Engineering of complex systems can sometimes be represented as a series of simplified approximations based on the performance of known subsystems. In the case of wind turbines, there are 4 subsystems, aerodynamic, gearing, generator and power conversion. In order to simplify the evaluation process of competing technologies in each category, they can be assigned an efficiency value and the system efficiency calculation is the product of values multiplied together. This turns out to be a fairly obvious and easy way to assess the technology tradeoffs in the wind turbine industry.
The system design decision in wind turbine technology is to start with a propeller-based rotor because of its efficiency. In part, this is due to the fact that a propeller blade has lift and will cause the rotor to turn faster than the actual wind velocity. An impulse rotor, sometimes called a Savonius rotor, only turns at the speed of the wind, like an anemometer, and has somewhat lower efficiency that a propeller.
So using a propeller based rotor seems to be sound engineering at first glance. The overall turbine efficiency is bounded by the aerodynamic solution. However, when the efficiencies are multiplied together, the aerodynamic efficiency does not change the overall performance quite as much. A side effect of the “obvious” focus on system efficiency is that the HWT solution has a number of technology issues that are becoming increasingly difficult to deal with.
Large horizontal wind turbines operate at wind speeds between 7 and 23 miles per hour, and depending on where they are sited, operate only 25% of the time on average. In addition HWTs require steering systems, starting motors, hydraulic blade pitch controls and long transmission lines.
If HWT operating times could arbitrarily be doubled to 50%, the gain in productivity would far outweigh efficiency issues. For this reason, the HWT industry is attempting to re-invent itself on the ocean. Unfortunately, the Bard 1 Project off the German coast is proving to be more difficult to complete than previously anticipated. Bard 1 has a project price tag over $3.5 billion dollars for 400MW of wind turbines, or $9.4 million per megawatt. The cost of the offshore wind has more than doubled making any productivity gains negligible.
Conspicuously, there is never any commentary on how profitable wind power is. If productivity cannot be increased, the only other option is to reduce the capital expense. To date, the Federal and State governments have been underwriting more than 30% of the cost of wind power projects through tax write offs.
It should be obvious that production time and capital costs are more important to profitable wind power than the esoteric and costly technology that is being created to solve fundamental problems with HWTs. Successful wind power will be a function of something that is inexpensive and can be sited in locations with constant prevailing winds. Efficiency is secondary.