In Structures & Energy Solutions, Wind

Rob and I were very happy to host Robs parents, Jon and Thea, for a few days here at the Folkecenter at the end of October. Keen to expose them to some of the neat things that we have experienced, we organized a field trip packed with renewable energy exhibitions. In the morning we visited (i) a straw-bale fired community district heating plant and (ii) a cooperatively owned biogas facility providing combined heat and power to the local community of Lemvig. We’ll discuss both of these in separate district heating and biogas blogs.

In the afternoon we concentrated on wind power. We first visited the large National wind turbine test site, followed by the Folkecenter’s 525kW turbine and the small-scale wind test site. 

The Risoe Large Wind Power Test Station, Hoevsoere

Within a one hour drive from the Folkecenter, several companies (Vestas, Micon, Bonus & Nordex) are testing out their mega-turbines at the Risoe Large Wind Power Test Station. These prototype turbines are the big boys ranging from 3-5MW. To give you an idea, the largest commercial turbine in the world today is a 5MW monster, with a 126m blade diameter! REpower is the manufacturer and this turbine is located in Germany.

Interestingly, several of the turbines were not operating. This certainly reflects the difficulty in constructing machines of this magnitude. Significant research is going into blade design to reduce the elasticity of longer blades. If the blades bend too much (from the force of the wind) they will hit the tower. In addition, the tall towers and long blades are difficult to transport (this can cost up to 20% of the equipment cost) and expensive to install requiring tall cranes and experienced operators.

Nonetheless, we enjoyed a picnic lunch at the base of these fantastic pieces of engineering.

The Folkecenter’s 525kW Turbine in Hanstholm


The next stop was Hanstholm, a small west coast village, an hour drive away. Here, the Folkecenter developed and installed a 525kW wind turbine back in 1992. At the time, this prototype turbine was the largest in Denmark. Over 25 years later, the wind turbine has produced an impressive 1.4MWh/yr, not required any component repairs or upgrades and has survived several severe North Sea storms. An impressive prototype indeed.

Here are some facts about this beauty: 

  • Hub height of 41m
  • 17m long blades with a fixed rotational speed of 31 RPM 
  • blade tips rotate at close to 200 kph and produce virtually no sound.
  • the three 1600 kg blades are connected to a hub of 3 tonnes
  • A gear ratio of 1:48 with an input shaft of 450mm

I think that Rob’s parents were most impressed that they were given the opportunity to climb the 41 meters and stand inside the nacelle. but a little shocked when we told them that they would be climbing a ladder, and not a staircase! But Jon and Thea were both up to the challenge and quite confidently threw on a pair of coveralls and started climbing. Thea was especially relieved to find a few resting platforms along the way. We all made it to the top, enjoyed the view, and caught our breaths as Rob keenly explained and pointed out all of the turbine components.

The Hanstholm turbine uses an integrated design which combines a heavy duty gear box to couple a 525 kW ABB generator and the rotor. The motive force of the machine comes from the three rotating blades. Over-torque is prevented in an ingenious way- the blades are aerodynamically designed to induce a stall condition above a certain rotational speed. As there is enormous momentum in a rotor of this size, it is not possible to stop the blades by applying a disk brake on the output shaft of the transmission. Air brakes, located on the blade tips are deployed to de-power the rotor to a specified RPM at which point a disk brake, located on the fast shaft of the gearbox, is applied.

Stall vs Pitch Control

The more recently designed wind turbines employ pitch control. Pitch control rotates the three blades along the blade axis in order to change the approach angle of the wind relative to the blade surface. This allows the turbine to vary the torque relative to the wind conditions. Pitch systems generally have a lower cut in speed which means that the turbine starts producing at lower wind speeds, 2 m/s as compared to 3 or 4m/s. Also, they don’t need air breaks as the whole blade surface can be rotated parallel to the direction of the wind. This increases efficiency slightly if controlled correctly.

Think about this: The wind measurement device (the anemometer) is located on the back of the turbine (behind the blades) and measures the wind speed after the wind has passed through the rotor. Therefore, the anemometer is not a good device to determine pitch angle. Another method has to be employed. Pitch angle is actually controlled by measuring input shaft torque. If the torque is too low a more aggressive pitch is used to maximize the power in the wind, if it is too high a less aggressive pitch is used to reduce the main shaft torque. The pitching system in the new turbines uses very fast motors which allow the blades to change at speeds of up to 300 Hz. All of the technical mumbo jumbo amounts to a turbine that produces high quality power – what the power companies demand!

After having been exposed to pitch and stall control wind turbines I would say that by no means have pitch control turbines made stall control machine obsolete. Although stall control versions are slightly less efficient, pitch control requires more mechanisms in the hub, more parts to repair, and more systems to monitor. Also, if something goes wrong with the pitching system the wind turbine can have an over speed situation causing catastrophic failure.

If you are interested in learning more about how a wind turbine works, wind turbine designs, over speed protection, the history of wind power in Denmark, turbine manufacture and installation, there is a fantastic movie by the Danish Wind Energy Association called “Out of the Blue”. You can watch it by clicking on the hyperlink.

The Folkecenter’s Small Scale Wind Test Station

We also visited the small scale test station here at the Folkecenter to show-off some of the models on display and currently being tested: a 1 kW Bergy, a 600W Lakota, and a small 1 kW wind turbine from Greece. In addition they have two Folkecenter design turbines operating: the 7.5kW Uni and the 75 kW “Potato” as it is affectionately called. The Uni and Potato are grid-connected and produce all of the annual electrical requirements (including electrical heat) for the Folkecenter.

The center provides a report and a power curve for the turbine manufactures based on established standards. This third party validation process is important for manufacturers in addition to testing their machines in some of the most severe European wind weather.

Interestingly, although the machines should be designed for 70m/s, many still end up quite battered after a trial period here. One in particular required some component replacement after only one year in service. Just two days ago, hurricane strength winds were predicted (+30m/s or +100kph) so we decided to forcefully yaw the machine out of the wind to protect it from further damage. Needless to say, we won’t be investing in that model when we install wind in our own home.

The Folkecenter has a small wind turbine catalogue where they have compiled a long list of small wind turbines that are available in the market. The book also provides technical, contact and cost information. As far as we know it is the only catalogue of its kind. You can find it here.

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