Rotor blade accelerations up to 200 times g

Mertens became responsible for the aerodynamics of the machine. He combined analytical flow calculations with data from the wind tunnel (using a computational fluid dynamics program) to continually improve the design. “I was able to calculate how high and how wide the rotor blades had to be. This resulted in a wind turbine with three inclined rotor blades, the whole assembly being three metres high and two metres wide. The chord of the blades is ten centimetres. A wind turbine of this size will produce 2.5 kilowatts.” This became the basis for the wind turbine that was to be named Turby. An extra complication for the design was that the wind turbine has to produce as little noise as possible. After all, it was to be operated in an urban environment, not in the open country. Mertens: “Keeping the noise level low means keeping the rotational speed down. The number characterising the rotation of the rotor blades is the tip speed ratio, which gives the ratio of the rotor blade tip speed over the wind speed. While for Turby the ratio sits at the relatively low value of 3, a propeller turbine in the open country can have a tip speed ratio of 9. The requirement for low-noise power production significantly reduces Turby’s tip speed ratio. As a result the Reynolds Number, that typifies the type of airflow, is relatively low, which makes model simulation harder to do. In addition, the lifting power acting on the rotor blades becomes more difficult to model, so the model simulation results could not be more than a guideline. We had to go on to testing models in a wind tunnel.” When the initial prototype of Turby was built, the company constructing the wind turbine underestimated the centrifugal forces acting on the blades, Since these can reach up to 200 times g, the blades must be securely fixed. As this was not the case in the first prototype, a second had to be built. This time the device was strong enough, but refused to turn. The third attempt proved to be better. After some fine-tuning and scaling up the dimensions, the result was the present prototype Turby, which consists of three inclined, elongated and curved rotor blades arranged around a virtual cylinder two metres in diameter and three metres high. The rotor blades are made of a lightweight, strong, and rigid carbon-fibre composite material, resulting in a weight of only 4 kg per blade. The weight of the entire turbine is 135 kg. Turby’s small diameter and the open nature of the rotor make it less intrusive than a normal wind turbine in an open field. Thanks to its lightweight construction, Turby can be installed without a lifting crane. “Each Turby located on the roof of a high-rise building in a town generates power exactly where it is needed, without transport losses,” Mertens says, 2 0 2005.2 Turby – Sustainable urban wind power from the roof top Graphic representation of the variation of the axial forces on the shaft as a function of the blade skew. The vertical dashed lines indicate the skew of the blades at which the variation in the axial force reaches zero on the shaft. Two graphs are shown. One shows the results of the theoretical model for a 3-bladed rotor, while the second gives the results of the theoretical model for a 5-bladed rotor. The first prototype Turby in front of the wind tunnel of the Wind Energy section at TU Delft’s Stevin Laboratory. It has racing bike wheels fitted with aerodynamic spokes at its top and bottom. The manufacturer of the first prototype thought the aerodynamic spokes would keep the drag figure down. However, the model disintegrated because the bolt used to secure the position of the two wheels broke under the strain of the centrifugal forces, causing the blades to bend outwards. A redesigned and stronger model, although it proved capable of coping with the centrifugal forces, required additional power to keep it in motion. It did show though that the required power input became less as the air speed increased. Apparently the model manage to extract energy from the wind, but this was too little to overcome the total drag. The readings obtained from tests on this model were used to determine the optimal aerodynamic dimensions. The third prototype Turby in front of the wind tunnel. This half-height model was built using the results of the tests on the early prototypes. The model was to run under its own power, and run it did! The model — the largest size that could be accommodated in the wind tunnel — was used for all the wind tunnel tests and the final design of Turby was based on these measurements. Visualisation of the air flow across the roof of a model building in a wind tunnel. The air flow path is made visible for photography by adding small particles. As can be clearly seen, the wind does not blow horizontally across the roof, but flows at an angle. The curly path of the particles close to the roof indicates that the local airflow constantly changes direction, in other words, it is turbulent. “That’s the beauty of the system. Turby is connected to the building’s power system behind the public utility meter. The power goes straight into the building, reimbursing the owner at full kilowatt hour rate. Whereas owners of wind turbines in the Dutch countryside have to go begging to a power company to offload their output. The power company, pays them only one third of the amount consumers are charged for a kilowatt-hour. In a reasonable wind location, not too close to the coastline, nor stuck between other high-rise blocks, a single Turby will provide about 3000 kilowatt-hours, enough to cover the power consumption of an average family. Turby prototypes have now been installed on the roof of an apartment building in Tilburg (the first installation dates from May 2004), on an apartment building in Breda, on the town hall in The Hague, and on the ChemTech faculty building of TU Delft. The only noise to be heard from the generators, and only along the top access gallery of the building, is a light whizzing sound as the wind gusts. Nothing is to be heard inside. These are all still single Turby installations, but Mertens is already thinking of roof-top wind turbine parks for the future. The Dutch Ministry of Transport has also installed a single Turby, along the A50 motorway. It supplements a pair of German-made wind turbines, whose purpose is to provide power for the matrix information signs and the street lighting along the road. At least, that is the idea. Mertens is not convinced that Turby is suitable for the purpose: “It was really designed to be used in a built-up environment, sitting on a high roof. Other types of turbine work much better along the motorway.” More power from angled flows Turby has a number of unique features. It produces little noise and can handle all the changes in wind direction the built-up environment can throw at it, thanks to its vertical shaft layout. “We were a little surprised by something else though,” Mertens recalls. “As the wind crosses the roof of a high-rise building, it flows at an angle, typically twenty degrees, to the flat roof-top. This angle varies with the wind speed, sometimes a little higher, at other times a little lower. As the angle of flow increases, the yield of a normal wind turbine will start to drop as the useful wind component decreases. Turby on the other hand can get more energy from the wind as the angle of flow increases, up to a certain maximum, of course. This was new to us. We had never before heard of a wind turbine managing such a feat, and I have found no previous reports of the phenomenon. As the angle of the wind flow increases, the surface area of the unhindered wind reaching Turby also increases. That is the secret of the design.” Mertens also drew up a number of rules of thumb to indicate how high the rotor should protrude above the roof. Typical values range from 5 to 7.5 metres above the roof-top, where the wind speed is about twenty percent higher than the unhindered wind speed at the same height in the open field. Mertens conclusion is that wind energy offers a viable alternative if you can make use of the highest buildings in a built-up area. “You have to be careful where you put the turbine, though. In Tokyo they once put a set of turbines just above the roof-top. That does not work. They really have to be up in the accelerated, angled airflow.” A normal wind turbine in the open field carries its own wind speed counter. Turby can do without. In fact it constantly measures the wind speed. Mertens: “At low wind speeds, the rotor blades will start to turn, but they will go too slowly to generate sufficient lifting power on the blades. This is also too slow to generate power. The initial movement is detected and a signal is sent to the generator which briefly acts as a motor, increasing the speed of the rotor blades to the point where they have sufficient airspeed to generate the required lifting force on the blades. At that point the rotor takes over and the turbine continues to rotate on wind power alone like any other wind turbine.” So it needs extra energy to get started? Sounds like a major drawback. Mertens: “It takes only very little energy, since the rotor is relatively light. What’s more, the system is monitored by a protocol to ensure that it does not get kick-started too often.”

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