There is a micro Kaplan hydro plant:
Flow rate: 0.58 mc/s Head: 4.85 m RPM: 625 Max electrical power: 16.5 kW Generator+gearbox efficiency: 0.89 Maximum Turbine efficiency: 0.67 Draft tube: vertical/conical, 1.5 m high
The problem is the decrease in electrical power (measured during operation) with the time up to an asymptotic value, although all the external conditions (head, flow rate, rpm, blades configurations) remain constant.
I summarize here better, by using a picture (in attach). The picture depicts the power in kW versus the time in minutes.
Let's suppose that the turbine is switched off. Then it is switched on and the ELECTRICAL power reaches in few seconds 16.5 kW. But, as you can see from the picture, then it starts to decrease approaching, over three days, 13.65 kW, and remains at 13.65 kW. Then, suppose that it is again switched off and switched on (process that last few minutes, not again plotted in the figure), again the ELECTRICAL POWER reaches 16.5 kW as at the beginning, but then it decreases approaching 13.65 kW (after 3 days) and remains 13.65 kW. If we repeat again the switch on/off (process done in few minutes), again it reaches 16.5 kW, and then decreases up to 13.65 kW. If the turbine is not switched on/off, the power remains 13.65 kW “forever” ….. until a new switched off/on is made. Therefore, the switched off/on process last few minutes, while the decreasing trend until the lowest power value of 13.65 kW lasts few days.
Have you got general ideas?
1) Formation of a big vortex in the draft tube. If this would occur, the decreasing trend should last few minutes (the time needed for the vortex development), and the power would immediately approach 13 kW. This phenomenun should last few minutes, not days. Furthermore, by CFD simulations, the vortex in the draft tube is not powerful, and at the outlet of the draft tube flow velocities are vertical.
2) Possible inclusion of air from the conical (vertical) draft tube towards the turbine. The draft tube is immersed of slightly more than 1.1 D into the downstream water level (D is maximum turbine diameter), as recommended by engineering practice. From CFD simulations flow velocities are downward, so this point should be ok.
3) Possible electro-mechanics/electrical load problem.
4) Cavitation: during operation there are not noise, neither appreciable vibrations, but there is hub erosion due to cavitation. But cavitation should reduce the maximum power output, not generate a decreasing trend during the time.
Thanks
Hi Emanuele Quaranta
If there is a guide blade, maybe their attack angle changed in 3 days and set in steady angle. If this problem cause is electrical, transient time is less than a minute. if it causes is hydrodynamic, transient time is less than a hour. so I think cause of that is mechanical mechanism.
Good Luck
Interesting! Where exactly are you measuring the power output? Is there some sort of electronic controller on the turbine? How are you measuring the power? Does the generator produce a lot of harmonics? Exactly what is the load made of? What is the power factor? Do any of these factors also change over time in the same way? You should look into these points and make sure that what you are reading on your instruments is exactly what you believe it to be! Best wishes!
Hi Emanuele Quaranta
If there is a guide blade, maybe their attack angle changed in 3 days and set in steady angle. If this problem cause is electrical, transient time is less than a minute. if it causes is hydrodynamic, transient time is less than a hour. so I think cause of that is mechanical mechanism.
Good Luck
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