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Ⅰ.I
NTRODUCTI
ON
The development of industry and urbanization have caused the worldwidedemandforenergyresourcestoincrease.Theworld’stotal energy consumption has increased by approximately 35% in 2013 compared with 1990,and ithasconsistently increased by 2.3% since 2000 (Enerdata, 2014). In particular, fossil fuels have been continuously used in mostindustries since the IndustrialRevolution, and demand for fossilfuels has been consistently increasing.The most commonly used energy is mainly supplied from fossilfuels, whichaccountfor88% ofthetotalenergyusedat2000,according to theInternationalEnergy Agency (IEA).However,energy supplied by fossilfuelisonly atemporarysolution;thisresourcealwaysinvolves risksofdepletionduetotheirlimitedreserves.Campbell(2011)noted that half of the total global reserves of oil have already been consumed,and thatthe world has reached peak oil,which implies decreasingoilproductioninthefuture.
South Korea is emerging as one of the world’s 15 largest economies on the basis ofGDP,as of2014.However,South Korea also consumes 0.252 TOE/million won ofenergy per unitofGDP. This consumption necessitates the world’s fourth-largest energy imports,and represents energy consumption thatis ranked fifth in Organization for Economic Cooperation and Development (OECD) countries.South Korea’s import dependence is 96%,making South Korea a country with very high energy dependence (Korea Energy EconomicsInstitute,2013).However,mostdomesticindustriesrequire significantenergy resources,and therefore,a stable energy supply baseexpansion isacrucialissueforthiscountry.Energy issuesare receiving attention worldwide and have increased the importance of renewable energy as a potential solution. While the average proportion of renewable energy to meet the primary energy
usedonly3.8% ofitsenergysourcesasrenewableenergy.Therefore, South Koreaisacountry withavery poorrenewableenergy system infrastructure.The renewable energy industry in Korea is in the beginningstagesascomparedwithdevelopedcountries,althoughitis expected to beincreased dueto expanding governmentsupport.The largestportion ofdomestic production ofrenewable energy is waste at65.8%,and the second-largestportion is bioenergy at15.8%.In particular,bioenergy isarenewableenergy sourcethathasbenefited from asignificantamountoffinancialandresearch supportand been studied to such an extentthatenergy production capacity has been increasedapproximately12-foldduring thelastdecade(KoreaEnergy ManagementCorporation,2014).
In domesticenergy usage,thedemand forlivestock production has been increasedby achangeofthepopulation’sdiettoconsumemore meat.The livestock industry has also grown to meetthis increased demand.Fig.1illustratesthisincreaseddemand.Energy consumption of livestock farms,including energy consumption for achieving an optimum environmentforlivestock development,has been increased according to the increasing scale of the livestock industry. The government has attempted to reduce the burden on farmers by applying tax exemptions for energy expenditures.The government haveafundamentalproblem intermsofenergyresources.
Fig.1Totalproductionoflivestockindustryandlivestockindustrial production(NationalInstituteofAnimalScience,2014)
According to the rise in the scale of the livestock industry, livestock wastewater volumes are increasing. The livestock wastewaterhasbeen treated by discarding itaslandfill,incineration, recycling and purification (Korea Organic Recycling Association, 2006). The treatment of livestock wastewater has increasingly changedfrom discarding aslandfilltocomposting andfertilization,as showninTable1.
Year 1999 2001 2003 2005 Generationrate (ton/day) 11,577 11,237 11,398 13,028 Treatment Landfill 6,803 3,856 2,836 333 Incineration 846 1,003 844 480 Recycling 3,928 6,378 7,718 12,215 Recycling Feed 2,400 3,524 3,832 5,349 Compost 1,457 2,598 3,391 5,470 Anaerobic digestion 71 256 495 1,396 Table 1 Currentstatus oflivestock waste water treatment(Korea OrganicResourceRecyclingAssociation,2006)
However,according to a ban on thedumping oflivestock manure atseathatwasimposedin2012,livestock farmshavehadtobeara largeburden.In particular,aswastewatertreatmentslikelandfilland incinerationcausedspatialproblemsandenvironmentalpollution,these treatments present limitations to the associated environmentaland economic problems.Therefore,the importance of the recycling of livestock manure has been increasing,as an alternative to disposal. As a result,studies on composting,fertilization and recycling of livestock manure have been increasingly conducted.Fertilization and composting remain significant techniques for addressing livestock manure,but the resource recycling method has come to the fore, according to thegrowing importanceoftheenergy resource.Oneof the resource recycling methods is that the livestock manure is converted into a fuel resource like methane gas by anaerobic digestion using microorganisms.Thismethod can significantly reduce odor,compared with othermethods like fertilization and composting. Becausethemethanegasthatisoutputfrom thismethodisafuel,it canbewidelyusedinheatingandelectricity-generationsystems.
As energy shortages and livestock manure treatment problems constantly cause a significant burden,a cogeneration plant using
recently.The cogeneration plant is an efficient system because it usesthewasteheatrecovered from an emittedhigh-temperaturegas following theelectricity-generation process.Becausethecogeneration plantproducesbothelectricityandvaluableheatenergyfrom asingle fuelsource,this plant achieves approximately 35% higher energy production efficiency than a conventionalpowerplant.Recently,the cogeneration plant has been introduced in the form of using a methanegasthatisproducedfrom anaerobicdigestionusinglivestock manure.Suchasystem canreducethelivestockmanuredisposalcost aswellascreating economicprofitsfrom producing fuel.Becausethe anaerobic digestion using livestock manure does not require food resources,incontrastwithothermethodsusingaplantandawooden pellet, this method has a significant advantage. Therefore, many domestic livestock farms have installed and managed anaerobic digestorsforlivestockmanure.
Meanwhile,when biogas is produced by anaerobic digestion,there are some factors that can affect the production of methane gas, including temperature,pH,andconcentrationofharmfulingredientsin the sludge.In particular,the internaltemperature ofthe anaerobic digestorisinfluencedbyheatlossduetoexternalweatherconditions, and itisaffected by the temperature ofthe slurry to be introduced into the anaerobic digestor. Therefore, when designing the Cogeneration Plant using Livestock Manure (CPLM),the real-time weather conditions at the installation site should be taken into account. Because many facilities composing the CPLM form a complex connection with other facilities, when designing the cogeneration plant,thedesign ofeach facility aswellastheenergy flow,such as energy transfer,which can be changed by variable situations, should be considered. However, most existing CPLMs presentlowerenergy production efficiency than the expected energy production efficiency on design process.As a result,many plants stopped operation orwere closed.Because ofthese problems,there
are many cases in which the energy flow ofeach facility was not abletobeaccurately predicted,involving metricssuch asamountof recovered waste heatenergy,energy losses during heat exchange, etc.
Therefore,theobjectiveofthisstudy isdevelop adynamicdesign method ofCPLM thatcan considerreal-timeweatherconditionsand energy flow foreach facility ofaplant.Accordingly,Building Energy Simulation,whichhasbeenwidelyusedtodesignrenewablesystems, was used in this study.A simulation modelofCPLM produced by the dynamic design method was verified to ensure reliability according to a comparison with data from a standard farm.We sought to reduce the difference of energy production efficiency between the design step and the implementation stage through the final simulation model. We estimated the proper scale of the cogeneration plant according to changing the capacity of major facility of the cogeneration plant. Additionally, we suggest some regulationsforthepropermanagementand operation ofcogeneration plants.
Ⅱ.Li
t
er
at
ur
er
evi
ew
2.
1 Cases st
udi
es on domest
i
c cogener
at
i
on pl
ant usi
ng
l
i
vest
ock manur
e
Asscaleoflivestockindustryandlivestockmanureemissionshave been increasing,many research efforts have been actively conducted to address how to treat organic waste.However,the method of dumping livestock manure into the sea, which had been rapidly increasing every year from 52,000 m3/year in 1997 to 2,745,000 m3/yearin 2005,hasbeen banned.Therefore,theproblem ofhow to
treat this huge volume of organic waste has become increasingly important(Kimetal.,2007).Iftheorganicwastewereto beneglected, itcouldcausesignificantenvironmentalproblems.However,itcan be recycled according to its treatment.Accordingly,there has recently been much effortinvested in recycling in Korea.A biogas plantis ableto generategasand digestivefluid asa byproductthrough the anaerobic digestion method to fermentorganic waste in a container that has been sealed;this represents one way to recycle organic waste (Hong etal.,2006).The gas can produce heatenergy and electricity thoughboilersandgenerators.Sincethedigestivefluidsdo notgenerateasignificantodor,thereistheadvantagethatitcan be immediatelyusedasaliquidfertilizer.Therefore,theuseofCPLM is increasingbasedontheinvestmentofcorporationsandthesupportof government. For example, Kolon Construction and the National Institute ofAgriculturalScience and Technology have developed an environmentally-friendly livestock manure treatment and biogas utilization technology thatinvolvesutilizing theanaerobicdigestion of livestock manure (IBES)through the research agenda ofthe Rural Development Administration in 2001. The main purpose of IBES techniques is to secure resources and alternative energy sources
fertilizeruseisnotpossible,IBES techniqueisabletosimultaneously fulfill two purposes of wastewater recycling and wastewater treatmentby being combinedwith advancedwastewatertreatment.A CPLM ofMojon farming estates,Icheon,Gyeonggi-dowascompleted in March 2007,and is currently operational(Jin etal.,2008).This plantisknown toproduce30kW ofelectricity and860Mcalofheat energyby utilizing the20tonsoflivestock manuregeneratedby their 2,550 head of pigs each day.Additionally,three regions of Muju, Jeongeup and Gochang have a plan to install CPLMs with the supportofthe Ministry ofCommerce.However,despite these many efforts,many cogeneration plantshavebeen found to haveproblems in theiroperation.A considerablenumberofcogeneration plantshave closedandceasedoperations.Thefollowingtextprovidesexamplesof theCPLMsattemptedinKorea.
(1)NationalInstituteofAgriculturalScienceandTechnologyofRural Development Administration, Seonghwan-eup, Cheonan-si, Chungcheongnam-do(Hwangetal.,2006)
This facility was designed and constructed with the objective of supplying this cogeneration plantto farmers in 2004.Ithad passed preliminary experiments and its construction was completed.Ithas bred 1,800 pigs,and approximately 10 tons ofmanure and washing waterperday havebeen used tofuelthecogeneration plant.Biogas produced in the anaerobic digestoris used as the heating sourceof the anaerobic digestorand fuelforthe generator.Accordingly,heat energy and electricity are used to operate the managementofthese facilities.Following the test operation,a technology transfer was attempted fordomesticsmalland medium-sized companies.However, it has not been developed into a commercialization stage. Nevertheless, it has been concluded that this facility developed automatic operations and an easy-to-manage system for the
(2)Eun-young Farm,Hongseoung-gun,Chungcheongnam-do(Hwang etal.,2006)
This facility was constructed by the Hongseoung Agricultural Technology Centerin 2000.Ithas an anaerobic digestorand simple gas storage facility, excepting its facility for treating livestock manurelikeadesulfurizationfacilityandawastestoragetank.Ithas bred 2,000 pigs,and approximately 8 tons ofmanure and washing waterperday havebeen used tofuelitscogeneration plant.Asthe livestock manure is introduced directly into the anaerobic digestor withoutpretreatment,hairandotherimpuritiesrelatedtothelivestock caused a generator failure. Because there is not desulfurization facilities,thiscould bethesourceofthegasdelivery tubeproblems and generator failure. In an attempt to increase the insulation, urethane has been foamed to the anaerobic digestor;however,this materialis difficultto uniformly foam.Therefore,its thermaleffect was reduced in winter,which caused a decrease in gas generation. Gas generated from the anaerobic digestion provides an economic advantage,as itcan beused forheating and feed production.Since the heating system of the anaerobic digestor was not working properly,theamountofgasgenerationvarieddrastically according to theseason,andthereweremanylimitationsofitsuse.Inthecaseof the anaerobic digester,because it does not take into account the thermal insulation of the external temperature, gas production quantitiesweresmallerthanexpected.
(3)Livestock Research InstituteofRuralDevelopmentAdministration (Hwangetal.,2006)
This plant has installed a livestock laboratory Suwon; it was completedin 2000.Ithasbred approximately 1,000pigs,and uses10 tonsoflivestock manureand wash waterperday.Thisplantisan experimentalplantforapplying theUpflow AnaerobicSludgeBlanket
(UASB),which involvesthehigh-concentration liquidwastefrom the anaerobicdigestorbeing precipitated up and down from thereaction vessel.Bottom-up anaerobic sludge blanket injury is a concurrent system method (Hong etal.,2006).Becausethisplantwasinstalled for development of the UASB method,a generator capacity was installed on asmallscalecompared with thegasgeneration amount. Therefore,surplus biogas would be incinerated,and digestive fluids would be used as liquid fertilizer.However,most of this plant’s productiondischargedintothesewagetreatmentplant.
(4)PublicTreatmentFacilitiesofLivestockManureandFoodWaste, Paju-si,Gyeonggi-do(Hwangetal.,2006)
Initially,a food wastecomposting facility wasinstalled in 1999.A plantusing a mixture oflivestock manure was additionally installed in 2004.The manure ofthe livestock farms in Paju is used as the fuelofthisplant;approximately63tonsoflivestockmanureandfood waste are used per day.Because the volume of food waste was larger than expected, the overall percentage of the treatment of livestock manureandfoodwastehasdecreased.A digestion solution, byproduct digestion, has been added to the processing because compostingmixedfoodwastehasbeenrecognizedasineffective. (5)PlantofLivestock andHorticultureCollegeofYonam (Hwang et al.,2006)
Thisplantwasinstalledin1983.Itwasdevelopedusingtechnology originating from Yonam Collegeand used thelivestock manurefrom externalfarms.Although theheatrequired fortheanaerobicdigestor wasreduced by mixing livestock manurewith hotwatermadeby a solar collectorplate,the energy production was much smaller than the initialcapitalinstallation costs.Because this plant experienced manydifficultiesinitsoperations,itwasclosedin1993.
(6)CogenerationPlant,Jeongeup-si,Jeonbuk(Lee,2012)
Thisplantwasdesigned to processlivestock manurein 2010,and itwasinstalledin 2012.Thisplantusedapproximately 70ton/day of livestock manureand 30ton/day offood wasteleachatesasfuelfor anaerobicdigestion.Thestabilizationperiodoftheanaerobicdigestion tank had an adverse effect on the prolonged operation of these facilities,andmanagementwasatthelow break-evenpoint.Although thisfacility stillopen,many problems arose in the operation ofthis facility.
Location Operation state
Capacity
(ton/day) Installationyear
Hongseong-gun,
Chungcheongnam-do Running 15 2003 Icheon-si,
Gyeonggi-do Running 20 2006 Cheongyang-gun,
Chungcheongnam-do Running 20 2006 Jeongeup-si,
Jeollabuk-do Running 50 2008 Gochang-gun,
Jeollabuk-do Running 50 2008 Muju-eup,
Jeollabuk-do Running 50 2008 Cheonan-si,
Chungcheongnam-do
Temporary
suspension 10 1999 Suwon-si,
Gyeonggi-do Running 10 2000 Cheonan-si, Chungcheongnam-do Closing 10 1979 Asan-si, Chungcheongnam-do Temporary suspension 25 1998 Gunwi-eup,
Gyeongsangbuk-do Closing 60 1997 Yeonggwang-gun,
Jeollanam-do Closing 130 1995 Cheongyang-gun,
Chungcheongnam-do Running 20 2005 Paju-si,
Gyeonggi-do Running 80 2004
Table 2 Installation and operation status of anaerobic digestion facilitiesforlivestockmanure(Huh,2007)
2.
2Desi
gn ofr
enewabl
eener
gy syst
em usi
ng BES
The TRaNsient Systems Simulation program (TRNSYS) commercialprogram is one of the BES commercialprograms that was used for the dynamic design of cogeneration systems using livestock manurein thisstudy.TRNSYS wasfirstdeveloped by the University ofWisconsin SolarEnergy Lab (SEL)in 1975 with the objectofanalyzing photovoltaicsystems.Many studieson renewable energysystem designutilizing aBES havebeenconductedusing this program.Initialstudies using BES were evaluated by Shariah and Lofin 1997 concerning the solarenergy system.They analyzed for annualeffectsoftheauxiliaryheatsourceequipmentinthermosyphon solarwaterheater,which isamethod comprising aphasechangeof thethermalenergy storagemedium in theprocessofstoring thermal energy obtained by a plate solar heat collector with a variety of system operating conditions.This study had a simple configuration and was carried outas a simulation fora relatively shortperiod of time.However,itwasneverthelessasignificantresearchmilestonein the analysis of solar energy.In addition,the effect of hot water storage tank volume and configuration ofefficiency was conducted using simulation.Thesefactorswereestimatedonadaily andannual scale (Hasan, 1997). Likewise, studies of solar energy systems comprisingthethermosiphonusingBES havebeensteadilyconducted. However, early studies evaluated only limited solar systems, according to the limited kinds of modules in the program. The performance ofthermalstorage tanks and a solarenergy collectors have been improved, and much research has been conducted to develop new modules implementing them. Carrillo and Cejudo developed a module forhotwatersystems (domestic waterheaters) for internalheating ofbuildings through solar thermalsystems in 2002, and they also conducted a verification experiment for this module.Asthepreceding research presented somelimitationsrelated
to the optimization ofthe design elements ofsome thermalsiphon solarenergy systems using BES,Abdunnabiand Loveday developed in 2012 a new module ofthermalsiphon solar energy systems to compensatefortheseproblems.Also,Ahmadrezaetal.(2014)defined anew moduleforfinding themostefficientvacuum tubeofsolarhot water systems under naturalweather conditions.Thus,research to extend the implementation range ofsolarenergy systems has been conducted through the development of new modules that have augmentedthebasicmodulesprovidedbytheBES program.
A numberofresearch effortson solarenergy systemsusing BES have been executed around the globe.Various studies using BES have been conducted since the 1990sKorea.However,early domestic studiesusing BES hadnotbeenusedtodesignsystemsandanalysis tools.Because the BES program had a complex configuration of simulation systems and required many input data fields,BES had been of limited utility for non-experts.Therefore,early domestic studies using BES had been mainly conducted to demonstrate a simple example provided by the BES program for non-experts, including thoseinterested in thesolarsystemsindustry.An analysis oftheenergy production efficiency ofsolarsystemswasstudied by changing theenvironmentalconditionsofeach modulecomprising the solar system.This constituted the basic research for implementing thesesolarsystems.
Likewise,as BES had been developed for the implementation of solarenergy systems,mostoftheearly studiesusing BES had been limitedtosolarpowersystems.However,arangeofresearch efforts using BES have been expanded from solarenergy systems to other renewable energy systems such as geothermal and wind power systems by defining new modules adapted forthe objectives ofthe user.Lee etal.(2012) calculated the periodic cooling and heating loads of greenhouses and estimated the periodic heatenergy from
(2006) analyzed the annual dynamic characteristics of the Ground-SourceHeatPump(GSHP)according tomajordesignfactors using TRNSYS.Accordingly,they presentedbasicresearch aboutthe physicalproperties ofthe GSHP (depth,spacing ofpipe,etc.)that affect its performance.Subsequent studies based on the preceding research reported thatthe performance prediction modelofthe heat pump system using solarpowerand geothermalhad been developed, and the efficiency ofgeothermalheatpump systems was estimated usingTRNSYS (Jangetal.,2011;Jo,2012;Nam etal.,2014).
As described above,renewable energy systems design using BES was initially limited to solar power systems,and itwas expanded into othervarious renewable energy systems by the developmentof new modulesadapted totheobjectivesofdifferentusergroups.BES has been evaluated as highly reliable for the prediction ofenergy productionfrom renewableenergysystems.
Ⅲ.MATERI
ALS AND METHODS
3.
1Cogener
at
i
on pl
antusi
ng bi
ogas
3.1.1Biogasplant
Biogases, organic substances that are synthesized by utilizing sunlight through a series of processes,may be used as fuelfor power generation to produce electricity.A technique for gasifying organicmaterialandgenerating powercan bedividedintotwosteps. First,using woodorhay,dry distillation ofwood-basedbiomasscan beperformed,including thestemsofagriculturalproducts.Sincethe biogas is passed through the gasification reaction using heat or catalyst,the biogas is used as fuelofgas engine or turbine for producing heat and electricity. Another method is that the organic-rich water is converted to methane gas through anaerobic digestion,and the methane gas is used to generate power.Fig.3 illustrates this latter technique.This method has been widely used through with livestock manure containing high concentrations of organic matter,and is used by adding organic solid waste such as foodwaste.
Fig.1Processofanaerobicdigestionusingorganicwastecomposting An anaerobic digestor is the facility that carries out anaerobic digestion using mixed livestock manure and food waste over the courseofapproximately 30days.In mostcases,thereaction vessels’ volumes are large,and high concentrations ofinfluentsludge would be introduced into the reaction vessels. Therefore, most of the reaction vessels require additionalstirring equipment.Ignoring the amountusedinthesynthesisofthecellduring anaerobicdigestionin the anaerobic digestion tank,the gas is generated from the organic wasteaccordingtothefollowingreactionformula(Parketal.,2008).
→
Accordingtotheaboveformula,thebiogascontainsCH4,CO2,NH3,
H2S afteranaerobicdigestion.AsH2S cancausecorrosionofthegas
pipe and the engine,a desulfurization facility for removing H2S is
desulfurization facility will be stored and will be used to fuel generatorsorheaters.
3.1.2Cogeneration plant
The cogeneration system is a generation process thatuses waste heat energy,which is the emitted high-temperature gas resulting from thegenerationprocess.Theconventionalpowerplantyields40% electricity production efficiency,and 56% and 4% ofthetotalenergy isreleasedthroughtheairintheform ofthermalenergyandthermal loss, respectively. In contrast, the cogeneration plant generates electricity and simultaneously produces heat energy according to recoveryofthewasteheatenergyfrom thegenerator.Accordingly,it boasts75% energyproductionefficiency.
Fig.2Efficiencyofenergyproduction inregardtocogenerationplantand
Cogeneration plants are classed as steam turbine plants, gas turbines,complex generation,gas engines,nuclearpowergeneration, utilizing waste heat from industrialprocesses,utilizing burning of waste,etc.Because the cogeneration system has to be supplied the energy ofeach load sizeand load pattern in realtime,moredetailed analysis isrequired to compareitsdesign with otherplantsystems. For example, the cogeneration system of the steam turbine has design elements such as a heat supply for generating steam,an absorption chiller for steam cooling, various pipes, and control devices,and outside weather data are also needed.Recently,much research has been carried out to investigate the conversion of livestock manure into biomass through an anaerobic digestor. Consequently, installations of anaerobic digestor facilities and cogeneration plants have increased. Most cogeneration plants on livestock farmsaresmallandmedium-sizedplantsthathaveadopted thegasturbinesystem and includesome facilities fortheanaerobic digestion process. The CPLM can reduce the cost of treating livestock manureand can alsolead tothereduction ofenvironmental pollution, as well as offering high-efficiency energy production. Because this plant involves the anaerobic digestion process, the biogas production ofwhich can be affected by pH,temperature of slurry,andotherfactors,thisplantmustbecarefullymanaged.
3.
2Ref
er
encef
aci
l
i
t
y
Thereferencefarm ofthisstudy isaGwang-ilfarm in Jinchon-ri, Samjuk-myeon, Anseong-si, Gyeonggi-do. This farm has been breeding approximately 4,000 pigs and hasa floorspaceof4000 m2
andasiteof16,500m2.Thiscogenerationplantinthereferencefarm
has been using livestock manure and food garbage from a nearby area for its anaerobic digestion,and ithas been producing biogas. Also,methanegasproduced atthisfacility isused asafuelforthe cogeneration planton the farm,and itproduces thermalenergy and electricalenergy.The produced energy from the cogeneration plant hasbeen used atthisfarm,and itcreateseconomicbenefitsforthe farm.The totalinstallation costofthis cogeneration plantwas 1.34 billion won. Of this cost, 1 billion won was supported by the governmentand 340 million won was covered by thebudgetofthe farm.Fig.4showsthefield installation diagram ofthiscogeneration plant.This CPLM stored livestock manure from a pig house ofthe referencefarm.Food wastefrom a nearby region wasstored in the wastestoragetankoverthecourseof2-3days,asshown in Fig.5. Subsequently,the livestock manure and the food waste were mixed and crushed. Next, biogas was produced through the anaerobic digestion process,and the biogas was used to fuelthe cogeneration plant.
Fig.3Installationfeatureofcogenerationplantin Gwang-ilfarm
(a)Entranceofwastestoragetankfacility
(b)Wholeview ofwastestoragetank
Approximately 12 tons oflivestock manure was generated on the reference farm during one day,and it was placed into the two 200-ton anaerobic digestors. Subsequently, this livestock manure would be used for anaerobic digestion for approximately 10 days. Biogaswould beplaced into thedesulfurization facility forremoving sulfuroxides.As a result,biogas with sulfur oxides removed was usedtofuelthecogenerationplant.
(a)Anaerobicdigestor (b)Gasstoragetank
(c)Innergasstoragetank (d)Desulfurizationfacility Fig.5AnaerobicdigestorandgasstoragetankofGwang-ilfarm
Methane gas stored in the gas tank was used to fuel two generators (presented in Fig.7).The generated electricity served as incomewhen itwas sold to the Korean Electric PowerCorporation.
However,theinstallation costofelectricpowertransmission and the registration tax were more than the margin ofgenerated electricity. Therefore,thegeneratedelectricity from thecogenerationplantofthe referencefarm hasbeenusedinthedormitoryandpighouse,instead. In addition,the recovered waste heatenergy was stored in a heat storage tank and was used forthe following two applications.The firstuse was to maintain the anaerobic digestor.The cogeneration plantofthe reference farm adopted a mesophilic anaerobic digestion process.In this case,the optimum temperature of the anaerobic digestorformethanegasproductionwasknowntobe35～36℃ (Choi etal.,2003).Therefore,theanaerobicdigestorwasalwaysmaintained within thistemperaturerangeby supplying artificialheatenergy.So, therecovered wasteheatenergy was preferentially used forheating the low temperature of the slurry introduced from outside and covering heat losses of the anaerobic digestor from the external weatherconditions.Ifrecoveredwasteheatenergywasinsufficientto warm theanaerobicdigestor,anadditionaloilboilerwouldbeused.If this energy was sufficient to warm the anaerobic digestor, the remaining amountofitwasusedforheating orhotwaterenergyfor thedormitory in thereferencefarm.Theremaining wastewaterand solids following anaerobic digestion were moved to the solid-liquid separation station (Fig.8),and weresupplied to theorchard orfields asaliquidfertilizer.
(a)Managementoffice (b)Generator
Fig.6ManagementofficeofplantandgeneratorofGwang-ilfarm
(a)Wholeview ofsolid-liquid separationfacility
(b)Solid-liquidseparation equipment
3.
3Bui
l
di
ng Ener
gy Si
mul
at
i
on(
BES)
Building Energy Simulation (BES) is a numerical technique to predictand calculateenergy loadsforcontrolofthermaland energy flows and designing appropriate environments of buildings. This technique is estimated to present high accuracy in the general architecturalfield,so itis widely used to design buildings and to evaluatetheoverallperformanceofbuildings.Itsfieldsofapplication include cooling and heating loads analysis of buildings,design of coolingandheatingairconditioning systems,anddesignofrenewable energy systems such as solar power and cogeneration plants.Its representativecommercialprogramsareBLAST,eQuest,EnergyPlus, TRNSYS,ESP-r,and DOE-2.In thisstudy,theTRNSYS (Ver.17, Wisconsin,USA)commercialsoftwareprogram hasbeenused.
TRNSYS hasbeen developed forthedynamicsimulation modeling ofsolar power systems,and ithas been widely used around the world (Lee etal.,2012).TRNSYS is an unsteady analysisprogram. As its simulation model is composed of a main program and subroutines called components, it has the advantage of being user-friendly and compatible with simulation models.Each module, called a component, receives as input physical information via parameters.Components are connected to each other and exchange inputvaluesandoutputvalues.Inparticular,theType56component, constituting the building model,calculates dynamic thermalenergy analysisconsidering heatradiation,complex heatgain ofthevarious formsthatoccurinthespace,convection,radiation,heatstorage,and otherfactors,basedonthetransferfunctionmethod.
Fig.8Overallprocessofestimatingbuilding’senergyloadsusing BES
TRNSYS performsthesimulationusing real-timeweatherdataand inputdatathathaveatime-dependentform.Consequently,theresults ofsimulation are also calculated in realtime.So,itis possible to estimatethemaximum orminimum values perunittimeaswellas to calculate the cumulative results during the period.This program canestimateunsteadystatesimulations,whicharetime-dependent,as well as calculate steady state simulations, which are time-independent. As TRNSYS has the advantage of creating user-defined components that are developed according to the objectivesoftheuserwritingtheFORTRAN code,ithasbeenhighly ratedforitsflexibility.
Fig.9Considerablefactorsforestimatingheatingandcoolingloads usingTRNSYS
TheTRNSYS program calculatesthecooling and heating loadsof thebuilding using adynamicsimulation modelbased on thetransfer function method.The reference building thatis analyzed forcooling and heating loads is divided into a numberofzones,which are the units of analysis.Each zone requires the input of data such as thermal conductivity, density, and heat capacity of the walls constituting the building.Based on this information,the conduction and theinsulation from thesurfaceofthebuilding iscalculated,and the cooling and heating loads of each zone are also estimated considering ventilation,filtration,and internalheat generation.The followingformulashowsthisconcept.
:theconvectivegainfrom surfaces(kJ)
:t heinfiltrationgainsfrom outsideonly∙ ∙ (kJ)
:theventilation gainsfrom a user-defined sourcelikea HVAC system
∙ ∙ (kJ)
: the internal convective gains by people, equipment, illumination,radiators,etc.(kJ)
:thegainsduetoconnectivefrom airnodeIorboundary condition ∙ ∙ (kJ)
: the fraction of solar radiation entering an airnode through external windows which is immediately transferredasaconvectivegaintotheinternalair(kJ)
: the absorbed solar radiation on allinternalshading devices ofzone and directly transferred as a convective gaintotheinternalair(kJ)
Thefollowing Fig.11presentanenergyloadcalculationflow chart of the target building considering solar radiation,convection and weatherconditionssuchasoutsidetemperature.
:angleofincidenceofbeam radiationonsurface(°) :solarzenithangle(°)
:slopeofsurface(°) :solarazimuthangle(°)
:azimuthangleofsurface(°)
:ratioofbeam radiationontiltedsurfacetobeam on
horizontal
:beam radiationontiltedsurface(kJ·h-1·m-2)
:beam radiationonhorizontalsurface(kJ·h-1·m-2)
:groundreflectedradiationonatiltedsurface (kJ·h-1·m-2)
:totalradiationonahorizontalsurface(kJ·h-1·m-2) Fig.10Flowchartofthedetailmechanism ofenergybalance
includingradiation,convection,skytemperature,etc. Where,
:ratioofreflectedradiationontiltedsurfacetototal
radiationonhorizontal :groundreflectance(kg·m-3)
:modulatingfactorforReindltitltedsurfacemodel
:diffuseradiationontiltedsurface(kJ·h-1·m-2)
:extraterrestrialradiation(kJ·h-1·m-2)
:anisotropyindex
:extraterrestrialradiation(kJ·h-1·m-2)
′,″ :hourangleatstartandendofdata(°)
:solarconstant
:factoraccountingfortheeccentricityoftheearth's orbit
:latitude(°)
:solardeclinationangle(°)
:meanhourangleoftimestep(°) :thetimeinhourscorrespondingto
:shiftinsolartimerelativetothenominaltimeofdata reading
:ambienttemperature(℃)
:emittanceoftheclearsky
:cloudinessfactorofthesky,
:diffuseradiationonthehorizontal(kJ·h-1·m-2)
:totalradiationonthehorizontal (kJ·h-1·m-2)
:dew pointtemperatureatambientconditions(℃)
:atmosphericpressure(atm)
:atmosphericpressureattheheight (atm) :gravitationalacceleration(m·s-2)
:elevationabovesealevel(m)
:airdensityattheheight (kg·m-3)
3.
4Resear
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One ofthe purposes ofthis study is to develop a BES dynamic designmethodfortheCPLM.Afterselecting thetargetfarm,various designdataandweatherdatawereacquiredforthedevelopmentofa simulationmodeltodesignacogenerationplant.Thereliabilityofthe developed simulation model was enhanced through the validation process.Thefinalvalidatedmodelwasusedtoanalyzeproperenergy production of the CPLM and to decide the proper scale of the cogeneration plant.Fig.12 is the flowchart of this study,which presentsthisprocess.
3.4.1Design ofthesimulation model
3.4.1.1Design ofthecogeneration plantusing livestockmanure TheCPLM atGwang-ilfarm,thetargetfarm,hastwogenerators and two anaerobic digestors.This plantalso includes a hotwater tank to store the heat energy,two types of boilers,electric and kerosene,forsupplying thethermalenergyrequiredinadditiontothe stored thermalenergy,and adesulfurization facility toremovesulfur from biogas.The targetfarm hasmonitoring equipmentthatchecks andmanagesvariouscontrolelements.
The BES dynamic design method is a technique thatimplements the energy flow between facilities of the system,and the design goals of this study also focused on the analysis of energy consumption and production.As a result,the facilities related to energy consumption and production were selected to design the simulation modelamong variousfacilitiesoftheCPLM atthetarget farm.Typically,a pulverizer,which crushes and mixes food waste withlivestockmanureforpromoting anaerobicdigestion,andawaste storage tank were classed as unrelated to the energy production process. A gas storage tank, which holds methane gas, and a desulfurizerfacility forimproving the quality ofmethane gas,were alsolabeledasunrelatedtotheenergyproductionprocess.Theothers excluding these facilities were separately designed according to the generation,thewasteheatrecovery,and theutilization ofthewaste heatsystem byconsideringtheenergyflow.
(1)Generationsystem
The generation system has the role ofgenerating electricity and consistsofan engine,fuelschedule,unitconvertermodule,andother components.Because mostofthe CPLMs,including the targetfarm ofthisstudy,maintain optimum conditions ofthe anaerobicdigestor for producing methane gas such as internaltemperature,pH,and other parameters, this system is designed so that the anaerobic digestorcan produce 12.5 m3ofmethane gas per1 m3oflivestock
manureinthecaseoftargetfarm.Theanaerobicdigestionprocessis influenced in its production ofmethane gas by pH,componentratio of the slurry, temperature of the anaerobic digestor, and other considerations.However,pH and the componentratio ofthe slurry are managed to maintain optimum conditions for producing the
methane gas at the target farm. Therefore, these factors were excluded from the variables included in this study.Another factor affecting the anaerobic digestion process is the internaltemperature of the anaerobic digestor. Its optimum range is divided into mesophilicanaerobicdigestionandthermophilicdigestion,according to the type of anaerobes.The CPLM at the target farm adopts the mesophilicanaerobicdigestion process.Theoptimum temperaturefor producing methane gas of this anaerobic digestion method is confirmedas35∼36℃ asmentionedin section3.2.TheCPLM atthe target farm has a heating system for maintaining optimum temperatureoftheanaerobicdigestor,andthisisalsoreflectedinthe simulation model. As a result, the internal temperature of the anaerobicdigestorwasneglected in the simulation model.Therefore, all factors described above were excluded from the methane production process,and it was assumed that 1 ton of livestock manureistransformed into afixed amountofmethanegaslike12.5 m3.
(2)Wasteheatrecoverysystem
The waste heat recovery system was designed to transfer the waste heat energy from the hotexhaustgas of the generator to waterintheheatstoragetank.Thissystem consistsoftwotypesof heat exchangers, a heat storage tank, pump modules, and other components.Eachtypeofheatexchangerperformstheheatexchange between exhaust gas and cooling water,and between the cooling water and the water of a storage tank. An efficiency of heat exchange is setat80% according to the design data.The specific heatlosscoefficientoftheheatstoragetankwassetto3kJ/hr·m2·K accordingtothedesigndata,andthevolumeoftheheatstoragetank
wassetto2.26m3basedonmeasureddata. (3)Utilizationofwasteheatsystem
In theutilization ofawasteheatsystem,thewasteheatisfirstly used to maintain optimum conditions ofthe anaerobic digestor for producing methane gas,and is then used to heat the dormitory. Calories for maintaining optimum temperature of the anaerobic digestor are estimated considering the heat loss of the anaerobic digestorand heating loadsfortheslurry inflow.Thisisdescribed in Section 3.4.1.3.Theequation (1)wasappliedtothesimulation model by a user-defined module.Theheating loads ofthe dormitory were estimated by various weather modules and building modules that imported the TRNBuild model made by the design data of the dormitory.
3.4.1.2Description ofcomponents
A design method ofa renewable energy system using TRNSYS has a basic unitofprogrammed components using the FORTRAN language. Each component consists of a mathematical model according totheimplementing facilities,and each hasinputs,outputs and parameters.Each componentis connected to the others by the user.Therefore,an overallunderstanding of the target system is essential to design a simulation model and to form a proper connectionbetweentheinputandoutputofeachcomponent.
Modules Specification
Generator (TYPE 907)
This component models an engine generator.A device used to generate electricitybyburningfuelinaninternal combustion engine.Themodelrelieson an external data file which contains efficiency, air flow rate and heat transferdataasafunctionoftheintake temperatureandthepartloadratio. Fuelschedule
(TYPE 14)
This models give on/off signal accordingtofuelschedule
Unitconvertor (Equation)
This calculator convert unit of electricityfrom generator
Thermal storagetank
(TYPE 4)
The thermal performance of a fluid-filledsensibleenergystoragetank. Fluid entering thehotsideofthe tank is added to the tank node below the firstauxiliary heater.Fluid entering the cold sideofthetank entersthebottom node Heat exchanger [Shelland tubemode] (TYPE 5)
Thissensibleheatexchangerisashell andtubetypedeviceGiventhehotand cold side inlet temperatures and flow rates,theeffectivenessiscalculated for a given fixed value ofthe overallheat Table1Descriptionofcomponentsofcogenerationplantmodelusing livestockmanure
transfercoefficient. Heat exchanger [Constant effectiveness] (TYPE 91) This sensible heat exchanger is modeled as a constant effectiveness device which is independent of the system configuration.
Pige (TYPE 31)
This component models the thermal behavioroffluidflow in apipeorduct usingvariablesizesegmentsoffluid. Heatingloads
foranaerobic digestor (Equation)
This component calculate the heating loads for anaerobic digestor using user-definedfunction.
Pump (TYPE 3)
This component models the thermal behavioroffluidflow in apipeorduct usingvariablesizesegmentsoffluid. Weatherdata
(TYPE 9)
This componentread a weatherfile in combinationwithothercomponents
Radiation processor (TYPE 16)
This component calculates several quantitiesrelated to theposition ofthe sun, and estimates insolation on a number of surfaces ofeither fixed or variableorientation
Psychometrics (TYPE 33)
This component calculate moist air taking as input the dry bulb temperatureandrelativehumidity
Sky temperature
calculator (TYPE 69)
This componentdetermine an effective sky temperature, which is used to calculate the long-wave radiation exchangebetween an arbitrary external surfaceandtheatmosphere
Multi-zone building [기숙사] (TYPE 56)
Thisbuilding modeldesign thethermal behaviourinsideamulti-zonebuilding
3.4.1.3Estimation oftheanaerobicdigestorheating calories The CPLM requires the mesophilic anaerobic digestion process using livestock manure and food waste,unlike generalcogeneration plants.As the mesophilic anaerobic digestion process functions to producethefuelofcogeneration plants,itmainly affectsthequantity ofgeneration and heatenergy production.Therefore,itis important tomaintaintheenvironmentalconditionsoftheanaerobicdigestorfor maximum biogas production, and it has to be equipped with monitoring devices.Mostofthe anaerobic digestion using livestock manure and food waste adopts to mesophilic anaerobic digestion method.Biogas production using this method is affected by pH, temperature,and contentsoftheslurry (Choietal.,2003).However, theinternaltemperature oftheanaerobic digestoris affected by the ambient temperature, and it is related to biogas production. An optimum temperature for maximum methane production in the mesophilic anaerobic digestion process is 35∼36℃ (Jung,2013).So, maintaining this temperature condition improves the efficiency of anaerobic digestion and minimizes the required volume of the
essential,and its scale has to be estimated to be the propersize. Heating loads can be divided into two types.One is a calory for heating inflow slurry and the otheris a calory forsupporting heat lossby outsideweatherconditions(Chaeetal.,2001).Therefore,the sum of two calories is the total required heating load for the anaerobicdigestor,anditisdescribedinTable4.
Heatingloadforinflow ofsludge (a)
Heatingloadforthermallosses ofanaerobicdigestor(b)
:Heatingvelocityforinflow of sludge,kcal·hr-1
:Flow rateofsludge,kg·hr-1
:Specificheatofsludge, kcal·kg-1·℃-1
:Sludgeinlettemperature,℃
:Sludgeoutlettemperature,℃
:Thermallosses ofanaerobic digestor,kcal·hr-1
:Heat transfer coefficient of anaerobicdigestor,
kcal·m-2·hr-1·K
:Ambienttemperature,℃
:Inside temperature ofanae-robicdigestor,℃
Totalrequiredheatenergy (c)=(a)+(b)
(1)
Table2Heating load ofanaerobicdigestorconsidering theinflow of sludgeandthermallossesofanaerobicdigestor
Author Researchcontents Year
Pons Strigari
Developmentofa conceptforthe autarkic energy and water supply ofa housing estate in an area withhighaveragesolarirradiation
2009
Justin Tamasausk
as
Modeling and optimization ofa solarassisted heat pumpusingiceslurryasalatentstoragematerial 2011
Table3Preceding researchesofsecuring simulation modelreliability bymoduleverification
3.4.2Validation ofsimulation model
3.4.2.1Majormodulevalidation ofsimulation model
The TRNSYS commercialprogram used to design the CPLM in this study is widely used to design renewable energy systems like geothermal,solar power,and wind power energy systems.These simulation models ofrenewable energy systems consistofvarious modules,which permitdiverse facilities,and each module exchanges itsinputsandoutputswithothermodules.Therefore,thereliabilityof a simulation modelisassociated with thereliabilitiesofeach ofthe modules thatcompose the simulation model.Therefore,Jiang (2011) conducted a module validation for the implementation of a hybrid geothermalheatpump system in a climate ofhigh temperature and humidity.Accordingly,he chose a heatstorage tank module as the major module of the entire model,and he compared the average temperatureoftheinflow watertotheheatpump.In addition,many research efforts addressing the design ofrenewable energy systems usingTRNSYS havebeenconducted,andarereviewedinTable5.
Johnetal. GeothermalHeatPumpsinK–12Schools 2000
Lacour Ayompe
Validated TRNSYS model for forced circulation solar water heating systems with flat plate and heatpipeevacuatedtubecollectors
2011
Abdunnabi etal.
Optimization of thermosyphon solar water heaters using TRNSYS.Part1Improved modeldevelopment andvalidation
2012
Miaomiao He
Numerical modelling of geothermal borehole heat exchangersystems 2012 Theodore
Wayne Hand
Hydrogenproductionusinggeothermalenergy 2008
Forrestet al.
Optimization of a geothermal heat pump system
withabovegroundwaterstorage 2003
In thepresentstudy,a heatstoragetank modulewasselected as the majormodule ofthe CPLM model,and simulation results ofits internal temperature were compared with measured results from outsideweatherconditions.A validation ofthesimulation modelwas conducted overthe period of12～19 Feb.2013.A portable weather station was installed atthe targetfarm and recorded weatherdata during this period. The target farm has monitoring equipment managing many facilities of the CPLM such as pH,the internal temperatureoftheanaerobicdigestor,andtheinternaltemperatureof theheatstoragetank.However,this monitoring equipmentcan only check data except recording.Therefore,the data recorded by the monitoring equipmentwere captured atintervals of5 minutes using separatecapturesoftware.
Fig.14Monitoringdevicescreenofcogenerationplantusing livestockmanureinGwang-ilfarm
3.4.2.2Resultvalidation ofsimulation model
Another validation method of renewable energy system models using TRNSYS has been conducted by comparing the totalenergy production ofthe simulation resultwith field data.The products of theCPLM areelectricityandheatenergy.Becausetheheatenergyis stored in the heat storage tank in the form of latent heat,it is difficult to measure quantitatively. In contrast, electricity can be quantifiedaccording tomeasuring equipmentlocatedatthegenerator. Therefore,the electricity energy production that can be quantified waschosen asthemetricforthisstudy.Accordingly,thesimulation resultofelectricity energy production was compared with field data during the objective period.This validation method was defined as theresultvalidationmethodinthisstudy.Fig.15showsanoverview document of the CPLM in the Gwang-il farm. This overview documentpresentstheoperatinghoursofthegenerator,theelectricity
thebiogasproduction process,andotherconsiderations.Consequently, the electricity production data were extracted from the overview documentin 2013,and these were analyzed and compared with the simulationresults.
Fig.15Overview documentofcogenerationplantusinglivestock manureinGwang-ilfarm
Ⅳ.RESULTS AND DI
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4.1.1 BES dynamic design of the cogeneration plant using livestockmanure
A simulation modelofthe CPLM was individually designed as a generationsystem,awasteheatrecoverysystem,andautilization of wasteheatrecovery system in thisstudy.Each system consisted of multiple modules,and each module was designed to exchange its input and output values with other modules.The modules of the library and the modules of TESS, which is additional library, provided by TRNSYS 17,were used for the design of the three systems. The user-defined modules were created to reflect the customizedequations.
(1)Generationsystem
The generation system, which realizes the energy generating performance, was composed of a generator module, fuel supply schedulemodule,andothercomponents.Theenginemodulewasused to generate electricity by burning fuelin an internalcombustion enginein thisstudy.Theinputvaluefortheenginemodulewasan energy dimension,notan amountofmethane gas.Accordingly,the fuelsupply scheduleoftheenginemodulewasdesignedby reflecting the realoperating conditions ofthe targetfarm.Thatis,1 m3 of methanegascouldbetransformedinto13kW ofelectricityaccording
Fig.16Designofgenerationsystem usingBES (2)Wasteheatrecoverysystem
Thereweretwotypesofheatexchangersinthissystem.Oneheat exchangerwasdesigned asashelland tubetypeofheatexchanger according to the situation ofthe reference farm.The exhaustgas exchanged itsheatenergy with the cooling waterthrough this heat exchanger.Anotherheatexchangerwasaconstanteffectivenesstype ofheatexchanger,and thecooling waterexchanged itsheatenergy with the waterofthe heatstorage tank.A heattransferfrom the constant effectiveness type of heat exchanger to the heat storage tankwasoperatedthroughthepipemodule,considering heatlossdue to theoutsidetemperature.Theheatenergy thathad been stored in the heatstorage tank was setto be used when the temperature of theheatstoragetank washigherthan 50℃.Using heatenergy from theheatstoragetankcontinuallyoperateduntilthetemperatureofthe heatstoragetankwasbelow 35℃.
Fig.16Designofwasteheatrecoverysystem usingBES (3)Utilizationofwasteheatsystem
The waste heat energy was firstly used to maintain optimum conditions of the anaerobic digestor for producing biogas.If any remained,itwas used to heatthe dormitory.The heating loads for maintaining optimum conditions of the anaerobic digestor for producing biogas were calculated by the equation (1)suggested in Section 3.4.2.3.This equation was applied to the simulation by the user-definedmodule.
Fig.17Designofutilizationofwasteheatrecoverysystem using BES
Lastly,the finalCPLM modelwas completed by combining the threesystems.ThismodelispresentedinFig.19.
Fig.18FinalcogenerationplantmodelusingBES dynamicdesign method
4.1.2Validation ofsimulation model (1)Modulevalidation
InordertoensurethereliabilityoftheCPLM,whichwasdesigned with TRNSYS software,modulevalidation wasconducted.Themain modules ofCPLM are the generator,the heatexchanger,the heat storagetank,andvariouspipmodules.Becausethesemoduleshavea
direct influence on the energy production and use, they were subjected to the module validation ofthe simulation modelin this study. However, most of the measuring equipment for collecting validation data werenotpresentatthereferencefarm,and thefield experiments for obtaining validation data were also limited.As a result,most measuring data could not be achieved,such as the internaltemperature of various pipes,temperature of exhaust gas from thegenerator,andtemperatureoftheheatexchanger.Therefore, the heat storage tank module,for which the field data could be obtained,was selected as the objective of module validation.The module validation was conducted from 12 to 19 February 2013,and Fig.20presentsitsresults.
Fig.19Modulevalidationresultaboutthermalstoragetankaccording tocomparingofcomputedinternaltemperaturewithexperimentdata
The internaltemperature ofthe heatstorage tank from the field data presented some fluctuations over time,as in Fig.20.These results were determined to be caused from the field experiment.
measured atonepoint,itcould bevariableaccording to theinternal flow oftheheatstoragetank.However,thesimulationresultspresent theaveragetemperatureoftheheatstoragetank.So,thesimulation results were very consistent. On the other hand, the average temperature of the internalheat storage tank from the field data presented as 42.39℃,and the average temperature ofthe simulation resultshowed42.02℃.Asaresult,asimpleerrorratewascalculated at2.1%,andthiswasdeterminedtobeareasonablelevel.
(2)Resultvalidation
The energy production results ofthe CPLM simulation modelcan be divided into two types,electricity and heatenergy.The result validation was performed on theresults ofelectricity production,for which thefield datacould beobtained.Table6showstheresultsof thisresultvalidation.
Month Jan. Feb. Mar. Apr. May Jun.
Simulationresult(MW) 8.26 6.75 10.02 8.50 9.36 8.42
Fileddata(MW) 9.16 7.48 10.70 8.81 10.00 7.53
Month Jul. Aug. Sept. Oct. Nov. Dec.
Simulationresult(MW) 9.32 10.17 10.22 10.26 9.62 10.76
Fileddata(MW) 9.02 14.15 15.41 15.32 14.36 16.04
Table 4 Comparison ofperiodicalcogeneration electricity production oftargetfarm andsimulationmodel
rangingfrom 0.31MW to5.26MW,dependingontheperiod.Because the electricity production per hour in the simulation model was affectedby thecapacity ofthegenerator,whichwasconstantly fixed in thisstudy,thetotalelectricity production ofthesimulation model wasonly influencedby theoperating timeofgenerator.Ontheother hand,the electricity production perhourofthe targetfarm showed fluctuations.The reason is thatthe generator-engine using biogas can beaffected in itselectricity production perhourby management conditionsoftheenginesuch asthenitrogen dilution ratioofbiogas and the ignition timing ofthe engine.However,the targetfarm did not manage the nitrogen dilution ratio of biogas and the ignition timing of the engine,nor did it take into account other similar factors.It is determined that it is necessary to installmeasuring equipment for the elements affecting the electricity production per hour for future studies. As mentioned above, there were some limitations related to reflecting the management conditions to the simulation.The simulation result of the electricity production was calibrated by theelectricity production,which had been described by thefielddatainthisstudy.ThisisdescribedinSection4.2.1.
4.
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4.2.1Electricity production resultofthegeneration system
Thecalibration ofthesimulation resultoftheelectricity production wasconducted using a correction valuethatwas determined by the field data. The calibration result of the electricity production is presentedin Table7,anditshowsahigheraccuracy than theresult described in Section 4.1.2. The results of monthly electricity production showed adifferencefrom itsfield datafrom 0.17 MW to 0.60MW andpresentedanaverage3% errorovertheentireperiod.
Month Jan. Feb. Mar. Apr. May Jun.
Fielddata
Generationtime
(hr) 635 519 771 654 720 648 Generationperhour
(kW) 14.4 3 14.4 1 13.88 13.4 7 13.8 9 11.6 2 Electricitygeneration (MW) 9.16 7.48 10.70 8.81 10.0 0 7.53 Corrected simulation result Electricitygeneration (MW) 9.58 7.76 11.09 9.24 10.4 2 8.13
Month Jul. Aug. Sept. Oct. Nov. Dec.
Fielddata
Generationtime
(hr) 717 782 786 789 740 828 Generationperhour
(kW) 12.5 8 18.0 9 19.61 19.4 2 19.4 1 19.3 7 Electricitygeneration (MW) 9.02 14.1 5 15.41 15.3 2 14.3 6 16.0 4 Corrected simulation result Electricitygeneration (MW) 9.51 14.3 6 15.73 15.4 9 14.5 6 16.2 7
Table 5 Corrected electricity production ofsimulation modelusing fielddata
4.2.2Heatenergy production resultofthegeneration system The heatenergy produced by the CPLM was generally used to heattheanaerobicdigestor,themain effectofwhich wastoproduce thefuelofthegenerator.Afterthatarestheatenergy wasused at many facilities ofthe targetfarm.Since heatenergy production is
designing thesystem.However,mostofthedesign casesofCPLMs did not consider the real-time weather conditions,nor the energy flow between each facility.Asa result,an overestimation ofenergy production caused a problem in management.Therefore,the present research aimedtoaccurately estimateheatenergy production through theBES dynamicdesign method.Becausetherewasno quantitative measuring equipmentofheatenergy production atthe targetfarm, thereliabilityoftheengine-generatormodulewasimprovedaccording to using the correction value via the field data,as in Section 4.1.2. Asaresult,theheatenergy production resultofthebasicsimulation modelwasestimatedaspresentedinTable8.
Month Ave. Ambient temp. (℃) Heat recovery energy(a) (GJ) Heatingloadsof anaerobic digestor(b) (GJ) Usableenergy (a)-(b) (GJ) 1 -2.6 5.17 14.15 -8.98 2 -0.4 4.63 12.60 -7.97 3 4.8 5.37 13.03 -7.67 4 9.7 5.11 11.06 -5.95 5 17.6 5.36 7.97 -2.61 6 23.5 5.12 5.09 0.03 7 25.6 5.36 4.33 1.02 8 27.3 5.50 3.48 2.02 9 21.7 5.37 5.86 -0.50 10 15.5 5.47 8.81 -3.34 11 6.0 5.26 12.14 -6.88 12 0.2 5.50 13.90 -8.40 Sum. - 63.22 112.44 -49.21 Table 6 Periodical heat recovery energy and heating load of anaerobicdigestorusingbasicsimulationmodel
As the calories for maintaining optimum temperature of the anaerobic digestor were mainly affected by heat loss via external temperature, this value was largely estimated during the winter season.Ontheotherhand,anamountofheatrecoverywasrelatively constantduring allseasons.BecausethescaleoftheCPLM wasnot large,mostpipelengthsforrecovering thewasteheatenergy ofthis
plantwere notlong and were located in the ground.Therefore,the heatloss ofpipes was relatively constantallyear.Accordingly,it was determined that the engine-generator mainly influenced the wasteheatgeneration.Theresultsofcomparing theamountofwaste heatrecovery with heating loads forthe anaerobicdigestorwereas follows:the heating loads for the anaerobic digestor were usually higherthan theamountofwasteheatrecovery,exceptin months6, 7,and 8.Thisindicatesthattheheatenergy autonomously produced by theCPLM could notcovertheheatload foroperating thisplant, and the plant required an additionalheating system.Actually,the target farm has had an additionalkerosene boiler for heating the anaerobicdigestor,andthisboilerhasbeenusedfrequently.
Fig.20Real-timeheatenergyproductionandheatingloadsfor anaerobicdigestorofbasicsimulationmodel
As the estimation aboutthe capacity ofan additionalboilercould significantly affectthe installation costand the operation expense,a
financial calculation of boiler capacity was important. The BES dynamic design method thatwas used by this study can estimate heatenergy production and heating loads foranaerobic digesters in realtime,as shown in Fig.21.Therefore,this method could also calculate the maximum instantaneous caloryof heating anaerobic digestors;the maximum calorie value was found to be 14.3 MJ. Consequently,itwas determined thatthe cogeneration plantofthe targetfarm requiredthe15MJscaleofaboilerorpropercapacityof ageneratorthatincreasestheamountofwasteheatrecovery.
4.
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TheBES dynamicdesign method,presentedin thisstudy,isbased on modulesand hasaform thatinvolvesconnecting many modules. Becauseeachmodulethatconstitutesthesimulationmodelexchanges its inputand outputvalues with othermodules,the performance of the entire system due to the exchange parameters ofeach module, like the capacity of facility module,the performance ofequipment module, etc., is easily simulated. Therefore, this method can be valuably usedtoestimatetheproperscaleofCPLM according tothe supply quantity of livestock manure or target energy production. Accordingly,theengineandanaerobicdigestormoduleswereselected as the main modules related to energy production in this research. The energy production thatresulted,according to changing capacity ofmainfacilities,wascomparedandanalyzed.
4.3.1 Estimation ofenergy production according to changing the capacity ofgenerator
Thecogenerationplantofthetargetfarm hasoperatedata50kW scale of generator.Although the generator had been designed to operate for22 hours perday,ithas actually been operated from 6 hours to 18 hours per day.In particular,ithad been operated an average of 12 hours per day during the year 2013.Thus,while engineoperation thatdiffersfrom thedesign can causefailurein the exact prediction of energy production, it can also affect energy production duetothedecreased operation timeofthegenerator.The electricity production and amountofheatrecovery werecalculatedto
beapproximately 25 and 35 kW capacity ofgenerator,which wasa smallerscale than therealcapacity aswellas50 kW.Additionally, therewas the assumption thattherequired fuelwas proportionalto thecapacityofthegenerator.Table9showstheresults.
Month Electricitygeneration(MW) Heatenergyproduction(GJ) 25kW 35kW 50kW 25kW 35kW 50kW 1 8.76 9.30 9.56 4.65 4.80 5.11 2 7.73 7.61 7.76 4.21 4.39 4.57 3 8.46 10.88 11.09 4.67 4.92 5.28 4 7.90 8.98 9.24 4.51 4.76 5.07 5 8.46 10.16 10.42 4.66 4.91 5.34 6 6.84 7.80 8.13 4.53 4.77 5.05 7 7.69 9.19 9.51 4.67 5.01 5.32 8 10.93 14.23 14.36 4.65 4.94 5.53 9 11.37 15.29 15.73 4.51 4.88 5.34 10 11.72 15.27 15.49 4.66 5.02 5.44 11 11.28 14.35 14.56 4.51 4.74 5.22 12 11.70 16.08 16.27 4.67 4.88 5.43 Sum. 112.84 139.14 142.12 54.90 58.02 62.69 Table 7 Monthly production of electricity and thermal energy dependingongeneratorcapacity(25,35,50kW)
Because the amount of fuel consumed was assumed to be proportionalto the scale of generator,the heat energy production
electricity production and total electricity production presented as relatively similar.However,a25kW scaleofthegeneratorpresented to produceapproximately 30MW lesselectricity than the35and 50 kW scales.Thereason wasdetermined by each operating time.The 25 kW scale ofthe generator could use twice the operation time compared with the50kW scalein termsofusing thesameamount offuel.However,the50kW scaleofthegeneratorhadbeenoperated over12hoursperdayduring3,8,9,10,11,12months.Althoughthe 25 kW scale of the generator was operated allday,it could not consume all of its fuel. Therefore, this led to decreasing the electricity production.Then,the heat energy production showed a tendency to be decreased according to reducing the scale of the generator.However,there was nota rapid change ofheatenergy production in the case ofthe 25 kW scale,unlike the case ofthe electricityproductionresult.
4.3.2 Estimation ofenergy production according to changing the capacity ofanaerobicdigestor
Anothermain facility thatdeterminesthesizeoftheCPLM isthe anaerobic digester.The anaerobic digestor supplies the fuelofthe generatorby producing methanegasusing livestock manure.Asthe fuel supply determines the energy production of the cogeneration plant,the anaerobic digestor is an important facility.Because the scale ofthe anaerobic digestor is increased,more methane gas is produced,and a largescaleofanaerobicdigestorenablestheenergy productionofthecogenerationplanttoincrease.However,theamount oflivestockmanurethatcan besuppliedfrom thesurroundingsmust be considered in the plant design. Although sufficient livestock
