Edited by Matt Grasson



Harnessing Tides


Twiflex VKSD-FL floating disc brakes have been installed on the large Proteus tidal turbine designed and built by Neptune Renewable Energy Ltd., which was formed in 2005 to develop intellectual properties in both tidal stream power and wave power projects in the United Kingdom.

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September 9, 2011

The massive steel-hulled Proteus – 150 tons,20m x 14m – is the first full-scale prototype deployed by Neptune.In July 2010, after five years in development, the Proteus NP 1000, from Neptune Renewable Energy, arrived at the Humber Estuary at Hull, England. The demonstrator is fitted with two Twiflex VKSD-FL caliper brakes and a hydraulic power pack that hold its vertical rotor in place for maintenance or in the event of a power failure.

Development of the turbine is for estuarine locations that can exhibit powerful currents, yet have the advantages of lower access, cabling, and maintenance costs when compared with deeper water ocean locations. The unit, moored in the free stream, minimizes environmental impact while operating efficiently for flood and ebb currents.

The Proteus turbine consists of a vertical axis, crossflow rotor operating at a speed of approximately 8rpm, mounted in a 6m x 6m venture diffuser duct beneath a steel deck and buoyancy chambers. The vertical shaft connects to a 1:200 gearbox and generator in the deck housing. The rotor maintains optimal power output via computer-controlled shutters within the duct and through variable electrical load.

The Twiflex caliper brakes met Neptune's key requirements for a floating brake with a high braking force that can be horizontally fitted.

Designed to generate power in nearshore estuaries, the Neptune Proteus 1000 is controlled by a pair of Twiflex VKSD-FL caliper brakes and hydraulic powerpacks.
"It was very useful to have a turnkey solution as the totality of the Proteus design is extremely complex," says Jack Hardisty, Neptune's technical director. "Twiflex also provided clear electronic interface inputs for our SCADA system."

The Twiflex VKSD-FL disc brake caliper consists of a single spring module forming the active side of the floating unit combined with a reactive side. It is ideal for use where space is limited, as in this application, or to accommodate axial disc float of ±6mm. Achievement of braking force ratings is through a combination of different springs, shims, and air gap settings. Two VKSD-FL brakes act on the vertical shaft-mounted disc, housed with the gen-set in the binnacle over the turbine shaft. The brakes provide both parking and emergency stop functions.

More Than Requested
While the initial request made by Neptune was for brakes, Twiflex engineers did a careful review and determined it could provide a complete braking solution including the brake calipers, disc, hub, and power pack.

"Our engineering team reacted quickly and efficiently to the original enquiry," says Phil Harvey, UK sales manager for Twiflex Ltd. "The requirement was for a high braking force, horizontally-fitted floating brake, and Twiflex was able to supply this.

The Neptune Proteus consists of a 6m x 6m vertical axis, crossflow turbine mounted within a patented, symmetrical Venturi diffuser duct and beneath a very simple steel deck and buoyancy chambers.In addition, we were able to offer engineering and application support," Harvey says, "Because much of the turbine design was already done, including ordering parts, it was a big help that Twiflex was able to offer a brake that we already supply and required little, all be it some, customization."

Twiflex Ltd., founded in 1946, and based in Twickenham, England, specializes in the design, manufacture, and supply of advanced braking technologies for industrial applications.

There are three major functions of a tidal turbine brake, all of which are critical to the turbine's operation:

Holding or parking brake – for when the turbine is to be inspected or parked due to impending floodtides.

Emergency stop – used only when there is power blackout and automatic control of the turbine speed is lost. One of Altra's custom power packs includes a soft-braking option in the design of the manifold slice, which allows deceleration of the turbine without inducing excessive loads in the system.

Provide controlled braking due to overspeed. The Twiflex brakes are spring-applied and hydraulically released. If there is a power failure, the brakes will automatically engage. In addition, application of the brakes for maintenance is through the Twiflex powerpack and customer's control system. During normal operation, the brakes are off. If the brakes are applied, the rotor comes to a stop and there will be no power generation.

Engineering Collaboration
"After an engineering review of the application, Twiflex supplied a VKSD-FL with a slight modification. A powerpack, disc, and hub were also supplied to suit the customer requirements after initial discussions between Twiflex and Neptune," Harvey says. "The hydraulic powerpack operates the spring-applied, hydraulically-released brakes. The MP powerpack is a standard product tailored to each application."

The customer requested a brake size at 50kNm torque, and, according to Harvey, calculations based on inertia seem to support this. "We have actually supplied two-off brakes, each giving a torque of 150kNm when pads are fully bedded," Harvey states.

The FL is a long established variant of the standard VKSD brake, accommodating any axial disc float, which permitting installation where space is limited. They are machined to suit the customers' disc thickness.

According to Tony George, energy market manager for Twiflex Ltd., housing of the gen-set and braking system are in the binnacle over the turbine shaft, and above the water line. To avoid any salt water/corrosion issues, the brake system is housed in a sealed environment, protecting it from water ingress, so standard factory finish was sufficient protection.

"Almost every brake supplied of this type is a special, based on the original design principle," George says. "There are a huge number of options, from varing the braking force, monitoring switches, paint finish, environment protection, pad material, disc thickness, etc., to be decided when specifying the final product."

Positioned for Success
Tidal turbines generate power as do wind turbines, by the rotating of blades powered by dynamic environmental forces. The low-profile Proteus NP1000 is aesthetically unobtrusive, a significant advantage over wind power alternatives, often viewed as intrusive to sea and landscapes.

Private industry and public agencies of the EU have led the way in the development of tidal turbine technology, with the United Kingdom leading the world in this emerging market. In recent years, the European Marine Energy Centre (EMEC) in Orkney, Scotland, was constructed as the world's first dedicated test center for commercial scale tidal and wave devices. It offers developers the opportunity to test full-scale, grid-connected prototype devices in unrivalled wave and tidal conditions.

"We selected the Humber Estuary for the first deployment of the Proteus for its depth and tidal flow, as it is considered one of the best locations in the British Isles for tidal stream power," Hardisty says.

The massive steel-hulled Proteus – 150 tons, 20m x 14m – is the first full-scale prototype deployed by Neptune. Once proven, expectations are for the deployment of multiple units in estuaries surrounding the United Kingdom, which feature strong peak spring currents. Neptune Renewable Energy's full-scale Proteus NP1000 tidal stream power generator has now generated its first electricity and is soon to become one of the first economically-viable devices in the United Kingdom.

"Neptune Renewable Energy can, potentially, deliver a continuously available source of power which is not reliant on the prevailing weather conditions," Hardisty states. "The predictability of tidal streams certainly has to be viewed as a major advantage for planners at a local, national, or governmental level, as it means that output can be reliably forecast many years into the future."

Twiflex Ltd. Twickenham, England twiflex.com

Neptune Renewable Energy Ltd North Ferriby, East Yorkshire, England neptunerenewableenergy.com