Renewable Energy Sources (RES) as alternative forms of energy supply are increasingly attracting the global interest since they are inexhaustible, pollution-free reducing the impact on natural environment. Wave and tidal energy have the highest energy densities among the other RES. However, despite that the electricity generation from marine technologies (i.e., wave and tidal) increased 33% from 2019 to 2020 [1], current and wave power conversion technologies are at an immature stage of development, remaining far from being aligned with the Sustainable Development Scenario [2] which requires annual growth of 23% through 2030.

Towards, inversing the situation and supporting the growth and development of the ocean energy sector, the European Commission launched a strategy [3] to harness the potential of offshore renewable energy. The strategy maps out a path to replace fossil fuels by offshore renewables, setting a framework for the development and uptake of the ocean energy technologies in order to contribute around 10% of EU power demand by 2050, which nowadays becomes increasingly important as Europe faces upon a possible energy supply reduction due to the current geopolitical situation.

The main obstacle in harvesting the wave power is the high energy cost. Especially, for the deep-water offshore locations in which the wave energy potential is much higher than the one at shoreline and nearshore locations, the high construction and maintenance costs due to the challenging ocean environment and the increased water depth, are hampering development efforts. Thus, technological advancements in improving the efficiency of Wave Energy Converter (WEC) technologies and optimizing the mooring characteristics, play a key role for the development of the offshore wave energy industry, aiming at reducing the Levelized Cost of Electricity (LCOE) value in order to be competitive among the other renewable technologies.

The ETHOS project aims at developing, optimizing, and assessing the main features of a novel type of an Oscillating Water Column (OWC) device for offshore wave energy exploitation, starting from a hollow cylindrical floater, moored with a TLP (tension-leg platform) mooring line system –this system is considered efficient in terms of WEC’s wave power absorption rate, but unprofitable in terms of the cost for the construction and installation of the mooring lines in deep waters– and evolving to alternative hull designs (consisting of several modular concentric chambers – see Figure 1), moored with multi-leg mooring arrangements. In this respect, the proposed study in ETHOS represents a departure from the classical designs developed so far for the offshore wave energy harvesting, by exploiting possible synergies and advances through the fluid’s oscillatory motion in multiple oscillating chambers of a common infrastructure in an efficient manner. In order to optimize the OWC components, the dynamics of the floating system will be evaluated through a coupled hydro-aero-dynamic analysis that accounts for the hydrodynamic analysis of the floater, the characteristics of the air turbine(s) placed in the oscillating chambers, along with the characteristics and specifications of the mooring system, seeking for maximum wave energy absorption at the installation location. All the above are validated under scaled down experiments of a physical OWC prototype within the framework of the ELIDEK basic research principles.

The work foreseen in ETHOS comprises the following steps:

• Considering a representative installation location in the Aegean Sea combining a high wave potential to maximize the power output, with the ability of different infrastructure needed to support the ETHOS project;
• Developing of a novel design of a WEC device based on the oscillating water column principle. The examined OWC device consists of several coaxial oscillating chambers. The wave action generates the oscillatory behaviour of the fluid inside the multiple chambers activating a Power Take Off –PTO mechanism (i.e., air turbine) converting the wave power into electrical;
• Optimizing the mooring-line-characteristics to maximize the captured wave energy by the floating structure;
• Constructing the integrated scaled down ETHOS model (indicative scale 1:20-1:30), and conducting scaled down physical model tests in the wave tank, to validate the theoretical formulation;
• Disseminating and communicating of the results of ETHOS in the scientific community and to the public to assess stakeholders’ acceptance and public preferences for offshore wave energy technologies.

The ETHOS project has a total duration of 18 months and is structured into two main phases. The overall methodology encompasses numerical simulations, experimental testing, and administrative tasks, including the project management and coordination. The time-schedule is appropriate structured to meet the requirements of the project, taking into account the interrelation of work packages and the scheduling of experiments, so that their completion is done a reasonable period of (4 months) before the end of the project. More specifically the proposal comprises:

• Improved design of a novel type of OWC device and its mooring system;
• Advanced numerical calculations to simulate the power efficiency of the OWC device under wave loading conditions;
• Manufacturing of the scaled-down physical model of the OWC to be tested in the wave tank, simulating its dynamic behavior for the operational and extreme sea state conditions at the selected installation location in the Aegean Sea;
• Dissemination and communication activities of the project outcomes.

ETHOS will advance the OWC hydrodynamic modelling using analytical and numerical methodologies to capture the wave-body interaction phenomena. The applied modelling will also allow a reliable estimation of the alternate pressure head inside the chambers, which in extreme environments, may lead to slackening conditions for a TLP mooring system configuration [4]. As a result, the definition of proper mooring characteristics shall be taken into account at the earliest phases of the design process.

Coupled hydro-aero-dynamic analysis of the complete OWC system (moored device with the PTO air turbine) will be conducted. The equations of motion of the system will be solved in the frequency domain to predict responses and rotations of the device, the air volume flow in the device, and the loads on the mooring lines. The effect of the OWC characteristics (e.g., pressure head in the chambers) on the motions of the moored system and on the dynamics of the mooring lines, which may affect the seaworthiness of the ETHOS structure will be carefully evaluated.

In the foreseen methodology the following computational tools will be applied:

Computational Tool – HAMVAB: HAMVAB (Hydrodynamic Analysis of Multiple Vertical Axisymmetric Bodies) is a potential flow solver primarily developed at National Technical University of Athens (see Ref. [5]-[8]) that tackles the linearized and quasi-static second-order hydrodynamic interaction problem between waves and vertical axisymmetric bodies of arbitrary shape being either free-floating or moored and encompassing OWC chambers in their interior. Analytical representations of the diffraction-, motion- and pressure- depended radiation-velocity-potentials around each body are established in form of Fourier-Bessel series, offering fast solutions of the associated hydrodynamic problems. The first-order loads, hydrodynamic parameters, inner air pressure, air flow rate inside the OWC, motions and mean drift-loads on the devices are calculated in monochromatic and irregular seas.

Computational Tool – MIKE 21 MA: MIKE 21 MA commercial software, powered by DHI [9], allows to perform highly precise moored vessel response assessment for single and multi-vessel systems. The software will be used to check and validate the analytical results of the mooring configuration for the OWC device.

Computational Tool – DESCABLE:  DESCABLE is a dynamic analysis solver for the design of multi-leg mooring systems with submerged attached buoys and combination of materials. The software evaluates the required mooring line length, diameter, and weight per length for given external forces and water depths. DESCABLE, which is developed at National Technical University of Athens, has been extensively used in numerous research projects over the last years, whereas its predictions in offshore applications have been verified successfully with other commercial tools [10, 11].

For the experimental tests in the wave basin, foreseen in ETHOS, the following instrumentation will be used:

• Two standard wave probes of wire type, one located near the wave maker while the other located in front of the OWC device will be used to measure the amplitudes of the waves generated by the wave maker;
• For the measurement of the water free surface, inside the OWC device, wave probes will be used, located at the top of the chamber. The elevation of the internal surface will be obtained on the basis of these elevation measurements;
• The pressures inside the OWC domes will be measured by differential pressure gauges, using a circular end-tube averaging the air pressures of four equally spaced points located at the perimeter of the OWC dome;
• The motions of the floating OWC system will be captured by tracking the positions of eight point-targets through the Qualysis Motion Capture optical recording system. The latter metric system has been successfully applied in a variety of research projects (i.e., [12] to name a few) with great measuring accuracy.