Keeping Solar Panels Secure on Unstable Ground

8/3/2019 Keeping Solar Panels Secure on Unstable Ground

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KeepingSolarPanelsSecureonUnstableGroundIntheworldofenergy,solarisgainingground.

Althoughsome

of

that

ground

may

not

be

as

Friendlyasthesolarindustrywouldlike.

Groundmounted solar power systems are being installed in commercial and residential

applications throughout the USA in increasing numbers. Whether the solar power system utilizes

Photovoltaic(PV)technologiessuchasfixedtilt,singleaxis,ordualaxistrackers,oraConcentratedSolar

Power(CSP)system,suchastheparabolictrough,there isonethingthateachofthesesystemshasin

common...theyareonlyasgoodastheirfoundation.Thesolarmodulesarethegreatestassetinanysolarpowerinstallation.Whilethecostofsolar

modulescanvarygreatlydependingon theirtype,quality,andsize,one thing iscertain, theyarenot

cheap.Solarmodulesalonecanaccountforbetween5060%ofthetotalsolarpowersystemcost. This

illustrateshowimportantitistomakesurethatthesolarmoduleshaveastable,securesupportsystem

and foundation regardlessof the type of installationmethod. When theonly thing protecting your

$30,000$40,000vehicle,nottomentionyourfamily,at70MPHisfourtires,theybetterbegood,high

qualitytires.Thatsameprincipleapplieswheninstallingsolarmodules.Protectthemostvaluableasset

byprovidingaquality,engineeredfoundation.

Whenitcomestosupportingsolarmodulesthereareseveraldifferentmethodsthatarebeing

usedinthesolarindustryincludingthefollowing:

Concrete

ballasted

footings

Drivenpiles Screwtypeearthauger Concretepiers Ballastmaterial(rocks)dumpedintoholdingtraysthatareattachedtothesolarmounting

structure.


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KNOWTHEENVIRONMENT

Choosing the right solar module support method involves understanding a variety of

environmentalfactorssuchassoilconditions,geographiclocation,andregionalwindspeedstonamea

few.Other importantfactorsthatneedtobeconsidered includesoilmechanicssuchasconsolidation,

permeability and seepage, bearing capacity, lateral earth pressure, slope stability, as well as wind

engineeringtodeterminepotentialforcesorloadsthatthesolarmoduleinstallationwillbesubjectto,

based on panel size, panel weight, geographic orientation, etc. Each of these factors needs to be

carefully considered for each and every solar system installation in order to minimize the risk of

downtimecausedbydamageorfailuretothesystemcomponentsandtheninturnmaximizetheROIor

returnoninvestmentofthesystem.

WithnorealSolarInstallationBestPracticeGuidelinesavailablefordesigners,manufacturers,

orinstallerstoreferenceorfollow,itfallstoeachindividualprojectpersonneltoensurethattheproper

environmentalandengineeringfactorsareconsideredduringthedesign,development,andinstallation

of the solar power system. This results in confusionwithin the solar industry as towhat minimum

requirements are andwhat installationmethod is best for a given application. Everyday there is a

likelihood that solar modules are being installed somewhere in the US without giving the proper

consideration to the environmental and engineering factors related to the project. This is a failure

waitingtohappen.Therightrainconditions,therightwindconditions,equalthewrongoutcome.

Whenemployingdrivenpiles,screwaugers,orconcretepiers,itisessentialtoconsiderallofthe

environmental factors involved to reduce the riskof holewideningor pole tiltingdue to ground

consolidation,panelrotationduetowindforces,soilsaturation,lateralearthpressure,etc.whichcan

directly affect the orientation and therefore the efficiency of the panels, andwhich can also cause

damage to the electrical connections below ground requiring costly repair. When utilizing precast

ballasted footings or ballasted trays it is imperative to understand wind loads, slope stability, soil

permeabilityandseepage,etc,toknowhowwindloadscancauseoverturningorsliding.

TheEnvironmentalProtectionAgency (EPA) isencouraging renewableenergy

developmentoncurrentandformerlycontaminated landandminingsites.TheEPAhas

identified thousands of properties that could potentially host solar,wind or biomass

energyproduction facilities.TheEPAused informationonproperties from several land

cleanupprograms, includingabandonedmine lands and landsunder EPAs Superfund,

Brownfields,andResourceConservationandRecoveryActprograms.

EnvironmentalProtectionAgency2009

President Obama and Congress are pushing to identify thousands of

contaminated landfillsandabandonedmines that couldbe repurposed tohousewind

farms,

solar

arrays

and

geothermal

power

plants.

WhiteHouse2009

Whilethedrivenpilemethodwastheoriginalpreferredmethodforinstallingsolarmodules,the

increasingrepurposeoflandfillsandcontaminatedsuperfundsitesashometosolarfarmshastheuseof

PrecastConcreteBallastedFootingsquicklygaininginpopularity. Landfillsandcontaminatedsuperfund

sites are ideal candidates for solar farms because they are considered already disturbed lands and


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thereby relieve thepressure todeveloponundisturbedoruncontaminated lands.However,manyof

thesesitesdonotpermitorallowgroundpenetrationforobviousreasons.Onceyougetpastthethree

feetof topcap soil,you reach thecontaminated soilbelow.This isoneareawhereprecastballasted

footingswinoutoverdrivenpilesbecausetheyprovideanonpenetratingsolution.Otherareaswhere

precast ballasted footings are finding success, is in solar installations going in over bedrock where

penetrationisdifficultifnotimpossible,solarinstallationswithhighwatertables,andinstallationswith

adversesoilconditionssuchascorrosivesoilsorsoilswithpoorpassiveearthpressurecharacteristics.In

additiontonotpenetratingtheground,precastballastedfootingsofferavarietyofotherbenefitssuch

as:

NosoilPenetration. Minimalsiteexcavation/preparationneeded. Speedofdeliveryandinstallation Eliminatestheneedforcastinplaceconcreteandallassociatedissuesincludingforming,

pouring,anddrytimewhichcandictatethepaceoftheinstallationprocess.

Accommodatesmostsitelocationsandconditions. Designperformanceisbasedonsolarassetweightwhichisknownprecisely. (onthe

contrarydrivenpilesrelyonassumedpassivesoilpressuresandotherassumptions)

If designed, andmanufactured properly, precast concrete ballasted footingswill outlast the

solar modules they support and should be viewed as a durable asset that in most cases can be

repurposedfromonedecommissionedsolarprojecttoanewsolarproject.Havingaclearunderstanding

of theenvironmentalandengineering factors involved inasolarprojectand thenmanufacturingand

installing the solarpower systembasedon those factors,will greatly increase the efficiency and life

expectancyoftheindividualcomponentsaswellasthesolarpowersystemasawhole.

POTENTIALPITFALLS&FAILURESTOCONSIDER

Whendesigningballastedconcretefootingsthereare3stabilityissuesthatmustbeconsidered:

Overturning(uplift)

Sliding

OvershadingOVERTURNING(UPLIFT)

Excessivewindloadforcescancauseoverturningmoments(whatiscommonlyreferredtoasuplift).

Figure1OverturningonBallastedFootingSystem Figure2OverturningonPostMountSystem


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Overturning on a precast ballasted system, FIGURE 1, results from a failure of the ballasted

footingtosuccessfullycounterexcessivewindloads.(Thismeansthefootingwasthewrongweightand

orsizefortheapplication.)

Overturning on a pile driven or postmount system, FIGURE 2, results from a failure of the

mountinghardware,rackingsystem,orpost/soilinteractionduetoexcessivewindloads.

HOWTOAVOIDOVERTURNING(UPLIFT)

Therearetwodesignoptionstoresisttheoverturningforces(uplift)causedbythewind. The

firstissimplybyaddingweightorwhatiscommonlyreferredtoasdeadload. Thisiscomprisedofthe

combinedweight

of

the

solar

electrical

system

(panels,

racking,

connections,

etc.)

plus

the

self

weight

of

theballastedfooting. Inadditionthereneedstobeenoughdeadloadtoallowforanadditionalsafety

factorthatmeetsorexceeds localbuildingcodestandards. Thesecondandmost importantoption is

howthedead load isdistributedgeometrically.Carefulthoughtandrigorousdesignprinciplesprovide

theexactlength,width,andthicknessoftheballastedfooting.

Theorientationoftheballastedfootingwithrespecttothesolarelectricalsystemalsoplaysa

keyroleindeterminingthesizeandgeometryofthefooting,whichhasadirectimpactonoverturning

potential. FIGURE 3 illustrates two different ballasted footing layouts for the same solar electrical

system. Althoughtheballasted footingsarethesameexactdimensionsandweighttheybehavevery

differentlyduetotheirorientation.

Figure

3

Varying

Ballasted

Footing

Orientation

Figure

4

Sliding

Of

Ballasted

Footings

SLIDING

Sliding is a potential failure that should be considered when designing ballasted footings.

Althoughnotascatastrophicasoverturning,slidingcanoccurdue to thewind induced forceson the

solar array system. Thewindnot only tries to push the SolarArray systemhorizontally, but it also

createsaliftwhichreducestheactualdeadloadoftheentiresystem,seeFIGURE4. Thisreduceddead

load coupledwith thehorizontalwind force iswhat creates thepossibilityofa solarmoduleand its

foundationslidinghorizontally. Taking intoconsiderationthetypeofSubgradematerialonwhichthe

ballastedfootingswillbeinstalledistheprimaryfactortoprovideresistancetosliding.Theidealtypeof

Subgradeisawelldrainedgranularcoursematerialsuchassand.

OVERSHADING

Anothercommonmistakeistoinstallrowsofsolarmodulestooclosetogetherwhichresultsin

overshading. Indrivenpileorpostmountapplicationsthiscanresult fromnot fullyunderstanding

theaffectthatachangeinheightcanhaveonthewayshadowsfallonthesolarsystemasawhole.With

precastballastedfootingsthiscanoccurasaresultofalteringthethicknessoftheballastedfootinginan


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attempttoaddweightwhenadditionalweightisrequiredduetoregionsofhigherwindspeeds. Thicker

ballastedfootingscouldincreasetheoverallheightofthesolarelectricalsystemwhichcouldrequirean

increaseindistancebetweenrowstoeliminatepossibleadjacentrowovershading,seeFigures5&6.

Figure5OvershadingonBallastedFootingSystem Figure6OvershadingonPostMount

System

HOWTOAVOIDOVERSHADING

Having precise knowledge of the path of the sun as well as a clear understanding of the

significantseasonal

47

degree

solar

elevation

angle

difference

above

the

horizon,

and

the

sunrise/sunset solar azimuth angle from summer to winter is essential to prevent mistakes when

positioningthesolarmodulesinasolarinstallation.Thiswillallowthecorrectheighttobecalculatedfor

eitheradrivenpileorballastedfootinginstallation.Whenadditionalweightisrequiredinthedesignof

aballastedfootingtoaccountforhigherwindswithinagivenregion,thedesigner/engineermusttake

intoaccountthepossibilityofaddingtheweighttothe lengthandorwidthoftheballastedfootingto

preventanunwantedincreaseinheight.OTHERFACTORSTOCONSIDER

BearingPressures:Once

the

footing

has

been

designed

(sized)

to

prevent

overturning

and

sliding

failure,

the

soil

bearing

pressuresshouldbecheckedtoensurethattheyare incompliancewithasoilsengineersreportthat

mayspecifymaximumbearingpressures.

ENGINEERINGANDDESIGNCONSIDERATIONSFORBALLASTEDFOOTINGS

One of the biggestmisconceptionswithin the solar industry in regards to precast concrete

ballastedfootings isthatthefootingsizeandtheunitcostaredirectlyrelatedtotheenergyoutputor

wattsofthesolarelectricalsystem.Thiscouldnotbefartherfromthetruth. Thefootingdesignshave

nothingtodowiththepoweroutputorpriceperwatt,andeverythingtodowiththefollowing:

Tiltangleandtrackingcharacteristicsofthesolarpowersystem. Localdesignwindspeedswherethesolarpowersystemistobeinstalled. Supportandrackingconfiguration. Overallsolarmodulesystemsizeandweight. Localdesigncodesandprojectrequirements. Soilcharacteristicsrelativetofriction,sliding,consolidation,slopestability,etc.


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TiltAngle:Tomaximizetheoutputofthesolarpowersystem,especiallyinPVSolarArrayapplications,the

optimaltiltangleistypicallyspecifiedfornontrackingsystems,andremainsfixed.

Structurally,highertiltanglesresultinanincreasedwindloadonthesolarmodulewhichwouldrequire

a largerballasted footing. Inaddition,higher tiltanglesmay requirean increase indistancebetween

rows to eliminate adjacentrowovershading, see FIGURE 5. A lower tilt angle isoftendesired to

minimize thewind forcesand reduce theballasted footing size. Thismaynotbe feasible inareasof

substantialsnowfallwhereanincreasedtiltanglemayberequiredtoshedsnowoffofthesolarmodule

effectively. Alowertiltanglealsoresults inreducedadjacentrowovershadingpotentialwhichallows

themodule

rows

to

be

spaced

closer

together.

A

cost

comparison

could

be

done

to

compare

the

tilt

angle versus the adjacentrow overshading versus the ballasted footing size. In addition, lower tilt

anglesallowformorecompactutilizationofthelandavailablebyminimizingtheunusableareathatisin

shadethusofferinganopportunityformoresolarmodules.

WindSpeed:The most significant impact on the ballastedfooting design is due to the local design wind

speed. The localdesignwindspeedcanbefoundfromtheInternationalBuildingCode, IBC,orfrom

the localbuildingdepartmentwherethesolarpowersystem istobe installed. It is imperativetoget

thecorrectdesignwindspeedas ithasadirect impacton the footingdimensions,and thereby the

economicsoftheproject. Theforceonthesolarpowersystemfromthewindisdirectlyrelatedtothe

windspeed

squared,

V2.

DesignCodes:Currently there are no design specifications or prescribed methodology that applies to the

designof foundations forsolarpowersystems. Dependingonwhichengineeryouask,youwillgeta

differentanswerbasedontheengineersacademicbackgroundandexperience,or lackthereof. The

mainproblemfacingdesignengineersofsolarpowersystemsishowwindloadsfactorintothefunction

andperformanceofthecomponents. TheACSEwindcodeisthemostapplicableandindustryapproved

standard. Thechallengeisidentifyingandinterpretingtherightpartofthecode,andthenapplyingitto

solarpowersystems.

CONCLUSION

At Oldcastle Precast, reliability is the cornerstone of everything we do and it is what

thousandsofsatisfiedcustomershavecometoexpect.Wehavetheequipmentandtheexpertiseto

handle even the most challenging jobs, from concept to install and every step in between. We

considerourselvestheleaderinthemarketplacewithregardstodeliveringafullyengineeredsystemto

accommodateanysolarpowerconfigurationandmeettheneedsoftheenduser.

The leadershipofOldcastlePrecasthas completedanR&D initiative related to thedesignof

riskmitigatedsolarfoundationproducts.Thisprojecthasculminatedinthedevelopmentofproprietary

softwarethatallowsustoprovideourcustomerswithproductsthathavebeenengineeredtoperform

atthehighestlevelsintheindustry.

OldcastlePrecastwillassist insmartdesignsmaking installationandfabricationatthejobsite

easierandfaster. Wewillprovideyouwithaninstallationmanualandbeatthejobsiteondayoneto

insureasuccessful

project.

We

have

ateam

devoted

to

supplying

you

with

awide

range

of

services

and

products.

At Oldcastle Precast we are committed to delivering the reliability that hasmade us the

IndustryLeader.


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SourceInformation

TheEnvironmentalProtectionAgency(EPA)2009The Environmental ProtectionAgency (EPA) isencouraging renewableenergy development on current and formerly

contaminated land andmining sites. The EPA has identified thousands of properties that could potentially host solar,wind or

biomass energy production facilities. The EPA used information on properties from several land cleanup programs, including

abandonedminelandsandlandsunderEPAsSuperfund,Brownfields,andResourceConservationandRecoveryActprograms.

http://www.epa.gov/renewableenergyland/

WhiteHouse009EnvironmentalStatementPresidentObamaandCongressarepushingtoidentifythousandsofcontaminatedlandfillsandabandonedminesthatcouldberepurposedto

housewindfarms,solararraysandgeothermalpowerplants.

http://wwwp.dailyclimate.org/tdcnewsroom/2009/10/greenshootsfrombrownfields

LarryJ.Miller,P.E.|JacksonBishop|OldcastlePrecast,Inc.,7921SouthParkPlaza,Suite200,Littleton,CO|8889653227|

www.Oldcastleprecast.com/energy

October2009V1

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Keeping Solar Panels Secure on Unstable Ground