By Tim Bowler BBC World Service business reporter, Alsleben, Germany

Germany is the world's biggest user of wind power, and it has ambitious plans to build even more wind turbines.

Germany has huge wind energy expansion plans

It has decided that generating nuclear power is not the way forward, and it has decided eventually to close all the country's existing nuclear power stations.

The country's great hope for is for a future of green energy, and in particular wind power.

However, some observers are now questioning whether all the investment in wind power makes economic sense.

Growing demands

Alsleben is a small market town in eastern Germany on the banks of the Saale river.

It's a quiet place surrounded by rolling farmland, but for the past few weeks the people here have been getting used to some new neighbours.

On the hills above them are 37 giant wind turbines. Alsleben is now the site of one of the biggest wind farms in the country.

Close up the engineering is impressive. The blades for these wind turbines are longer than the wing of a Boeing 747 jumbo jet. They are all built in the shape of aerofoils, in order to withstand speeds of up to 270 km an hour.

The site is owned by the US industrial conglomerate, General Electric. It is convinced that wind energy makes economic sense. GE reckons the demand for wind power in other European countries will grow in the same way that it has in Germany.

Under the Kyoto Protocol, Germany is committed to cutting its greenhouse gas emissions. Wind power has obvious advantages as the electricity it generates is non-polluting.

Cost estimates challenged

Germany's politicians plan to have 20% of the country's energy coming from renewable sources like wind by 2020.

More of the costs will be pushed to the domestic sector Professor Wolfgang Pfaffenberger, Bremen International University Energy calculator: nuclear, fossil or renewable energy?

But a row is brewing over the cost of building the power lines which will be needed.

Germany's energy agency says this will cost 1.1bn euros ($1.4bn; £750m) or an extra 17 euros a year for each household.

But energy specialist Professor Wolfgang Pfaffenberger, of Bremen International University, says these figures are too low and it will be domestic customers who will foot the bill.

"It is a big problem for industrial users to pay these extra prices because other countries have cheaper energy. To keep the jobs here, and stop businesses from leaving, more of the costs will be pushed to the domestic sector."

Wind versus conventional power

Alsleben's new wind farm is designed to supply electricity to 30,000 homes, but when the wind stops blowing, the blades stop turning and the power output falls to zero.

Critics say this underlines one essential drawback: you can't depend on wind for energy. Even if you build wind farms you still need conventional power plants in case the wind fails.

GERMANY'S 2020 ENERGY GOALS 20% of all energy to come from renewable sources 28,000 megawatts of power from onshore windfarms 20,000 megawatts of power from offshore windfarms in the Baltic and North Sea

"We face many hours a year with more or less no wind," says Martin Fuchs, chief executive of one of Germany's biggest electricity grid operators, E.On Netz. "We can save only a very small number of conventional power stations."

Surges of wind-generated electricity risk overloading the grid, he adds, causing power blackouts.

These are charges the wind power industry robustly rejects. Christian Kjaer, of the European Wind Energy Association, says all electricity grids are designed to cope with power fluctuations.

"Fossil fuel or nuclear power stations are truly intermittent," he argues. "You never see 1000 megawatts of wind energy shutting down in a second, yet that's what conventional power stations do."

For now, few in Germany are questioning the country's wind energy programme.

The savings in terms of greenhouse gas emissions are politically popular.

Yet there is a lingering question-mark over the cost of all this, and whether building so many wind turbines truly makes economic sense.