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What Makes a Great Wind Turbine for Distributed Applications?

When investing considerable resources in a distributed wind energy project, doesn’t it make sense to at least evaluate the minimum criteria?

Let’s start with this list:

  • Good Value
  • Quiet
  • Attractive
  • Durable

Good Value

There are three initial factors to consider in terms of value: (1) installed cost; (2) estimated annual power production; and (3) confidence in the estimated annual power production. As a gut-check on a turbine supplier’s output estimate, a very good aerodynamic lift-based turbine will be rated between 300-400 W/m² and a drag-based turbine’s rating shall be less than 200 W/m² of swept area. Any claims outside of these norms should be thoroughly investigated. Other factors like operating costs and social concerns are more subtle and require deeper exploration.

Quiet

Who wants their quiet enjoyment of the outdoors disturbed by a relentless noisy machine? In order for distributed wind to gain broad acceptance, this reality must be addressed. A wind turbine design, as in any machine, is a balancing act. Every decision is a tradeoff and every designer a chef looking for that winning flavor. So far, most successful distributed-scale turbines have mimicked their utility-scale counterparts as 3-bladed horizontal-axis wind turbines (HAWTs). A key tradeoff in this scenario is noise vs. storm wind resistance.

In a storm, a HAWT’s blades must bear the forces of the wind from any direction. Storm resistance being a key design driver, minimizing loads on the machine in high winds is important. It is this requirement that drives a designer to want to utilize thin blades. Here something called tip speed ratio comes into play. A turbine’s tip speed ratio is the optimal velocity of its blade relative to the wind speed. The higher the tip speed ratio, the thinner the blades can be and still produce optimal power. Designers of traditional HAWT’s normally hone in on a tip speed ratio of about eight, which equates to blade speeds of 200 mph in a 25-mph wind. This is the point at which storm wind blade forces and noise levels are ‘tolerable’. Is tolerable good enough for your environment?

The key to quiet is lowering blade speeds without increasing storm wind loads. Is there a way to do this without sacrificing efficiency of the turbine? Yes, but it does require thinking outside the box. By combining full pitch control with a straight bladed VAWT, aka Pitch Controlled H-VAWT (PCH-VAWT), blades can be feathered into the wind. If blades are appropriately positioned, the rotor can even point itself into the wind like a weathervane. When appropriately implemented, this approach can have a much higher factor of safety than the locked rotor approach of a traditional HAWT. When each mph of wind speed results in thousands of pounds of structural forces, the added margin is meaningful. With straight (non-twisted) blades that can more effectively shed wind load, these blades can grow in sectional area to reduce the optimal tip speed ratio. At a tip speed ratio of three, for example, blades are traveling about 75 mph in that same 25-mph wind. This can be a quiet turbine! 

Attractive

Unlike solar panels, where architects, project planners, and owners typically try to hide on rooftops and within landscape, wind turbines are big and tall and must be seen if used. If you are thinking about mounting something to the roof of your structure and ‘hiding’ it in your architecture, you will have some challenges to overcome. The physics of incorporating a high wind load dynamic machine into a dwelling or other structure not designed for the task is potentially dangerous. Solutions typically sacrifice power output to minimize impact on structural integrity and occupant safety.

So if it’s going to be big, what makes it attractive? Beauty is normally in the eye of the beholder, but keep in mind customers, neighbors, and the general public will be the eyes beholding your new turbine. It needs to be aesthetically pleasing.

Durable

Operating costs are directly impacted by turbine durability and design again rules the day in a durability match. Turbines have several systems that must be able to withstand the mechanical, electrical, and environmental stresses to which they are exposed. Structural systems like the blades, tower, frame, hub, etc.; mechanical systems like drivetrains and bearings; electromechanical systems like the generator and pitch/yaw control drives; and control systems are all in this category. Every system must also be environmentally durable for its own operating environment. Material selections and interfaces should therefore be appropriate.

Structural systems are designed around modeled (software simulated) load cases. For traditional HAWT designs, software packages are commercially available to generate this load data. One such software, ADAMS, uses an aerodynamic library originally developed by NREL called AeroDyn. The designer must run the software for different cases and use the load outputs to check the structural design. FEA, or finite element analysis, is often used to analyze the structural elements to insure they will withstand the loads.

Problem: composites like the materials used in traditional blade construction don’t simulate well by this method. Material variations, manufacturing process inconsistencies, and software limitations all contribute to inaccuracies in simulated results. These are key reasons why Airbus has major structural issues with the wings of its new A380 and Boeing’s 787 is years behind schedule. It also is why most of the major utility-scale turbine manufacturers are having blade failures.

#rewpage#

Lightning can also cause structural failure in a composite blade. IEC 61400-24 defines blade lightning protection systems for utility-scale turbines, but even with these elaborate systems lightning damage remains the leading cause of blade failure. The primary reason for this damage is that while the blade has conductors to discharge the energy through the machine, there is localized heating within the composites themselves. Often, the heating is enough to cause moisture within the blades to vaporize and the resulting increase in pressure to delaminate materials. Perhaps the most dangerous attribute of blade lightning damage is that it can go unnoticed until the weakened structure has a fatigue failure days or weeks later.

One way around these problems is to make blades out of materials that are more predictable, and are more resistant to lightning, like metals. With traditional HAWT blade geometries — having twisted airfoils and variable chords — this is certainly impractical, but what about the straight airfoils on the PCH-VAWT turbine? In this machine the blades can be constructed more like a typical aluminum airplane wing with spar sections, ribs, and sheet skin. Very well-established and consistent manufacturing processes exist and FEA simulations are accurate and validated. The biggest drawback is the need for a unique mathematical model to generate simulated machine loads.

Drivetrains are critical elements of a wind turbine. The best drivetrain is no drivetrain, or direct-drive. Any components or systems between the mechanical energy takers (blades) and the mechanical to electrical energy converters (generators) are opportunities for failure. For this reason major utility-scale turbine manufacturers are widely adopting direct-drive topologies in their latest machines.

Variable speed synchronous permanent magnet generators are the state-of-the-art, and further coreless designs are the best of the best. A coreless PM generator has no steel laminations, thus eliminating a potential corrosion failure point and the associated eddy current and hysteresis losses that occur in the iron of these laminations. Also eliminated is the attraction force between stator and rotor, a design element requiring considerable mechanics and friction to deal with. The drawback is the need for more magnet material.

Pitch and yaw drives are often necessary hardware elements to meet turbine design objectives. VAWT machines have the advantage of not needing yaw control, as it would be the same axis as the main hub rotation. Similar to the drivetrain scenario, the fewer parts the better for these systems. Direct drive is the best and most reliable way to perform these motions, although the vast majority of HAWT’s still have gears on these actuators.

Controllers provide another opportunity for balance. A good turbine control system should have enough reach to thoroughly monitor and track the condition of the machine. The most technically relevant comparison of a state-of-the-art control system today is an automobile. With distributed nodes all performing relatively simple functions and communicating on a CANbus interface, reliability, redundancy, and safety can be insured. A modern wind turbine should at a minimum have a controller that monitors vibration, bearing temperature, generator coil temperatures, and pitch control actuator performance as well as overall turbine output performance. A comprehensive lightning protection system is also critical to a machine controller’s durability.

The lack of any one of these sub-systems could allow a catastrophic circumstance to occur. I believe this to be even more important in the case of a distributed generation machine that might be near people than a utility-scale machine that is more likely in the middle of a field or ocean.

Conclusion

The same factors that make a durable turbine will also make a low-maintenance one. A great wind turbine can bring a facility owner many benefits. Clean and cost-effective power, local jobs, a sense of pride, and an icon of commitment to protecting natural resources are some. Along with the benefits comes a responsibility to invest wisely.

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