Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE credits
The expansion of the Swedish hydro power has contributed positively to the reduction of the environmental impacts from the fossil fueled power production. In the same time the extensive expansion has been proved to cause negative ecological and local environment impacts of a stream. This, in turn, has been affecting the biodiversity and fish stock negatively, which has been reduced. One of the underlying problems is the lack of possibility for the fish to pass the plants, but it is also directly correlated to the mortality rate of the fish passing through the turbine, which this study is investigating.
The mortality rate has been computed for three different locations in Sweden, Untra, Tännfallet and a third one whose location is confidential, and is denoted as Case 1, 2 and 3. The model used to calculate the mortality rate in this investigation is based by existing blade strike models and knowledge and are investigating the death due to mechanical injuries from collision with a runner blade in the turbine. The model is valid for Kaplan- and Francisturbines and is taking technical design, physiological and behavioral parameters, in consideration.
In the first case, Untra, the mortality rate is lower for the Kaplan turbines than for the Francis turbine that has been under investigation. However the Francis turbine has lower mortality rate at low loads and the result also indicates that a Kaplan turbine with greater diameter has lower mortality rate than a smaller one. In case 2, Tännfallet, the Kaplan turbine would give 50 % lower mortality rate than a Francis turbine with a corresponding drop and water intake and at full load. At part load the mortality rate is 42 % less.
Case 1 and 2 indicates that a Kaplan turbine is more fish friendly than a Francis turbine, but Case 3, indicates the opposite. The mortality rate in the old Francis turbine was 0 % according to the calculations. This could be explained to the low relative velocity, which is partly a result of the small scale turbine. The relative velocity is in this case is well below 10 m/s and a collision with the runner blades has not necessary a deadly outcome.
In all cases the trash rack is of great importance to prevent a fish with high probability to die in the turbine to pass. In Untra a rack with a gap width of 20 mm is recommended. This would prevent 25 cm long fish and 80 cm long eels to pass. In Tännfallet the current rack that has a gap of 25 mm is considered to be sufficient and will prevent a 30 cm long fish and 90 cm long eels. In Case 3 an 18 mm wide rack is recommended and if choosing that the mortality rate in Case 3 will be at most 26 % if a fish is passing and 70 % for the eel, these with a length of 25 and 70 cm respectively.
The results from the case study and the analysis indicates that the mortality rate due to fish passing through the turbine can be reduced. Partly by choosing a turbine that is more fish friendly designed, but also by using optimally designed diverters as trash racks. In general the mortality rate increases with the length of the fish and with a reduction in the turbine dimensions, but at relative velocities below 10 m/s the mortality rate are reduced. This occurs in more extent in Francis turbines, but in general the Kaplan turbine has a lower mortality rate. Also to be mentioned, there are a lot of uncertainties when calculating the mortality and the technical parameter that has the most influence to the mortality rate is the relative flow angle (runner blade angle). Another uncertainty is the behavior of the fish. For example, the mortality rate is much greater for a fish passing at the top of the impeller in a Francis turbine than one passing at the bottom.
Hopefully, the knowledge given in this report will contribute to future investigations where the best possible technique to reduce the mortality of a fish passing through the turbine is to be done, and thereby promote both the environmental and economic sustainability of the hydro power.
2014. , 89 p.