Seeing Deep Green with Low-velocity Tidal Power

Resembling an underwater kite and comprised of a carbon-fibre wing with a turbine slung underneath rather like a gondola, the so-called Deep Green device is secured to the seabed with a tether and moves in a figure of eight-shaped path in the tidal or ocean current.

DeepGreen is the brainchild of Magnus Landberg and is being developed by Gothenburg, Sweden-based Minesto, founded in 2007 with backing from BGA Invest, Midroc New Technology, Saab Group and Chalmers University of Technology.

Anders Jansson, Minesto’s CEO, explains that hydrodynamic forces on the kite caused by the ocean current create lift but also make the kite move transverse to the flow at a velocity around 10 times higher than the actual flow. The relative velocity entering the turbine is thereby considerably increased and, Jansson tells REW, the energy output could potentially be increased by a factor of 1,000 — since the velocity and energy has a cubic relationship.

By apparently making it possible to imitate a fast-moving stream in a low-velocity location, advocates claim it to be the only marine power technology that is able to cost-effectively produce electricity from low velocity tidal and ocean currents (1-2.5 m/s). Conversely, other technologies compete for tidal hot spot locations, where velocities are in excess of 2.5 m/s.

Weighing in at seven tonnes, its developers say this is also 20-30 times less than competing technologies located in high velocity areas and makes it possible to handle the device with smaller vessels and cranes.

Buoyant Technology

During January 2013 Minesto conducted prototype tests with a quarter scale turbine in the cavitation tunnel at SSPA in Gothenburg. The tests were part financed by the Swedish Energy Agency. The focus of the tests was on cavitation properties of the turbine and drive train performance of the device.

Sea trials are now underway using this 1:4 scale model in Strangford Lough for a period of up to two years to validate the technology having received final approvals for the installation in 2012, including from the UK’s Crown Estate.

This 3-kW test device is not grid-connected — the output is discharged at a nearby floating ‘load’ platform — but by next summer the expectation is that enough data, information and experience will have been gathered to move up to a full scale 500-kW version.

The large difference between the output of the quarter scale and the full-scale versions is a result of the cubic relationship between rotor diameter and power output, but the move nonetheless represents a big step, Jansson says.

By mid-2015 the company expects to install its first grid-connected full-scale device in a pre-commercial set up with an as yet unnamed utility backer.

With the 3-kW device installed during the spring of 2013, initial testing is underway covering elements such as retrieval, sensor operations, health and safety and so on. Testing will continue for at least one year but the company hopes to begin generating electricity any day now, assuming that there will be verified data available from the machine during the fall of 2013.

As part of the development process, a new simulator — called HAMoS (Hydrodynamic Analysis and Motion Simulation) — has been developed in-house by Minesto’s R&D department. In essence the simulator is based on two existing open source programs: one for commercial flight simulation and one for marine vehicle simulation. HAMoS combines CFD analysis with a flight simulator and a simulator for marine vehicles. The CFD analysis is used to calculate lift, drag and added mass acting on the body while the flight simulator is used as the main simulation platform formulating the equations of motion.

The end result will be used to predict how Deep Green, moves and performs in various subsea ocean environments.

“The new simulator is a very valuable tool for us as a supplement to real life sea tests since it speeds up the development of Deep Green,” said Jansson. “It is of great commercial value to be able to estimate the cost of energy more precisely at a specific location,” he added.

The quarter-scale machine was manufactured by a number of different supply companies, with the wing coming from Marstrom, a manufacturer working in carbon fibre and the turbine coming from the test facility SSPA and manufacturer Modell Teknik, also based in Gothenburg.

Operating at a relatively high velocity – the quarter scale machine operates at around 1300 rpm – eliminates the need for a gearing system, reducing the generator size and thereby total size and overall cost of the entire assembly. The full-scale version will run at approximately 650 rpm though the final choice of generator design has not yet been decided. “Changes to the generator design significantly affect the hydrodynamics, a wider generator increases drag for example,” explains Jansson. However, while the company has not confirmed the final design it is working with a leading manufacturer on this element.

Electricity is transmitted onshore through a cable integrated into the tether, which also incorporates power feed and control cables. The tether — comprising a Dyneema stress component, copper for power and control systems and a streamlined fairing in polyurethane — came from Netherlands company DSM and UW Elast, respectively.

Extensive efforts have gone into addressing the durability of the tether to prevent failure, with Jansson explaining that “Security is built into system to ensure fatigue is not an issue, we have very high standards of security.”

In addition, by operating at depth and in relatively high current speeds, growth of marine organisms is slowed allowing the use of environmentally friendly silicone paint to prevent fouling.

Meanwhile, the foundations were designed and manufactured locally by Northern Ireland’s McLaughlin and Harvey.

Neutrally buoyant while operating and typically situated roughly in the middle of the water column, the machine has active buoyancy located in the wing. For servicing or retrieval water ballast is pumped out to allow the device to surface during a period of slack tide.

All the power electronics are also located in the wing structure.

Flying Hydro

Its developers claim that Deep Green, with its relatively low weight and ability to function in low velocity currents, has several advantages compared to other tidal and ocean current power plants. In particular, the design can operate across a much wider catchment area of lower-speed currents. Furthermore, in areas with high velocity tidal currents boats can typically only operate during slack tide, a period of a few hours a day. Because of the relatively lower current speeds, Deep Green sites are accessible for much longer. Service and maintenance is therefore more cost-efficient and the capital expenditure for offshore operations are decreased.

Jansson explains that, to date, some EUR12-14 million has been invested in the company and its technology. Of this, around EUR10 million has been sourced from private equity with the remainder coming from various state sources, notably the UK’s Carbon trust, the largest state funding source.

But he has high expectations of DeepGreen: “We’re aiming to produce commercially viable electricity without governmental subsidiaries at a cost comparable with onshore wind at a cost less than £100/MWh after full industrialisation. But that will be at the point when we have several hundred MWs installed. It will cost, say, £300/MWh to generate for first few machines, but we anticipate a rapid reduction thereafter,” he says.

Read more hydropower news here.

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David Appleyard is a contributing editor. Formerly Editor in Chief of Renewable Energy World and sister renewable energy magazines Wind Technology, Large Scale Solar and HRW - Hydro Review Worldwide, now a freelance journalist and photographer contributing to a wide range of on-line and print publications. David has some 20 years' experience of writing about the renewable energy sector and is based in Europe.

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