Offshore lays claim to base load in Germany

Improving forecasting accuracy key to system management

Pro-offshore wind voices in Germany claim that the country’s emerging offshore wind industry will become, over time, a supplier of base load electricity.

Offshore wind is "capable, to a degree, of supplying base-load electricity generation," asserted Pieter Wasmuth, of Vattenfall Europe (VE), recently, at the Branchentag Windenergie conference in Cologne.

Average base-load electricity demand in Germany over the year is about 40GW. By 2020, offshore wind stations in German waters could supply 15-20% of base-load needs, says Michael Buijzen of energy company, WSW Energie & Wasser.

However, claims that offshore wind will be capable of contributing to the continuous, reliable, stable flow of electricity that is base load generation "needs some qualification," warns Matthias Lange, managing director at energy&meteo systems, a firm specialising in energy meteorology and wind power prediction.

Knowledge about how and when winds blow at sea remains inadequate, making forecasting difficult. "We have little experience concerning wind-still periods at sea. What we do know is that there are roughly half as many of those calm periods offshore compared to onshore," says Lange.

Forecasting offshore output

Far more data collected at sea is needed. "If we had good observation data from the Atlantic, we could use these as a kind of early weather-warning system for forecasting. But there are virtually no weather stations towards Iceland and Greenland," says Lange. Satellite pictures help, but these provide no information about air pressure and weather models cannot deliver the accuracy that is needed.

In addition, rapidly-changing conditions at sea mean that offshore stations tend to operate either at low or no output or at full rated-capacity, with very fast ramping up and ramping down.

Forecasting electricity generation when stations are operating at full output is comparatively reliable, with a relatively small prediction error, says Lange. "But when a weather front flows across an offshore station, there is a very swift increase in output. If this comes earlier or later than expected it can create problems for integration of the electricity," he adds.

This issue is compounded for Germany because its North Sea offshore wind stations will be concentrated within a relatively small area and, thus, are expected to act often as a single, synchronised generation unit. Such a scenario suggests that any prediction errors apply to a large amount of capacity," notes Lange.

Offshore stations can ramp up about 80% of their capacity within 30 minutes. Provided German offshore wind development targets are fulfilled, and 7GW of North Sea capacity is online by 2020 (supplemented by 3GW in the Baltic Sea), this sets the stage for something in the region of 5.6GW ramping up within half an hour in the not-too-distant future, according to Lange.

Looking ahead to a time when multi-gigawatt offshore wind capacity off German shores is likely to be highly synchronised, there is clearly a pressing need to improve forecasting. Until this is achieved, offshore wind’s potential to supply base load electricity will not be realised.

Integrating offshore wind

Last year, Germany’s Federal Environment Office (Umweltbundesamt) commissioned energy&meteo, in conjunction with consultancy Consentec, to explore the impact of planned offshore wind capacity on the national electricity system’s secondary reserve (power that can be called up within 5 minutes) and tertiary reserve (within 15 minutes). Research conclusions are due next year.

It is already clear that one method for managing steep increases in offshore wind-generated electricity is to curtail the output of offshore stations during rapid ramp-up periods.  Existing conventional power stations can be used for balancing wind power gradients by supplying secondary and tertiary reserve. Ideally, the role of conventional power station units will be merely to supply "residual load," says Lange.

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