LONDON The extreme variability of wind and solar power in Germany is laid bare by 2012 data that also illustrates a particular vulnerability when power demand surges on cold winter days - they are often both weak at the same time.
Intermittent renewable energy requires grid balancing services, primarily from stand-by baseload fossil fuel capacity or battery and pumped storage, but also via interconnectors bringing power from other countries and regions.
The cost of that balancing need is still unclear, as European countries ramp up wind and solar capacity with an eye to meeting 2020 targets, with ambitious goals including Britain's aim to meet 15 percent of its primary energy demand from renewables, compared with less than 4 percent now.
Germany provides a good litmus test, given its large economy and rapid shift to renewable energy in the past five years.
Data from Germany's Fraunhofer Institute provide a detailed picture for the first 40 weeks of the year, from January 1 to October 7, and indicates the scale of grid balancing already needed.
For example, while the renewable energy lobby cheered a solar power peak at 22.4 gigawatts (GW) (or about a third of total lunchtime demand) on Friday, May 25, perhaps more relevant was a slump in combined wind and solar power during a surge in demand in an early February cold spell.
A COLD WEEK IN FEBRUARY
Superficially, wind and solar in Europe complement each other seasonally, with summer peaks for solar power coinciding with a lull in wind power and the reverse holding true.
For example, aggregate solar power generation in the first week of January was only 7 percent of peak production in the final week of May, while for wind power generation the same week saw eight times the level in the first week of July, the minimum for the year so far.
However, that only applies when considering aggregate weekly or monthly data. In fact the two do not average out.
At a more refined hourly and daily level, wind power is seen to be equally unpredictable at any time of the year, recording minimum generation of 1 percent or less of installed capacity in each of the first nine months of the year.
Lulls in wind power in winter especially coincide with anticyclones bringing clear skies and cold weather when power demand is also high, revealing an Achilles heel for renewables, when sunlight is still relatively weak and the wind drops.
A northwest European cold snap which hit a nadir in the week starting February 6 is illustrative.
The week recorded the second lowest combined German wind and solar power generation of the 40 weeks in 2012 to date, at 0.77 terawatt hours (TWh) compared with a weekly average of 1.48 TWh, Fraunhofer Institute data show.
In the same week, French power demand hit at an all-time record of 102 GW, on the evening of February 8, which affected Germany as it is a major exporter to France in February.
French power demand is especially high in winter because of a reliance on electric over gas heating.
Following this combination of factors, German conventional power production (fossil fuels and nuclear) also hit a 2012 high on the evening of February 8, at 65.9 gigawatts.
Natural gas in particular supplied the gap left by renewable energy, with peak production of 22.4 GW, very close to total gas-fired capacity of 24 GW.
While the grid survived, the risk of blackouts is clearly elevated without substantial fossil-fuelled back-up capacity.
And the narrowness of the margin suggests the confidence of the European Commission, in its "Quarterly report on European electricity markets", for the first quarter this year, may be misplaced:
"Despite harsh winter conditions, there was not a single serious disturbance in the European electricity system. This underlines the strength of the European internal electricity market and the effectiveness of related European policies."
Such volatile power output, where troughs in renewable energy are plugged by flexible gas-fired capacity, has major consequences for traders and policymakers.
For traders, volatility creates opportunities if market participants can buy power ahead using superior weather forecasting resources.
For example, on February 8, when French peak demand spiked and German renewable power generation slumped, baseload day-ahead German power prices hit their highest so far this year of 88 euros per megawatt hour, according to ICAP data, compared with a 2012 average of 46 euros.
It was a similar picture for day-ahead peak power prices, which hit a 2012 high on February 9, with February 8 a close second.
The large standard deviation of wind and solar power illustrates the volatility.
The Fraunhofer Institute data show average weekly solar output of 635 gigawatt hours (GWh) with a standard deviation (capturing the central two thirds or so of the 40 observations) of 289 GWh.
For wind power, the average weekly output is 843 GWh and the standard deviation 480 GWh.
The trend towards more renewable energy generation in Europe makes more regulation inevitable, including capacity mechanisms or strategic reserves to ensure gas power headroom for cold days in February, for example, and other publicly funded resources including battery storage and interconnectors.
The aim for policymakers must be to achieve this intervention most cost-effectively, and to be transparent with voters about the combined impact on power bills. (Reporting by Gerard Wynn; Editing by Anthony Barker)