When targeting substantial, potentially dominant optimisation and cost reduction, the categories that contribute the most to the overall techno-economic viability of a wave energy converter (WEC) are of a particular interest.
Overall, the main structure and power take-off system are widely recognised as the main contributors to the Levelised Cost of Energy (LCOE) and the devices’ capital expenditure (CAPEX). The creation of a testing platform addressing the key subsystems of all WEC types and standardised testing procedures is therefore essential for accelerating the devices’ development and reducing their cost.
Drivetrain test rig
The drivetrain rig tests the entire drivetrain of a wave energy converter (WEC), from the input mechanical power to the output electrical, grid-compliant power. This conversion chain includes the following subsystems:
- mechanical drives (gearbox, ball/roller/lead screw, rack-pinion, belt pulley, slider-crank, sprocket chain),
- electrical generators,
- power converters,
- storage systems,
- grid interface units, and
- the control system.
The drivetrain test rig is able to provide either linear or rotary input power with a flexible set-up, in order to adapt the load-speed ratio of the actuation systems to the ones of the device under test.
The rig can also be used in a hardware-in-the-loop (HIL) setup to emulate the interaction of the subsystem/component under test with the rest of the WEC.
The WEC consists of different components: some, like the mooring line, are fixed to the seabed. Some, like the buoy, move freely. The Power Take-Off – or PTO – restricts the movement of the buoy by absorbing the kinetic energy, like a damper in a car. This energy is then turned into electricity, which can be sent to the grid for powering our houses.
The drivetrain test rig tests the PTO by turning the movement between the buoy and the mooring line into data. The real-time data is fed into the drivetrain test rig as actuation input, which replicates the motion that the buoy exerts on the PTO input shaft. The mechanical load from the PTO is measured and sent to the real-time simulator as feedback data.
Structural components test rig
The structural components test rig tests several wave energy converter components/subsystems:
- Structural components (as part of the hull),
- Mechanical interfaces,
- Power cables, and
- Sealing systems.
The structural components test rig can actuate one linear degree of freedom at one end and up to five degrees of freedom at the other end (three linear and two rotary). Thanks to the test cell, loads, bending moments and torque can be provided by on-purpose arrangements according to the load application.
Environmental conditions can be replicated by submerging the test sample in synthetic sea water. These features allow for the dynamic testing of large-scale components with realistic motions or loads in a wet environment.
The rig can also be used in a HIL setup to emulate the interaction of the subsystem/component under test with the rest of the WEC.
In a real environment, WECs need to be able to endure many years of operating under external variable forces, such as waves, tidal currents and wind. The structural components test rig assesses the structural integrity of different parts of the WEC. As with the previous test, actuation data is fed into the rig and turned into mechanical loads that recreate the stresses coming from the sea environment.
These tests can focus on different components: the structural parts of the freely moving buoy, including sealing systems, the mooring lines and the power cables.
As with the drivetrain test rig, the results are turned into measurements and fed back into the real-time simulator.
Dual Hardware-in-the-Loop (DHIL)
The ocean is a renewable energy resource with many possible ways of exploitation. In wave energy applications, the energy naturally contained in waves is converted into green electricity. To do this, we use wave energy converters (WECs). However, while WECs have developed a lot in recent years, more work is still needed for the sector to reach its full potential.
The IMPACT project aims to develop and demonstrate a new testing approach for WECs called “dual hardware-in-the-loop” (DHIL).
We start with a high-performance PC, called a real-time simulator. We input environmental data to simulate the environmental conditions, and then WEC data to simulate the type and characteristics of the WEC we want to test.
Testing both rigs simultaneously
Individually, the tests on each rig are both examples of hardware-in-the-loop (HIL). However, in IMPACT, these two tests are run simultaneously, and the data collected from both is used collectively to close the simulation loop and influence the next round of tests. This is known as dual hardware-in-the-loop (DHIL).
This technique is an innovative and beyond start-of-the-art approach that makes use of the advantages of hardware-in-the-loop while incorporating the interdependencies between subsystems as a fundamental feature in order to de-risk WEC development. Therefore, the Horizon 2020 IMPACT project created standardised testing procedures involving the dual hardware-in-the-loop that address the development of key parts for all WEC types while increasing their technical maturity as a whole.
This technique is essential for accelerating WEC development, reducing technology costs, and meeting the energy goals of the European Commission.
