Tidal turbine development in Ireland and Canada


Novel sensors aid tidal turbine development

A few months ago we reported on an application on harnessing electrical power from the sea out in Galway Bay on the west coast of Ireland. Today we have a report from the other side of the country, Greenore, at the mouth of Carlingford Lough in Co Louth. This company is also working on using the tides but is different in that their generators are completly submerged at the sea bed.

Non-contact torque sensors from Sensor Technology are playing a key role in the development of commercial-scale in-stream tidal turbines produced by OpenHydro. The company is using these novel sensors, which are based on surface acoustic wave (SAW) technology, to accurately measure rotational speed and frictional forces in a simulator for the turbine bearings, thereby allowing it to optimise the performance and reliability of its innovative products.

OpenHydro is a technology company that designs and manufactures marine turbines to generate renewable energy from tidal streams. The company’s vision is to deploy farms of tidal turbines under the world’s oceans, where they will dependably generate electricity with no cost to the environment. This method of producing electricity has many benefits.

Because the turbines are submerged, they are invisible and they produce no noise. And because they are submerged at a considerable depth, they present no hazard to shipping. An advantage that is possibly the most important, however, is that the tides are completely predictable, which means that the energy output of the turbines is equally 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. Clearly, installation in such an inaccessible location makes reliability a prime consideration in the design and construction of the turbines. For this reason, OpenHydro carefully and comprehensively 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 provided an ideal solution.

Like all TorqSense sensors, the RWT320 units depend for their operation on surface acoustic wave (SAW) transducers. These transducers comprise two thin metal electrodes, in the form of interlocking “fingers”, on a piezoelectric substrate such as quartz.

When an RF signal of the correct frequency is applied to the transducer, surface acoustic waves 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 drive shaft, 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 radio frequencies, it is easy to couple signals to them wirelessly. Hence 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.

“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 set up,” said Kevin Harnett, Mechanical Engineer at OpenHydro.  “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. This is a very straightforward and convenient arrangement.”

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, RPM, temperature and power. It also provides facilities for realtime plotting and for data recording, and can output stored results as files that are compatible with Matlab and Excel.

“We have found both the sensor and the software very easy to work with,” said Kevin Harnett, “and the sensor has proved itself to be well able to withstand the tough operating conditions in our laboratory. We’ve also received excellent technical support from Sensor Technology, which was very helpful as we have never previously worked with sensors of this type. Overall, we’re very happy with product and the service we’ve received, and the sensor is providing invaluable data for our development work.”

Proof that this development work is yielding dividends was amply provided late in 2009, when OpenHydro deployed the first commercial-scale in-stream tidal turbine in the Bay of Fundy, Canada, on behalf of its customer, Nova Scotia Power.

This 1 MW unit was arrived on site on 11 November and was operational, rotating with the tides, collecting data and producing energy by 17 November.

Power from the sea


Floating off the Conamara Coast looking west on Galway Bay

Prototyping a Wave Farm Energy Converter Using LabVIEW, Compact FieldPoint and CompactRIO

By Eugene Doogan, Wavebob.

Ní minic a bhíonn seans againn tuairisc ar thógra inár áit dúchas. Ach seo tógra i gCois Fhairrige atá thar a bheith spéisiúil.

Just eleven miles east of the Read-out offices is a winking presence bobbing on the waves of Galway Bay a little distance from the shore. This article from Eugene Doogan of Wavebob tells us what’s its doing and how they keep tabs on what is happening out there.

How Wavebob works!

The Challenge:
Developing a control and data acquisition system for a wave energy converter (WEC) to achieve efficient power extraction in varying sea conditions.

The Solution:
Rapidly creating a highly integrated, rugged system for real-time control and data acquisition for a WEC prototype using NI LabVIEW, Compact FieldPoint, and CompactRIO.
” In particular, the versatility, speed, and simplicity of coding in LabVIEW, as well as excellent diagnostic and debugging tools, made it an obvious choice.”

Wave Energy
Since 1999, Wavebob Ltd, one of the world’s leading wave energy technology companies, has been developing a prototype WEC for deployment in offshore “wave farms” that are similar to wind farms. Our goal is to develop a commercial WEC that can produce significant electrical power for the onshore grid on coastlines with a suitable wave climate.

Invented by Irish physicist William Dick, the Wavebob WEC is a unique dual-body point absorber in which the two bodies move relative to ocean waves and to each other. The two bodies are coupled by hydraulic cylinder pumps, which are used to extract power from the relative motion. This part of the WEC is known as the power takeoff (PTO).

WEC development involves trials at different scales: at small-scale in-wave generating tanks (one one-hundredth to one-tenth) and then a larger scale (one-fifth to one-half) with fully operating PTO systems. The development team administers trials with the small-scale WECs in a sink, bath, or pond. When all trials are complete and successful, the team builds a full-scale WEC prototype.

To control the PTO in extreme sea conditions while maintaining efficient power extraction, the WEC requires a rugged and sophisticated control system. In addition, each stage of product development has its own requirements for the data acquisition and supervisory control system, which changes throughout the development cycle.

Hardware and Software Selection
The WEC prototype trials aim to successfully demonstrate the Wavebob WEC technology and gather data, which would inform the design of a full-scale Wavebob WEC.

A control and data acquisition system for the trials required real-time control of hydraulic valve switching according to sensor input, as well as data acquisition from a variety of sensors at appropriate sample rates. The requirements are similar to those of many industrial controller applications, but also include the unique challenges inherent to operating in varying sea conditions. These include operating in a marine environment, consequent dynamic effects on equipment, operation from DC source (24 VDC batteries with charging systems), the need for deterministic control (real-time OS), and relatively high-channel-count data acquisition and digital I/O.

In addition, the WEC prototypes include a variety of sensors, and the digital I/O includes solenoid switching with significant power requirements. Rapid code development, easy-to-modify control software, code versatility, and standard interfacing are essential.

After extensive research into the various options on the market, LabVIEW graphical design software was a natural fit for the PTO control system. In particular, the versatility, speed, and simplicity of coding in LabVIEW, as well as excellent diagnostic and debugging tools, made it an obvious choice. In addition, the range of hardware available from NI and its seamless integration with LabVIEW offered real benefits to the project.

The team selected LabVIEW coupled with Compact FieldPoint and CompactRIO for the control and data acquisition system to achieve the following benefits:

  • Hardware/software integration
  • Rapid development using LabVIEW
  • Real-time hardware and OS
  • Compact, rugged, and adaptable hardware
  • Upgrade path and distributed system capability
  • Excellent technical backup, particularly via National Instruments website

Control and Acquisition System – WB 06-07
During the first two trial phases for the Wavebob WEC, the team used Compact FieldPoint with LabVIEW and LabVIEW Real-Time. While the system performed extremely well, both the complexity of the control requirements and channel count increased in the subsequent development phase (MK3). As a result, the limits of the test system were approached on the second prototype. The MK3 prototype would require more processor power and faster acquisition rates.

MK3 Prototype
The scale of the WEC prototype in this phase of development, also known as the MK3 prototype, is a quarter of full scale with a fully operable PTO. Given the increasing complexity of control and additional sensors, the team selected CompactRIO hardware, LabVIEW, LabVIEW Real-Time, and the LabVIEW FPGA Module. In addition, they constructed a one-seventeenth scale model for rapid trials of structural and other changes. We had to build the new control and acquisition system into this model for data acquisition.

We selected CompactRIO for its integration with LabVIEW as well as its processing power and acquisition rates. The hardware offers the ability to run control and data acquisition loops at much faster sample rates without compromising the timing integrity of the system due to processor overload. All control and I/O functions can be programmed on the field-programmable gate array (FPGA) in the CompactRIO backplane and can run simultaneously. The controller only has to read the resulting data when logging. The small footprint and low power consumption of the CompactRIO system also facilitated incorporation into the one-seventeenth scale model.

Just as in the previous prototypes, LabVIEW was the ideal choice due to its tight integration with the selected hardware. The graphical programming language is easy to use, versatile, has a myriad of modules and tools available (including the ability to target a real-time embedded OS), and very good support.