Alternative Sources

Sections: Facts | Aeroturbine | Selection

Facts and Figures

Wind is air in motion, which is the manifestation of solar energy. Wind blows at a rate of 1.67 x 10¹⁵kWh/year.

On wind farms, where there could be 50 to 100 aeroturbines, can generate between 25 to 65 kW, 75 to 150 kW or 250 to 4000 kW, with load factors ranging from 0.15 to 0.35 and availability of 95% or more.

Power Density, P_w= kv³ watts per unit area, where k is the aeroturbine constant, usually indicated in the nameplate, and v is the wind speed.

k	Wind Speed Units
.	Miles per hour	KM per min	Feet per sec	Meter per sec	knots
ft²	5.3 x 10-3	1.272 x 10-3	1.68 x 10-3	5.934 x 10-2	8.08 x 10-3
m²	5.7 x 10-2	1.3687 x 10-2	1.807 x 10-2	0.6386	8.70 x 10-2

Accordingly, there is a limit to the value of k called the Glauert's limit or the Betz coefficient, which is equal to 16/27 = 0.5926.

Efficiency. The theoretical maximum of aerodynamic efficiency is equal to about 70%, while the conversion efficiency is pegged at 40%.

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Sections: Facts | Aeroturbine | Selection

Aeroturbines

A. Horizontal Axis Aeroturbines. This includes the bamboo-poles-and-cloth contraptions to cartwheels. Tip-speed ratio (specific speed), the ratio of peripheral speed to wind speed, is from 2 to 10. Smaller ratio, less than 4, requires several blades but not greater than 3 blades, which results to high starting torque and low rotational speeds. Higher ratios, greater than 4, require fewer blades but produces low starting torque and high rotational speeds.

High tip-speed ratio rotors are inherently less efficient because of frictional losses.

The catch-phrase in Wind Power is Economics rather than Aerodynamics.

Power Coefficient, c_p, is the fraction of the power of the wind converted to mechanical shaft power. It is a function of tip-speed ratio and has a maximum value dependent upon the pitch angle of the blades.

Blades can be single or a number. Single-blade aeroturbine requires a dead weight for counterbalancing. Most of the modern aeroturbines have two or more blades, which is more cost-effective than single-blade, but suffer from vibrations especially during orientation. Wooden blades for small, but high speed machines, are popular because wood provides excellent strength while maintaining the weight at minimum. For large machines, however, metal blades usually glass-reinforced or carbon-fiber-strengthened are commonly used.

B. Vertical-Axis Aeroturbines provide symmetry, i.e., operational with wind from any angle, can deliver mechanical power at ground level, less weight aloft, simple construction and economical operation. There are four basic types under this classification: Savonius rotor, Darrieus rotor, Giromill vertical blades, and Tormely-type wind energy system.

B.1 Savonius Rotor Vertical-Axis Aeroturbines, debuted in 1929, is constructed of an S-shaped metal or airfoil, supported between two circular end plates. Tip-speed ratio is about 1 to 2, efficiency is about 15%, self-starting rotor, Height to Diameter ratio, H:D, is less than three (3).

B.2 Darrieus Rotor Vertical-Axis Aeroturbines, debuted in 1931, consists of two or more curved aerofoil, resembles the lower section of an egg beater. Tip-speed ratio is about 6 to 8, efficiency is about 35 - 40%, non-self-starting rotor.

B.3 Giromill Vertical Blades Aeroturbines, consists of a set of vertical blades attached to the axis by mills of support arms at the top, bottom, and middle (if necessary). As the rotor rotates these arms are in circular paths, the orientation of the blades is changed to achieve maximum force from the wind. Efficiency is about 60%.

B.4 Tornado Wind Energy System, wind energy is collected by a stationary tower and a vortex is formed in the center of the tower by properly directing the wind by opening vanes in the windward side and closing in the back side. This vortex creates a low-pressure core directly above a horizontal turbine located at the throat of an inlet that is open at the bottom with a bellmouth shape.

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Sections: Facts | Aeroturbine | Selection

Selection

1. Type of Output
a. Direct Current
b. Variable Frequency Alternating Current
c. Constant Frequency Alternating Current

2. Aeroturbine Rotational Speed
a. Constant Speed - use variable pitch blade.
b. Nearly Constant Speed - use pitch-changing blades.
c. Variable Speed - use fixed blade.

3. Utilization of Electrical energy output
a. Direct to Battery Storage.
b. Interconnection with utility grid.

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1. Author. TitleBook Title Publisher, Place, Year, page.

Alternate Sources: Wave & Tidal Power

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