Tidal generator’s low maintenance

Written by: Tom Shelley | Published:

Dean Palmer reports on a unique, compact design of motor, adapted to operate in a water-powered generator for river and tidal-powered turbines



Researchers from Southampton University have designed a marine generator that could reduce the cost of producing hydroelectricity.
The team has adapted the motor to operate in a compact, water-powered generator, aiming to develop river- and tide-powered turbines.
”We’ve been working on this generator for the last 10 years. Currently, we’re still developing the prototype,” explains Steve Turnock, senior lecturer and expert in hydrodynamics at Southampton University’s School of Engineering Sciences. “We’re caught between a really good idea and an actual industrial application – so we need further funding to help us develop larger prototypes that we can test, then take the product to market.”
The idea came from a motor that was originally designed for underwater vehicle thrusters. Back in 2000, Dr Turnock and Dr Suleiman Abu-Sharkh used funding from the Engineering and Physical Sciences Research Council (EPSRC) and industry to create an electric motor design for tethered underwater vehicles. Electricity turned a ducted propeller, providing thrust to control the vehicle’s position and speed.
Tethered underwater vehicles are used extensively in the offshore industry for conducting underwater inspections and robotic manipulation. An overall propulsion system based on electrical thrusters is much smaller and lighter than the traditional hydraulic thrusters used in tethered underwater vehicles. Therefore, using the new ones reduced the weight of the vehicles, so that they consumed less power to move and were cheaper to operate.
“The concept of an electricity generator sprang out of the fundamental research we had to undertake in the hydrodynamic and electrical design of the integrated electric thrusters,” says Dr Turnock. “These thrusters are manufactured under licence by a local Hampshire company called TSL, and are now being used worldwide in a variety of underwater applications.”
He told Eureka: “We’ve adapted the original motor so that the tidal power generation version is very compact and eliminates many of the moving parts found in current marine turbines. I suppose you could say that it’s a new take on tidal energy generation.”
According to Dr Turnock, most current tidal stream generators are essentially wind turbines that are turned upside down and made to work underwater. They often include complex gearboxes and move the entire assembly to face the flow of the water. Their gears and moving parts are relatively high maintenance, especially when they are used underwater. This, says Turnock, pushes up the cost of running the turbines – which then has to be passed on to the electricity consumer. This design uses less than 20 parts.
Aside from its simplicity, the Southampton design does not need to turn around in the water because the design of its turbine blades means that they turn equally well regardless of which way the water flows past them. The blades are also placed in a specially shaped housing that helps channel the water smoothly through the turbine.
Another advantage of the design is that everything is wrapped in a single package that can be prefabricated so there will be lower on-site construction costs.
“You can simply drop the generator into flowing water and it will start generating electricity,” says Dr Turnock. “It will work best in fast-flowing, shallow water, around 40 to 50 metres deep. I foresee rows of these devices secured to sea floors and riverbeds across the UK in the next three to five years,” he adds.
The current prototype is undergoing further tests and is just 25cm in diameter. The research team plans to design a larger model, up to around 3-5m in diameter, with improved propeller blades, that will further increase the efficiency of generating electricity.
The principle of tidal power generation is simple enough: water flows over a propeller causing it to turn. This spins a shaft that is connected to a generator, which in turn, generates electricity. However, typical tidal turbines are complex pieces of kit, with lots of gears, bearings and other moving parts that need maintaining and replacing over time.
This new design promises a lower build cost and reduced underwater maintenance. It would also mean less downtime, since the generators do not need to be moved to face the direction of the tidal flow. The prototype is also designed so that all the components are in a single package, making it much cheaper and easier to install.
The motor was originally designed to power submarine thrusters and has subsequently gone into production in a range of sizes from a 50mm bore, 15W version, up to a 15kW, 300mm diameter model.
The thruster was designed by Dr Turnock and electromagnetics scientist Dr Abu-Sharkh. At the time, they were looking for a way to remove the bulk of a conventional motor and gearbox from inside the duct of existing thrusters.
“We worked pretty closely, matching the speed-torque characteristics of the propeller and motor parts of the design,” says Dr Turnock.
The result was a motor design of large diameter with its windings placed in the duct wall and its permanent magnets arranged in a ring joining the thruster propeller’s tips, leaving only the propeller bearings to obstruct the flow of water. Such a large diameter allows more poles to be squeezed in, making for a more efficient gearbox-less operation at low speed, as either a motor or a generator. According to Dr Abu-Sharkh, the motor in the large thrusters is 90-92% efficient.
As part of the University’s MSc programme in Maritime Engineering Science, the early prototype was placed in a flow test tank to see how well it performed. The generator rotated at 1,000rpm and, according to Dr Abu-Sharkh, the overall efficiency as a turbine was 35%, which is close to the maximum theoretical efficiency of 40%.
But further funding is now required. Several companies are proposing 1MW tidal generators of around 20m diameter. A generating turbine this large could be made using this new design, but both researchers favour a smaller design of 3-5m diameter.
“I think a several-kilowatt design up to 1m in diameter for operating in fast-flowing rivers would be the most practical solution,” said Dr Turnock. “The large diameter-to-length ratio of the motor, coupled with its large water-filled air gap, make it a challenge to design.”

Relatively large end windings compared with slot winding length and the large proportion of flux that ends up in axial fringes are just two problems that have to be overcome.

Pointers

The key to the generator’s high efficiency is the turbine blades, which are designed to cope equally well with the water flow, regardless of which direction it is coming from

In tank tests on a 25cm diameter prototype at 1,000rpm, the overall efficiency of the turbine was 35%, which is close to the maximum theoretical efficiency of 40%.

Funding is now needed to further develop the turbine, which could be ready for market in 3-5 years


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