Using LabVIEW and NI Data Acquisition Hardware to Prototype Tidal Stream Turbines for Power Generation

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"We chose LabVIEW as the interface because it is regarded as a data acquisition industry standard. As with all computational programs, there is a steep learning curve; however, once the basics are understood, development and productivity rapidly accelerate."

- Pascal Galloway, Sustainable Energy Research Group, University of Southampton

The Challenge:
Synchronising real-time strain-gauge data with a digital shaft encoder to characterise tidal stream turbine loading in dynamic ocean conditions.

The Solution:
Using NI LabVIEW software and an NI USB-6210 M Series multifunction DAQ device to acquire analogue and digital counter data and provide a user interface for real-time load and acceleration visualisation.

Author(s):
Pascal Galloway - Sustainable Energy Research Group, University of Southampton
Dr. Luke Myers - Sustainable Energy Research Group, University of Southampton

Tidal Stream Power

Our seas and oceans contain a huge untapped kinetic energy resource in tidal streams. New technology broadly based on the operational characteristics of wind turbines is presently being developed to exploit these streams for electricity generation.

Tidal stream turbine (TST) technology, is currently at the prototype stage, where unique devices are being deployed at specific sites or marine energy testing centres. The co-existence of waves and currents is a common feature in a marine environment, but there is little detailed knowledge of flow field properties at highly energetic tidal sites. It is unclear what effect uncharacterised fluid forces have on TSTs, which may lead to prototype devices being installed at sheltered locations where these effects are minimised. If this timid installation strategy becomes a trend with developers, it may result in reduced energy capture, as blade diameters are constrained at these locations and potentially higher energy flows are not utilised.

Operating a turbine driven by a tidal stream presents major advantages over those driven by air due to the water density of the water (and, therefore, the power it can deliver. It also presents significant challenges. TSTs typically experience four times the thrust of a wind turbine rated equally in power, even though TSTs are significantly smaller in diameter. It is expected that rotor loading and general structural integrity could be a driving factor in TST device design. Therefore, blade and rotor loading caused by wave-current interaction must be quantitatively assessed.

Experimental Process

At present, few experimental wave-current studies have been conducted in the presence of TSTs. A 1/20th scale tidal turbine model during early development stages to prove the concepts and pave the way for large-scale models and sea trials. The model can measure rotor thrust and torque (using a custom waterproof dynamometer), blade root bending (with a built-in strain gauge) and rotation rate (using a shaft encoder). Varying parameters include tip-speed ratio, turbine yaw, and turbine submergence depth.

We used an NI USB-6210 to acquire the data and calibrated it using LabVIEW. We applied external filtering to remove extraneous signal noise then compared this data to a numerical code developed for load prediction.

Using the USB-6210 and LabVIEW made the data acquisition process straightforward. USB-6210 counter inputs enable easy connection of a digital shaft encoder, where the alternative was interpreting binary or hexadecimal information (via another USB connection) and synchronising several COM ports on a bench PC. LabVIEW acquired, calibrated, synchronised, displayed and recorded amplified voltage signals from the strain gages and wave probe. We chose LabVIEW as the interface because it is regarded as a data acquisition industry standard. As with all computational programs, there is a steep learning curve; however, once the basics are understood, development and productivity rapidly accelerate. This is facilitated by the high level of technical support which is provided by the engineers at National Instruments, both on-site and via phone and email.

The prototype showed that waves are likely to have a detrimental impact on TSTs. This can be largely mitigated in terms of power output by tuning the on board power electronics using the predictive numerical model created with this prototype to smooth the power/flicker. The main issue with wave-current interaction around a TST is the cyclic loading, which will likely result in accelerated rotor and blade fatigue, especially with flapwise blade bending. Another important consideration is whether a rotor yaw drive is required at any specific tidal site. Large amounts of directional swing occurs around headlands and can cause a significant power reduction and increase in dynamic loading if a yaw drive is omitted. Our continuing research will assist in the structural design of TST blades and maximise rotor diameter to achieve a robust, high energy yield device.

Author Information:
Pascal Galloway
Sustainable Energy Research Group, University of Southampton
University Road
Southampton SO17 1BJ
United Kingdom
p.w.galloway@soton.ac.uk

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