Product development

Background

Since 2005, TGL (now part of Alstom) has been developing a tidal stream turbine in order to generate electricity from the flow of the tides. The TGL concept was developed with several objectives in mind: minimising complex and hazardous operations at sea, minimising the use of costly large offshore marine vessels and exploiting tidal stream resource reliably, invisibly and economically.

We have designed, assembled, tested and deployed concept demonstrator machines at the EMEC tidal energy site, Orkney. The first demonstrator was sufficient to power about 300 homes when the tide was flowing.  In its first 10 weeks of operation it generated over 50MWh of power and produced over 250 MWh in total during its test phase. Over the course of the machine’s commissioning, a significant amount was learned about the operation and characteristics of the machine, the nature of the tidal flow, waves and weather impact at EMEC, as well as corrosion and bio-fouling issues on the turbine.

 Why Alstom’s tidal stream turbine?

 Simple, efficient to transport – its buoyancy means that it is easily installed and retrieved in a single tidal cycle using small vessels, reducing installation and maintenance costs

Intelligent nacelles – thruster intelligently rotates the nacelle to face the direction of the tide, managing ebb and flood tides seamlessly and maximising energy production

Efficient blades – turbine blade pitching can be altered to control load on the turbine and optimise use of the tidal conditions locally

Features of the tidal turbine

• Rotor diameter 18 metres
• Turbine length 21 metres
• Service life of 30 years with minor maintenance every 2 years
• Turbine weight: 135 tonnes (not including seabed support structure)
• Turbine width: 2.6 to 3.5 metres
• Turbine height: 5 metres
• Installed water depth 35 to 80 metres
• Water speed at rated power: 2.7 m/s
• Cut-in velocity: 1m/s
• Maximum operating water speed: 3.4m/s
• Power exported via subsea cable to grid at 6.6kV

The turbine consists of a three-bladed, upstream pitch-controlled rotor with a relatively standard drivetrain and power electronics inside the nacelle. The nacelle is mounted onto a separate seabed mounted foundation. The nacelle rotates on the foundation every time the tide changes, to face the new oncoming tide.

The foundation for the prototype is a relatively lightweight steel tripod support structure attached to the sea bed secured to the sea bed by drilling and piling.

The turbine nacelle itself is buoyant, allowing the turbine to be towed to and from the foundation site. As this site can be difficult to access a Remotely Operated Vehicle (ROV) is used to connect the machine to the foundation via a winch rope and the machine is winched down under water and connected to the foundation using a patented clamping mechanism. High voltage electrical and control connections are then made automatically and the machine is ready to generate electricity.

 Deployment

We have designed a 1 MW turbine, in partnership with the Energy Technologies Institute (ETI), known as ReDAPT (Reliable Data Acquisition Platform for Tidal). This project aims to collect and publish significant information into the public domain in order to benefit the entire tidal industry. Information obtained will include data about the environmental impact of tidal machines, bio-fouling and corrosion, computer modelling of turbine arrays and instrumentation application on tidal turbines. The 1 MW machine, using design processes and assembly techniques that will be used for volume production units in the future.

We have replaced the 500 kW device installed at EMEC with our pre-production 1 MW device.  This has been mounted on the same tripod support structure and is undergoing a 18-month test schedule.  Now part of Alstom, we plan to deploy demonstration arrays as a precursor to full commercial production.