Persistent wind speed lows in high-resolution global climate models: future projections and validation with lidars

The North Sea is becoming increasingly densely populated with offshore wind farms and
this trend will continue in the future. In the coming decades, the power production of these
wind farms will depend on how the wind resources in the North Sea are affected by climate
change. The frequency and intensity of extremely low or high wind speeds could change, which
would significantly impact a renewables-based electricity system. This work focuses on prolonged
periods of low wind speed, so-called persistent lows, since they cause long-lasting drops in the
wind energy production.
In this study, persistent lows are defined in terms of the capacity factor, which is the ratio of
the actual energy generated by the wind turbine to the maximal energy that could be generated
under ideal circumstances. A persistent low of, for example, two days with a capacity factor
below 10% is a period of at least two days in a row during which the daily mean capacity factor
was below 10% each day. This work aims to answer the following three research questions related
to these phenomena.
Firstly, how often do persistent lows occur in the North Sea? To answer this question,
profiling lidar data from 10 stations in the North Sea are analyzed for persistent lows over a
range of capacity factors, which are estimated from the wind speeds using a reference wind
turbine.
Secondly, are persistent lows also captured by reanalysis data and climate models? Here,
CERRA and ERA5 are the reanalysis datasets of interest, and for the climate models the CMIP6
HighResMIP ensemble is chosen. The representation of persistent lows is investigated by first
validating the reanalysis data using lidar data during the active period of the lidar at its location.
Afterwards, the reanalysis data is used to validate the historical climate models. The findings
show that the reanalyses recreate the persistent lows found in the lidar data well, while the
results are model-dependent for the climate models.
Lastly, how does the frequency of these persistent lows evolve in the future? Here, the
future and historical simulations from the HighResMIP climate models are compared. Instead
of focusing on the locations of the lidars, the locations of current and planned wind farms in
the North Sea are selected to make a direct link to the impact of persistent lows on energy
production. Preliminary analysis shows no statistically significant climate signal.

 

 

Acknowledgements
This work has been financially supported by the Flemish Government through the Agency for
Innovation and Entrepreneurship (Vlaams Agentshap Innoveren en Ondernemen, VLAIO), in
the context of the CORE project. This study has also been partially supported by the DTWO
project, funded by Horizon Europe grant no. 101146689.