Albuquerque-based Sandia National Laboratories is conducting comprehensive research into the viability of vertical axis wind turbines (VAWTs) for offshore use. The design, previously considered impractical for large-scale applications, has the potential to transform offshore wind technology, making it a more economically viable energy source.

The research is being conducted under a 2011 Department of Energy (DOE) solicitation for advanced rotor technologies for wind power generation in the United States. The US$4.1 million research project began in January of this year and will continue for five years. The first stage will last two years, during which time, several concept designs will be created and run through modern modelling software. The most workable of these will then be selected and undergo a three-year construction period before completing a rigorous test regime, measuring its effectiveness against the most extreme conditions that turbines must endure in an offshore environment.

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In the 1970s and 80s, VAWTs were actively developed as wind power generators, exhibiting simpler designs than their horizontal-axis cousins and proving generally more reliable. However, once wind turbines began to be scaled up in size, the lower cost of rotors for horizontal axis wind turbines (HAWTs) seemingly relegated VAWTs to the history books.

The project will reassess the economic implications of large-scale VAWTs, with the goal of making them a cost effective and viable method for generating energy from offshore breezes. In doing so, it aims to address the national energy challenge of increasing the use of low-carbon generation.

The costs associated with offshore wind power differs significantly from land-based turbines. Offshore turbines are significantly less accessible, making the initial installation and ongoing operational challenges more expensive. VAWTs offer a number of significant advantages that make maintenance easier, and may well reduce the economic footprint of offshore wind energy. They exhibit reduced machine complexity and better scalability to very large sizes, while a lower center of gravity affords them improved stability when afloat. Additionally, the drivetrain is closer to the surface on a VAWT, making maintenance easier and less time-consuming.

VAWT turbines offer a number of significant advantages over the more common HAWT design

However, there are numerous and significant challenges that must be overcome before VAWTs can be used for large-scale offshore power generation. The first of these is the problem involved in manufacturing the complex, curved blades. Iowa State University and TPI Composites will address this issue, exploring new techniques to allow for the production of geometrically complex VAWT blade shapes on an unprecedented scale, without incurring unacceptable costs.

The second problem that must be addressed is the issue surrounding the cyclic loading on the drivetrain. VAWTs have two “pulses” of torque and power for each blade, determined by its upwind or downwind position. This “torque ripple” produces unsteady loading, leading to drivetrain fatigue. The project will have to have to evaluate new rotor designs in order to smooth out the amplitude of the torque oscillations, while once again keeping an eye on the cost.

The project is currently in the prototype stage, with a number of project partners working on the various problems. Josh Paquette, one of Sandia's two principle investigators on the project, explained the overriding impetus of the project, stating that “Ultimately, it's all about the cost of energy. All these decisions need to lead to a design that's efficient and economically viable”.

Source: Sandia National Laboratories

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