Capacity Densities of European Offshore Wind Farms

Questions this practice may help answer

• What are the factors influencing the design of an Offshore Wind Farm (OWF)?
• What is the capacity density (installed capacity per km2) of OWFs in the North Sea and Baltic Sea regions?
• What is the influence of technical-economic issues on the capacity density and design of an OWF?
• What is the influence of political and institutional issues on the capacity density and the design of an OWF?

Implementation Context

The Baltic Sea countries aim at achieving greater transnational coherence of energy infrastructure in order to ensure efficient and sustainable use of the Baltic Sea space. One of the questions to solve in this context is the extent of sea space that is required to install a certain capacity of offshore wind energy and related decision factors for the capacity density of offshore wind farm (OWFs). To address this issue the Federal Maritime and Hydrographic Agency of Germany, BSH, tendered a study dealing with the prospective capacity density of European offshore wind farms. The study has been conducted by Deutsche WindGuard GmbH.

Aspects / Objectives

• Estimate the capacity density (MW per km2) for future offshore wind farms in the Baltic Sea Region
• Analyze the influence of the different national regulatory frameworks on the capacity density

Method

This report analyzes capacity density on three levels:

• Reviews of international assumptions for the determination of capacity densities at a theoretical level.
• The planning and national regulatory frameworks within the Baltic Sea and North Sea regions.
• The implementation level and analysis of existing offshore wind farms with respect to their capacity densities.

Main Outputs / Results

Current international assumptions / research

The report describes the most important definitions and mathematical formulas for capacity density:

• The capacity density of a wind farm is defined as the ratio of the wind farm’s rated capacity to its ground area. Capacity density is expressed in megawatts per square kilometer.
• A wind farm’s capacity density alone does not lead to conclusions on its energy production. This requires the wind farm’s capacity factor to be taken into account, which is usually calculated as the yearly averaged power production divided by the rated power production.
• Specific power defines a turbine’s specific rated power capacity per rotor area. It is expressed in watts per square meter. It depends on the power capacity of the wind turbine and the rotor diameter (m).

• The distances between neighbouring wind turbines in a wind farm defines a wind farm’s turbine spacing. Usually, turbine spacing depends on the prevailing wind direction. Turbine spacing is a critical issue because of the wake effect. Wind turbines that are placed within the wake of a neighbouring wind turbine will produce less power than under free-stream conditions. The wake of an offshore wind farm can extend for tens of kilometers (see picture).

• Studies in European sea regions conclude that the capacity density is in the range of5 MW/km2 to 5.4 MW/km2
• Future industry trends, such as taller wind turbines and better placing and reduction of the specific power per wind turbines (reducing the influence of the wake), will increase the capacity factor of the wind parks.
• Policy frameworks and offshore regulations, on for example subsidies per MW or MW production objectives for a certain area are of influence on the future developments.

Current national approaches

Capacity density choice is not a purely techno-economical decision. Instead, it is driven by the regulatory framework defined by the national authorities. Wind farm capacity densities show high variances and significant differences exist between national averages. The following figure provides an overview of the regulatory frameworks in European countries currently having Offshore Wind Farms.

Layout of Realized Offshore Wind Farms

Because of wake effects, the layout of a wind farm and the positioning of the wind turbines are important factors for the capacity and the final energy production. The lay-out takes into consideration several factors, such as the wind direction and changes in the wind direction, the size of the turbines (as explained above) and connection to the grid.

Conclusions

• The following figure shows the corrected capacity densities for European wind farms. As can be seen, the wind farms in the North Sea region (6.0 MW/km2) and the Baltic Sea region (5.5 MW/km2) are close to the assumptions of scientific literature (see report for overview of research) (5– 5.4 MW2).
• The difference in the average capacity densities of the North Sea region and the Baltic Sea region can be partly explained by the strong influence of national regulatory frameworks (e.g. countries require a certain amount of MW energy production for a certain area as a criterion for a successful bid in a tender). Another reason might be lower specific power ratings as a consequence of the slightly lower wind speeds in the Baltic Sea.

• Turbine spacing demonstrated to be the dominant driver of capacity density. Hence wind farms with high capacity densities are characterized by low distances between wind turbines.

Infographic

The report produced an ‘infographic’ as a summary of the capacity density of the current offshore wind energy farms.

Transferability

The analysis of the technical-economic assumptions of the design of an offshore wind farm, as well as the regularity frameworks and their consequences for the capacity density, are relevant for other to other sea basins or countries working or starting to work on zoning for offshore wind in their Maritime Spatial Plans. Although no detailed recommendations have been developed, the report provides some key analytical insights which are relevant for any Maritime Spatial Planner dealing with offshore wind.

Contractor

Deutsche WindGuard GmbH

Costs / Funding Source

The study is part of the BalticLINes project, which is funded by the Baltic Sea Region Interreg VB Programme

Contact person

Annika Koch

annika.kochbsh.de (annika[dot]koch[at]bsh[dot]de)