon-fibre sail to provide thrust.

On the high seas, German-based skysails are developing a kite-like system to assist cargo ships in reducing their fuel consumption by an estimated 30%. Essentially harnessing the power of the wind to pull them along when favourable, assisting their conventional drive system. Even better, it can be used on the current fleet not requiring a radical change in the current ystem.

For transport applications, using wind energy directly rather than converting to electricty and then back to kinetic is definitely a more efficient approach however the practicalities remain to be demonstrated. This could be a great step forwards for reducing CO2 emissions, with transport accounting for around 20% of the total, ship-transport being around a quarter of this. It will take some radical thinking to get past our reliance on cheap and readily available fossil fuels, this could form a piece of the puzzle.

350.org - taking a fresh look at CO2 emissions

2008-10-29T14:27:00.006+11:00

Read a recent article at Scientific American, that a good friend of mine forwarded to me, that takes another look at CO2 emissions. It brings up some issues that are... well, disconcerting to say the least.

The article suggests that the commonly-touted concensus CO2 target of 450ppm (how did we exactly come to that value anyway?) may not be the safe level just off the current political horizon that we were expecting. It's no secret that the planet-level simulation of global warming is not an exact science, something that many governments and interest groups cynically used to their advantage to discredit the whole theory - check out the Competitive Enterprise Institute's take for a laugh. However, work done by James Hansen at NASA's Goddard Institute (the same guys who started up the initial CO2 models for use in analysing Jupiter's atmosphere) suggests that the CO2 levels we are at now (around 380ppm) are already dangerously high - and could lead to a dynamic runaway of temperatures, bad news for the biome of this planet. He suggests that the planet needs to get down to 350ppm, and fast.

More doom and gloom? Easy to interpret like that, however his research has inspired the 350.org movement, aimed at raising global awareness for returning the planet to 350ppm of CO2 in the atmosphere. Well worth having a look at it, although you would be forgiven to believe that the recent financial crisis has reversed global warming, as coverage in the media has fallen off the radar. The test of our species is upon us: comfortable retirement for the lucky few % of the world's population; or the planet - you be the judge.

Some interesting implications for wind power, I'm planning on talking a little about what this could mean for the industry in practical terms in a future post, stay tuned.

Vortex Generators

2008-10-20T15:35:00.002+11:00

The first and most obvious aerodynamic device I thought I’d write about is the vortex generator.As its name suggests, vortex generators generate a vortex. I won’t go too much into what a vortex is (check out Wikipedia), however it’s basically the rotation of the air flow – the core of which acts almost like a solid rotating cylinder as it moves with the surrounding flow. As it’s moving with the surrounding flow, and rotating, there is more energy in the vortex than the surrounding flow – and can be thought of as ‘stiffer’ than the surrounding flow. This higher-energy ‘stiffer’ vortex core effectively delays the boundary layer detaching from the airfoil: thus, delaying stall. In practical terms, this means that the CL_max for a blade section is increased, however as more energy is put into the flow drag is also increased: reducing L/D. Some test data is shown here for the NACA 63-415 section:

So, in essence vortex generators are fit to a blade in order to delay the stall and get more lift out of the blade - however knowing that we are going to increase drag and reduce our L/D ratio (something we don't want to be doing). A perfectly designed blade shouldn't have vortex generators, however are typically fitted to optimise the aerodynamics after the prototype blade has been power-tested. They are always fitted on the suction side of the blade, and typically from about 1/3 to 2/3 of the blade's span at 10 to 30% chord, as stall begins inboard.

For pitch-controlled turbines, vortex generators are typically fitted when the power performance of the rotor isn't as good as expected. What happens here, is that the turbine will be running at higher pitch settings to try and get the right power. This can result in stall (starting inboard and moving out), to prevent this vortex generators are fitted to delay the stall and squeeze a little more power out. For stall-controlled turbines, vortex generators are again fitted if the power production is less than expected and the turbine isn't hitting rated power (as it's stalling before rated is achieved). Again, vortex generators in this application will delay stall of the rotor and allow more power to be squeezed out of the turbine - although by a different mechanism. From a power-curve perspective, this will increase the power around rated speed, however the increased drag will reduce the Cp at lower speeds due to the increased drag in the system.

Here is a picture of a stall-controlled blade I took, vortex generators clearly visible with vortex paths clearly visible as dust buildup on the surface.

Check more info out at a great Risoe report here: Risø-R-1193(EN) ‘Wind Tunnel Tests of the NACA 63-415 and a Modified NACA 63-415 Airfoil’. 2000.

Wind turbine blade aerodynamic devices

2008-08-05T19:05:00.003+10:00

Cyril recently responded to one of my earlier posts below, asking about a recent article on MIT's technology review titled Whale-inspired Wind Turbines that discussed the hydrodynamic 'design' of whale fins - and possible use of these concepts in the design of wind turbine blades. Namely, the article the discussed the irregulatities in the front of the whale's fins (the leading edge) and the use of this to vary the onset of stall in different sections for better control of the fin. It's a pretty good article, with some good concepts, some of which are already used in the wind industry.

As one of the commenters rightly pointed out, these concepts have been used in the aircraft industry for quite some time - particularly in fixed-wing design. As I sat on a recent 737-800 flight pondering this the other day, I noticed that most of the aerodynamic device used on the wing were also used in one way or another in wind turbine blade design. Looking at the picture below, you can see: leading-edge and fowler flaps, vortex generators, spoilers, ailerons, and winglets (or tip extension).

The first aerodynamic device used in wind turbine blade design (that I am aware of) was the vortex generator. Vortex generators were brought in to delay the stall, and often used as a remedy when the power curve wasn't performing as expected. Now, we see the use of: stall-strips (very similar to spoilers) to damp out vibrations; the use of flaps for pitch control (on a concept level anyway); winglets to increase lift; zig-zag tape to increase roughness, delay ing separation; and rotating blade tips to act as an aerodynamic brake.

I'll planning on posting on each of these aerodynamic devices over the next few months, and will be posting up pictures I have taken over the last few years and reference some interesting test data to support it.

The realities of nuclear energy

2008-05-20T08:11:00.005+10:00

I'm on vacation in southern California at the moment, and couldn't help noticing the San Onofre Nuclear Generating Station on a road trip - I had to take a picture. You can see the outer concrete containment structure of one of the three generators here, each one rated at around 1100MW, and cooled by the Pacific ocean. After coincidentally reading an article in Scientific American about the nuclear industry, felt compelled to investigate a little further.

The average 1 GW nuclear power plant generates 10 tonne of nuclear waste a year, and as the industry hasn't worked out what to do with it yet, are either dropping the spent fuel rods in cooling pools or storing it in casks costing the government $3m a year for every 10 tonne. What about recycling the fuel? Attempts to recycle the fuel by reprocessing it in a dedicated facility have proven extremely costly, and estimated at $1m per tonne - that's $10m per year per facility. Therefore, the answer is storage.

The US industry had proposed to store it at the apparently geologically stable Yucca Mountain, however as the chief waste product is plutonium-239 with a half life of 24,000 years the details of developing a facility to store this safely for one million years are understandably quite complex - hence throwing the rods into a swimming pool until someone else figures it out!

Additionally, when these plants are decommissioned a huge amount of waste has to be disposed of, and it's not exactly clear how this will be done. In the US for example no nuclear power stations have ever been decommissioned, with the costs expected in the billions for each station. This is without even considering the possibility of a nuclear accident, and the horrendous environmental, social, and financial costs involved. Operators in the nuclear industry have of course not factored these into their costs, and will by default be carried by the tax payer - in what looks like could be the most heavily subsidised industry of all time.

There has been a resurgence in interest in nuclear energy recently, which on a simplistic level appears very attractive with zero carbon emissions. However, as always the devil is in the details, and for countries that are considering 'going nuclear' (like my home of Australia) these issues should be clearly understood, identified in the planning phase, and clearly factored into the project finances. With transparent and realistic project costs, nuclear energy would be alarmingly more expensive than wind energy, and it would be a tragic short-sighted mistake to defer these costs to our future generations to manage so that we can have 'cheap' electricity today.

Read the full article here

It's... wind turbine base jumping dude!

2008-04-16T04:28:00.003+10:00

Heh heh, check these guys out! It looks like someone has found the quick way down from some Vestas V90-2MW turbines in Germany (they have the distinctive tip marking) - or maybe they are pioneering some innovative maintenance access techniques? Either way, I'm sure the wind farm owner wouldn't be too happy if they found out! If you really gotta do it, points to note:

Make sure the manual rotor lock is on (it's bright red man!)
Don't kick off the trailing edge like those guys did (it's a delicate laminate dude!)

Stall-control basics

2008-03-07T03:03:00.017+11:00

Following on from the airfoil aerodynamics post, here is an overview of the important aerodynamic concepts for stall-controlled wind turbines. Stall control was the first really practical control system for large wind turbines, and was a logical development of the disc-area-regulation concept used on small wind turbines that regulated the power through varying the frontal disc area (typically pitching or yawing the rotor out of the wind after rated speed). A typical stall-control turbine is shown here, which is an NM52-900 that I took a picture of in Spain a few years ago (Pena Amada in Gallicia). It has a 52m rotor diameter, with the blades directly mounted to the hub with a fixed pitch.

The concept of stall control is that the power is regulated through stalling the blades after rated speed is achieved. As the rotational speed of the rotor is effectively constant, the AoA of the blades increase with increasing wind speed. As the wind speed increases the blades begin to stall, the lift drops, and the drag increases to a more inefficient L/D relationship at a higher AoA and thereby reducing the driving torque. There is little control of the blade aerodynamics, which means that the blade is typically designed such that stall occurs at rated wind speed with the most optimal AoA setting (the L/D sweet spot) occurring much earlier (around average wind speed if you are lucky!). You can really see this in the power curve when you have a good look at it (that will be a future post!).

As the blade pitch is fixed, the turbine rotor is quite simple as there is no blade pitch system required (no additional hydraulics, electrics, or pitch bearings!) making the turbine cheap and simple - these are very important characteristics for a turbine in my opinion, as the cost per kW.hr should really be the market driver.

Some of the drawbacks I think with this type are:

Turbines have reduced efficiency (Cp) towards rated speeds due to the higher AoA (reduced L/D)
Higher thrust coefficient (Ct) due to high AoA means:

Higher loads, particularly on blades and tower
Larger wake deficits resulting in greater array losses for the farm
Increased wake turbulence

Noisier due to higher AoA during operation
Can induce severe vibrations due to transition period around rated speed where blades are coming in and out of stall during each rotation
Sensitive to the blade's initial pitch setting (must be correctly set to account for local density)
Sensitive to dirty blades (rated power can be significantly reduced)

A derivative of this control system is the active-stall system, where the blades have a small amount of pitch control to allow power regulation after rated power is achieved; effectively the control system utilises the negative lift-curve slope characteristic of the airfoil in the post-stall condition, increasing the stall will reduce the power. This is a very popular system, and was first introduced on the NEG-Micon 54-950 and has been further optimised for use on the popular NM82-1650. Here's a picture of some NM82-1650 turbines I checked in Australia (Wattle point, if you are really interested!).

Turbine destroyed during overspeed

2008-03-02T23:00:00.006+11:00

Here's a video of a small Nordtank turbine (Nordtank were bought and integrated into Vestas) being destroyed during some of the heavy wind activity we had here in Denmark last month.

According to the Danish engineering publication Ingeniøren (the engineer), the turbine was an old Nordtank N500/41 - a 500kW stall turbine with a 41m rotor diameter and 43m hub height tower. As stall turbines have a fixed blade pitch, the safety system relies on pitching the blade tips to fully stop the turbine and then yawing into the wind.

Apparently the blade-tips were sheared off during operation some time before the accident, and there was therefore no way to shut down the turbine. Luckily, the local council (kommune) was aware of the problem and cordoned off the surrounding area. Good decision, as it was reported that large pieces of the blade flew up to 400m - not suprising when you see the video.

In the video you can see the turbine spinning at high RPM, and then something thrown off the blade (kicking up dirt on the ground) and then all the blades disintegrating. The huge imbalance that occured during the blades failure sheared the bolted tower section as well. No suprise the turbine failed here, with incredibly high centrifugal loading on the blades, needless to say the turbine is a total write off.

You can read the article (in Danish) here.