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Research Papers: Gas Turbines: Microturbines and Small Turbomachinery

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

[+] Author and Article Information
Carlo De Servi

e-mail: carlo.deservi@polimi.it

Stefano Campanari

e-mail: stefano.campanari@polimi.it
Politecnico di Milano,
Department of Energy,
Via Lambruschini 4,
20156 Milano, Italy

Alessio Tizzanini

Enel Ingegneria e Ricerca SpA,
Via Mantova 24,
00198 Roma, Italy
e-mail: alessio.tizzanini@enel.com

Claudio Pietra

Turboden s.r.l.,
Via Cernaia 10,
25124 Brescia, Italy

The occupational exposure limit, the threshold below which a toxic substance has no effect on the health of the workers for an exposure period of 8 h per day, is 1 ppm for benzene, against 100 ppm of toluene and xylene, and 300 ppm of R245fa. Since toluene is currently employed in ORC systems and the toxicity level of the other considered aromatic hydrocarbons, when available, is comparable to that one of toluene (and also of some refrigerants), it is assumed that only benzene is affected by a level of toxicity incompatible with ORC applications.

The maximum allowable operating pressure of the turbine is calculated as the nominal operating pressure plus a safety margin of 3 bar.

1Corresponding author.

Contributed by the International Gas Turbine Institute of (IGTI) ASME for publication in the Journal of Engineering for Gas Turbines andPower. Manuscript received February 29, 2012; final manuscript received October 21, 2012; published online March 18, 2013. Assoc. Editor: Piero Colonna.

J. Eng. Gas Turbines Power 135(4), 042309 (Mar 18, 2013) (11 pages) Paper No: GTP-12-1063; doi: 10.1115/1.4023119 History: Received February 29, 2012; Revised October 21, 2012

Among the various fuel cell (FC) systems, molten carbonate fuel cells (MCFC) are nowadays one of the most promising technologies, thanks to the lower specific costs and a very high electrical efficiency (net low heating value (LHV) electric efficiency in the range 45%–50% at MWel scale using natural gas as fuel). Despite this high performance, MCFC rejects to the ambient almost half of the fuel energy at about 350–400 °C. Waste heat can be exploited in a recovery Rankine cycle unit, thereby enhancing the electric efficiency of the overall system. Due to the temperature of the heat source and the relatively small power capacity of MCFC plants (from few hundred kWel to 10 MWel), 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 of the integration between a MCFC system (topping unit) and an ORC turbogenerator (bottoming unit). The potential benefits of the 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 of the overall system. Finally, the economical benefits 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 first one, the ORC power output is approximately 500 kWel; in the latter, about 1 MWel. 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.

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Figures

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Fig. 1

Saturation curves of selected fluids

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Fig. 2

Layout of the integrated plant

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Fig. 3

The composite temperature thermal power curve (T-Q curve) of the exhaust gases and the working medium. Pinch point does not occur at the ends of PHE.

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Fig. 4

Composite temperature thermal power curve (T-Q curve) of the exhaust gases and of the working medium. Pinch point occurs at the inlet of the working fluid in PHE.

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Fig. 5

Temperature–entropy diagram for cyclohexane

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Fig. 7

Temperature–entropy diagram for MM

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Fig. 8

ORC capital cost (€/kW)

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Fig. 10

LCOE reduction obtained by implementing the ORC unit

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