Minesto DG100 Project
Renewable Energy / Tidal Energy
Minesto was in the process of developing a 100kW marine energy converter in the form of a subsea kite tethered to the sea bed. Minesto’s Deep Green technology generates electricity from low-flow tidal streams and ocean currents by a unique and patented principle, similar to a stunt kite flying in the wind. The underwater current creates a hydrodynamic lift force on the wing, which pushes the kite forwards and the kite is autonomously steered in an eight-shaped trajectory. As the kite flies across the current, it pushes the turbine through the water at a speed several faster than the actual flow rate and times electricity is produced in the on-board generator. The power has to be transmitted through a cable in the tether attached to the wing and continues via seabed cables to a converter on shore.
The generator had to be robust, compact, light-weight and efficient. The equipment, machine and frame also had to be of a very high and reliable marine standard in order to minimise the cost of maintenance and repairs. The rotor and bearings had to be capable of withstanding over-speeds in excess of 5000rpm caused by load rejections. The generator enclosure also had to withstand continuous pressures of up to 10bar under water whilst handling the continuous fatigue cycles of the kite’s movement.
The decision was made to design an Induction Generator running at a very specific rated frequency to allow for optimum energy transfer and utilising a 3kV mush winding to reduce the cable losses to the shore. The torque, slip, frequency and efficiency had to be accurately modelled and mapped to allow the converter drive to select the optimum load point for power export. The dynamic torque cycling and high overspeed requirement also meant that an accurate FE model had to be created to analyse the stresses of the frame, rotor and bearings. The generator’s thermal design also had to ensure the heat produced inside the machine during running will conduct to the generator frame first and then dissipate to the sea by convection method.
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