Toward a New Energy ParadigmJOSEP M. GUERRERO,FREDE BLAABJERG,TOSHKO ZHELEV,KAS HEMMES,ERIC MONMASSON,SAMIR JEMEI¨,MARIA P. COMECH,RAMO´ N GRANADINO,and JUAN I. FRAU

Distributed generation

(DG) is emerging asa new paradigm toproduce on-sitehighly reliable andgood quality electricalpower. Thus, the DGsystems are presented as a suitableform to offer highly reliable electricalpower supply. The concept is particularlyinteresting when different kindsof energy resources are available, suchas photovoltaic (PV) panels, fuel cells(FCs), or wind turbines. The DG of differentkinds of energy systems allowfor the integration of renewable andnonconventional energy resources.Hence, the DG is becoming a part ofthe strategic plans of most countriesto address current challenges associatedwith energy management.Today, electrical and energy engineeringhave to face a new scenario inwhich small distributed power generatorsand dispersed energy storagedevices have to be integrated togetherinto the grid. The new electrical gridwill deliver electricity from suppliersto consumers using digital technologyto control appliances at consumer’shomes to save energy, reducing costand increasing reliability and transparency.The idea behind this concept isDigital Object Identifier 10.1109/MIE.2010.935862 to have devices that plug into your© COMSTOCK

52 IEEE INDUSTRIAL ELECTRONICS MAGAZINE n MARCH 2010 1932-4529/10/$26.00&2010IEEE

outlet and you would plug your applianceinto this device. These deviceswould communicate and report to theelectric companies at what time yourappliance used energy and how muchenergy was used. This will in turncharge youmore for the electricity thatyou use during peak hours of late afternoon

and early evening. In this sense,the expected whole energy system willbe more interactive, intelligent, anddistributed. Using DG of energy systemsmakes no sense without usingdistributed storage (DS) systems tocope with the energy balances.This article deals with the integrationof renewable distributed energyresources as depicted in Figure 1.Concepts about the DG combinedwith storage energy systems will bepresented. The article highlights theapplication of hydrogen generationsystems, FCs, and its combinationwith renewable energy resources,especially solar and wind, to integratethese systems into the grid.The trends in power electronics as aninterface between those prime sourcesand the electrical grid will beexplained. Then, the control implementationfor these electrical interfacesby using advanced digital deviceswill be examined. Finally, examples ofthose topics applied in the electricalgrid of Spain will be given, showingthis country as a paradigmatic caseof high penetration of the DG andrenewable energy.Integrated Energy SystemsThe challenges theworld is facingwithrelation to energy supply, sustainability,and climate change are huge.However, the question arises whetheror not our present organization ofresearch and development (R&D)funding is appropriate for dealing withthose huge challenges. The call forinnovation and breakthroughs in scienceand technology is loud, but wetend to forget about innovation andbreakthroughs in the way we organizeinstitutions and R&D funding to reachthose objectives. Six forms of integrationcan be distinguished [1], [2].Integration of Componentsinto a SystemMuch of present energy research isfocused on the component level. It isobvious that this is necessary, becausewithout components with along endurance and good performancespecifications, it is impossible toTransformerFuelPower StationSolar CellsWind TurbineMotorPumpRoboticsRefrigeratorTelevisionLightIndustry=Power Supply

ac dcCompensatorFCsCommunicationSolarEnergyTransport ~PrimaryFuelEnergyStoragesFacts/CupsHeatLoadsEnergyStorageCHPCombustionEnginedcacdcac3 333 1-3FIGURE 1 – The distributed electrical generation systems.

The integration of new technology in existinginstallations can significantly speed up theprocess of adaption of new technology.MARCH 2010 n IEEE INDUSTRIAL ELECTRONICS MAGAZINE 53construct well-functioning energy systems.Still, the perception and relativeimportance of the problems on thecomponent level is often differentfrom those from a system perspective.Therefore, it is important to coupleR&D and also long-term R&D todirect implementation as fast as possiblewithout losing a long-term vision,depending on the specific developmentphase of the technology.Integration of Energy Sourcesinto Multisource MultiproductEnergy SystemsThe super-wind concept is an interestingexample of a multisource multiproductenergy system. The fluctuatingrenewable electricity production fromwind or solar is compensated by theflexible coproduction of hydrogen andelectric power by a high-temperatureinternal-reforming FC fueled by naturalgas or biogas (see Figure 2). It offersthe possibility to continuously producevaluable economic products allthe time [2]. If less electricity is neededfrom the FC (because of increasedwind), the operation can be changedto produce more hydrogen. This conceptthat we called super-wind is apromising alternative if a hydrogenmarket is developing in the future, e.g.,for the automotive sector. Other examplesare superheating, where steam inan existing power plant can be superheatedby an external source (e.g.,waste heat from a FC) and integrationof fossil energy with solar energy in the

thermal decomposition of natural gasinto hydrogen and carbon.Integration of Industries intoEcoparks (Industrial Ecology)Industrial ecology has already developedinto a recognized area of researchin the science and technology.The slogan ‘‘cradle to cradle’’ reflectsthe principles of industrial ecology inmimicking the closed cycles that existin nature.Integration of New Technologyinto the Existing TechnologyThe replacement of old, less-efficienttechnology by a new and more-efficienttechnology is a long process.This is not only a consequence of thelong-development times and often necessarycost reductions of new technologybut also because of the longdepreciation time of existing largeindustrial (power) plants. The integrationof new technology in existinginstallations can significantly speedup the process of adaption of newtechnology. An example is the applicationof high-temperature FCs in a toppingcycle in existing power plants.Integration of SectorsThe integration of the waste and energysectors is a form of integration onthe highest system level on a largescale. This is an example of a successfulintegration realized in waste incinerationplants. In the vehicle-to-grid (V2G)concept, electrical vehicles charge uptheir batteries from the electricity grid.When the electrical vehicles becomehybrid FC-battery vehicles, it is alsolikely that the FCs will deliver electricalpower to the electricity grid. This is anexample of the integration of the transportand energy sector.Integration of FunctionsThe development of a product dedicatedfor just one function often leadsto an expensive product. Synergy andcost reduction can be achieved bymultifunctional products. A very illustrativeexample is the use of solar panelsor solar heat collectors instead ofroof tiles. Architects and industrialdesigners are all educated to apply theprinciples of integration of functionsto their designs. The engineers in thefield of energy technology usually donot workwith those concepts.Microreactor Technology forDistributed Fuel GenerationThe recent tendency to seek the bestway to capture CO2 green-house gas(GHG) and to store it undergroundseems to be a sustainable solution.Mitigation of CO2 emissions is a majorconcern because of the severe climatechanges resulting from uncontrolled

GHG emission. In parallel, thedepletion of fossil fuel reserves areforcing scientists to look for alternativefuel resources with minimalchanges and investments (if possible)in new infrastructure.Imagine returning home from college,plugging your car into a socket,and producing overnight your 10 L offuel for the next day. This may not bea dream, but one of the options oftomorrow’s distributed fuel generation.The focus of this approach is:n on the development of the conceptof distributed fuel-generationtechnologyn on the parallel utilization of unwantedCO2, which can derivefrom thermal combustion, gasification,or chemical conversion.The background of the actualprocess is CO2 capturing and its catalyticconversion back to hydrocarbons.This process includes absorption ofCO2 from the surrounding air or its separationfrom combustion/pyrolysis/gasificationprocesses followed by reversewater gas-shift process (CO2 reactswith H2 to form CO and H2O—syngas)and Fischer-Tropsch (FTS) process ofIR-FCNGE-PowerCO/H2

HeatE-PowerFIGURE 2 – The super-wind concept. An example of integrated multisource multiproductenergy system.54 IEEE INDUSTRIAL ELECTRONICS MAGAZINE n MARCH 2010

conversion of syngas (hydrogenation ofCO) to a series of hydrocarbons andwater. These processes are well knownand well established. The novel conceptin our case is the intent to performthese catalytic processes in microreactors.There are two main expectedadvantages of this novel concept:n the development of fundamentalsfor possible distributed fuel generationandn the utilization of microprocessingadvantages, i.e., better heat andmass transfer, higher reaction rates,more controllable product quality,lower temperatures and pressures,better resources management, reducedmissions and cost, enhancedsafety and reduced environmentalimpact.Figure 3 presents the block diagramof the process of CO2 conversion tohydrocarbons. One should note thatthere are four microreactors in a seriesattributed to the reverse water gasshiftreaction. The reason is that thisreaction is reversible and to secure COgeneration and prevent its reconversion

back to CO2, at least one of theproducts (here it is the water) has tobe removed as soon as possible. Thereactor is followed by a cooler/condenserto secure water-vapor condensationand separation.Another important issue is theprovision of cheap, concentrated, andpure CO2 for efficient conversion. Currenttechnologies are estimating thecost of CO2 scrubbing at about US$30per ton of CO2, which corresponds toabout six cents per kilogram. Theenergy required for supplying therequired hydrogen is estimated to bein the range of 5 kWhel/Nm3H2. Relyingon renewable energy resources(wind) for hydrogen provision, thecurrent estimate shows that a liter ofliquid petroleum produced throughpromoted technology would cost inthe range of e3. The authors of [3]report that energy resynthesis penaltyis 82% ideally and 95% on a practicalbasis. As the cost of offshore windpower is predicted to be reduced bySteam ReformingSteamFTSReactorrWGSReactorSyngasCarbonDioxideHydrogenHotWaterDistillationReboilerHydrocarbonsHydrocarbonsHydrocrackingHeaterRWGSFTSPetrolDieselHeavy OilBypassC1–C4 to Self-Powered CombustionO Heating Cooling 2 FeedCO2 FeedH2OSeparationFuel Gases 5.6 1.6Naphtha 33.0 24.6Kerosene 44.8 49.2Gas Oil 16.6 24.6BoilerWater OxygenElectrolyzerResynthesizedFuel ProductsUndesirableShort ChainHydrocarbonsCoolersandCondensers(a)(b)

MicroreactorsStackFIGURE 3 – (a) Block diagram of fuel resynthesis process and (b) microreactor system.MARCH 2010 n IEEE INDUSTRIAL ELECTRONICS MAGAZINE 552020, this resyn jet fuel would be competitivewith conventional jet fuel,especially if carbon taxes apply.The described idea gives a radicaland promising way for reducing thenegative environmental impact of fuelcombustion and CO2 emission generation.It adds the missing link amongbiofuel generation, fuel combustion,and CO2 emission mitigation [4], [5].New Trends in FC SystemsAmong the different technologicalalternatives, FC power generation becomesa more and more interestingand promising solution for both automotiveindustry and stationary powerplants. However, many technologicalhurdles must still be overcome beforethe development of industrial andcompetitive products in these fields.The FCs convert fuel and air directlyto electricity, heat, and water in anelectrochemical process. Figure 4 depictsthe general scheme of an FC, aswell as the power electronics convertersand the related controller.There are several different typesof FCs, but they are all based around acentral design, which consists of twoelectrodes, a negative anode and apositive cathode. These are separatedby a solid or liquid electrolytethat carries electrically charged ionsbetween the two electrodes. A catalystsuch as platinum is often used tospeed up the reactions at the electrodes.The FCs are classified accordingto the nature of the electrolyte andtheir operating temperature.Considering automotive applications,proton exchange membraneFCs (PEMFCs) are the most appropriate.Compared with other types ofFCs, PEMFCs generate more power fora given volume or weight of FC. Thishigh-power density characteristicmakes them compact and lightweight.In addition, the operating temperatureis less than 100 _C, which allows rapidstart-up. These traits and the ability torapidly change power output aresome of the characteristics that makethe PEMFC the top candidate for automotivepower applications.To produce power, an FC system ismade of several components thatworktogether. The basic system requirementsfor a PEMFC system include fourmajor circuits. Considering the air circuit,a compressor must be placed atthe inlet of the FC so as to provide oxygenfor the electrochemical reactionand to raise its efficiency by increasing

the air pressure. Considering the hydrogencircuit, hydrogen can be directlystored on board or suppliedthrough a reforming system. Specialcare must also be taken on the watermanagement system. In fact, the PEMmust always be maintained in a wellhydrationstate so as to ensure themigration of protons Hþ from theanode to the cathode. Moreover, inmost cases, a special water-coolant circuitexists within the stack. The aim isto obtain a self-sufficient water system.Finally, an electrical convertermust beplaced at the output of the FC, providingthe electrical power to the dcelectrical bus of the vehicle. It has tobe noticed that the electrical responseof the FC does not only depend on thestack but also on the time response ofthe ancillaries that are located aroundthe FC.One of the othermain objectives ofFC research is to reduce costs in theproduction of FC systems and theirrelated components. During the lastdecade, a cost reduction of a factor often has been achieved, but moreresearch is needed to reduce the costof FC systems to a competitive level(e.g., the amount of noble catalyst hasto be reduced_0.5mg/cm2 today).The general technological barriersto be overcome include the cost, thefuel choice, the need tomake improvementsto achieve higher specificpowers and power densities, and theelectrical engineering part.Different barriers considering anFC system have been presented previously.Among all these challenges, theauthors would point out one of themajor priority concerning FCs. Indeed,an FC system has to be demonstratedas durable and reliable. To succeed inthis task, highly efficient models haveto be developed to fully control theFC’s behavior and/or to improve theperformances of the FC system. Furthermore,real-time diagnosis and efficientcontrol laws have to beimplemented to increase the durabilityand lifetime of the FC system. TheFC lifetime requirements vary significantly,from 5,000 h for light-vehicleapplications to 30,000 h for trucks.The operation of an FC is usually subjectto inherent uncertainty in variousoperating parameters (fuel and oxidantstack starvation, FC temperature,membrane hydration, or pressure variations),which can cause severe transientsin the performance of the cell.That is why it is necessary to designsome robust control laws for the FCparameters. Furthermore, the technological

choices made for various FCancillaries have to be evaluated andadapted to increase the lifetime anddurability of the FC systems [6].One way to optimize simultaneouslythe performances, lifetime, anddurability is to performan online diagnosisof the PEMFC. Narjiss et al. [7]developed an online detection of theFC dysfunction in embedded applicationswithout additional hardwarePEM Fuel Cell dc-Link−Inverter HF Transformer Rectifierdc/dc ConverterDiagnosis ControlDSP ControllerFIGURE 4 – Block diagram of an FC system, including the power electronics converter andthe digital controller.56 IEEE INDUSTRIAL ELECTRONICS MAGAZINE n MARCH 2010

(Hw) component. Indeed, the powerconverter usually coupled with the FCis, here, used to performthe diagnosisstrategies.In addition, many different kinds ofphysical phenomena are involved inthe performances of an FC. Independentlyof the problems of cost, reliability,and durability, the use of PEMFCin a vehicle requires also to fully controlits behavior and thus to have atone’s disposal a highly efficient model.Many different kinds of models aredeveloped in our research groupbased on electrochemical impedancemeasurements, polarization curves,and semiempirical models.Jemei et al. [8] explored a powerfulsolution provided by an artificial neuralnetwork. Indeed, black box modelsallow getting a behavioral model withoutidentifying all the FC parameters.They are based on a set of easilymeasurable inputs such as temperature,current, or pressures within thestack and are able to predict the outputvoltage of the FC stack. The firststep to achieve this kind of model isto study the behavior of the FC todetermine the most relevant parametersthat govern the FC behavior.Thanks to a methodology based onfast Fourier transform, a recurrentneural network dynamic model of theFC is performed. This model coversthe whole range of possible FC currentfrequency solicitations. Thismodeling strategy provides an interestingand powerful solution even indynamical operating modes (errorson the computed voltage versusmeasured ones is below 2.5%).Different technological challengesconsidering the FC power generationhave been presented in this section.The main research objective is costreduction in all aspects of FC production,

materials, systems, and applications,together with those of relatedcomponents. To overcome these barriers,our research group proposesdifferent technical and engineeringchallenges to investigate, which willallow improving FC performancesand decreasing the FC system cost.To reach these objectives, manypoints still have to be studied: watermanagement issues, cold start-up,efficient and real-time control, onlinediagnosis, power electronics topologies,real and large-scale experimentaltests and evaluation, and power electronicstopologies.Power Electronics inRenewable Energy SystemsThe global electrical energy consumptionis still rising, and there is a steadydemand to increase the power capacity.It is expected that it has to be doubledwithin 20 years. The production, distribution,and use of the energy should beas technological efficient as possible,and incentives to save energy at theend user should also be set up. Deregulationof energy has lowered the investmentin larger power plants, whichmeans the need for new electricalpower sources may be very high in thenear future. Two major technologieswill play important roles to solve thefuture problems. One is to change theelectrical power production sourcesfrom the conventional, fossil (and shortterm) based energy sources to renewableenergy resources. Another is touse high-efficient power electronics inpower generation, power transmission/distribution, and end-user application.In classical power systems, largepower-generation plants located atadequate geographical places producemost of the power, which is thentransferred to large consumption centersover long distance transmissionlines. The system control centers monitorand regulate the power system continuouslyto ensure the quality of thepower, such as frequency and voltage.However, now the overall power systemis changing, a large number of DG units,including both renewable and nonrenewablesources such as wind turbines,wave generators, PV generators, minihydro,FCs, and gas/steam-poweredcombined heat and power stations, arebeing developed and installed.A wide-spread use of renewableenergy sources in distribution networksand a high-penetration level willbe seen in the near future in many places.Denmark has a high-power capacitypenetration (>20%) of windenergy, having presence inmajor areas

of the country, and today, 18% of thewhole electrical energy consumptionis covered by wind energy. The mainadvantages of using renewable energysources are the elimination of harmfulemissions and inexhaustible resourcesof the primary energy. However, themain disadvantage, apart from thehigher costs, e.g., PV, is the uncontrollability[30]. The availability of renewableenergy sources has strong dailyand seasonal patterns, and the powerdemand by the consumers could havea very different characteristic. Therefore,it is difficult to operate a powersystem installed with only renewablegeneration units because of thecharacteristic differences and the highuncertainty in the availability of therenewable energy sources.Wind turbine technology is one ofthe most important emerging renewabletechnologies. It started in the1980s with a few tens of kilowattproduction power to today with multimegawatt-size wind turbines that arebeing installed. It also means that, inthe beginning,wind power productiondid not have any impact on the powersystem control, but now because oftheir size, they have to play an activepart in the grid. Earlier, the technologyused in wind turbines was basedon a squirrel-cage induction generatorconnected directly to the grid. By that,power pulsations in the wind arealmost directly transferred to theelectrical grid. Furthermore, there isno control of the active and reactivepower, which typically are importantcontrol parameters to regulate thefrequency and voltage [9], [10]. As thepower range of the turbines increases,

Real-time diagnosis and efficient control lawshave to be implemented to increase thedurability and lifetime of the FC system.MARCH 2010 n IEEE INDUSTRIAL ELECTRONICS MAGAZINE 57these control parameters becomemore important, and it is necessary tointroduce power electronics as aninterface between the wind turbineand grid. Power electronics is changingthe basic characteristic of thewind turbine from being an energysource to be an active power source.The electrical technology used inwind turbine is not new. It has beendiscussed for several years, but now,the price per produced kilowatt houris so low, that solutions with powerelectronics are very attractive.Power electronics has changedrapidly over the last 30 years, and thenumber of applications has been increasing,mainly because of the developments

of the semiconductor devicesand the microprocessor technology.For both cases, higher performance issteadily given for the same area of silicon,and at the sametime, they are continuouslyreducing in price. Figure 5shows a typical power electronic systemconsisting of a power converter,load/source, and control unit. The powerconverter is the interface betweenthe load/generator and grid. The powermay flow in both directions, of course,dependent on topology and applications[11], [12].Three important issues are of concernusing such a system: reliability,efficiency, and cost. For the moment,the cost of power semiconductordevices is decreasing between 1 and5% every year for the same outputperformance, and the price perkilowatt for a power electronic systemis also decreasing. The trend ofpower electronic conversion is shrinkingin volume and weight. This showsthat integration is an important key tobe competitive, as more available functionscan be implemented in such aproduct. Accordingly, power electronicswill be a key point to allow thechange from the traditional centralizedgrid to a more distributed and smartgrid, as depicted in Figure 6.Digital Control ofPower ElectronicsAnother important topic is regardingthe control of the DG energy systems.Indeed, these new systems are requiringvery performing and flexiblecontrollers. In this context, digitaltechnology is of big interest since itallows implementing complex controlstrategies easily. On the other hand,analog controllers, despite of theirdrawbacks such as parameter driftingor lack of integration, still remain thereference in terms of rapidity andbandwidth.That is the reason why digital controllerexecution times must be reducedwhile keeping the inherentflexibility of all digital solution. Thiscan only be achieved with the help ofefficient digital platforms. Today, suchdigital platforms exist, some of themalso integrate analog functions such asanalog-to-digital conversion, and theycan be developed by the use of performingdesign tools. This concept isalso known as system-on-programmable-chip (SoPC). In this area, amature technology is the digital signalprocessor (DSP) controller [13]. Thisplatform has the great advantage tointegrate within the same componenta performing 32-b DSP core and a collection

of peripherals. This architectureis clearly optimized for thecontrol of electrical systems, but it suffersfrom a lack of customization, thuslimiting the achievable bandwidth ofthe controlled systems. For example,no concurrency is allowed betweenthe tasks.Another possibility is the use offield programmable gate arrays(FPGAs) [14]. These componentstake benefits of a high-integrationrate and, as DSP controllers, they canbe programmed by software (Sw)even if, in this case, the developed Swis only devoted to the description ofthe architecture that will support thefinal application (VHDL or verilog languages).Thus, by using an FPGAbasedcontroller, the designer is ableto build a fully dedicated digital systemthat is perfectly adapted to thealgorithm to implement. Dependingof the cases, the resulting architecturescan be all Sw, Hw, or mixed Sw/Hw. This customization of the architectureallows reducing significantlythe execution time of the control algorithm.The obtained performancesare then closed of their analog counterparts.However, to better understandthe effect of this new degree offreedom in the design of the controller,one has to examine in details thedifferent possible cases that exist inthe field of power converter control.These cases can be divided in twomain groups: high-demanding applicationsand constrained switchingfrequency applications. The highdemandingapplications consists ofapplications where timing constraintsare so stringent that it is the digitalcontroller that represents the mainlimitation of the whole control loop.This group can be classified in twosubtypes. The first subtype concernsthe control of static converters wherepower is segmented to reduce thestress of the power switches, such asApplianceIndustryCommunicationRenewableSourcesLoad/Generator2/3 2/3Power ConverterBidirectional Power FlowElectricalNetworkReference(Local/Centralized)ControlFIGURE 5 – Power electronic system with the grid, load/source, power converter,and control.58 IEEE INDUSTRIAL ELECTRONICS MAGAZINE n MARCH 2010

interleaved choppers and multilevelconverters [15]. These topologies ofconverters imply to control concurrentlymultiple channels of the process[see Figure 7(a)]. The secondsubtype concerns applications wherethe sampling frequency is very high(equal or above megahertz) same asthat for switch-mode power supplies[16]. In such case, no option but havinga performing Hw architecture[see Figure 7(b)].The constrained switching frequencyapplications consists of applicationwhere sampling is not criticalbecause of switching frequency limitation(acceptable level of switchinglosses in the power converter). In thiscase, Sw implementation is possible.However, even in this case, usingdedicated Hw architecture can be ofgreat interest, since this way, controlprocessing time can be drasticallyreduced up to a fraction of the switchingperiod. The reduction of processingtime, beyond its immediate improvementof the closed-loop bandwidth, isalso interesting for three other reasons.All of them take benefits of the remainingtime between the end of the controltasks and the next sampling time. Thefirst reason is the possibility to simplifythe complexity of the control algorithmby accurately choosing the momentwhere the different control tasks areexecuted within the switching period.For example, in [17], no filter is requiredsince the exact average current ismeasured [see Figure 7(c)]. The secondreason is the possibility of sharing thecontroller between different processeslike in [18] where a single FPGA-basedcontroller is used to control up to fourac-drives in 50 ls [see Figure 7(d)].Finally, the third reason is the possibilityof adding a new control functionalityduring this remaining time such ashealth monitoring [19] or predictivecontrol [20] [see Figure 7(e)].To conclude, FPGA-based customizeddigital control platforms aredefinitely an attractive solution forimplementing always ever more complexDG energy systems. The mainadvantages are the ability to reachquasi-analog control performancesby means of flexible digital solutionswith the addition of new control functionalitiessuch as health-monitoringand performing communications.NuclearOilCoalCentral Power PlantsTransmission NetworkDistribution NetworkCommercial

Buildings IndustryCentralPowerPlantsCentralPower PlantsHousesHousesLocalCHP PlantCommercialBuildingsPVPower PlantStorageIndustryPowerQualityDevicePowerQualityDeviceStorageTransmissionLinesStorageWindPowerPlantElectricityElectricityHeatHousesGeneratorsGeneratorsTransmission NetworkTransmission NetworkDistribution NetworkDistributionNetworkConsumersConsumersInformational FlowInformational FlowPower FlowPower Flow Green PowerDGPowerFlow(a)(b)FIGURE 6 – Power plants toward distributed power generation: (a) traditional power systems and (b) decentralized future power systems.MARCH 2010 n IEEE INDUSTRIAL ELECTRONICS MAGAZINE 59

Integrating Renewable Energyinto Transmission GridsSpain is a paradigm example of renewableenergy integration into the electricalgrid, with the following features:high external-energy dependency (84%in 2008), high electricity consumptiongrowth rates, 70.6% between 1996and 2008: significant growth of theeconomic activity (57.8% between1996 and 2008). Electrically speaking,Spain is a peninsula with weak electricalinterconnections with the EuropeanUnion.Red Electrica de Espan˜a (REE) wasthe first company in the world dedicatedexclusively to power transmissionand the operation of electricalsystems [system operator and transmission

network owner (TSO)]. Asthe system operator guarantees thecontinuity and security of the powersupply and the proper coordinationof the production and transmissionsystem, performing its functionsbased on the principles of transparency,objectiveness, and independence(see Figure 8). In addition, REEis the manager of the transmissiongrid and acts as the sole transmissioncompany on an exclusive basis. Overthe last decade, wind generation inSpain has experienced an extraordinarygrowth, continuously increasingits contribution to demand supply.This external requirement togetherwith the unique properties associatedwith wind power management,which may affect the secure operationof the power system, has encouragedREE to create a Control Centreof Renewable Energies (CECRE).CECRE allows the maximum amountof production from renewable energysources, especially wind energy, tobe integrated into the Spanish powersystem under secure conditions.The particular features of theSpanish system within the Europeancontext and its electrical connectionemphasize the ambitious objectivesin renewable energy:n formerly in 2010: 12% in primaryenergy (_30% in electric energy)n currently in 2020: 20% in finalenergy (_40% in electric energy).CECRE is an operation unit integratedinto the Electrical Control Center(CECOEL) of REE. The generationof the renewable energy producers,which have been set up in our countryare managed and controlled byCECRE. In addition, this center is thesole interlocutor in real time betweenCECOEL and each one of the authorizedgeneration control centers, towhich the wind farms are connected.Its main function is to supervise andcontrol the renewable energy generators,mainly wind power. It also articulatesthe integration of its productionto the power system in a way compatiblewith its security.Figure 9 shows an excellent applicationexample of DG and storage,(k)Ts (k + 1) Ts

(k + 1) Ts

(k – 1) Ts (k + 1) Ts

(k + 1)Ts

(k)Ts

(k)Ts

(k)Ts

First Plant ControlSecond Plant ControlAddedTreatment(a)

(b)(c)(d)(e)FIGURE 7 – Performing FPGA-based digital controllers. (a)–(e) Timing charts (Ts is thesampling period, which is also equal to the switching period, in blue: A/D conversion,in orange: control tasks).

The main advantages of using renewable energysources are the elimination of harmful emissionsand inexhaustible resources of theprimary energy.60 IEEE INDUSTRIAL ELECTRONICS MAGAZINE n MARCH 2010

allowing the global optimization ofthe electrical system, thus delayingthe building of conventional infrastructures,i.e., lines and substations.It consist of a 3-MW PV system combinedwith a 4-MW 3 3-h battery setlocated in a certain area of the Spanishoperator Empresa Nacional deElectricidad S.A. (ENDESA), in whichit is optimized the demand curve,shaving the peak and delaying theneed for a new substation of 66/15 kV.Low-Voltage Ride Throughfor Wind TurbinesThe present wind energy penetrationinto the electrical network has forcedsystem operators to adapt their gridcodes to this new generation, preventingan unacceptable effect on the systemsafety and reliability [21]–[26].One of these new connection requirementsregarding wind energy is faultride-through capability. In the past[27], wind generators were not allowedto remain connected to the utilitywhenvoltage at the point of common coupling(PCC) fell below 85%, forcingtheir disconnection even when thefault happened far from the wind farm[28]. This is the reason why, in gridswith significant wind energy penetration,the voltage dip and subsequentwind farm disconnections would createan important stability problem [29].System operators of the differentcountries have established diverse voltage-limit curves for fault ride through.Wind turbines must remain connectedto the grid when facing voltage dips, aslong as voltage at the PCC remainsabove the specified voltage profile.Compliance with grid codes can bechecked by means of simulation of validatedmodels.Today, there are modelsfor the different generator types requiredfor grid stability studies throughdynamic simulation. However, to certifythe validity of the simulation modelswhen testing voltage dip ride-throughcapability, the obtained results must bevalidated by the ones measured on afield test. Once this validation is made,compliance with grid requirements can

be certified using simulations of thevalidated model.Taking into account the requirementsexposed previously, the maincharacteristics that a voltage dip generatorshould have are variable sagdepth, adjustable sag duration, andflexible input. The last requirement isneeded to present high-input impedanceduring the dip, compared withthe short-circuit power of the grid, preventingthe system from affecting significantlyits voltage.The voltage-dip generators arebased on the use of two impedances[Figure 10(a)]. The parallel impedanceenables the generation of the faultwhile the series impedance immunizesthe grid from the dip, and thetest can be performed without affectingother systems connected to it. InSpecial Regime International Exchanges—REETransmission Grid—REE System Operator—REEMarket Operator—OMELConsumers UnderApproved Tariffs Distribution CompaniesSuppliers andMarket PriceConsumers Demand BidsCommunicationsEnergy FlowsDemand BidsDistributionNetwork < 132 kVOutagesAccepted BidsTechnical ConstraintsTSO (Foreign)GeneratorsFIGURE 8 – Electricity sector in Spain (courtesy of Red El_ectrica de Espan˜ a).

Wind turbine technology is one of the mostimportant emerging renewable technologies.MARCH 2010 n IEEE INDUSTRIAL ELECTRONICS MAGAZINE 61the proposed voltage-dip generator[Figure 10(b)], the series impedance ismade up of a three-phase seriesimpedance at the system input andthe parallel branch consisting of a taptransformer and a three-phase impedancegrounded through a controlswitch in the secondary of the transformer.The system includes someother control elements to perform thevoltage dip generation, a 100% voltagedip can be achieved, and the impedancebanks have single-phase switchesto have the possibility of performingsingle-phase, two-phase, and threephasetests in wind turbines up to20 kV and 5 MW.ConclusionsThis article has shown the concept ofDG as a way to integrate energy systems.As an example, it was illustratedusing microreactors for the DG CO2 orhydrogen, which can be used toempower FC systems. DG of electricityby using FCs and dispersed microreactors

can be used to support renewableenergy systems integration. The windenergy integration was presented asan application of large-scale integrationof DG and DS concepts. However,it was pointed out that to enhance thepenetration of renewable energy systemsinto the grid, power electronicsand digital control systems are required.Finally, the electrical grid inSpain was presented as a paradigmfor renewable energy integration.Voltage Dip GeneratorNetworkSeries ImpedanceReducing the Impact ofthe Fault on the Gridand Bypass SwitchParallel ImpedanceAllowing theGeneration of theFault by Means ofthe Series SwitchWind TurbineRSTRSTTransformerRSTRSTMV NetworkWind TurbineMV PositionImpedancesImpedances(a) (b)FIGURE 10 – Voltage-dip generator scheme: (a) general system and (b) proposed system.25,000Low LoadStorage fromGrid SystemDemandPV Generationto GridStorageDischargingGridDemandCustomersDemandNet Loadfrom Grid(Shaved)BatteryDischargingto GridPVGenerationto GridStoragefrom PVStoragefrom GridPV (to Battery and Grid)(a)(b)20,00015,00010,0005,00000123456789101112

131415161718192021222325,000(kVA) (kVA)20,00015,00010,0005,000001234567891011121314151617181920212223SINEU Zone (Day: Peak 2025–Winter)Peak 2025

vFIGURE 9 – (a) Demand curve and (b) load curves of the PV plus the storage system.62 IEEE INDUSTRIAL ELECTRONICS MAGAZINE n MARCH 2010

BiographiesJosep M. Guerrero received theB.S. degree in telecommunicationsengineering, the M.S. degree in electronicsengineering, and the Ph.D.degree in power electronics fromthe Technical University of Catalonia,Barcelona, Spain, in 1997, 2000,and 2003, respectively. He is anassociate professor with the Departmentof Automatic Control Systemsand Computer Engineering, TechnicalUniversity of Catalonia, Barcelona,where he currently teachescourses on DSP, FPGAs, microprocessors,and renewable energy. Since2004, he has been responsible forthe Renewable Energy Laboratory,Escola Industrial de Barcelona. He isa Senior Member of the IEEE. He isthe editor-in-chief of InternationalJournal of Integrated Energy Systems.He is an associate editor for IEEETransactions on Industrial Electronics,IEEE Transactions on Power Electronics,International Journal of PowerElectronics, and International Journalof Industrial Electronics and Drives.

His research interests include PVs,wind energy conversion, uninterruptiblepower supplies, storage energysystems, and microgrids.Frede Blaabjerg received theM.Sc.E.E. degree from Aalborg University,Denmark, in 1987, and thePh.D. degree from the Institute ofEnergy Technology, Aalborg University,in 1995. He is currently withAalborg University, where he becamethe dean of the Faculty of Engineeringand Science in 2006. He is anassociate editor of Journal of PowerElectronics and the Danish journal Elteknik.He has authored or coauthoredmore than 300 papers and is theauthor of Control in Power Electronics.He is an associate editor of IEEETransactions on Industry Applicationsand IEEE Transactions on Power Electronics.In 2006, he became the editorin-chief of IEEE Transactions on PowerElectronics. He received the 1995Angelos Award and the Annual TeacherPrize at Aalborg University in 1995.In 1998, he received the OutstandingYoung Power Electronics EngineerAward from the IEEE PowerElectronics Society. He received fiveIEEE Prize Paper Awards over the lastsix years. He is a Fellow of the IEEE.His research interests include powerelectronics, static power converters,ac drives, switched reluctance drives,modeling and characterization of powersemiconductor devices and simulation,wind turbines, and greenpower inverters.Toshko Zhelev received his bachelor’sdegree in chemical engineeringand master’s degree in chemicalcybernetics from Moscow Universityof Chemical Technology, and his Ph.D.degree in energy conservation fromthe Bulgarian Academy of Sciences.He is a professor at the Departmentof Chemical and Environmental Sciences,University of Limerick, Ireland,where he is a director of thepostgraduate program known asGraduate Diploma in Chemical Engineering.He holds a position of a visitingprofessor at the University ofSurrey, United Kingdom and a positionof extraordinary professor atthe University of Pretoria, South Africa.He is a member of the CAPE Workingparty—branch of the EuropeanFederation of Chemical Engineersand a member of the executive boardof its educational branch EURECHA.He is a member of the executiveboard of the Charles Parsons Institutein Energy and Sustainable Environment.His research interests

include process systems engineering,process integration, and sustainableindustrial resources management.Kas Hemmes received his master’sdegree in experimental andtheoretical physics from GroningenUniversity in 1983 and his Ph.D. degreein perpendicular magnetic recordingfrom Twente University in1986. He became assistant professorand then associate professor in theDepartment of Materials Science ofTU Delft, where he was responsiblefor the molten carbonate fuel cellprojects carried out within the frameworkof the National Fuel Cell ResearchProgram for 15 years. InNovember 2001, he joined the Energyand Industry sections of Technology,Policy, and Management responsiblefor the Dutch Greening of Gas project,which is a large feasibility studyon the addition of hydrogen into thenatural gas grid in The Netherlands.In October 2005, he joined the sectionTechnology Dynamics and SustainableDevelopment of the samefaculty, working on innovative energysystems in the wider context ofsustainability.Eric Monmasson is currently afull professor and the head of theInstitut Universitaire Professionnalis_e de G_enie Electrique et d’InformatiqueIndutrielle, University of Cergy-Pontoise, Cergy-Pontoise, France. Heis also with the Syste`mes et Applicationsdes Technologies de l’Informationet de l’Energie (SATIE, UMRCNRS8029). At SATIE, his currentresearch interests include the advancedcontrol of electrical motorsand generators and the use of FPGAsfor energy control systems. He is thechair of the technical committee onelectronic systems-on-chip of theIEEE Industrial Electronics Society.He is also a member of the steeringcommittee of the European PowerElectronics Association and of thenumber one technical committee ofthe International Association forMathematics and Computers in Simulation(IMACS). He is an associateeditor of IEEE Transactions on IndustrialElectronics. He is the author orcoauthor of two books and morethan 100 scientific papers. He is aSenior Member of the IEEE.Samir Jeme€ı received his master’sdegree from the University ofFranche Comt_e, Belfort, France, in2001, and his Ph.D. degree in engineeringsciences from the Universityof Franche Comt_e and the Universityof Technology of Belfort-Montb_eliard,

France, in 2004. Since 2005, he hasbeen a research engineer and workson fuel cell systems for transportationin the Fuel Cell LaboratoryInstitute of Belfort, France, with theEnergy team (ENISYS/FEMTO-ST).His research interests include fuelcell systems dedicated to automotiveapplications, modeling, fuelcell system characterization, andcompressors.MARCH 2010 n IEEE INDUSTRIAL ELECTRONICS MAGAZINE 63Maria P. Comech received herM.S. and Ph.D. degrees in electricalengineering from the University ofZaragoza, Zaragoza, Spain, in 2003and 2008, respectively. Her Ph.D.thesis was on the subject of modellingand testing wind turbinesbefore network contingencies.Ramo´n Granadino earned hisdegree in industrial engineering in1990 at the Polytechnic University ofMadrid, Spain, and his M.Sc. ECEdegree in 1993 from the Universityof Massachusetts, Amherst, UnitedStates. He has worked with REEsince 1994, managing projects forthe development of the 220 and 400kV Spanish networks. He is currentlythe director of REE in the BalearicElectric System and project managerfor the HVDC Submarine Interconnectionbetween the Spanish Peninsulaand Mallorca. He has been theproject manager of significant transmissionprojects in the Spanish electricgrid. He was a member of CIGREWG B1-19. He is currently a memberof the Tutorial Advisory Group of SCB1 (Insulated Cables) and is the Spanishrepresentative for CIGRE SC B1(Insulated Cables).Juan I. Frau received the M.S. andPh.D degrees in electrical engineeringfrom the Polytechnic University ofCatalonia, Barcelona, Spain, in 1988and 1991, respectively. From 1988 to1992, he was an associate professorwith the Department of AutomaticControl Systems and Computer Engineering,Polytechnic University of Catalonia,and in 1992, he joined ENDESA.From 1993 to 2001, he collaboratedwith the University of the BalearicIslands as an assistant professor,teaching courses on instrumentationsystems and power electronics. Heis the director of Network Planningin Balearic Islands (ENDESA Distribution)since 2002 and has been involvedin several RþDþi projects.He has been a consultant of theUNFCCC (Bonn) to develop the cleandevelopment mechanism (CDM) methodology(AM0067) to estimate CO2

emissions reduction due to energyefficient transformers. He will leadfrom ENDESA the ECCOFLOW project(2010–2013), cofinanced by theEC, to develop and perform the fieldtest of a superconducting fault currentlimiter (HTS based on coatedconductor YBCO tape) for operationin electricity networks.References[1] K. Hemmes, ‘‘Towards multi-source multiproductand other integrated energy systems,’’Int. J. Integr. Energy Syst., vol. 1,no. 1, pp. 1–15, Jan.–June 2009.[2] K. Hemmes, A. Patil, and N. Woudstra,‘‘Flexible coproduction of hydrogen andpower using an internal reforming SOFCsystem,’’ J. Fuel Cell Sci. Technol., vol. 5,no. 4, pp. 1–6, Nov. 2008.[3] K. J. Winch, P. N. Sharratt, and R. Mann,‘‘Carbon-neutral jet-fuel re-synthesisedfrom sequestrated CO2,’’ Int. J. Sustain.Eng., vol. 1, no. 2, pp. 1–9, 2008.[4] D. Graham-Rowe, N. Scientist, vol. 197,no. 2645, pp. 32–34, 2008.[5] T. Zhelev and O. Strelow, ‘‘Heat integrationin micro-fluidic devices,’’ in Proc. 16thEuropean Symp. on Computer AidedProcess Engineering and 9th Int. Symp. onProcess Systems Engineering, W. Marquardtand C. Pantelides, Eds. New York: Elsevier,2006, pp. 1863–1868.[6] B. Wahdame, D. Candusso, X. Francois,F. Harel, M.-C. Pera, D. Hissel, and J. M.Kauffmann, ‘‘Analysis of a fuel cell durabilitytest based on design of experimentapproach,’’ IEEE Trans. Energy Conversion,vol. 23, no. 4, pp. 1093–1104, 2008.[7] A. Narjiss, D. Depernet, D. Candusso,F. Gustin, and D. Hissel, ‘‘Online diagnosisof PEM fuel cell,’’ in Proc. 13thEPE-PEMC Conf., 2008, pp. 734–739.[8] S. Jeme€ı, D. Hissel, M.-C. Pera, and J. M.Kauffmann, ‘‘A new modeling approach ofembedded fuel cell power generatorsbased on artificial neural network,’’ IEEETrans. Ind. Electron., vol. 55, no. 1, pp. 437–447, 2008.[9] Z. Chen, J. M. Guerrero, and F. Blaabjerg,‘‘A review of the state of the art of powerelectronics for wind turbines,’’ IEEE Trans.Power. Electron., vol. 25, no. 8, pp. 1859–1875,Aug. 2009.[10] J. M. Guerrero, L. Hang, and J. Uceda,‘‘Control of distributed uninterruptiblepower supply systems,’’ IEEE Trans. Ind.Electron., vol. 55, no. 8, pp. 2845–2859,Aug. 2008.[11] J. M. Guerrero, L. Garcia De Vicuna, and J.Uceda, ‘‘Uninterruptible power supply systemsprovide protection,’’ IEEE Ind. Electron.Mag., vol. 1, no. 1, pp. 28–38, 2007.[12] F. Blaabjerg, Z. Chen, and S. B. Kjaer,‘‘Power electronics as efficient interface indispersed power generation systems,’’IEEE Trans. Power Electron, vol. 19, no. 5,pp. 1184–1194, Sept. 2004.[13] H. A. Toliyat and S. G. Campbell, DSPBasedElectromechanical Motion Control.Boca Raton, FL: CRC, 2004.[14] E. Monmasson and M. Cirstea, ‘‘FPGA designmethodology for industrial control systems—A review,’’ IEEE Trans. Ind. Electron.,vol. 54, no. 4, pp. 1824–1842, Aug. 2007.[15] O. Lopez, J. Alvarez, J. Doval-Gandoy, andF. D. Freijedo, ‘‘Multilevel multiphasespace vector PWM algorithm,’’ IEEE Trans.Ind. Electron., vol. 55, no. 5, pp. 1933–1942,May 2008.[16] L. Corradini, P. Mattavelli, E. Tedeschi,and D. Trevisan, ‘‘High bandwidth multisampleddigitally controlled DC/DC converters

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