Wind turbine design technology has not converged. Unlike the automobile industry, where underframes have long given way to the cheap and robust monocoque construction, design options for wind turbines have multiplied steadily over the past decade or so. The most convergent era was probably the early 1980s, when most of the prototype megawatt size machines were built. Since then, the growing market for wind turbines has thrown down challenge after challenge to test the imagination of wind energy's design engineers.
The result has been an increasing diversity of technology options and the near completion of an evolutionary step by the wind industry. Today, wind companies are increasingly becoming wind turbine assemblers rather than manufacturers, selecting off-the-shelf combinations of components tailored to the needs of each individual order. Some companies have even reduced their manufacturing divisions while at the same time increasing turnover -- a sure sign of the times.
Wind power plant development is, by nature, site specific. While some sites are highly noise sensitive, others are not; while some sites have steady winds, others are buffeted by ground level turbulent gusts, with tall towers becoming a must; remote sites demand technology with minimal maintenance requirements (as a trade-off for higher capital cost); site access for construction demands solutions such as telescopic towers, or even crane-free erection. The list of market demands goes on. Problems with wind turbines disturbing communications signals again returned designers to the drawing board, as did the demand for sealed nacelles in saline humidity. Local electrical network characteristics, especially weak grids, also have to be catered for to ensure power quality: on small sites, two or three small machines might provide smoother output than a single big one; and some utilities impose charges for supplying the reactive power, which most electrical generators need. Here, electronically innovative wind turbines have the edge since most of these do not consume reactive power -- and many can supply it -- winning a financial bonus as a result.
As design options have multiplied, talk of a standardised concept has faded into wind energy history. The one common design feature remaining of note has been the steady growth in machine size -- and perhaps the steady disappearance of the vertical axis concept. Meantime, other options once written off as impractical have recently been resurrected. In Europe, the last few years have seen the emergence, or re-emergence, of several novel concepts, including three types of gearless machine, and a flexible downwind rotor. Wind turbines on the market today mix and match numerous possibilities for almost every component (table 1).
With such diversity, any classification of types of wind turbine can only be arbitrary. It can be argued, however, that there are four principal design themes with associated advantages and disadvantages (table 2): stall regulated and pitch regulated models, which equally make up about 80% of the world's wind turbines; and two broad types of innovative machine, the lightweight designs, which strive to reduce cost by reducing weight, and the electrically innovative designs which concentrate more on improving compatibility with the grid than on reducing weight. The gearless and direct drive machines come in this category.
As a rule of thumb, the heavier the wind turbine, the more costly it is to make. Yet manufacturers of heavier machines argue forcibly that a more robust machine does not necessarily mean increased price per unit of output. High availability, low running costs, low noise (in most of Europe), and long machine lifetime are all positive features of heavier more traditional machines, they argue.
On the other hand, proponents of the "slimmer is healthier" approach say reducing weight is the key to bringing down costs and can be achieved without paying penalties elsewhere. Likewise, the electrically innovative camp argues that the extra cost and complexity of electronic wizardry does not add to machine downtime and is more than compensated for in the production of quality electricity.
Again, the site specific nature of wind development means there is no design consensus for a standard concept. Weight is, nevertheless, a crucial parameter in distinguishing between the pitch controlled, stall regulated and light weight innovative design classes, though for electrically innovative machines it is not a factor for debate. Their concentration on electrical compatibility allows them to stand outside the weight argument. They generally weigh in around the average weight of pitch controlled machines (Fig 1). Broadly speaking, the tower head weights of stall regulated machines are about 20% higher than those of pitch regulated machines, and these, in turn, are around 30% heavier than the lightweight innovative designs.
Pitching and stalling
Stall regulation possibly represents the simplest and most rugged design option, factors which are promoted as its principal advantages. Although some means must be provided of controlling rotors when they are not connected to the grid, such controls are generally straightforward. The essence of a stall controlled machine is its ability to run with a simple control system, safe in the knowledge that power limitation will be by passive means and, contrary to early fears, operation in high winds with a significant proportion of the blade in stall does not produce high dynamic loading. Although the weight trend line (Fig 1) confirms the widely held view that such rotors are often heavier than their pitch controlled counterparts, this is not always the case. Four out of the seven stall controlled nacelle assemblies are no heavier than those of pitch regulated machines of equivalent size.
Pitch controlled machines are as numerous as stall controlled machines and achieve similarly high availability. This indicates that "active" pitch control, rather than "passive" stall, is not a drawback in practice. As the power output can be controlled, these machines generally yield slightly better energy productivity per unit of weight. As electronic controls generally become more sophisticated the science of power limitation for wind turbines is becoming more refined and the wind turbine designer can exploit these advances to ensure that rotor loads are kept to as low a level as possible, thus ensuring longer life for the whole machine.
The right of light
Lightweight innovative machines mostly have rotors with two blades or just one. These include designs marketed by Riva Calzoni in Italy, Dutch companies Lagerwey and Nedwind and the Swedish Zephyr Energy. Not only are these rotors intrinsically light but the fact that they are teetered and/or flexible means the loads are alleviated, enabling further weight savings to be made. Some blades twist passively in high winds, dispensing with the need for pitch controls. An array of 36 one blade rotors has recently been commissioned in Italy and it will be interesting to track their fortunes. Design data show that the weight of the one blade nacelles is similar to that of two blade machines in the same lightweight class.
The latest manifestation of the lightweight philosophy is, significantly, a three blade machine from Britain's Wind Energy Group (WEG). The company has long championed the benefits of a slim line approach and several years ago switched from three blades to two, although for aesthetic reasons these rotors proved unpopular in conservative Britain where the more visually pleasing appearance of regular rotation afforded by three blades is preferred. The decision to opt for three blades in the new design will inevitably incur a slight weight penalty, but the prototype weighs about the same as that of a pitch regulated machine of the equivalent size and the expectation is that later versions will be significantly lighter.
WEG's new design incorporates several innovative features including a down wind rotor configuration, with three highly flexible blades, active stall regulation, nacelle pitching and two speed operation. None of these are new or unique but the combination is a major step forward, and the particular use of active stall is unique. Active stall control simply means that the blades are rotated so as to increase the airfoil angle of attack rather than reducing it, as with most pitch controlled machines. Even more novel, pitch angle changes to limit power during normal operation are not continuous, but selective in response to need on time scales which may vary from seasonal to minute-by-minute. "Pragmatic pitch control" might be a better description.
Variable speed and direct drive concepts have both been around for some time, but a radical change of direction by German wind company Enercon in 1992 brought both concepts together in a range of machines which has put the firm in a strong competitive position. The advantages of variable speed rotors have been discussed at length; although the quantity of extra energy realisable is debatable and may be small, there are undoubted advantages in the reduced noise levels at low rotational speeds when wind speeds -- and hence wind noise -- is low.
The other key advantage of variable speed is that it enables power conditioning equipment to be incorporated, thus giving the machine operator the control of power factor. Since utilities everywhere are wary of applications which draw too much reactive power -- and reflect this concern by charging for it in many instances -- control of this quantity is important. Furthermore, many export markets outside Europe include locations where grids are weak, again making power factor control important.
Elimination of the (heavy) gearbox has also been an objective long sought and Enercon has been successful in simplifying the drive train and cutting down the number of components in a wind turbine. The direct drive generator weighs more than the conventional type, but overall an Enercon nacelle weighs roughly the same as that of a pitch controlled machine of equivalent size. The use of variable speed also enables rotor loads to be alleviated. Enercon claims that the extra capital cost of one its machines is repaid in about nine years by lower replacement costs for mechanical components.
Two more recent innovative machines also use direct drives with permanent magnet excitation, an inherently simpler concept much discussed but difficult to realise at large scale as the magnetism cannot be "switched off" during assembly. These are the Swedish Pitch Wind (Windpower Monthly, May 1997), from the Zephyr stable, and the German Genesys 600 which use permanent magnets in the generator, in place of the more usual electromagnets and their need for windings which add cost and weight. Both manufacturers claim higher generator efficiency as a result of this innovation, coupled with lower weight.
One trend which seems set to continue is that towards larger rotor sizes. The dramatic reduction in wind energy prices over the years has been due, in part, to the savings realised by the use of progressively larger machines. Most of the machines installed in the early 1980s were around 50 kW in size, whereas most modern wind farms use machines in the 600 to 750 kW range and some use machines around the one megawatt mark (Fig 2). The claim that bigger machines mean higher yields is also supported by the evidence (Fig 3). The productivity of 600 kW machines is around 50% higher than that of the 55 kW machines of yesteryear.
The trend towards larger sizes cannot continue indefinitely. Weight per unit area increases steadily with size (Fig 1) and the extra energy yields are becoming smaller. The trend in size, however, is upwards in the short to medium term, with a feeling that it may level out around the 70 metre rotor diameter mark. Only time will tell.
Looking for clues to the future, the design concepts employed in the machines in the European Commission's "Large Wind Turbine Scientific Evaluation Project" (Table 3) might have revealed a secret or two. But all these machines provide is confirmation that the range of design options is still wide. The wind turbines in this programme are all operational; several grew in size between their inception and commissioning.
Mix and match
There is tremendous scope for careful selection of innovative design concepts which make machines more marketable. Enercon's choice of variable speed and direct drive was a shrewd choice and is reflected in growing worldwide sales. Generally, however, manufacturers have retained their initial design concepts. In Denmark, its industry heavyweights Bonus and NEG Micon have stayed faithful to stall regulation and Vestas has stayed with pitch regulation.
The history of manufacturers who have changed track has not always been a success story. WEG's switch from three to two blades in the 1980s marked the beginning of a difficult patch from which the company is now just recovering. There are two difficulties with major design changes. In the first place, the new design inevitably comes up with minor problems which need to be rectified. Secondly, and perhaps more important, the manufacturer loses the benefits accrued from long production runs. In any technology, price falls with the accumulated number of units sold and wind energy is no exception. A recent study, "The Future for Renewable Energy," carried out by the European Association of Renewable Energy Research and Development Centres (EUREC) on behalf of the European Commission, identified improved manufacturing techniques as a key objective in bringing down wind turbine costs.
It seems that diversity will remain in wind turbine design. It may even increase as the variety of site increases with global expansion of wind development. Pressures for the particular needs of the developing world and of off-shore machines may well increase the diversity. Radical breakthroughs in overall performance are unlikely: the much discussed and long awaited improvements in basic airfoil design have yet to realise the promised 5-20% increase in output. Evolution will therefore be more gradual, backed by industrial research and development and shared-cost programmes of the type favoured by the European Union and increasingly by the United States.