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RESEARCH PAPERS

The Thermodynamic Evaluation and Optimization of Fuel Cell Systems

[+] Author and Article Information
N. Woudstra

Section Energy Technology, Faculty of Mechanical Engineering and Marine Technology, Delft University of Technology, Mekelweg 2, 2628 CD Delft, The Netherlandsn.woudstra@wbmt.tudelft.nl

T. P. van der Stelt, K. Hemmes

Section Energy Technology, Faculty of Mechanical Engineering and Marine Technology, Delft University of Technology, Mekelweg 2, 2628 CD Delft, The Netherlands

J. Fuel Cell Sci. Technol 3(2), 155-164 (Jan 04, 2006) (10 pages) doi:10.1115/1.2174064 History: Received July 26, 2005; Revised January 04, 2006

Energy conversion today is subject to high thermodynamic losses. About 50% to 90% of the exergy of primary fuels is lost during conversion into power or heat. The fast increasing world energy demand makes a further increase of conversion efficiencies inevitable. The substantial thermodynamic losses (exergy losses of 20% to 30%) of thermal fuel conversion will limit future improvements of power plant efficiencies. Electrochemical conversion of fuel enables fuel conversion with minimum losses. Various fuel cell systems have been investigated at the Delft University of Technology during the past 20 years. It appeared that exergy analyses can be very helpful in understanding the extent and causes of thermodynamic losses in fuel cell systems. More than 50% of the losses in high temperature fuel cell (molten carbonate fuel cell and solid oxide fuel cell) systems can be caused by heat transfer. Therefore system optimization must focus on reducing the need for heat transfer as well as improving the conditions for the unavoidable heat transfer. Various options for reducing the need for heat transfer are discussed in this paper. High temperature fuel cells, eventually integrated into gas turbine processes, can replace the combustion process in future power plants. High temperature fuel cells will be necessary to obtain conversion efficiencies up to 80% in the case of large scale electricity production in the future. The introduction of fuel cells is considered to be a first step in the integration of electrochemical conversion in future energy conversion systems.

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Copyright © 2006 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Exergy flow diagram (Grassmann diagram) of a 600MW conventional steam turbine plant

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Figure 2

Exergy flow diagram (Grassmann diagram) of a 379MW combined cycle plant

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Figure 3

Exergy losses in a molten carbonate fuel cell system with external reforming

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Figure 4

Exergy losses in a solid oxide fuel cell system with external reforming

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Figure 5

Causes of exergy losses in a molten carbonate fuel cell system with external reforming

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Figure 6

Exergy losses in a micro CHP system based on solid polymer fuel cells (PEMFCs) and ATR

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Figure 7

Pressurized MCFC-CHP system with external reformer

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Figure 8

Atmospheric MCFC-CHP plant with internal reforming

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Figure 9

Cell voltages in the case of single-stage (left) and two-stage (right) operation

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Figure 10

Multistage oxidation in a single stack

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Figure 11

Some options for a series connection of stacks

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Figure 12

SOFC subsystem at atmospheric pressure with external reforming

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Figure 13

SOFC subsystem with internal reforming and off-gas recycle

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Figure 14

Cell voltage and net system efficiency versus the equivalent cell resistance

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