MXPA05013615A - Method and installation for extracting energy from a flowing fluid - Google Patents

Abstract

Turbine farm comprising at least a first turbine (1) and at least a second turbine (2) by means of which energy can be extracted from a flowing fluid, wherein when the second turbine is on the lee side of the first turbine, under nominal power, the axial induction of the first turbine is lowered with respect to the second turbine, to reduce turbulence mainly at the location of the at least second turbine.

Description

METHOD AND INSTALLATION FOR EXTRACTING ENERGY FROM A FLUID OF FLUID The present invention relates to a turbine farm according to the pre-characterizing clause of claim 1.

The invention also relates to a method for this. In addition, the invention relates to a control system and a program for the control system, to implement the method. More generally, the invention relates to a method and / or installation by which energy can be extracted from a fluid flow. The term fluid flow is used to refer to both the wind and the water (sea) that flows. It is understood that the installation is a turbine system with a control system (in particular a turbine farm). It is generally known that energy can be extracted from wind using wind turbines, the size of wind turbines and number of these has been increasing rapidly in recent years. More and more often they are being installed next to one another in the so-called wind farms. Due to the lack of space on earth (especially in Europe), turbines are also installed more frequently on the coasts. Now, wind farms are planned on the coasts that have ten turbines or more. Although the opinion of the experts is very divergent with regard to this, wind energy is being seen as one of the greatest sources of energy of the future.

If this becomes a reality, many farms with hundreds of turbines will be needed. These types of farms are expensive and therefore it would be very important that the production of the farms be very high, in order to justify their cost. Because a wind turbine extracts kinetic energy from the wind, the wind speed will have decreased behind the turbine. This effect is often called with the term wake effect or shadow effect, and also with the term interference; The loss suffered by the turbines on the leeward side is called loss of shadow or loss of wake. The loss of wake in the wind farms, is often taken into account introducing the figure of efficiency of the farm. This figure provides the rate between performance with wake losses, compared with the yield without wake losses. Typical values are between 0.70 and 0.99. Virtually in all parts of the world, certain wind directions appear more frequently than others. This is what is called the dominant direction of the wind, which is defined here as the direction of the wind in which the main proportion of the annual production in operation with partial load is harvested. The undisturbed wind direction is defined as the wind direction at the location of a turbine or farm, without the influence of that turbine or that farm. Incidentally, the wind direction varies substantially on a small time scale (from seconds to minutes); therefore, the term wind direction is used to refer not to the instantaneous value, but to the averaged value, for example, for 10 minutes. According to the present theory, turbines extract the maximum amount of energy from a fluid if the fluid decelerates to approximately 2/3 of the original speed at the turbine location, and up to approximately 1/3 of diameter 1 behind of the turbine. The fractional reduction in the speed of 1/3 of the original speed at the location of the rotor is called axial induction, which is indicated by the letter a. In the case of extraction of maximum energy, a is equal to 1/3. By choosing the axial induction to be less than 1/3, the turbine decelerates the wind to a lesser extent, and the affected turbine extracts less energy from the wind, which according to the prior art can be beneficial for the turbine behind it . Current wind turbines are often designed to have an axial induction of approximately 0.28. The value is less than optimal, because a substantial load reduction is achieved by these means, while the drop in energy production is relatively slight. If a wind turbine reaches its maximum or nominal power at the nominal wind speed, then provision is made in one form or another, so that the energy does not increase additionally with the increasing wind speed (above the nominal speed of the wind). wind), the control can be passive or active, and in both cases has the result that the axial induction decreases with the increase in wind speed. At wind speeds of 20 m / s to 25 m /, the axial induction can fall to below 0.1. After the axial induction, an axial force is defined as the force in the direction of the rotor axis, exerted by the wind in the turbine. The axial force (Fa?) Is associated with the axial induction by means of the relation Fgx = 4a (1-a) Fnorm, where Fnorm is a force used for normalization. This force is equal to 1 / 2pVzA, where p is the density of the fluid, V the velocity of the fluid and A the surface area of the rotor that is traversed. If the surface area of the rotor and the density are known, the axial induction can be determined, therefore, from the measurement of the axial force and the velocity of the fluid. If a first wind turbine extracts the maximum amount of energy from the wind, it is normal for the wind speed to fall to less than 50% of the original speed at a short distance behind the turbine (for example, a diameter). Since the energy that can be obtained from the wind is proportional to the third energy of the wind speed, the drop in speed means a second turbine that could be installed in that position behind the first wind turbine, as much as it would be able to achieve only one eighth of the energy, compared to the first turbine on the windward side. In practice, these dramatic drops in energy rarely occur because the wind turbines are placed far enough away. The distance between the turbines is usually 3 to 10 times the diameter of the turbine. Over that distance, the slow wind in the wake mixes with the faster wind around it, as a result of which the wind speed at the location of a subsequent turbine has not decreased too much compared to the original wind speed. In summary, the shadow effect decreases as the distance between the turbines increases. The wake problem is not restricted only to an adverse interaction between two wind turbines, one after the other in the wind direction, but occurs to a more significant extent in wind farms in particular. The energy extracted by wind turbines on the windward side of a farm, together with the loss of kinetic energy as a result of mixing (this concept is explained below), inevitably leads to a decrease in speed, in the layer atmospheric boundary in which the rest of the farm is located. It is said to be energy depletion in the boundary atmospheric layer. In the broadest sense, it can also be said that it is a shadow effect between different wind farms. A whole farm that is located leeward with respect to another farm may be subject to a substantial reduction in production. Apart from the aforementioned drops in output, operation on the wake can also lead to more fatigue damage for wind turbines. If the number of turbines located one after the other becomes large, increasingly large distances between the turbines are needed to keep wake losses at an acceptable level. This means that a large surface area is needed, and that the cable lengths between the turbines, and therefore the cost, increase. In the case of installation on earth, a greater distance between the turbines also means that longer roads have to be built, which means an additional increase in costs. While placing the wind turbines additionally apart, helps against shade losses, a noticeable drop in production by the turbines on the leeward side on large farms will be inevitable. The decrease can be so great that a farm becomes uneconomical as a result. Losses of 30% or more are generally known from the literature. In the state of the art, a wind farm is often designed in such a way that it extends mainly perpendicular to the prevailing wind direction, as a result of which the effects of the shadow can be reduced. In practice, however, the placement of wind turbines is also dictated by numerous other interests, such as: what area of land or sea surface has been assigned to the wind turbine operator, what are the other functions of the area, which disturbance is caused by the turbines, how the existing power lines run, etc. Consequently, this option will only be able to offer a limited solution to the previously mentioned problems.

The publication by Steinbuch, M., Boer, of WW, and coauthors, entitled "Optimal Control of Wind Power Plants' in Journal of Wind Engineering and Industrial Aerodynamics", (27), Amsterdam, 1988, describes that the operation of wind turbines On the windward side of a farm with a lower speed at the tip of the blades less than the one at which the maximum amount of energy is extracted, can lead to an increase in the total production of the farm. No physical explanation is given for the result confirmed by simulation. In Corten's thesis, G. P., entitled "Flow Separation on Wind Turbine Blades ", ISBN 90-393-2592-0, dated January 8, 2001, states that during the mixing of the slow air in the wake, with the rapid air from the outside, this momentum of the two streams of mass together is maintained, but some of the kinetic energy is lost as heat.In the case of a solitary wind turbine, which is running at optimum operation, the loss in the mixture is approximately 50% of the energy generated by the turbine, so that the kinetic energy extracted by a wind turbine from the flow is not equal to the energy generated, but is one and a half times as much.In this publication, it is proposed to choose the axial induction of the turbines on the windward side in a turbine farm to be 10% below the optimum value of 0.33 (ie, a = 0.30), so that the production of the whole farm increases.Despite the previous literature, the prevailing opinion is that wake effects can be mod They are better, but they can not be reduced. This can be seen, for example, in Hutting, H., "Samenvatting technisch onderzoek SEP-Proefwindcentrale" (Technical study summary on the test SEP wind power station), Kema-lndustriele energie systemen, Arnhem, November 1994, at which leads to the following conclusion: "increasing production with a farm control system taking into account the interaction with the wake, does not seem to be feasible". The most recent confirmation of this point of view can be seen from the minutes of a meeting held on May 23, 2002 at Riso National Laboratory, Denmark. Twenty experts, some of whom had been working on this topic since 1980, were in this meeting, and all the attention was focused on the modeling of wake losses. According to the minutes, the effects are great, but it is not yet known how big, and what precisely determines them. By improving the modeling one can estimate more accurately in advance how much a large turbine farm will produce in a specific position. This information is, of course, extremely relevant for investors. During the meeting, no attention was paid to the options to reduce the effects of wake by operating the turbines in a different way. To summarize, the current thinking is that the shadow effect gives rise to substantial drops in production, that placing the wind turbines further away, is a remedy that leads to high costs (longer cable length, and, on the ground, longer paths) and a low energy per unit surface area. Because space is expensive, this is a major disadvantage. Not only can less be generated on a given surface, but many areas (ie locations) will also lose in competition with other purposes, if only low production is expected. The prevailing view is that while the problem can be modeled better, it can not be solved. A further problem of the state of the art is as follows: as the axial induction of a turbine increases, the turbulence in the wake also increases. Turbines in the wake of other turbines can record this (for example, from anemometer measurements or fluctuating loads on the blades). As the turbulence increases, there is an increasing fluctuating load in the turbines, which is a disadvantage. An object of the present invention is to provide one. turbine for farm that combats the appearance of turbulence, and that somehow will solve the problem of load fatigue of the turbines. For this purpose, the invention provides a turbine farm according to the pre-characterizing clause of claim 2, characterized in that when the second turbine is on the leeward side of the first turbine, under nominal energy, the axial induction of the first turbine it is decreased with respect to the second turbine, to reduce turbulence mainly at the location of the at least one second turbine. Inseparably, however, progress is made toward a solution by means of the present invention. In accordance with the invention, this problem is combated. If the turbulence increases to undesirable loads (which can be seen from the anemometer measurement records or from the fluctuating loads in the blades), the turbine that generates the turbulence can be set at a lower axial induction. A turbine system according to the invention could advantageously be controlled in this way. It is proposed to decrease the axial induction of one or more turbines in a turbine farm to values of less than 0.25, for example, 0.2, or even 0.15. These values for axial induction are averages over the surface area traversed by the turbine. In the case of a turbine with horizontal axis, the values are averages on the part of the surface between 40% f? and 95% R occupied by the rotor, where R is the radius of the rotor, in such a way that substantial deviations from the average in the center of a turbine with horizontal axis, and the tips can be excluded. The low values are comparable to making the turbine more transparent to the fluid, so that the fluid velocity behind the turbine falls to a lesser extent, and as a complementary aspect, the power supply for the turbines in the leeward increases, for therefore. From the numerical values it can be seen that the measurement goes further than the reduction proposed in the aforementioned thesis. In addition, it indicates how the fall in the induction can be achieved. An advantageous modality that can be used with current wind turbines is the reduction of the speed of revolution and / or rotation of the blade angles towards the position offering the least resistance. These measures can also be combined with the reduction of the blade of the blades. To indicate the extent to which the rope can be reduced according to the invention, we define the rope characteristic as Nct? R2 / r. In this expression, N is the number of vanes, c, the chord in a specific radial position ry? R is the measure of local velocity, which, in turn, is defined as the radius between the velocity of the local vane and the Wind speed without disturbance. In the case of turbines (with radius R), which (without taking into account the losses of the farm) extract optimum energy from the wind, the rope characteristic between 0.5f? and 0.8R does not reach any value below 4.0. The normal values are between 4.2 and 5.0 for turbines with a rotor diameter of more than 50 m. For smaller turbines, the string feature increases more. Several advantages are achieved by designing such a turbine, that the string feature is smaller. According to an embodiment according to the invention, this rope characteristic reaches values lower than 3.75 or less, for example lower than 3.5 or even lower than 3.0. According to one embodiment of a turbine according to the invention, the integral of Ncr / (0.3rR) between 0.5R and 0.8R is less than 0.04, for example less than 0.036 and even less than 0.03. The formula 0.8Ü í -Ve, dr 03rR of this integral is as follows: <; > • ** The advantages obtained with these turbines are that the axial induction is low, as a result of which the wake losses decrease, and that the loads on the blades are smaller because the rope of the blades is relatively small . The latter also brings an advantage in wind speed survival, the maximum wind speed at which a turbine can survive. Additionally, the invention provides a method for operating a turbine farm, characterized by lowering of the axial induction of the first turbine with respect to the second turbine, when the second turbine is on the leeward side of the first turbine, under power nominal, to reduce turbulence mainly in the location of the at least one second turbine. In addition, the invention provides a control system for operating the turbine farm as described above, characterized in that the control system is capable, when the second turbine is on the leeward side of the first turbine, under nominal power, of decreasing the axial induction of the first turbine with respect to the second turbine to reduce turbulence mainly at the location of the at least one second turbine. The present invention also provides design software for a turbine farm as described above, characterized in that the software is capable of adding guiding elements to the installation, where the turbines have a guiding function, when the second turbine is on the leeward side of the first turbine, under nominal power, the axial induction of the first turbine is decreased with respect to the second turbine to reduce turbulence, mainly at the location of the at least second turbine, to calculate the influence of she on the turbine farm. Additionally, the present invention provides control software for a turbine farm as described above, characterized in that the control software is capable, when the second turbine is on the leeward side of the first turbine, under nominal power, of decreasing the axial induction of the first turbine with respect to the second turbine to reduce turbulence mainly at the location of the at least one second turbine. Finally, the present invention provides a turbine provided with a control system as described above. An advantage can be achieved by equipping a turbine farm with a control system that controls the axial induction of the turbines as a function of wind direction: as a general rule, the turbines that increase the loss of the farm (the turbines that are on the windward side) are set to decrease the axial induction values. A turbine farm can also be built with a control system that reduces the axial induction of at least one turbine, if turbulence in the undisturbed wind is high. The advantage obtained in this way is that in those situations where the turbines are subjected to a relatively substantial fluctuating load, the turbines add less turbulence, so that there is a relative decrease in the fluctuating loads. A turbine farm can be constructed with a control system that sets the axial induction of at least one turbine based on the number of turbines in the wake. According to one embodiment according to the invention, a turbine farm can be equipped with turbines with a lower axial induction on the leeward side of the farm, based on a dominant wind direction. The configuration of the axial induction can then be independent of the wind direction. The turbine farm according to the present invention has a width and a length. The width is measured perpendicular to the dominant direction of fluid flow and the length is measured in the dominant direction of the flow. The width is the width of the largest separation between two turbines, and the length the length of the largest separation between two turbines in the turbine farm. The turbines between which the distances are measured have to be in that part of the turbine farm within which there is an essentially regular pattern in the positions of the turbines. The surface area of the turbine farm then follows from the product of length and width. By adding all surface areas traversed by the turbines in the farm (in the case of two turbines with horizontal axis with a diameter of 100 m, the occupied surface area is 2-p / 4-1002m2), the surface area is obtained occupied by the turbine farm. In a turbine farm in which turbines with horizontal axis are at a distance 8D (eight diameters) apart, the occupied surface area is approximately 1.3% of the surface area of the farm. A turbine farm can be constructed more compactly using the present invention. In turbine farms with more than 50 turbines, according to one embodiment of the invention, the percentage of occupied surface area may increase to more than 3%, in particular more than 5% and even more than 10%. A preferred embodiment according to the invention is one in which an additional control system is not needed, but in which the windward side turbines are configured for axial induction lower than that of the wind turbines in leeward, in such a way what part of the expected benefit is still achieved. This can be a good mode, especially if there is a strongly dominant wind direction. The turbine farm operating in accordance with the invention will be subject to less wake loss than an installation in accordance with the state of the art. Because the traditional way of limiting wake losses is to increase the distance between the elements that draw energy (in particular wind turbines), a turbine farm becomes more expensive and gives a less efficient use of the surface area. By using the present invention, a turbine farm can be made with a more compact design, while the loss of wake remains acceptable. As each knower of the subject understands, in addition to the turbines with horizontal axis and turbines with vertical axis, the turbines can also be stairwell turbines, but also, for example, flying turbines, turbines that move, turbines in combination with concentrators such as pointed blades or diffusers, electrostatic turbines, turbines in vehicles lighter than air, turbines with multiple rotors, in a tower, and groups of turbines. As has been seen, an advantageous installation and an associated advantageous method for extracting energy from the flow depends on many factors. When designing these, you must carry out calculations to select the various elements and place them in advantageous locations. This is, of course, a function of the characteristics of the passive or active elements used, their mutual positions, the terrain, the meteorological parameters and a wide variety of other aspects, such as financial aspects and insurance aspects. The complexity and the large number of possible solutions provides the incentive to support this design process with design software. The design software with the special feature of advising the elements that can be added to the installation and / or where the turbines can have a recommended function, and where the influence of these elements on the farm can be predicted, can be part of the invention. Once an installation for energy extraction (ie a turbine farm) has been designed, there is a large number of associated variables, such as axial induction, revolution speed, blade angle, tilt angle , the circulation scale and the positions of the turbines, which have to be selected. The optimal combination of all these variables is difficult to determine in advance. Thus, control software is needed to test a large number of combinations of configurations, optionally based on specific physical revelations. According to one modality of this software, the parameters for those variables are varied according to a specific strategy. The performance of the turbine farm is stored as a function of the parameters that can be established, and then the optimum is sought for each wind speed and for each direction of the wind. Other meteorological data, such as the temperature distribution or the stability of the atmosphere, also appear as possible parameters here. Starting from a local optimum point found, the parameters can be changed in order to find a better optimal point.

The program can be self-learning, and therefore able to control the installation better and better. In this way, a good image of the control strategy is obtained in the course of time, and a database is compiled which, in turn, can be functional for the adjustment of other facilities to extract energy, such as a farm of turbines. With this knowledge it is also possible to improve the design process for new turbine farms. Additional characteristics and features will be explained with reference to two figures. Figure 1 shows, diagrammatically, a plan view of a small wind farm with only two wind turbines; Figure 2 shows, diagrammatically, a plan view of a small wind farm according to the present invention. A view in the plan of a small wind farm with only two turbines, that is, a first turbine 1 and a second turbine 2, can be seen in figure 1. The wind 5 has a given force and direction, as indicated by the arrow 5. In this particular case, the direction is parallel to the line from the front turbine 1 to the rear turbine 2. In figure 1 a representation of the situation where the first turbine 1 extracts the maximum amount of energy has been drawn of the wind, that is to say, with an axial induction of 1/3 in theory (and approximately of 0.28 in practice).

The wind 5 has a uniform velocity profile (6) before passing the first turbine 1. After passing the first turbine 1, the speed of the wind 5 blowing through the turbine, decreases substantially in speed, which can be seen from the uniform velocity profile 6 which, after passing the turbine 1, changes to the wind velocity profile 7, 8, in which the central part 8 of the profile represents the substantially decelerated weak air, which extends from the first turbine 1 within the contour 3 in the wind direction, and the outer part 7 of the profile indicates the flow which is essentially not influenced by the first turbine. The difference in speed between parts 7 and 8 of the wind speed profile is large, as a result of which a large amount of turbulence is created. This is disadvantageous because it produces higher fluctuating loads in the second turbine 2 and because more kinetic energy of the wind is lost as heat. The air stream in the central part 8 of the wind speed profile serves as a supply for the second turbine 2 in leeward, which has also been configured to extract energy from the wind in the maximum form. However, it is much smaller because the wind speed in the central part 8 is much smaller than the original uniform speed 6. Behind the second turbine 2 an additional speed profile (9-10-11) is produced in the which outer portions 9 show the last loss of speed, intermediate portions some loss of speed, and additional central portion 11, shows where the velocity has fallen substantially. The additional central portion 11 of the profile represents the substantially decelerated air trail, which extends from the second turbine 2 within the contour 4 in the wind direction. Figure 2 shows, diagrammatically, a plan view of a small wind farm of only two wind turbines. In Figure 2, elements that are identical to the elements in Figure 1 are indicated by the same reference numbers. The same situation as in Figure 1 is shown in Figure 2, but the axial induction of the first turabine 1 has now been reduced according to an illustrative embodiment of the invention. The wind 5 has a uniform speed profile (6) before passing the first turbine 1. After passing the first turbine 1, the speed of the wind 5 blowing through the turbine decreases substantially, which can be seen from of the uniform speed profile 6, which after having passed the turbine 1 changes to the profile of wind speed 7 ', 8', in which the central portion 8 'of the profile represents the steep air gap, which extends from the first turbine 1 within the contour 3 'in the direction of the wind, and the outer portion 7' of the profile indicates the flow which is essentially not influenced by the first turbine. The consequence of reduced axial induction is that the velocity in the central portion 8 'in Fig. 2 is higher than the velocity in the central portion 8 in Fig. 1. The difference in velocity between the portion 7' and the portion central 8 'is also smaller, as a result of which less turbulence is created. The supply for the second turbine 2 is therefore more advantageous because the wind speed is higher and the turbulence is lower. This means a better relationship between performance and loads. In addition, less kinetic energy has been lost from the wind as heat, which is beneficial to the performance of the turbine farm. The second turbine 2 in Figure 2 has been established in such a way that the maximum amount of wind energy is extracted, because there is no additional turbine behind the second turbine 2. An additional speed profile 9 ', 10', 11 'thus occurs behind the second turbine 2. If this profile is compared with the undisturbed supply of the profile with uniform velocity 6, it can be established (although it can not be seen in the figure), compared to the situation in the Figure 1, more useful energy may have been extracted from flow 5 and / or less wind kinetic energy lost. While the invention has been described above with reference to an example, a person skilled in the art will immediately realize that the advantage can also be achieved in other ways that fall within the scope of the appended claims. A person skilled in the art will further understand that the disclosed invention extends to facilities such as turbine farms with underwater turbines that draw energy from a water flow. This flow of water can be a flowing river, a tidal flow and any other water flow that is in the ground from which energy can be extracted. Additionally, it should be understood that the invention may also be employed in conjunction with the technology that has been described in patent application NL 1021078 of the Netherlands. In that case, in an advantageous embodiment according to the invention, the decrease of the axial induction can be combined with the application of transverse forces on the flow, in such a way that rapid air is guided through the farm. Transverse forces, for example, are generated with wind turbines placed at an angle. Another obvious combination is with the angle adjustment of the blades. Both an adjustment that results in the induction in the upper part of the rotor greater than the bottom (stele more advantageous) and a negative cyclic adjustment (lower loads) can be advantageous.

Claims (20)

1. Turbine farm comprising at least a first turbine (1) and at least a second turbine (2) by means of which energy can be extracted from a fluid flow (5), characterized in that when the second turbine (2) is on the leeward side of the first turbine (I), under nominal power, the axial induction (a) of the first turbine (1) decreases with respect to the second turbine (2), rotating the angles of the rotor blades of the first turbine (1) to a position of minimum resistance.

2. Turbine farm according to the preceding claim, further characterized in that the axial induction (a) of the first turbine (1) is reduced to 0.25 or less. 3. Turbine farm according to one of the preceding claims, further characterized in that the axial induction (a) of the first turbine (1) is reduced to 0.25 or less.

3. Turbine farm according to one of the preceding claims, further characterized in that the reduction of the axial induction (a) is further effected by reducing the revolution speed of the rotor.

4. Turbine farm according to one of the preceding claims, further characterized in that the reduction of the axial induction (a) is carried out by reducing the rope of the blades.

5. Turbine farm according to claim 4, further characterized in that at least the first turbine has blades, each blade has a rope characteristic, r, of less than 3.75, where r is a radial distance running between 0.5R and 0.8R, where R is the radius of the rotor.

6. Turbine farm according to one of the preceding claims, further characterized in that a control system is provided, wherein this control system establishes the axial induction of (a) at least one first turbine (1) in the farm as a function of wind direction. Turbine farm according to claim 6, further characterized in that the control system establishes the axial induction of the first turbine (1) on the basis of a turbulence measurement determined in the second turbine (2) which is located essentially on the leeward side of the first turbine (1). 8. Turbine farm according to claim 6 or 7, characterized in that the control system establishes the axial induction (a) of at least one first turbine as a function of the distance to at least one second turbine located in leeward. 9. Turbine farm according to claim 7, further characterized in that the control system optimizes the performance of the farm measured in terms of maximum yield and / or minimum loads by adjusting the axial induction (a) of the individual turbines. 10. Turbine farm according to claim 9, further characterized in that the control system is self-learning. Turbine farm according to one of the preceding claims, characterized in that at least one wind speed in at least one first turbine, essentially located on the windward side of the farm, based on the dominant direction of the wind, differs in terms of axial induction of at least one second turbine, essentially located on the windward side of the farm, on average by more than 0.05. 12. Turbine farm according to one of the preceding claims, further characterized in that the axial force of the entire farm is reduced in such a way that the energy of another farm located to the leeward increases. 13. Turbine farm according to one of the preceding claims, characterized in that the fluid is water, and the turbines are water turbines that extract energy from a water flow. 14. Method for a turbine farm comprising at least one first turbine (1), and at least one second turbine (2) by means of which energy can be extracted from a fluid flow (5), characterized in that the axial induction (a) of the first turbine (1) with respect to the second turbine (2), when the second turbine (2) is on the leeward side of the first turbine (1), under nominal power, turning the angles of the rotor blades of the first turbine (1) to a position of minimum resistance. 15. Design software for a turbine farm comprising at least a first turbine (1) and at least a second turbine (2) by means of which energy can be extracted from a fluid flow (5). 16. Control software for a turbine farm comprising at least a first turbine (1) and at least a second turbine (2) by means of which energy can be extracted from a fluid flow (5), characterized in that the Control software is capable of determining at least one meteorological parameter comprising wind speed and wind direction, temperature distribution and stability of the atmosphere, and determining and setting the power of the turbine farm as a function of at least one of the parameters that can be established, comprising axial induction, revolution speed, rotor blade angle, tilt angle, circulation scale and turbine positions, further characterized because the control software is capable, when the second turbine ( 2) is on the leeward side of the first turbine (1), under nominal power, to decrease the axial induction (a) of the first turbine (1) with respect to the second turbine (2) turning the angles of the rotor blades of the first turbine (1) to a position of minimum resistance. 1

7. Control system for a turbine farm comprising at least one first turbine (1) and at least one second turbine (2) by means of which energy can be extracted from a fluid flow (5), characterized in that the control system is capable, when the second turbine (2) is on the leeward side of the first turbine (1) under niminal power, to decrease the axial induction (a) of the first turbine (1) with respect to the second turbine (2), turning the angles of the rotor blades of the first turbine (1) to a position of minimum resistance. Control system according to claim 17, further characterized in that the control system establishes the axial induction of at least one first turbine in the farm, as a function of wind direction. Control system according to claim 17 or 18, provided with control software according to claim 16. 20. Turbine provided with a control system according to claim 17.