DESCRIPTION

Technical Field

This invention relates to an improved vertical-axis aerogenerator. More specifically, the invention regards a high-performance Darrieus type aerogenerator.

Prior Art

It is known that aerogenerators of this type (hereinafter also referred to as "wind turbines" or simply "turbines") comprise two or more rectilinear or curved aerodynamic members (hereinafter referred to as "blades") - operating principally by the effect of aerodynamic lift - positioned vertically and rotating about a common vertical axis.

It is also known that the efficiency of a Darrieus turbine is, in practice, reduced compared to its theoretical efficiency on account of structural constraints and mechanical stresses. In particular, on account of the inconstant wind conditions typical of most urban, suburban and rural areas, the efficiency (defined by the coefficient of power Cp given by the ratio between the power extracted from the wind and the total energy available in the wind) is hardly ever, in practice, greater than 25%.

Another limitation of Darrieus turbines is the difficulty of their being set in motion at wind speeds under 3.5 m/sec, which means that auxiliary motors or resistance reduction devices may be required.

Disclosure of the Invention

The main aim of this invention, therefore, is to provide an improved, Darrieus type turbine whose shape and structure are such as to provide maximum energy efficiency under medium wind conditions typical of most urban, suburban and rural areas.

Another aim of the invention is to provide a vertical-axis aerogenerator which starts automatically at reduced wind speeds.

Yet another aim of the invention is to guarantee a high level of safety in the event of lightning or accidental detachment of turbine parts.

The above aims are achieved by an aerogenerator according to the accompanying claims.

Experts in the field will better appreciate the features and advantages of the invention from the following description with reference to the accompanying drawings illustrating a preferred non-limiting embodiment of it.

Brief Description of the Drawings

In the drawings:

- Figure 1 is a schematic perspective view of a turbine according to the invention;

- Figure 2 is a side elevation view of the dorsal surface of a blade of the turbine of Figure 1;

- Figure 3 is a longitudinal section of the blade of Figure 2 through the cutting plane III-III of Figure 2;

- Figure 4 shows three wing profiles of the blade;

- Figure 5 illustrates the turbine in a plan view from above.

Preferred Embodiment of the Invention

With reference to the accompanying drawings, an improved, Darrieus type vertical- axis aerogenerator according to the invention comprises three equispaced blades 1 mechanically connected by arms 3a,3b to a central rotary member 5, advantageously consisting of an alternator for converting rotational kinetic energy to electrical energy to be stored or sent to a user.

In each blade 1 (see Figure 2) the leading edge 10 and the trailing edge 12 are arcuate, essentially in the shape of circular arcs, where the curvature radius Rl of the leading edge is greater than the curvature radius R2 of the trailing edge. The blade (see Figure 3) also has a curved longitudinal section which is symmetrical about a horizontal midplane M.

According to the invention, the longitudinal section is substantially in the shape of a circular arc whose radius Rc is essentially equal to 4 times the average radius Rr of rotation of the blade about the axis of the turbine.

Further (see Figures 2 and 4), the blade has a symmetrical wing profile, preferably of the NACA 0018 type, where:

- the length of the chord decreases symmetrically from the midplane M towards the upper edge 14 and the lower edge 16;

- the points on each chord which are located at a distance from the leading edge equal aerodynamic performance and reduction of torsional stresses induced by aerodynamic forces;

- the length of the maximum chord Cmax (of the wing profile at the midplane M) is 5 - 15%, and preferably 10%, greater than the length of the mid chord Cmed of the profiles; similarly, the length of the minimum chord Cmin (of the wing profile at the upper and lower edges) is 5 - 15%, and preferably 10%, smaller than the length of the mid chord Cmed; this, together with what is stated in the previous point, gives the blade the tapered profile illustrated in Figure 2;

- the solidity of the turbine, σ = 3Cmed/Rr, is between 0.6 and 0.9 and preferably equal to about 0.85 for a turbine of 1 kW of power and to about 0.63 for a 3 kW turbine.

Advantageously, each blade 1 has an angle of incidence of between 2° and 6° and preferably approximately 3° (see Figure 5) for a 1 kW turbine and approximately 5° for a 3 kW turbine.

As illustrated in Figure 1, each blade is mounted on a pair of arms 3a,3b whose wing profiles (for example of the NACA 0028 type) are symmetrical about the midplane. Advantageously, the connection point 18a, 18b at which each arm is joined to the respective blade is located at a distance from the midplane substantially equal to ¼ of the height H of the blade, so as to minimize the bending moment. Further, each connection point intercepts the chord of the corresponding wing profile substantially at a distance from the leading edge equal to 30% of the chord, in order to avoid torsional stresses induced by aerodynamic forces.

Advantageously, the height H of the blade is substantially equal to 2 times the average radius of rotation Rr for a 1 kW turbine and to 2.2 times for a 3 kW turbine. In order to improve blade efficiency, end winglets 20a,20b are advantageously applied to the upper and lower edges 14 and 16 of each blade 1.

Tests were conducted under inconstant wind conditions using a turbine having blades made of a composite material consisting of vinyl ester resin and glass carbon fabric, with a height H = 200 cm, blade curvature radius Rc = 400 cm and the preferred features stated above. The measured efficiency Cp was approximately 35%, much higher than the efficiency of prior art turbines under similar conditions. Moreover, the turbine proved capable of starting automatically at a wind speed as low as 2.5 m/sec.

In a further preferred embodiment of the invention, in order to protect the turbine from lightning and also to minimize the risks of detachment of parts of it, the blades 1 and the arms 3a, 3b are provided with metal cables connected to each other at the connection points where the arms and blades are joined. In each blade, the ends of the cable are connected to the winglets 20a,20b, which are also made of metal. In the arms 3a,3b the ends of the cables on the alternator 5 side are connected at the point of connection between arm and alternator. The cables are made from materials characterized by high tensile strength, such as BAYCO, by Bayer, or steel braiding of suitable diameter. Running parallel with the cables there is a copper wire or cable which acts as a lightning conductor and which must be electrically connected to earth.

The embodiment described above is provided purely by way of an example and it will be understood that other equivalent embodiments are imaginable without departing from the scope of protection of the invention.