* Power system requires around 1.2 trillion euros 2020-30
* Innovative funds, such as bonds, could help provide funds
* Gas infrastructure largely adequate to 2040
By Christopher Le Coq and Barbara Lewis
BRUSSELS, Nov 7 (Reuters) - European Union nations must nearly double investment in grid building in the decade after 2020 if they are to get on the path to carbon-free electricity by the middle of the century, the European Climate Foundation (ECF) think-tank said on Monday.
The European Commission raised the goal of virtually emissions-free electricity in its 2050 road map towards a low carbon economy, published earlier this year.
Carbon-free generation is regarded as the easiest means to an 80-95 percent cut in carbon that scientists say is needed by the middle of the century to stave off the worst effects of global warming.
"Only by setting a clear policy direction to 2030 will it be possible to achieve the decarbonisation goal," said Johannes Meier, CEO of the European Climate Foundation, speaking at the launch of the report in Brussels.
"What this report finds is that the grid is the glue that will hold together our decarbonised power system. We must be more efficient to deliver this in the most effective way."
Already to meet 2020 goals to cut carbon by 20 percent and increase the share of renewable energy to 20 percent, member states need to increase transmission lines by 14 percent, or 42,000 km.
The total investment in the system, including grid and generation, would be 628 billion euros ($872.7 billion) over this decade.
For the next 10 years, investment climbs to 1.2 trillion, said the ECF's report -- Power perspectives 2030: on the road to a decarbonised power sector in Europe.
In what it labels an "on track" scenario, based on the EU managing fully to implement its plans up to 2020 and projects in 2030 in line with targeted emissions cuts, it anticipates a power mix in 2030 made up of 50 percent renewable energy.
The rest would include 34 percent fossil fuels, plus a further 8 percent abated by carbon capture and storage, and 17 percent nuclear.
Perfecting the transmission system is crucial to optimising available renewable energy, which tends to be concentrated on the fringes of Europe.
The ever steeper challenge could require innovative approaches, especially to financing: for instance, selling asset-backed bonds to institutional investors.
The Commission has confirmed plans to launch European project bonds as part of its infrastructure financing plans to the end of this decade.
While power grids require huge investments, gas infrastructure could be adequate until around 2040 -- provided the planned investments to 2020 go ahead -- as renewable power, rather than carbon-emitting gas provides the solution to emissions reduction.
Beyond 2030, the report finds gas can only be "a significant destination fuel" if commercially viable ways of reducing its carbon emissions are in place.
For gas supplies, the European Union is looking at bringing in alternatives to Russian gas through a southern corridor where rival projects include the highly ambitious Nabucco pipeline with an annual capacity of 31 billion cubic metres.
Philipe Lowe, director general of the European Commission's energy department, said future demand estimates for gas varied.
"We may have a different view on that (from the ECF)," he told Reuters.
"In any case, from a strategic point of view, the European Union is extremely concerned that there should be a diversification of supplies, as well as of routes."
The ECF's report was researched in collaboration with Imperial College in London, consultants, Royal Dutch Shell and a range of utilities and technology manufacturers.