Torque 2012 - The Science of making Torque from Wind, Oldenburg, 9-11 October 2012 200 300 400 500 600 700 800 0 2 4 6 8 10 12 rotor speed ωωωω [rpm] r o to r to rq u e ττττ [ N m ] D nozzle = 1.24 mm D nozzle = 1.32 mm D nozzle = 1.35 mm D nozzle = 1.42 mm D nozzle = 1.51 mm D nozzle = 2.07 mm
As alternative to geared and direct drive solutions, fluid power drive trains are being developed by several institutions around the world. The common configuration is where the wind turbine rotor is coupled to a hydraulic pump. The pump is connected through a high pressure line to a hydraulic motor and (synchronous) generator. However, in the concept presented here the high pressure line connects the pump to a nozzle.
Introduction
Transient Results
The response of the experimental power transmission system, i.e. rotor and hydraulic circuit using the passive control method is inherently stable for all nozzles used in these experiments. However, the pump used in the experiments was not designed for speed below 500rpm. Its start-up torque makes it unsuited for this type of application outside of a test environment. The effect of hydraulic losses is minimal at pump rotation speeds greater than 500rpm
Steady-State Results
Conclusion & Outlook
The presented concept of passive control for variable speed rotors using fluid power technology works as predicted. The torque envelope of a conventional variable speed wind turbine drive train can be achieved for a hydraulic transmission through solely passive control. An optimally sized constant cross-sectional area of the nozzle results in optimal performance below rated wind speed. The activation pressure of the relief valve effectively determines the rated wind speed.
The presented and proven concept of passive torque control could lead to innovations in other wind energy technologies such as water desalination and water pumping applications where the need of converting wind energy into electricity is eliminated.
In June 2012 patent request P31185NL00/MVE was filed by the TU Delft for this control method.
If one connects a horizontal-axis wind turbine rotor with fixed blade pitch angle to a fixed displacement hydraulic pump and then connects the high pressure line to a nozzle with fixed cross-section, the torque and speed of the rotor are passively controlled through the nozzle-induced pressure. By sizing the nozzle correctly, the rotor will operate at (or near) maximum efficiency (CP,max).
N.F.B. Diepeveen, A. Jarquin Laguna
Offshore Engineering Section, Delft University of Technology
Wind tunnel experiments to prove a hydraulic passive
torque control concept for variable speed wind turbines
Acknowledgements
We thank Dennis Uneken, Anton Kempenaar, Koen Hermans, Nico van Beek, Nando Timmer, Ruud van Rooij, Joost Sterenborg and Nienke Hosman for their assistance with the experiments at the Open Jet Facility of the TU Delft.
Experimental Setup at the Open Jet Facility of the TU Delft
The volumetric flow created by the pump is considered to be proportional to the rotational speed of the rotor. The nozzle-induced pressure is proportional to the square of the flow. Thus the pressure across the pump, and consequently the transmitted torque is ideally described as a function of the square of the rotational speed of the rotor. For variable speed turbines, the same relationship of aerodynamic torque and rotor speed is required to extract the maximum energy from the wind.
A pressure relief valve is used to limit the pressure in the line above certain limit. In this way the transmitted torque is kept constant above rated wind speed. Hydraulic braking or shutting down the rotor is done by means of a manual valve which completely cuts off the flow and increases the pressure.
Results
What We Set Out To Prove
The nozzle converts the high pressure/low speed flow into low pressure/high speed jet, i.e. hydrostatic to hydrodynamic power. Converting the power in the jet to electricity can be done using an impulse hydro turbine, such as a Pelton turbine. However, this was beyond the scope of these experiments.
200 300 400 500 600 700 800 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 rotor speed ωωωω [rpm] p o w e r c o e ff . C P [ -] Hydraulic Brake
To demonstrate an emergency shutdown the manual valve was used to stop the flow in the high pressure line. When the valve is being closed the pressure in the system builds up. The required rotor torque thus increases and the rotor slows down to an eventual stop.
Preset Torque Limit
The significance of this demonstration is that such a simple and highly reliable component is able to limit the pressure (i.e. torque) to a preset maximum (here around 35 bar). By setting the activation pressure of the relief valve correctly, the torque envelope of the hydrostatic drive train can be rendered the same as for an electrically controlled drive train.
Experimental setup in operation
Example of a measurement sequence
The lid of the water reservoir The experimental setup in the wind tunnel
Hydraulic diagram of the experimental setup
Rotor
Diameter: 1.8m airfoil: NACA 4212
Water pump
Vol. displ.: 12.5cc/rev max pressure: 160bar speed: 500-1800rpm 4 6 8 10 12 200 300 400 500 600 700 0 5 10 wind speed U ∞ ∞∞ ∞ [m/s] rotor speed ωωωω [rpm] r o to r to rq u e ττττ [ N m ]
rotor torque envelope
transmission torque envelope line through filtered data points
