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Onshore turbines are installed in northern and eastern Germany, while new offshore wind farms are mushrooming along the North and Baltic Sea coastlines.
Dena claims some 2,240 miles of new transmission lines are needed to transport the fluctuating renewable electricity south and southwest to Germany's industrial hubs, where most of the power is needed. Dena forecasts renewables to account for 39 percent of the German power mix in 2020, up from around 16 percent today.
"Germany is right to bank on renewables," Dena head Stephan Kohler said in a statement. "But the expansion of renewables puts the energy system before great challenges. We must bring wind power from the North and Baltic Sea to consumers in the South. Conventional power plants need to be modernized and operated in a way that they complement renewables well and remain economic at the same time."
Grid expansion in Germany has been slow and tedious, with massive public opposition to new high-voltage transmission lines. In 2005 Dena urged the government to build some 530 miles of new infrastructure, but less than 60 miles have actually been built.
Kohler said Germany needs an "acceptance offensive" that communicates to the population that Germany "needs a network expansion."
Building new sub-ground lines would be less controversial, but also more costly -- up to $38 billion, Dena says.
The government's recent decision to prolong the life of the German nuclear reactors by an average of 12 years won't affect the grid expansion plan, the think tank added.
Conducted by scientists from Cologne University and the Fraunhofer Institute, the study was funded by the German government, the energy industry and the network operators.
It comes a few weeks after the European Union outlined its energy infrastructure priorities for the next two decades, saying some $270 billion has to be invested into "smart" power grids, electricity highways as well as new oil and gas pipelines.