EE535: Renewable Energy: Systems, Technology & Economics

EE535: Renewable Energy:
Systems, Technology &
Session 7: Wave Energy (2) Devices
Device Types
• These devices need to convert the wave motion
into electricity
• Classifications:
– Location
• Shore and Bottom Mounted Near Shore Devices
• Off-shore Devices
– Geometry and Orientation
• Terminators (principal axis parallel to the incident wavefront,
and physically intercept the waves)
• Attenuators (principal axis perpendicular to the wavefront, so
wave energy drawn to the device as the wave moves past it)
• Point Absorbers
Fixed Device: Oscillating Water
Air out
Water rising
Air in
Water Falling
Oscillating Water Column
• Partly submerged structure with a chamber
having 2 openings
• Air trapped in the chamber above the water
surface is alternately pushed out of and then
drawn back into the chamber
• The motion of the air drives a turbine (such as a
Wells turbine), which generates electricity
• A Wells turbine rotates in one direction,
regardless of the direction of low of air
Floating Devices: Point Absorber or
• The rising and falling of waves can move
buoy-like structures, creating mechanical
energy – which is then converted to
The Salter Duck
• A wave entering from the
left sets the beak of the
duck into oscillation
• The back of the duck is
circular to ensure no
wave is propagated to the
• With very little energy
transmitted or reflected
there is a very high
conversion rate – over a
broad frequency band to
match the conditions
Duck Motion in Waves
Wave Motion
The Salter Duck
• Originally envisaged as
many cam shaped bodies
linked together on a long
flexible floating spine
• Spine oriented towards
the principal wave,
making the duck a
• Scale of a duck is circa
• Benefits of duck design:
– Can flip over and recover
again after an unusually
large wave
– Mooring relatively
straightforward due to
flexible spine and number
of ducks oriented along
• Disadvantages of the
duck design:
– Slow oscillatory motion is
difficult to couple to
electrical generators
– Extracting energy from a
‘randomly’ rocking body
The Circular Clam
• 12 interconnect chambers
arranged around the
circumference of a toroid
• The chambers are
interconnected, separated by
Wells turbines.
• The chamber is sealed
against the sea water by a
flexible reinforced membrane
• Movement of the sea against
the membrane forces air to
pass through the Wells turbinegenerating electricity
Offshore devices
• The Wave dragon
– Is an overtopping device, which
elevates ocean waves to a
reservoir above sea level
– Water is let out through a
number of turbines and in this
way transformed into electricity
– The prototype is deployed in
Nissum Bredning, an inlet in the
northern part of Denmark
Tethered Devices
• Main body of device
floating on the surface
but moored to the
seabed via a pump
• The Wavebob is an axisymmetric, self-reacting
point absorber, primarily
operating in the heave
mode. It is specifically
designed to recover
useful power from ocean
wave energy, and to be
deployed in large arrays
Wavebob Key Features
Survivability: The Wavebob is an axi-symmetric buoy structure on slack moorings which makes it inherently seaworthy. Its ability to de-tune in seconds is vitally important in a resonating energy absorber.
Response to long period and high waves: Unlike all other self-reacting heaving buoys, the Wavebob’s natural
frequency may be set to match the typical ocean swell (Atlantic 10”, or Pacific 15”), facilitating good energy
absorption. It can ride very large waves and still recover useful power.
Tuning and control: The Wavebob has exceptional facilities for almost instantaneous tuning and longer period
adjustment of natural frequencies and bandwidth. On-board autonomous control is a feature, and there is
considerable scope for intelligent systems, for example individual units co-operating in arrays. These are highly
significant attributes in changing wave climates, so typical of the North Atlantic.
Accessibility: The outer torus has a diameter of the order of 20metres, and an overall height of 8 metres, allowing
adequate space for the power train and control systems below decks. As a large floating structure, Wavebob is
relatively stable in all but the most severe storms.
Low operating and maintenance costs, high availability: O&M costs have a massive bearing on the costs /
kWh delivered. Only well-proven and standard marine hydraulic components and generators are installed. The
Wavebob typically carries three or four motor-alternator sets, all or some of which may be entrained, depending on
incident wave energy. In-built redundancy facilitates remote switching and high availability when weather
conditions might preclude maintenance visits. The main device remains on site (for up to 25 years), with individual
components being replaced and taken ashore for servicing as necessary.
Low capital costs: The main hull structures will be assembled from smaller pre-cast and extruded concrete units
manufactured using widely available and standard processes. There is no requirement for deep water facilities or
dry docks. The main hull structures would be towed to site and attached to prepared moorings.
High power output: Average electrical power 500kW and greater is expected from North Atlantic sites. Power
output will be synchronous with low VAr
Wave Power Imperatives
Survival: Especially challenging for the North Atlantic. We design for the 100-year extremes, the greatest hazard
being a freak ‘wall-of-water’ presently estimated to be ~24metres. Certification by Det Norske Veritas or similar is
necessary for marine insurance. Fail-safe modes are essential during extreme events and breakdowns (eg failure
within the device or of grid connection).
Deep water: Ocean waves lose energy and become steeper as the water shoals; losses become significant as
the depth becomes less than half a wave-length. The North Atlantic energy ‘hot spot’ West of Ireland is centred on
~178 metre wavelengths, ie longer wavelengths are important. The equivalent off West Coast USA is over 300
metres. As might be expected, the bathymetry is ideal in the regions mentioned above, deep water is available
within a few kilometres of the shore.
25+ year life on site: The main hull structures should be capable of remaining on site for at least 25 years, and be
readily decommissioned thereafter. The costs of recovering and re-deploying a device at any intermediate stage
should be avoided completely.
Self-reacting point absorbers: Oscillating systems capable of resonant energy absorption have been the subject
of a great deal of attention since the 1970’s. The theory is now well established but, until recently, a number of
technical challenges limited the prospects of commercial success. Self-reacting point absorbers have two
advantages, - independence from the sea-bed (other than slack moorings and grid connection) thus minimising
installation and maintenance costs and, secondly, if axi-symmetric, can respond to waves from any direction.
Arrays: The energy density of ocean waves is considerably greater than wind and consequently closer spacing is
possible. Theoretically defined by each unit’s absorption or capture width, in practice an array layout will be
dictated by moorings (slack, for self-reacting devices), the prevailing wave direction, and foreshore consents.
Wave Power Imperatives
Tuning and control: Ocean waves are typically a mix of wind-waves and swell. Most of the time the wave climate
is far from regular, and varies very significantly. North Atlantic wave periods and wave heights can more than
double within 24 hours as depressions pass over. It is essential that any commercial device will have autonomous
control (on-board ‘intelligence’) allowing it to tune to changing conditions and to maximise useful power output. An
ability to vary bandwidth is desirable.
Significant installed capacity: Installed capacities should be greater than 1MW, otherwise per unit costs of
moorings, grid connection, operations and maintenance become excessive.
Power capacity: The amount of wave energy that an oscillating system can in theory absorb is a function of the
prevailing wavelength and the oscillating mode(s). For a North Atlantic site the theoretical limits are well above
1MW, averaged across the expected distribution, ie there are many occasions when the theoretical limit is much
higher. A good point absorber, if ‘run backwards’, becomes a good wave generator. To do so requires that a
suitably large volume of water is displaced each wave period, and that is a function of water-plane area and stroke
Fabrication: Low cost / long life / low maintenance materials such as concrete are to be preferred over steel or
polymers, other things being equal. Similarly, any need for large dry docks, deep water harbours, jack-up barges,
etc., will add to costs and limit the number of suitable facilities for the construction and deployment stages.
Cost / kWh: This is a matter of minimising costs (capital, opex) and maximising useful electrical power delivered
to the grid.
Health and safety: Although not expected to carry permanent crew or volatile hydrocarbons, access for routine
and un-planned on-board basic maintenance requires clear procedures. The installed equipment should be safely
housed and accessible above the water-line. Boarding and dis-embarking via a rib, small service craft or
helicopter should be well within acceptable standards up to at least Force 5. The device must be capable of being
switched remotely to a non-operational safe mode, and of failing safe.
Offshore devices: The McCabe
Wave Pump
Offshore devices
• The McCabe Wave Pump
– The device consists of three
rectangular steel pontoons,
which are hinged together
across their beam
– The MWP was primarily
designed to produce potable
water although it can also be
used to produce electricity
– A 40 m long prototype was
deployed in 1996 off the
coast of Kilbaha, County
Clare, Ireland
Offshore devices
• The Pelamis
– Is a semi-submerged
structure composed of
cylindrical sections linked
by hinged joints
– The wave induced motion of
these joints is resisted by
hydraulic rams which pump
high pressure oil through
hydraulic motors via
smoothing accumulators
– The hydraulic motors drive
electrical generators to
produce electricity
Offshore devices
• The Pelamis
– Several devices can be
connected together
and linked to shore
through a single
seabed cable
– A typical 30MW
installation would
occupy a square
kilometre of ocean and
provide sufficient
electricity for 20,000
• Reducing operation and maintenance costs is key to
successful economic implementation of wave energy
• Capital costs per kW of wave energy estimated to be
double that of fossil fuel installations
• Load Factor (average power divided by peak power)
lower than conventional due to variability of the wave
• Wave energy costs can only be competitive if running
costs are significantly lower than for a conventional
• Fuel costs are zero, therefore operation and
maintenance costs are determining factors
• A typical efficiency of a wave energy
device is 30%
• If the average annual electricity
consumption in Ireland is 26000 GWh,
what is the required scale of a wave
energy device to meet this demand? (Giga
= 109)
• Assume storage capability etc are already
in place