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Session: Parallel Session: Systems Design, Optimization and Applications II
Session starts: Friday 23 September, 09:00
Presentation starts: 09:40
Room: Senaatszaal

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Carlo De Servi (Politecnico di Milano)
Alessio Tizzanini (Enel Ingegneria e Innovazione SpA)
Roberto Bini (Turboden s.r.l.)
Claudio Pietra (Turboden s.r.l.)
Stefano Campanari (Politecnico di Milano)

Abstract:
Fuel cells (FC) are a promising technology for distributed electricity production, especially for power applications in the few hundred kW to 10 MW size range, but they have not yet achieved significant penetration into energy market, mainly due to their high specific costs compared to other conventional technologies. They can be applied to combined heat and power, recovering heat dissipated by stack exhaust gases, when the power plant can be installed in presence of a heat demand, and they can use natural gas as primary fuel as well as biogas (for instance from wastewater treatment) or fuel blends. A possible way to improve a FC power plant economics consists in enhancing the electrical efficiency of the overall system as much as possible, exploiting the waste heat to generate additional electricity by means of an Organic Rankine Cycle (ORC) used as a heat recovery bottom cycle [1]. In this paper, the potential benefits of the integration between a fuel cell (topping cycle) and an ORC (bottoming cycle) are assessed in relation to a specific case study, related to a fuel cell unit which is in an early commercialization stage: the molten carbonate fuel cell (MCFC) unit recently proposed by Fuel Cell Energy [2]. This kind of fuel cell has been selected due to its well established performances, its increasingly competitive cost of electricity and its availability on the market. On the other hand, the relatively low temperature of the exhaust heat generated by the fuel cells (the exhaust gases are released from the MCFC at about 370°C) is particularly suitable for recovery through a bottoming cycle based on the ORC technology. ORC are nowadays more and more applied in many fields, for example in the exploitation of low entalphy geothermal sources [3] or waste heat from biomass. In this study, to enhance the performances of the fuel cell+ORC combined cycle, both subcritical and supercritical technology for the ORC are considered and optimized for the chosen working fluid. Thanks to the modular features of the fuel cell system, it is possible to analyze two different power sizes, with the ORC resulting at about 500 kWel and 1 MWel power output, evidencing scale effects on the integration. Simulation of the integrated plant of the bottoming cycle are performed in Aspen Plus® environment and optimized by means of a Matlab® code. Results show that recovering waste heat from a MCFC unit could increase the electrical power and efficiency of the plant by more than 10%, well exceeding a 50% overall electrical efficiency. A preliminary economic analysis investigates the feasibility of the proposed solution, showing a reduction of about 8% of the levelized cost of electricity (LCOE) of the MCFC plant. On the other hand, an environmental comparison shows the extremely low pollutant emissions which could be achieved by this power plant with respect to competitive conventional technologies.