Green energy from 'underwater kites'!

Underwater kite from MINESTO

Remember the days when you owned a kite and looked forward to windy weather to try it out?

The stronger the wind, the faster your kite flew. It was great fun!

Now imagine capturing that energy by attaching a turbine to the kite – then plunging it into the ocean.

Replacing wind power with the strength of water currents is the basis of a renewable energy-producing concept developed by a marine technology company based in Sweden.

Minesto’s Deep Green power plant technology converts renewable energy from tidal and ocean currents into electricity using an innovative process similar to that of the wind kite.

The prototype marine version, which consists of a wing, turbine, nacelle, rudder, control system and struts, is attached to a fixed point on the seabed. It’s designed to fly in a figure eight pattern using the kinetic energy from the waves combined with an automatic steering system.

As water flows over the wing, the current creates a lift force which pushes the kite forward through the water. The kite is steered by the rudder in a figure-eight trajectory and reaches speeds 10 times faster than the water current just as a conventional kite travels much quicker than the wind which powers it. As the kite moves, water flows through the turbine and electricity is produced in the gearless generator. The kite speed has a cubic relationship to the power so movement at 10 times faster than the tidal or ocean current produces 1,000 times more power.

Electricity is transferred from the plant generator through the tether to the seabed where it continues in a seafloor cable. Minesto plans to connect several power plants together in an array to increase the energy production potential from each site. In years to come it’s envisaged that an array of up to 100 Deep Green energy-producing devices will supply enough power for more than 33,000 homes.

These tidal and ocean current power plants seemingly offer several key advantages over other tidal energy devices. Being light and small, Deep Green power plants can be used in waters where no other known technologies operate cost-effectively in low velocities. In this respect Minesto is expanding the marine energy potential and offering what it describes as “a step change in cost for tidal energy”.

Other competitive advantages include:

* high efficiency – the relative flow factor is increased by a factor of 10-plus

* small and lightweight – plants weigh, on average, seven tons, 10-25 times less per megawatt than technologies developed for high velocity currents

* low-cost offshore operations – power plants require only small boats and equipment for installation, service and maintenance

* plant technology maximises power potential at low velocity – the eco-friendly devices operate cost-effectively in tidal and ocean currents.

minesto kite
The Kite produces electricity by a unique principle illustrated in the figure below. The water current creates a hydrodynamic lift force on the wing which pushes the kite forward (1). The kite is steered in an 8-shaped trajectory by a rudder and reaches a speed ten times the water current speed (2). As the kite moves, water flows through the turbine and electricity is produced in the gearless generator (3). The electricity is transmitted through a cable in the tether attached to the wing (4). The electricity continues in sub-sea cables on the seabed to the shore (5).

Five variations of the proposed Deep Green power plants are under development. They range from the two-ton, eight metres (26 feet) wing span device operating at depths of 50-65 metres (164-213 feet) with the potential to provide power for 80 air conditioner systems for a year, to the considerably more powerful 11-ton, 14 metres (46 feet) wing span version designed to operate in depths of 90-110 metres (295-360 feet). Developers estimate that two arrays of 100 large power plants could supply almost one third of Malta’s annual electricity needs.

The first tests in authentic ocean conditions took place in 2011. Buoyed by encouraging results, a 25 per cent scale prototype has been undergoing extensive sea trials at Strangford Lough, Northern Island. Further development of installation, operation and maintenance procedures is expected to achieve significant cost savings.

Extensive assessments are also carried out to ensure the presence of Deep Green power plants have no impact on subsea environments or disturb local ecosystems. Mammal surveillance is carried out before, during and after each trial.

Development of the Deep Green technology has now entered a new, exciting phase following an agreement between Minesto and Florida Atlantic University (FAU) to assess the feasibility of using the power plant devices to harness ocean power from the Florida Current, a thermal ocean current linking the Gulf of Mexico with the Atlantic Ocean. The agreement will examine the technical, environmental and economic case for installing demonstration and commercial power plants in the Florida Current. The long-term aim is to develop a partnership with mutual research, testing and educational activities within the terms of a memorandum of understanding.

Minesto is hoping to demonstrate that one ‘underwater kite’ alone will produce enough energy in one hour to supply an average home for two weeks. The company hopes to make Deep Green products commercially available within four years.

Read more about the Deep Green technology development here.

Source: OceanHub

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