Exergy represents the quality of energy. Unlike the first law of thermodynamics, which considers only the quantity of energy, exergy considers the usefulness (or value) of energy [87]. Moran, et al. define exergy as follows:
Exergy is the maximum theoretical work obtainable from an overall system consisting of a system and the environment as the system comes into equilibrium with the environment (passes to the dead state). [49, p. 362, emphasis in the original]
Calise, et al. [61] performed perhaps one of the earliest exergy studies on a hybrid system. The authors performed an exergy analysis of a 1.5 MW hybrid system consisting of an internal reforming fuel cell stack, gasifier turbine, power turbine, and balance-of-plant com- ponents. During operation, the authors found that the largest exergy destruction occurred in the SOFC stack, followed by the afterburner. The authors varied the fuel cell’s current density and operating pressure to improve the system’s performance. They found that de- creasing the current density reduced the system’s total exergy destruction, thus increasing the system’s electric and overall efficiencies. Increasing the fuel cell’s operating pressure further improved the system’s performance, but only up to a certain pressure. Gandiglio, et al. [60] performed an exergy analysis of a large (280 MW) hybrid system. Similar to Calise, et al. [61], these authors found that the largest exergy destruction occurred in the
SOFCstack and afterburner. Calise, et al. [62] also performed an exergy analysis of a hybrid system, this time considering both full-load and part-load operation. The authors found that the system achieved an exergetic efficiency of 62.6% at full-load operation. The sys- tem’s part-load performance, on the other hand, depended largely on the control strategy. In particular, reducing the fuel flow rate (while maintaining a constant air flow rate) reduced the system’s exergetic efficiency to less than 45% at low loads. Alternatively, maintaining a constant fuel-to-air ratio allowed for a higher efficiency but reduced the system’s operating envelope.
Economic analyses generally fall into one of the following categories: thermoeconomic or techno-economic analyses. A thermoeconomic analysis integrates energy and exergy balances to calculate the product streams’ costs (e.g., electricity and steam costs). Techno-economic analyses, on the other hand, incorporate only energetic performance data into life cycle cost calculations [49,60]. Gandiglio, et al. [60], for instance, performed a thermoeconomic com- parison between multi-MW hybrid (pressurized) and non-hybrid (atmospheric) power plants. The authors first defined each system’s productive structure, thus specifying the resources (e.g., fuel) and products (e.g., electricity) associated with each system. The authors then
applied cost balances to each system component to calculate the levelized cost of electric- ity, or LCOE (defined as the ratio of the life cycle cost to net power output), as well as the thermoeconomic cost of electricity, or TCOE (defined as the ratio of the total exergetic cost to the net power output). The authors found that the hybrid and non-hybrid systems exhibited similar LCOEs, but in terms of TCOE, the hybrid system performed superior to the non-hybrid system (47.71$/MWh vs. 64.19 $/MWh, respectively). Franzoni, et al. [88] performed a techno-economic analysis of a hybrid system with variousCO2 capture configu- rations. The authors found thatCO2 capture with steam condensation yielded a lowerLCOE and capital cost than a system with amine-based capture. Santin, et al. [89] performed a techno-economic analysis of a hybrid system with different fuels. The authors found that methanol-fueled systems yielded higher internal rates of return on investment (IRRs) than kerosene-fueled systems. Cheddie and Murray [90] performed a techno-economic analysis of a multi-MW hybrid system, finding that the system yielded a minimumLCOEof 4.65 ¢/kWh. In addition to hybrid systems, authors have considered advanced SOFC power plants to address environmental concerns surrounding central (baseload) power generation. Siefert, et al. [91] performed a techno-economic analysis of an integrated gasification fuel cell (IGFC) power plant with carbon capture and sequestration (CCS). The power plant yielded an
IRRof approximately 4%. The authors compared theIGFC-CCS’s economic performance to that of more conventional fossil fuel power plants, including natural gas combined cycle and pulverized coal combustion power plants. The authors found that advanced power plants (IGFC and integrated gasification combined cycle (IGCC) power plants with CCS) yielded lowerLCOEs than more conventional power plants, assuming sufficiently high natural gas and
CO2 emission prices. Siefert, et al. [92] modeled an IGFC-CCS with a CaO-looping gasifier. The authors found that the power plant yielded anIRR of approximately 5%, depending on the SOFC’s operating point. Siefert and Litster [93] and Trendewicz and Braun [94] both performed techno-economic analyses of biogas-fueledSOFCsystems. These studies found the
SOFCsystems to be economically competitive with more conventional technologies, including microturbines and reciprocating engines. Becker, et al. [95] performed a techno-economic analysis of an SOFCpolygeneration system, which produced thermal energy, H2, and power, all from the same fuel source. The authors determined the cost ofH2 production (4.4 $/kg) to
be competitive with alternative H2 production pathways, such as steam-methane reforming and electrolysis.
The present study perhaps draws most heavily on the work of Braun [56,96,97], who con- sidered small (residential-scale) non-hybrid, SOFCsystems. Braun [97] performed a techno- economic analysis of differentSOFCsystem configurations, considering such features as anode gas recycle, cathode gas recycle, internal reforming, external reforming, H2-fuel, and CH4- fuel. He found that theCH4-fueled system with anode gas recycle, cathode gas recycle, and 100% internal reforming yielded the greatest life cycle savings. Braun also found that even lower life cycle costs could be achieved by varying the fuel cell’s voltage, fuel utilization, temperature, and air temperature rise. In his thesis and related work [56], Braun provides a thorough discussion of his modeling methodology and results. Hawkes and Leach [98–100] and Hawkes, et al. [101–104] also considered residential-scaleCHP systems. Similar to many of the studies reviewed herein, the present work calculates the LCOEs of hybrid and non- hybrid systems. The present study also investigates the sensitivity of theLCOEto variations in operating parameters. The present study, however, does not implement a formal opti- mization method. More formal techniques may be found in the literature [98–107].