Tapping the Power of Low-Velocity Tidal Currents

A novel tidal power generation device has moved a step closer to commercialization with the development of a simulator to support sea trials, currently under way in Ireland’s Strangford Lough. Developers of this technology hope it will become a substantial part of the new energy mix by the end of the decade.

By David Appleyard

David Appleyard is chief editor of HRW-Hydro Review Worldwide.

Resembling an underwater kite and comprised of a carbon-fiber wing with a turbine slung underneath rather like a gondola, the Deep Green device is secured to the seabed with a tether and moves in a figure eight-shaped path in the tidal or ocean current. Deep Green is the brainchild of Magnus Landberg and is being developed by Gothenburg, Sweden-based Minesto, which was founded in 2007 with backing from BGA Invest, Midroc New Technology, Saab Group and Chalmers University of Technology.

Anders Jansson, Minesto’s chief executive officer, 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 about 10 times higher than the actual flow. The relative velocity entering the turbine is thereby considerably increased and, Jansson tells HRW-Hydro Review Worldwide, the energy output could potentially be increased by a factor of 1000 – since the velocity and energy have a cubic relationship.

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

The Deep Green device weighs in at 7 tons, which its developers say is 20 to 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

In January 2013, Minesto conducted prototype tests with a quarter-scale turbine in the cavitation tunnel at SSPA in Gothenburg. The tests were financed in part 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 under way using this 1:4 scale model in Strangford Lough in Ireland for a period of up to two years to validate the technology. The company received final approvals for the installation in 2012, including from the UK’s Crown Estate.

The Deep Green tidal energy unit consists of a wing that operates in a figure eight pattern, a turbine, nacelle, rudder, struts and tether.
The Deep Green tidal energy unit consists of a wing that operates in a figure eight pattern, a turbine, nacelle, rudder, struts and tether. One of these units is undergoing testing in the waters of Strangford Lough in Ireland.

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 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 setup, working with an as-yet-unnamed utility backer.

With the 3 kW device installed during the spring of 2013, initial testing is under way, covering elements such as retrieval, sensor operations, and health and safety. 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 research and development 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 computational fluid dynamics (CFD) analysis with these two simulators. 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 fiber, 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 about 1,300 rpm – eliminates the need for a gearing system, reducing the generator size and thereby total size and overall cost of the assembly. The full-scale version will run at about 650 rpm, although 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 DSO and UW plastics, respectively.

Extensive efforts have gone into addressing the durability of the tether to prevent failure, with Jansson explaining that, “Security is built into the 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 for the unit 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 is decreased.

Jansson explains that, to date, about €12 million to €14 million (US$16.1 million to $18.8 million) has been invested in the company and its technology. Of this, about €10 million ($13.4 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 Deep Green: “We’re aiming to produce commercially viable electricity without governmental subsidies at a cost comparable with onshore wind, and at a cost less than £100/MWh ($157/MWh) after full industrialization. But that will be at the point when we have several hundred megawatts installed. It will cost, say, £300/MWh ($470) to generate for the first few machines, but we anticipate a rapid reduction thereafter.”

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