Making marine energy viable

Justin Cunningham finds out about the engineering and innovation breakthroughs that are helping to make the economic case for renewable marine energy.

Burning fossil fuel to create electricity is a well known process and, barring emission caps, is limited only by the resource. This is in strange contrast with renewable power generation. With abundant natural resources, the limiting factor for this seems to be technology. It is not that it can't be done, it can't be done at the right price. Converting energy from the wind, sea or sun in to useable electricity economically is now the biggest challenge for the renewable energy industry, particularly those focusing on offshore. The use of wind power has risen significantly in the last decade, especially in the UK. Yet the other resource that the UK has in abundance – waves and tides – remains largely untapped. The potential for renewable marine power generation is huge. Natural water systems contain vast amounts of energy and the energy embedded in waves around the globe dwarfs the world's electricity consumption. Harnessing all of that energy would be near impossible, but even the extractable wave energy has the potential to make a substantial contribution to the overall energy mix. The other big advantage is that, unlike wind power, wave power – and tidal power in particular – is predictable. "The UK has got the best tidal resource in the world and some of the best wave resources," says Stephen Wyatt, Marine Energy Accelerator manager at The Carbon Trust. "Our analysis suggests we can generate up to 20% of the UK's electricity from wave and tidal energy." But the engineering challenge to extract energy from these systems economically is immense. In 2005 The Carbon Trust launched its Marine Energy Challenge in order to better understand the engineering and economic challenges facing wave and tidal energy. This has been followed by its Marine Energy Accelerator Programme, which aims to facilitate solutions. "We have to make wave and tidal energy commercially viable," says Wyatt. "Much of the technology has been conceptual from early stage inventors and universities." This is why the market is mostly occupied by small companies and start ups. But larger engineering companies have a role to play by supporting the implementation of an effective supply chain. Part of The Carbon Trust programme involves identifying the generic component technologies that can be used in different wave and tidal energy devices. "For example, getting hydraulic systems to a stage where they are reliable enough for subsurface applications," says Wyatt. "A number of technologies work well in the lab, but need a lot of work to 'marinise' them. Part of the programme is to engage with suppliers which are not necessarily in the marine energy industry, but which may have technology that could provide some of the answers." And the UK is well placed to facilitate the solutions. A number of large prototypes are currently being trialled at the European Marine Energy Centre (EMEC). Set up around the Orkney Islands, EMEC has five tidal berths and four wave berths for full size prototype devices to be deployed, tested and connected to the grid. Tidal stream devices have seen a reasonable amount of design conversion, all broadly using an underwater horizontal axis turbine. The key engineering challenge is to make the structures robust enough to last in excess of 20 years with a minimum need for maintenance. Wave power development is at a more fundamental stage. The two main challenges here are the ability to harness the energy and to cope with the potential for huge storm waves that will put significant stress and loading through the structures. "There are a number of different designs out there that all look quite different," says Wyatt. "We are still trying to understand the best way of harnessing wave energy." Pelamis Wave Power uses a series of cylindrical sections linked by hinged joints. This uses the waves to induce motion in the joints, which is resisted by hydraulic rams. This, in turn, pumps high pressure fluid through hydraulic motors to drive electrical generators. Ocean Power Technologies, meanwhile, uses floating buoys, with the resulting vertical motion used to drive an electric generator. And Aquamarine Power uses a hydraulic flap to pump water ashore to drive a hydroelectric power station. Aquamarine Power's Oyster concept aims to capture the energy of waves in near shore applications and the company is testing a large scale prototype and completing tests at the EMEC. "The device would typically be deployed in water between 10 and 20m deep," says Dr Ronan Doherty chief technical officer at Aquamarine Power. "The concept is principally pinned to the seabed and has a large buoyant flap that oscillates in the waves of a near shore environment." These waves have the advantage of producing large horizontal forces. "It is that magnified surge in the waves that our device exploits," says Dr Doherty. The company wanted to keep its design as simple as possible and the offshore component of the device has no active controls, electronics or gearboxes on the buoyant flap. The flap is connected to a closed loop hydraulic system. The waves induce motion in the flap and that essentially drives two large pistons. These pump water ashore through a high pressure pipeline to power the hydroelectric power station before the water returns to the buoyant flap through a low pressure pipeline. "This keeps the offshore part as simple as possible," Dr Doherty continues. "The generators, the control system, the transformers; all this is onshore where we can get at it and maintain it in the usual way." Because the offshore part of the Oyster device has very few moving parts, maintenance is a lot easier. Maintenance is a huge issue for marine renewable devices in general and was a key driver for the Aquamarine concept. And the device is capable dealing with the unpredictability and ferocity of storm waves. "If an extremely large wave hits, it will cause the bottom hinge, along with the whole device, to rotate up to 70°," says Dr Doherty, "so it literally ducks out of the largest waves and doesn't have to absorb massive amounts of energy. "Having said that, we still have to design for the extreme loads and have done a lot of testing on that aspect." Another key design challenge for Aquamarine Power is the extreme cycling that the device, and its component parts, will experience. Coping with anything from 3 to 4million waves a year will take its toll on valves, pipelines and pistons. As a result the company has carried out its own component testing – and some development – on many standard products to make sure they will stand up to the application. Like many major players in the marine renewable sector, the company looks toward the oil and gas sector as well as the defence industry for reliable engineering in the offshore environment. "That is something our industry needs to tap in to," says Dr Doherty. "But there can be this clash of cultures. We want to get these things built and deployed, but those sectors tend to be very conservative, with long term strategic out looks." Another company well into the development phase is OpenHydro. The Dublin based firm also has a prototype platform of its tidal stream device in EMEC and has been carrying out test and research on the rig since 2006. OpenHydro, which uses an open centre turbine, has also chased a philosophy of keeping everything simple and straightforward. James Ives, managing director, says: "This has got to operate in a harsh, corrosive and aggressive marine environment. So our design approach is to keep the turbine as simple as possible, limit the need for maintenance and minimise the possible failure modes. We have to produce something that is very robust." The turbine uses no oil or lubrication on its moving parts: a feature intended to minimise the maintenance requirement as lubrication fluids would need to be replenished. It also earns green credentials and removes the risk of pollution. The turbine is made out of a combination of glass fibre with a reinforcing steel structure to give strength and rigidity. Glass fibres are well known for being good in marine environments and are commonly used in boat hulls. The OpenHydro structure at EMEC is fundamentally a research structure, so is heavily instrumented. Mounted on a tower that protrudes from the water, the turbine can be lowered, tested underwater and then recovered. This allows the company's design engineers to analyse and inspect it. "After a time, we can take that turbine off the structure and put another one in its place, so we can have a continual improvement process," says Ives. "The commercial deployment, though, will be mounted on the seabed so you can't see them or hear them and they will be deep enough to not interfere with shipping." The company has also developed a system to mount the whole device and its support structure on the seabed quickly and easily. The basic principle is to assemble and mount the turbine on a frame so it can be fully commissioned onshore, after which it is taken out to sea on a specialist deployment barge that submerges it. This ensures as little as possible is done offshore. This system has also undergone extensive trialling at EMEC, with great success. Another big challenge for underwater systems is sealing mechanical, hydraulic and electronic equipment from the marine environment. Here, the oil and gas, offshore wind and even boat building industries have been able to offer possible solutions. "They have become suppliers," says Ives, "and have been able to provide products and components off the shelf in some instances. That has been very useful." OpenHydro has recently shipped its first commercial 1MW unit to its first customer; Canadian based Nova Scotia Power. It is now in the process of scaling its business to meet demand with interest for commercial units expressed by the UK, Ireland, Europe and the US.