3rd December 2013

Underwater kites: a new generation of tidal power

A new tidal energy device can operate cost-effectively in deep waters with low-velocity currents. A full-scale demonstration of this innovative technology is now planned for 2015.

Founded in 2007, Minesto is a marine energy company based in Sweden and Ireland. Their patented technology aims to provide green electricity production from tidal and ocean currents. Known as "Deep Green", it is based on a system of underwater kites, consisting of a wing and turbine, attached by a tether to a fixed point on the ocean bed. As water flows over the hydrodynamic wing, a lift force is generated which allows the device to move smoothly through the water and for the turbine to rotate – hence generating electricity.

The wing is designed to create high loads, requiring a stiff structure and light weight with sufficient fatigue and material properties and has to include watertight compartments ensuring a lifetime of 20 years. It contains a buoyancy system, batteries and pressure sensors.

The tether is mainly a force-bearing element designed to take the high loads created by the wing, but will also accommodate power cables from the generator and signal cable to the control system.

Minesto claims that Deep Green is the only marine power plant in the world that will operate cost-effectively in areas with low-velocity currents – as opposed to other technologies, which are restricted to tidal "hot spot" locations. Since it can operate economically in deep waters at velocities below 2.5 m/s, the number of potentially suitable sites is huge.

Other advantages of Deep Green include:

– A robust anchorage system, since no tower is needed

– Low maintenance cost, since only attachment and detachment has to be done offshore

– Minimal visual and environmental impact; Deep Green is always at 20 metres or more below the water surface

– Predictability. Tidal currents are extremely regular

– Small and low weight, less than 7 tons per unit, 500kW

– Gearless turbine-generator system

Vinnova, the Swedish Innovation Agency, yesterday awarded Minesto a grant for a pre-study of its planned ocean trials. Miniature versions of Deep Green have already undergone trials in scale 1:4 at Strangford Lough, Northern Ireland, and the next step is to develop a full-size prototype and test it in the ocean. The project funded by Vinnova will enable Minesto to assess the feasibility and budget in preparation for these full-scale trials in 2015.

Anders Jansson, CEO and Co-Founder: "Ocean currents are the hidden treasure of renewable energy sources. With their almost continuous water flows they carry large amounts of renewable energy over the globe, and with a high load factor compared to weather-dependent sources like wind or solar power. This resource is predictable and feasible for providing base grid power, and has minimal environmental impact."

"The challenge has been that the currents are too slow and the sites are too deep for most available marine power plants. Deep Green solves that problem. Minesto's technology will contribute to making countries like the USA, Japan and Taiwan carbon neutral and independent energy producers, instead of hugely dependent on fossil-based and imported energy."

"Just to take one example: Taiwan claims that 50 per cent of their energy can be supplied from the ocean currents along the coast if they just find a viable technology, and we believe that Deep Green is that technology. Today Taiwan depends on 98 per cent imported energy, which is a threat to the country's economy."



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