1、2011 ASHRAE 961ABSTRACTCombined heat and power (CHP) is an efficient and cleanmethod of providing energy services at the point of use. Insteadof purchasing electricity from the local utility and burning fuelin an on-site furnace or boiler to produce needed thermalenergy, an industrial or commercial
2、user can use CHP toprovide both energy services in one energy-efficient step.Consequently, CHP can provide significant energy efficiencyand environmental advantages over separate heat and power. It is becoming increasingly critical that a commonapproach be established to estimate the fuel and CO2emi
3、s-sions savings of CHP. This approach will need to recognizeboth outputs of the CHP system, and be able to compare the fueluse and emissions of the CHP system to the fuel use and emis-sions that would have normally occurred in providing energyservices to the site through separate heat and power. A k
4、ey factor in estimating the energy and CO2emissionssavings for CHP is determining the nature of the avoidedcentral station generation. Should the calculation of thedisplaced energy and CO2emissions be based on the all-gener-ation average of the region the facility is located in, the all-fossil avera
5、ge, the average for some specific fuel type, an esti-mate of marginal generation, or a projection of future installedgeneration? This paper provides a suggested methodology for calcu-lating fuel and CO2emissions savings from CHP, and devel-ops recommendations on the appropriate nature of avoidedcent
6、ral station generation and the level of regional aggrega-tion for accurate estimates of energy and emissions savings.The methodology for calculating fuel savings is consistentwith and equivalent to the calculation of primary energysavings (PES) included in the European Union CogenerationDirective (E
7、U 2004).INTRODUCTIONCombined heat and power (CHP) is an efficient and cleanapproach to generating power and useful thermal energy froma single fuel source. CHP is used either to replace or supple-ment conventional separate heat and power (SHP). Instead ofpurchasing electricity from the local utility
8、 and burning fuel inan on-site furnace or boiler to produce needed thermal energy,an industrial or commercial user can use CHP to provide bothenergy services in one energy-efficient step. Every CHP appli-cation involves the recovery of otherwise wasted thermalenergy to produce additional power or us
9、eful thermal energy;as such, CHP can provide significant energy efficiency andenvironmental advantages over SHP. CHP can be configured either as a topping or bottomingcycle. In a topping cycle, fuel is combusted in a prime moversuch as a gas turbine or reciprocating engine, generating elec-tricity o
10、r mechanical power. Energy normally lost in the primemovers hot exhaust and/or cooling systems is recovered toprovide process heat, hot water, or space heating/cooling forthe site.1In a bottoming cycle, also referred to as waste heatrecovery, heat energy is recovered from the hot exhaust of afurnace
11、 or kiln to generate mechanical power or electricitythough a Rankine power cycle (ASHRAE 2008). 1.In another version of a topping cycle, fuel is burned in a boiler toproduce high-pressure steam. That steam is fed to a steam turbine,generating mechanical power or electricity before exiting theturbine
12、 at lower pressure and temperature and used for process orheating applications at the site.Fuel and CO2Emissions Savings Calculation Methodology for Combined Heat and Power (CHP) SystemsBruce A. Hedman, PhD Anne C. HampsonBruce A. Hedman is a vice president and Anne C. Hampson is a senior associate
13、at ICF International, Washington, DC.LV-11-0292011. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions, Volume 117, Part 1. For personal use only. Additional reproduction, distribution, or transmission in either print or
14、 digital form is not permitted without ASHRAES prior written permission.962 ASHRAE TransactionsThe advantages of CHP broadly include the following:The simultaneous production of useful thermal and elec-trical energy in CHP systems leads to increased fuelefficiency.CHP units can be strategically loca
15、ted at the point ofenergy use. Such on-site generation avoids the transmis-sion and distribution losses associated with electricitypurchased via the grid from central stations. The increase in fuel use efficiency generally translates toreductions in greenhouse gas emissions.Figure 1 shows the effici
16、ency advantage of CHPcompared with conventional central station power generationand on-site boilers.2By avoiding line losses and capturingmuch of the heat energy normally wasted in power generation,CHP systems typically achieve total system efficiencies of60% to 80% compared to only about 45% to 50%
17、 for conven-tional separate electricity and thermal energy generation. Byefficiently providing electricity and thermal energy from thesame fuel source at the point of use, CHP significantly reducesthe total primary fuel needed to supply energy services to abusiness or industrial plant. As shown in F
18、igure 1, CHP systems not only reduce theamount of total fuel required to provide electricity and thermalenergy services to a user, but also shift where that fuel is used.Installing a CHP system onsite will generally increase theamount of fuel that is used at the site because additional fuelis requir
19、ed to operate the CHP system as compared to theequipment that would have otherwise been required onsite toserve the local thermal loads. For the example shown inFigure 1, the on-site fuel use increases from 56 units in theconventional generation case to 100 units in the CHP case.However, despite thi
20、s increase in on-site fuel use, the total fueluse needed to deliver the required electrical and thermalenergy services to the facility drops from 154 units in theconventional generation case to 100 units in the CHP case.Using less fuel to provide the same amount of energyservices generally translate
21、s into reduced emissions of carbondioxide (CO2) and other stack emissions. Figure 2 shows theCO2 emissions savings of a natural gas combustion turbineCHP system compared with conventional central stationpower generation and on-site natural gas boilers.3In this case,the CHP system produces less than
22、half the annual CO2emis-sions of SHP providing the same energy services. CALCULATING FUEL AND CO2EMISSIONS SAVINGS FROM CHPThe energy savings benefit of a CHP system is found inthe aggregate reduction in overall fuel consumption. A CHPsystem replaces both a separate on-site thermal system(furnace or
23、 boiler) and purchased power (typically electricityfrom a central station power plant) with a single, integratedsystem producing both thermal energy and power concur-rently. To calculate the fuel use or CO2emissions avoided bya CHP system, both outputs of the CHP system must beaccounted for. The CHP
24、 systems thermal output displaces thefuel normally consumed in and emissions from on-site thermal2.Conventional power plant delivered efficiency of 31% (HHV) isbased on eGRID 2005 data and reflects the national average all-fossil generating efficiency of 33.7% and 7% transmission anddistribution los
25、ses. eGRID provides information on emissions andfuel resource mix for individual power plants, generating compa-nies, states, and regions of the power grid. eGRID is available atwww.epa.gov/cleanenergy/energy-resources/egrid/index.html.Figure 1 CHP versus separate heat and power (SHP) productionener
26、gy efficiency.3.Based on the efficiencies included in Figure 1 and 7000 annualoperating hours. Central station CO2 emissions based on eGRID2005 average all fossil generation average.2011 ASHRAE 963generation in a boiler or other equipment, and the poweroutput displaces the fuel consumed and emission
27、s from grid-connected power plants. To quantify the fuel or CO2emissionssavings of a CHP system, the fuel use or emissions releasedfrom the CHP system must be subtracted from the fuel use oremissions that would normally occur without the system (i.e.,using conventional separate heat and power). An e
28、quationshowing this relationship for fuel savings isFS= (FT+ FG) FCHP ,whereFS= total fuel savings,FT = fuel use from avoided on-site thermal production,FG= fuel use from avoided purchased grid electricity, andFCHP= fuel use by the CHP system. The equation for CO2emissions savings isCS= (CT+ CG) CCH
29、P , whereCS= total CO2 savings, CT = CO2 emissions from avoided on-site thermal production,CG= CO2 emissions from avoided purchased grid electricity, andCCHP= CO2 emissions from the CHP system.CHP Fuel Use and CO2Emissions CalculationsThe energy content of the fuel consumed by the CHPsystem can be m
30、easured directly as the higher heating value ofthe fuel consumed (typically in MMBtu or GJ) or by the fuelvolume or weight, which can then be converted to the energyvalue through fuel-specific energy factors or heating values.Fuel consumption can also be estimated based on the electricor power outpu
31、t of the CHP system and the net generation effi-ciency. The CO2emissions from the CHP system are a func-tion of the type and amount of fuel consumed. CO2emissionsrates are commonly presented as pounds of emissions perMMBtu of fuel input (lb/MMBtu or kg/GJ). Table 1 showsenergy and CO2emissions facto
32、rs for common fuels.Fuel Use and CO2Emissions Avoided at the SiteThe fuel use and CO2emissions avoided at the site resultfrom the displacement by the CHP system of some or all ofthe fuel otherwise combusted in boilers or other thermalequipment to provide required heating or cooling services.The amou
33、nt of energy and emissions represented by thisavoided fuel can be calculated from the thermal output of theCHP system and the efficiency characteristics of the avoidedthermal equipment. The amount of avoided fuel use can beestimated by measuring the annual useful thermal output ofthe CHP system and
34、applying an efficiency factor represen-tative of the avoided equipment (e.g., 80% efficiency for anatural gas boiler). Once the fuel used to produce the equiv-alent amount of avoided thermal energy is estimated, theavoided energy and CO2emissions can be calculated throughthe fuel-specific factors th
35、at are listed in Table 1. Relevantequations for calculating avoided thermal fuel use and CO2emissions areFT= CHPT/ T,whereFT= avoided thermal fuel savings, MMBtu (GJ);CHPT= CHP system useful thermal output, MMBtu (GJ);T= avoided thermal equipment efficiency, %; andCT= FT EFFFigure 2 CHP versus SHP p
36、roductionCO2 emissions.964 ASHRAE TransactionswhereCT= avoided thermal CO2emissions savings, CO2lb (kg);FT= avoided thermal fuel savings, MMBtu (GJ); andEFF= fuel specific emission factor, CO2lb/MMBtu (kg/GJ).Fuel Use and CO2Emissions Avoided at the Central Station Power PlantTo estimate the fuel an
37、d emissions reduction fromavoided electricity purchases, one must first determine theamount of grid generation avoided by the generation of powerfrom the CHP system. When power produced from a CHPsystem is consumed onsite, close to or at the point of genera-tion, the overall grid savings also includ
38、es the avoided deliverylosses for power that occur along the transmission and distri-bution systems. A portion of the electricity that is transmittedover power lines is lost due to resistance and other forms ofdissipation, commonly referred to as transmission losses. Theamount of electricity actuall
39、y delivered to consumers is there-fore less than the amount generated at central station powerplants, usually by about 7% to 10% (WRI 2007). Conse-quently, avoiding 1 MWh of purchased electricity onsitemeans that more than 1 MWh of electricity no longer needs tobe generated at the central station po
40、wer plant. In order to esti-mate fuel and emissions savings from displaced central stationgeneration, the electricity displaced on-site must be convertedto a corresponding amount of avoided grid generation. Thiscan be calculated using the following equation:EG= CHPE/ (1 LT state electricity generati
41、on may not serve all ofthe consumption within the state. The eGRID subregion emis-sions and resource mix (based on generation, not consump-tion) uniformly attribute power in a specific region of thecountry and minimize these issues. Using the eGRID subregion emission factors is therecommended approa
42、ch by EPAs Climate Leaders program(EPA 2008b). eGRID subregions are identified and defined byEPA using the NERC regions and PCAs as a guide. An eGRIDsubregion is often, but not always, equivalent to an IntegratedPlanning Model (IPM) subregion. The 26 eGRID subregionsin eGRID2007 are subsets of the N
43、ERC regions (see Figure 3).A plants associated PCA determines the plants associatedeGRID subregion, which is defined as a subset of the NERCregion and is composed of entire PCAs, with the exception ofPJM Interconnection and New York Independent SystemOperator PCAs (each is associated with three eGRI
44、D subre-gions). eGRID subregions consist of one or a portion of apower control area and generally represent sections of the gridthat have similar emissions and resource mix characteristics.eGRID subregions may also be partially isolated by transmis-sion constraints. The total fuel use and CO2emissio
45、ns represented byavoided central station power are calculated through thefollowing equations:FG= EG HRGwhereFG= fuel use from avoided purchased grid electricity, Btu (kJ)EG= total grid generation avoided, kWhHRG= central station heat rate, Btu/kWh (kJ/kWh), and CG= EG EFGwhereCG=CO2emissions from av
46、oided purchased grid electricity, lb (kg)EG= total grid generation avoided, MWhEFG= central station emission factor, CO2lb/MWh (kg/MWh)Special Case: Waste Heat RecoveryIn the case of bottoming cycle or waste heat recovery forpower CHP systems, power is generated onsite from the hotexhaust of a furna
47、ce or kiln with no additional fuel require-ment, and no thermal energy is displaced at the site. Therefore,the fuel use and CO2emissions for both the CHP system anddisplaced thermal energy are both zero; CHP system fuel andemissions savings are simply equal to the displaced centralstation power fuel
48、 use and CO2emissions. 5.The PJM Interchange is a regional transmission organization(RTO), which is part o the Eastern Interconnection Grid, operat-ing an electric transmissions system serving all or parts of Dela-ware, Illinois, Indiana, Kentucky, Maryland, Michigan, NewJersey, North Carolina, Ohio
49、, Pennsylvania, Tennessee, Virginia,West Virginia, and the District of Columbia.Figure 3 eGRID subregion map.966 ASHRAE TransactionsON WHAT DISPLACED CENTRAL STATION POWER CHARACTERISTICS SHOULD THE ENERGY AND EMISSIONS SAVINGS CALCULATION BE BASED?A key factor in estimating the energy and CO2emissionssavings for CHP is determining the nature of the avoidedcentral station generation. Should the calculation of thedisplaced energy and CO2emissions be based on the all-gener-ation average of the region the facility is located in, the all-fossil average, the average for some s
