by bruce shafer

1077-2618/10/$26.00©2010 IEEE

B Y B R U C E S H A F E R

A case study of electrical wind generationfor cement manufacturing

GENERAL BACKGROUND INFORMA-

tion on wind generation and factors leading

to the genesis of the Mojave plant wind

project is presented here. The case study

reviews the project scope; technical, physical, and regula-

tory challenges; technical innovations; business model

innovation; and the positive impacts of the project.

Finally, it provides a primer for those considering similar

on-site renewable energy projects.

The Modern Wind Turbine

The evolution of the modern wind turbine is a story of

innovation through the application of engineering and

scientific ingenuity. The concept of a wind-driven rotor is

ancient, with the traditional Dutch windmill (Figure 1)

having been popularly envisioned as a historical anchor toDigital Object Identifier 10.1109/MIAS.2010.936116

© DIGITAL VISION

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Authorized licensed use limited to: Univesity of Witswatersrand. Downloaded on July 20,2010 at 12:03:52 UTC from IEEE Xplore. Restrictions apply.

the more modern development ofwind turbines. This technology hadproliferated to the extent of about100,000 machines throughout Europeat their peak usage in the late 19thcentury. These machines predated elec-trical generation technology and wereused for grinding grain. The early use ofwind for water pumping also became acommon application as seen across theplain states of America.

In contrast, the function of a mod-ern power-generating wind turbineis to generate high-quality, networkfrequency electricity. Each wind tur-bine must function as an automati-cally controlled independent powerstation. A modern wind turbine is required to workautonomously, with low maintenance, continuously inexcess of 20 years. In the last 20 years, turbines haveincreased in size by a factor of 100 (from 30 to 3,000kW), and the cost of energy has been reduced by a factorof more than five. Figure 2 provides an end-stop imageto frame the dramatic evolution of wind turbinetechnology. It depicts the school-bus-sized nacelle (struc-ture that houses all of the generating components) of the3,000 kW turbine chosen for the project.

Figure 3 provides another illustration of the dramaticscaling resulting from increased turbine capacity by pre-senting the relationship between swept rotor diameter andkilo rating.

The recent focus on renewable energy development inresponse to climate change has helped advance windturbine technology in the areas of efficiency, scale, and

cost. These advances have catapultedthe industry in the last ten years, afterhitting a plateau subsequent to initialgrowth in the early 1980s (Figure 4).

Background—Mojave PlantWind ProjectThe genesis for the eventual Mojaveplant wind project was kindled duringthe California electrical grid crisis of2001. The fallout from the crisis includednumerous electrical service interruptions,skyrocketing electricity prices, and long-term stranded expenses deemed as exitfees for many large electrical consumerswho chose to remain in the crippledderegulated power market. The exit fees

covered the long-term power contracts the state entered into toresolve the crisis.

This period was reminiscent of the energy crisis of the1970s, to which a number of cement manufacturersresponded with another form of on-site electrical genera-tion: waste heat boilers on long dry kilns. In the present

90 m

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1,000kW

2,000kW

3,000kW

3Swept rotor diameter relationship with capacity.

1Dutch windmill. (Photo courtesy of Wikimedia Commons.)

2Nacelle cut-away view. (Photo courtesy of WikimediaCommons.)

THE LACK OF NETMETERING ANDTHE ABILITY TO

EXPORTCONSTRAINEDTHE PROJECT

SCALE.

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case, the response with a wind genera-tion project was influenced primarilyby natural synergies.

The first and foremost natural asso-ciation with wind relates to the loca-tion of the Mojave plant. The plant islocated in the heart of the most devel-oped wind resource area of California.The Tehachapi mountain pass, locatedvery near the plant, acts as a funnel forthe movement of cool onshore coastalair moving east out to the Mojavedesert where hot air rises, creating anatural pull of cooler air across thepass. This reliable wind resource hasresulted in a mature wind generationindustry in the area, with roots goingback to the early 1980s. This base ofexisting wind operations providedincentives beyond a basic wind re-source for wind development at thecement plant. Local turbine manufacturers, wind develop-ers, wind project constructors, and operation and mainte-nance groups provided ready and economical talent for thecement plant project. Specifically, a relationship wasestablished with a wind developer who happened to haveexisting operations physically adjacent to the cementplant property. This relationship, the natural resource, andthe established industrial base created synergy for pursu-ing the project.

Public Policy and Regulatory IssuesPublic policy providing tax incentives for wind powerdevelopment has played a role in the industry for decades.These federal subsidies in the form of production tax cred-its (PTCs) were critical for the industry to grow (or not),as seen in Figure 4. PTCs are based on the actual produc-tion of a project, cannot be enjoyed by the consumer of thepower, and typically flow to an equity investor in theproject with an appetite for tax credits.

Public policy providing tax incentives for wind powerdevelopment has played a role in the industry for decades.

More recently, public utilities have implemented incen-tives for renewable energy projects. However, to date, theseprograms focus on small projects typically constrained in

capacity to less than 1–5 MW and tendto be slanted toward small consumers.Often, the utility allows for net meter-ing that is particularly applicable towind and solar renewable projects. Netmetering is the sum of all energyproduction minus energy consumedand allows for the energy to flow inboth directions through the meter,providing the generator the full creditfor all power produced.

In the absence of a net meteringarrangement, a potential solution foraddressing intermittent surplus gener-ation is to sell the power. This typi-cally requires a deregulated powermarket, governmental approvals, util-ity agreements, and a power marketer.Conventional wind projects are mostoften built with the power flowingdirectly to the utility through a power

purchase agreement (PPA) and thus avoid much of thecomplexity of a behind-the-meter project (behind themeter is defined as an on-site generation that is connecteddirectly to the plant electrical infrastructure and does notflow through the electrical distribution provider’s reve-nue meter).

Additional considerations regarding self-generationand utility are standby charges and departing loadcharges. Particularly, intermittent generation relies uponthe utility for electrical supply when the resource (windor sun) is not present. The utility must be compensatedfor the transmission line capacity and generation capacityeven if it is not used all the time. Likewise, the utilitymay require ongoing compensation for the generatingand/or transmission investment it has made to serve theplant load, which has now departed from the grid due toself-generation.

Unfortunately, net metering was not allowed in thecase at hand, because the requirements of the utility arenot met in a large scale. Additionally, in this particularcase, the utility’s electrical transmission capacity is con-strained because of the acceleration of new wind genera-tion coming online. This blocked the option to sell anyexcess power. In fact, the project is prohibited fromexporting power onto the grid (a technical aspectexplored further below). However, once a planned trans-mission upgrade to facilitate further wind developmentin the area is completed, selling excess power will bepossible, albeit with the complex requirements fortransacting into a deregulated market and its associ-ated requirements.

Project ScalingThe lack of net metering and the ability to export con-strained the project scale. A discussion of the nature of thecement plant load and the profile of the wind generation isimportant in understanding the scaling evaluation.

Electrical loads are often characterized by calculating acapacity factor that is simply the average load over aperiod of time expressed as a percentage. From the pointof view of the utility, the electrical load of a cement plant

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Historical growth of wind generation in the United States.

THE EVOLUTIONOF THE MODERN

WIND TURBINE IS ASTORY OF

INNOVATIONTHROUGH THE

APPLICATION OFENGINEERING

AND SCIENTIFICINGENUITY.

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has a relatively high capacity factordue to a large 24/7 base load. Indeed,a cement plant capacity factor ismuch higher than commercial orresidential loads if viewed on amonthly or yearly basis. However, acement plant is also characterized byintermittent operations. Extendedoverhaul periods of the pyroprocess-ing unit result in a lengthy andsubstantial load reduction and canoccur unplanned. Additionally, rawgrinding operations with a verticalroller mill are characterized by peri-odic maintenance. Finally, finishedgrinding operations tend to havegreater productive capacity becauseof the seasonally cyclical nature ofthe industry. A high-capacity finishoperation is more base loaded inpeak months, but in nonpeak months is more intermit-tently constrained by inventory and more intense main-tenance activities.

Electricity generated from wind power is also variableat several different time scales: from hour to hour, daily,and seasonally. Annual variation also exists but is not assignificant. Typical capacity factors are 20–40%, withvalues at the upper end of the range in a particularlyfavorable site. The capacity factor is important in deter-mining the economic feasibility of a wind project. Giventhe large initial capital outlay (sitework, foundations, towers, tur-bines), relatively low variable ex-pense of operation (no fuel), andmaintenance (reliable generator andgearbox), the simplistic economicviability of a project is based on theinvestment required for a givennameplate capacity and the capacityfactor of the wind resource. Statedanother way, a wind site with a 20%capacity factor will have double theunit cost of a site with a 40%capacity factor as it would requirethe same investment, but the highercapacity factor site would delivertwice as much power.

The Mojave site has a very favor-able capacity factor of 35%. How-ever, the intermittent nature ofboth the cement plant load andwind generation required a de-tailed analysis to determine theoptimum project scale. Figure 5compares the cement plant loadand wind generation (based on acapacity of 24 MW) on a monthlybasis for a calendar year. The sea-sonal nature of this wind site is evi-dent, with 50% of annual powerbeing generated in the months ofApril through July, which have

monthly capacity factors rangingfrom 50 to 60%.

Clearly on a monthly basis, theplant load is substantially greaterthan the generation. However, as dis-cussed earlier, the intermittency isbetter seen on a shorter time scale.Comparing historical hourly load andhourly forecasted generation data overan entire year demonstrated the amountof time in which generation would begreater than plant load. Given theprohibition of exporting power, thispower would be lost. The project evalu-ation team defined this condition ascurtailment. To analyze the amount ofcurtailment for a range of capacities, amodel was developed to compare thehourly data for load and generation.Figure 6 shows the relationship be-

tween the wind project scale and the anticipated curtail-ment. It should be noted that since historical load data wasused for the model it represented no load shifting by thecement plant to track wind generation (this is discussedlater in the area of innovation).

The nonlinear function provides an optimum at theknee of the curve, where the greatest capacity can beinstalled without transitioning to a steep relationshipwith curtailment. This analysis provided the basis forestablishing the project with a capacity of 24 MW [often,

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Monthly historical plant loading and project wind generation.

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PUBLIC POLICYPROVIDING TAXINCENTIVES FOR

WIND POWERDEVELOPMENTHAS PLAYED A

ROLE IN THEINDUSTRY FOR

DECADES.

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as in this case with 3-MW turbines, theunit size of the turbine plays into thespecific capacity of a project as it mustbe a multiple (8) of the turbine size].

Technical InnovationBecause of the unconventional behind-the-meter configuration, utility non-export requirements, and plant-spe-cific factors, the efforts of the developerresulted in a number of technicalinnovations to achieve a success-ful project.

Typically, the utility meter is onthe primary or secondary side of theutility’s transformer(s), feeding themain switchgear for a load system. Inthe case at hand, two 66-kV/4-kVtransformers supply the plant load.The metering is on the 4-kV secondarysides of the transformers prior to themain plant switchgear. The plantoperates with two independent 4-kV circuits. Multipleutility circuits are common for redundancy or sizing con-straints and must always be considered when generationsources are added.

It was determined that the wind turbines must be con-figured to feed two generation circuits, mirroring the twoutility circuits. This compounded the challenge of avoid-ing export and minimizing curtailment.

The basic infrastructure included a dual transmissionline from an array of eight turbines, dual switchgear tie-ins to the 4-kV system, and an innovative set of relays onthe collection grid of the turbines to switch generatorsbetween circuits. This switching allows for balancingthe number of turbines on each circuit to match theload conditions.

This coarse load/generation matching is not suffi-cient in controlling the generation to avoid an exportcondition, short of a binary system of turning turbineson and off. This would result in excessive curtailment.Modern wind turbines are designed with variable pitchblades that control the wind force input to the machineand thus govern the power generated. Although pitch-ing is conventional, the active load tracking demandedby this project required a sophisticated control systemand coordination with the turbine manufacturer to con-figure the turbines for a quick response that could fullyfeather out the blades and thus load in less than 30 s.The utility provided a challenging reverse power relayspecification that would trip the generation circuit if abuffer of 600 kW (utility still delivering 600 kW) wasviolated for more than 30 s or an export of 10 MW formore than 2 s.

The switching capability of the turbine array and fine-tuned control system have avoided nuisance trips andoptimized the amount of generation. Another innovationof this unique project was in the area of communication.The day-to-day operation of a cement plant presentsmany challenges, such as planned and unplanned mainte-nance activities, production upsets, production schedul-ing for inventory control, and kiln overhauls. All of these

impact plant electrical load. Thewind resource is uncontrollable: it iswhat it is. However, a very helpfultool is to utilize meteorological datato predict it. Given the challenge ofvariable load and generation, it wasdetermined that heightened com-munication could optimize evenfurther beyond the engineered con-trol system.

A communication protocol wasestablished between the wind projectoperations and the cement plant oper-ations. To minimize curtailment, theoperator of the wind project provideshourly, daily, and weekly forecasts ofthe generation. This provides theplant production and maintenancemanagers with a framework to sched-ule activities. For example, if the rawmill is scheduled for routine mainte-nance on Wednesday in the following

week and a wind forecast predicts that that particular daywould require the extra load due to high wind, then, per-haps the maintenance schedule would be shifted to alower wind day. Likewise, on a longer-term basis, infor-mation is exchanged between the plant and the projectfor major maintenance plans, both for wind turbinerepairs as well as significant maintenance activities in thecement plant. Finally, periodic meetings are held toimprove communication protocol and review how theproject is performing.

Business Model InnovationThe typical business model for a cement facility to procureelectricity (one of its largest expense items) is to simplybuy it from the utility that services the plant. Acceptingthe prevailing utility rate is a sound business approach, asit can be fair to assume that within a limited geographicalarea the competition will pay the same rate. The industryhas also soundly focused on energy efficiency through bestpractices and capital investment in more efficient technol-ogy. With regard to self-generation as a business strategy,the industry has infrequently pursued cogeneration throughtraditional steam cycle systems or more recently throughlow-temperature Rankine cycles. Beyond a 10.2-MW windproject (behind the meter) at a cement plant in Moroccocompleted in 2004 and a large wind PPA (250 MW off site,wheeled to multiple locations) executed by a Mexicancement company, the Mojave project is the only other local-ized wind generation project in the industry.

The business case for this specific wind project consid-ered the following factors: leveraging an on-site naturalwind resource, not getting in the wind business by limit-ing the business relationship to a long-term PPA (25years), letting others make the investment, and lettingothers operate and maintain the facility. There is obviousrisk in fixing the procurement of power in the form ofon-site generation. The structure of the business modelmitigated this risk to a great extent.

The overriding business strategy of the project is one ofhedging. Tapping another form of electricity supply by

THE FUNCTION OFA MODERN

POWER-GENERATING

WIND TURBINE ISTO GENERATE

HIGH-QUALITY,NETWORK

FREQUENCYELECTRICITY.

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means of different risk variables cre-ates a window of opportunity in acompetitive environment. This is illus-trated by the unique risk mitigationassociated with the wind generation’srenewable nature. Wind has a highercertainty for the cost of power over thelife of the project compared to conven-tional fossil fuel-based power genera-tion that must predict the ever changingcost of fuel. This provides both theproject investor and the power purchaserwith more certainty and therefore re-duced the risk as the initial investmentanchors project economics.

The wind project required assign-ment of risk associated with curtail-ment. It was accepted that there wouldbe uncontrollable events where windgeneration would be greater than theplant load. A reasonable annual allot-ment of curtailment was agreed upon, which would resultin a nonpenalty. The cement plant accepted that any addi-tional curtailment beyond the allotment would be on atake or pay basis. This equitably assigned risk to both par-ties and provided incentive to both parties to maximizegeneration and mitigate curtailment.

To summarize the business model, the burden ofinvestment and project performance resides with thewind developer and associated equity investors, leavingthe cement manufacturer with a long-term commitmentto purchase the generation. In this particular case, thedelivered purchase price of electricity was intentionallynegotiated as a flat rate for the 25-year term to backloadthe savings (and thus frontload the hedging risk) againstutility rates that will very likely increase over time dueto inflation.

Project BenefitsIt is helpful to summarize the project benefits into twocategories: benefits for the cement manufacturer and

benefits for the environment. Thecement manufacturer has created along-term hedge for 35% of itselectricity needs and is very likely tobe at a competitive advantage onthis large expense item. Effortstoward sustainable cement manufac-turing are virtually guaranteed on agoing-forward basis for the industry,and this project catapults this facil-ity into the forefront of sustainabilityby converting a significant portionof its power consumption to a renew-able source. The environmental bene-fits include the reduction in fossilfuel consumption and related reduc-tion in green house gas emissions.One additional benefit often notnoted is the reduction in electricaltransmission line losses by serving aload with local generation with very

little transmission loss.

ConclusionsThe review of this innovative sustainability project pro-vides a primer for those considering similar projects. Itprovides insight to the wind generation industry and itsapplication to the cement industry through a detailedcase study of a completed project (Figure 7). This projectclearly demonstrates the challenges of such an endeavorand the resulting innovations that made it successful.Finally, it is a shining example that sustainability effortsin their best form are not necessarily an added businessexpense but can realize a more sustainable and profitablebusiness model.

References[1] (2009, Jan. 12). Another record year for new wind installations

[Online]. Available: http://www.awea.org/pubs/factsheets/Market_

Update.pdf

[2] (2009, Jan. 16). Wind energy production tax credit (PTC) [Online].

Available: http://www.awea.org/pubs/factsheets/PTC_Fact_Sheet.pdf

[3] (2009, Jan. 16). Wind energy—The facts [Online]. Available: http://

www.ewea.org/index.php?id=91

[4] (2009, Feb. 3). Clean development mechanism simplified project

design document for small scale project activities [Online]. Available:

http://www.cdmmorocco.ma/download/projet/PDD-parc-eolien-tetouan-

LAFARGE.pdf

[5] (2009, Feb. 6). ACCIONA energy constructs 250 WM wind in Mex-

ico [Online]. Available: http://www.renewableenergyfocus.com/

articles/windother/bus_news/090203_acciona.html

[6] L. Royan. (2009, Jan. 27). The California power crisis 2000–2001

[Online]. Available: http://www.erisk.com/Learning/CaseStudies/

CaliforniaPowerCrisis2000.asp

Bruce Shafer is with CalPortland Company in Mojave, Cali-fornia. This article first appeared as “Electrical Wind Gener-ation for Cement Manufacturing, a Case Study: CalPortlandMojave Plant’s 24 MW Wind Project” at the 2009 CementIndustry IEEE Conference.

7Completed project: two of the eight turbines with cementplant in background.

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THE FIRST ANDFOREMOSTNATURAL

ASSOCIATIONWITH WIND

RELATES TO THELOCATION OFTHE MOJAVE

PLANT.

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by bruce shafer

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