There has been a massive boom in wind power capacity both in Europe and worldwide. In 2015 global installed capacity was around 350 gigawatt (GW), with 135 GW installed in Europe, distributed across some 87,000 wind turbines. Wind power now provides a bigger share (13 percent) of electricity than nuclear power stations. In countries such as Spain, Denmark and Germany, the amount of wind power already installed is in theory enough to cover nationwide demand for electricity under ideal conditions, i.e. maximum wind power output and low consumer demand.

However, the amount of installed capacity says very little about how much electricity is actually fed into the national grid by a country's wind fleet. Unlike nuclear power, wind is by nature harder to predict. This makes it difficult to connect wind farms to existing power grids.

Both energy researchers and providers therefore need to simulate electricity production across very short time intervals to accurately predict how high the load could be at any given point in time.

Recently, researchers have started performing such simulations with the help of "reanalysis" models: global meteorological models fed with measured data such as from weather stations and satellites, which process these measurements into a coherent world-wide simulation of atmospheric conditions.

Critical review of weather models
However, there is one major drawback with data from reanalysis: meteorological models simplify the real world do not provide adequately detailed simulation of factors that are important for wind power, such as the topology around a wind farm. So, if data from reanalysis models is used to simulate wind power production without further correction, the models are liable to produce a systematically distorted picture. Despite this, a number of studies have been published on wind power generation that are based on uncorrected data.

This inspired the energy researcher Stefan Pfenninger from ETH Zurich and his colleague Iain Staffell from Imperial College London to create a large database of recorded electricity output from wind farms across Europe, as well as country-wide production data reported by transmission network operators, and to use that database to derive correction factors for each European country. They then use their Virtual Wind Farm Model (VWF) to simulate wind power production in Europe over the course of 20 years.

Fresh simulation of output
By adopting a rigorous approach, the two researchers have managed to create a more realistic picture of wind energy output in Europe. Their corrected simulations show that the uncorrected simulations used in other studies have overestimated wind power output in north-western Europe by up to 50 percent, while underestimating it by as much as 30 percent in southern Europe.

The researchers also recalculated the capacity factors for Europe: the current European average is 24.2 percent, compared with 32.4 percent in the UK and 19.5 percent in Germany. The European average only varies by a few percent from one year to the next. "This fluctuation is much less than the deviation observed in individual countries", says Pfenninger. "The bigger the wind fleet and the wider the geographical footprint, the smaller the fluctuations on the supply side". It is therefore important for national grids to be interconnected more efficiently so as to be able to offset power outages in one region with surplus output in another country.

The simulation also shows that capacity factors are improving, partly thanks to technological advances and better offshore locations. Britain's wind parks are now 25 percent more productive than they were 10 years ago.

North Sea countries in expansion mode
Given the current state of planning, Pfenninger and Staffell predict that the average capacity factor for Europe could rise by a third, to more than 31 percent. "Countries adjoining the North Sea should experience particularly strong growth in the near future", says Pfenninger. The UK could achieve a capacity factor of almost 40 percent, and Germany close to 30 percent.

But in order for planners, network operators, utility companies and other scientists to be able to continue using the simulations developed by the energy researchers, Pfenninger and Staffell have devised an interactive web application, where the European data sets are also available as a download. The web platform also gives access to data from a study, published at the same time, which develops simulation of Europe's photovoltaic power output. Pfenninger and Staffell have been beta testing Renewables.ninja for six months and now have users from 54 institutions across 22 countries, including the International Energy Agency and IRENA.

Staffell I, Pfenninger S. Using Bias-corrected Reanalysis to Simulate Current and Future Wind Power Output. Energy, published online 5th September 2016. doi: 10.1016/j.energy.2016.08.068