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To make wave power a viable energy source, large clusters of wave energy converters should be deployed. For most of the farms the output power of the WECs should be aggregated in a marine substation and then transmitted to the grid. The need for cost effective underwater cable connection operations is one of the main issues in offshore operations. Underwater connections can be conducted with wet- or dry-mateable connectors, performed by divers or ROVs. Although there are existing solutions used by the oil and gas industry that could be employed, the capital expenditure needed is not compatible with the offshore renewable energy industry.

The objective of this research is to decrease costs and minimize working hazards associated with sub-sea work when performing these underwater electrical connections. This article presents a solution using small ROV’s instead of divers to execute the task. The main idea is to perform the connection underwater, but using dry-mateable connectors. A solution to make this possible is to install air pockets at the substation enclosing the connectors. These boxes are meant to be filled with air and hence create a dry environment in which to perform the connections. This is achieved with help of two tools. First a docking system allows the operator to fix the ROV at the substation before doing the connection. Then a gripper tool added to the ROV grasps the cable and connects it to the substation in the air pocket. The procedure and design of this low-cost solution are described, and the different prototypes that have been tested for offshore operation are also shown.

This paper provides a summarized status update ofthe Lysekil wave power project. The Lysekil project is coordinatedby the Div. of Electricity, Uppsala University since 2002, with theobjective to develop full-scale wave power converters (WEC). Theconcept is based on a linear synchronous generator (anchored tothe seabed) driven by a heaving point absorber. This WEC has nogearbox or other mechanical or hydraulic conversion systems,resulting in a simpler and robust power plant. Since 2006, 12 suchWECs have been build and tested at the research site located atthe west coast of Sweden. The last update includes a new andextended project permit, deployment of a new marine substation,tests of several concepts of heaving buoys, grid connection,improved measuring station, improved modelling of wave powerfarms, implementation of remote operated vehicles forunderwater cable connection, and comprehensive environmentalmonitoring studies.

This study presents the dynamic modelling of a linear permanent magnet generator for extracting energy from ocean waves. Translator position, calculated from measured generator voltage, is used as input for the simulation model. Instantaneous power from the simulation model has been compared with the measurements from the Lysekil research site. The power output from the model considering the air gap flux variation is precisely matching with the measured values before core saturation. The generator dynamic model is modified by including the saturation effect. Although a simple mathematical expression is considered for representing the saturation, the model gives accurate power spectrum close to the experimental results. The presented model is a first step towards the system model that can simulate the entire electric system including electric grid. As such, the generator model can be used for further analysis of the wave energy conversion system.

Wave energy comes in pulses and is unsuitable for direct conversion and transmission to the grid. One method to smooth the power is to deploy arrays of wave energy converters (WECs), the geometrical layout and damping optimisation of which many have studied analytically and numerically, but very few by experiments at sea. In this study, the standard deviation of electrical power as function of various parameters is investigated. Two offshore experiments have been conducted. During the longer run, three WECs were operated in linear damping during 19.7 days. It is shown that the standard deviation reduces with the number of WECs in the array up to three WECs. The reduction compared to single WEC operation was found here to be 30 and 80% with two and three WECs, respectively, as a mean for an arbitrary array member. It is found that in sea states above ~2 kW/m, the standard deviation is independent of sea state parameters. This is contradictory to a previous study on the same device. The results are, however, in accordance with numerical results of the SEAREV device but show larger reduction in standard deviation with number of WECs. This could be because of suboptimal damping conditions.

This paper analyzes temperature measurements acquired in the offshore operation of a wave energy converter array. The three directly driven wave energy converters have linear generators and are connected to a marine substation placed on the seabed. The highly irregular individual linear generator voltages are rectified and added on a common dc-link and inverted to 50 Hz to facilitate future grid-connection. The electrical power is transmitted to shore and converted to heat in a measuring station. The first results of temperature measurements on substation components and on the stator of one of the linear generators are presented based on operation in linear and in nonlinear damping. The results indicate that there might be some convective heat transfer in the substation vessel. If high power levels are extracted from the waves, this has to be considered when placing components in the substation vessel in order to avoid heating from neighboring components. The results also indicate that the temperature increase in the linear generator stator is very small. Failure due to excessive heating of the stator winding polyvinyl chloride cable insulation is unlikely to occur even in very energetic sea states. Should this conclusion be incorrect, the thermal conductivity between the stator and the hull of the wave energy converter could be enhanced. Another suggested alteration is to lower the resistive losses by reducing the linear generator current density.

Renewable energy must be stored in order to make itreliable. Flywheels are capable of storing high amounts ofenergy and can also be used as power buffers, due to theirhigh power densities. This paper investigates a way tosmooth the power output from renewable energy converters(wave, wind and marine current) by adding a doublewoundflywheel energy storage to the system. Simulationsshow that a ramp-controlled flywheel energy storage woulddrastically smooth the short time power from a wave energyconverter but not be that appropriate for longer termenergy storage. The power quality enhancement producedby the addition of the flywheel to the system is alsosimulated and discussed.