On Friday morning, Europe went dark. As the moon swept across its face, the sun was hidden for about three hours in an eclipse that allowed only 20 percent of its usual light to reach the Earth. It's a reminder that, in the drive to switch more of the world to renewable energy sources, solar panels don't light up when it's dark, and turbines don't rotate when the wind doesn't blow.

The sea, though, is constant, reliable - and scandalously underutilized.

Europe leads the world in solar-power installations, with about 60 percent of the global total. Germany's decision to shun nuclear power has propelled the industry's growth there: On a particularly bright day in the middle of last year, the sun provided half the nation's energy needs. A government drive resulted in Germany having a third of Europe's solar total, according to the European Photovoltaic Industry Association.

Europe has also been swift to adopt wind power, with sufficient capacity to meet more than 10 percent of the region's energy needs, according to the European Wind Energy Association. That reflects a global embrace of turbines, which has seen the world's wind capacity double since 2009, according to the Global Wind Energy Council.

Wave and tidal energy, though, remain negligible contributors to renewable energy output. Marine power companies raised just $74 million last year by selling shares or taking investments from venture-capital and private-equity companies, according to Bloomberg New Energy Finance. While that's double the 2013 total, it's down from a peak of $271 million in 2007.

Britain is ideally suited to water technology, being a small island surrounded by crashing seas - and rich enough to fund investments. There are more marine-energy experiments in Britain than in the rest of the world combined, according to the lobby group RenewableUK. Even so, the 10 megawatts of power those test sites are capable of producing is the equivalent of a single offshore wind turbine.

There are some encouraging signs of progress, though. In Australia, Carnegie Wave Energy Ltd. has connected the world's first wave-power station to a national grid, using submerged buoys to drive pressurized seawater through a pipeline to drive a turbine. In February, the Scottish government awarded 14.3 million pounds to Wave Energy Scotland to expand wave-power studies there.

Plans are under way to build the world's first tidal-lagoon power plant in Swansea, on the southwest coast of Wales. The developers claim a second scheme currently on the drawing board has the potential to power every household in Wales. The six facilities tentatively planned around Britain could supply 8 percent of the nation's electricity needs. Chancellor of the Exchequer George Osborne signaled in his budget last week his willingness to consider subsidizing the 1-billion-pound ($1.5 billion) Swansea project.

And there's the rub. Earlier this week, my Bloomberg colleague Louise Downing detailed the support the tidal lagoon will need:

"A subsidy of 168 pounds a megawatt-hour will be needed for 35 years, according to a report by Poyry Oyj. In comparison, the U.K. agreed on a rate of 89.5 pounds to 92.5 pounds for Electricite de France SA's planned Hinkley Point C nuclear station. Onshore wind projects have won deals at about 82.5 pounds for 15 years."

The sea is a harsh environment and corralling waves requires expensive and complicated technology, while the relative paucity of investment in recent years has left the industry far behind renewable-energy rivals. Just as nuclear, solar and wind energy all needed government money to get off the ground, marine energy will need subsidies, with public funding augmenting private investment.

That high price is worth paying to harvest the power of the sea and further safeguard our environmental future.

Mark Gilbert is a Bloomberg View columnist.

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