* Green power plans to 2050 require backing by fossil fuels
* New fossil fuels plants needed to make up for closures
* Otherwise, Germany may turn net power importer
By Vera Eckert
FRANKFURT, Aug 22 (Reuters) - Germany must keep a safe layer of fossil fuels-based power capacity in the decades to 2050 by when it hopes to derive 80 percent of electricity from renewable sources, said a study released by Deutsche Energie Agentur (Dena) on Wednesday.
Coal and gas-fired power plants at that stage would still have to provide 60 percent of the secure capacity in place in 2010, said the study, which was carried out by the Aachen Technical University and commissioned by utility RWE.
Capacity is considered secure when it is available around the clock, typical of fuels-based generators, while operations at wind and solar power installations depend entirely on weather patterns.
"In order to ensure secure power supply, the installed total of conventional power plants may only drop by 14 percent (over 2010) to 83 gigawatt (GW) in the year 2030, and by 37 percent to 61 GW in 2050," the study concluded.
Dena said that while the power produced from renewable sources in 2050 might actually reach the desired 80 percent of the total, green power's contribution to secure capacity would only represent 24 percent of the total by then.
The study assumed constant power consumption over the next decades in Europe's biggest economy.
Having closed a big chunk of its nuclear capacity after Japan's Fukushima disaster in 2011, Germany wants to scale down coal and gas plants as well and go increasingly green.
At least 35 percent of its power is to come from green energy by 2020, up from 25 percent now.
Dena said according to its calculations, some 49 GW of new fossil-fuels capacity must still be built by 2030 at the latest, to compensate for ageing plants going offline.
It also highlighted more consequences of the planned changes.
Given the strong expansion of renewable capacity, there would be many oversupply scenarios in the time after 2020. As power storage so far can only absorb a paltry volume, experts would have to step up efforts to develop more storage options.
Utilities operating conventional power may be relegated to providing reserve power for renewables. But this would not be cost-efficient as returns and profitability were calculated on n full-time production runs.
Efforts to tailor demand more to supply would require huge investments in making networks more flexible, for example building more transmission lines to remote renawable sites and equipping households with smart meters.
If there was no additional construction of new conventional power plants while old ones retired, Germany could turn a net power importer of some 22 percent of its usage by 2050.
"We need a new power market design," said Dena's managing director Stephan Kohler.
"Part of that would be a Europe-wide capacity market so that it is worth the operators' while to ensure that secure load is ready, and a totally reformed renewable energy law which integrates them better in the market and the existing system."
Dena is funded to 50 percent by German government ministries while the other half is made up by banks and insurers including Deutsche Bank and Allianz. (Reporting by Vera Eckert; editing by Keiron Henderson)