here - Power Electronics

DESIGNfeature
SAM DAVIS, Editor-in-Chief, PET
ENERGY
FROM OCEAN
A system for converting wave energy
into electricity employs a staged
power conversion system. The
system performs hydrodynamic-tomechanical,mechanical-to-hydraulic,
hydraulic to mechanical, and
mechanical to electrical energy.
A
quamarine Power will begin
deployment of three wave
energy converters (WECs)
at European Marine Energy
Centre (EMEC) in Orkney,
Scotland in summer 2011.
Three 800 kW devices will
be linked to a pair of onshore 1.2 MW
hydro-electric turbines. When completed,
the WECs near the shore can provide enough
energy to power up to 12,000 homes.
The new 2.4MW system will deliver 700%
more power than the original WEC, called
Oyster 1, that was successfully deployed at
TO SUPPLY
SCOTLAND
HOMES
WITH
UTILITY
POWER
18 Power Electronics Technology | September 2010 www.powerelectronics.com

EMEC in Orkney 2009 (Fig.
1). The new version, Oyster 2,
incorporates design improvements,
enabling it to produce
more energy while being easier to
install and maintain. Fig. 2 shows
Oyster 1 in operation.
The basic Oyster WEC consists
of an oscillator flap mounted
on a sub-frame support structure
(Fig. 3). The oscillating action of
waves on the flap drives hydraulic
pistons to pressurize fresh
water, which is then pumped to
shore through the high pressure
pipelines. At shore, a hydroelectric
plant converts the hydraulic
pressure into electrical power via
a Pelton wheel turbine, which
turns an electrical generator tied to the local utility
grid.
Design of the Oyster WEC required a balance
between capital cost and wave power conversion
efficiency efficiency. The key wave power conversion parameters
are water depth, flap geometry, buoyancy
and damping. Its design objective was to maximize
captured wave power per ton of device and foundations.
Oy st ster t er 1 e eext
xtra ract cts po powe w r from the waves by har-
ne ness ssing g th the ho hori rizo zo zont nt n al s ssur
urge ge ccom
om ompo po p ne n nt o oof
f th the h e wave
mo moti ti tion on ttha
ha hat pr p od oduc uc u es f ffor
or o ce ces on o its fla f la l p. p Forces a re r
ma maxi xi ximi m zed d by mmak
ak akin in i g th the e fl f ap p ppen
e etrate the ful ul ull l
wa wate te ter r co colu l mn a
a nd d mmin
in inim imiz izin in ing g leak akag ag a e un unde de dern rn rnea ea eath th
or o from m ov over er erto to topp pp ppin in i g th t e de devi vi vice ce ce. . Wi W de ni ng t tthe
t he fla la lap p
in incr cr crea ea ease se ses th the e fo forc rc r es a pp ppro ro roxi xi xima ma mate te tely ly in n pr prop op opor or orti ti tion o
to the squar a e of o t tthe
he ffla
la lap p wi w dt dth. h.
Howe weve ver, r, aas
s th the e wi w dt dth h in incr cr crea ea ease se ses, s, Fig Fig. 2 . O OOyst
yst y er 1 1i 1 i in
o
th the ma maxi xi ximu mu mum m wa w ve force rrea
ea each ches h
a li limi mi m t du due to phase incoher-
en ence across the flap. Water depth
reduces the water’s horizontal
amplitude, so motion increases
but overall power reduces.Besides
optimizing the power take off,
there are other design considerat
ations that affect installation and
op operating costs:
an and d se s ab abed ed ccon
o ditions
to the seabe bed
removal of the WEC EC E
Fig. 1. Wave action on Oyster 1 causes pressurized fresh water to be pumped to shore through high pressure pipelines
where it produces hydroelectric power. The low pressure return-water passes back to the device in a closed loop.
■ WHAT IS A PELTON WHEEL?
THE PELTON WHEEL is among the most efficient types of water turbines.
The wheel (Fig. 5) 5 extracts energy from the impulse ( momentum)
of moving water, as opposed to its weight like the traditional
overshot water wheel whee . With Pelton Pelton’ss paddle geometry geometry, when the rim
runs at one-half the speed of the water jet, the water leaves the wheel
with very little speed, extracting almost all of its energy, allowing a
very efficient turbine.
Invention of the wheel by Lester Pelton apparently originated from
an accidental observation. Pelton was watching a spinning water turbine
when the key holding its wheel onto its shaft slipped, causing it to
become misaligned. Instead of the jet hitting the cups in their middle,
the slippage made it hit near the edge, rather than stopping the water
flow, it was deflected into a half-circle, coming out again with reversed
direction. Surprisingly, the turbine now moved faster.
n operation n off o ff th t e coas oas oa o t oof
f S Scot
cotlan a d.
www.powerelectronics.com September 2010 | Power Electronics Technology 19

OC OCEA OC OCEANWAVEenergy
EA EANW NW NWAV NW AV AVE V
Fig. 3. Oyster 1 consists of an oscillator flap mounted on a sub-frame structure. The action of waves on the
flap drives hydraulic pistons which pressurize fresh water that is sent to the shore where it is converted to
electricity.
DC
Bus
Fuses
SPMC
Controlled
Rectifier for
DC Bus
Pre Charge
Fuses
DC
Bus
Fuses
SPMD
Master Main
Drive Module
DC to AC
Converter
Input
Chokes
Input
Fuses
PWM Filter
DC
Bus
Fuses
SPMD
Slave Main
Drive Module
DC to AC
Converter
Input
Chokes
Input
Fuses
To Utility Power
Rectifier to Inverter
Control Wiring
DC
Bus
Fuses
SPMD
Master Main
Drive Module
DC to AC
Converter
Input
Chokes
20 Power Electronics Technology y | September 2010 www.powerelectronics.com
DC
Bus
Fuses
SPMD
Slave Main
Drive Module
DC to AC
Converter
Water Input
Input
Chokes
Pelton Wheel Turbine
Direct-Coupled to
Flywheel and
Variable Speed
Induction Generator
Fig. 4. The electrical system drive components that produce electricity and synchronizes it with the associated
electric utility grid.

OC OCEA OC OCEANWAVEenergy
EA EANW NW NWAV NW AV AVE V
Water hitting
….aligned
blade
misaligned
blade
dump the flow directly to an underground tank, bypassing
the Pelton wheel.
ELECTRICAL SYSTEM
The electrical system obtains mechanical power on the
common shaft between the Pelton wheel, generator and
the grid connection at EMEC. The electrical energy is
transferred to the grid via an 11kV interface at EMEC.
The key components of the electrical power system are:
Oyster 1 power conversion included a variable frequency
inverter coupled to an induction generator. The Fig. 6. An artist’s rendering of the new version of Oyster wave energy converter
(WEC) that will be deployed starting in 2011.
characteristics of the Pelton turbine. Fig. 4 shows the key
drive components. The flywheel is used to store energy, allowing the gen-
■ WAVE ENERGY CONVERSION
WHEN A BODY OSCILLATES in still water, it creates waves at
the frequency of oscillation. The amplitude of this radiated
wave depends on the size of the body and the amplitude
of its motion. You can consider wave energy absorption as
a destructive interference of this radiated wave with an
incident wave when the correct phase relationship is present.
Wave energy is proportional to the square of the wave
amplitude. Similarly, the reduction in the incident wave
amplitude is related to the energy absorbed by the device.
A simple method of absorbing wave surge energy is to use
a hinge or bearing to allow the body to oscillate in pitch, which
then turns the surge force into a moment, or wave torque.
Moreover, a purely surging device has no inherent spring
force to aid in tuning and to resist nonlinear drift forces.
However pitching devices do, whether it is excess buoyancy
for a bottom hinged device, such as Oyster.
The dynamics of bottom hinged pitching flap type WECs
tend to be inertia dominated and therefore the natural pitchp ing period is generally well above that of the most common
wave energy periods, which is about 5 to 15 seconds.
Fig. 5. The Pelton wheel
(photo) provides an effi-
cient means for converting
the incoming pressurized
fresh water into a rotat-
ing turbine that drives the
generator.
generator to have a rating of about half the Pelton wheel
power rating.
the variable speed requirements of the generator with the
other two providing the grid interface. One advantage of
this modular arrangement is that it provides fault tolerance.
If one drive unit should fail, the others can continue
to operate at a reduced power until repaired.
With Oyster 2 (Fig. 6
dynamic performance of clusters of flaps in different geometric
patterns, and modeling the array of devices with
and spacing of onshore hydroelectric plants. It was always
and operate, as it does not have the benefit of smoothing
economies of scale of the onshore hydroelectric plant.
The future wave energy development program also
includes wave-powered desalination by feeding the high
pressure water directly into reverse osmosis tubes.
22 Power Electronics Technology y | September 2010 www.powerelectronics.com