As well as being the world’s first commercial scale tidal turbine, SeaGen is also more efficient, safer, easier to maintain and best suited to the challenging marine environment than anything that has been proposed to date. SeaGen is leading the way to unlocking the potential of tidal stream and marine current energy on a commercial scale.

SeaGen is based on MCT’s experience with its predecessor, the 300kW Seaflow system installed off Lynmouth Devon in May 2003 and still thriving in open sea conditions.  Hence a wealth of experience with large scale technology in authentic offshore operational conditions lies behind the design process that led to the development of SeaGen.

The Rotors

The Structure

Comparisons

The Rotors 

 

 

 

Apart from the substantial experience behind its development, SeaGen is more efficient both in the amount of energy it can extract from the current and in cost. This is because it uses a pair of pitch-controlled axial flow rotors, which for good reason are the technology of choice in the closely analogous low-head hydro and wind generator industries.  In fact virtually 100% of all commercial sized wind turbines use pitch controlled open axial flow rotors.  Many other, sometimes “weird and wonderful”, rotor concepts have been tried both as wind turbines and as hydro-turbines but in the end the elegant simplicity and unsurpassed efficiency of the axial flow pitch-controlled rotor has been shown to make it superior in all respects to any other method of kinetic energy conversion.  What applied in the fields of hydro and wind power seems unlikely to be significantly different in the field of water current kinetic energy conversion because similar laws of physics apply.  

The rotor design also provides the capability for controlling a large power system as the rotor blades may be pitched into a neutral position to stop the turbine gently even at full flow – an essential requirement for any power generation system; by comparison fixed pitch turbines require a powerful brake to stop them and if the brake fails they cannot be stopped.

Moreover SeaGen’s rotor blades can be pitched to limit the power to a pre-chosen “rated power” at times when high velocities are experienced; this greatly reduces the loads on the turbine structure, the rotor blades and the power take-off – and reduced and controlled loads translate into reduced costs and safer and more reliable operation. These days no major wind turbine manufacturers use fixed pitch rotors for the aforementioned reasons.

What is more SeaGen's rotor blades can be pitched through 180 degrees so that the rotor can run efficiently in a bi-directional flow (on both the ebb and the flood tides) – this allows high lift/drag ratios to be developed (through using cambered high lift foils) which in turn are essential for high rotor efficiencies. MCT’s patented rotor blade pitching system has been shown to permit rotor efficiencies of similar quality to the best in the wind turbine business.

Fixed pitch rotors and rotors in ducts, as have been proposed by others, can never compete effectively for such high levels of efficiency or energy capture as can be gained from conventional pitch-regulated axial-flow rotors. This has been comprehensively demonstrated over several decades by the wind industry under closely analogous flow conditions.

The Structure


Apart from using the rotor technology of choice, MCT’s SeaGen system is mounted on a structure securely seated on the seabed. This makes it easier to maintain and more steadfast in the challenging marine environment.

The technology for placing monopiles at sea is well developed. The patented design of the SeaGen turbine is able to be installed and maintained entirely without the use of costly underwater operations. A unique, and also patented feature of MCT's technology is that the turbines and accompanying power units can be raised bodily up the support pile clear above sea-level to permit access for maintenance from small service vessels. This is an important feature because underwater intervention using divers or ROVs (Remotely Operated Vehicles) is virtually impossible in locations with such strong currents as are needed for effective power generation. The artist's impression indicates a row of turbines such as MCT is planning to install (subject to consents) off the north Anglesey coast and shows one raised for maintenance from a small workboat.

 

Inexpensive access is particularly important for minimising the risks of running up unexpected maintenance costs.

 

How do you fix the system so it cannot move? It is often not well understood that the act of taking energy out of a flowing water current generates a major thrust reaction, typically in the order of 100 tonnes per MW, which in turn demands competent foundations. Anchors, relying on gravity and friction with the seabed, and other such seemingly simple solutions are generally inadequate for commercially sized tidal turbines. Perhaps the most difficult engineering problems are the high structural loads to be dealt with and in effect solving the question of “how do you nail it to the floor?” The weakest material to which the turbine is attached is the seabed itself, even if it is rock, so the foundations need to be sized so as not to overstress the seabed and cause the turbine to move or break loose. This is where piles drilled deep into the bedrock of the seabed, as used by MCT are probably the only reliable solution.

 

Comparisons with other technologies


MCT’s SeaGen is the only tidal current turbine world-wide to be ready for deployment in commercial projects. There are many web sites claiming to have developed such technology, but in reality there are very few other designs which have even been tested in the sea, and with one exception these are all small-scale demonstration projects that will need to be scaled up significantly before they could be capable of commercially viable operation. This is because the fixed overhead costs of an offshore project are high and therefore a large system is essential so as to collect enough energy to cover its costs and make a profit. Just as no wind farm developer these days will consider using windturbines of less than 1MW similarly it is uneconomic to consider using very small tidal turbines.



These devices can also be compared in terms of the rotor swept area (or where a duct is used, the entry cross sectional area of the duct) since the swept area or entry area governs the cross-section of current energy that can be captured. Given devices of similar efficiency, the energy capture at any given site is proportional to swept area just as solar panels deliver in proportion to their array area and windturbines in proportion to their rotor area. MCT’s economic analysis suggests that at least 300 square meters of rotor area are about the minimum that could realistically generate commercially competitive power; this is regardless of the kind of technology. For reference, MCT’s SeaGen system with 16m rotors has a swept area of 402 square meters. Most of the few competitors who have demonstrated systems in the sea have devices with rotor diameters in the order of 5 to 9m, which would be 15 to about 60 square meters.