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Energy Storage Systems for Transport and Grid Applications

Article  in  IEEE Transactions on Industrial Electronics · January 2011

DOI: 10.1109/TIE.2010.2076414 · Source: IEEE Xplore

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Energy Storage Systems for Transport and GridApplications

Sergio Vazquez, Member, IEEE, Srdjan M. Lukic, Member, IEEE, Eduardo Galvan, Member, IEEE, Leopoldo G.Franquelo, Fellow, IEEE and Juan M. Carrasco, Member, IEEE.

Abstract— Energy storage systems (ESSs) are enabling tech-nologies for well established and new applications such as powerpeak shaving, electric vehicles, integration of renewable energies,etc. This paper presents a review of ESSs for transport and gridapplications, covering several aspects as the storage technology,the main applications and the power converters used to operatesome of the energy storage technologies. Special attention is givento the different applications, providing a deep description of thesystem and addressing the most suitable storage technology. Themain objective of this paper is to introduce the subject and to givean updated reference to non-specialist, academic and engineersin the field of power electronics.

I. INTRODUCTION

DUE to environmental and geopolitical concerns, therehas been a renewed push to minimize the use of hy-

drocarbons for electric energy generation and transportation.These concerns have lead to the proliferation of electricitygeneration using both grid-tied and stand alone renewable en-ergy resources such as wind turbines and photovoltaic arrays.However, the intermittent nature of these resources introducesissues with system stability, reliability and power quality. Theissue of sporadic availability of renewable resources can beaddressed by introducing energy storage systems to (partially)decouple energy generation from demand. In addition, theenergy storage system (ESS) can be used to address the powerquality issues by providing ancillary services to the grid. LargeESSs are routinely used alongside renewable generation suchas wind to stabilize the power output.

Transportation electrification is seen as an effective way tosubstantially reduce the overall use of hydrocarbons. Elec-trified vehicles with plug-in capability contain an energystorage element capable of storing power from the grid. Ifthis power is produced using renewable energy sources, theoverall reduction in the use of hydrocarbons is substantial.In addition to diminish the use of hydrocarbons to propelvehicles, the introduction of plug-in vehicles present numeroussmall distributed energy storage resources that can be used to

Manuscript received July 28, 2010. Accepted for publication August 27,2010. This work was supported by the Spanish Science and EducationMinistry under project TEC2007-61879. Copyright c© 2010 IEEE. Personaluse of this material is permitted. However, permission to use this material forany other purposes must be obtained from the IEEE by sending a request topubs-permissions@ieee.org. S. Vazquez, E. Galvan, L. G. Franquelo and J.M. Carrasco are with the Electronic Engineering Department, University ofSeville (Spain), (e-mail: sergi@us.es). S. M. Lukic is with the Departmentof Electrical and Computer Engineering at North Carolina State University(USA), (e-mail: smlukic@unity.ncsu.edu).

stabilize the grid locally. The main objective of this paper is toreview the technologies used for energy storage in utility andtransportation applications and to present the latest advancesand developments at the component and system level as wellas to discuss some implementation issues.

The paper is organized as follows: Section II presents areview of the energy storage technologies that are consideredfor use in utility and transportation applications. In certaincases, no single energy storage technology is capable ofsatisfying application requirements effectively. In such a casehybrid energy storage systems are used, they electronicallycombine two or more energy storage technologies. This kindof systems are discussed in Section III. Section IV presentsthe energy storage requirements of transportation and utilityapplications. A link is made between the energy storagecapabilities and applications requirements. Section V reviewsthe power converters that are used to interface the energystorage systems with the application and finally Section VIconcludes the paper.

II. REVIEW OF ENERGY STORAGE TECHNOLOGIES

ESSs discussed in this paper convert electrical energy tosome form of energy that can be stored and released as needed.The choice of the energy storage system for an applicationwill depend on the application power and energy ratings,response time, weight, volume and operating temperature.Table I summarizes the characteristic parameters of differentenergy storage technologies; these values have been extractedfrom [1], [2] and [3].

A. Batteries

1) Lead-acid: The use of lead acid batteries for energystorage dates back to mid-1800’s. Lead acid battery cellconsists of spongy lead as the negative active material, leaddioxide as the positive active material, immersed in dilutedsulfuric acid electrolyte, with lead as the current collector.

During discharge, lead sulfate is the product on both elec-trodes. If the batteries are over-discharged or kept at a dis-charged state, the sulfate crystals become larger and are moredifficult to break up during recharge. In addition, the large leadsulfite crystals disjoin the active material from the collectorplates. Due to the production of hydrogen at the positiveelectrode lead acid batteries suffer from water loss duringovercharge. Distilled water is sometimes added to floodedlead acid batteries to mitigate this problem. Maintenance free

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TABLE IENERGY STORAGE SYSTEMS.

Type EnergyEfficiency

(%)

EnergyDensity

(Wh/kg)

PowerDensity(W/kg)

Cycle Life(cycles)

Self Discharge

Pb-Acid 70–80 20–35 25 200–2000 LowNi-Cd 60–90 40–60 140–180 500–2000 LowNi-MH 50–80 60–80 220 < 3000 HighLi-Ion 70–85 100–200 360 500–2000 Med

Li-polymer 70 200 250–1000 > 1200 MedNaS 70 120 120 2000 –VRB 80 25 80–150 > 16000 Negligible

EDLC 95 < 50 4000 > 50000 Very highPumped hydro 65–80 0.3 – > 20 years Negligible

CAES 40–50 10–30 – > 20 years –Flywheel (steel) 95 5–30 1000 > 20000 Very high

Flywheel (composite) 95 > 50 5000 > 20000 Very high

versions use a valve to minimize the water loss by allowinghydrogen and oxygen recombination. Current collectors in leadacid batteries are made of lead, leading to the low energydensity (Wh/kg). In addition the lead is prone to corrosionwhen exposed to the sulfuric acid electrolyte.

Lead acid batteries are still prevalent in cost sensitiveapplications where the low energy density and limited cyclelife is not an issue and where ruggedness and abuse toleranceis required, [4], [5]. Such applications include automotivestarting, lighting and ignition (SLI), and battery-powered un-interruptible power supplies (UPS). SLI applications makeuse flat plate grid designs as current collectors, while moreadvanced batteries use tubular designs. Recent advances aimto replace lead with lighter materials such as carbon to increasepower and energy density.

2) Li-Ion: In lithium-ion (Li-ion) batteries the lithium ionsmove between the anode and cathode to produce a currentflow [6].

The main advantages of this battery technology are highenergy-to-weight ratios, no memory effect and a low self-discharge. Main applications include portable equipment, lap-tops, cameras, mobile telephones and portable tools. Due to itshigh energy density, li-ion is proving to be the most promisingbattery technology for plug-in hybrid and electric vehicleapplications. However, the price of li-ion batteries is still highfor many applications. The price issue may become morecontentions given the limited lithium resources. Although thereis an ongoing debate regarding the available worldwide lithiumreserves, its widespread use for vehicle batteries will graduallydeplete the known resources leading to increasing raw materialcosts, [7], [8].

The technical characteristics of the basic cell, its density andvoltage, depend on the chemistry used. The specific energydensity is about 200 Wh/kg, double the energy density ofnickel metal hydride or nickel-cadmium batteries.

The cell of this battery can be operated with higher currentlevel than other cells, but some problems have to be solved.The internal resistance can produce internal heat up andfailure. Therefore, to ensure safe operation, it is mandatory touse a battery management system (BMS) to at least provideover-voltage, under-voltage, over-temperature and over-current

protection. In addition, more advanced systems provide cellvoltage balancing that ensures all batteries operate at the samevoltage and therefore state of charge, [9]-[11].

3) NiCd/NiMH: NiCd batteries were the chemistry ofchoice for a wide range of high performance applicationsbetween 1970 and 1990. Recently they have been replaced byLi-ion and NiMH chemistries in many applications. The NiCdbattery uses nickel oxyhydroxide for the positive electrode andmetallic cadmium for the negative electrode.

NiCd batteries have a higher energy density and longer cyclelife than lead acid batteries, but are inferior to chemistries suchas Li-ion and NiMH. Other disadvantages of NiCd batteriescompared to NiMH include: (1) shorter life cycle (2) morepronounced ”memory effect” (3) toxicity of Cd requires acomplex recycling procedure (4) lower energy density (5)flat discharge curve and negative temperature coefficient maycause thermal runaway in voltage controlled charging.

For the reasons mentioned above, nickel metal hydridebatteries have gained prominence over NiCd batteries in therecent past. NiMH batteries use nickel oxyhydroxide for thepositive electrode and metallic cadmium for the negativeelectrode.

NiMH batteries have been the chemistry of choice for EVand HEV applications in the 1990’s and 2000’s respectivelydue to their relatively high power density, proven safety, goodabuse tolerance and very long life at a partial state of charge,[4], [5]. One of the disadvantages of NiMH chemistry is therelatively high self discharge rate, though the introduction ofnovel separators has mitigated this problem.

When overcharged, NiMH batteries use excess energy tosplit and recombine water. Therefore, the batteries are main-tenance free. However, if the batteries are charged at anexcessively high charge rate, hydrogen buildup can cause cellrupture. If the battery is over-discharged, the cell can bereverse-polarized, leading to capacity reduction.

4) NaS: Sodium sulfur batteries consist of molten sulfurat the positive electrode and molten sodium at the negativeelectrode separated by a solid beta alumina ceramic electrolyte.

The electrolyte allows only the positive sodium ions togo through it and combine with the sulfur to form sodiumpolysulfides. During discharge, positive sodium ions flow

3

Termal enclosure

Termal enclosure

Gas Tight Seal

Insulator

Sodium

Safety Tube

Beta-Alumina

Sulfur Electrode

Sulfur Housing

Fig. 1. NaS battery cell and package.

Electrolyte

Tank 1

Electrolyte

Tank 2

Ele

ctro

de

-

Ele

ctro

de

+

Flow Reactor

Membrane

Fig. 2. Flow battery cell.

TABLE IIFLOW BATTERIES CHARACTERISTICS.

Technology Potential Efficiency

Zinc Bromide (ZnBr), [17] 1.8 70 %

Vanadium Redox (VRB), [18], [19] 1.2 – 1.6 80 %

Polysulphide Bromide (PSB), [20], [21] 1.5

Zinc-Air, [22] 1.6 50 %

through the electrolyte. The battery operating temperature isin the 300 oC to 360 oC range. Therefore NaS batteries needto be heated externally for optimal operation.

NaS batteries exhibit high power and energy density (overfour times that of the lead-acid battery), high Coulombicefficiency, good temperature stability, long cycle life, low costand good safety [12]-[14]. The batteries are made of abundantand low cost materials, making them suitable for high volumemass production. Great achievements have been made duringthe last two decades, especially under the collaboration ofTokyo Electric Power Company (TEPCO) and NGK Insulator,Ltd., (NGK). A NaS cell design developed by NGK is illus-trated in Fig. 1, [15]. These batteries can be used for loadleveling, emergency power supply or uninterruptible powersupply applications, being suitable to a number of markets,including industrial applications, commercial owners and windpower generating systems. For example, they are used in asubstation update demonstration project at Charleston, Vir-ginia, by American Electric Power (AEP) [16]. The batteriesgenerate up to 1.2 megawatt of power for up to seven hours,easing the strain on an overloaded substation.

5) Flow Batteries: Flow Batteries (FB) are a promisingtechnology that decouples the total stored energy from therated power. The rated power depends on the reactor size whilethe stored capacity depends on the auxiliary tank volume.These characteristics make the FB suitable for providing largeamounts of power and energy required by electrical utilities.FB work in a similar way as hydrogen fuel cells, as theyconsume two electrolytes that are stored in different tanks(no self-discharge) and there is a microporous membranethat separates both electrolytes but allows selected ions tocross through, creating an electrical current. There are manypotential electrochemical reactions, usually called reduction-oxidation reaction or REDOX, but only a few of them seemto be useful in practice. The main technologies that are usedcurrently are summarized in Table II, [17]- [22].

Fig. 2 shows a schematic of a FB. The power rating isdefined by the flow reactants and the area of the membranes,while the electrolyte tank capacity defines the total storedenergy. Note that in a classical battery, the electrolyte is storedin the cell itself, so there is a strong coupling between thepower and energy rating. In the cell (flow reactor) a reversibleelectrochemical reaction takes place producing (or consuming)electric dc current. At this time, several large scale and smallscale demonstration and commercial products utilize the flowbattery (FB) technology.

The main advantages of the technology include: (1) highpower and energy capacity; (2) fast recharge by replacingexhaust electrolyte; (3) long life enabled by easy electrolytereplacement; (4) full discharge capability; (5) use of non toxicmaterials; (6) low temperature operation. The main disadvan-tage of the system is the need for moving mechanical partssuch as pumping systems that make system miniaturizationdifficult. Therefore, the commercial uptake to date has beenlimited.

B. Electrochemical double-layer Capacitors

Electrochemical double-layer capacitor (EDLC) works inmuch the same way as conventional capacitor in that thereis no ionic or electronic transfer resulting in a chemicalreaction (there is no Faradic process), [23], [24]. In otherwords, energy is stored in the electrochemical capacitor bysimple charge separation. Therefore, the energy stored in theelectrochemical capacitor can be calculated using the samewell-known equation that is used for conventional capacitors:

Q = CV =Aε

dV (1)

As for the conventional capacitor, the capacitance C isproportional to the area A of the plates and the permitivityof the dielectric ε and is inversely proportional to the distanced between the plates. EDLCs are designed to have a very highelectrode surface area and use high permitivity dielectric. Theelectrode surface area is maximized by using porous carbonas the current collector allowing a relatively large amountof energy to be stored at the collector surface. ThereforeEDLC attain very high capacitance ratings (kilo-Farads versusmili- and micro- Farads for conventional capacitors). The two

4

electrodes are separated by a very thin porous separator andimmersed in an electrolyte such as propylene carbonate. Dueto the high permeability and close proximity of the electrodes,EDLCs have a low voltage withstand capability (typically 2V-3V).

EDLC stores energy by physically separating unlikecharges. This has profound implications on cycle life, effi-ciency, energy and power density. EDLCs have a long cyclelife due to the fact that (ideally) there are no chemical changeson the electrodes in normal operation. EDLCs have superiorefficiency: it is only a function of the ohmic resistance of theconducting path. EDLCs also provide exceptional power den-sity, since the charges are physically stored on the electrodes.Conversely, energy density is low since the electrons are notbound by chemical reactions. This lack of chemical bondingalso implies that the EDLC can be completely discharged,leading to larger voltage swings as a function of the state-of-charge.

C. Regenerative Fuel Cells

Fuel cells (FCs) are electrochemical conversion devicesthat consume hydrogen and oxygen to produce water andelectricity. FCs are a critical component of the proposed“hydrogen economy”, a concept wherein hydrogen would beproduced by some process, for instance electrolysis of water,and then used as fuel [25]. Regenerative FCs or unitizedregenerative FCs are devices that combine the function of thefuel cell and the electrolyser into one device. The hydrogen isstored as gaseous fuel for future use to generate electricity. Inprinciple all FCs can work as regenerative FCs, but they aretypically optimized to perform only one function. Combiningthe two functions reduces the system size for applicationsthat require both energy storage (production of hydrogen) andenergy production (production of electricity).

Current research aims to use polymer electrolyte membrane(PEM) FCs with hydrogen or methanol as the main fuel.The issue is to design a system that is efficient in bothhydrogen and electricity production; current FCs designs areless efficient in hydrogen production than other methodssuch as conventional electrolysis [1]. Unitized fuel cells havebeen proposed for aerospace applications that are not as costsensitive and require the highest possible energy density.

Like conventional FCs, regenerative FCs, experience lifedegradation in dynamic applications. Therefore these devicesare often coupled with EDLCs or other ESS to smooth thechanges that the regenerative fuel cell suffers.

D. Compressed Air Energy Storage (CAES)

CAES is a technology that stores energy as compressed airfor later use. Energy is extracted using a standard gas turbine,where the air compression stage of the turbine is replacedby the CAES, thus eliminating the use of natural gas fuelfor air compression. System design is complicated by thefact that air compression and expansion are exothermic andendothermic processes respectively. With this in mind, threetypes of systems are considered to manage the heat exchange:(1) Isothermal storage, which compresses the air slowly, thus

AC

AC

AC

AC

Power Converter Power ConverterFlywheel

Motor GeneratorP P

(a)

AC

AC

Power ConverterFlywheel

Motor/Generator P

(b)

Fig. 3. Different parts of a FESS: a) General scheme with two machines,b) Typical construction with only one machine

allowing the temperature to equalize with the surroundings[26]. Such a system works well for small systems where powerdensity is not paramount.(2) Adiabatic systems, which store the released heat duringcompression and feed it back into the system during air re-lease. Such a system needs a heat storing device, complicatingthe system design.(3) Diabatic storage systems, which uses external powersources to heat or cool the air to maintain a constant systemtemperature. Most commercially implemented systems are ofthis kind due to high power density and great system flexibility,albeit at the expense of cost and efficiency.

CAES have been considered for numerous applications,most notably for electric grid support for load leveling ap-plications [26]-[28]. In such systems energy is stored duringperiods of low demand, and then converted back to electricitywhen the electricity demand is high. Commercial systems usenatural caverns as air reservoirs in order to store large amountsof energy; installed commercial system capacity ranges from35-300 MW.

E. Flywheel

Flywheel energy storage systems (FESS) store energy in arotatory mass. This concept has been utilized for some time tostabilize the output voltage in synchronous generators. Recentadvances in power electronics and material engineering havemade this technology attractive for a number of other appli-cations such as transportation and power quality improvement[29].

Flywheel systems are characterized by being able to pro-vide very high peak power. In fact the input/output peakpower is limited only by the power converter. FESS havehigh power density and high energy density and virtuallyinfinite number of charge-discharge cycles. Therefore, theyare typically employed in transportation and power qualityapplications that require a large number of charge-dischargecycles, [30], [31]. In addition, FESS enable relatively simplestate monitoring, as ”state-of-charge” is a function of readilymeasurable parameters such as flywheel inertia and speed [32].

Fig. 3a shows the different parts of a FESS, designed witha separate motor and generator. However, the typical FESS

5

has only one machine that serves as a motor/generator unit asshown in Fig. 3b.

The key factor that determines the technology used to buildeach component, is the maximum rotational speed of theflywheel. Depending on this speed the FESS can be classifiedas low speed FESS and high speed FESS [33]-[38]. Theborder between the two systems is around 10000rpm. Therotational speed not only determines the material, geometryand length of the flywheel, but the type of electrical machineand the type of bearing [39]-[40]. Due to the technologicalrequirements the high speed systems are more complex but,as the total energy stored in the flywheel depends on thesquare of the rotational speed, high speed flywheels providehigher energy density. Other design considerations includeperformance, safety and reliability of the system [41]-[43].

F. Superconductive Magnetic Energy Storage (SMES)

SMES consists of storing energy in the magnetic fieldcreated by a direct current flowing through a superconductingcoil. The coil is cryogenically cooled to a temperature belowits superconducting critical temperature. The first SMES sys-tem was proposed in [44]. The construction can be a solenoidor a group of two or more solenoids in order to cancel themagnetic field around them.

SMES provides one of the highest densities of any powerstorage method. Its main advantage is high storage efficiency,above 90 % (not including the refrigeration system, whichrequires approximately 1.5 kW continuously per MWh ofstorage capacity), [45]. An additional advantage is the highdynamic response that permits response time in the range ofmilliseconds.

G. Thermo-Electric Energy Storage (TESS)

TEES for solar thermal power plants consists of a syntheticoil or molten salt that stores energy in the form of heatcollected by solar thermal power plants to enable smoothpower output during daytime cloudy periods and to extendpower production for 1-10 hours after sunset, [46]. End-useTEES stores electricity from off-peak periods through the useof hot or cold storage in underground aquifers, water or icetanks, or other storage materials and uses this stored energy toreduce the electricity consumption of building heating or airconditioning systems during times of peak demand.

III. HYBRID ENERGY STORAGE SYSTEMS

Certain applications require a combination of energy, powerdensity, cost, and life cycle specifications that can not be metby a single energy storage device.

To implement such applications, hybrid energy storagedevices (HESD) have been proposed. HESDs electronicallycombine the power output of two or more deviceswith com-plementary characteristics. HESDs all share a common traitcombining high-power devices (devices with quick response)and high-energy devices (devices with slow response).

Proposed HESD are listed below with the energy-supplyingdevice listed first followed by the power-supplying device:

Energy

Storage

Device

ESD1

Power

Electronic

Unit

PEU1

Energy

Storage

Device

ESD2

Power

Electronic

Unit

PEU2

(a) (b) (c)

Energy

Storage

Device

ESD1

Energy

Storage

Device

ESD2

Energy

Storage

Device

ESD1

Power

Electronic

Unit

PEU1

Energy

Storage

Device

ESD2

Fig. 4. Topologies for hybrid energy storage devices.

1) Battery and EDLC [47]-[53]2) Fuel Cell and Battery or EDLC [54]-[56]3) CAES and Battery or EDLC [26]4) Battery and flywheel [57]5) Battery and SMES [58]

For HESD applications, batteries can serve as either theenergy or power-supplying device, as shown in the list above.Also, note that [54]-[56] consider FCs rather than regenerativeFCs. However, the system operation principle would be iden-tical for the regenerative FC, with the difference that the FCwould be bi-directional. However, HESDs have been proposedfor utilization as an energy source for propulsion applications[47]-[52] or grid support [54]-[56], [26], [58]-[59].

In order to combine two or more energy storage devicesacting as a single power source, more complex conditioningcircuitry is required. Numerous topologies have been proposedto achieve this task ranging from simple to very flexible. Ingeneral, the proposed topologies can be grouped into threecategories as shown in Fig. 4. A discussion of the merits ofeach topology and typical uses follows.

Fig. 4a shows direct parallel connection of two energystorage devices. This topology requires that the voltage outputsof the two power sources match (V1=V2). Direct parallelconnection of batteries and EDLC has been proposed [51]-[53] for low voltage, cost-sensitive applications, such as theautomotive 42 V PowerNet system [52]. This system consistsof a high power pulse (engine cranking) followed by a constantlow power demand over a longer time period (while the vehicleis in operation). A direct parallel connection of batteries andEDLC makes use of the source impedance mismatch causingthe low-impedance ultracapacitor to provide power during highpower pulses, while the high-energy battery supplies the longterm lower power demand. The output voltage (Vout) varies asthe system charges and discharges. The range of power that isused from either energy source is limited by the voltage swingof the other. In other words, individual maximum power pointtracking is not possible for each source.

A more complex but flexible solution is to place an addi-tional converter between the two power sources as shown inFig. 4b. PEU1 controls the current output of ESD1 allowingits voltage to vary while ESD2 supplies the remaining powerrequirement to the load. Therefore this system allows for thedecoupling of the two power sources. Typically, the energystorage device with larger voltage fluctuation is utilized as

6

ESD1. Another criterion may be to put the more sensitivedevice in place of ESD1 to prolong the life of the system byconditioning the current output of ESD1. Systems that makeuse of this topology include battery and EDLC [49], [50];fuel cell and battery or EDLC [55], [56]; SMES and battery[57]. The commonly used topology for combining the twosystems is the single leg (two switches in series) converterwhich can act as a boost in the forward and as a buck in thereverse operation mode [50], [55]. In [49], authors propose touse a variation of this system, where the battery and EDLCare connected to the load one at a time allowing the systemcontroller to choose which source should power the load.Source switching results in step changes of the bus voltage,requiring an appropriate flexible modulation strategy. In [56]the authors propose the use of an isolated topology to allowfor a larger voltage gain between the input and output. In [57]the SMES device is connected to the middle points of twoconverter legs, allowing SMES to charge or discharge. Thebattery is connected to the bus to make use of the relativelyinvariant battery voltage.

In [48], [54], [26], [59]-[61] researchers have looked at usingthe topology shown in Fig. 4c where each power source isconnected to a dedicated power converter with the convertersconnected to the common output bus. Such a system providesthe highest level of flexibility, since each power source isallowed to operate at its optimal conditions - in essencemaximum power point tracking can be implemented for eachsource. Having dedicated converters for each power sourceallows a wide range of topologies and control strategies to beimplemented. The simplest topology that allows an acceptabledegree of flexibility is to use the single leg (two switchesin series) converter which can act as a boost in the forwardand as a buck in the reverse operation mode [59]-[61]. Othertopologies have been proposed that introduce a transformereither for isolation, or to allow efficient voltage boosting. Anoverview of the proposed topologies is presented in [59].

IV. APPLICATIONS OF ENERGY STORAGE SYSTEMS

ESSs can improve the performance of several applications.They are especially suitable for transport and utility scaleapplications and in some cases are the key factor that willdetermine the adoption of a technology, for example, electricvehicles. Fig. 5 shows the time vs. power operational rangeof the different energy storage technologies. The figure alsoshows the suitability of various ESS for both transport andutility applications. In case of transport applications, time andpower ranges are from seconds to hundred of minutes andfrom tens of kW to tens of MW, while in the case of utilityscale applications, time and power ranges are from tens ofminutes to hours and from MW to GW.

For utility or renewable energy integration, energy storagecapacity, power output, and life cycle are key performancecriteria. The need for long life cycle has motivated the useof storage systems from reversible physics such as CAES orpumped hydro as an alternative to electrochemical batteriesthat present problems of ageing and are difficult to recycle. Intransportation applications, portability, scalability and energy

0.01 0.1 1 10 100 10000.1

0.3

1

3

10

30

100

300

1000

Power(MW)

Tim

e(m

in)

Utility scale applications

Flywheels

Li-Ion

Ni-Cd

Lead acid

Sodium sulfur

Vanadium redox !owPumped

hydroCompressed airTESS

V2G

SMES

EDLC

Transport applications

Fig. 5. Storage technology.

0.01 0.1 1 10 100 10000.1

0.3

1

3

10

30

100

300

1000

Power(MW)

Tim

e(m

in)

Full HEV

PHEV

BEV

Mild HEV

Micro HEV Rail - on trainRail - stationary

peak shaving

Rail - stationary

!atteningDiesel - electric

rail

Fig. 6. Transport applications.

0.01 0.1 1 10 100 10000.1

0.3

1

3

10

30

100

300

1000

Power(MW)

Tim

e(m

in)

End-user

peak shaving

Primary frequency

regulation

Arbitrage

Increasing of renewables

penetration

Load leveling

T&D postponement

Utility scale applications

Fig. 7. Utility applications.

7

and power density are key performance criteria. Therefore,due to their modularity and portability, and in spite of thenumerous issues including limited life, batteries are still con-sidered the most viable option for transport applications. Inthe following paragraphs it is discussed transport and utilityapplications in more detail.

A. Transport Applications

1) Road transport: Due to environmental, geopolitical andeconomical concerns, recently there has been a push to di-versify the energy supply for road vehicles. This push haspromoted alternatives to the internal combustion engine (ICE),such as fuel cell vehicles (FCV), hybrid electric vehicles(HEV), plug-in hybrid electric vehicles (PHEV) and batteryelectric vehicles (BEV). All these alternatives have in commonthat ICE is replaced or augmented by electric propulsion. Inthe case of FCV, on-board fuel cells consume hydrogen toproduce electricity and power the electric motor. The FCVconcept is considered as an integral part of the proposed”hydrogen economy” [25]. HEV and PHEV combine the ICEand electric propulsion to propel the vehicle. PHEV providesthe option of recharging the on-board energy storage from thepower grid. Finally BEV uses battery energy storage as theonly energy source on the vehicle.

Fig. 6 represents the power magnitude and duration require-ments for various road transport applications. The plot showsthat the energy requirements from the storage system increasefrom HEV to PHEV to EV. HEVs can be further differentiatedby the size of their electric system as will be discussed later inthe text. FCV energy storage requirements are similar to thatof a HEV. The remainder of this section will look at thesevehicles and their energy storage requirements in more detail.

FCV make use of fuel cells that convert hydrogen andoxygen into electricity and steam. These vehicles are con-sidered as environmentally benign since hydrogen can beproduced using electrolysis - a process that could be poweredby renewable energy sources. However, even with substantialgovernment and private sector investment to commercializethe technology, no FCV is commercially available. Some ofthe long standing technological issues include relatively lowenergy density of hydrogen (even in the liquid form) comparedto petroleum, nonexistent refueling infrastructure, cost and lifeof the fuel cell, and the detrimental effect of vehicle powerprofile on the life of the fuel cell [62], [63]. To address thelast issue researchers propose to hybridize the fuel cell witha battery [63], [64], EDLC [64], [65], or a battery/EDLChybrid [64]-[66]. In such vehicles, the fuel cell would providethe average power demand while the battery or EDLC wouldprovide peak power.

HEV and PHEV use both ICE and an electric motor topropel the vehicle. The ICE provides the average power whilethe electric motor meets the peak power demand. HEV gainsefficiency by employing a smaller ICE operating at higher loadwhere the engine is more efficient. The peak power demandis then augmented by the electric motor powered by electricenergy storage. The electric energy storage is replenished viaregeneration of kinetic energy during braking or by the ICE

during low vehicle power demands. Choice of electric energystorage depends on the hybridization factor (HF): relativepower of the electric subsystem as a fraction of the total systempower [67], [24]. Therefore, HF of 0 represents a conventionalICE-powered vehicle, while a HF of 1 represents a BEV. Thisdefinition allows for the classification of Micro HEV, MildHEV, Full HEV, Plug-in HEV and BEV listed in order ofincreasing HF. Fig. 6 shows that the energy requirements fromthe battery increase with the HF. With the improvement inthe capabilities of electric energy storage devices, the autoindustry has been moving to higher levels of hybridization.Currently, full HEV such as Toyota Prius and Ford Escapehybrid are common. PHEV and BEV are already availablefrom a number of smaller innovative vehicle manufacturerssuch as Tesla and Aptera; larger automakers such as Nissanand GM plan to sell the GM Volt PHEV and Nissan Leaf EVin the coming year. In the following paragraphs we look atenergy storage requirements of Micro HEV, Mild HEV, FullHEV, Plug-in HEV and BEV.

Micro-HEVs typically use an integrated starter generator(ISG) technology to allow for automatic engine stop/start,limited propulsion assist, and regenerative braking resultingin moderate fuel economy improvements in city driving.Advanced lead acid batteries are considered to be sufficientto supply this application [68]-[70].

Mild-HEV are fitted with a more powerful electric propul-sion system totaling up to 0.25 HF. They are capable of enginestop/start as well as moving the vehicle at low speeds andcapturing more regenerative braking energy. Due to the higherpower capabilities of the electrochemical system, the energythroughput in the batteries is much higher than in micro-HEV.Therefore advanced lead acid [69], NiMH, battery/EDLChybrids [71] are considered for the mild HEV application. In[72] the author proposes the use of EDLC as the sole electricpower source for a mild hybrid. Examples of vehicles in themild HEV category include Honda Civic Hybrid, and ToyotaCamry Hybrid, all of which use NiMH batteries.

Full HEV or power assist HEV offer substantial electricpropulsion assistance and limited electric-only range. Electricdrive and battery typically operate at voltages above 200 V.Examples of these vehicles include Toyota Prius or HondaInsight. Both vehicles employ high power NiMH batteries. Thebatteries are used in a narrow state of charge range, but seepeak currents of over ten times the amp-hour rating. Therefore,full HEV applications can be served by EDLC alone [72],or by a battery/EDLC hybrid energy source [73]-[75], [49].A number of researchers have focused on optimizing thebattery/EDLC control strategy [74] and have proposed novelpower electronics topologies for these applications such asmultilevel converters [75] and source swapping [49]. Highpower Li-ion batteries are also suitable for full HEV appli-cations. For larger vehicles such as busses, the flywheel/ICEhybrid energy source has also been considered [29].

PHEV is a full HEV characterized by batteries that canbe recharged from the power grid. Compared to HEVs thesevehicles provide a much longer all electric range and therefore,higher overall fuel economy. Thus PHEVs are essentiallyBEVs with an on-board ICE generator. Lithium ion batteries

8

are considered as the chemistry of choice for this applicationdue to the relatively high energy density and high powercapability [70], [72].

BEV solely relies on on-board battery energy storage topropel the vehicle. These vehicles have the advantage of asimpler drivetrain than the hybrid electric vehicle. However,the power and energy throughput requirements from the bat-tery pack become substantially more demanding than for HEVand PHEV vehicles. Therefore there is a major concern aboutthe life of the batteries in these vehicles. The other issuewith these vehicles is the long battery recharge time at theprescribed recharge rates as well as the effect on the powergrid and the need for a new charging infrastructure [76], [77].Lithium ion batteries are considered for this application due tothe relatively high energy density and high power capability[70], [72].

2) Rail Transport: Due to periodic acceleration and de-celeration as trains move from station to station, their powerconsumption is very uneven. Therefore, electrically poweredrailway systems such as trams, subways and high-speed mag-netic levitation trains can benefit from electric energy storageto smooth out the train power demand. ESSs can be installedeither at the substation supplying the train network, or on thetrain itself. In addition, when placed at the substation, energystorage systems can serve as peak shaving units or as demandflattening units. As show in Fig. 6, these constraints greatlyaffect the energy storage requirements.

Assuming no energy storage or brake resistors on the train,there will be a large power draw from the substation duringvehicle acceleration and a similarly large power surge back tothe substation when the vehicle decelerates. Attempts weremade to minimize the power spikes and flatten the powercurve by coordinating train accelerations and decelerations tooffset each other [78]; however, managing train movement isdifficult, especially in urban systems. Therefore, the power thatmust be supplied by the substation is highly irregular.

Installing an energy storage unit minimizes the substationpeak power requirements improves efficiency by storing theenergy that would otherwise be dissipated in the resistor banks.A number of systems have been proposed to serve as theenergy sources at the substation level. Typically these arehigh power density, high cycle sources such as EDLC andSMES [79]-[84]. In [79] researchers propose the use of SMESand flow batteries to flatten out the power demand curve ofa high speed railway system. The SMES provides the pulsepower, while the flow battery supplies the bulk of the energyto completely flatten out the energy demand. Another commonapproach uses EDLC to minimize the power surge at thesubstation and to minimize the use of resistor banks at thesubstation [80]-[82].

Rather than smoothing out the power profile at the sub-station on-board energy systems can smooth out the powerdemand from the train itself. This approach is typically consid-ered for systems that do not have the ability to feed power backto the supply lines. Such trains must have resistor banks on thetrain itself to be able to slow down the train. An alternative tothe resistive bank is an energy storage system that will providepower to the vehicle during acceleration and would charge up

during deceleration. Since this is a high power application,EDLC systems are commonly used [84]-[87], though otherhigh power density storage devices such as flywheels havebeen proposed [88].

Even though most advanced high speed train systems areelectrically powered, there exists a large infrastructure thatuses trains powered by diesel generators. To improve theefficiency of these systems, hybridization with energy storagehas been proposed. The system operates in a very similar wayto the series roadway hybrid electric vehicle: a diesel enginesupplies the train average demand, while the ESS suppliesthe peak demand. As shown in Fig. 6, the energy storagerequirements for this application are the same as for the HEVin terms of pulse duration; however, these systems are muchlarger than road vehicles. Proposed systems [91], [92] combinediesel generators with EDLC [93], lead acid batteries andflywheels [87], [88]. Finally in [89], [90] researchers suggestreplacing of the diesel generator with a fuel cell hybridizedwith EDLC [94] or batteries [95].

B. Utility applications

ESSs are increasing their impact on the utility grid asa solution to stability problems. The main advantage of astorage plant is to contribute to the quality of the grid bymaintaining the power constant [46], [96]–[97]. The main roleof these ESSs are to increase the Renewable Energy Sources(RES) penetration, to level load curve, to contribute to thefrequency control, to upgrade the transmission lines capability,to mitigate the voltage fluctuations and to increase the powerquality and reliability, Fig. 7 represents the power magnitudeand duration requirements for these applications.

1) Increasing RES Penetration: Although renewable energysources are environmentally beneficial, the intermittent natureof two fast growing energies, wind and solar, causes voltageand frequency fluctuations on the grid. That represents asignificant barrier to widespread penetration and replacementof fossil-fuel source base-load generation, because integratingrenewable sources introduces some new issues on the op-eration of the power system, such as potential unbalancingbetween generation and demand, [98].

However, intermittent RES, such as solar and wind, needto be supported with other conventional utility power plants[99]. It is estimated that, for every 10% wind penetration, abalancing power from other generation sources equivalent to2-4% of the installed wind capacity is always required fora stable power system operation. So, with more penetrationof intermittent renewable energy like wind power, the systemoperation will be more complex and it will require additionalbalancing power. This is critical in countries with a largepenetration of solar and wind systems, as Denmark or Spain,where it is estimated that approximately 20 % and 10 % of theelectricity generation comes from wind power, respectively. Alarge storage capacity will allow a high percentage of wind[100], [101], photovoltaic [102] and others power plants inthe electrical mix contributing to fulfill the objectives for amore sustainable future. In order to integrate renewable energysources, it is necessary to propose a suitable storage system

9

that offers capacities of several hours and power level from 1to 100 MW.

Nowadays, high temperature thermo-solar power plants areincluding a Thermo-Electric Energy Storage (TEES) and itis expected that other storage systems will be included in thenew generation of RES and the Distributed Generation Sources(DGS) in general.

Recently, the concept of Vehicle-to-grid (V2G) has beenintroduced. It describes a system in which electric or plug-in hybrid vehicles communicate with the power grid to selldemand respond services by either delivering electricity intothe grid or by throttling their charging rate. When coupledto an electricity network, electric vehicles can act as acontrollable load and energy storage in power systems withhigh penetration of renewable energy sources. The reliabilityof the renewable electricity will be enhanced with the vastuntapped storage of electric vehicle fleets when connected tothe grid. In Fig. 5, market area for V2G represents 1 millionvehicles with 20-50 kWh capacity, where 10% of this capacityis available for utility applications, including integration ofRES. The benefits of energy storage applications in renewableenergy sources have been deeply studied in the bibliography.In [103], the case of a wind farm placed in Portugal with a144 MW installed capacity has been evaluated. This particularinstallation produces more energy in off-peak hours than inpeak hours, which is a drawback in terms of the renewablePortuguese tariff. A 5 MW and 30 MWh capacity ESS hasbeen proposed, in order to transfer part of the generation to thepeak hours and improve the wind farms economical payback.The resultant average wind farm tariff with storage is 2,1%higher than the original one and represents additional 250 keincome of the annual wind farm turnover. A target price of60 e/kWh as the storage device purchase cost, will allow apayback time of 7 years of this system.

2) Load leveling: Load leveling refers to the use of electric-ity stored during times of low demand to supply peak electric-ity demand, which reduces the need to draw on electricity frompeaking power plants or increase the grid infrastructure. Thisapplication is represented in Fig. 8. Pmax is the maximumpower that can be delivered to the load by the electricalgrid through the existing transmission line. To deliver morepower to the load there are two possibilities, increase theinfrastructure and the generator capacity or install an ESS. TheESS allows to postpone a large infrastructure investment intransmission and distribution (T&D) network. New technolo-gies, which are not restricted by their geographic limitations,have been proposed as more suitable for load leveling such asTESS and BSS, [104]. Rechargeable battery technologies likesodium-sulfur (NAS) technology are attractive candidates foruse in many utility-scale energy-storage applications. Theseadvanced battery systems can be utilized with existing infras-tructure, helping energy providers to meet peak demands andcritical load [105].

3) Energy Arbitrage: Energy arbitrage refers to earning aprofit by charging EES with cheap electricity when demandis low and selling the stored energy at a higher price whendemand is high, as it is shown in Fig. 9. This activity can alsobe used to influence in the demand side, such as using higher

Time(h)

Po

we

r (M

W)

0 3 6 9 12 15 18 21 24

ESS

ESS

Fig. 8. Basic concept of load leveling through ESS.

Time(h)

Po

we

r (M

W)

0 3 6 9 12 15 18 21 24

ESS

ESS

Fig. 9. Basic concept of arbitrage through ESS.

peak prices to induce a reduction in peak demand throughdemand charges, real time pricing, or other market measures.This function has been traditionally performed by pumpedhydro storage (PHS). PHS is appropriate for energy arbitragebecause it can be constructed at large capacities over 100MWrange and discharged over periods of time from 100 to 1000min. These installations allow storage when the demand is lowand the energy is cheap. This ESS is the most widely used

10

Time(h)

Po

we

r (M

W)

0 3 6 9 12 15 18 21 24

ESS

Fig. 10. Basic concept of primary frequency regulation through ESS.

energy storage technology at utility scale (100 GW installedworldwide). CAES is also appropriate for energy arbitragebecause it can be constructed in capacities of a few hundredMW and can be discharged over long periods of time.

A new trend for this application is to use the ancillaryservices that offer the battery of electrical V2G. The largequantity of this V2G expected in a next future could contributeto a new concept of the energy marker [106].

4) Primary Frequency Regulation: This application is rep-resented in Fig. 10. The technical application of ESS includestransient and permanent grid frequency stability support. Tocontribute to the frequency stabilization during transient,called Grid Angular Stability (GAS) in [107], low and mediumcapacity ESS is needed. This low energy storage requirementis because GAS operation consists on injection and absorptionof real power during short periods of time, 1 to 2 seconds. Thisapplication contributes for example to the frequency stabilityof isolated utilities based on diesel generators [108].

Modern variable speed wind turbines and large photovoltaicpower plants connected to the utility grid do not contributeto the frequency stability as the synchronous generators ofthe conventional gas or steam turbine do. This creates a newapplication of ESS that is to be used to emulate the inertiaof these steam turbines generators to complement this angularstability deficit [109]. Another solution is to use the powerelectronic converter of variable speed wind turbines to emulatethe steam turbines inertia using the inertial energy storage ofthe rotors of these wind turbines [110].

SMESs are getting increasing acceptance in variation ap-plications of damping frequency oscillations [111] because oftheir higher efficiency and faster response. EDLC, FESS andBSS are also very suitable for this application.

5) End User Peak Saving: There are several undesiredgrid voltage effects at the end user level, depending on theduration and variability. Typical voltage effects are long periodinterruptions (blackouts), short period interruptions (voltagesags) voltage peaks and variable fluctuation (flicker).

To perform peak shaving and prevent against blackouts,the typical approach involves installing uninterruptible powersupplies (UPSs). If an on-line UPS is installed in series, this

isolates the load from the grid and fluctuations produced by theutility have no effect on the users. But this solution may notbe optimal for all applications. One solution, presented as gridvoltage stability (GVS) in [107], involves mitigating againstdegraded voltage by providing additional reactive power andinjecting real power for durations of up to 2 seconds. Theenergy storage needed to protect the load against this voltagedegradation is low. The energy storage demanded is even lowerin applications with ride-through capability, where the electricload or the generator stays connected during the systemdisturbance, because part of the energy can be obtained fromthe grid during the undervoltage period. The voltage flicker iscaused by rapid changes of RES and industrial or domesticloads, such as electric arc furnaces, rolling mills, weldingequipments and pumps operating periodically. An ESS canhelp to reduce voltage fluctuations at the point of common-coupling produced by these transitory generators and loads.These applications can be supplied by lower energy storagedevices such as EDCL [112], [113] and batteries managingthe active power plus additional reactive power produced bySTATCOM which includes BESS or FESS, [114], [35].

V. POWER CONVERTERS FOR ESS

Some energy storage technologies need an additional equip-ment to adapt their output voltage or current to the requiredoutput voltage level or waveform. This is the case of a BESSsystem connected to the grid, that has to adapt its dc output tothe ac voltage level of the grid. Other technologies that havethis necessity are EDCL, SMES, FESS and regenerative FCs.

The device used to perform this task is a power converter.Depending on the storage technology and the application, thepower converter has to allow the connection between twodifferent dc voltage level buses, a dc voltage bus and an acvoltage bus or even connect a current source to a voltage bus.For this reason, the topology used for the power converterdepends on both, the technology and the application.

In general, power converters applied to ESSs have to presentthe following features:

• To manage the energy flow in a bidirectional way, con-trolling the charging and discharging process of the ESS.

• To have high efficiency.Additionally, depending on the application they have to

fulfill the following characteristics:• To provide fast response (frequency regulation applica-

tions).• To have small size and weight (transport applications).• To stand high peak power (peak shaving applications).• To manage High rated power (load leveling applications).To connect batteries, EDCLs or regenerative fuel cells

between two different dc voltage level buses, the most popu-larly used topology has been the bidirectional boost convertershown in Fig. 11a. This topology enables connections to ahigher voltage bus and can operate properly against voltagefluctuations coming from the ESS, [115], [116], as for examplethe EDLC voltage reduction that halves its voltage to deliverthe maximum energy. Another version, the bidirectional buck

11

Fig. 11. Conventional buck-boost dc-dc converter. a) Battery in the lowvoltage side, [115]. b) Battery in the high voltage side, [117].

Fig. 12. Isolated dc-dc converter, [118].

Fig. 13. Conventional ac-dc converter, [119]-[120].

Fig. 14. Power converter to connect a SMES system to the grid, [124].

converter represented in Fig. 11b, can be suitable for connect-ing a lower voltage dc bus to these storage technologies, [117].If isolation is needed between the ESS and another stage,a transformer option can be chosen using the bidirectionaltopology shown in Fig. 12, [118]. This topology is alsosuitable for high frequency applications and can be combinedwith resonant techniques that permit lower size and volumemaintaining a good efficiency.

On the other hand, to connect batteries, EDCLs or regener-ative fuel cells directly to an ac motor or generator an inverterhas to be used, [119]-[120]. In Fig. 13 it is depicted the caseof a BESS connected to the grid through a conventional three-phase two level converter.

The energy flow in a FESS is controlled by the electrical

machine attached to the flywheel. Usually this machine is athree-phase ac motor/generator unit. To control the torque andthe speed of this unit an actuator is needed as was presentedin Fig. 3b. In general, the motor drive is set up by two ac-dcpower converters connected through a common dc-link in aback-to-back fashion, [121]-[122], although other topologiesare possible as presented in [123] where authors use a matrixconverter.

The SMES technology presents special conditions because itbehaves as a current source. Besides, the current inside the coilflows permanently in only one direction [124]. Therefore, thepower converter used for the SMES conditioning system has tobe an especial topology. Although other topologies are possibleto handle this current, the most suitable form is represented inFig. 14. Here, an asymmetric H-bridge is used to manage thecurrent coming from the dc-link of the three phase converterunit, because only two switches and two diodes are needed tocontrol the current.

Finally, large power systems can be formed by a combina-tion of the topologies described. For instance, a multi-MWhBESS can be built by several strings and these strings canbe connected in parallel to a common dc bus by step up dc-dc converters. Then, the dc bus is connected to the ac gridby an inverter. This application is presented in Fig. 15 whereconventional dc-dc boost converters and conventional three-phase two-level dc-ac converter have been chosen as the powerconditioning system, [16]. It should be noticed that for a largerated power system the dc-ac converter can be implementedusing a multilevel topology which are more suitable to managehigh amount of power, [125]-[126].

VI. CONCLUSIONS

Energy storage systems are the key enabling technologiesfor transport and utility applications. In particular, the prolifer-ation of energy storage will enable the integration and dispatchof renewable generation and will facilitate the emergence ofsmarter grids with less reliance on inefficient peak-powerplants. In the transportation sector, the emergence of viable on-board electric energy storage devices such as high power andhigh energy lithium ion batteries will enable the widespreadadoption of plug-in electric and hybrid electric vehicles whichwill also interact with the smart grids of the future. Maturestorage technologies can be used in several applications, butin other situations these technologies can not fulfill with theapplication requirements. Thus, new storage systems haveappeared, opening new challenges that have to be solved by theresearch community. Transport and utility applications operatewith a wide range of time vs. power storage requirements. Thebenefits obtained in transport and utility go from technicalaspects to economic objectives. For instance, a reductionof CO2 emissions can be achieved by the use of electricvehicles or increase of profits can be obtained with a loadleveling application in a power transmission line. Anyway, thecontinuous development of the storage technologies and theevolution of their applications will motivate further researchto solve the existing issues and improve the energy storagesystems.

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Fig. 15. Connection of several battery modules to the grid, [16].

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Sergio Vazquez (S’04, M’08) was born in Seville,Spain, in 1974. He received the B.S, M.S. and PhDdegrees in industrial engineering from the Universityof Seville (US) in 2003, 2006 and 2010, respectively.In 2002, he was with the Power Electronics Group,US, working in R&D projects. He is currently an As-sistant Professor with the Department of ElectronicEngineering in the US. His research interests includepower electronic systems, modelling, modulationand control of power electronic converters appliedto renewable energy technologies.

Srdjan M. Lukic (S’02, M’07) received his M.S.and Ph.D. degrees in Electrical Engineering fromthe Illinois Institute of Technology, Chicago, in 2004and 2007, respectively. Currently he is an AssistantProfessor in the Department of Electrical and Com-puter Engineering at North Carolina State University,Raleigh, NC. He serves as a faculty member andtestbed leader for the plug in hybrid electric vehiclesin the Future Renewable Electric Energy Deliveryand Management (FREEDM) Systems EngineeringResearch Center (ERC). From 2002 to 2004 he was

with Firefly Energy Inc. where he was responsible for optimizing certainaspects of carbon/graphite foam-based lead acid batteries for novel automotiveapplications. His research interests are broadly in the field of power electronicsand motor drives applied to electric energy storage systems, electric vehicledrivetrains, and inductive power transfer systems.

E. Galvan (M’97) was born in Aracena (Huelva),Spain, in 1964. He received the M.Sc. degree inelectrical engineering and the Ph.D. degree in in-dustrial engineering from the University of Sevilla,Sevilla, Spain, in 1991 and 1994, respectively. He isan Associate Professor of electronic engineering atthe Escuela Superior de Ingenieros, Sevilla. He hasbeen working for several years in the power elec-tronic field where he was involved in the industrialapplication for the design and development of powerconverters applied to renewable energy technologies.

His research interests include control of power converters (wind turbineapplications, active filters, and electric machines).

L. G. Franquelo (M’84, SM’96, F’05) was bornin Malaga, Spain. He received the M.Sc. and Ph.D.degrees in electrical engineering from the Universityde Seville (US), Seville, Spain, in 1977 and 1980,respectively. He is currently with the Departmentof Electronics Engineering, University of Seville.His current research interests include modulationtechniques for multilevel inverters and its applicationto power electronic systems for renewable energysystems. Dr. Franquelo has been a DistinguishedLecturer since 2006 and an Associated Editor for

the IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS since 2007.He was the Vice-President of the Industrial Electronics Society (IES) SpanishChapter (2002-2003), a member at large of the IES AdCom (2002-2003),the Vice-President for Conferences (2004-2007), and the President Elect ofthe IES (2008-2009). He is the President of the IEEE Industrial ElectronicsSociety (2010).

Juan M. Carrasco (M’97) was born in San Roque,Spain. He received the M.Eng. and Dr.Eng. degreesin industrial engineering from the University ofSeville (US), Seville, Spain, in 1989 and 1992,respectively. From 1990 to 1995, he was an As-sistant Professor with the Department of ElectronicEngineering in the US where he is currently anFull Professor. He has been working for severalyears in the power electronic field where he wasinvolved in the industrial application of the designand development of power converters applied to

renewable energy technologies. His current research interests are in distributedpower generation and the integration of renewable energy sources.

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