Power Management of a Stand-Alone Wind/Photovoltaic/Fuel Cell Energy System

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This paper proposes an ac-linked hybrid wind/ photovoltaic (PV)/fuel cell (FC) alternative energy system for stand-alone applications. Wind and PV are the primary power sources of the system, and an FC–electrolyzer combination is used as a backup and a long-term storage system. An overall power management strategy is designed for the proposed system to manage power flows among the different energy sources and the storage unit in the system.Asimulationmodel for the hybrid energy system has been developed using MATLAB/Simulink. The system performance under different scenarios has been verified by carrying out simulation studies using a practical load demand profile and real weather data.
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  IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 23, NO. 3, SEPTEMBER 2008 957 Power Management of a Stand-AloneWind/Photovoltaic/Fuel Cell Energy System Caisheng Wang , Senior Member, IEEE , and M. Hashem Nehrir  , Senior Member, IEEE  Abstract  —This paper proposes an ac-linked hybrid wind/ photovoltaic (PV)/fuel cell (FC) alternative energy system forstand-alone applications. Wind and PV are the primary powersources of the system, and an FC–electrolyzer combination is usedas a backup and a long-term storage system. An overall powermanagement strategy is designed for the proposed system to man-agepowerflowsamongthedifferentenergysourcesandthestorageunitinthesystem.Asimulationmodelforthehybridenergysystemhas been developed using MATLAB/Simulink. The system perfor-mance under different scenarios has been verified by carrying outsimulation studies using a practical load demand profile and realweather data.  Index Terms —Alternative energy, electrolyzer, fuel cell (FC),hybrid, photovoltaic (PV), power management, stand-alone, wind. I. I NTRODUCTION T HE EVER increasing energy consumption, the soaringcost and the exhaustible nature of fossil fuel, and theworseningglobalenvironmenthavecreatedincreasedinterestingreen [renewable and/or fuel celll (FC)-based energy sources]power generation systems. Wind and solar power generation aretwo of the most promising renewable power generation tech-nologies. The growth of wind and photovoltaic (PV) power generation systems has exceeded the most optimistic estima-tion [1]–[3]. FCs also show great potential to be green power sources of the future because of many merits they have (suchas high efficiency, zero or low emission of pollutant gases, andflexible modular structure) and the rapid progress in FC tech-nologies. However, each of the aforementioned technologieshas its own drawbacks. For instance, wind and solar power arehighlydependentonclimatewhileFCsneedhydrogen-richfuel.Nevertheless, because different alternative energy sources cancomplementeachothertosomeextent,multisourcehybridalter-native energy systems (with proper control) have great potentialto provide higher quality and more reliable power to customersthan a system based on a single resource. Because of this fea-ture, hybrid energy systems have caught worldwide researchattention [4]–[28]. ManuscriptreceivedAugust14,2006;revisedDecember27,2006.Thisworkwas supported in part by the National Science Foundation (NSF) Grant ECS-0135229 and in part by the HiTEC fuel cell project at Montana State University,funded by the United States Department of Energy, as a subcontract from Bat-telle Memorial Institute and Pacific Northwest National Laboratory (PNNL)under Award DE-AC06-76RL01830. Paper No. TEC-00399-2006.C. Wang is with the Division of Engineering Technology, Wayne State Uni-versity, Detroit, MI 48202 USA (e-mail: caisheng.wang@gmail.com).M. H. Nehrir is with the Electrical and Computer Engineering Department,Montana State University, Bozeman, MT 59717 USA (e-mail: hnehrir@ece.montana.edu).Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/TEC.2007.914200 Many alternative energy sources including wind, PV, FC,diesel system, gas turbine, and microturbine can be used tobuild a hybrid energy system [4]–[28]. Nevertheless, the major renewable energy sources used and reported are wind and PVpower [4]–[28]. Due to the intermittent nature of wind andsolarenergy,stand-alonewindandPVenergysystemsnormallyrequireenergystoragedevicesorsomeothergenerationsourcesto form a hybrid system. The storage device can be a batterybank, supercapacitor bank, superconducting magnetic energystorage (SMES), or an FC–electrolyzer system.In this paper, a stand-alone hybrid alternative energy systemconsistingofwind,PV,FC,electrolyzer,andbatteryisproposed.Wind and PV are the primary power sources of the system totakefulladvantageofrenewableenergy,andtheFC–electrolyzer combinationisusedasabackupandalong-termstoragesystem.A battery bank is also used in the system for short-time backupto supply transient power. The different energy/storage sourcesin the proposed system are integrated through an ac link bus.The details of the system configuration, system unit-sizing, andthe characteristics of the major system components are alsodiscussedinthepaper.Anoverallpowermanagementstrategyisdesignedforthesystemtocoordinatethepowerflowsamongthedifferent energy sources. Simulation studies have been carriedout to verify the system performance under different scenariosusing practical load profile and real weather data.The paper is organized as follows. The system configurationand system unit-sizing are discussed in Section II. The systemcomponent characteristics are given in Section III. Section IVdiscussestheoverallpowermanagementstrategyforthesystem.Section V gives the simulation results. Section VI concludes thepaper.II. S YSTEM C ONFIGURATION AND U NIT -S IZING  A. System Configuration Fig. 1 shows the system configuration for the proposed hy-brid alternative energy system. In the system, the renewablewind and PV power are taken as the primary source while theFC–electrolyzer combination is used as a backup and storagesystem. This system can be considered as a complete “green”power generation system because the main energy sources andstorage system are all environmentally friendly. When there isexcess wind and/or solar generation available, the electrolyzer turns on to begin producing hydrogen, which is delivered to thehydrogen storage tanks. If the H 2 storage tanks become full,the excess power will be diverted to the dump load shown inFig. 1. When there is a deficit in power generation, the FC stackwill begin to produce energy using hydrogen from the reservoir  0885-8969/$25.00 © 2008 IEEE Authorized licensed use limited to: IEEE Xplore. Downloaded on October 14, 2008 at 17:33 from IEEE Xplore. Restrictions apply.  958 IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 23, NO. 3, SEPTEMBER 2008 Fig. 1. System configuration of the proposed multisource alternative hybridenergy system (coupling inductors are not shown). tanks, or in case they are empty, from the backup H 2 tanks. Abattery bank is also used in the system to supply transient power to load transients, ripples, and spikes. There are several ways tointegrate different alternative energy sources to form a hybridsystem.Eachmethodhasitsownadvantagesanddisadvantages.In this paper, a 60 Hz ac link is used due to its high reliability,modular and scalable structure, and readiness for grid connec-tion [25], [27]. Different energy sources are connected to the acbus through appropriate power electronic interfacing circuits.The system can be easily expended, i.e., other energy sourcescan be integrated into the system when they are available, asshown in Fig. 1. The main system unit-sizing is discussed in thefollowing section.  B. System Unit-Sizing Theunit-sizingprocedurediscussedinthissectionisassumedfor a stand-alone hybrid system with the proposed structure(Fig. 1) for residential electricity supply in the southwesternpart of Montana. The purpose of the study is to properly size thesystem components to assure reliable electricity supply. Hence,the system’s economic aspect is not considered in the paper.Some details on the economics of similar wind/PV/FC systemsare given in another paper by the authors [5]. Fig. 2. Hourly average demand of five typical homes in the Pacific Northwestarea. The hybrid systemisdesigned tosupply power tofive homes.A typical hourly average residential load demand for a home inthe Pacific Northwest regions, reported in [29], is used in thissimulation study. The total hourly average load demand of thefive homes is shown in Fig. 2. A 50 kW wind turbine is assumedto be available for the hybrid system. The following unit-sizingprocedure is used to determine the size of the PV array, FCstack, electrolyzer, and the battery.Before the discussion of unit-sizing, the following concept isapplied for indicating the overall efficiency and the availabilityof a renewable energy source.Capacity factor  ( k cf  ) of a renewable energy source is definedas k cf  =¯ P P  rated (1)where ¯ P  is the actual average output power over a period of time and P  rated is the nominal power rating of the renewableenergy source.Forthewindandsolardatareportedin[5]and[20],thecapac-ityfactorofthewindturbine( k cf wtg )andthePVarray( k cf PV )used in the proposed hybrid system for the southwestern part of Montana are taken as 13% and 10%, respectively.The purpose of unit-sizing is to minimize the difference be-tween the generated power ( ¯ P  gen ) from the renewable energysource and the demand ( ¯ P  dem ) over a period of time T  . T  istaken as one year in this study: ∆ P  =¯ P  gen − ¯ P  dem = k cf wtg × P  wtg , rated + k cf PV × P  PV , rated − ¯ P  dem (2)where P  wtg , rated is the power rating of the wind turbine gener-ator and P  PV , rated is the power rating of the PV array.To balance the generation and demand, the rated power for the PV array is P  PV , rated =¯ P  dem − k cf wtg × P  wtg , rated k cf PV . (3)From Fig. 2, the average load demand is 9.76 kW. Then,according to (3), the size of the PV array is calculated to be32.6 kW. Authorized licensed use limited to: IEEE Xplore. Downloaded on October 14, 2008 at 17:33 from IEEE Xplore. Restrictions apply.  WANG AND NEHRIR: POWER MANAGEMENT OF A STAND-ALONE WIND/PHOTOVOLTAIC/FUEL CELL ENERGY SYSTEM 959 TABLE IS YSTEM C OMPONENT P ARAMETERS The FC–electrolyzer combination provides backup for thesystem. The FC needs to supply the peak load demand (Fig. 2)when there is no wind and solar power. Therefore, the size of the FC stack is 14.6 kW. To leave some safe margin (20% usedin this paper), an 18 kW FC array is used.The electrolyzer should be able to handle the excess power from the wind and solar power source. The maximum possibleexcess power is P  gen , max − P  dem , min = 50 + 32 . 6 − 5 . 85 = 76 . 75 kW . (4)However,thepossibilitythatbothwindandsolarpowerreachtheir maximum points while the load demand is at its lowestvalue is very small. According to the data reported in [26], theexcess available power normally is less than half of the maxi-mumpossiblevalue.Andtheelectrolyzerisalsoveryexpensive.Therefore, a 50 kW electrolyzer [over 60% of the maximumavailable given in (4)] is used in this paper. Fig. 3. C   p  –  λ characteristics of the WECS at different pitch angles ( θ ). Battery capacity can be determined based on the transientpower at the load site. In this study, a 10 kWh battery bankis used. In single-phase systems, a larger size battery may beneeded for reactive power compensation purposes. In three-phase systems, as discussed in this paper, reactive power com-pensation can be achieved by proper control of power electronicswitching devices [39], and only a small size battery is neededfor this purpose [40].The details of the system component parameters are listed inTable I.III. S YSTEM C OMPONENT C HARACTERISTICS Todevelopanoverallpowermanagementstrategyforthesys-temandtoinvestigatethesystemperformance,dynamicmodelsfor the main components in the proposed hybrid system havebeen developed using MATLAB/Simulink [27]. The models arefor the following: wind energy conversion system (WECS), PV,FC, and electrolyzer.Inthissection,thecharacteristicsoftheaforementionedmainsystem components are discussed. For the details of model de-velopment, the reader is referred to [27].  A. Wind Energy Conversion System The power  P  wind (in watts) extracted from wind is P  wind =12 ρAv 3 C   p ( λ ,θ ) (5)where ρ is the air density in kilogram per cubic meter, A is thearea swept by the rotor blades in square meter, and v is the windvelocity in meters per second. C   p is called the power coefficientor the rotor efficiency and is a function of tip speed ratio (TSRor  λ ) and pitch angle ( θ ) [30], [31].A variable-speed pitch-regulated wind turbine is consideredinthispaper,wherethepitchanglecontrollerplaysanimportantrole.Fig.3showsthegroupsof  C   p  –  λ curvesofthewindturbineused in this study at different pitch angles [31]. It is noted fromthe figure that the value of  C   p can be changed by changing thepitch angle ( θ ). In other words, the output power of the windturbine can be regulated by pitch angle control.A self-excited induction generator (SEIG) model [27], [37],[38] was developed and used as a part of the WECS model. Theratings of the SEIG are given in Table I. Authorized licensed use limited to: IEEE Xplore. Downloaded on October 14, 2008 at 17:33 from IEEE Xplore. Restrictions apply.  960 IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 23, NO. 3, SEPTEMBER 2008 Fig. 4. Wind turbine output power characteristic. Fig. 4 shows the output power of the WECS vs. wind speed.It can be observed that the output power is kept constant whenwind speed is higher than the rated wind velocity even thoughthe wind turbine has the potential to produce more power. Thisis done through the pitch angle control to protect the electricalsystem and to prevent over speeding of the rotor. When windspeed is higher than the cutout speed (25 m/s), the system istaken out of operation for protection of its components.  B. Photovoltaic PV effect is a basic physical process through which solar energy is converted directly into electrical energy. The physicsofaPVcell,orasolarcell,issimilartotheclassical  p-n  junctiondiode [32]. The relationship between the output voltage V  andthe load current I  of a PV cell or a module can be expressedas [15], [32] I  = I  L − I  0  exp  V  + IR s α  − 1  (6)where I  L is the light current of the PV cell (in amperes), I  0 isthe saturation current, I  is the load current, V  is the PV outputvoltage (in volts), R s is the series resistance of the PV cell (inohms), and α is the thermal voltage timing completion factor of the cell (in volts).The I   –  V  characteristic curves of the PV model used in thisstudy under different irradiances (at 25 ◦ C) are given in Fig. 5[27]. It is noted from the figure that the higher the irradiance,the larger are the short-circuit current ( I  sc ) and the open-circuitvoltage( V  oc ).Asaresult,thelargerwillbetheoutputPVpower.Temperature plays an important role in the PV performancebecause the four parameters ( I  L , I  0 , R s , and α ) in (6) are allfunctions of temperature. The effect of the temperature on thePVmodelperformanceisillustratedinFig.6.Itisnotedfromthefigurethatthelowerthetemperature,thehigheristhemaximumpower and the larger the open circuit voltage. C. Fuel Cell Two types of FCs have been modeled for this study. Theyare low-temperature proton-exchange membrane FC (PEMFC)[33] and high-temperature solid oxide FC (SOFC) [34]. Both of  Fig. 5. I   –  V  characteristic curves of the PV model at different irradiances.Fig. 6. P  –  V  characteristic curves of the PV model at different operating tem-peratures. them show great potential in hybrid energy system applications.For the purpose of simplicity, only the PEMFC application isdiscussed in this paper.The PEMFC model is based on the validated dynamic modelfor a PEMFC stack reported in [33]. It is an autonomous modeloperated under constant channel pressure with no control onthe input fuel flow into the FC. The model was validated byexperimental data measured from an Avista Labs (ReliOn now)SR-12 500 W PEMFC stack. The FC will adjust the input fuelflow according to its load current to keep the channel pressureconstant. Fig. 7 shows the output voltage vs. load current ( V   –   I  )characteristic curve of the 500 W PEMFC model comparedwith the experimental data [33]. This characteristic curve canbe divided into three regions. The voltage drop across the FCassociated with low currents is due to the activation loss insidethe FC; the voltage drop in the middle of the curve (which isapproximately linear) is due to the ohmic loss in the FC stack;andasaresultoftheconcentrationloss,theoutputvoltageattheend of the curve will drop sharply as the load current increases. Authorized licensed use limited to: IEEE Xplore. Downloaded on October 14, 2008 at 17:33 from IEEE Xplore. Restrictions apply.
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