The present invention relates generally to hydroelectric turbine installations. More particularly, this invention pertains to hydroelectric installations with means for introducing oxygen containing gas into the water passing through the installation.

Dissolved oxygen is required to sustain aquatic life. Depending on the species, the threshold value may be 6mg/l of dissolved oxygen or greater. In warmer periods with little river flow the dissolved oxygen stratifies in the reservoir upstream of the turbine and turbine inlet flow could be very low in dissolved oxygen (in some extreme cases levels of 0 mg/I have been recorded).

The level of dissolved oxygen in water passing a hydroelectric turbine installation depends of course on the amount of oxygen containing gas which is introduced into the water passing through the installation. Therefore the focus of prior art solutions for enhancing the amount of dissolved oxygen is on establishing means to ensure that the amount of introduced gas is high enough. WO 2019/179742 A1 describes a runner of a hydroelectric turbine or pump with improved level of dissolved oxygen when backpressure increases. This is achieved by altering the geometry near the trailing edge of the runner to create a local drop in pressure on the trailing edge surface. The described runner comprises openings in the trailing edge surface to admit gas to the passing fluid during operation of the runner. The profile of the suctions side surface at the location of the openings is concave.

The inventors realized that there are other factors besides the amount of introduced gas which can influence the level of dissolved oxygen. When the gas is introduced into the water bubbles are formed. Oxygen is dissolved into the water by crossing the surface of the bubbles. For a given amount of gas introduced into the water the dissolving rate of oxygen will be proportional to the total surface of the generated bubbles. It is clear that the total surface of the generated bubbles is larger for smaller bubbles compared to larger bubbles. But the task is not done, when the generated bubbles are small at the point of time when the bubbles are formed. Bubbles in uniform water flows have the tendency to coalesce and to form in this way larger bubbles. Since the water flow in hydroelectric turbine installations at flow rates close to the optimum efficiency point is very uniform the effect of bubbles coalescing has a big negative impact on the level of dissolved oxygen. Since the main purpose of such hydroelectric turbine installations is to maximize electric power production, the operation conditions are normally controlled in a way to be as close to the optimum efficiency point as possible.

The objective of the present invention is to increase the level of dissolved oxygen downstream of the turbine over the level of dissolved oxygen achieved by state of the art when the turbine is operating at flow rates close to the optimum efficiency point. The problem is solved by a hydroelectric turbine installation and a method according to the independent claims. Other favorable implementations of the invention are disclosed in the depended claims.

The invention will hereinafter be described in conjunction with the appended drawings:

Fig.1 Hydroelectric turbine installation according to the present invention;

Fig.2 Another embodiment of the present invention;

Fig.3 Perforated cover plates;

Fig. 4 Flow chart of the method according to the present invention; Fig. 5 Data carrier.

Figure 1 displays very schematically a hydroelectric turbine installation according to the present invention. The installation comprises a turbine designated as 1, a water passage located upstream of the turbine 1 and designated as 2, a draft tube located downstream of the turbine 1 and designated as 3 and means for controlling the operation conditions of the turbine 1 designated as 4. The installation comprises means for introducing oxygen containing gas into the water passing through the installation during operation of the turbine 1 . These means are designated as 6. Means 6 can be located in any part of the installation such as turbine runner, wicked gates, draft tube 3 etc. The installation comprises further means for injecting water into the draft tube 3, whereas the injected water passes through one or more openings in the wall of the draft tube. In Figure 1 the means for injecting water into the draft tube 3 are designated as 7. The one or more openings are located downstream of the means 6 for introducing oxygen containing gas into the water passing through the installation.

The installation comprises further means for controlling the flowrate of the water injected into the draft tube 3. These means comprise a control-unit designated by 5 and a valve. The control-unit 5 is designed to control the flowrate of the water injected into the draft tube 3 by controlling the valve position. By changing the valve position the control-unit 5 is varying the flowrate of the water injected into the draft tube 3. The control-unit 5 is further designed to control the flowrate of the water injected into the draft tube 3 in a way that the flowrate is a function of the operation conditions of the turbine, whereas the flowrate of the water injected into the draft tube 3 is higher when the operation conditions of the turbine are at or near the optimum efficiency point compared to operation conditions being away from the optimum efficiency point. To establish such a control of the flowrate of the water injected into the draft tube 3 the control-unit 5 is connected to the means 4 for controlling the operation conditions of the turbine 1 . Of course means 4 and control-unit 5 can also be combined forming a single steering unit of a hydroelectric turbine installation according to the present invention.

The inventors have realized that by injecting a jet of water into the draft tube at operation conditions at or near the optimum efficiency point the smooth flow that occurs at these operation conditions is disturbed in order to shear air bubbles into smaller bubbles and cause re-circulations that will increase bubble travel time and thus increasing dissolved oxygen for a given gas/water volume fraction.

Figure 2 shows another embodiment of the present invention. The means 7 for injecting water into the draft tube 3 comprise a water channel linking the water passage 2 located upstream of the turbine 1 to the draft tube 3. The water channel is designated as 9. The means 7 for injecting water into the draft tube 3 comprise further a valve, which is designated by 8 and which is located near the branching point from the water passage 2. The valve 8 can be located anywhere within the water channel 9. The purpose of the valve 8 is to control the water flow through the water channel 9. At the outlet of the water channel 9 into the draft tube a perforated cover plate is located, which is designated as 10. For the sake of simplicity means 4 for controlling the operation conditions of the turbine 1 and the control-unit 5 are not shown in figure 2.

When the valve 8 is opened high pressure water from the water passage 2 enters the water channel 9 and is injected into the draft tube 3 via the one or more perforations of the cover plate 10. In this way one or more crossflow water jets are directed into the draft tube flow to cause the desired flow disturbance.

It is of advantage that the outlet of the water channel 9 is located on the upper part of the draft tube 3 since this generates a larger volume of disturbed flow in the draft tube by impacting the downstream flow characteristics and thus has a larger impact on bubble shear and bubble travel time within the draft tube.

The water flowing through the water channel 9 into the draft tube 3 does not participate in the production of electric energy. It is therefore of advantage that the valve 8 is only opened when crossflow water jets are needed to increase the level of dissolved oxygen above the desired value. This is the case when the operation conditions of the turbine 1 are at or near the optimum efficiency point. The control-unit 5 is designed to control the flowrate of the water injected into the draft tube 3 in a way that the flowrate is a function of the operation conditions of the turbine. The simplest function achieving the desired behavior is therefore a plateau-shaped function varying from zero flowrate to a maximal flowrate (plateau value), whereas the plateau is located in a region of operation conditions around the optimum efficiency point. The function could also be a smooth function varying from zero flowrate to a maximal flowrate, whereas the maximal flowrate is reached at least at the optimum efficiency point. Alternatively the maximal flowrate could also be located at an operation condition corresponding not exactly to the optimal efficiency point of the turbine (that means, that the flowrate exactly at the optimal efficiency point could be slightly smaller than the maximal flowrate). In any case according to the present invention there has to be one or more operation conditions of the turbine corresponding not to the optimal efficiency point of the turbine, where the flowrate set by the control-unit 5 is lower than the flowrate set by the control-unit 5 at the optimal efficiency point of the turbine.

Figure 3 shows cover plates in two different embodiments according to the present invention. In the embodiment shown on the upper part of figure 3 the cover plate comprises a plurality of perforations which are shaped like conical nozzles whereas the axes of the cones (dashed lines) are oriented perpendicular to the inner surface of the cover plate. The phrase inner surface relates to the surface of the cover plate which is orientated towards the inner of the draft tube. One of the perforations is designated as 11. The injected water jets will enter the draft tube in the direction of the axes of the cones. In the embodiment shown on the lower part of figure 3 the axes of the cones are inclined to the perpendicular direction by a non-zero angle. In this way the injected water jets can have a velocity-component which is orientated up- or downstream of the water flow within the draft tube. It is also possible that the injected water jets can have a tangential velocity-component related to the water flow within the draft tube. The embodiments shown in figure 3 are examples of a large number of possible embodiments according to the present invention. In other embodiments the perforations 11 could be for example shaped differently. In other embodiments the axes of the perforations 11 could be non-uniform ly orientated to establish different entrance angles for the different injected water jets. Of course the surface of the cover plate facing inside the draft tube has not to be perfectly flat. It is of advantage that this surface of the cover plate forms a smooth junction with the inner surface of the draft tube.

Figure 4 shows a flow chart of the method according to the present invention. The method is a method for operating a hydraulic turbine installation according to the present invention as described in the passages above. The method comprises at least two steps which are designated in figure 4 as S1 and S2. In step S1 the operation conditions of the turbine are set by the means for controlling the operation conditions of the turbine. In step S2 the control-unit sets the flowrate of the water injected into the draft tube whereas the flowrate is a function of the operation conditions set in step S1 as described above.

The present application is also related to a computer program designed to perform the steps of the described operation method. The present application is also related to a data carrier on which the computer program is stored. Figure 5 shows a data carrier, which is designated by 12.

List of references

1 Turbine

2 Water passage located upstream of the turbine 3 Draft tube

4 Means for controlling the operation conditions of the turbine

5 Control-unit

6 Means for introducing oxygen containing gas into the passing water

7 Means for injection water into the draft tube 8 Valve

9 Water channel

10 Cover plate

11 Perforation

12 Data carrier