Thermodynamic, economic and environmental evaluation of solid oxide fuel cell hybrid power generation systems

A Solid‐oxide fuel‐cell (SOFC) based power generation system generates electricity at a higher efficiency, consequently emits lower CO2 being compared with most other conventional power generation technologies. And an SOFC system also emits less gaseous pollutants such as CO, NOx, SOx, and particulate matter; therefore the SOFC system can be regarded as one of the most efficient and clean power generation technologies, particularly in the decentralized power generation area. In spite of the progress of SOFC technology, achieved during the recent decades, several barriers still prevent the full commercialization of SOFC systems; first, performance degradation should be minimized; second, robust operation should be guaranteed; finally and most importantly, the cost should be reduced not only from the manufacturing but also from the operational standpoint. In addition, the SOFC system should be environmentally friendly during its entire life‐cycle; i.e. from cradle to grave. Among the various ways of improving efficiency, economics, and environmental characteristics of the SOFC system, hybridization of SOFC with other power units has been obtaining great interest. In this thesis, a novel hybrid system, a combination of an SOFC and an internal combustion engine, has been proposed and the thermodynamic, economic, and environmental performances of the system have been evaluated using exergy‐based methods: exergetic, exergoeconomic, and exergoenvironmental analyses. For comparison purposes, a conventional simple SOFC system and an SOFC/GT hybrid system have been also assessed together with the proposed system. Through the exergetic analysis, the location, magnitude and sources of thermodynamic inefficiencies in the SOFC power generation systems are identified; chemical reactors and heat exchangers are the main sources of the exergy destruction; however, contrary to common expectation, very little exergy is destroyed within the SOFC stack. This is because the generated heat is effectively re‐utilized within the SOFC stack for reforming, internal reforming. An exergoeconomic analysis is an appropriate combination of an exergetic analysis and an economic analysis; through the analysis, the information of the cost in the SOFC power generation systems have been assessed in terms not only of capital investment but also of the cost of exergy. Results reveal that a large portion of the cost is associated with the capital investment, showing high exergoeconomic factors. Consideration should be on how to reduce the capital cost, even at the expense of the thermodynamic efficiency. The environmental impact of an energy conversion system can be assessed using a life cycle assessment (LCA); the LCA can be effectively combined with the exergetic analysis in an exergoenvironmental analysis. Through the exergoenvironmental analysis on the SOFC power generation systems, it can be concluded that improving the operational efficiency and reducing the fossil fuel use are the key factors of improving the environmental characteristics of SOFC power generation systems.