Final LiftWEC wave energy concept moves forward to tank trials

The partners in the EU-funded €3.4 million LiftWEC project have selected the final concept for a wave energy device, based on hydrodynamic lift forces, that will be put through its paces at the wave tank in Ecole Centrale de Nantes.

The LiftWEC project aims to develop a novel type of wave energy converter whose primary coupling with the waves is through the generation of hydrodynamic lift on a series of rotating hydrofoils.

The system consists of one or more hydrofoils, driven by the waves to rotate around an axis orthogonal to the direction of waves.

Through the interaction with lift forces, the LiftWEC device takes advantage of its ability to exploit the unidirectional motion of the forces. In addition, the lift-force can easily be reduced so that the concept can survive storms in the same way that modern wind turbines survive, by stopping turning, according to developers.

Following the experimental testing of a two-dimensional LiftWEC concept and analysis of the results, the final concept has now been developed and is due to be tested in the Hydraulic and Offshore Engineering wave tank (HOET) in Ecole Centrale de Nantes (ECN), the project partners have informed.

The project is being developed by Queen’s University Belfast, acting as project coordinator, with partners Hamburg University of Technology, National University of Ireland Maynooth, Innosea, Aalborg University, University College Cork, LHEEA Centrale Nantes, Julia F Chozas Consulting Engineer, WavEC, and the University of Strathclyde.

The project partners have developed a physical model of the device, whose specifications were derived through discussions with the partners involved in the numerical modelling work package, considerations of the prototype characteristics, testing facilities, and ECN’s physical model design and testing know-how.

The main purpose of the experimental model is to represent the behavior of the final LiftWEC concept.

The test model is designed at 1:20 scale, which is the largest scale that can be tested in the tank, based on tank dimensions and wave capabilities.

The 3D nature of the tests implies that the most appropriate testing facility is a wide tank, such as HOET, which is 50 meters long, 30 meters wide and has a depth of 5 meters.

The tank is also equipped with several gantries, to allow staff access to models and support instrumentation, signal conditioning and acquisition materials.

The test setup installed at HOET includes the tripod structure, six-degree motion hexapod and the LiftWEC 3D model assembly.

The model allows testing of configurations with one or two foils and with adjustments of the foil angle of attack between tests, according to project partners.

The model is to be tested in the HOET under a hexapod capable of six-degree actuation. The idea behind this approach is to use the hexapod to test a fixed model with variable depth and orientation and to test a model moving in a controlled motion cycle to simulate a floating model.

Beaneath the water surface, the rotor is held horizontal, and in the main configuration, parallel to the wave crest.

The motion of the rotor is controlled using a power take-off (PTO) system consisting of an electrical machine which can be operated in position, speed and torque control.

The quantities measured are the PTO torque, radial and tangential loads on the axis of each foil, absolute angular position of the rotor and wave elevation upstream and downstream of the model, according to LiftWEC project partners.

The foils are made of a urethane machinable tooling block material Labelite, produced by the company Sika. Using this material, the foils are said to be structurally strong enough to minimize deformation while enabling the mass to be as close as possible to the displaced mass of water, so that it is neutrally buoyant.

From the measured quantities, rotor velocity and acceleration as well as captured power can be inferred, according to project partners.

The objective of the LiftWEC project, running until the end of November 2022, is to determine the potential of a wave energy device based on hydrodynamic lift forces concept to produce renewable energy at a commercially competitive price while ensuring minimal environmental and social impact.

This will be achieved by a combination of numerical and physical modelling and desk-based studies of the structural design, the operational and maintenance requirements, and the environmental and social impacts of the technology.

The numerical and physical modelling are expected to demonstrate the concept’s performance, thereby taking the concept to technology readiness lever (TRL) 4 – the level confirming the technology has been validated in lab, while the desk-based studies will allow the socially-acceptable commercial potential to be determined.

Wave energy is one of the few untapped sources of renewable energy that could make a significant contribution to the future energy system.

However, a study of the literature and a patent search indicates that of the hundreds of concepts that have been developed, only four couple to the waves through lift forces, whilst the rest couple through diffraction or buoyancy forces.

Coupling through lift forces is said to have the significant advantage of reducing extreme loads (by reducing the circulation like a wind turbine) which facilitates survivability, and produces unidirectional rotation, which simplifies power extraction.

Unfortunately, none of the current lift-based wave energy concepts have a high efficiency in all sea-states due to difficulties in maintaining a good lift-to-drag ratio, according to project partners.

The novel ideas developed in the LiftWEC project are designed to achieve this, and therefore could accelerate the commercial development of wave energy, the project partners expect.

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