Power Performance of the Wave Energy Converter in a Combined Wind and Wave Concept and its Survivability
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The spar torus combination (STC) concept consists of a spar floating wind turbine and a torusshaped heaving-body wave energy converter (WEC). The torus can move along the spar cylinder to absorb wave energy. The 5 MW NREL reference wind turbine is supported by the spar floater, and a delta mooring system is used in the STC model. Model tests on the STC concept in extreme conditions have been performed in the towing tanks at MARINTEK, Norway, as well as at INSEAN, Italy. Two survival modes were proposed and tested in both tanks. In one survival mode, the torus WEC was fixed to the spar, and the whole structure was floated at the mean water level (designed draft) of the WEC, named the MWL mode. In the other mode, the torus WEC was fixed to the spar, and then, the whole body was fully submerged to a specified position by additional ballast, named the SUB mode. Large motion and interface forces between the two bodies were observed in the MWL mode with strongly nonlinear phenomena, i.e., slamming and green water. In the SUB mode, the motion and interface forces were significantly reduced. A detailed power assessment of the WEC is necessary under operating conditions. The concept of a torus WEC on a fixed cylinder is first studied in the frequency domain to obtain optimum power take off (PTO) damping under different regular wave conditions, and then, the time domain model of the STC is studied. In this paper, the power performance of the WEC in the STC is investigated by applying different PTO parameters to obtain the annual mean absorbed wave power. Several performance indicators for the WEC, such as the capture width ratio, annual mean power and annual mean power per characteristic mass, are selected. Survival criteria for the WEC performance in severe sea states are defined, and the influences of the survival criteria on the WEC power and performance indicators are also studied. The wind power should also be considered in the cost indicators, but this is not covered in this paper.