Lower uncertainties for North Sea offshore wind energy with accurate wind information

DOWA

The Dutch Offshore Wind Atlas (DOWA) provides accurate wind information at the North Sea for offshore wind energy use. It is an innovation in the wind energy sector in the Netherlands. With this accurate wind information offshore wind developers can make more accurate business plans with lower costs and more favourable financing.

Three parties worked on the DOWA-project: KNMI created the atlas using state-of-the art weather models including millions of measurements every day and including satellite and aircraft measurements over the North Sea. TNO was responsible for the project management and worked closely together with KNMI to validate the atlas with high quality measurements from various offshore stations. Whiffle has created very high-resolution models for detailed wind turbine loading and wake analyses. TKI Wind op Zee supported the consortium to create the new wind atlas and encouraged the parties to combine their knowledge.

The DOWA-project was an innovative project. KNMI focused its innovation work on including wind farm wake effects in HARMONIE and made it possible to nest Whiffle’s GRASP-model in HARMONIE. Whiffle focused on improving the turbine parametrisation in GRASP. TNO focused on improving knowledge of wind climatology on the North Sea at heights relevant for wind energy.

By doing so, the consortium was able to deliver a very accurate wind climatology on the North Sea. This was validated against satellite measurements, floating and mast mounted LiDAR measurements and anemometers mounted on meteorological masts, both offshore and onshore.

Data

The output of DOWA is enormous. There are more than 80 terabytes of data available that are interesting for researchers, wind energy developers and producers of offshore wind energy. They can use the data, eg. for making a business case when they are planning a new wind farm. In the DOWA they can find accurate ‘free stream’ wind climatology. When they combine this information with local measurements, they can assess local wake effects and get a full picture of the local wind climate. This information can be used to decide on the design criteria for the wind turbines or the optimal wind farm layout.

The DOWA dataset is an open dataset which is stored in the KNMI Data Centre (KDC) until May 2020. There is a special website from where users can download all the data to make their calculations. There are two types of datasets: daily 3D files and 10-year time series. They contain exactly the same information: ‘free stream’ wind information from 10 to 600 meter height on a 2.5 by 2.5 km grid. They are only ‘organised’ differently to accommodate different types of users. In the daily 3D files users can select a day and download all available information for that day. The 10-year time series files are organised per location. Users of this set can select a grid cell location and download all available time series for that location. There are also complementary high-resolution datasets that include the effects of existing wind farms on the wind.

Novelties

The DOWA is the successor of the KNMI North Sea Wind (KNW) atlas which is also targeted at wind energy users. Both the DOWA and the KNW-atlas are a ‘downscaled’ global re-analysis, but they are made with improved versions of the models in a fundamentally different and unprecedented way. The very high-resolution modelling, provided by Whiffle (10 min, 64 m) is unique. Not only because of its resolution, but also because it takes the effect of existing windfarms into account on the individual turbine level, which is very useful for wind energy developers.

Challenges

Of course there were challenges the consortium was facing during developing the DOWA. For instance, even the fastest computer in Europe needed 500 full days to calculate 10 DOWA years, preparation of the runs and validation of it not included. For Whiffle, it was challenging to embed the wind turbine models in their weather model in a way that is practically feasible and user-friendly. And TNO as the project coordinator, had the challenge to make sure that all results were publically available in time. After all, the information was required during and not after the ongoing offshore wind tenders.

Future development steps

During the project the consortium did some explorative work too. The results were very promising, and the DOWA partners are pursuing them further. One of the innovations in the DOWA-project was to include wind farm wake effects, which is important information for wind energy. With the extensive growth of wind energy on the North Sea, their effect becomes significant. The first results are very promising and in the WINS50 project (Whiffle, TU Delft and KNMI) this will be further explored.

Because offshore wind turbines are getting bigger and bigger, they will harvest wind from a larger area. The traditional approach of a single inflow wind field at hub height that is representative for the whole rotor is clearly insufficient. Vertical, horizontal and temporal variations should definitely be taken into account. Especially when considering that the larger wind turbines designs are slenderer and more flexible, which means that the impact of excitations and loadings becomes increasingly important. These are obviously driven by the incoming wind fields. As such, the project partners have clearly seen the value of coupling detailed wind fields to aero-elastic codes in order to improve the loading on modern and future offshore wind turbines. This will support the design of very large wind turbine, making them structurally and economically feasible. TNO and Whiffle are defining a project to tackle this.

Project partners and TKI Wind op Zee

The DOWA project is executed by the project partners TNO, Whiffle and KNMI. They are supported with TKI Wind op Zee, part of Topsector Energy subsidy from the Ministry of Economic Affairs and Climate Policy (SDE+ Hernieuwbare Energie Call).

Find out more about DOWA