"There is a huge technological potential for each type of renewable energy," said Ottmar Edenhofer, chief economist of the Potsdam Institute of Climate Impact Research. "But coal remains more attractive" unless there is a big swing in policy.

"Renewables will become competitive by the end of the century, but not by 2050" unless governments force up the price of burning coal, oil and natural gas by penalising their emissions of greenhouse gases, he added.

This year may see new impetus towards renewables when the scientific panel that advises the United Nations on climate change releases a report in February pointing to a stronger link between human use of fossil fuels and warming.

The chairman of the panel, Rajendra Pachauri, has told Reuters that evidence of the link is "far more robust" than in the last report in 2001.

That could put pressure for tougher curbs on use of fossil fuels and a shift to renewables, when a first period of restraints under the UN's Kyoto Protocol runs out in 2012.

But the United States is the main uncertainty. President George W. Bush pulled out of Kyoto in 2001 and called it an economic straitjacket that wrongly excludes poor nations. US coal is also a reason for opposition.

COAL OR IMPORT

"Signing up for Kyoto would mean a shift from relying on domestic coal to imported energy such as natural gas from Mexico and Canada," a US official said. That goes down badly when Bush is trying to cut dependence on energy imports.

Kyoto binds 35 rich nations to cut emissions of greenhouse gases, mainly from burning fossil fuels, by 5 percent below 1990 levels by 2008-12.

Dependence on coal also partly explains why Australia, the only other developed nation outside Kyoto, also pulled out. China, the United States, India and Australia are the world's top coal producers.

And China is opening a coal-fired power plant at a rate of almost one a week.

Despite coal, renewable industry groups are optimistic.

By one reckoning by Renewable Energy Corp., a solar power group, the market capitalisation of more than 30 leading renewable energy firms surged 700 percent in the past three years to 53 billion euros (US$70 billion).

And there are breakthroughs -- Denmark already generates a fifth of its electricity from wind.

The Global Wind Energy Council, for instance, projects that wind could supply 34 percent of the world's electricity by 2050.

SOLAR SURGE

The American Council of Renewable Energy is looking into ways to ensure that renewables make up 25 percent of global primary energy use by 2025 and the German Solar Industry Association reckons the solar power will eclipse fossil fuel use near the end of the century.

But many of the scenarios assume rising oil prices. Yet many investors remember that a barrel cost US$10 as recently as 1998.

The International Energy Agency's 2006 reference scenario forecasts that renewables' share of global energy demand will gain only slightly, to 13.74 percent in 2030 from 13.17 in 2004.

Biomass -- firewood, charcoal and dung that supply energy for about 2.5 billion people in the Third World and makes up about 10 percent of world primary energy use -- will slip because of widening electricity use in poor nations.

Hydropower will gain slightly to 2.4 percent as big projects such as China's Three Gorges Dam are completed while other renewables -- solar, geothermal, wind, tidal -- will surge most strongly but only to a 1.7 percent share from 0.5, it says.

"The IEA projections underestimate oil prices," said Sven Teske of Greenpeace. The IEA assumes that oil prices would dip and then rise to about US$55 a barrel by 2030.