Wind farm layout

The decision for a wind farm layout is an optimization problem that depends on different constraints. The optimal layout must be a trade-off between those constraints, which are typically technical feasibility, lowest cost, least power losses and the compliance with specific site requirements. The main factors that influence the decision are described next.

Wake is one of the primary concerns in the design of the offshore wind farm layout. It can be defined as the region of circulating wind flow behind the moving rotor blades of a wind turbine. By the rotation of the blades energy is extracted from the incoming wind stream and a turbulent slowed down wind is released in the wake area. The extracted energy is called wake loss and is a typical phenomenon of a rotating wind turbine. The wake effect is highly important for the layout of a wind farm since the turbines are affected by each other. Each turbine will slow down the incoming wind by extracting energy and pass a lower wind speed to next turbine in downwind direction.

The affected turbine will further extract energy of the wind, which causes a sequential reduction in wind energy along a wind flow direction and thus causes a decrease of power generation from one wind turbine to the next. Besides the power loss, wake increases also the turbulences in the rotor with corresponding consequences for the fatigue strength of the turbines [34]. A visualization of the wake effect is presented in Figure 2.18.

Figure 2.18: Wake visualization [54].

The blue colored wind streams represent the areas affected by turbulences and reduced wind speed. The turbines in a wind farm are not online dis- turbed by neighbor wind turbines but also by other wind farms nearby. This will become more important in the light of a denser construction of offshore wind farms. The wake effect and associated power losses can be reduced by increasing the distance between turbines and optimizing the operation of the wind turbines. It is common for BOWTs to have a distance of 5 to 9 rotor diameters to each other in the prevailing wind direction and about 3 to 5 rotor diameters in perpendicularly wind direction [55]. For floating offshore wind the distance between turbines will not only be influenced by wake but also by the applied mooring system. The catenary and taut-leg system that possess a larger footprint will certainly require a greater distance between the substructures than the tight tension-leg system. However, an increased distance between turbines will lead, on the other hand, to more cost for inter-array cables [29].

The site conditions are of great importance and need to be taken into account in the wind farm layout. For example wind turbines are certified according to extreme wind speeds. Therefore, the wind speeds need to be measured in order to select the required class of wind turbines [34]. Fur- thermore, the soil conditions are crucial in order to identify the required anchor system. In particular, for FOWTs the met-ocean conditions, such as wind and waves, are important to consider for a proper design of the floating concept and the wind farm layout [56]. The direction of the wind speed is also an essential factor for the layout definition. The orientation of the wind farm is set towards the prevailing wind direction, which is the direction of the highest occurrence of winds. This will enable generally the highest power production [57].

Permits have a large impact on the layout design especially in the develop- ment phase of a project. They may restrict the dimensions and boundaries of the site or place limits to the maximum capacity, tip height or rotor di- ameter of the wind turbines. Furthermore, restrictions may occur regarding the use of certain technologies or type of foundations. Permits are generally based on an impact assessment that evaluates for example environmental issues, grid constraints, and maritime requirements. It is essential for the success of a project to know well the permits in order to develop a fully compliant layout [34].

Finally, cost matters as always in commercial projects. Most of the con- straints can be translated into cost. For example, if different water depths are present at a site different mooring lengths or technologies need to be ap- plied, which causes a variation in costs. Another example of cost is the case of increasing power cable length due to the wake effect reduction. This would result in a higher investment as well as an increase in operation and mainte- nance cost. Nowadays, computer software such as Windpro and Windfarmer are available that can design wind farm layouts taking into account different constraints and produce cost optimal solutions [17].

Despite the above mentioned constrains, time has also an influence on the design. At the beginning of a project a simple layout is designed based on the current technology, which is used for evaluating the feasibility of the project and applying for tender. It may take several years from the first offer to the final decision in the tender process and the permission to manufacture and construct. During this time, new technologies have been developed or requirements have been changed. Hence, an updated layout is required, which comes typically with a detailed engineering of the wind farm. The layout will be improved regarding the latest know-how and requirements in order to gain an optimal solution.

A further requirement to the layout is the consideration of shipping lines for the maritime traffic as well as the flying routes of birds and crossing routes of sea animals [17]. Finally, the location of the offshore substation has to be considered in the layout, which has an influence on the inter-array cable layout and installation costs. The substation can be located inside or outside of the wind farm collection grid. Project developers will likely look for the most economical solution since offshore platforms are large and involve high transport and installation cost [5].