Cian Murtagh outlines the engineering behind Sea Power's scale wave-energy converter platform, as the company progresses to quarter-scale testing in open-water trials in Galway Bay, with support from SEAI
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A small Irish engineering company, Sea Power Ltd, has just launched its 1:4 scale wave-energy converter (WEC) test platform in open-water sea trials in Galway Bay (see Figure 1). The Sea Power Platform has been under development since its inception in 2008 and has received over €1 million in R&D funding support from the Sustainable Energy Authority of Ireland (SEAI).

Fig 1 Sea Power Platform

Figure 1: The Sea Power Platform

Directors Tom Lyne and Joe Murtagh have been pioneers in the wave-energy industry and have been involved with other wave-energy projects since the 1990s. The Sea Power Platform itself has had an Irish patent granted for some time, but has now been recently patented in a number of countries where there is also great wave-energy resource.

The WEC has gone through many rigorous levels of numerical analysis and small-scale tank testing. It is designed to harness the energy in deep water waves, and the company is committed to achieving this at the lowest levelised cost of energy (LCOE) possible, making it more competitive than other renewable, and non-renewable, energy sources.

The Sea Power Platform is a stable, hinged platform that is ideal for development of onboard power take-off (PTO) systems, and for accessibility during testing. It is a long machine with a low visual profile and further optimisation work will lead to machines with even lower visual profile. Pontoon-size optimisation is due in later stages to optimise the pontoon draughts and add curved features where hydro-dynamically appropriate, all aimed to further lower capital expenditure levels.

Relative angular motion and high torque is developed around the hinge (see Figure 2) and the machine tends to pitch and heave in this degree of freedom in a wide range of sea states. This hinge suits the integration of a rotary PTO, which is under development with a number of Sea Power’s partners including Limerick Wave Ltd and Romax Technologies. The WEC has a good power curve that has been established by a third party in tank-testing environments.

Sea Power: testing for success


Fig 2

Figure 2: The hinge

This medium-scale Sea Power Platform was towed from Foynes Port in Limerick, where it was constructed, to Galway Harbour. There, the Cork-based marine operations company Atlantic Towage Ltd hooked up with and installed the WEC and her mooring system into the Smartbay test site. This took place at the start of November 2016, which demonstrated a winter deployment. SmartBay offers multidisciplinary expertise to wave-energy developers and Sea Power Ltd has benefited tremendously from this. The site is pre-fitted with weather- and wave-measuring equipment, sub-sea data and power cables and a dedicated radio telemetry, which facilitates data capture back to shore.

The WEC is built from steel pontoons, which are supported by a beam or lattice chassis structure. It is known as a wave-following attenuator and its performance is influenced mainly by its overall length. The pontoons can be removed and new curved pontoons can be fitted if these prove to be better during the tank-testing campaigns that are also ongoing.

Cylindrical pontoons are currently being tested in Scotland. Overall length variation is also something that the engineers are keen to investigate. Slow adjustments in pontoon positions may shift the power-curve peak response and, in doing so, allow the bandwidth of the WEC’s power curve to widen further.

Power is measured by mechanical means such as an onboard rectilinear dynamometer (see Figure 3). The mechanical power is measured across a wide range of sea states, which enables the power curve to be plotted. In the safe and small-scale environment of the wave tank, these mean wave periods typically range between one and two seconds. For the medium-scale device, these correspond to a range between 2.25 seconds and 4.5 seconds for Galway Bay, while for full-scale systems, these correspond to approximately five- and nine-second mean periods, which typically occur off the coast of Mayo.

Since the launch date, the WEC has already demonstrated survival in extreme conditions. For this 1:4 scale of device, it has already survived maximum instantaneous wave heights in excess of of four metres (Hs=2.05m at the test site). For an equivalent full-scale Sea Power Platform, this is survival in Hs=8.2 metres conditions – or 16.4 metres instantaneous wave heights.

Fig 3 onbard brake dynamometer

Figure 3: The onboard brake dynamometer

Sea Power expects even larger seas at the test site during winter months. Wave conditions are monitored using the Marine Institute’s digital ocean portal and accurate sea-state predictions are also available, which has proved to be invaluable.

Throughout the testing period in Galway Bay, a number of onboard parameters are being continuously measured. These are: tension forces in the front mooring lines, bow panel pressures due to wave slapping and wave slam events, multi-axis hinge load in both hinge pins, accelerations of each body and, of course, mechanical power in the representative PTO system. The WEC also has a number of condition monitoring systems in place – including temperature monitoring of the control system and brake, water ingress sensors and GPS location.

The engineers at Sea Power also monitor their device using a remote camera located on the mast (see Figure 4). So far, all mechanical, electrical and control systems are performing exactly as expected and the data is streaming from the onboard DAQ at a very high rate. Post processing of this data is ongoing and the Marine Institute and MaREI are also assisting in this regard.

Learning for future development


Fig 4

Figure 4: Remote IP camera monitoring

The onboard parameters being measured can directly inform future structural and mechanical design calculations for optimised full-scale platforms. For example, mooring loads in survival conditions at the medium scale can be scaled up to full-scale loads. These loads are the main driver for cost. This means that design loads at full scale determine the size of mooring components and, hence, determine the capital cost involved.

Once these costs are established, they are inputted into a detailed LCOE calculation to determine the feasibility of the platform as a method of converting ocean energy. Sea Power needs to target cost of energy in the 15c to 20c range per kWh for now, and despite seeing a way forward to such levels, the company is under no illusion that it is there yet.

In parallel with all this, Sea Power Ltd is also designing a water-hydraulic energy delivery system that can deliver non-electric products such as energy storage of sea water in elevated reservoirs or drinking water from pressurised reverse-osmosis desalination.

Apart from aims to lower LCOE to suitable levels, another key challenge will be the quality of the power that is being produced and how the end user (grid or energy storage facility) will accept the power quality. At the moment, the Sea Power Platform is demonstrating that mechanical power is being produced from incident wave power at the site. Even 2kW of power measured at this scale can equate to 43.5 times this amount at full scale (=512kW).

Larger devices produce even more power. The next step will be to demonstrate that efficient electrical power production at this scale is also possible, and this will be done by installing the rotary PTO on the hinge mid way through the project.

Sea Power plans to remain testing at the site for an initial six-month continuous period, with additional trials to follow should this be successful. We continue to watch this space!

Author:
Cian Murtagh MEng MIEI is the project engineer for Sea Power Ltd. For more information, visit www.seapower.ie.

You can see the evolution of the device; from tank testing, through small-scale testing, to construction of the 1:4 scale device in Foynes dockyard, and deployment in Foynes Dock in the following video: https://youtu.be/Nnv_dHUksUs.

http://www.engineersjournal.ie/wp-content/uploads/2017/02/Sea-Power-wave-energy-converter-1024x580.jpghttp://www.engineersjournal.ie/wp-content/uploads/2017/02/Sea-Power-wave-energy-converter-300x300.jpgMary Anne CarriganElecenergy,renewables,SEAI,tidal,wave
A small Irish engineering company, Sea Power Ltd, has just launched its 1:4 scale wave-energy converter (WEC) test platform in open-water sea trials in Galway Bay (see Figure 1). The Sea Power Platform has been under development since its inception in 2008 and has received over €1 million in...