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By Dr Graham Foster, Chief Technology Officer,  Marine Power Systems (MPS)

In this month’s blog, CTO Graham Foster takes some time to consider the challenges of energy generation at sea and explains how the unique mechanics of the WaveSub device address them.

How do you successfully generate energy from moving water? This question has occupied the mind of many an engineer over the course of time. From water-powered mills in early Hellenistic times, to the first hydroelectric power plant in 1879, history is replete with fascinating accounts of how hydropower has inspired the design and build of machines to improve human life.

Over 800 times denser than air, water is the most abundant resource on our planet. The world’s oceans, which cover more than 70% of the earth’s surface, are an untapped, powerful, highly concentrated clean energy resource. Ocean-generated energy holds huge potential on both an environmental and economic level. As outlined in our report launched in June, Making Wave Power Work, global wave power resource is estimated to be as much as 80,000 TWh / year, of which up to 4,000 TWh / year is considered economically exploitable. Industry body Ocean Energy Europe has stated that 337GW of marine renewables could be deployed globally by 2050 with the global marine renewables industry set to grow to £76 billion.

Evidently there is great potential in wave energy generation. Why, then, has it taken so long to create a device that is capable of harnessing it? The answer is that a device designed to generate energy from the movement of ocean waves must be able to contend with a temperamental, inhospitable and unruly environment. From the extreme forces exerted on to it, to how components survive with constant exposure to sea water, the smallest details need to be considered in order for the device to be fully functional and commercially viable.

Over the past decade MPS have been working on an ambitious project to develop and bring to market the WaveSub. To date we have received over £5 million in finance from a number of private investors and public grants to finance this project. From the early R&D stages to proof of principle device and small scale prototype, the WaveSub journey has employed a ‘design, test and scale-up’ approach to development. This has ensured that the design concepts remains robust and continue to address the challenge of energy generation at sea.

So how does the WaveSub contend with the inhospitable environment it is set to work in? MPS identified four main challenges of energy generation at sea. Below I outline what they are and how we have overcome them, as well as providing an overview on the key components of the WaveSub.

The WaveSub – an overview 

The WaveSub is a wave energy generation device which comprises two main components: a spherical-shaped power capture float and a rectangular reactor barge. The power capture float sits atop the reactor barge and all machinery, power and control equipment is housed on or in this barge. Four power take-off (PTO) lines and a central tether connect the power capture float to the reactor barge. Taut mooring lines connect the reactor barge to the seabed anchors.

Survivability

The first challenge, survivability, was one that we knew needed to be addressed right from the start of our project. It outlines an imperative for the device to withstand the brutality of life at sea, particularly during storms. To contend with the forces that storms could place on the device, the WaveSub is fitted with a depth adjustability mechanism. This mechanism enables the WaveSub to adopt a survivability mode, during which it descends to the sea-bed. Here it is protected from the loads that storms would place on the system. 

Energy capture

The second challenge identifies the need for a device to capture wave energy efficiently and accommodate all sea conditions. The subsurface orbital flow of waves is powerful but difficult to harness – most of the energy is underwater and the orbital energy flow necessitates a more complex approach to exploit efficiently. In the case of the WaveSub, the power capture float can track the orbital energy path, allowing it to harness energy through the entire wave cycle. The same depth adjustability mechanism used to protect it from storms also enables it to move to an optimum generation depth in the sea.  The depth adjustability is operated by a control system, which regularly monitors the sea state (wave height and period), thereby adjusting the float and reactor depth to an optimum location for energy generation.

Transportability / operation and maintenance

One of the main considerations when developing a wave energy generation device is the overall ease of deployment. No matter how fantastic a concept, the economic propositions of a device will be undermined if the costs to deploy, recover and maintain it at sea are too high. With this in mind, the WaveSub was designed to have a dedicated surface configuration in which the entire machine is a self-contained barge.  In this configuration, the machine is easily towed without the need for specialist vessels, and can be easily accessed for routine maintenance. 

Capital costs

Overall costs need to be considered throughout the development of any device being prepared for market. The WaveSub has been designed so that it is cost-effective to build, largely using off-the-shelf components with minimal bespoke engineering, thereby significantly de-risking it. Furthermore it is small, in relative terms, for the power it produces, reducing the ‘per megawatt’ cost of the structure.

The WaveSub is the only device that currently addresses these four challenges of wave energy generation. Our economic forecasting suggests that in time, it will be in a position to compete with other renewable energy sources such as wind and solar. The UK, currently a global leader in the marine energy sector would see a new maritime-based renewables sector emerge that brings new jobs, particularly to coastal areas of the UK which are often in need of an economic boost. 

Of course, there is an additional obstacle which no amount of engineering and design can solve, which is securing the right long-term outlook and stable policy framework from Government. The future for wave power and large-scale wave farms is promising if these considerations are put in place. As outlined in our report, we believe wave power technology has the potential to supply around 10% of the world’s electricity demand by 2050. With the right mentality and strong support from government, this vision can be realised.

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