Photograph: Photo by go7o, used under CC BY

On the edge of the broad, bucolic Lueneburg Heath in northern Germany they're building a high-performance waterwheel as big as a house. Conservationists are not amused.

Hydro-electric systems usually require a dam, where water rushes down a penstock and turns a turbine. On flat rivers which can't be dammed, a wheel turned by the natural current is the next-best alternative.

The 11-million-euro (12 million dollar) project, a co-production of the Braunschweig University of Technology (TU-BS) and German steelmaker Salzgitter AG, is aimed at demonstrating the potential of hydropower in Germany alongside other renewable energy sources such as wind and solar power. Conservationists are still not amused.

With a vertical diameter of 11 metres, a width of 12 metres and 60 blades, it will be the world's most powerful waterwheel when it begins operation on the Aller River near the town of Winsen at the end of 2017, the partners say.

Its output will be enough to supply 1,000 households with electricity.

"If the project proves successful, it'll be a technological revolution," said Christian Seidel, an engineer at TU-BS's Institute for Structural Analysis.

Although rivers like the Aller have high flow volumes, he said, they lack the "head," or height from which the water flows, necessary for the large-scale use of hydropower. TU-BS engineers have sought to work round the problem for the past 10 years.

Assisting them with construction material is Salzgitter AG, Germany's second-largest steelmaker. The project's general contractor, Salzgitter is paying half of the costs and will sell the electricity that is eventually generated.

The other half of the costs is being picked up by government.

Seidel said the maximum flow rate by waterwheels to date was about six cubic metres per second. The rate of the Aller waterwheel will be 10 times greater, and its torque will equal that of 3,200 Porsche 911 sports cars.

According to Germany's Fraunhofer Institute for Wind Energy and Energy System Technology, hydroelectric power output in Germany has changed little since 1990. Its share of the renewable-energy mix has fallen to 13 per cent, however, as wind and solar energy have grown.

"Renewables" are being pushed worldwide amid efforts to cut down on the combustion of fossil fuels to produce electricity, which releases large amounts of the greenhouse gas carbon dioxide, blamed for global warming.

Adding to renewables' importance is Germany's commitment to phase out nuclear power plants. 

An advantage of hydro over wind and solar power is that it is not much affected by vagaries of weather.

Its "baseload capability" means it could actually replace conventional, fossil-fuel-fired power plants and not just supplement them as onshore wind power and solar energy do when the weather allows.

Fraunhofer scientists say hydropower even tops offshore wind power in its full-load hours per year by a factor of about 1.5.

Despite these pluses, conservationist organizations such as Friends of the Earth Germany and the Aller-Oker Salmon Association have reservations about the project. They worry that it will stress or harm fish.

The Lower Saxony State Anglers Association opposes it outright.

Some reassurance has come from the licensing authority, the Lower Saxony State Office for Water Management, Coastal and Environmental Protection (NLWKN), which said that waterwheels are safer for fish than turbines.

"According to current scientific evidence," it pointed out, fish - in particular endangered salmon and sea trout - are at little risk from the slow rotation of waterwheel blades.

To make certain this is so, there are plans to monitor the fish for three years once the waterwheel begins operation. Seidel has already conducted tests himself using a model waterwheel and rubberized polystyrene fish.

Fewer than one in a hundred were abraded by the blades.

Fish ladders are to be built at the waterwheel and its weir, which dams the river and increases the head of water.

Since they're not ideal solutions, engineers and conservationists are now wrangling over gap widths allowing fish to pass the waterwheel, and over the size of lock-out gratings - features that usually diminish a waterwheel's output.

Conservationists fear that political backing for the project is so strong that only technical problems could stop it now.

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