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Development and validation of technical and economic feasibility of a multi MW Wave Dragon offshore wave energy converter

Final Report Summary - WAVE DRAGON MW (Development and validation of technical and economic feasibility of a multi MW Wave Dragon offshore wave energy converter)

The WAVEDRAGON MW project aimed to improve a pre-existing offshore wave energy converter device, developing it from the already tested scale prototype to a full size composite 7 MW unit and to validate its technical and economic feasibility through testing. In that way a technological basis for an innovative, commercially viable solution to the generation of renewable power would be provided.

The Wave dragon is the largest wave energy converter known today. Its design consists of two parabolic arms that reflect and enlarge waves towards a ramp. Wave energy is absorbed passively by the overtopping water which is collected and stored temporarily in a reservoir behind the ramp. The device could, because of its large size, act as a floating foundation for MW wind turbines, thus contributing to annual power production at a marginal cost. Among its numerous novelties is that the turbines are the only moving part of the structure, thus facilitating maintenance. Large scale devices could be produced and installed resulting in power stations of increased capacity.

The quantified expected results of the converter launch, referring to a 24 kW/m wave climate and compared to the available technology at the project beginning, were the following:
1. higher energy production of each unit to a total of 10 GWh/y resulting in a 12 % total improvement;
2. 5 % reduction of the overall installation capacity cost and
3. 5 % reduction of the operation and maintenance costs.

WAVEDRAGON MW had the following targets in terms of the converter improvement:
1. development of the optimal construction way;
2. finalisation of the power takeoff system;
3. combination of steel and reinforced concrete for the device construction;
4. deployment of the device in relatively calm water in order to monitor its hydraulic behaviour prior to final deployment;
5. development of an operation and maintenance scheme and actual operation of a device;
6. performing of extensive tests on the device to acquire information and scientific knowledge;
7. establishment of the social and economic impact of the technology.

Tests on the pre-existing scale model proved that the conceptual design was sound and therefore the 300 m wide demonstrator could be constructed; problems related to project financing occurred though and resulted to the construction being postponed. Attention was hence given to the improvement of the conceptual design. Manufacturing on a floating barge appeared as the most appropriate construction process, with some device parts being cast in-situ and others being prefabricated and floated to the assembly site. Other construction methods could also be applied, depending on the site conditions. In order to reduce the forces acting on the Wave dragon, a slack mooring solution was adopted, consisting of six uniform and evenly distributed mooring chains at a circular mooring spread. The solution was evaluated using an artificial load time series due to waves, current and wind. A significant reduction of the forces applied on the structure occurred.

The lack of marine experience was an important hurdle during the scale-up of the power take-off (PTO) system. A direct drive asynchrony generator along with traditional gearboxes was studied, but gearboxes turned out as being neither practical nor reliable. Further customisation appeared necessary in order to fulfil Wave dragon control requirements. In terms of the connection grid, various alternatives were examined and efforts aimed at solving problems due to limited export capacity of the grid connection.

The evolvement of the existing operational and maintenance schemes so as to meet the device requirements was necessary. Work included deployment of the necessary instruments, outline of the data acquisition, outline of monitoring and management systems and development of improved algorithms for buoyancy control. Given that no major problems occurred throughout the process it was estimated that the systems could be successfully implemented once the device was built. In addition, technical solutions were found for all minor component failures that were observed during the scale prototype real sea testing.

Estimates regarding productivity were based on the prototype data. A production of around 6.50 GWh/y seemed feasible in any case, with possibility of reaching a value of 12 GWh/y.

The necessity to have continuous data stream, along with the fact that various transducers of the prototype failed, led to the redundancy being defined as of vital importance for the monitoring system. It was proposed to be ensured by either using the available information in numerous estimates or by using numerous identical transducers, connected to different electrical groups.

An environmental impact assessment (EIA) was carried out, considering potential impacts on the physical, biological and human environment. No significant impacts have been identified, with the exception of potential harm on some marine seabed habitat due to cable installation. Through careful selection of the installation sites though the overall impact could be reduced to an acceptable level.

Construction of the Wave dragon, once promoted, is expected to take place at a port close to the deployment site. Various alternative civil engineering methods could form an option, such as using a sufficiently large dry dock, using a floating steel barge moored in the harbour, employing onshore sites for components manufacturing and then assembling them using pre-stress techniques or building the entire unit on a slip way and launching it into the sea. After the assembly of the device mechanical and electrical parts could be installed.

The operational system was not finalised, given that the construction and operation were postponed. A standalone monitoring system, separate from the main SCADA and management tools was considered beneficial for scientific monitoring purposes. Further development of an existing software tool began as part of the project, in order to support continuous data logging and analysis.

Power production and grid connection were examined during the project execution, with research related to other neighbouring fields being postponed. The effect of the buoyancy control performance on the power production proved to be important; thus a new dynamic model and a control algorithm were developed and tested. Non-symmetrical voltage dips were the most doubtful issue and several compensation methods were presented to overcome the problem through use of numerous instruments. The connection to the existing infrastructure was studied and, at the closure of the project, was under evaluation by the local grid operator at the site of future construction. A conclusion was to be reached in relation to the design of the PTO system.

Data from the wind power industry was employed to investigate the potential for jobs creation. Studies showed that the jobs per MW did not deviate much between different countries.

Finally, a life cycle analysis (LCA) was carried out for the project, with the environmental footprint of the converter being calculated. Its outcome, currently based on estimated and prototype data, will be revised after the deployment of the Wave dragon.

Dissemination of the project results was overall successful, with intense interest from research institutes and the press. The delay in converter construction resulted however in postponing the technology exploitation, even though this was among the initial project objectives.