Wind turbines for the large offshore wind farms of the near future will have a power of 5 to 6 MW. A comparison of the six current multi-megawatt prototypes provides clues as to how the manufacturers are aiming to achieve this.
The offshore planners are having to explore new territory in almost all aspects.
Firstly, the wind farms of the future will be larger than ever before, and secondly, wind turbines much larger than the current series models, tried and tested a thousand-fold, will have to be erected in water. Thirdly and fourthly, financing and insurance will be beyond all boundaries known so far. Project developers and planners thus have lots to keep them busy.
It is the technology which is foremost in public perception, however, i.e. the construction and design of turbines which should one day provide a large portion of Europe’s electricity from far out on the seas.
The relatively large distances to the coast make grid connections more expensive and the deep water makes expensive foundations necessary. The result is that one must get the highest possible yield out of every single turbine.
Offshore wind farm Scroby Sands, North Sea, UK
© Chinese Wind Energy Association CWEA
What does this mean for the rated power? The largest offshore prototypes, at 5 MW, apparently haven’t yet reached the limits of what is feasible1. The upper power limit is sure to move up to 6 MW. This target was set by Enercon in mid 2005, when it announced details of the test operation of an E-112. The generator cooling system of the originally 4.5 MW wind turbine has been improved so much that this turbine can now generate 6 MW continuously. Three type E-112 wind turbines, each with 6 MW, were erected in Germany at the end of 2005.
If 6 MW is possible with an E-112, then in principle this should also be possible with the Repower 5M, which has a much larger rotor. Competition between the big manufacturers will do the rest to push power boundaries towards 6 MW.
A glance at the table shows that the “world market rankings” are not reflected one-to-one. There are currently six manufacturers with prototypes in the race. From the top five companies, Gamesa is missing, however. In the running instead are the relatively small manufacturer Repower Systems AG and the still-smaller company Multibrid.
In the near future things will stay at just these six companies, as both remaining candidates (apart from Gamesa, Nordex would also be possible) apparently do not wish to enter this race.
| Prototype | Repower 5M | Vestas V120 | Multibrid M5000 | Enercon E-112 | Siemens 3.6 MW | GE Energy 3.6s |
| Rotor diameter | 126 m | 120 m | 116 m | 114 m | 107 m | 104 m *1 |
| Swept area | 12.469 m2 | 11.310 m2 | 10.568 m2 | 10.207 m2 | 8.992 m2 | 8.495 m2 |
| Rated capacity | 5,0 MW | 4,5 MW | 5,0 MW | 4,5 MW *2 | 3,6 MW | 3,6 MW |
| Gearbox | 3-step | 3-step | 1-step | - | 3-step | 3-step |
| Generator | df ASG | df ASG | pm SG | SG | ASG | df ASG |
| Masses: | ||||||
| Rotor blade | 17,8 t | 12,3 t | 16,5 t | 21 t | 16 t | k.A. |
| Rotor with hub | 120 t | 65 t | 110 t | k.A. | 90 t | 85 t |
| Nacelle | 290 t | 145 t | 200 t | k.A. | 120 t | 210 t |
| Nacelle + Rotor | 410 t | 210 t | 310 t | 500 t | 210 t | 295 t |
| Tower | 750 t | 220 t | 1.138 t *3 | 2.500 t *4 | 250 t | 250 t |
| Hub height | 120 m | 90 m | 102 m | 124 m | 80 m | 76,5 m |
| In operation since | Nov 2004 | 2007 *5 | Dec 2004 | Aug 2002 | Dec 2004 | June 2004 *6 |
| specific power | 401 W/m2 | 398 W/m2 | 473 W/m2 | 441 W/m2 | 400 W/m2 | 424 W/m2 |
| specific mass | 32,9 kg/m2 | 18,6 kg/m2 | 29,3 kg/m2 | 49,0 kg/m2 | 23,4 kg/m2 | 34,7 W/m2 |
Table: Technical data for prototypes with more than 3 MW rated power (multi-megawatt prototypes)
Source: Manufacturer’s publications
Abbreviations:
SG = synchronous generator
pm SG = permanent-magnet synchronous generator
ASG = asynchronous generator
df ASG = asynchronous generator (doubly fed)
Explanations:
*1 = in planning: 3.6sl with 111 m rotor diameter (9.677 m2)
*2 = prototype with 6 MW in operation since Nov 2005
*3 = Multibrid M5000: 313 t steel + 825 t concrete
*4 = Enercon E-112: concrete
*5 = in planning
*6 = prototype with 100 m rotor diameter in operation since April 2002 in Barrax (Spain)
Here, diversity is growing. There are several ways of building up the drive train. In the 2 MW class only two concepts have won out: either a fast-running transmission and a doubly-fed asynchronous generator (ASG) with a partial inverter or a directly coupled synchronous generator (SG) with a full inverter.
The six offshore prototypes represent three different drive train concepts and two generator-inverter systems.
Wind power turbines with stepped transmission and fast-running generator. The four manufacturers who have decided on this concept (GE Energy, Repower, Siemens, Vestas), use three-step transmissions, each consisting of two planetary and one spur gear steps. The generator-inverter systems differ, however. Although all four manufacturers use asynchronous generators, only three use a doubly-fed ASG whose stator is directly connected to the grid. Only part of the generator load (typically 20 to 30%) must be inverted.
Siemens has gone its own way and is the first to use a squirrel-cage ASG, which feeds into the grid via a full inverter (100%). GE Energy has now also moved to using a full inverter. The 2.X turbine type is already fitted with a SG and a full inverter, and the more powerful successor to the offshore turbine 3.6s is also expected to have this generator-inverter system.
This is because the full inverter has considerable advantages with a view to running an offshore wind farm as a power station.
Wind power turbines without a transmission, with a slow-turning SG – the well-known Enercon concept. The synchronous machine of the E-112 consists of four single, independent generators. The alternating current is fully inverted into electricity at the grid frequency by several inverter modules. Both the ring generator and the inverter thus have a high redundancy.
The third drive train concept lies between the two others: the Multibrid 5000 makes do with a single step planetary transmission, which drives a permanent-magnet SG with medium rotation speed. Electric magnetisation is not required and losses are lower. The drive train and generator form a very compact unit. The inversion of the electricity generated by the Multibrid 5000 is via a full inverter.
Four of the six prototypes have a fast-running transmission. This suggests that components with conventional engineering can still keep up with the large growth which has taken place. Giving up on one or two gear steps (Multibrid) or even the whole transmission as such (Enercon) is thus not yet essential.
In terms of grid infeed, however, a change is more likely to come about. Three of the six prototypes feed electricity into the grid via a full inverter and secure control advantages through this higher technical effort. This concept looks like it will win out. It is also preferred by the energy suppliers, who are going to invest ever-more strongly in offshore wind farms in the future.
The ratio of rated power to rotor swept area (specific power, W/m2) enables a comparison to be made despite differing sizes and concepts. It is striking that the specific powers of the six prototypes are not far apart. The largest value (473 W/m2) is just 20% higher than the smallest (398 W/m2) 2. This narrow corridor suits the requirements, as at sea there are practically only “strong-wind locations”. In the future, however, one will have to view this term more selectively, as the conditions in the Baltic North Sea and Atlantic can be very different from one another.
The specific power figures presented here are not going to stay this way, however. GE Energy will shortly increase the rotor diameter to 111 m; the 3.6s will then become the 3.6sl, the rotor swept area will rise to 9,676 m2 and the specific power will thus sink to 372 W/m2. The first examples of this turbine type are to be installed in the British offshore wind farm Gunfleet Sands (30 x 3.6 MW).
Other manufacturers are going to increase the power while maintaining the same rotor diameter. Generally the manufacturers will wish to keep their options open in order to be able to offer the right specific power for each location. Wind farm planners will then have the choice of several rotor diameters and maybe even several generators.
The six prototypes vary a lot more in terms of their specific mass than in their specific power, however, i.e. in the ratio of tower head mass to rotor swept area (kg/m2). The E-112 still has the most up top, so to speak, at 50 kg/m2, whereas Vestas manages to get the specific mass down to significantly lower than 20 kg/m2.
According to the simple rule that “every kilogram costs money”, saving weight will inevitably bring cost savings sooner or later. In the short-term, however, saving weight costs a lot of money. Heavy, but cheap materials must be replaced by light, but expensive ones. To produce an extremely light rotor blade at the required size, expensive carbon-fibre reinforced plastic is now an essential material, for example.
Each parameter is relative, including the specific mass and how much in costs may potentially be saved. Only total costs are important to the customer and these are not determined just by the price of the turbine but also by the costs of the foundation, of the cable track and lastly by operating costs. This brings reliability into the game, something which from the customer’s point of view is usually more likely to be achieved with a conservative and heavy construction than with a “highly evolved lightweight”.
The Repower 5M is still the world’s largest wind turbine. The relatively large rotor makes a better use of the location possible, i.e. the number of full load hours is higher. At an average annual wind speed of 10 m/s the 5M should be able to achieve 4,000 full load hours. The prototype has been running at Brunsbüttel (Germany) since November 2004. In summer 2006 one further turbine was erected offshore for the first time. The site lies 25 kilometres off the Scottish coast (“Beatrice wind farm”).
The Repower 5 M at Beatrice wind farm, © Repower Systems AG
The Repower 5 M at Beatrice wind farm, © Repower Systems AG
The Repower 5 M wind turbine
With friendly permission of © Repower Systems AG
The Vestas V120 is based on the NM 110 developed back then by NEG Micon. Vestas has managed to increase the size of the turbine without substantially increasing the weight. This was made possible by, among other things, making the rotor blades individually adjustable (single-pitch), so that the loads on the drive train are reduced – a principle which other manufacturers have been using for quite a while. There is a lot of carbon-fibre reinforced plastic in the extremely lightweight rotor blades. Vestas is obviously determined to produce the lightest prototype regardless of the cost. The installation of the prototype has been put back several times, however.
The Enercon E-112 has already been put up eight times, once even in shallow water at Emden (Germany), in Germany’s first “nearshore project”. Enercon has announced that a newer, larger prototype is to follow soon. The expected size is 6 MW. The rotor diameter will probably lie around 125 m, which means an increase in swept area of 20%. Thanks to the new blade geometry, generation is even set to increase by 35% over the E-112.
The Multibrid 5000 is still unique among the prototypes with over 3 MW of power 3. Because this wind turbine makes do with only one transmission step, the manufacturer hopes to achieve a higher reliability. The special construction has the disadvantage, however, that practically no larger repairs are possible at sea, and certainly not a replacement of the transmission. In most cases, and in order to keep things simple, one will replace the whole tower head in order to make the repairs onshore.
Siemens gets extra marks with its special generator (squirrel-cage, see above) and the full inverter. Both rated power and rotor diameter will have to improve drastically, however, in order to keep up with the competition.
GE Energy has been testing seven type 3.6s turbines for two years now in the Irish Sea, and thus has the greatest offshore experience in the class above 3 MW. Although the 3.6s is now going to be produced with a larger rotor diameter (and will be called 3.6sl), there is the question of which direction GE Energy will take in the long term. The development of the 2.X suggests that the next offshore prototype will also be equipped with a synchronous generator and a full inverter.
This report is based on an article which appeared in the volume 1 / 2006 issue of Sun, Wind and Energy. The article was updated in September 2006 and is republished with permission of the authors and publisher.
1 For three of the multi-megawatt turbine types presented in this article there is so far just one prototype of each, a prototype for a fourth is in planning. The GE Energy 3.6s, the E-112, the Repower 5M and the Siemens 3.6 MW have already been put up more than once. To keep things simple, in the following we will always speak simply of prototypes.
2 The E-112 with 6 MW has an even higher specific power (588 W/m2). It is not taken into account here because it is expected to soon receive a larger rotor (probably 125 m).
3 The small Finnish manufacturer Winwind erected a 3 MW prototype (90 m rotor diameter) in November 2004, which also uses the Multibrid principle (www.winwind.fi).