New thinking key to viable water and wave power

Tom Shelley reports on simple ideas that give a dramatic improvement to the efficiencies of water based renewable energy sources.

Power can be harnessed from flowing rivers and tidal streams by using a novel German water wheel with greater efficiency than hitherto by allowing the vanes to partially rotate, Romanians have come up with a novel wave to pneumatic generator, which also uses free to rotate vanes and UK ventures press forward with yet more ideas.

The water wheel and Romanian concept were both were on show at this year's Hannover Fair, the water wheel in the form of a large, working model, and the wave power generator, in the form of the component parts of a 3.5kW demonstration unit. The novel water wheel has been developed by consulting engineer, Hartmuth Drews, who is based in Pinneberg, and runs a company that makes water wheels commercially.

He calls his new design an, "Energy floater water power plant". His breakthrough ideas are to mount the water wheel on a pontoon in the flowing river and include a framework, to support the blade against the force of the incoming water flow, but which allows them to freely pivot back when withdrawn.

He claims that this increases mechanical efficiency by around 40%, rediscovering and adapting an idea made use of by the designers of nineteenth century paddle steamers, who found that efficiency was greatly improved if blades were turned vertically as they entered and left the water.

Having the water wheel float on the river gets round a whole host of problems. Drews water wheels are normally installed in existing old mill sites, but in a new installation, introducing dams for small and large scale hydro installation means dealing with environmental legislation that requires free flow of the main body of water to allow fish to reach their spawning grounds.

Hence, new installations requiring a dam across a river from one bank to the other have difficulty obtaining approval, even if they include fish ladders. Furthermore, there is a considerable cost requirement for civil engineering work in order to provide head for a traditional type of installation, whether using water wheels or turbines.

Traditional water wheels rotate slowly, typically 5 to 10 rpm, producing torques of 8 to 25kNm, and it is necessary to gear them up by 150:1 to 300:1 to turn a generator at 1,500 rpm. Drews observes that while planetary gear trains can transmute such loads and achieve suitable gear ratios with more than 90% efficiency, they are not cost effective for this type of application.

For this reason, he proposes that the Energy Floater should be equipped with gear ring segments round the wheel rim that engage directly with a pinion attached to the shaft of the generator. Much of the untapped hydroelectric power potential in Europe is small scale – "Microhydro". A Scottish government report, "Employment Potential of Scotland's Hydro Resource", published in September 2009 suggested that there could be 1.204 GW of potential new hydro capacity in 7,043 schemes across that part of the UK. 4,419 of these are less than 100kW.

On the stand of the National Institute for Research and Development in Electrical Engineering ICPE-RA, which is based in Bucharest, we saw a small, submersible generator with a directly connected, four bladed turbine. It is designed to rotate at 350rpm in a water flow of 1.6m/s. While producing only 200W at 11V, it is more than enough to power a remote monitoring system, and if made a little larger, could be made to power a mobile phone base station, the latest designs of which only consume about 500W.

The same laboratory also had on show a 750rpm , 3kW permanent magnet generator optimised for small scale hydroelectric and wind power generation and a hydro-pneumatic converter in which waves compress air in a narrowing chamber and drive a turbine. The problem that has to be overcome with such devices is that the air moves back and forth, but the generator needs to keep rotating in the same direction.

One solution is the Wells turbine, but its efficiency is only about 40% to 70%, which is significantly less than for traditional turbines, which in large units, come in at more than 90%. The Romanian solution is to direct the airflow into a narrow, annular space, and have blades that can pivot. This requires that the blades be short and stubby rather than long and narrow.

The experimental unit we were shown seemed robustly made and we were told that it produced a rated power of 3.5kW at 2,000 rpm from a 50m/s air flow. A suitable way of producing large amounts of compressed air from wave power is John Kemp's OWEL (Offshore Wave Energy Limited) converter, which has waves compressing air as they move along a narrowing chamber in a free floating vessel.

Thanks to £2.5 million from the Technology Strategy Board, OWEL, working with IT Power is now at the design and build stage of a 42m long, 600 tonne, 0.5MW demonstrator. The machine is to be tested at the Wave Hub facility off the north coast of Cornwall. A similar commercial machine deployed in open seas would have a rating of 1MW.

IT Power is also the lead company in an EU funded project to develop the Pulse Tidal Device in which oscillating horizontal hydrofoils extract power from tidal or other water flows. The company has begun a year long environmental study ahead of an application to Marine Scotland for a licence to start producing 1.2MW of renewable electricity in 2012 in Kyle Rhea, a narrow, fast flowing strait between the Isle of Skye and the Scottish Mainland.

The company has made extensive use of Autodesk Inventor to simulate the transmission mechanism in the Pulse Tidal Device and the mechanical and structural design of the OWEL.

Design Pointers
• Water wheels can extract power from water flows by being mounted on a floating pontoon. Allowing the blades to pivot as they are lifted out of the water greatly improves efficiency
• In wave power to pneumatic systems, turbine efficiency in a bidirectional turbine can be greatly enhanced by allowing the blades to pivot
• Both the Offshore Wave Energy (OWEL) and Pulse Tidal Devices have reached the large prototype design and build stage

Tom Shelley

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