By Muriel Boselli and Vera Eckert
PARIS/FRANKFURT (Reuters) - Sun and wind in western Europe will be crucial this summer to ensure enough renewable electricity supplies to Germany and France, where nuclear power may fail to deliver, traders and analysts say.
Demand for power rises in the summer when homes and businesses turn on cooling devices but with a big chunk of Germany's nuclear capacity out and a possible cut in French nuclear capacity due to a severe drought, things may get tight.
While summer officially starts on June 21, temperatures have been well above seasonal averages since the start of April, pushing water levels in rivers and mountain reservoirs and squeezing hydroelectric supplies in the region.
"There are some question marks over the summer (power supply)," said Konstantin Lenz of Lenz Energy in Berlin.
"If the (German) nuclear plants are missing, solar power can compensate for some of it but the question is can renewables really cope with all scenarios," he said.
Germany has decided its eight suspended reactors, or 40 percent of its capacity, would be idled permanently, totaling 8,800 megawatts (MW), and to get rid of its remaining 12,700 MW by 2022 in response to Japan's nuclear disaster.
Anti-nuclear voters have succeeded in demanding an unprecedented and singular approach to drop a low-cost and nil-carbon producing energy source at the heart of Europe.
GREEN POWER RISKS, DRY FRANCE
Germany has big wind and solar power capacity. In peak midday hours, there can be 17,000 MW of solar, or the equivalent capacity of 17 second-generation nuclear reactors.
On windy days, there is up to 27,000 MW of renewable utilization rates, making the concern over the missing German nuclear capacity look manageable.
A sunny summer could in theory bring oversupply of solar power, negative prices and margin pressure for thermal plants.
But while thermal power supply can last 24 hours, renewables often produce an unwanted supply push or can stop producing at little notice -- and only a fraction of power can be stored.
Unlike Germany, France relies on stable nuclear energy to supply 75 percent of electricity needs and actively trades with Germany, depending on resources on both sides of the border.
But if river water used to cool reactors dries or gets too hot, authorities can ban producers from using the water.
"It's still early in the year but if in June or early July we start getting prolonged hot weather periods, it will definitely start causing problems," a trader said.
During heatwaves in 2003 and 2006, key producer EDF was forced to stop some reactors for this reason, creating panic in the market and causing wholesale prices rallies.
River levels are key for water supply but also for the safety of transport to carry coal to power stations.
For Thomas Houdre, head of nuclear reactors at France's nuclear watchdog (ASN), potential nuclear supply cuts are more likely to be a result of the drought than heat.
"Heatwaves and droughts are not the same thing," Houdre said. During the 2003 heatwave, ASN granted EDF exemptions to continue using river water despite excess temperatures.
"We are in a drought situation that could prolong...and in case of a severe drought, if water levels go below set limits, power output has to be cut or completely stopped," Houdre added.
The most sensitive region in France is the southwest, where rivers are narrow and dry out more quickly than say the Rhone in southweastern France, officials said.
Local authorities in the Poitou-Charentes region said the 2,900 megawatt Civaux nuclear plant could continue operating until mid-July in the absence of rainfall.
EDF is also taking wider precautions, stopping only three of its 14 seaside reactors for annual repairs this summer.
WHERE TO FIND POWER?
If German renewables fall short of requirements this summer, Germany will have to burn more coal and import more electricity from France and the Czech republic to stay afloat.
Such a situation would likely hit its neighbors too, but Czech utility CEZ says its cooling towers would help keep its nuclear capacity intact, enabling exports to Germany.
Nuclear reactors equipped with cooling towers suffer less from droughts than closed-circuit reactors since the water used to cool the reactors then mostly evaporates in the air.
French traders also pointed out France's record nuclear availability for this time of year, allowing plenty of margin in case of drought-related capacity cuts.
So while the likelihood of black-outs this summer are on people's minds, many traders believe this is very unlikely.
"I'd be surprised if you saw black-out problems because for governments they should never happen," one trader said.
(Additional reporting by Michael Kahn)