Modeling GHG Emissions for a MCFC Tri-Generation System - Steward, Melaina, Webster and Joseck IAEE Conference, Calgary, October 2010, Page 1 of 15 MODELING GREENHOUSE GAS EMISSIONS FOR A MOLTEN CARBONATE FUEL CELL TRI-GENERATION (HEAT, HYDROGEN, AND POWER) SYSTEM Darlene M. Steward, Senior Engineer, National Renewable Energy Laboratory, 1617 Cole Blvd, Golden, CO, 80401, U.S. A., 303-275-3837, [email protected]Marc W. Melaina, National Renewable Energy Laboratory, 303-275-3836, [email protected]Karen Webster, National Renewable Energy Laboratory, 303-275-3712, [email protected]Fred Joseck, U.S. Department of Energy, 202-586-7932, [email protected]Abstract During the early transition to hydrogen vehicles, relatively small volumes of hydrogen will be dispensed from a sparse network of fueling stations. Convenient and low-cost hydrogen refueling can accelerate vehicle adoption during this transition phase. Hydrogen fuel cell vehicles have the potential to greatly reduce greenhouse gas emissions from the transportation sector. Integrating hydrogen production with combined heat and power applications also has the potential to reduce overall greenhouse gas emissions associated with supplying buildings with heat and power. We analyze greenhouse gas emissions implications of tri-generation stationary fuel cells, providing combined heat, hydrogen and power (CHHP), with respect to variations in geography and building demand profiles. The NREL Fuel Cell Power (FCPower) model is employed to analyze system performance across yearly demand profiles, using 8760 hourly time steps for a range of climates and building types. Supplying some of the building’s electrical and heat demand as well as hydrogen from tri-generation fuel cells is compared to more traditional means of producing hydrogen and supplying heat and power. Our analysis suggests that CHHP systems may prove to provide a greater greenhouse gas (GHG) benefit when located in mild or warmer climates where electricity loads are more uniform throughout the year. Introduction One of the key barriers to the commercialization of hydrogen vehicles is the lack of hydrogen refueling infrastructure. Though proven technologies exist for delivering hydrogen to stations or producing hydrogen at stations, such as onsite steam methane reforming or electrolysis, stakeholder engagement proves challenging due to the high capital costs of refueling stations and uncertainty around future demand for hydrogen vehicles and therefore hydrogen fuel (Greene, Leiby et al. 2008; NRC 2008). One of the complicating requirements for infrastructure rollout is the need for small stations to serve early vehicle markets. Early stations will likely be underutilized as local hydrogen fuel cell electric vehicle (FCEV) fleets increase in size over time. The uncertain rate of growth in demand from FCEVs increases the financial risks associated with large stations, which are capital intensive. Deployment of smaller stations also results in a larger number of refueling locations serving early vehicle markets for a given capital outlay, stimulating demand for more FCEVs by removing the consumer adoption barrier of limited refueling availability (Melaina 2003; Melaina and Bremson 2006). Small stations also address hesitancy on the part of key stakeholders to make large commitments to novel technological systems perceived as having high technical risk (IPHE 2010). Convenient, low-cost and small-scale hydrogen refueling can therefore facilitate the investment process and accelerate vehicle adoption during the transition phase of hydrogen infrastructure development. Combined heat and power (CHP) applications are an established technology capable of utilizing energy resources more efficiently than conventional centralized electricity systems while reducing overall emissions. A typical combined efficiency for electricity from centralized power plants and heat generation in natural gas boilers is 45 percent, compared to up to 80 percent for a comparable CHP system (Shipley, Hampson et al. 2008). Carbon dioxide emissions from CHP systems can be half those from traditional systems, depending upon the source of the electricity being displaced (ICF International 2008). Of the 85 GW of CHP installed in the U.S., providing about 12 percent of all electricity, 88 percent is in industrial applications and 12 percent is in commercial and institutional building applications. The majority of CHP applications utilize reciprocating engines (46 percent of facilities), and the majority of installed CHP capacity is large combined cycle natural gas turbines (53 percent of total installed capacity) (EEA 2007). In the present study we examine a technological strategy to produce excess hydrogen from high temperature fuel cells installed in combined heat and power configurations. The resulting tri-generation systems, referred to here as combined heat, hydrogen and power (CHHP), benefit from sustained operation and relatively stable revenue streams associated with heat and power while also providing hydrogen at low volumes. We focus on building applications where hydrogen fueling capability maybe advantageous for either private FCEV fleets (“behind the fence”) or through a public outlet. If the building location is not suitable as a refueling location, hydrogen delivery costs can be relatively small if installations are located near suitable
Transcript
Modeling GHG Emissions for a MCFC Tri-Generation System - Steward, Melaina, Webster and Joseck IAEE Conference, Calgary, October 2010, Page 1 of 15
MODELING GREENHOUSE GAS EMISSIONS FOR A MOLTEN CARBONATE FUEL CELL TRI-GENERATION
(HEAT, HYDROGEN, AND POWER) SYSTEM
Darlene M. Steward, Senior Engineer, National Renewable Energy Laboratory, 1617 Cole Blvd, Golden, CO, 80401, U.S. A., 303-275-3837, [email protected]
Marc W. Melaina, National Renewable Energy Laboratory, 303-275-3836, [email protected] Karen Webster, National Renewable Energy Laboratory, 303-275-3712, [email protected]
Fred Joseck, U.S. Department of Energy, 202-586-7932, [email protected]
Abstract During the early transition to hydrogen vehicles, relatively small volumes of hydrogen will be dispensed from a sparse network of fueling stations. Convenient and low-cost hydrogen refueling can accelerate vehicle adoption during this transition phase. Hydrogen fuel cell vehicles have the potential to greatly reduce greenhouse gas emissions from the transportation sector. Integrating hydrogen production with combined heat and power applications also has the potential to reduce overall greenhouse gas emissions associated with supplying buildings with heat and power. We analyze greenhouse gas emissions implications of tri-generation stationary fuel cells, providing combined heat, hydrogen and power (CHHP), with respect to variations in geography and building demand profiles. The NREL Fuel Cell Power (FCPower) model is employed to analyze system performance across yearly demand profiles, using 8760 hourly time steps for a range of climates and building types. Supplying some of the building’s electrical and heat demand as well as hydrogen from tri-generation fuel cells is compared to more traditional means of producing hydrogen and supplying heat and power. Our analysis suggests that CHHP systems may prove to provide a greater greenhouse gas (GHG) benefit when located in mild or warmer climates where electricity loads are more uniform throughout the year.
Introduction One of the key barriers to the commercialization of hydrogen vehicles is the lack of hydrogen refueling infrastructure. Though proven technologies exist for delivering hydrogen to stations or producing hydrogen at stations, such as onsite steam methane reforming or electrolysis, stakeholder engagement proves challenging due to the high capital costs of refueling stations and uncertainty around future demand for hydrogen vehicles and therefore hydrogen fuel (Greene, Leiby et al. 2008; NRC 2008). One of the complicating requirements for infrastructure rollout is the need for small stations to serve early vehicle markets. Early stations will likely be underutilized as local hydrogen fuel cell electric vehicle (FCEV) fleets increase in size over time. The uncertain rate of growth in demand from FCEVs increases the financial risks associated with large stations, which are capital intensive. Deployment of smaller stations also results in a larger number of refueling locations serving early vehicle markets for a given capital outlay, stimulating demand for more FCEVs by removing the consumer adoption barrier of limited refueling availability (Melaina 2003; Melaina and Bremson 2006). Small stations also address hesitancy on the part of key stakeholders to make large commitments to novel technological systems perceived as having high technical risk (IPHE 2010). Convenient, low-cost and small-scale hydrogen refueling can therefore facilitate the investment process and accelerate vehicle adoption during the transition phase of hydrogen infrastructure development. Combined heat and power (CHP) applications are an established technology capable of utilizing energy resources more efficiently than conventional centralized electricity systems while reducing overall emissions. A typical combined efficiency for electricity from centralized power plants and heat generation in natural gas boilers is 45 percent, compared to up to 80 percent for a comparable CHP system (Shipley, Hampson et al. 2008). Carbon dioxide emissions from CHP systems can be half those from traditional systems, depending upon the source of the electricity being displaced (ICF International 2008). Of the 85 GW of CHP installed in the U.S., providing about 12 percent of all electricity, 88 percent is in industrial applications and 12 percent is in commercial and institutional building applications. The majority of CHP applications utilize reciprocating engines (46 percent of facilities), and the majority of installed CHP capacity is large combined cycle natural gas turbines (53 percent of total installed capacity) (EEA 2007). In the present study we examine a technological strategy to produce excess hydrogen from high temperature fuel cells installed in combined heat and power configurations. The resulting tri-generation systems, referred to here as combined heat, hydrogen and power (CHHP), benefit from sustained operation and relatively stable revenue streams associated with heat and power while also providing hydrogen at low volumes. We focus on building applications where hydrogen fueling capability maybe advantageous for either private FCEV fleets (“behind the fence”) or through a public outlet. If the building location is not suitable as a refueling location, hydrogen delivery costs can be relatively small if installations are located near suitable
Modeling GHG Emissions for a MCFC Tri-Generation System - Steward, Melaina, Webster and Joseck IAEE Conference, Calgary, October 2010, Page 2 of 15
retail locations. In CHHP applications co-located with centrally controlled vehicle fleets, financial risks can be reduced compared to public retail hydrogen stations due to increased certainty around future demand. These installations may involve multiple stakeholder groups, including building managers, utilities, and one or more fleet managers. In addition to facilitating early hydrogen infrastructure development, the CHHP strategy may also result in a durable, long-term market outside the public light duty vehicle sector, especially in cases where buildings or building owners have or are partnered with “captive” hydrogen demand from forklifts, ground equipment, buses, or hydrogen vehicles. Figure 1 provides a simple depiction of a combined heat, hydrogen and power (CHHP) stationary fuel cell system. The upper portion of the schematic indicates the baseline or conventional means of supplying a building with electricity from the grid and heat for space and water heating from natural gas (or bio-methane) combusted in a boiler. The lower portion of the schematic shows a stationary fuel cell converting natural gas (or bio-methane) into electricity and heat for the building as well as a side stream of hydrogen for use in vehicular applications. A variety of distributed generation technologies are capable of being deployed exclusively in CHP applications, including reciprocating engines, gas turbines, steam turbines (given an appropriate source of steam), and micro turbines. Fuel cells are unique among these distributed energy technologies in their capability to produce excess hydrogen in addition to heat and power. Some fuel cells produce hydrogen though external reforming of methane, including polymer electrolyte membrane (PEM) and phosphoric acid fuel cells (PAFC), while other higher temperature systems produce hydrogen internally, including solid oxide (SOFC) and molten carbonate fuel cells (MCFC). Molten carbonate fuel cells are modeled in this study, though future analyses will include SOFC systems.
Fuel Cellwith CHP
Electricity
Natural Gas
Power
Heat
Natural Gasor Biogas
Hydrogen
Figure 1. Schematic of the combined heat, hydrogen and power energy system
The Fuel Cell Power model (FCPower) has been developed for the Department of Energy by the National Renewable Energy Laboratory, and is available for download as an Excel spreadsheet (DOE 2010). In the FCPower model, CHHP system performance is resolved across an entire year, including 8760 hourly time steps with the fuel cell responding to specified building demands for electricity and economic calculations taking into account hourly electricity prices structures. The FCPower model is configured to be electricity load following, meaning that the fuel cell automatically ramps up and down within its operational range in response to the electricity load. Heat and hydrogen are produced proportionally to electricity; more heat and hydrogen are produced when more electricity is being produced. In cases where the CHHP system does not provide sufficient electricity or heat to meet building demands, electricity and natural gas are purchased. Economic analyses of CHHP systems are ongoing, and preliminary results have been presented elsewhere (Steward 2009). Stationary fuel cell system costs are a moving target as systems become commercially viable. In the present paper we focus on the GHG reduction potential of CHHP systems. The CHHP system GHG emissions are calculated with reference to a baseline or pre-fuel cell configuration, in which the building relies exclusively on the electricity grid and natural gas supply system for electricity and heating. We assume that in addition to a particular building application, the CHHP system is being considered for a location where FCEVs or some other source of hydrogen demand (e.g., fuel cell forklifts) either exists or is anticipated. Hydrogen demand is therefore included in the baseline system, with the hydrogen being produced using a small, onsite steam methane reformer (SMR), as shown in Figure 2. Energy products from the CHHP system are shown being supplied to the building and refueling equipment in Figure 3, with supplemental electricity assumed to be derived both from the US average grid mix and the respective local grid mix for comparison. In the current study, commercial (non-renewable) natural gas is the assumed feedstock, though biogas is also being considered for stationary CHP and CHHP applications (DOE 2009) and the FCPower model is capable of analyzing systems relying upon this feedstock. The sections below review the FCPower model and the influences of climate and building type on potential GHG reductions from CHHP applications. The final section presents a summary and conclusions.
Modeling GHG Emissions for a MCFC Tri-Generation System - Steward, Melaina, Webster and Joseck IAEE Conference, Calgary, October 2010, Page 3 of 15
Figure 2. Boundaries and energy flows for the baseline system.
Figure 3. Boundaries and energy flows for the CHHP system.
Modeling GHG Emissions for a MCFC Tri-Generation System - Steward, Melaina, Webster and Joseck IAEE Conference, Calgary, October 2010, Page 4 of 15
Modeling CHP and CHHP molten carbonate fuel cell systems In this section we review the methodology used to represent the technical and economic performance of CHHP fuel cell systems. Details pertinent to the unique characteristics of the FCPower model are highlighted. Other economic and operational details are provided in the online documentation for the H2A models and the FCPower model in particular (DOE 2010). Fuel cells have just begun to be used in CHP applications and hydrogen production as a byproduct has only been demonstrated in the field for molten carbonate fuel cells (Heydorn 2010). Therefore, much of the performance information summarized below is derived from modeling and performance characteristics of fuel cell CHP systems rather than CHHP systems.
Performance of CHHP stationary fuel cell systems
A basic schematic diagram of the molten carbonate fuel cell CHHP configuration is presented in Figure 4. The system concept was modeled using ASPEN Plus, a steady-state thermodynamics simulation software. The CHHP systems models use conventional industrial unit operations integrated into a novel system. The detailed model was then used to create a simplified linear model of system performance within the FCPower Model framework so that FCPower results approximate the ASPEN results within a reasonable range of system performance. Unlike alkaline, phosphoric acid, and polymer electrolyte membrane fuel cells, MCFCs do not require an external reformer to convert fuels to hydrogen. Owing to the high operating temperature, fuels are converted to hydrogen within the fuel cell itself via SMR, which reduces system complexity. In addition, MCFCs are not prone to carbon monoxide (CO) poisoning; CO is used as fuel along with hydrogen (H2). Molten carbonate fuel cells can be more efficient than PAFCs, with efficiencies approaching 50%, compared with 37%–42% for PAFCs. For MCFC configurations in which waste heat is used for additional electricity generation, electrical efficiencies greater than 60% are possible, and overall fuel efficiencies can be as high as 85% (EG&G 2005; DOE 2010). The following description of MCFC operation is taken from the Fuel Cell Handbook, 7th edition (EG&G 2005), which should be referenced for additional technology details. The following are the half-cell electrochemical reactions:
(1) )(222232 anodeeCOOHCOH −− ++→+
(2) )(2 23222
1 cathodeCOeCOO −− →++
The following is the overall cell reaction: (3) 22222
12 COOHCOOH +→++
Besides the reaction involving H2 and O2 to produce H2O, this equation shows a transfer of CO2 from the cathode gas stream to the anode gas stream via the CO3
2- ion. The need for CO2 at the cathode requires that either CO2 is transferred from the anode exit gas to the cathode inlet gas, CO2 is produced by combusting the anode exhaust gas (which is mixed directly with the cathode inlet gas), or CO2 is supplied from an alternate source. It is usual practice in an MCFC system that the CO2 generated at the anode (right side of Equation 1) be routed (external to the cell) to the cathode (left side of Equation 2). The primary disadvantage of current MCFC technology is durability. The high temperatures at which these cells operate and the corrosive electrolyte used accelerate component breakdown and corrosion, decreasing cell life. Corrosion-resistant component materials are being developed along with fuel cell designs that increase cell life without decreasing performance. Another disadvantage of MCFCs is their slow responsiveness to demand fluctuations. Because the system must balance the fuel cell temperature while maintaining an even temperature distribution, long times are required for the temperature to distribute as the output level changes. The MCFC-CHHP system developed in ASPEN uses fuel, air, and water inputs to produce electricity, heat, and hydrogen. Hydrogen from the anode exhaust gas is enriched, separated, and compressed for storage and dispensing. The system operates in two modes. In the hydrogen production mode, 70% of the caloric content of the fuel mixture entering the anode is used to make electricity (as in the CHP system), and 70% of the hydrogen in the anode exhaust gas is recovered and stored. In the hydrogen over-production mode, 60% of the caloric content of the fuel mixture entering the anode is used to make electricity, and 75% of the hydrogen in the anode exhaust gas is recovered and stored. Both modes reduce the amount of
Modeling GHG Emissions for a MCFC Tri-Generation System - Steward, Melaina, Webster and Joseck IAEE Conference, Calgary, October 2010, Page 5 of 15
energy available for heating the building by producing excess hydrogen. Detailed ASPEN process flow diagram and accompanying energy and material flows are provided in the FCPower model documentation (DOE 2010).
Figure 4. Schematic of the molten carbonate fuel cell CHHP system
Increased hydrogen production can be achieved by utilizing useful heat from the electricity production process. Hydrogen production is therefore a means of cooling the fuel cell, and available useful heat is inversely proportional to hydrogen output. Due to this direct relationship between hydrogen production and heat generation, more hydrogen can be produced at higher electricity output levels. However, the electrical efficiency of the fuel cell is slightly reduced.
Hypothetical hospital case studies
Case study data for the Fuel Cell Power (FCPower) Model include building energy load profiles for U.S. cities in eight climate zones. These data were developed by the National Renewable Energy Laboratory's Electricity, Resources, and Building Systems Integration Center for the U.S. Department of Energy's Building Technologies Program. Two hospitals, one in Chicago and one in Miami, were selected from the database and compared to illustrate the effect of climate on CHHP performance and GHG emissions. Even in the same climate, demand profiles differ between different building types because of differences in use patterns for the buildings. Hospitals were selected for this comparison because of their high overall electricity and heat demand profiles and 24-hour per day operation. Two additional building types, hotels and office buildings, are included in the GHG emissions analysis to illustrate the effect of different building types on GHG emissions.
The NREL buildings database relies upon the climate zones shown in Figure 5 to generate building electricity and heat demands. The six climate zones are indicated in the map, as well as the zones corresponding to the two building locations analyzed in the present study.
Modeling GHG Emissions for a MCFC Tri-Generation System - Steward, Melaina, Webster and Joseck IAEE Conference, Calgary, October 2010, Page 6 of 15
Figure 5. U.S. Climate zones with state and county borders and six representative cities
Geographic considerations for CHHP systems Examples of how these climate zones influence electricity and heat demands are shown for the cases of CHHP systems installed at hospitals located in Chicago and Miami in Figure 6 and Figure 7. In Chicago the hospital has significant seasonal demand fluctuations, with large heat demands in the early hours of the winter day and high electricity loads for air conditioning during the summer day. In contrast, the Miami hospital heat demands are relatively flat during the day and electricity demand is higher in the summer due to air conditioning. The fuel cell in each case follows the electricity load during a given day. During the winter in northern climates the electricity demand is well below the fuel cell capacity, while the fuel cell heat output meets less than 10% of the heat load. During the summer, the fuel cell operates at near peak electricity output, providing sufficient electricity except during peak demand in the middle of the day, and heat output supplies nearly half of the heat demand. The demands and outputs of electricity for the mild climate are similar to results for the northern climate with electricity output from the fuel cell being sufficient all day in the winter but not meeting peak demand in the summer. In the mild climate, heat supply meets a greater fraction of the heat demand in both winter and summer. The result of these demand profiles tends to be higher capacity of hydrogen production in hotter climates where electricity demand is high due to air conditioning. In these cases, hydrogen production can be increased (on average) as excess heat is used to produce hydrogen. Resulting fuel cell sizes and hydrogen production capacities for hospitals in these cities are shown in Table 1. In each case, the fuel cell size is set to the average electricity demand of the hospital plus 1 standard deviation of the demand profile, based upon previous analyses conducted with multiple runs to identify optimal system sizes (Steward 2009). Table 1. Fuel cell sizes, hydrogen production capacity and fuel cell utilization as a function of hospital location
FC Heat Output Heat Load Electricity Load FC Electrical Output
(B)
Figure 7. Miami hospital loads and fuel cell outputs for a day in winter (A) and summer (B).
Modeling GHG Emissions for a MCFC Tri-Generation System - Steward, Melaina, Webster and Joseck IAEE Conference, Calgary, October 2010, Page 9 of 15
Building type considerations for CHHP systems This section examines the influence of building type on CHHP energy use and emissions. Figure 8 summarizes the energy use for CHHP systems in comparison to the baseline system for the hospitals in Chicago and Miami discussed above as well as large offices and large hotels in the two cities. Energy use includes both onsite energy use and the upstream energy that is required to produce and transport the grid electricity and natural gas. The primary difference in the overall energy use between the baseline system (conventional supply of energy) and the CHHP system is the energy use for production of electricity. For the baseline system, approximately 3.6 kWh of energy are used for every kWh of electricity delivered to the building (~28% energy efficiency including production of fuels). In contrast, approximately 2.6 kWh of energy are used for every kWh of electricity produced by the fuel cell (approximately 38% energy efficiency including production of the natural gas). The orange bars in Figure 8 represent natural gas fuel use for the small on-site SMR unit. The SMR unit is assumed to operate at an efficiency of 68.5%, with 94% of its energy supplied with natural gas and 6% supplied with electricity, primarily for compression of the hydrogen. As noted earlier, hydrogen production is higher for the buildings in Miami than for buildings in Chicago. The compression electricity load for the CHHP system is added to the overall building load for the CHHP system and may be supplied either by the fuel cell or from supplemental electricity. Direct natural gas use for space and hot water heating (shown as red bars in Figure 8) is only slightly reduced for the CHHP systems (18% to 29%) in Chicago, indicating that the fuel cell is only supplying a small fraction of the heat load for all the building types. The fuel cell system supplies a greater fraction of the heat load in Miami, in part because the heat loads are smaller in the milder climate. The electricity loads are higher for all building types in Miami than in Chicago. Overall energy savings from the CHHP system in comparison to the baseline system for the three building types in Miami and Chicago range from 24 to 34 percent.
-1,000,000
4,000,000
9,000,000
14,000,000
19,000,000
24,000,000
29,000,000
34,000,000
39,000,000
Base Hospital
CHHP Hospital
Base Large Office
CHHP Large Office
Base Large Hotel
CHHP Large Hotel
Base Hospital
CHHP Hospital
Base Large Office
CHHP Large Office
Base Large Hotel
CHHP Large Hotel
kWh/
y
NG for SMR H2 Production
FC Fuel (NG)
Grid Electricity
Heating Fuel (NG)
Electricity sold to the Grid
Chicago Miami
Figure 8. Baseline and CHHP energy use, by source, in three building types in Chicago and Miami.
Modeling GHG Emissions for a MCFC Tri-Generation System - Steward, Melaina, Webster and Joseck IAEE Conference, Calgary, October 2010, Page 10 of 15
Figure 9 presents overall GHG intensity for the three building types including upstream energy use and emissions, such as the extraction, transportation, and refining phases, as defined for the ‘well-to-outlet’ fuel cycle in Argonne’s GREET model (Wang 2007) . The difference between the baseline and the CHHP systems reflects both the increased overall energy efficiency discussed above and the switch from the US grid mix to natural gas for supply of some of the building electricity.
0
50
100
150
200
250
300
350
400
450
Chicago Hospital
Chicago Large Office
Chicago Large Hotel
Miami Hospital
Miami Large Office
Miami Large Hotel
Tota
l Bui
ldin
g Ca
rbon
Inte
nsit
y (g
CO2e
q/M
J) Baseline CHP/H2
Figure 9. Carbon intensity for baseline and CHHP systems for three building types in Chicago and Miami, assuming a national grid mix.
The advantages observed for the CHHP systems in comparison to the baseline systems vary as a function of the electricity grid mix and carbon intensity. The results shown in Figure 8 assume a national average grid mix, with approximately 54% coal generation, 18% natural gas, 19% nuclear, 7% residual oil and 2% biomass for an electricity carbon intensity of 216 g/MJ. When state-level grid mixes are assumed, baseline building carbon intensities declines and the respective GHG advantages of CHHP systems declines because the carbon intensity of the grid mixes in Chicago and Miami are lower than the national average. In Illinois the average grid mix is 50% coal, 48% nuclear, and less than 2% natural gas and biomass, for an electricity carbon intensity of 157 g/MJ. Florida’s average grid mix is 29% coal, 37 % natural gas, 15% nuclear, 17% residual oil, and 2% biomass, for an electricity carbon intensity of 182 g/MJ. These electricity carbon intensities are based upon grid mix values from EPA and fuel cycle values from GREET (Wang 2007; EPA 2010). Results from the FCPower model, which utilize the state grid carbon intensities for Illinois and Florida, are shown in Figure 9. Comparing this figure to Figure 8 illustrates how CHHP installations at locations with relatively low carbon intensity electricity will have diminished capacity to reduce overall GHG emissions.
Modeling GHG Emissions for a MCFC Tri-Generation System - Steward, Melaina, Webster and Joseck IAEE Conference, Calgary, October 2010, Page 11 of 15
0
50
100
150
200
250
300
350
400
Chicago Hospital
Chicago Large Office
Chicago Large Hotel
Miami Hospital
Miami Large Office
Miami Large Hotel
Tota
l Bui
ldin
g Ca
rbon
Inte
nsit
y (g
CO2e
q/M
J) Baseline CHP/H2
Figure 10. Carbon intensity for baseline and CHHP systems for three building types in Chicago and Miami, assuming respective grid mixes for each state.
The FCPower model was used to model the effects of changing the size of the fuel cell in relationship to the building electricity demand for the three building types. The fuel cell size (output in kW of electricity) was varied between the average building electricity load – ½ standard deviation and the building average + 2 standard deviations. Results of this analysis are shown in Figure 11, 12 and 13 for hospitals, large offices and hotels, respectively. In these figures, the baseline energy supply includes hydrogen supply via SMR equivalent to the amount of hydrogen produced by the fuel cell system. More hydrogen is produced for larger fuel cells, so the emissions for the baseline system likewise increase slightly for the same building with fuel cells of increasing size. All of the electricity produced by the fuel cell displaces grid electricity for all building types in all locations. The fuel cell is controlled to be electricity load following, so if the electricity load is always high in relationship to the fuel cell size, all or nearly all of the electricity produced by the fuel cell will displace electricity that otherwise would have been purchased for the building. A small amount of “excess” electricity may be generated if the fuel cell cannot turn down production fast enough to match a rapid change in the building load. “Excess” electricity will also be generated if the building electricity demand falls below the minimum production rate of the fuel cell (20% of its rated power in this case). Excess electricity is routed back onto the grid and assumed to displace grid electricity generated by conventional means. Heat produced by the fuel cell displaces on-site heat production from a furnace or boiler also using natural gas. Heat that the fuel cell produces that cannot be used on-site is not counted toward emissions reductions. Figure 11, 12 and 13 illustrate the effects of the fuel cells’ interaction with different types of building load profiles on total GHG emissions. Hospitals have a relatively flat 24-hour electricity load profile, consuming nearly as much electricity during the night as during the day. In contrast, offices have a very bimodal electricity load profile, in which electricity demand fall dramatically during the night. Hotels have an intermediate daily load profile with reduced electricity demand during the night that is not as dramatic as the drop in demand for offices. For hospitals in both climates (Figure 11), even large fuel cells do not ramp up and down significantly and are never turned down to their minimum level. Therefore, little if any excess electricity is generated and the fuel cell CHHP system looks very similar to the baseline system, but shifted down because of the increased efficiency of the CHHP system in comparison to the baseline system. For the office buildings, even small fuel cells must be turned down to their
Modeling GHG Emissions for a MCFC Tri-Generation System - Steward, Melaina, Webster and Joseck IAEE Conference, Calgary, October 2010, Page 12 of 15
minimum operating level during the night. In Figure 10, only the smallest fuel cell, sized at the average electricity load -1/2 standard deviation, does not have to be turned down to its minimum operating level much of the time. If the fuel cell size is increased to the average electricity load, the fuel cell must be turned down to its minimum level and produces electricity that must be routed to the grid during the night. The displacement of grid electricity by excess production from the fuel cell, reduces the overall GHG emissions for the overall system. As the fuel cell size is increased above the average electricity load, the total emissions begin to rise due to the use of additional fuel to produce additional hydrogen. Hotels, as expected, show an intermediate profile. The average reduction in GHG emissions for CHHP systems compared to the baseline systems for all the buildings was about 26%. However, hospitals and hotels in Miami show a larger GHG reduction than their counterparts in Chicago. For hospitals the difference was about 8% and the difference was about 12% for hotels. This is due largely to the lower and more seasonally uniform heat demand in all buildings in Miami, which could supplied more effectively by the fuel cell. Office buildings showed the smallest reduction in GHG emissions and the reduction was lower than either hospitals or hotels.
0
1,000,000
2,000,000
3,000,000
4,000,000
5,000,000
6,000,000
7,000,000
8,000,000
9,000,000
0 200 400 600 800 1,000 1,200
Emis
sion
s, k
g CO
2eq/
yea
r
Fuel Cell Size (kW)
Hospital
Chicago - CHHP Miami - CHHP Chicago - Base Miami - Base
Figure 11. GHG Emissions as a function of fuel cell size for hospitals in Chicago and Miami.
Modeling GHG Emissions for a MCFC Tri-Generation System - Steward, Melaina, Webster and Joseck IAEE Conference, Calgary, October 2010, Page 13 of 15
0
1,000,000
2,000,000
3,000,000
4,000,000
5,000,000
6,000,000
0 200 400 600 800 1,000 1,200 1,400 1,600
Emis
sion
s, k
g CO
2eq/
yea
r
Fuel Cell Size (kW)
Large Office
Chicago - CHHP Miami - CHHP Chicago - Base Miami - Base
Figure 12. GHG Emissions as a function of fuel cell size for large offices in Chicago and Miami.
0
500,000
1,000,000
1,500,000
2,000,000
2,500,000
3,000,000
3,500,000
4,000,000
0 100 200 300 400 500 600
Emis
sion
s, k
g CO
2eq/
yea
r
Fuel Cell Size (kW)
Large Hotel
Chicago - CHHP Miami - CHHP Chicago - Base Miami - Base
Figure 13. GHG Emissions as a function of fuel cell size for large hotels in Chicago and Miami.
Modeling GHG Emissions for a MCFC Tri-Generation System - Steward, Melaina, Webster and Joseck IAEE Conference, Calgary, October 2010, Page 14 of 15
Summary and Conclusions High temperature fuel cells in combined heat and power (CHP) applications can be configured to produce excess hydrogen for use in vehicles. In producing three energy products, these tri-generation systems are referred to here as combined heat, hydrogen and power (CHHP) systems. We have modeled the greenhouse gas implications of this technological strategy for facilitating efforts to establish a fueling infrastructure to support early hydrogen vehicle markets. The analysis is conducted using the Fuel Cell Power (FCPower) model, developed by the National Renewable Energy Laboratory. This article explains some of the basic modeling assumptions underlying the representation of molten carbonate fuel cell (MCFC) systems in the FCPower model, and reviews the total energy use and emissions for CHHP installations in comparison to conventional supply of energy to buildings and small-scale dedicated production of hydrogen. Through these analyses we determine effects of climate, geographical location, grid carbon intensity, building type and fuel cell size on CHHP efficacy. In general, the building electricity load dictates the behavior of the CHHP system. Consistently high electricity demands such as those in milder warm climates better utilize fuel cell capacity and are able to generate more excess heat and hydrogen with reduced emissions. In harsher climates that fluctuate from season to season, fuel cell capacity is only fully utilized in the summer, and there is less excess heat available for the building and hydrogen generation, resulting in a lower GHG emissions reduction advantage. In terms of building types, building types like hospitals or hotels with more consistent electricity loads and fuel cell utilizations see greater emissions advantages than buildings with fluctuating loads, like office buildings. In addition, assumptions regarding local grid carbon intensity affect the apparent advantage of CHHP systems. As the carbon intensity of the displaced electricity supply increases, the GHG emission reductions achieved by CHHP system also increase. Considering the U.S. average grid GHG intensity, and taking into account all three energy products from a CHHP system, we generally find GHG emission reductions of approximately 24 to 34 percent.
References DOE (2009). Delivering Renewable Hydrogen: A Focus on Near-Term Applications, Palm Springs, CA, U.S.
Department of Energy, National Renewable Energy Laboratory, California Fuel Cell Partnership, Retrieved 9-10-2010, from http://www1.eere.energy.gov/hydrogenandfuelcells/delivering_hydrogen_wkshp.html.
DOE. (2010). "DOE Fuel Cell Power Analysis." Retrieved 9-10-2010, from http://www.hydrogen.energy.gov/fc_power_analysis.html.
DOE. (2010). "DOE H2A Analysis." Department of Energy Retrieved March 30, 2010, from http://www.hydrogen.energy.gov/h2a_analysis.html.
EEA (2007). CHP Installation Database, Energy and Environmental Analysis Inc., developed for Oak Ridge National Laboratory.
EG&G (2005). Fuel Cell Handbook, 7th Edition, EG&G Technical Services, Inc., University Press of the Pacific. EPA. (2010). "eGRID." Retrieved 9-10-2010, from http://www.epa.gov/cleanenergy/energy-
resources/egrid/index.html. Greene, D. L., P. N. Leiby, et al. (2008). Analysis of the Transition to Hydrogen Fuel Cell Vehicles & the Potential
Hydrogen Energy Infrastructure Requirements, Oak Ridge National Laboratory. Heydorn, E. (2010). Validation of an Integrated Hydrogen Energy Station. Hydrogen Program Annual Merit
Review, Washington, D.C., June 10, 2010, U.S. Department of Energy, Retrieved 9-10-2010, from http://www.hydrogen.energy.gov/pdfs/review10/tv006_heydorn_2010_o_web.pdf.
ICF International (2008). Availability, Economics, and Production Potential of North American Unconventional Natural Gas Supplies.
IPHE (2010). IPHE Infrastructure Workshop: Workshop Proceedings, Sacramento, CA, February 25-26, 2010, International Partnership for the Hydrogen Economy, California Fuel Cell Partnership, National Renewable Energy Laboratory. Retrieved 9-10-2010, from http://www.iphe.net/docs/Events/iphe_infrastructure_workshop_feb2010.pdf
Melaina, M. W. (2003). "Initiating hydrogen infrastructures: preliminary analysis of a sufficient number of initial hydrogen stations in the U.S." International Journal of Hydrogen Energy 28(7): 743-755.
Melaina, M. W. and J. Bremson (2006). Regularities in early hydrogen station size distributions. Energy in a World of Changing Costs and Technologies, 26th North American Conference, International Association of Energy Economics, Ann Arbor, Michigan.
Modeling GHG Emissions for a MCFC Tri-Generation System - Steward, Melaina, Webster and Joseck IAEE Conference, Calgary, October 2010, Page 15 of 15
NRC (2008). Transitions to Alternative Transportation Technologies: A Focus on Hydrogen. Washington, D.C., National Academies Press. National Research Council of the National Academies, Committee on Assessment of Resource Needs for Fuel Cell and Hydrogen Technologies.
Shipley, A., A. Hampson, et al. (2008). Combined Heat and Power: Effective Solutions for a Sustainable Energy Future, Oak Ridge National Laboratory.
Steward, D. (2009). The Influence of Building Location on Combined Heat and Power / Hydrogen (Tri-Generation) System Cost, Hydrogen Output and Efficiency. National Hydrogen Association Conference and Exposition. Columbia, SC, March 30 to April 3, 2009.
Wang, M. (2007). "GREET Model and Documentation." Retrieved April 30, 2007, from http://www.transportation.anl.gov/software/GREET/index.html.
