ADVANCED WIND TURBINE FOR MULTI-MEGAWATT POWER GENERATION
 
The Engineering and Economical Practicality of Multi-Megawatt Vertical-Axis Wind Turbines

Synopsis

Economy-of-scale is a widely misused term. When a particular plant manufactures diverse products requiring multiple component sources and different distribution channels the advantages of economy-of-scale is largely lost. Small plants strategically placed are often far more economical than a single large plant. In regard to single-product plants however the larger the installation generally translates into greater economy-of-scale. This approach is particularly recognized in the power-generating industry. Enlargement of power-generating facilities translated directly into economy-of-scale, whether fossil, fission, or hydro. This concept however has been, to a large extent, neglected by the wind-power industry.

The direction that this industry has taken is diametrically opposed to this concept. Rather wind turbine farms extending over immense areas covered with relatively small turbines is the concept embraced. Serial manufacturing of identical turbines is thought to be the path to economical power production. According to this scenario power stations would occupy many square kilometers with numerous turbines wired together and combined into a local network. The shortcomings of this approach are manifest.

For on-shore installations availability of suitable terrain near locales of power consumption will always be limited. For off-shore installations the extended areas required for power generation would constitute hazards to navigation.

A particular solution to this dilemma would be to extend the power generating facility vertically rather than horizontally. A smaller number of identical multi-megawatt turbines extending upwards into the smoother faster winds aloft could accomplish all that is required of present widely-extended wind farms. The horizontal-axis wind turbines embraced by the American proponents of wind power however have an intrinsic limitation in this regard. This limitation involves the need for large cantilevered blades attached to relatively heavy generators perched atop tall towers whose overturning moments are such that extensive foundations are required for each turbine. Perhaps five megawatt turbines would probably be a limiting possibility, particularly as imposed stresses would increase much faster than capacity. Moreover maintenance would be a ongoing concern.

In this report the practicality of a one-gigawatt wind power generating station comprising identical ten-megawatt vertical-axis wind turbines is considered.

Abstract

At present less that two percent of the total electric power generating capacity of the United States is derived from wind power, despite operation, maintenance and fuel costs competitive with more conventional fossil and fission power sources. With the de-emphasis of research and development efforts directed towards alternative power source as an integral part of Federal power policy during the 1980's, this continued reliance of conventional power sources can be expected to continue. Nevertheless, almost 200 megawatts of electric power is presently being generated from over 10,000 turbines, essentially all in California. Recently eleven turbines with an aggregate output of some six megawatts were installed at Searburg, VT, the largest such concentration east of the Mississippi.

The present reliance on fractional-megawatt turbines is based on both mechanical practicality and economic feasibility. Although the power rating of the conventional horizontal-axis wind turbine increases roughly with the square of the propeller diameter, propeller blade axial deflection increases with the cube of the rotor diameter. Accordingly, for the same allowable axial deflection blade rigidity and essentially weight increases proportionally. Moreover both blade-pitch control and yaw-control motors are required whose power requirements increase with blade diameter. However the principal deterrent to multi-megawatt horizontal axis turbines remains the integrity of the rotor blades. The blades are cantilever loaded and pass through a vertical wind gradient, with the wind speed increasing upwards from ground level. Hence the blades are subject to at least 1/rev fluctuations in loading. Blade twist, whose angular magnitude is set for a uniform inflow speed, can be significantly greater or lesser than optimum depending on the azmuthial position of the blades. Accordingly both the blades and blade roots are subject to low-cycle fatigue and unless extremely over-designed, are subject to potential fatigue failure.

Accordingly, presently operating wind turbines have average capacities of roughly 500 to 750 kilowatts with diameters of some 30 to 60 meters, with the gearbox and generator light enough to be mounted on simple towers at heights on the order of 1.5 rotor diameters above the surface so as to minimize the vertical wind-speed differential. Although such units are still subject to fatigue loading considerable improvements have been made in this regard and consequently down-time due to blade or hub crack-detection has a minimal effect on aggregate power production and moreover component replacement can be readily accomplished.

Because of this reliance on fractional-megawatt turbines, most often situated in large numbers at power generating farms located at favorable sites, volume manufacture of identical units becomes feasible, and is probably the single most important factor in the reduction of wind-generated power costs to competitive levels. Accordingly, the cost of manufacture, site fabrication, erection, and operation of identical fractional-megawatt turbines is significantly less than the equivalent costs of a single multi-megawatt unit on a per kilowatt basis. Essentially, the economy of scale that normally should favor many multi-megawatt units is not nearly as important as the savings that accrue to volume production of many more fractional-megawatt units.

Thus, as long as wind-power generation remains a minor contributor to the power requirements of the United States, this reliance on small turbines is mechanically practical and economically feasible, a strategy that should remain valid well for the next several decades.

Although the choice in the United States had been between a limited number of multi-megawatt turbines versus a large number of fractional-megawatt turbines, in the low countries and Scandinavia apparently the choice eventually will be between a large number of multi-megawatt turbines versus an overwhelmingly vast number of fractional-megawatt turbines. Thus, the same economic advantages that accrue to the fractional-megawatt units in the United States should be able to come into play in Scandinavia as far as multi-megawatt units are concerned: series manufacture of identical units. However, the mechanical obstacles remain.

Ultimately, the large multi-megawatt wind turbine must be considered seriously for wind power itself to considered seriously. The goal should be aimed upwards towards ten-megawatt units. If this goal appears preposterous, then also must be wind-power itself as a major source of electric power in competition with present sources: fossil and fission fuel. One-hundred identical turbines would produce a gigawatt of power: among the largest power stations in operation. Conservatively, a ten-megawatt horizontal-axis wind turbine would require a tower some 190 meters tall with blades each at least 120 meters in length. Erection of such units might appear to be a major engineering task in its own right, but this is not necessary so. These capacities are totally impractical as far as the horizontal-axis wind turbine is concerned.

The greatest potential for multi-megawatt turbines resides with the vertical-axis wind turbine, of which a four megawatt unit has operated in Canada. It is the vertical-axis wind turbine towards which our attention must be drawn. To meet these practical objections to multi-megawatt turbines an economical advanced vertical-axis wind turbine is proposed which includes

1) light-weight composite components readily fabricated and erected,
2) ease of maintenance with significant component redundancy,
3) non-oscillating structural loading in a vertical wind gradient, and
4) rotor blade reefing under severe wind conditions.

To achieve these goals several difficulties associated with conventional vertical-axis wind turbines are addressed, including the requirement for guy-wire stabilizers, blade resonance, and main bearing replacement.

Unlike horizontal-axis wind turbines however, the blade-sections of the vertical-axis turbines pass through a relatively uniform wind-velocity portion of the vertical wind gradient and therefore are subject to minimum fatigue loading. Moreover, rather than being cantilever loaded as are horizontal-axis turbines, the blades are catenary loaded, and hence secured at both ends. In addition the blades are neither twisted nor subject to oscillating loads, permitting employment of simple composite fabrication techniques using high strength carbon fiber construction rather than lower strength glass fiber construction. A ten-megawatt turbine will be examined, an example of the power potential of vertical-axis turbines utilizing advanced structural-composite construction.

The rotor mast construction utilizes polymer-matrix carbon-fiber winding on an enclosed steel mandrel with integral end plates. In this manner identical masts can be constructed in sections and bolted together. Such highly symmetrical construction is both light-weight and stiff, permitting ease of erection, and is economical in even moderate-volume production. The vertical-axis rotor blades are subject primarily to hoop loads rather than the cantilever loads to which horizontal-axis turbines are subject. Because the blade orientation is vertical the blade sections do not pass through significant alternating wind velocities. Accordingly, rather than requiring the massive construction required for very large propellers the vertical-axis rotor blades can use light polymer-matrix construction with glass filament winding and carbon longitudinal lay-up with fiberglass covering as in helicopter blade construction. Thus, the blades can be fabricated to be flexible in the bending mode, yet stiff in the in-plane mode. Because the aerodynamic blade section is symmetrical, upper and lower blade construction is identical, as is the deployment mechanism, providing component redundancy.

CONTENTS
1 Introduction
2 Background
3 Analysis
4 Alternative Configuration
5 Turbine Capacity
6 Mechanical and Aerodynamical Loading
7 Blade Loading
8 Rotor Characteristics
9 Composite Blade Configuration
10 Composite Mast Construction
11 Ground Erection
12 Blade Deployment
13 Maintenance
14 Economy of Scale
15 Summary

 
VERTICAL-AXIS WIND TURBINE FOR MULTI-MEGAWATT POWER GENERATION
 
For detailed description please contact
Moishe Garfinkle
Aeolus Associates
(215) 235-5042