aerogenerator sitting in complex terrains

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RESOLVING DIFFICULT ISSUES OF WIND POWER MICROSITING IN COMPLEXTERRAIN

SESSION 11BRESOURCE ASSESSMENT AND MICROSITING STRATEGIES AND EXPERIENCE

WEDNESDAY, MARCH 31, 2004

1:45 P.M. - 3:15 P.M.

RUSSELL G. DERICKSON A MICHAEL McDIARMID B

BRAD C. COCHRAN C JON A. PETERKA D

A,C ASSOCIATE, CPP INC., FORT COLLINS, COLORADO 80524 USAB MECHANICAL ENGINEER, STATE OF NEW MEXICO, SANTA FE, NEW MEXICO 87505D VICE PRESIDENT, CPP INC., FORT COLLINS, COLORADO 80524 USA


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AWEA Global WINDPOWER 2004 Conference -- March 28-31, 2004 -- Chicago, Illinois

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Resolving Difficult Issues of Wind Power Micrositing in Complex Terrain

Russell G. Derickson a Michael McDiarmid b

Brad C. Cochran c Jon A. Peterka d

a,c Associate, CPP Inc., Fort Collins, Colorado 80524 USAb Mechanical Engineer, State of New Mexico, Santa Fe, New Mexico 87505 USA d Vice President, CPP Inc., Fort Collins, Colorado 80524 USA

ABSTRACTMicrositing of wind turbines in complex terrain istricky game, and many of the current sitingmethods and tools, while useful and improving,remain inadequate to the task in extreme terrain.As a consequence, there are numerous windturbine installations that are buffeted by damagingturbulence or are faced with suboptimal windenergy performance.

Flow separation at mountain peaks and the leadingedge of cliffs and escarpments is a primary culpritthat can lead to poor performance or damagingturbulence. Many practitioners poorly understandthese phenomena, but the blame can also be laid atthe feet of assessment tools that are not suitablefor complex, separating flow conditions.

Terrain geometries can do strange things to flow

patterns. Geophysical phenomena such as thermalstratification and Earths rotation can add to thecomplexity. Thus, mountain peaks, terrain saddle

points, steep escarpments, and other intricateterrain shapes can play havoc with flowconditions.

A combined approach utilizing wind tunneltesting, a refined mesoscale numerical model,and effective field testing is shown to be aviable tool for the accurate micrositing of windturbines in these complex terrain settings.

INTRODUCTIONMicrositing for wind power in complex terrainremains a formidable task subject to potentiallysignificant error. Two effects may be at play:misconceptions about basic wind phenomena, andassessment tools that degrade in extreme terrain.Consequently, many commercial projects havefallen victim to suboptimal siting that undermines

wind power output or introduces severe, damagingturbulence. Three scenarios leading to poor

performance are explored:

1) It may not be a good strategy to place turbinesnear the leading edge of steep escarpments ormesas, where flow separation occurs,turbulence is high, a large vertical windcomponent exists, and turbines encounterextreme wind shear. Several commercial

projects have faced these unfavorableconditions. Other locations on a mesa maynot be appropriate, either, depending onterrain character and atmospheric thermalstability.

2) Assessing high points or ridge tops for wind power siting requires analysis of anyupstream terrain features that may produce

flow separation regions that encompass thedownwind sites, undermining performance.Subtle, hard-to-quantify effects can occur.Thus, initial qualitative site assessmentsutilizing simple rule-of-thumb methods forcharacterizing a site may inadvertentlydisqualify a good site or accept a bad one. Itappears that many numerical simulation toolsin existence may not accurately account forthe downwind influence of flow separation,either. Lastly, it may be difficult or expensiveto fully account for disruptive upstream

separation effects with field measurementsalone.

3) Flow in complex terrain varies remarkably,depending on daily and seasonal variations inthe thermal stability of flowing air masses.Consequently, peaks and ridges may be goodcandidates under some thermal conditions butnot others. In some cases, locations within


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valleys or at their openings to plains may beideal sites. It is important to assess an area forseasonal variations in thermal stratification inchoosing a site for overall wind powerviability.

A hybrid tool, incorporating mesoscale numericalmodeling, boundary-layer wind tunnel testing, andfield measurements, provides a powerful meansfor improving micrositing in the complex terrainscenarios described above.

FLOW SEPARATIONFig. 1 displays the multifaceted and complicated

phenomena of flow separation and reattachmentover mountainous terrain. The picture is of flowvisualization of a scale model in a boundary-layerwind tunnel. Flow separates within a range ofvarious points either upwind or downwind of the

peak, depending on precise terrain geometry andupwind flow turbulence. Reattachment occurs atsome point on the lee side of the peak. The

processes of separation and reattachment arehighly transient and erratic (and flip-flop innature), as is the character of turbulent flow ingeneral. At the leading edge of an escarpment,flow separation and reattachment are similar, yetdistinct from that over a peak. Exact flow behavioris a critical function of detailed peak orescarpment features, and it is difficult togeneralize flow response. Thus, each case isunique. Each candidate site therefore would haveto be explored independently. Fig. 2 shows asimplified schematic of flow over two types ofsteep escarpment.

DESCRIPTION OF MICROSITING TOOLS

A hybrid tool incorporating boundary layer windtunnel testing and mesoscale modeling (i.e., anumerical simulation software program foratmospheric analysis and study) for micrositing in

flow reattachmenton lee of slope

(no recirculation)1


(no recirculation)


(no recirculation)


(no recirculation)1

reverse flow inseparation region

(recirculation)

Wind

flow separationoccurs at peak

1

reverse flow inseparation region

(recirculation)

Wind

flow separationoccurs at peak

1

Fig.1. Capturing the transient nature of flow separation and reattachment in the wind tunnel ata peak. The peak corresponds to Location 1 in the

scale model.

Fig. 2. Schematic of two types of steep escarpment showing flow separation recirculation bubbles and

low reattachment.


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complex terrain was detailed in Derickson andPeterka [1]. The potential pitfalls and complexitiesof field measurements were also presented in thatreference. In this paper, we summarize key pointsof our previous efforts and move on to newinformation based in more recent wind tunnelresults with escarpments of various leading edgeslopes.

Boundary-Layer Wind Tunnels

A boundary layer wind tunnel is well suited tocapture the physics of flow separation andreattachment around bluff bodies of extremegeometry. These include highly articulated terrainfeatures such as cliffs and escarpments, as well asthe intricate building shapes and arrays of

buildings found in city environments. Currentnumerical models cannot deal accurately with citygeometries and are limited in the level ofcomplexity treated accurately in complex terrain.On the other hand, wind tunnels are limited incertain geophysical realities such as thermallystratified atmospheric conditions and effects ofEarths rotation. Numerical models can addressthose geophysical effects. Hence, the combineduse of the wind tunnel and a comprehensivemesoscale model represents an impressive tool.

A pictorial of two types of wind tunnel are shown

in Fig. 3. Scale physical models of terrain are placed in the tunnel and tested with flows thatcorrectly simulate the characteristics of theatmospherics boundary layer (i.e., the mean windstructure and turbulence characteristics that varywith elevation as influenced by surrounding terrainfeatures and various geophysical forcingmechanisms. Fig. 1 suggests the power of the windtunnel to simulate these phenomena and capturethe essential character of transient flow behavior.

Atmospheric Numerical Model

In our numerical simulations of flow over complexterrain, we employ the Advanced RegionalPrediction System (ARPS), which was developedat the Center for the Analysis and Prediction ofStorms (CAPS) at the University of Oklahoma [2,3,4]. ARPS is a comprehensive, multi-scalemodel, originally developed for the mesoscale(i.e., intermediate scale), particularly to forecastsevere storms. It can simulate synoptic scale (i.e.,

large scale) atmospheric forcing and allintermediate scales down to the micro-scale (i.e.,small scale) through a process of nesting thenumerical grids.

Our efforts with ARPS entailed furtherdevelopments and software refinements that

enhanced its ability to simulate flow separation incomplex terrain. The process of refinement wasenabled by synergistic applications with the windtunnel. Further explanation of this procedure andthe hybridization of the wind tunnel and ARPS are

presented in Derickson and Peterka [1].

Field MeasurementsUltimately, field site measurements andcomprehensive atmospheric observations areindispensable for micrositing and for validation ofmicrositing tools such as a wind tunnel, a

mesoscale numerical model, or their hybrid use.However, field measurements are limited byexpense, time, and scale. Most critically, the vastvariability of nature and its myriad of interactive

parameters (weather systems, terrain and surfacefeatures, solar insolation, thermal stratification,Coriolis forces, etc.) create a formidableimpediment to the effective comparison ofnumerical modeling output or wind tunnel results

Fig. 3. Pictorial of boundary layer wind

Open-Circuit Wind Tunnel


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to field data. Correlation between neighboringmeasurements and determination of actual speed-up effects of terrain sites can break down,depending on diurnal meteorological conditions asdiscussed by Meroney [5]. Hence, ultimatevalidation is a tricky, elusive process.

For example, uncertainties and seasonal variationsin surface roughness produce equal uncertaintyand variation in wind speeds, particularly in theatmospheric surface layer where wind turbines arelocated. It is difficult to quantify a precise valuefor z o in the field at a specific location, let alone itsspatial variation for upwind fetches that influenceoverall flow characteristics. If surface roughnesschanges with season as vegetation either grows orchanges by various processes, the task ofidentifying proper values for z o becomesincreasingly convoluted. The consequences onwind power assessment are profound for turbinesnear the surface and not insignificant for largerscale devices with higher hub heights.

We recognize, however, the crucial need forcomparisons to field measurements as a final stepin the validation process, and have planned for thatactivity. However, we emphasize that anycomparative study between ARPS (or anynumerical model) and the field is replete withcomplex issues that may be difficult to resolve.

FLOW OVER LEADING EDGE OF ANESCARPMENT

Recent wind tunnel studies have complementedour previous explorations [1] of flow overescarpments and mountainous terrain in general.Due to unanswered questions from the previousstudy, we chose to look in greater spatial detail atflow in the vicinity of the leading edge of anescarpment. These questions included uncertaintyin the magnitudes of vertical wind componentsnear the leading edge. We used a refined

measurement devise, a 5-hole probe (brieflydescribed in [1]) to help answer our questions.

Fig. 4. is a schematic of a scale model of anescarpment used in our wind tunnel study. Wefocused on the region between Locations 1 & 2 inthe figure (shown as a detail), and measured bothhorizontal and vertical wind velocity components.

Our previous study [1] examined Locations 1-7,looking at horizontal wind speed only andturbulent gusts.

Results are presented in Figs. 5-10. With steepapproach slopes (60 and 90 degrees), the verticalvelocity component remains large compared to thehorizontal component for distances up to 20 to 30meters from the edge. Even at 40 meters, thevertical component is large. This would inducenegative consequences on the aerodynamics of awind turbine experiencing this condition. Notice

also that the point of maximum vertical windspeed is displaced upward as the distance from theedge increases. At 30 degree and 45 degree slopes,the vertical wind component is much less.

At Location 2, the vertical wind speed hasdecreased substantially for the 60 and 90 degreecases, but the horizontal wind speed manifests alarge vertical gradient that would induce strongasymmetrical loading on a wind turbine. Thus forsteep escarpment slopes, one is faced with eitherlarge upward incursions of wind flow or strongasymmetrical turbine loading, depending ondistance from the edge.

In cases with shallow slopes, there is horizontalwind speed-up relative to an approach flowupwind of the escarpment, as shown in the figures.With the steep slopes, the situation is morecomplicated with regard to speed-up at variousheights.

Fig. 4. Schematic of escarpment with windward slopes of 30, 45, 60, and 90. Wind profiles areindicated at Locations 1-7. In this study, weexplore the details between locations 1 & 2, asdepicted in the subsequent figures.

wind1 3 4 5 6 72

3600 m

190 m

300 m


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The lesson is clear, locations in the vicinity of theleading edge of a steep escarpment presentundesirable wind conditions for essentially all hubheights. On the other hand, escarpments withshallow slopes that produce little or no flowseparation are excellent candidates for wind powersiting. Thus, a solid recommendation would be to

select those sites for turbine siting and seriouslyconsider avoiding locations that produce highlevels of flow separation.

Wind Components: Location 1 (leading edge)

0

50

100

150

200

0 2 4 6 8 10 12 14 16

Wind Speed, m/s

E l e v a

t i o n , m

30 045 045 0

60 0

60 0

90 0

90 030 0

vertical wind horizontal wind

approachprofile

1 2

wind

Fig. 5. Flow parameters at Location 1: at leading edge of escarpment.

Wind Components: Location A (10m from edge )

0

50

100

150

200

0 2 4 6 8 10 12 14 16

Wind Speed, m/s

E l e v a

t i o n , m

30 045 0

45 0

60 0 60 0

90 0

90 030 0


approachprofile

1 2

windA

Fig.6. Flow parameters at Location A: 10 m downwind of escarpment leading edge.


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Wind Components: Location B (20m from edge)

0

50

100

150

200

0 2 4 6 8 10 12 14 16

Wind Speed, m/s

E l e v a

t i o n , m

30 045 0

45 0

60 0

60 0

90 0 90 030 0


approachprofile

1 2

wind B

Fig.7. Flow parameters at Location B: 20 m downwind of escarpment leading edge.

Wind Components: Location C (30m from edge)

0

50

100

150

200

-2 0 2 4 6 8 10 12 14 16

Wind Speed, m/s

E l e v a

t i o n , m

30 0

45 0

45 0 60 0

60 0

90 0

90 030 0


approachprofile

1 2

windC

Fig. 8. Flow parameters at Location C: 30 m downwind of escarpment leading edge.


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Wind Components: Location D (40m from edge)

0

50

100

150

200

-2 0 2 4 6 8 10 12 14 16

Wind Speed, m/s

E l e v a

t i o n , m

30 045 0

45 0 90 0

90 030 0


note : no 60 0

caseapproachprofile

1 2

windD

Fig.9. Flow parameters at Location D: 40 m downwind of escarpment leading edge.

Wind Components: Location 2 (190m from edge)

0

50

100

150

200

-2 0 2 4 6 8 10 12 14 16

Wind Speed, m/s

E l e v a

t i o n , m

30 045 0

45 090

0

90 0

30 0vertical wind horizontal wind

60 0

60 0

approachprofile 1 2

wind

Fig. 10. Flow parameters at Location 2: 190 m downwind of escarpment leading edge.


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In our previous study [1], we explored wind behavior on the full extent of the top ofescarpments of varying approach slopes. Thatstudy revealed the high levels of turbulence nearthe leading edge of steep escarpments. It alsoshowed that the lee regions of escarpmentsmight induce a slight speedup, if the approachslope is not steep. This has been confirmed by awind tunnel study performed by Bowen andLindley [6].

Figs. 11 & 12 are pictures of installed windturbines in the vicinity of an escarpment edge.Without knowing the details of the site or anyanalysis that was performed in site selection, it isuncertain as to what sort of total windenvironment these turbines face. However, theresults and discussion presented above indicatethe possibility that the turbines may be subjectedto adverse wind conditions.

MULTIPLE PEAKS AND VALLEYS INMOUNTAINOUS TERRAIN

A scenario of mountainous terrain with multiple

peaks and valleys is shown in Fig. 13. This isactually a scale model that was employed in aseries of wind tunnel studies (Fig. 1 shows anisolated peak, denoted as Location 1, of theentire model.). For a given wind condition, any

peak can be either upwind or downwind ofanother peak, complicating the matter of which

peak influences other downwind peaks orvalleys. A detailed study would be necessary to

Fig. 11. Wind turbines near escarpment edge.

Fig 12. Wind turbines near an escarpmentedge. (Photo by Michael McDiarmid).

1

3

2

1

3

2

Fig.13. Scale model of mountainous terrain(Lantau Island) with multiple peaks andvalleys. For wind tunnel study.


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evaluate specific peak sites or valley locationsfor effective wind power utilization. This wouldrequire knowledge of the regional windclimatology and all other relevantmeteorological conditions. Many existingmicrositing tools can address certain of theissues surrounding such a siting evaluation, butmost tools would be inadequate in exploring theeffects of flow separation and reattachment inthe regions with extreme terrain geometry.

SUMMARY AND CONCLUSIONSMicrositing in complex terrain is a tricky affair,

particularly in various escarpment scenarios andmountainous areas with multiple peaks andintervening valleys. The ability to accuratelyassess the effects of flow separation andreattachment is crucial to the task. However,most current tools are inadequate when it comesto extreme, detailed terrain geometry. Fieldmeasurements, while crucial, are replete withtheir own limitations and difficulties. Thecombined use of wind tunnel testing andmesoscale numerical modeling represents a

powerful hybrid tool for wind power siteassessment in highly complex terrain.

REFERENCES1] Derickson, R.G., and Peterka. Jon A. (2004),Development of a Powerful Hybrid Tool for

Evaluating Wind Power in Complex Terrain:Atmospheric Numerical Models and WindTunnels, Proceedings of the 23 rd ASME Wind

Energy Symposium , Reno, Nevada (in press).

2] Xue, M., Wang, D., Gao J., Brewster, K, andDroegemeir, K. K., 2003, The AdvancedRegional Prediction System (ARPS), Storm-Scale Numerical Weather Prediction and DataAssimilation, Meteorol. Atmos. Phys ., 82 , 139-170.

[3] Xue, M., Droegemeir, K. K., and Wong, V.,2000, The Advanced Regional PredictionSystem (ARPS)-A Multi-Scale NonhydrostaticAtmospheric Simulation and Prediction Model.Part I: Model Dynamics and Verification,

Meteorol. Atmos. Phys ., 75 , 161-193.

[4] Xue, M., Droegemeir, K. K., and Wong, V.,Shapiro, A., Brewster, K., Carr, F., Weber, D.,

Liu, Y., and Wang, D., 2001, The AdvancedRegional Prediction System (ARPS)-A Multi-Scale Nonhydrostatic Atmospheric Simulationand Prediction Model. Part II: Model Physicsand Applications, Meteorol. Atmos. Phys ., 76 ,143-165.

[5] Meroney, R.N., 1990, "Fluid Dynamics ofFlow over Hills/Mountains Insights Obtainedthrough Physical Modeling", Chapter 7 in

Atmospheric Processes Over Complex Terrain, ed. William Blumen, American MeteorologicalSociety, 145-171.

[6] Bowen. A.J. and Lindley, D., 1976, AWind-Tunnel Investigation of the Wind Speedand Turbulence Characteristics Close to theGround Over Various Escarpment Shapes,

Boundary-Layer Meteorology , 12 , 259-271.

ACKNOWLEDGEMENTSWe wish to extend our appreciation to the CPPcrew responsible for wind tunnel testing, datareduction, and computer administration.Particular thanks go to Bill Roeder, Aki Hosoya,Morgan Downing, Steve Mike, KurtFleckenstein, Mike Schoonover, AdrianManzanares, Brian James, Tim Vice, and MattPetersen. We also thank our colleagues Daryl

Boggs, Leighton Cochran, Gary Larrew, andDave Banks for their helpful discussions andtechnical insights during the study leading to this

paper.

aerogenerator sitting in complex terrains

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