Designing and Testing Composite Energy Storage Systems for ... - MDPI

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

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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

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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

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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.

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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

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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

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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.

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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.

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4. A Composite Energy Storage System

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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

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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.

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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

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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

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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

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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

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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

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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

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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

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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

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