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Technology
The PTO technology, pictured below, is designed to harness energy from the ocean in a protected environment of an enclosed nacelle. It is suited to be integrated into most common Wave Energy Converter (WEC) types including hinged attenuators and point absorber buoys, or anything with relative motion at sea.

The PTO mechanism and transmission is devised and patented by Limerick Wave (The LWPTO) and uses a combination of existing technologies currently in use in offshore wind turbine market. The function of the LWPTO is to enable standard direct drive generator technology to suit existing wave energy converters, and is the much needed link between the two.  This brings supply chain economics into the forefront of considerations for developers. The goal of deploying such a technology on board a device, is to lower the overall cost of energy of such machines. Choosing and deploying a LWPTO as a PTO will lead to lower overall CAPEX for a machine than any other competing technology streams – especially compared to hydraulic oil based systems. By its nature, the module can be integrated into any wave energy converter and is designed to do so even while a machine is on station. Operations and maintenance of such a PTO module can therefore be carried out by removal or replacement of the PTO module if desired.

The forward and backward rotation of a bidirectional sealed cylindrical unit (shown in grey above) from the motion of the water surface feeds energy to a flywheel within the cylinder. The   unidirectional rotation of the flywheel within the bidirectional cylinder allows the unit to have more than one generator, thus enabling it to deal with changing energy input levels. This allows the WEC to capture energy from irregular, multi-directional and low-speed waves in a wide range of sea states. 

The PTO mechanism devised by Limerick Wave uses a combination of existing technologies that include a gearbox and a flywheel. This arrangement eliminates the problems experienced through the use of hydraulic oil, as the only oil used in this array is used in the gearbox to prevent the gears from sticking or overheating. According to HMRC (2011) the flywheel produces smooth energy, with an output standard deviation output of 15% (maximum) from a 60% (maximum) input standard deviation and a maximum efficiency of 74%. Over travel is something that most WEC experience difficulties with, this not an issue for Limerick Wave’s power take off unit as the device was designed to rotate 360° with no end stop required, ensuring limited damage to the PTO system during rough seas.

 

How it works
The forward and backward rotation of a bidirectional sealed cylindrical unit (shown in grey above) from the motion of the water surface feeds energy to the transmission system. There, it turns a rotor/flywheel unit in one direction, with a considerable amount of power smoothing from this component. There is considerable smoothing and torque reduction achieved upfront in the transmission which lowers greatly the capital cost of the generator required. We believe this technology is a game changer for wave energy, and it has been demonstrated to convert the extreme high loads and slow speeds seen in wave energy, into grid compatible electrical power.

Over-travel is something that most WEC types experience difficulties with, this not an issue for the LWPTO unit as it was designed to rotate 360° with no end-stop required, ensuring no damage to the PTO system during extreme seas, unlike other systems or hydraulic systems in particular.

 

Testing
The LWPTO has been tested at HMRC and demonstrated excellent efficiency of 74% even at small scale (HMRC,2011). It also demonstrated considerable power smoothing capabilities. It is expected that at larger scale, the units would be even more efficient both mechanically and electrically. The LWPTO has been tested by integrating into an attenuator called the Seapower Platform and using actual motions and loads experienced by this attenuator on the PTO module transmission. It has been tested to proof of concept level, and has demonstrated the use of direct drive generator technology in a small scale laboratory environment.