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The U.S. Department of Energy (DOE) has released the 2011 Critical Materials Strategy - a report that examines the role that rare-earth metals and other key materials play in clean energy technologies such as wind turbines.

According to the report, several clean energy technologies use materials that are at risk of supply disruptions in the short term, with risks generally decreasing in the medium and long terms. Supply challenges for five rare-earth metals - dysprosium, neodymium, terbium, europium and yttrium - may affect clean energy technology deployment in the years ahead, the DOE says.

This is the DOE's second report on this topic and provides an update to last year's analysis. Using a methodology adapted from the National Academy of Sciences, the report includes criticality assessments for 16 elements based on their importance to clean energy and supply risk.

The DOE’s critical materials research and development (R&D) plan is aligned with the three pillars of the DOE strategy: diversifying supply, developing substitutes and improving recycling.

While the use of rare-earth permanent magnets (PMs) in these applications is growing due to the significant performance benefits PMs provide, a number of technical, economic and policy factors may influence future trends, the report says.

In 2009, turbines smaller than 2.5 MW made up more than 90% of the market. However, in 2012, this share is projected to drop to 62%, the report notes. Larger turbines are more likely to use rare-earth PMs, which can dramatically reduce the size and weight of the generator compared to non-PM designs, such as induction or synchronous generators.

A second trend is toward turbines that run at slower speeds, allowing electricity generation at much lower wind speeds than traditional high-speed turbines, the DOE says. The slowest turbine speeds are achieved through a direct-drive arrangement, in which the rotating turbine blades are directly coupled to the generator. Higher-speed turbines, on the other hand, use one or more gearing stages between the rotating blades and the generator.

In addition to being highly efficient, the lack of gearing in direct-drive turbines reduces maintenance requirements, providing a life-cycle cost advantage in remote or offshore locations.

Despite their advantages, however, slow-speed turbines require larger PMs for a given power rating, translating into greater rare-earth content. Arnold Magnetics estimates that direct-drive turbines require 600 kg of PM material per megawatt, which translates to several hundred kilograms of rare-earth content per megawatt.

Alternative design options
As manufacturers seek to reduce rare-earth content in wind turbines, they have turned to a range of design options.

For instance, hybrid-drive PM turbines, which use a PM generator in conjunction with a geared drive, have received increasing interest. These turbines operate at higher speeds than direct-drive turbines and require a more complicated gearing system, but require PMs that are one-third the weight of direct-drive turbines, with correspondingly less rare-earth content, the DOE report explains.

Hybrid-drive turbines currently represent just a small fraction of the wind turbine market but could represent more than half of wind power generation over the next decade, the DOE says.

Concerns over critical-materials scarcity could also accelerate the development of superconducting generator turbines, which do not use permanent magnets and show promise for turbines in the 10 MW+ range. For instance, AMSC has been developing a 10 MW Sea Titan turbine prototype that uses a direct-drive high-temperature superconducting generator.

There is also evidence that rare-earth export quotas and price premiums have led to a disparity between the use of PM designs inside and outside of China, the DOE notes in its report. While PM wind turbines only account for about 5% of the market outside of China, their market share inside of China is estimated at 25% or higher, according to the report.

To proactively address possible rare-earth shortages, ARPA-E’s Rare Earth Alternatives in Critical Technologies for Energy program is currently seeking to fund early-stage technology alternatives that reduce or eliminate dependence on rare-earth materials by developing substitutes for rare-earth PMs in wind generators.

The technical scope of the research includes the development of materials-level alternatives through research into alternative magnetic materials that exhibit equivalent or superior magnetic properties as compared to the existing rare-earth materials, or component or system-level alternatives that eliminate the need for rare-earth PMs while achieving equivalent or superior cost and functionality. These alternatives include superconducting or advanced electric machine topologies.

In addition, the DOE’s wind program is supporting several next-generation drivetrain technology projects. For example, research is being led by Florida-based Advanced Magnet Lab to develop a superconducting direct-drive generator for large wind turbines. The project will employ a new technology for the drivetrain coil configuration to address technical challenges of large torque electric machines.

The full DOE report is available here.

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