WO2017026894A1

WO2017026894A1 - Wind turbine

Info

Publication number: WO2017026894A1
Application number: PCT/NL2016/050573
Authority: WO
Inventors: Freerk JAARSMA; Harm Antonius VREDENBURG
Original assignee: Jaarsma Freerk; Vredenburg Harm Antonius
Priority date: 2015-08-11
Filing date: 2016-08-11
Publication date: 2017-02-16
Also published as: NL2015290B1; EP3334927A1

Abstract

The invention relates to a wind turbine comprising a rotor and a stator placed upstream thereof. The rotor and the stator are here accommodated in a tunnel having a diverging form downstream of the rotor, particularly a progressively diverging form. The stator can comprise a number of fixed stator vanes with a section for generating a vortex flow in the tunnel. The rotor can comprise a number of rotatable rotor blades having a section and a twist adapted to this vortex flow. The wind turbine can be provided with a streamlined central body extending from the inlet to the rotor at the position of the longitudinal axis and carrying the stator and the rotor, and a passage extending in axial direction can be formed in the central body.

Description

Wind turbine

The invention relates to a wind turbine. Wind turbines are generally known and are attracting increasing interest as means for generating energy in sustainable manner.

As known, a large number of the energy sources currently in use are in principle not inexhaustible. At current levels of consumption the proven reserves of mineral fuels such as oil and natural gas are thus still sufficient for a maximum of several tens or hundreds of years, while with the increasing level of development in the world population this consumption is more likely to increase than decrease. The use of these energy sources moreover causes a number of problems, of which air pollution and the warming of the earth's atmosphere are the best-known.

Sustained efforts are therefore being made to find alternatives, these having been found up to this point in the form of nuclear power and different natural or sustainable energy sources such as wind energy, solar energy and the like. These alternative energy sources still have a large number of drawbacks however. Generation of energy by means of nuclear fission thus results in the release of dangerous radiation and radioactive waste for which no processing method has yet been found, while the technique of nuclear fusion, although promising in theory, has not yet been found usable in practice.

Of the renewable energy sources wind energy appears in the short term to be the most promising. Wind energy can in principle be produced almost anywhere in the world. The most significant drawback of wind energy is that the speed of the wind just above the ground is limited so that wind turbines at high locations are necessary for an efficient energy generation. Furthermore, the power generated increases with the surface area covered by the blades of a wind turbine, so by the square of the length of the blades, this being an additional reason for opting for large wind turbines. Various factors result in the wind turbines being very conspicuously present in the landscape, thereby limiting their social acceptance. The rotating blades of a wind turbine moreover cause a considerable noise nuisance, and an annoying cast shadow when the sun shines.

The invention therefore has for its object to provide an improved wind turbine, wherein these drawbacks do not occur, or at least do so to lesser extent. According to the invention, this is achieved by a wind turbine comprising a rotor and a stator placed upstream thereof, wherein the rotor and the stator are accommodated in a tunnel having a diverging form downstream of the rotor. Such a ducted wind turbine produces a higher specific power than conventional, unducted or "bare" wind turbines, and can therefore take a more compact form while the power remains the same, whereby visual pollution and related problems decrease and acceptance is expected to increase. Because of the diverging form, the tunnel has a surface area which increases in flow direction. The How in the tunnel is thereby decelerated, and the pressure thereof increases. In other words: the kinetic energy is converted into potential energy. A ducted wind turbine with a tunnel or diffuser diverging downstream of the rotor is also referred to in the English literature as DAWT, which stands for Diffuser Augmented Wind Turbine. Different aerodynamic laws apply to a DAWT than to a bare wind turbine, as demonstrated in a presentation "One Dimensional Flow Theory for Diffuser Augmented Wind Turbines" during the EAWE TORQUE conference of June 2010 in Crete by BJ. Konijn, H.W.M. Hoeijmakers, F. Jaarsma, B.I. Sumarwoto and K.M. de Cock. It was demonstrated here that when a diffuser functions very well, wherein the flow through the turbine practically comes to a standstill when it has reached the ambient pressure again at a downstream position, the power coefficient C_P of such a wind turbine is equal to:

wherein C_T is the pressure drop coefficient over the turbine, related to the local dynamic pressure. This means that high power coefficient values can be achieved in the case of a DAWT when the pressure drop coefficient is very small (which goes against human intuition), in any case several times higher than is possible with bare wind turbines. This is because the maximum value of the power coefficient is here the known Betz's coefficient, which is equal to about 0.6, or more precisely: 16/27. Betz's coefficient is reached at a pressure drop coefficient of 2, which means that there is very little point to providing a bare wind turbine with a duct.

In an embodiment of the wind turbine according to the invention the tunnel has a progressively diverging form downstream of the rotor. A diffuser is thus obtained with a form similar to a trumpet bell with increasing divergence. It can be achieved in this manner that, due to the radial impulse in situ, including that of the outer flow, the effective outlet area of the flow is much greater than the physical surface area, which is favourable for the performance of the turbine.

A well-functioning diffuser requires a reasonably uniform approach flow, which is why a combination of a stator with a rotor downstream thereof has been chosen for the wind turbine according to the invention.

The stator preferably comprises a number of fixed stator vanes extending substantially radially and each having a section for generating a vortex flow in the tunnel.

When the section of each stator vane is substantially constant in radial direction here, a vortex flow in the form of a helical vortex, also referred to as free vortex, is created downstream of the stator. The strength of the vortex flow is inversely proportional to the distance from the longitudinal axis, apart from the flow in the boundary layers along the inner side of the tunnel and along the periphery of the hub. This vortex flow provides for an optimal approach flow to the rotor. Upstream of the rotor the tunnel preferably has a converging inlet in which the stator is accommodated. The stator is thus situated some distance upstream of the rotor, whereby the vortex flow can develop to sufficient extent before reaching the rotor, and wakes coming from the stator vanes have already weakened to some extent because of helix formation.

It is preferred for this purpose for a distance in flow direction between the stator and the rotor to amount to at least three times a chord length of a stator vane.

The rotor of the wind turbine according to the invention can comprise a number of rotor blades rotatable about the longitudinal axis of the wind turbine and each having a section and a twist adapted to the vortex flow generated by the stator vanes in order to minimize a tangential component of the vortex flow. The rotor is thus set into rotation optimally.

The section and the twist of each rotor blade thus preferably run in radial direction such that the vortex flow is converted by the rotor into a flow substantially parallel to the longitudinal axis. The energy in the vortex flow is in this way converted optimally into rotation of the rotor.

Because the tangential aerodynamic load on the rotor blades is practically uniform over their whole span, i.e. from the hub to the tip, the chord length of each rotor blade can be substantially constant in radial direction (span direction), which results in a higher efficiency of the rotor than can be achieved with a bare rotor.

For an optimal efficiency of the wind turbine it is preferred for the stator and rotor to be formed such that the overall aerodynamic load of the wind turbine, expressed in the above stated pressure drop coefficient C_T, to be relatively small. The value of this pressure drop coefficient depends on the vortex strength in the tunnel generated by the stator and the tip speed of the rotor in relation to the wind speed - also referred to as tip-speed ratio in wind turbines.

In a preferred embodiment of the wind turbine according to the invention the stator and rotor are therefore formed such that the resulting pressure drop coefficient C_T of the wind turbine is smaller than 0.25, preferably smaller than 0.20, and more preferably smaller than 0.15, wherein:

C_T = Ap_R/q_R , (2) wherein Δρ_κ is the pressure drop over the wind turbine (stator and rotor) and q_R the local dynamic pressure: q_R = ½-p-U_R ² (3) wherein p is the air density and U_R the flow speed in axial direction of the air at the position of the wind turbine. This optimal value of C_T is about one tenth of the optimal value for an unducted rotor.

Tn order to limit the noise production of the wind turbine it is preferred for the rotor in combination with the stator to be configured such that it rotates at relatively low speed.

The turbine can preferably be configured for this purpose such that the ratio of the rotation speed of the tips of the rotor blades and the wind speed upstream of the wind turbine - the above stated tip-speed ratio - amounts to a maximum of 0.6 times, preferably a maximum of 0.5 times and more preferably a maximum of 0.4 times the value of this ratio for an unducted rotor. The noise impact thus remains considerably lower than in the case of a wind turbine with unducted rotor.

In this respect it is preferred for the wind turbine to be provided with means for controlling the rotation speed of the rotor such that the tip-speed ratio is kept substantially constant within an operative range of wind speeds.

A simple control is achieved when the speed control means are configured to control a power taken off from the rotor.

The speed control means can additionally or instead comprise flaps on the rear edges of the stator vanes. The strength of the vortex rotation can be influenced by adjusting the flaps.

A relatively low rotation speed of the rotor has the result that the degree of filling of the rotor, defined as the overall surface area of the rotor blades divided by the surface area covered by the rotor blades, may be relatively high. A rotor is thus obtained with a large effectively operating surface area. In a preferred embodiment of the wind turbine according to the invention this degree of filling of the rotor amounts to in the order of 0.5.

In order to further limit the noise production of the wind turbine the ratio of the number of stator vanes n_s and the number of rotor blades N_R preferably complies with one of the relations: n_s = 1 .5 x N_R ± 1 (4a) or

N_R = 1.5 x n_s ± l , (4b) wherein n_s and N_R do not have a common denominator. The wind turbine can advantageously be provided with a streamlined central body extending from the inlet to the rotor at the position of the longitudinal axis and carrying the stator and the rotor. Such a streamlined body, also referred to as hub, has no or hardly any disruptive effect on the flow through the wind turbine.

A further point of concern is the downstream-bound boundary layer of the central body. At an outer end of a solid shaft on which a rotor is mounted the boundary layer will concentrate in a dead flow zone downstream of this end. Back-flow results hereby on the central axis of the diffuser, whereby the operation of the diffuser is seriously disrupted and the efficiency declines sharply.

In a preferred embodiment of the wind turbine according to the invention at least one passage extending in axial direction is therefore formed in the central body. The formation of a wake downstream of the body, which would disrupt the flow in the diffuser, can thus be lessened or even prevented.

In order to ensure good passage of air with high energy to the diffuser, the at least one passage can comprise a channel having a substantially constant diameter and extending from a position close to the inlet to a position slightly downstream of the rotor. By situating the opening of the channel on the side of the central body lying furthest upstream, a part of the centrally entering flow can flow centrally into the diffuser practically undisturbed downstream of the turbine or the rotor in order to activate the boundary layer coming from the central body, whereby back-flow is prevented. The downstream end of the channel can here form a sharp edge with the periphery of the central body, whereby the so-called Kutta condition can be met for the downstream-bound flow.

If the wind turbine is provided with speed control means, these speed control means can in that case comprise among other things a controllable valve accommodated in the passage. The flow through the passage and thereby the development of the boundary layer on the central body can be influenced by controlling the valve. This boundary layer in turn determines the operation of the diffuser and thereby the flow through the rotor and the rotation speed thereof.

A compact embodiment of the wind turbine is obtained when a power converter driven by the rotor is accommodated in the central body. This power converter can be situated in a thickened portion of the central body upstream of the rotor, for instance at the position where the stator is mounted. As power converter it is possible to envisage an electric generator, but another type of converter, for instance a hydraulic pump, can also be envisaged. If the power converter is a generator, it is preferably embodied as a multipolar generator so that it is possible to dispense with the use of a reducing gearbox, which would require a lot of space and is vulnerable. The power converter can be operated at a substantially constant speed by making use of speed control means which keep the tip-speed ratio almost constant in an operative range of wind speeds.

Tf the hub is provided with a passage in order to give the boundary layer a renewed impulse, the power converter is preferably connected to the rotor by means of a hollow shaft arranged around the passage. The various functions can thus be integrated in the central body in efficient manner.

As stated, downstream of the rotor the tunnel has a diverging form which defines a diffuser. This diffuser is important for the proper functioning of the turbine. In the case of a diffuser with fixed walls a boundary layer however develops along the wall, this layer having a thickness which increases rapidly and tends to separate from the wall, whereby hardly any diffuser action takes place anymore.

In order to prevent flow separation in the diffuser, the tunnel is in a preferred embodiment of the invention constructed downstream of the rotor from a number of tunnel segments partially overlapping in flow direction and defining in each case an annular gap therebetween, these segments together forming the diffuser. A part of the undisturbed outer flow, i.e. air which has not flowed through the turbine, can thus be supplied with a higher energy to the boundary layer along the inner wall of the tunnel through the annular gaps. The boundary layer is hereby given a new axial impulse, whereby separation can be postponed or even prevented for the first subsequent part.

In this embodiment of the invention the diffuser is therefore formed by means of one, preferably two or, if necessary, more preferably three or more closed wings placed one behind the other in stepwise manner in flow direction, with gaps therebetween through which outer air can flow in.

The tunnel thus in fact forms a system in longitudinal section which is similar to a flap system of an aircraft wing during the landing stage.

It is further noted that all surfaces of parts of the wind turbine which are covered by the flow, i.e. the tunnel, hub, stator and rotor, do not have steps, bends or greatly varying radii of curvature in flow direction. On the downstream side these parts are also slender and they have a sharp end, so that the Kutta condition can be met for the downstream-bound flow.

If the tunnel is mounted for pivoting about a vertical axis, the wind turbine can be brought into an optimal position relative to the wind in simple manner.

The wind turbine is preferably provided for this purpose with a streamlined element which is located close to the upstream end of the tunnel, extends substantially transversely thereof, and is mounted pivotally on a carrier. The forward position of the vertical mounting on the tunnel and the length of the tunnel with the diverging part downstream will ensure that the whole of the turbine and tunnel is self -aligning.

The invention is now elucidated on the basis of an example, wherein reference is made to the accompanying drawing, in which:

Fig. 1 is a longitudinal section through a wind turbine according to the invention,

Fig. 2 is a view corresponding with fig. 1 in which the flow through the wind turbine is shown,

Fig. 3 shows a cross-section through the stator vanes and rotor blades at a representative radius of the stator and rotor and the flow vectors associated therewith,

Fig. 4 shows a front view of a stator vane which shows schematically the progression of the vortex flow,

Fig. 5A and 5B show the deflection of the airflow between two stator vanes, respectively in the vicinity of the hub and close to the tip,

Fig. 6 shows a diagram of the taken-off power as a function of the aerodynamic load for a number of possible configurations of the wind turbine according to the invention, wherein an ideal unducted wind turbine is shown as reference, and

Fig. 7A and 7B show the results of calculations of the flow by a model of the wind turbine when the passage in the central body is respectively opened and closed.

A wind turbine 1 comprises a rotatable rotor 2 and a stator 3 placed some distance upstream thereof (fig. 1). Stator 3 comprises a number of fixed stator vanes 4 extending substantially radially from a streamlined, rotation-symmetrical central body 5, which is substantially droplet-shaped here. Stator vanes 4 are distributed evenly over the periphery of central body 5. Rotor 2 is correspondingly provided with a number of blades 20 which are likewise evenly peripherally distributed and which extend substantially radially from central body 5. The central axis of this central body 5 coincides with the longitudinal axis L of wind turbine 1. This central body 5, also referred to as hub, is provided with an axially running passage or flow channel 22, the purpose of which will be elucidated below. Rotor 2 is mounted rotatably on the downstream end of central body 5 and is connected at the position of stator 3 via a hollow shaft 6 to a power converter accommodated in central body 5, here in the form of an electric generator 7. A transmission can optionally also be placed between rotor 2 and generator 7, although use is preferably made of a multipolar generator 7 which is directly connected to rotor 2.

Tips 8 of stator vanes 4 are connected to a first annular segment 10 of a tunnel 9 in which rotor 2 and stator 3 are accommodated. In the shown embodiment tunnel 9 consists of two additional further segments or closed wings 11, 12 which are arranged downstream of rotor 2 and form a diverging outlet or diffuser 17. The different tunnel segments 10, 11, 12 overlap each other in flow direction and define annular gaps 13, 14 therebetween, the purpose of which will be elucidated below. Segments 10, 11, 12 are mutually connected by a number of peripherally distributed tunnel supports, which are not shown here. The tunnel segment 10 carried by stator 3 and lying furthest upstream defines an inlet 15 which has a form converging toward rotor 2. Rotor 2 is thus situated in the narrowest part of tunnel 9, the throat 16.

Tunnel 9 is mounted for pivoting about a vertical axis V so that it can adapt to the wind direction. The vertical axis V is streamlined close to the tunnel by means of a cover 18 in order to prevent wakes in the outer flow upstream of gaps 13 and 14. The symmetrical section P of cover 18 is shown schematically in cross-section. The whole ensures that tunnel 9 is always directed into the wind, also because the whole is finally mounted pivotally on a carrier 19.

Cabling for leading away the electric power generated by generator 7 and for controlling and monitoring the operation of wind turbine 1 can be guided through carrier 19, control element 18 and one or more stator vanes 4.

Due to the form of tunnel 9 air is drawn in from an area wider than the periphery of tunnel 9 and is accelerated and expanded while flowing through first tunnel segment 10 to rotor 2 (fig. 2, fig. 7A). After passing rotor 2, wherein the pressure drops below the ambient pressure, the airflow in the diverging tunnel segments 11, 12 is decelerated and the pressure rises once again, almost to the ambient pressure.

In order to prevent the boundary layer along the inner wall of tunnel 9 separating as a result of this pressure increase, air from the outer flow, which has a higher energy, is supplied through annular gaps 13, 14. A part of the air approaching freely upstream of the wind turbine is further guided directly to the downstream side of central body 5 through the channel 22 with sharp end edge. New energy is thereby also supplied to the boundary layer which has formed on central body 5, so that flow separation and formation of a wake are prevented.

Formation of a wake W downstream of central body 5 can otherwise also be induced intentionally. This can be done by completely or partially closing channel 22 using a shut- off valve which forms part of the above discussed speed control means. The thus formed wake W considerably reduces the effective throughflow area of diffuser 17, whereby the flow through tunnel 9 is partially blocked (fig. 7B). The load on rotor 2 and generator 7 is hereby limited. This is important in order to prevent overload of wind turbine 1 at high wind speeds, the so-called overspeed protection. The power of wind turbine 1 can optionally also be controlled in this way during normal use.

The fact that the flow tube S running through tunnel 9 has a greater surface area upstream of tunnel 9 than rotor 2 is one of the reasons that rotor 2 is more efficient in tunnel 9 and produces a higher specific power than an unducted rotor. In addition, rotor 2 loses no or hardly any flow to tips 21 of its blades 20 owing to the presence of tunnel 9, this in contrast to an unducted rotor, where these losses can be considerable.

The airflow flowing into first tunnel segment 10 is set into rotation by stator vanes 4, whereby a natural vortex flow results in this tunnel segment 10. Each stator vane 4 has for this purpose an axial section, the chord c_s of which encloses an angle a with the longitudinal axis L of wind turbine 1 (fig. 3). Stator vanes 4 generate a helical vortex, i.e. a vortex flow with a strength which is almost inversely proportional to the distance from the longitudinal axis L (fig. 4) according to the relation: r.V = constant , (5) wherein r is the radial distance from longitudinal axis L and V the tangential component of the speed at that position. The angle of incidence a of the local chords c_s of each stator vane 4 can here be substantially constant in radial direction or span direction. Due to the radially increasing distance between adjacent stator vanes 4 the deflection of the flow after all decreases as the distance to longitudinal axis L increases, as can be seen in figures 5 A and 5B. Stator vanes 4 thus have no or hardly any twist, as shown schematically by the section p₅ drawn in stator vane 4 in figure 1.

Rotor blades 20 also each have a section, the chord C_R of which encloses an angle β with longitudinal axis L. This angle of incidence β is indirectly also adjusted to the angle of incidence a of the section of stator vane 4 in order to set rotor 2 into rotation in response to the vortex flow and to once again convert the vortex flow into a flow which is essentially parallel to longitudinal axis L. Each rotor blade 20 must be twisted for this purpose, so that the angle of incidence β and/or the curvature of the section in radial or span direction varies between the hub and blade tip 21. This is shown schematically by sections p_R1, p_R2 and p_R3 in a rotor blade 20 in figure 1. The length of the blade chord C_R can otherwise be constant in radial or span direction, as can the length of the vane chord c_s

The air which enters tunnel 9 at the axially directed wind speed U₀ (compare the numbers at the top of Fig. 2) has an axial speed Uj when it reaches stator vanes 4. This air is deflected by stator vanes 4 and accelerated to (Ui² + V₂ ²), wherein V₂ is the tangential component of the speed. As can be seen in the bottom half of figure 3, the axial component of the speed does not change when passing the stator: U₂ = Ui.

After the air has flowed through first tunnel segment 10 and the vortex flow has fully developed therein, it reaches rotor 2, which is set into rotation by the passing airflow. The speed of the air is V(U₃ ² + V₃ ²) at the position of rotor 2, as can be seen in the upper part of figure 3. Relative to rotor blades 20, which rotate at a speed Ωτ, wherein Ω represents the rotational speed (rad/sec) of rotor 2 and r the local radius, the relative speed of the airflow amounts to (U₃ + (Ω·_Γ - ν₃)²).

Rotor blades 20 deflected the airflow back again and decelerate it to a speed U₄. This speed is equal to the axial component U₃ of the speed while flowing into rotor 2: U₄ = U₃. The rotational speed Ω of rotor 2 is in each case controlled here such that the air leaves rotor 2 as parallel flow. In the shown example this control takes place by controlling the power taken off from generator 7.

Aerodynamic calculations have shown that the power which can be extracted from the airflow by rotor 2 in the wind turbine 1 according to the invention is considerably higher than the power which can be generated by an unducted or bare turbine with corresponding dimensions.

It is usual in wind turbines to express the power P extracted from the airflow by rotor 2 in relation to the power P₀ in the flow which occurring in the flow tube flowing undisturbed through the turbine or rotor surface. The dimensionless power coefficient C_P is then defined as:

C_P = P/P₀ = P/(½-p-A_R-U_w ³) , (6) wherein p is the air density, A_R is the surface area covered by rotor 2 and U_w is the undisturbed flow speed of the wind.

The thus defined power coefficient C_P can be derived as indicated in the above stated publication:

Cp - (C_T- C_D)-A₅R³/(C_T-A_5R ² + 1)^3/2 , (7) wherein C_T is the aerodynamic load coefficient of the turbine, C_D is the resistance coefficient on the internal flow through wind turbine 1 , and A_5R is the ratio of the surface area A₅ of the flow tube S at the point downstream of rotor 2 where the airflow once again reaches the ambient pressure and the surface area A_R which is covered by rotor 2: A_5R = A₅/A_R.

As stated above, C_T is defined as the pressure drop Ap_R over rotor 2 divided by the local dynamic pressure q_R:

C_T = Ap_R/q_R (2) which local dynamic pressure can be expressed as: q_R = ½-p-U_R ² , (3) wherein p is once again the air density and U_R is the flow speed of the air at the position of rotor 2 in axial direction, while C_D is defined as:

C_D = D/(½-p-U_R ²-A_R) . (8)

D is here the overall internal resistance of the wind turbine, thus for a wind turbine 1 according to the invention the sum of the resistance of stator 3, rotor 2, central body 5 and the inner side of tunnel 9.

For an ideal unducted wind turbine the power coefficient can be reduced to:

Cp = Cr-(4/(4 + C_T))³ . (9) This coefficient reaches its maximum value when the aerodynamic load coefficient C_T = 2. This maximum value, which is reached when the airflow is decelerate to a third of its undisturbed speed, then amounts to:

Cpmax— Pmax/Pf) 16/27.

In other words, a maximum of about 60 percent of the overall kinetic energy can be obtained from the wind with an unducted wind turbine. This is known as Betz's law. In a diagram of the power coefficient C_P as a function of the aerodynamic load coefficient C_T (fig- 6) this value is indicated by a horizontal line which is designated as B. In this diagram the power extraction which is possible with an ideal unducted turbine is shown as reference by the line FT.

Also shown in the same figure are the values of the power coefficient C_P which can be reached with the ducted wind turbine 1 according to the invention. This power coefficient Cp is calculated here as a function of the aerodynamic load coefficient C_T on the basis of the above stated relation (7) for three different values of the resistance coefficient C_D and for four different values of the area ratio A_SR. The considered values of the resistance coefficient C_D are, successively, C_D = 0.00 (designated with thick lines in the diagram), C_D = 0.02 (designated with normal lines) and C_D = 0.04 (thin lines). Values of A_¾ = 2 (dotted lines in fig. 6), A_SR = 4 (dashed lines), A_5R = 6 (dash-dot lines) and A_5R = 8 (full lines) have been considered for the area ratio A_5R (= A₅/A_R). Further indicated in the diagram is the threshold value C_Pi_im of the power coefficient, which follows from the relation: Cpiim— 1/ C (10)

The diagram of fig. 6 shows clearly that considerably higher powers can be achieved with the wind turbine according to the invention than with an unducted or bare turbine with the same rotor dimensions. At a resistance coefficient of C_D = 0.02 - which can certainly be achieved in practice - it is already possible to extract three times as much power from the airflow, for instance with a diffuser with an outflow area A5 which amounts to six times the surface area AR covered by the rotor, as is possible with a bare turbine. This figure further shows that the power coefficient C_P is at a maximum when the aerodynamic load per unit of area of rotor 2 - also referred to as disk loading - is relatively low. The maximums occur particularly at relatively low values of the load coefficient, in the order of C_T ~ 0.1 - 0.2. A range of promising combinations of C_D and A_5R is shown hatched in the diagram.

In order to limit the noise production of wind turbine 1, and thereby improve its social acceptance, the rotor can be designed for relatively low tip speeds in comparison to the wind speed. The tip speed of rotor 2 can for instance remain limited to about 50 percent of the value which is usual for unducted wind turbines. In order to then still be able to extract sufficient power from the passing airflow, the degree of filling of rotor 2 is relatively high. This degree of filling is defined as the ratio of the overall surface area of the rotor blades divided by the surface area covered by the rotor blades, so: degree of filling = N_R x A_b / A_R , (11) wherein N_R represents the number of blades 20 of rotor 2, A_b the surface area of a rotor blade 20 and A_R the surface area covered by rotor blades 20, so nR², when R is the radius or span of a rotor blade 20, calculated from longitudinal axis L. The degree of filling can amount to in the order of

0.5 in the wind turbine 1 according to the invention.

In the wind turbine 1 according to the invention the ratio of the number of stator vanes n_s and the number of rotor blades N_R is further also chosen such that noise production is minimized. This is done by having this ratio comply with one of the relations: n_s = 1.5 x N_R ± l (4a) or N_R = 1.5 x n₅ ± l , (4b) wherein n₃ and N_R do not have a common denominator.

Rotor 2 can for instance be provided with eight blades 20, with which is then associated a stator 3 with eleven or thirteen vanes 4.

The wind turbine 1 according to the invention is considerably more efficient than conventional, unducted wind turbines and can produce up to four times as much power per unit of rotor area. The greatest diameter of the rear tunnel segment 12 is still considerably smaller here than the diameter of an equivalent unducted wind turbine. The wind turbine 1 according to the invention is aimed particularly at small-scale energy generation, for instance in private households, for farmers and in small and medium-sized companies. In such applications the diameter of rotor 2 can vary from 1 to 4 metres, and the generated power will lie between 1 and 14 kW at wind force 5 Bf.

Although the invention has been elucidated above on the basis of an embodiment, this can be varied in many ways. The turbine can thus also be applied in order to extract power from flowing media other than only air or wind. The turbine could for instance be placed in a river or in tidal waters. The form and dimensions of the stator and the rotor can further be chosen differently than shown here, with more or fewer vanes and blades. The form and dimensions of the tunnel can also be modified, with more or fewer tunnel segments or closed wings, which can moreover have a different progression than shown here. The wind turbine could additionally be combined with an external generator. The scope of the invention is therefore defined solely by the following claims.

Claims

1. Wind turbine, comprising a rotor and a stator placed upstream thereof, wherein the rotor and the stator are accommodated in a tunnel having a diverging form downstream of the rotor.

2. Wind turbine as claimed in claim 1, characterized in that the tunnel has a progressively diverging form downstream of the rotor.

3. Wind turbine as claimed in claim 1 or 2, characterized in that the stator comprises a number of fixed stator vanes extending substantially radially and each having a section for generating a vortex flow in the tunnel.

4. Wind turbine as claimed in claim 3, characterized in that the section of each stator vane is substantially constant in radial direction.

5. Wind turbine as claimed in claim 3 or 4, characterized in that upstream of the rotor the tunnel has a converging inlet in which the stator is accommodated.

6. Wind turbine as claimed in claim 5, characterized in that a distance in flow direction between the stator and the rotor amounts to at least three times a chord length of a stator vane.

7. Wind turbine as claimed in any of the claims 3 to 6, characterized in that the rotor comprises a number of rotor blades rotatable about the longitudinal axis of the wind turbine and each having a section and a twist adapted to the vortex flow generated by the stator vanes in order to minimize a tangential component of the vortex flow.

8. Wind turbine as claimed in claim 7, characterized in that the section and the twist of each rotor blade run in radial direction such that the vortex flow is converted by the rotor into a flow substantially parallel to the longitudinal axis.

9. Wind turbine as claimed in claim 7 or 8, characterized in that the chord length of each rotor blade is substantially constant in radial direction.

10. Wind turbine as claimed in any of the foregoing claims, characterized in that the stator and rotor are formed such that the overall aerodynamic load of the wind turbine is relatively small.

11. Wind turbine as claimed in claim 10, characterized in that the stator and rotor are formed such that the pressure drop coefficient C_T of the wind turbine is smaller than 0.25, preferably smaller than 0.20, and more preferably smaller than 0.15, wherein:

C_T = Ap_R/q_R , (2) wherein Ap_R is the pressure drop over the wind turbine (stator and rotor) and q_R the local dynamic pressure: q_R = ½-p-U_R ² , (3) wherein p is the air density and U_R the flow speed in axial direction of the air at the position of the rotor.

12. Wind turbine as claimed in any of the foregoing claims, characterized in that the rotor in combination with the stator is configured such that it rotates at relatively low speed.

13. Wind turbine as claimed in claim 12, characterized in that the ratio of the rotation speed of the tips of the rotor blades and the wind speed upstream of the wind turbine amounts to a maximum of 0.6 times, preferably a maximum of 0.5 times and more preferably a maximum of 0.4 times the value of this ratio for an unducted rotor.

14. Wind turbine as claimed in claim 12 or 13, characterized by means for controlling the rotation speed of the rotor such that the tip-speed ratio is kept substantially constant within an operative range of wind speeds.

15. Wind turbine as claimed in claim 14, characterized in that the speed control means are configured to control a power taken off from the rotor.

16. Wind turbine as claimed in any of the foregoing claims, characterized in that the degree of filling of the rotor, defined as the overall surface area of the rotor blades divided by the surface area covered by the rotor blades, is relatively high.

17. Wind turbine as claimed in claim 16, characterized in that the degree of filling of the rotor amounts to in the order of 0.5.

18. Wind turbine as claimed in any of the foregoing claims, characterized in that the ratio of the number of stator vanes n_s and the number of rotor blades N_R complies with one of the relations: n_s = 1.5 x N_R ± l (4a) or

N_R = 1.5 x n_s ± l , (4b) wherein n_s and N_R do not have a common denominator.

19. Wind turbine as claimed in any of the foregoing claims, characterized by a streamlined central body extending from the inlet to the rotor at the position of the longitudinal axis and carrying the stator and the rotor.

20. Wind turbine as claimed in claim 19, characterized in that at least one passage extending in axial direction is formed in the central body.

21. Wind turbine as claimed in claim 20, characterized in that the at least one passage comprises a channel having a substantially constant diameter and extending from a position close to the inlet to a position slightly downstream of the rotor.

22. Wind turbine as claimed in claims 13 and 20 or 21, characterized in that the speed control means comprise a controllable shut-off valve accommodated in the passage.

23. Wind turbine as claimed in any of the claims 19-22, characterized in that a power converter driven by the rotor is accommodated in the central body.

24. Wind turbine as claimed in claim 23, characterized in that the power converter is an electric generator embodied as a multipolar generator.

25. Wind turbine as claimed in claims 20 and 23 or 24, characterized in that the power converter is connected to the rotor by means of a hollow shaft arranged around the passage.

26. Wind turbine as claimed in any of the foregoing claims, characterized in that the tunnel is constructed downstream of the rotor from a number of tunnel segments partially overlapping in flow direction and defining in each case an annular gap therebetween, these segments together forming a diffuser.

27. Wind turbine as claimed in any of the foregoing claims, characterized in that the tunnel is mounted for pivoting about a vertical axis.

28. Wind turbine as claimed in claim 27, characterized by a streamlined element which is arranged close to the upstream end of the tunnel, extends substantially transversely thereof, and is mounted pivotally on a carrier.

29. Wind turbine as claimed in any of the foregoing claims, characterized in that all the surfaces covered by the flow are slender and have a sharp end, such that the Kutta condition is met for downstream-bound flow.