Digital offshore transmission offers capex saving to challenge conventional systems

MHI subsidiary Artemis Intelligent Power is among a group of international companies developing alternative technology in the UK which suits increasing turbine capacity and will drive down offshore wind costs, Jamie Taylor, Artemis’ Senior Project Manager, told Wind Energy Update.

The DDT system up close. Credit: MHI Vestas Offshore Wind

The UK offshore market is closing in on an industry production cost target of GBP100/MWh by 2020 as it looks to emerge as the leading offshore market in the world.

Earlier this year, Mainstream Renewable Power and ScottishPower Renewables agreed Contracts for Difference (CfDs) with the UK government at £114/MWh and £119/MWh, respectively.

These prices are over £20/MWh lower than UK CfDs signed in 2014 and the developers said costs had been reduced through the deployment of the latest high capacity turbines, grid connection systems and installation equipment.

Wind Energy Update spoke to Edinburgh-based Artemis Intelligent Power, the subsidiary of Mitsubishi Heavy Industries (MHI) that is working with MHI Vestas Offshore Wind to bring Digital Displacement Transmission (DDT) to the wind energy market and further reduce project costs.

Artemis has created a system that uses two generators to enhance turbine gearbox and direct drive-transmission efficiency, by allowing unrequired capacity to be turned off in low and moderate winds.

The DDT replaces the port plates and swash plates in conventional hydraulic machines with computer-controlled high speed solenoid valves. This enables very high operating efficiency in part-load conditions, and unprecedented controllability, especially in multi-service and synchronized applications.

“The Digital Displacement Technology is owned by MHI and it will be developed, tested and certified with the aim of commercializing it for use in the offshore wind market. MHI Vestas Offshore Wind, as a future customer of this technology, is looking forward to closely follow the development,” said Matthew Whitby, Communications Officer at MHI Vestas Offshore Wind.

The DDT installed in a motor vehicle

Because of the inherently balanced load distribution and pressure-limiting nature of hydraulic machines, the transmission is extremely robust. The Digital Displacement Pump (DDP) and the DD Motor are built in modular fashion and most components can be swapped out from within the nacelle using the internal crane.

Gains for large turbines

Artemis’ technology has already proved its value in tests on large scale turbines, according to Taylor.

The Hunterston wind-turbine is a hydraulic driven demonstration machine. The current tests at this onshore site are a "verification test of the world’s first digital and variable speed controlled hydraulic driven 7 MW offshore wind turbine," he said.

The Hunterston turbine’s Digital Displacement Transmission is also the largest capacity hydrostatic transmission ever built.

The Hunterston turbine. In the center of the group is Artemis co-founder Professor Stephen Salter, whose 1984 paper Hydraulics for Wind set out 'what hydraulics can do for wind'.

“Artemis believes that Digital Displacement Transmissions for wind turbines will be particularly appropriate as sizes go up to and beyond 10 megawatts,” Taylor said.

Tests have shown that the technology will reduce costs associated with the electrical output of the synchronous generators having to meet grid code requirements. The system does not require further harmonic reduction equipment or reactive compensation.

“The power is clean and compatible with Low Voltage Ride-Through [LVRT] and grid codes. Artemis understands that compared with wind-turbines having power converter electronics, this typically implies savings of up to 5% saving on the overall wind-turbine installation capex through the elimination of harmonics control equipment,” said Taylor.

The LVRT is a widely-used grid connection requirement which responds to the mismatch between the generated active power and the active power delivered to the grid, present when there is a dip in grid voltage.

Commissioning work has shown that the components can be relatively easily swapped, due to their inherent modularity, Taylor said.

“This promises well for the avoidance of heavy lifting equipment at sea after the initial installation work has completed,” he said.

The technology also has proven fault tolerance and adaptability.

“If an individual cylinder is disabled when the transmission is under load, by removing its electrical connector, the software detects and adapts automatically by distributing lost capacity amongst the remaining cylinders,” Taylor said.

Welcoming foreign firms

To date, international technology collaboration has massively aided the UK offshore wind sector, despite local content requirements aimed at favoring domestic development.

Major offshore wind turbine companies like Germany’s Siemens, Senvion and Denmark’s Vestas have been working hard on ways to bring down project costs in order to sustain or increase market share and make offshore wind competitively priced compared with other generation types.

Siemens, for example, remains the UK’s leading offshore wind market player in 2014, boasting a 76.2% share of the country’s cumulative capacity. Vestas comes in second, with 19.9%, followed by Senvion and South Korea’s Samsung with 3.8% and 0.2%, respectively.

After the successful testing at Hunterston in the UK, MHI is installing this year a second and similar hydraulically-driven wind turbine in Japan, this time at sea. The system is being installed on a floating platform in a wind farm research project off the coast of Fukushima Prefecture.

When MHI has completed commercialisation phase of the hydraulic drive train, the company will call for an early market launch of a turbine based on the V164-8 MW platform which will incorporate MHI’s hydraulic drivetrain, according to news reports.

Sustainable sector

Open market international collaboration on projects, as opposed to countries requiring locally sourced resources, has a positive impact on long-term employment levels, according to a recent paper by the London School of Economics.

LSE economist Dr Tony Hockley argues in the paper that current UK local content policies intended to create jobs in offshore wind energy may have the opposite effect.

The UK Government demands that bidders for wind farm development must have approved “supply chain plans”, showing their local jobs and local content, and this will undermine the sector’s viability once current subsidies come to an end, the paper said.

Dr Hockley compares the energy policy to previous agendas in state aid and state protectionism in the airline industry, what he coins as risky “techno-nationalism” in industrial policy.

“Offshore wind energy is an attractive option for the UK, but it must make dramatic cost reductions if it is to play a significant part in the long-term energy mix, once the current subsidies are removed. Current government policies send mixed messages that discourage the investments required to innovate and reduce costs.”

The paper provides arguments supporting a Government commitment to offshore wind energy beyond 2020, as a spur to private investment, and a new emphasis on a competitive supply chain, instead of a local supply chain.

“In the end the subsidised and protected flag carriers’ airlines had to shed jobs or close. But competition enabled cost reductions that increased the capacity for air travel. Employment growth came about because of competition. I would expect a similar result from a stable, transparent and open market in offshore wind energy,” said Hockley.

“It is a young industry, with considerable potential for innovation free from well-intentioned, but harmful, techno-nationalism.”

By Katherine Steiner-Dicks


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