Massive production of low-cost and environmentally friendly energy
An ORE farm consists of a set of energy production units exploiting one or more offshore renewable energy sources on an offshore concession area. The main objective of a farm is to maximise energy harvesting in space and time, while ensuring availability at minimum cost. To achieve this, the ORE farm should be considered as a system, and not as a simple aggregation of several production units. Its design requires a multidisciplinary scientific approach, called systems engineering, which aims to establish an overall architecture that allows the best compromise between cost and availability while respecting predefined constraints.
Offshore farms, a complex challenge of design and optimisation
There are many disciplines involved in designing a ORE farm. Engineering disciplines include the mechanics, aerodynamics, control, electrotechnics, materials science and civil engineering. In the hard sciences, this includes mathematics, physics, chemistry, geotechnics, atmospheric dynamics, oceanography, and biology. Finally, in the social sciences, economics, politics, law and sociology are needed to understand the interactions between the farm, society and the energy market. All these disciplines are involved in the design of subsystems of an ORE farm. To illustrate the concept of competing design objectives, consider maximising annual energy production (AEP) and minimising the levelized cost of energy (LCOE). To maximise AEP, it is tempting to define a high density of turbines in the area to be operated, which significantly increases the cost of the farm, and ultimately the LCOE. Conversely, minimising the LCOE to the extreme can lead to almost zero energy production. It is therefore necessary to find a compromise using optimisation methods. France Energies Marines, its members and partners are working on the optimisation of the inter-array cables and the possibilities of mutualisation of anchors (MUTANC project).
Towards joint optimisation methods and tools
A state of the art of the optimisation methods currently used and the software available today was carried out as part of an R&D project led by France Energies Marines (VALARRAY project). The easiest method is sequential optimisation. It consists in optimising the subsystems one after the other, each one taking into account the constraints inherited from the previous design. The open source software suite DTOcean+, developed within the framework of a European project (DTOCEANPLUS project) involving 17 partners including France Energies Marines, is based on this method. This digital tool aims to design, optimise and evaluate ORE systems, from the component to the farm scale. Our team has developed the modules dealing with site characterisation, design of foundations and moorings as well as evaluation of societal and environmental acceptability. Sequential optimisation is intuitive and easy to implement. However, it has the limitation of not systematically exploring all possible design solutions. For this, the use of algorithms allowing a joint optimisation, leading to a global optimum, becomes necessary. This is one of the reasons why the DTOcean+ software suite is developed in a modular way: this allows the code of the different elements of the suite to be adapted to apply it to joint optimisation methods. The quantification of uncertainties in the models used in the design must also be controlled and documented. The DTOcean+ software suite meets this need by taking into account different levels of analysis complexity.
Environmental integration, a goal of integrated design at the farm scale
Beyond the optimization method, environmental integration is also a major theme to consider when designing an ORE farm. One of the modules of the DTOcean+ suite developed by France Energies Marines addresses this issue. It allows the environmental and socio-economic evaluation of the different technological choices and array configurations. For each operation in the life cycle of a project, the potential impact is assessed in terms of existing pressure (e.g. risk of collision with the marine megafauna), life cycle analysis (e.g. carbon footprint assessment) and social acceptability (e.g. economic benefits through job creation). It is an original tool in this field, which is subject to future developments in order to gain in precision. The theme of environmental integration applied to the design of ORE farms should continue to grow strongly in the future.
Photo credit: Anna Axelsson / AdobeStock
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