Wind Turbines

The Excel is a 7 meter (23 ft) diameter three-blade upwind turbine that achieves high reliability through rugged construction and a minimum of moving parts.

The rotor on the Excel has three pultruded fiber reinforced plastic (FRP) blades which are rigidly attached at their hubs. The Excel blades are flexible in torsion and have a unique pitch weight attached to their outboard leading edge. The patented Powerflex® blade system provides passive blade pitching as a function of rotor speed. Thus, the blades are set for a higher pitch to aid start-up, but "flatten out" at speed to improve operating aerodynamic efficiency. The airfoil used on the Excel is a proprietary design (BW-3) developed specifically for low Reynolds Number conditions. This thin, highly cambered section is not affected by normal dust and residue build-up on the blade leading edge. BWC offers an Extra-Stiff Blade Option for high-turbulence or high icing conditions. These blades are often painted black for ice-shedding.

The blades attach directly to the outside shell of the Excel's purpose-built direct drive 38-pole permanent magnet (PM) alternator. The alternator has an "inside-out" configuration in that the outer shell (containing the magnets) rotates about the fixed internal stator structure. Thus, the alternator incorporates the rotor hub, has no central rotating shaft, puts the front bearing in the rotor plane, and has no brushes. The output is a sinusoidal three-phase alternating current that varies in voltage and frequency with wind speed.

The Excel wind turbine series is nominally rated at 10 kW. In a battery charging application, however, there is an inherent load matching problem between permanent magnet alternators and battery banks. The Excel-R is optimized for low wind speed performance, which maximizes energy production. But providing the best performance in low winds results in some reduction in peak power under high wind conditions. Therefore, the Excel-R is rated at 7.5 kW.

The turbine is aligned into the wind by a tail assembly. The tail boom and and integrated rotor/alternator assembly attach to the mainframe assembly, which incorporates the yaw-axis slip-rings and the tower interface. The geometry of the mainframe creates the passive Autofurl® high wind speed protection. The mainframe offsets the rotor and yaw axes such that rotor thrust produces a furling moment about the yaw-axis. The weight and inclined pivot of the hinged tail provides a preset resistance to the rotor furling moment. Overspeed control is initiated at 13-15 m/s (30-34 mph) when rotor thrust overcomes the tail resistance and restoration is caused by gravity as the wind speed subsides. The Excel has no shut-down wind speed. The turbine can be manually shut-down using a furling winch installed at the base of the tower.

Corrosion protection for the Excel is provided by hot-dip galvanizing (mainframe, tower adapter, and tail boom), electro-zinc plating, and polyurethane paint systems. FRP components, such as the blades, are protected by ultraviolet-inhibiting resin additives and sub-surface “scrim cloth” UV barrier. The blades have polyurethane leading edge tape for protection against erosion. The Excel’s four sealed bearings are provided with static drain brushes to protect against discharge pitting. A Marine Corrosion Option is available which substitutes stainless steel hardware and provide full encapsulation of the stator windings.

The Excel has only four moving parts, no adjustable elements, and no grease fittings. No scheduled maintenance is required beyond biannual inspections and replacement of the outer three feet of blade leading edge tape every 4-5 years. Static components of the Excel are designed for a 50 year life and dynamic components are designed for a 30 year life. Predicted major overhaul requirements include blade refinishing and alternator bearing replacement at 15 years or 225,000 kWh’s.

The VCS-10 controller, which comes with the Excel-R wind turbine, provides controlled rectification through a three-phase semi-converter. The semi-converter uses phase modulation to control charging voltage and current based upon the battery bank voltage. Safe operation of the Excel turbine is independent of the load conditions, allowing the controller to unload the turbine during regulation. No auxiliary load is required. The VCS-10 is fully solid-state and is passively cooled. It incorporates an LCD display of DC bus voltage and status lights to indicate the charging mode of the unit.

Since its introduction in 1983 the Excel has distinguished itself as the one of the most reliable wind turbines ever produced. The Wisconsin Power & Light SWECS test program, the largest of its kind (involving SWECS from UTRC (Windtech), Windworks, Jacobs, Enertech, and Carter), showed a 99.1% availability (9.0% higher than any other unit) and an O&M cost of $0.0026/kWh for the Excel over a five year test program. This is less than half the O&M costs that have been typically reported for grid-intertied photovoltaic systems (eg, PG&E test program).

Another utility, Pacific Power & Light, through its subsidiary OnSite Energy, installed leading remote wind turbine products (3 kW Northern Power Systems HR-3, 10 kW BWC Excel, and 17.5 kW Jacobs 10-23) on a telecommunications site at Duncan Mountain, Idaho in November, 1984 for the purposes of product and technology evaluation. The site is very remote and accessible only by helicopter during the winter. Since installation, over fourteen years now, the BWC Excel has achieved nearly 100% availability. The next most reliable turbine in the PP&L test program had an 84% availability in the first five years. The other turbines at the site have now been decommissioned and the Excel is providing prime power to the telecommunications facility.

The BWC Excel is the best selling wind turbine in the size range of 2-20 kW in the world. Over 600 units have been installed in more than 30 countries.

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