Wind and solar power are well established and, as their commercialisation continues apace, there is a degree of technology convergence in each case. This is not the case with wave and tidal power, where the field for fundamental innovation is still wide open. Ideas range from the straightforward to the fanciful and, although several concepts are in pilot and demonstration phases, commercialisation is still some way off for most. Wave power, in particular, is still characterised by diversity and experimentation. Here we outline some of the prevailing concepts and give an example of each:
Surface point absorber (buoy)
Buoy-based solutions, also called “surface point absorbers,” come in several guises and rely on the differential motion between a floating buoy (which rises and falls with the passage of waves) and a base resting on the sea bed. The two elements are joined by a mechanism that converts the kinetic energy due to the buoy’s motion into electrical energy.
A notable example of this approach is the PowerBuoy® system from Ocean Power Technologies, which utilises a sliding spar and power take-off approach. As the buoy contours the waves, it rises and falls inside a cylindrical tube which terminates in a weighted element on the sea bed. In doing so, it causes electricity to be generated via a power take-off and generator.
In ocean trials carried out off Scotland in 2011, an OPT Mk3 system incorporating a simulated grid connection proved able to maintain generation, averaging over 400kW, in waves ranging from slight to very high in storm sea states. A peak capacity of 866kW was recorded.
This ocean test run of a third-generation system followed previous trials of earlier models. A prototype utility PowerBuoy deployed at a US Marine Corps base in Hawaii was the first grid-connected wave energy device in US territory. Another prototype, developed with support from the US Navy and utilities in New Jersey, was trialled in the period 2005-08; yet another trial took place off Spain. Results from these early deployments led to the OPT Mk3 that featured in the arduous Scottish evaluation.
Surface point absorbers have been a strong development focus and other devices include CETO Wave Power’s Aqua buoy (Australia), the Flansea (Flanders Electricity from the Sea) device (Belgium), the SE Sea Waves Power Plant (Israel), the OE Buoy from Ocean Energy (Ireland), the Wavebob (Ireland), Sea Raser (UK) and Upsala University’s Lysekit Project (Sweden). Differences between these lie mainly in the way kinetic energy is converted to electricity, with various arrangements of piston pumps, linear generators, turbines, etc.
As the name suggests, a multi-point absorber uses multiple floats instead of a single buoy. These are attached to a fixed platform that is standing on the sea bed via legs. Wave-induced up-and-down motion of the floats is converted to angular motion in the arms that connect to the floats to the platform and thence (via power take-offs) into hydraulic power which drives a generator. Advantages over a single point device include smoother power delivery, since the spaced floats experience passage of a wave at different times, higher aggregate power and redundancy in case of failure at any given float. In addition, the platform can be used to support a wind turbine so that combined wave and wind power generation is possible.
A two-float prototype Wave Star machine from Wave Star Energy A/S in Denmark has exported electricity to the grid since September 2009. Currently the device is being extended so that more power can be generated when it is redeployed. A much larger machine with up to 20 floats is planned. Wave Star incorporates a storm protection system under which floats are automatically raised out of the water in high sea states.
An attenuator comprises two or more floating elements that are hinged together in a “chain” which is aligned at right angles to the waves. As a wave passes beneath and along the chain, the elements move in the vertical plane such that their hinged joints rise and fall creating a constantly varying angle between each pair of elements. This angular motion is captured by power take-off struts and converted (via hydraulically driven generators) to electricity. In extracting energy from the waves, the device also reduces their amplitude — that is, it attenuates them.
The best known example, and arguably the nearest to becoming commercial, is the Pelamis Wave Energy Converter from Pelamis Wave Power in Scotland. In a Pelamis device, the elements are cylindrical floats that are linked by articulating joints. The first Pelamis deployment at sea took place in 2004 at the European Marine Energy Centre (EMEC) off Orkney, Scotland. A derivative machine deployed off Orkney six years later was claimed as the world’s first wave energy device to deliver electricity from an offshore WEC to an onshore grid. In 2008 the world’s first multiple machine wave farm was commissioned, off Agucadoura, Portugal, with three Pelamis machines giving 2.25MW total capacity. This farm was subsequently decommissioned for corporate financial reasons.
Currently, Pelamis Wind Power is involved in a joint venture with Vatenfall to develop a project off the Shetland Isles that would deliver power to the mainland via an HVDC link. Another part of this wind and wave-rich area, the Outer Hebrides, could be in line for Pelamis power, too — given that there are plans with Lewis Castle College and an industrial consortium to field a 10MW, 14-machine farm 10km off Lewis. A further Pelamis farm could be deployed off Scotland’s north coast, specifically at Farr Point, Sunderland, where Pelamis Wave Power has a seabed lease option.
Richard Yemm, founder and CEO of Pelamis Wave power, believes that 15 years of development that have seen six full-scale machines deployed, collaborative work with power utilities since 2005 and more than 10,000 hours of at-sea grid-connected experience, have opened up real commercial prospects for Pelamis. The Scottish government seems to agree, having last autumn awarded a share of its Marine Renewables Commercialisation Fund to the company to support further progress, including the development of an enhanced version of Pelamis. This adds to a £1.4m award from the Energy Technologies Institute to fund further development and trials.
Oscillating wave surge converter
Basically, a buoyant flap or plate is attached to the near-shore seabed by a horizontal hinge that allows it to pitch down towards the horizontal when a passing wave exerts surge pressure, and become vertical again — under its own buoyancy — once the wave has passed. The continual oscillating motion caused by the passage of waves is used to drive a pump which pushes water through a hydro-electric turbine.
Championing this oscillating plate approach is Edinburgh-based Aquamarine Power Ltd with its Oyster WEC. Oyster is designed to be located in depths of 10-15m, typically about half a kilometre from the shore. A double-acting hydraulic pump operated by the pitching motion of the device pushes water through a pipeline to a turbine ashore where electricity is generated. Keeping the turbine ashore (together with its power take-off and control system) keeps them out of the aggressive near-shore marine environment, prolonging life and facilitating operational adjustments and maintenance.
A 315kW proof-of-concept first-generation Oyster was deployed in 13m of water at EMEC in 2009. This has subsequently been replaced by a later improved model, the Oyster 800. Aquamarine CEO Martin McAdam reports that following an extensive product improvement programme intended to strengthen the device against the ravages of the sea, the resident Oyster had notched up 6MW of delivered power in 60 hours of operation by November. The present trials programme should lay the groundwork for a further-developed Oyster 801 successor, which will be a major milestone on the path to a fully commercial model.
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About George Marsh--Engineering roles in high-vacuum physics, electronics, flight testing and radar led George Marsh, via technology PR, to technology journalism. He is a regular contributor to Renewable Energy Focus magazine.