Weather forecasts calling for bright sun today across Europe drove up tensions in advance of the partial solar eclipse that blocked the sun’s rays and plunged much of the continent into a brief period of darkness this morning. Grid operators were bracing for record swings in solar power generation because of the celestial phenomenon. Some power distributors in Germany had warned of fluctuations in frequency, notifying customers and suggesting that they shut down sensitive equipment.
In the end, while clear weather made for some excellent eclipse viewing, the electrical story ultimately felt more like Monty Python’s radio coverage of the 1972 eclipse. As if audio coverage of a quintessentially visual event isn’t absurd enough, the Pythons closed their fictitious report in the ultimate anticlimax, as a sudden rainstorm swept in to spoil the solar spectacle. Europe’s interconnected power grid brought about an equally anticlimactic ending today by delivering rock-solid stability throughout the 2.5-hour eclipse.
In fact, according to Enrico Maria Carlini, Head of Electric System Engineering for National Dispatching at Rome-based transmission system operator Terna, the grid was more stable than normal. Carlini had joked last week about doing a rain dance to dampen solar output during before and after the eclipse. Today he took satisfaction from the fact that the frequency of Europe’s power barely budged from its 50 hertz standard.
If transmission system operators had been struggling to keep power capacity and demand in balance during the eclipse, the frequency would have diverged strongly from 50 hertz. But according to Carlini, it strayed only ±25 millihertz all morning, which he says is about the half of normal variability in Europe’s grid frequency.
The grid rolled over the solar swings partly because they were smaller than the worst case scenarios for which operators had been preparing for many months. In all, Europe lost and regained about 17 gigawatts of solar power generation this morning, according to the European Network for Transmission System Operators for Electricity (ENTSO-E) in Brussels. That is a lot of power—just shy of the total solar capacity installed across the United States as of the start of 2015—but just half of the swing that ENTSO-E had warned of in its February eclipse analysis.
TSOs thus confronted the possibility of solar swings with overwhelming force. German TSOs had double the normal personnel on hand in their control rooms. They also had enough gas- and coal-fired power capacity on standby to double the effectiveness of their standard contingency plans for keeping the grid balanced, says Bruno Burger, an expert in renewable energy integration at the Fraunhofer Institute for Solar Energy Systems in Freiburg. “The TSOs really massively prepared,” says Burger.
Today’s solar swings still represented the biggest power gyrations that Germany’s TSOs have ever confronted, according to Burger. Germany’s solar generation was at nearly 13 GW when the lunar shading began. At the peak of the eclipse, at around 10:30 CET, it had crashed to just 5.4 GW. Then, by noon, it had rushed back up to about 20 GW. That post-eclipse surge would have soaked up about 50 percent more than the maximum negative power reserve capacity that Germany’s TSOs had in place in 2013 and 2014. “This was a big challenge for the TSOs,” says Burger.
Still, they could have handled plenty more, claims an analysis (in German) by Burger’s group, which estimates that Germany’s power installations can provide up to 25.6 GW of regulating capacity per hour. Bloomberg reported today that TSOs called up only about 30 percent of the balancing power that plant owners tendered.
Terna, whose Italian power plants are less conventional than the ones its German TSO counterparts can call upon, took an additional measure to balance this morning’s eclipse-driven solar swings: For the first time ever, it exercised its authority to order the nation’s largest, most advanced solar power plants to regulate themselves.
“Those plants were ordered to limit their output to about 30 percent of total capacity from 7 am to 2 pm CET,” says Carlini. That action, he says, reduced the solar power deflation during the eclipse by about 500 megawatts and trimmed the post-eclipse rebound by 2 GW. Under Italian law, solar plant owners will absorb the lost revenue because this was an “emergency procedure,” says Carlini, who doesn’t expect that authority to be wielded again before the next eclipse in 2027..
Burger says Germany’s TSOs could have also shut down solar capacity, but chose not to because German law requires TSOs to compensate the solar plants. An analysis by Berlin-based research group Energy Brainpool estimated that would be more expensive than ramping fossil-fired power plants up and down. As reported by PV Magazine yesterday, shutting down solar generation could nearly triple the cost of handling the eclipse to €9.95 million (US$10.7 million).
This post was created for Energywise, IEEE Spectrum’s blog on green power, cars and climate