Minesto's Deep Green subsea tidal kite

The future of renewable energy

Imagine that you are standing on a beach, flying a kite in the wind. You feel the strong lift force from the kite in the rope as the wind tries to carry the kite away. As you move the kite sideways, you notice that it flies fast – way faster than the wind is blowing.

If you would attach a turbine to the kite and put it in the ocean, where a water current flows instead of the wind blowing, you would have the concept of Deep Green, Minesto’s patented and awarded ocean energy power plant.

Expanding the ocean energy potential

What makes Deep Green different from other tidal energy technologies is the wing, the size of the turbine and the fact that the power plant is “flying” under water. The wing pushes the turbine through the water in an eight-shaped trajectory, sweeping a large area at a relative speed that is several times the actual speed of the underwater current.

The speed has a cubic relationship to the power production. This means that when Deep Green multiplies the relative speed which the turbine is pushed through the water, the electricity produced by the power plant's generator is several hundred times greater compared to if the turbine would be stationary.

By adding this step of energy conversion, Minesto expands the global ocean energy potential.

Competetive advantages

"Monopoly" on a natural resource

Deep Green is the only known power plant that cost-effectively produces electricity at sites with velocities between 1.2–2.4 m/s and depths between 60–120 meters.

Small in size and lightweight

The power plant weighs only ten tonnes which is up to 15 times less per MW than competing technologies.

Low-cost offshore operations

Small vessels and equipment are used for installation, service and maintenance. Detachable design concept enables service and maintenance on shore.

No visual, minimal environmental impact

Deep Green operates completely submerged at least 20 meter below the water surface.

Predictable electricity production

Tides are generated by the relative motion of the Earth, sun and moon, which can be calculated with almost 100% accuracy. Ocean currents are nearly constant.

Utilisation of ocean currents

The ability to operate at low velocities makes Minesto's Deep Green the only technology to be cost-efficient in both tidal and ocean currents.

Technology and project development

Deep Green has been undergoing extensive ocean testing in scale model for more than five years, during which operational functionality and power production have been verified and gradually improved. The project to manufacture and commission the first commercial-scale device is well underway. Read more under Projects.

How it works

Deep Green produces electricity by a unique principle illustrated in the figure below. The underwater current creates a hydrodynamic lift force on the wing (1) which pushes the kite upwards. The kite is steered in an eight-shaped trajectory by a rudder and reaches a speed several times the actual stream speed (2). As the kite flies in the current, water flows through the turbine at the same speed and electricity is produced in the generator (3). The electricity is transmitted through a cable in the tether attached to the wing (4). The electricity continues via cables on the seabed to grid on shore (5).

Illustration showing the principle behind how Deep Green produce electricity
Illustration of the Deep Green power plant and its subsystems

Power plant parts

The power plant consists of a wing (1), which carries a turbine (2) directly coupled to a generator in a nacelle (3). Rudders (4) and servo and control system steers the kite in the predetermined trajectory. The struts (5) are via a top joint connected to the tether (6), which is connected to a bottom joint at the seabed foundation. The tether accommodates the tether rope and cables for communication and power distribution.

Technical specifications

Rated power0.5MW
Water current velocity1.2–2.4 m/s
Installation depth60–120 m
Wingspan12 m
Height (top of rudder to end of top joint)9.8 m
Turbine diameter1.5 m
Nacelle length9 m
Nacelle diameter0.9 m
Tether length80–120 m
Dry weight10 t

A word from the inventor

Portrait of Magnus Landberg, original inventor of Deep Green.
As an engineer, I always have been interested in renewable energy and technical inventions. In year 2001, I was the project leader for a pre-study on long carbon fiber blade for large wind turbines at the Swedish aircraft manufacturer Saab. In parallel, I studied the economics of scale of wind turbines and different turbine concepts, especially vertical axis configurations, H-rotors and the Giromill concept from McDonnell Aircraft.

During this period, I came across a study which predicted the weight for a MW machine to hundreds of tonnes due to the need of large swept area and the low air density. Because of these limitations, I began to look for alternative solutions with light-weight construction. As carbon fiber is compatible with salt water and water is about 800 times denser than air, I found that the H-rotor machine would be smaller, more weight and cost efficient in tidal currents. Then I realised that the cross arm and tower could be even lighter if a high-speed turbine and a generator was attached directly to the blade.

The outcome was a compact, efficient tidal power plant – able to sweep large areas, much more efficient than rotors on static structures.

No mechanical moment was then transmitted through the cross arm and tower. In the next step, I replaced the cross arm and the tower with a wire attached to the bottom. This concept was possible if the blade or wing moved across the current all the time in a circular or an eight-shaped path, steered by a rudder (in the same way as the blades move on a horizontal axis wind turbine).

After discussions with hydrodynamic specialists the outcome was a compact, efficient tidal power plant – able to sweep large areas, much more efficient than rotors on static structures. The design offered a significant decrease in electricity generating cost.

In year 2004, I presented the invention called the Enerkite (the former name of Deep Green) for Saab Ventures. During the following years, I supervised two students in their master thesis works on the Enerkite, which showed the potential of the invention theoretically and led to a proof of concept in sea tests. The thesis also led to the conclusion that the invention is highly suitable for low-velocity tidal and ocean stream.

Magnus Landberg
Deep Green Inventor