Orbital’s breakthrough design for tidal power generator uses Xsens motion sensor to help maintain stable position in fast-moving waters

The exploitation of renewable power sources is an essential response to the climate crisis. Solar and wind energy are the giants of the renewable world, but their output is unpredictable and intermittent. From one week to the next, it is difficult to forecast reliably how strongly the wind will blow or how brightly the sun will shine. This is a problem for national electricity grid operators, which are responsible for dynamically balancing the energy supply with the load. 

There is, though, a renewable energy source which is naturally very predictable: tidal power. The relative distances and positions of the sun, moon and Earth all affect the size and magnitude of the Earth’s two tidal bulges. At a local level, the magnitude of tides is influenced by the shape of the shoreline, as well as by weather conditions. Because the tides are driven by planetary movement, their timing can be precisely predicted, enabling power system operators to plan confidently for a tidal power generator’s contribution to the grid. 

This helps explain the growing interest in Orbital Marine Power, a UK-based innovator in tidal power generation. Succeeding where many tidal turbine manufacturers have failed, Orbital has developed a unique floating structure with turbines on each of two retractable legs. Its system off the coast of Orkney in Scotland generates a nominal 2MW output which helps the UK’s national grid to reduce its reliance on fossil fuel power sources. 

The north Atlantic is an unforgiving operating environment for electrical and electronics systems, however – and this is why Orbital relies on a robust Xsens MTi inertial motion sensor to keep the floating turbine structure stable. 
The MTi motion sensor is incorporated in the system as part of a Gable IMU (Inertial Measurement Unit) from Gable Technologies, which created an EtherCAT interface to integrate Xsens sensors seamlessly into industrial applications such as Orbital’s tidal power generator. This combination takes advantage of the accuracy of Xsens sensors, ensuring reliable performance in even the harshest conditions.

Free from the seabed: the key Orbital innovation
The breakthrough that the Orbital design team made came from its observation of previous attempts to fix tidal turbines to the seabed: many companies that adopted this approach ran out of money, strained by the massive cost of hiring construction vessels and building a fixed underwater structure. 
The floating structure of the Orbital O2 generator eliminates these construction costs, and uses a simpler mooring system to hold the hull in place. On each side of the hull is a retracting leg featuring a turbine: this part of the structure can be lowered to the depth at which the most energetic tidal flows are to be found, or raised to protect the structure when extreme weather is forecast.  

For the turbines to work effectively, the structure needs to be stable both in its geoposition, and in its attitude: excessive movement – roll or pitch – reduces the flow through the turbines, and thus affects total power output. It must maintain this stable position in extremely challenging conditions: floating in a narrow channel between islands, where the tidal flows are particularly strong, the Orbital hull is exposed to wind, saves, salt water ingress, and sunlight. 

Stable station keeping– maintaining a fixed position and attitude – in all conditions calls for an array of sensors, including a Doppler sensor to measure the speed of the tidal flow, load gauges on both legs, a GPS satellite sensor for geopositioning, and various weather sensors such as anemometers. 
Alongside these, the job of continuously measuring the hull’s pitch and roll, as well as low-frequency vibration, is performed by an Xsens MTi motion sensor. MTi sensors’ accuracy and performance are renowned – these devices are used in many applications which measure motion with millimetre precision, more than enough for the Orbital application. 

Angel Rua Santaclara, Orbital’s Powertrain Manager, was convinced that the performance of the MTi sensor would meet the tidal power generator’s needs, backed by the comprehensive magnetic calibration capability provided in the MT Software Suite from Movella.  

But he says that equally important was the sensor’s ability to operate reliably in the harsh marine environment. He says: ‘The interior of the hull is dry by design – this facilitates repair and maintenance. But inevitably a floating marine structure is always going to be exposed to salt water and a humid atmosphere, and so every electrical and electronic component we use has to be rigorously specified to withstand these harsh conditions. 
‘That’s why Orbital’s first instinct was to use the specialist motion sensors that are marketed to the oil and gas industry for use in rigs and drilling platforms. But we found that they are over-specified for our requirements – for instance, they can handle operation at great depths. That meant they were also hugely expensive. We were glad to discover the MTi motion sensor family, because it gives us the performance we need. The Gable IMU has an IP67-rated enclosure, which means that it can survive the damp atmosphere in the generator and repeated contact with salt water. The sensor has operated reliably ever since it was installed in our generator off the coast of Orkney.’

Promising future for tidal power generation
Angel Rua Santaclara says that having proved the feasibility of tidal power generation with the installation of the O2, the company is preparing to scale up its operation, building more generators for use in UK waters and elsewhere. He says, ‘Tidal power has its part to play in a carbon-free future for the electricity grid, and it can be deployed anywhere that has strong tidal flows. Our design, including the sensor systems for controlling the motion of the hull, is now proven, and we are looking forward to offering our design all around the world.’ 

To find out more about the Xsens MTi family of motion sensors from Movella, go to www.movella.com/products/sensor-modules. 

To learn about Orbital Marine Power, visit www.orbitalmarine.com. 

To learn more about Gable, visit www.gable-imu.nl