The Council for Scientific and Industrial Research (CSIR) in Pretoria launched a revolutionary vertical axis wind turbine recently. Although still in the early stages of prototype development, the wind turbine has already been subjected to comprehensive scrutiny and has passed feasibility trials at the Faculty of Engineering at the University of Stellenbosch, and wind tunnel testing at the CSIR.
The turbine is based on the Brayfoil, a “morphing” wing designed and developed by Robert Bray, an architect and entrepreneur supported by the Climate Innovation Centre South Africa (CICSA).
The Brayfoil is an auto-setting aerofoil which can reverse lift from one surface to the other, and can adopt any section required by means of a simple cam driven actuation method. The variable shape allows the wing to create optimal lift at low rotational wind speeds, giving a low cut-in speed and the ability to self-start. Its strength lies in its simplicity as the seamless wing works without hinges, joints, panel sections or flaps.
The concept of an aerofoil-based vertical-shaft wind turbine is nothing new, and a number of systems have appeared on the market in recent years. A vast amount of research has been done on the design of the air foils used. Most existing vertical aerofoil wind turbine (VAWT) designs use fixed blades, and unlike the operation of a more common horizontal axis wind turbine (HAWT) rotor, a fixed VAWT rotor blade sees an inconsistent angle of attack which changes as the turbine rotates.
Variable pitch designs, where the blade is not fixed but pivots around an axis have been investigated in the past, but no commercial design has yet been produced.
Thickness and camber are two parameters which have a major influence on aerodynamic performance, and these are used as design variables. Most existing VAWT designs have fixed camber and thickness, which limits the efficiency of the wind turbine and its ability to self-start.
Variable camber aerofoil designs have been developed, but most make use of changing the camber on one side only, and are aimed at aircraft wings. Some also make use of trailing edge flaps attached to a basic aerofoil shape, the angle of which is adjusted to vary the overall camber. No other design allows bilateral camber alteration.
The main difference between this design and previous ones is that the aerofoil is not fixed to the frame but pivots around an axis central to the foil. This allows it to “weathercock” into the apparent wind and also allows the angle of attack to change as the position of the aerofoil changes with rotation of the frame.
In addition, the camber and thickness of the blade are automatically changed as the blade rotates and as the frame rotates, driven by a cam mechanism inside the blade. Variation of wing geometry is made possible by the use of a transparent flexible polycarbonate shell comprising the surface of the aerofoil.
The design allows the turbine to respond rapidly to changes in wind direction and to work well in turbulent conditions, while the slow rotation speed ensures minimal noise. This design produces a significantly higher energy yield compared to a conventional horizontal axis turbine with similar area footprint.
Furthermore, it is quieter in operation and does not result in the bird and bat mortalities associated with large, high speed turbines or existing small wind turbines in urban areas.
The demonstration model consists of a three aerofoil construction (see Fig. 1), but a commercial version with 11 blades is expected to be 12,5 m in diameter, erected at a height of 6 m. It is expected to produce 60 kW with 10 m/s wind speed.
Besides wind turbines, it is anticipated that the concept could be applied anywhere where wing sections are used, opening up greener energy solutions which were previously impossible. Examples include maritime power, automatic sailing, and fuel savings in aviation. Engineering drawings and tooling have already been completed for an automatic wing sail production model, while aviation wing design is in a conceptual stage.
The automatic wing sail for yachts and other wind-driven craft automatically adjusts to the optimum position, leaving the steering of the craft to be the only operation required by the crew. This may not prove popular with yachting enthusiasts, but would be useful in pleasure cruisers which could enjoy the feeling of wind powered travel without all the drama of the normal yacht.
The wing sail cannot be used for cargo vessels, as a possible answer to the age-old challenge of using modern wind power for cargo vessels, as it would still require tacking, making the journey longer and more difficult. A more viable solution could be to place wind turbine generators on the ship’s decks to supplement the power of the engines.
This turbine, unlike traditional horizontal axis wind turbines, does not require a high tower, which lowers its visual impact, and also allows advantage to be taken of laminar wind flow over ridges, hills, and around and over buildings.
WorleyParsons is supplying design support in the form of engineering drawings and engineering analysis for the Brayfoil turbine prototype, and has been working closely with Brayfoil on optimising the design of the moving wing mechanism, as well as the external skin of the wing which requires flexion and morphing abilities.
The Brayfoil turbine prototype was manufactured by Diesel Electric Services, a local company which specialises in the design, manufacture, delivery, installation, commissioning and maintenance of generator sets, distribution boards, UPS and associated products. Bray says the project would have been impossible without the assistance rendered by Diesel Electric Services.
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Source: EE plublishers