|The Rotech Tidal Turbine|
Bosch Rexroth, the drive, control and linear motion specialty subsidiary of the Bosch Group, is working with UK firms Lunar Energy and Rotech on a prototype tidal turbine for a tidal power project to be tested this year off the Orkney Islands.
Rotech, a specialist in tools for the oil & gas, geothermal, subsea and renewable energy markets, developed the bi-directional Rotech Tidal Turbine (RTT) that has been exclusively licensed to Lunar Energy. Rexroth is providing the scale of equipment needed to bring the concept from small scale water tank model to prototype scale.
Power is generated from tidal water movement that flows through the turbine which turns a large fixed-displacement hydrostatic pump to produce hydraulic pressure and flow.
The hydraulic flow and pressure varies with the state of the tidal stream, from minute to minute, due to the nature of the hydrodynamics of the location of the turbine. The turbine also reverses direction when the tidal flow reverses as it does twice a day.
The output to drive the generator is needs to be at a constant speed and direction. Rexroth provides a duel overcenter swash plate axial piston motor to carry out this task of converting the flow and pressure into mechanical shaft power.
The duct captures a large area of the tidal stream and accelerates the flow through a narrowing channel into the turbine. Thus, a smaller turbine can be used for a given power output, or alternatively, a larger amount of power can be generated by a turbine of given blade diameter.
The capability for bi-directional operation obviates the need for a pitch or yaw control thus keeping the design simple and more cost effective. Tidal flow can be offset by as much as 40 degrees to the duct axis without affecting the performance. In fact, when operating in flows that come from this ±40 degree sector, the ducted system extracts more power than when the flow is perfectly aligned with the turbine axis.
The bi-directional turbines, each weighing about 1,000 tonnes, will be mounted on the seabed. The core elements of the RTT are a large bi-directional venturi casing with an inlet diameter of 21m, narrowing to 14m, and a five-bladed fixed-pitch propeller of innovative blade form. All components are to be designed for a four-year maintenance period.
The prototype unit, designed to produce about 1MW of power, will be placed on the seabed off the Orkney Islands during Summer 2006.
The Orkney tidal power project is backed by a £5-million (US$9.2-million) grant from the Department of Trade and Industry.
Rexroth is also a prime supplier to the major wind turbine manufacturers. The company is also working on hydraulic hybrid drive components.