ARCHITECTURE - The aim of the LWG design was to find a mechanical arrangement of blades that would produce a better power return when compared to the associated cost of the support structure, leading to a more efficient and cheaper machine overall.
The power produced by an airfoil is proportional to air density, blade area and air velocity squared. It follows that the part of the blade that travels fastest has the capability of producing the most power (other parameters being equal).
In a conventional wind turbine generator (WTG), a blade experiences maximum air velocity at one end and zero at the hub, thereby limiting it’s power return figure. In a LWG machine, the blades are placed across two endless cables that run around pulleys at the ends of the machine. As both cables run at the same speed, all blades and every part of each blade experiences the same velocity, thereby maximizing the power return figure. With this blade arrangement, the power return per blade is increased by up to 20%.
OPERATION - In operation, the blades move from one end to the other, each time interacting with the wind. Blades on the windward side of the machine (first stage) interact with the air when it’s velocity is near that of the undisturbed flow. After negotiating the first stage blades, the air velocity has significantly decreased and has changed direction somewhat. The second stage blades need to interact with this slower air stream, while still travelling as fast as the first stage blades. To do this, the second stage blades must be set to a different angle because the apparent wind vector is different to that of the first stage.
CHANGING BLADE ANGLES - In order to operate as just described, the angles of the blades need to be locked in relation to the cables during their linear travel between pulleys then need to be unlocked and set to a new angle while travelling around the pulleys in preparation for their next linear section.
On the LWG machine, each blade has a brake mechanism which locks the blade angle to the cable. As the blade moves around the end pulley, the brake is automatically unlocked by a rotating mechanism. Once unlocked, the angle of the blade is changed gradually then the brake is reapplied prior to the blade leaving the pulley. All of this happens at full operational speed by mechanical means alone.
The rotating mechanism responsible for unlocking and changing the angle of the blades has an axis which can be offset from the pulley/drive shaft axis. The degree of offset is infinitely variable via electronic control permitting the required angle to be set in real time.
CABLE SYNCHRONIZATION - The LWG does not use traditional methods to ensure cable synchronization of both tracks as this would require tooth belts or chains which are not feasible over the required distance. Instead, the LWG uses low cost conventional multi-stranded cables. Synchronization is achieved at the lower pulley end where both drive discs are keyed to the output shaft.
When a blade negotiates the pulley, providing the blade attachment engages with the slot in the drive disc, synchronization occurs and any misalignment is transferred through the cable resulting in the far end pulley walking on its shaft. This system is tolerant of cable stretching and cable length mismatch between the two tracks.
ALIGNMENT TO THE WIND - For smaller implementations of the LWG a castor angle is incorporated into the mounting of the structure and together with side cowlings acting as wind vanes, the LWG is self aligning into the wind. For larger machines without a castor angle or cowlings, the wind direction is sensed and electric motors independently drive the top axel and bottom axels to align the machine into the wind.
WIND ENERGY EXTRACTION - Air passing over the first stage blades produces a lift force, a component of which is in the direction of cable travel. The second stage blades also produce a lift force component in the direction of cable travel thereby adding to the first stage’s contribution. The blades are always angled to the apparent wind to utilise the blade’s optimum lift/drag ratio. See Wind Energy Extraction diagram.
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