Here is a quote from an April 2014 Siemens brochure on their new 6 MW turbine:
“With a tower head mass of less than 60 tons per megawatt, the D6 wind turbine is genuinely lean. This new low-weight standard for offshore wind turbines offers significant cost benefits in terms of substructure requirements, shipping, and installation. All of this is made possible thanks to Siemens’ proven direct drive technology.”
American Superconductor is developing high temperature superconductors for direct drive generators which they believe will substantially reduce tower top weight:
These are just two examples of a serious effort in the industry to reduce tower top weight. In their presentation, AMSC estimates that the tower top weight of a conventional 8MW direct drive turbine would be 800 tons, while their superconducting design would weigh 480 tons.
But in both of these cases, the decision to use direct drive has prevented them from achieving even greater TTW reduction. A geared turbine will always have a lower TTW than a direct drive machine. But those who advocate direct drive invariably claim that the elimination of the gearbox is necessary to increase reliability.
Gearboxes do have a bad reputation in the industry, but it is too soon to conclude that they will always be a problem. Recent research has discovered some likely causes for these failures. This will allow for improved designs. One discovery is that the gearbox is greatly overloaded in both directions during braking. Not the fault of the gearbox, just fix the braking system. Another discovery concerns bearings. They are failing sooner than expected, and the focus of research now is on “white etching area.” Fix that problem and the gearbox becomes even more reliable. Distortion of the gear case from external loads is also a problem, again not due to design of the gearbox, but to inferior design of the rotor shaft support.
Not that I want to start a new controversy here, but the planetary gearbox design must be abandoned if gearbox reliability is to achieve an acceptable level. The planetary design is inherently inferior to the multi-branch parallel shaft type. In the planetary design it is impossible to make the planets share the load equally. The planet carrier will distort under load, misaligning he gears. Gravity aggravates these effects, and I have not exhausted my objections. Here is a good study of the subject, although it avoids the conclusion that the planetary design itself is the main problem:
There are many other important papers on gearboxes where that one is listed:
Further to the question of replacing gearboxes with direct drive, I cannot agree with the commonly heard claim that fewer parts necessarily mean greater reliability. What would you rather have in your car, a one-lunger or a V-8? Werner von Braun made this point as well as anyone (quoted in an earlier thread on this forum):
“ …when he was asked how something as complex as the moon rocket for the Apollo mission could possibly be reliable with so many parts involved. He replied that in his garage, he had a car and a lawn mower. The lawn mower was much simpler than the car, but the car started every time he pulled the starter, and the lawn mower didn't.”
In fact, power electronics fail twice as often as gearboxes, as shown in the following:
Note that this study was conducted before any gearbox improvements indicated by recent research have been put into practice. If power electronics are the main cause of turbine downtime, how can direct drive advocates justify their claim that they are intent on increasing reliability? Direct drive cannot work without them. If they are worried about part count, maybe they have a point after it reaches a certain level. Power electronics have many trillions of moving parts.
One of the foremost advocates of direct drive, Sandy Butterfield of Boulder Power, formerly chief engineer for many years at the national wind research organization NREL, has finally succumbed to industry pressure by providing a generator for geared turbines:
Neither can direct drive achieve what DeWind has been doing with ever greater success, using a geared turbine to eliminate the power electronics. Their website explains it well.
Terry, the proposition that most wind turbine manufacturers have made the best possible choice by settling on a planetary geared turbine, whether low, medium or high speed, with a DFIG and power electronics, and everything mounted in the nacelle, reminds me of the attitude of the major American car companies when the Japanese introduced fuel efficient small cars. There was no end of ridicule from the majors. At first these small cars were not very good, but the Japanese kept at it. That is why Toyota is the biggest car company today. I regard the advocates of direct drive and conventional geared turbines as the modern version of General Motors circa 1972, before the first big oil shock.
With the arrangement I propose, tower top weight would be reduced to a minimum. Most of the equipment would be located at the base for easier erection and maintenance. It may even be possible to replace the power electronics in this arrangement. All of these changes would be highly advantageous. Direct drive cannot achieve any of them. The only question I have is the behavior of the vertical drive shaft. If it is controllable, I think the arrangement should be studied in detail.
Concerning your failure to interest any wind turbine manufacturers in your CVT design, which I believe would require at least some gearing, which again would be in opposition to direct drive, I am familiar with the problem. I first proposed a vertical drive shaft more than ten years ago to Vestas with no reply. For more than twenty years I have been working on an alternative to the planetary gearbox. At least two leaders in this field have found that my work is “unique and has technical merit” but have declined to develop it. There is a wise saying about this:
“Establishments like nothing so little as progress not established by themselves.”
Gearbox development is extremely expensive. Unless the inventor has millions of his own money to spend to demonstrate his concepts, he will almost certainly get nowhere. But no one who gives up can call himself an inventor, even when he goes to the grave a forgotten pauper while others profit greatly from his work. There is no better example of this than John Fitch, inventor of the steamboat. I cannot recommend a more interesting story than his. See
