Most of the SWTs(Small wind turbines) that are currently deployed around the world have three blades, but there are also models with two, four or more at the micro-scale. Rotor diameter is below 20m but most of the commercial small wind turbines have a rotor diameter below 10m. These turbines are mounted typically on 12 to 24m towers.
For the rotor, technology trends are towards advanced blade manufacturing methods based mainly on alternative manufacturing techniques such as injection moulding, compression moulding, and reaction injection moulding. The advantages are shorter fabrication time, lower parts cost, and increased repeatability and uniformity, but tooling costs are higher.
Most of the current SWTs use a synchronous permanent magnet generator based on rare earth permanent magnets as the electromechanical converter for the following reasons:
• Rare earth permanent magnets are now taking over from ferrite magnets: they have superior magnetic properties, and there has been a steady decline in prices; and
• They result in more compact and lighter-weight generators.
An important characteristic to achieve in permanent magnet generators is a reduced generator ‘cogging’ torque, which enhances low wind speed start-up.
Some manufacturers still continue to use induction generators. However, in the recent past, no turbines of less than 50 kW rated power have used induction generators directly connected to the grid. Currently, though, designs utilising induction generators are re-emerging to avoid power electronics in order to achieve reduced cost and improved reliability.
A costly component for grid connected SWTs is the inverter, or DC/AC converter. Most of the inverters used come from the PV market and are being adapted for use with wind turbines, installed downstream of voltage control devices.
Lately, wind-turbine-specific inverters have also started to appear in both single- and three-phase configurations. These can be certified against International Power Quality and EMC (electromagnetic compatibility) standards.
As a general tendency, SWTs are currently designed for low wind speeds, which means larger rotors, taller towers and precise regulation devices for gust events. Usually the turbine is protected against high winds by yawing or ‘furling’, in other words the rotor is turned out of the wind passively, by aerodynamic forces. Some alternatives to furling, such as stall control, dynamic brakes, mechanical brakes, and pitch control (both centrifugal and active) have also been developed.
In order to reduce noise emissions, reduced operating and peak rotor speeds are being pursued. Because of this, the typical design tip speed ratio is 5:1.
New standards for SWT design (IEC 61400-02, second edition) were published in 2006 for turbines with a rotor area <200m2 (~ 16m rotor diameter), with slowly increasing use of this standard by the industry.
The industry is diverse and manufacturers vary widely in degree of maturity. Over 300 different models (in various stages of development) exist worldwide, of which 100 are engineered by US manufacturers.
The most recent developments in the field of small wind turbines can be summarised as follows:
• Active pitch controls to maintain energy capture at very high wind speeds;
• Vibration isolators to dampen sound;
• Advanced blade design and manufacturing methods;
• Alternative means of self-protection in extreme winds;
• Adapting a single model to either on-grid or off-grid use;
• Software and wireless display units;
• Inverters integrated into the nacelle (rotor hub);
• Electronics designed to meet stronger safety and durability standards;
• Systems wired for turnkey interconnection;
• Attempts to make SWT more visually attractive; and
• Integrating turbines into existing tower structures, such as utility or lighting poles.