The instrument promises to improve how we understand the effects of the marine environment on the performance of Ocean Renewable Energy (ORE) technologies and operations and therefore contributes to our progress towards a future powered by sustainable energy.
The University of Edinburgh's involvement in the research is led by Dr Brian Sellar and IDCORE EngD student Mairi Dorward, both based in the School's Institute for Energy Systems.
Measuring tidal currents more accurately
This new research seeks to further develop instruments that are presently in use to measure currents in the marine environment which are known as a ‘acoustic doppler current profilers’.
These divergent beam instruments generate acoustic beams that spread out from the sensor. The returning echoes carry information about the current velocity along these beams. Importantly, they rely on the assumption that there is no difference between the marine current velocity measured by each beam at a given water depth. This is typically not the case in the high-speed flow marine environments that are commonly targeted for harnessing tidal energy.
The researchers have developed an arrangement of single acoustic beam instruments – collectively known as a ‘convergent beam instrument’ – to improve the measurement of high-speed currents.
This convergent beam instrument works by generating acoustic beams which meet at a distant focal point. Since the returned signals come from the same 'point' in the flow, it is valid to assume that the velocity measured by each beam is the same. The system’s focal point can be remotely adjusted, a novel feature that enables researchers to measure different regions of the current throughout the 'water column'.
Figure: Divergent acoustic beam instrument (left) and Convergent beam instrument (centre and right) (after Harding et al., 2019 )
The research will help those working on Ocean Renewable Energy (ORE) technologies and operations to better understand and characterise the environmental conditions in which they are used. It will help ORE engineering teams understand how the marine environment affects the performance and survivability of technologies being deployed, enabling improvements in their efficiency and resilience.
More accurate measurements of current will also improve the calibration and validation of numerical and physical models at different scales, giving ORE project developers and other stakeholders better tools to understand and replicate highly energetic marine environments.
Testing the new instrument
The convergent system was built, integrated and dry-tested at PNNL’s workshop facilities in Richland, Washington, USA, before being transported to PNNL’s Marine Sciences Laboratory, in Sequim, where it was deployed in the ocean for two weeks of testing.
Figure: Dry testing of the convergent current measurement system, PNNL, Richland (Image credit: Andrea Starr for Pacific Northwest National Laboratory)
The findings from the field test campaign are being fed into the RealTide project, an EU Horizon 2020 funded research programme which will obtain measurements in high energy tidal flow environments to characterise the environmental stresses that affect tidal turbines. A subsequent version of the convergent measurement system deployed in Sequim will be deployed in the Fromveur Strait, France in 2020.
Commenting on the project, Dr Brian Sellar of the University of Edinburgh said, “A developed ORE sector would bring wide-ranging economic benefits and decarbonised energy systems. The measurement systems developed through the European H2020 project RealTide and our collaboration with PNNL will allow us to observe the marine environment’s complex flows in new ways.”
Figure: Deployment of the convergent current measurement system, PNNL Marine Sciences Laboratory, Sequim
This research was enabled by funding from the Laboratory Directed Research and Development Program at Pacific Northwest National Laboratory, a multi-program national laboratory operated by Battelle for the U.S. Department of Energy.
The research was also supported by The Energy Technology Institute and RCUK Energy programme as part of the IDCORE programme (EP/J500847/1), the RealTide project – a European Union Horizon 2020 research and innovation program (Grant Agreement 727689) – and an International Exchange Grant (2018) from the UK Energy Technology Partnership’s Postgraduate and Early Career Researchers Exchanges scheme.
The research deployed Nortek single-beam Doppler profilers and Arctic Rays Hammerhead Pan & Tilt units.
1. Harding S, Sellar B, Dorward M (2019) Implications of asymmetric beam geometry for convergent acoustic Doppler profilers. 12th CWTMA conference San Diego