How Direct Drive helps in the Transportation of Large Wind Turbines
Some years ago the NewGen generator was presented as a way of making direct-drive wind turbines more economic. Here, the originator, Staffan Engstrom discusses the features of NewGen generators and demonstrates how they can be used to make very large wind turbines transportable on ordinary roads.
By Staffan Engstrom, Ägir Konsult AB, Sweden
The NewGen concept was developed in order to obtain a more economical generator design. It implies that the generator rotor is supported by steel wheels around the periphery of the stator of the generator. This is a radical way to reduce the load paths, and reduces the generator weight to half of that of today’s typical direct-drive permanent magnet (PM) generators. In comparison with generators with electrical excitation (Enercon) the reduction is even larger. In a practical sense it means that railway technology is introduced into generator design, since it is based on steel wheels rolling on a steel rail.
A couple of years ago VG-Power in Vasteras, Sweden, designed and built a small (2-metre diameter, 160kW) generator in order to verify the technical solution. It worked as foreseen. A project to develop a 3MWgenerator, in cooperation with the Norwegian manufacturer Scanwind, was supported by both the EU and by the Swedish Energy Administration. Vattenfall agreed to purchase the first installation. Unfortunately, Scanwind had to withdraw for economic reasons, and the project was abandoned.
The Transportation Problem
There are strong economic incentives to put wind turbines on tall towers, especially in areas where the height exponent is larger than the standard 0.14–0.20 values. For example, above forests the exponent often reaches 0.30–0.40. However, the most economic way to get a tall tower is to build a large turbine. Large turbines also have other benefits, such as less ground intrusion and less impact on the landscape. Unfortunately, one complication to this solution is that it is not possible to transport most turbines in the 5MW class on ordinary roads, because the components are just too big. However, the NewGen generator already had to be built in smaller-sized sections, because it was designed with such a large diameter. Scaling the design up to a 5MW size does not present any new problems.
The turbine blades are normally built as composite structures of plastics laminated with glass or carbon fibre. The usual method is to make the turbine blade in one piece and to flange it to the turbine hub, which is usually a casting. The blade bearing performs the transition. The blade pitch mechanism, used for controlling the power level at high wind speeds and for starting and stopping the turbine, is also situated at this point. A turbine blade for a 5MW turbine is just over 60 metres, which is longer than can be transported on normal roads (with special permits 50–55 metres may be allowable). It is possible to make composite blades in two sections, but this presents technical complications, and will add to the blade cost. There is, however, another solution to the transportation problem, which is also economically superior.
Control of the outer Blade Sections
The new solution proposes that the load carrying structure of the inner section of each blade should be made up of a cylinder of welded steel plate, similar to a tower section. The inner end is flanged to the turbine hub and the outer blade section is connected to the blade pitch bearing and its pitch mechanism. Outside the spinner the structure is enclosed by panels which provide the aerodynamic shape. These are most easily manufactured using bent and welded aluminium plate.
It seems reasonable to let the steel inner part make up about a third of the length of the blade. This will give it a length of about 20 metres with an outer composite section of around 40 metres. For each blade about 10 tonnes of composite material, at roughly 20€/kg, will be substituted by possibly double that weight of steel, at 2€/kg. Thus it will be a more economic solution. Also the blade bearing and the blade pitch mechanism will get cheaper, since they will be exposed to less load stress.
The reduction in cost can be achieved by using so-called partial blade pitch control; this has been used before on wind turbines in the megawatt class. Typically, it was then used to control the outer 15–40%. Only controlling the outer part of the blade has no apparent aerodynamic drawbacks. One of the main problems with this configuration, however, was that the section diameter at the transition was rather small, which meant that it was difficult to get access to bearings, etc, for maintenance. By putting the transition closer to the hub, and by building a very large wind turbine, the blade bearing can have a diameter of about 2 metres. After putting the blade in a horizontal position, it is possible to walk freely all the way from the hub to the bearing and the blade pitch mechanism. This solution is protected by a patent application.
No Increase in Fatigue
Due to the high weight of steel a turbine blade made entirely of steel is problematic from a fatigue standpoint, since the direction of the forces from the weight of the blade changes direction during every revolution. However, the moment from an inner blade made of steel is not that significant, since the length is less.
The choice of a fixed inner part to the blade is especially well adapted for the NewGen generator, since it then is possible to transfer the torque of the turbine blades directly from the blade inner parts to the generator rotor, which encloses the stator of the generator (‘outer rotor’). Thus one no longer has to transfer the torque all the way to the turbine shaft and thereafter again to the rotor, which in this case is at a radius of about 7 metres. This saves several tonnes of structural weight.
Larger Production
The large nacelle, which follows with the use of a NewGen generator, may seem to be a drawback, but instead it turns out to be an asset. The tradition is to end the aerodynamically active part of the turbine blade inwards at 10% of the blade radius, since the area inside only accounts geometrically for 1% of the turbine disc, which is considered reasonable to sacrifice. However the losses through this hole may be much larger – according to one source as much as 10%. By letting the active aerofoil extend all the way towards the spinner it is also possible to utilise this energy. Among the manufacturers of large wind turbines today, only Enercon has introduced this type of design, which is easier to accomplish if the inner part of the turbine blade is fixed.
Conclusions
The NewGen generator in itself is designed for easy transportation in large sizes. Partial blade pitch control, and building the load carrying sections of the inner blade of steel, provides an economical solution for solving the transportation problem of very large wind turbines. The benefits are especially prominent in combination with the NewGen generator
Further Reading
Engstrom, S., 2008 NewGen. A new type of direct-drive generator. Windtech International January/February.
Biography of the Author
Staffan Engstrom, MSc, Mech. Eng., has been working in wind power since 1975, initially at the National Swedish Board for Energy Source Development (NE) and the National Swedish Energy Administration (Statens Energiverk). He has also been active in Nordic Windpower. Today he runs his own consultancy in Ägir Konsult AB. He is the holder of several patents.{/access}
Some years ago the NewGen generator was presented as a way of making direct-drive wind turbines more economic. Here, the originator, Staffan Engstrom discusses the features of NewGen generators and demonstrates how they can be used to make very large wind turbines transportable on ordinary roads.By Staffan Engstrom, Ägir Konsult AB, Sweden
{access view=!registered}Only logged in users can view the full text of the article.{/access}{access view=registered}The problem with gearboxes in wind turbines seems to be a never-ending story. It also appears to be the most straightforward explanation for the increase in interest, from both customers and manufacturers, in direct-drive wind turbines. Today at least half of the top-ten wind turbine manufacturers are working on direct-drive technology (see box).
For a direct-drive generator the function of the gearbox has to be substituted by more active material and by a larger diameter generator. The electrically and magnetically active material consists of the copper windings, the iron sheets and the permanent magnets (if used). Since the amount of active material is inversely proportional to the diameter at the air gap between the rotor and the stator, it is possible to decrease this by increasing the diameter. On the other hand, in conventional designs, the mechanical structure then gets very heavy. This is due to the long load paths between the central bearings and the air gap, as well as the need to keep the structure very rigid in order to maintain the air gap at a few millimetres.The NewGen concept was developed in order to obtain a more economical generator design. It implies that the generator rotor is supported by steel wheels around the periphery of the stator of the generator. This is a radical way to reduce the load paths, and reduces the generator weight to half of that of today’s typical direct-drive permanent magnet (PM) generators. In comparison with generators with electrical excitation (Enercon) the reduction is even larger. In a practical sense it means that railway technology is introduced into generator design, since it is based on steel wheels rolling on a steel rail.
A couple of years ago VG-Power in Vasteras, Sweden, designed and built a small (2-metre diameter, 160kW) generator in order to verify the technical solution. It worked as foreseen. A project to develop a 3MWgenerator, in cooperation with the Norwegian manufacturer Scanwind, was supported by both the EU and by the Swedish Energy Administration. Vattenfall agreed to purchase the first installation. Unfortunately, Scanwind had to withdraw for economic reasons, and the project was abandoned.
The Transportation Problem
There are strong economic incentives to put wind turbines on tall towers, especially in areas where the height exponent is larger than the standard 0.14–0.20 values. For example, above forests the exponent often reaches 0.30–0.40. However, the most economic way to get a tall tower is to build a large turbine. Large turbines also have other benefits, such as less ground intrusion and less impact on the landscape. Unfortunately, one complication to this solution is that it is not possible to transport most turbines in the 5MW class on ordinary roads, because the components are just too big. However, the NewGen generator already had to be built in smaller-sized sections, because it was designed with such a large diameter. Scaling the design up to a 5MW size does not present any new problems.
The turbine blades are normally built as composite structures of plastics laminated with glass or carbon fibre. The usual method is to make the turbine blade in one piece and to flange it to the turbine hub, which is usually a casting. The blade bearing performs the transition. The blade pitch mechanism, used for controlling the power level at high wind speeds and for starting and stopping the turbine, is also situated at this point. A turbine blade for a 5MW turbine is just over 60 metres, which is longer than can be transported on normal roads (with special permits 50–55 metres may be allowable). It is possible to make composite blades in two sections, but this presents technical complications, and will add to the blade cost. There is, however, another solution to the transportation problem, which is also economically superior.
Control of the outer Blade Sections
The new solution proposes that the load carrying structure of the inner section of each blade should be made up of a cylinder of welded steel plate, similar to a tower section. The inner end is flanged to the turbine hub and the outer blade section is connected to the blade pitch bearing and its pitch mechanism. Outside the spinner the structure is enclosed by panels which provide the aerodynamic shape. These are most easily manufactured using bent and welded aluminium plate.
It seems reasonable to let the steel inner part make up about a third of the length of the blade. This will give it a length of about 20 metres with an outer composite section of around 40 metres. For each blade about 10 tonnes of composite material, at roughly 20€/kg, will be substituted by possibly double that weight of steel, at 2€/kg. Thus it will be a more economic solution. Also the blade bearing and the blade pitch mechanism will get cheaper, since they will be exposed to less load stress.
The reduction in cost can be achieved by using so-called partial blade pitch control; this has been used before on wind turbines in the megawatt class. Typically, it was then used to control the outer 15–40%. Only controlling the outer part of the blade has no apparent aerodynamic drawbacks. One of the main problems with this configuration, however, was that the section diameter at the transition was rather small, which meant that it was difficult to get access to bearings, etc, for maintenance. By putting the transition closer to the hub, and by building a very large wind turbine, the blade bearing can have a diameter of about 2 metres. After putting the blade in a horizontal position, it is possible to walk freely all the way from the hub to the bearing and the blade pitch mechanism. This solution is protected by a patent application.
No Increase in Fatigue
Due to the high weight of steel a turbine blade made entirely of steel is problematic from a fatigue standpoint, since the direction of the forces from the weight of the blade changes direction during every revolution. However, the moment from an inner blade made of steel is not that significant, since the length is less.
The choice of a fixed inner part to the blade is especially well adapted for the NewGen generator, since it then is possible to transfer the torque of the turbine blades directly from the blade inner parts to the generator rotor, which encloses the stator of the generator (‘outer rotor’). Thus one no longer has to transfer the torque all the way to the turbine shaft and thereafter again to the rotor, which in this case is at a radius of about 7 metres. This saves several tonnes of structural weight.
Larger Production
The large nacelle, which follows with the use of a NewGen generator, may seem to be a drawback, but instead it turns out to be an asset. The tradition is to end the aerodynamically active part of the turbine blade inwards at 10% of the blade radius, since the area inside only accounts geometrically for 1% of the turbine disc, which is considered reasonable to sacrifice. However the losses through this hole may be much larger – according to one source as much as 10%. By letting the active aerofoil extend all the way towards the spinner it is also possible to utilise this energy. Among the manufacturers of large wind turbines today, only Enercon has introduced this type of design, which is easier to accomplish if the inner part of the turbine blade is fixed.
Conclusions
The NewGen generator in itself is designed for easy transportation in large sizes. Partial blade pitch control, and building the load carrying sections of the inner blade of steel, provides an economical solution for solving the transportation problem of very large wind turbines. The benefits are especially prominent in combination with the NewGen generator
Further Reading
Engstrom, S., 2008 NewGen. A new type of direct-drive generator. Windtech International January/February.
| 50% of turbine manufacturers are working on direct-drive Of the ten largest wind turbine manufacturers in the world, at least half are currently developing or are already manufacturing direct-drive wind turbines.
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Biography of the Author
Staffan Engstrom, MSc, Mech. Eng., has been working in wind power since 1975, initially at the National Swedish Board for Energy Source Development (NE) and the National Swedish Energy Administration (Statens Energiverk). He has also been active in Nordic Windpower. Today he runs his own consultancy in Ägir Konsult AB. He is the holder of several patents.{/access}
