Enhancement of the Electrical Efficiency of Commercial Fuel Cell Units by Means of an Organic Rankine Cycle: A Case Study

Abstract

Among the various fuel cell (FC) systems, molten carbonate fuel cells (MCFC) are nowadays one ofthe most promising technologies, thanks to the lower specißc costs and a very high electrical efficiency (net low heating value (LHV) electric efficiency in the range 45%-50% at MWei scale using natural gas as fuel). Despite this high performance, MCFC rejects to the ambient almost half of the fuel energy at about 350^00° C. Waste heat can be exploited in a recovery Rankine cycle unit, thereby enhancing the electric efflciency ofthe overall system. Due to the temperature ofthe heat source and the relatively small power capacity of MCFC plants (from few hundred kW^i to ¡OMW^i), steam Rankine cycle technology is uneconomical and less efficient compared to that of the organic Rankine cycle (ORC). The objective of this work is to verify the practical feasibility ofthe integration betn'een a MCFC system (topping unit) and an ORC turbogenerator (bottoming unit). The potential beneflts ofthe combined plant are assessed in relation to a commercial MCFC stack. In order to identify the most suitable working fluids for the ORC system, organic substances are considered and compared. The figure of merit is the maximum net power ofthe overall system. Finally, the economical beneflts of the integration are determined by evaluating the levelized cost of electricity (LCOE) of the combined plant, with respect to the standalone MCFC system. In order to assess the economic viability of the bottoming power unit, two cases are considered. In the flrst one, the ORC power output is approximately 500 kW^¡; in the latter, about I h/TW^i. Results show that the proposed solution can increase the electrical power output and efficiency of the plant by more than 10%, well exceeding 50% overall electrical efficiency. In addition, the LCOE of the combined power plant is 8% lower than the standalone MCFC system.

Organic Rankine Cycle Power Systems: From the Concept to Current Technology, Applications, and an Outlook to the Future

“…They have not reached widespread market introduction because of the very high cost per kilowatt and because of long-term reliability and maintenance issues. The first commercialized MCFC units are in the 300 kW e -3 MW e power range, and an ORC power system is the heat recovery system of choice, not only because of technical reasons, but also because its cost per kilowatt is bound to be lower than that of the fuel cell, thus inherently improving the return on investment of the total system [201][202][203].…”

Toward the Integrated Design of Organic Rankine Cycle Power Plants: A Method for the Simultaneous Optimization of Working Fluid, Thermodynamic Cycle, and Turbine

“…For waste heat recovery, internal recuperation has either no impact or a positive effect on the ORC power output, despite the reduced amount of thermal energy recovered, as demonstrated in Ref. [47]. In both types of applications, the adoption of a recuperated cycle configuration, thus, allows the identification of the solutions with the highest net power output.…”

“…Common working fluids adopted for high-temperature ORC systems belong to the family of siloxanes, alkanes, cyclic-alkanes, and aromatic hydrocarbons, see, e.g., Refs. [1,9,10], and [47]. A special mention is also deserved for perfluorocarbons, which exhibit remarkable thermal stability and have been successfully investigated in the 1980s for small-scale ORC turbogenerators [66] but are affected by a high value of the Global Warming Potential.…”

“…The organic working fluids that appear more suitable for the selected temperature levels based on the literature results in Refs. [47] and [67] are toluene, cyclohexane, cyclopentane, and the linear siloxanes MM and MDM. These fluids are here considered to generate benchmark solutions for 1-stage CoMT-CAMD.…”

Economic analysis of CO2 capture from natural gas combined cycles using Molten Carbonate Fuel Cells

“…The specific gas turbine equipment cost was taken from reference literature [40,41] and consistently with [31,36]. The MCFC, whose Total Equipment Cost (TEC) was assumed equal to 2700 €/kW el according to indications for current costs in [43,44]. A large number of parallel MCFC modules have been assumed to achieve the total power output required in the plants, provided that the current largest commercial module is 2.8 MW.…”