WO2003029644A1

WO2003029644A1 - Blade design method involving iteration calculations and processing of drawings

Info

Publication number: WO2003029644A1
Application number: PCT/NO2002/000333
Authority: WO
Inventors: Harald Johansen; Morten Kjeldsen
Original assignee: Hammerfest Strøm As
Priority date: 2001-09-21
Filing date: 2002-09-20
Publication date: 2003-04-10
Also published as: NO20014597D0; NO20014597L

Abstract

A method for design of wing blade for a horizontal axis free flow turbine is described, where the method comprises to determine desired efficiency for the turbine and to decide a load distribution function expressed as T(r)= x &ast; f(r), where x is a variable and f(r) is a function of the radius for the wing blade. Design angle and chord length is calculated among other things on the basis of these beforehand set numbers. In the end finished detailed drawings for the wing blades are generated on the basis of chord length and design angle. The wing blades may be designed without use of experience data from earlier wing blades. It is placed relatively small demands on flow technical competence to use the method.

Description

BLADE DESIGN METHOD INVOLVING ITERATION CALCULATIONS AND

PROCESSING OF DRAWINGS

INTRODUCTION The present invention concerns a method for design of turbine blades, and especially the design of wing profiles in free flow turbines, which are to be placed under water for exploiting the energy in flood tides. The invention also comprises a computer program product and use of the method.

GENERAL DESCRIPTION OF TECHNOLOGY

A free flow turbine is a turbine exploiting the kinetic effects of a water stream. The turbines function in approximately the same way as a wind turbine. The principle for the turbine is to convert the kinetic effect of the water into electric effect. This takes place through two processes. First, the kinetic effect of the water is transfor- med to mechanical effect, and thereafter the mechanical effect is transformed to electrical effect in the generator.

The transition from kinetic effect to mechanical effect takes place in the turbines wing blades. Forces act on the wing blades of the turbine, and a torque is created around the rotational axis of the turbine. The design of the wing blades is thereby vital for the efficiency in the conversion from kinetic effect to mechanical effect and thereby also vital for the efficiency of the whole turbine.

Fluid mechanical basis When a water current passes a turbine, some of the kinetic effect in the current is transferred to mechanical effect in the wing blades. Thereby the kinetic effect in the water is reduced after having passed the turbine, and the current also has a lower speed.

The power extraction through the turbine takes place in reduction of the speed of flow in front of the turbine and in that the static pressure increases. Through the turbine plane itself a sudden reduction of the pressure takes place and immediately after the turbine the static pressure in the water is lower than in the surround- ings. This pressure difference is smoothed out downstream of the turbine in that water speed is further retarded.

The reason for increase in the static pressure and retardation of the velocity up- stream of the turbine is the forces from the wing blades acting opposite of the current direction. The magnitude of the forces acting from the turbine is dependent of how much effect is being taken out of the turbine, and how the load distribution is on the wing and the chord length of the wing blade parallel with the relative velocity of the current. Retardation of the current velocity through the turbine is called axial induction factor and is important to be able to calculate the correct design angle of the wing blades. These terms are visualized in Figure 2.

Upstream of the turbine the flow will also be influenced by the rotation of the wing blade. A rotation on the flow is induced leading to other relationships between the wing speed and flow velocity, and thereby a difference in design angle. This factor is called tangential induction factor.

Neither axial nor tangential induction factor is even over the radius of the turbine and it must be calculated for each section of the wing blade.

It is thus a mutual independency between forces acting on the turbine, flow and wing speed, chord length and design angle.

The concept of free flow turbines has not been developed earlier. Design of similar wing blades has on the other hand been made in the windmill industry. The differ- ence between ordinary windmill wings and free flow turbines is relatively small. The fluid dynamical basis is the same and it is the same equations for power transmission on the wing blade, which found the basis. The difference on the turbines is the fluid they are operated in, the rate of flow in the fluid and the rotation speed of the turbine. Water density is approximately 1000 times that of air, the flow velocity of the fluid at the design point is 3-6 times greater on a wind turbine, and the rotation speed on a non-chokable turbine is lower than for a wind turbine. Previous methods for design of wing blades for wind turbines are also based on the existence of experience data. These differences have made it necessary to develop an automated computer program which can calculate wing blades for free flow turbines, but that also may be used in designing wind turbine blades. On the basis of this it is developed a method presenting the geometry on the wing blades for a free flow turbine with a hori- zontal axis. The method uses data from the flow, wing profiles and other design desires regarding the size and form of the turbine, to calculate the design of the wing blade and generate finished construction drawings for the wing blade. The method designs a wing blade for a certain flow velocity. By design is meant the twist of the blade and the chord length as a function of radius.

SUMMARY OF THE INVENTION

The objects of the invention are achieved in a first aspect by a method for design of a wing blade for a horizontal axis free flow turbine for placement in a fluid flow, which is characterized in that the method comprises: in a first step: to select parameters connected to the desired characteristics of the free flow turbine; to decide wanted efficiency η_de_sir_ed for the turbine; to select a distribution function f(r) where r is the radius of the wing blade, and a load variable x; in a calculation step: to calculate the load distribution, T for all wing blade sections i, where Tj= x ^* f(η); and based on said values, and the characteristics of the fluid flow, to calculate the design angle βι for each wing blade section; to calculate the chord length for all wing blade sections; and in a generating step: to generate finished construction drawings for the wing blades, and dimensioning criteria for the free flow turbine.

In a preferred embodiment the method is further characterized in, in the calculation step: to calculate for each wing blade section, on the basis of the design angle, said values, and the characteristics of the fluid flow, lifting force perpendicular to the wing profile, drag force parallel with absolute flow velocity, the drag force coefficient of the turbine, axial induction factor, the flow velocity of the fluid through the turbine, induced tangential speed, radial induction factor; and based on these values to calculate a new design angle for each wing blade section; and if the new design angle is different from the design angle, to set the design angle equal the new design angle, to repeat the calculation step, and to perform said steps until the new design angle is equal to the design angle. In an even further preferred embodiment the method is characterized in that it calculates for each wing blade section, and for the whole turbine, the drag force parallel with absolute flow velocity, and the torque about the rotational axis of the turbine, and the efficiency η_caicuiated for the turbine, and

- if the efficiency η_cai_cuiated for the turbine is different from the desired efficiency η_desir_ed. to change the value of the load variable x; to repeat the calculation step; and to perform said steps until the calculated turbine efficiency is equal to the desired turbine efficiency.

If the calculated turbine efficiency is less than the desired turbine efficiency, the program will increase the load variable x, while the calculation program will lower the value of the load variable x if the calculated turbine efficiency is larger than the desired turbine efficiency. The wing blade sections are designed such that all the calculation operations on the wing blade are performed 2-dimensionally.

In a second aspect the invention concerns a wing blade designed according to the method given above. The wing blades are manufactured on the basis of the working directions and the dimensioning criteria for the free flow turbine. This can be performed in a computer controlled process on the basis of the coordinate specifications for the wing blade generated in a program and stored in a data file. The dimensioning criteria decide the choice of material, which again decides what manufacturing process that must be used.

In a third aspect the invention concerns a computer program product comprising a computer readable storage media comprising a computer readable code device performing the method described above when loaded into a main memory in a data processing system.

In a fourth aspect the invention concerns a computer program product comprising a computer readable code device performing the method above, when it is loaded into the main memory in a data processing system. The wing blades are especially designed for free flow turbines in a tidal power station. Known methods for design of wind turbine blades within the wind turbine industry usually use experienced data to find the most suitable wing blade. In the method according to the invention it is performed calculations on forces and efficiency on the wing blade.

Compared with methods known from the wind turbine industry the invented method is turned upside down. It is decided before hand which load distribution that shall act on the wing blade and which efficiency one desires on the power conver- sion from kinetic effect to mechanical effect. Some other properties connected to the physics in the flow, physical dimensions on the turbine and the 2-dimensional characteristic of the wing profile are also used as input data in the calculations in the method. From these data the method designs, through an iteration process, a wing blade for the turbine and it is also calculated total forces and torques on the wing blade and the whole construction.

What is special about the method is the load distribution function, T(r). It consists of a load variable, x, which varies so that the forces on the turbine and thereby the torque about the rotational axis and the efficiency on the turbine shall satisfy the desired efficiency. The second part of the load distribution function is a distribution function deciding the shape of the load distribution. load distribution function: T(r) = x * f(r) where f(r) is a function of r selected by the user of the program, and which decides the shape of the load distribution. The distribution function is a random function which can be linear, exponential, logarithmic, a combination of several types etc., as long as it is a function where r is the only variable. It is this distribution function which is decisive for all the further calculations in the automated method according to the invention.

The method can be used without experience data from earlier wing blades to be able to design a new wing blade. It is also put relatively small demands on the flow technical competence of the users of the method. BRIEF DESCRIPTION OF DRAWINGS

Embodiments of the invention will now be described with reference to the appending drawings, where

Figure 1a shows an example of a wing blade designed in accordance with the invention;

Figure 1b shows an example of a free flow turbine with wing blades designed by the use of the method according to the invention; Figure 2 shows a wing profile with visualized known concepts; Figure 3 is a flow chart showing the calculations being performed to design the wing blade according to the invention;

Figure 4 is a graph showing chord length as a function of the radius of the wing blade calculated according to an embodiment of the invention; Figure 5 is a graphical presentation of the design angle / attack angle as a function of the radius of the wing blade calculated according to an embodiment of the invention;

Figure 6a is a graphical representation of radial induction factor as a function of the radius of the wing blade calculated according to an embodiment of the invention; Figure 6b is a graphical presentation of axial induction factor as a function of the radius of the wing blade calculated according to an embodiment of the invention; Figure 7 is a graphical representation of drag force parallel with absolute flow velocity as a function of the radius of the wing blade calculated according to an embodiment of the invention; Figure 8 is a graphical representation of torque around the rotational axis of the turbine as a function of the radius of the wing blade calculated according to an embodiment of the invention; and Figure 9 is a graphical representation of the thrust coefficient as a function of the radius of the wing blade calculated according to an embodiment of the invention.

DETAILED DESCRIPTION

In Figure 1a it is shown a wing blade designed by the use of the method according to the invention. A free flow turbine with wing blades designed by the use of the method according to the invention is shown in Figure 1 b. The free flow turbine in Figure 1b is placed on the sea bottom and is a part of a tidal power plant. Definitions of the concepts being used in the further description of the invention are listed in the following. It is also referred to Figure 2 in that connection.

Definitions

Wing blade: A wing blade is the whole appearance of the wing; i.e. how the wing, which uses a certain wing profile, is twisted and how the chord length varies along the radius of the wing. Wing blade partitioning: Wing blade partitioning is how the wing is partitioned in smaller elements along the radius. The wing blade is partitioned such that all the calculating operations can be performed 2-dimensionally.

Wing profile: The wing profile is a 2-dimensional profile in a radial section of the wing on the turbine. Most wing profiles have a known 2-dimensional characteristic, i.e. the hydraulic properties of the wing profile at 2-dimensional stationary flow conditions.

Free flow velocity: The flow velocity of the fluid that would have passed through the sweep area of the turbine if the turbine was absent. The free flow velocity is constant along the radius of a wing blade. Sweep area: The sweep area is the area of the circle defined by the wing tip movement in one whole revolution.

Speed of rotation: The rotation speed is the speed with which the turbine is rotating (rad/s).

Wing speed: The wing speed is the speed with which the wing blade rotates in a section at a certain radius. The speed varies as a function of the radius of the wing. Wing speed = speed of rotation x radius.

Relative speed: The relative speed is the speed the wing blade "experiences" that the current has. The relative speed is the vector sum of the free flow velocity and the wing speed, and varies according to the radius of the wing blade. Design angle: The design angle is the angle between the directions of the free flow velocity and the relative speed. The design angle varies along the radius of the wing blade. Wing blade torsion: The wing blade torsion is how the torsion of the wing blade varies along the radius. The torsion of the wing blade correspond to the design angle for each section of the wing. Chord length: Chord length is the length of a wing profile parallel with the relative speed. The chord length varies along the radius dependent on the load distribution on the wing as it is the chord length which decides the forces that are being transferred to each wing blade section.

TSR: TSR stands for Tip Speed Ratio and is a dimensionless number describing the connection between free flow velocity, the turbine's speed of rotation and wing radius. TSR is of experience decided to be put in the interval 3-7. Variation in TSR is made to achieve a speed of rotation on the turbine satisfying generator and gear system needs.

Desired efficiency: The efficiency of the free flow turbine is decided before the design of the turbine. Theoretical maximum efficiency on a free flow turbine is 59%. In the method and in reality, it has shown to be difficult to design wing blades with efficiency higher than 45%.

Wing length: By wing length is meant the radius of the free flow turbine. Number of blades: The number of blades means how many wing blades one wants on the turbine. Hub radius: Hub radius is the desired radius of the hub, i.e. the length from the rotational centre of the turbine to the start of the wing blade. 2D characteristic of a wing profile: 2D characteristic of a wing profile is the hydraulic properties of a wing profile calculated at 2-dimensional experiments. By hydraulic properties is meant the lift and drag coefficient of a wing profile at different angles of attack between the direction of the flow and the centre line of the wing profile.

Distribution function: Distribution function is a function deciding the shape of the load distribution of a wing blade. It provides possibilities for choosing between different types of functions.

Overview of parameters and equations being used in the calculations Parameters a axial induction factor a tangential induction factor

A the sweep area of the turbine ci 2-dimensional lift coefficient of the wing profile c_d 2-dimensional drag coefficient of the wing profile c_2x induced tangential speed

C_T drag coefficient / thrust coefficient of the turbine dr length of wing blade element along the radi D drag force parallel with the wing profile dA area element f(r) distribution function i random wing element, i™ [ 1 ,n] k chord length L lift force perpendicular to the wing profile

M torque around the rotational axis of the turbine

M_N perpendicular force torque, moment of flexion on the wing blade from the perpendicular forces n number of blade elements in the calculation N drag force parallel with absolute flow velocity

P_nature nature effect of the flow

Pturbine turbine effect

R turbine radius r running radius R_hub hub radius (measured between the centre of rotation and wing tip)

T force parallel with the rotational direction of the turbine (tangential direction)

T(r) load distribution function

TSR tip speed ratio

U_∞ the free flow velocity of the fluid (free flow velocity) U_D the flow velocity of the fluid through the turbine

W relative speed x load variable in the load distribution function z number of wing blades on the turbine β design angle of the wing blade (the attack angle of the relative speed on the free flow velocity) β_new new calculated design angle ηdcsire desired turbine efficiency

C_alculated the efficiency of the turbine η_reaι the actual efficiency of the turbine included secondary loss effects ω the rotational speed of the turbine p fluid density π constant (3,1415)

?i index "i" means that values are calculated for each wing section

?ι blade index "1 blade" means calculations for one wing blade ?,y_t index "tot" means calculations for the whole turbine Equations

_dr=(^R-^R,

TSR-U„ ω =

R

3- P_na,_ure=^~p^R²

4. r =R_hub+(i-\)-dr

5. β_t = arctan(— ) ωr.

6- T, =x-f(r,)

7. M =r - T

^\

9. N, =L,-(sin( ?,--) + ^-sin ?,)

2

3-V. to. C_τ =

1

/>£/*^■ <*

C_r,, =4(l-«,)² (12a)

13. [/_Dι =[/_∞.(1-_Ω,) T,

14 c =

2pττr\J _Dl dr

^C2

15. rt, =

<D/ ^UD, , 180 16. p_newι = arctan( ) ; calculated in degrees cor +c 2xι π

17. k. =

-ctp-Vl +in β>(l + α,^'))² )^■</-

18- N_lbhde=∑N,

19. N,_ol=z-N_lblade

20. M 1 blade = ∑ ,

21. M_lot = z-M_Wade

22. 'calculated _^ nature _c _N_tot- _∞ N_lol

23.

^T _m, / ^λ ₂pUlπR²

24. M_N, =N r_t

25- M _blωlc=∑M_Nl

²⁶- Αbladc

n

Figure 3 is a flow chart showing the different steps in the method. The method demands that the user first must provide some parameters as a basis for the calculations that are being performed. This is denoted as input in Figure 3, and is connected to the physics in the fluid flow in which the turbine is to be placed, some desired turbine specifications, and specifications connected to the size and profile of the wing blades.

What is in the input is listed below:

Physics in flow:

• Free flow velocity, U_ro • Fluid (database with various properties of the fluids put into the method) Turbine specifications:

• Tip Speed Ratio, TSR

• Desired efficiency, η_desir_ed (Theoretical maximum value 59%)

Wing blade specifications:

• Wing length, R

• Number of blades, z

• Hub radius, Rhub

• Number of wing blade elements, n • Distribution function, f(r)

2D characteristic of the wing profile:

• Drag coefficient, c_d

• Lift coefficient, q

The user of the program must thus decide in which flow the free flow turbine is to be placed, example given in a special inlet in salt water, and the physical dimensions on the desired turbine. Thereafter it is decided, by the program, constants for the whole calculation on the basis of these parameters and the desired properties, and on the basis of the fluid data base, and all the variables are initialized. The following calculations and initializations are being made in this input step:

The density of the fluid, p, is found from the fluid data base. The constants which shall be used in the rest of the calculations are calculated. These are: - length of wing blade element along the radi, dr (Equation 1 ),

- the rotational speed of the turbine, ω (Equation 2),

- nature effect on the flow, P_nat (Equation 3),

- running radius for each wing section, r,for all i (Equation 4), and

- area element for each wing section, dA, for all i (Equation 11 ).

The initial design angle of the wing blade for each wing section, β,'s initial value, is calculated from Equation (5) for all i, i.e. for each wing section. The value, for each wing section, for the variables axial and tangential induction factor, new calculated design angle, the drag coefficient of the turbine, induced tangential speed, drag force parallel with absolute flow velocity, lift and torque around the rotational axis of the turbine, a-

βnew , C_τ,i, c_2x,i, Nj, L, and Mj, res- pectively, is set to 0 for all i, and for the load distribution function the load variable, x, is set to an initial value equal to P_nat /10000, in the computer program.

The iteration process

The method is based on that two procedures are being used, as in Figure 3 is de- nominated and shown as procedure 1 and procedure 2. The two procedures have feedbacks to be able to perform new calculations if not certain criteria are fulfilled with regard to the calculated values. Procedure 2 is connected to the design angle and lies within procedure 1 , which is connected to the efficiency of the turbine. This will be further explained in the following.

Procedure 1

Procedure 1 is performed as long as the calculated efficiency η_cai_cuiate_d is different from the desired turbine efficiency η_de_si_red which was chosen in the input step. The following calculations are performed at least once:

1. Load distribution is calculated for all sections from the distribution function, f(r) and use of the load variable, x from Equation (6).

2. Procedure 2 is performed.

3. Chord length from all sections, kj, is calculated from Equation (17). 4. Normal force on one blade, N_1biad_e, and all blades, N _ot, is calculated after

Equation (18) and (19).

5. Torque on one blade, Mibiade, and for all blades, M_tot, is calculated from Equation (20) og (21 ).

6. Calculated efficiency, η_Caicuiated, is calculated from Equation (22). 7. If Calculated ≠ ηdesired the load variable, x, is changed.

8. If ηcaicuiated < ηdesired , is increased, if ηcaicuiated > η esire , the value on x is decreased.

9. If ηcaicuiated = η esired procedure 1 is ended and an output step is started. Procedure 2

Procedure 2 is a loop which can be run through as long as the design angle β, which is used as input value in these calculations are different from the new de- 5 sign angle β_new,ι which is calculated on the basis of among other things the value for β,. As long as β_πew,ι ≠ βι. the following calculations are performed for all wing blade sections:

1. β, is set equal to β_new,ι- ιo 2. Forces perpendicular to the wing profile section, L,, is calculated from

Equation (8).

3. Forces on the turbine parallel with absolute flow velocity, N,, is calculated from Equation (9).

4. Perpendicular force coefficient on the turbine, C_τ,ι, is calculated from i5 Equation (10).

5. Axial induction factor, a,, is calculated from Equation (12).

6. Actual flow velocity through the turbine, U_D,ι, is calculated from Equation (13).

7. Factor c_2x,, is calculated after Equation (14) .

20 8. Radial induction factor, a',, is calculated from Equation (15).

9. New angle of attack, β_new,ι, is calculated from Equation (16).

When procedure 1 and 2 fulfils the criteria for stopping the iteration, the following values are calculated: 25 • C_τ from Equation (23)

• M_N|ι from Equation (24)

• M_N,ibiade from Equation (25)

• T_1biade from Equation (26)

Secondary terms of loss as "tip-loss" on the wing tip is included in the calculation 30 in the end. The loss function for "tip-loss" is an empirical function, g(r). This provides and actual efficiency which is somewhat lower than the desired efficiency. If the desired efficiency is too large such that C_τ,i > 1 , for one or more i, the desired efficiency is too large and it is then not possible to make a design for the turbine with the desired input criteria. (C-r > 1 means that the stagnation energy of the flow is larger than the total energy in the flow.) The user is then given a message about this, e.g. on a display device, and must then possibly input new parameters for the free flow turbine, before calculations on the basis of these new numbers are performed in the program.

From the program it is generated three different types of results which are denomi- nated as output in Figure 3:

1. Detailed drawing of the wing blade.

When the calculations are fulfilled, it is generated detailed drawings for the wing blade. Chord length, design angle for each section and the appearance of the wing profile is used for this.

In the calculations on the end of the wing blade it is entered a function into the method which provides that the chord length at r,=R approaches 0.

2. Dimensioning criteria for turbine, blades and other parts. Values for N_1b|_ade, N_tot, M-ι_bι_ade, _tot, ηcaicuiated, CT, T _biade, N_M,ι_biade are given.

3. Graphs visualizing result from the calculations.

It is also generated graphs showing diverse variables for the wing blade as a function of the radius. The variables which are shown graphically are: Chord length, the design angle of the wing blade, forces axially and tangentially to the blade, torque axially tangentially, tangential and radial induction factor, and thrust coefficient. An example of such graphs is shown in Figures 4-9.

EXAMPLE The example contains data assisted calculations performed with a method according to the invention. For design of wing blade the following variables are decided: Physics in flow:

• Free flow velocity, U∞ 6m/s

• Fluid: Salt water

Turbine specifications:

• Tip Speed Ratio, TSR 8

• Desired efficiency, ηde_sired 0,3

Wing blade specifications:

• Wing length, R 3,5m

• Number of blades, z 4

• Hub radius, Rhub 0,5m

• Number of wing blade elements, n 70

• Distribution function f(r) = r-0,3

2D-characteristic of the wing profile:

• Drag coefficient cd 0,02

• Lift coefficient, cl 1 ,00

The program calculates then constants, which shall apply for the further calculations, and collect the necessary information for the fluid database stored in a memory device:

Density, for salt water: p = 1025 kg/m3.

Nature effect after equation: Pnature = 1274465 W.

The rotational speed of the turbine, ω: = 13,71 rad/s.

Wing partitioning dr = 0,04286 m. η for all i. dA, is calculated for all sections. The program starts the calculation by starting procedure 1 , which again starts procedure 2 as explained above and which is shown in the flow chart in Figure 3. When the calculations have been fulfilled, three different types of results are generated:

• The appearance of the wing blade as detailed drawing.

When the calculations are fulfilled, detailed drawings for the wing blade are generated. The drawings are generated on the basis of chord length, design angle and chosen wing profile. They are printed out in normal manner, but are also genera- ted as a computer file with coordinates for the wing blade.

• Values for forces and torques on one wing blade and the whole turbine. ηcaicuiated 0,298 ηreal 0,290

Cτ,tot 0,538

Ml blade 23232 Nm

Ni blade 75623 N

Mtot 92930 Nm

Ntot 302494 N

' turbine 1274465 W ω 131 o/min,

NM,1 blade 96062 N

MN,1blade 1675792 Nm

• Graphs showing the distribution of forces and torques.

The program is generating graphs showing diverse variables as function of the radius. The variables shown graphically are: Chord length, the design angle of the wing blade, forces axially and tangentially on the blade, torque axially and tangen- tially, tangential and radial induction factor. Graphs generated by the number values used above are shown in Figures 4-9, respectively. The graphs can both be displayed on screen and follow the detailed drawings. The graphs do not show the iterations process itself, but visualizes the results from this. The program is run in a data processing unit with CPU and memory units. The program gets the information necessary from an underlying fluid data base which is stored together with the design program itself in a storage device. Example given on a hard disc or CD-ROM. The user communicates with the program through a user interface, the necessary instructions to the user may be laid out visually on a display device connected to the processing unit. The user may then input data via for example a keyboard, mouse or by use of voice, press on screen or similar known techniques. The iteration process may also be laid out visually for the user on the display device. After ended simulation, the program will generate detailed drawings which together with the calculated values for forces and torque, which both can be stored as a data file and written out on paper for the user, via a suitable printer unit connected to the processing unit. The program may example given be run on a today normal windows based computer with normal yields on memory and processor, example given a machine of the type Pentium.

The dimensioning criteria where the size on the tensioning and forces calculated on the wing blade take part, will decide the choice of material. If steel has to be used, the wing blade may be manufactured in a computer controlled process where the wing blade is shaped directly on the basis of the coordinate specification which the program has calculated and stored on the data file. In other circumstances where it is regarded as adequate with a wing blade in fibre, the coordinate specifications on the data file may be used in a data controlled process for the manufacturing of the mould for the wing blade. The choice of material will decide which process to be used for manufacturing of the wing blade itself. This will in any case be a computer controlled process, based on the generated coordinates for the wing profile.

In the previous embodiments of the invention have been described. It should how- ever be clear to a skilled person in the art that these are only to be regarded as examples and that the scope of protection for the invention is defined in the appended patent claims.

Claims

C L A I S

1. Method for design/manufacturing of wing blade for a horizontal axis free flow turbine for placement in a fluid flow, c h a r a c t e r i z e d i n that the method comprises: in a first step:

- to choose parameters connected to the desired properties of the free flow turbine;

- to decide the desired efficiency ηd_esir_ed for the turbine; - to select a distribution function f(r) where r is the radius of the wing blade, and a load variable x; in a calculation step:

- to calculate the load distribution, Tj, for all wing blade sections i, where Tι= x * f(η); and based on said values, and the properties of the fluid flow

- to calculate the design angle βj for each wing blade section;

- to calculate the chord length for all wing blade sections; and in a generating step:

- to generate finished detailed drawings for the wing blades, and dimensioning criteria for the free flow turbine.

2. Method according to claim 1 , c h a r a c t e r i z e d i n that in the calculating step

- to calculate for each wing blade section, on the basis of the design angle, said values, and the properties of the fluid flow, lift force perpendicular to the wing profile, drag force parallel with absolute flow velocity, the drag force coefficient of the turbine, axial induction factor, the flow velocity for the fluid through the turbine, induced tangential speed, radial induction factor; and

- based on these values to calculate a new design angle for each wing blade section; and if the new design angle is different from the design angle

- to set the design angle equal to the new design angel,

- to repeat the calculating step, and

- to perform said steps until the new design angle is equal to the design angle.

3. Method according to claim 1 , characterized in

- to calculate for each wing blade section, and for the whole turbine, the drag force 5 parallel with absolute flow velocity, and torque on the rotation axis of the turbine, and

- efficiency ηcai_cui_ated for the turbine is different from the desired efficiency ηdesired. to change the value on the load variable x;

- to repeat the calculating step; and ιo - to perform said previous steps until the calculated efficiency of the turbine is equal to the desired efficiency of the turbine.

4. Method according to claim 3, characterized in increasing the value on the load variable x if the calcu- i5 lated efficiency of the turbine is less than desired turbine efficiency.

5. Method according to claim 3, characterized in decreasing the value on the load variable x if the calculated turbine efficiency is larger than desired turbine efficiency.

20

6. Method according to claim 1 , characterized in that the wing blade sections are designed in such a way that all the calculating operations on the wing blade is performed 2-dimensionally.

25 7. Wing blade for free flow turbine designed in accordance with the method according to one of claims 1-6.

8. Computer program product for a data processing system comprising a computer readable storage medium comprising a computer readable code device per- 30 forming the method according to claims 1-5, when loaded into the main memory in a data processing system.

9. Computer program product for a data processing system comprising a computer readable code device performing the method according to claims 1-6, when loaded into the main memory in a data processing system.

10. Use of free flow turbine with wing blades manufactured by the method in the independent claim 1 in a tidal power plant.