Water in a glass

Taking hydro to a new low

While the UK turns to ancient technology to tap energy from its waterways, German engineers have been working on a modern approach to meet their 2020 renewables targets.

The scrapping of plans to build the £30bn Severn Estuary barrage in October last year were a problem for the UK. The decision followed decades of debate on the impact on biodiversity and wildlife, flood management, local geology and water quality, with energy secretary Chris Huhne concluding that there is 'no strategic case at this time for public funding of the scheme'.

Such environmental issues, coupled with a dwindling number of suitable sites, are duplicated across Europe. The result is a revival in interest in small-scale hydroelectric schemes. With a relatively benign impact on local habitats, small plants also make a good option for decentralised power generation. So why haven't we seen more small schemes? To date, many constructions have stalled over the high civil-engineering costs needed to recover even a modest power output from small-scale installations (see 'Cost pressures'). But this is set to change.

Earlier this year, researchers from the Oskar von Miller Institute at the Technical University of Munich, Germany, unveiled a small hydroelectric power plant design that promises to 'enable renewable power generation at thousands of unused sites across Europe'.

Pioneered by Professor Peter Rutschmann and TUM civil engineer Albert Sepp, from the department of hydraulic and water resources engineering, the construction is described as simple and thereby cost-efficient so that the hydroelectric plant can operate profitably at dam heights as small as 1m to 1.5m. Achieving affordable power from a mere metre or so of water is quite an accomplishment, so how exactly have the engineers achieved this?

Shaft design

The new design comprises a concrete box housing a generator that is concealed in a shaft dug into the riverbed just in front of a weir. Water drops vertically into the concrete box, turns the turbine of the submersible generator and then returns to the river below via a suction pipe.

While a small transformer station sits on the river bank, most of the system is submerged, minimising the visual and noise impact on the landscape and waterways. Also, the generator sits in flowing water so doesn't require any cooling equipment.

Crucially, the shaft design can be duplicated, unlike many existing hydroelectric plants that are costly one-off projects. In fact, Rutschmann believes pre-fabricated modules could be made, enabling the creation of a 'power plant kit'.

Wider bodies of water could also house several shafts dug side by side. Importantly, the modules can be installed using standard piling techniques, avoiding the huge expense of draining the weir.

As Rutschmann points out: 'Cost savings of up to 50 per cent can be made depending on the site. What's more, it needs only 20 per cent of the concrete volume of traditional designs and is very rapidly built.'

Rutschmann and colleagues have already constructed a one-fifth functional model of the shaft plant, equipped with a turbine and generator producing 2kW of power. This model allowed the researchers to test the hydraulic characteristics of the design, which, according to Rutschmann, are better than expected, thanks to the addition of a gate sited just above the power plant shaft.

'It is important to have a smooth velocity distribution [within the water] across the intake plane, to optimise machine performance and minimise wear,' he explains. 'We have really very even velocities across the whole cross-section of the shaft.'

The gate can also be operated at times of flooding to divert sediment around the installation. At other times, particles less than 20mm diameter will fall into the shaft, although Rutschmann is certain these will not damage the turbine blades.

Of course, injury to fish must be prevented, and researchers incorporated a trash-rack at the power plant intake to prevent fish from entering the installation. Relatively low water velocity at the intake also ensures fish can lift themselves from the steel bars if pressed against them. An automated paddle has also been added to the rack to clear heavy debris.

'We have minimised the diversion of the water-course,' Rutschmann adds. 'This is important as you don't want to have areas of water with very little current. [With this installation], the impact is minimal as you will only change the river flow over the weir.'

The researchers are now busy building a 30kW power plant with a 2.5m head, scheduled to start operating in March 2011. While Rutschmann jokes 'this is not bad for a lab', he is serious about the future.

'I imagine a large power plant, something like 500kW, will be in operation in two years time,' he says. 'Around five sites [in Bavaria] have switched from building a traditional design to a shaft design plant. These sites still have to go through planning issues, but [the developers] have selected this design as it is more ecological and not visible.'

Turbine developments

The design is only possible thanks to the development of commercial generators capable of underwater operation. Rutschmann's shaft plant currently uses a 'dive turbine', developed by Germany-based hydropower machinery manufacturer Fella Maschinenbau, motor supplier Oswald Electromotoren and renewable energy consultancy, Schodo, specifically for small hydroelectric power plants.

This variable-speed turbine is similar to a Kaplan turbine but has fixed rather than rotatable blades, making it more robust. With a comparable efficiency of around 81 per cent, the turbine is directly connected to a permanent magnet synchronous generator while an adaptive control unit handles plant operation and transfers power to the grid.

However, Rutschmann is keen to highlight that the shaft design is not dependent on a specific turbine. 'We simply need a compact turbine with a generator that can sit in the water flow,' he says. 'Besides the current collaboration with Fella, a further collaboration with [German hydro and wave turbine manufacturer] Voith, is planned in which... a universal hydro concept would be developed with a flexible range of applications.'

One such application is large hydroelectric plants, with Rutschmann hoping eventually to scale up the shaft design and construct much simpler installations, again saving money. However, given the maximum power output from today's suitable units is 1MW, his grand designs must first wait for manufacturers to develop larger turbines.

In the meantime, the shaft design could play a role in developing countries. 'Major portions of the world's population have no access to electricity, and distributed, local power generation by lower cost, easy-to-operate, low maintenance power plants is the only solution,' says Rutschmann.

But surely the cost of the turbine would prohibit the use of a small hydroelectric shaft plant? 'It would be possible to take a cheap submersible pump and run it in reverse,' says Rutschmann.

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