The tidal energy device developed by Minesto, named Deep Green, converts energy from tidal stream flows into electricity by way of a novel principle, somewhat similar to the posture of a wind kite. The kite assembly, consisting of a wing and turbine, is attached by a tether to a fixed point on the ocean bed. As water flows over the hydrodynamic wind, a lift force is generated which allows the device to move smoothly through the water and for the turbine to rotate hence generating electricity.
Parts
The main parts characterizing the Deep Green technology is the wing, tether and swivel. These design elements is the “motor” of the device ensuring that the driving forces are large enough to overcome the drag forces achieving a sufficient speed ensuring high flow velocities into the turbine for efficient power production.
The wing is by design made 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. The wing will accommodate systems for buoyancy, 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. For the best possible performance the tether is designed considering hydrodynamic forces to minimize the overall drag of the device.
The swivel mounted on the foundation is the anchoring point for the tether at the seabed and as such essential for the system assuring that the tether can move smoothly in all directions depending of the current velocity direction and the kite motions.
Device Description
The kite consists of a wing (1), which carries a nacelle (4) and turbine (6), which is direct coupled to a generator inside the nacelle. The wing is attached to the seabed by struts and a tether (7). The tether accommodates power cables to shore but also cables for communication. By means of a rudder (3) and servo system in the rear cone (5) of the nacelle and a control system the kite is steered in a predestinated trajectory. The tether attaches the kite to a swivel mounted on a foundation at the seabed.
Technology Advantages
The Deep Green technology utilises low velocity currents as opposed to other technologies that compete for tidal hot spot locations. A tidal hot spot can be defined as an area where velocities are in excess of 2.5 m/s. Other stationary tidal energy devices cannot operate in the low velocity locations, as the flow of water is not sufficient to generate a useful flow of electricity. However, the hydrodynamic principle on which the technology is based allows for the kite to move at speeds of up to ten times that of the flow of water it is operating in thus unlocking many more potential tidal farm sites than its competitors.
Deep Green offers several competitive advantages:
• Low current velocities; Deep Green operates economically in deep waters at low velocities, thus the number of suitable sites are huge;
• 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; Minesto is always at 20 meters 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
|
|
|
|
The data sheets for the four Deep Green devices can be downloaded here.
Minesto DG-8
Minesto DG-10
Minesto DG-12
Minesto DG-14
|
The Beginning 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 Saab AB (the aircraft manufacturer). 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 a come 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 lightweight construction. As carbon fiber is compatible with salt water and water is much denser than air (appr. 800 times) I found that the H-rotor machine would be smaller, more weight and cost efficient in tidal current. 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.
No mechanical moment was then transmitted through the cross arm and tower and no three stage gearbox was needed. 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 digit 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, and the design offered a 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 lower-energy sites
Magnus Landberg
Deep Green Inventor
|
|
|
|
