The specific features of the VLH turbogenerator unit make it on principle particularly respectful of fish populations. As we were aware of this advantage, we have centred the mechanical and hydraulic development of the VLH on an optimization of these intrinsic features.
Thus, from the beginning of the design by Pr. Kueny of the INPG (refer to professional partners) of the hydraulic profile of the wheel and distributor assembly, fish friendliness criteria to be optimised have been introduced and given a degree of priority over pure energetic performances.
We have thus tried to identify and quantify objective criteria enabling defining where and how a hydroelectric turbine can be fish friendly. We have for this purpose used existing scientific studies and especially those carried out by the USA Department of Energy (Idaho National Engineering and Environmental Laboratory) which has selected a list of criteria likely to qualify the degree of fish friendliness of a turbine.
At the end of the CFD studies, the values of the criteria obtained by the VLH turbo-generator unit have been compared with those retained by the laboratory.
Runner mesh of a 3 D CFD
Diagram of pressure variation
As a summary, a comparison between the value of the retained criteria and the corresponding value obtained by the CFD simulation is disclosed hereafter.
|Speed at the blade periphery||from 6 to 12 m/s||4.5 to 8 m/s|
|Minimum encountered pressure||69 kPa||94 kPa|
|Maximum pressure gradient||550 kPa/s||80 kPa/s|
|Maximum speed gradient||180 m/s/m||10 m/s/m|
|Clearance between blade and mantle||2.0 mm||4.5 mm|
The VLH turbo-generator does not fulfil this last criterion. However, this search for a minimum clearance between blades and wheel mantle is only justified inasmuch as wheel crossing velocities are high and as the fish cannot use its directional power. The latter essentially depends on its swimming velocity as compared to flow velocities in the machine. Further, the new hydraulic design implemented from the end of 2009 incorporates a discharge ring and a hub of spherical shape, which enables to decrease the blade tip clearance and to maintain it whatever the blade opening position.
It should be reminded that the wheel crossing velocity in the VLH turbine is, in the case of the Millau prototype, 1.85 m/s at the maximum flow rate. It exceeds twice this value in the IDAHO laboratory machine (> 4 m/s). In these last conditions, the fish which is swept along into the hydraulic passage can only follow the flow threads, its travel path being imposed to it. If it is close to the wall of the runner discharge ring, it will have more difficulty to draw away from it than in the case of the VLH turbo-generator. Hence the relative importance of the blade/discharge ring clearance criterion. Live fish crossing tests have proven the relevance of this assumption (refer to fish friendliness test).
The analysis of the fish friendliness criteria shows that the VLH turbine is, from a qualitative point of view, far below the maximum values considered as acceptable (except for the blade end clearance criterion).
This study and its conclusions have been incorporated to the request for administrative authorization for the Millau demonstration site. They have been accepted by the relevant administrations. Due to the results of the downstream migration tests carried out in situ (refer to fish friendliness test), the Millau site is definitely exempt from downstream migration devices and fine rakes.