While wind turbines are primarily seen as mechanical engineering devices, there is a definite trend towards electrification of their major systems and components. At the forefront of this development is Midlothian-based Artemis Intelligent Power, which has developed a digital displacement technology to a stage where it is ready to challenge wind-turbine gearbox and direct-drive transmissions on cost and performance.
Digital displacement machines contain multiple cylinders arranged radially in banks around a single-lobed or a multi-lobed camshaft. The key operational concepts are cylinder ‘enabling’ and cylinder ‘idling’. Pump and motor cylinders are enabled, on a just-in-time basis at bottom- or top-dead-centre if their contribution is required to satisfy the high-level needs of the system. Otherwise, they continue to idle – breathing oil in and out at low-pressure and thus placing very little parasitic burden on the active cylinders.
The basic components of the system are variable-displacement, low speed, ring-cam pump; variable-displacement, high-speed, generator drive-motor; medium or high voltage, wound-rotor, synchronous generator; hydraulic accumulators; low-level pump and motor controllers; and high-level transmission controller.
The decoupling provided by the accumulators allows pump torque to be instantaneously varied so as to capture the most energy from changing winds and gusts while the generator is driven at a steady rate. The industry-standard synchronous generator connects directly to the network without the need for a power converter and has low-voltage fault ride-through and network support performance. The natural controllability of digital displacement machines allows the use of more than one generator, so that system efficiency in low winds can be further enhanced by turning off unrequired capacity. Because of the inherently balanced load distribution and pressure limiting nature of hydraulic machines, the transmission is extremely robust. However, should partial or full rebuild be necessary, it can be carried out within the nacelle using the internal crane.
Artemis, along with its parent company Mitsubishi Heavy Industries, is refining its technology with onshore verification testing of a large digital hydraulically-driven offshore wind turbine at the Hunterston Test Centre by the Firth of Clyde (shown above). Equipped with this system, the 7MW wind turbine converts wind energy by a combination of pumps and motors to produce a constant speed irrespective of the blade rotation speed, eliminating the need for a step-up gearbox, complex generator technology and power inverter.
“In the past two weeks, the turbine has been generating power to the grid,” says Win Rampen, research director and chairman of the board at Artemis. “We are now following a process of increasing the power levels and gaining confidence as we go. We are getting some very encouraging results. It’s only been synchronised to the grid for a short time, but it has already generated 2MW peak power. And the turbine is behaving exactly as the simulation models predict. We are very confident the transmission can run at a sustained power of 7MW.”
Based on the results of the onshore verification testing at Hunterston, Mitsubishi plans to install a second and similar hydraulically driven wind turbine during 2015 on a floating platform within a wind farm verification research project being implemented offshore from Fukushima Prefecture in Japan. Ultimately, Artemis and Mitsubishi will supply the new hydraulic drive to MHI Vestas Offshore Wind, a joint venture that will apply the technology. “Mitsubishi will make the heavy machinery in Japan. However, the Artemis parts – the control elements and software – will be delivered as a turnkey system built in Scotland.”