A list of recent significant developments in distributed wind technology (DWT) systems less than 20 kW include the following:

• Alternative power and load control strategies: Furling inherently increases sound levels because the cross-wind operation creates a helicopter-type chopping noise. Aerodynamic models available today cannot accurately predict the rotor loads in the highly skewed and unsteady flows that occur during the furling process, complicating design and analysis. Alternative development approaches include soft-stall rotor-speed control, constant-speed operation, variable-pitch blades, hinged blades, mechanical brakes, and centrifugally actuated blade tips. These concepts offer safer, quieter turbines that respond more predictably to high winds, gusts, and sudden wind direction changes.
• Advanced blade manufacturing methods: Blades for small turbines have been made primarily of fiberglass by hand lay-up manufacturing or pultrusion. The industry is now pursuing alternative manufacturing techniques, including injection, compression, and reaction injection molding. These methods often provide shorter fabrication time, lower parts costs, and increased repeatability and uniformity, although the tooling costs are typically higher.
• Rare-earth permanent magnets: Ferrite magnets have long been the staple in permanent-magnet generators for small wind turbines. Rare-earth permanent magnets are now taking over the market with Asian suppliers offering superior magnetic properties and a steady decline in price. This enables more compact and lighter weight generator designs.
• Reduced generator cogging: Concepts for generators with reduced cogging torque (the force needed to initiate generator rotation) are showing promise to reduce cut-in wind speeds. This is an important advancement to improve low-wind-speed turbine performance and increase the number of sites where installation is economical.
• Induction generators: Small turbine designs that use induction generators are under development. This approach, common in the early 1980s, avoids the use of power electronics that increase cost and complexity, and reduce reliability.
• Grid-connected inverters: Inverters used in the photovoltaics market are being adapted for use with wind turbines. Turbine-specific inverters are also appearing in both single- and three-phase configurations. Another new trend is obtaining certification of most inverters by Underwriters Laboratories and others for compliance with national interconnection standards.
• Reduced rotor speeds: To reduce sound emissions, turbine designs with lower tip-speed ratios and lower peak-rotor speeds are being pursued.
• Design standards and certification: The industry is increasing the use of consensus standards in its turbine design efforts for machines with rotor swept areas under 200 m2 (about 65 kW rated power). In particular, IEC Standard 61400-2 Wind Turbines – Part 2: Design Requirements of Small Wind Turbines. Currently, however, a limited number of wind turbines have been certified in compliance with this standard because of the high cost of the certification process. To address this barrier, a Small Wind Certification Council has been formed in North America to certify that small wind turbines meet the requirements of the draft AWEA standard that is based on the IEC standard (AWEA 1996–2007).

Source: NREL

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