Feasibility Study For Wind Power At SAB Newlands
Feasibility Study forWind Power at SABNewlandsAuthor:Walter Brosius2009
The author wishes to thank the following for contributing to thisproject:At Stellenbosch University: Professor Van Niekerk for offering me a studyposition and this project as thesis subject. Professor Von Backstrom for supervising me duringthis project and for his guidance and patience. The team of lecturers affiliated with the Centre forRenewable Energy Studies Mr. Francis Jackson for his clear lecturing on the topicof wind energy.At SAB: Mr. Tony Cole for his interest and support in thisproject. Mr. Josiah Mpofu for his great support at the SABNewlands plant.And last but not least my loving parents for their everlastingsupport.
1.ABSTRACTThis paper describes a MEng thesis project for a MEng inRenewable Energy Systems program at the Centre for RenewableEnergy Studies at the University of Stellenbosch, South Africa. Theaim of this paper was to offer to SAB (South African Breweries), inCape Town, a feasibility study for the possibilities of the usage ofwind energy on site.The small scale wind power technology has a long historyand has been in South Africa for more than a hundred years in theform of water pump wind mills. All wind mills have an absolutemaximum power output defined by the Betz limit. The choice of awind turbine depends not only on this, but also on the wind speeddistribution, the power curve, the location and financing. The smallscale turbines have many different design which are predominantlygrouped in horizontal axis (HAWT) and vertical axis (VAWT)machines.The choice of turbine for SAB depends on the available windenergy, the available budget, the available space and theapplication. The aim of the measurements on site was threefold;find a correlation with existing weather stations in the area like atCape Town International Airport, propose a turbine for SAB’sbudget and research the possibility for installing the turbine on oneof the buildings. This is also known as building integrated windturbines.Wind speeds can increase over buildings due to venturieffects and it could therefore be viable to locate these accelerationzones and install a turbine there. The data analysis shows that thewind above the brewery is very well correlated with the wind at theairport. We can therefore use the average speed values of thisstation to predict average power production. This leads to theproposal of a 1kW or 3kW turbine from a South Africanmanufacturer: Kestrel. Building integration is however not a goodidea. The wind is too turbulent and can therefore not be used. Thisis mainly caused by the fact that the surroundings of the breweryare too high and irregularly shaped. This makes it difficult for thewind to “lower” in between the buildings and accelerate.
The wind turbine for SAB is proposed to be installed on thehighest point of the roof and based on the neighbouring averagewind speed values. The wind turbine should be connected directlyto the brewery’s grid with an inverter and would then solelyfunction as an energy saver. Another important aspect is thepromotional value in the energy efficiency strategy of SAB.
Table of Contents1.2.3.4.Abstract . iiiIntroduction . 7Theory and background . 8Introduction . 8History . 8Wind Energy . 9Betz Law . 10The Power Curve of a Wind Turbine . 10The Variability of Wind. . 11Wind Speed Distribution . 11Variation over long periods . 13Local obstructions and wind shear . 13Wind Maps . 16Annual Energy Production . 18Capacity factor . 19Capacity Credit . 20Electricity Generation . 21Voltage . 21Current . 21Three Phase Alternating Current . 22Generation . 22Small-scale wind power . 24Introduction . 24Definition and types . 24Definition . 24Horizontal axis wind turbines. 24Vertical-axis wind turbines. 25Systems . 26Stand alone wind power . 26Grid tied wind power . 27Hybrid wind power . 28Inverter . 29Site assessment for small scale systems . 30Wind measurement . 30Consumption monitoring . 31Batteries . 32
5.6.7.8.9.Charge controller . 34Market research . 34Building Integrated Wind Turbines . 35Building Integrated Wind . 35Examples: (large and small) . 36The Swift. 36The Bahrain World Trade Centre . 36The Aeroturbine . 38The Windpods . 40Feedback . 43SAB Brewery Cape town . 44Introduction: a feasibility study . 44Problem Formulation . 44Proposed Approach. 44Results . 45Location Analysis . 45Graphs . 49Calculations . 54Wind roses . 57Difficulties . 62Conclusion . 63Main Conclusion . 64Summary . 64Recommendations . 64Bibliography . 65APPENDIX . 66
2.INTRODUCTIONCape Town has a reputation of a very windy city. It istherefore logical to investigate the potential of this wind power forharvesting it in the form of electric power by use of wind turbines.The dominating winds in the region are the infamous south easternwind and the north western wind. These are seasonal and areresponsible for the reputation of Cape Town as a good wind sportssite.Since several years, the effects of climate change have beenincreasing and the general interest in renewable energytechnologies has been growing steadily. Wind energy in particularseems to be the fastest growing energy technology at the moment.European countries like Germany are aiming at a penetration levelof 20% and higher. The main objective there is to use wind energyas a fuel saver and for replacing a percentage of conventionalenergy technology.In South Africa, the interest in wind energy is not less thanelsewhere although the legislative mechanisms are lagging behind.Many private investors like companies and households don’t wishto wait for this and are prepared to take the initiative to invest inwind energy. However, the knowledge of wind turbines is stillquite low. SAB is one of the companies that would like to invest inwind turbines to be placed on the premises of its factories. Thequestion arises then where to place the turbine, which system tochoose and how big it should be. The purpose of this report is toanswer these questions.The first half of this report includes a history, summary anddescription of existing technologies, types and more.The second half comprises a feasibility study for SAB CapeTown. The site was surveyed by measuring the wind speeds anddirections with two measuring devices. Based on that and otherlimitations, a list of recommendations will be made in order toprovide a clear conclusion to SAB.7
3.THEORY AND BACKGROUNDIntroductionThe theory and background described in this chaptercontains a general overview of wind energy systems. First thehistory of wind energy and turbines is briefly described. After that,the topic of wind energy is described by starting from the definitionof wind energy and its limitation by the Betz law. The latterinfluences directly the power curve and performance of a windturbine. Then, the variability of wind energy is discussed becausethis is probably the most important topic to understand. The nextparagraphs discuss how to calculate the annual energy production(AEP) of wind turbines and the related capacity factor and capacitycredit. The main objective of wind turbines is electricity generationand we have added a paragraph regarding this subject whichtouches on electricity and generators.HistoryThroughout the human history the wind has been used topower sailboats and sail ships and to ventilate buildings or houses.The applications where wind power is used to generate mechanicalpower or shaft power are relatively young. As early as the 17thcentury BC, in Babylon, there are traces of the use of wind mills topower irrigation systems. The oldest practical wind mills have beenfound in Afghanistan dating back to the 7th century. These werevertical-axis windmills, which had long vertical drive shafts withrectangular blades. The materials used were wood, reed matting,cloth and limestone. These windmills were used to grind corn anddraw up water. Horizontal-axis windmills were later usedextensively in North-western Europe and Greece to grind flour. Themost famous are found in The Netherlands which date back to the1180s.During the 1800s there was a major boom in thedevelopment and distribution of the well known multi-bladedturbine which was mounted on a wooden structure. These wereused by farms and railroads to pump ground water used forirrigation, cattle and steam locomotives and were rapidlydistributed all over the world.The first modern wind turbines were built in the early 1980s,and have been subject to increasingly efficient design.8
Wind EnergyWind is generated by atmospheric pressure differences. Thesolar energy that falls upon the earth warms the surface byradiation. The surface warms up and transmits a large part of theheat back to the air by convection and this causes the air to rise inthe warmer regions near the equator. This mechanism causes themajor wind systems that govern global wind patterns. They arequite well understood, but on the more local level there are manyparameters still unknown. Most of the energy stored in these windmovements can be found at high altitudes where continuous windspeeds of over 160 km/h occur. Eventually, the wind energy isconverted back through friction into diffuse heat throughout theearth's surface and the atmosphere. A good example of local windpatterns occurs in high altitude mountain ranges like the Alps inEurope. Here, aside from the meteorological wind, there is also awind force generated by the difference in temperature of themountain flanks and valleys. This wind is also known as a valleybreeze. Another example of local wind force is the acceleration ofwind between buildings in built-up areas in cities or betweenfunnel shaped hills.Figure 1: Global wind systems; trade winds(http://ww2010.atmos.uiuc.edu)The total wind power that is present in the earth’satmosphere is estimated to be considerably more that the presenttotal human power usage. Of this total power, it is estimated thatabout 72 TW can be commercially exploited compared to the 15TW average global power consumption. This number is incentiveenough to allow investments and research to continue.9
Betz LawThe Betz law allows us to calculate the maximum energy thatcan be converted by a wind turbine. It was developed in 1919 byGerman physicist Albert Betz (Jackson, 2009).The essence of the Betz law the power extracted from thewind by the rotor is proportional to the product of the wind speedtimes the pressure drop across the rotor.Figure 2: A rotor in a wind stream (Jackson, 2009)If the rotor has a higher flow resistance, the pressure drop isincreased, but less flow goes through the disc and more goesaround it. The Betz law shows that there is a maximum efficiencyfor the extraction of power which is at 59.3%. See Appendix A fora derivation of the Betz law.Based on the law, it is common practice to say that a turbinecan’t capture more than 59.3 % of the kinetic energy in wind.Because some modern wind turbines approach this potentialmaximum efficiency, once practical engineering obstacles areconsidered, the Betz law shows a limiting factor for this form ofrenewable energy. Engineering constraints, energy storagelimitations and transmission losses cause for even the best modernturbines to operate at efficiencies substantially below the Betzlimit.The Power Curve of a Wind TurbineThe power curve of a wind turbine is a graph that indicateshow large the electrical power output will be for the turbine atdifferent wind speeds. Usually, wind turbines are designed to startgenerating at wind speeds somewhere around 3 to 5 metres per10
second. This is called the cut-in wind speed. The wind turbine willbe programmed or designed to stop at high wind speeds in order toavoid damaging the turbine or its surroundings. This speed is calledthe cut-out wind speed. In Figure 3 this speed is 10, 5 m/s.Figure 3: Example of a power curve (Kestrel Wind)The Variability of WindWind Speed DistributionThe limitation stated by the Betz law is not the only factorthat counters the easy implementation of wind turbines. Anotherimportant one is the non-consistency of wind speed regardless oflocation. Common practice these days is to measure wind speedscontinuously and map these data on graphs and analyze thesestatistically with the Weibull distribution formulas. These give us agood graphical representation of the behaviour of the wind at acertain location. An example is given underneath.Figure 4 shows two Weibull curves with the same meanwind speed, but different shape factors. When the shape factor is 2,the distribution is called a Raleigh distribution.This wind speed distribution is a very accurate display of theshortcoming of wind energy: the energy is not reliably availablewhen needed and not constant. Unlike fossil fuel plants which canrun 24 hrs regardless of weather conditions.11
Figure 4: Two Weibull curves with different shapefactors but same mean wind speed. (Jackson, 2009)12
Variation over long periodsThe effects of day and night, of seasonal differences andannual variation trends are all visible when wind speeds are plottedversus a time scale. The wind speed at a certain location can differbetween night and day for instance when a daily sea breeze exists.When data of many years is available, one can also detecttrends over the years. In Europe, the wind has been monitored formore than a hundred years which has enabled to find certain trends.One can for example find that the average annual wind speeddiffers with a standard mean deviation of 6% between the years.Local obstructions and wind shearWind is air in motion. The air behaves like a fluid and itsflow path is not only defined by a pressure difference and a flowrate, but also by the obstructions it meets. When air flows over oraround an obstruction, there is always a certain amount ofturbulence in the vicinity of this obstruction. The turbulencedepends on the wind speed and can affect a wind turbineperformance negatively. This can be understood from the fact thatthe force on the blades is a result of a smooth well ordered laminarflow over them. The lift that is generated by the blade results in adisplacement or rotation of the rotor. When the incoming airflow isturbulent, the flow around the blade is disturbed and this causes a‘stall’ of the blade resulting in very little to no lift force. There aretwo ways to stall a blade. One is to increase the angle of attack α(Fig 7) too much and the second one is to vary α too fast andirregularly which is the case with turbulent flow.13
Figure 5: Laminar flow and stalling due to turbulence(Jackson, 2009)The positioning of a wind turbine will be heavily influencedby its surrounding characteristics as is shown in the next figure.The reader can understand that the placement of a turbine in anopen field is much different from the placement in a built up areawhere the turbulences are very difficult to predict.Figure 6: turbulence effect due to building (Jackson,2009)Another variation due to obstructions is the variation of thewind speed between the ground level and higher up in the air. Atany location, a shear profile can be measured which shows theeffect of the ground on the wind speed. Like any normal fluid, air issensitive to the roughness of its surroundings. This roughnesscreates a shear effect depending on its speed and viscosity. Whenmeasurements are made at a location, the best thing to do is tomeasure the wind shear profile by using wind vanes at differentheights. This profile tends to be an exponential function withfollowing relationship:14
Where the velocities U(x) are related to the heights zx. (SeeFig 7)Figure 7: wind shear profile (Jackson, 2009)It is clear that the wind speeds are generally higher at greaterelevation from the surface where the measurement was taken. Thisis why commercial wind farms reach for higher wind speeds byusing higher turbines. There is of course a maximum to this whichis a balance between cost and benefit.15
Wind MapsThe effects of geography and pressure zones and thepreviously mentioned disturbances cause an average variabilitybetween regions. This is easily shown in the next figures which arewind maps for South Africa. These windmaps show the averagewind speeds in all the areas in South Africa. Although this shouldnot be the only decisive indicator for the choice of a wind farmlocation, this certainly indicates where the best regions will befound. These windmaps show for example that the coastal regionsof the Western Cape have a higher mean annual wind speed thanthe more inland regions of for example the Northern Cape. (Fig 8and 9)Figure 8: wind speed variation with region (DME)16
Figure 9: Another wind map (Hageman, 2008)17
Annual Energy ProductionThe total maximum energy that a wind turbine can producein a year is calculated by multiplying the nominal capacity by thenumber of hours in a year. The total annual energy production(AEP) of a wind turbine is never as high because of the varyingwind speed.The easiest way to explain how to find the AEP is by usingthe following graphs:Figure 10: wind speed distribution vs. power curve(Jackson, 2009)Figure 10 shows the effect of the available power from acertain wind speed distribution in combination with a certainturbine’s power curve. When overlapping these, one can see theavailable power underneath the intersection of the curves for eachwind speed window.18
Figure 11: wind speed, power and AEP (Jackson, 2009)This available power is then used to calculate the AEP foreach window which results in a graph like figure 11. The total AEPis then the sum of each speed window’s AEP. For each wind speed bin (u) a certain number of hoursper year occur tu [h] The turbine puts out power for that bin Pu according tothe power curve. Pu [W] Energy captured in that bin is the number of hourstimesEu Pu x tu [WH](1) Total energy captured is sum of all binsAEP ΣEu [WH](2)Capacity factorThe ratio of actual productivity in a year to the theoreticalmaximum is called the capacity factor. Typical capacity factors are20–40%, with values at the upper end of the range in particularlyfavourable sites.For example: a 1MW turbine with a capacity factor of 35%will not produce 8,760 MWh in a year (1 24 365), but only 1 0.35 24 365 3,066 MWh, averaging to 0.35 MW.(3)19
In comparison, the capacity factors of conventional plantscan be as high as 90% if they are used to provide base load. Someexpensive plants that run on natural gas are mostly used for peakload production and therefore have capacity factors of around 5 to25 %.Capacity CreditDue to the intermittency of wind energy, the grid will neverbe supplied with wind power only. That is the reason why windenergy is mostly seen as a conventional fuel saver and agreenhouse gas reducer. However, there is always a certain amountof conventional capacity that can be displaced by wind and this isexpressed as a percentage of the installed wind capacity.Capacity Credit Displaceable Conventional Capacity[MW ]Installed Wind Capacity[MW ](4)This capacity credit can also be expressed in GW capacity,meaning the exact displaceable capacity. This so-calledshortcoming of wind power is often used as an argument to say thatfor each wind farm, one needs to build as much conventionalcapacity. This statement has been found incorrect by recent studies.In the UK for example, history suggests that the capacity creditshould be about 30% at low wind penetrations. (Marsh, 2009)At low penetration this capacity credit is usually equal to thecapacity credit. (Jackson, 2009) At higher penetration, this relationis not correct and the capacity credit follows a square root curvelike in following figure:Figure 12: wind capacity in the UK related to thenational grid (Boyle, 2007)20
Electricity GenerationWind turbines can generate mechanical power which caneither be used directly in pumping applications or for running agenerator to create electric power. Wind turbines that feed thenational grid are usually grouped in a wind farm and generateelectrical power of several kVolts and are connected to the gridthrough a transmission system. The small scale wind turbines thatare discussed further in this report operate at a much lower voltageof around 12 to 200 V. On large wind turbines (above 100-150 kW)the voltage generated by the turbine is usually 690 V three-phasealternating. Turbines usually produce alternating current or ACsince they are mostly directly coupled to the generator. With thesmall turbines this is not always the case as will be seen later.Following paragraphs are a summary of the theory ofelectricity generation since it is important for understanding thebasic characteristics of wind turbines.VoltageA voltage is the same as a potential difference between twoends of any electrical circuit or conducting material. The voltage islike the driving force for the current and is comparable to waterpressure. This pressure is needed to overcome resistances in thecircuits that can be compared to the electrical loads. The power thatcan be delivered increases directly with voltage. Voltage isexpressed as number with unit Volt and has typical values of 220 Vor 110 V in household networks. Lower values are typically 12 Vor 24 V for battery applications. The grid networks operate atextremely high voltages ranging between 10 kV to 400 kV.CurrentThe electricity that comes out of a battery is direct current(DC), i.e. the electrons flow in one direction only. Most electricalgrids in the world are alternating current (AC) grids, however. Onereason for using alternating current is that it is possible to directlytransform the electricity up and down to different voltages. This isneeded because the transport of electricity over long distancesrequires a high voltage to minimize losses. Once delivered, thevoltage has to be reduced to household values.21
Three Phase Alternating CurrentThe AC networks mentioned above consist normally of threephases. This means that the electricity is generated and transportedin three different lines at the same time in parallel which is shownon figure 13. We will not go into details since that is not the scopeof this paper. Important to mention is the frequency of thealternating voltage which is usually 50 or 60 Hz.Figure 13: Three phase electricity (DWTMA)GenerationAll electrical generation from a mechanical source like awind turbine uses electromagnetic induction. Basically, the turbineaxis rotates a set of copper windings in a static magnetic field. Thiscauses an electric field across these windings and if these windingsare connected to a load in a circuit, a current will flow. The voltageacross the windings is therefore dependent of the rotor speed and isalso alternating or AC. In common generators that are run by steadyspeed turbines, the voltage are controlled by the speed of theturbine. When the load increases, the turbine must deliver moretorque at the same speed and therefore consume more fuel. With awind turbine, the speed of the generator varies continuously so inorder to produce a useable voltage which is constant, the turbineneeds a voltage control.There are two basic types of generators used on windturbines: induction generators and alternators. The inductiongenerators are basically the same as induction motors. They createelectrical energy in a rotor which is carrying a set of copperwindings while rotating in a changing magnetic field exerted by thestator. The conversion from AC to DC is done by a commutator onthe rotor. This commutator is connected by brushes to the outerelectrical circuit. The consequence of this system is that the rotor22
becomes quite heavy and the generator is not efficient at low rotorspeeds. Also, the brushes need preventive maintenance. This is whythis type of generator is used mainly in larger turbines.The one that is used mostly in small scale turbines is basedon the alternator type. This is also the generator type that is used inautomobiles. The difference here is that the magnetic field isexerted by the rotor while the electrical power is generated in thestator. This makes the rotor much lighter and enables powergeneration in much lower rotor speeds. The magnetic field can becreated by electromagnets which require a small electric fieldprovided to the shaft by brushes. Another option is the use ofpermanent magnets which eliminated the use of brushes. This isvery favorable for small scale wind turbines since it reducesmaintenance a lot. The rectification of the AC to the DC is done inthis case by diode bridges.Figure 14: a typical wind turbine: propeller, gearbox,synchronous generator, transformer connected to the grid(Jackson, 2009)23
4.SMALL-SCALE WIND POWERIntroductionThe reason for choosing small scale turbines is because boththe budget and site surface size don’t allow for large turbines. Thischapter reviews the small scale wind turbines that are in use todayall over the world. The different types are discussed as well as theiradvantages and disadvantages. In most cases, the turbinesthemselves are part of an integral system. Stand alone, grid tied orhybrid systems are integral systems that include wind turbines,batteries, inverters, other power generators etc.In order to be able to assess the wind conditions at a certainsite for small scale wind turbines, there are a few recommendationsat the end of this chapter along with a short market research reportfor South Africa.Definition and typesDefinitionWith small scale wind power we understand those systemsthat have a nominal electrical power below 50 kW. These aretypically turbines with rotor diameter from 1m to 3m and are usedin small local electrical networks. There are many types andapplications for these turbines and we will try to give an overviewof these in a generalized way with examples.Horizontal axis wind turbinesThe most well known windmill has a horizontal axis. Thesehori
history of wind energy and turbines is briefly described. After that, the topic of wind energy is described by starting from the definition of wind energy and its limitation by the Betz law. The latter influences directly the power curve and performance of a wind turbine. Then, the variability of wind energy is discussed because this is .
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