Hybrid Solar-Wind Power Generation System

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Journal of Industrial Technology • Volume 16, Number 3 • May 2000 to July 2000 • www.nait.org

The Official Electronic Publication of the National Association of Industrial Technology • www.nait.org© 2000

A Hybrid Solar-Wind Power GenerationSystem as an Instructional Resource

for Industrial Technology StudentsBy Dr. Recayi Pecen, Dr. MD Salim, & Dr. Marc Timmerman

Volume 16, Number 3 - May 2000 to July 2000

Reviewed Article

ElectricityElectronics

EnergyEnvironmental Issues

Teaching Methods

KEYWORD SEARCH

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Dr. Recayi Pecen holds a B.S.E.E. and an M.S. in Con-trols and Computer Engineering from the IstanbulTechnical University, an M.S.E.E. from the Universityof Colorado at Boulder, and a Ph.D. in Electrical Engi-neering from the University of Wyoming. Dr. Pecenis an Assistant Professor in the Department of Indus-trial Technology at the University of Northern Iowa.Dr. Pecen is the recipient of University of Wyoming,G. Nicholson Power System Lab Scholarship in 1996,recipient of Provost�s two Mini-Grants for AchievingEducational Excellence, in 1999 and 2000, and re-cipient of UNI Summer Fellowship in wind/PV powersystems. Dr. Pecen is a member of Tau Beta Pi-National Engineering Honor Society, IEEE, and NAIT.His research interests and publications are in theareas of AC/DC power systems, HVDC transmission,power system control, power quality, and grid-con-nected wind/PV renewable energy systems.

Dr. MD Salim holds a B.S. in Civil Engineering fromthe Bangladesh Institute of Technology, an M.S. inConstruction Engineering from the University ofLeeds (UK) and a Ph.D. in Civil Engineering from theNorth Carolina State University. He has served innumerous industrial positions and on the faculty ofthe University of Northern Iowa. His research inter-ests and publications are in the applications of arti-ficial intelligence to construction management.

Dr. Marc A. Timmerman holds a B.S.E.E. from theSanta Clara University, a M.Eng.E.E. from theRensselaer Polytechnic Institute, and a Ph.D. fromthe George W. Woodruff School of Mechanical Engi-neering at Georgia Tech. He has served on the fac-ulties of the University of Tulsa, the Louisiana StateUniversity at Baton Rouge, and the University ofNorthern Iowa. His research publications are in thearea of embedded microprocessors and DSP,Mechatronics, vibrations, and magnetic bearings.

A Hybrid Solar-Wind PowerGeneration System as an In-structional Resource for In-dustrial Technology StudentsBy Dr. Recayi Pecen, Dr. MD Salim, & Dr. Marc Timmerman

AbstractThe detailed study of electrical

power systems is a key element of manycurricula in Industrial Technology. Anovel laboratory set-up has beendesigned and implemented at theUniversity of Northern Iowa as aninstructional resource for teachingelectrical power system and renewableenergy concepts. The set-up consists ofa photo-voltaic solar-cell array, a mastmounted wind generator, lead-acidstorage batteries, an inverter unit toconvert DC power to AC power,electrical lighting loads and electricalheating loads, several fuse and junctionboxes and associated wiring, and testinstruments for measuring voltages,currents, power factors, and harmoniccontamination data throughout thesystem. This hybrid solar-wind powergenerating system is extensively used toillustrate electrical concepts in hands-onlaboratories and demonstrations in theIndustrial Technology curriculum.

IntroductionElectricity cannot be seen, felt,

tasted, smelled, heard or (safely)touched. Providing Industrial Technol-ogy students with vivid, memorable,hands-on learning experiences in thearea of electricity is a challenge for alleducators in the Industrial Technologyarea. The traditional measuring instru-ments for electricity (oscilloscopes,voltmeters, ammeters, and powermeters) and traditional signal sources(motors, transformers, resistors, induc-tors) are fine but have some drawbacks:

• They have a tendency to becomerepetitive and boring.

• They have an artificial, educa-tional non-real-world feel.

• They may fail to convey the notionof the true complexity and interre-lations of industrial electricalpower systems.

• They may not capture thestudent’s attention and motivatelearning.

• They may not give an intuitivefeeling for what electrical quanti-ties really mean physically.

Iowa is a geographically large statewith a low population density. Electri-cal power needs are supplied by a largenumber of local power companies.Due to the isolation of many dwellings,agricultural sites, and industrial sites,there is considerable interest in novelforms of electricity production. Twosuch forms of production are solarphoto-voltaic (PV) cells based on DC-power generating arrays and wind-turbines based on propeller-driven DC-power generators. In fact, Iowa is nowthe home of the largest wind-turbinepower installation in the world (Pecen,1999). Electrical power generation andspecial sources of electric power, likewind-turbines, are frequently discussedin the public media and are veryvividly in the minds of IndustrialTechnology students from daily-lifeexperiences. The additional factor ofthe general concern and interest forenvironmental issues is a furtherenticement to attract the student’sinterest in these “green-technology”forms of electricity generation.

Special Educational Issues of Wind-turbines and Photo-Voltaic Cells

As the wind does not blow all thetime nor does the sun shine all the time,solar and wind power alone are poor

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power sources. Hybridizing solar andwind power sources together withstorage batteries to cover the periods oftime without sun or wind provides arealistic form of power generation.

This variable feature of wind-turbine power generation is differentfrom conventional fossil fuel, nuclear,or hydro-based power generation.Wind energy has become the leastexpensive renewable energy technol-ogy in existence and has peaked theinterest of scientists and educators theworld over. A simple relationship existsrelating the power generated by awind-turbine and the wind parameters:

P = 0.5ρA Cpv3η

g η

b(1)

where,ρ = air density (about 1.225 kg/m3 at

sea level, less at higher eleva-tions).

Α = rotor swept area, exposed to thewind (m2).

Cp = Coefficient of performance(.59 to .35 depending on turbine).

v = wind speed in meters/secη

g = generator efficiency

ηb = gearbox/bearings efficiency

A mast-mounted anemometer(wind meter) allows the students todirectly measure wind speed and tovividly relate this easily felt force-of-nature to electrical measurements.

Photo-Voltaic or PV cells, knowncommonly as solar cells, convert theenergy from sunlight into DC electricity.PVs offer added advantages over otherrenewable energy sources in that theygive off no noise and require practicallyno maintenance. PV cells are a familiarelement of the scientific calculatorsowned by many students. Their operat-ing principles and governing relation-ships are unfortunately not as pedagogi-cally simple as that of wind-turbines.However, they operate using the samesemiconductor principles that governdiodes and transistors and the explana-tion of their functioning is straightfor-ward and helps to make more intuitivemany of the principles covered insemiconductor electronic classes.

Most industrial uses of electricityrequire AC or alternating 60 Hz power.

Wind-turbines and PV cells provideDC power. A semiconductor-baseddevice known as a power inverter isused to convert the DC power to ACpower. This device has a relativelysimple operation that is a vivid illustra-tion of many topics traditionallycovered in power electronics classes.

The inverter also introduces theproblem of power quality. Powerquality is an extremely important issuein real-life industrial electric powersystems. Power quality is the contami-nation of the voltage or frequencycharacteristics of electric power. Thesystem exhibits many common prob-lems of power quality such as voltagesag (sudden drops in voltage due toover demand of battery capacity and/orloss of wind or sun), harmonic con-tamination (errors in the 60 Hz fre-quency due to nonlinear loads such ascomputers, energy efficient light bulbs,laser printers, scanners), and voltageregulation problems (prolonged dropsin voltage due to interactions of systemelements). Power quality is anextremely important problem inindustrial electricity applications andthis setup offers unique opportunitiesfor the students to study power qualityproblems in a real system.

Three major phases of the develop-ment and implementation of thisfacility have been completed. First, theset-up itself has been constructed anddebugged. Second instruments forhands-on measurement of electricalquantities in the system have beenacquired and deployed for hands-on labuse. And third, numerous simulationshave been performed on PSCAD/EMTDC (1996), a well-known powersystem computer simulation packageused by industry and universities. Thissoftware allows the students to make anintuitive link between the physicalsystem present in the hands-on labs andthe more abstract mathematicalequations presented in their lecturenotes and texts. Two such simulationsare presented in this paper.

Liter atur e ReviewThe literature in the subject areas

of this paper is very extensive. Anexcellent textbook for instructional use

is Wind and Solar Power Systems byPatel (1999) that covers the specificissues in this project in a style appro-priate for Industrial Technologystudents. Sabin (1999) and coworkershave summarized the various standardsand benchmarks used in large-scalepower quality, and Koval (1999) andcoworkers have presented similarfinding for rural (small-scale) powerquality problems. Many articles haveappeared on the impact of new elec-tronics technologies on power qualitymanagement, for example Poisson(1999) and coworkers have describedthe impact of DSP chips on the prob-lem. Barbosa (1998) and coworkershave described the use of PWM (pulse-width-modulation) control schemes topower quality control.

Numerous studies have appeareddescribing the impact of power qualityproblems caused by PV systems fromearly work by McNeil (1983) andcoworkers in to more recent work byOliva (1988) and coworkers and mostrecently by Chowdhury (1999). Theextensive literature on power qualityaspects of wind generation includeswork by Demoulias and Dokopoulos(1996) on transient power measurementand by Thiringer (1996) on harmoniccontamination measurement issues.Taylor (1987) is responsible for some ofthe early practical work on powerquality measurements in wind genera-tion. Kariniotakis and Stavrakakis(1995) have written extensively onsimulation problems in wind generatorand power grid interactions.

Finally, many papers have beenwritten on the electronics regulation/control aspects of the problem includ-ing a recent study by Neris and co-workers (1999) proposing an IGBT(Integrated-Base-Bipolar-Transistor)based regulator.

MethodologyIn order to address the shortcom-

ings of existing instructional tech-niques for electrical power systems, ahybrid wind-turbine and solar cellsystem has been implemented at theUniversity of Northern Iowa. Thesystem was designed and implementedwith the following goals:

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• To be completely different fromtraditional electricity labs and tobe fresh and interesting.

• To be intimately related to real-world industrial power issues suchas power quality.

• To show a complex, interrelatedsystem that is closer to the “realworld” than the usual simplesystems covered in educational labs.

• To motivate learning by introduc-ing such elements as environmen-tal and economic concerns ofpractical interest to the students.

Establishment of a Wind/PV Hybr idUnit

The hybrid unit contains twocomplete generating plants, a PV solar-cell plant and a wind-turbine system.These sources are connected in parallelto a 12V DC line. The power is nextconnected to a DC to AC inverter andis then supplied from the inverter’soutput to a single-phase 60 HZ, 120VAC load. The overall project structureis presented in Figure 1. The windturbine is installed at the top of a steeltower that has a height of 18.3 metersand a diameter of 8.9 cm. The windturbine depicted is a 0.7 kW unit andthe solar panels depicted number fourin all with a capacity of 50 Watts each.The instrumentation panel depictedmonitors the outputs of the generatorusing digital panel meters.

A small wind turbine was chosenfor its low maintenance and manysafety features. One of the lowmaintenance features is the turbine’sbrushless alternator and an internalgovernor. The turbine generates 0.4kW when turning at its rated speed of47 km/hr and it is capable of generat-ing up to 0.7 kW at its peak wind speedof 72 km/hr. The actual system’spictures are shown in Figure 2. Theturbine’s blades are made of a carbonfiber reinforced composite that willintentionally deform as the turbinereaches its rated output. This deforma-tion effect changes the shape of theblade, causing it to go into a stallmode, thus limiting the rotation speedof the alternator and preventingdamage in high winds. Anotherfeature of the wind turbine is a sophis-

ticated internal regulator that periodi-cally checks the line voltage andcorrects for low voltage conditions.The solar panels are 12 VDC units andwere chosen for their ultra cleartempered glass that is manufactured forlong term durability. Figure 3 showsthe DC voltage measured across the 12volt DC bus where the wind turbineand PV arrays outputs are connected.A slight ripple in power regulation canclearly be seen. This ripple is a func-tion of the unpredictable nature of windand sunshine along with the dynamiceffects of the electrical load.

As mentioned earlier, one of thelargest problems in systems containingpower inverters is power quality. Thisproblem becomes serious if the inverterused in the system does not have a

good sinusoidal waveform output andcauses problems such as harmoniccontamination and poor voltageregulation. According to the IEEE (aprofessional society which codifiessuch issues) standards, a maximum of 3to 4% total harmonic distortion (this isa quantitative measure of how bad theharmonic contamination is) may beallowed from inverter outputs. How-ever, many inverter outputs have muchmore harmonic distortion than isallowed. The inverter used in thissystem has a power rating of a 1.5KVA and was manufactured by TraceTechnologies ®. The battery bankscontain 4 deep-cycle lead-acid batteriesconnected in parallel. High powercapacity heating resistors, energyefficient light bulbs, incandescent light

Figure 1. Established Wind/PV hybrid power generation unit.

Figure 2. Actual pictures of wind/PV hybrid power station.

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bulbs, and two small AC motorsconstitute electrical loads that can beapplied to the system.

To monitor and store the voltage,current, power, and harmonic contami-nation data, two Fluke ® power qualityanalyzers (types 39 and 41) are used inthe system. In addition, permanentlymounted AC/DC digital panel metersform part of the system’s instrumenta-tion. A laptop computer is interfaced tothe system via the Fluke power qualityanalyzers to store data in real-time.Figure 4 illustrates such data as voltageand current waveforms on the load sideof the inverter in the case of small heatand lighting loads connected to thesystem. As shown in Figure 4, thecurrent waveform is much more dis-torted than voltage waveform. Sincecurrent harmonic contamination has amore detrimental effects on industrialpower quality problems than voltageharmonic contamination, the currentharmonic spectrum of the inverteroutput is a vivid illustration of thepitfalls present in the measurement ofreal power quality problems in industrialsimulations. Figure 5 illustrates thecurrent harmonic spectrum in the caseof nonlinear loads. A harmonic spec-trum is a graphical plot of harmoniccontamination on the y-axis and thefrequency where this contaminationoccurs on the x-axis. This plot iscommonly used in industry to studyharmonic contamination and its possibleremedies.

Voltage sags may cause a crucialdamage to high precision measurementand protection devices, especiallycomputer equipment present in manyhighly-automated industrial plants. Avoltage sag example for the system isshown in Figure 6.

Simulation of the Hybrid SystemFigure 7 presents the overall system

including a passive RLC filter on the ACside of inverter. This filter is a circuitmade up of a resistor (R), inductor (L),and a capacitor (C). Such filters arecommonly installed in industrial situa-tions to remedy power quality problems.

The inverter is of a twelve pulsetype and the inverter and the controlcircuit models are both standard

Vdc (V)

50

-50

t(min)1 2 3 4 5 6 7

0

Figure 3. DC voltage generated by hybrid wind/PV power system.

Figure 4. AC load voltage and current waveforms.

Figure 5. Current harmonic spectrum of the system when linear/nonlinear loads are connected.

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models in the PSCAD/EMTDCsoftware package. Two simulationcases were run, with and without thisRLC filter. Figures 8 presents raw andanalyzed data for the system with andwithout the RLC filter. The studentscan vividly see from such simulationsthe practical remedies for powerquality problems used in industry.

FindingsTo date, the experience gained

from using this setup in numeroushands-on labs and demonstrations hasshown the following:

• The students find the system to behighly interesting and an enhance-ment to even simple tasks likelearning how to use a voltmeter.

• The students gain a better intuitiveknowledge of the generation anddistribution of electric power thanthey can from textbooks andtraditional lab experiments alone.

• The students’ interest in electricalpower systems is raised and theyare able to gain a better physicalunderstanding of the quantitiesand units involved.

• The system’s use of a powerinverters and of PV cells provides avivid illustration of semiconductorissues and the students are betterable to assimilate semiconductorconcepts in their electronics classes.

Futur e StudyFigure 9 show the future direction

of this project. A computer measure-ment and control bus will be added tothe system. Computer controlledrelays will be added to allow all themajor elements of the system to beswitched in and out of the systemthrough computer programs. Themeasurement bus will be connected toall the major signals in the system andwill allow for computerizes dataacquisition simultaneously of all themajor signals in the system. Theseimprovements will allow for the studyof more complex issues like powerfaults caused by sudden over voltageslike lightning. These improvementswill also allow the same benefits toinstruction realized in electricity and

Figure 6. Voltage sags recorded in the wind/PV system.

Figure 7. PSCAD/EMTDC Simulation of system with load and RLC filter.

electronics classes to be extended tocontrol and instrumentation classes.

Conclusions andRecommendations

Obviously, a complete hybridpower system of this nature may be tooexpensive and too labor intensive formany Industrial Technology Depart-ments. However, many of the samebenefits could be gleaned from havingsome subset of the system, for examplea PV panel, batteries, and an inverter, oreven just a PV panel and a DC motor.The enhancements to instruction,especially in making electrical powermeasurements more physical, intuitive,and real-world are substantial and thecosts and labor involved in someadaptation of the ideas in this paper to asmaller scale setup are reasonable.

The use of solar and wind hybridpower generation is an especially vividand relevant choice for students ofIndustrial Technology in Iowa as theseare power sources of technological,political, and economic importance intheir state. In other places, other powersources could be used. For examplehybrid combinations of wind power,solar power, geothermal power,hydroelectric power, tidal power,biomass generated power, power fromincineration of solid wastes, and manyother technologies could be considereddepending on local interests andresources. The key elements of thistest bed concept presented in this paperare two or more renewable powersources connected to a power grid withcomplex electrical interactions.

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ReferencesAmerican Wind Energy Association.

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Chowdhury, B. H. (1999, June).Designing an innovative laboratoryto teach concepts in grid-tiedrenewable and other dispersedresources. ASEE Annual Confer-ence and Exhibition.

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Pecen R., Timmerman M., (1999,November). A novel power qualityscheme to improve a utility interfacein a small-sized hybrid solar/windgeneration unit. 10th. InternationalPower Quality Conference.

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Figure 9. Block diagram of future research.

Figure 8. Harmonic contamination (a) without RLC filter and (b) with RLC filter.

Hybrid Solar-Wind Power Generation System

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