Over the past few years, the U.S. Environmental Protection Agency (EPA) has proposed
CO2 emission standards for existing, modified/reconstructed, and new power pants. On September 20, 2013, the EPA proposed emission standards for new power plants, targeting utility boilers, IGCC power plants, and natural gas-fired stationary combustion turbines. The EPA proposed that utility boilers and IGCC plant be required to meet a standard of 1,000 lbmCO2/MWh (0.45 kgCO2/kWh) over a one year period, or 1,000–1,050 lbmCO2/MWh (0.45–0.48 kg CO2/kWh) over a seven year period. New combustion turbines with a heat input greater than 850 MMBtu/h (approximately 100 MWe) would need to limit their emissions to 1,000 lbm CO2/MWh (0.45 kgCO2/kWh), while those with a heat input less than or equal to 850 MMBtu/h would need to limit their emissions to 1,100 lbm CO2/MWh
(0.50 kgCO2/kWh) [27]. On June 2, 2014, theEPAproposed emission standards for modified and reconstructed power plants. These standards established rate-based limits depending on the system type and time of modification [28]. On this same date, the EPA proposed the Clean Power Plan to establish state-specific, rate-based emission goals for existing power plants. The proposed guidelines would allow states to formulate their own plans toward meeting emission targets [29].
Previous life cycle assessment (LCA) studies indicate that CO2 emissions from SOFC systems may be a concern, particularly during their use phase. LCA is a methodology for assessing a system’s environmental impact over the course of its life cycle [108,109]. Staffell, et al. [110] conducted a comparativeLCA between a 1 kW residentialSOFC-CHPsystem and a conventional system. The conventional system drew on electricity from the power grid and a condensing boiler to meet the thermal energy demand. The authors simulated both the
SOFC and conventional systems in 1,000 residential buildings in the United Kingdom. The authors found that theSOFCsystem emitted significantly moreCO2during its use stage than during its manufacturing and disposal stages. Osman and Ries [30] developedLCAmodels of various energy systems for commercial buildings, including an SOFC system, microturbine, and internal combustion engine. The authors found that the SOFC system produced higher global warming emissions across its life cycle compared to the other systems. These higher emissions resulted from the steam reforming of natural gas. Karakoussis, et al. [31] performed a life cycle inventory study on the manufacturing of planar and tubularSOFCsystems. These authors compared key air emissions associated with theSOFCsystem manufacturing process with those during its use stage. The authors determined that the SOFC systems emitted significantly more CO2 emissions during their use stage than during their manufacturing stage.
The present work calculates the CO2 emissions during the operation of hybrid and non- hybrid systems. The hybrid system’s emissions are compared to the EPA’s proposed reg- ulations for new power plants (described above). These regulations serve as a benchmark for future power generation. The hybrid system’s emissions are also compared to those of more conventional CHP technologies. In particular, emissions data is taken from the EPA’s Catalog ofCHP Technologies [5] for reciprocating internal combustion engines, gas turbines,
and microturbines. These more mature technologies provide a benchmark for current power generation. Lastly, the hybrid system’s emissions are compared to those of coal and natural gas sources in theU.S. TheU.S. Energy Information Administration provides emissions data for coal and natural gas sources [111,112]. Clearly, the environmental prospect of SOFC
systems depends on a number of different factors. The present study addresses only a subset of these factors.
SOFC systems impact the environment during other life cycle stages as well. Osman and Ries [30] found that over 99% of the SOFCsystem’s NOxemissions originated upstream of the use phase, during manufacturing and fuel production. The SOFC system emitted 1.41 × 10−3 kg NOx/kWh, compared to 2.34 × 10−3 kg/kWh emitted by the microturbine. Pehnt [113] similarly found that the fuel cell system’s acidification impact originated pri- marily from upstream manufacturing and fuel production processes. The electricity used to manufacture the stack, in particular, contributed significantly to the system’s environmental impact. Exergetic life cycle assessment (ExLCA) is another technique for evaluating a sys- tem’s environmental impact. ExLCAquantifies the mass, energy, and exergy flows associated with each life cycle stage. The system’s (or process’s) total exergy destruction equals the sum of its component exergy destructions, and the exergetic efficiency equals the total exergy output divided by the total exergy input [114]. Ozbilen, et al. [115] performed an ExLCA
of an H2 production process involving the thermochemical splitting of water. These authors applied exergy balance equations to the fuel processing, thermal energy production, and water-splitting stages. The fuel processing stage exhibited the highest exergy destruction (corresponding to the lowest exergetic efficiency), whereas the water-splitting stage exhibited the lowest exergy destruction (corresponding to the highest exergetic efficiency). Because the fuel processing stage exhibited the lowest exergetic efficiency, the authors recommended that this stage receive the most attention when improving system performance in future work. While the present study does not consider upstream life cycle stages, such as fuel processing or manufacturing, the integration of exergy with LCA is a possible direction for future work.
2.5 SUMMARY
The present work builds upon previous studies. Numerous studies, in particular, have inves- tigated hybrid system control strategies. The present work complements these system-level studies by focusing specifically on theSOFCstack’s behavior. Furthermore, numerous studies indicate that electrochemical dynamics inside fuel cells diminish within milliseconds follow- ing a load change. It seems reasonable, then, to neglect electrochemical processes inside an
SOFCduring dynamic operation. Few (if any) studies, however, have verified this assumption across a wide range of operating conditions, as done in the present study. Moreover, exer- getic and economic studies demonstrate the potential of SOFCsystems to operate efficiently and cost-effectively. The present study focuses specifically on comparing kW-scale hybrid and non-hybrid systems. Lastly, the EPA’s proposed regulations provide a benchmark for future power generation, and emissions from more mature technologies provide a benchmark for current power generation. The next chapter develops theSOFCmodel. TheSOFCmodel is incorporated into a larger system model in later chapters.