Green energy from green water


Wave energy represents a significant untapped


energy source. As Professor Jo Hermans from the University of Leiden says: “Worldwide the economi- cally exploitable amount of wave energy is estimated at 2,000 TWh/year, an


average power of 200GW over a year. This is quite a lot: the equivalent of 200 large power stations.” But the challenge is to gener- ate a predictable amount of energy, in a reliable way, at a reasonable cost.


T


he challenges of wave energy are very similar to those of the offshore industry: safe and economic design,


production, transportation, installation, maintenance, repair and removal. That is why MARIN decided to use its expertise to further develop this type of renewable en- ergy. For this MARIN used an ‘Inverse con- cept’ - inversing the objectives of offshore engineering. Instead of reducing the mo- tions and green water of ships and offshore structures, the concept maximises the mo- tions and green water as a means of ex- tracting energy from waves. This initiative was dubbed the “Green Water Concept”.


The concept works following these steps: 1. Through maximised pitch motions, the bow makes large vertical motions relative to the seabed, to which it is connected with a wire. The wire moves relative to the structure and can be attached to an electrical generator (first Power Take Off).


2. At the same moment, waves exceed the freeboard and green water flows onto the deck. Green water comes from the front and sides and forms a high velocity water jet. This concentrated jet, together with the upward pitch motions, allows the green water to flow into a higher reservoir at the centre of the structure.


3. The green water in the reservoir then flows back into the sea through low water head turbines (second Power Take Off).


The concept was initially developed using diffraction theory and VOF simulations with the ComFLOW method. In May, the first successful model tests were carried out, including the modelling of an electrical and hydraulic Power Take Off (PTO).


With the Green Water Concept’ MARIN wants to stimulate the development of wave energy. Therefore, the Dutch maritime and offshore industry was invited to take over the further development in the RENEW-ABLE project. Now the complete range of companies needed to make wave energy a success are working together, including Bluewater En- ergy Services, Damen Shipyards, Heerema Marine Contractors, Huisman Equipment, Imtech Marine & Offshore, Meteoconsult and TU Delft.


MARIN will stay involved as advisor: apply- ing its knowledge but also learning more about the challenges of wave energy. This way we will be ready to give an independ- ent advice to any company that wants to bring this form of renewable energy a step further.


Bas Buchner b.buchner@marin.nl 16 report Phases of the ‘Green water concept’ and the model before it went into the basin


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