Europe has big plans for greatly expanding its renewable energy sources, but there's a problem: weak connections between a patchwork of national power grids. The situation is particularly problematic for wind power, because smaller, isolated grids have more difficulty absorbing the variable power generated by wind farms.

Last month a Dublin-based wind-farm developer, Airtricity, and Swiss engineering giant ABB began promoting a bold solution to the continent's power grid bottlenecks: a European subsea supergrid running from Spain to the Baltic Sea, in which high-voltage DC power lines link national grids and deliver power from offshore wind farms. When the wind is blowing over a wind farm on the supergrid, the neighboring cables would carry its power where most needed. When the farms are still, the cables will serve a second role: opening up Europe's power markets to efficient energy trading.

The result would be a more integrated and thus more competitive European market, delivering power at lower prices. And it would enable Europe's grid to safely accommodate even more clean, but highly variable wind power. That accommodation will be needed because the European Union has set a target of 21 percent of electricity from renewable sources by 2010, and much of this will come from wind farms. "The primary benefit of the supergrid is that it aggregates wind power across geographically dispersed areas, and, by doing so, it smoothes the output of those wind farms," says Chris Veal, the Airtricity director promoting the supergrid. "If the wind isn't blowing in the Irish seas, it's likely to be blowing in the North Sea or the Baltic. The wind is always blowing somewhere."

By solving two problems at once -- interlinking grids and providing hookups for more offshore wind farms -- Veal thinks Airtricity has found a solution that's economically feasible. "It's something the market can do," he says.

Airtricity proposes to start by building a massive 20 billion euro ($23.8 billion) project in the North Sea. Last November, Swiss-based ABB completed a study mapping out the power links for a group of wind farms that Airtricity would like to build in the southern half of the North Sea. (Airtricity is vague on the exact location, since it is still staking claim to the seabed, which lies in the U.K., German, and Dutch waters.) The wind farms would produce 10,000 megawatts of electricity -- 50 times more than today's biggest offshore farms.

A 5,000 megawatt DC power line would carry power west to the U.K., and a second 5,000 megawatt line would run east to continental Europe, perhaps to the Netherlands. When the wind is too calm to produce power -- about 60 percent of the time at Airtricity's North Sea sites -- the lines would go into interconnect mode, carrying 5,000 megawatts of electricity in either direction. This would, for example, more than double the U.K.'s energy-trading capacity, making that country's grid more stable and giving its consumers access to a wider range of power producers.