Torque Sensors Aid Tidal Turbine Development
The company aims to deploy farms of tidal turbines under the world's oceans, where they will generate electricity with no cost to the environment. This method of producing electricity is said to have many benefits. For instance, as the turbines are submerged, they are invisible and they produce no noise. In addition, since they are submerged at a considerable depth, they present no hazard to shipping. The tides are completely predictable, which means that the energy output of the turbines is equally as predictable.
There are no large seasonal variations and no dependence on the vagaries of the weather, as there are with many other renewable energy sources. Reliably and efficiently harvesting energy from the tides, however, requires the use of novel technology and, in the case of Openhydro, this takes the form of open-centre turbines that can be deployed directly on the seabed. Installation in such an inaccessible location makes reliability a prime consideration in the design and construction of the turbines.
For this reason, Openhydro evaluates the performance of all of the components used in its turbines. For the bearings, this evaluation involves the use of a simulator that allows the company's engineers to determine how frictional forces in the bearings vary with different loads and rotational speeds. Central to the operation of this simulator is the measurement of torque in a shaft from the motor that drives the bearing under test. With conventional sensors, it is hard to carry out this type of torque measurement accurately and reliably, but Openhydro found that Sensor Technology's Torqsense RWT320 series sensor was suitable. Like all Torqsense sensors, the RWT320 units depend on SAW transducers.
These transducers comprise two thin metal electrodes, in the form of interlocking 'fingers', on a piezoelectric substrate such as quartz. When a radio-frequency (RF) signal of the correct frequency is applied to the transducer, SAWs are set up and the transducer behaves as a resonant circuit. If the substrate is deformed, however, the resonant frequency changes. When the transducer is attached to a driveshaft, the deformation of the substrate and hence the change in resonant frequency will be related to the torque applied to the shaft. In other words, the transducer operates as a frequency-dependent strain gauge.
Since the transducers operate at RFs, it is simple to couple signals to them wirelessly, according to Sensor Technology. Torqsense sensors can be used on rotating shafts and can provide data continuously without the need for the inherently unreliable and inconvenient brushes and slip rings often found in traditional torque measurement systems. Kevin Harnett, mechanical engineer at Openhydro, said: 'We chose the RWT320 because of its convenient wireless operation and because it was easy for us to fix in line with an existing shaft in our experimental setup.
'In addition, this model of sensor has integral electronics and a serial output, which means that we can link it directly to a laptop computer in our test laboratory,' he added. Openhydro uses the RWT320 sensor in conjunction with Sensor Technology's Torqview software. This offers a choice of dial, digital-bar and chart-graph format display for torque, revolutions per minute, temperature and power.
It also provides facilities for real-time plotting and for data recording and can output stored results as files that are compatible with Matlab and Excel. 'The sensor has proved itself to be well able to withstand the tough operating conditions in our laboratory,' said Harnett. In late 2009, Openhydro successfully deployed the first commercial-scale in-stream tidal turbine in the Bay of Fundy, Canada, on behalf of Nova Scotia Power. This 1MW unit arrived on site on 11 November and was operational, rotating with the tides, collecting data and producing energy, by 17 November.
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