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.
Multi-point absorber
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.
Attenuator
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.
Subscribe to Renewable Energy Focus magazine to read this
article in its entirety.
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.
 |
http://www.renewableenergyfocus.com/view/37023/innovation-rules-the-tidal-waves/