Download Designing and Testing Composite Energy Storage Systems for

energies
Article
Designing and Testing Composite Energy Storage
Systems for Regulating the Outputs of Linear Wave
Energy Converters
Zanxiang Nie 1,2, *, Xi Xiao 1, *, Pritesh Hiralal 3 , Xuanrui Huang 1 , Richard McMahon 4 ,
Min Zhang 5 and Weijia Yuan 5
1
2
3
4
5
*
Department of Electrical Engineering, Tsinghua University, Beijing 100084, China; [email protected]
Zinergy Shenzhen Ltd., Taohuayuan Science and Technology Innovation Park, Baoan,
Shenzhen 518101, China
Zinergy UK Ltd., Future Businesss Centre, Cambridge CB4 2HY, UK; [email protected]
The Warwick Manufacturing Group, University of Warwick, Coventry CV4 7AL, UK;
[email protected]
Department of Electronic and Electrical Engineering, Bath University, Bath BA2 7AY, UK;
[email protected] (M.Z.); [email protected] (W.Y.)
Correspondence: [email protected] (Z.N.); [email protected] (X.X.); Tel.: +86-139-1009-0731 (X.X.)
Academic Editor: Stephen Nash
Received: 7 August 2016; Accepted: 6 January 2017; Published: 18 January 2017
Abstract: Linear wave energy converters generate intrinsically intermittent power with variable
frequency and amplitude. A composite energy storage system consisting of batteries and super
capacitors has been developed and controlled by buck-boost converters. The purpose of the
composite energy storage system is to handle the fluctuations and intermittent characteristics of
the renewable source, and hence provide a steady output power. Linear wave energy converters
working in conjunction with a system composed of various energy storage devices, is considered as
a microsystem, which can function in a stand-alone or a grid connected mode. Simulation results
have shown that by applying a boost H-bridge and a composite energy storage system more power
could be extracted from linear wave energy converters. Simulation results have shown that the
super capacitors charge and discharge often to handle the frequent power fluctuations, and the
batteries charge and discharge slowly for handling the intermittent power of wave energy converters.
Hardware systems have been constructed to control the linear wave energy converter and the
composite energy storage system. The performance of the composite energy storage system has been
verified in experiments by using electronics-based wave energy emulators.
Keywords: wave energy; linear machine; energy storage; power conversion; renewable energy
1. Introduction
As a large untapped renewable source, electricity can be harnessed from ocean waves [1–4], and
this could contribute a significant portion of the electricity needed to meet world-wide consumption.
Most current research into harnessing energy from ocean waves has focused on developing the
mechanical system for the capture of energy from the ocean sources, rather than the conversion methods
of the irregular electrical power from wave energy converters. Conventional wave energy converters,
such as the oscillating-water-column system and the pendulum-type, usually need intermediate
mechanical systems for kinetic energy conversion from the low frequency of ocean waves to the high
frequency of conventional generators. This reduces the overall system efficiency. Linear wave energy
converters (LWECs) directly use the kinetic energy of the floating body driven by ocean waves. As the
energy conversion from low frequency to high frequency is not needed, it is an attractive technology
Energies 2017, 10, 114; doi:10.3390/en10010114
www.mdpi.com/journal/energies
Energies 2017,
2017, 10,
10, 114
114
Energies
of 16
15
22 of
an attractive technology with advantages of reduced complexity, reduced maintenance and system
with
of reduced
complexity,
reduced
maintenance
andconversion
system costmethods
[5–13]. At
the same
cost advantages
[5–13]. At the
same time,
appropriate
control
and power
need
to be
time,
appropriate
control
and
power
conversion
methods
need
to
be
developed
for
linear
wave
energy
developed for linear wave energy converters, to increase the efficiency of electrical power extraction
converters,
to increase the efficiency of electrical power extraction and conversion.
and conversion.
Traditional
generator
oror
a
Traditionalpoint
pointabsorbing
absorbingwave
waveenergy
energyconverters
convertersemploy
employeither
eithera alinear
linearelectric
electric
generator
linear-to-rotary
mechanism.
Rack-pinion,
slider-crank
or
ball-screw
mechanisms
are
usually
employed
a linear-to-rotary mechanism. Rack-pinion, slider-crank or ball-screw mechanisms are usually
in
linear-to-rotary
systems [8]. systems
Among the
mechanisms, the
ball-screw the
mechanism
is
employed
in linear-to-rotary
[8]. linear-to-rotary
Among the linear-to-rotary
mechanisms,
ball-screw
useful
in
transforming
a
slow
linear
motion
into
a
fast
rotary
motion
with
a
high
efficiency
(more
than
mechanism is useful in transforming a slow linear motion into a fast rotary motion with a high
90%),
and this
is suitable
for use
point
absorbing
It is well-known
efficiency
(more
than 90%),
andin
this
is suitable
forwave
use inenergy
point converters
absorbing [14,15].
wave energy
converters
that
linear
outweigh
rotational
ones
becauserotational
of the simplicity
and of
effectiveness
of and
the
[14,15].
It isgenerators
well-known
that linear
generators
outweigh
ones because
the simplicity
total
structure
[16].
In
a
LWEC,
a
linear
electrical
machine
is
directly
coupled
to
the
driving
source,
effectiveness of the total structure [16]. In a LWEC, a linear electrical machine is directly coupled to
for
a floating
buoy. When
the linear
is linear
coupled
to the is
motion
of to
a floating
buoy,
the example
driving source,
for example
a floating
buoy.machine
When the
machine
coupled
the motion
of
aa characteristic
force
(EMF) is induced
in the machine’s
ocean
waves
with
floating buoy,electromotive
a characteristic
electromotive
force (EMF)
is inducedcoils.
in theVarying
machine’s
coils.
Varying
slow
cyclic
motions
generate
EMFcorresponding
waveforms. An
example
of the usual
electrical
oceanand
waves
with
slow and
cycliccorresponding
motions generate
EMF
waveforms.
An example
of
output
is shown
in Figure
is similar
to that
presented
in to
[6,7].
the usual
electrical
output1isand
shown
in Figure
1 and
is similar
that presented in [6,7].
Figure 1.
1. Electromotive
wave
energy
converter
(LWEC)
(1
Figure
Electromotive force
force(EMF)
(EMF)waveform
waveformofofa atubular
tubularlinear
linear
wave
energy
converter
(LWEC)
phase).
(1 phase).
When the translator of a LWEC reaches the ends of its stroke with a frequency of twice that of
When the translator of a LWEC reaches the ends of its stroke with a frequency of twice that of
ocean waves, the linear machine is being reset and the power output is zero at these two instants.
ocean waves, the linear machine is being reset and the power output is zero at these two instants.
Hence, a large fluctuation occurs in the output power of linear wave energy converters, and with a
Hence, a large fluctuation occurs in the output power of linear wave energy converters, and with a
varying time period of seconds. In addition, like wind power and all other renewable sources, ocean
varying time period of seconds. In addition, like wind power and all other renewable sources, ocean
power may vary significantly according to sea states, resulting in a long term variation of the output
power may vary significantly according to sea states, resulting in a long term variation of the output
power of wave energy converters, varying from minutes to hours.
power of wave energy converters, varying from minutes to hours.
A wave energy converter ideally should provide steady power to the local load or grid, and its
A wave energy converter ideally should provide steady power to the local load or grid, and its
output power should be regulated to maintain a desired value over a time of hours or more. This
output power should be regulated to maintain a desired value over a time of hours or more. This
paper describes the methods for extracting powers from a LWEC and conditioning the fluctuating
paper describes the methods for extracting powers from a LWEC and conditioning the fluctuating
output power. A composite energy storage system composing of super capacitors and batteries is
output power. A composite energy storage system composing of super capacitors and batteries is
presented to operate in conjunction with the wave energy converter. The composite energy storage
presented to operate in conjunction with the wave energy converter. The composite energy storage
system overcomes the variations of powers outputted from wave energy converters, by providing
system overcomes the variations of powers outputted from wave energy converters, by providing
temporary storage ability over a few wave cycles and long term storage ability up to hours, thereby
temporary storage ability over a few wave cycles and long term storage ability up to hours, thereby
meeting the goal of providing a steady output power. Moreover, the batteries are prevented to handle
meeting the goal of providing a steady output power. Moreover, the batteries are prevented to handle
the frequent fluctuations of LWECs, and are like have longer life and less maintenance. The control
the frequent fluctuations of LWECs, and are like have longer life and less maintenance. The control
systems of linear wave energy converters and the composite energy storage system are simulated by
systems of linear wave energy converters and the composite energy storage system are simulated
MATLAB software and the experimental results will be given to verify the performance of the
by MATLAB software and the experimental results will be given to verify the performance of the
composite energy storage system by using an electrical wave energy emulator.
composite energy storage system by using an electrical wave energy emulator.
2. Proposed System
2. Proposed System
Linear wave energy converters can be considered a variable frequency generator, and the output
Linear wave energy converters can be considered a variable frequency generator, and the output
is conventionally conditioned by controlled rectification. Therefore, a stable DC link voltage could be
is conventionally conditioned by controlled rectification. Therefore, a stable DC link voltage could be
provided for supplying an inverter bridge to feed the grid. The DC link bus is a suitable location for
Energies 2017, 10, 114
3 of 16
Energies 2017, 10, 114
3 of 15
provided for supplying an inverter bridge to feed the grid. The DC link bus is a suitable location for
the connection
connection of
of energy
energy storage
storage to be used as a buffer for leveling the power output. A composite
the
output power
power fluctuations
fluctuations of
of linear
linear wave
wave energy
energy
energy storage system is presented for handling the output
converters and
andachieving
achievingthe
theconstant
constant
power-flow
requirement.
proposed
system
includes
converters
power-flow
requirement.
TheThe
proposed
system
includes
a higha
high
power
density
device
(super
capacitor)
and
a
high
energy
density
device
(battery),
as
shown
in
power density device (super capacitor) and a high energy density device (battery), as shown in Figure
Figure 2.
2.
tra nsla tor
win ding s
Sta tor
PW
ele ctro m
-agn etic
for ce F
g
PG
AC-DC
buo yanc y
Fh
for ce
gra vity G
flo ater
wav es
Wave Energy
Converters
储
能
系
统
DC - AC
PE
PL
PLi
Li
PSC
Grid
uDC
SC
DC bus
Load
Hybrid Energy
Storage System
Figure
Figure 2.
2. A
A linear
linear wave
wave converter
converter and
and composite
composite energy
energy storage
storage microsystem.
microsystem.
The operation of a linear wave energy converter is expected to last for many years in the offshore
The operation of a linear wave energy converter is expected to last for many years in the offshore
marine environment, as performing maintenance in the offshore marine environment is expensive,
marine environment, as performing maintenance in the offshore marine environment is expensive,
hazardous and time consuming. The power generated from wave power can be transmitted to the
hazardous and time consuming. The power generated from wave power can be transmitted to the
shore either by underwater AC or high voltage DC transmission cables, depending on the distance,
shore either by underwater AC or high voltage DC transmission cables, depending on the distance,
as planned in the UK for the connection of offshore wind farms. Hence, it is a feasible option to place
as planned in the UK for the connection of offshore wind farms. Hence, it is a feasible option to place
the super capacitor system offshore with the LWEC while the battery system resides onshore, as
the super capacitor system offshore with the LWEC while the battery system resides onshore, as super
super capacitors have much longer lifetimes than batteries [4].
capacitors have much longer lifetimes than batteries [4].
3.
(LWECs)
3. Controls
Controls of
of Linear
Linear Wave
Wave Energy
Energy Converters
Converters (LWECs)
A
numberofoflinear
linear
electrical
machines
been designed
fordrive
direct
drive ofmethods
of
A number
electrical
machines
have have
been designed
for direct
methods
harnessing
harnessing
power [5,6,10–12,17]
The comparatively
large
internalofinductance
of the
generator
wave powerwave
[5,6,10–12,17].
The comparatively
large internal
inductance
the generator
coils
of some
coils
of
some
linear
machines
results
in
a
low
power
factor,
as
noted
by
Ran
and
Hodgins
[11,17].
linear machines results in a low power factor, as noted by Ran and Hodgins [11,17]. Hence,
these
Hence,
these
machines
require
substantial
reactive
power
compensation
in
order
to
increase
their
machines require substantial reactive power compensation in order to increase their output power
output
factors. An
alternative
machine
design,
the machine
tubular linear
machine
[10],
can
factors. power
An alternative
linear
machinelinear
design,
the tubular
linear
[10], can
achieve
power
achieve
power
factors
above
0.95
in
wave
application.
However,
there
is
a
tradeoff
between
improved
factors above 0.95 in wave application. However, there is a tradeoff between improved power factor
power
factor
and power
density,
as tubular
linear
has large
internal resistance
and power
density,
as tubular
linear
machines
has machines
large internal
resistance
The
modelled
as as
a mass-spring-damper
system,
as
The energy
energy harvesting
harvestingsystem
systemofofa aLWEC
LWECcan
canbebe
modelled
a mass-spring-damper
system,
F
described
in [18],
andand
as as
shown
in in
Figure
3, 3,
where
as described
in [18],
shown
Figure
where eFe isis the
the excitation
excitation force;
force; U
U is
is the
the device
device
K ee isisthe
translational velocity;
velocity;M
Misisthe
thedevice
devicemass;
mass;11/K
force constant;
constant;
translational
theinverse
inverse of
of the spring stiffness force
and
R
is
the
total
resistance
of
the
mechanical
damping.
e
and Re is the total resistance of the mechanical damping.
The load impedance seen by a LWEC depends upon the control strategy of the generator, which
The load impedance seen by a LWEC depends upon the control strategy of the generator, which
can be purely resistive or can present some capacitive reactance to cancel that of the energy harvesting
can be purely resistive or can present some capacitive reactance to cancel that of the energy harvesting
system. The resistive and capacitive-resistive loadings are represented in Figure 3a,b, respectively.
system. The resistive and capacitive-resistive loadings are represented in Figure 3a,b, respectively. A
A linear system could output maximum power by damping the system in resonance [18]. In this
linear system could output maximum power by damping the system in resonance [18]. In this
approach, maximum power extraction takes place when the load damping equals the mechanical losses.
approach, maximum power extraction takes place when the load damping equals the mechanical
losses.
Energies 2017, 10, 114
Energies 2017, 10, 114
4 of 16
4 of 15
M
Energies 2017, 10, 114
Re
1 Ke
Re
1 Ke
M
Excitation
force
Fe
U
4 of 15
U
Rg
Excitation
force
Fe
Rg
(a) Resistive loading
M
(a) Resistive1loading
Ke
Re
M
Re
1 Ke
U
U
1 Kg
Excitation
force
Fe
1K
Rgg
Excitation
force
Fe
Rg
(b) Capacitive-resistive loading
(b) Capacitive-resistive loading
Figure 3.
3. System
System model
model and
and loading
loading of
of LWEC.
LWEC.(a)
(a)Resistive
Resistiveloading;
loading;(b)
(b)Capacitive-resistive
Capacitive-resistiveloading.
Figure
Figure 3. System model and loading of LWEC. (a) Resistive loading; (b) Capacitive-resistive
loading.
loading.
In other words, all impedance terms in the generator loading system are controlled to achieve
In other
words,
all all
impedance
terms
in
generatorloading
loading system are controlled to achieve
other
words,
impedance
terms
in the
the generator
maximumInpower
transfer,
as
expressed
by Equations
(1) and (2): system are controlled to achieve
maximum
power
transfer,
as
expressed
by
Equations
(1)
and
(2):
maximum power transfer, as expressed by Equations (1) and (2):
 RReee
R gR=
gg 
2
K gKK=g ωω
Ke e
ω22·M
M
M−
KK
g
(1)
(1)
(1)
(2)
(2)
(2)
e
EvenEven
taking
the the
uncertainty
and
into consideration,
a maximum
power
taking
uncertainty
anderrors
errorsof
of parameters
parameters into
a maximum
power
Even taking
the uncertainty
and
errors
of
parameters intoconsideration,
consideration,
a maximum
power
pointpoint
tracking
(MPPT)
control
strategy
can
be
employed
to
achieve
maximum
power
extraction
tracking (MPPT) control strategy can be employed to achieve maximum power extraction [19]. [19].
point tracking (MPPT) control strategy can be employed to achieve maximum power extraction [19].
Two AC/DC
power
converter
topologieshave
have been
been proposed
thethe
power
extraction
fromfrom
Two AC/DC
power
converter
topologies
proposedtotocontrol
control
power
extraction
Two LWECs:
AC/DCone
power
converter topologies
have been
to control
the
powerespecially
extraction
is a unidirectional
topology[11,20]
[11,20]
basedproposed
on
and
designed
for from
LWECs:
one is a unidirectional
topology
based
ondiode
dioderectifiers
rectifiers
and
designed
especially
for
LWECs:
one
is
a
unidirectional
topology
[11,20]
based
on
diode
rectifiers
and
designed
especially
for
the
high
power
factor
machines
as
mentioned
before;
the
other
topology
is
based
on
an
H-bridge
or
the high power factor machines as mentioned before; the other topology is based on an H-bridge or full
the high
power
factor
machines
as
mentioned
before;
the
other
topology
is
based
on
an
H-bridge
or
full bridge which allows bidirectional power flow [6,21–24]. Although the unidirectional topology
bridge
which allows bidirectional power flow [6,21–24]. Although the unidirectional topology offers
full bridge
which
allows
[6,21–24].
topology
offers cost
savings,
thebidirectional
bidirectional power
topologyflow
is necessary
forAlthough
cancellingthe
theunidirectional
machine’s dynamic
cost savings, the bidirectional topology is necessary for cancelling the machine’s dynamic reactance,
and to allow
a reversible energy
exchange
for tuning afor
resonant
arrangement
of mechanical
offersreactance,
cost savings,
the bidirectional
topology
is necessary
cancelling
the machine’s
dynamic
and to
allowtake-off.
a reversible
energy
exchange for
tuning topology
a resonant
arrangement
of mechanical
power
powerand
Therefore,
the bidirectional
H-bridgefor
in Figure
4 is adopted
in this
reactance,
to allow
a reversible
energy exchange
tuningshown
a resonant
arrangement
of mechanical
take-off.
Therefore,
the
bidirectional
H-bridge
topology
shown
in
Figure
4
is
adopted
in
this
paper.
paper.
power
take-off. Therefore, the bidirectional H-bridge topology shown in Figure 4 is adopted in this
paper.
Z1
D1
D2
Z2
Z1
i
D1
D2
Z2
i
Z1
E
Cout
D2
Z2
D1
Z1
E
i
i
i
D1
Z2
Cout
Cout
Cout
E
Z4
D4
Z3
D3
D2
i
E
D4
Z4
i
D3
Z3
i
(a)
D4
Z4
Z1
Z3
D1
Z2
(a)
i
D3
D2
Z1
D1
Z2
i
Cout
E
(b)
D4
Z4
D3
Z3
D2
(b)
Cout
E
i Z1
i
D1
Z4
D4
Z2
Z3
i
D2
i
D3
Cout
E
Z1
Z4
D1
D4
D2
Z2
Z3
D3
Cout
E
i
Figure 4. Current
i
(c)
Z4
paths
of
D4 Z3boost
AC/DC
D3
H-bridge (1 phase): (a)
(d)
Z2 Z4
and
Z4
D4 Z3
switched
D3
on; (b)
Z2 and Z4
Figure 4. Current paths of AC/DC boost H-bridge (1 phase): (a) Z2 and Z4 switched on; (b) Z2 and Z4
switched off; (c) Z1 and Z3 switched on; and (d) Z1 and Z3 switched off.
switched off; (c) Z1 and Z3 switched
(c) on; and (d) Z1 and Z3 switched off.
(d)
Figure 4. Current paths of AC/DC boost H-bridge (1 phase): (a) Z2 and Z4 switched on; (b) Z2 and Z4
switched off; (c) Z1 and Z3 switched on; and (d) Z1 and Z3 switched off.
Energies 2017, 10, 114
Energies 2017, 10, 114
5 of 15
5 of 16
The AC power from LWECs with slowly varying amplitude and frequency, must be entirely
controlled
bidirectional
H-bridge
converter
improve
the power
extraction
from
LWECs,
and
The by
ACa power
from LWECs
with
slowlyto
varying
amplitude
and
frequency,
must
be entirely
is controlled
required toby
bearectified
and
regulated
to
a
desired
DC
link
voltage.
Moreover,
the
DC
link
voltage
bidirectional H-bridge converter to improve the power extraction from LWECs, and
value
is
generally
than
inducedtoEMF
in the DC
generator
coils ofMoreover,
LWECs sothe
a boost
function
is required to behigher
rectified
andthe
regulated
a desired
link voltage.
DC link
voltage
is value
required.
The
generator
coil
L
coil could be exploited as boost inductor, for the purpose of achieving
is generally higher than the induced EMF in the generator coils of LWECs so a boost function is
lower
powerThe
lossgenerator
and avoiding
an could
additional
component.
The inductor,
free boostfor
inductor
or theof
generator
required.
coil Lcoil
be exploited
as boost
the purpose
achieving
coil
is
connected
in
front
of
the
H-bridge.
IGBTs
and
fast
recovery
diodes
could
be
adopted
for the
lower power loss and avoiding an additional component. The free boost inductor or the generator
H-bridges,
but
the
switching
frequency
is
supposed
to
be
carefully
controlled
below
the
generator
coil is connected in front of the H-bridge. IGBTs and fast recovery diodes could be adopted for the
coil’s
self-resonance
H-bridges,
but thefrequency.
switching frequency is supposed to be carefully controlled below the generator
Theself-resonance
currents of generator
coil’s
frequency.coils are controlled to track proportionally to the open loop output
EMF ofThe
LWECs
by
switching
pairs
and Z4)
and (Z1
and Z3) diagonally,
and
this
aims EMF
to
currents of generatorthe
coils
are (Z2
controlled
to track
proportionally
to the open
loop
output
optimize
theby
power
flow from
LWECs.
In the
waveform,
coil is
of LWECs
switching
the pairs
(Z2 and
Z4)positive
and (Z1cycle
and of
Z3)EMF
diagonally,
andthe
thisgenerator
aims to optimize
charging
when
Z2
and
Z4
are
turned
on
as
shown
in
Figure
4a,
otherwise
it
is
discharging
as
shown
the power flow from LWECs. In the positive cycle of EMF waveform, the generator coil is charging
inwhen
Figure
shows
current
flows4a,
of otherwise
the H-bridge
converter when
EMFinisFigure
in the4b.
Z24b.
andFigure
Z4 are4c,d
turned
on asthe
shown
in Figure
it is discharging
as shown
negative
cycle.shows the current flows of the H-bridge converter when EMF is in the negative cycle.
Figure 4c,d
The
free
boost
inductor
has
a value
of of
more
than
1010
times
thethe
minimum
inductance
required
forfor
The
free
boost
inductor
has
a value
more
than
times
minimum
inductance
required
ensuring
the
continuous
conduction
mode
(CCM)
operation
of
the
mentioned
boost
H-bridge
[25–28].
In
ensuring the continuous conduction mode (CCM) operation of the mentioned boost H-bridge [25–28].
theIncontinuous
mode,mode,
the inductor
current
never reaches
zero during
any part
of the
cycle.
the continuous
the inductor
current
never reaches
zero during
any
partswitching
of the switching
There
is
no
dead
time
gap
between
cycles,
as
shown
in
Figure
5.
During
the
period
t
on, the IGBT
cycle. There is no dead time gap between cycles, as shown in Figure 5. During the period ton , the IGBT
switches
are
onon
and
, replacingthe
switches
are
andthe
theinductor
inductorcurrent
currentincreases
increasesfrom
froman
aninitial
initialvalue
value to
to aa peak
peak value
value i i p, replacing
p
theenergy
energy
given
cycle,
energy
being
drawn
the input.
When
the IGBT
switches
given
upup
in in
thethe
lastlast
cycle,
energy
being
drawn
fromfrom
the input.
When
the IGBT
switches
are off,
i
arethe
off,
the
diode
conducts
for
the
rest
of
the
cycle
t
off
,
and
falls
to
a
lower
value
than
.
diode conducts for the rest of the cycle toff , and i L falls to
value
iL a lower value than i p . This lower
p This
is
the
initial
value
of
the
next
cycle,
but
never
becomes
zero
unless
V
reaches
zero.
The
inductor’s
in
lower value is the initial value of the next cycle, but never becomes zero unless Vin reaches zero. The
energy isenergy
transferred
to the output
.
inductor’s
is transferred
to theduring
outputtoff
during
toff.
Vin
ip
iL
t
iinitial
(a)
iL
ip
(b)
iQ
t
ton
(c)
t
id
toff
T
(d)
t
Figure
of of
boost
converter
in in
continuous
mode:
(a)(a)
Voltage
and
inductor
current
Figure5. 5.Waveforms
Waveforms
boost
converter
continuous
mode:
Voltage
and
inductor
current
waveforms;
(b)(b)
Inductor
current;
(c)(c)
Switch
current;
and
(d)(d)
Diode
current.
waveforms;
Inductor
current;
Switch
current;
and
Diode
current.
4. A Composite Energy Storage System
Energies 2017, 10, 114
6 of 16
Energies 2017, 10, 114
6 of 15
4. A Composite Energy Storage System
The intermittent
intermittent and
and fluctuating
fluctuating power
power produced
produced by
by aa linear
linear wave
wave energy
energy converter
converter could
could be
be
The
considered as
as low
low quality
quality electricity,
electricity, which
which is
is supposed
supposed to
to be
be regulated
regulated to
to aa continuous
continuous and
and steady
steady
considered
form to
to meet
meet the
the connection
connection requirements
requirements of
of the
the local
local load
load or
orpower
powergrid.
grid. These
These power
power quality
quality
form
concerns can
can be
be allayed
allayed by
by appropriate
appropriateenergy
energystorage
storagedevices.
devices.
concerns
4.1.
4.1. Energy
Energy Storage
Storage Components
Components
Diverse
Diverse energy
energy storage
storage techniques
techniques are
are summarized
summarized in
in [29–32],
[29–32], and
and they
they have
have various
various relative
relative
disadvantages
advantages. Referring
ReferringtotoFigure
Figure6 [29–32],
6 [29–32],
advantages
of super
capacitors
disadvantages and advantages.
thethe
advantages
of super
capacitors
are
are
summarized
as
having
a
high
power
density,
high
efficiency
and
tolerance
to
a
large
number
summarized as having a high power density, high efficiency and tolerance to a
number of
of
charging
charging cycles.
cycles. Comparatively,
Comparatively, batteries
batteries are
are characterised
characterised as
as having
having aa large
large energy
energy capacity,
capacity, but
but the
the
number
number of
of charging
charging cycles
cycles is
is limited
limited and
and the
the power
power density
density is
is relatively
relatively low.
low. Batteries
Batteries are
are not
not suited
suited
to
to handling
handling frequent
frequent fluctuations
fluctuations in
in power
power systems
systems and
and devices
devices with
with aa high
high power
power density
density are
are more
more
capable
capable of
of handling
handling frequent
frequentvarying
varyingfluctuations
fluctuationsin
inpower
powersystems.
systems.
Composite
Energy storage system
Figure 6. Energy density versus power density of different storage technologies.
Figure 6. Energy density versus power density of different storage technologies.
In the application of wave energy conversion, performing maintenance is difficult and
In the application
of wave
energy conversion,
performing
maintenance
andenergy
uneconomical,
uneconomical,
especially
for offshore
installations.
This paper
proposesisa difficult
composite
storage
especially
for
offshore
installations.
This
paper
proposes
a
composite
energy
storage
system
working
system working with wave energy converters. The composite system not only combines the benefits
with
wave
energyand
converters.
The composite
system
not only
combines life
the of
benefits
of both
batteries
of both
batteries
super capacitors,
but also
extends
the operation
batteries,
as the
super
and
super
capacitors,
but
also
extends
the
operation
life
of
batteries,
as
the
super
capacitor
is
used to
capacitor is used to handle the frequent fluctuations of the system. Hence, the frequency
of
handle
the
frequent
fluctuations
of
the
system.
Hence,
the
frequency
of
maintenance
of
the
overall
maintenance of the overall system can be decreased.
system
be decreased.
In can
consideration
of device maturity and system cost, a composite energy storage system
In
consideration
of deviceand
maturity
and systemiscost,
a composite
storage
composed of Li-ion batteries
super capacitors
proposed.
Superenergy
capacitors
are system
appliedcomposed
to handle
of
Li-ion
batteries
and
super
capacitors
is
proposed.
Super
capacitors
are
applied
to
handle
the frequently varying power fluctuations within a time period of less than a minute, for examplethe
the
frequently
varying power
fluctuations
within
a timeinperiod
of less
than areferring
minute, to
forFigure
example
the
typical fluctuations
of power
outputs from
LWECs
one wave
period,
2. This
typical
fluctuations
of power outputs
from LWECs
inlimited
one wave
period,
referring
to Figure
2. This
goal is achieved
to economise
the consumption
of the
charging
cycles
of batteries.
Long
term
goal
is
achieved
to
economise
the
consumption
of
the
limited
charging
cycles
of
batteries.
Long
term
power stability is provided by the Li-ion batteries over time periods of hours. The batteries absorb
power
stabilitypower
is provided
the Li-ion
batteries over
timepower
periods
of hours.
Theby
batteries
absorb
the excessive
whenby
LWECs
are generating
more
than
required
the local
loadsthe
or
excessive
power
when
LWECs
are
generating
more
power
than
required
by
the
local
loads
or
grids.
grids. The batteries supply power to meet the load demand when the LWECs are generating
The
batteriespower.
supplyApower
to meet
the storage
load demand
the LWECs
are generating
insufficient
insufficient
composite
energy
systemwhen
combines
the advantages
of the high
energy
power.
A
composite
energy
storage
system
combines
the
advantages
of
the
high
energy
density
density and high power density characteristics of different devices as shown in Figure 6, andand
the
high
power
density
characteristics
of
different
devices
as
shown
in
Figure
6,
and
the
super
capacitors
super capacitors and batteries are complementary.
and batteries are complementary.
4.2. Interface Circuits
In Figure 2, the terminal voltage values of the batteries and super capacitors are generally smaller
than the voltage value at DC link. A buck-boost DC/DC interface topology [33,34] is used for
regulating the energy exchanges between energy storage systems and the DC link. The same
Energies 2017, 10, 114
7 of 16
4.2. Interface Circuits
In Figure 2, the terminal voltage values of the batteries and super capacitors are generally smaller
than
the2017,
voltage
value at DC link. A buck-boost DC/DC interface topology [33,34] is used for regulating
Energies
10, 114
7 of 15
Energies 2017, 10, 114
7 of 15
the energy exchanges between energy storage systems and the DC link. The same converter topology
is
applied
for topology
both the super
capacitors
andthe
batteries,
as shown in
Figure
7. Theasinterface
circuit
operates
converter
is applied
for both
super capacitors
and
batteries,
shown in
Figure
7. The
converter topology is applied for both the super capacitors and batteries, as shown in Figure 7. The
ininterface
two different
In the
buckdifferent
mode, the
energyInstorage
device
is charged
by controlling
S1 , andis
circuitmodes.
operates
in two
modes.
the buck
mode,
the energy
storage device
interface circuit operates in two different modes. In the buck mode, the energy storage device
is
extracts
powerSfrom
the
DC link.
When Spower
current
through
S S, 1the
inductor
L and
chargedexcessive
by controlling
1, and
extracts
excessive
the goes
DC link.
When
is on,
current
goes
1 is on,from
charged by controlling S1, and extracts excessive power
from the DC link. When1S1 is on, current goes
the
electrical
energy
storage
device
are charged
by storage
the electrical
from the
DCelectrical
link. When
S1
through
S1, the
inductor
L and
the electrical
energy
devicepower
are charged
by the
power
through S1, the inductor L and the electrical energy storage device are charged by the electrical power
isfrom
off, the
turnsSto
discharge
mode, current
through Dmode,
,
the
electrical
energy
storage
the inductor
DC link. LWhen
1 is
off, the inductor
L turnsgoes
to discharge
current
goes
through
D2,
2
from the DC link. When S1 is off, the inductor L turns to discharge mode,
current goes through D2,
device
is maintaining
boost mode,
the terminal
of energy
storage
device
the electrical
energy charge
storagemode.
deviceInisthe
maintaining
charge
mode. voltage
In the boost
mode,
the terminal
the electrical energy storage device is maintaining charge mode. In the boost mode, the terminal
isvoltage
stepped
and release
link. up
When
is
on,
the
energy
storage
device
charges
ofup
energy
storageenergy
deviceto
is DC
stepped
andSrelease
energy
to
DC
link.
When
S
2
is
on, up
the
2
voltage of energy storage device is stepped up and release
energy to DC link. When S2 is on, the
inductor
L. When
S2 is off,
the energy
storage
inductor
L works
in conjunction
both
energy storage
device
charges
up inductor
L. device
When Sand
2 is off,
the energy
storage
device andand
inductor
energy storage device
charges up inductor L. When S2 is off, the energy storage device and inductor
discharge
energies
to theand
DC both
link and
maintain
the DC
voltage.
L works in
conjunction
discharge
energies
to link
the DC
link and maintain the DC link voltage.
L works in conjunction and both discharge energies to the DC link and maintain the DC link voltage.
L
L
u
uii
s
s11
iL
iL
s
s22
D1
D1
C
C
u
uDCDC
D2
D2
L
L
u
uii
s
s11
iL
iL
s
s22
(a)
(a)
s
s11
L iL
L iL
u
uii
s
s22
D1
D1
C
C
u
uDCDC
D2
D2
(b)
(b)
D1
D1
C
C
u
uDCDC
D2
D2
L
L
u
uii
s
s11
i
iLL
s
s22
D1
D1
C
C
u
uDCDC
D2
D2
(c)
(d)
(c)
(d)
Figure 7. Current paths of the interface circuit of super capacitors and batteries. (a) Buck mode, S1 is
Figure
Figure7.7.Current
Currentpaths
pathsof
ofthe
theinterface
interfacecircuit
circuitof
ofsuper
supercapacitors
capacitorsand
andbatteries.
batteries.(a)
(a)Buck
Buckmode,
mode,SS11isis
on;
(b)
Buck
mode,
D
2 is on; (c) Boost mode, S2 is on; and (d) Boost mode, D1 is on.
on;
is on.
on.
on;(b)
(b)Buck
Buckmode,
mode,DD22 is
is on;
on; (c)
(c) Boost
Boost mode,
mode, SS22 is
is on;
on; and
and (d)
(d) Boost
Boost mode,
mode, D
D11 is
4.3. Control of Super Capacitors and Batteries
4.3.
4.3. Control
Control of
of Super
Super Capacitors
Capacitorsand
andBatteries
Batteries
The super capacitors are connected to a DC Link by DC/DC bidirectional converters as shown
The
The super
supercapacitors
capacitorsare
areconnected
connectedto
toaaDC
DCLink
Linkby
byDC/DC
DC/DC bidirectional
bidirectional converters
converters as
as shown
shown
in Figure 7. The voltage value of the DC link is maintained by the control method shown in Figure 8
in
inFigure
Figure7.
7. The
The voltage
voltage value
value of
of the
the DC
DC link
link is
is maintained
maintained by
by the
the control
control method
method shown
shown in
in Figure
Figure 88
using a proportional-integral controller. A feedback signal ufd is taken from the DC link. An error
using
using aa proportional-integral
proportional-integral controller.
controller. A
A feedback
feedback signal
signal uufdfd is
is taken
taken from
from the
the DC
DC link.
link. An
An error
error
signal is obtained by subtracting ufd from a designed value of DC link voltage uref, and then fed to a
signal
signal isisobtained
obtainedby
bysubtracting
subtractinguufdfd from
from aa designed
designed value
value of
ofDC
DClink
linkvoltage
voltageuurefref,, and
and then
then fed
fed to
to aa
proportional/integral stages (PI). A current reference iref is used to compare with iL the instant inductor
proportional/integral
proportional/integralstages
stages(PI).
(PI).A
Acurrent
currentreference
referenceiref
iref is
is used
used to
to compare
comparewith
withiiLL the
the instant
instant inductor
inductor
currents of interface circuits shown in Figure 7, and then an error signal is generated and adjusted
currents
7, 7,
and
then
an an
error
signal
is generated
andand
adjusted
for
currentsof
ofinterface
interfacecircuits
circuitsshown
shownininFigure
Figure
and
then
error
signal
is generated
adjusted
for generating pulse width modulation (PWM) signals to control the inductor currents.
generating
pulse
width
modulation
(PWM)
signals
to control
the inductor
currents.
for generating
pulse
width
modulation
(PWM)
signals
to control
the inductor
currents.
uref
uref +
-
iref
PI iref ++PI
+
-
ufd
ufd
PI
PI
PWM
PWM
DC/DC
DC/DC
converter
converter
iL
iL
uDC
uDC
Figure 8. Controls of the interface circuit of super capacitor.
Figure 8. Controls of the interface circuit of super capacitor.
Figure 8. Controls of the interface circuit of super capacitor.
The inductor current is fully controlled as shown in Figure 8, and the super capacitors and
The inductor current is fully controlled as shown in Figure 8, and the super capacitors and
inductor are connected in series; this means the flows of currents or energy from the from the super
inductor are connected in series; this means the flows of currents or energy from the from the super
capacitors are also fully regulated to maintain a steady DC link voltage. When the measured value of
capacitors are also fully regulated to maintain a steady DC link voltage. When the measured value of
the DC link voltage is higher than the designed value u , the super capacitors are controlled to
the DC link voltage is higher than the designed value urefref , the super capacitors are controlled to
operate in the buck mode for absorbing excess energy, thereby preventing the rise of the DC link
operate in the buck mode for absorbing excess energy, thereby preventing the rise of the DC link
Energies 2017, 10, 114
8 of 16
The inductor current is fully controlled as shown in Figure 8, and the super capacitors and
Energies 2017, 10, 114
8 of 15
inductor
are connected in series; this means the flows of currents or energy from the from the super
capacitors are also fully regulated to maintain a steady DC link voltage. When the measured value
voltage values. When the measured value of DC link voltages is smaller than uref , the circuit operates
of the DC link voltage is higher than the designed value ure f , the super capacitors
are controlled to
in the boost
mode
and
supplies
energy to excess
maintain
a steady
DC link
voltage. the rise of the DC link
operate
in the
buck
mode
for absorbing
energy,
thereby
preventing
Thevalues.
AC power
generated
from value
LWECs
extracted
and converted
DCupower.
Low pass filters
voltage
When
the measured
of is
DC
link voltages
is smallerto
than
re f , the circuit operates
or the
moving
filters
can beenergy
used totoseparate
the
andDC
AClink
components
in
boostaverage
mode and
supplies
maintain
a DC
steady
voltage. of the power at the DC
link. The
TheAC
ACpower
powergenerated
components
(the
frequent
fluctuations)
are
suitably
addressed
by using
super
from LWECs is extracted and converted to DC
power. Low
pass filters
capacitors.
LWECs
are
presumed
to
continuously
supply
a
designed
steady
power
to
the
PG
or moving average filters can be used to separate the DC and AC components of the power
at thelocal
DC
link.
The
ACinpower
(the
frequent
fluctuations)
are suitably
addressed
usingpower
super
load or
grid
a timecomponents
intervals over
hours,
where
determined
as the by
average
PG is normally
capacitors.
LWECs
are at
presumed
continuously
supplyvalues
a designed
steady
power
the local
from LWECs
predicted
a specificto
location.
The different
between
PW (tP
) Gistoaddressed
PG and
load
or
grid
in
a
time
intervals
over
hours,
where
P
is
normally
determined
as
the
average
G
by batteries as shown in Equation (3), where PW (t ) represents
the varying DC component of power
power
from LWECs predicted at a specific location. The different values between PG and PW (t) is addressed
produced by LWECs.
by batteries as shown in Equation (3), where PW (t) represents the varying DC component of power
PE (t )  PW (t )  PG
produced by LWECs.
(3)
PE (t) = PW (t) − PG
(3)
When PE (t ) is positive, the DC/DC converter is controlled to work in the buck mode as
When PE (t) is positive, the DC/DC converter is controlled to work in the buck mode as
represented in Figure 7, and batteries absorb the excess power from LWECs. When P (t ) is negative,
represented in Figure 7, and batteries absorb the excess power from LWECs. When PEE (t) is negative,
the DC/DC
DC/DC converter
to DC
DC
the
converter is
is controlled
controlled to
to function
function in
in the
the boost
boost mode,
mode, and
and batteries
batteries release
release energy
energy to
link,
aiming
to
retain
steady
values
of
power
output
and
DC
link
voltage.
Figure
9
describes
the
P
link, aiming to retain steady values of power output PGG and DC link voltage. Figure 9 describes the
the interface
interface circuit
circuit of
of batteries.
batteries.
controls of the
PE,ref
LPF
PW (t )
+
-
PG


PI
+-
PWM
uBattery iL
PSC,ref
DC/DC
converter
Figure 9. Control of the interface circuit of battery.
battery.
4.4. Capacity
Capacity of
of Energy
Energy Storage
Storage Component
Component
4.4.
In the
the application
application of
of harnessing
harnessing ocean
ocean waves
waves for
for electricity
electricity generation,
generation, the
the available
available amount
amount of
of
In
ocean
power
over
a
certain
period
can
be
forecasted.
Accordingly,
the
power
output
amount
P
(
ocean power over a certain period can be forecasted. Accordingly, the power output amount PWW(tt ))
can be estimated for LWECs. The total energy capacity of batteries required in a composite energy
storage system can therefore be determined by PPEE ,, which
which is
is the
the difference
difference between
between the
the maximum
maximum
power
power and
and the
the average
average power
power from
from LWECs
LWECs at
at aa particular
particular location
location over
over aa certain
certain time.
time. The maximum
energy required to
be
stored
by
batteries
can
be
evaluated
as
the
integral
of
P
over
to be stored by batteries can be evaluated as the integral of EPE overa atime
timeinterval
intervalt.
Equation
(4)(4)
shows
thethe
evaluation
ofof
Cbat
·h,h,where
max
t. Equation
shows
evaluation
Cbatthe
therequired
requiredcapacity
capacityofofbatteries
batteriesininAA·
whereVV
max is
is the
the
maximum
terminal
voltage
and
V
is
the
related
minimum
terminal
voltage:
maximum terminal voltage and Vmin
min is the related minimum terminal voltage:
VVmax++
VV
Cbat
) )
Ebat E=batCbat
( ( max min min
22
(4)
(4)
determined
byby
a similar
evaluation
method.
As
The required capacity
capacityof
ofsuper
supercapacitors
capacitorscan
canbebe
determined
a similar
evaluation
method.
described,
the the
super
capacitors
generally
respond
faster
than
batteries,
andand
hence
areare
chosen
to
As
described,
super
capacitors
generally
respond
faster
than
batteries,
hence
chosen
address
frequent
fluctuations
of
output
powers
from
LWECs
in
a
relatively
short
time
period.
to address frequent fluctuations of output powers from LWECs in a relatively short time period.
the evaluation
evaluation of
of C
Ccap
cap the
Equation (5) shows the
the required
required capacitance
capacitance of super capacitors:
Vmax ++
VV 2 2
1 1 CcapV
max min
min
)
=cap C2cap
Ecap E
((
)
2
2
2
(5)
(5)
5. Hardware Implementation
A complete wave energy converter system could be tested in a wave tank facilities or at the sea,
but this would be complicated, risky and costly. The investigation of control methods and power
conversion strategies for LWECs requires repeatable wave conditions. Laboratory scale tests can
Energies 2017, 10, 114
9 of 16
5. Hardware Implementation
A complete wave energy converter system could be tested in a wave tank facilities or at the sea,
but this would be complicated, risky and costly. The investigation of control methods and power
Energies 2017, 10, 114
9 of 15
conversion strategies for LWECs requires repeatable wave conditions. Laboratory scale tests can imitate
the characteristics
of an individual
energy
converter,
and provide
savings
convenience.
Energies 2017,
114
9 and
of 15
imitate
the 10,
characteristics
of an wave
individual
wave
energy converter,
andcost
provide
cost and
savings
A
power
electronics-based
wave
energy
emulator
is
capable
of
emulating
the
impedances,
power
convenience.
imitate
the
characteristics
of
an
individual
wave
energy
converter,
and
provide
cost
savings
and
A voltage
power electronics-based
wave energy
emulatoroutputs
is capable
emulating LWEC,
the impedances,
losses and
waveforms reflecting
the expected
of aofparticular
as proposed
convenience.
power
and could
voltage
reflecting
expected and
outputs
of a particular
LWEC, as
in [35].
Thislosses
emulator
bewaveforms
used to excite
powerthe
converters
the power
train of LWECs
for the
A power
electronics-based
wave
energy
is
capable
of emulatingthethe
impedances,
proposed
in [35].
This emulator
could of
be conditions.
usedemulator
to excite
power
converters
assessment
of performances
over a range
Referring
to Figure and
10, eachpower
phase train
of theofwave
power losses
voltage of
waveforms
reflecting
expected
outputs of a particular
LWEC,
as
LWECs
for theand
assessment
performances
over athe
range
of conditions.
to Figure
10, each
energy
emulator
consists
of a DC
supply, a low
pass
filter,
a section ofReferring
coils from
a particular
linear
proposed
in
[35].
This
emulator
could
be
used
to
excite
power
converters
and
the
power
train
of
phase of the wave energy emulator consists of a DC supply, a low pass filter, a section of coils from
machine,
an
H-bridge
inverter
and
related
control
circuitries.
The
H-bridge
is
controlled
by
signals
for the
assessment
performances
over a range
of conditions.
Figure
10, each
aLWECs
particular
linear
machine,ofan
H-bridge inverter
and related
control Referring
circuitries.toThe
H-bridge
is
produced
from
a
field
programmable
gate
array
(FPGA)
based
controller
with
PWM
methods
[36–38].
phase
of
the
wave
energy
emulator
consists
of
a
DC
supply,
a
low
pass
filter,
a
section
of
coils
controlled by signals produced from a field programmable gate array (FPGA) based controller from
with
The low
pass
filter
is machine,
usedThe
to low
remove
the inverter
high
PWM
from
the output
a particular
linear
an H-bridge
and
related
circuitries.
The
H-bridge
is of
PWM
methods
[36–38].
pass filter
is usedfrequency
to
remove
thecontrol
highcomponents
frequency
PWM
components
controlled
by
signals
produced
from
a
field
programmable
gate
array
(FPGA)
based
controller
with
the H-bridge.
from the output of the H-bridge.
PWM methods [36–38]. The low pass filter is used to remove the high frequency PWM components
generator coil
from the output of the H-bridge.
H-bridge and gate
drive circuitry
H-bridge and gate
drive circuitry
DC
DC
generator coil
LC filter
LC filter
FPGA controller
FPGA
Figurecontroller
10. Wave Energy Emulator (1 phase).
Figure 10. Wave Energy Emulator (1 phase).
Controller
Controller
FPGAFPGA
Controller
FPGAFPGA
Controller
for for
for Emulator
boost converter
for Emulator
Ac/DcAc/Dc
boost converter
Figure 10. Wave
Energy
Emulator
(1 phase).
Figure 11 shows the experimental
setup,
which
includes:
two FPGA controllers, one for the
Figure
shows
the experimental
setup,
which
includes:
two FPGA
one
for the
AC/DC 11
boost
H-bridge
and another one
for the
emulator;
the H-bridges
sharecontrollers,
the same heat
sink.
FigureH-bridge
11
shows and
the
setup,
which
includes:
two
FPGA
controllers,
one of
forheat
the sink.
The boost
AC/DC
converters
areexperimental
built withone
four
fastthe
recovery
diodes
four
IGBTs,
withthe
ratings
1200
AC/DC
another
for
emulator;
theand
H-bridges
share
same
AC/DC
boost H-bridge
and
another
one for the
emulator;
thefor
H-bridges
share
same
heat sink.
V.
A generator
coil section
from
a small
machine
isdiodes
used
replicating
thethe
source
impedances
The AC/DC
converters
are built
with
fourlinear
fast recovery
and
four IGBTs,
with ratings
of 1200 V.
The
AC/DC
converters
are
built
with
four
fast
recovery
diodes
and
four
IGBTs,
with
ratings
of 1200
of
a
particular
generator.
The
coil
has
a
resistance
20
Ω
and
an
inductance
0.4
H.
As
the
system
is of
A generator coil section from a small linear machine is used for replicating the source impedances
V. A emulating
generator coil
fromof
a small
linear
machine
is used
replicating
the source is
impedances
only
onesection
coil section
a small
prototype
LWEC,
thefor
power
level considered
low.
a particular
generator.
The coil
resistance
20 Ω20and
an inductance
0.40.4H.H.As
of a particular
generator.
Thehas
coilahas
a resistance
Ω and
an inductance
Asthe
thesystem
system is
is only
emulating
one
coil
section
of
a
small
prototype
LWEC,
the
power
level
considered
is
low.
Generator
only emulating one coil section of a small prototype LWEC, the power
levelcoil
considered is low.
Generator coil
H-Bridge and Ac/Dc boost converter
Low pass LC filter
mounted on the same heat sink
H-Bridge and Ac/Dc boost converter
Low pass LC filter
Figure 11. Experimental setup of wave energy emulator and AC/DC power conversion.
mounted on the same heat sink
Figure
11. Experimental
setup
waveenergy
energy
emulator and
AC/DC
power
conversion.
Figure
1211.
shows
the experimental
setup
of
the composite
energy
storage
system,
including LiFigure
Experimental
setup
ofofwave
emulator
and
AC/DC
power
conversion.
ion batteries with 51.2 V voltage rating and 6 A·h capacity, super capacitors with a total 52 F
Figure 12and
shows
experimental
setup of theand
composite
storage
including
Licapacitance,
the the
related
power converters
controlenergy
circuits.
The system,
bi-directional
dc-dc
ion batteries with 51.2 V voltage rating and 6 A·h capacity, super capacitors with a total 52 F
capacitance, and the related power converters and control circuits. The bi-directional dc-dc
Energies 2017, 10, 114
10 of 16
Figure 12 shows the experimental setup of the composite energy storage system, including Li-ion
batteries with 51.2 V voltage rating and 6 A·h capacity, super capacitors with a total 52 F capacitance,
Energies 2017, 10, 114
10 of 15
and the related power converters and control circuits. The bi-directional dc-dc converters are built
with converters
IGBTs andare
fast
recovery
diodes,
which
are rated
at 1200
Table
lists
the
built
with IGBTs
and fast
recovery
diodes,
whichV.are
rated1at
1200
V. parameter
Table 1 lists setup
the of
the experiments.
parameter setup of the experiments.
DSP
Controller
Interface power
converter
Voltage and current
sensers
Li-ion battery
simulator
DC source
load
inductor
super-capacitor
Figure 12. Experimental setup of the composite energy storage system.
Figure
12. Experimental setup of the composite energy storage system.
Table 1. Parameters of experimental platform.
Table 1. Parameters of experimental platform.
Parameters
DC supply
Parameters
DC bus filtering capacitor
DC
IGBTsupply
used for H-bridge
DCRecovery
bus filtering
diodecapacitor
used for H-bridge
IGBT used
for H-bridge
Low-pass
LC filter
Recovery
diodecoil’s
usedresistance/inductance
for H-bridge
Generator
Low-pass
LCvoltage/capacity
filter
Li-batteries
Generator coil’s resistance/inductance
Super capacitors
Li-batteries
voltage/capacity
Buck/boost
filtering inductor
Super capacitors
Load
Buck/boost filtering inductor
Load
6. Results and Discussion
Values
500 V/10
A
Values
2200 μF
500 V/10 A
IRG4BC20U
2200 µF
HFA08TB60
IRG4BC20U
1.4 mH/20
μF
20HFA08TB60
Ω/0.4 H
1.4V/6
mH/20
51.2
A·h µF
20
Ω/0.4
H
52 F/15 V (3 in series)
51.2
V/6
A
·h
5 mH
52 F/15
33.3VΩ(3 in series)
5 mH
33.3 Ω
MATLAB is used to simulate the power converters and related control methods represented in
this paper. These converters extract power from LWECs, and condition the output to a steady voltage
and hence constant
power
for connecting
to the
load or grid.
opencontrol
loop EMF
waveforms
are
MATLAB
is used to
simulate
the power
converters
and The
related
methods
represented
simulated
for
a
LWEC,
when
it
is
driven
by
a
particular
ocean
wave
with
0.8
m
amplitude
and
s
in this paper. These converters extract power from LWECs, and condition the output to a5steady
period, as shown in Figure 1. It can be seen that the expected voltage waveforms vary in both
voltage and hence constant power for connecting to the load or grid. The open loop EMF waveforms
frequency and magnitude. The low frequency envelopes of voltage waveforms correspond to the
are simulated for a LWEC, when it is driven by a particular ocean wave with 0.8 m amplitude and
input wave with a 5 s period. The frequency modulated EMFs are within the range 0–13 Hz. Since
5 s period,
as shown
Figure
It can
be seen that rest
the at
expected
waveforms
in both
the translator
of a in
LWEC
will1.come
to instantaneous
the ends voltage
of its stroke,
it is clearvary
that the
frequency
and
magnitude.
The
low
frequency
envelopes
of
voltage
waveforms
correspond
mechanical power extracted must drop to zero at these positions, and power flow will be periodicto the
inputwith
wave
with
5 s period.
The frequency modulated EMFs are within the range 0–13 Hz. Since
twice
theawave
frequency.
the translator
a LWEC
will passive
come to
instantaneous
at thefrom
ends
of its are
stroke,
it is clearand
that the
After of
passing
through
diode
rectifiers, therest
currents
LWECs
discontinuous
small, and
hence
the extracted
and
output
voltage
are relatively
low, as
shown
2. In with
mechanical
power
extracted
must power
drop to
zero
at these
positions,
and power
flow
willinbeTable
periodic
thefrequency.
AC/DC boost topology is capable of achieving quasi-continuous currents and higher DC
twicecontrast,
the wave
link voltages. This results in the increase of output powers from 88 to 236 W. A small one-phase test
6. Results and Discussion
Energies 2017, 10, 114
11 of 16
After passing through passive diode rectifiers, the currents from LWECs are discontinuous and
small, and hence the extracted power and output voltage are relatively low, as shown in Table 2.
In contrast, the AC/DC boost topology is capable of achieving quasi-continuous currents and higher
DC link voltages. This results in the increase of output powers from 88 to 236 W. A small one-phase
test rig is simulated at this stage, and 367 W is generated. However, a large amount of power is wasted
due to the large intrinsic resistance of generator coils, and this result in only 236 W is extracted from
the LWEC. The power level of a LWEC could be scaled up to kW or MW in real applications [10].
LWECs generate electrical power of varying frequency and amplitude, regardless of the state
Energies 2017, 10, 114
11 of 15
of the input driving ocean waves. Moreover, the power variation results a large ratio of peak to
average
As shown
Table
DCWlink
voltages with
largea ripples
are generated,
rigpower.
is simulated
at this in
stage,
and2,367
is generated.
However,
large amount
of power iswhich
wastedmeans
due
to
the
large
intrinsic
resistance
of
generator
coils,
and
this
result
in
only
236
W
is
extracted
from
using a capacitor even as large as 3 mF is ineffective. A large voltage ripple at the DC link
is likely
the
LWEC.
The
power
level
of
a
LWEC
could
be
scaled
up
to
kW
or
MW
in
real
applications
[10].
to cause challenges for maintaining the system’s stability and cause stresses on power electronics.
LWECs generate electrical power of varying frequency and amplitude, regardless of the state of
The composite
energy storage system of batteries and super capacitors has the ability of maintaining
the input driving ocean waves. Moreover, the power variation results a large ratio of peak to average
steady values of DC link voltage and power, but with a small voltage ripple of only 7 V.
power. As shown in Table 2, DC link voltages with large ripples are generated, which means using a
capacitor even as large as 3 mF is ineffective. A large voltage ripple at the DC link is likely to cause
Table 2. Comparing performances of various system topologies.
challenges for maintaining the system’s stability and cause stresses on power electronics. The
composite energy storage system of batteries and super capacitors has the ability of maintaining
Average
Average
Output
steady values
of DC link voltage and power, but
with aDC
small voltage
ripple
of only 7 DC
V. Link Voltage
Topologies
Link Voltage (V)
Power (W)
Ripples (V)
Table 2. Comparing performances of various system topologies.
Rectifier diodes
192
88
(without battery-super capacitor system)
Topologies
Boost AC/DC converter
Rectifier
diodes system)
(without battery-super
capacitor
(without battery-super capacitor system)
Boost AC/DC converter
Boost AC/DC converter
(with battery-super capacitor system)
(without battery-super capacitor system)
Boost AC/DC converter
(with
battery-super
capacitor
system)long
Varying sea conditions
will cause
120
Average DC Link
Voltage (V)
Average Output
Power (W)
DC Link Voltage
Ripples (V)
192
88
120
315
315
236
236
133
315
236
133
315
236
7
7
term power variations from the LWECs. These can be
handled by large energy density devices, in this case the batteries. Due to the limitations of computer
Varying sea conditions will cause long term power variations from the LWECs. These can be
resources,
the by
simulation
wasdensity
performed
for
fourthe
wave
cycles
astoshown
in Figure
The LWEC
handled
large energy
devices,
inonly
this case
batteries.
Due
the limitations
of 13.
computer
is desired
to
harnesses
electricity
from
waves
with
amplitudes
of
1.2
m
and
0.8
m.
As
can
be seen in
resources, the simulation was performed for only four wave cycles as shown in Figure 13. The LWEC
Figureis13,
the
value
of
DC
link
voltage
is
maintained
at
425
V
by
the
composite
system
of
desired to harnesses electricity from waves with amplitudes of 1.2 m and 0.8 m. As can be seen various
in
13, means.
the valueWhen
of DCthe
linkLWEC
voltageisisdriven
maintained
at 425 with
V by the
system
of various
energyFigure
storage
by waves
1.2 composite
m amplitude,
a 758
W average
storagefrom
means.
the LWEC
is driven
by waves
m amplitude,
758 Wload
average
powerenergy
is obtained
theWhen
generator,
a steady
power
of 422with
W is1.2
required
by thea local
and grid,
power
is
obtained
from
the
generator,
a
steady
power
of
422
W
is
required
by
the
local
load
and
grid,
and the excess amount of power is stored by the batteries. When the LWEC is driven by waves with
and the excess amount of power is stored by the batteries. When the LWEC is driven by waves with
0.8 m amplitude, a 208 W average power is produced by the generator, and the batteries release energy
0.8 m amplitude, a 208 W average power is produced by the generator, and the batteries release
for meeting
the demand value of 422 W load.
energy for meeting the demand value of 422 W load.
Figure 13. Output voltages of LWEC for four wave cycles.
Figure 13. Output voltages of LWEC for four wave cycles.
The composite system of various energy storages is controlled to smooth the fluctuation of
powers from LWECs and meet the steady power-flow requirement. As shown in Figure 14, the super
capacitor is controlled according to the arrangement shown in Figure 8. When the green line or red
line is positive, the super capacitors or batteries are in discharging mode, respectively. When the two
lines are negative, the energy storage components are in charging mode. The charging and
discharging processes of the super capacitors (indicated by green lines) are much more frequently
Energies 2017, 10, 114
12 of 16
The composite system of various energy storages is controlled to smooth the fluctuation of powers
from LWECs and meet the steady power-flow requirement. As shown in Figure 14, the super capacitor
is controlled according to the arrangement shown in Figure 8. When the green line or red line is
positive, the super capacitors or batteries are in discharging mode, respectively. When the two lines
are
negative,
Energies
2017, 10,the
114 energy storage components are in charging mode. The charging and discharging
12 of 15
processes of the super capacitors (indicated by green lines) are much more frequently than that of
batteries
by red
line). This
that This
capacitor’s
are being
exploitedare
to cover
than that(indicated
of batteries
(indicated
by means
red line).
meansadvantages
that capacitor’s
advantages
being
the
disadvantages
batteries
includingoflimited
charging/discharging
rates and charging/discharging
exploited
to coverofthe
disadvantages
batteries
including limited charging/discharging
rates and
Energies
10, 114 hand,
12could
of energy
15
cycles.
On 2017,
the other
theOn
batteries
have
advantage
of a large
energy
density,
be
charging/discharging
cycles.
the other
hand,
the batteries
have
advantage
ofwhich
a large
applied
to
handle
long
term
fluctuations
of
power.
density, which could be applied to handle long term fluctuations of power.
than that of batteries (indicated by red line). This means that capacitor’s advantages are being
exploited to cover the disadvantages of batteries including limited charging/discharging rates and
charging/discharging cycles. On the other hand, the batteries have advantage of a large energy
density, which could be applied to handle long term fluctuations of power.
Figure 14.
14. Power
Power of
of battery
battery and
and super
super capacitor
capacitor responding
responding to
to the
the power
power variation
variation of
of aa LWEC.
LWEC.
Figure
Figure 14. Power of battery and super capacitor responding to the power variation of a LWEC.
The wave energy emulator produces the expected EMF envelope with similar amplitude and
The wave energy emulator produces the expected EMF envelope with similar amplitude and
the same frequency,
as compared
to
the simulation
results
shown
in Figure
1 and the experimental
wave energy
emulatorto
produces
the expected
EMFshown
envelope
with similar
and
the same The
frequency,
as compared
the simulation
results
in Figure
1 andamplitude
the experimental
results
shown
in
Figure
15.
The
green
line
indicates
the
EMF
to
be
100
V
for
each
the blue
theshown
same frequency,
as compared
theindicates
simulation
results
in Figure
1 and
the division;
experimental
results
in Figure 15.
The greentoline
the
EMFshown
to be 100
V for each
division;
the blue line
line indicates
currents
as
1
A
per
division.
The
time
scale
is
0.5
per
division.
The
generator
coil
current
results
shown as
in 1Figure
The green
line
indicates
the
be 100 VThe
for each
division;
blue
indicates
currents
A per15.
division.
The
time
scale is
0.5EMF
perto
division.
generator
coilthe
current
can
linefully
indicates
currents as
per division.
time scale isfrom
0.5 perthe
division.
The generator
coil current
can be
controlled
in1 A
tracking
EMFThe
waveforms
emulator
and keep
the current
be fully controlled in tracking EMF waveforms from the emulator and keep the current waveforms in
can be in
fully
controlled
in of
tracking
EMF as
waveforms
from
the emulator
keep the
waveforms
phase
with that
the voltage
can be seen
in Figure
15. Thisand
improves
thecurrent
low power
phasewaveforms
with that in
of phase
the voltage
as of
can
bevoltage
seen in
Figure
15. This
improves
the
low power
factor
despite
with
that
the
as
can
be
seen
in
Figure
15.
This
improves
the
low
power
factor despite the large intrinsic inductance. This correction of the current waveforms results in an
the large
intrinsic
inductance.
This
correction
of
the
current
waveforms
results
in
an
increment
of
factor of
despite
the large
intrinsic
increment
extracted
power
frominductance.
LWECs. This correction of the current waveforms results in an
extracted
power
from
LWECs.
increment of extracted power from LWECs.
Figure 15. Generator coil current tracks the voltage waveform (one phase).
Figure 15. Generator coil current tracks the voltage waveform (one phase).
Figure 15. Generator coil current tracks the voltage waveform (one phase).
Figure 16 shows the voltage outputs of the emulator imitating the electrical outputs of a
particular
driven by
multiple
ocean imitating
waves withthecontinuously
varying of a
Figure
16 LWEC
showswhen
the voltage
outputs
of random
the emulator
electrical outputs
amplitude.
As
shown,
the
AC
power
output
fluctuates
with
voltage
envelopes
varying
from
50
to 175
particular LWEC when driven by multiple random ocean waves with continuously
varying
V.
The
period
of
each
driving
wave
is
adjusted
to
2.5
s
for
measurement
convenience,
but
it
should
amplitude. As shown, the AC power output fluctuates with voltage envelopes varying from 50 to 175
be noted that the real wave periods could be longer than 10 s. The DC link bus is controlled to retain
V. The period of each driving wave is adjusted to 2.5 s for measurement convenience, but it should
Energies 2017, 10, 114
13 of 16
Figure 16 shows the voltage outputs of the emulator imitating the electrical outputs of a particular
LWEC when driven by multiple random ocean waves with continuously varying amplitude. As shown,
the AC power output fluctuates with voltage envelopes varying from 50 to 175 V. The period of each
driving wave is adjusted to 2.5 s for measurement convenience, but it should be noted that the real
wave periods could be longer than 10 s. The DC link bus is controlled to retain at a designed voltage of
2017, 10, 114
13 of 15
100 V.Energies
This verified
the performances of composite energy storage system as that shown by simulation
results,
and shows that the proposed system is capable of regulating the outputs from LWECs and
that shown by simulation results, and shows that the proposed system is capable of regulating the
supply
a desired
to meet
the load
demand.
outputs
from output
LWECs power
and supply
a desired
output
power to meet the load demand.
Figure 16. Experimental results of DC link voltage and the emulated electrical outputs for LWEC
Figure 16. Experimental results of DC link voltage and the emulated electrical outputs for LWEC
driven with different wave amplitudes.
driven with different wave amplitudes.
7. Conclusions
7. Conclusions
A microsystem composed of LWECs, a composite energy storage system, and related control
A
microsystem
composed
of LWECs,
a composite
storage
system,
andArelated
control
and
power electronics
is proposed.
The LWECs
generateenergy
electricity
from ocean
waves.
proposed
AC/DCelectronics
converter with
boost function
manageselectricity
the resonant
operation
the mechanics
and power
is proposed.
The effectively
LWECs generate
from
ocean of
waves.
A proposed
of LWECs,
andwith
fully boost
controls
the coil effectively
currents tracking
the EMF
waveforms
to improve
AC/DC
converter
function
manages
the resonant
operation
ofthe
theextraction
mechanics of
of
power
from
the
LWECs.
Simulations
and
experiments
are
performed
to
evaluate
the
performance
LWECs, and fully controls the coil currents tracking the EMF waveforms to improve the extraction of
of the AC/DC boost H-bridge and its control methods. The composite energy storage system
power from the LWECs. Simulations and experiments are performed to evaluate the performance of
consisting of batteries and super capacitors is capable of flattening the fluctuations of power output
the AC/DC boost H-bridge and its control methods. The composite energy storage system consisting
from LWECs, and maintaining a steady value of DC link voltage, hence providing a steady power
of batteries
capacitors isand
capable
of flattening
the fluctuations
power
output from
LWECs,
output.and
Bothsuper
the experimental
simulation
results have
demonstratedofthat
the proposed
control
and maintaining
a
steady
value
of
DC
link
voltage,
hence
providing
a
steady
power
output.
method for the composite energy storage system is capable of making full use of the relative Both
the experimental
simulation
have demonstrated
that
the the
proposed
control
method
advantages ofand
super
capacitorsresults
and batteries,
allowing them to
cover
drawbacks
for each
other.for the
composite
energy
storage
system
is
capable
of
making
full
use
of
the
relative
advantages
of super
Acknowledgment: This work was supported by the National Natural Science Foundation of China under Grant
capacitors
and
batteries,
allowing
them
to
cover
the
drawbacks
for
each
other.
51577095.
Author Contributions:
Zanxiang
is the mainby
author,
involved in
all related
research
work and preparations
Acknowledgments:
This work
wasNie
supported
the National
Natural
Science
Foundation
of China under
the manuscript. Xi Xiao gave guidance for the research of applying composite energy storage to wave energy
Grantof
51577095.
conversions, and for writing the whole manuscript. Xuanrui Huang was involved in the experimentations of
Author
Contributions:
Zanxiang
Nie is the
main author,
involved
all related
researchand
work
and preparations
composite
energy storage
working
in conjunction
with
wave in
energy
conversions,
prepared
some
of theexperimental
manuscript. results.
Xi XiaoPritesh
gave guidance
for
the
research
of
applying
composite
energy
storage
to wave
energy
Hiralal and Richard McMahon gave guidance for the research of wave
energy
conversions,
and
for
writing
the
whole
manuscript.
Xuanrui
Huang
was
involved
in
the
experimentations
of
converters; they were involved in the development of the hardware system and controls to wave energy
composite energy storage working in conjunction with wave energy conversions, and prepared some experimental
converters. Min Zhang and Weijia Yuan gave guidance for the research of super capacitor, batteries and
results. Pritesh Hiralal and Richard McMahon gave guidance for the research of wave energy converters; they
composite energy storage system, and prepared some related simulation results.
were involved
in the development of the hardware system and controls to wave energy converters. Min Zhang
and Weijia
Yuan
guidance
for thedeclare
research
of super
Conflicts
ofgave
Interest:
The authors
no conflict
of capacitor,
interest. batteries and composite energy storage system,
and prepared some related simulation results.
Nomenclature
Conflicts
of Interest: The authors declare no conflict of interest.
PLi
Power output of Li-ion battery
Psc
Power output of super-capacitor
PW
Power output of LWEC
PE
Total power output of energy storage
PG
Steady power output
PL
Load
Mass of moving parts of LWEC
Wave excitation force
Device translational velocity
M
Fe
U
Energies 2017, 10, 114
14 of 16
Nomenclature
PLi
Psc
PW
PE
PG
PL
M
Fe
U
Ke
Re
Kg
Rg
ui
L
iL
ire f
ufd
Power output of Li-ion battery
Power output of super-capacitor
Power output of LWEC
Total power output of energy storage
Steady power output
Load
Mass of moving parts of LWEC
Wave excitation force
Device translational velocity
Mechanical spring stiffness
Mechanical resistance
Generator spring stiffness
Generator resistance
Input voltage of DC-DC converter
Inductance of DC-DC converter
Inductance current of DC-DC converter
Current reference of DC-DC converter
Feedback voltage of DC-DC converter
References
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
Schoen, M.P.; Hals, J.; Moan, T. Wave prediction and robust control of heaving wave energy devices for
irregular waves. IEEE Trans. Energy Convers. 2011, 26, 627–638. [CrossRef]
Zhang, H.; Nie, Z.; Xiao, X.; Aggarwal, R.; Kang, Q.; Ainslie, M.; Zhu, J.; Coombs, T.; Yuan, W. Design
and simulation of SMES system using YBCO tapes for direct drive wave energy converters. IEEE Trans.
Appl. Superconduct. 2013, 23, 5700704. [CrossRef]
Zhang, J.; Yu, H.; Chen, Q.; Hu, M.; Huang, L.; Liu, Q. Design and experimental analysis of AC linear
generator with Halbach PM arrays for direct-drive wave energy conversion. IEEE Trans. Appl. Superconduct.
2014, 24, 1–4. [CrossRef]
Murray, D.B.; Hayes, J.G.; O’Sullivan, D.L.; Egan, M.G. Supercapacitor testing for power smoothing in a
variable speed offshore wave energy converter. IEEE J. Ocean. Eng. 2012, 37, 301–308. [CrossRef]
Cappelli, L.; Marignetti, F.; Mattiazzo, G.; Giorcelli, E.; Bracco, G.; Carbone, S.; Attaianese, C. Linear tubular
permanent-magnet generators for the inertial sea wave energy converter. Ind. Appl. IEEE Trans. 2014, 50,
1817–1828. [CrossRef]
Shek, J.K.H.; Macpherson, D.E.; Mueller, M.A. Experimental verification of linear generator control for direct
drive wave energy conversion. IET Renew. Power Gener. 2010, 4, 395–403. [CrossRef]
Boström, C.; Leijon, M. Operation analysis of a wave energy converter under different load conditions.
IET Renew. Power Gener. 2011, 5, 245–250. [CrossRef]
Rhinefrank, K.; Schacher, A.; Prudell, J.; Brekken, T.K.A.; Stillinger, C.; Yen, J.Z.; Ernst, S.G.; von Jouanne, A.;
Amon, E.; Paasch, R.; et al. Comparison of direct-drive power takeoff systems for ocean wave energy
applications. IEEE J. Ocean. Eng. 2012, 37, 35–44. [CrossRef]
Du, Y.; Chau, K.T.; Cheng, M.; Fan, Y.; Zhao, W.; Li, F. A linear stator permanent magnet vernier HTS machine
for wave energy conversion. IEEE Trans. Appl. Superconduct. 2012, 22, 5202505. [CrossRef]
Gargov, N.P.; Zobaa, A.F. Multi-phase air-cored tubular permanent magnet linear generator for wave energy
converters. IET Renew. Power Gener. 2012, 6, 171–176. [CrossRef]
Ran, L.; Mueller, M.A.; Ng, C.; Tavner, P.J.; Zhao, H.; Baker, N.J.; McDonald, S.; McKeever, P. Power
conversion and control for a linear direct drive permanent magnet generator for wave energy. IET Renew.
Power Gener. 2011, 5, 1–9. [CrossRef]
Crozier, R.; Bailey, H.; Mueller, M.; Spooner, E.; McKeever, P. Analysis, design and testing of a novel
direct-drive wave energy converter system. IET Renew. Power Gener. 2013, 7, 565–573. [CrossRef]
Vermaak, R.; Kamper, M.J. Design aspects of a novel topology air-cored permanent magnet linear generator
for direct drive wave energy converters. IEEE Trans. Ind. Electron. 2012, 59, 2104–2115. [CrossRef]
Matsuoka, T.; Omata, K.; Kanda, H.; Tachi, K. A study of wave energy conversion systems using ball
screws—Comparison of output characteristics of the fixed type and the floating type. In Proceedings of the
Twelfth International Offshore and Polar Engineering Conference, Kitakyushu, Japan, 26–31 May 2002.
Energies 2017, 10, 114
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
27.
28.
29.
30.
31.
32.
33.
34.
35.
15 of 16
Agamloh, E.B.; Wallace, A.K.; Jouanne, A.V. A novel direct-drive ocean wave energy extraction concept with
contact-less force transmission system. Renew. Energy 2008, 37, 520–529. [CrossRef]
Kimoulakis, N.M.; Kakosimos, P.E.; Kladas, A.G. Power generation by using point absorber wave energy
converter coupled with linear permanent magnet generator. In Proceedings of the Power Generation,
Transmission, Distribution and Energy Conversion, Agia Napa, Cyprus, 7–10 November 2010.
Hodgins, N.; Keysan, O.; McDonald, A.S.; Mueller, M.A. Design and testing of a linear generator for
wave-energy applications. IEEE Trans. Ind. Electron. 2012, 59, 2094–2103. [CrossRef]
Szarka, G.D.; Stark, B.H.; Burrow, S.G. Review of power conditioning for kinetic energy harvesting systems.
IEEE Trans. Power Electron. 2012, 27, 803–815. [CrossRef]
Xiao, X.; Huang, X.; Kang, Q. A hill-climbing-method-based maximum-power-point-tracking strategy for
direct-drive wave energy converters. IEEE Trans. Ind. Electron. 2016, 63, 257–267. [CrossRef]
Nie, Z.; Xiao, X.; Yi, H.; Kang, Q. Direct drive wave energy converters integrated with a composite energy
storage system. In Proceedings of the 2011 International Conference on Electrical Machines and Systems,
Beijing, China, 20–23 August 2011.
Nie, Z.; Xiao, X.; Kang, Q.; Aggarwal, R.; Zhang, H.; Yuan, W. SMES-battery energy storage system for
conditioning outputs from direct drive linear wave energy converters. IEEE Trans. Appl. Superconduct. 2013,
23, 5000705.
Wu, F.; Ju, P.; Zhang, X.; Qin, C.; Peng, G.J.; Huang, H.; Fang, J. Modeling, control strategy, and power
conditioning for direct-drive wave energy conversion to operate with power grid. Proc. IEEE 2013, 101,
925–941. [CrossRef]
Su, M.; Pan, P.; Long, X.; Sun, Y.; Yang, J. An active power-decoupling method for single-phase AC–DC
converters. IEEE Trans. Ind. Inform. 2014, 10, 461–468. [CrossRef]
Narimani, M.; Moschopoulos, G. An AC-DC Single-stage full-bridge converter with improved output
characteristics. IEEE Trans. Ind. Inform. 2015, 11, 27–32. [CrossRef]
Dayal, R.; Dwari, S.; Parsa, L. Design and implementation of a direct AC–DC boost converter for low-voltage
energy harvesting. IEEE Trans. Ind. Electron. 2011, 58, 2387–2396. [CrossRef]
Lai, Y.; Yeh, C.; Ho, K. A family of predictive digital-controlled PFC under boundary current mode control.
IEEE Trans. Ind. Inform. 2012, 8, 448–458. [CrossRef]
Amin; Bambang, R.T.; Rohman, A.S.; Dronkers, C.J.; Ortega, R.; Sasongko, A. Energy management of fuel
cell/battery/supercapacitor hybrid power sources using model predictive control. IEEE Trans. Ind. Inform.
2014, 10, 1992–2002. [CrossRef]
Tani, A.; Camara, M.B.; Dakyo, B.; Azzouz, Y. DC/DC and DC/AC converters control for hybrid electric
vehicles energy management-ultracapacitors and fuel cell. IEEE Trans. Ind. Inform. 2013, 9, 686–696.
[CrossRef]
Xu, Z.; Guan, X.; Jia, Q.; Wu, J.; Wang, D.; Chen, S. Performance analysis and comparison on energy storage
devices for smart building energy management. IEEE Trans. Smart Grid 2012, 3, 2136–2147. [CrossRef]
Wee, K.W.; Choi, S.S.; Vilathgamuwa, D.M. Design of a least-cost battery-supercapacitor energy storage
system for realizing dispatchable wind power. IEEE Trans. Sustain. Energy 2013, 4, 786–796. [CrossRef]
Zhou, H.; Bhattacharya, T.; Tran, D.; Siew, T.S.T.; Khambadkone, A.M. Composite energy storage system
involving battery and ultracapacitor with dynamic energy management in microgrid applications. IEEE Trans.
Power Electron. 2011, 26, 923–930. [CrossRef]
Shim, J.W.; Cho, Y.; Kim, S.; Min, S.W.; Hur, K. Synergistic control of SMES and battery energy storage for
enabling dispatchability of renewable energy sources. IEEE Trans. Appl. Superconduct. 2013, 23, 5701205.
[CrossRef]
Zhang, Z.; Ouyang, Z.; Thomsen, O.C.; Andersen, M.A.E. Analysis and design of a bidirectional isolated
DC–DC converter for fuel cells and supercapacitors hybrid system. IEEE Trans. Power Electron. 2012, 27,
848–859. [CrossRef]
Kabir, M.N.; Mishra, Y.; Ledwich, G.; Dong, Z.Y.; Wong, K.P. Coordinated control of grid-connected
photovoltaic reactive power and battery energy storage systems to improve the voltage profile of a residential
distribution feeder. IEEE Trans. Ind. Inform. 2014, 10, 967–977. [CrossRef]
Nie, Z.; Xiao, X.; McMahon, R.; Clifton, P.; Wu, Y.; Shao, S. Emulation and control methods for direct drive
linear wave energy converters. IEEE Trans. Ind. Inform. 2013, 9, 790–798. [CrossRef]
Energies 2017, 10, 114
36.
37.
38.
16 of 16
Buccella, C.; Cecati, C.; Latafat, H. Digital control of power converters—A survey. IEEE Trans. Ind. Inform.
2012, 8, 437–447. [CrossRef]
Sanchez, P.M.; Machado, O.; Peña, E.J.B.; Rodriguez, F.J.; Meca, F.J. FPGA-based implementation of a
predictive current controller for power converters. IEEE Trans. Ind. Inform. 2013, 9, 1312–1321. [CrossRef]
Lupon, E.; Busquets-Monge, S.; Nicolas-Apruzzese, J. FPGA implementation of a PWM for a three-phase
DC–AC multilevel active-clamped converter. IEEE Trans. Ind. Inform. 2014, 10, 1296–1306. [CrossRef]
© 2017 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC-BY) license (http://creativecommons.org/licenses/by/4.0/).